note: project gutenberg also has an html version of this file which includes the original illustrations. see -h.htm or -h.zip: (http://www.gutenberg.net/dirs/ / / / / / -h/ -h.htm) or (http://www.gutenberg.net/dirs/ / / / / / -h.zip) american addresses, with a lecture on the study of biology by thomas h. huxley. london: macmillan and co. london: r. clay, sons, and taylor, printers, bread street hill, queen victoria street. "naturæ leges et regulæ, secundum quas omnia fiunt et ex unis formis in alias mutantur, sunt ubique et semper eadem." b. de spinoza, _ethices_, pars tertia, præfatio. contents. i. three lectures on evolution (new york, september , , , ). lecture i. the three hypotheses respecting the history of nature lecture ii. the hypothesis of evolution. the neutral and the favourable evidence lecture iii. the demonstrative evidence of evolution ii. an address on the occasion of the opening of the johns hopkins university (baltimore, september , ) iii. a lecture on the study of biology, in connection with the loan collection of scientific apparatus (south kensington museum, december , ) new york. lectures on evolution. lecture i. the three hypotheses respecting the history of nature. we live in and form part of a system of things of immense diversity and perplexity, which we call nature; and it is a matter of the deepest interest to all of us that we should form just conceptions of the constitution of that system and of its past history. with relation to this universe, man is, in extent, little more than a mathematical point; in duration but a fleeting shadow; he is a mere reed shaken in the winds of force. but, as pascal long ago remarked, although a mere reed, he is a thinking reed; and in virtue of that wonderful capacity of thought, he has the power of framing for himself a symbolic conception of the universe, which, although doubtless highly imperfect and inadequate as a picture of the great whole, is yet sufficient to serve him as a chart for the guidance of his practical affairs. it has taken long ages of toilsome and often fruitless labour to enable man to look steadily at the shifting scenes of the phantasmagoria of nature, to notice what is fixed among her fluctuations, and what is regular among her apparent irregularities; and it is only comparatively lately, within the last few centuries, that the conception of a universal order and of a definite course of things, which we term the course of nature, has emerged. but, once originated, the conception of the constancy of the order of nature has become the dominant idea of modern thought. to any person who is familiar with the facts upon which that conception is based, and is competent to estimate their significance, it has ceased to be conceivable that chance should have any place in the universe, or that events should depend upon any but the natural sequence of cause and effect. we have come to look upon the present as the child of the past and as the parent of the future; and, as we have excluded chance from a place in the universe, so we ignore, even as a possibility, the notion of any interference with the order of nature. whatever may be men's speculative doctrines, it is quite certain, that every intelligent person guides his life and risks his fortune upon the belief that the order of nature is constant, and that the chain of natural causation is never broken. in fact, no belief which we entertain has so complete a logical basis as that to which i have just referred. it tacitly underlies every process of reasoning; it is the foundation of every act of the will. it is based upon the broadest induction, and it is verified by the most constant, regular, and universal of deductive processes. but we must recollect that any human belief, however broad its basis, however defensible it may seem, is, after all, only a probable belief, and that our widest and safest generalizations are simply statements of the highest degree of probability. though we are quite clear about the constancy of the order of nature, at the present time, and in the present state of things, it by no means necessarily follows that we are justified in expanding this generalisation into the infinite past, and in denying, absolutely, that there may have been a time when nature did not follow a fixed order, when the relations of cause and effect were not definite, and when extra-natural agencies interfered with the general course of nature. cautious men will allow that a universe so different from that which we know may have existed; just as a very candid thinker may admit that a world in which two and two do not make four, and in which two straight lines do inclose a space, may exist. but the same caution which forces the admission of such possibilities demands a great deal of evidence before it recognises them to be anything more substantial. and when it is asserted that, so many thousand years ago, events occurred in a manner utterly foreign to and inconsistent with the existing laws of nature, men, who without being particularly cautious, are simply honest thinkers, unwilling to deceive themselves or delude others, ask for trustworthy evidence of the fact. did things so happen or did they not? this is a historical question, and one the answer to which must be sought in the same way as the solution of any other historical problem. * * * * * so far as i know, there are only three hypotheses which ever have been entertained, or which well can be entertained, respecting the past history of nature. i will, in the first place, state the hypotheses, and then i will consider what evidence bearing upon them is in our possession, and by what light of criticism that evidence is to be interpreted. upon the first hypothesis, the assumption is, that phenomena of nature similar to those exhibited by the present world have always existed; in other words, that the universe has existed from all eternity in what may be broadly termed its present condition. the second hypothesis is, that the present state of things has had only a limited duration; and that, at some period in the past, a condition of the world, essentially similar to that which we now know, came into existence, without any precedent condition from which it could have naturally proceeded. the assumption that successive states of nature have arisen, each without any relation of natural causation to an antecedent state, is a mere modification of this second hypothesis. the third hypothesis also assumes that the present state of things has had but a limited duration; but it supposes that this state has been evolved by a natural process from an antecedent state, and that from another, and so on; and, on this hypothesis, the attempt to assign any limit to the series of past changes is, usually, given up. it is so needful to form clear and distinct notions of what is really meant by each of these hypotheses that i will ask you to imagine what, according to each, would have been visible to a spectator of the events which constitute the history of the earth. on the first hypothesis, however far back in time that spectator might be placed, he would see a world essentially, though perhaps not in all its details, similar to that which now exists. the animals which existed would be the ancestors of those which now live, and similar to them; the plants, in like manner, would be such as we know; and the mountains, plains, and waters would foreshadow the salient features of our present land and water. this view was held more or less distinctly, sometimes combined with the notion of recurrent cycles of change, in ancient times; and its influence has been felt down to the present day. it is worthy of remark that it is a hypothesis which is not inconsistent with the doctrine of uniformitarianism, with which geologists are familiar. that doctrine was held by hutton, and in his earlier days by lyell. hutton was struck by the demonstration of astronomers that the perturbations of the planetary bodies, however great they may be, yet sooner or later right themselves; and that the solar system possesses a self-adjusting power by which these aberrations are all brought back to a mean condition. hutton imagined that the like might be true of terrestrial changes; although no one recognised more clearly than he the fact that the dry land is being constantly washed down by rain and rivers and deposited in the sea; and that thus, in a longer or shorter time, the inequalities of the earth's surface must be levelled, and its high lands brought down to the ocean. but, taking into account the internal forces of the earth, which, upheaving the sea-bottom give rise to new land, he thought that these operations of degradation and elevation might compensate each other; and that thus, for any assignable time, the general features of our planet might remain what they are. and inasmuch as, under these circumstances, there need be no limit to the propagation of animals and plants, it is clear that the consistent working-out of the uniformitarian idea might lead to the conception of the eternity of the world. not that i mean to say that either hutton or lyell held this conception--assuredly not; they would have been the first to repudiate it. nevertheless, the logical development of their arguments tends directly towards this hypothesis. the second hypothesis supposes that the present order of things, at some no very remote time, had a sudden origin, and that the world, such as it now is, had chaos for its phenomenal antecedent. that is the doctrine which you will find stated most fully and clearly in the immortal poem of john milton--the english _divina commedia--paradise lost_. i believe it is largely to the influence of that remarkable work, combined with the daily teachings to which we have all listened in our childhood, that this hypothesis owes its general wide diffusion as one of the current beliefs of english-speaking people. if you turn to the seventh book of _paradise lost_, you will find there stated the hypothesis to which i refer, which is briefly this: that this visible universe of ours came into existence at no great distance of time from the present; and that the parts of which it is composed made their appearance, in a certain definite order, in the space of six natural days, in such a manner that, on the first of these days, light appeared; that, on the second, the firmament, or sky, separated the waters above, from the waters beneath the firmament; that, on the third day, the waters drew away from the dry land, and upon it a varied vegetable life, similar to that which now exists, made its appearance; that the fourth day was signalised by the apparition of the sun, the stars, the moon, and the planets; that, on the fifth day, aquatic animals originated within the waters; that, on the sixth day, the earth gave rise to our four-footed terrestrial creatures, and to all varieties of terrestrial animals except birds, which had appeared on the preceding day; and, finally, that man appeared upon the earth, and the emergence of the universe from chaos was finished. milton tells us, without the least ambiguity, what a spectator of these marvellous occurrences would have witnessed. i doubt not that his poem is familiar to all of you, but i should like to recall one passage to your minds, in order that i may be justified in what i have said regarding the perfectly concrete, definite picture of the origin of the animal world which milton draws. he says:-- "the sixth, and of creation last, arose with evening harps and matin, when god said, 'let the earth bring forth soul living in her kind, cattle and creeping things, and beast of the earth, each in their kind!' the earth obeyed, and, straight opening her fertile womb, teemed at a birth innumerous living creatures, perfect forms, limbed and full-grown. out of the ground uprose, as from his lair, the wild beast, where he wons in forest wild, in thicket, brake, or den; among the trees in pairs they rose, they walked; the cattle in the fields and meadows green; those rare and solitary; these in flocks pasturing at once, and in broad herds upsprung. the grassy clods now calved; now half appears the tawny lion, pawing to get free his hinder parts--then springs, as broke from bonds, and rampant shakes his brinded mane; the ounce, the libbard, and the tiger, as the mole rising, the crumbled earth above them threw in hillocks; the swift stag from underground bore up his branching head; scarce from his mould behemoth, biggest born of earth, upheaved his vastness; fleeced the flocks and bleating rose as plants; ambiguous between sea and land, the river-horse and scaly crocodile. at once came forth whatever creeps the ground, insect or worm." there is no doubt as to the meaning of this statement, nor as to what a man of milton's genius expected would have been actually visible to an eye-witness of this mode of origination of living things. the third hypothesis, or the hypothesis of evolution, supposes that, at any comparatively late period of past time, our imaginary spectator would meet with a state of things very similar to that which now obtains; but that the likeness of the past to the present would gradually become less and less, in proportion to the remoteness of his period of observation from the present day; that the existing distribution of mountains and plains, of rivers and seas, would show itself to be the product of a slow process of natural change operating upon more and more widely different antecedent conditions of the mineral framework of the earth; until, at length, in place of that framework, he would behold only a vast nebulous mass, representing the constituents of the sun and of the planetary bodies. preceding the forms of life which now exist, our observer would see animals and plants not identical with them, but like them; increasing their differences with their antiquity and, at the same time, becoming simpler and simpler; until, finally, the world of life would present nothing but that undifferentiated protoplasmic matter which, so far as our present knowledge goes, is the common foundation of all vital activity. the hypothesis of evolution supposes that in all this vast progression there would be no breach of continuity, no point at which we could say "this a natural process," and "this is not a natural process;" but that the whole might be compared to that wonderful process of development which may be seen going on every day under our eyes, in virtue of which there arises, out of the semi-fluid, comparatively homogeneous substance which we call an egg, the complicated organization of one of the higher animals. that, in a few words, is what is meant by the hypothesis of evolution. * * * * * i have already suggested that in dealing with these three hypotheses, in endeavouring to form a judgment as to which of them is the more worthy of belief, or whether none is worthy of belief--in which case our condition of mind should be that suspension of judgment which is so difficult to all but trained intellects--we should be indifferent to all _à priori_ considerations. the question is a question of historical fact. the universe has come into existence somehow or other, and the problem is, whether it came into existence in one fashion, or whether it came into existence in another; and, as an essential preliminary to further discussion, permit me to say two or three words as to the nature and the kinds of historical evidence. the evidence as to the occurrence of any event in past time may be ranged under two heads which, for convenience' sake, i will speak of as testimonial evidence and as circumstantial evidence. by testimonial evidence i mean human testimony; and by circumstantial evidence i mean evidence which is not human testimony. let me illustrate by a familiar example what i understand by these two kinds of evidence, and what is to be said respecting their value. suppose that a man tells you that he saw a person strike another and kill him; that is testimonial evidence of the fact of murder. but it is possible to have circumstantial evidence of the fact of murder; that is to say, you may find a man dying with a wound upon his head having exactly the form and character of the wound which is made by an axe, and, with due care in taking surrounding circumstances into account, you may conclude with the utmost certainty that the man has been murdered; that his death is the consequence of a blow inflicted by another man with that implement. we are very much in the habit of considering circumstantial evidence as of less value than testimonial evidence, and it may be that, where the circumstances are not perfectly clear and intelligible, it is a dangerous and unsafe kind of evidence; but it must not be forgotten that, in many cases, circumstantial is quite as conclusive as testimonial evidence, and that, not unfrequently, it is a great deal weightier than testimonial evidence. for example, take the case to which i referred just now. the circumstantial evidence may be better and more convincing than the testimonial evidence; for it may be impossible, under the conditions that i have defined, to suppose that the man met his death from any cause but the violent blow of an axe wielded by another man. the circumstantial evidence in favour of a murder having been committed, in that case, is as complete and as convincing as evidence can be. it is evidence which is open to no doubt and to no falsification. but the testimony of a witness is open to multitudinous doubts. he may have been mistaken. he may have been actuated by malice. it has constantly happened that even an accurate man has declared that a thing has happened in this, that, or the other way, when a careful analysis of the circumstantial evidence has shown that it did not happen in that way, but in some other way. we may now consider the evidence in favour of or against the three hypotheses. let me first direct your attention to what is to be said about the hypothesis of the eternity of the state of things in which we now live. what will first strike you is, that it is a hypothesis which, whether true or false, is not capable of verification by any evidence. for, in order to obtain either circumstantial or testimonial evidence sufficient to prove the eternity of duration of the present state of nature, you must have an eternity of witnesses or an infinity of circumstances, and neither of these is attainable. it is utterly impossible that such evidence should be carried beyond a certain point of time; and all that could be said, at most, would be, that so far as the evidence could be traced, there was nothing to contradict the hypothesis. but when you look, not to the testimonial evidence--which, considering the relative insignificance of the antiquity of human records, might not be good for much in this case--but to the circumstantial evidence, then you find that this hypothesis is absolutely incompatible with such evidence as we have; which is of so plain and so simple a character that it is impossible in any way to escape from the conclusions which it forces upon us. you are, doubtless, all aware that the outer substance of the earth, which alone is accessible to direct observation, is not of a homogeneous character, but that it is made up of a number of layers or strata, the titles of the principal groups of which are placed upon the accompanying diagram. each of these groups represents a number of beds of sand, of stone, of clay, of slate, and of various other materials. [illustration: fig. .--ideal section of the crust of the earth.] on careful examination, it is found that the materials of which each of these layers of more or less hard rock are composed are, for the most part, of the same nature as those which are at present being formed under known conditions on the surface of the earth. for example, the chalk, which constitutes a great part of the cretaceous formation in some parts of the world, is practically identical in its physical and chemical characters with a substance which is now being formed at the bottom of the atlantic ocean, and covers an enormous area; other beds of rock are comparable with the sands which are being formed upon sea-shores, packed together, and so on. thus, omitting rocks of igneous origin, it is demonstrable that all these beds of stone, of which a total of not less than seventy thousand feet is known, have been formed by natural agencies, either out of the waste and washing of the dry land, or else by the accumulation of the exuviæ of plants and animals. many of these strata are full of such exuviæ--the so-called "fossils." remains of thousands of species of animals and plants, as perfectly recognisable as those of existing forms of life which you meet with in museums, or as the shells which you pick up upon the sea-beech, have been imbedded in the ancient sands, or muds, or limestones, just as they are being imbedded now, in sandy, or clayey, or calcareous subaqueous deposits. they furnish us with a record, the general nature of which cannot be misinterpreted, of the kinds of things that have lived upon the surface of the earth during the time that is registered by this great thickness of stratified rocks. but even a superficial study of these fossils shows us that the animals and plants which live at the present time have had only a temporary duration; for the remains of such modern forms of life are met with, for the most part, only in the uppermost or latest tertiaries, and their number rapidly diminishes in the lower deposits of that epoch. in the older tertiaries, the places of existing animals and plants are taken by other forms, as numerous and diversified as those which live now in the same localities, but more or less different from them; in the mesozoic rocks, these are replaced by others yet more divergent from modern types; and in the palæozoic formations the contrast is still more marked. thus the circumstantial evidence absolutely negatives the conception of the eternity of the present condition of things. we can say with certainty that the present condition of things has existed for a comparatively short period; and that, so far as animal and vegetable nature are concerned, it has been preceded by a different condition. we can pursue this evidence until we reach the lowest of the stratified rocks, in which we lose the indications of life altogether. the hypothesis of the eternity of the present state of nature may therefore be put out of court. we now come to what i will term milton's hypothesis--the hypothesis that the present condition of things has endured for a comparatively short time; and, at the commencement of that time, came into existence within the course of six days. i doubt not that it may have excited some surprise in your minds that i should have spoken of this as milton's hypothesis, rather than that i should have chosen the terms which are more customary, such as "the doctrine of creation," or "the biblical doctrine," or "the doctrine of moses," all of which denominations, as applied to the hypothesis to which i have just referred, are certainly much more familiar to you than the title of the miltonic hypothesis. but i have had what i cannot but think are very weighty reasons for taking the course which i have pursued. in the first place, i have discarded the title of the "doctrine of creation," because my present business is not with the question why the objects which constitute nature came into existence, but when they came into existence, and in what order. this is as strictly a historical question as the question when the angles and the jutes invaded england, and whether they preceded or followed the romans. but the question about creation is a philosophical problem, and one which cannot be solved, or even approached, by the historical method. what we want to learn is, whether the facts, so far as they are known, afford evidence that things arose in the way described by milton, or whether they do not; and, when that question is settled, it will be time enough to inquire into the causes of their origination. in the second place, i have not spoken of this doctrine as the biblical doctrine. it is quite true that persons as diverse in their general views as milton the protestant and the celebrated jesuit father suarez, each put upon the first chapter of genesis the interpretation embodied in milton's poem. it is quite true that this interpretation is that which has been instilled into every one of us in our childhood; but i do not for one moment venture to say that it can properly be called the biblical doctrine. it is not my business, and does not lie within my competency, to say what the hebrew text does, and what it does not signify; moreover, were i to affirm that this is the biblical doctrine, i should be met by the authority of many eminent scholars, to say nothing of men of science, who, at various times, have absolutely denied that any such doctrine is to be found in genesis. if we are to listen to many expositors of no mean authority, we must believe that what seems so clearly defined in genesis--as if very great pains had been taken that there should be no possibility of mistake--is not the meaning of the text at all. the account is divided into periods that we may make just as long or as short as convenience requires. we are also to understand that it is consistent with the original text to believe that the most complex plants and animals may have been evolved by natural processes, lasting for millions of years, out of structureless rudiments. a person who is not a hebrew scholar can only stand aside and admire the marvellous flexibility of a language which admits of such diverse interpretations. but assuredly, in the face of such contradictions of authority upon matters respecting which he is incompetent to form any judgment, he will abstain, as i do, from giving any opinion. in the third place, i have carefully abstained from speaking of this as the mosaic doctrine, because we are now assured upon the authority of the highest critics, and even of dignitaries of the church, that there is no evidence that moses wrote the book of genesis, or knew anything about it. you will understand that i give no judgment--it would be an impertinence upon my part to volunteer even a suggestion--upon such a subject. but, that being the state of opinion among the scholars and the clergy, it is well for the unlearned in hebrew lore, and for the laity, to avoid entangling themselves in such a vexed question. happily, milton leaves us no excuse for doubting what he means, and i shall therefore be safe in speaking of the opinion in question as the miltonic hypothesis. now we have to test that hypothesis. for my part, i have no prejudice one way or the other. if there is evidence in favour of this view, i am burdened by no theoretical difficulties in the way of accepting it; but there must be evidence. scientific men get an awkward habit--no, i won't call it that, for it is a valuable habit--of believing nothing unless there is evidence for it; and they have a way of looking upon belief which is not based upon evidence, not only as illogical, but as immoral. we will, if you please, test this view by the circumstantial evidence alone; for, from what i have said, you will understand that i do not propose to discuss the question of what testimonial evidence is to be adduced in favour of it. if those whose business it is to judge are not at one as to the authenticity of the only evidence of that kind which is offered, nor as to the facts to which it bears witness, the discussion of such evidence is superfluous. but i may be permitted to regret this necessity of rejecting the testimonial evidence the less, because the examination of the circumstantial evidence leads to the conclusion, not only that it is incompetent to justify the hypothesis, but that, so far as it goes, it is contrary to the hypothesis. the considerations upon which i base this conclusion are of the simplest possible character. the miltonic hypothesis contains assertions of a very definite character relating to the succession of living forms. it is stated that plants, for example, made their appearance upon the third day, and not before. and you will understand that what the poet means by plants are such plants as now live, the ancestors, in the ordinary way of propagation of like by like, of the trees and shrubs which flourish in the present world. it must needs be so; for, if they were different, either the existing plants have been the result of a separate origination since that described by milton, of which we have no record, nor any ground for supposition that such an occurrence has taken place; or else they have arisen by a process of evolution from the original stocks. in the second place, it is clear that there was no animal life before the fifth day, and that, on the fifth day, aquatic animals and birds appeared. and it is further clear that terrestrial living things, other than birds, made their appearance upon the sixth day, and not before. hence, it follows that, if, in the large mass of circumstantial evidence as to what really has happened in the past history of the globe we find indications of the existence of terrestrial animals, other than birds, at a certain period, it is perfectly certain that all that has taken place since that time must be referred to the sixth day. in the great carboniferous formation, whence america derives so vast a proportion of her actual and potential wealth, in the beds of coal which have been formed from the vegetation of that period, we find abundant evidence of the existence of terrestrial animals. they have been described, not only by european but by your own naturalists. there are to be found numerous insects allied to our cockroaches. there are to be found spiders and scorpions of large size, the latter so similar to existing scorpions that it requires the practised eye of the naturalist to distinguish them. inasmuch as these animals can be proved to have been alive in the carboniferous epoch, it is perfectly clear that, if the miltonic account is to be accepted, the huge mass of rocks extending from the middle of the palæozoic formations to the uppermost members of the series, must belong to the day which is termed by milton the sixth. but, further, it is expressly stated that aquatic animals took their origin upon the fifth day, and not before; hence, all formations in which remains of aquatic animals can be proved to exist, and which therefore testify that such animals lived at the time when these formations were in course of deposition, must have been deposited during or since the period which milton speaks of as the fifth day. but there is absolutely no fossiliferous formation in which the remains of aquatic animals are absent. the oldest fossils in the silurian rocks are exuviæ of marine animals; and if the view which is entertained by principal dawson and dr. carpenter respecting the nature of the _eozoön_ be well founded, aquatic animals existed at a period as far antecedent to the deposition of the coal as the coal is from us; inasmuch as the _eozoön_ is met with in those laurentian strata which lie at the bottom of the series of stratified rocks. hence it follows, plainly enough, that the whole series of stratified rocks, if they are to be brought into harmony with milton, must be referred to the fifth and sixth days, and that we cannot hope to find the slightest trace of the products of the earlier days in the geological record. when we consider these simple facts, we see how absolutely futile are the attempts that have been made to draw a parallel between the story told by so much of the crust of the earth as is known to us and the story which milton tells. the whole series of fossiliferous stratified rocks must be referred to the last two days; and neither the carboniferous, nor any other, formation can afford evidence of the work of the third day. not only is there this objection to any attempt to establish a harmony between the miltonic account and the facts recorded in the fossiliferous rocks, but there is a further difficulty. according to the miltonic account, the order in which animals should have made their appearance in the stratified rocks would be this: fishes, including the great whales, and birds; after them, all varieties of terrestrial animals except birds. nothing could be further from the facts as we find them; we know of not the slightest evidence of the existence of birds before the jurassic, or perhaps the triassic, formation; while terrestrial animals, as we have just seen, occur in the carboniferous rocks. if there were any harmony between the miltonic account and the circumstantial evidence, we ought to have abundant evidence of the existence of birds in the carboniferous, the devonian, and the silurian rocks. i need hardly say that this is not the case, and that not a trace of birds makes its appearance until the far later period which i have mentioned. and again, if it be true that all varieties of fishes and the great whales, and the like, made their appearance on the fifth day, we ought to find the remains of these animals in the older rocks--in those which were deposited before the carboniferous epoch. fishes we do find, in considerable number and variety; but the great whales are absent, and the fishes are not such as now live. not one solitary species of fish now in existence is to be found in the devonian or silurian formations. hence we are introduced afresh to the dilemma which i have already placed before you: either the animals which came into existence on the fifth day were not such as those which are found at present, are not the direct and immediate ancestors of those which now exist; in which case either fresh creations of which nothing is said; or a process of evolution must have occurred; or else the whole story must be given up, as not only devoid of any circumstantial evidence, but contrary to such evidence as exists. i placed before you in a few words, some little time ago, a statement of the sum and substance of milton's hypothesis. let me now try to state as briefly, the effect of the circumstantial evidence bearing upon the past history of the earth which is furnished, without the possibility of mistake, with no chance of error as to its chief features, by the stratified rocks. what we find is, that the great series of formations represents a period of time of which our human chronologies hardly afford us a unit of measure. i will not pretend to say how we ought to estimate this time, in millions or in billions of years. for my purpose, the determination of its absolute duration is wholly unessential. but that the time was enormous there can be no question. it results from the simplest methods of interpretation, that leaving out of view certain patches of metamorphosed rocks, and certain volcanic products, all that is now dry land has once been at the bottom of the waters. it is perfectly certain that, at a comparatively recent period of the world's history--the cretaceous epoch--none of the great physical features which at present mark the surface of the globe existed. it is certain that the rocky mountains were not. it is certain that the himalaya mountains were not. it is certain that the alps and the pyrenees had no existence. the evidence is of the plainest possible character, and is simply this:--we find raised up on the flanks of these mountains, elevated by the forces of upheaval which have given rise to them, masses of cretaceous rock which formed the bottom of the sea before those mountains existed. it is therefore clear that the elevatory forces which gave rise to the mountains operated subsequently to the cretaceous epoch; and that the mountains themselves are largely made up of the materials deposited in the sea which once occupied their place. as we go back in time, we meet with constant alternations of sea and land, of estuary and open ocean; and, in correspondence with these alternations, we observe the changes in the fauna and flora to which i have referred. but the inspection of these changes give us no right to believe that there has been any discontinuity in natural processes. there is no trace of general cataclysms, of universal deluges, or sudden destructions of a whole fauna or flora. the appearances which were formerly interpreted in that way have all been shown to be delusive, as our knowledge has increased and as the blanks which formerly appeared to exist between the different formations have been filled up. that there is no absolute break between formation and formation, that there has been no sudden disappearance of all the forms of life and replacement of them by others, but that changes have gone on slowly and gradually, that one type has died out and another has taken its place, and that thus, by insensible degrees, one fauna has been replaced by another, are conclusions strengthened by constantly increasing evidence. so that within the whole of the immense period indicated by the fossiliferous stratified rocks, there is assuredly not the slightest proof of any break in the uniformity of nature's operations, no indication that events have followed other than a clear and orderly sequence. that, i say, is the natural and obvious teaching of the circumstantial evidence contained in the stratified rocks. i leave you to consider how far, by any ingenuity of interpretation, by any stretching of the meaning of language, it can be brought into harmony with the miltonic hypothesis. there remains the third hypothesis, that of which i have spoken as the hypothesis of evolution; and i purpose that, in lectures to come, we should discuss it as carefully as we have considered the other two hypotheses. i need not say that it is quite hopeless to look for testimonial evidence of evolution. the very nature of the case precludes the possibility of such evidence, for the human race can no more be expected to testify to its own origin, than a child can be tendered as a witness of its own birth. our sole inquiry is, what foundation circumstantial evidence lends to the hypothesis, or whether it lends none, or whether it controverts the hypothesis. i shall deal with the matter entirely as a question of history. i shall not indulge in the discussion of any speculative probabilities. i shall not attempt to show that nature is unintelligible unless we adopt some such hypothesis. for anything i know about the matter, it may be the way of nature to be unintelligible; she is often puzzling, and i have no reason to suppose that she is bound to fit herself to our notions. i shall place before you three kinds of evidence entirely based upon what is known of the forms of animal life which are contained in the series of stratified rocks. i shall endeavour to show you that there is one kind of evidence which is neutral, which neither helps evolution nor is inconsistent with it. i shall then bring forward a second kind of evidence which indicates a strong probability in favour of evolution, but does not prove it; and, lastly, i shall adduce a third kind of evidence which, being as complete as any evidence which we can hope to obtain upon such a subject, and being wholly and strikingly in favour of evolution, may fairly be called demonstrative evidence of its occurrence. lecture ii. the hypothesis of evolution. the neutral and the favourable evidence. in the preceding lecture i pointed out that there are three hypotheses which may be entertained, and which have been entertained, respecting the past history of life upon the globe. according to the first of these hypotheses, living beings, such as now exist, have existed from all eternity upon this earth. we tested that hypothesis by the circumstantial evidence, as i called it, which is furnished by the fossil remains contained in the earth's crust, and we found that it was obviously untenable. i then proceeded to consider the second hypothesis, which i termed the miltonic hypothesis, not because it is of any particular consequence to me whether john milton seriously entertained it or not, but because it is stated in a clear and unmistakable manner in his great poem. i pointed out to you that the evidence at our command as completely and fully negatives that hypothesis as it did the preceding one. and i confess that i had too much respect for your intelligence to think it necessary to add that the negation was equally clear and equally valid, whatever the source from which that hypothesis might be derived, or whatever the authority by which it might be supported. i further stated that, according to the third hypothesis, or that of evolution, the existing state of things is the last term of a long series of states, which, when traced back, would be found to show no interruption and no breach in the continuity of natural causation. i propose, in the present, and the following lecture, to test this hypothesis rigorously by the evidence at command, and to inquire how far that evidence can be said to be indifferent to it, how far it can be said to be favourable to it, and, finally, how far it can be said to be demonstrative. from almost the origin of the discussions about the existing condition of the animal and vegetable worlds and the causes which have determined that condition, an argument has been put forward as an objection to evolution, which we shall have to consider very seriously. it is an argument which was first clearly stated by cuvier in his criticism of the doctrines propounded by his great contemporary, lamarck. the french expedition to egypt had called the attention of learned men to the wonderful store of antiquities in that country, and there had been brought back to france numerous mummified corpses of the animals which the ancient egyptians revered and preserved, and which, at a reasonable computation, must have lived not less than three or four thousand years before the time at which they were thus brought to light. cuvier endeavoured to test the hypothesis that animals have undergone gradual and progressive modifications of structure, by comparing the skeletons and such other parts of the mummies as were in a fitting state of preservation, with the corresponding parts of the representatives of the same species now living in egypt. he arrived at the conviction that no appreciable change had taken place in these animals in the course of this considerable lapse of time, and the justice of his conclusion is not disputed. it is obvious that, if it can be proved that animals have endured, without undergoing any demonstrable change of structure, for so long a period as four thousand years, no form of the hypothesis of evolution which assumes that animals undergo a constant and necessary progressive change can be tenable; unless, indeed, it be further assumed that four thousand years is too short a time for the production of a change sufficiently great to be detected. but it is no less plain that if the process of evolution of animals is not independent of surrounding conditions; if it may be indefinitely hastened or retarded by variations in these conditions; or if evolution is simply a process of accommodation to varying conditions; the argument against the hypothesis of evolution based on the unchanged character of the egyptian fauna is worthless. for the monuments which are coeval with the mummies testify as strongly to the absence of change in the physical geography and the general conditions of the land of egypt, for the time in question, as the mummies do to the unvarying characters of its living population. the progress of research since cuvier's time has supplied far more striking examples of the long duration of specific forms of life than those which are furnished by the mummified ibises and crocodiles of egypt. a remarkable case is to be found in your own country, in the neighbourhood of the falls of niagara. in the immediate vicinity of the whirlpool, and again upon goat island, in the superficial deposits which cover the surface of the rocky subsoil in those regions, there are found remains of animals in perfect preservation, and among them, shells belonging to exactly the same species as those which at present inhabit the still waters of lake erie. it is evident, from the structure of the country, that these animal remains were deposited in the beds in which they occur at a time when the lake extended over the region in which they are found. this involves the conclusion that they lived and died before the falls had cut their way back through the gorge of niagara; and, indeed, it has been determined that, when these animals lived, the falls of niagara must have been at least six miles further down the river than they are at present. many computations have been made of the rate at which the falls are thus cutting their way back. those computations have varied greatly, but i believe i am speaking within the bounds of prudence, if i assume that the falls of niagara have not retreated at a greater pace than about a foot a year. six miles, speaking roughly, are , feet; , feet, at a foot a year, gives , years; and thus we are fairly justified in concluding that no less a period than this has passed since the shell-fish, whose remains are left in the beds to which i have referred, were living creatures. but there is still stronger evidence of the long duration of certain types. i have already stated that, as we work our way through the great series of the tertiary formations, we find many species of animals identical with those which live at the present day, diminishing in numbers, it is true, but still existing, in a certain proportion, in the oldest of the tertiary rocks. furthermore, when we examine the rocks of the cretaceous epoch, we find the remains of some animals which the closest scrutiny cannot show to be, in any important respect, different from those which live at the present time. that is the case with one of the cretaceous lamp-shells (_terebratula_), which has continued to exist unchanged, or with insignificant variations, down to the present day. such is the case with the _globigerinæ_, the skeletons of which, aggregated together, form a large proportion of our english chalk. those _globigerinæ_ can be traced down to the _globigerinæ_ which live at the surface of the present great oceans, and the remains of which, falling to the bottom of the sea, give rise to a chalky mud. hence it must be admitted that certain existing species of animals show no distinct sign of modification, or transformation, in the course of a lapse of time as great as that which carries us back to the cretaceous period; and which, whatever its absolute measure, is certainly vastly greater than thirty thousand years. there are groups of species so closely allied together that it needs the eye of a naturalist to distinguish them one from another. if we disregard the small differences which separate these forms and consider all the species of such groups as modifications of one type, we shall find that, even among the higher animals, some types have had a marvellous duration. in the chalk, for example, there is found a fish belonging to the highest and the most differentiated group of osseous fishes, which goes by the name of _beryx_. the remains of that fish are among the most beautiful and well preserved of the fossils found in our english chalk. it can be studied anatomically, so far as the hard parts are concerned, almost as well as if it were a recent fish. but the genus _beryx_ is represented, at the present day, by very closely allied species which are living in the pacific and atlantic oceans. we may go still farther back. i have already referred to the fact that the carboniferous formations, in europe and in america, contain the remains of scorpions in an admirable state of preservation, and that those scorpions are hardly distinguishable from such as now live. i do not mean to say that they are not different, but close scrutiny is needed in order to distinguish them from modern scorpions. more than this. at the very bottom of the silurian series, in beds which are by some authorities referred to the cambrian formation, where the signs of life begin to fail us--even there, among the few and scanty animal remains which are discoverable, we find species of molluscous animals which are so closely allied to existing forms that, at one time, they were grouped under the same generic name. i refer to the well-known _lingula_ of the _lingula_ flags, lately, in consequence of some slight differences, placed in the new genus _lingulella_. practically, it belongs to the same great generic group as the _lingula_, which is to be found at the present day upon your own shores and those of many other parts of the world. the same truth is exemplified if we turn to certain great periods of the earth's history--as, for example, the mesozoic epoch. there are groups of reptiles, such as the _ichthyosauria_ and the _plesiosauria_, which appear shortly after the commencement of this epoch, and they occur in vast numbers. they disappear with the chalk and, throughout the whole of the great series of mesozoic rocks, they present no such modifications as can safely be considered evidence of progressive modification. facts of this kind are undoubtedly fatal to any form of the doctrine of evolution which postulates the supposition that there is an intrinsic necessity, on the part of animal forms which have once come into existence, to undergo continual modification; and they are as distinctly opposed to any view which involves the belief, that such modification as may occur, must take place, at the same rate, in all the different types of animal or vegetable life. the facts, as i have placed them before you, obviously directly contradict any form of the hypothesis of evolution which stands in need of these two postulates. but, one great service that has been rendered by mr. darwin to the doctrine of evolution in general is this: he has shown that there are two chief factors in the process of evolution: one of them is the tendency to vary, the existence of which in all living forms may be proved by observation; the other is the influence of surrounding conditions upon what i may call the parent form and the variations which are thus evolved from it. the cause of the production of variations is a matter not at all properly understood at present. whether variation depends upon some intricate machinery--if i may use the phrase--of the living organism itself, or whether it arises through the influence of conditions upon that form, is not certain, and the question may, for the present, be left open. but the important point is that, granting the existence of the tendency to the production of variations; then, whether the variations which are produced shall survive and supplant the parent, or whether the parent form shall survive and supplant the variations, is a matter which depends entirely on those conditions which give rise to the struggle for existence. if the surrounding conditions are such that the parent form is more competent to deal with them and flourish in them, than the derived forms, then, in the struggle for existence, the parent form will maintain itself and the derived forms will be exterminated. but if, on the contrary, the conditions are such as to be more favourable to a derived than to the parent form, the parent form will be extirpated and the derived form will take its place. in the first case, there will be no progression, no change of structure, through any imaginable series of ages; in the second place, there will be modification and change of form. thus the existence of these persistent types, as i have termed them, is no real obstacle in the way of the theory of evolution. take the case of the scorpions to which i have just referred. no doubt, since the carboniferous epoch, conditions have always obtained, such as existed when the scorpions of that epoch flourished; conditions in which scorpions find themselves better off, more competent to deal with the difficulties in their way, than any variation from the scorpion type which they may have produced; and, for that reason, the scorpion type has persisted, and has not been supplanted by any other form. and there is no reason, in the nature of things, why, as long as this world exists, if there be conditions more favourable to scorpions than to any variation which may arise from them, these forms of life should not persist. therefore, the stock objection to the hypothesis of evolution, based on the long duration of certain animal and vegetable types, is no objection at all. the facts of this character--and they are numerous--belong to that class of evidence which i have called indifferent. that is to say, they may afford no direct support to the doctrine of evolution, but they are capable of being interpreted in perfect consistency with it. there is another order of facts belonging to the class of negative or indifferent evidence. the great group of lizards, which abound in the present world, extends through the whole series of formations as far back as the permian, or latest palæozoic, epoch. these permian lizards differ astonishingly little from the lizards which exist at the present day. comparing the amount of the differences between them and modern lizards, with the prodigious lapse of time between the permian epoch and the present age, it may be said that the amount of change is insignificant. but, when we carry our researches farther back in time, we find no trace of lizards, nor of any true reptile whatever, in the whole mass of formations beneath the permian. now, it is perfectly clear that if our palæontological collections are to be taken, even approximately, as an adequate representation of all the forms of animals and plants that have ever lived; and if the record furnished by the known series of beds of stratified rock, covers the whole series of events which constitute the history of life on the globe, such a fact as this directly contravenes the hypothesis of evolution; because this hypothesis postulates that the existence of every form must have been preceded by that of some form little different from it. here, however, we have to take into consideration that important truth so well insisted upon by lyell and by darwin--the imperfection of the geological record. it can be demonstrated that the geological record must be incomplete, that it can only preserve remains found in certain favourable localities and under particular conditions; that it must be destroyed by processes of denudation, and obliterated by processes of metamorphosis. beds of rock of any thickness, crammed full of organic remains, may yet, either by the percolation of water through them, or by the influence of subterranean heat, lose all trace of these remains, and present the appearance of beds of rock formed under conditions in which living forms were absent. such metamorphic rocks occur in formations of all ages; and, in various cases, there are very good grounds for the belief that they have contained organic remains, and that those remains have been absolutely obliterated. i insist upon the defects of the geological record the more because those who have not attended to these matters are apt to say, "it is all very well, but when you get into a difficulty with your theory of evolution, you appeal to the incompleteness and the imperfection of the geological record;" and i want to make it perfectly clear to you that this imperfection is a great fact, which must be taken into account in all our speculations, or we shall constantly be going wrong. [illustration: fig. .--tracks of brontozoum.] you see the singular series of footmarks, drawn of its natural size in the large diagram hanging up here (fig. ), which i owe to the kindness of my friend professor marsh, with whom i had the opportunity recently of visiting the precise locality in massachusetts in which these tracks occur. i am, therefore, able to give you my own testimony, if needed, that the diagram accurately represents what we saw. the valley of the connecticut is classical ground for the geologist. it contains great beds of sandstone, covering many square miles, which have evidently formed a part of an ancient sea-shore, or, it may be, lake-shore. for a certain period of time after their deposition, these beds have remained sufficiently soft to receive the impressions of the feet of whatever animals walked over them, and to preserve them afterwards, in exactly the same way as such impressions are at this hour preserved on the shores of the bay of fundy and elsewhere. the diagram represents the track of some gigantic animal, which walked on its hind legs. you see the series of marks made alternately by the right and by the left foot; so that, from one impression to the other of the three-toed foot on the same side, is one stride, and that stride, as we measured it, is six feet nine inches. i leave you, therefore, to form an impression of the magnitude of the creature which, as it walked along the ancient shore, made these impressions. of such impressions there are untold thousands upon these sandstones. fifty or sixty different kinds have been discovered, and they cover vast areas. but, up to this present time, not a bone, not a fragment, of any one of the animals which left these great footmarks has been found; in fact, the only animal remains which have been met with in all these deposits, from the time of their discovery to the present day--though they have been carefully hunted over--is a fragmentary skeleton of one of the smaller forms. what has become of the bones of all these animals? you see we are not dealing with little creatures, but with animals that make a step of six feet nine inches; and their remains must have been left somewhere. the probability is, that they been dissolved away, and absolutely lost. i have had occasion to work out the nature of fossil remains, of which there was nothing left except casts of the bones, the solid material of the skeleton having been dissolved out by percolating water. it was a chance, in this case, that the sandstone happened to be of such a constitution as to set, and to allow the bones to be afterward dissolved out, leaving cavities of the exact shape of the bones. had that constitution been other than what it was, the bones would have been dissolved, the layers of sandstone would have fallen together into one mass, and not the slightest indication that the animal had existed would have been discoverable. i know of no more striking evidence than these facts afford, of the caution which should be used in drawing the conclusion, from the absence of organic remains in a deposit, that animals or plants did not exist at the time it was formed. i believe that, with a right understanding of the doctrine of evolution on the one hand, and a just estimation of the importance of the imperfection of the geological record on the other, all difficulty is removed from the kind of evidence to which i have adverted; and that we are justified in believing that all such cases are examples of what i have designated negative or indifferent evidence--that is to say, they in no way directly advance the hypothesis of evolution, but they are not to be regarded as obstacles in the way of our belief in that doctrine. i now pass on to the consideration of those cases which, for reasons which i will point out to you by and by, are not to be regarded as demonstrative of the truth of evolution, but which are such as must exist if evolution be true, and which therefore are, upon the whole, evidence in favour of the doctrine. if the doctrine of evolution be true, it follows, that, however diverse the different groups of animals and of plants may be, they must all, at one time or other, have been connected by gradational forms; so that, from the highest animals, whatever they may be, down to the lowest speck of protoplasmic matter in which life can be manifested, a series of gradations, leading from one end of the series to the other, either exists or has existed. undoubtedly that is a necessary postulate of the doctrine of evolution. but when we look upon living nature as it is, we find a totally different state of things. we find that animals and plants fall into groups, the different members of which are pretty closely allied together, but which are separated by definite, larger or smaller, breaks from other groups. in other words, no intermediate forms which bridge over these gaps or intervals are, at present, to be met with. to illustrate what i mean: let me call your attention to those vertebrate animals which are most familiar to you, such as mammals, birds, and reptiles. at the present day, these groups of animals are perfectly well defined from one another. we know of no animal now living which, in any sense, is intermediate between the mammal and the bird, or between the bird and the reptile; but, on the contrary, there are many very distinct anatomical peculiarities, well-defined marks, by which the mammal is separated from the bird, and the bird from the reptile. the distinctions are obvious and striking if you compare the definitions of these great groups as they now exist. the same may be said of many of the subordinate groups, or orders, into which these great classes are divided. at the present time, for example, there are numerous forms of non-ruminant pachyderms, or what we may call broadly, the pig tribe, and many varieties of ruminants. these latter have their definite characteristics, and the former have their distinguishing peculiarities. but there is nothing that fills up the gap between the ruminants and the pig tribe. the two are distinct. such also is the case in respect of the minor groups of the class of reptiles. the existing fauna shows us crocodiles, lizards, snakes, and tortoises; but no connecting link between the crocodile and lizard, nor between the lizard and snake, nor between the snake and the crocodile, nor between any two of these groups. they are separated by absolute breaks. if, then, it could be shown that this state of things had always existed, the fact would be fatal to the doctrine of evolution. if the intermediate gradations, which the doctrine of evolution requires to have existed between these groups, are not to be found anywhere in the records of the past history of the globe, their absence is a strong and weighty negative argument against evolution; while, on the other hand, if such intermediate forms are to be found, that is so much to the good of evolution; although, for reasons which i will lay before you by and by, we must be cautious in our estimate of the evidential cogency of facts of this kind. it is a very remarkable circumstance that, from the commencement of the serious study of fossil remains; in fact, from the time when cuvier began his brilliant researches upon those found in the quarries of montmartre, palæontology has shown what she was going to do in this matter, and what kind of evidence it lay in her power to produce. i said just now that, in the existing fauna, the group of pig-like animals and the group of ruminants are entirely distinct; but one of the first of cuvier's discoveries was an animal which he called the _anoplotherium_, and which proved to be, in a great many important respects, intermediate in character between the pigs, on the one hand, and the ruminants on the other. thus research into the history of the past did, to a certain extent, tend to fill up the breach between the group of ruminants and the group of pigs. another remarkable animal restored by the great french palæontologist, the _palæotherium_, similarly tended to connect together animals to all appearance so different as the rhinoceros, the horse, and the tapir. subsequent research has brought to light multitudes of facts of the same order; and, at the present day, the investigations of such anatomists as rütimeyer and gaudry have tended to fill up, more and more, the gaps in our existing series of mammals, and to connect groups formerly thought to be distinct. but i think it may have an especial interest if, instead of dealing with these examples, which would require a great deal of tedious osteological detail, i take the case of birds and reptiles; groups which, at the present day, are so clearly distinguished from one another that there are perhaps no classes of animals which, in popular apprehension, are more completely separated. existing birds, as you are aware, are covered with feathers; their anterior extremities, specially and peculiarly modified, are converted into wings, by the aid of which most of them are able to fly; they walk upright upon two legs; and these limbs, when they are considered anatomically, present a great number of exceedingly remarkable peculiarities, to which i may have occasion to advert incidentally as i go on, and which are not met with, even approximately, in any existing forms of reptiles. on the other hand, existing reptiles have no feathers. they may have naked skins, or be covered with horny scales, or bony plates, or with both. they possess no wings; they neither fly by means of their fore-limbs, nor habitually walk upright upon their hind-limbs; and the bones of their legs present no such modifications as we find in birds. it is impossible to imagine any two groups more definitely and distinctly separated, notwithstanding certain characters which they possess in common. as we trace the history of birds back in time, we find their remains, sometimes in great abundance, throughout the whole extent of the tertiary rocks; but, so far as our present knowledge goes, the birds of the tertiary rocks retain the same essential characters as the birds of the present day. in other words, the tertiary birds come within the definition of the class constituted by existing birds, and are as much separated from reptiles as existing birds are. not very long ago no remains of birds had been found below the tertiary rocks, and i am not sure but that some persons were prepared to demonstrate that they could not have existed at an earlier period. but in the course of the last few years, such remains have been discovered in england; though, unfortunately, in so imperfect and fragmentary a condition, that it is impossible to say whether they differed from existing birds in any essential character or not. in your country the development of the cretaceous series of rocks is enormous; the conditions under which the later cretaceous strata have been deposited are highly favourable to the preservation of organic remains; and the researches, full of labour and risk, which have been carried on by professor marsh in these cretaceous rocks of western america, have rewarded him with the discovery of forms of birds of which we had hitherto no conception. by his kindness, i am enabled to place before you a restoration of one of these extraordinary birds, every part of which can be thoroughly justified by the more or less complete skeletons, in a very perfect state of preservation, which he has discovered. this _hesperornis_ (fig. ), which measured between five and six feet in length, is astonishingly like our existing divers or grebes in a great many respects; so like them indeed that, had the skeleton of _hesperornis_ been found in a museum without its skull, it probably would have been placed in the same group of birds as the divers and grebes of the present day.[ ] [illustration: fig. .--hesperornis regalis (marsh).] but _hesperornis_ differs from all existing birds, and so far resembles reptiles, in one important particular--it is provided with teeth. the long jaws are armed with teeth which have curved crowns and thick roots (fig. ), and are not set in distinct sockets, but are lodged in a groove. in possessing true teeth, the _hesperornis_ differs from every existing bird, and from every bird yet discovered in the tertiary formations, the tooth-like serrations of the jaws in the _odontopteryx_ of the london clay being mere processes of the bony substance of the jaws, and not teeth in the proper sense of the word. in view of the characteristics of this bird we are therefore obliged to modify the definitions of the classes of birds and reptiles. before the discovery of _hesperornis_, the definition of the class aves based upon our knowledge of existing birds, might have been extended to all birds; it might have been said that the absence of teeth was characteristic of the class of birds; but the discovery of an animal which, in every part of its skeleton, closely agrees with existing birds, and yet possesses teeth, shows that there were ancient birds which, in respect of possessing teeth, approached reptiles more nearly than any existing bird does, and, to that extent, diminishes the _hiatus_ between the two classes. [illustration: fig. .--hesperornis regalis (marsh). (side and upper views of half the lower jaw; side and end views of a vertebra and a separate tooth.)] the same formation has yielded another bird _ichthyornis_ (fig. ), which also possesses teeth; but the teeth are situated in distinct sockets, while those of _hesperornis_ are not so lodged. the latter also has such very small, almost rudimentary, wings, that it must have been chiefly a swimmer and a diver, like a penguin; while _ichthyornis_ has strong wings and no doubt possessed corresponding powers of flight. _ichthyornis_ also differed in the fact that its vertebræ have not the peculiar characters of the vertebræ of existing and of all known tertiary birds, but were concave at each end. this discovery leads us to make a further modification in the definition of the group of birds, and to part with another of the characters by which almost all existing birds are distinguished from reptiles. [illustration: fig. .--ichthyornis dispar (marsh). (side and upper views of half the lower jaw; and side and end views of a vertebra.)] apart from the few fragmentary remains from the english greensand, to which i have referred, the mesozoic rocks, older than those in which _hesperornis_ and _ichthyornis_ have been discovered have afforded no certain evidence of birds, with the remarkable exception of the solenhofen slates. these so-called slates are composed of a fine grained calcareous mud which has hardened into lithographic stone, and in which organic remains are almost as well preserved as they would be if they had been imbedded in so much plaster of paris. they have yielded the _archæopteryx_, the existence of which was first made known by the finding of a fossil feather, or rather of the impression of one. it is wonderful enough that such a perishable thing as a feather, and nothing more, should be discovered; yet, for a long time, nothing was known of this bird except its feather. but, by and by a solitary skeleton was discovered, which is now in the british museum. the skull of this solitary specimen is unfortunately wanting, and it is therefore uncertain whether the _archæopteryx_ possessed teeth or not. but the remainder of the skeleton is so well preserved as to leave no doubt respecting the main features of the animal, which are very singular. the feet are not only altogether bird-like, but have the special characters of the feet of perching birds, while the body had a clothing of true feathers. nevertheless, in some other respects, _archæopteryx_ is unlike a bird and like a reptile. there is a long tail composed of many vertebræ. the structure of the wing differs in some very remarkable respects from that which it presents in a true bird. in the latter, the end of the wing answers to the thumb and two fingers of my hand; but the metacarpal bones, or those which answer to the bones of the fingers which lie in the palm of the hand, are fused together into one mass; and the whole apparatus, except the last joints of the thumb, is bound up in a sheath of integument, while the edge of the hand carries the principal quill-feathers. in the _archæopteryx_, the upper-arm bone is like that of a bird; and the two bones of the fore-arm are more or less like those of a bird, but the fingers are not bound together--they are free. what their number may have been is uncertain; but several, if not all, of them were terminated by strong curved claws, not like such as are sometimes found in birds, but such as reptiles possess; so that, in the _archæopteryx_, we have an animal which, to a certain extent, occupies a midway place between a bird and a reptile. it is a bird so far as its foot and sundry other parts of its skeleton are concerned; it is essentially and thoroughly a bird by its feathers; but it is much more properly a reptile in the fact that the region which represents the hand has separate bones, with claws resembling those which terminate the fore-limb of a reptile. moreover, it had a long reptile-like tail with a fringe of feathers on each side; while, in all true birds hitherto known, the tail is relatively short, and the vertebræ which constitute its skeleton are generally peculiarly modified. like the _anoplotherium_ and the _palæotherium_, therefore, _archæopteryx_ tends to fill up the interval between groups which, in the existing world, are widely separated, and to destroy the value of the definitions of zoological groups based upon our knowledge of existing forms. and such cases as these constitute evidence in favour of evolution, in so far as they prove that, in former periods of the world's history, there were animals which overstepped the bounds of existing groups, and tended to merge them into larger assemblages. they show that animal organisation is more flexible than our knowledge of recent forms might have led us to believe; and that many structural permutations and combinations, of which the present world gives us no indication, may nevertheless have existed. but it by no means follows, because the _palæotherium_ has much in common with the horse, on the one hand, and with the rhinoceros on the other, that it is the intermediate form through which rhinoceroses have passed to become horses, or _vice versâ_; on the contrary, any such supposition would certainly be erroneous. nor do i think it likely that the transition from the reptile to the bird has been effected by such a form as _archæopteryx_. and it is convenient to distinguish these intermediate forms between two groups, which do not represent the actual passage from the one group to the other, as _intercalary_ types, from those _linear_ types which, more or less approximately, indicate the nature of the steps by which the transition from one group to the other was effected. i conceive that such linear forms, constituting a series of natural gradations between the reptile and the bird, and enabling us to understand the manner in which the reptilian has been metamorphosed into the bird type, are really to be found among a group of ancient and extinct terrestrial reptiles known as the _ornithoscelida_. the remains of these animals occur throughout the series of mesozoic formations, from the trias to the chalk, and there are indications of their existence even in the later palæozoic strata. most of these reptiles at present known are of great size, some having attained a length of forty feet or perhaps more. the majority resembled lizards and crocodiles in their general form, and many of them were, like crocodiles, protected by an armour of heavy bony plates. but, in others, the hind limbs elongate and the fore limbs shorten, until their relative proportions approach those which are observed in the short-winged, flightless, ostrich tribe among birds. the skull is relatively light, and in some cases the jaws, though bearing teeth, are beak-like at their extremities and appear to have been enveloped in a horny sheath. in the part of the vertebral column which lies between the haunch bones and is called the sacrum, a number of vertebræ may unite together into one whole, and in this respect, as in some details of its structure, the sacrum of these reptiles approaches that of birds. but it is in the structure of the pelvis and of the hind limb that some of these ancient reptiles present the most remarkable approximation to birds, and clearly indicate the way by which the most specialized and characteristic features of the bird may have been evolved from the corresponding parts in the reptile. in fig. , the pelvis and hind limbs of a crocodile, a three-toed bird, and an ornithoscelidan are represented side by side; and, for facility of comparison, in corresponding positions; but it must be recollected that, while the position of the bird's limb is natural, that of the crocodile is not so. in the bird, the thigh-bone lies close to the body, and the metatarsal bones of the foot (ii., iii., iv., fig. ) are, ordinarily, raised into a more or less vertical position; in the crocodile, the thigh-bone stands out at an angle from the body, and the metatarsal bones (i., ii., iii., iv., fig. ) lie flat on the ground. hence, in the crocodile, the body usually lies squat between the legs, while, in the bird, it is raised upon the hind legs, as upon pillars. in the crocodile, the pelvis is obviously composed of three bones on each side: the ilium (il.), the pubis (pb.), and the ischium (is.). in the adult bird there appears to be but one bone on each side. the examination of the pelvis of a chick, however, shows that each half is made up of three bones, which answer to those which remain distinct throughout life, in the crocodile. there is, therefore, a fundamental identity of plan in the construction of the pelvis of both bird and reptile; though the differences in form, relative size, and direction of the corresponding bones in the two cases are very great. but the most striking contrast between the two lies in the bones of the leg and of that part of the foot termed the tarsus, which follows upon the leg. in the crocodile, the fibula (f) is relatively large and its lower end is complete. the tibia (t) has no marked crest at its upper end, and its lower end is narrow and not pulley-shaped. there are two rows of separate tarsal bones (as., ca., &c.) and four distinct metatarsal bones, with a rudiment of a fifth. in the bird, the fibula is small and its lower end diminishes to a point. the tibia has a strong crest at its upper end and its lower extremity passes into a broad pulley. there seem at first to be no tarsal bones; and only one bone, divided at the end into three heads for the three toes which are attached to it, appears in the place of the metatarsus. in a young bird, however, the pulley-shaped apparent end of the tibia is a distinct bone, which represents the bones marked as., ca., in the crocodile; while the apparently single metatarsal bone consists of three bones, which early unite with one another and with an additional bone, which represents the lower row of bones in the tarsus of the crocodile. in other words, it can be shown by the study of development that the bird's pelvis and hind limb are simply extreme modifications of the same fundamental plan as that upon which these parts are modelled in reptiles. [illustration: fig. .--bird. ornithoscelidan. crocodile. (the letters have the same signification in all the figures. il., ilium; a, anterior end; b, posterior end; is., ischium; pb., pubis; t, tibia; f, fibula; as., astragalus; ca., calcaneum; , distal portion of the tarsus; i., ii., iii., iv.; metatarsal bones.)] on comparing the pelvis and hind limb of the ornithoscelidan with that of the crocodile, on the one side, and that of the bird, on the other (fig. ), it is obvious that it represents a middle term between the two. the pelvic bones approach the form of those of the birds, and the direction of the pubis and ischium is nearly that which is characteristic of birds; the thigh bone, from the direction of its head, must have lain close to the body; the tibia has a great crest; and, immovably fitted on to its lower end, there is a pulley-shaped bone, like that of the bird, but remaining distinct. the lower end of the fibula is much more slender, proportionally, than in the crocodile. the metatarsal bones have such a form that they fit together immovably, though they do not enter into bony union; the third toe is, as in the bird, longest and strongest. in fact, the ornithoscelidan limb is comparable to that of an unhatched chick. taking all these facts together, it is obvious that the view, which was entertained by mantell and the probability of which was demonstrated by your own distinguished anatomist, leidy, while much additional evidence in the same direction has been furnished by professor cope, that some of these animals may have walked upon their hind legs, as birds do, acquires great weight. in fact, there can be no reasonable doubt that one of the smaller forms of the _ornithoscelida_, _compsognathus_, the almost entire skeleton of which has been discovered in the solenhofen slates, was a bipedal animal. the parts of this skeleton are somewhat twisted out of their natural relations, but the accompanying figure gives a just view of the general form of _compsognathus_ and of the proportions of its limbs; which, in some respects, are more completely bird-like than those of other _ornithoscelida_. [illustration: fig. .--restoration of compsognathus longipes.] we have had to stretch the definition of the class of birds so as to include birds with teeth and birds with paw-like fore-limbs and long tails. there is no evidence that _compsognathus_ possessed feathers; but, if it did, it would be hard indeed to say whether it should be called a reptilian bird or an avian reptile. as _compsognathus_ walked upon its hind legs, it must have made tracks like those of birds. and as the structure of the limbs of several of the gigantic _ornithoscelida_, such as _iguandon_, leads to the conclusion that they also may have constantly, or occasionally, assumed the same attitude, a peculiar interest attaches to the fact that, in the wealden strata of england, there are to be found gigantic footsteps, arranged in order like those of the _brontozoum_, and which there can be no reasonable doubt were made by some of the _ornithoscelida_, the remains of which are found in the same rocks. and, knowing that reptiles that walked upon their hind legs and shared many of the anatomical characters of birds did once exist, it becomes a very important question whether the tracks in the trias of massachusetts, to which i referred some time ago, and which formerly used to be unhesitatingly ascribed to birds, may not all have been made by ornithoscelidan reptiles; and whether, if we could obtain the skeletons of the animals which made these tracks, we should not find in them the actual steps of the evolutional process by which reptiles gave rise to birds. the evidential value of the facts i have brought forward in this lecture must be neither over nor under estimated. it is not historical proof of the occurrence of the evolution of birds from reptiles, for we have no safe ground for assuming that true birds had not made their appearance at the commencement of the mesozoic epoch. it is, in fact, quite possible that all these more or less avi-form reptiles of the mesozoic epoch are not terms in the series of progression from birds to reptiles at all but simply the more or less modified descendants of palæozoic forms through which that transition was actually effected. we are not in a position to say that the known _ornithoscelida_ are intermediate in the order of their appearance on the earth between reptiles and birds. all that can be said is that, if independent evidence of the actual occurrence of evolution is producible, then these intercalary forms remove every difficulty in the way of understanding what the actual steps of the process, in the case of birds, may have been. that intercalary forms should have existed in ancient times is a necessary consequence of the truth of the hypothesis of evolution; and, hence, the evidence i have laid before you in proof of the existence of such forms, is, so far as it goes, in favour of that hypothesis. there is another series of extinct reptiles, which may be said to be intercalary between reptiles and birds, in so far as they combine some of the characters of both these groups; and, which, as they possessed the power of flight, may seem, at first sight, to be nearer representatives of the forms by which the transition from the reptile to the bird was effected, than the _ornithoscelida_. [illustration: fig. .--pterodactylus spectabilis (von meyer).] these are the _pterosauria_, or pterodactyles, the remains of which are met with throughout the series of mesozoic rocks, from the lias to the chalk, and some of which attained a great size, their wings having a span of eighteen or twenty feet. these animals, in the form and proportions of the head and neck relatively to the body, and in the fact that the ends of the jaws were often, if not always, more or less extensively ensheathed in horny beaks, remind us of birds. moreover, their bones contained air cavities, rendering them specifically lighter, as is the case in most birds. the breast-bone was large and keeled, as in most birds and in bats, and the shoulder girdle is strikingly similar to that of ordinary birds. but, it seems to me, that the special resemblance of pterodactyles to birds ends here, unless i may add the entire absence of teeth which characterizes the great pterodactyles (_pteranodon_), discovered by professor marsh. all other known pterodactyles have teeth lodged in sockets. in the vertebral column and the hind limbs there are no special resemblances to birds, and when we turn to the wings they are found to be constructed on a totally different principle from those of birds. there are four fingers. these four fingers are large, and three of them, those which answer to the thumb and two following fingers in my hand--are terminated by claws, while the fourth is enormously prolonged and converted into a great jointed style. you see at once, from what i have stated about a bird's wing, that there could be nothing less like a bird's wing than this is. it concluded by general reasoning that this finger had the office of supporting a web which extended between it and the body. an existing specimen proves that such was really the case, and that the pterodactyles were devoid of feathers, but that the fingers supported a vast web like that of a bat's wing; in fact, there can be no doubt that this ancient reptile flew after the fashion of a bat. thus though the pterodactyle is a reptile which has become modified in such a manner as to enable it to fly, and therefore, as might be expected, presents some points of resemblance to other animals which fly; it has, so to speak, gone off the line which leads directly from reptiles to birds, and has become disqualified for the changes which lead to the characteristic organization of the latter class. therefore, viewed in relation to the classes of reptiles and birds, the pterodactyles appear to me to be, in a limited sense, intercalary forms; but they are not even approximately linear, in the sense of exemplifying those modifications of structure through which the passage from the reptile to the bird took place. lecture iii. the demonstrative evidence of evolution. the occurrence of historical facts is said to be demonstrated, when the evidence that they happened is of such a character as to render the assumption that they did not happen in the highest degree improbable; and the question i now have to deal with is, whether evidence in favour of the evolution of animals of this degree of cogency is, or is not, obtainable from the record of the succession of living forms which is presented to us by fossil remains. those who have attended to the progress of palæontology are aware that evidence of the character which i have defined has been produced in considerable and continually-increasing quantity during the last few years. indeed, the amount and the satisfactory nature of that evidence are somewhat surprising, when we consider the conditions under which alone we can hope to obtain it. it is obviously useless to seek for such evidence except in localities in which the physical conditions have been such as to permit of the deposit of an unbroken, or but rarely interrupted, series of strata through a long period of time; in which the group of animals to be investigated has existed in such abundance as to furnish the requisite supply of remains; and in which, finally, the materials composing the strata are such as to ensure the preservation of these remains in a tolerably perfect and undisturbed state. it so happens that the case which, at present, most nearly fulfils all these conditions is that of the series of extinct animals which culminates in the horses; by which term i mean to denote not merely the domestic animals with which we are all so well acquainted, but their allies, the ass, zebra, quagga, and the like. in short, i use "horses" as the equivalent of the technical name _equidæ_, which is applied to the whole group of existing equine animals. the horse is in many ways a remarkable animal; not least so in the fact that it presents us with an example of one of the most perfect pieces of machinery in the living world. in truth, among the works of human ingenuity it cannot be said that there is any locomotive so perfectly adapted to its purposes, doing so much work with so small a quantity of fuel, as this machine of nature's manufacture--the horse. and, as a necessary consequence of any sort of perfection, of mechanical perfection as of others, you find that the horse is a beautiful creature, one of the most beautiful of all land-animals. look at the perfect balance of its form, and the rhythm and force of its action. the locomotive machinery is, as you are aware, resident in its slender fore and hind limbs; they are flexible and elastic levers, capable of being moved by very powerful muscles; and, in order to supply the engines which work these levers with the force which they expend, the horse is provided with a very perfect apparatus for grinding its food and extracting therefrom the requisite fuel. without attempting to take you very far into the region of osteological detail, i must nevertheless trouble you with some statements respecting the anatomical structure of the horse; and, more especially, will it be needful to obtain a general conception of the structure of its fore and hind limbs, and of its teeth. but i shall only touch upon those points which are absolutely essential to our inquiry. let us turn in the first place to the fore-limb. in most quadrupeds, as in ourselves, the fore-arm contains distinct bones called the radius and the ulna. the corresponding region in the horse seem at first to possess but one bone. careful observation, however, enables us to distinguish in this bone a part which clearly answers to the upper end of the ulna. this is closely united with the chief mass of the bone which represents the radius, and runs out into a slender shaft which may be traced for some distance downwards upon the back of the radius, and then in most cases thins out and vanishes. it takes still more trouble to make sure of what is nevertheless the fact, that a small part of the lower end of the bone of the horse's fore-arm, which is only distinct in a very young foal, is really the lower extremity of the ulna. what is commonly called the knee of a horse is its wrist. the "cannon bone" answers to the middle bone of the five metacarpal bones, which support the palm of the hand in ourselves. the "pastern," "coronary," and "coffin" bones of veterinarians answer to the joints of our middle fingers, while the hoof is simply a greatly enlarged and thickened nail. but if what lies below the horse's "knee" thus corresponds to the middle finger in ourselves, what has become of the four other fingers or digits? we find in the places of the second and fourth digits only two slender splint-like bones, about two-thirds as long as the cannon bone, which gradually taper to their lower ends and bear no finger joints, or, as they are termed, phalanges. sometimes, small bony or gristly nodules are to be found at the bases of these two metacarpal splints, and it is probable that these represent rudiments of the first and fifth toes. thus, the part of the horse's skeleton, which corresponds with that of the human hand, contains one overgrown middle digit, and at least two imperfect lateral digits; and these answer, respectively, to the third, the second, and the fourth fingers in man. corresponding modifications are found in the hind limb. in ourselves, and in most quadrupeds, the leg contains two distinct bones, a large bone, the tibia, and a smaller and more slender bone, the fibula. but, in the horse, the fibula seems, at first, to be reduced to its upper end; a short slender bone united with the tibia, and ending in a point below, occupying its place. examination of the lower end of a young foal's shin-bone, however, shows a distinct portion of osseous matter, which is the lower end of the fibula; so that the, apparently single, lower end of the shin-bone is really made up of the coalesced ends of the tibia and fibula, just as the, apparently single, lower end of the fore-arm bone is composed of the coalesced radius and ulna. the heel of the horse is the part commonly known as the hock. the hinder cannon bone answers to the middle metatarsal bone of the human foot, the pastern, coronary, and coffin bones, to the middle toe bones; the hind hoof to the nail; as in the fore-foot. and, as in the fore-foot, there are merely two splints to represent the second and the fourth toes. sometimes a rudiment of a fifth toe appears to be traceable. the teeth of a horse are not less peculiar than its limbs. the living engine, like all others, must be well stoked if it is to do its work; and the horse, if it is to make good its wear and tear, and to exert the enormous amount of force required for its propulsion, must be well and rapidly fed. to this end, good cutting instruments and powerful and lasting crushers are needful. accordingly, the twelve cutting teeth of a horse are close-set and concentrated in the fore part of its mouth, like so many adzes or chisels. the grinders or molars are large, and have an extremely complicated structure, being composed of a number of different substances of unequal hardness. the consequence of this is that they wear away at different rates; and, hence, the surface of each grinder is always as uneven as that of a good millstone. i have said that the structure of the grinding teeth is very complicated, the harder and the softer parts being, as it were, interlaced with one another. the result of this is that, as the tooth wears, the crown presents a peculiar pattern, the nature of which is not very easily deciphered at first; but which it is important we should understand clearly. each grinding tooth of the upper jaw has an _outer wall_ so shaped that, on the worn crown, it exhibits the form of two crescents, one in front and one behind, with their concave sides turned outwards. from the inner side of the front crescent, a crescentic _front ridge_ passes inwards and backwards, and its inner face enlarges into a strong longitudinal fold or _pillar_. from the front part of the hinder crescent, a _back ridge_ takes a like direction, and also has its _pillar_. the deep interspaces or _valleys_ between these ridges and the outer wall are filled by bony substance, which is called _cement_, and coats the whole tooth. the pattern of the worn face of each grinding tooth of the lower jaw is quite different. it appears to be formed of two crescent-shaped ridges, the convexities of which are turned outwards. the free extremity of each crescent has a _pillar_, and there is a large double _pillar_ where the two crescents meet. the whole structure is, as it were, imbedded in cement, which fills up the valleys, as in the upper grinders. if the grinding faces of an upper and of a lower molar of the same side are applied together, it will be seen that the apposed ridges are nowhere parallel, but that they frequently cross; and that thus, in the act of mastication, a hard surface in the one is constantly applied to a soft surface in the other, and _vice versâ_. they thus constitute a grinding apparatus of great efficiency, and one which is repaired as fast as it wears, owing to the long-continued growth of the teeth. some other peculiarities of the dentition of the horse must be noticed, as they bear upon what i shall have to say by and by. thus the crowns of the cutting teeth have a peculiar deep pit, which gives rise to the well-known "mark" of the horse. there is a large space between the outer incisors and the front grinder. in this space the adult male horse presents, near the incisors on each side, above and below, a canine or "tush," which is commonly absent in mares. in a young horse, moreover, there is not unfrequently to be seen in front of the first grinder, a very small tooth, which soon falls out. if this small tooth be counted as one, it will be found that there are seven teeth behind the canine on each side; namely, the small tooth in question, and the six great grinders, among which, by an unusual peculiarity, the foremost tooth is rather larger than those which follow it. i have now enumerated those characteristic structures of the horse which are of most importance for the purpose we have in view. to any one who is acquainted with the morphology of vertebrated animals, they show that the horse deviates widely from the general structure of mammals; and that the horse type is, in many respects, an extreme modification of the general mammalian plan. the least modified mammals, in fact, have the radius and ulna, the tibia and fibula, distinct and separate. they have five distinct and complete digits on each foot, and no one of these digits is very much larger than the rest. moreover, in the least modified mammals, the total number of the teeth is very generally forty-four, while in horses, the usual number is forty, and in the absence of the canines, it may be reduced to thirty-six; the incisor teeth are devoid of the fold seen in those of the horse: the grinders regularly diminish in size from the middle of the series to its front end; while their crowns are short, early attain their full length, and exhibit simple ridges or tubercles, in place of the complex foldings of the horse's grinders. hence the general principles of the hypothesis of evolution lead to the conclusion that the horse must have been derived from some quadruped which possessed five complete digits on each foot; which had the bones of the fore-arm and of the leg complete and separate; and which possessed forty-four teeth, among which the crowns of the incisors and grinders had a simple structure; while the latter gradually increased in size from before backwards, at any rate in the anterior part of the series, and had short crowns. and if the horse has been thus evolved, and the remains of the different stages of its evolution have been preserved, they ought to present us with a series of forms in which the number of the digits becomes reduced; the bones of the fore-arm and leg gradually take on the equine condition; and the form and arrangement of the teeth successively approximate to those which obtain in existing horses. let us turn to the facts, and see how far they fulfil these requirements of the doctrine of evolution. in europe abundant remains of horses are found in the quaternary and later tertiary strata as far as the pliocene formation. but these horses, which are so common in the cave-deposits and in the gravels of europe, are in all essential respects like existing horses. and that is true of all the horses of the latter part of the pliocene epoch. but, in deposits which belong to the earlier pliocene and later miocene epochs, and which occur in britain, in france, in germany, in greece, in india, we find animals which are extremely like horses--which, in fact, are so similar to horses, that you may follow descriptions given in works upon the anatomy of the horse upon the skeletons of these animals--but which differ in some important particulars. for example, the structure of their fore and hind limbs is somewhat different. the bones which, in the horse, are represented by two splints, imperfect below, are as long as the middle metacarpal and metatarsal bones; and, attached to the extremity of each, is a digit with three joints of the same general character as those of the middle digit, only very much smaller. these small digits are so disposed that they could have had but very little functional importance, and they must have been rather of the nature of the dew-claws, such as are to be found in many ruminant animals. the _hipparion_, as the extinct european three-toed horse is called, in fact, presents a foot similar to that of the american _protohippus_ (fig. ), except that, in the _hipparion_, the smaller digits are situated farther back, and are of smaller proportional size, than in the _protohippus_. the ulna is slightly more distinct than in the horse; and the whole length of it, as a very slender shaft, intimately united with the radius, is completely traceable. the fibula appears to be in the same condition as in the horse. the teeth of the _hipparion_ are essentially similar to those of the horse, but the pattern of the grinders is in some respects a little more complex, and there is a depression on the face of the skull in front of the orbit, which is not seen in existing horses. in the earlier miocene, and perhaps the later eocene deposits of some parts of europe, another extinct animal has been discovered, which cuvier, who first described some fragments of it, considered to be a _palæotherium_. but as further discoveries threw new light upon its structure, it was recognised as a distinct genus, under the name of _anchitherium_. in its general characters, the skeleton of _anchitherium_ is very similar to that of the horse. in fact, lartet and de blainville called it _palæotherium equinum_ or _hippoides_; and de christol, in , said that it differed from _hipparion_ in little more than the characters of its teeth, and gave it the name of _hipparitherium_. each foot possesses three complete toes; while the lateral toes are much larger in proportion to the middle toe than in _hipparion_, and doubtless rested on the ground in ordinary locomotion. the ulna is complete and quite distinct from the radius, though firmly united with the latter. the fibula seems also to have been complete. its lower end, though intimately united with that of the tibia, is clearly marked off from the latter bone. there are forty-four teeth. the incisors have no strong pit. the canines seem to have been well developed in both sexes. the first of the seven grinders, which, as i have said, is frequently absent, and, when it does exist, is small in the horse, is a good-sized and permanent tooth, while the grinder which follows it is but little larger than the hinder ones. the crowns of the grinders are short, and though the fundamental pattern of the horse-tooth is discernible, the front and back ridges are less curved, the accessory pillars are wanting, and the valleys, much shallower, are not filled up with cement. seven years ago, when i happened to be looking critically into the bearing of palæontological facts upon the doctrine of evolution, it appeared to me that the _anchitherium_, the _hipparion_, and the modern horses, constitute a series in which the modifications of structure coincide with the order of chronological occurrence, in the manner in which they must coincide, if the modern horses really are the result of the gradual metamorphosis, in the course of the tertiary epoch, of a less specialised ancestral form. and i found by correspondence with the late eminent french anatomist and palæontologist, m. lartet, that he had arrived at the same conclusion from the same data. that the _anchitherium_ type had become metamorphosed into the _hipparion_ type, and the latter into the _equine_ type, in the course of that period of time which is represented by the latter half of the tertiary deposits, seemed to me to be the only explanation of the facts for which there was even a shadow of probability.[ ] and, hence, i have ever since held that these facts afford evidence of the occurrence of evolution, which, in the sense already defined, may be termed demonstrative. all who have occupied themselves with the structure of _anchitherium_, from cuvier onwards, have acknowledged its many points of likeness to a well-known genus of extinct eocene mammals, _palæotherium_. indeed, as we have seen, cuvier regarded his remains of _anchitherium_ as those of a species of _palæotherium_. hence, in attempting to trace the pedigree of the horse beyond the miocene epoch and the anchitheroid form, i naturally sought among the various species of palæotheroid animals for its nearest ally, and i was led to conclude that the _palæotherium minus (plagiolophus)_ represented the next step more nearly than any form then known. i think that this opinion was fully justifiable; but the progress of investigation has thrown an unexpected light on the question, and has brought us much nearer than could have been anticipated to a knowledge of the true series of the progenitors of the horse. you are all aware that, when your country was first discovered by europeans, there were no traces of the existence of the horse in any part of the american continent. the accounts of the conquest of mexico dwell upon the astonishment of the natives of that country when they first became acquainted with that astounding phenomenon--a man seated upon a horse. nevertheless, the investigations of american geologists have proved that the remains of horses occur in the most superficial deposits of both north and south america, just as they do in europe. therefore, for some reason or other--no feasible suggestion on that subject, so far as i know, has been made--the horse must have died out on this continent at some period preceding the discovery of america. of late years there has been discovered in your western territories that marvellous accumulation of deposits, admirably adapted for the preservation of organic remains, to which i referred the other evening, and which furnishes us with a consecutive series of records of the fauna of the older half of the tertiary epoch, for which we have no parallel in europe. they have yielded fossils in an excellent state of conservation and in unexampled number and variety. the researches of leidy and others have shown that forms allied to the _hipparion_ and the _anchitherium_ are to be found among these remains. but it is only recently that the admirably conceived and most thoroughly and patiently worked-out investigations of professor marsh have given us a just idea of the vast fossil wealth, and of the scientific importance, of these deposits. i have had the advantage of glancing over the collections in yale museum; and i can truly say that, so far as my knowledge extends, there is no collection from any one region and series of strata comparable, for extent, or for the care with which the remains have been got together, or for their scientific importance, to the series of fossils which he has deposited there. this vast collection has yielded evidence bearing upon the question of the pedigree of the horse of the most striking character. it tends to show that we must look to america, rather than to europe, for the original seat of the equine series; and that the archaic forms and successive modifications of the horse's ancestry are far better preserved here than in europe. professor marsh's kindness has enabled me to put before you a diagram, every figure in which is an actual representation of some specimen which is to be seen at yale at this present time (fig. ). [illustration: fig. .] the succession of forms which he has brought together carries us from the top to the bottom of the tertiaries. firstly, there is the true horse. next we have the american pliocene form of the horse (_pliohippus_); in the conformation of its limbs it presents some very slight deviations from the ordinary horse, and the crowns of the grinding teeth are shorter. then comes the _protohippus_, which represents the european _hipparion_, having one large digit and two small ones on each foot, and the general characters of the fore-arm and leg to which i have referred. but it is more valuable than the european _hipparion_ for the reason that it is devoid of some of the peculiarities of that form--peculiarities which tend to show that the european _hipparion_ is rather a member of a collateral branch, than a form in the direct line of succession. next, in the backward order in time, is the _miohippus_, which corresponds pretty nearly with the _anchitherium_ of europe. it presents three complete toes--one large median and two smaller lateral ones; and there is a rudiment of that digit, which answers to the little finger of the human hand. the european record of the pedigree of the horse stops here; in the american tertiaries, on the contrary, the series of ancestral equine forms is continued into the eocene formations. an older miocene form, termed _mesohippus_, has three toes in front, with a large splint-like rudiment representing the little finger; and three toes behind. the radius and ulna, the tibia and the fibula, are distinct, and the short crowned molar teeth are anchitherioid in pattern. but the most important discovery of all is the _orohippus_, which comes from the eocene formation, and is the oldest member of the equine series, as yet known. here we find four complete toes on the front-limb, three toes on the hind-limb, a well-developed ulna, a well-developed fibula, and short-crowned grinders of simple pattern. thus, thanks to these important researches, it has become evident that, so far as our present knowledge extends, the history of the horse-type is exactly and precisely that which could have been predicted from a knowledge of the principles of evolution. and the knowledge we now possess justifies us completely in the anticipation, that when the still lower eocene deposits, and those which belong to the cretaceous epoch, have yielded up their remains of ancestral equine animals, we shall find, first, a form with four complete toes and a rudiment of the innermost or first digit in front, with, probably, a rudiment of the fifth digit in the hind foot;[ ] while, in still older forms, the series of the digits will be more and more complete, until we come to the five-toed animals, in which, if the doctrine of evolution is well founded, the whole series must have taken its origin. * * * * * that is what i mean by demonstrative evidence of evolution. an inductive hypothesis is said to be demonstrated when the facts are shown to be in entire accordance with it. if that is not scientific proof, there are no merely inductive conclusions which can be said to be proved. and the doctrine of evolution, at the present time, rests upon exactly as secure a foundation as the copernican theory of the motions of the heavenly bodies did at the time of its promulgation. its logical basis is precisely of the same character--the coincidence of the observed facts with theoretical requirements. the only way of escape, if it be a way of escape, from the conclusions which i have just indicated, is the supposition that all these different equine forms have been created separately at separate epochs of time; and, i repeat, that of such an hypothesis as this there neither is, nor can be, any scientific evidence; and, assuredly, so far as i know, there is none which is supported, or pretends to be supported, by evidence or authority of any other kind. i can but think that the time will come when such suggestions as these, such obvious attempts to escape the force of demonstration, will be put upon the same footing as the supposition made by some writers, who are, i believe, not completely extinct at present, that fossils are mere simulacra, are no indications of the former existence of the animals to which they seem to belong; but that they are either sports of nature, or special creations, intended--as i heard suggested the other day--to test our faith. in fact, the whole evidence is in favour of evolution, and there is none against it. and i say this, although perfectly well aware of the seeming difficulties which have been built up upon what appears to the uninformed to be a solid foundation. i meet constantly with the argument that the doctrine of evolution cannot be well founded, because it requires the lapse of a very vast period of time; the duration of life upon the earth, thus implied, is inconsistent with the conclusions arrived at by the astronomer and the physicist. i may venture to say that i am familiar with those conclusions, inasmuch as some years ago, when president of the geological society of london, i took the liberty of criticising them, and of showing in what respects, as it appeared to me, they lacked complete and thorough demonstration. but, putting that point aside, suppose that, as the astronomers, or some of them, and some physical philosophers, tell us, it is impossible that life could have endured upon the earth for as long a period as is required by the doctrine of evolution--supposing that to be proved--i desire to be informed, what is the foundation for the statement that evolution does require so great a time? the biologist knows nothing whatever of the amount of time which may be required for the process of evolution. it is a matter of fact that the equine forms which i have described to you occur, in the order stated, in the tertiary formations. but i have not the slightest means of guessing whether it took a million of years, or ten millions, or a hundred millions, or a thousand millions of years, to give rise to that series of changes. a biologist has no means of arriving at any conclusion as to the amount of time which may be needed for a certain quantity of organic change. he takes his time from the geologist. the geologist, considering the rate at which deposits are formed and the rate at which denudation goes on upon the surface of the earth, arrives at more or less justifiable conclusions as to the time which is required for the deposit of a certain thickness of rocks; and if he tells me that the tertiary formations required , , years for their deposit, i suppose he has good ground for what he says, and i take that as a measure of the duration of the evolution of the horse from the _orohippus_ up to its present condition. and, if he is right, undoubtedly evolution is a very slow process, and requires a great deal of time. but suppose, now, that an astronomer or a physicist--for instance, my friend sir william thomson--tells me that my geological authority is quite wrong; and that he has weighty evidence to show that life could not possibly have existed upon the surface of the earth , , years ago, because the earth would have then been too hot to allow of life, my reply is: "that is not my affair; settle that with the geologist, and when you have come to an agreement among yourselves i will adopt your conclusion." we take our time from the geologists and physicists; and it is monstrous that, having taken our time from the physical philosopher's clock, the physical philosopher should turn round upon us, and say we are too fast or too slow. what we desire to know is, is it a fact that evolution took place? as to the amount of time which evolution may have occupied, we are in the hands of the physicist and the astronomer, whose business it is to deal with those questions. * * * * * i have now, ladies and gentlemen, arrived at the conclusion of the task which i set before myself when i undertook to deliver these lectures. my purpose has been, not to enable those among you who have paid no attention to these subjects before, to leave this room in a condition to decide upon the validity or the invalidity of the hypothesis of evolution; but i have desired to put before you the principles upon which all hypotheses respecting the history of nature must be judged; and furthermore, to make apparent the nature of the evidence and the amount of cogency which is to be expected and may be obtained from it. to this end, i have not hesitated to regard you as genuine students and persons desirous of knowing the truth. i have not shrunk from taking you through long discussions, that i fear may have sometimes tried your patience; and i have inflicted upon you details which were indispensable, but which may well have been wearisome. but i shall rejoice--i shall consider that i have done you the greatest service, which it was in my power to do--if i have thus convinced you that the great question which we have been discussing is not one to be dealt with by rhetorical flourishes, or by loose and superficial talk; but that it requires the keen attention of the trained intellect and the patience of the accurate observer. when i commenced this series of lectures, i did not think it necessary to preface them with a prologue, such as might be expected from a stranger and a foreigner; for during my brief stay in your country, i have found it very hard to believe that a stranger could be possessed of so many friends, and almost harder that a foreigner could express himself in your language in such a way as to be, to all appearance, so readily intelligible. so far as i can judge, that most intelligent, and, perhaps, i may add, most singularly active and enterprising body, your press reporters, do not seem to have been deterred by my accent from giving the fullest account of everything that i happen to have said. but the vessel in which i take my departure to-morrow morning is even now ready to slip her moorings; i awake from my delusion that i am other than a stranger and a foreigner. i am ready to go back to my place and country; but, before doing so, let me, by way of epilogue, tender to you my most hearty thanks for the kind and cordial reception which you have accorded to me; and let me thank you still more for that which is the greatest compliment which can be afforded to any person in my position--the continuous and undisturbed attention which you have bestowed upon the long argument which i have had the honour to lay before you. [ ] the absence of any keel on the breast-bone and some other osteological peculiarities, observed by professor marsh, however, suggest that _hesperornis_ may be a modification of a less specialised group of birds than that to which these existing aquatic birds belong. [ ] i use the word "type" because it is highly probable that many forms of _anchitherium_-like and _hipparion_-like animals existed in the miocene and pliocene epochs, just as many species of the horse tribe exist now; and it is highly improbable that the particular species of _anchitherium_ or _hipparion_, which happen to have been discovered, should be precisely those which have formed part of the direct line of the horse's pedigree. [ ] since this lecture was delivered, professor marsh has discovered a new genus of equine mammals (_eohippus_) from the lowest eocene deposits of the west, which corresponds very nearly to this description.--_american journal of science_, november, . baltimore. address on university education.[ ] the actual work of the university founded in this city by the well-considered munificence of johns hopkins commences to-morrow, and among the many marks of confidence and good-will which have been bestowed upon me in the united states, there is none which i value more highly than that conferred by the authorities of the university when they invited me to deliver an address on such an occasion. for the event which has brought us together is, in many respects, unique. a vast property is handed over to an administrative body, hampered by no conditions save these;--that the principal shall not be employed in building: that the funds shall be appropriated, in equal proportions, to the promotion of natural knowledge and to the alleviation of the bodily sufferings of mankind; and, finally, that neither political nor ecclesiastical sectarianism shall be permitted to disturb the impartial distribution of the testator's benefactions. in my experience of life a truth which sounds very much like a paradox has often asserted itself; namely, that a man's worst difficulties begin when he is able to do as he likes. so long as a man is struggling with obstacles he has an excuse for failure or shortcoming; but when fortune removes them all and gives him the power of doing as he thinks best, then comes the time of trial. there is but one right, and the possibilities of wrong are infinite. i doubt not that the trustees of the johns hopkins university felt the full force of this truth when they entered on the administration of their trust a year and a half ago; and i can but admire the activity and resolution which have enabled them, aided by the able president whom they have selected, to lay down the great outlines of their plan, and carry it thus far into execution. it is impossible to study that plan without perceiving that great care, forethought, and sagacity, have been bestowed upon it, and that it demands the most respectful consideration. i have been endeavouring to ascertain how far the principles which underlie it are in accordance with those which have been established in my own mind by much and long-continued thought upon educational questions. permit me to place before you the result of my reflections. under one aspect a university is a particular kind of educational institution, and the views which we may take of the proper nature of a university are corollaries from those which we hold respecting education in general. i think it must be admitted that the school should prepare for the university, and that the university should crown the edifice, the foundations of which are laid in the school. university education should not be something distinct from elementary education, but should be the natural outgrowth and development of the latter. now i have a very clear conviction as to what elementary education ought to be; what it really may be, when properly organised; and what i think it will be, before many years have passed over our heads, in england and in america. such education should enable an average boy of fifteen or sixteen to read and write his own language with ease and accuracy, and with a sense of literary excellence derived from the study of our classic writers: to have a general acquaintance with the history of his own country and with the great laws of social existence; to have acquired the rudiments of the physical and psychological sciences, and a fair knowledge of elementary arithmetic and geometry. he should have obtained an acquaintance with logic rather by example than by precept; while the acquirement of the elements of music and drawing should have been pleasure rather than work. it may sound strange to many ears if i venture to maintain the proposition that a young person, educated thus far, has had a liberal, though perhaps not a full, education. but it seems to me that such training as that to which i have referred may be termed liberal, in both the senses in which that word is employed, with perfect accuracy. in the first place, it is liberal in breadth. it extends over the whole ground of things to be known and of faculties to be trained, and it gives equal importance to the two great sides of human activity--art and science. in the second place, it is liberal in the sense of being an education fitted for free men; for men to whom every career is open, and from whom their country may demand that they should be fitted to perform the duties of any career. i cannot too strongly impress upon you the fact that, with such a primary education as this, and with no more than is to be obtained by building strictly upon its lines, a man of ability may become a great writer or speaker, a statesman, a lawyer, a man of science, painter, sculptor, architect, or musician. that even development of all a man's faculties, which is what properly constitutes culture, may be effected by such an education, while it opens the way for the indefinite strengthening of any special capabilities with which he may be gifted. in a country like this, where most men have to carve out their own fortunes and devote themselves early to the practical affairs of life, comparatively few can hope to pursue their studies up to, still less beyond, the age of manhood. but it is of vital importance to the welfare of the community that those who are relieved from the need of making a livelihood, and still more, those who are stirred by the divine impulses of intellectual thirst or artistic genius, should be enabled to devote themselves to the higher service of their kind, as centres of intelligence, interpreters of nature, or creators of new forms of beauty. and it is the function of a university to furnish such men with the means of becoming that which it is their privilege and duty to be. to this end the university need cover no ground foreign to that occupied by the elementary school. indeed it cannot; for the elementary instruction which i have referred to embraces all the kinds of real knowledge and mental activity possible to man. the university can add no new departments of knowledge, can offer no new fields of mental activity; but what it can do is to intensify and specialise the instruction in each department. thus literature and philology, represented in the elementary school by english alone, in the university will extend over the ancient and modern languages. history, which, like charity, best begins at home, but, like charity, should not end there, will ramify into anthropology, archæology, political history, and geography, with the history of the growth of the human mind and of its products in the shape of philosophy, science, and art. and the university will present to the student libraries, museums of antiquities, collections of coins, and the like, which will efficiently subserve these studies. instruction in the elements of social economy, a most essential, but hitherto sadly-neglected part of elementary education, will develop in the university into political economy, sociology, and law. physical science will have its great divisions of physical geography, with geology and astronomy; physics; chemistry and biology; represented not merely by professors and their lectures, but by laboratories, in which the students, under guidance of demonstrators, will work out facts for themselves and come into that direct contact with reality which constitutes the fundamental distinction of scientific education. mathematics will soar into its highest regions; while the high peaks of philosophy may be scaled by those whose aptitude for abstract thought has been awakened by elementary logic. finally, schools of pictorial and plastic art, of architecture, and of music, will offer a thorough discipline in the principles and practice of art to those in whom lies nascent the rare faculty of æsthetic representation, or the still rarer powers of creative genius. the primary school and the university are the alpha and omega of education. whether institutions intermediate between these (so-called secondary schools) should exist, appears to me to be a question of practical convenience. if such schools are established, the important thing is that they should be true intermediaries between the primary school and the university, keeping on the wide track of general culture, and not sacrificing one branch of knowledge for another. such appear to me to be the broad outlines of the relations which the university, regarded as a place of education, ought to bear to the school, but a number of points of detail require some consideration, however briefly and imperfectly i can deal with them. in the first place, there is the important question of the limitations which should be fixed to the entrance into the university; or, what qualifications should be required of those who propose to take advantage of the higher training offered by the university. on the one hand, it is obviously desirable that the time and opportunities of the university should not be wasted in conferring such elementary instruction as can be obtained elsewhere; while, on the other hand, it is no less desirable that the higher instruction of the university should be made accessible to every one who can take advantage of it, although he may not have been able to go through any very extended course of education. my own feeling is distinctly against any absolute and defined preliminary examination, the passing of which shall be an essential condition of admission to the university. i would admit to the university any one who could be reasonably expected to profit by the instruction offered to him; and i should be inclined, on the whole, to test the fitness of the student, not by examination before he enters the university, but at the end of his first term of study. if, on examination in the branches of knowledge to which he has devoted himself, he show himself deficient in industry or in capacity, it will be best for the university and best for himself, to prevent him from pursuing a vocation for which he is obviously unfit. and i hardly know of any other method than this by which his fitness or unfitness can be safely ascertained, though no doubt a good deal may be done, not by formal cut and dried examination, but by judicious questioning, at the outset of his career. another very important and difficult practical question is, whether a definite course of study shall be laid down for those who enter the university; whether a curriculum shall be prescribed; or whether the student shall be allowed to range at will among the subjects which are open to him. and this question is inseparably connected with another, namely, the conferring of degrees. it is obviously impossible that any student should pass through the whole of the series of courses of instruction offered by a university. if a degree is to be conferred as a mark of proficiency in knowledge, it must be given on the ground that the candidate is proficient in a certain fraction of those studies; and then will arise the necessity of insuring an equivalency of degrees, so that the course by which a degree is obtained shall mark approximately an equal amount of labour and of acquirements, in all cases. but this equivalency can hardly be secured in any other way than by prescribing a series of definite lines of study. this is a matter which will require grave consideration. the important points to bear in mind, i think, are that there should not be too many subjects in the curriculum, and that the aim should be the attainment of thorough and sound knowledge of each. one half of the johns hopkins bequest is devoted to the establishment of a hospital, and it was the desire of the testator that the university and the hospital should co-operate in the promotion of medical education. the trustees will unquestionably take the best advice that is to be had as to the construction and administration of the hospital. in respect to the former point, they will doubtless remember that a hospital may be so arranged as to kill more than it cures; and, in regard to the latter, that a hospital may spread the spirit of pauperism among the well-to-do, as well as relieve the sufferings of the destitute. it is not for me to speak on these topics--rather let me confine myself to the one matter on which my experience as a student of medicine, and an examiner of long standing, who has taken a great interest in the subject of medical education, may entitle me to a hearing. i mean the nature of medical education itself, and the co-operation of the university in its promotion. what is the object of medical education? it is to enable the practitioner, on the one hand, to prevent disease by his knowledge of hygiene; on the other hand, to divine its nature, and to alleviate or cure it, by his knowledge of pathology, therapeutics, and practical medicine. that is his business in life, and if he has not a thorough and practical knowledge of the conditions of health, of the causes which tend to the establishment of disease, of the meaning of symptoms, and of the uses of medicines and operative appliances, he is incompetent, even if he were the best anatomist, or physiologist, or chemist, that ever took a gold medal or won a prize certificate. this is one great truth respecting medical education. another is, that all practice in medicine is based upon theory of some sort or other; and therefore, that it is desirable to have such theory in the closest possible accordance with fact. the veriest empiric who gives a drug in one case because he has seen it do good in another of apparently the same sort, acts upon the theory that similarity of superficial symptoms means similarity of lesions; which, by the way, is perhaps as wild an hypothesis as could be invented. to understand the nature of disease we must understand health, and the understanding of the healthy body means the having a knowledge of its structure and of the way in which its manifold actions are performed, which is what is technically termed human anatomy and human physiology. the physiologist again must needs possess an acquaintance with physics and chemistry, inasmuch as physiology is, to a great extent, applied physics and chemistry. for ordinary purposes a limited amount of such knowledge is all that is needful; but for the pursuit of the higher branches of physiology no knowledge of these branches of science can be too extensive, or too profound. again, what we call therapeutics, which has to do with the action of drugs and medicines on the living organism, is, strictly speaking, a branch of experimental physiology, and is daily receiving a greater and greater experimental development. the third great fact which is to be taken into consideration in dealing with medical education, is that the practical necessities of life do not, as a rule, allow aspirants to medical practice to give more than three, or it may be four years to their studies. let us put it at four years, and then reflect that, in the course of this time, a young man fresh from school has to acquaint himself with medicine, surgery, obstetrics, therapeutics, pathology, hygiene, as well as with the anatomy and the physiology of the human body; and that his knowledge should be of such a character that it can be relied upon in any emergency, and always ready for practical application. consider, in addition, that the medical practitioner may be called upon, at any moment, to give evidence in a court of justice in a criminal case; and that it is therefore well that he should know something of the laws of evidence, and of what we call medical jurisprudence. on a medical certificate, a man may be taken from his home and from his business and confined in a lunatic asylum; surely, therefore, it is desirable that the medical practitioner should have some rational and clear conceptions as to the nature and symptoms of mental disease. bearing in mind all these requirements of medical education, you will admit that the burden on the young aspirant for the medical profession is somewhat of the heaviest, and that it needs some care to prevent his intellectual back from being broken. those who are acquainted with the existing systems of medical education will observe that, long as is the catalogue of studies which i have enumerated, i have omitted to mention several that enter into the usual medical curriculum of the present day. i have said not a word about zoology, comparative anatomy, botany, or materia medica. assuredly this is from no light estimate of the value or importance of such studies in themselves. it may be taken for granted that i should be the last person in the world to object to the teaching of zoology, or comparative anatomy, in themselves; but i have the strongest feeling that, considering the number and the gravity of those studies through which a medical man must pass, if he is to be competent to discharge the serious duties which devolve upon him, subjects which lie so remote as these do from his practical pursuits should be rigorously excluded. the young man, who has enough to do in order to acquire such familiarity with the structure of the human body as will enable him to perform the operations of surgery, ought not, in my judgment, to be occupied with investigations into the anatomy of crabs and starfishes. undoubtedly the doctor should know the common poisonous plants of his own country when he sees them; but that knowledge may be obtained by a few hours devoted to the examination of specimens of such plants, and the desirableness of such knowledge is no justification, to my mind, for spending three months over the study of systematic botany. again, materia medica, so far as it is a knowledge of drugs, is the business of the druggist. in all other callings the necessity of the division of labour is fully recognised, and it is absurd to require of the medical man that he should not avail himself of the special knowledge of those whose business it is to deal in the drugs which he uses. it is all very well that the physician should know that castor oil comes from a plant, and castoreum from an animal, and how they are to be prepared; but for all the practical purposes of his profession that knowledge is not of one whit more value, has no more relevancy, than the knowledge of how the steel of his scalpel is made. all knowledge is good. it is impossible to say that any fragment of knowledge, however insignificant or remote from one's ordinary pursuits, may not some day be turned to account. but in medical education, above all things, it is to be recollected that, in order to know a little well, one must be content to be ignorant of a great deal. let it not be supposed that i am proposing to narrow medical education, or, as the cry is, to lower the standard of the profession. depend upon it there is only one way of really ennobling any calling, and that is to make those who pursue it real masters of their craft, men who can truly do that which they profess to be able to do, and which they are credited with being able to do by the public. and there is no position so ignoble as that of the so-called "liberally-educated practitioner," who, as talleyrand said of his physician, "knows everything, even a little physic;" who may be able to read galen in the original; who knows all the plants, from the cedar of lebanon to the hyssop upon the wall; but who finds himself, with the issues of life and death in his hands, ignorant, blundering, and bewildered, because of his ignorance of the essential and fundamental truths upon which practice must be based. moreover, i venture to say, that any man who has seriously studied all the essential branches of medical knowledge; who has the needful acquaintance with the elements of physical science; who has been brought by medical jurisprudence into contact with law; whose study of insanity has taken him into the fields of psychology; has _ipso facto_ received a liberal education. having lightened the medical curriculum by culling out of it everything which is unessential, we may next consider whether something may not be done to aid the medical student toward the acquirement of real knowledge by modifying the system of examination. in england, within my recollection, it was the practice to require of the medical student attendance on lectures upon the most diverse topics during three years; so that it often happened that he would have to listen, in the course of a day, to four or five lectures upon totally different subjects, in addition to the hours given to dissection and to hospital practice: and he was required to keep all the knowledge he could pick up, in this distracting fashion, at examination point, until, at the end of three years, he was set down to a table and questioned pell-mell upon all the different matters with which he had been striving to make acquaintance. a worse system and one more calculated to obstruct the acquisition of sound knowledge and to give full play to the "crammer" and the "grinder" could hardly have been devised by human ingenuity. of late years great reforms have taken place. examinations have been divided so as to diminish the number of subjects among which the attention has to be distributed. practical examination has been largely introduced; but there still remains, even under the present system, too much of the old evil inseparable from the contemporaneous pursuit of a multiplicity of diverse studies. proposals have recently been made to get rid of general examinations altogether, to permit the student to be examined in each subject at the end of his attendance on the class; and then, in case of the result being satisfactory, to allow him to have done with it; and i may say that this method has been pursued for many years in the royal school of mines in london, and has been found to work very well. it allows the student to concentrate his mind upon what he is about for the time being, and then to dismiss it. those who are occupied in intellectual work, will, i think, agree with me that it is important, not so much to know a thing, as to have known it, and known it thoroughly. if you have once known a thing in this way it is easy to renew your knowledge when you have forgotten it; and when you begin to take the subject up again, it slides back upon the familiar grooves with great facility. lastly comes the question as to how the university may co-operate in advancing medical education. a medical school is strictly a technical school--a school in which a practical profession is taught--while a university ought to be a place in which knowledge is obtained without direct reference to professional purposes. it is clear, therefore, that a university and its antecedent, the school, may best co-operate with the medical school by making due provision for the study of those branches of knowledge which lie at the foundation of medicine. at present, young men come to the medical schools without a conception of even the elements of physical science; they learn, for the first time, that there are such sciences as physics, chemistry, and physiology, and are introduced to anatomy as a new thing. it may be safely said that, with a large proportion of medical students, much of the first session is wasted in learning how to learn--in familiarising themselves with utterly strange conceptions, and in awakening their dormant and wholly untrained powers of observation and of manipulation. it is difficult to overestimate the magnitude of the obstacles which are thrown in the way of scientific training by the existing system of school education. not only are men trained in mere book-work, ignorant of what observation means, but the habit of learning from books alone begets a disgust of observation. the book-learned student will rather trust to what he sees in a book than to the witness of his own eyes. there is not the least reason why this should be so, and, in fact, when elementary education becomes that which i have assumed it ought to be, this state of things will no longer exist. there is not the slightest difficulty in giving sound elementary instruction in physics, in chemistry, and in the elements of human physiology, in ordinary schools. in other words, there is no reason why the student should not come to the medical school, provided with as much knowledge of these several sciences as he ordinarily picks up, in the course of his first year of attendance, at the medical school. i am not saying this without full practical justification for the statement. for the last eighteen years we have had in england a system of elementary science teaching carried out under the auspices of the science and art department, by which elementary scientific instruction is made readily accessible to the scholars of all the elementary schools in the country. commencing with small beginnings, carefully developed and improved, that system now brings up for examination as many as seven thousand scholars in the subject of human physiology alone. i can say that, out of that number, a large proportion have acquired a fair amount of substantial knowledge; and that no inconsiderable percentage show as good an acquaintance with human physiology as used to be exhibited by the average candidates for medical degrees in the university of london, when i was first an examiner there twenty years ago; and quite as much knowledge as is possessed by the ordinary student of medicine at the present day. i am justified, therefore, in looking forward to the time when the student who proposes to devote himself to medicine will come, not absolutely raw and inexperienced as he is at present, but in a certain state of preparation for further study; and i look to the university to help him still further forward in that stage of preparation, through the organisation of its biological department. here the student will find means of acquainting himself with the phenomena of life in their broadest acceptation. he will study not botany and zoology, which, as i have said, would take him too far away from his ultimate goal; but, by duly arranged instruction, combined with work in the laboratory upon the leading types of animal and vegetable life, he will lay a broad, and at the same time solid, foundation of biological knowledge; he will come to his medical studies with a comprehension of the great truths of morphology and of physiology, with his hands trained to dissect and his eyes taught to see. i have no hesitation in saying that such preparation is worth a full year added on to the medical curriculum. in other words, it will set free that much time for attention to those studies which bear directly upon the student's most grave and serious duties as a medical practitioner. up to this point i have considered only the teaching aspect of your great foundation, that function of the university in virtue of which it plays the part of a reservoir of ascertained truth, so far as our symbols can ever interpret nature. all can learn; all can drink of this lake. it is given to few to add to the store of knowledge, to strike new springs of thought, or to shape new forms of beauty. but so sure as it is that men live not by bread, but by ideas, so sure is it that the future of the world lies in the hands of those who are able to carry the interpretation of nature a step further than their predecessors; so certain is it that the highest function of a university is to seek out those men, cherish them, and give their ability to serve their kind full play. i rejoice to observe that the encouragement of research occupies so prominent a place in your official documents, and in the wise and liberal inaugural address of your president. this subject of the encouragement, or, as it is sometimes called, the endowment of research, has of late years greatly exercised the minds of men in england. it was one of the main topics of discussion by the members of the royal commission of whom i was one, and who not long since issued their report, after five years' labour. many seem to think that this question is mainly one of money; that you can go into the market and buy research, and that supply will follow demand, as in the ordinary course of commerce. this view does not commend itself to my mind. i know of no more difficult practical problem than the discovery of a method of encouraging and supporting the original investigator without opening the door to nepotism and jobbery. my own conviction is admirably summed up in the passage of your president's address, "that the best investigators are usually those who have also the responsibilities of instruction, gaining thus the incitement of colleagues, the encouragement of pupils, and the observation of the public." at the commencement of this address i ventured to assume that i might, if i thought fit, criticise the arrangements which have been made by the board of trustees, but i confess that i have little to do but to applaud them. most wise and sagacious seems to me the determination not to build for the present. it has been my fate to see great educational funds fossilise into mere bricks and mortar, in the petrifying springs of architecture, with nothing left to work the institution they were intended to support. a great warrior is said to have made a desert and called it peace. administrators of educational funds have sometimes made a palace and called it a university. if i may venture to give advice in a matter which lies out of my proper competency, i would say that whenever you do build, get an honest bricklayer, and make him build you just such rooms as you really want, leaving ample space for expansion. and a century hence, when the baltimore and ohio shares are at one thousand premium, and you have endowed all the professors you need, and built all the laboratories that are wanted, and have the best museum and the finest library that can be imagined; then, if you have a few hundred thousand dollars you don't know what to do with, send for an architect and tell him to put up a façade. if american is similar to english experience, any other course will probably lead you into having some stately structure, good for your architect's fame, but not in the least what you want. it appears to me that what i have ventured to lay down as the principles which should govern the relations of a university to education in general, are entirely in accordance with the measures you have adopted. you have set no restrictions upon access to the instruction you propose to give; you have provided that such instruction, either as given by the university or by associated institutions, should cover the field of human intellectual activity. you have recognised the importance of encouraging research. you propose to provide means by which young men, who may be full of zeal for a literary or for a scientific career, but who also may have mistaken aspiration for inspiration, may bring their capacities to a test, and give their powers a fair trial. if such a one fail, his endowment terminates, and there is no harm done. if he succeed, you may give power of flight to the genius of a davy or a faraday, a carlyle or a locke, whose influence on the future of his fellow-men shall be absolutely incalculable. you have enunciated the principle that "the glory of the university should rest upon the character of the teachers and scholars, and not upon their numbers or buildings constructed for their use." and i look upon it as an essential and most important feature of your plan that the income of the professors and teachers shall be independent of the number of students whom they can attract. in this way you provide against the danger, patent elsewhere, of finding attempts at improvement obstructed by vested interests; and, in the department of medical education especially, you are free of the temptation to set loose upon the world men utterly incompetent to perform the serious and responsible duties of their profession. it is a delicate matter for a stranger to the practical working of your institutions, like myself, to pretend to give an opinion as to the organisation of your governing power. i can conceive nothing better than that it should remain as it is, if you can secure a succession of wise, liberal, honest, and conscientious men to fill the vacancies that occur among you. i do not greatly believe in the efficacy of any kind of machinery for securing such a result; but i would venture to suggest that the exclusive adoption of the method of co-optation for filling the vacancies which must occur in your body, appears to me to be somewhat like a tempting of providence. doubtless there are grave practical objections to the appointment of persons outside of your body and not directly interested in the welfare of the university; but might it not be well if there were an understanding that your academic staff should be officially represented on the board, perhaps even the heads of one or two independent learned bodies, so that academic opinion and the views of the outside world might have a certain influence in that most important matter, the appointment of your professors? i throw out these suggestions, as i have said, in ignorance of the practical difficulties that may lie in the way of carrying them into effect, on the general ground that personal and local influences are very subtle, and often unconscious, while the future greatness and efficiency of the noble institution which now commences its work must largely depend upon its freedom from them. * * * * * i constantly hear americans speak of the charm which our old mother country has for them, of the delight with which they wander through the streets of ancient towns, or climb the battlements of mediæval strongholds, the names of which are indissolubly associated with the great epochs of that noble literature which is our common inheritance; or with the blood-stained steps of that secular progress, by which the descendants of the savage britons and of the wild pirates of the north sea have become converted into warriors of order and champions of peaceful freedom, exhausting what still remains of the old berserk spirit in subduing nature, and turning the wilderness into a garden. but anticipation has no less charm than retrospect, and to an englishman landing upon your shores for the first time, travelling for hundreds of miles through strings of great and well-ordered cities, seeing your enormous actual, and almost infinite potential, wealth in all commodities, and in the energy and ability which turn wealth to account, there is something sublime in the vista of the future. do not suppose that i am pandering to what is commonly understood by national pride. i cannot say that i am in the slightest degree impressed by your bigness, or your material resources, as such. size is not grandeur, and territory does not make a nation. the great issue, about which hangs a true sublimity, and the terror of overhanging fate, is what are you going to do with all these things? what is to be the end to which these are to be the means? you are making a novel experiment in politics on the greatest scale which the world has yet seen. forty millions at your first centenary, it is reasonably to be expected that, at the second, these states will be occupied by two hundred millions of english-speaking people, spread over an area as large as that of europe, and with climates and interests as diverse as those of spain and scandinavia, england and russia. you and your descendants have to ascertain whether this great mass will hold together under the forms of a republic, and the despotic reality of universal suffrage; whether state rights will hold out against centralisation, without separation; whether centralisation will get the better, without actual or disguised monarchy; whether shifting corruption is better than a permanent bureaucracy; and as population thickens in your great cities, and the pressure of want is felt, the gaunt spectre of pauperism will stalk among you, and communism and socialism will claim to be heard. truly america has a great future before her; great in toil, in care, and in responsibility; great in true glory if she be guided in wisdom and righteousness; great in shame if she fail. i cannot understand why other nations should envy you, or be blind to the fact that it is for the highest interest of mankind that you should succeed; but the one condition of success, your sole safeguard, is the moral worth and intellectual clearness of the individual citizen. education cannot give these, but it may cherish them and bring them to the front in whatever station of society they are to be found; and the universities ought to be, and may be, the fortresses of the higher life of the nation. may the university which commences its practical activity to-morrow abundantly fulfil its high purpose; may its renown as a seat of true learning, a centre of free inquiry, a focus of intellectual light, increase year by year, until men wander hither from all parts of the earth, as of old they sought bologna, or paris, or oxford. and it is pleasant to me to fancy that, among the english students who are drawn to you at that time, there may linger a dim tradition that a countryman of theirs was permitted to address you as he has done to-day, and to feel as if your hopes were his hopes and your success his joy. [ ] delivered at the formal opening of the johns hopkins university at baltimore, u.s., september . the total amount bequeathed by johns hopkins is more than , , dollars. the sum of , , dollars is appropriated to a university, a like sum to a hospital, and the rest to local institutions of education and charity. london. lecture on the study of biology. it is my duty to-night to speak about the study of biology, and while it may be that there are many of my audience who are quite familiar with that study, yet as a lecturer of some standing, it would, i know by experience, be very bad policy on my part to suppose such to be extensively the case. on the contrary, i must imagine that there are many of you who would like to know what biology is; that there are others who have that amount of information, but would nevertheless gladly hear why it should be worth their while to study biology; and yet others, again, to whom these two points are clear, but who desire to learn how they had best study it, and, finally, when they had best study it. i shall, therefore, address myself to the endeavour to give you some answer to these four questions--what biology is; why it should be studied; how it should be studied; and when it should be studied. in the first place, in respect to what biology is, there are, i believe, some persons who imagine that the term "biology" is simply a new-fangled denomination, a neologism in short, for what used to be known under the title of "natural history;" but i shall try to show you, on the contrary, that the word is the expression of the growth of science during the last years, and came into existence half a century ago. at the revival of learning, knowledge was divided into two kinds--the knowledge of nature and the knowledge of man; for it was the current idea then (and a great deal of that ancient conception still remains) that there was a sort of essential antithesis, not to say antagonism, between nature and man; and that the two had not very much to do with one another, except that the one was oftentimes exceedingly troublesome to the other. though it is one of the salient merits of our great philosophers of the seventeenth century, that they recognised but one scientific method, applicable alike to man and to nature, we find this notion of the existence of a broad distinction between nature and man in the writings both of bacon and of hobbes of malmesbury; and i have brought with me that famous work which is now so little known, greatly as it deserves to be studied, "the leviathan," in order that i may put to you in the wonderfully terse and clear language of thomas hobbes, what was his view of the matter. he says:-- "the register of knowledge of fact is called history. whereof there be two sorts, one called natural history; which is the history of such facts or effects of nature as have no dependence on man's will; such as are the histories of metals, plants, animals, regions, and the like. the other is civil history; which is the history of the voluntary actions of men in commonwealths." so that all history of fact was divided into these two great groups of natural and of civil history. the royal society was in course of foundation about the time that hobbes was writing this book, which was published in ; and that society was termed a "society for the improvement of natural knowledge," which was then nearly the same thing as a "society for the improvement of natural history." as time went on, and the various branches of human knowledge became more distinctly developed and separated from one another, it was found that some were much more susceptible of precise mathematical treatment than others. the publication of the "principia" of newton, which probably gave a greater stimulus to physical science than any work ever published before, or which is likely to be published hereafter, showed that precise mathematical methods were applicable to those branches of science such as astronomy, and what we now call physics, which occupy a very large portion of the domain of what the older writers understood by natural history. and inasmuch as the partly deductive and partly experimental methods of treatment to which newton and others subjected these branches of human knowledge, showed that the phenomena of nature which belonged to them were susceptible of explanation, and thereby came within the reach of what was called "philosophy" in those days; so much of this kind of knowledge as was not included under astronomy came to be spoken of as "natural philosophy"--a term which bacon had employed in a much wider sense. time went on, and yet other branches of science developed themselves. chemistry took a definite shape; and since all these sciences, such as astronomy, natural philosophy, and chemistry, were susceptible either of mathematical treatment or of experimental treatment, or of both, a broad distinction was drawn between the experimental branches of what had previously been called natural history and the observational branches--those in which experiment was (or appeared to be) of doubtful use, and where, at that time, mathematical methods were inapplicable. under these circumstances the old name of "natural history" stuck by the residuum, by those phenomena which were not, at that time, susceptible of mathematical or experimental treatment; that is to say, those phenomena of nature which come now under the general heads of physical geography, geology, mineralogy, the history of plants, and the history of animals. it was in this sense that the term was understood by the great writers of the middle of the last century--buffon and linnæus--by buffon in his great work, the "histoire naturelle générale," and by linnæus in his splendid achievement, the "systema naturæ." the subjects they deal with are spoken of as "natural history," and they called themselves and were called "naturalists." but you will observe that this was not the original meaning of these terms; but that they had, by this time, acquired a signification widely different from that which they possessed primitively. the sense in which "natural history" was used at the time i am now speaking of has, to a certain extent, endured to the present day. there are now in existence in some of our northern universities, chairs of "civil and natural history," in which "natural history" is used to indicate exactly what hobbes and bacon meant by that term. the unhappy incumbent of the chair of natural history is, or was, supposed to cover the whole ground of geology, mineralogy, and zoology, perhaps even botany, in his lectures. but as science made the marvellous progress which it did make at the latter end of the last and the beginning of the present century, thinking men began to discern that under this title of "natural history" there were included very heterogeneous constituents--that, for example, geology and mineralogy were, in many respects, widely different from botany and zoology; that a man might obtain an extensive knowledge of the structure and functions of plants and animals, without having need to enter upon the study of geology or mineralogy, and _vice versâ_; and, further as knowledge advanced, it became clear that there was a great analogy, a very close alliance, between those two sciences of botany and zoology which deal with living beings, while they are much more widely separated from all other studies. it is due to buffon to remark that he clearly recognised this great fact. he says: "ces deux genres d'êtres organisés [les animaux et les végétaux] ont beaucoup plus de propriétés communes que de différences réelles." therefore, it is not wonderful that, at the beginning of the present century, in two different countries, and so far as i know, without any intercommunication, two famous men clearly conceived the notion of uniting the sciences which deal with living matter into one whole, and of dealing with them as one discipline. in fact, i may say there were three men to whom this idea occurred contemporaneously, although there were but two who carried it into effect, and only one who worked it out completely. the persons to whom i refer were the eminent physiologist bichat, and the great naturalist lamarck, in france; and a distinguished german, treviranus. bichat[ ] assumed the existence of a special group of "physiological" sciences. lamarck, in a work published in ,[ ] for the first time made use of the name "biologie" from the two greek words which signify a discourse upon life and living things. about the same time it occurred to treviranus, that all those sciences which deal with living matter are essentially and fundamentally one, and ought to be treated as a whole; and, in the year , he published the first volume of what he also called "biologie." treviranus's great merit lies in this, that he worked out his idea, and wrote the very remarkable book to which i refer. it consists of six volumes, and occupied its author for twenty years--from to . that is the origin of the term "biology;" and that is how it has come about that all clear thinkers and lovers of consistent nomenclature have substituted for the old confusing name of "natural history," which has conveyed so many meanings, the term "biology" which denotes the whole of the sciences which deal with living things, whether they be animals or whether they be plants. some little time ago--in the course of this year, i think--i was favoured by a learned classic, dr. field of norwich, with a disquisition, in which he endeavoured to prove that, from a philological point of view, neither treviranus nor lamarck had any right to coin this new word "biology" for their purpose; that, in fact, the greek word "bios" had relation only to human life and human affairs, and that a different word was employed by the greeks when they wished to speak of the life of animals and plants. so dr. field tells us we are all wrong in using the term biology, and that we ought to employ another; only he is not quite sure about the propriety of that which he proposes as a substitute. it is a somewhat hard one--"zootocology." i am sorry we are wrong, because we are likely to continue so. in these matters we must have some sort of "statute of limitations." when a name has been employed for half-a-century, persons of authority[ ] have been using it, and its sense has become well understood, i am afraid that people will go on using it, whatever the weight of philological objection. now that we have arrived at the origin of this word "biology," the next point to consider is: what ground does it cover? i have said that, in its strict technical sense, it denotes all the phenomena which are exhibited by living things, as distinguished from those which are not living; but while that is all very well, so long as we confine ourselves to the lower animals and to plants, it lands us in considerable difficulties when we reach the higher forms of living things. for whatever view we may entertain about the nature of man, one thing is perfectly certain, that he is a living creature. hence, if our definition is to be interpreted strictly, we must include man and all his ways and works under the head of biology; in which case, we should find that psychology, politics, and political economy would be absorbed into the province of biology. in fact, civil history would be merged in natural history. in strict logic it may be hard to object to this course, because no one can doubt that the rudiments and outlines of our own mental phenomena are traceable among the lower animals. they have their economy and their polity, and if, as is always admitted, the polity of bees and the commonwealth of wolves fall within the purview of the biologist proper, it becomes hard to say why we should not include therein human affairs, which in so many cases resemble those of the bees in zealous getting, and are not without a certain parity in the proceedings of the wolves. the real fact is that we biologists are a self-sacrificing people; and inasmuch as, on a moderate estimate, there are about a quarter of a million different species of animals and plants to know about already, we feel that we have more than sufficient territory. there has been a sort of practical convention by which we give up to a different branch of science what bacon and hobbes would have called "civil history." that branch of science has constituted itself under the head of sociology. i may use phraseology which, at present, will be well understood and say that we have allowed that province of biology to become autonomous; but i should like you to recollect that that is a sacrifice, and that you should not be surprised if it occasionally happens that you see a biologist apparently trespassing in the region of philosophy or politics; or meddling with human education; because, after all, that is a part of his kingdom which he has only voluntarily forsaken. having now defined the meaning of the word biology, and having indicated the general scope of biological science, i turn to my second question, which is--why should we study biology? possibly the time may come when that will seem a very odd question. that we, living creatures, should not feel a certain amount of interest in what it is that constitutes our life will eventually, under altered ideas of the fittest objects of human inquiry, appear to be a singular phenomenon; but, at present, judging by the practice of teachers and educators, biology would seem to be a topic that does not concern us at all. i propose to put before you a few considerations with which i dare say many will be familiar already, but which will suffice to show--not fully, because to demonstrate this point fully would take a great many lectures--that there are some very good and substantial reasons why it may be advisable that we should know something about this branch of human learning. i myself entirely agree with another sentiment of the philosopher of malmesbury, "that the scope of all speculation is the performance of some action or thing to be done," and i have not any very great respect for, or interest in, mere knowing as such. i judge of the value of human pursuits by their bearing upon human interests; in other words, by their utility; but i should like that we should quite clearly understand what it is that we mean by this word "utility." in an englishman's mouth it generally means that by which we get pudding or praise, or both. i have no doubt that is one meaning of the word utility, but it by no means includes all i mean by utility. i think that knowledge of every kind is useful in proportion as it tends to give people right ideas, which are essential to the foundation of right practice, and to remove wrong ideas, which are the no less essential foundations and fertile mothers of every description of error in practice. and inasmuch as, whatever practical people may say, this world is, after all, absolutely governed by ideas, and very often by the wildest and most hypothetical ideas, it is a matter of the very greatest importance that our theories of things, and even of things that seem a long way apart from our daily lives, should be as far as possible true, and as far as possible removed from error. it is not only in the coarser practical sense of the word "utility," but in this higher and broader sense, that i measure the value of the study of biology by its utility; and i shall try to point out to you that you will feel the need of some knowledge of biology at a great many turns of this present nineteenth century life of ours. for example, most of us attach great importance to the conception which we entertain of the position of man in this universe and his relation to the rest of nature. we have almost all been told, and most of us hold by the tradition, that man occupies an isolated and peculiar position in nature; that though he is in the world he is not of the world; that his relations to things about him are of a remote character; that his origin is recent, his duration likely to be short, and that he is the great central figure round which other things in this world revolve. but this is not what the biologist tells us. at the present moment you will be kind enough to separate me from them, because it is in no way essential to my present argument that i should advocate their views. don't suppose that i am saying this for the purpose of escaping the responsibility of their beliefs; indeed, at other times and in other places, i do not think that point has been left doubtful; but i want clearly to point out to you that for my present argument they may all be wrong; and, nevertheless, my argument will hold good. the biologists tell us that all this is an entire mistake. they turn to the physical organisation of man. they examine his whole structure, his bony frame and all that clothes it. they resolve him into the finest particles into which the microscope will enable them to break him up. they consider the performance of his various functions and activities, and they look at the manner in which he occurs on the surface of the world. then they turn to other animals, and taking the first handy domestic animal--say a dog--they profess to be able to demonstrate that the analysis of the dog leads them, in gross, to precisely the same results as the analysis of the man; that they find almost identically the same bones, having the same relations; that they can name the muscles of the dog by the names of the muscles of the man, and the nerves of the dog by those of the nerves of the man, and that, such structures and organs of sense as we find in the man such also we find in the dog; they analyse the brain and spinal cord, and they find that the nomenclature which fits the one answers for the other. they carry their microscopic inquiries in the case of the dog as far as they can, and they find that his body is resolvable into the same elements as those of the man. moreover, they trace back the dog's and the man's development, and they find that, at a certain stage of their existence, the two creatures are not distinguishable the one from the other; they find that the dog and his kind have a certain distribution over the surface of the world, comparable in its way to the distribution of the human species. what is true of the dog they tell us is true of all the higher animals; and they assert that they can lay down a common plan for the whole of these creatures, and regard the man and the dog, the horse and the ox as minor modifications of one great fundamental unity. moreover, the investigations of the last three-quarters of a century have proved, they tell us, that similar inquiries, carried out through all the different kinds of animals which are met with in nature, will lead us, not in one straight series, but by many roads, step by step, gradation by gradation, from man, at the summit, to specks of animated jelly at the bottom of the series. so that the idea of leibnitz, and of bonnet, that animals form a great scale of being, in which there are a series of gradations from the most complicated form to the lowest and simplest; that idea, though not exactly in the form in which it was propounded by those philosophers, turns out to be substantially correct. more than this, when biologists pursue their investigations into the vegetable world, they find that they can, in the same way, follow out the structure of the plant, from the most gigantic and complicated trees down through a similar series of gradations, until they arrive at specks of animated jelly, which they are puzzled to distinguish from those specks which they reached by the animal road. thus, biologists have arrived at the conclusion that a fundamental uniformity of structure pervades the animal and vegetable worlds, and that plants and animals differ from one another simply as diverse modifications of the same great general plan. again, they tell us the same story in regard to the study of function. they admit the large and important interval which, at the present time, separates the manifestations of the mental faculties observable in the higher forms of mankind, and even in the lower forms, such as we know them, from those exhibited by other animals; but, at the same time, they tell us that the foundations, or rudiments, of almost all the faculties of man are to be met with in the lower animals; that there is a unity of mental faculty as well as of bodily structure, and that, here also, the difference is a difference of degree and not of kind. i said "almost all," for a reason. among the many distinctions which have been drawn between the lower creatures and ourselves, there is one which is hardly ever insisted on,[ ] but which may be very fitly spoken of in a place so largely devoted to art as that in which we are assembled. it is this, that while, among various kinds of animals, it is possible to discover traces of all the other faculties of man, especially the faculty of mimicry, yet that particular form of mimicry which shows itself in the imitation of form, either by modelling or by drawing, is not to be met with. as far as i know, there is no sculpture or modelling, and decidedly no painting or drawing, of animal origin, i mention the fact, in order that such comfort may be derived therefrom as artists may feel inclined to take. if what the biologists tell us is true, it will be needful to get rid of our erroneous conceptions of man, and of his place in nature, and to substitute right ones for them. but it is impossible to form any judgment as to whether the biologists are right or wrong, unless we are able to appreciate the nature of the arguments which they have to offer. one would almost think this to be a self-evident proposition. i wonder what a scholar would say to the man who should undertake to criticise a difficult passage in a greek play, but who obviously had not acquainted himself with the rudiments of greek grammar. and yet, before giving positive opinions about these high questions of biology, people not only do not seem to think it necessary to be acquainted with the grammar of the subject, but they have not even mastered the alphabet. you find criticism and denunciation showered about by persons, who, not only have not attempted to go through the discipline necessary to enable them to be judges, but who have not even reached that stage of emergence from ignorance in which the knowledge that such a discipline is necessary dawns upon the mind. i have had to watch with some attention--in fact i have been favoured with a good deal of it myself--the sort of criticism with which biologists and biological teachings are visited. i am told every now and then that there is a "brilliant article"[ ] in so-and-so, in which we are all demolished. i used to read these things once, but i am getting old now, and i have ceased to attend very much to this cry of "wolf." when one does read any of these productions, what one finds generally, on the face of it, is that the brilliant critic is devoid of even the elements of biological knowledge, and that his brilliancy is like the light given out by the crackling of thorns under a pot of which solomon speaks. so far as i recollect, solomon makes use of the image for purposes of comparison; but i will not proceed further into that matter. two things must be obvious: in the first place, that every man who has the interests of truth at heart must earnestly desire that every well-founded and just criticism that can be made should be made; but that, in the second place, it is essential to anybody's being able to benefit by criticism, that the critic should know what he is talking about, and be in a position to form a mental image of the facts symbolised by the words he uses. if not, it is as obvious in the case of a biological argument, as it is in that of a historical or philological discussion, that such criticism is a mere waste of time on the part of its author, and wholly undeserving of attention on the part of those who are criticised. take it then as an illustration of the importance of biological study, that thereby alone are men able to form something like a rational conception of what constitutes valuable criticism of the teachings of biologists.[ ] next, i may mention another bearing of biological knowledge--a more practical one in the ordinary sense of the word. consider the theory of infectious disease. surely that is of interest to all of us. now the theory of infectious disease is rapidly being elucidated by biological study. it is possible to produce, from among the lower animals, examples of devastating diseases which spread in the same manner as our infectious disorders, and which are certainly and unmistakably caused by living organisms. this fact renders it possible, at any rate, that that doctrine of the causation of infectious disease which is known under the name of "the germ theory" may be well-founded; and, if so, it must needs lead to the most important practical measures in dealing with those terrible visitations. it may be well that the general, as well as the professional, public should have a sufficient knowledge of biological truths to be able to take a rational interest in the discussion of such problems, and to see, what i think they may hope to see, that, to those who possess a sufficient elementary knowledge of biology, they are not all quite open questions. let me mention another important practical illustration of the value of biological study. within the last forty years the theory of agriculture has been revolutionised. the researches of liebig, and those of our own lawes and gilbert, have had a bearing upon that branch of industry the importance of which cannot be overestimated; but the whole of these new views have grown out of the better explanation of certain processes which go on in plants; and which, of course, form a part of the subject-matter of biology. i might go on multiplying these examples, but i see that the clock won't wait for me, and i must therefore pass to the third question to which i referred: granted that biology is something worth studying, what is the best way of studying it? here i must point out that, since biology is a physical science, the method of studying it must needs be analogous to that which is followed in the other physical sciences. it has now long been recognised that, if a man wishes to be a chemist, it is not only necessary that he should read chemical books and attend chemical lectures, but that he should actually perform the fundamental experiments in the laboratory for himself, and thus learn exactly what the words which he finds in his books and hears from his teachers, mean. if he does not do so, he may read till the crack of doom, but he will never know much about chemistry. that is what every chemist will tell you, and the physicist will do the same for his branch of science. the great changes and improvements in physical and chemical scientific education, which have taken place of late, have all resulted from the combination of practical teaching with the reading of books and with the hearing of lectures. the same thing is true in biology. nobody will ever know anything about biology except in a dilettante "paper-philosopher" way, who contents himself with reading books on botany, zoology, and the like; and the reason of this is simple and easy to understand. it is that all language is merely symbolical of the things of which it treats; the more complicated the things, the more bare is the symbol, and the more its verbal definition requires to be supplemented by the information derived directly from the handling, and the seeing, and the touching of the thing symbolised:--that is really what is at the bottom of the whole matter. it is plain common sense, as all truth, in the long run, is only common sense clarified. if you want a man to be a tea merchant, you don't tell him to read books about china or about tea, but you put him into a tea-merchant's office where he has the handling, the smelling, and the tasting of tea. without the sort of knowledge which can be gained only in this practical way, his exploits as a tea merchant will soon come to a bankrupt termination. the "paper-philosophers" are under the delusion that physical science can be mastered as literary accomplishments are acquired, but unfortunately it is not so. you may read any quantity of books, and you may be almost as ignorant as you were at starting, if you don't have, at the back of your minds, the change for words in definite images which can only be acquired through the operation of your observing faculties on the phenomena of nature. it may be said:--"that is all very well, but you told us just now that there are probably something like a quarter of a million different kinds of living and extinct animals and plants, and a human life could not suffice for the examination of one-fiftieth part of all these." that is true, but then comes the great convenience of the way things are arranged; which is, that although there are these immense numbers of different kinds of living things in existence, yet they are built up, after all, upon marvellously few plans. there are certainly more than , species of insects, and yet anybody who knows one insect--if a properly chosen one--will be able to have a very fair conception of the structure of the whole. i do not mean to say he will know that structure thoroughly, or as well as it is desirable he should know it; but he will have enough real knowledge to enable him to understand what he reads, to have genuine images in his mind of those structures which become so variously modified in all the forms of insects he has not seen. in fact, there are such things as types of form among animals and vegetables, and for the purpose of getting a definite knowledge of what constitutes the leading modifications of animal and plant life, it is not needful to examine more than a comparatively small number of animals and plants. let me tell you what we do in the biological laboratory which is lodged in a building adjacent to this. there i lecture to a class of students daily for about four-and-a-half months, and my class have, of course, their text-books; but the essential part of the whole teaching, and that which i regard as really the most important part of it, is a laboratory for practical work, which is simply a room with all the appliances needed for ordinary dissection. we have tables properly arranged in regard to light, microscopes, and dissecting instruments, and we work through the structure of a certain number of animals and plants. as, for example, among the plants, we take a yeast plant, a _protococcus_, a common mould, a _chara_, a fern, and some flowering plant; among animals we examine such things as an _amoeba_, a _vorticella_, and a fresh-water polype. we dissect a star-fish, an earth-worm, a snail, a squid, and a fresh-water mussel. we examine a lobster and a cray-fish, and a black beetle. we go on to a common skate, a cod-fish, a frog, a tortoise, a pigeon, and a rabbit, and that takes us about all the time we have to give. the purpose of this course is not to make skilled dissectors, but to give every student a clear and definite conception, by means of sense-images, of the characteristic structure of each of the leading modifications of the animal kingdom; and that is perfectly possible, by going no further than the length of that list of forms which i have enumerated. if a man knows the structure of the animals i have mentioned, he has a clear and exact, however limited, apprehension of the essential features of the organisation of all those great divisions of the animal and vegetable kingdoms to which the forms i have mentioned severally belong. and it then becomes possible for him to read with profit; because every time he meets with the name of a structure, he has a definite image in his mind of what the name means in the particular creature he is reading about, and therefore the reading is not mere reading. it is not mere repetition of words; but every term employed in the description, we will say, of a horse, or of an elephant, will call up the image of the things he had seen in the rabbit, and he is able to form a distinct conception of that which he has not seen, as a modification of that which he has seen. i find this system to yield excellent results; and i have no hesitation whatever in saying, that any one who has gone through such a course, attentively, is in a better position to form a conception of the great truths of biology, especially of morphology (which is what we chiefly deal with), than if he had merely read all the books on that topic put together. the connection of this discourse with the loan collection of scientific apparatus arises out of the exhibition in that collection of certain aids to our laboratory work. such of you as have visited that very interesting collection may have noticed a series of diagrams and of preparations illustrating the structure of a frog. those diagrams and preparations have been made for the use of the students in the biological laboratory. similar diagrams and preparations illustrating the structure of all the other forms of life we examine, are either made or in course of preparation. thus the student has before him, first, a picture of the structure he ought to see; secondly, the structure itself worked out; and if with these aids, and such needful explanations and practical hints as a demonstrator can supply, he cannot make out the facts for himself in the materials supplied to him, he had better take to some other pursuit than that of biological science. i should have been glad to have said a few words about the use of museums in the study of biology, but i see that my time is becoming short, and i have yet another question to answer. nevertheless i must, at the risk of wearying you, say a word or two upon the important subject of museums. without doubt there are no helps to the study of biology, or rather to some branches of it, which are, or may be, more important than natural history museums; but, in order to take this place in regard to biology, they must be museums of the future. the museums of the present do not, by any means, do so much for us as they might do. i do not wish to particularise, but i dare say many of you, seeking knowledge, or in the laudable desire to employ a holiday usefully, have visited some great natural history museum. you have walked through a quarter of a mile of animals, more or less well stuffed, with their long names written out underneath them; and, unless your experience is very different from that of most people, the upshot of it all is that you leave that splendid pile with sore feet, a bad headache, and a general idea that the animal kingdom is a "mighty maze without a plan." i do not think that a museum which brings about this result does all that may be reasonably expected from such an institution. what is needed in a collection of natural history is that it should be made as accessible and as useful as possible, on the one hand to the general public, and on the other to scientific workers. that need is not met by constructing a sort of happy hunting-ground of miles of glass cases; and, under the pretence of exhibiting everything, putting the maximum amount of obstacle in the way of those who wish properly to see anything. what the public want is easy and unhindered access to such a collection as they can understand and appreciate; and what the men of science want is similar access to the materials of science. to this end the vast mass of objects of natural history should be divided into two parts--one open to the public, the other to men of science, every day. the former division should exemplify all the more important and interesting forms of life. explanatory tablets should be attached to them, and catalogues containing clearly-written popular expositions of the general significance of the objects exhibited should be provided. the latter should contain, packed into a comparatively small space, in rooms adapted for working purposes, the objects of purely scientific interest. for example, we will say i am an ornithologist. i go to examine a collection of birds. it is a positive nuisance to have them stuffed. it is not only sheer waste, but i have to reckon with the ideas of the bird-stuffer, while, if i have the skin and nobody has interfered with it, i can form my own judgment as to what the bird was like. for ornithological purposes, what is needed is not glass cases full of stuffed birds on perches, but convenient drawers into each of which a great quantity of skins will go. they occupy no great space and do not require any expenditure beyond their original cost. but for the edification of the public, who want to learn indeed, but do not seek for minute and technical knowledge, the case is different. what one of the general public walking into a collection of birds desires to see is not all the birds that can be got together. he does not want to compare a hundred species of the sparrow tribe side by side; but he wishes to know what a bird is, and what are the great modifications of bird structure, and to be able to get at that knowledge easily. what will best serve his purpose is a comparatively small number of birds carefully selected, and artistically, as well as accurately, set up; with their different ages, their nests, their young, their eggs, and their skeletons side by side; and in accordance with the admirable plan which is pursued in this museum, a tablet, telling the spectator in legible characters what they are and what they mean. for the instruction and recreation of the public such a typical collection would be of far greater value than any many-acred imitation of noah's ark. lastly comes the question as to when biological study may best be pursued. i do not see any valid reason why it should not be made, to a certain extent, a part of ordinary school training. i have long advocated this view, and i am perfectly certain that it can be carried out with ease, and not only with ease, but with very considerable profit to those who are taught; but then such instruction must be adapted to the minds and needs of the scholars. they used to have a very odd way of teaching the classical languages when i was a boy. the first task set you was to learn the rules of the latin grammar in the latin language--that being the language you were going to learn! i thought then that this was an odd way of learning a language, but did not venture to rebel against the judgment of my superiors. now, perhaps, i am not so modest as i was then, and i allow myself to think that it was a very absurd fashion. but it would be no less absurd, if we were to set about teaching biology by putting into the hands of boys a series of definitions of the classes and orders of the animal kingdom, and making them repeat them by heart. that is so very favourite a method of teaching, that i sometimes fancy the spirit of the old classical system has entered into the new scientific system, in which case i would much rather that any pretence at scientific teaching were abolished altogether. what really has to be done is to get into the young mind some notion of what animal and vegetable life is. in this matter, you have to consider practical convenience as well as other things. there are difficulties in the way of a lot of boys making messes with slugs and snails; it might not work in practice. but there is a very convenient and handy animal which everybody has at hand, and that is himself; and it is a very easy and simple matter to obtain common plants. hence the general truths of anatomy and physiology can be taught to young people in a very real fashion by dealing with the broad facts of human structure. such viscera as they cannot very well examine in themselves, such as hearts, lungs, and livers, may be obtained from the nearest butcher's shop. in respect to teaching something about the biology of plants, there is no practical difficulty, because almost any of the common plants will do, and plants do not make a mess--at least they do not make an unpleasant mess; so that, in my judgment, the best form of biology for teaching to very young people is elementary human physiology on the one hand, and the elements of botany on the other; beyond that i do not think it will be feasible to advance for some time to come. but then i see no reason why, in secondary schools, and in the science classes which are under the control of the science and art department--and which i may say, in passing, have, in my judgment, done so very much for the diffusion of a knowledge of science over the country--we should not hope to see instruction in the elements of biology carried out, not perhaps to the same extent, but still upon somewhat the same principle as here. there is no difficulty, when you have to deal with students of the ages of or , in practising a little dissection and in getting a notion of, at any rate, the four or five great modifications of the animal form; and the like is true in regard to the higher anatomy of plants. while, lastly, to all those who are studying biological science with a view to their own edification merely, or with the intention of becoming zoologists or botanists; to all those who intend to pursue physiology--and especially to those who propose to employ the working years of their lives in the practice of medicine--i say that there is no training so fitted, or which may be of such important service to them, as the discipline in practical biological work which i have sketched out as being pursued in the laboratory hard by. * * * * * i may add that, beyond all these different classes of persons who may profit by the study of biology, there is yet one other. i remember, a number of years ago, that a gentleman who was a vehement opponent of mr. darwin's views and had written some terrible articles against them, applied to me to know what was the best way in which he could acquaint himself with the strongest arguments in favour of evolution. i wrote back, in all good faith and simplicity, recommending him to go through a course of comparative anatomy and physiology, and then to study development. i am sorry to say he was very much displeased, as people often are with good advice. notwithstanding this discouraging result, i venture, as a parting word, to repeat the suggestion, and to say to all the more or less acute lay and clerical "paper-philosophers"[ ] who venture into the regions of biological controversy--get a little sound, thorough, practical, elementary instruction in biology. [ ] see the distinction between the "sciences physiques" and the "sciences physiologiques" in the "anatomic générale," . [ ] "hydrogeologie," an. x. ( ). [ ] "the term _biology_, which means exactly what we wish to express, _the science of life_, has often been used, and has of late become not uncommon, among good writers."--whewell, "philosophy of the inductive sciences," vol. i. p. (edition of ). [ ] i think that my friend professor allman was the first to draw attention to it. [ ] galileo was troubled by a sort of people whom he called "paper philosophers," because they fancied that the true reading of nature was to be detected by the collation of texts. the race is not extinct, but, as of old, brings forth its "winds of doctrine" by which the weathercock heads among us are much exercised. [ ] some critics do not even take the trouble to read. i have recently been adjured with much solemnity, to state publicly why i have "changed my opinion" as to the value of the palæontological evidence of the occurrence of evolution. to this my reply is, why should i, when that statement was made seven years ago? an address delivered from the presidential chair of the geological society, in , may be said to be a public document, inasmuch as it not only appeared in the _journal_ of that learned body, but was re-published, in , in a volume of "critiques and addresses," to which my name is attached. therein will be found a pretty full statement of my reasons for enunciating two propositions: ( ) that "when we turn to the higher _vertebrata_, the results of recent investigations, however we may sift and criticise them, seem to me to leave a clear balance in favour of the evolution of living forms one from another;" and ( ) that the case of the horse is one which "will stand rigorous criticism." thus i do not see clearly in what way i can be said to have changed my opinion, except in the way of intensifying it, when in consequence of the accumulation of similar evidence since , i recently spoke of the denial of evolution as not worth serious consideration. [ ] writers of this stamp are fond of talking about the baconian method. i beg them therefore to lay to heart these two weighty sayings of the herald of modern science:-- "syllogismus ex propositionibus constat, propositiones ex verbis, verba notionum tesseræ sunt. itaque si notiones ipsæ (_id quod basis rei est_) confusæ sint et temere a rebus abstractæ, nihil in iis quæ superstruuntur est firmitudinis."--"novum organon," ii. . "huic autem vanitati nonnulli ex modernis summa levitate ita indulserunt, ut in primo capitulo geneseos et in libro job et aliis scripturis sacris, philosophiam naturalem fundare conati sint; _inter vivos quærentes mortua_."--_ibid._, . the story of the living machine a review of the conclusions of modern biology in regard to the mechanism which controls the phenomena of living activity by h.w. conn professor of biology in wesleyan university author of the story of germ life, evolution of to-day, the living world, etc. _with fifty illustrations_ new york d. appleton and company copyright, , by d. appleton and company. preface. that the living body is a machine is a statement that is frequently made without any very accurate idea as to what it means. on the one hand it is made with a belief that a strict comparison can be made between the body and an ordinary, artificial machine, and that living beings are thus reduced to simple mechanisms; on the other hand it is made loosely, without any special thought as to its significance, and certainly with no conception that it reduces life to a mechanism. the conclusion that the living body is a machine, involving as it does a mechanical conception of life, is one of most extreme philosophical importance, and no one interested in the philosophical conception of nature can fail to have an interest in this problem of the strict accuracy of the statement that the body is a machine. doubtless the complete story of the living machine can not yet be told; but the studies of the last fifty years have brought us so far along the road toward its completion that a review of the progress made and a glance at the yet unexplored realms and unanswered questions will be profitable. for this purpose this work is designed, with the hope that it may give a clear idea of the trend of recent biological science and of the advances made toward the solution of the problem of life. middletown, conn., u.s.a. _october , _. contents. page introduction--biology a new science--historical biology--conservation of energy--evolution--cytology--new aspects of biology--the mechanical nature of living organisms--significance of the new biological problems--outline of the subject part i. _the running of the living machine._ chapter i. is the body a machine? what is a machine?--a general comparison of a body and a machine--details of the action of the machine--physical explanation of the chief vital functions--the living body is a machine--the living machine constructive as well as destructive--the vital factor chapter ii. the cell and protoplasm. vital properties--the discovery of cells--the cell doctrine--the cell--the cellular structure of organisms--the cell wall--protoplasm--the reign of protoplasm--the decline of the reign of protoplasm--the structure of protoplasm--the nucleus--centrosome--function of the nucleus--cell division or karyokinesis--fertilization of the egg--the significance of fertilization--what is protoplasm?--reaction against the cell doctrine--fundamental vital activities as located in cells--summary part ii. _the building of the living machine_. chapter iii. the factors concerned in the building of the living machine. history of the living machine--evidence for this history--historical--embryological--anatomical--significance of these sources of history--forces at work in the building of the living machine--reproduction--heredity--variation--inheritance of variations--method of machine building--migration and isolation--direct influence of environment--consciousness--summary of nature's power of building machines--the origin of the cell machine--general summary list of illustrations. figure page _amoeba polypodia_ in six successive stages of division _frontispiece_ . figure illustrating osmosis . figure illustrating osmosis . diagram of the intestinal walls . diagram of a single villus . enlarged figure of four cells in the villus membrane . a bit of muscle showing blood-vessels . a bit of bark showing cellular structure . successive stages in the division of the developing egg . a typical cell . cells at a root tip . section of a leaf showing cells of different shapes . plant cells with thick walls, from a fern . section of potato . various shaped wood cells from plant tissue . a bit of cartilage . frogs' blood . a bit of bone . connective tissue . a piece of nerve fibre . a muscle fibre . a complex cell, vorticella . an amoeba . a cell as it appears to the modern microscope . a cell cut into pieces, each containing a bit of nucleus . a cell cut in pieces, only one of which contains any nucleus . different forms of nucleii and . two stages in cell division and . stages in cell division and . latest stages in cell division . an egg and . stages in the process of fertilization of the egg and . stages in the process of fertilization of the egg , , and . stages in fertilization of the egg and . latest stages in the fertilization of the egg and . two stages in the division of the egg . a group of cells resulting from division, the first step in machine building . a later step in machine building, the gastrula . the arm of a monkey . the arm of a bird . the arm of an ancient half-bird, half-reptile animal . diagram to illustrate the principle of heredity the story of the living machine. introduction. ==biology a new science==.--in recent years biology has been spoken of as a new science. thirty years ago departments of biology were practically unknown in educational institutions. to-day none of our higher institutions of learning considers itself equipped without such a department. this seems to be somewhat strange. biology is simply the study of living things; and living nature has been studied as long as mankind has studied anything. even aristotle, four hundred years before christ, classified living things. from this foundation down through the centuries living phenomena have received constant attention. recent centuries have paid more attention to living things than to any other objects in nature. linnæus erected his systems of classification before modern chemistry came into existence; the systematic study of zoology antedated that of physics; and long before geology had been conceived in its modern form, the animal and vegetable kingdoms had been comprehended in a scientific system. how, then, can biology be called a new science when it is older than all the others? there must be some reason why this, the oldest of all, has been recently called a _new_ science, and some explanation of the fact that it has only recently advanced to form a distinct department in our educational system. the reason is not difficult to find. biology is a new science, not because the objects it studies are new, but because it has adopted a new relation to those objects and is studying them from a new standpoint. animals and plants have been studied long enough, but not as we now study them. perhaps the new attitude adopted toward living nature may be tersely expressed by saying that in the past it has been studied as _at rest_, while to-day it is studied as _in motion_. the older zoologists and botanists confined themselves largely to the study of animals and plants simply as so many museum specimens to be arranged on shelves with appropriate names. the modern biologist is studying these same objects as intensely active beings and as parts of an ever-changing history. to the student of natural history fifty years ago, animals and plants were objects to be _classified_; to the biologist of to-day, they are objects to be _explained_. to understand this new attitude, a brief review of the history of the fundamental features of philosophical thought will be necessary. when, long ago, man began to think upon the phenomena of nature, he was able to understand almost nothing. in his inability to comprehend the activities going on around him he came to regard the forces of nature as manifestations of some supernatural beings. this was eminently natural. he had a direct consciousness of his own power to act, and it was natural for him to assume that the activities going on around him were caused by similar powers on the part of some being like himself, only superior to him. thus he came to fill the unseen universe with gods controlling the forces of nature. the wind was the breath of one god, and the lightning a bolt thrown from the hands of another. with advancing thought the ideas of polytheism later gave place to the nobler conception of monotheism. but for a long time yet the same ideas of the supernatural, as related to the natural, retained their place in man's philosophy. those phenomena which he thought he could understand were looked upon as natural, while those which he could not understand were looked upon as supernatural, and as produced by the direct personal activity of some divine agency. as the centuries passed, and man's power of observation became keener and his thinking more logical, many of the hitherto mysterious phenomena became intelligible and subject to simple explanations. as fast as this occurred these phenomena were unconsciously taken from the realm of the supernatural and placed among natural phenomena which could be explained by natural laws. among the first mysteries to be thus comprehended by natural law were those of astronomy. the complicated and yet harmonious motions of the heavenly bodies had hitherto been inexplicable. to explain them many a sublime conception of almighty power had arisen, and the study of the heavenly bodies ever gave rise to the highest thoughts of deity. but newton's law of gravitation reduced the whole to the greatest simplicity. through the law and force of gravitation these mysteries were brought within the grasp of human understanding. they ceased to be looked upon as supernatural, and became natural phenomena as soon as the force of gravitation was accepted as a part of nature. in other branches of natural phenomena the same history followed. the forces and laws of chemical affinity were formulated and studied, and physical laws and forces were comprehended. as these natural forces were grasped it became, little by little, evident that the various phenomena of nature were simply the result of nature's forces acting in accordance with nature's laws. phenomena hitherto mysterious were one after another brought within the realm of law, and as this occurred a smaller and smaller portion of them were left within the realm of the so-called supernatural. by the middle of this century this advance had reached a point where scientists, at least, were ready to believe that nature's forces were all-powerful to account for nature's phenomena. science had passed from the reign of mysticism to the reign of law. but after chemistry and physics, with all the forces that they could muster, had exhausted their powers in explaining natural phenomena, there apparently remained one class of facts which was still left in the realm of the supernatural and the unexplained. the phenomena associated with living things remained nearly as mysterious as ever. life appeared to be the most inexplicable phenomena of nature, and none of the forces and laws which had been found sufficient to account for other departments of nature appeared to have much influence in rendering intelligible the phenomena of life. living organisms appeared to be actuated by an entirely unique force. their shapes and structure showed so many marvellous adaptations to their surroundings as to render it apparently certain that their adjustment must have been the result of some intelligent planning, and not the outcome of blind force. who could look upon the adaptation of the eye to light without seeing in it the result of intelligent design? adaptation to conditions is seen in all animals and plants. these organisms are evidently complicated machines with their parts intricately adapted to each other and to surrounding conditions. apart from animals and plants the only other similarly adjusted machines are those which have been made by human intelligence; and the inference seemed to be clear that a similar intelligence was needed to account for the _living machine_. the blind action of physical forces seemed inadequate. thus the phenomena of life, which had been studied longer than any other phase of nature, continued to stand aloof from the rest and refused to fall into line with the general drift of thought. the living world seemed to give no promise of being included among natural phenomena, but still persisted in retaining its supernatural aspect. it is the attempt to explain the phenomena of the living world by the same kind of natural forces that have been adequate to account for other phenomena, that has created modern biology. so long as students simply studied animals and plants as objects for classification, as museum objects, or as objects which had been stationary in the history of nature, so long were they simply following along the same lines in which their predecessors had been travelling. but when once they began to ask if living nature were not perhaps subject to an intelligent explanation, to study living things as part of a general history and to look upon them as active moving objects whose motion and whose history might perhaps be accounted for, then at once was created a new department of thought and a new science inaugurated. ==historical geology==.--preparation had been made for this new method of studying life by the formulation of a number of important scientific discoveries. prominent among these stood historical geology. that the earth had left a record of her history in the rocks in language plain enough to be read appears to have been impressed upon scientists in the last of the century. that the earth has had a history and that man could read it became more and more thoroughly understood as the first decades of this century passed. the reading of that history proved a somewhat difficult task. it was written in a strange language, and it required many years to discover the key to the record. but under the influence of the writings of lyell, just before the middle of the century, it began to appear that the key to this language is to be found by simply opening the eyes and observing what is going on around us to-day. a more extraordinary and more important discovery has hardly ever been made, for it contained the foundation of nearly all scientific discoveries which have been made since. this discovery proclaimed that an application of the forces still at work to-day on the earth's surface, but continued throughout long ages, will furnish the interpretation of the history written in the rocks, and thus an explanation of the history of the earth itself. the slow elevation of the earth's crust, such as is still going on to-day, would, if continued, produce mountains; and the washing away of the land by rains and floods, such as we see all around us, would, if continued through the long centuries, produce the valleys and gorges which so astound us. the explanation of the past is to be found in the present. but this geological history told of a history of life as well as a history of rocks. the history of the rocks has indeed been bound up in the history of life, and no sooner did it appear that the earth's crust has had a readable history than it appeared that living nature had a parallel history. if the present is a key to the past in interpreting geological history, should not the same be true of this history of life? it was inevitable that problems of life should come to the front, and that the study of life from the dynamical standpoint, rather than a statical, should ensue. modern biology was the child of historical geology. but historical geology alone could never have led to the dynamical phase of modern biology. three other conceptions have contributed in an even greater degree to the development of this science. ==conservation of energy==.--the first of these was the doctrine of conservation of energy and the correlation of forces. this doctrine is really quite simple, and may be outlined as follows: in the universe, as we know it, there exists a certain amount of energy or power of doing work. this amount of energy can neither be increased nor decreased; energy can no more be created or destroyed than matter. it exists, however, in a variety of forms, which may be either active or passive. in the active state it takes some form of motion. the various forces which we recognize in nature--heat, light, electricity, chemism, etc.--are simply forms of motion, and thus forms of this energy. these various types of energy, being only expressions of the universal energy, are convertible into each other in such a way that when one disappears another appears. a cannon ball flying through the air exhibits energy of motion; but it strikes an obstacle and stops. the motion has apparently stopped, but an examination shows that this is not the case. the cannon ball and the object it strikes have been heated, and thus the motion of the ball has simply been transformed into a different form of motion, which we call heat. or, again, the heat set free under the locomotive boiler is converted by machinery into the motion of the locomotive. by still different mechanism it may be converted into electric force. all forms of motion are readily convertible into each other, and each form in which energy appears is only a phase of the total energy of nature. a second condition of energy is energy at rest, or potential energy. a stone on the roof of a house is at rest, but by virtue of its position it has a certain amount of potential energy, since, if dislodged, it will fall to the ground, and thus develop energy of motion. moreover, it required to raise the stone to the roof the expenditure of an amount of energy exactly equal to that which will reappear if the stone is allowed to fall to the ground. so in a chemical molecule, like fat, there is a store of potential energy which may be made active by simply breaking the molecule to pieces and setting it free. this occurs when the fat burns and the energy is liberated as heat. but it required at some time the expenditure of an equal amount of energy to make the molecule. when the molecule of fat was built in the plant which produced it, there was used in its construction an amount of solar energy exactly equivalent to the energy which may be liberated by breaking the molecule to pieces. the total sum of the active and potential energy in the universe is thus at all times the same. this magnificent conception has become the cornerstone of modern science. as soon as conceived it brought at once within its grasp all forms of energy in nature. it is primarily a physical doctrine, and has been developed chiefly in connection with the physical sciences. but it shows at once a possible connection between living and non-living nature. the living organism also exhibits motion and heat, and, if the doctrine of the conservation of energy be true, this energy must be correlated with other forms of energy. here is a suggestion that the same laws control the living and the non-living world; and a suspicion that if we can find a natural explanation of the burning of a piece of coal and the motion of a locomotive, so, too, we may find a natural explanation of the motion of a living machine. ==evolution==--a second conception, whose influence upon-the development of biology was even greater, was the doctrine of evolution. it is true that the doctrine of evolution was no new doctrine with the middle of this century, for it had been conceived somewhat vaguely before. but until historical geology had been formulated, and until the idea of the unity of nature had dawned upon the minds of scientists, the doctrine of evolution had little significance. it made little difference in our philosophy whether the living organisms were regarded as independent creations or as descended from each other, so long as they were looked upon as a distinct realm of nature without connection with the rest of nature's activity. if they are distinct from the rest of nature, and therefore require a distinct origin, it makes little difference whether we looked upon that origin as a single originating point or as thousands of independent creations. but so soon as it appeared that the present condition of the earth's crust was formed by the action of forces still in existence, and so soon as it appeared that the forces outside of living forces, including astronomical, physical and chemical forces, are all correlated with each other as parts of the same store of energy, then the problem of the origin of living things assumed a new meaning. living things became then a part of nature, and demanded to be included in the same general category. the reign of law, which was claiming that all nature's phenomena are the result of natural rather than supernatural powers, demanded some explanation of the origin of living things. consequently, when darwin pointed out a possible way in which living phenomena could thus be included in the realm of natural law, science was ready and anxious to receive his explanation. ==cytology.==--a third conception which contributed to the formulation of modern biology was derived from the facts discovered in connection with the organic cell and protoplasm. the significance of these facts we shall notice later, but here we may simply state that these discoveries offered to students simplicity in the place of complexity. the doctrine of cells and protoplasm appeared to offer to biologists no longer the complicated problems which were associated with animals and plants, but the same problems stripped of all side issues and reduced to their lowest terms. this simplifying of the problems proved to be an extraordinary stimulus to the students who were trying to find some way of understanding life. ==new aspects of biology==.--these three conceptions seized hold of the scientific world at periods not very distant from each other, and their influence upon the study of living nature was immediate and extraordinary. living things now came to be looked upon not simply as objects to be catalogued, but as objects which had a history, and a history which was of interest not merely in itself, but as a part of a general plan. they were no longer studied as stationary, but as moving phases of nature. animals were no longer looked upon simply as beings now existing, but as the results of the action of past forces and as the foundation of a different series of beings in the future. the present existing animals and plants came to be regarded simply as a step in the long history of the universe. it appeared at once that the study of the present forms of life would offer us a means of interpreting the past and perhaps predicting the future. in a short time the entire attitude which the student assumed toward living phenomena had changed. biological science assumed new guises and adopted new methods. even the problems which it tried to solve were radically changed. hitherto the attempt had been made to find instances of _purpose_ in nature. the marvellous adaptations of living beings to their conditions had long been felt, and the study of the purposes of these adaptations had inspired many a magnificent conception. but now the scientist lost sight of the purpose in hunting for the _cause._ natural law is blind and can have no purpose. to the scientist, filled with the thought of the reign of law, purpose could not exist in nature. only cause and effect appeal to him. the present phenomena are the result of forces acting in the past, and the scientist's search should be not for the purpose of an adaptation, but for the action of the forces which produced it. to discover the forces and laws which led to the development of the present forms of animals and plants, to explain the method by which these forces of nature have acted to bring about present results, these became the objects of scientific research. it no longer had any meaning to find that a special organ was adapted to its conditions; but it was necessary to find out how it became adapted. the difference in the attitude of these two points of view is world-wide. the former fixes the attention upon the end, the latter upon the means by which the end was attained; the former is what we sometimes call _teleological_, the latter _scientific;_ the former was the attitude of the study of animals and plants before the middle of this century, the latter the spirit which actuates modern biology. ==the mechanical nature of living organisms.==--this new attitude forced many new problems to the front. foremost among them and fundamental to them all were the questions as to the mechanical nature of living organisms. the law of the correlation of force told that the various forms of energy which appear around us--light, heat, electricity, etc.--are all parts of one common store of energy and convertible into each other. the question whether vital energy is in like manner correlated with other forms of energy was now extremely significant. living forces had been considered as standing apart from the rest of nature. _vital force_, or _vitality_, had been thought of as something distinct in itself; and that there was any measurable relation between the powers of the living organism and the forces of heat and chemical affinity was of course unthinkable before the formulation of the doctrine of the correlation of forces. but as soon as that doctrine was understood it began to appear at once that, to a certain extent at least, the living body might be compared to a machine whose function is simply to convert one kind of energy into another. a steam engine is fed with fuel. in that fuel is a store of energy deposited there perhaps centuries ago. the rays of the sun, shining on the world in earlier ages, were seized upon by the growing plants and stored away in a potential form in the wood which later became coal. this coal is placed in the furnace of the steam engine and is broken to pieces so that it can no longer hold its store of energy, which is at once liberated in its active form as heat. the engine then takes the energy thus liberated, and as a result of its peculiar mechanism converts it into the motion of its great fly-wheel. with this notion clearly in mind the question forces itself to the front whether the same facts are not true of the living animal organism. it, too, is fed with food containing a store of energy; and should we not regard it, like the steam engine, simply a machine for converting this potential energy into motion, heat, or some other active form? this problem of the correlation of vital and physical forces is inevitably forced upon us with the doctrine of the correlation of forces. plainly, however, such questions were inconceivable before about the middle of the nineteenth century. this mechanical conception of living activity was carried even farther. under the lead of huxley there arose in the seventh decade of the century a view of life which reduced it to a pure mechanism. the microscope had, at that time, just disclosed the universal presence in living things of that wonderful substance, _protoplasm._ this material appeared to be a homogeneous substance, and a chemical study showed it to be made of chemical elements united in such a way as to show close relation to albumens. it appeared to be somewhat more complex than ordinary albumen, but it was looked upon as a definite chemical compound, or, perhaps, as a simple mixture of compounds. chemists had shown that the properties of compounds vary with their composition, and that the more complex the compound the more varied its properties. it was a natural conception, therefore, that protoplasm was a complex chemical compound, and that its vital properties were simply the chemical properties resulting from its composition. just as water possesses the power of becoming solid at certain temperatures, so protoplasm possesses the power of assimilating food and growing; and, since we do not doubt that the properties of water are the result of its chemical composition, so we may also assume that the vital properties of protoplasm are the result of its chemical composition. it followed from this conclusion that if chemists ever succeeded in manufacturing the chemical compound, protoplasm, it would be alive. vital phenomena were thus reduced to chemical and mechanical problems. these ideas arose shortly after the middle of the century, and have dominated the development of biological science up to the present time. it is evident that the aim of biological study must be to test these conceptions and carry them out into details. the chemical and mechanical laws of nature must be applied to vital phenomena in order to see whether they can furnish a satisfactory explanation of life. are the laws and forces of chemistry sufficient to explain digestion? are the laws of electricity applicable to an understanding of nervous phenomena? are physical and chemical forces together sufficient to explain life? can the animal body be properly regarded as a machine controlled by mechanical laws? or, on the other hand, are there some phases of life which the forces of chemistry and physics cannot account for? are there limits to the application of natural law to explain life? can there be found something connected with living beings which is force but not correlated with the ordinary forms of energy? is there such a thing as _vital energy_, or is the so-called vital force simply a name which we have given to the peculiar manifestations of ordinary energy as shown in the substance protoplasm? these are some of the questions that modern biology is trying to answer, and it is the existence of such questions which has made modern biology a new science. such questions not only did not, but could not, have arisen before the doctrines of the conservation of energy and evolution had made their impression upon the thought of the world. ==significance of the new biological problems==--it is further evident that the answers to these questions will have a significance reaching beyond the domain of biology proper and affecting the fundamental philosophy of nature. the answer will determine whether or not we can accept in entirety the doctrines of the conservation of energy and evolution. plainly if it should be found that the energy of animate nature was not correlated with other forms of energy, this would demand either a rejection or a complete modification of our doctrine of the conservation of energy. if an animal can create any energy within itself, or can destroy any energy, we can no longer regard the amount of energy of the universe as constant. even if that subtile form of force which we call nervous energy should prove to be uncorrelated with other forms of energy, the idea of the conservation of energy must be changed. it is even possible that we must insist that the still more subtile form of force, mental force, must be brought within the scope of this great law in order that it be implicitly accepted. this law has proved itself strictly applicable to the inanimate world, and has then thrust upon us the various questions in regard to vital force, and we must recognize that the real significance of this great law must rest upon the possibility of its application to vital phenomena. no less intimate is the relation of these problems to the doctrine of evolution. evolution tries to account for each moment in the history of the world as the result of the conditions of the moment before. such a theory loses its meaning unless it can be shown that natural forces are sufficient to account for living phenomena. if the supernatural must be brought in here and there to account for living phenomena, then evolution ceases to have much meaning. it is undoubtedly a fact that the rapidly developing ideas along the above mentioned lines of dynamical biology have, been potent factors in bringing about the adoption of evolution. certain it is that, had it been found that no correlation could be traced between vital and non-vital forces, the doctrine of evolution could not have stood, and even now the special significance which we shall in the end give to evolution will depend upon how we succeed in answering the questions above outlined. the fact is that this problem of the mechanical explanation of vital phenomena forms the capstone of the arch, the sides of which are built of the doctrines of the conservation of energy and the theory of evolution. to the presentation of these problems the following pages will be devoted. the fact that both the doctrine of the conservation of energy and that of evolution are practically everywhere accepted indicates that the mechanical nature of vital forces is regarded as proved. but there are still many questions which are not so easily answered. it will be our purpose in the following discussion to ascertain just what are these problems in dynamical biology and how far they have been answered. our object will be then in brief to discover to what extent the conception of the living organism as a machine is borne out by the facts which have been collected in the last quarter century, and to learn where, if anywhere, limits have been found to our possibility of applying the forces of chemistry and physics to an explanation of life. in other words, we shall try to see how far we have been able to understand living phenomena in terms of natural force. ==outline of the subject==.--the subject, as thus presented, resolves itself at once into two parts. that the living organism is a machine is everywhere recognized, although some may still doubt as to the completeness of the comparison. in the attempt to explain the phenomena of life we have two entirely different problems. the first is manifestly to account for the existence of this machine, for such a completed piece of mechanism as a man or a tree cannot be explained as a result of simple accident, as the existence of a rough piece of rock might be explained. its intricacy of parts and their purposeful interrelation demands explanation, and therefore the fundamental problem is to explain how this machine came into existence. the second problem is simpler, for it is simply to explain the running of the machine after it is made. if the organism is really a machine, we ought to be able to find some way of explaining its actions as we can those of a steam engine. of these two problems the first is the more fundamental, for if we fail to find an explanation for the existence of the machine, our explanation of its method of action is only partly satisfactory. but the second question is the simpler, and must be answered first. we cannot hope to explain the more puzzling matter of the origin of the machine unless we can first understand how it acts. in our treatment of the subject, therefore, we shall divide it into two parts: i. _the running of the living machine_. ii. _the origin of the living machine_. part i. _the running of the living machine._ * * * * * chapter i. is the body a machine? the problem before us in this section is to find out to what extent animals and plants are machines. we wish to determine whether the laws and forces which regulate their activities are the same as the laws and forces with which we experiment in the chemical and physical laboratory, and whether the principles of mechanics and the doctrine of the conservation of energy apply equally well in the living machine and the steam engine. it might be inferred that the proper method of study would be to confine our attention largely to the simplest forms of life, since the problems would be here less complicated, and therefore of easier solution. this, however, has not been nor can it be the method of study. our knowledge of the processes of life have been derived largely from the most rather than the least complex forms. we have a better knowledge of the physiology of man and his allies than any other animals. the reason for this is plain enough. in the first place, there is a value in the knowledge of the life activities of man entirely apart from any theoretical aspects, and hence human physiology has demanded attention for its own sake. the practical utility of human physiology has stimulated its study for centuries; and in the last fifty years of scientific progress it has been human physiology and that of allied animals that has attracted the chief attention of physiologists. the result is that while the physiology of man is tolerably well known, that of other animals is less understood the farther we get away from man and his allies. for this reason most of our knowledge of the living body as a machine must be derived from the study of man. this is, however, fortunate rather than otherwise. in the first place, it enables us to proceed from the known to the unknown; and in the second place, more interest attaches to the problem as connected with human physiology than along any other line. in our discussion, therefore, we shall refer chiefly to the physiology of man. if we find that the functions of human life are amenable to a mechanical explanation we cannot hesitate to believe that this will be equally true of the lower orders of nature. for similar reasons little reference will be made to the mechanism of plant life. the structure of the plant is simpler and its activities are much more easily referable to mechanical principles than are those of animals. for these reasons it will only be necessary for us to turn our attention to the life activities of the higher animals. ==what is a machine?==--turning now to our more immediate subject of the accuracy of the statement that the body is a machine, we must first ask what is meant by a machine? a brief definition of a machine might be as follows: _a machine is a piece of apparatus so designed that it can change one kind of energy into another for a definite purpose_. energy, as already noticed, is the power of doing work, and its ordinary active forms are heat, motion, electricity, light, etc.; but it may be in a passive or potential form, and in this form stored within a chemical molecule. these various forms of energy are readily convertible into each other; and any form of apparatus designed for the purpose of producing such a conversion is called a machine. a dynamo is thus a machine so adjusted that when mechanical motion is supplied to it the energy of motion is converted into electricity; while an electromotor, on the other hand, is a piece of apparatus so designed that when electricity is applied to it, it is converted into motion. a steam engine, again, is designed to convert potential or passive energy into active energy. potential energy in the form of chemical composition (coal) is supplied to the engine, and this energy is first liberated in the active form of heat and then is converted into the motion of the great fly-wheel. in all these cases there is no energy or power created, for the machine must be always supplied with an amount of energy equal to that which it gives back in another form. indeed, a larger amount of energy must be furnished the machine than is expected back, for there is always an actual loss of available energy. in the process of the conversion of one form of energy into another some of the energy, from friction or other cause, takes the form of heat, and is then radiated into space beyond our reach. it is, of course, not destroyed, for energy cannot be destroyed; but it has assumed a form called radiant heat, which is not available for our uses. a machine thus neither creates nor destroys energy. it receives it in one form and gives it back in another form, with an inevitable loss of a portion of the energy as radiant heat. with this understanding, we may now ask if the living body can be properly compared with a machine. ==a general comparison of a body and a machine==.--that the living body exhibits the ordinary types of energy is of course clear enough when we remember that it is always in motion and is always radiating heat--two of the most common types of physical energy. that this energy is supplied to the body as it is to other machines, in the form of the energy of chemical composition, will also need no further proof when it is remembered that it is necessary to supply the body with appropriate food in order that it may do work. the food we eat, like coal, represents so much solar energy which is stored up by the agency of plant life, and the close comparison between feeding the body to enable it to work and feeding the engine to enable it to develop energy is so evident that it demands no further demonstration. the details of the problem may, however, present some difficulties. the first question which presents itself is whether the only power the body possesses is, as in the case with other machines, to _transform_ energy without being able to create or destroy it? can every bit of energy shown by the living organism be accounted for by energy furnished in the food, and conversely can all the energy furnished in the food be found manifested in the living organism? the theoretical answer to this question in terms of the law of the conservation of energy is clear enough, but it is by no means so easy to answer it by experimental data. to obtain experimental demonstration it would be necessary to make an accurate determination of the amount of energy an individual receives during a given period, and at the same time a similar measurement of the amount of energy liberated in his body either as motion or heat. if the body is a machine, these two should exactly balance, and if they do not balance it would indicate that the living organism either creates or destroys energy, and is therefore not a machine. such experiments are exceedingly difficult. they must be performed usually upon man rather than other animals, and it is necessary to inclose an individual in an absolutely sealed space with arrangements for furnishing him with air and food in measured quantity, and with appliances for measuring accurately the work he does and the heat given off from his body. in addition, it is necessary to measure the exact amount of material he eliminates in the form of carbonic acid and other excretions. such experiments present many difficulties which have not yet been thoroughly overcome, but they have been attempted by several investigators. for the purpose of such an experiment scientists have allowed themselves to be shut up in a small chamber six or eight feet in length, in which their only communication with the outer world is by telephone and through a small opening in the side of the chamber, occasionally opened for a second or two to supply the prisoner with food. in such a chamber they have remained as long as twelve days. in these experiments it is necessary to take account not only of the food eaten, but of the actual amount of this food which is used by the body. if the person gains in weight, this must mean that he is storing up in his body material for future use; while if he loses in weight, this means that he is consuming his own tissues for fuel. careful daily records of his weight must therefore be taken. estimates of the solids, liquids, and gases given off from his body must be obtained, for to carry out the experiment an exact balance must be made between the income and the outgo. the apparatus devised for such experiments has been made very delicate; so delicate, indeed, that the rising of the individual in the box from his chair is immediately seen in a rise in temperature of the apparatus. but even with this delicacy the apparatus is comparatively coarse, and can measure only the most apparent forms of energy. the more subtle types of energy, such as nervous force, if this is to be regarded as energy, do not make any impression on the apparatus. the obstacles in the way of these experiments do not particularly concern us, but the general results are of the greatest significance for our purpose. while, for manifest reasons, it has not been possible to carry on these experiments for any great length of time, and while the results have not yet been very accurately refined, they are all of one kind and teach unhesitatingly one conclusion. so far as concerns measurable energy or measurable material, the body behaves just like any other machine. if the body is to do work in this respiration apparatus, it does so only by breaking to pieces a certain amount of food and using the energy thus liberated, and the amount of food needed is proportional to the amount of work done. when the individual simply walks across the floor, or even rises from his chair, this is accompanied by an increase in the amount of food material broken up and a consequent increase in the amount of refuse matter eliminated and the heat given off. the income and outgo of the body in both matter and energy is balanced. if, during the experimental period, it is found that less energy is liberated than that contained in the food assimilated, it is also found that the body has gained in weight, which simply means that the extra energy has been stored in the body for future use. no more energy can be obtained from the body than is furnished, and for all furnished in the food an equivalent amount is regained. there is no trace of any creation or destruction of energy. while, on account of the complexity of the experimenting, an absolutely strict balance sheet cannot be made, all the results are of the same nature. so far as concerns measurable energy, all the facts collected bear out the theoretical conception that the living body is to be regarded as a machine which converts the potential energy of chemical composition, stored passively in its food, into active energy of motion and heat. it is found, however, that the body is a machine of a somewhat superior grade, since it is able to convert this potential energy into motion with less loss than the ordinary machine. as noticed above, in all machines a portion of the energy is converted into heat and rendered unavailable by radiating into space. in an ordinary engine only about one-fifteenth of the energy furnished in the coal can be regained in the form of motive power, the rest being radiated from the machine as heat. some of our better engines to-day utilize a somewhat larger part, but most of them utilize less than one-tenth. the experiments with the living body in the respiration apparatus above described, give a means of determining the proportion of the energy furnished in the form of food which can be utilized in the form of motive force. this figure appears to be decidedly larger than that obtained by any machine yet devised by man. the conclusion of the matter up to this point is then clear. if we leave out of account the phenomena of the nervous system, which we shall consider presently, _the general income and outgo of the body as concerns matter and energy is such that the body must be regarded as a machine, which, like other machines, simply transforms energy without creating or destroying it. to this extent, at least, animals conform to the law of the conservation of energy and are veritable machines_. ==details of the action of the machine.==--we turn next to some of the subordinate problems concerning the details of the action of the living machine. we have a clear understanding of the method of action of a steam engine. its mechanism is simple, and, moreover, it was designed by human intelligence. we can understand how the force of chemical affinity breaks up the chemical composition of the coal, how the heat thus liberated is applied to the water to vapourize it; how the vapour is collected in the boiler under pressure; how this pressure is applied to the piston in the cylinder, and how this finally results in the revolution of the fly-wheel. it is true that we do not understand the underlying forces of chemism, etc., but these forces certainly exist and are the foundation of science. but the mechanism of the engine is intelligible. our understanding of it is such that, with the forces of chemistry and physics as a foundation, we can readily explain the running of the machine. our next problem, therefore, is to see if we can in the same way reach an understanding of the phenomena of the living machine. can we, by the use of these same chemical and physical forces, explain the activities taking place in the living organism? can the motion of the body, for example, be made as intelligible as the motion of the steam engine? ==physical explanation of the chief vital functions.==--the living machine is, of course, vastly more complicated than the steam engine, and there are many different processes which must be considered separately. there is not space in a work of this size to consider them all carefully, but we may select a few of the vital functions as illustrations of the method which is pursued. it will be assumed that the fundamental processes of human physiology are understood by the reader, and we shall try to interpret some of them in terms of chemical and physical force. _digestion._--the first step in this transformation of fuel is the process of digestion. now this process of digestion is nothing mysterious, nor does it involve any peculiar or special forces. digestion of food is simply a chemical change therein. the food which is taken into the body in the form of sugar, starch, fat or protein, is acted upon by the digestive juices in such a way that its chemical nature is slightly changed. but the changes that thus occur are not peculiar to the living body, since they will take place equally well in the chemist's laboratory. they are simply changes in the molecular structure of the food material, and only such changes as are simple and familiar to the chemist. the forces which effect the change are undoubtedly those of chemical affinity. the only feature of the process which is not perfectly intelligible in terms of chemical law is the nature of the digestive juices. the digestive fluids of the mouth and stomach contain certain substances which possess a somewhat remarkable power, inasmuch as they are able to bring about the chemical changes which occur in the digestion of food. an example will make this clearer. one of the digestive processes is the conversion of starch into sugar. the relation of these two bodies is a very simple one, starch being readily converted into sugar by the addition to its molecule of a molecule of water. the change can not be produced by simply adding starch to water, but the water must be introduced into the starch molecule. this change can be brought about in a variety of ways, and is undoubtedly effected by the forces of chemical affinity. chemists have found simple methods of producing this chemical union, and the manufacture of sugar out of starchy material has even become something of a commercial industry. one of the methods by which this change can be produced is by adding to the starch, along with some water, a little saliva. the saliva has the power of causing the chemical change to occur at once, and the molecule of water enters into the starch molecule and forms sugar. now we do not understand how this saliva possesses this power to induce the chemical change. but apparently the process is of the simplest character and involves no greater mystery than chemical affinity. we know that the saliva contains a certain material called a ferment, which is the active agent in bringing about the change. this ferment is not alive, nor does it need any living environment for its action. it can be separated from the saliva in the form of a dry amorphous powder, and in this form can be preserved almost indefinitely, retaining its power to effect the change whenever put under proper conditions. the change of starch into sugar is thus a simple chemical change occurring under the influence of chemical affinity under certain conditions. one of the conditions is the presence of this saliva ferment. if we can not exactly understand how the ferment produces this action, neither do we exactly understand how a spark causes a bit of gunpowder to explode. but we can not doubt that the latter is a purely natural result of the relation of chemical and physical forces, and there is no more reason for doubting it in the former case. what is true of the digestion of starch by saliva is equally true of the digestion of other foods in the stomach and intestine. each of the digestive juices contains a ferment which brings about a chemical change in the food. the changes are always chemical changes and are the result of chemical forces. apart from the presence of these ferments there is really little difference between laboratory chemistry and living chemistry. _absorption of food_.--the next function of this machine to attract our attention is the absorption of food from the intestine into the blood. the digested food is carried down the alimentary canal in a purely mechanical fashion by muscular action, and when it reaches the intestine it begins to pass through its walls into the blood. in this absorption we find engaged another set of forces, the chief of which appears to be the physical force of _osmosis_. the force of osmosis has no special connection with life. if a membrane separates two liquids of different composition (fig. i), a force is exerted on the liquids which cause them to pass through the membrane, each passing through the membrane into the other compartment. the force which drives these liquids through the membrane is considerable, and may sometimes be exerted against considerable pressure. a simple experiment will illustrate this force. in fig. is represented a membranous bag tightly fastened to a glass tube. the bag is filled with a strong solution of sugar, and is immersed in a vessel containing pure water. under these conditions some of the sugar solution passes through the bag into the water, and some of the water passes from the vessel into the bag. but if the solution of sugar is inside the bag and the pure water outside, the amount of liquid passing into the bag is greater than the amount passing out; the bag soon becomes distended and the water even rises in the tube to a considerable height at _a_(fig. ). the force here concerned is a force known as _osmosis_ or _dialysis_, and is always exerted when two different solutions of certain substances are separated from each other by a membrane. the substances in solution will, under these conditions, pass from the dense to the weaker solution. the process is a purely physical one. [illustration: fig. .--to illustrate osmosis. in the vessel _a_ is a solution of sugar; in _b_, is pure water. the two are separated by the membrane _c_. the sugar passes through the membrane into _b_.] [illustration: fig. .--in the bladder _a_ is a sugar solution. in the vessel _b_ is pure water. sugar passes out and water into the bladder until it rises in the tube to a.] this process of osmosis lies at the basis of the absorption of food from the alimentary canal. in the first place, most of the food when swallowed is not soluble, and therefore not capable of osmosis. but the process of digestion, as we have seen, changes the chemical nature of the food. the food, as the result of chemical change, has become soluble, and after being dissolved it is _dialyzable_--i.e., capable of osmosis. after digestion, therefore, the food is dissolved in the liquids in the stomach and intestine, and is in proper condition for dialysis. furthermore, the structure of the intestine is such as to produce conditions adapted for dialysis. this can be understood from fig. , which represents diagrammatically a cross section through the intestinal wall. within the intestinal wall, at _a_, is the food mass in solution. at _b_ are shown little projections of the intestinal wall, called _villi_ extending into this food and covered by a membrane. one of these _villi_ is shown more highly magnified in fig. , in which _b_ shows this membrane. inside of these villi are blood-vessels, _c_, and it will be thus seen that the membrane, _b_, separates two liquids, one containing the dissolved food outside the villus, and the other containing blood inside the villus. here are proper conditions for osmosis, and this process of dialysis will take place whenever the intestinal contents holds more dialyzable material than the blood. under these conditions, which will always occur after food has been digested by the digestive juices, the food will begin to pass through this membranous wall of the intestine into the blood under the influence of the physical force of osmosis. thus the primary factor in food absorption is a physical one. we must notice, however, that the physical force of osmosis is not the only factor concerned in absorption. in the first place, it is found that the food during its passage through the intestinal wall, or shortly afterwards, undergoes a further change, so that by the time it has fairly reached the blood it has again changed its chemical nature. these changes are, however, of a chemical nature, and, while we do not yet know very much about them, they are of the same sort as those of digestion, and involve probably nothing more than chemical processes. [illustration: fig. --diagram of the intestinal walls. _a_, lumen of intestine filled with digested food. _b_, villi, containing blood vessels. _c_, larger blood vessel, which carries blood with absorbed food away from the intestine.] secondly, we notice that there is one phase of absorption which is still obscure. part of the food is composed of fat, and this fat, as the result of digestion, is mechanically broken up into extremely minute droplets. although these droplets are of microscopic size they are not actually in solution, and therefore not subject to the force of osmosis which only affects solutions. the osmotic force will not force fat drops through membranes, and to explain their passage through the walls of the intestine requires something additional. we are as yet, however, able to give only a partial explanation of this matter. the inner wall of the intestine is not an inert, lifeless membrane, but is made of active bits of living matter. these bits of living matter appear to seize hold of the droplets of oil by means of little processes which they thrust out, and then pass them through their own bodies to excrete them on their inner surface into the blood vessels. fig. shows a few of these living bits of the membrane, each containing several such fat droplets. this fat absorption thus appears to be a _vital_ process, and not one simply controlled by physical forces like osmosis. here our explanation runs against what we call _vital power_ of the ultimate elements of the body. the consideration of this vital feature we must, of course, investigate further; but this will be done later. at present our purpose is a general comparison of the body and a machine, and we may for a little postpone the consideration of this vital phenomenon. [illustration: fig. .--diagram of a single villus enlarged. _b_ represents the membranous surface covering the villus; _c_, the blood-vessels within the villus.] [illustration: fig. .--an enlarged figure of four cells of the membrane _b_ in fig. . the free surface is at _a_; _f_ shows fat droplets in process of passage through the cells.] _circulation_.--the next piece of mechanism for us to consider in this machine is the device for distributing this fuel to the various parts of the machine where it is to be used as a source of energy, corresponding in a sense to the fireman of a locomotive. this mechanism we call the circulatory system. it consists of a series of tubes, or blood vessels, running to every part of the body and supplying every bit of tissue. within the tubes is the blood, which, from its liquid nature, is easily forced around the body through the tubes. at the centre of the system is a pump which keeps the blood in motion. the tubes form a closed system, such that the pump, or heart, may suck the blood in from one side to force it out into the tubes on the other side; and the blood, after passing over the body in this closed set of tubes, is finally brought back again to be forced once more over the same path. as this blood is carried around the body it conveys from one part of the machine to another all material that needs distribution. while in the intestine, as already noticed (fig. ), it receives the food, and now this food is carried by the circulation to the muscles or the other organs that need it. while in the lungs the blood receives oxygen, and this oxygen is then carried to those parts of the body that need it. the circulatory system is thus simply a medium by which each part of the machine may receive its proper share of the supplies needed for its action. now in this circulation we have again to do with chemical and physical forces. all of its general phenomena are based upon purely mechanical principles. the action of the heart--leaving out of consideration for a moment its muscular power--is that of a simple pump. it is provided with valves whose action is as simple and as easy to understand as those of any water pump. by the action of these valves the blood is kept circulating in one direction. the blood vessels are elastic, and the study of the effect of a liquid pumped rhythmically into elastic tubes explains with simplicity the various phenomena associated with the circulation. for example, the rhythmically contracting heart forces a small quantity of blood into the arteries at short intervals. these tubes are large near the heart, but smaller at their ends, where they flow into the veins, so that the blood does not flow out into the veins so readily as it flows in from the heart. the jet of blood that is sent in with every beat of the heart slightly stretches the artery, and the tension thus produced causes the blood to continue to flow between the beats. but the heart continues beating, and there is an accumulation of the blood in the arteries until it exists under some pressure--a pressure sufficient to force it rapidly through the small ends of the arteries into the veins. after passing into the veins the pressure is at once removed, since the veins are larger than the arteries, and there is no resistance to the flow of the blood. hence the blood in the arteries is under pressure, while there is little or no pressure in the veins. into the details of this matter we need not go, but this will be sufficient to indicate that the whole process is a mechanical one. we must not fail to see, however, that in this problem of circulation there are two points at least where once more we meet with that class of phenomena which we still call vital. the beating of the heart is the first of these, for this is active muscular power. the second is a contraction of the smaller blood-vessels which regulates the blood supply. both of these phenomena are phases of muscular activity, and will be included under the discussion of other similar phenomena later. [illustration: fig. .--a bit of muscle with its blood-vessels: _a_, the muscle fibres; _b_, the minute blood-vessels. the fibres and vessels are bathed in lymph (not shown in the figure), and food material passes through the walls of the blood-vessels into this lymph.] we next notice that not only is the distribution of the blood explained upon mechanical principles, but the supplying of the active parts of the body with food is in the same way intelligible. as we have seen, the blood coming from the intestine contains the food material received from the digested food. now when this blood in its circulation flows through the active tissues--for instance, the muscles--it is again placed under conditions where osmosis is sure to occur. in the muscles the thin-walled blood-vessels are surrounded and bathed by a liquid called lymph. figure shows a bit of muscle tissue, with its blood-vessels, which are surrounded by lymph. the lymph, which is not shown, fills all the space outside the blood-vessels, thus bathing both muscles and blood-vessels. here again we have a membrane (i.e., the wall of the blood-vessel) separating two liquids, and since the lymph is of a different composition from the blood, dialysis between them is sure to occur, and the materials which passed into the blood in the intestine through the influence of the osmotic force, now pass out into the lymph under the influence of the same force. the food is thus brought into the lymph; and since the lymph lies in actual contact with the living muscle fibres, these fibres are now able to take directly from the lymph the material needed for their use. the power which enables the muscle fibre to take the material it needs, discarding the rest, is, again, one of the _vital_ processes which we defer for a moment. _respiration_.--pursuing the same line of study, we turn for a moment to the relation of the circulatory system to the function of supplying the body with oxygen gas. oxygen is absolutely needed to carry on the functions of life; for these, like those of the engine, are based upon the oxidation of the fuel. the oxygen is derived from the air in the simplest manner. during its circulation the blood is brought for a fraction of a second into practical contact with air. this occurs in the lungs, where there are great numbers of air cells, in the walls of which the blood-vessels are distributed in great profusion. while the blood is in these vessels it is not indeed in actual contact with the air, but is separated from it by only a very thin membrane--so thin that it forms no hindrance to the interchange of gases. these air-cells are kept filled with air by simple muscular action. by the contraction of the muscles of the thorax the thoracic cavity is enlarged, and as a result air is sucked in in exactly the same way that it is sucked into a pair of bellows when expanded. then the contraction of another set of muscles decreases the size of the thoracic cavity, and the air is squeezed out again. the action is just as truly mechanical as is that of the blacksmith's bellows. the relation of the air to the blood is just as simple. in the blood there are various chemical ingredients, among which is one known as hæmoglobin. it does not concern us at present to ask where this material comes from, since this question is part of the broader question, the origin of the machine, to be discussed in the second part of this work. the hæmoglobin is a normal constituent of the blood, and, being red in colour, gives the red colour to the blood. this hæmoglobin has peculiar relations to oxygen. it can be separated from the blood and experimented upon by the chemist in his laboratory. it is found that when hæmoglobin is brought in contact with oxygen, under sufficient pressure it will form a chemical union with it. this chemical union is, however, what the chemist calls a loose combination, since it is readily broken up. if the oxygen is above a certain rather low pressure, the union will take place; while if the pressure be below this point the union is at once destroyed, and the oxygen leaves the hæmoglobin to become free. all of this is a purely chemical matter, and can be demonstrated at will in a test tube in the laboratory. but this union and disassociation is just what occurs as the foundation of respiration. the blood coming to the lungs contains hæmoglobin, and since the oxygen pressure in the air is quite high, this hæmoglobin unites at once with a quantity of oxygen while the blood is flowing through the air-vessels. the blood is then carried off in the circulation to the active tissues like the muscles. these tissues are constantly using oxygen to carry on their life processes, and consequently at all times use up about all the oxygen within their reach. the result is that in these tissues the oxygen pressure is very low, and when the oxygen-laden hæmoglobin reaches them the association of the hæmoglobin with oxygen is at once broken up and the oxygen set free in the tissue. it passes at once to the lymph, from which the active tissues seize it for the purpose of carrying on the oxidizing processes of the body. this whole matter of supplying the body with oxygen is thus fundamentally a chemical one, controlled by chemical laws. _removal of waste_.--the next step in this life process is one of difficulty. after the food and oxygen have reached the tissues it is seized by the living cell. the food material is now oxidized by the oxygen and its latent energy is liberated, and appears in the form of motion or heat or some other vital function. herein is the really mysterious part of the life process; but for the present we will overlook the mystery of this action, and consider the results from a purely material standpoint. in a steam engine the fundamental process by which the latent energy of the fuel is liberated is that of oxidation. the oxygen of the air unites with the chemical elements of the fuel, and breaks up that fuel into simple compounds--which may be chiefly considered as three--carbonic dioxide (co_{ }), water (h_{ }o), and ash. the energy contained in the original compound can not be held by these simpler bodies, and it therefore escapes as heat. just the same process, with of course difference in details, is found in the living machine. the food, after reaching the living cell, is united with the oxygen, and, so far as chemical results are concerned, the process is much the same as if it occurred outside the body. the food is broken into simpler compounds and the contained energy is liberated. the energy is, by the mechanism of the machine, changed into motion or nervous impulse, etc. the food is broken into simple compounds, which are chiefly carbonic dioxide, water, and ash; the ash being, however, quite different from the ash obtained from burning coal. now the engine must have its chimney to remove the gases and vapours (the co_{ } and h_{ }o) and its ashpit for the ashes. in the same way the living machine has its excretory system for removing wastes. in the removal of the carbonic acid and water we have to do once more with the respiratory system, and the process is simply a repetition of the story of gas diffusion, chemical union, and osmosis. it is sufficient here to say that the process is just as simple and as easily explained as those already described. the elimination of these wastes is simply a problem of chemistry and mechanics. in the removal of the ash, however, we have something more, for here again we are brought up against the vital action of the cell. this ash takes chiefly the form of a compound known as urea, which finds its way into the general circulatory system. from the blood it is finally removed by the kidneys. in the kidneys are a large number of bits of living matter (kidney cells), which have the power of seizing hold of the urea as the blood is flowing over them, and after thus taking it out of the blood they deposit it in a series of tubes which lead to the bladder and hence to the exterior. the bringing of this ash to the kidney cell is a mechanical matter, based simply upon the flow of the blood. the seizing of the urea by the kidney cell is a vital phenomenon which we must waive for the moment. up to this point in the analysis there has been no difficulty, and no one can fail to agree with the conclusions. the position we reach is as follows: so far as relates to the general problems of energy in the universe the body is a machine. it neither creates nor destroys energy, but simply transforms one form into another. in attempting to explain the action of the machine, we find that for the functions thus far considered (sometimes called the vegetative functions) the laws of chemistry and physics furnish adequate explanation. we must now look a little further, and question some of the functions the mechanical nature of which is less obvious. the whole operation thus far described is under the control of the nervous system, which acts somewhat like the engineer of an engine. can this phase of living activity be included within the conception of the body as a machine? _nervous system_.--when we come to try to apply mechanical principles to the nervous system, we meet with what seems at first to be no thoroughfare. while dealing with the grosser questions of chemical compounds, heat, and motion, there is little difficulty in applying natural laws to the explanation of living phenomena. but the problem with the nervous system is very different. it is only to-day that we are finding that the problem is open to study, to say nothing of solution. it is true that mental and other nervous phenomena have been studied for a long time, but this study has been simply the study of these phenomena by themselves without a thought of their correlation with other phenomena of nature. it is a matter of quite recent conception that nervous phenomena have any direct relation to the other realms of nature. our first question must be whether we can find any correlation between nervous energy and other types of energy. for our purpose it will be convenient to distinguish between the phenomena of simple nervous transmission and the phenomena of mental activity. the former are the simpler, and offer the greatest hope of solution. if we are to find any correlation between nervous energy and other physical energy, we must do so by finding some way of measuring nervous energy and comparing it with the latter. this has been very difficult, for we have no way of measuring a nervous impulse directly. in the larger experiments upon the income and outgo of the body, in the respiration apparatus mentioned above, nervous phenomena apparently leave no trace. so far as experiments have gone as yet, there is no evidence of an expenditure of extra physical energy when the nervous system is in action. this is not surprising, however, for this apparatus is entirely too coarse to measure such delicate factors. that there is a correlation between nervous energy and physical energy is, however, pretty definitely proved by experiments along different lines. the first step in this direction was to find that a nervous stimulus can be measured at least indirectly. when the nerve is stimulated there passes from one end to the other an impulse, and the rapidity with which it travels can be accurately measured. when such an impulse reaches the brain it may give rise to a conscious sensation, and a somewhat definite estimation can be made of the amount of time required for this. the periods are very short, of course, but they are not instantaneous. the nervous impulse, can be studied in still other ways. we find that the impulse can be started by ordinary forms of energy. a mechanical shock, a chemical or an electrical shock will develop nervous energy. now these are ordinary forms of physical energy, and if, when they are applied to a nerve, they give rise to a nervous stimulus, the inference is certainly a legitimate one that the nerve is simply a bit of machinery adapted to the conversion of certain kinds of physical energy into nervous energy. if this is the case, then it is necessary to regard nervous energy as correlated with other forms of energy. other facts point in the same direction. not only can the nervous stimulus be developed by an electric shock, but the strength of the stimulus is within certain limits proportional to the strength of the shock which produces it. again, not only is it found that an electrical shock can develop a nervous stimulus, but conversely a nervous stimulus develops electrical energy. in ordinary nerves, even when not active, slight electric currents can be detected. they are extremely slight, and require the most delicate instruments for their detection. now when a nerve is stimulated these currents are immediately affected in such a way that under proper conditions they are increased in intensity. the increase is sufficient to make itself easily seen by the motion of a galvanometer. the motion of the galvanometer under these conditions gives a ready means of studying the character of the nervous impulse. by its use it can be determined that the nerve impulse travels along the nerve like a wave, and we can approximately determine the length and shape of the wave and its relative height at various points. now what is the significance of all these facts for our discussion? together they point clearly to the conclusion that nervous energy is correlated with other forms of physical energy. since the nervous stimulus is started by other forms of energy, and since it can, in turn, modify ordinary forms of energy, we can not avoid the conclusion that the nervous impulse is only a special form of energy developed within the nerve. it is a form of wave motion peculiar to the nerve substance, but correlated with and developed from other types of energy. this, of course, makes the nerve simply a bit of machinery. if this conclusion is true, the development of a nerve impulse would mean that a certain portion of food is broken to pieces in the body to liberate energy, and this should be accompanied by an elimination of carbonic dioxide and heat. this is easily shown to be true of muscle action. when we remove a muscle from the body it may remain capable of contracting for some time. by studying it under these conditions we find that it gives rise to carbonic dioxide and other substances, and liberates heat whenever it contracts. as already noticed, in the respiration experiments, whenever the individual experimented upon makes any motions, there is an accompanying elimination of waste products and a development of heat. but this does not appear to be demonstrable for the actions of the nervous system. although very careful experiments have been made, it has as yet been found impossible to detect any rise in temperature when a nerve impulse is passing through a nerve, nor is there any demonstrable excretion of waste products. this would be a serious objection to the conception of the nerve as a machine were it not for the fact that the nerve is so small that the total sum of its nervous energy must be very slight. the total energy of this minute machine is so slight that it can not be detected by our comparatively rough instruments of measurement. in short, all evidence goes to show that the nerve impulse is a form of motion, and hence of energy, correlated with other forms of physical energy. the nerve is, however, a very delicate machine, and its total amount of energy is very small. a tiny watch is a more delicate machine than a water-wheel, and its actions are more dependent upon the accuracy of its adjustment. the water-wheel may be made very coarse and yet be perfectly efficacious, while the watch must be fashioned with extreme delicacy. yet the water-wheel transforms vastly more energy than the watch. it may drive the many machines in a factory, while the watch can do no more than move itself. but who can doubt that the watch, as well as the water-wheel, is governed by the law of the correlation of forces? so the nervous system of the living machine is delicately adjusted and easily put out of order, and its action involves only a small amount of energy; but it is just as truly subject to the law of the conservation of energy as is the more massive muscle. _sensations_.--pursuing this subject further, we next notice that it is possible to trace a connection between physical energy and _sensations_. sensations are excited by certain external forms of motion. the living machine has, for example, one piece of apparatus capable of being affected by rapidly vibrating waves of air. this bit of the machine we call the ear. it is made of parts delicately adjusted, so that vibrating waves of air set them in motion, and their motion starts a nervous stimulus travelling along the auditory nerve. as a result this apparatus will be set in motion, and an impulse sent along the auditory nerve whenever that external type of motion which we call sound strikes the ear. in other words, the ear is a piece of apparatus for changing air vibrations into nervous stimulation, and is therefore a machine. apparently the material in the ear is like a bit of gunpowder, capable of being exploded by certain kinds of external excitation; but neither the gunpowder nor the material in the ear develops any energy other than that in it at the outset. in the same way the optic nerve has, at its end, a bit of mechanism readily excited by light vibrations of the ether, and hence the optic nerve will always be excited when ether vibrations chance to have an opportunity of setting the optic machinery in motion. and so on with the other senses. each sensory nerve has, at its end, a bit of machinery designed for the transformation of certain kinds of external energy into nervous energy, just as a dynamo is a machine for transforming motion into electricity. if the machine is broken, the external force has no longer any power of acting upon it, and the individual becomes deaf or blind. _mental phenomena_.--thus far in our analysis we need not hesitate in recognizing a correlation between physical and nervous energy. even though nervous energy is very subtle and only affects our instruments of measurements under exceptional conditions, the fact that nervous forces are excited by physical forces, and are themselves directly measurable, indicates that they are correlated with physical forces. up to this point, then, we may confidently say that the nervous system is part of the machine. but when we turn to the more obscure parts of the nervous phenomena, those which we commonly call mental, we find ourselves obliged to stop abruptly. we may trace the external force to the sensory organ, we may trace this force into a nervous stimulus, and may follow this stimulus to the brain as a wave motion, and therefore as a form of physical energy. but there we must stop. we have no idea of how the nervous impulse is converted into a sensation. the mental side of the sensation appears to stand in a category by itself, and we can not look upon it as a form of energy. it is true that many brave attempts have been made to associate the two. sensations can be measured as to intensity, and the intensity of a sensation is to a certain extent dependent upon the intensity of the stimulus exciting it. the mental sensation is undoubtedly excited by the physical wave of nervous impulse. in the growth of the individual the development of its mental powers are found to be parallel to the development of its nerves and brain--a fact which, of course, proves that mental power is dependent upon brain structure. further, it is found that certain visible changes occur in certain parts of the brain--the brain cells--when they are excited into mental activity. such series of facts point to an association between the mental side of sensations and physical structure of the machine. but they do not prove any correlation between them. the unlikeness of mental and physical phenomena is so absolute that we must hesitate about drawing any connection between them. it is impossible to conceive the mental side of a sensation as a form of wave motion. if, further, we take into consideration the other phenomena associated with the nervous system, the more distinctly mental processes, we have absolutely no data for any comparison. we can not imagine thought measured by units, and until we can conceive of such measurement we can get no meaning from any attempt to find a correlation between mental and physical phenomena. it is true that certain psychologists have tried to build up a conception of the physical nature of mind; but their attempts have chiefly resulted in building up a conception of the physical nature of the brain, and then ignoring the radical chasm that exists between mind and matter. the possibility of describing a complex brain as growing parallel to the growth of a complex mind has been regarded as equivalent to proving their identity. all attempts in this direction thus far have simply ignored the fact that the stimulation of a nerve, a purely physical process, is not the same thing as a mental action. what the future may disclose it is hazardous to say, but at present the mental side of the living machine has not been included within the conception of the mechanical nature of the organism. ==the living body is a machine.==--reviewing the subject up to this point, what must be our verdict as to our ability to understand the running of the living machine? in the first place, we are justified in regarding the body as a machine, since, so far as concerns its relations to energy, it is simply a piece of mechanism--complicated, indeed, beyond any other machine, but still a machine for changing one kind of energy into another. it receives the energy in the form of chemical composition and converts it into heat, motion, nervous wave motion, etc. all of this is sure enough. whether other forms of nervous and mental activity can be placed under the same category, or whether these must be regarded as belonging to a realm by themselves and outside of the scope of energy in the physical sense, can not perhaps be yet definitely decided. we can simply say that as yet no one has been able even to conceive how thought can be commensurate with physical energy. the utter unlikeness of thought and wave motion of any kind leads us at present to feel that on the side of mentality the comparison of the body with a machine fails of being complete. in regard to the second half of the question, whether natural forces are adequate to explain the running of the machine, we have again been able to reach a satisfactory positive answer. digestion, assimilation, circulation, respiration, excretion, the principal categories of physiological action, and at least certain phases of the action of the nervous system are readily understood as controlled by the action of chemical and physical forces. in the accomplishment of these actions there is no need for the supposition of any force other than those which are at our command in the scientific laboratory. ==the living machine constructive as well as destructive.==--in one respect the living machine differs from all others. the action of all other machines results in the _destruction_ of organized material, and thus in a _degradation of matter_. for example, a steam engine receives coal, a substance of high chemical composition, and breaks it into _more simple_ compounds, in this way liberating its stored energy. now if we examine all forms of artificial machines, we find in the same way that there is always a destruction of compounds of high chemical composition. in such machines it is common to start with heat as a source of energy, and this heat is always produced by the breaking of chemical compounds to pieces. in all chemical processes going on in the chemist's laboratory there is similarly a destruction of organic compounds. it is true that the chemist sometimes makes complex compounds out of simpler ones; but in order to do this he is obliged to use heat to bring about the combination, and this heat is obtained from the destruction of a much larger quantity of high compounds than he manufactures. the total result is therefore _destruction_ rather than manufacture of high compounds. thus it is a fact, that in all artificial machines and in all artificial chemical processes there is, as a total result, a degradation of matter toward the simpler from the more complex compounds. as a result of the action of the living machine, however, we have the opposite process of _construction_ going on. all high chemical compounds are to be traced to living beings as their source. when green plants grow in sunlight they take simple compounds and combine them together to form more complex ones in such a way that the total result is an increase of chemical compounds of high complexity. in doing this they use the energy of sunlight, which they then store away in the compounds formed. they thus produce starches, oils, proteids, woods, etc., and these stores of energy now may be used by artificial machines. the living machine builds up, other machines pull down. the living machine stores sunlight in complex compounds, other machines take it out and use it. the living organism is therefore to be compared to a sun engine, which obtains its energy directly from the sun, rather than to the ordinary engine. while this does not in the slightest militate against the idea of the living body as a machine, it does indicate that it is a machine of quite a different character from any other, and has powers possessed by no other machine. _living machines alone increase the amount of chemical compounds of high complexity._ we must notice, however, that this power of construction in distinction from destruction, is possessed only by one special class of living machines. _green plants_ alone can thus increase the store of organic compounds in the world. all colourless plants and all animals, on the other hand, live by destroying these compounds and using the energy thus liberated; in this respect being more like ordinary artificial machines. the animal does indeed perform certain constructive operations, manufacturing complex material out of simpler bodies; as, for example, making fats out of starches. but in this operation it destroys a large amount of organic material to furnish the energy for the construction, so that the total result is a degradation of chemical compounds rather than a construction. constructive processes, which increase the amount of high compounds in nature, are confined to the living machine, and indeed to one special form of it, viz., the green plant. this constructive power radically separates the living from other machines; for while constructive processes are possible to the chemist, and while engines making use of sunlight are possible, the living machine is the only machine that increases the amount of high chemical compounds in the world. ==the vital factor.==--with all this explanation of life processes it can not fail to be apparent that we have not really reached the centre of the problem. we have explained many secondary processes, but the primary ones are still unsolved. in studying digestion we reach an understanding of everything until we come to the active vital property of the gland-cells in secreting. in studying absorption we understand the process until we come to what we have called the vital powers of the absorptive cells of the alimentary canal. the circulation is intelligible until we come to the beating of the heart and the contraction of the muscles of the blood-vessels. excretion is also partly explained, but here again we finally must refer certain processes to the vital powers of active cells. and thus wherever we probe the problem we find ourselves able to explain many secondary problems, while the fundamental ones we still attribute to the vital properties of the active tissues. why a muscle contracts or a gland secretes we have certainly not yet answered. the relation of the actions to the general problems of correlation of force is simple enough. that a muscle is a machine in the sense of our definition is beyond question. but the problem of _why_ a muscle acts is not answered by showing that it derives its energy from broken food material. there are plainly still left for us a number of fundamental problems, although the secondary ones are soluble. what can we say in regard to these fundamental vital powers of the active tissues? firstly, we must notice that many of the processes which we now understand were formerly classed as vital, and we only retain under this term those which are not yet explained. this, of course, suggests to us that perhaps we may some day find an explanation for all the so-called vital powers by the application of simple physical forces. is it a fact that the only significance to the term vital is that we have not yet been able to explain these processes to our entire satisfaction? is the difference between what we have called the secondary processes and the primary ones only one of degree? is there a probability that the actions which we now call vital will some day be as readily understood as those which have already been explained? is there any method by which we can approach these fundamental problems of muscle action, heart beat, gland secretion, etc.? evidently, if this is to be done, it must be by resolving the body into its simple units and studying these units. our study thus far has been a study of the machinery of the body as a whole; but we have found that the various parts of the machine are themselves active, that apart from the action of the general machine as a whole, the separate parts have vital powers. we must, therefore, get rid of this complicated machinery, which confuses the problem, and see if we can find the fundamental units which show these properties, unencumbered by the secondary machinery which has hitherto attracted our attention. we must turn now to the problem connected with protoplasm and the living cell, since here, if anywhere, can we find the life substance reduced to its lowest terms. chapter ii. the cell and protoplasm. ==vital properties.==--we have seen that the general activities of the body are intelligible according to chemical and mechanical laws, provided we can assume as their foundation the simple vital properties of living phenomena. we must now approach closer to the centre of the problem, and ask whether we can trace these fundamental properties to their source and find an explanation of them. in the first place, what are these properties? the vital powers are varied, and lie at the basis of every form of living activity. when we free them from complications, however, they may all be reduced to four. these are: ( ) _irritability_, or the property possessed by living matter of reacting when stimulated. ( ) _movement_, or the power of contracting when stimulated. ( ) _metabolism_, or the power of absorbing extraneous food and producing in it certain chemical changes, which either convert it into more living tissue or break it to pieces to liberate the inclosed energy. ( ) _reproduction_, or the power of producing new individuals. from these four simple vital activities all other vital actions follow; and if we can find an explanation of these, we have explained the living machine. if we grant that certain parts of the body can assimilate food and multiply, having the power of contraction when irritated, we can readily explain the other functions of the living machine by the application of these properties to the complicated machinery of the body. but these properties are fundamental, and unless we can grasp them we have failed to reach the centre of the problem. as we pass from the more to the less complicated animals we find a gradual simplification of the machinery until the machinery apparently disappears. with this simplification of the machinery we find the animals provided with less varied powers and with less delicate adaptations to conditions. but withal we find the fundamental powers of the living organisms the same. for the performance of these fundamental activities there is apparently needed no machinery. the simple types of living bodies are simple in number of parts, but they possess essentially the same powers of assimilation and growth that characterize the higher forms. it is evident that in our attempt to trace the vital properties to their source we may proceed in two ways. we may either direct our attention to the simplest organisms where all secondary machinery is wanting, or to the smallest parts into which the tissues of higher organisms can be resolved and yet retain their life properties. in either way we may hope to find living phenomena in its simplest form independent of secondary machinery. but the fact is, when we turn our attention in these two directions, we find the result is the same. if we look for the lowest organisms we find them among forms that are made of a single _cell_, and if we analyze the tissues of higher animals we find the ultimate parts to be _cells_. thus, in either direction, the study of the cell is forced upon us. before beginning the study of the cell it will be well for us to try to get a clear notion of the exact nature of the problems we are trying to solve. we wish to explain the activities of life phenomena in such a way as to make them intelligible through the application of natural forces. that these processes are fundamentally chemical ones is evident enough. a chemical oxidation of food lies at the basis of all vital activity, and it is thus through the action of chemical forces that the vital powers are furnished with their energy. but the real problem is what it is in the living machine that controls these chemical processes. fat and starch may be oxidized in a chemist's test tubes, and will there liberate energy; but they do not, under these conditions, manifest vital phenomena. proteid may be brought in contact with oxygen without any oxidation occurring, and even if it is oxidized no motion or assimilation or reproduction occurs under ordinary conditions. these phenomena occur only when the oxidation takes place _in the living machine_. our problem is then to determine, if possible, what it is in the living machine that regulates the oxidations and other changes in such a way as to produce from them vital activities. why is it that the oxidation of starch in the living machine gives rise to motion, growth, and reproduction, while if the oxidation occurs in the chemist's laboratory, or even in a bit of dead protoplasm, it simply gives rise to heat? one of the primary questions to demand attention in this search is whether we are to find the explanation, at the bottom, a _chemical_ or a _mechanical_ one. in the simplest form of life in which vital manifestations are found are we to attribute these properties simply to chemical forces of the living substance, or must we here too attribute them to the action of a complicated machinery? this question is more than a formal one. that it is one of most profound significance will appear from the following considerations: chemical affinity is a well recognized force. under the action of this force chemical compounds are produced and different compounds formed under different conditions. the properties of the different compounds differ with their composition, and the more complex are the compounds the more varied their properties. now it might be assumed as an hypothesis that there could be a chemical compound so complex as to possess, among other properties, that of causing the oxidation of food to occur in such a way as to produce assimilation and growth. such a compound would, of course, be alive, and it would be just as true that its power of assimilating food would be one of its physical properties as it is that freezing is a physical property of water. if such an hypothesis should prove to be the true one, then the problem of explaining life would be a chemical one, for all vital properties would be reducible to the properties of a chemical compound. it would then only be necessary to show how such a compound came into existence and we should have explained life. nor would this be a hopeless task. we are well acquainted with forces adequate to the formation of chemical compounds. if the force of chemical affinity is adequate under certain conditions to form some compounds, it is easy to conceive it as a possibility under other conditions to produce this chemical living substance. our search would need then to be for a set of conditions under which our living compound could have been produced by the known forces of chemical affinity. but suppose, on the other hand, that we find this simplest bit of living matter is not a chemical compound, but is in itself a complicated machine. suppose that, after reducing this vital substance to its simplest type, we find that the substance with which we are dealing not only has complex chemical structure, but that it also possesses a large number of structural parts adapted to each other in such a way as to work together in the form of an intricate mechanism. the whole problem would then be changed. to explain such a machine we could no longer call upon chemical forces. chemical affinity is adequate to the explanation of chemical compounds however complicated, but it cannot offer any explanation for the adaptation of parts which make a machine. the problem of the origin of the simplest form of life would then be no longer one of chemical but one of mechanical evolution. it is plain then that the question of whether we can attribute the properties of the simplest type of life to chemical composition or to mechanical structure is more than a formal one. ==the discovery of cells.==--it is difficult for us to-day to have any adequate idea of the wonderful flood of light that was thrown upon scientific and philosophical study by the discoveries which are grouped around the terms cells and protoplasm. cells and protoplasm have become so thoroughly a part of modern biology that we can hardly picture to ourselves the vagueness of knowledge before these facts were recognized. perhaps a somewhat crude comparison will illustrate the relation which the discovery of cells had to the study of life. imagine for a moment, some intelligent being located on the moon and trying to study the phenomena on the earth's surface. suppose that he is provided with a telescope sufficiently powerful to disclose moderately large objects on the earth, but not smaller ones. he would see cities in various parts of the world with wide differences in appearance, size, and shape. he would see railroad trains on the earth rushing to and fro. he would see new cities arising and old ones increasing in size, and we may imagine him speculating as to their method of origin and the reasons why they adopt this or that shape. but in spite of his most acute observations and his most ingenious speculation, he could never understand the real significance of the cities, since he is not acquainted with the actual living unit. imagine now, if you will, that this supramundane observer invents a telescope which enables him to perceive more minute objects and thus discovers human beings. what a complete revolution this would make in his knowledge of mundane affairs! we can imagine how rapidly discovery would follow discovery; how it would be found that it was the human beings that build the houses, construct and run the railroads, and control the growth of the cities according to their fancy; and, lastly, how it would be learned that it is the human being alone that grows and multiplies and that all else is the result of his activities. such a supramundane observer would find himself entering into a new era, in which all his previous knowledge would sink into oblivion. something of this same sort of revolution was inaugurated in the study of living things by the discovery of cells and protoplasms. animals and plants had been studied for centuries and many accurate and painstaking observations had been made upon them. monumental masses of evidence had been collected bearing upon their shapes, sizes, distribution, and relations. anatomy had long occupied the attention of naturalists, and the general structure of animals and plants was already well known. but the discoveries starting in the fourth decade of the century by disclosing the unity of activity changed the aspect of biological science. ==the cell doctrine==.--the cell doctrine is, in brief, the theory that the bodies of animals and plants are built up entirely of minute elementary units, more or less independent of each other, and all capable of growth and multiplication. this doctrine is commonly regarded as being inaugurated in by schwann. long before this, however, many microscopists had seen that the bodies of plants are made up of elementary units. in describing the bark of a tree in , robert hooke had stated that it was composed of little boxes or cells, and regarded it as a sort of honeycomb structure with its cells filled with air. the term cell quite aptly describes the compartments of such a structure, as can be seen by a glance at fig. , and this term has been retained even till to-day in spite of the fact that its original significance has entirely disappeared. during the last century not a few naturalists observed and described these little vesicles, always regarding them as little spaces and never looking upon them as having any significance in the activities of plants. in one or two instances similar bodies were noticed in animals, although no connection was drawn between them and the cells of plants. in the early part of the century observations upon various kinds of animals and plant tissues multiplied, and many microscopists independently announced the discovery of similar small corpuscular bodies. finally, in , these observations were combined together by schwann into one general theory. according to the cell doctrine then formulated, the parts of all animals and plants are either composed of cells or of material derived from cells. the bark, the wood, the roots, the leaves of plants are all composed of little vesicles similar to those already described under the name of cells. in animals the cellular structure is not so easy to make out; but here too the muscle, the bone, the nerve, the gland are all made up of similar vesicles or of material made from them. the cells are of wonderfully different shapes and widely different sizes, but in general structure they are alike. these cells, thus found in animals and plants alike, formed the first connecting link between animals and plants. this discovery was like that of our supposed supramundane observer when he first found the human being that brought into connection the widely different cities in the various parts of the world. [illustration: fig. .--a bit of bark showing cellular structure.] schwann and his immediate followers, while recognizing that the bodies of animals and plants were composed of cells, were at a loss to explain how these cells arose. the belief held at first was that there existed in the bodies of animals and plants a structureless substance which formed the basis out of which the cells develop, in somewhat the same way that crystals arise from a mother liquid. this supposed substance schwann called the _cytoblastema_, and he thought it existed between the cells or sometimes within them. for example, the fluid part of the blood is the cytoblastema, the blood corpuscles being the cells. from this structureless fluid the cells were supposed to arise by a process akin to crystallization. to be sure, the cells grow in a manner very different from that of a crystal. a crystal always grows by layers being added upon its outside, while the cells grow by additions within its body. but this was a minor detail, the essential point being that from a structureless liquid containing proper materials the organized cell separated itself. this idea of the cytoblastema was early thrown into suspicion, and almost at the time of the announcement of the cell doctrine certain microscopists made the claim that these cells did not come from any structureless medium, but by division from other cells like themselves. this claim, and its demonstration, was of even greater importance than the discovery of the cells. for a number of years, however, the matter was in dispute, evidence being collected which about equally attested each view. it was a scotchman, dr. barry, who finally produced evidence which settled the question from the study of the developing egg. the essence of his discovery was as follows: the ovum of an animal is a single cell, and when it begins to develop into an embryo it first simply divides into two halves, producing two cells (fig, , _a_ and _b_). each of these in turn divides, giving four, and by repeated divisions of this kind there arises a solid mass of smaller cells (fig. , _b_ to _f_,) called the mulberry stage, from its resemblance to a berry. this is, of course, simply a mass of cells, each derived by division from the original. as the cells increase in number, the mass also increases in size by the absorption of nutriment, and the cells continue dividing until the mass contains thousands of cells. meantime the body of the animal is formed out of these cells, and when it is adult it consists of millions of cells, all of which have been derived by division from the original cell. in such a history each cell comes from pre-existing cells and a cytoblastema plays no part. [illustration: fig. .--successive stages in the division of the developing egg.] it was impossible, however, for barry or any other person to follow the successive divisions of the egg cell through all the stages to the adult. the divisions can be followed for a short time under the microscope, but the rest must be a matter of simple inference. it was argued that since cell origin begins in this way by simple division, and since the same process can be observed in the adult, it is reasonable to assume that the same process has continued uninterruptedly, and that this is the only method of cell origin. but a final demonstration of this conclusion was not forthcoming for a long time. for many years some biologists continued to believe that cells can have other origin than from pre-existing cells. year by year has the evidence for such "free cell" origin become less, until the view has been entirely abandoned, and to-day it is everywhere admitted that new cells always arise from old ones by direct descent, and thus every cell in the body of an animal or plant is a direct descendant by division from the original egg cell. ==the cell==.--but what is this cell which forms the unit of life, and to which all the fundamental vital properties can be traced? we will first glance at the structure of the cell as it was understood by the earlier microscopists. a typical cell is shown in fig. . it will be seen that it consists of three quite distinct parts. there is first the _cell wall (cw)_ which is a limiting membrane of varying thickness and shape. this is in reality lifeless material, and is secreted by the rest of the cell. being thus produced by the other active parts of the cell, we will speak of it as _formed_ material in distinction from the rest, which is _active_ material. inside this vesicle is contained a somewhat transparent semifluid material which has received various names, but which for the present we will call _cell substance_ (fig. , _pr_). it may be abundant or scanty, and has a widely varying consistency from a very liquid mass to a decidedly thick jellylike substance. lying within the cell substance is a small body, usually more or less spherical in shape, which is called the _nucleus_ (fig. , _n_). it appears to the microscope similar to the cell substance in character, and has frequently been described as a bit of the cell substance more dense than the remainder. lying within the nucleus there are usually to be seen one or more smaller rounded bodies which have been called _nucleoli_. from the very earliest period that cells have been studied, these three parts, cell wall, cell substance, and nucleus have been recognized, but as to their relations to each other and to the general activities of the cell there has been the widest variety of opinion. [illustration: fig. .--a cell; _cw_ is the cell wall; _pr_, the cell substance; _n_, the nucleus.] ==cellular structure of organisms==.--it will be well to notice next just what is meant by saying that all living bodies are composed of cells. this can best be understood by referring to the accompanying figures. figs. - , for instance, show the microscopic appearance of several plant tissues. [illustration: fig. .--cells at a root tip.] [illustration: fig. .--section of a leaf showing cells of different shapes.] at fig. will be seen the tip of a root, plainly made of cells quite similar to the typical cell described. at fig. will be seen a bit of a leaf showing the same general structure. at fig. is a bit of plant tissue of which the cell walls are very thick, so that a very dense structure is formed. at fig. is a bit of a potato showing its cells filled with small granules of starch which the cells have produced by their activities and deposited within their own bodies. at fig. are several wood cells showing cell walls of different shape which, having become dead, have lost their contents and simply remain as dead cell walls. each was in its earlier history filled with cell substance and contained a nucleus. in a similar way any bit of vegetable tissue would readily show itself to be made of similar cells. in animal tissues the cellular structure is not so easily seen, largely because the products made by the cells, the formed products, become relatively more abundant and the cells themselves not so prominent. but the cellular structure is none the less demonstrable. in fig. , for instance, will be seen a bit of cartilage where the cells themselves are rather small, while the material deposited between them is abundant. this material between the cells is really to be regarded as an excessively thickened cell wall and has been secreted by the cell substance lying within the cells, so that a bit of cartilage is really a mass of cells with an exceptionally thick cell wall. at fig. is shown a little blood. here the cells are to be seen floating in a liquid. the liquid is colourless and it is the red colour in the blood cells which gives the blood its red colour. the liquid may here again be regarded as material produced by cells. at fig. is a bit of bone showing small irregular cells imbedded within a large mass of material which has been deposited by the cell. in this case the formed material has been hardened by calcium phosphate, which gives the rigid consistency to the bone. in some animal tissues the formed material is still greater in amount. at fig. , for example, is a bit of connective tissue, made up of a mass of fine fibres which have no resemblance to cells, and indeed are not cells. these fibres have, however, been made by cells, and a careful study of such tissue at proper places will show the cells within it. the cells shown in fig. (_c_) have secreted the fibrous material. fig. shows a cell composing a bit of nerve. at fig. is a bit of muscle; the only trace of cellular structure that it shows is in the nuclei (_n_), but if the muscle be studied in a young condition its cellular structure is more evident. thus it happens in adult animals that the cells which are large and clear at first, become less and less evident, until the adult tissue seems sometimes to be composed mostly of what we have called formed material. [illustration: fig. .--plant cells with thick walls, from a fern.] [illustration: fig. .--section of a potato showing different shaped cells, the inner and larger ones being filled with grains of starch.] [illustration: fig. .--various shaped wood cells from plant tissue.] [illustration: fig. .--a bit of cartilage.] [illustration: fig. .--frog's blood: _a_ and _b_ are the cells; _c_ is the liquid.] [illustration: fig. .--a bit of bone, showing the cells imbedded in the bony matter.] it must not be imagined, however, that a very rigid line can be drawn between the cell itself and the material it forms. the formed material is in many cases simply a thickened cell wall, and this we commonly regard as part of the cell. in many cases the formed material is simply the old dead cell walls from which the living substance has been withdrawn (fig. ). in other cases the cell substance acquires peculiar functions, so that what seems to be the formed material is really a modified cell body and is still active and alive. such is the case in the muscle. in other cases the formed material appears to be manufactured within the cell and secreted, as in the case of bone. no sharp lines can be drawn, however, between the various types. but the distinction between formed material and cell body is a convenient one and may well be retained in the discussion of cells. in our discussion of the fundamental vital properties we are only concerned in the cell substance, the formed material having nothing to do with fundamental activities of life, although it forms largely the secondary machinery which we have already studied. [illustration: fig. .--connective tissue. the cells of the tissue are shown at _c_, and the fibres or formed matter at _f_.] in all higher animals and plants the life of the individual begins as a single ovum or a single cell, and as it grows the cells increase rapidly until the adult is formed out of hundreds of millions of cells. as these cells become numerous they cease, after a little, to be alike. they assume different shapes which are adapted to the different duties they are to perform. thus, those cells which are to form bone soon become different from those which are to form muscle, and those which are to form the blood are quite unlike those which are to produce the hairs. by means of such a differentiation there arises a very complex mass of cells, with great variety in shape and function. [illustration: fig. . a piece of nerve fibre, showing the cell with its nucleus at _n_.] it should be noticed further that there are some animals and plants in which the whole animal is composed of a single cell. these organisms are usually of extremely minute size, and they comprise most of the so-called animalculæ which are found in water. in such animals the different parts of the cell are modified to perform different functions. the different organs appear within the cell, and the cell is more complex than the typical cell described. fig. shows such a cell. such an animal possesses several organs, but, since it consists of a single mass of protoplasm and a single nucleus, it is still only a single cell. in the multicellular organisms the organs of the body are made up of cells, and the different organs are produced by a differentiation of cells, but in the unicellular organisms the organs are the results of the differentiation of the parts of a single cell. in the one case there is a differentiation of cells, and in the other of the parts of a cell. [illustration: fig. .--a muscle fibre. the nucleii are shown at _n_.] [illustration: fig. .--a complex cell. it is an entire animal, but composed of only one cell.] such, in brief, is the cell to whose activities it is possible to trace the fundamental properties of all living things. cells are endowed with the properties of irritability, contractibility, assimilation and reproduction, and it is thus plainly to the study of cells that we must look for an interpretation of life phenomena. if we can reach an intelligible understanding of the activities of the cell our problem is solved, for the activities of the fully formed animal or plant, however complex, are simply the application of mechanical and chemical principles among the groups of such cells. but wherein does this knowledge of cells help us? are we any nearer to understanding how these vital processes arise? in answer to this question we may first ask whether it is possible to determine whether any one part of the cell is the seat of its activities. ==the cell wall.==--the first suggestion which arose was that the cell wall was the important part of the cell, the others being secondary. this was not an unnatural conclusion. the cell wall is the most persistent part of the cell. it was the part first discovered by the microscope and is the part which remains after the other parts are gone. indeed, in many of the so-called cells the cell wall is all that is seen, the cell contents having disappeared (fig. ). it was not strange, then, that this should at first have been looked upon as the primary part. the idea was that the cell wall in some way changed the chemical character of the substances in contact with its two sides, and thus gave rise to vital activities which, as we have seen, are fundamentally chemical. thus the cell wall was regarded as the most essential part of the cell, since it controlled its activities. this the belief of schwann, although he also regarded the other parts of the cell as of importance. [illustration: fig. .--an amoeba. a single cell without cell wall. _n_ is the nucleus; _f_, a bit of food which the cell has absorbed.] this conception, however, was quite temporary. it was much as if our hypothetical supramundane observer looked upon the clothes of his newly discovered human being as forming the essential part of his nature. it was soon evident that this position could not be maintained. it was found that many bits of living matter were entirely destitute of cell wall. this is especially true of animal cells. while among plants the cell wall is almost always well developed, it is very common for animal cells to be entirely lacking in this external covering--as, for example, the white blood-cells. fig. shows an amoeba, a cell with very active powers of motion and assimilation, but with no cell wall. moreover, young cells are always more active than older ones, and they commonly possess either no cell wall or a very slight one, this being deposited as the cell becomes older and remaining long after it is dead. such facts soon disproved the notion that the cell wall is a vital part of the cell, and a new conception took its place which was to have a more profound influence upon the study of living things than any discovery hitherto made. this was the formulation of the doctrine of the nature of _protoplasm_. protoplasm.--(a) _discovery_. as it became evident that the cell wall is a somewhat inactive part of the cell, more attention was put on the cell contents. for twenty years after the formulation of the cell doctrine both the cell substance and the nucleus had been looked upon as essential to its activities. this was more especially true of the nucleus, which had been thought of as an organ of reproduction. these suggestions appeared indefinitely in the writings of one scientist and another, and were finally formulated in into a general theory which formed what has sometimes been called the starting point of modern biology. from that time the material known as _protoplasm_ was elevated into a prominent position in the discussion of all subjects connected with living phenomena. the idea of protoplasm was first clearly defined by schultze, who claimed that the real active part of the cell was the cell substance within the cell wall. this substance he proved to be endowed with powers of motion and powers of inducing chemical changes associated with vital phenomena. he showed it to be the most abundant in the most active cells, becoming less abundant as the cells lose their activity, and disappearing when the cells lose their vitality. this cell substance was soon raised into a position of such importance that the smaller body within it was obscured, and for some twenty years more the nucleus was silently ignored in biological discussion. according to schultze, the cell substance itself constituted the cell, the other parts being entirely subordinate, and indeed frequently absent. a cell was thus a bit of protoplasm, and nothing more. but the more important feature of this doctrine was not the simple conclusion that the cell substance constitutes the cell, but the more sweeping conclusion that this cell substance is in _all_ cells essentially _identical._ the study of all animals, high and low, showed all active cells filled with a similar material, and more important still, the study of plant cells disclosed a material strikingly similar. schultze experimented with this material by all means at his command, and finding that the cell substance in all animals and plants obeys the same tests, reached the conclusion that the cell substance in animals and plants is always identical. to this material he now gave the name protoplasm, choosing a name hitherto given to the cell contents of plant cells. from this time forth this term protoplasm was applied to the living material found in all cells, and became at once the most important factor in the discussion of biological problems. the importance of this newly formulated doctrine it is difficult to appreciate. here, in protoplasm had been apparently found the foundation of living phenomena. here was a substance universally present in animals and plants, simple and uniform--a substance always present in living parts and disappearing with death. it was the simplest thing that had life, and indeed the only thing that had life, for there is no life outside of cells and protoplasm. but simple as it was it had all the fundamental properties of living things--irritability, contractibility, assimilation, and reproduction. it was a compound which seemingly deserved the name of "_physical basis of life_", which was soon given to it by huxley. with this conception of protoplasm as the physical basis of life the problems connected with the study of life became more simplified. in order to study the nature of life it was no longer necessary to study the confusing mass of complex organs disclosed to us by animals and plants, or even the somewhat less confusing structures shown by individual cells. even the simple cell has several separate parts capable of undergoing great modifications in different types of animals. this confusion now appeared to vanish, for only _one_ thing was found to be alive, and that was apparently very simple. but that substance exhibited all the properties of life. it moved, it could grow, and reproduce itself, so that it was necessary only to explain this substance and life would be explained. (b) _nature of protoplasm_.--what is this material, protoplasm? as disclosed by the early microscope it appeared to be nothing more than a simple mass of jelly, usually transparent, more or less consistent, sometimes being quite fluid, and at others more solid. structure it appeared to have none. its chief peculiarity, so far as physical characters were concerned, was a wonderful and never-ceasing activity. this jellylike material appeared to be endowed with wonderful powers, and yet neither physical nor microscopical study revealed at first anything more than a uniform homogeneous mass of jelly. chemical study of the same substance was of no less interest than the microscopical study. of course it was no easy matter to collect this protoplasm in sufficient quantity and pure enough to make a careful analysis. the difficulties were in time, however, overcome, and chemical study showed protoplasm to be a proteid, related to other proteids like albumen, but one which was more complex than any other known. it was for a long time looked upon by many as a single definite chemical compound, and attempts were made to determine its chemical formula. such an analysis indicated a molecule made up of several hundred atoms. chemists did not, however, look with much confidence upon these results, and it is not surprising that there was no very close agreement among them as to the number of atoms in this supposed complex molecule. moreover, from the very first, some biologists thought protoplasm to be not one, but more likely a mixture of several substances. but although it was more complex than any other substance studied, its general characters were so like those of albumen that it was uniformly regarded as a proteid; but one which was of a higher complexity than others, forming perhaps the highest number of a series of complex chemical compounds, of which ordinary proteids, such as albumen, formed lower members. thus, within a few years following the discovery of protoplasm there had developed a theory that living phenomena are due to the activities of a definite though complex chemical compound, composed chiefly of the elements carbon, oxygen, hydrogen, and nitrogen, and closely related to ordinary proteids. this substance was the basis of living activity, and to its modification under different conditions were due the miscellaneous phenomena of life. (c) _significance of protoplasm_.--the philosophical significance of this conception was very far-reaching. the problem of life was so simplified by substituting the simple protoplasm for the complex organism that its solution seemed to be not very difficult. this idea of a chemical compound as the basis of all living phenomena gave rise in a short time to a chemical theory of life which was at least tenable, and which accounted for the fundamental properties of life. that theory, the _chemical theory of life_, may be outlined somewhat as follows: the study of the chemical nature of substances derived from living organisms has developed into what has been called organic chemistry. organic chemistry has shown that it is possible to manufacture artificially many of the compounds which are called organic, and which had been hitherto regarded as produced only by living organisms. at the beginning of the century, it was supposed to be impossible to manufacture by artificial means any of the compounds which animals and plants produce as the result of their life. but chemists were not long in showing that this position is untenable. many of the organic products were soon shown capable of production by artificial means in the chemist's laboratory. these organic compounds form a series beginning with such simple bodies as carbonic acid (co_{ }), water (h_{ }o), and ammonia (nh_{ }), and passing up through a large number of members of greater and greater complexity, all composed, however, chiefly of the elements carbon, oxygen, hydrogen, and nitrogen. our chemists found that starting with simple substances they could, by proper means, combine them into molecules of greater complexity, and in so doing could make many of the compounds that had hitherto been produced only as a result of living activities. for example, urea, formic acid, indigo, and many other bodies, hitherto produced only by animals and plants, were easily produced by the chemist by purely chemical methods. now when protoplasm had been discovered as the "physical basis of life," and, when it was further conceived that this substance is a proteid related to albumens, it was inevitable that a theory should arise which found the explanation of life in accordance with simple chemical laws. if, as chemists and biologists then believe, protoplasm is a compound which stands at the head of the organic series, and if, as is the fact, chemists are each year succeeding in making higher and higher members of the series, it is an easy assumption that some day they will be able to make the highest member of the series. further, it is a well-known fact that simple chemical compounds have simple physical properties, while the higher ones have more varied properties. water has the property of being liquid at certain temperatures and solid at others, and of dividing into small particles (i.e., dissolving) certain bodies brought in contact with it. the higher compound albumen has, however, a great number of properties and possibilities of combination far beyond those of water. now if the properties increase in complexity with the complexity of the compound, it is again an easy assumption that when we reach a compound as complex as protoplasm, it will have properties as complex as those of the simple life substance. nor was this such a very wild hypothesis. after all, the fundamental life activities may all be traced to the simple oxidation of food, for this results in movement, assimilation, and growth, and the result of growth is reproduction. it was therefore only necessary for our biological chemists to suppose that their chemical compound protoplasm possessed the power of causing certain kinds of oxidation to take place, just as water itself induces a simpler kind of oxidation, and they would have a mechanical explanation of the life activities. it was certainly not a very absurd assumption to make, that this substance protoplasm could have this power, and from this the other vital activities are easily derived. in other words, the formulation of the doctrine of protoplasm made it possible to assume that _life_ is not a distinct force, but simply a name given to the properties possessed by that highly complex chemical compound protoplasm. just as we might give the name _aquacity_ to the properties possessed by water, so we have actually given the name _vitality_ to the properties possessed by protoplasm. to be sure, vitality is more marvelous than aquacity, but so is protoplasm a more complex compound than water. this compound was a very unstable compound, just as is a mass of gunpowder, and hence it is highly irritable, also like gunpowder, and any disturbance of its condition produces motion, just as a spark will do in a mass of gunpowder. it is capable of inducing oxidation in foods, something as water induces oxidation in a bit of iron. the oxidation is, however, of a different kind, and results in the formation of different chemical combinations; but it is the basis of assimilation. since now assimilation is the foundation of growth and reproduction, this mechanical theory of life thus succeeded in tracing to the simple properties of the chemical compound protoplasm, all the fundamental properties of life. since further, as we have seen in our first chapter, the more complex properties of higher organisms are easily deduced from these simple ones by the application of the laws of mechanics, we have here in this mechanical theory of life the complete reduction of the body to a machine. ==the reign of protoplasm.==--this substance protoplasm became now naturally the centre of biological thought. the theory of protoplasm arose at about the same time that the doctrine of evolution began to be seriously discussed under the stimulus of darwin, and naturally these two great conceptions developed side by side. evolution was constantly teaching that natural forces are sufficient to account for many of the complex phenomena which had hitherto been regarded as insolvable; and what more natural than the same kind of thinking should be applied to the vital activities manifested by this substance protoplasm. while the study of plants and animals was showing scientists that natural forces would explain the origin of more complex types from simpler ones through the law of natural selection, here in this conception of protoplasm was a theory which promised to show how the simplest forms may have been derived from the non-living. for an explanation of the _origin_ of life by natural means appeared now to be a simple matter. it required now no violent stretch of the imagination to explain the origin of life something as follows: we know that the chemical elements have certain affinities for each other, and will unite with each other under proper conditions. we know that the methods of union and the resulting compounds vary with the conditions under which the union takes place. we know further that the elements carbon, hydrogen, oxygen, and nitrogen have most remarkable properties, and unite to form an almost endless series of remarkable bodies when brought into combination under different conditions. we know that by varying the conditions the chemist can force these elements to unite into a most extraordinary variety of compounds with an equal variety of properties. what more natural, then, than the assumption that under certain conditions these same elements would unite in such a way as to form this compound protoplasm; and then, if the ideas concerning protoplasm were correct, this body would show the properties of protoplasm, and therefore be alive. certainly such a supposition was not absurd, and viewed in the light of the rapid advance in the manufacture of organic compounds could hardly be called improbable. chemists beginning with simple bodies like co_{ } and h_{ }o were climbing the ladder, each round of which was represented by compounds of higher complexity. at the top was protoplasm, and each year saw our chemists nearer the top of the ladder, and thus approaching protoplasm as their final goal. they now began to predict that only a few more years would be required for chemists to discover the proper conditions, and thus make protoplasm. as late as the prediction was freely made that the next great discovery would be the manufacture of a bit of protoplasm by artificial means, and thus in the artificial production of life. the rapid advance in organic chemistry rendered this prediction each year more and more probable. the ability of chemists to manufacture chemical compounds appeared to be unlimited, and the only question in regard to their ability to make protoplasm thus resolved itself into the question of whether protoplasm is really a chemical compound. we can easily understand how eager biologists became now in pursuit of the goal which seemed almost within their reach; how interested they were in any new discovery, and how eagerly they sought for lower and simpler types of protoplasm since these would be a step nearer to the earliest undifferentiated life substance. indeed so eager was this pursuit for pure undifferentiated protoplasm, that it led to one of those unfounded discoveries which time showed to be purely imaginary. when this reign of protoplasm was at its height and biologists were seeking for even greater simplicity a most astounding discovery was announced. the british exploring ship challenger had returned from its voyage of discovery and collection, and its various treasures were turned over to the different scientists for study. the brilliant prof. huxley, who had first formulated the mechanical theory of life, now startled the biological world with the statement that these collections had shown him that at the bottom of the deep sea, in certain parts of the world, there exists a diffused mass of living _undifferentiated protoplasm_. so simple and undifferentiated was it that it was not divided into cells and contained no nucleii. it was, in short, exactly the kind of primitive protoplasm which the evolutionist wanted to complete his chain of living structures, and the biologist wanted to serve as a foundation for his mechanical theory of life. if such a diffused mass of undifferentiated protoplasm existed at the bottom of the sea, one could hardly doubt that it was developed there by some purely natural forces. the discovery was a startling one, for it seemed that the actual starting point of life had been reached. huxley named his substance _bathybias_, and this name became in a short time familiar to every one who was thinking of the problems of life. but the discovery was suspected from the first, because it was too closely in accord with speculation, and it was soon disproved. its discoverer soon after courageously announced to the world that he had been entirely mistaken, and that the bathybias, so far from being undifferentiated protoplasm, was not an organic product at all, but simply a mineral deposit in the sea water made by purely artificial means. bathybias stands therefore as an instance of a too precipitate advance in speculation, which led even such a brilliant man as prof. huxley into an unfortunate error of observation; for, beyond question, he would never have made such a mistake had he not been dominated by his speculative theories as to the nature of protoplasm. but although bathybias proved delusive, this did not materially affect the advance and development of the doctrine of protoplasm. simple forms of protoplasm were found, although none quite so simple as the hypothetical bathybias. the universal presence of protoplasm in the living parts of all animals and plants and its manifest activities completely demonstrated that it was the only living substance, and as the result of a few years of experiment and thought the biologist's conception of life crystallized into something like this: living organisms are made of cells, but these cells are simply minute independent bits of protoplasm. they may contain a nucleus or they may not, but the essence of the cell is the protoplasm, this alone having the fundamental activities of life. these bits of living matter aggregate themselves together into groups to form colonies. such colonies are animals or plants. the cells divide the work of the colony among themselves, each cell adopting a form best adapted for the special work it has to do. the animal or plant is thus simply an aggregate of cells, and its activities are the sum of the activities of its separate cells; just as the activities of a city are the sum of the activities of its individual inhabitants. the bit of protoplasm was the unit, and this was a chemical compound or a simple mixture of compounds to whose combined physical properties we have given the name vitality. ==the decline of the reign of protoplasm.==--hardly had this extreme chemical theory of life been clearly conceived before accumulating facts began to show that it is untenable and that it must at least be vastly modified before it can be received. the foundation of the chemical theory of life was the conception that protoplasm is a definite though complex chemical compound. but after a few years' study it appeared that such a conception of protoplasm was incorrect. it had long been suspected that protoplasm was more complex than was at first thought. it was not even at the outset found to be perfectly homogeneous, but was seen to contain minute granules, together with bodies of larger size. although these bodies were seen they were regarded as accidental or secondary, and were not thought of as forming any serious objection to the conception of protoplasm as a definite chemical compound. but modern opticians improved their microscopes, and microscopists greatly improved their methods. with the new microscopes and new methods there began to appear, about twenty years ago, new revelations in regard to this protoplasm. its lack of homogeneity became more evident, until there has finally been disclosed to us the significant fact that protoplasm is to be regarded as a substance not only of chemical but also of high mechanical complexity. the idea of this material as a simple homogeneous compound or as a mixture of such compounds is absolutely fallacious. protoplasm is to-day known to be made up of parts harmoniously adapted to each other in such a way as to form an extraordinarily intricate machine; and the microscopist of to-day recognizes clearly that the activities of this material must be regarded as the result of the machinery which makes up protoplasm rather than as the simple result of its chemical composition. protoplasm is a machine and not a chemical compound. [illustration: fig. .--a cell as it appears to the modern microscope. _a_, protoplasmic reticulum; _b_, liquid in its meshes; _c_, nuclear membrane; _d_, nuclear reticulum; _e_, chromatin reticulum; _f_, nucleolus; _g_, centrosome; _h_, centrosphere; _i_, vacuole; _j_, inert bodies.] ==structure of protoplasm==.--the structure of protoplasm is not yet thoroughly understood by scientists, but a few general facts are known beyond question. it is thought, in the first place, that it consists of two quite different substances. there is a somewhat solid material permeating it, usually, regarded as having a reticulate structure. it is variously described, sometimes as a reticulate network, sometimes as a mass of threads or fibres, and sometimes as a mass of foam (fig. , _a_). it is extremely delicate and only visible under special conditions and with the best of microscopes. only under peculiar conditions can it be seen in protoplasm while alive. there is no question, however, that all protoplasm is permeated when alive by a minute delicate mass of material, which may take the form of threads or fibres or may assume other forms. within the meshes of this thread or reticulum there is found a liquid, perfectly clear and transparent, to whose presence the liquid character of the protoplasm is due (fig. , _b_). in this liquid no structure can be determined, and, so far as we know, it is homogeneous. still further study discloses other complexities. it appears that the fibrous material is always marked by the presence of excessively minute bodies, which have been called by various names, but which we will speak of as _microsomes_. sometimes, indeed, the fibres themselves appear almost like strings of beads, so that they have been described as made up of rows of minute elements. it is immaterial for our purpose, however, whether the fibres are to be regarded as made up of microsomes or not. this much is sure, that these microsomes --granules of excessive minuteness--occur in protoplasm and are closely connected with the fibres (fig. , _a_). ==the nucleus.==--(a) _presence of a nucleus_.--if protoplasm has thus become a new substance in our minds as the result of the discoveries of the last twenty years, far more marvelous have been the discoveries made in connection with that body which has been called the nucleus. even by the early microscopists the nucleus was recognized, and during the first few years of the cell doctrine it was frequently looked upon as the most active part of the cell and as especially connected with its reproduction. the doctrine of protoplasm, however, so captivated the minds of biologists that for quite a number of years the nucleus was ignored, at least in all discussions connected with the nature of life. it was a body in the cell whose presence was unexplained and which did not fall into accord with the general view of protoplasm as the physical basis of life. for a while, therefore, biologists gave little attention to it, and were accustomed to speak of it simply as a bit of protoplasm a little more dense than the rest. the cell was a bit of protoplasm with a small piece of more dense protoplasm in its centre appearing a little different from the rest and perhaps the most active part of the cell. as a result of this excessive belief in the efficiency of protoplasm the question of the presence of a nucleus in the cell was for a while looked upon as one of comparatively little importance. many cells were found to have nucleii while others did not show their presence, and microscopists therefore believed that the presence of a nucleus was not necessary to constitute a cell. a german naturalist recognized among lower animals one group whose distinctive characteristic was that they were made of cells without nucleii, giving the name _monera_ to the group. as the method of studying cells improved microscopists learned better methods of discerning the presence of the nucleus, and as it was done little by little they began to find the presence of nucleii in cells in which they had hitherto not been seen. as microscopists now studied one after another of these animals and plants whose cells had been said to contain no nucleus, they began to find nucleii in them, until the conclusion was finally reached that a nucleus is a fundamental part of all active cells. old cells which have lost their activity may not show nucleii, but, so far as we know, all active cells possess these structures, and apparently no cell can carry on its activity without them. some cells have several nucleii, and others have the nuclear matter scattered through the whole cell instead of being aggregated into a mass; but nuclear matter the cell must have to carry on its life. [illustration: fig. .--a cell cut into three pieces, each containing a bit of the nucleus. each continues its life indefinitely, soon acquiring the form of the original as at _c_.] later the experiment was made of depriving cells of their nucleii, and it still further emphasized the importance of the nucleus. among unicellular animals are some which are large enough for direct manipulation, and it is found that if these cells are cut into pieces the different pieces will behave very differently in accordance with whether or not they have within them a piece of the nucleus. all the pieces are capable of carrying on their life activities for a while. the pieces of the cell which contain the nucleus of the original cell, or even a part of it, are capable of carrying on all its life activities perfectly well. in fig. is shown such a cell cut into three pieces, each of which contains a piece of the nucleus. each carries on its life activities, feeds, grows and multiplies perfectly well, the life processes seeming to continue as if nothing had happened. quite different is it with fragments which contain none of the nucleus (fig. ). these fragments ( and ), even though they may be comparatively large masses of protoplasm, are incapable of carrying on the functions of their life continuously. for a while they continue to move around and apparently act like the other fragments, but after a little their life ceases. they are incapable of assimilating food and incapable of reproduction, and hence their life cannot continue very long. facts like these demonstrate conclusively the vital importance of the nucleus in cell activity, and show us that the cell, with its power of continued life, must be regarded as a combination of protoplasm with its nucleus, and cannot exist without it. it is not protoplasm, but cell substance, plus cell nucleus, which forms the simplest basis of life. [illustration: fig. .--a cell cut into three pieces, only one of which, no. , contains any nucleus. this fragment soon acquires the original form and continues its life indefinitely, as shown at _b_. the other two pieces though living for a time, die without reproducing.] as more careful study of protoplasm was made it soon became evident that there is a very decided difference between the nucleus and the protoplasm. the old statement that the nucleus is simply a bit of dense protoplasm is not true. in its chemical and physical composition as well as in its activities the nucleus shows itself to be entirely different from the protoplasm. it contains certain definite bodies not found in the cell substance, and it goes through a series of activities which are entirely unrepresented in the surrounding protoplasm. it is something entirely distinct, and its relations to the life of the cell are unique and marvelous. these various facts led to a period in the discussion of biological topics which may not inappropriately be called the reign of the nucleus. let us, therefore, see what this structure is which has demanded so much attention in the last twenty years. (b) _structure of the nucleus_.--at first the nucleus appears to be very much like the cell substance. like the latter, it is made of fibres, which form a reticulum (fig. ), and these fibres, like those of protoplasm, have microsomes in intimate relation with them and hold a clear liquid in their meshes. the meshes of the network are usually rather closer than in the outer cell substance, but their general character appears to be the same. but a more close study of the nucleus discloses vast differences. in the first place, the nucleus is usually separated from the cell substance by a membrane (fig. , _c_). this membrane is almost always present, but it may disappear, and usually does disappear, when the nucleus begins to divide. within the nucleus we find commonly one or two smaller bodies, the nucleoli (fig. , _f_). they appear to be distinct vital parts of the nucleus, and thus different from certain other solid bodies which are simply excreted material, and hence lifeless. further, we find that the reticulum within the nucleus is made up of two very different parts. one portion is apparently identical with the reticulum of the cell substance (fig. , _d_). this forms an extremely delicate network, whose fibres have chemical relations similar to those of the cell substance. indeed, sometimes, the fibres of the nucleus may be seen to pass directly into those of the network of the cell substance, and hence they are in all probability identical. this material is called _linin_, by which name we shall hereafter refer to it. there is, however, in the nucleus another material which forms either threads, or a network, or a mass of granules, which is very different from the linin, and has entirely different properties. this network has the power of absorbing certain kinds of stains very actively, and is consequently deeply stained when treated as the microscopist commonly prepares his specimens. for this reason it has been named _chromatin_ (fig, , _e_), although in more recent times other names have been given to it. of all parts of the cell this chromatin is the most remarkable. it appears in great variety in different cells, but it always has remarkable physiological properties, as will be noticed presently. all things considered, this chromatin is probably the most remarkable body connected with organic life. [illustration: fig. .--different forms of nucleii.] the nucleii of different animals and plants all show essentially the characteristics just described. they all contain a liquid, a linin network, and a chromatin thread or network, but they differ most remarkably in details, so that the variety among the nucleii is almost endless (fig. ). they differ first in their size relative to the size of the cell; sometimes--especially in young cells--the nucleus being very large, while in other cases the nucleus is very small and the protoplasmic contents of the cell very large; finally, in cells which have lost their activity the nucleus may almost or entirely disappear. they differ, secondly, in shape. the typical form appears to be spherical or nearly so; but from this typical form they may vary, becoming irregular or elongated. they are sometimes drawn out into long masses looking like a string of beads (fig. ), or, again, resembling minute coiled worms (fig. ), while in still other cells they may be branching like the twigs of a tree. the form and shape of the chromatin thread differs widely. sometimes this appears to be mere reticulum (fig. ); at others, a short thread which is somewhat twisted or coiled (fig. ); while in other cells the chromatin thread is an extremely long, very much twisted convolute thread so complexly woven into a tangle as to give the appearance of a minute network. the nucleii differ also in the number of nucleoli they contain as well as in other less important particulars. fig. will give a little notion of the variety to be found among different nucleii; but although they thus do vary most remarkably in shape in the essential parts of their structure they are alike. ==centrosome.==--before noticing the activities of the nucleus it will be necessary to mention a third part of the cell. within the last few years there has been found to be present in most cells an organ which has been called the _centrosome._ this body is shown at fig. , _g_. it is found in the cell substance just outside the nucleus, and commonly appears as an extremely minute rounded dot, so minute that no internal structure has been discerned. it may be no larger than the minute granules or microsomes in the cell, and until recently it entirely escaped the notice of microscopists. it has now, however, been clearly demonstrated as an active part of the cell and entirely distinct from the ordinary microsomes. it stains differently, and, as we shall soon see, it appears to be in most intimate connection with the center of cell life. in the activities which characterize cell life this centrosome appears to lead the way. from it radiate the forces which control cell activity, and hence this centrosome is sometimes called the dynamic center of the cell. this leads us to the study of cell activity, which discloses to us some of the most extraordinary phenomena which have come to the knowledge of science. ==function of the nucleus.==--to understand why it is that the nucleus has taken such a prominent position in modern biological discussion it will be only necessary to notice some of the activities of the cell. of the four fundamental vital properties of cell life the one which has been most studied and in regard to which most is known is reproduction. this knowledge appears chiefly under two heads, viz., _cell division_ and the _fertilization of the egg_. every animal and plant begins its life as a simple cell, and the growth of the cell into the adult is simply the division of the original cell into parts accompanied by a differentiation of the parts. the fundamental phenomena of growth and reproduction is thus cell division, and if we can comprehend this process in these simple cells we shall certainly have taken a great step toward the explanation of the mechanics of life. during the last ten years this cell division has been most thoroughly studied, and we have a pretty good knowledge of it so far as its microscopical features are concerned. the following description will outline the general facts of such cell division, and will apply with considerable accuracy to all cases of cell division, although the details may differ not a little. [illustration: fig. .--this and the following figures show stages in cell division. fig. shows the resting stage with the chromatin, _cr_, in the form of a network within the nuclear membrane and the centrosome, _ce_, already divided into two.] [illustration: fig. .--the chromatin is broken into threads or chromosomes, _cr._ the centrosomes show radiating fibres.] ==cell division or karyokinesis.==--we will begin with a cell in what is called the resting stage, shown at fig. . such a cell has a nucleus, with its chromatin, its membrane, and linin, as already described. outside the nucleus is the centrosome, or, more commonly, two of them lying close together. if there is only one it soon divides into two, and if it has already two, this is because a single centrosome which the cell originally possessed has already divided into two, as we shall presently see. this cell, in short, is precisely like the typical cell which we have described, except in the possession of two centrosomes. the first indication of the cell division is shown by the chromatin fibres. during the resting stage this chromatin material may have the form of a thread, or may form a network of fibres (see fig. ). but whatever be its form during the resting stage, it assumes the form of a thread as the cell prepares for division. almost at once this thread breaks into a number of pieces known as _chromosomes_ (fig. ). it is an extremely important fact that the number of these chromosomes in the ordinary cells of any animal or plant is always the same. in other words, in all the cells of the body of animal or plant the chromatin material in the nucleus breaks into the same number of short threads at the time that the cell is preparing to divide. the number is the same for all animals of the same species, and is never departed from. for example, the number in the ox is always sixteen, while the number in the lily is always twenty-four. during this process of the formation of the chromosomes the nucleoli disappear, sometimes being absorbed apparently in the chromosomes, and sometimes being ejected into the cell body, where they disappear. whether they have anything to do with further changes is not yet known. the next step in the process of division appears in the region of the centrosomes. each of the two centrosomes appears to send out from itself delicate radiating fibres into the surrounding cell substance (fig. ). whether these actually arise from the centrosome or are simply a rearrangement of the fibres in the cell substance is not clear, but at all events the centrosome becomes surrounded by a mass of radiating fibres which give it a starlike appearance, or, more commonly, the appearance of a double star, since there are two centrosomes close together (fig. ). these radiating fibres, whether arising from the centrosomes or not, certainly all centre in these bodies, a fact which indicates that the centrosomes contain the forces which regulate their appearance. between the two stars or asters a set of fibres can be seen running from one to the other (fig. ). these two asters and the centrosomes within them have been spoken of as the dynamic centre of the cell since they appear to control the forces which lead to cell division. in all the changes which follow these asters lead the way. the two asters, with their centrosomes, now move away from each other, always connected by the spindle fibres, and finally come to lie on opposite sides of the nucleus (figs. , ). when they reach this position they are still surrounded by the radiating fibres, and connected by the spindle fibres. meantime the membrane around the nucleus has disappeared, and thus the spindle fibres readily penetrate into the nuclear substance (fig. ). [illustration: fig. .--the centrosomes are separating but are connected by fibres.] [illustration: fig. .--the centrosomes are separate and the equatorial plate of chromosomes, _cr_, is between them.] during this time the chromosomes have been changing their position. whether this change in position is due to forces within themselves, or whether they are moved around passively by forces residing in the cell substances, or whether, which is the most probable, they are pulled or pushed around by the spindle fibres which are forcing their way into the nucleus, is not positively known; nor is it, for our purposes, of special importance. at all events, the result is that when the asters have assumed their position at opposite poles of the nucleus the chromosomes are arranged in a plane passing through the middle of the nucleus at equal distances from each aster. it seems certain that they are pulled or pushed into this position by forces radiating from the centrosomes. fig. shows this central arrangement of the chromosomes, forming what is called the _equatorial plate_. the next step is the most significant of all. it consists in the splitting of each chromosome into two equal halves. the threads _do not divide in their middle but split lengthwise_, so that there are formed two halves identical in every respect. in this way are produced twice the original number of chromosomes, but all in pairs. the period at which this splitting of the chromosomes occurs is not the same in all cells. it may occur, as described, at about the time the asters have reached the opposite poles of the nucleus, and an equatorial plate is formed. it is not infrequent, however, for it to occur at a period considerably earlier, so that the chromosomes are already divided when they are brought into the equatorial plate. at some period or other in the cell division this splitting of the chromosomes takes place. the significance of the splitting is especially noteworthy. we shall soon find reason for believing that the chromosomes contain all the hereditary traits which the cell hands down from generation to generation, and indeed that the chromosomes of the egg contain all the traits which the parent hands down to the child. now, if this chromatin thread consists of a series of units, each representing certain hereditary characters, then it is plain that the division of the thread by splitting will give rise to a double series of threads, each of which has identical characters. should the division occur _across_ the thread the two halves would be unlike, but taking place as it does by a _longitudinal splitting_ each unit in the thread simply divides in half, and thus the resulting half threads each contain the same number of similar units as the other and the same as possessed by the original undivided chromosome. this sort of splitting thus doubles the number of chromosomes, but produces no differentiation of material. [illustration: fig. .--stage showing the two halves of the chromosomes separated from each other.] [illustration: fig. .--final stage with two nucleii in which the chromosomes have again assumed the form of a network. the centrosomes have divided preparatory to the next division, and the cell is beginning to divide.] the next step in the cell division consists in the separation of the two halves of the chromosomes. each half of each chromosome separates from its fellow, and moves to the opposite end of the nucleus toward the two centrosomes (fig. ). whether they are pulled apart or pushed apart by the spindle fibres is not certain, although it is apparently sure that these fibres from the centrosomes are engaged in the matter. certain it is that some force exerted from the two centrosomes acts upon the chromosomes, and forces the two halves of each one to opposite ends of the nucleus, where they now collect and form two _new nucleii_, with evidently exactly the same number of chromosomes as the original, and with characters identical to each other and to the original (fig. ). the rest of the cell division now follows rapidly. a partition grows in through the cell body dividing it into two parts (fig. ), the division passing through the middle of the spindle. in this division, in some cases at least, the spindle fibres bear a part--a fact which again points to the importance of the centrosomes and the forces which radiate from them. now the chromosomes in each daughter nucleus unite to form a single thread, or may diffuse through the nucleus to form a network, as in fig. . they now become surrounded by a membrane, so that the new nucleus appears exactly like the original one. the spindle fibres disappear, and the astral fibres may either disappear or remain visible. the centrosome may apparently in some cases disappear, but more commonly remains beside the daughter nucleii, or it may move into the nucleus. eventually it divides into two, the division commonly occurring at once (fig. ), but sometimes not until the next cell division is about to begin. thus the final result shows two cells each with a nucleus and two centrosomes, and this is exactly the same sort of structure with which the process began. (_see frontispiece_.) viewed as a whole, we may make the following general summary of this process. the essential object of this complicated phenomena of _karyokinesis_ is to divide the chromatin into equivalent halves, so that the cells resulting from the cell division shall contain an exactly equivalent chromatin content. for this purpose the chromatic elements collect into threads and split lengthwise. the centrosome, with its fibres, brings about the separation of these two halves. plainly, we must conclude that the chromatin material is something of extraordinary importance to the cell, and the centrosome is a bit of machinery for controlling its division and thus regulating cell division. ==fertilization of the egg.==--this description of cell division will certainly give some idea of the complexity of cell life, but a more marvelous series of changes still takes place during the time when the egg is preparing for development. inasmuch as this process still further illustrates the nature of the cell, and has further a most intimate bearing upon the fundamental problem of heredity, it will be necessary for us to consider it here briefly. the sexual reproduction of the many-celled animals is always essentially alike. a single one of the body cells is set apart to start the next generation, and this cell, after separating from the body of the animal or plant which produced it, begins to divide, as already shown in fig. , and the many cells which arise from it eventually form the new individual this reproductive cell is the egg. but before its division can begin there occurs in all cases of sexual reproduction a process called fertilization, the essential feature of which is the union of this cell with another commonly from a different individual. while the phenomenon is subject to considerable difference in details, it is essentially as follows: [illustration: fig. --an egg showing the cell substance and the nucleus, the latter containing chromosomes in large number and a nucleolus] the female reproductive cell is called the egg, and it is this cell which divides to form the next generation. such a cell is shown in fig. . like other cells it has a cell wall, a cell substance with its linin and fluid portions, a nucleus surrounded by a membrane and containing a reticulum, a nucleolus and chromatic material, and lastly, a centrosome. now such an egg is a complete cell, but it is not able to begin the process of division which shall give rise to a new individual until it has united with another cell of quite a different sort and commonly derived from a different individual called the male. why the egg cell is unable to develop without such union with male cell does not concern us here, but its purpose will be evident as the description proceeds. the egg cell as it comes from the ovary of the female individual is, however, not yet ready for union with the male cell, but must first go through a series of somewhat remarkable changes constituting what is called _maturation_ of the egg. this phenomenon has such an intimate relation to all problems connected with the cell, that it must be described somewhat in detail. there are considerable differences in the details of the process as it occurs in various animals, but they all agree in the fundamental points. the following is a general description of the process derived from the study of a large variety of animals and plants. [illustration fig. .--this and the following figures represent the process of fertilization of an egg. in all figures _cr_ is the chromosomes; _cs_ represents the cell substance (omitted in the following figures); _mc_ is the male reproductive cell lying in contact with the egg; _mn_ is the male nucleus after entering the egg.] [illustration: fig. .--the egg centrosome has divided, and the male cell with its centrosome has entered the egg.] in the cells of the body of the animal to which this description applies there are four chromosomes this is true of all the cells of the animal except the sexual cells. the eggs arise from the other cells of the body, but during their growth the chromatin splits in such a way that the egg contains double the number of chromosomes, i.e., eight (fig. ). if this egg should now unite with the other reproductive cell from the male, the resulting fertilized egg would plainly contain a number of chromosomes larger than that normal for this species of animal. as a result the next generation would have a larger number of chromosomes in each cell than the last generation, since the division of the egg in development is like that already described and always results in producing new cells with the same number of chromosomes as the starting cell. hence, if the number of chromosomes in the next generation is to be kept equal to that in the last generation, this egg cell must get rid of a part of its chromatin material. this is done by a process shown in fig. . the centrosome divides as in ordinary cell division (fig. ), and after rotating on its axis it approaches the surface of the egg (figs. and ). the egg now divides (fig. ), but the division is of a peculiar kind. although the chromosomes divide equally the egg itself divides into two very unequal parts, one part still appearing as the egg and the other as a minute protuberance called the polar cell (_pc'_ in fig. ). the chromosomes do not split as they do in the cell division already described, but each of these two cells, the egg and the polar body, receives four chromosomes (fig. ). the result is that the egg has now the normal number of chromosomes for the ordinary cells of the animal in question. but this is still too many, for the egg is soon to unite with the male cell; and this male cell, as we shall see, is to bring in its own quota of chromosomes. hence the egg must get rid of still more of its chromatin material. consequently, the first division is followed by a second (fig. ), in which there is again produced a large and a small cell. this division, like the first, occurs without any splitting of the chromosomes, one half of the remaining chromosomes being ejected in this new cell, the second polar cell (_pc"_) leaving the larger cell, the egg, with just one half the number of chromosomes normal for the cells of the animal in question. meantime the first pole cell has also divided, so that we have now, as shown in fig. , four cells, three small and one large, but each containing one half the normal number of chromosomes. in the example figured, four is the normal number for the cells of the animal. the egg at the beginning of the process contained eight, but has now been reduced to two. in the further history of the egg the smaller cells, called _polar cells_, take no part, since they soon disappear and have nothing to do with the animal which is to result from the further division of the egg. this process of the formation of the polar cells is thus simply a device for getting rid of some of the chromatin material in the egg cell, so that it may unite with a second cell without doubling the normal number of chromosomes. [illustration: fig. --first division complete and first polar cell formed, _pc'_.] [illustration: fig. .--formation of the second polar cell, _pc"_.] [illustration: fig. .--completion of the process of extrusion of the chromatic material; _fn_ shows the two chromosomes retained in the egg forming the female pronucleus. the centrosome has disappeared.] previously to this process the other sexual cell, the _spermatozoon_, or male reproductive cell, has been undergoing a somewhat similar process. this is also a true cell (fig. , _mc_), although it is of a decidedly smaller size than the egg and of a very different shape. it contains cell substance, a nucleus with chromosomes, and a centrosome, the number of chromosomes, as shown later, being however only half that normal for the ordinary cells of the animals. the study of the development of the spermatozoon shows that it has come from cells which contained the normal number of four, but that this number has been reduced to one half by a process which is equivalent to that which we have just noticed in the egg. thus it comes about that each of the sexual elements, the egg and the spermatozoon, now contains one half the normal number of chromosomes. [illustration: fig. --the egg centrosomes have changed their position. the male cell with its centrosome remains inactive until the stage represented in fig. .] [illustration: fig. --beginning of the first division for removing superfluous chromosomes.] now by some mechanical means these two reproductive cells are brought in contact with each other, shown in fig. , and as soon as they are brought into each other's vicinity the male cell buries its head in the body of the egg. the tail by which it has been moving is cast off, and the head containing the chromosomes and the centrosome enters the egg, forming what is called the male pronucleus (figs. - , _mn_). this entrance of the male cell occurs either before the formation of the polar cells of the egg or afterward. if, however, it takes place before, the male pronucleus simply remains dormant in the egg while the polar cells are being protruded, and not until after that process is concluded does it begin again to show signs of activity which result in the cell union. the further steps in this process appear to be controlled by the centrosome, although it is not quite certain whence this centrosome is derived. originally, as we have seen, the egg contained a centrosome, and the male cell has also brought a second into the egg (fig. , _ce_). in some cases, and this is true for the worm we are describing, it is certain that the egg centrosome disappears while that of the spermatozoon is retained alone to direct the further activities (fig. ). possibly this may be the case in all eggs, but it is not sure. it is a matter of some little interest to have this settled, for if it should prove true, then it would evidently follow that the machinery for cell division, in the case of sexual reproduction, is derived from the father, although the bulk of the cell comes from the mother, while the chromosomes come from both parents. in the cases where the process has been most carefully studied, the further changes are as follows: the head of the spermatozoon, after entrance into the egg, lies dormant until the egg has thrown off its polar cells, and thus gotten rid of part of its chromosomes. close to it lies its centrosomes (fig. , _ce_), and there is thus formed what is known as the _male pronucleus_ (fig. - , _mn_). the remains of the egg nucleus, after having discharged the polar cells, form the _female nucleus_ (fig. , _fn_). the chromatin material, in both the male and female pronucleus, soon breaks up into a network in which it is no longer possible to see that each contains two chromosomes (fig. ). now the centrosome, which is beside the male pronucleus, shows signs of activity. it becomes surrounded by prominent rays to form an aster (fig. , _ce_), and then it begins to move toward the female pronucleus, apparently dragging the male pronucleus after it. in this way the centrosome approaches the female pronucleus, and thus finally the two nucleii are brought into close proximity. meantime the chromatin material in each has once more broken up into short threads or chromosomes, and once more we find that each of the nucleii contains two of these bodies (fig. ). in the subsequent figures the chromosomes of the male nucleus are lightly shaded, while those of the female are black in order to distinguish them. as these two nucleii finally come together their membranes disappear, and the chromatic material comes to lie freely in the egg, the male and female chromosomes, side by side, but distinct forming the _segmentation nucleus_. the egg plainly now contains once more the number of chromosomes normal for the cells of the animal, but half of them have been derived from each parent. it is very suggestive to find further that the chromosomes in this _fertilized egg_ do not fuse with each other, but remain quite distinct, so that it can be seen that the new nucleus contains chromosomes derived from each parent (fig. ). nor does there appear to be, in the future history of this egg, any actual fusion of the chromatic material, the male and female chromosomes perhaps always remaining distinct. [illustration: fig. .--the chromosomes in the male and female pronucleii have resolved into a network. the male centrosome begins to show signs of activity.] [illustration: fig. .--the centrosome has divided, and the two pronucleii have been brought together. the network in each nucleus has again resolved itself into two chromosomes which are now brought together near the centre of the egg but do not fuse; _mcr_, represents the chromosomes from the male nucleus; _fcr_, the chromosomes from the female nucleus.] [illustration: fig. .--an equatorial plate is formed and each chromosome has split into two halves by longitudinal division.] [illustration: fig. .--the halves of the chromosomes have separated to form two nucleii, each with male and female chromosomes. the egg has divided into two cells.] while this mixture of chromosomes has been taking place the centrosome has divided into two parts, each of which becomes surrounded by an aster and travels to opposite ends of the nucleus (fig. ). there now follows a division of the nucleus exactly similar to that which occurs in the normal cell division already described in figs. - . each of the chromosomes splits lengthwise (fig. ), and one half of each then travels toward each centrosome to form a new nucleus (fig. ). since each of the four chromosomes thus splits, it follows that each of the two daughter nucleii will, of course, contain four chromosomes; two of which have been derived from the male and two from the female parent. from now the divisions of the egg follow rapidly by the normal process of cell division until from this one egg cell there are eventually derived hundreds of thousands of cells which are gradually moulded into the adult. all of these cells will, of course, contain four chromosomes; and, what is more important, half of the chromosomes will have been derived directly from the male and half from the female parent. even into adult life, therefore, the cells of the animal probably contain chromatin derived by direct descent from each of its parents. ==the significance of fertilization.==--from this process of fertilization a number of conclusions, highly important for our purpose, can be drawn. in the first place, it is evident that the chromosomes form the part of the cell which contain the hereditary traits handed down from parent to child. this follows from the fact that the chromosomes are the only part of the cell which, in the fertilized egg, is derived from both parents. now the offspring can certainly inherit from each parent, and hence the hereditary traits must be associated with some part of the cell which is derived from both. but the egg substance is derived from the mother alone; the centrosome, at least in some cases and perhaps in all, is derived only from the father, while the chromosomes are derived from _both_ parents. hence it follows that the hereditary traits must be particularly associated with the chromosomes. with this understanding we can, at least, in part understand the purpose of fertilization. as we shall see later, it is very necessary in the building of the living machine for each individual to inherit characters from more than one individual. this is necessary to produce the numerous variations which contribute to the construction of the machine. for this purpose there has been developed the process of sexual union of reproductive cells, which introduces into the offspring chromatic material from _two_ parents. but if the two reproductive cells should unite at once the number of chromosomes would be doubled in each generation, and hence be constantly increasing. to prevent this the polar cells are cast out, which reduces the amount of chromatic material. the union of the two pronucleii is plainly to produce a nucleus which shall contain chromosomes, and hence hereditary traits from each parent and the subsequent splitting of these chromosomes and the separation of the two halves into daughter nucleii insures that all the nucleii, and hence all cells of the adult, shall possess hereditary traits derived from both parents. thus it comes that, even in the adult, every body cell is made up of chromosomes from each parent, and may hence inherit characters from each. the cell of an animal thus consists of three somewhat distinct but active parts--the cell substance, the chromosomes, and the centrosome. of these the cell substance appears to be handed down from the mother; the centrosome comes, at least in some cases, from the father, and the chromosomes from both parents. it is not yet certain, however, whether the centrosome is a constant part of the cell. in some cells it cannot yet be found, and there are some reasons for believing that it may be formed out of other parts of the cell. the nucleus is always a direct descendant from the nucleus of pre-existing cells, so that there is an absolute continuity of descent between the nucleii of the cells of an individual and those of its antecedents back for numberless generations. it is not certain that there is any such continuity of descent in the case of the centrosomes; for, while in the process of fertilization the centrosome is handed down from parent to child, there are some reasons for believing that it may disappear in subsequent cells, and later be redeveloped out of other parts. the only part of the cell in which complete continuity from parent to child is demonstrated, is the nucleus and particularly the chromosomes. all of these facts simply emphasize the importance of the chromosomes, and tell us that these bodies must be regarded as containing the most important features of the cell which constitute its individuality. ==what is protoplasm?==--enough has now been given of disclosures of the modern microscope to show that our old friend protoplasm has assumed an entirely new guise, if indeed it has not disappeared altogether. these simplest life processes are so marvelous and involve the action of such an intricate mass of machinery that we can no longer retain our earlier notion of protoplasm as the physical basis of life. there can be no life without the properties of assimilation, growth, and reproduction; and, so far as we know, these properties are found only in that combination of bodies which we call the cell, with its mixture of harmoniously acting parts. _life, at least the life of a cell, is then not the property of a chemical compound protoplasm, but is the result of the activities of a machine._ indeed, we are now at a loss to know how we can retain the term protoplasm. as originally used it meant the contents of the cell, and the significance in the term was in the conception of protoplasm as a somewhat homogeneous chemical compound uniform in all types of life. but we now see that this cell contains not a single substance, but a large number, including solids, jelly masses, and liquids, each of which has its own chemical composition. the number of chemical compounds existing in the material formerly called protoplasm no one knows, but we do know that they are many, and that the different substances are combined to form a physical structure. which of these various bodies shall we continue to call protoplasm? shall it be the linin, or the liquids, or the microsomes, or the chromatin threads, or the centrosomes? which of these is the actual physical basis of life? from the description of cell life which we have given, it will be evident that no one of them is a material upon which our chemical biologists can longer found a chemical theory of life. that chemical theory of life, as we have seen, was founded upon the conception that the primitive life substance is a definite chemical compound. no such compound has been discovered, and these disclosures of the microscope of the last few years have been such as to lead us to abandon hope of ever discovering such a compound. it is apparently impossible to reduce life to any simpler basis than this combination of bodies which make up what was formerly called protoplasm. the term protoplasm is still in use with different meanings as used by different writers. sometimes it is used to refer to the entire contents of the cell; sometimes to the cell substance only outside the nucleus. plainly, it is not the protoplasm of earlier years. with this conclusion one of our fundamental questions has been answered. we found in our first chapter that the general activities of animals and plants are easily reduced to the action of a machine, provided we had the fundamental vital powers residing in the parts of that machine. we then asked whether these fundamental properties were themselves those of a chemical compound or whether they were to be reduced to the action of still smaller machines. the first answer which biologists gave to this question was that assimilation, growth, and reproduction were the simple properties of a complex chemical compound. this answer was certainly incorrect. life activities are exhibited by no chemical compound, but, so far as we know, only by the machine called the cell. thus it is that we are again reduced to the problem of understanding the action of a machine. it may be well to pause here a moment to notice that this position very greatly increases the difficulties in the way of a solution of the life problem. if the physical basis of life had proved to be a chemical compound, the problem of its origin would have been a chemical one. chemical forces exist in nature, and these forces are sufficient to explain the formation of any kind of chemical compound. the problem of the origin of the life substance would then have been simply to account for certain conditions which resulted in such chemical combination as would give rise to this physical basis of life. but now that the simplest substance manifesting the phenomena of life is found to be a machine, we can no longer find in chemical forces efficient causes for its formation. chemical forces and chemical affinity can explain chemical compounds of any degree of complexity, but they cannot explain the formation of machines. machines are the result of forces of an entirely different nature. man can manufacture machines by taking chemical compounds and putting them together into such relations that their interaction will give certain results. bits of iron and steel, for instance, are put together to form a locomotive, but the action of the locomotive depends, not upon the chemical forces which made the steel, but upon the relation of the bits of steel to each other in the machine. so far as we have had any experience, machines have been built under the guidance of intelligence which adapts the parts to each other. when therefore we find that the simplest life substance is a machine, we are forced to ask what forces exist in nature which can in a similar way build machines by the adjustment of parts to each other. but this topic belongs to the second part of our subject, and must be for the present postponed. ==reaction against the cell doctrine.==--as the knowledge of cells which we have outlined was slowly acquired, the conception of the cell passed through various modifications. at first the cell wall was looked upon as the fundamental part, but this idea soon gave place to the belief that it was the protoplasm that was alive. under the influence of this thought the cell doctrine developed into something like the following: the cell is simply a bit of protoplasm and is the unit of living matter. the bodies of all larger animals and plants are made up of great numbers of these units acting together, and the activities of the entire organism are simply the sum of the activities of its cells. the organism is thus simply the sum of the cells which compose it, and its activities the sum of the activities of the individual cells. as more facts were disclosed the idea changed slightly. the importance of the nucleus became more and more forcibly impressed upon microscopists, and this body came after a little into such prominence as to hide from view the more familiar protoplasm. the marvellous activities of the nucleus soon caused it to be regarded as the important part of the cell, while all the rest was secondary. the cell was now thought of as a bit of nuclear matter surrounded by secondary parts. the marvellous activities of the nucleus, and above all, the fact that the nucleus alone is handed down from one generation to the next in reproduction, all attested to its great importance and to the secondary importance of the rest of the cell. this was the most extreme position of the cell doctrine. the cell was the unit of living action, and the higher animal or plant simply a colony of such units. an animal was simply an association together for mutual advantage of independent units, just as a city is an association of independent individuals. the organization of the animals was simply the result of the combination of many independent units. there was no activity of the organism as a whole, but only of its independent parts. cell life was superior to organized life. just as, in a city, the city government is a name given to the combined action of the individuals, so are the actions of organisms simply the combined action of their individual cells. against such an extreme position there has been in recent years a decided reaction, and to-day it is becoming more and more evident that such a position cannot be maintained. in the first place, it is becoming evident that the cell substance is not to be entirely obliterated by the importance of the nucleus. that the nucleus is a most important vital centre is clear enough, but it is equally clear that nucleus and cell substance must be together to constitute the life substance. the complicated structure of the cell substance, the decided activity shown by its fibres in the process of cell division, clearly enough indicate that it is a part of the cell which can not be neglected in the study of the life substance. again the discovery of the centrosome as a distinct morphological element has still further added to the complexity of the life substance, and proved that neither nucleus nor cell substance can be regarded as the cell or as constituting life. it is true that we may not yet know the source of this centrosome. we do not know whether it is handed down from generation to generation like the nucleus, or whether it can be made anew out of the cell substance in the life of an ordinary cell. but this is not material to its recognition as an organ of importance in the cell activity. thus the cell proves itself not to; be a bit of nuclear matter surrounded by secondary parts, but a community of several perhaps equally important interrelated members. another series of observations weakened the cell doctrine in an entirely different direction. it had been assumed that the body of the multicellular animal or plant was made of independent units. microscopists of a few years ago began to suggest that the cells are in reality not separated from each other, but are all connected by protoplasmic fibres. in quite a number of different kinds of tissue it has been determined that fine threads of protoplasmic material lead from one cell to another in such a way that the cells are in vital connection. the claim has been made that there is thus a protoplasmic connection between all the cells of the body of the animal, and that thus the animal or plant, instead of consisting of a large number of separate independent cells, consists of one great mass of living matter which is aggregated into little centres, each commonly holding a nucleus. such a conclusion is not yet demonstrated, nor is its significance very clear should it prove to be a fact; but it is plain that such suggestions quite decidedly modify the conception of the body as a community of independent cells. there is yet another line of thought which is weakening this early conception of the cell doctrine. there is a growing conviction that the view of the organism, simply as the sum of the activities of the individual cells, is not a correct understanding of it. according to this extreme position, a living thing can have no organization until it appears as the result of cell multiplication. to take a concrete case, the egg of a starfish can not possess any organization corresponding to the starfish. the egg is a single cell, and the starfish a community of cells. the egg can, therefore, no more contain the organization of a starfish than a hunter in the backwoods can contain within himself the organization of a great metropolis. the descendants of individuals like the hunter may unite to form a city, and the descendants of the egg cell may, by combining, give rise to the starfish. but neither can the man contain within himself the organization of the city, nor the egg that of the starfish. it is, perhaps, true that such an extreme position of the cell doctrine has not been held by any one, but thoughts very closely approximating to this view have been held by the leading advocates of the cell doctrine, and have beyond question been the inspiration of the development of that doctrine. but certainly no such conception of the significance of cell structure would longer be held. in spite of the fact that the egg is a single cell, it is impossible to avoid the belief that in some way it contains the starfish. we need not, of course, think of it as containing the structure of a starfish, but we are forced to conclude that in some way its structure is such that it contains the starfish potentially. the relation of its parts and the forces therein are such that, when placed under proper conditions, it develops into a starfish. another egg placed under identical conditions will develop into a sea urchin, and another into an oyster. if these three eggs have the power of developing into three different animals under identical conditions, it is evident that they must have corresponding differences in spite of the fact that each is a single cell. each must in some way contain its corresponding adult. in other words, the organization must be within the cells, and hence not simply produced by the associations of cells. over this subject there has been a deal of puzzling and not a little experimentation. the presence of some sort of organization in the egg is clear--but what is meant by this statement is not quite so clear. is this adult organization in the whole egg or only in its nucleus, and especially in the chromosomes which, as we have seen, contain the hereditary traits? when the egg begins to divide does each of the first two cells still contain potentially the organization of the whole adult, or only one half of it? is the development of the egg simply the unfolding of some structure already present; or is the structure constantly developing into more and more complicated conditions owing to the bringing of its parts into new relations? to answer these questions experimenters have been engaged in dividing developing eggs into pieces to determine what powers are still possessed by the fragments. the results of such experiments are as yet rather conflicting, but it is evident enough from them that we can no longer look upon the egg cell as a simple undifferentiated cell. in some way it already contains the characters of the adult, and when we remember that the characters of the adult which are to be developed from the egg are already determined, even to many minute details--such, for instance, as the inheritance of a congenital mark--it becomes evident that the egg is a body of extraordinary complexity. and yet the egg is nothing more than a single cell agreeing with other cells in all its general characters. it is clear, then, that we must look upon organization as something superior to cells and something existing within them, or at least within the egg cell, and controlling its development. we are forced to believe, further, that there may be as important differences between two cells as there are between two adult animals or plants. in some way there must be concealed within the two cells which constitute the egg of the starfish and the man differences which correspond to the differences between the starfish and the man. organization, in other words, is superior to cell structure, and the cell itself is an organization of smaller units. as the result of these various considerations there has been, in recent years, something of a reaction against the cell doctrine as formerly held. while the study of cells is still regarded as the key to the interpretation of life phenomena, biologists are seeing more and more clearly that they must look deeper than simple cell structure for their explanation of the life processes. while the study of cells has thrown an immense amount of light upon life, we seem hardly nearer the centre of the problem than we were before the beginning of the series of discoveries inaugurated by the formulation of the doctrine of protoplasm. ==fundamental vital activities as located in cells.==--we are now in position to ask whether our knowledge of cells has aided us in finding an explanation of the fundamental vital actions to which, as we have seen, life processes are to be reduced. the four properties of irritability, contractibility, assimilation, and reproduction, belong to these vital units--the cells, and it is these properties which we are trying to trace to their source as a foundation of vital activity. we may first ask whether we have any facts which indicate that any special parts of the cell are associated with any of these fundamental activities. the first fact that stands out clearly is that the nucleus is connected most intimately with the process of reproduction and especially with heredity. this has long been believed, but has now been clearly demonstrated by the experiments of cutting into fragments the cell bodies of unicellular animals. as already noticed, those pieces which possess a nucleus are able to continue their life and reproduce themselves, while those without a nucleus are incapable of reproduction. with greater force still is the fact shown by the process of fertilization of the egg. the egg is very large and the male reproductive cell is very small, and the amount of material which the offspring derives from its mother is very great compared with that which it derives from its father. but the child inherits equally from father and mother, and hence we must find the hereditary traits handed down in some element which the offspring obtains equally from father and mother. as we have seen (figs. - ), the only element which answers this demand is the nucleus, and more particularly the chromosomes of the nucleus. clearly enough, then, we must look upon the nucleus as the special agent in reproduction of cells. again, we have apparently conclusive evidence that the _nucleus_ controls that part of the assimilative process which we have spoken of as the constructive processes. the metabolic processes of life are both constructive and destructive. by the former, the material taken into the cell in the form of food is built up into cell tissue, such as linin, microsomes, etc., and, by the latter, these products are to a greater or less extent broken to pieces again to liberate their energy, and thus give rise to the activities of the cell. if the destructive processes were to go on alone the organism might continue to manifest its life activities for a time until it had exhausted the products stored up in its body for such purposes, but it would die from the lack of more material for destruction. life is not complete without both processes. now, in the life of the cell we may apparently attribute the destructive processes to the cell substance and the constructive processes to the nucleus. in a cell which has been cut into fragments those pieces without a nucleus continue to show the ordinary activities of life for a time, but they do not live very long (fig. ). the fragment is unable to assimilate its food sufficiently to build up more material. so long as it still retains within itself a sufficiency of already formed tissue for its destructive metabolism, it can continue to move around actively and behave like a complete cell, but eventually it dies from starvation. on the other hand, those fragments which retain a piece of the nucleus, even though they have only a small portion of the cell substance, feed, assimilate, and grow; in other words, they carry on not only the destructive but also the constructive changes. plainly, this means that the nucleus controls the constructive processes, although it does not necessarily mean that the cell substance has no share in these constructive processes. without the nucleus the cell is unable to perform those processes, while it is able to carry on the destructive processes readily enough. the nucleus controls, though it may not entirely carry on, the constructive metabolism. it is equally clear that the _cell substance_ is the seat of most of the destructive processes which constitute vital action. the cell substance is irritable, and is endowed with the power of contractility. cell fragments without nucleii are sensitive enough, and can move around as readily as normal cells. moreover, the various fibres which surround the centrosomes in cell division and whose contractions and expansions, as we have seen, pull the chromosomes apart in cell division, are parts of the cell substance. all of these are the results of destructive metabolism, and we must, therefore, conclude that destructive processes are seated in the cell substance. the _centrosome_ is too problematical as yet for much comment. it appears to be a piece of the machinery for bringing about cell division, but beyond this it is not safe to make any statements. in brief, then, the cell body is a machine for carrying on destructive chemical changes, and liberating from the compounds thus broken to pieces their inclosed energy, which is at once converted into motion or heat or some other form of active energy. this chemical destruction is, however, possible only after the chemical compounds have become a part of the cell. the cell, therefore, possesses a nucleus which has the power of enabling it to assimilate its food--that is, to convert it into its own substance. the nucleus further contains a marvellous material--chromatin--which in someway exercises a controlling influence in its life and is handed down from one generation to another by continuous descent. lastly, the cell has the centrosome, which brings about cell division in such a manner that this chromatin material is divided equally among the subsequent descendants, and thus insures that the daughter cells shall all be equivalent to each other and to the mother cell. we must therefore look upon the organic cell as a little engine with admirably adapted parts. within this engine chemical activity is excited. the fuel supplied to the engine is combined by chemical forces with the oxygen of the air. the vigour of the oxidation is partly dependent upon temperature, just as it is in any other oxidation process, and is of course dependent upon the presence of fuel to be oxidized, and air to furnish the oxygen. unless the fuel is supplied and the air has free access to it, the machine stops, the cell _dies_. the energy liberated in this machine is converted into motion or some other form. we do not indeed understand the construction of the machine well enough to explain the exact mechanism by which this conversion takes place, but that there is such a mechanism can not be doubted, and the structure of the cell is certainly complex enough to give plenty of room for it. the irritability of the cell is easily understood; for, since it is made of very unstable chemical compounds, any slight disturbance or stimulation on one part will tend to upset its chemical stability and produce reaction; and this is what is meant by irritability. or, again, we may look upon the cell as a little chemical laboratory, where chemical changes are constantly occurring. these changes we do not indeed understand, but they are undoubtedly chemical changes. the result is that some compounds are pulled to pieces and part of the fragments liberated or excreted, while other parts are retained and built into other more complex compounds. the compounds thus manufactured are retained in the cell body, and it grows in bulk. this continues until the cell becomes too big, and then it divides. if a machine is broken it ceases to carry on its proper duties, and if the parts are badly broken it is ruined. so with the cell. if it is broken by any means, mechanical, thermal, or otherwise, it ceases to run--we say it dies. it has within itself great power of repairing injury, and therefore it does not cease to act until the injury is so great as to be beyond repair. thus it only stops its motion when the machinery has become so badly injured as to be beyond hope of repair, and hence the cell, after once ceasing its action, can never resume it again. there are, of course, other functions of living things besides the few simple ones which we have considered. but these are the fundamental ones; and if we can reduce them to an intelligible explanation, we may feel that we have really grasped the essence of life. if we understand how the cell can move and grow and reproduce itself, we may rest assured that the other phenomena of life follow as a natural consequence. if, therefore, we have obtained an understanding of these fundamental vital phenomena, we have accomplished our object of comprehending the life phenomena in our chemical and mechanical laws. but have we thus reduced these fundamental phenomena to an intelligible explanation? it must be acknowledged that we have not. we have reduced them to the action of chemical forces acting in a machine. but the machine itself is unintelligible. the organic cell is no more intelligible to us than is the body as a whole. the chemical understanding which we thought we had a few years ago in protoplasm has failed us, and nothing has taken its place we have no conception of what may be the primitive life substance. all we can say is that this most marvellous of all natural phenomena occurs only within that peculiar piece of machinery which we call the cell, and that it is the result of the action of physical forces in that machine. how the machine acts, or even the structure of the machine, we are as far from understanding as we were fifty years ago. the solution has retreated before us even faster than we have advanced toward it. ==summary.==--we may now notice in a brief summary the position which we have reached. in our attempt to explain the living organism on the principle of the machine, we are very successful so far as secondary problems are concerned. digestion, circulation, respiration, and motion are readily solved upon chemical and mechanical principles. even the phenomena of the nervous system are, in a measure, capable of comprehension within a mechanical formula, leaving out of account the purely mental phenomena which certainly have not been touched by the investigation. all of these phenomena are reducible to a few simple fundamental activities, and these fundamental activities we find manifested by simple bits of living matter unincumbered by the complicated machinery of organisms. with the few fundamental properties of these bits of organic matter we can construct the complicated life of the higher organism. when we come, however, to study these simple bits of matter, they prove to be anything but simple bits of matter. they, too, are pieces of complicated mechanism whose action we do not even hope to understand. that their action is dependent upon their machinery is evident enough from the simple description of cell activity which we have noticed. that these fundamental vital properties are to be explained as the result of chemical and mechanical forces acting through this machinery, can not be doubted. but how this occurs or what constitutes the guiding force which corresponds to the engineer of the machine, we do not know. thus our mechanical explanation of the living machine lacks a foundation. we can understand tolerably well the building of the superstructure, but the foundation stones upon which that structure is built are unintelligible to us. the running of the living machine is thus only in part understood. the living organism is a machine or, it is better to say, it is a series of machines one within the other. as a whole it is a machine, and its parts are separate machines. each part is further made up of still smaller machines until we reach the realm of the microscope. here still we find the same story. even the parts formerly called units, prove to be machines, and when we recognize the complexity of these cells and their marvellous activities, we are ready to believe that we may find still further machines within. and thus vital activity is reduced to a complex of machines, all acting in harmony with each other to produce together the one result--life. part ii. _the building of the living machine_. * * * * * chapter iii. the factors concerned in the building of the living machine. having now outlined the results of our study into the mechanism of the living machine, we turn our attention next to the more difficult problem of the method by which this machine was built. from the facts which we have been considering in the last two chapters it is evident that the problem we have before us is a mechanical rather than a chemical one. of course, chemical forces lie at the bottom of vital activity, and we must look upon the force of chemical affinity as the fundamental power to which the problems must be referred. but a chemical explanation will evidently not suffice for our purpose; for we have absolutely no reason for believing that the phenomena of life can occur as the results of the chemical properties of any compound, however complex. the simplest known form of matter which manifests life is a machine, and the problem of the origin of life must be of the origin of that machine. are there any forces in nature which are of a sort as to enable us to use them to explain the building of machines? plants and animals are the only machines which nature has produced. they are the only instances in nature of a structure built with its parts harmoniously adjusted to each other to the performance of certain ends. all other machines with which we are acquainted were made by man, and in making them intelligence came in to adapt the parts to each other. but in the living organism is a similarly adapted machine made by natural means rather than artificial. how were they built? does nature, apart from human intelligence, possess forces which can achieve such results? here again we must attack the problem from what seems to be the wrong end. apparently it would be simpler to discover the method of the manufacture of the simplest machine rather than the more complex ones. but this has proved contrary to the fact. perhaps the chief reason is that the simplest living machine is the cell whose study must always involve the use of the microscope, and for this reason is more difficult. perhaps it is because the problem is really a more difficult one than to explain the building of the more complex machines out of the simpler ones. at all events, the last fifty years have told us much of the method of the building of the complex machines out of the simpler ones, while we have as yet not even a hint as to the solution of the building of the simplest machine from the inanimate world. our attention must, therefore, be first directed to the method by which nature has constructed the complex machines which we find filling the world to-day in the form of animals and plants. ==history of the living machine.==--in the first place, we must notice that these machines have not been fashioned suddenly or rapidly, but have been the result of a very slow growth. they have had a history extending very far back into the past for a period of years which we can only indefinitely estimate, but certainly reaching into the millions. as we look over this past history in the light of our present knowledge we see that whatever have been the forces which have been concerned in the construction of these machines they have acted very slowly. it has taken centuries, and, indeed, thousands of years, to take the successive steps which have been necessary in this construction. secondly, we notice that the machines have been built up step by step, one feature being added to another with the slowly progressing ages. thirdly, we notice that in one respect this construction of the living machine by nature's processes has been different from our ordinary method of building machines. our method of building puts the parts gradually into place in such a way that until the machine is finished it is incapable of performing its functions. the half-built engine is as useless and as powerless as so much crude iron. its power of action only appears after the last part is fitted into place and the machine finished. but nature's process in machine building is different. every step in the process, so far as we can trace it at least, has produced a complete machine. so far back as we can follow this history we find that at every point the machine was so complete as to be always endowed with motion and life activity. nature's method has been to take simpler types of machines and slowly change them into more complicated ones without at any moment impairing their vigour. it is something as if the steam engine of watt should be slowly changed by adding piece after piece until there was finally produced the modern quadruple expansion engine, but all this change being made upon the original engine without once stopping its motion. [illustration: fig. . a group of cells resulting from division, representing the first step in machine making.] this gradual construction of the living machines has been called _organic evolution_, or the _theory of descent_. it will be necessary for us, in order to comprehend the problem which we have before us, to briefly outline the course of this evolution. our starting point in this history must be the cell, for such is the earliest and simplest form of living thing of which we have any trace. this cell is, of course, already a machine, and we must presently return to the problem of its origin. at present we will assume this cell as a starting point endowed with its fundamental vital powers. it was sensitive, it could feel, grow, and reproduce itself. from such a simple machine, thus endowed, the history has been something as follows: in reproducing itself this machine, as we have already seen, simply divided itself into two halves, each like the other. at first all the parts thus arising separated from each other and remained independent. but so long as this habit continued there could be little advance. after a time some of the cells failed to separate after division, but remained clinging together (fig. ). the cells of such a mass must have been at first all alike; but, after a little, differences began to appear among them. those on the outside of the mass were differently affected by their surroundings from those in the interior, and soon the cells began to share among themselves the different duties of life. the cells on the outside were better situated for protection and capturing food, while those on the inside could not readily seize food for themselves, and took upon themselves the duty of digesting the food which was handed to them by the outer cells. each of these sets of cells could now carry on its own special duties to better advantage, since it was freed from other duties, and thus the whole mass of cells was better served than when each cell tried to do everything for itself. this was the first step in the building of the machine out of the active cells (fig. ). from such a starting point the subsequent history has been ever based upon the same principle. there has been a constant separation of the different functions of life among groups of cells, and as the history went on this division of labor among the different parts became greater and greater. group after group of cells were set apart for one special duty after another, and the result was a larger and ever more complicated mass of cells, with a greater and greater differentiation among them. in this building of the machine there was no time when the machine was not active. at all points the machine was alive and functional, but each step made the total function of the machine a little more accurately performed, and hence raised somewhat the totality of life powers. this parcelling out of the different duties of life to groups of cells continued age after age, each step being a little advance over the last, until the result has been the living machine as we know it in its highest form, with its numerous organs, all interrelated in such a way as to form a harmoniously acting whole. [illustration: fig. . a later step in machine building in which the outer cells have acquired different form and function from the inner cells: _ec_, the outer cells, whose duties are protective; _en_, the inner cells engaged in digesting food.] but a second principle in this growth of the machine was needed to produce the variety which is found in nature. as the different cells in the multicellular mass became associated into groups for different duties, the method of such division of labor was not alike in all machines. a city in china and one in america are alike made up of individuals, and the fundamental needs of the chinaman and the american are alike. but differences in industrial and political conditions have produced different combinations and associations, so that pekin is wonderfully unlike new york. so in these early developing machines, quite a variety of method of organization was adopted by the different groups. now as soon as any special type of organization was adopted by any animal or plant, the principle of heredity transmitted the same kind of organization to its descendants, and there thus arose lines of descent differing from each other, each line having its own method of organization. as we follow the history of each line the same thing is repeated. we find that the representatives of each line again separate into groups, each of which has acquired some new type of organization, and there has thus been a constant divergence of these lines of descent in an indefinite number of directions. the members of the different lines of descent all show a fundamental likeness with each other since they retain the fundamental characters of their common ancestor, but they show also the differences which they have themselves acquired. and thus the process is repeated over and over again. this history of the growth of these different machines has thus been one of divergence from common centres, and is to be diagrammatically expressed after the fashion of a branching tree. the end of each branch represents the highest state of perfection to which each line has been carried. one other point in this history must be noted. as the development of the complication of the machine progressed the possibility of further progress has been constantly narrowed. when the history of these machines began as a simple mass of cells, there was a possibility of an almost endless variety of methods of organization. but as a distinct type of organization was adopted by one and another line of descendants all subsequent productions were limited through the law of heredity to the general line of organization adopted by their ancestors. with each age the further growth of such machines must consist in the further development in the perfection of its parts, and not in the adoption of any new system of organization. hence it is that the history of the living machine has shown a tendency toward development along a few well-marked lines, and although this complication becomes greater, we still see the same fundamental scheme of organization running through the whole. as the ages have progressed the machines have become more perfect in the adjustment of their parts, i.e., they have become more perfect machines, but the history has been simply that of perfecting the early machines rather than the production of new types. ==evidence for this history.==--as just outlined, we see that the living machines have been gradually brought into their present condition by a process which has been called organic evolution. but we must pause for a moment to ask what is our evidence that such has been the history of the living machine. the whole possibility of understanding living nature depends upon our accepting this history and finding an explanation of it. at the outset we have the question of fact, and we must notice the grounds upon which we stand in assuming this history to be as outlined. this problem is the one which has occupied such a prominent place in the scientific world during the last forty years, and which has contributed so largely toward making modern biology such a different subject from the earlier studies of natural history. it is simply the evidence for organic evolution, or the theory of descent. the subject has for forty years been thoroughly sifted and tested by every conceivable sort of test. as a result of the interest in the question there has been disclosed an immense mass of evidence, relevant and irrelevant. as the evidence has accumulated it has become more and more evident that the evolution theory must be recognized as the only one which is in accord with the facts, and the outcome has been a practical unanimity among thinkers that the theory of descent must be the foundation of our further study. the evidence which has forced this conclusion upon scientists we must stop for a moment to consider, since it bears very directly upon the subject we are studying. ==historical.==--the first source of evidence is naturally a historical one. this long history of the construction of the living machine has left its record in the rocks which form the earth's surface. during this long period the rocks of the earth's crust have been deposited, and in these rocks have been left samples of many of the steps in this history of machine building. the history can be traced by the study of these samples just as the history of any machine might be traced from a study of the models in a patent office. one might very easily trace, with most strict accuracy and minute detail, the history of the printing machine from the models which are preserved in the patent offices and elsewhere. so is it with the history of the living machine. to be sure, the history is rather incomplete and at times difficult to read. many a period in the development has left no samples for our inspection and must be interpreted in our history between what went before and what comes after. many of the machines, especially the early ones, were made of such fragile material that they could not be preserved in the rocks. in many a case, too, the rocks in which the specimens were deposited have been subjected to such a variety of heatings and pressures, that they have been twisted out of shape and even crushed out of recognizable form. but in spite of this the record is showing itself more complete each year. our paleontologists are opening layer after layer of these rocks, and thus examining each year new pages in nature's history. the more recent epochs in the history have been already read with almost historic accuracy. from them we have learned in great detail how the finishing touches were given to these machines, and are able to trace with accuracy how the somewhat more generalized forms of earlier days were changed to produce our modern animals. this fossil record has given us our best knowledge of the course by which the present living world has been brought into its existing condition. but its accuracy is largely confined to the recent periods. of the very early history fossils tell us little or nothing. all the early rocks, which we may believe were formed during the period when the first steps in this machine building were taken, have been so changed by heat and pressure that whatever specimens they may have originally contained have been crushed out of shape. furthermore, the earliest organisms had no hard skeletons, and it was not until living beings had developed far enough to have hard parts that it was possible for them to leave traces of themselves in the rocks. hence, so far as concerns this earliest history, we can get no record of it in the rocks. ==embryological.==--but here comes in another source of evidence which helps to fill up the gap. in its development every animal to-day begins as an egg. this is a simple cell, and the animal goes through a series of changes which eventually lead to the adult. now these changes appear for the most part to be parallel to the changes through which the earlier forms of life passed in their development from the simple to the more complicated forms. where it is possible to follow the history of the groups of animals from their fossil remains and compare it with the history of the individual animal as it progresses from the egg to the adult, there is found a very decided parallelism. this parallelism between embryology and past history has been of great service in helping us toward the history of the past. at one time it was believed that it was the key which would unlock all doors, and for a decade biologists eagerly pursued embryology with the expectation that it would solve all problems in connection with the history of animals. the result has been somewhat disappointing. embryology has, it is true, been of the utmost service in showing relationships of forms to each other, and in thus revealing past history. but while this record is a valuable one, it is a record which has unfortunately been subject to such modifying conditions that in many cases its original meaning has been entirely obliterated and it has become worthless as a historical record. these imperfections in regard to the record were early seen after the attention of biologists was seriously turned to the study of embryology, but it was expected that it would be possible to correct them and discover the true meaning underlying the more apparent one. indeed, in many cases this has been found possible. but many of the modifications are so profound as to render it impossible to untangle them and discover the true meaning. as a result the biologist to-day is showing less confidence in embryology, and is turning his attention in different directions as more promising of results in the line desired. but although the teachings of embryology have failed to realize the great hopes that were placed upon them, their assistance in the formulation of this history of the machine has been of extreme value. many a bit of obscurity has been cleared up when the embryology of puzzling animals has been studied. many a relationship has been made clear, and this is simply another way of saying that a portion of this history of life has been read. this aid of embryology has been particularly valuable in just that part of the history where the evidence from the study of fossils is wanting. the study of fossils, as we have seen, gives little or no data concerning the early history of living machines; and it is just here that embryology has proved to be of the most value. it is a source of evidence that has told us of most of the steps in the progress from the single-celled animal to the multicellular organisms, and gives us the clearest idea of the fundamental principles which have been concerned in the evolution of life and the construction of the complicated machine out of the simple bit of protoplasm. in spite of its limits, therefore, embryology has contributed a large quota of the evidence which we have of the evolution of life. ==anatomical.==--a third source of this history is obtained from the facts of comparative anatomy. the essential feature of this subject is the fact that animals and plants show relationships. this fact is one of the most patent and yet one of the most suggestive facts of biology. it has been recognized from the very beginning of the study of animals and plants. one cannot be even the most superficial observer without seeing that certain forms show great likeness to each other while others are much more unlike. the grouping of animals and plants into orders, genera, and species is dependent upon this relationship. if two forms are alike in everything except some slight detail, they are commonly placed in the same genus but in different species, while if they show a greater unlikeness they may be placed in separate genera. by thus grouping together forms according to their resemblance the animal and vegetable kingdoms are classified into groups subordinate to groups. the principle of relationship, i.e., fundamental similarity of structure, runs through the whole animal and vegetable kingdom. even the animals most unlike each other show certain points of similarity which indicates a relationship, although of course a distant one. the fact of such a relationship is too patent to demand more words, but its significance needs to be pointed out. when we speak of relationship among men we always mean historical connection. two brothers are closely related because they have sprung from common parents, while two cousins are less closely related because their common point of origin was farther back in time. more widely we speak of the relationship of the indo-european races, meaning thereby that back in the history of man these races had a common point of origin. we never speak of any real relation of objects unless thereby we mean to imply historical connection. we are therefore justified in interpreting the manifest relationships of organisms as pointing to history. particularly are we justified in this conclusion when we find that the relationships which we draw between the types of life now in existence run parallel to the history of these types as revealed to us by fossils and at the same time disclosed by the study of embryology. this subject of comparative anatomy includes a consideration of what is called homology, and perhaps a concrete example may be instructive both in illustration and as suggesting the course which nature adopts in constructing her machines. we speak of a monkey's arm and a bird's wing as homologous, although they are wonderfully different in appearance and adapted to different duties. they are called homologous because they have similar parts in similar relations. this can be seen in figs. and , where it will be seen that each has the same bones, although in the bird's wing some of the bones have been fused together and others lost. their similarity points to a relationship, but their dissimilarity tells us that the relationship is a distant one, and that their common point of origin must have been quite far back in history. now if we follow back the history of these two kinds of appendages, as shown to us by fossils, we find them approaching a common point. the arm can readily be traced to a walking appendage, while the bird's wing, by means of some interesting connecting links, can in a similar way be traced to an appendage with its five fingers all free and used for walking. fig. shows one of these connecting links representing the earliest type of bird, where the fingers and bones of the arm were still distinct, and yet the whole formed a true wing. thus we see that the common point of origin which is suggested by the likenesses between an arm and a wing is no mere imaginary one, for the fossil record has shown us the path leading to that point of origin. the whole tells us further that nature's method of producing a grasping or flying organ was here, not to build a new organ, but to take one that had hitherto been used for other purposes, and by slow changes modify its form and function until it was adapted to new duties. [illustration: fig. .--the arm of a monkey, a prehensile appendage.] [illustration: fig. .--the arm of a bird, a flying appendage. in life covered with feathers.] [illustration: fig. .--the arm of an ancient half-bird half-reptile animal. in life covered with feathers and serving as a wing.] ==significance of these sources of history.==--the real force of these sources of evidence comes to us only when we compare them with each other. they agree in a most remarkable fashion. the history as disclosed by fossils and that told by embryology agree with each other, and these are in close harmony with the history as it can be read from comparative anatomy. if archæologists were to find, in different countries and entirely unconnected with each other two or more different records of a lost nation, the belief in the actual existence of that nation would be irresistible. when researches at nineveh, for example, unearth tablets which give the history of ancient nations, and when it proves that among the nations thus mentioned are some with the same names and having the same facts of history as those mentioned in the bible, it is absolutely impossible to avoid the conclusion that such a nation with such a history did actually exist. two independent sources of record could not be false in regard to such a matter as this. now, our sources of evidence for this history of the living machine prove to be of exactly this kind. we have three independent sources of evidence which are so entirely different from each other that there is almost no likeness between them. one is written in the rocks, one in bone and muscle, while the third is recorded in the evanescent and changing pages of embryology and metamorphosis. yet each tells the same story. each tells of a history of this machine from simple forms to more complex. each tells of its greater and greater differentiation of labour and structure as the periods of time passed. each tells of a growing complexity and an increasing perfection of the organisms as successive periods pass. each tells us of common points of origin and divergence from these points. each tells us how the more complicated forms have arisen as the results of changes in and modifications of the simpler forms. each shows us how the individual parts of the organisms have been enlarged or diminished or changed in shape to adapt them to new duties. each, in short, tells the same story of the gradual construction of the living machine by slow steps and through long ages of time. when these three sources of history so accurately agree with each other, it is as impossible to disbelieve in the existence of such history as it is to disbelieve in the existence of the ancient hittite nation, after its history has been told to us by two different sources of record. now all this is very germane to our subject. we are trying to learn how this living machine, with its wonderful capabilities, was built. the history which we have outlined is undoubtedly the history of the building of this machine, and the knowledge that these complicated machines have been produced as the result of slow growth is of the utmost importance to us. this knowledge gives us at the very start some idea of the nature of the forces which have been at work. it tells us that in searching for these forces we must look for those which have been acting constantly. we must look for forces which produce their effects not by sudden additions to the complication of the machine. they must be constant forces whose effect at any one time is comparatively slight, but whose total effect is to increase the complexity of the machine. they must be forces which produce new types through the modification of the old ones. we must look for forces which do not adapt the machine for its future, but only for its present need. each step in the history has been a complete animal with its own fully developed powers. we are not to expect to find forces which planned the perfect machine from the start, nor forces which were engaged in constructing parts for future use. each step in the building of the machine was taken for the good of the machine at the particular moment, and the forces which we are to look for must therefore be only such as can adapt the organisms for its present needs. in other words, nothing has been produced in this machine for the purpose of being developed later into something of value, but all parts that have been produced are of value at the time of their appearance. we must, in short, look for forces constantly in action and always tending in the same direction of greater complexity of structure. is it possible to discover these forces and comprehend their action? before the modern development of evolution this question would unhesitatingly have been answered in the negative. to-day, under the influence of the descent theory, stimulated, in the first place, by darwin, the question will be answered by many with equal promptness in the affirmative. at all events, we have learned in the last forty years to recognize some of the factors which have been at work in the construction of this machine. we must turn, therefore, to the consideration of these factors. ==forces at work in the building of the living machine.==--there are three primary factors which lie at the bottom of the whole process. they are-- . _reproduction_, which preserves type from generation to generation. . _variation_, which modifies type from generation to generation. . _heredity_, which transmits characters from generation to generation. each must be considered by itself. ==reproduction.==--reproduction is the primary factor in this process of machine building, heredity and variation being simply phases of reproduction. the living machine has developed by natural processes, all other machines by artificial methods. reproduction is the one essential point of difference between the living machine and the others which has made their construction by natural processes a possibility. what, then, is reproduction? reproduction is in all cases at the bottom simple division. whether we consider the plant that multiplies by buds or the unicellular animal that simply divides into two equal parts, or the larger animal that multiplies by eggs, we find that in all cases the fundamental feature of the process is division. in all cases the organism divides into two or more parts, each of which becomes in time like the original. moreover, when we trace this division further we find that in all cases it is to be referred back to the division of the cell, such as we have described in a previous chapter. the egg is a single cell which has come from the parent by the division of one of the cells in the body of the parent. a bud is simply a mass of cells which have all arisen from the parent cells by division. the foundation of reproduction is thus in all cases cell division. now, this process of division is dependent upon the properties of the cell. firstly, it is a result of the assimilative powers of the cell, for only through assimilation can the cell increase in size, and only as it increases in size can it gain sustenance for cell division. secondly, it is dependent, as we have seen, upon the mechanism of the cell body, and especially the nucleus and centrosome. these structures regulate the cell division, and hence the reproduction of all animals and plants. we can not, therefore, find any explanation of reproduction until we have explained the mechanism of the cell. the fundamental feature, of nature's machine building is thus based upon the machinery of the nucleus and centrosome of the organic cell. aside from the simple fact that it preserves the race, the most important feature connected with this reproduction is its wonderful fruitfulness. since it results from division, it always tends to increase the offspring in geometrical ratio. in the simplest case, that of the unicellular animals, the cell divides, giving rise to two animals, each of which divides again, producing four, and these again, giving eight, etc. the rapidity of this multiplication is sometimes inconceivable. it depends, of course, upon the interval of time between the successive divisions, but among the lower organisms this interval is sometimes not more than half an hour, the result of which is that a single individual could give rise in the course of twenty-four hours to sixteen million offspring. this is doubtless an extreme case, but among all the lower animals the rate is very great. among larger animals the process is more complicated; but here, too, there is the same tendency to geometrical progression, although the intervals between the successive reproductions may be quite long and irregular. but it is always so great that if allowed to progress unhindered at its normal rate the offspring would, in a few years, become so numerous as to crowd other life out of existence. even the slow-breeding elephant would, if allowed to breed unhindered for seven hundred and fifty years, produce nineteen million offspring--a rate of increase plainly incompatible with the continued existence of other animals. here, then, we have the foundation of nature's method of building animals and plants of the higher classes. in the machinery of the cell she has a power of reproduction which produces an increase in geometrical ratio far beyond the possibility for the surface of the earth to maintain. ==heredity.==--the offspring which arise by these processes of division are like each other, and like the parent from which they sprung. this is the essence of what is called heredity. its significance in the process of machine building is evident at once. it is the conserving force which preserves the forms already produced and makes it possible for each generation to build upon the structures of the earlier ones. without it each generation would have to begin anew at the beginning, and nothing could be accomplished. but since this principle brings each individual to the same place where its parents stand, and thus always builds the offspring into a machine like the parent, it makes it possible for the successive generations to advance. heredity is thus like the power of memory, or better still, like the invention of printing in the development of civilization. it is a record of past achievements. by means of printing each age is enabled to benefit by the discoveries of the previous age, and without it the development of civilization would be impossible. in the same way heredity enables each generation to benefit by the achievements of its ancestors in the process of machine building, and thus to devote its own energies to advancement. the fact of heredity is patent enough. it has been always clearly recognized that the child has the characters of its parents, and this belief is so well attested as to need no proof. it is still a question as to just what characters may be inherited, and what influences may affect the inheritance. there are plenty of puzzling problems connected with heredity, but the fact of heredity is one of the foundation stones of biological science. upon it must be built all theories which look toward the explanation of the origin of the living machine. this factor of heredity again we must trace back to the machinery of the cell. we have seen in the previous pages evidence for the wonderful nature of the chromosomes of the cells. we can not pretend to understand them, but they must be extraordinarily complex. we have seen proof that these chromosomes are probably the physical basis of heredity, since they are the only parts of each parent which are handed down to subsequent generations. with these various facts of cell division and cell fertilization in mind, we can reach a very simple explanation of fundamental features of heredity. the following is an outline of the most widely accepted view of the hereditary process. recognizing that the chromosomes are the physical basis of hereditary transmission, we can picture to ourselves the transmission of hereditary characters something as follows: as we have seen, the fertilized egg contains an equal number of chromosomes from each parent (fig. ). now when this fertilized cell divides, each of the rods splits lengthwise, half of each entering each of the two cells arising from the cell division. from this method of division of the chromosomes it follows that the daughter cells would be equivalent to each other and equivalent also to the undivided egg. if the original chromosomes contained potentially all the hereditary traits handed down from parent to child, the chromosomes of each daughter cell will contain similar hereditary traits. if, therefore, the original fertilized egg possessed the power of developing into an adult like the parent, each of the daughter cells should likewise possess the power of developing into a similar adult. and thus each cell which arises as the result of such division should possess similar characters so long as this method of division continues. but after a little in the development of the egg a differentiation among the daughter cells arises. they begin to acquire different shapes and different functions. this we can only believe to be the result of a differentiation in their chromatin material. in the cell division the chromosomes no longer split into equivalent halves, but some characters are portioned off to some cells and others to other cells. those cells which are to carry on digestive functions when they are formed receive chromatin material which especially controls them in the performance of this digestive function, while those which are to produce sensory organs receive a different portion of the chromatin material. thus the adult individual is built up as the cells receive different portions of this hereditary substance contained in the original chromosomes. the original chromosomes contained _all_ hereditary characters, but as development proceeds these are gradually portioned out among the daughter cells until the adult is formed. from this method of division it will be seen that each cell of the adult does not contain all the characters concealed in the original chromosomes of the egg, although each contains a part which may have been derived from each parent. it is thought, however, that a part of the original chromatin material does not thus become differentiated, but remains entirely unchanged as the individual is developing. this chromatin material may increase in amount by assimilation, but it remains unchanged during the entire growth of the individual. it thus follows that the adult will contain, along with its differentiated material, a certain amount of the original physical basis of heredity which still retains its original powers. this undifferentiated chromatin material originally possessed powers of producing a new individual, and of course it still possesses these powers, since it has remained dormant without alteration. further, it will follow that if this dormant undifferentiated chromatin should start into activity and produce a new individual, the new individual thus produced would be identical in all characters with the one which actually did develop from the egg, since both individuals would have come from a bit of the same chromatin. the child would be like the parent. this would be true no matter how much this undifferentiated material should increase in amount by assimilation, _so long as it remained unaltered in character_, and it hence follows that every individual carries around a certain amount of undifferentiated chromatin material in all respects identical with that from which he developed. now whether this undifferentiated _germ plasm_, as we will now call it, is distributed all over the body, or is collected at certain points, is immaterial to our purpose. it is certain that portions of it find their way into the reproductive organs of the animal or plant. thus we see that part of the chromatin material in the egg of the first generation develops into the second generation, while another part of it remains dormant in that second generation, eventually becoming the chromatin of its eggs and spermatozoa. thus each egg of the second generation receives chromosomes which have come directly from the first generation, and thus it will follow that each of these eggs will have identical properties with the egg of the first generation. hence if one of these new eggs develops into an adult it will produce an adult exactly like the second generation, since it contains chromosomes which are absolutely identical with those from which the second generation sprung. there is thus no difficulty in understanding why the second generation will be like the first, and since the process is simply repeated again in the next reproduction, the third generation will be like the second, and so on, generation after generation. a study of the accompanying diagram will make this clear. in other words, we have here a simple understanding of at least some of the features of heredity. this explanation is that some of the chromatin material or germ plasm is handed down from one generation to another, and is stored temporarily in the nucleii of the reproductive cells. during the life of the individual this germ plasm is capable of increasing in amount without changing its nature, and it thus continues to grow and is handed down from generation to generation, always endowed with the power of developing into a new individual under proper conditions, and of course when it does thus give rise to new individuals they will all be alike. we can thus easily understand why a child is like its parent. it is not because the child can inherit directly from its parent, but rather because both child and parent have come from the unfolding of two bits of the same germ plasm. this fact of the transmission of the hereditary substance from generation to generation is known as the theory of the _continuity of germ plasm_. such appears to be, at least in part, the machinery of heredity. this understanding makes the germ substance perpetual and continuous, and explains why successive generations are alike. it does not explain, indeed, why an individual inherits from its parents, but why it is like its parents. while biologists are still in dispute over many problems connected with heredity, all are agreed to-day that this principle of the continuity of the heredity substance must be the basis of all attempts to understand the machinery of heredity. but plainly this whole process is a function of the cell machinery. while, therefore, the idea of the continuity of germ substance greatly simplifies our problem, we must acknowledge that once more we are thrown back upon the mysteries of the cell. until we can more fully explain the cell machine we must recognize our inability to solve the fundamental question of why an individual is like its parents. [illustration: fig. .--diagram illustrating the principle of heredity. _a_ represents an egg of a starfish. from one half, the unshaded portion, develops the starfish of the next generation, _b_. the other is distributed without change in the ovaries, _ov_, of the individual, _b_. from these ovaries arises the next egg, _a'_, with its germ plasm. this germ plasm is evidently identical with that in _a_, since it is merely a bit of the same handed down through the individual, _b_. in the development of the next generation the process is repeated, and hence _b'_ will be like _b_, and the third generation of eggs identical with the first and second. the undifferentiated part of the germ plasm is thus simply handed on from one generation to the next.] but plainly reproduction and heredity, as we have thus far considered them, will be unable to account for the slow modification of the machine; for in accordance with the facts thus far outlined, each generation would be _precisely like the last_, and there would be no chance for development and change from generation to generation. if the individual is simply the unfolding of the powers possessed by a bit of germ plasm, and if this germ plasm is simply handed on from generation to generation, the successive generations must of necessity be identical. but the living machine has been built by changes in the successive generation, and hence plainly some other factor is needed. this factor is _variation_. ==variation.==--variation is the principle that produces _modification of type_. heredity, as just explained, would make all generations alike. but nothing is more certain than that they are not alike. the fact of variation is patent on every side, for no two individuals are alike. successive generations differ from each other in one respect or another. birds vary in the length of their bills or toes; butterflies, in their colours; dogs, in their size and shape and markings; and so on through an endless category. plants and animals alike throughout nature show variations in the greatest profusion. it is these variations which must furnish us with the foundation of the changes which have gradually built up the living machine. of the fact of these variations there is no question, and the matter need not detain us. every one has had too many experiences to ask for proof. of the nature of the variations, however, there are some points to be considered which are very germane to our subject. in the first place, we must notice that these variations are of two kinds. there is one class which is born with the individual, so that they are present from the time of birth. in saying that these variations are born with the individual we do not necessarily mean that they are externally apparent at birth. a child may inherit from its parents characters which do not appear till adult life. for example, a child may inherit the colour of its father's hair, but this colour is not apparent at birth. it appears only in later life, but it is none the less an inborn character. in the same way, we may have many inborn variations among individuals which do not make themselves seen until adult life, but which are none the less innate. the offspring of the same parents may show decided differences, although they are put under similar conditions, and such differences are of course inherent in the nature of the individual. such variations are called _congenital variations_. there is, however, a second class of variations which are not born in the individual, but which arise as the result of some conditions affecting its after-life. the most extreme instances of this kind are mutilations. some men have only one leg because the other has been lost by accident. here is a variation acquired as the result of circumstances. a blacksmith differs from other members of his race in having exceptionally large arm muscles; but here, again, the large muscles have been produced by use. a european who has lived under a tropical sun has a darkened skin, but this skin has evidently been darkened by the action of the sun, and is quite a different thing from the dark skin of the dark races of men. in such instances we have variations produced in individuals as the result of outside influences acting upon them. they are not inborn, but are secondarily acquired by each individual. we call them _acquired variations_. it is not always possible to distinguish between these two types of variation. frequently a character will be found in regard to which it is impossible to determine whether it is congenital or acquired. if a child is born under the tropical sun, how can we tell whether its dark skin was the result of direct action of the sun on its own skin, or was an inheritance from its dark-skinned parents? we might suppose that this could be answered by taking a similar child, bringing it up away from the tropical sun, and seeing whether his skin remained dark. this would not suffice, however; for if such a child did then develop a white skin, we could not tell but that this lighter-coloured skin had been produced by the direct bleaching effect of the northern climate upon a skin which otherwise would have been dark. in other words, a conclusive answer can not here be given. it is not our purpose, however, to attempt to distinguish between these two kinds of variations, but simply to recognize that they occur. our next problem must be to search for an explanation of these variations. with the acquired variations we have no particular trouble, for they are easily explained as due to the direct action of the environment upon animals. one of the fundamental characters of the living protoplasm (using the word now in its widest sense) is its extreme instability. so unstable is it that any disturbing influence will affect it. if two similar unicellular organisms are placed under different conditions they become unlike, since their unstable protoplasm is directly affected by the surrounding conditions. with higher animals the process is naturally a little more complicated; but here, too, they are easily understood as part of the function of the machine. one of the adjustments of the machine is such that when any organ is used more than usual the whole machine reacts in such a way as to send more blood to this special organ. the result is a change in the nutrition of the organ and a corresponding variation in the individual. thus acquired variations are simply functions of the action of the machine. congenital variations, however, can not receive such an explanation. being born with the individual, they can not be produced by conditions affecting him, but rather to something affecting the germ plasm from which he sprung. the nature of the germ plasm controls the nature of the individual, and congenital variations must consequently be due to its variations. but it is not so easy to see how this germ plasm can undergo variation. the conditions which surround the individual would affect its body, but it is not easy to believe that they would affect the germinal substance. indeed, it is not easy to see how any external conditions can have influence upon this germinal material if it is not an active part of the body, but is simply stored within it for future use in reproduction. how could any changes in the environment of the individual have any effect upon this dormant material stored within it? but if we are correct in regarding this germ material in the reproductive bodies as the basis of heredity and the guiding force in development, then it follows that the only way in which congenital variations can occur is by some variations in the germ plasm. if a child developed from germ plasm _identical_ with that from which its parents developed, it would inherit identical characters; and if there are any congenital variations from its parents, they must be due to some variations in the germ plasm. in other words, in order to explain congenital variations we must account for variations in the germ plasm. now, there are two methods by which we may suppose that these variations in the germ may arise. the first is by the direct influence upon the germ plasm of certain unknown external conditions. the life substance of organisms is always very unstable, and, as we have seen, acquired variations are caused by external influences directly affecting it. now, the hereditary material is also life substance, and it is plainly a possibility for us to imagine that this germ material is also subject to influences from the conditions surrounding it. that such variations do occur appears to be hardly doubtful, although we do not know what sort of influences can produce them. if the germ plasm is wholly stored within the reproductive gland, it is certainly in a position to be only slightly affected by surrounding conditions which affect the animal. we can readily understand that the use of an organ like the arm will affect it in such a way as to produce changes in its protoplasm, but we can hardly imagine that such use of the _arm_ would produce any change in the hereditary substance which is stored in the reproductive organs. external conditions may thus readily affect the body, but not so readily the germ material. even if such material is distributed more or less over the body instead of being confined to the reproductive glands, as some believe, the difficulty is hardly lessened. this difficulty of understanding how the germ plasm can be affected by external conditions has led one school of biologists to deny that it is subject to any variation by external conditions, and hence that all modification of the germ plasm must come from some other source. probably no one, however, holds this position to-day, and it is the general belief that the germ plasm may be to some slight extent modified by external conditions. of course, if such variations do occur in the germ plasm they will become congenital variations of the next generation, since the next generation is the unfolding of the germ plasm. the second method by which the variations of germ plasm may arise is apparently of more importance. it is based upon the fact that, with all higher animals and plants at least, each individual has two parents instead of one. in our study of cells we have seen that the machinery of the cell is such that it requires in the ordinary process of reproduction the union of germinal material from two different individuals to produce a cell which can develop into a new individual. as we have seen, the egg gets rid of half its chromosomes in order to receive an equal number from a male parent; and thus the fertilized egg contains chromosomes, and hence hereditary material, from two different individuals. now, this sexual reproduction occurs very widely in the organic world. among some of the lowest forms of unicellular organisms it is not known, but in most others some form of such union is universal. now, here is plainly an abundant opportunity for congenital variations; for it is seen that each individual does not come from germ material _identical with that from which either parent came, but from some of this material mixed with a similar amount from a different parent_. now, the two parents are never exactly alike, and hence the germ plasm which each contributes to the offspring will not be exactly alike. the offspring will thus be the result of the unfolding of a bit of germ plasm which will be different from that from which either of its parents developed, and these differences will result in _congenital variations_. sexual reproduction thus results in congenital variations; and if congenital variations are necessary for the evolution of the living machine--and we shall soon see reason for believing that they are--we find that sexual reproduction is a device adopted for bringing out such congenital variations. ==inheritance of variations.==--the reason why congenital variations are needed for the evolution of the living machine is clear enough. evanescent variations can have no effect upon this machine, for they would disappear with the individual in which they appeared. in order that they should have any influence in the process of machine building they must be permanent ones; or, in other words, they must be inherited from generation to generation. only as such variations are transmitted by heredity can they be added to the structure of the developing machine. therefore we must ask whether the variations are inherited. in regard to the congenital variations there can be no difficulty. the very fact that they are congenital shows us that they have been produced by variations in the germ plasm, and as such they must be transmitted, not only to the next generation, but to all following generations, until the germ plasm becomes again modified. this germ plasm is handed on from generation to generation with all its variations, and hence the variations will be added permanently to the machine. congenital variations are thus a means for permanently modifying the organism, and by their agency must we in large measure believe that evolution through the ages has taken place. with the acquired variations the matter stands quite differently. we can readily understand how influences surrounding an animal may affect its organs. the increase in the size of the muscles of the blacksmith's arm by use we understand readily enough. but with our understanding of the machinery of heredity we can not see how such an effect can extend to the next generation. it is only the organ directly affected that is modified by external conditions. acquired variations will appear in the part of the body influenced by the changed conditions. but the germ plasm within the reproductive glands is not, so far as we can see, subject to the influence of an increased use, for example, in the arm muscles. the germ material is derived from the parents, and, if it is simply stored in the individual, how could an acquired variation affect it? if an individual lose a limb his offspring will not be without a corresponding limb, for the hereditary material is in the reproductive organs, and it is impossible to believe that the loss of the limb can remove from the hereditary material in the reproductive glands just that part of the germ plasm which was designed for the production of the limb. so, too, if the germ plasm is simply stored in the individual, it is impossible to conceive any way that it can be affected by the conditions around the individual in such a way as to explain the inheritance of acquired variations. if acquired variations do not affect the germ plasm they cannot be inherited, and if the germ plasm is only a bit of protoplasmic substance handed down from generation to generation, we can not believe that acquired variations can influence it. from such considerations as these have arisen two quite different views among biologists; and, while it is not our purpose to deal with disputed points, these views are so essential to our subject that they must be briefly referred to. one class of biologists adhere closely to the view already outlined, and insist for this reason that acquired variations _can not_ under any conditions be inherited. they insist that all inherited variations are congenital, and due therefore to direct variations in the germ plasm, and that all instances of seeming inheritance of acquired variations are capable of other explanation. the other school is equally insistent that there are abundant instances of the inheritance of acquired characters, claiming that these proofs are so strong as to demand their acceptance. hence this class of biologists insist that the explanation of heredity given as a simple handing down from generation to generation of a germ plasm is not complete, and that while it is doubtless the foundation of heredity, it must be modified in some way so as to admit of the inheritance of acquired characters. there is no question that has excited such a wide interest in the biological world during the last fifteen years as this one of the inheritance of acquired characters. until about the question was not seriously raised. heredity was known to be a fact, and it was believed that while congenital characters are more commonly inherited, acquired characters may also frequently be handed down from generation to generation. the facts which we have noted of the continuity of germ plasm have during the last fifteen years led many biologists to deny the possibility of the latter. the debate which arose has continued vigorously, and can not be regarded as settled at the present time. one result of this debate is clear. it has been shown beyond question that while the inheritance of congenital characters is the rule, the inheritance of acquired characters is at all events unusual. at the present time many naturalists would be inclined to think that the balance of evidence indicates that under certain conditions certain kinds of acquired characters may be inherited, although this is still disputed by others. into this discussion we cannot enter here. the reason for referring to it at all is, however, evident. we are searching for nature's method of building machines. it is perfectly clear that variations among animals and plants are the foundations of the successive steps in advance made in this machine building, but of course only such variations as can be transmitted to posterity can serve any purpose in this development. if therefore it should prove that acquired characters can not be inherited, then we should no longer be able to look upon the direct influence of the surroundings as a factor in the machine building. we should then have nothing left except the congenital variations produced by sexual union, or the direct variation of the germ plasm as a factor for advance. if, however, it shall prove that acquired characters may even occasionally be inherited, then the direct effect of the environment upon the individual will serve as a decided assistance in our problem. here, then, we have before us the factors which have been concerned in the building of the living machine under nature's hands. reproduction keeps in existence a constantly active, unstable, readily modified organism as a basis upon which to build. variation offers constantly new modifications of the type, while heredity insures that the modifications produced in the machine by the influences which give rise to the variations shall be permanently fixed. ==method of machine building.==--_natural selection._ the method by which these factors have worked together to build up the living machines is easily understood in its general aspects, although there are many details as yet unsolved. the general facts connected with the evolution of animals are matters of common knowledge. we need do no more than outline the subject, since it is well understood by all. the basis of the method is _natural selection_, which acts in this machine building something as follows: the law of reproduction, as we have seen, produces new individuals with extraordinary rapidity, and as a result more individuals are born than can possibly find sustenance in the world. hence only a few of the offspring of any animal or plant can live long enough to produce offspring in turn. the many must die that the few may live; and there is, therefore, a constant struggle among the individuals that are born for food or for room in the world. in this _struggle for existence_ of course the weakest will go to the wall, while those that are best adapted for their place in life will be the ones to get food, live, and reproduce their kind. this is at all events true among the lower animals, although with mankind the law hardly applies. now, among the individuals that are born there will be no two exactly alike, since variations are universal, many of which are congenital and thus born with the individual and transmitted by inheritance. clearly enough those animals that have a variation which makes them a little better adapted for the struggle will be the ones to live and hence to produce offspring, while those without such advantage will be the ones to die. we may suppose, for example, that some of the individuals had longer necks than the average. in time of scarcity of food these individuals would be able to get food that the short-necked individuals could not reach. hence in times of famine the long-necked individuals would be the ones to survive. now if this peculiarity were a congenital variation it would be already represented in the germ plasm, and consequently it would be inherited by the next generation. the short-necked individuals being largely destroyed in this struggle for food, it would follow that the next generation would be a little better off than the last, since all would inherit this tendency toward a long neck. a few generations would then see the disappearance of all individuals which did not show either this or some other corresponding advantage, and in this way the lengthened neck would be added permanently as a _part of the machine_. when this time came this peculiarity would no longer give its possessors any advantage over its rivals, since all would possess it. now, therefore, some new variation would in the same way determine which animals should live and which should die in the struggle, and in time a new modification would be added to the machine. and thus this process continues, one variation after another being added, until the machine is slowly built into a more and more complicated structure, always active but with a constantly increasing efficiency. the construction is a natural one. a mixing of germ plasm in sexual reproduction or some other agencies produce congenital variations; natural selection acting upon the numerous progeny selects the best of the new variations, and heredity preserves and hands them down to posterity. all students of whatever school recognize the force of this principle and look upon natural selection as an efficient agency in machine building. it is probably the most fundamental of the external laws that have guided the process. there are, however, certain other laws which have played a more or less subordinate part. the chief of these are the influence of migration and isolation, and the direct influence of the environment. each of these laws has its own school of advocates, and each has been given by its advocates the chief role in the process of machine building. ==migration and isolation.==--the production of the various types of machines has been undoubtedly facilitated by the migrations of animals and the isolation of different groups of descendants from each other by various natural barriers. the variations which occur in organisms are so great that they would sometimes run into abnormal structures were it not for the fact that sexual reproduction constantly tends to reduce them. in an open country where animals and plants interbreed freely, it will commonly happen that individuals with certain peculiarities will mate with others without such peculiarities, and the offspring will therefore inherit the peculiarity not in increased degree but in decreased degree. this constant interbreeding of individuals will tend to prevent the formation of many modifications in the machine which become started by variations. now plainly if some such individuals, with a peculiar variation, should migrate into a new territory or become isolated from their relatives which do not have similar variations, these individuals will be obliged to breed with each other. the result will be that the next generation, arising thus from two parents each of which shows the same variation, will show it also in equal or increased degree. migrations and isolations will thus tend to _fix_ in the machine variations which sexual union or other influences inaugurate. now in the history of the earth's surface there have been many changes which tend to bring about such migration and isolations, and this factor has doubtless played a more or less important part in the building of the machines. how great a part we cannot say, nor is it necessary for our purpose to decide; for in all these cases the machine building has only been the result of the hereditary transmission of congenital variation under certain peculiar conditions. the fundamental process is the same as already considered, only the details of its working being in question. ==direct influence of the environment.==--under this head we have a subject of great importance. it is an undoubted fact that the environment has a very decided effect upon the machine. these direct effects of the environment are very positive and in great variety. the tropical sun darkens the human skin; cold climate stunts the growth of plants; lack of food dwarfs all animals and plants, and hundreds of other similar examples could be selected. another class of similar influences are those produced by _use_ and _disuse_. beyond question the use of an organ tends to increase its size, and disuse to decrease it. combats of animals with each other tend to increase their strength, flight from enemies their running powers, etc. now all these effects are direct modifications of the machine, and if they are only transmitted to following generations so as to become _permanent_ modifications, they will be most important agencies in the machine building. if, on the other hand, they are not transmitted by heredity, they can have no permanent effect. we have here thus again the problem of the inheritance of acquired characters. we have already noticed the uncertainty surrounding this subject, but the almost universal belief in the inheritance of such characters requires us to refer to it again. it is uncertain whether such direct effects have any influence upon the offspring, and therefore whether they have anything to do with this machine building. still, there are many facts which point strongly in this direction. for example, as we study the history of the horse family we find that an originally five-toed animal began to walk more and more on its middle toe, in such a way that this toe received more and more use, while the outer toes were used less and less. now that such a habit would produce an effect upon the toes in any generation is evident; but apparently this influence extended from generation to generation, for, as the history of the animals is followed, it is found that the outer toes became smaller and smaller with the lapse of ages, while the middle one became correspondingly larger, until there was finally produced the horse with its one toe only on each foot. now here is a line of descent or machine building in the direct line of the effects of use and disuse, and it seems very natural to suppose that the modification has been produced by the direct effect of the use of the organs. there are many other similar instances where the line of machine building has been quite parallel to the effects of use and disuse. if, therefore, acquired characters can be inherited to _any_ extent, we have, in the direct influences of the environment an important agency in machine building. this direct effect of the conditions is apparently so manifest that one school of biologists finds in it the chief cause of the variations which occur, telling us that the conditions surrounding the organism produce changes in it, and that these variations, being handed down to subsequent generations, constitute the basis of the development of the machine. if this factor is entirely excluded, we are driven back upon the natural selection of congenital variations as the only kind of variations which can permanently effect the modification of the machine. ==consciousness.==--it may be well here to refer to one other factor in the problem, because it has somewhat recently been brought into prominence. this factor is consciousness on the part of the animal. among plants and the lower animals this factor can have no significance, but consciousness certainly occurs among the higher animals. just when or how it appeared are questions which are not answered, and perhaps never will be. but consciousness, after it had once made its appearance, became a controlling factor in the development of the machine. it must not be understood by this that animals have had any consciousness of the development of their body, or that they have made any conscious endeavours to modify its development. this has not always been understood. it has been frequently supposed that the claim that consciousness has an influence upon the development of an animal means that the animal has made conscious efforts to develop in certain directions. for example, it has been suggested that the tiger, conscious of the advantage of being striped, had a desire to possess stripes, and the desire caused their appearance. this is absurd. consciousness has been a factor in the development of the machine, but an _indirect_ one. consciousness leads to effort, and effort has a direct influence in development. for example, an animal is conscious of hunger, and this leads to efforts on his part to obtain food. his efforts to obtain food may lead to migration or to the adoption of new kinds of food or to conflicts with various kinds of rivals, and all of these efforts are potent factors in determining the direction of development. consciousness, again, may lead certain animals to take pleasure in each other's society, or to recognize that in mutual association they have protection against common enemies. such a consciousness will give rise to social habits, and social habits are a very potent factor in determining the direction in which the inherited variations will tend; not, perhaps, because it effects the variations themselves, but rather because it determines which variations among the many shall be preserved and which rejected by natural selection. consciousness may lead the antelope to recognize that he has no chance in a combat with a lion, and this will induce him to flee. the _habit_ of flight would then develop the _power_ of flight, not because the antelope desired such power, but because the animals with variations which gave increased power of flight would be the ones to escape the lion, while the slower ones would die without offspring. thus consciousness would indirectly, though not directly, result in the lengthening of the legs of the animal and in the strengthening of his running muscles. beyond a doubt this factor of consciousness has been a factor of no little moment in the development of the higher types of organic machines. we can as yet only dimly understand its action, but it must hereafter be counted as one of the influences in the evolution of the living machine. but, after all, these are only questions of the method of the action of certain well demonstrated, fundamental factors. whether by natural selection, or by the inheritance of acquired characters produced by the environment, or whether by the effect of isolation of groups of individuals, the machine building has always been produced in the same way. a machine, either through the direct influence of the environment, or as a result of sexual combination of germ plasm, shows a variation from its parents. this variation proves of value to its possessor, who lives and transmits it permanently to posterity. thus step by step, one part is added to another, until the machine has grown into the intricately adapted structure which we call the animal or plant. this has been nature's method of building machines, all based upon the three properties possessed by the living cell--reproduction, variation, and heredity. ==summary of nature's power of building machines.==--let us now notice the position we have reached. our problem in the present chapter has been to find out whether nature possesses forces adequate to explain the building of machines with their parts accurately adapted to each other so as to act harmoniously for certain ends. astronomy has shown that she has forces for the building of worlds; geology, that she has forces for making mountain and valley; and chemistry, that she has forces for building chemical compounds. but the organism is neither a world, nor a mass of matter, nor a chemical compound. it is a machine. has nature any forces for machine building? we have found that by the use of the three factors, reproduction, variation, and heredity, nature is able to produce a machine of ever greater and greater complexity, with the parts all adapted to each other. now the difference between a machine and a mass of matter is simply in the adaptation of parts to act harmoniously for definite ends. hence if we are allowed these three factors, we can say that nature _does possess forces adequate to the manufacture of machines_. these forces are not chemical forces, and the construction of the machine has thus been brought about by forces entirely different from those which produced the chemical molecule. but we have plainly not reached the bottom of the matter in our attempt to explain the machinery of living things. we have based the whole process upon three factors. reproduction, variation, and heredity are the properties of all living matter; but they are not, like gravity and chemism, universal forces of nature. they occur in living organisms only. why should they occur in living organisms, and here alone? these three properties are perhaps the most marvellous properties of nature; and surely we have not finished our task if we have based the whole process of machine building upon these mysterious phenomena, leaving them unintelligible. we must therefore now ask whether we can proceed any farther and find any explanation of these fundamental powers of the living machine. it must be confessed that here we are at present forced to stop. we can proceed no further with any certainty, or even probability. we may say that variation and heredity are only phases of reproduction, and reproduction is a property of the living cell. we may say that this power of reproduction is dependent upon the power of assimilation and growth, for cell division is a result of cell growth. we may further say that growth and assimilation are chemical processes resulting from the oxidation of food, and that thus all of these processes are to be reduced to chemical forces. in this way we may seem to have a chemical foundation for life phenomena. but clearly this is far from satisfactory. in the first place, it utterly fails to explain why the living cell has these properties, while no other body possesses them, nor why they are possessed by living protoplasms _alone_, ceasing instantly with death. indeed it does not tell us what death can be. secondly, it utterly fails to explain the marvels of cell division with resulting hereditary transmission. for all this we must fall back upon the structure of protoplasm, and say that the cell machinery is so adjusted that the machine, when acting as a whole, is capable of transforming the energy of chemical composition in certain directions. these fundamental properties are then the properties of the cell _machine_ just as surely as printing is the property of the printing press. we can no more account for the life phenomena by chemical powers than we can for printing by chemical forces manifested in the burning of the coal in the engine room. to be sure, it is the chemical forces in the engine room that furnishes the energy, but it is the machinery of the press that explains the printing. so, while chemical forces supply life energy, it is the cell machinery that must explain the fundamental living factors. so long as this machine is intact it can continue to run and perform its duties. but it is a very delicate machine and is easily broken. when it is broken its activities cease. a broken machine can not run. it is dead. in short, we come back once more to the idea of the machinery of protoplasm, and must base our understanding of its properties upon its structure. it is proper to state that there are still some biologists who insist that the ultimate explanation of protoplasm is purely chemical and that life phenomena may be manifested in mixtures of compounds that are purely physical mixtures and not machines. it is claimed that much of this cell structure described above is due to imperfection in microscopic methods and does not really exist in living protoplasm, while the marvellous activities described are found only in the highly organized cell, but do not belong to simple protoplasm. it is claimed that simple protoplasm consists of a physical mixture of two different compounds which form a foam when thus mixed, and that much of the described structure of protoplasm is only the appearance of this foam. this conception is certainly not the prevalent one to-day; and even if it should be the proper one, it would still leave the cell as an extremely complicated machine. under any view the cell is a mechanism and must be resolved into subordinate parts. it may be uncertain whether these subordinate parts are to be regarded simply as chemical compounds physically mixed, or as smaller units each of which is a smaller mechanism. at all events, at the present time we know of no such simple protoplasm capable of living activities apart from machinery, and the problem of explaining life, even in the simplest form known, remains the problem of explaining a mechanism. ==the origin of the cell machine.==--we have thus set before us another problem, which is after all the fundamental one, namely, to ask whether we can tell anything of nature's method of building the protoplasmic machine. the building of the higher animal and plant, as we have seen, is the result of the powers of protoplasm; but protoplasm itself is a machine. what has been its history? we must first notice that no notion of _chemical evolution_ helps us out. it has been a favourite thought with some that the origin of the first living thing was the result of chemical evolution. as the result of physical forces there was produced, from the original nebulous mass, a more and more complicated system until the world was formed. then chemical phenomena became more and more complicated until, with the production of more and more complicated compounds, protoplasm was finally produced. a few years ago, under the impulse of the idea that protoplasm was a compound, or at least a simple mixture of compounds, this thought of protoplasm as the result of chemical evolution was quite significant. _physical forces_, _chemical forces_, and _vital forces_, explain successively the origin of _worlds_, _protoplasm_, and _organisms_. this conception has, however, no longer much significance. we know of no such living chemical compound apart from cell machinery. a new conception of protoplasm has arisen which demands a different explanation of its origin. since it is a machine rather than a compound, mechanical rather than chemical forces are required for its explanation. have we then any suggestion as to the method of the origin of this protoplasmic machine? our answer must, at the present, be certainly in the negative. the complexity of the cell tells us plainly that it can not be the ultimate living substance which may have arisen from chemical evolution. it is made up of parts delicately adapted to act in harmony with each other, and its activity depends upon the relation of these parts. whatever chemical forces may have accomplished, they never could have combined different bodies into linin, centrosomes, chromosomes, etc., which, as we have seen, are the basis of cell life. to account for this machine, therefore, we are driven to assume either that it was produced by some unknown intelligent power in its present condition of complex adjustment, or to assume that it has had a long history of building by successive steps, just as we have seen to be the case with the higher organisms. the latter assumption is, of course, in harmony with the general trend of thought. to-day protoplasm is produced only from other protoplasm; but, plainly, the first protoplasm on the earth must have had a different origin. we must therefore next look for facts which will enable us to understand its origin. we have seen that the animal and plant machines have been built up from the simple cell as the result of its powers acting under the ordinary conditions of nature. now, in accordance with this general line of thought, we shall be compelled to assume that previous to the period of building machinery which we have been considering, there was another period of machine building during which this cell machine was built by certain natural forces. but here we are forced to stop, for nothing which we yet know gives even a hint as to the method by which this machine was produced. we have, however, seen that there are forces in nature efficient in building machines, as well as those for producing chemical compounds; and this, doubtless, suggests to us that there may be similar forces at work in building protoplasm. if we can find natural forces by which the simplest bit of living matter can be built up into a complicated machine like the ox, with its many delicately adjusted parts, it is certainly natural to imagine that the same forces may have built this simpler machine with which we started. but such a conclusion is for a simple reason impossible. we have seen that the essential factor in this machine building is reproduction, with the correlated powers of variation and heredity. without these forces we could not have advanced in this machine building at all. but these properties are themselves the result of the machinery of protoplasm. we have no reason for thinking that this property of reproduction can occur in any other object in nature except this protoplasmic machine. of course, then, if reproduction is the result of the structure of protoplasm we can not use this factor in explaining the origin of this protoplasm. the powers of the completed machine can not be brought forward to account for its origin. thus the one fundamental factor for machine building is lacking, and if we are to explain nature's method of producing protoplasm from simpler structures, we must either suppose that the _parts_ of the cell are capable of reproduction and subject to heredity, or we must look for some other method. such a road has however not yet been found, nor have we any idea in what direction to look. but the fact that nature has methods of machine building, as we have seen, may hold out the possibility that some day we may discover her method of building this primitive living machine, the cell. it is useless to try to go further at present. the origin of living matter is shrouded in as great obscurity as ever. we must admit that the disclosures of the modern microscope have complicated rather than simplified this problem. while a few years ago chemists and biologists were eagerly expecting to discover a method of manufacturing a bit of living matter by artificial means, that hope has now been practically abandoned. the task is apparently hopeless. we can manipulate chemical forces and produce an endless series of chemical compounds. but we can not manipulate the minute bits of matter which make up the living machine. since living matter is made of the adjustment of these microscopic parts of matter, we can not hope to make a bit of living matter until we find some way of making these little parts and adjusting them together. most students of protoplasm have therefore abandoned all expectation of making even the simplest living thing. we are apparently as far from the real goal of a natural explanation of life as we were before the discovery of protoplasm. ==general summary.==--it is now desirable to close this discussion of seemingly somewhat unconnected topics by bringing them together in a brief summary. this will enable us to see more clearly the position in which science stands to-day upon this matter of the natural explanation of living phenomena, and to picture to ourselves more concisely our knowledge of the living machine. the problem we have set before us is to find out to what extent it is possible to account for vital phenomena by the application of ordinary natural laws and forces, and therefore to find out whether it is necessary to assume that there are forces needed to explain life which are different from those found in other realms of nature, or whether vital forces are all correlated with physical forces. it has been evident at a glance that the living body is a machine. like other machines it consists of parts adjusted to each other for the accomplishment of definite ends, and its action depends upon the adjustment of its parts. like other machines, it neither creates nor destroys energy, but simply converts the potential energy of its foods into some form of active energy, and, like other machines, its power ceases when the machine is broken. with this understanding the problem clearly resolved itself into two separate ones. the first was to determine to what extent known physical and chemical laws and forces are adequate to an explanation of the various phenomena of life. the second was to determine whether there are any known forces which can furnish a natural explanation of the origin of the living machine. manifestly, if the first of these problems is insolvable, the second is insolvable also. in the study of the first problem we have reached the general conclusion that the secondary phenomena of life are readily explained by the application of physical and chemical forces acting in the living machine. these secondary phenomena include such processes as the digestion and absorption of food, circulation, respiration, excretion, bodily motion, etc. nervous phenomena also doubtless come under this head, at least so far as concerns nervous force. we have been obliged, however, to exclude from this correlation the mental phenomena. mental phenomena can not as yet be measured, and have not yet been shown to be correlated with physical energy. in other words, it has not yet been proved that mental force is energy at all; and if it is not energy, then of course it can not be included in the laws which govern the physical energy of the universe. although a close relation exists between physical changes in the brain cells and mental phenomena, no further connection has yet been drawn between mental power and physical force. all other secondary phenomena, however, are intelligently explained by the action of natural forces in the machinery of the living organism. while we have thus found that the secondary phenomena of life are intelligible as the result of the structure of the machine, certain other fundamental phenomena have been constantly forcing themselves upon our attention as a _foundation_ of these secondary activities. the power of contraction, the power of causing certain kinds of chemical change to occur which result in metabolism, the property of sensibility, the property of reproduction--these are fundamental to all living activity, and are, after all, the real phenomena which we wish to explain. but these are not peculiar to the complicated machines. we can discard all the apparent machinery of the animal or plant and find these properties still developed in the simplest bit of living matter. to learn their significance, therefore, we have turned to the study of the simplest form of matter in which these fundamental properties are manifested. this led us at once to the study of the so-called protoplasm, for protoplasm is the simplest known form of matter that is alive. protoplasm itself at first seemed to be a homogeneous body, and was looked upon as a chemical compound of high complexity. if this were true its properties would depend upon its composition and would be explained by the action of chemical forces. such a conception would have quickly solved the problem, for it would reduce living properties to chemical powers. but the conception proved to be delusive. protoplasm, at least the simplest form known to possess the fundamental life properties, soon showed itself to be no chemical compound, but a machine of wonderful intricacy. the fundamental phenomena of life and of protoplasm have proved to be both chemical and mechanical. metabolism is the result of the oxidation of food, and motion is an instance of transference of force. our problem then resolved itself into finding the power that guides the action of these natural forces. food will not undergo such an oxidation except in the presence of protoplasm, nor will the phenomena of metabolism occur except in the presence of _living_ protoplasm. clearly, then, the living protoplasm contains within itself the power of guiding this play of chemical force in such a way as to give rise to vital phenomena, and our search must be not for chemical force but for this guiding principle. our study of protoplasm has told us clearly enough that we must find this guiding principle in the interaction of the machinery within the protoplasm. the microscope has told us plainly that these fundamental principles are based upon machinery. the cell division (reproduction) is apparently controlled by the centrosomes; the heredity by the chromosomes; the constructive metabolism by the nucleus in general, while the destructive metabolism is also seated in the cell substance outside the nucleus. whether these statements are strictly accurate in detail does not particularly affect the general conclusion. it is clearly enough demonstrated that the activities of the protoplasmic body are dependent upon the relation of its different parts. although we have got rid of the complicated machinery of the organism in general, we are still confronted with the machinery of the cell. but our analysis can not, at present, go further. our knowledge of this machine has not as yet enabled us to gain any insight as to its method of action. we can not yet conceive how this machine controls the chemical and physical forces at its disposal in such a way as to produce the orderly result of life. the strict correlation between the forces of the physical universe and those manifested by this protoplasm tells us that a transformation of energy occurs within it, but of the method of that transformation we as yet know nothing. irritability, movement, metabolism, and reproduction appear to be not chemical properties of a compound, but mechanical properties of a machine. our mechanical analysis of the living machine stops short before it reaches any foundation in the chemical forces of nature. it is thus clearly apparent that the phenomena of life are dependent upon the machinery of living things, and we have therefore the second question of the _origin_ of this machinery to answer. chemical forces and mechanical forces have been laboriously investigated, but neither appear adequate to the manufacture of machines. they produce only chemical compounds and worlds with their mountains and seas. the construction of artificial machines has demanded intelligence. but here is a natural machine--the organism. it is the only machine produced by natural methods, so far as we know; and we have therefore next asked whether there are, in nature, simple forces competent to build machines such as living animals and plants? in pursuance of this question we have found that the complicated machines have been built out of the simpler ones by the action of known forces and laws. the factors in this machine building are simply those of the fundamental vital properties of the simplest protoplasmic machine. reproduction, heredity, and variation, acting under the ever-changing conditions of the earth's surface, are apparently all that are needed to explain the building of the complex machines out of the simpler ones. nature _has_ forces adequate to the building of machines as well as forces adequate to the formation of chemical compounds and worlds. but here again we are unable to base our explanation upon chemical and physical forces. reproduction, heredity, and variation are properties of the cell machine, and we are therefore thrown back upon the necessity of explaining the origin of this machine. can we find a mechanical or chemical explanation of the origin of protoplasm? a chemical explanation of the cell is impossible, since it is not a chemical compound, but a piece of mechanism. the explanation given for the origin of animals and plants is also here apparently impossible. the factors upon which that explanation depended are factors of this completed machine itself, and can not be used to explain its origin. we are left at present therefore without any foundation for further advance. the cells must have had a history of construction, but we do not as yet conceive any forces which may be looked upon as contributing to that history. whether life phenomena can be manifested by any mixture of compounds simpler than the cell we do not yet know. the great problems still remaining for solution, which have hardly been touched by modern biology in all its endeavours to find a mechanical explanation of the living machine, are, therefore, three. first, the relation of mentality to the general phenomena of the correlation of force; second, the intelligible understanding of the mechanism of protoplasm which enables it to guide the blind chemical and physical forces of nature so as to produce definite results; third, the kind of forces which may have contributed to the origin of that simplest living machine upon whose activities all vital phenomena rest--the living cell. index. a. absorption of food, . acquired characters, inheritance of, , , , , . --variations, , . amoeba, . anatomical evidence for evolution, . aquacity, . arm compared with wing, . aristotle, . assimilation, , , , . asters of dividing cells, . b. barry, , . bathybias, . biology a new science, , , . blood, , , , , . blood-vessels, , . body as a machine, , , . bone cells, . building of the living machine, , , , , , , . c. cartilage cells, . cell as a machine, , . --description of, . --division, , , . --discovery of, . --doctrine, . --substance, , . cells, , , , , . cellular structure of organisms, . cell wall, , . centrosome, , , , , , , . challenger expedition, . chemical evolution, . chemical theory of vitality, ; of life, , . chemism or mechanism, , . chemistry of digestion, , ; of protoplasm, ; of respiration, . chromatin, , , , , , . chromosomes, , , , , , , , . circulation, . colonies of cells, . comparison of the body and a machine, . congenital variations, , , ; inheritance of, . connective-tissue cells, . conservation of energy, , . consciousness as a factor in machine building, . constructive chemical processes, , , , . continuity of germ plasm, . correlation of vital and physical forces, , , , , , . cytoblastema. . cytology, . d. darwin, . death of the cell, . decline of the reign of protoplasm, . destructive chemical processes, , , , . dialysis, , , . digestion, . e. egg, , , . division of, . egg, fertilization of, . embryological evidence for evolution, . energy of nervous impulse, , . environment, . evidence for evolution as a method of machine building, , . evolution, , , , . experiments with developing eggs, . f. fat, absorption of, . female pronucleus, . fern cells, section of, . fertilization of the egg, , ; significance of, . fibres in protoplasm, ; --in spindle, , . forces at work in machine building, , , . formed material, . free cell formation, . g. geological evidence for evolution, . germ plasm, . h. heart as a pump, . heat, , , . heredity, , , ; --explanation of, . hereditary traits, , . historical geology, . history of the living machine, , . horses' toes, loss of, . huxley, , , , . i. irritability, . isolation, theory of, . k. karyokinesis, , . kidneys, . l. leaf, section of, . life the result of a mechanism, , . linin, , . linnæus, . lyell, . lymph, , . m. machine defined, . machines the result of mechanical forces, . male cell, , . ---- pronucleus, . maturation of the egg, . mechanical nature of living organisms, . mechanical theory of life, , . membrane of the nucleus, , . mental phenomena, , . metabolism, . microsomes, . migration, theory of, . monera, . movement, . muscle, , . n. natural selection, . nerve-fibre cell, . nervous energy, , . ---- system, . new biological problems, . nucleolus, , , . nucleus, , , , , , , , , ; formation of new, . ---- function of, , , . ---- presence of, , , . ---- structure of, . o. organic chemistry, . organic compounds, artificial manufacture of, , . origin of cell machine, , , . origin of life, , . osmosis, . oxidation, , . ---- as a vital process, , . p. philosophical biology, . physical basis of life, . polar cells, . potato, section of cells, . properties of chemical compounds, . protoplasm, , , , , , , , . ---- artificial manufacture of, . ---- as a machine, , . ---- discovery of, . ---- nature of, . ---- structure of, , . purpose _vs._ cause, , . r. reaction against the cell doctrine, . reign of law, . ---- of the nucleus, . ---- of protoplasm, , . relationship, significance of, . removal of waste, , . reproduction, , , , , ; --rapidity of, . respiration, . reticulum of cell, ; --of nucleus, . root tip, section of, . s. schultze, , . schwann, , , . secretion, , . segmentation nucleus, . sensations, . separation of chromosomes, . sexual reproduction, . spermatozoan, , , . splitting of chromosomes, . spindle fibres, . struggle for existence, . summary of part i, . ---- general, . u. undifferentiated protoplasm, . unicellular animals, . units of vital activity, . use and disuse, , . v. variation, , , , . variation from sexual union, . variation in germ plasm, . vegetative functions, . villi, . vital force, vitality, , , , , , , . vital properties, ; --located in cells, . w. wing compared with arm, . wood cells, . the end. ==the library of useful stories.== illustrated. mo. cloth, cents net per volume; postage, cents per volume additional. the story of a grain of wheat. by w.c. edgar. the story of alchemy. by m.m. pattison muir. the story of animal life. by b. lindsay. the story of the art of music. by f.j. crowest. the story of the art of building. by p.l. waterhouse. the story of king alfred. by sir walter besant. the story of books. by gertrude b. rawlings. the story of the alphabet. by edward clodd. the story of eclipses. by g.f. chambers, f.r.a.s. the story of the living machine. by h.w. conn. the story of the british race. by john munro, c.e. the story of geographical discovery. by joseph jacobs. the story of the cotton plant. by f. wilkinson, f.g.s. the story of the mind. by prof. j. mark baldwin. the story of photography. by alfred t. story. the story of life in the seas. by sydney j. hickson. the story of germ life. by prof. h.w. conn. the story of the earth's atmosphere. by douglas archibald. the story of extinct civilizations of the east. by robert anderson, m.a., f.a.s. the story of electricity. by john munro, c.e. the story of a piece of coal. by e.a. martin, f.g.s. the story of the solar system. by g.f. chambers, f.r.a.s. the story of the earth. by h.g. seeley, f.r.s. the story of the plants. by grant allen. the story of "primitive" man. by edward clodd. the story of the stars. by g.f. chambers, f.r.a.s. others in preparation. d. appleton and company, new york. new edition of huxley's essays. ==collected essays.== by thomas h. huxley. new complete edition, with revisions, the essays being grouped according to general subject. in nine volumes, a new introduction accompanying each volume. mo. cloth, $ . per volume. volume. ==i. methods and results. ii. darwiniana. iii. science and education. iv. science and hebrew tradition. v. science and christian tradition. vi. hume. vii. man's place in nature. viii. discourses, biological and geological. ix. evolution and ethics, and other essays.== "mr. huxley has covered a vast variety of topics during the last quarter of a century. it gives one an agreeable surprise to look over the tables of contents and note the immense territory which he has explored. to read these books carefully and studiously is to become thoroughly acquainted with the most advanced thought on a large number of topics."--_new york herald._ d. appleton and company, new york. d. appleton and company's publications. _pioneers of evolution, from thales to huxley_ by edward clodd, president of the folk-lore society; author of "the story of creation," "the story of 'primitive' man," etc. with portraits, mo. cloth, $ . . "the mass of interesting material which mr. clodd has got together and woven into a symmetrical story of the progress from ignorance and theory to knowledge and the intelligent recording of fact is prodigious.... the 'goal' to which mr. clodd leads us in so masterly a fashion is but the starting point of fresh achievements, and, in due course, fresh theories. his book furnishes an important contribution to a liberal education."--_london daily chronicle._ "we are always glad to meet mr. clodd. he is never dull; he is always well informed, and he says what he has to say with clearness and precision.... the interest intensifies as mr. clodd attempts to show the part really played in the growth of the doctrine of evolution by men like wallace, darwin, huxley, and spencer.... we commend the book to those who want to know what evolution really means."--_london times._ "this is a book which was needed.... altogether, the book could hardly be better done. it is luminous, lucid, orderly, and temperate. above all, it is entirely free from personal partisanship. each chief actor is sympathetically treated, and friendship is seldom or never allowed to overweight sound judgment"--_london academy._ "we can assure the reader that he will find in this work a very useful guide to the lives and labors of leading evolutionists of the past and present. especially serviceable is the account of mr. herbert spencer and his share in rediscovering evolution, and illustrating its relations to the whole field of human knowledge. his forcible style and wealth of metaphor make all that mr. clodd writes arrestive and interesting."--_london literary world._ "can not but prove welcome to fair-minded men.... to read it is to have an object-lesson in the meaning of evolution.... there is no better book on the subject for the general reader.... no one could go through the book without being both refreshed and newly instructed by its masterly survey of the growth of the most powerful idea of modern times."--_the scotsman._ d. appleton and company, new york. ==books on social science.== ==socialism new and old.== by prof. william graham, mo. cloth, $ . . "professor graham's book may be confidently recommended to all who are interested in the study of socialism, and not so intoxicated with its promises of a new heaven and a new earth as to be impatient of temperate and reasoned criticism."_--london times._ "professor graham presents an outline of the successive schemes of three writers who have chiefly influenced the development of socialism, and dwells at length upon the system of rousseau, that of st. simon, and on that of karl marx, the founder of the new socialism, 'which has gained favor with the working classes in all civilized countries,' which agrees with rousseau's plan in being democratic, and with st. simon's in aiming at collective ownership.... the professor is an independent thinker, whose endeavor to be clear has resulted in the statement of definite conclusions. the book is a remarkably fair digest of the subject under consideration."_--philadelphia ledger._ ==dynamic sociology:== _or, applied social science, as based upon statical sociology and the less complex sciences._ by lester f. ward, a.m. in vols. mo. cloth, $ . . "a book that will amply repay perusal.... recognizing the danger in which sociology is, of falling into the class of dead sciences or polite amusements, mr. ward has undertaken to 'point out a method by which the breath of life can be breathed into its nostrils.'"--_rochester post-express._ "mr. ward has evidently put great labor and thought into his two volumes, and has produced a work of interest and importance. he does not limit his effort to a contribution to the science of sociology.... he believes that sociology has already reached the point at which it can be and ought to be applied, treated as an art, and he urges that 'the state' or government now has a new, legitimate, and peculiar field for the exercise of intelligence to promote the welfare of men."--_new york times._ ==criminal sociology.== by prof. e. ferri. a new volume in the criminology series, edited by w. douglas morrison, mo. cloth, $ . . in this volume professor ferri, a distinguished member of the italian parliament, deals with the conditions which produce the criminal population, and with the methods by which this anti-social section of the community may be diminished. he divides the causes of crime into two great classes, individual and social. the individual causes consist of physical and mental defects; the social causes consist of social disadvantages of every description. his view is that the true remedy against crime is to remove individual defects and social disadvantages where it is possible to remove them. he shows that punishment has comparatively little effect in this direction, and is apt to divert attention from the true remedy--the individual and social amelioration of the population as a whole. d. appleton and company, new york. books for nature lovers. ==insect life. (new edition in colors.)== by john henry comstock, professor of entomology in cornell university. with full-page plates reproducing butterflies and various insects in their natural colors, and with many wood engravings by anna botsford comstock, member of the society of american wood engravers, mo. cloth, $ . net; postage, cents additional. "the volume is admirably written, and the simple and lucid style is a constant delight.... it is sure to serve an excellent purpose in the direction of popular culture, and the love of natural science which it will develop in youthful minds can hardly fail to bear rich fruit."--_boston beacon._ ==familiar fish: their habits and capture.== a practical book on fresh-water game fish. by eugene mccarthy. with an introduction by dr. david starr jordan, president of leland stanford junior university, and numerous illustrations, mo. cloth, $ . . "one of the handsomest, most practical, most informing books that we know. the author treats his subject with scientific thoroughness, but with a light touch that makes the book easy reading.... the book should be the companion of all who go a-fishing."--_new york mail and express._ ==the art of taxidermy.== by john rowley, chief of the department of taxidermy in the american museum of natural history. illustrated, mo. cloth, $ . . "mr. rowley will long be gratefully remembered by taxidermists, amateurs, and others, for the care he has used in thus meeting a long-felt want."--_bangor, me., sportsman._ "the book is not an elaborate treatise upon the abstract principles which lie at the foundation of artistic taxidermy, but is rather a compendium full of practical hints and suggestions, recipes, and formulas for the working taxidermist."--_the dial._ ==plants. (plant relations and plant structures in one volume.)== by john m. coulter, a.m., ph.d., head of department of botany, university of chicago, mo. cloth, $ . net. (one of the twentieth century text-books.) d. appleton and company, new york. _evolution of man and christianity._ new edition. by the rev. howard macqueary. with a new preface, in which the author answers his critics, and with some important additions, mo. cloth, $ . . "this is a revised and enlarged edition of a book published last year. the author reviews criticisms upon the first edition, denies that he rejects the doctrine of the incarnation, admits his doubts of the physical resurrection of christ, and his belief in evolution. the volume is to be marked as one of the most profound expressions of the modern movement toward broader theological positions."--_brooklyn times._ _history of the conflict between religion and science._ by dr. john william draper. mo. cloth, $ . . "the keynote to this volume is found in the antagonism between the progressive tendencies of the human mind and the pretensions of ecclesiastical authority, as developed in the history of modern science. no previous writer has treated the subject from this point of view, and the present monograph will be found to possess no less originality of conception than vigor of reasoning and wealth of erudition."--_new york tribune._ _a critical history of free thought in reference to the christian religion._ by rev. canon adam storey farrar, d.d., f.r.s., etc. mo. cloth, $ . . "a conflict might naturally be anticipated between the reasoning faculties of man and a religion which claims the right, on superhuman authority, to impose limits on the field or manner of their exercise. it is the chief of the movements of free thought which it is my purpose to describe, in their historic succession and their connection with intellectual causes. we must ascertain the facts, discover the causes, and read the moral."--_the author._ _creation or evolution? a philosophical inquiry._ by george ticnor curtis. mo. cloth, $ . . "a treatise on the great question of creation or evolution by one who is neither a naturalist nor theologian, and who does not profess to bring to the discussion a special equipment in either of the sciences which the controversy arrays against each other, may seem strange at first sight; but mr. curtis will satisfy the reader, before many pages have been turned, that he has a substantial contribution to make to the debate, and that his book is one to be treated with respect. his part is to apply to the reasonings of the men of science the rigid scrutiny with which the lawyer is accustomed to test the value and pertinency of testimony, and the legitimacy of inferences from established facts."--_new york tribune._ d. appleton and company, new york. transcriber's note: text enclosed by underscores is in italics (_italics_). a single underscore nh_ introduces a subscript, and a caret ^a a superscript. * * * * * the principles of biology by herbert spencer [illustration] in two volumes volume i new york and london d. appleton and company copyright, , , by d. appleton and company. preface to the revised and enlarged edition. rapid in all directions, scientific progress has during the last generation been more rapid in the direction of biology than in any other; and had this work been one dealing with biology at large, the hope of bringing it up to date could not have been rationally entertained. but it is a work on the _principles_ of biology; and to bring an exposition of these up to date, seemed not impossible with such small remnant of energy as is left me. slowly, and often interrupted by ill-health, i have in the course of the last two years, completed this first volume of the final edition. numerous additions have proved needful. what was originally said about vital changes of matter has been supplemented by a chapter on "metabolism." under the title "the dynamic element in life," i have added a chapter which renders less inadequate the conception of life previously expressed. a gap in preceding editions, which should have been occupied by some pages on "structure," is now filled up. those astonishing actions in cell-nuclei which the microscope has of late revealed, will be found briefly set forth under the head of "cell-life and cell-multiplication." further evidence and further thought have resulted in a supplementary chapter on "genesis, heredity, and variation"; in which certain views enunciated in the first edition are qualified and developed. various modern ideas are considered under the title "recent criticisms and hypotheses." and the chapter on "the arguments from embryology" has been mainly rewritten. smaller increments have taken the shape of new sections incorporated in pre-existing chapters. they are distinguished by the following section-marks:--§ a, § a, § a, § a, § a, § a, §§ a- d. there should also be mentioned a number of foot-notes of some significance not present in preceding editions. of the three additional appendices the two longer ones have already seen the light in other shapes. after these chief changes have now to be named the changes necessitated by revision. in making them assistance has been needful. though many of the amendments have resulted from further thought and inquiry, a much larger number have been consequent on criticisms received from gentlemen whose aid i have been fortunate enough to obtain: each of them having taken a division falling within the range of his special studies. the part concerned with organic chemistry and its derived subjects, has been looked through by mr. w. h. perkin, ph.d., f.r.s., professor of organic chemistry, owens college, manchester. plant morphology and physiology have been overseen by mr. a. g. tansley, m.a., f.l.s., assistant professor of botany, university college, london. criticisms upon parts dealing with animal morphology, i owe to mr. e. w. macbride, m.a., fellow of st. john's college, cambridge, professor of zoology in the mcgill university, montreal, and mr. j. t. cunningham, m.a., late fellow of university college, oxford. and the statements included under animal physiology have been checked by mr. w. b. hardy, m.a., fellow of gonville and caius college, cambridge, demonstrator of physiology in the university. where the discoveries made since have rendered it needful to change the text, either by omissions or qualifications or in some cases by additions, these gentlemen have furnished me with the requisite information. save in the case of the preliminary portion, bristling with the technicalities of organic chemistry (including the pages on "metabolism"), i have not submitted the proofs, either of the new chapters or of the revised chapters, to the gentlemen above named. the abstention has resulted partly from reluctance to trespass on their time to a greater extent than was originally arranged, and partly from the desire to avoid complicating my own work. during the interval occupied in the preparation of this volume the printers have kept pace with me, and i have feared adding to the entailed attention the further attention which correspondence and discussion would have absorbed: feeling that it was better to risk minor inaccuracies than to leave the volume unfinished: an event which at one time appeared probable. i make this statement because, in its absence, one or other of these gentlemen might be held responsible for some error which is not his but mine. yet another explanation is called for. beyond the exposition of those general truths constituting the principles of biology as commonly accepted, the original edition of this work contained sundry views for which biological opinion did not furnish any authority. some of these have since obtained a certain currency; either in their original forms or in modified forms. misinterpretations are likely to result. readers who have met with them in other works may, in the absence of warning, suppose, to my disadvantage, that i have adopted them without acknowledgment. hence it must be understood that where no indication to the contrary is given the substance is unchanged. beyond the corrections which have been made in the original text, there are, in some cases, additions to the evidence or amplifications of the argument; but in all sections not marked as new, the essential ideas set forth are the same as they were in the original edition of . brighton, _august, _. preface. the aim of this work is to set forth the general truths of biology, as illustrative of, and as interpreted by, the laws of evolution: the special truths being introduced only so far as is needful for elucidation of the general truths. for aid in executing it, i owe many thanks to prof. huxley and dr. hooker. they have supplied me with information where my own was deficient;[ ] and, in looking through the proof-sheets, have pointed out errors of detail into which i had fallen. by having kindly rendered me this valuable assistance, they must not, however, be held committed to any of the enunciated doctrines that are not among the recognized truths of biology. the successive instalments which compose this volume, were issued to the subscribers at the following dates:--no. (pp. - ) in january, ; no. (pp. - ) in april, ; no. (pp. - ) in july, ; no. (pp. - ) in january, ; no. (pp. - ) in may, ; and no. (pp. - ) in october, . _london, september th, ._ contents of vol. i. ---- chapter page i. organic matter ii. the actions of forces on organic matter iii. the re-actions of organic matter on forces iii^a. metabolism iv. proximate conception of life v. the correspondence between life and its circumstances vi. the degree of life varies as the degree of correspondence vi^a. the dynamic element in life vii. the scope of biology part ii.--the inductions of biology. i. growth ii. development ii^a. structure iii. function iv. waste and repair v. adaptation vi. individuality vi^a. cell-life and cell-multiplication vii. genesis viii. heredity ix. variation x. genesis, heredity, and variation x^a. genesis, heredity, and variation--_concluded_ xi. classification xii. distribution part iii.--the evolution of life. i. preliminary ii. general aspects of the special-creation-hypothesis iii. general aspects of the evolution-hypothesis iv. the arguments from classification v. the arguments from embryology vi. the arguments from morphology vii. the arguments from distribution viii. how is organic evolution caused? ix. external factors x. internal factors xi. direct equilibration xii. indirect equilibration xiii. the co-operation of the factors xiv. the convergence of the evidences xiv^a. recent criticisms and hypotheses appendices. a. the general law of animal fertility b. the inadequacy of natural selection, etc. c. the inheritance of functionally-wrought modifications: a summary d. on alleged "spontaneous generation" and on the hypothesis of physiological units part i. the data of biology. chapter i. organic matter. § . of the four chief elements which, in various combinations, make up living bodies, three are gaseous under all ordinary conditions and the fourth is a solid. oxygen, hydrogen, and nitrogen are gases which for many years defied all attempts to liquefy them, and carbon is a solid except perhaps at the extremely high temperature of the electric arc. only by intense pressures joined with extreme refrigerations have the three gases been reduced to the liquid form.[ ] there is much significance in this. when we remember how those redistributions of matter and motion which constitute evolution, structural and functional, imply motions in the units that are redistributed; we shall see a probable meaning in the fact that organic bodies, which exhibit the phenomena of evolution in so high a degree, are mainly composed of ultimate units having extreme mobility. the properties of substances, though destroyed to sense by combination, are not destroyed in reality. it follows from the persistence of force, that the properties of a compound are _resultants_ of the properties of its components--_resultants_ in which the properties of the components are severally in full action, though mutually obscured. one of the leading properties of each substance is its degree of molecular mobility; and its degree of molecular mobility more or less sensibly affects the molecular mobilities of the various compounds into which it enters. hence we may infer some relation between the gaseous form of three out of the four chief organic elements, and that comparative readiness displayed by organic matters to undergo those changes in the arrangement of parts which we call development, and those transformations of motion which we call function. considering them chemically instead of physically, it is to be remarked that three out of these four main components of organic matter, have affinities which are narrow in their range and low in their intensity. hydrogen, it is true, may be made to combine with a considerable number of other elements; but the chemical energy which it shows is scarcely at all shown within the limits of the organic temperatures. of carbon it may similarly be said that it is totally inert at ordinary heats; that the number of substances with which it unites is not great; and that in most cases its tendency to unite with them is but feeble. lastly, this chemical indifference is shown in the highest degree by nitrogen--an element which, as we shall hereafter see, plays the leading part in organic changes. among the organic elements (including under the title not only the four chief ones, but also the less conspicuous remainder), that capability of assuming different states called allotropism, is frequent. carbon presents itself in the three unlike conditions of diamond, graphite, and charcoal. under certain circumstances, oxygen takes on the form in which it is called ozone. sulphur and phosphorus (both, in small proportions, essential constituents of organic matter) have allotropic modifications. silicon, too, is allotropic; while its oxide, silica, which is an indispensable constituent of many lower organisms, exhibits the analogue of allotropism--isomerism. no other interpretation being possible we are obliged to regard allotropic change as some change of molecular arrangement. hence this frequency of its occurrence among the components of organic matter is significant as implying a further kind of molecular mobility. one more fact, that is here of great interest for us, must be set down. these four elements of which organisms are almost wholly composed, exhibit certain extreme unlikenesses. while between two of them we have an unsurpassed contrast in chemical activity; between one of them and the other three, we have an unsurpassed contrast in molecular mobility. while carbon, until lately supposed to be infusible and now volatilized only in the electric arc, shows us a degree of atomic cohesion greater than that of any other known element, hydrogen, oxygen, and nitrogen show the least atomic cohesion of all elements. and while oxygen displays, alike in the range and intensity of its affinities, a chemical energy exceeding that of any other substance (unless fluorine be considered an exception), nitrogen displays the greatest chemical inactivity. now on calling to mind one of the general truths arrived at when analyzing the process of evolution, the probable significance of this double difference will be seen. it was shown (_first principles_, § ) that, other things equal, unlike units are more easily separated by incident forces than like units are--that an incident force falling on units that are but little dissimilar does not readily segregate them; but that it readily segregates them if they are widely dissimilar. thus, the substances presenting these two extreme contrasts, the one between physical mobilities, and the other between chemical activities, fulfil, in the highest degree, a certain further condition to facility of differentiation and integration. § . among the diatomic combinations of the three elements, hydrogen, nitrogen and oxygen, we find a molecular mobility much less than that of these elements themselves; at the same time that it is much greater than that of diatomic compounds in general. of the two products formed by the union of oxygen with carbon, the first, called carbonic oxide, which contains one atom[ ] of carbon to one of oxygen (expressed by the symbol co) is a gas condensible only with great difficulty; and the second, carbonic acid, containing an additional atom of oxygen (co_{ }) assumes a liquid form also only under a pressure of about forty atmospheres. the several compounds of oxygen with nitrogen, present us with an instructive gradation. nitrous oxide (n_{ }o), is a gas condensible only under a pressure of some fifty atmospheres; nitric oxide (no) is a gas which although it has been liquefied does not condense under a pressure of atmospheres at . ° f. ( ° c.): the molecular mobility remaining undiminished in consequence of the volume of the united gases remaining unchanged. nitrogen trioxide (n_{ }o_{ }) is gaseous at ordinary temperatures, but condenses into a very volatile liquid at the zero of fahrenheit; nitrogen tetroxide (n_{ }o_{ }) is liquid at ordinary temperatures and becomes solid at the zero of fahrenheit; while nitrogen pentoxide (n_{ }o_{ }) may be obtained in crystals which melt at ° and boil at °. in this series we see, though not with complete uniformity, a decrease of molecular mobility as the weights of the compound molecules are increased. the hydro-carbons illustrate the same general truth still better. one series of them will suffice. marsh gas (ch_{ }) is gaseous except under great pressure and at very low temperatures. olefiant gas (c_{ }h_{ }) and ethane (c_{ }h_{ }) may be readily liquefied by pressure. propane (c_{ }h_{ }) becomes liquid without pressure at the zero of fahrenheit. hexane (c_{ }h_{ }) is a liquid which boils at °. and the successively higher multiples, heptane (c_{ }h_{ }), octane (c_{ }h_{ }), and nonane (c_{ }h_{ }) are liquids which boil respectively at °, °, and °. pentadecan (c_{ }h_{ }) is a liquid which boils at °, while paraffin-wax, which contains the still higher multiples, is solid. there are three compounds of hydrogen and nitrogen that have been obtained in a free state--ammonia (nh_{ }) is gaseous, but liquefiable by pressure, or by reducing its temperature to - ° f., and it solidifies at - ° f.; hydrazine (nh_{ }--nh_{ }) is liquid at ordinary temperatures, but hydrozoic acid (n_{ }h) has so far only been obtained in the form of a highly explosive gas. in cyanogen, which is composed of carbon and nitrogen, (cn)_{ }, we have a gas that becomes liquid at a pressure of four atmospheres and solid at - ° f. and in paracyanogen, formed of the same proportions of these elements in higher multiples, we have a solid which does not fuse or volatilize at ordinary temperatures. lastly, in the most important member of this group, water (h_{ }o), we have a compound of two difficultly-condensible gases which assumes both the fluid state and the solid state within ordinary ranges of temperature; while its molecular mobility is still such that its fluid or solid masses are continually passing into the form of vapour, though not with great rapidity until the temperature is raised to °. considering them chemically, it is to be remarked of these diatomic compounds of the four chief organic elements, that they are, on the average, less stable than diatomic compounds in general. water, carbonic oxide, and carbonic acid, are, it is true, difficult to decompose. but omitting these, the usual strength of union among the elements of the above-named substances is low considering the simplicity of the substances. with the exception of acetylene and possibly marsh gas, the various hydro-carbons are not producible by directly combining their elements; and the elements of most of them are readily separable by heat without the aid of any antagonistic affinity. nitrogen and hydrogen do not unite with each other immediately save under very exceptional circumstances; and the ammonia which results from their union, though it resists heat, yields to the electric spark. cyanogen is stable: not being resolved into its components below a bright red heat. much less stable, however, are several of the oxides of nitrogen. nitrous oxide, it is true, does not yield up its elements below a red heat; but nitrogen tetroxide cannot exist if water be added to it; nitrous acid is decomposed by water; and nitric acid not only readily parts with its oxygen to many metals, but when anhydrous, spontaneously decomposes. here it will be well to note, as having a bearing on what is to follow, how characteristic of most nitrogenous compounds is this special instability. in all the familiar cases of sudden and violent decomposition, the change is due to the presence of nitrogen. the explosion of gunpowder results from the readiness with which the nitrogen contained in the nitrate of potash, yields up the oxygen combined with it. the explosion of gun-cotton, which also contains nitrogen, is a substantially parallel phenomenon. the various fulminating salts are all formed by the union with metals of a certain nitrogenous acid called fulminic acid; which is so unstable that it cannot be obtained in a separate state. explosiveness is a property of nitro-mannite, and also of nitro-glycerin. iodide of nitrogen detonates on the slightest touch, and often without any assignable cause. and the bodies which explode with the most tremendous violence of any known, are the chloride of nitrogen (ncl_{ }) and hydrazoic acid (n_{ }h). thus these easy and rapid decompositions, due to the chemical indifference of nitrogen, are characteristic. when we come hereafter to observe the part which nitrogen plays in organic actions, we shall see the significance of this extreme readiness shown by its compounds to undergo changes. returning from these facts parenthetically introduced, we have next to note that though among the diatomic compounds of the four chief organic elements, there are a few active ones, yet the majority of them display a smaller degree of chemical energy than the average of diatomic compounds. water is the most neutral of bodies: usually producing little chemical alteration in the substances with which it combines; and being expelled from most of its combinations by a moderate heat. carbonic acid is a relatively feeble acid: the carbonates being decomposed by the majority of other acids and by ignition. the various hydro-carbons are but narrow in the range of their comparatively weak affinities. the compounds formed by ammonia have not much stability: they are readily destroyed by heat, and by the other alkalies. the affinities of cyanogen are tolerably strong, though they yield to those of the chief acids. of the several oxides of nitrogen, it is to be remarked that, while those containing the smaller proportions of oxygen are chemically inert, the one containing the greatest proportion of oxygen (nitric acid) though chemically active, in consequence of the readiness with which one part of it gives up its oxygen to oxidize a base with which the rest combines, is nevertheless driven from all its combinations by a red heat. these diatomic compounds, like their elements, are to a considerable degree characterized by the prevalence among them of allotropism; or, as it is more usually called when displayed by compound bodies--isomerism. professor graham finds reason for thinking that a change in atomic arrangements of this nature, takes place in water, at or near the melting point of ice. in the various series of hydro-carbons, differing from each other only in the ratios in which the elements are united, we find not simply isomerism but polymerism occurring to an almost infinite extent. in some series of hydro-carbons, as, for example, the terpenes, we find isomerism and at the same time a great tendency to undergo polymerisation. and the relation between cyanogen and paracyanogen is, as we saw, a polymeric one. there is one further fact respecting these diatomic compounds of the chief organic elements, which must not be overlooked. those of them which form parts of the living tissues of plants and animals (excluding water which has a mechanical function, and carbonic acid which is a product of decomposition) belong for the most part to one group--the carbo-hydrates.[ ] and of this group, which is on the average characterized by comparative instability and inertness, these carbo-hydrates found in living tissues are among the most unstable and inert. § . passing now to the substances which contain three of these chief organic elements, we have first to note that along with the greater atomic weight which mostly accompanies their increased complexity, there is, on the average, a further marked decrease of molecular mobility. scarcely any of them maintain a gaseous state at ordinary temperatures. one class of them only, the alcohols and their derivatives, evaporate under the usual atmospheric pressure; but not rapidly unless heated. the fixed oils, though they show that molecular mobility implied by an habitually liquid state, show this in a lower degree than the alcoholic compounds; and they cannot be reduced to the gaseous state without decomposition. in their allies, the fats, which are solid unless heated, the loss of molecular mobility is still more marked. and throughout the whole series of the fatty acids, in which to a fixed proportion of oxygen there are successively added higher equimultiples of carbon and hydrogen, we see how the molecular mobility decreases with the increasing sizes of the molecules. in the amylaceous and sugar-group of compounds, solidity is the habitual state: such of them as can assume the liquid form, doing so only when heated to ° or ° f.; and decomposing when further heated, rather than become gaseous. resins and gums exhibit general physical properties of like character and meaning. in chemical stability these triatomic compounds, considered as a group, are in a marked degree below the diatomic ones. the various sugars and kindred bodies, decompose at no very high temperatures. the oils and fats also are readily carbonized by heat. resinous and gummy substances are easily made to render up some of their constituents. and the alcohols, with their allies, have no great power of resisting decomposition. these bodies, formed by the union of oxygen, hydrogen, and carbon, are also, as a class, chemically inactive. formic and acetic are doubtless energetic acids; but the higher members of the fatty-acid series are easily separated from the bases with which they combine. saccharic acid, too, is an acid of considerable power; and sundry of the vegetable acids possess a certain activity, though an activity far less than that of the mineral acids. but throughout the rest of the group, there is shown but a small tendency to combine with other bodies; and such combinations as are formed have usually little permanence. the phenomena of isomerism and polymerism are of frequent occurrence in these triatomic compounds. starch and dextrine are probably polymeric. fruit-sugar and grape-sugar, mannite and sorbite, cane-sugar and milk-sugar, are isomeric. sundry of the vegetal acids exhibit similar modifications. and among the resins and gums, with their derivatives, molecular re-arrangements of this kind are not uncommon. one further fact respecting these compounds of carbon, oxygen and hydrogen, should be mentioned; namely, that they are divisible into two classes--the one consisting of substances that result from the destructive decomposition of organic matter, and the other consisting of substances that exist as such in organic matter. these two classes of substances exhibit, in different degrees, the properties to which we have been directing our attention. the lower alcohols, their allies and derivatives, which possess greater molecular mobility and chemical stability than the rest of these triatomic compounds, are rarely found in animal or vegetal bodies. while the sugars and amylaceous substances, the fixed oils and fats, the gums and resins, which have all of them much less molecular mobility, and are, chemically considered, more unstable and inert, are components of the living tissues of plants and animals. § . among compounds containing all the four chief organic elements, a division analogous to that just named may be made. there are some which result from the decomposition of living tissues; there are others which make parts of living tissues in their state of integrity; and these two groups are contrasted in their properties in the same way as are the parallel groups of triatomic compounds. of the first division, certain products found in the animal excretions are the most important, and the only ones that need be noted; such, namely, as urea, kreatine, kreatinine. these animal-bases exhibit much less molecular mobility than the average of the substances treated of in the last section: being solid at ordinary temperatures, fusing, where fusible at all, at temperatures above that of boiling water, and having no power to assume a gaseous state. chemically considered, their stability is low, and their activity but small, in comparison with the stabilities and activities of the simpler compounds. it is, however, the nitrogenous constituents of living tissues, that display most markedly those characteristics of which we have been tracing the growth. albumen, fibrin, casein, and their allies, are bodies in which that molecular mobility exhibited by three of their components in so high a degree is reduced to a minimum. these substances are known only in the solid state. that is to say, when deprived of the water usually mixed with them, they do not admit of fusion, much less of volatilization. to which add, that they have not even that molecular mobility which solution in water implies; since, though they form viscid mixtures with water, they do not dissolve in the same perfect way as do inorganic compounds. the chemical characteristics of these substances are instability and inertness carried to the extreme. how rapidly albumenoid matters decompose under ordinary conditions, is daily seen: the difficulty of every housewife being to prevent them from decomposing. it is true that when desiccated and kept from contact with air, they may be preserved unchanged for long periods; but the fact that they can be only thus preserved, proves their great instability. it is true, also, that these most complex nitrogenous principles are not absolutely inert, since they enter into combinations with some bases; but their unions are very feeble. it should be noted, too, of these bodies, that though they exhibit in the lowest degree that kind of molecular mobility which implies facile vibration of the molecules as wholes, they exhibit in high degrees that kind of molecular mobility resulting in isomerism, which implies permanent changes in the positions of adjacent atoms with respect to each other. each of them has a soluble and an insoluble form. in some cases there are indications of more than two such forms. and it appears that their metamorphoses take place under very slight changes of conditions. in these most unstable and inert organic compounds, we find that the molecular complexity reaches a maximum: not only since the four chief organic elements are here united with small proportions of sulphur and sometimes phosphorus; but also since they are united in high multiples. the peculiarity which we found characterized even diatomic compounds of the organic elements, that their molecules are formed not of single equivalents of each component, but of two, three, four, and more equivalents, is carried to the greatest extreme in these compounds, which take the leading part in organic actions. according to lieberkühn, the formula of albumen is c_{ }h_{ }sn_{ }o_{ }. that is to say, with the sulphur there are united seventy-two atoms of carbon, one hundred and twelve of hydrogen, eighteen of nitrogen, and twenty-two of oxygen: the molecule being thus made up of more than two hundred ultimate atoms. § . did space permit, it would be useful here to consider in detail the interpretations that may be given of the peculiarities we have been tracing: bringing to their solution, the general mechanical principles which are now found to hold true of molecules as of masses. but it must suffice briefly to indicate the conclusions which such an inquiry promises to bring out. proceeding on these principles, it may be argued that the molecular mobility of a substance must depend partly on the inertia of its molecules; partly on the intensity of their mutual polarities; partly on their mutual pressures, as determined by the density of their aggregation; and (where the molecules are compound) partly on the molecular mobilities of their component molecules. whence it is to be inferred that any three of these remaining constant, the molecular mobility will vary as the fourth. other things equal, therefore, the molecular mobility of molecules must decrease as their masses increase; and so there must result that progression we have traced, from the high molecular mobility of the uncombined organic elements, to the low molecular mobility of those large-moleculed substances into which they are ultimately compounded. applying to molecules the mechanical law which holds of masses, that since inertia and gravity increase as the cubes of the dimensions while cohesion increases as their squares, the self-sustaining power of a body becomes relatively smaller as its bulk becomes greater; it might be argued that these large, aggregate molecules which constitute organic substances, are mechanically weak--are less able than simpler molecules to bear, without alteration, the forces falling on them. that very massiveness which renders them less mobile, enables the physical forces acting on them more readily to change the relative positions of their component atoms; and so to produce what we know as re-arrangements and decompositions. further, it seems a not improbable conclusion, that this formation of large aggregates of elementary atoms and resulting diminution of self-sustaining power, must be accompanied by a decrease of those dimensional contrasts to which polarity is ascribable. a sphere is the figure of equilibrium which any aggregate of units tends to assume, under the influence of simple mutual attraction. where the number of units is small and their mutual polarities are decided, this proclivity towards spherical grouping will be overcome by the tendency towards some more special form, determined by their mutual polarities. but it is manifest that in proportion as an aggregate molecule becomes larger, the effects of simple mutual attraction must become relatively greater; and so must tend to mask the effects of polar attraction. there will consequently be apt to result in highly compound molecules like these organic ones, containing hundreds of elementary atoms, such approximation to the spherical form as must involve a less distinct polarity than in simpler molecules. if this inference be correct, it supplies us with an explanation both of the chemical inertness of these most complex organic substances, and of their inability to crystallize. § . here we are naturally introduced to another aspect of our subject--an aspect of great interest. professor graham has published a series of important researches, which promise to throw much light on the constitution and changes of organic matter. he shows that solid substances exist under two forms of aggregation--the _colloid_ or jelly-like, and the _crystalloid_ or crystal-like. examples of the last are too familiar to need specifying. of the first may be named such instances as "hydrated silicic acid, hydrated alumina, and other metallic peroxides of the aluminous class, when they exist in the soluble form; with starch, dextrine and the gums, caramel, tannin, albumen, gelatine, vegetable and animal extractive matters." describing the properties of colloids, professor graham says:--"although often largely soluble in water, they are held in solution by a most feeble force. they appear singularly inert in the capacity of acids and bases, and in all the ordinary chemical relations." * * * "although chemically inert in the ordinary sense, colloids possess a compensating activity of their own arising out of their physical properties. while the rigidity of the crystalline structure shuts out external impressions, the softness of the gelatinous colloid partakes of fluidity, and enables the colloid to become a medium of liquid diffusion, like water itself." * * * "hence a wide sensibility on the part of colloids to external agents. another and eminently characteristic quality of colloids is their mutability." * * * "the solution of hydrated silicic acid, for instance, is easily obtained in a state of purity, but it cannot be preserved. it may remain fluid for days or weeks in a sealed tube, but is sure to gelatinize and become insoluble at last. nor does the change of this colloid appear to stop at that point; for the mineral forms of silicic acid, deposited from water, such as flint, are often found to have passed, during the geological ages of their existence, from the vitreous or colloidal into the crystalline condition (h. rose). the colloid is, in fact, a dynamical state of matter, the crystalloidal being the statical condition. the colloid possesses _energia_. it may be looked upon as the primary source of the force appearing in the phenomena of vitality. to the gradual manner in which colloidal changes take place (for they always demand time as an element) may the characteristic protraction of chemico-organic changes also be referred." the class of colloids includes not only all those most complex nitrogenous compounds characteristic of organic tissues, and sundry of the carbo-hydrates found along with them; but, significantly enough, it includes several of those substances classed as inorganic, which enter into organized structures. thus silica, which is a component of many plants, and constitutes the spicules of sponges as well as the shells of many foraminifera and infusoria, has a colloid, as well as a crystalloid, condition. a solution of hydrated silicic acid passes in the course of a few days into a solid jelly that is no longer soluble in water; and it may be suddenly thus coagulated by a minute portion of an alkaline carbonate, as well as by gelatine, alumina, and peroxide of iron. this last-named substance, too--peroxide of iron--which is an ingredient in the blood of mammals and composes the shells of certain _protozoa_, has a colloid condition. "water containing about one per cent. of hydrated peroxide of iron in solution, has the dark red colour of venous blood." * * * "the red solution is coagulated in the cold by traces of sulphuric acid, alkalies, alkaline carbonates, sulphates, and neutral salts in general." * * * "the coagulum is a deep red-coloured jelly, resembling the clot of blood, but more transparent. indeed, the coagulum of this colloid is highly suggestive of that of blood, from the feeble agencies which suffice to effect the change in question, as well as from the appearance of the product." the jelly thus formed soon becomes, like the last, insoluble in water. lime also, which is so important a mineral element in living bodies, animal and vegetal, enters into a compound belonging to this class. "the well-known solution of lime in sugar forms a solid coagulum when heated. it is probably, at a high temperature, entirely colloidal." generalizing some of the facts which he gives, professor graham says:--"the equivalent of a colloid appears to be always high, although the ratio between the elements of the substance may be simple. gummic acid, for instance, may be represented by c^{ }h^{ }o^{ }; but, judging from the small proportions of lime and potash which suffice to neutralize this acid, the true numbers of its formula must be several times greater. it is difficult to avoid associating the inertness of colloids with their high equivalents, particularly where the high number appears to be attained by the repetition of a small number. the inquiry suggests itself whether the colloid molecule may not be constituted by the grouping together of a number of smaller crystalloid molecules, and whether the basis of colloidality may not really be this composite character of the molecule." § . a further contrast between colloids and crystalloids is equally significant in its relations to vital phenomena. professor graham points out that the marked differences in volatility displayed by different bodies, are paralleled by differences in the rates of diffusion of different bodies through liquids. as alcohol and ether at ordinary temperatures, and various other substances at higher temperatures, diffuse themselves in a gaseous form through the air; so, a substance in aqueous solution, when placed in contact with a mass of water (in such way as to avoid mixture by circulating currents) diffuses itself through this mass of water. and just as there are various degrees of rapidity in evaporation, so there are various degrees of rapidity in diffusion: "the range also in the degree of diffusive mobility exhibited by different substances appears to be as wide as the scale of vapour-tensions." this parallelism is what might have been looked for; since the tendency to assume a gaseous state, and the tendency to spread in solution through a liquid, are both consequences of molecular mobility. it also turns out, as was to be expected, that diffusibility, like volatility, has, other things equal, a relation to molecular weight--other things equal, we must say, because molecular mobility must, as pointed out in § , be affected by other properties of atoms, besides their inertia. thus the substance most rapidly diffused of any on which professor graham experimented, was hydrochloric acid--a compound which is of low molecular weight, is gaseous save under a pressure of forty atmospheres, and ordinarily exists as a liquid, only in combination with water. again, "hydrate of potash may be said to possess double the velocity of diffusion of sulphate of potash, and sulphate of potash again double the velocity of sugar, alcohol, and sulphate of magnesia,"--differences which have a general correspondence with differences in the massiveness of their molecules. but the fact of chief interest to us here, is that the relatively small-moleculed crystalloids have immensely greater diffusive power than the relatively large-moleculed colloids. among the crystalloids themselves there are marked differences of diffusibility; and among the colloids themselves there are parallel differences, though less marked ones. but these differences are small compared with that between the diffusibility of the crystalloids as a class, and the diffusibility of the colloids as a class. hydrochloric acid is seven times as diffusible as sulphate of magnesia; but it is fifty times as diffusible as albumen, and a hundred times as diffusible as caramel. these differences of diffusibility manifest themselves with nearly equal distinctness, when a permeable septum is placed between the solution and the water. the result is that when a solution contains substances of different diffusibilities, the process of dialysis, as professor graham calls it, becomes a means of separating the mixed substances: especially when such mixed substances are partly crystalloids and partly colloids. the bearing of this fact on the interpretation of organic processes will be obvious. still more obvious will its bearing be, on joining with it the remarkable fact that while crystalloids can diffuse themselves through colloids nearly as rapidly as through water, colloids can scarcely diffuse themselves at all through other colloids. from a mass of jelly containing salt, into an adjoining mass of jelly containing no salt, the salt spread more in eight days than it spread through water in seven days; while the spread of "caramel through the jelly appeared scarcely to have begun after eight days had elapsed." so that we must regard the colloidal compounds of which organisms are built, as having, by their physical nature, the ability to separate colloids from crystalloids, and to let the crystalloids pass through them with scarcely any resistance. one other result of these researches on the relative diffusibilities of different substances has a meaning for us. professor graham finds that not only does there take place, by dialysis, a separation of _mixed_ substances which are unlike in their molecular mobilities; but also that _combined_ substances between which the affinities are feeble, will separate on the dialyzer, if their molecular mobilities are strongly contrasted. speaking of the hydrochloride of peroxide of iron, he says, "such a compound possesses an element of instability in the extremely unequal diffusibility of its constituents;" and he points out that when dialyzed, the hydrochloric acid gradually diffuses away, leaving the colloidal peroxide of iron behind. similarly, he remarks of the peracetate of iron, that it "may be made a source of soluble peroxide, as the salt referred to is itself decomposed to a great extent by diffusion on the dialyzer." now this tendency to separate displayed by substances which differ widely in their molecular mobilities, though usually so far antagonized by their affinities as not to produce spontaneous decomposition, must, in all cases, induce a certain readiness to change which would not else exist. the unequal mobilities of the combined atoms must give disturbing forces a greater power to work transformations than they would otherwise have. hence the probable significance of a fact named at the outset, that while three of the chief organic elements have the greatest atomic mobilities of any elements known, the fourth, carbon, has the least atomic mobility of known elements. though, in its simple compounds, the affinities of carbon for the rest are strong enough to prevent the effects of this great difference from clearly showing themselves; yet there seems reason to think that in those complex compounds composing organic bodies--compounds in which there are various cross affinities leading to a state of chemical tension--this extreme difference in the molecular mobilities must be an important aid to molecular re-arrangements. in short, we are here led by concrete evidence to the conclusion which we before drew from first principles, that this great unlikeness among the combined units must facilitate differentiations. § . a portion of organic matter in a state to exhibit those phenomena which the biologist deals with, is, however, something far more complex than the separate organic matters we have been studying; since a portion of organic matter in its integrity, contains several of these. in the first place no one of those colloids which make up the mass of a living body, appears capable of carrying on vital changes by itself: it is always associated with other colloids. a portion of animal-tissue, however minute, almost always contains more than one form of protein-substance: different chemical modifications of albumen and gelatine are present together, as well as, probably, a soluble and insoluble modification of each; and there is usually more or less of fatty matter. in a single vegetal cell, the minute quantity of nitrogenous colloid present, is imbedded in colloids of the non-nitrogenous class. and the microscope makes it at once manifest, that even the smallest and simplest organic forms are not absolutely homogeneous. further, we have to contemplate organic tissue, formed of mingled colloids in both soluble and insoluble states, as permeated throughout by crystalloids. some of these crystalloids, as oxygen,[ ] water, and perhaps certain salts, are agents of decomposition; some, as the saccharine and fatty matters, are probably materials for decomposition; and some, as carbonic acid, water, urea, kreatine, and kreatinine, are products of decomposition. into the mass of mingled colloids, mostly insoluble and where soluble of very low molecular mobility or diffusive power, we have constantly passing, crystalloids of high molecular mobility or diffusive power, that are capable of decomposing these complex colloids, or of facilitating decompositions otherwise caused; and from these complex colloids, when decomposed, there result other crystalloids (the two chief ones extremely simple and mobile, and the rest comparatively so) which diffuse away as rapidly as they are formed. and now we may clearly see the necessity for that peculiar composition which we find in organic matter. on the one hand, were it not for the extreme molecular mobility possessed by three out of the four of its chief elements; and were it not for the consequently high molecular mobility of their simpler compounds; there could not be this quick escape of the waste products of organic action; and there could not be that continuously active change of matter which vitality implies. on the other hand, were it not for the union of these extremely mobile elements into immensely complex compounds, having relatively vast molecules which are made comparatively immobile by their inertia, there could not result that mechanical fixity which prevents the components of living tissue from diffusing away along with the effete matters produced by decomposition. § a. let us not omit here to note the ways in which the genesis of these traits distinguishing organic matter conforms to the laws of evolution as expressed in its general formula. in pursuance of the belief now widely entertained by chemists that the so-called elements are not elements, but are composed of simpler matters and probably of one ultimate form of matter (for which the name "protyle" has been suggested by sir w. crookes), it is to be concluded that the formation of the elements, in common with the formation of all those compounds of them which nature presents, took place in the course of cosmic evolution. various reasons for this inference the reader will find set forth in the addenda to an essay on "the nebular hypothesis" (see _essays_, vol. i, p. ). on tracing out the process of compounding and re-compounding by which, hypothetically, the elements themselves and afterwards their compounds and re-compounds have arisen, certain cardinal facts become manifest. . considered as masses, the units of the elements are the smallest, though larger than the units of the primordial matter. later than these, since they are composed of them, and since they cannot exist at temperatures so high as those at which the elements can exist, come the diatomic compounds--oxides, chlorides, and the rest--necessarily larger in their molecules. above these in massiveness come the molecules of the multitudinous salts and kindred bodies. when associated, as these commonly are, with molecules of water, there again results in each case increase of mass; and unable as they are to bear such high temperatures, these molecules are necessarily later in origin than those of the anhydrous diatomic compounds. within the general class of triatomic compounds, more composite still, come the carbohydrates, which, being able to unite in multiples, form still larger molecules than other triatomic compounds. decomposing as they do at relatively low temperatures, these are still more recent in the course of chemical evolution; and with the genesis of them the way is prepared for the genesis of organic matter strictly so called. this includes the various forms of protein-substance, containing four chief elements with two minor ones, and having relatively vast molecules. unstable as these are in presence of heat and surrounding affinities, they became possible only at a late stage in the genesis of the earth. here, then, in that chemical evolution which preceded the evolution of life, we see displayed that process of integration which is the primary trait of evolution at large. . along with increasing integration has gone progress in heterogeneity. the elements, regarding them as compound, are severally more heterogeneous than "protyle." diatomic molecules are more heterogeneous than these elements; triatomic more heterogeneous than diatomic; and the molecules containing four elements more heterogeneous than those containing three: the most heterogeneous of them being the proteids, which contain two other elements. the hydrated forms of all these compounds are more heterogeneous than are the anhydrous forms. and most heterogeneous of all are the molecules which, besides containing three, four, or more elements, also exhibit the isomerism and polymerism which imply unions in multiples. . this formation of molecules more and more heterogeneous during terrestrial evolution, has been accompanied by increasing heterogeneity in the aggregate of compounds of each kind, as well as an increasing number of kinds; and this increasing heterogeneity is exemplified in an extreme degree in the compounds, non-nitrogenous and nitrogenous, out of which organisms are built. so that the classes, orders, genera, and species of chemical substances, gradually increasing as the earth has assumed its present form, increased in a transcendent degree during that stage which preceded the origin of life. § . returning now from these partially-parenthetic observations, and summing up the contents of the preceding pages, we have to remark that in the substances of which organisms are composed, the conditions necessary to that re-distribution of matter and motion which constitutes evolution, are fulfilled in a far higher degree than at first appears. the mutual affinities of the chief organic elements are not active within the limits of those temperatures at which organic actions take place; and one of these elements is especially characterized by its chemical indifference. the compounds formed by these elements in ascending grades of complexity, become progressively less stable. and those most complex compounds into which all these four elements enter, together with small proportions of two other elements which very readily oxidize, have an instability so great that decomposition ensues under ordinary atmospheric conditions. among these elements out of which living bodies are built, there is an unusual tendency to unite in multiples; and so to form groups of products which have the same chemical elements in the same proportions, but, differing in their modes of aggregation, possess different properties. this prevalence among them of isomerism and polymerism, shows, in another way, the special fitness of organic substances for undergoing re-distributions of their components. in those most complex compounds that are instrumental to vital actions, there exists a kind and degree of molecular mobility which constitutes the plastic quality fitting them for organization. instead of the extreme molecular mobility possessed by three out of the four organic elements in their separate states--instead of the diminished, but still great, molecular mobility possessed by their simpler combinations, the gaseous and liquid characters of which unfit them for showing to any extent the process of evolution--instead of the physical properties of their less simple combinations, which, when not made unduly mobile by heat, assume the unduly rigid form of crystals; we have in these colloids, of which organisms are mainly composed, just the required compromise between fluidity and solidity. they cannot be reduced to the unduly mobile conditions of liquid and gas; and yet they do not assume the unduly fixed condition usually characterizing solids. the absence of power to unite together in polar arrangement, leaves their molecules with a certain freedom of relative movement, which makes them sensitive to small forces, and produces plasticity in the aggregates composed of them. while the relatively great inertia of these large and complex organic molecules renders them comparatively incapable of being set in motion by the ethereal undulations, and so reduced to less coherent forms of aggregation, this same inertia facilitates changes of arrangement among their constituent molecules or atoms; since, in proportion as an incident force impresses but little motion on a mass, it is the better able to impress motion on the parts of the mass in relation to one another. and it is further probable that the extreme contrasts in molecular mobilities among the components of these highly complex molecules, aid in producing modifiability of arrangement among them. lastly, the great difference in diffusibility between colloids and crystalloids, makes possible in the tissues of organisms a specially rapid re-distribution of matter and motion; both because colloids, being easily permeable by crystalloids, can be chemically acted on throughout their whole masses, instead of only on their surfaces; and because the products of decomposition, being also crystalloids, can escape as fast as they are produced: leaving room for further transformations. so that while the composite molecules of which organic tissues are built up, possess that low molecular mobility fitting them for plastic purposes, it results from the extreme molecular mobilities of their ultimate constituents, that the waste products of vital activity escape as fast as they are formed. to all which add that the state of warmth, or increased molecular vibration, in which all the higher organisms are kept, increases these various facilities for re-distribution: not only as aiding chemical changes, but as accelerating the diffusion of crystalloid substances. chapter ii. the actions of forces on organic matter. § . to some extent, the parts of every body are changed in their arrangement by any incident mechanical force. but in organic bodies, and especially in animal bodies, the changes of arrangement produced by mechanical forces are usually conspicuous. it is a distinctive mark of colloids that they readily yield to pressures and tensions, and that they recover, more or less completely, their original shapes, when the pressures or tensions cease. evidently without this pliability and elasticity, most organic actions would be impossible. not only temporary but also permanent alterations of form are facilitated by this colloid character of organic matter. continued pressure on living tissue, by modifying the processes going on in it (perhaps retarding the absorption of new material to replace the old that has decomposed and diffused away), gradually diminishes and finally destroys its power of resuming the outline it had at first. thus, generally speaking, the substances composing organisms are modifiable by arrested momentum or by continuous strain, in far greater degrees than are inorganic substances. § . sensitiveness to certain forces which are quasi-mechanical, if not mechanical in the usual sense, is seen in two closely-related peculiarities displayed by organic matter as well as other matter which assumes the same state of molecular aggregation. colloids take up by a power called "capillary affinity," a large quantity of water: undergoing at the same time great increase of bulk with change of form. conversely, with like readiness, they give up this water by evaporation; resuming, partially or completely, their original states. whether resulting from capillarity, or from the relatively great diffusibility of water, or from both, these changes are to be here noted as showing another mode in which the arrangements of parts in organic bodies are affected by mechanical actions. in what is termed osmose, we have a further mode of an allied kind. when on opposite sides of a permeable septum, and especially a septum of colloidal substance, are placed miscible solutions of different densities, a double transfer takes place: a large quantity of the less dense solution finds its way through the septum into the more dense solution; and a small quantity of the more dense finds its way into the less dense--one result being a considerable increase in the bulk of the more dense at the expense of the less dense. this process, which appears to depend on several conditions, is not yet fully understood. but be the explanation what it may, the process is one that tends continually to work alterations in organic bodies. through the surfaces of plants and animals, transfers of this kind are ever taking place. many of the conspicuous changes of form undergone by organic germs, are due mainly to the permeation of their limiting membranes by the surrounding liquids. it should be added that besides the direct alterations which the imbibition and transmission of water and watery solutions by colloids produce in organic matter, they produce indirect alterations. being instrumental in conveying into the tissues the agents of chemical change, and conveying out of them the products of chemical change, they aid in carrying on other re-distributions. § . as elsewhere shown (_first principles_, § ) heat, or a raised state of molecular vibration, enables incident forces more easily to produce changes of molecular arrangement in organic matter. but besides this, it conduces to certain vital changes in so direct a way as to become their chief cause. the power of the organic colloids to imbibe water, and to bring along with it into their substance the materials which work transformations, would not be continuously operative if the water imbibed were to remain. it is because it escapes, and is replaced by more water containing more materials, that the succession of changes is maintained. among the higher animals and higher plants its escape is facilitated by evaporation. and the rate of evaporation is, other things equal, determined by heat. though the current of sap in a tree is partly dependent on some action, probably osmotic, that goes on in the roots; yet the loss of water from the surfaces of the leaves, and the consequent absorption of more sap into the leaves by capillary attraction, must be a chief cause of the circulation. the drooping of a plant when exposed to the sunshine while the earth round its roots is dry, shows us how evaporation empties the sap-vessels; and the quickness with which a withered slip revives on being placed in water, shows us the part which capillary action plays. in so far, then, as the evaporation from a plant's surface helps to produce currents of sap through the plant, we must regard the heat which produces this evaporation as a part-cause of those re-distributions of matter which these currents effect. in terrestrial animals, heat, by its indirect action as well as by its direct action, similarly aids the changes that are going on. the exhalation of vapour from the lungs and the surface of the skin, forming the chief escape of the water that is swallowed, conduces to the maintenance of those currents through the tissues without which the functions would cease. for though the vascular system distributes nutritive liquids in ramified channels through the body; yet the absorption of these liquids into tissues, partly depends on the escape of liquids which the tissues already contain. hence, to the extent that such escape is facilitated by evaporation, and this evaporation facilitated by heat, heat becomes an agent of re-distribution in the animal organism.[ ] § . light, which is now known to modify many inorganic compounds--light, which works those chemical changes utilized in photography, causes the combinations of certain gases, alters the molecular arrangements of many crystals, and leaves traces of its action even on substances that are extremely stable,--may be expected to produce marked effects on substances so complex and unstable as those which make up organic bodies. it does produce such effects; and some of them are among the most important that organic matter undergoes. the molecular changes wrought by light in animals are of but secondary moment. there is the darkening of the skin that follows exposure to the sun's rays. there are those alterations in the retina which cause in us sensations of colours. and on certain eyeless creatures that are semi-transparent, the light permeating their substance works some effects evinced by movements. but speaking generally, the opacity of animals limits the action of light to their surfaces; and so renders its direct physiological influence but small.[ ] on plants, however, the solar rays that produce in us the impression of yellow, are the immediate agents of those molecular changes through which are hourly accumulated the materials for further growth. experiments have shown that when the sun shines on living leaves, they begin to exhale oxygen and to accumulate carbon and hydrogen--results which are traced to the decomposition, by the solar rays, of the carbonic acid and water absorbed. it is now an accepted conclusion that, by the help of certain classes of the ethereal undulations penetrating their leaves, plants are enabled to separate from the associated oxygen those two elements of which their tissues are chiefly built up. this transformation of ethereal undulations into certain molecular re-arrangements of an unstable kind, on the overthrow of which the stored-up forces are liberated in new forms, is a process that underlies all organic phenomena. it will therefore be well if we pause a moment to consider whether any proximate interpretation of it is possible. researches in molecular physics give us some clue to its nature. the elements of the problem are these:--the atoms[ ] of several ponderable matters exist in combination: those which are combined having strong affinities, but having also affinities less strong for some of the surrounding atoms that are otherwise combined. the atoms thus united, and thus mixed among others with which they are capable of uniting, are exposed to the undulations of a medium that is so rare as to seem imponderable. these undulations are of numerous kinds: they differ greatly in their lengths, or in the frequency with which they recur at any given point. and under the influence of undulations of a certain frequency, some of these atoms are transferred from atoms for which they have a stronger affinity, to atoms for which they have a weaker affinity. that is to say, particular orders of waves of a relatively imponderable matter, remove particular atoms of ponderable matter from their attachments, and carry them within reach of other attachments. now the discoveries of bunsen and kirchoff respecting the absorption of particular luminiferous undulations by the vapours of particular substances, joined with prof. tyndall's discoveries respecting the absorption of heat by gases, show very clearly that the atoms of each substance have a rate of vibration in harmony with ethereal waves of a certain length, or rapidity of recurrence. every special kind of atom can be made to oscillate by a special order of ethereal waves, which are absorbed in producing its oscillations; and can by its oscillations generate this same order of ethereal waves. whence it appears that immense as is the difference in density between ether and ponderable matter, the waves of the one can set the atoms of the other in motion, when the successive impacts of the waves are so timed as to correspond with the oscillations of the atoms. the effects of the waves are, in such case, cumulative; and each atom gradually acquires a momentum made up of countless infinitesimal momenta. note, further, that unless the members of a chemically-compound molecule are so bound up as to be incapable of any relative movements (a supposition at variance with the conceptions of modern science) we must conceive them as severally able to vibrate in unison or harmony with those same classes of ethereal waves that affect them in their uncombined states. while the compound molecule as a whole will have some new rate of oscillation determined by its attributes as a whole; its components will retain their original rates of oscillation, subject only to modifications by mutual influence. such being the circumstances of the case we may partially understand how the sun's rays can effect chemical decompositions. if the members of a diatomic molecule stand so related to the undulations falling on them, that one is thrown into a state of increased oscillation and the other not; it is manifest that there must arise a tendency towards the dislocation of the two--a tendency which may or may not take effect, according to the weakness or strength of their union, and according to the presence or absence of collateral affinities. this inference is in harmony with several significant facts. dr. draper remarks that "among metallic substances (compounds) those first detected to be changed by light, such as silver, gold, mercury, lead, have all high atomic weights; and such as sodium and potassium, the atomic weights of which are low, appeared to be less changeable." as here interpreted, the fact specified amounts to this; that the compounds most readily decomposed by light, are those in which there is a marked contrast between the atomic weights of the constituents, and probably therefore a marked contrast between the rapidities of their vibrations. the circumstance, too, that different chemical compounds are decomposed or modified in different parts of the spectrum, implies that there is a relation between special orders of undulations and special orders of molecules--doubtless a correspondence between the rates of these undulations and the rates of oscillation which some of the components of such molecules will assume. strong confirmation of this view may be drawn from the decomposing actions of those longer ethereal waves which we perceive as heat. on contemplating the whole series of diatomic compounds, we see that the elements which are most remote in their atomic weights, as hydrogen and the noble metals generally, will not combine at all, or do so with great difficulty: their vibrations are so unlike that they cannot keep together under any conditions of temperature. if, again, we look at a smaller group, as the metallic oxides, we see that whereas those metals which have atoms nearest in weight to the atoms of oxygen, cannot be separated from oxygen by heat, even when it is joined by a powerful collateral affinity; those metals which differ more widely from oxygen in their atomic weights, can be de-oxidized by carbon at high temperatures; and those which differ from it most widely combine with it very reluctantly, and yield it up if exposed to thermal undulations of moderate intensity. here indeed, remembering the relations among the atomic weights in the two cases, may we not suspect a close analogy between the de-oxidation of a metallic oxide by carbon under the influence of the longer ethereal waves, and the de-carbonization of carbonic acid by hydrogen under the influence of the shorter ethereal waves? these conceptions help us to some dim notion of the mode in which changes are wrought in light in the leaves of plants. among the several elements concerned, there are wide differences in molecular mobility, and probably in the rates of molecular vibration. each is combined with one of the others, but is capable of forming various combinations with the rest. and they are severally in presence of a complex compound into which they all enter, and which is ready to assimilate with itself the new compound molecules they form. certain of the ethereal waves falling on them when thus arranged, cause a detachment of some of the combined atoms and a union of the rest. and the conclusion suggested is that the induced vibrations among the various atoms as at first arranged, are so incongruous as to produce instability, and to give collateral affinities the power to work a rearrangement which, though less stable under other conditions, is more stable in the presence of these particular undulations. there seems, indeed, no choice but to conceive the matter thus. an atom united with one for which it has a strong affinity, has to be transferred to another for which it has a weaker affinity. this transfer implies motion. the motion is given by the waves of a medium that is relatively imponderable. no one wave of this imponderable medium can give the requisite motion to this atom of ponderable matter: especially as the atom is held by a positive force besides its inertia. the motion required can hence be given only by successive waves; and that these may not destroy each other's effects, it is needful that each shall strike the atom just when it has completed the recoil produced by the impact of previous ones. that is, the ethereal undulations must coincide in rate with the oscillations of the atom, determined by its inertia and the forces acting on it. it is also requisite that the rate of oscillation of the atom to be detached, shall differ from that of the atom with which it is united; since if the two oscillated in unison the ethereal waves would not tend to separate them. and, finally, the successive impacts of the ethereal waves must be accumulated until the resulting oscillations have become so wide in their sweep as greatly to weaken the cohesion of the united atoms, at the same time that they bring one of them within reach of other atoms with which it will combine. in this way only does it seem possible for such a force to produce such a transfer. moreover, while we are thus enabled to conceive how light may work these molecular changes, we also gain an insight into the method by which the insensible motions propagated to us from the sun, are treasured up in such ways as afterwards to generate sensible motions. by the accumulation of infinitesimal impacts, atoms of ponderable matter are made to oscillate. the quantity of motion which each of them eventually acquires, effects its transfer to a position of unstable equilibrium, from which it can afterwards be readily dislodged. and when so dislodged, along with other atoms similarly and simultaneously affected, there is suddenly given out all the motion which had been before impressed on it. speculation aside, however, that which it concerns us to notice is the broad fact that light is an all-important agent of molecular changes in organic substances. it is not here necessary for us to ascertain _how_ light produces these compositions and decompositions. it is necessary only for us to observe that it _does_ produce them. that the characteristic matter called chlorophyll, which gives the green colour to leaves, makes its appearance whenever the blanched shoots of plants are exposed to the sun; that the petals of flowers, uncoloured while in the bud, acquire their bright tints as they unfold; and that on the outer surfaces of animals, analogous changes are induced; are wide inductions which are enough for our present purpose. § . we come next to the agency of chief importance among those that work changes in organic matter; namely, chemical affinity. how readily vegetal and animal substances are modified by other substances put in contact with them, we see daily illustrated. besides the many compounds which cause the death of an organism into which they are put, we have the much greater number of compounds which work those milder effects termed medicinal--effects implying, like the others, molecular re-arrangements. indeed, most soluble chemical compounds, natural and artificial, produce, when taken into the body, alterations that are more or less manifest in their results. after what was shown in the last chapter, it will be manifest that this extreme modifiability of organic matter by chemical agencies, is the chief cause of that active molecular re-arrangement which organisms, and especially animal organisms, display. in the two fundamental functions of nutrition and respiration, we have the means by which the supply of materials for this active molecular re-arrangement is maintained. the process of animal nutrition consists partly in the absorption of those complex substances which are thus highly capable of being chemically altered, and partly in the absorption of simpler substances capable of chemically altering them. the tissues always contain small quantities of alkaline and earthy salts, which enter the system in one form and are excreted in another. though we do not know specifically the parts which these salts play, yet from their universal presence, and from the transformations which they undergo in the body, it may be safely inferred that their chemical affinities are instrumental in working some of the metamorphoses ever going on. the inorganic substance, however, on which mainly depend these metamorphoses in organic matter, is not swallowed along with the solid and liquid food, but is absorbed from the surrounding medium--air or water, as the case may be. whether the oxygen taken in, either, as by the lowest animals, through the general surface, or, as by the higher animals, through respiratory organs, is the immediate cause of those molecular changes which are ever going on throughout the living tissues; or whether the oxygen, playing the part of scavenger, merely aids these changes by carrying away the products of decompositions otherwise caused; it equally remains true that these changes are maintained by its instrumentality. whether the oxygen absorbed and diffused through the system effects a direct oxidation of the organic colloids which it permeates, or whether it first leads to the formation of simpler and more oxidized compounds, which are afterwards further oxidized and reduced to still simpler forms, matters not, in so far as the general result is concerned. in any case it holds good that the substances of which the animal body is built up, enter it in either an unoxidized or in a but slightly oxidized and highly unstable state; while the great mass of them leave it in a fully oxidized and stable state. it follows, therefore, that, whatever the special changes gone through, the general process is a falling from a state of unstable chemical equilibrium to a state of stable chemical equilibrium. whether this process be direct or indirect, the total molecular re-arrangement and the total motion given out in effecting it, must be the same. § . there is another species of re-distribution among the component matters of organisms, which is not immediately effected by the affinities of the matters concerned, but is mediately effected by other affinities; and there is reason to think that the re-distribution thus caused is important in amount, if not indeed the most important. in ordinary cases of chemical action, the two or more substances concerned themselves undergo changes of molecular arrangement; and the changes are confined to the substances themselves. but there are other cases in which the chemical action going on does not end with the substances at first concerned, but sets up chemical actions, or changes of molecular arrangement, among surrounding substances that would else have remained quiescent. and there are yet further cases in which mere contact with a substance that is itself quiescent, will cause other substances to undergo rapid metamorphoses. in what we call fermentation, the first species of this communicated chemical action is exemplified. one part of yeast, while itself undergoing molecular change, will convert parts of sugar into alcohol and carbonic acid; and during its own decomposition, one part of diastase "is able to effect the transformation of more than times its weight of starch into sugar." as illustrations of the second species, may be mentioned those changes which are suddenly produced in many colloids by minute portions of various substances added to them--substances that are not undergoing manifest transformations, and suffer no appreciable effects from the contact. the nature of the first of these two kinds of communicated molecular change, which here chiefly concerns us, may be rudely represented by certain visible changes communicated from mass to mass, when a series of masses has been arranged in a special way. the simplest example is that furnished by the child's play of setting bricks on end in a row, in such positions that when the first is overthrown it overthrows the second, the second the third, the third the fourth, and so on to the end of the row. here we have a number of units severally placed in unstable equilibrium, and in such relative positions that each, while falling into a state of stable equilibrium, gives an impulse to the next sufficient to make the next, also, fall from unstable to stable equilibrium. now since, among mingled compound molecules, no one can undergo change in the arrangement of its parts without a molecular motion that must cause some disturbance all round; and since an adjacent molecule disturbed by this communicated motion, may have the arrangement of its constituent atoms altered, if it is not a stable arrangement; and since we know, both that the molecules which are changed by this so-called catalysis _are_ unstable, and that the molecules resulting from their changes are _more_ stable; it seems probable that the transformation is really analogous, in principle, to the familiar one named. whether thus interpretable or not, however, there is good reason for thinking that to this kind of action is due a large amount of vital metamorphosis. let us contemplate the several groups of facts which point to this conclusion.[ ] in the last chapter (§ ) we incidentally noted the extreme instability of nitrogenous compounds in general. we saw that sundry of them are liable to explode on the slightest incentive--sometimes without any apparent cause; and that of the rest, the great majority are very easily decomposed by heat, and by various substances. we shall perceive much significance in this general characteristic when we join it with the fact that the substances capable of setting up extensive molecular changes in the way above described are all nitrogenous ones. yeast consists of vegetal cells containing nitrogen,--cells that grow by assimilating the nitrogenous matter contained in wort. similarly, the "vinegar-plant," which greatly facilitates the formation of acetic acid from alcohol, is a fungoid growth that is doubtless, like others of its class, rich in nitrogenous compounds. diastase, by which the transformation of starch into sugar is effected during the process of malting, is also a nitrogenous body. so too is a substance called synaptase--an albumenous principle contained in almonds, which has the power of working several metamorphoses in the matters associated with it. these nitrogenized compounds, like the rest of their family, are remarkable for the rapidity with which they decompose; and the extensive changes produced by them in the accompanying carbo-hydrates, are found to vary in their kinds according as the decompositions of the ferments vary in their stages. we have next to note, as having here a meaning for us, the chemical contrasts between those organisms which carry on their functions by the help of external forces, and those which carry on their functions by forces evolved from within. if we compare animals and plants, we see that whereas plants, characterized as a class by containing but little nitrogen, are dependent on the solar rays for their vital activities; animals, the vital activities of which are not thus dependent, mainly consist of nitrogenous substances. there is one marked exception to this broad distinction, however; and this exception is specially instructive. among plants there is a considerable group--the fungi--many members of which, if not all, can live and grow in the dark; and it is their peculiarity that they are very much more nitrogenous than other plants. yet a third class of facts of like significance is disclosed when we compare different portions of the same organism. the seed of a plant contains nitrogenous substance in a far higher ratio than the rest of the plant; and the seed differs from the rest of the plant in its ability to initiate, in the absence of light, extensive vital changes--the changes constituting germination. similarly in the bodies of animals, those parts which carry on active functions are nitrogenous; while parts that are non-nitrogenous--as the deposits of fat--carry on no active functions. and we even find that the appearance of non-nitrogenous matter throughout tissues normally composed almost wholly of nitrogenous matter, is accompanied by loss of activity: what is called fatty degeneration being the concomitant of failing vitality. one more fact, which serves to make still clearer the meaning of the foregoing ones, remains--the fact, namely, that in no part of any organism where vital changes are going on, is nitrogenous matter wholly absent. it is common to speak of plants--or at least all parts of plants but the seeds--as non-nitrogenous. but they are only relatively so; not absolutely. the quantity of albumenoid substance in the tissues of plants, is extremely small compared with the quantity contained in the tissues of animals; but all plant-tissues which are discharging active functions have some albumenoid substance. in every living vegetal cell there is a certain part that includes nitrogen as a component. this part initiates those changes which constitute the development of the cell. and if it cannot be said that it is the worker of all subsequent changes undergone by the cell, it nevertheless continues to be the part in which the independent activity is most marked. looking at the evidence thus brought together, do we not get an insight into the actions of nitrogenous matter as a worker of organic changes? we see that nitrogenous compounds in general are extremely prone to decompose: their decomposition often involving a sudden and great evolution of energy. we see that the substances classed as ferments, which, during their own molecular changes, set up molecular changes in the accompanying carbo-hydrates, are all nitrogenous. we see that among classes of organisms, and among the parts of each organism, there is a relation between the amount of nitrogenous matter present and the amount of independent activity. and we see that even in organisms and parts of organisms where the activity is least, such changes as do take place are initiated by a substance containing nitrogen. does it not seem probable, then, that these extremely unstable compounds have everywhere the effect of communicating to the less unstable compounds associated with them, molecular movements towards a stable state, like those they are themselves undergoing? the changes which we thus suppose nitrogenous matter to produce in the body, are clearly analogous to those which we see it produce out of the body. out of the body, certain carbo-hydrates in continued contact with nitrogenous matter, are transformed into carbonic acid and alcohol, and unless prevented the alcohol is transformed into acetic acid: the substances formed being thus more highly oxidized and more stable than the substances destroyed. in the body, these same carbo-hydrates, in continued contact with nitrogenous matter, are transformed into carbonic acid and water: substances which are also more highly oxidized and more stable than those from which they result. and since acetic acid is itself resolved by further oxidation into carbonic acid and water; we see that the chief difference between the two cases is, that the process is more completely effected in the body than it is out of the body. thus, to carry further the simile used above, the molecules of carbo-hydrates contained in the tissues are, like bricks on end, not in the stablest equilibrium; but still in an equilibrium so stable, that they cannot be overthrown by the chemical and thermal forces which the body brings to bear on them. on the other hand, being like similarly-placed bricks that have very narrow ends, the nitrogenous molecules contained in the tissues are in so unstable an equilibrium that they cannot withstand these forces. and when these delicately-poised nitrogenous molecules fall into stable arrangements, they give impulses to the more firmly-poised non-nitrogenous molecules, which cause them also to fall into stable arrangements. it is a curious and significant fact that in the arts, we not only utilize this same principle of initiating extensive changes among comparatively stable compounds, by the help of compounds much less stable, but we employ for the purpose compounds of the same general class. our modern method of firing a gun is to place in close proximity with the gunpowder which we wish to decompose or explode, a small portion of fulminating powder, which is decomposed or exploded with extreme facility, and which, on decomposing, communicates the consequent molecular disturbance to the less-easily decomposed gunpowder. when we ask what this fulminating powder is composed of, we find that it is a nitrogenous salt.[ ] thus, besides the molecular re-arrangements produced in organic matter by direct chemical action, there are others of kindred importance produced by indirect chemical action. indeed, the inference that some of the leading transformations occurring in the animal organism, are due to this so-called catalysis, appears necessitated by the general aspect of the facts, apart from any such detailed interpretations as the foregoing. we know that various amylaceous and saccharine matters taken as food do not appear in the excreta, and must therefore be decomposed in their course through the body. we know that these matters do not become components of the tissues, but only of the contained liquids and solids; and that thus their metamorphosis is not a direct result of tissue-change. we know that their stability is such that the thermal and chemical forces to which they are exposed in the body, cannot alone decompose them. the only explanation open to us, therefore, is that the transformation of these carbo-hydrates into carbonic acid and water, is due to communicated chemical action. § . this chapter will have served its purpose if it has given a conception of the extreme modifiability of organic matter by surrounding agencies. even were it possible, it would be needless to describe in detail the immensely varied and complicated changes which the forces from moment to moment acting on them, work in living bodies. dealing with biology in its general principles, it concerns us only to notice how specially sensitive are the substances of which organisms are built up to the varied influences that act upon organisms. their special sensitiveness has been made sufficiently manifest in the several foregoing sections. chapter iii. the re-actions of organic matter on forces. § . re-distributions of matter imply concomitant re-distributions of motion. that which under one of its aspects we contemplate as an alteration of arrangement among the parts of a body, is, under a correlative aspect, an alteration of arrangement among certain momenta, whereby these parts are impelled to their new positions. at the same time that a force, acting differently on the different units of an aggregate, changes their relations to one another; these units, reacting differently on the different parts of the force, work equivalent changes in the relations of these to one another. inseparably connected as they are, these two orders of phenomena are liable to be confounded together. it is very needful, however, to distinguish between them. in the last chapter we took a rapid survey of the re-distributions which forces produce in organic matter; and here we must take a like survey of the simultaneous re-distributions undergone by the forces. at the outset we are met by a difficulty. the parts of an inorganic mass undergoing re-arrangement by an incident force, are in most cases passive--do not complicate those necessary re-actions that result from their inertia, by other forces which they themselves originate. but in organic matter the re-arranged parts do not re-act in virtue of their inertia only. they are so constituted that an incident force usually sets up in them other actions which are much more important. indeed, what we may call the indirect reactions thus caused, are so great in their amounts compared with the direct re-actions, that they quite obscure them. the impossibility of separating these two kinds of reaction compels us to disregard the distinction between them. under the above general title, we must include both the immediate re-actions and those re-actions mediately produced, which are among the most conspicuous of vital phenomena. § . from organic matter, as from all other matter, incident forces call forth that re-action which we know as heat. more or less of molecular vibration necessarily results when, to the forces at work among the molecules of any aggregate, other forces are added. experiment abundantly demonstrates this in the case of inorganic masses; and it must equally hold in the case of organic masses. in both cases the force which, more markedly than any other, produces this thermal re-action, is that which ends in the union of different substances. though inanimate bodies admit of being greatly heated by pressure and by the electric current, yet the evolutions of heat, thus induced are neither so common, nor in most cases so conspicuous, as those resulting from chemical combination. and though in animate bodies there are certain amounts of heat generated by other actions, yet these are secondary to the heat generated by the action of oxygen on the substances composing the tissues and the substances contained in them. here, however, we see one of the characteristic distinctions between inanimate and animate bodies. among the first there are but few which ordinarily exist in a condition to evolve the heat caused by chemical combination; and such as are in this condition soon cease to be so when chemical combination and genesis of heat once begin in them. whereas, among the second there universally exists the ability, more or less decided, thus to evolve heat; and the evolution of heat, in some cases very slight and in no cases very great, continues as long as they remain animate bodies. the relation between active change of matter and re-active genesis of molecular vibration, is clearly shown by the contrasts between different organisms, and between different states and parts of the same organism. in plants the genesis of heat is extremely small, in correspondence with their extremely small production of carbonic acid: those portions only, as flowers and germinating seeds, in which considerable oxidation is going on, having decidedly raised temperatures. among animals we see that the hot-blooded are those which expend much force and respire actively. though insects are scarcely at all warmer than the surrounding air when they are still, they rise several degrees above it when they exert themselves; and in mammals, which habitually maintain a temperature much higher than that of their medium, exertion is accompanied by an additional production of heat. this molecular agitation accompanies the falls from unstable to stable molecular combinations; whether they be those from the most complex to the less complex compounds, or whether they be those ultimate falls which end in fully oxidized and relatively simple compounds; and whether they be those of the nitrogenous matters composing the tissues or those of the non-nitrogenous matters diffused through them. in the one case as in the other, the heat must be regarded as a concomitant. whether the distinction, originally made by liebig, between nitrogenous substances as tissue-food and non-nitrogenous substances as heat-food, be true or not in a narrower sense, it cannot be accepted in the sense that tissue-food is not also heat-food. indeed he does not himself assert it in this sense. the ability of carnivorous animals to live and generate heat while consuming matter that is almost exclusively nitrogenous, suffices to prove that the nitrogenous compounds forming the tissues are heat-producers, as well as the non-nitrogenous compounds circulating among and through the tissues: a conclusion which is indeed justified by the fact that nitrogenous substances out of the body yield heat, though not a large amount, during combustion. but most likely this antithesis is not true even in the more restricted sense. the probability is that the hydrocarbons and carbo-hydrates which, in traversing the system, are transformed by communicated chemical action, evolve, during their transformation, not heat alone but also other kinds of force. it may be that as the nitrogenous matter, while falling into more stable molecular arrangements, generates both that molecular agitation called heat and such other molecular movements as are resolved into forces expended by the organism; so, too, does the non-nitrogenous matter. or perhaps the concomitants of this metamorphosis of non-nitrogenous matter vary with the conditions. heat alone may result when it is transformed while in the circulating fluids, but partly heat and partly another force when it is transformed in some active tissue that has absorbed it; just as coal, though producing little else but heat as ordinarily burnt, has its heat partially transformed into mechanical motion if burnt in a steam-engine furnace. in such case the antithesis of liebig would be reduced to this--that whereas nitrogenous substance is tissue-food _both_ as material for building-up tissue and as material for its function; non-nitrogenous substance is tissue-food _only_ as material for function. there can be no doubt that this thermal re-action which chemical action from moment to moment produces in the body, is from moment to moment an aid to further chemical action. we before saw (_first principles_, § ) that a state of raised molecular vibration is favourable to those re-distributions of matter and motion which constitute evolution. we saw that in organisms distinguished by the amount and rapidity of such re-distributions, this raised state of molecular vibration is conspicuous. and we here see that this raised state of molecular vibration is itself a continuous consequence of the continuous molecular re-distributions it facilitates. the heat generated by each increment of chemical change makes possible the succeeding increment of chemical change. in the body this connexion of phenomena is the same as we see it to be out of the body. just as in a burning piece of wood, the heat given out by the portion actually combining with oxygen, raises the adjacent portion to a temperature at which it also can combine with oxygen; so, in a living animal, the heat produced by oxidation of each portion of organized or unorganized substance, maintains the temperature at which the unoxidized portions can be readily oxidized. § . among the forces called forth from organisms by re-action against the actions to which they are subject, is light. phosphorescence is in some few cases displayed by plants--especially by certain fungi. among animals it is comparatively common. all know that there are several kinds of luminous insects; and many are familiar with the fact that luminosity is a characteristic of various marine creatures. much of the evidence is supposed to imply that this evolution of light, like the evolution of heat, is consequent on oxidation of the tissues or of matters contained in them. light, like heat, is the expression of a raised state of molecular vibration: the difference between them being a difference in the rates of vibration. hence it seems inferable that by chemical action on substances contained in the organism, heat or light may be produced, according to the character of the resulting molecular vibrations. some experimental evidence supports this view. in phosphorescent insects, the continuance of the light is found to depend on the continuance of respiration; and any exertion which renders respiration more active, increases the brilliancy of the light. moreover, by separating the luminous matter, prof. matteucci has shown that its emission of light is accompanied by absorption of oxygen and escape of carbonic acid. the phosphorescence of marine animals has been referred to other causes than oxidation; but it may perhaps be explicable without assuming any more special agency. considering that in creatures of the genus _noctiluca_, for example, to which the phosphorescence most commonly seen on our own coasts is due, there is no means of keeping up a constant circulation, we may infer that the movements of aerated fluids through their tissues, must be greatly affected by impulses received from without. hence it may be that the sparkles visible at night when the waves break gently on the beach, or when an oar is dipped into the water, are called forth from these creatures by the concussion, not because of any unknown influence it excites, but because, being propagated through their delicate tissues, it produces a sudden movement of the fluids and a sudden increase of chemical action. nevertheless, in other phosphorescent animals inhabiting the sea, as in the _pyrosoma_ and in certain _annelida_, light seems to be produced otherwise than by direct re-action on the action of oxygen. indeed, it needs but to recall the now familiar fact that certain substances become luminous in the dark after exposure to sunlight, to see that there are other causes of light-emission. § . the re-distributions of inanimate matter are habitually accompanied by electrical disturbances; and there is abundant evidence that electricity is generated during those re-distributions of matter that are ever taking place in organisms. experiments have shown "that the skin and most of the internal membranes are in opposite electrical states;" and also that between different internal organs, as the liver and the stomach, there are electrical contrasts: such contrasts being greatest where the processes going on in the compared parts are most unlike. it has been proved by du bois-reymond that when any point in the longitudinal section of a muscle is connected by a conductor with any point in its transverse section, an electric current is established; and further, that like results occur when nerves are substituted for muscles. the special causes of these phenomena have not yet been determined. considering that the electric contrasts are most marked where active secretions are going on--considering, too, that they are difficult to detect where there are no appreciable movements of liquids--considering, also, that even when muscles are made to contract after removal from the body, the contraction inevitably causes movements of the liquids still contained in its tissues; it may be that they are due simply to the friction of heterogeneous substances, which is universally a cause of electric disturbance. but whatever be the interpretation, the fact remains the same:--there is throughout the living organism, an unceasing production of differences between the electric states of different parts; and, consequently, an unceasing restoration of electric equilibrium by the establishment of currents among these parts. besides these general, and not conspicuous, electrical phenomena common to all organisms, vegetal as well as animal, there are certain special and strongly marked ones. i refer, of course, to those which have made the _torpedo_ and the _gymnotus_ objects of so much interest. in these creatures we have a genesis of electricity which is not incidental on the performance of their different functions by the different organs; but one which is itself a function, having an organ appropriate to it. the character of this organ in both these fishes, and its largely-developed connexions with the nervous centres, have raised in some minds the suspicion that in it there takes place a transformation of what we call nerve-force into the force known as electricity. perhaps, however, the true interpretation may rather be that by nervous stimulation there is set up in these animal-batteries that particular transformation of molecular motion which it is their function to produce. but whether general or special, and in whatever manner produced, these evolutions of electricity are among the reactions of organic matter called forth by the actions to which it is subject. though these re-actions are not direct, but seem to be remote consequences of changes wrought by external agencies on the organism, they are yet incidents in that general re-distribution of motion which these external agencies initiate; and as such must here be noticed. § . to these known modes of motion, has next to be added an unknown one. heat, light, and electricity are emitted by inorganic matter when undergoing changes, as well as by organic matter. but there is manifested in some classes of living bodies a kind of force which we cannot identify with any of the forces manifested by bodies that are not alive,--a force which is thus unknown, in the sense that it cannot be assimilated to any otherwise-recognized class. i allude to what is called nerve-force. this is habitually generated in all animals, save the lowest, by incident forces of every kind. the gentle and violent mechanical contacts, which in ourselves produce sensations of touch and pressure--the additions and abstractions of molecular vibration, which in ourselves produce sensations of heat and cold, produce in all creatures that have nervous systems, certain nervous disturbances: disturbances which, as in ourselves, are either communicated to the chief nervous centre, and there arouse consciousness, or else result in mere physical processes set going elsewhere in the organism. in special parts distinguished as organs of sense, other external actions bring about other nervous re-actions, that show themselves either as special sensations or as excitements which, without the intermediation of distinct consciousness, beget actions in muscles or other organs. besides neural discharges following the direct incidence of external forces, others are ever being caused by the incidence of forces which, though originally external, have become internal by absorption into the organism of the agents exerting them. for thus may be classed those neural discharges which result from modifications of the tissues wrought by substances carried to them in the blood. that the unceasing change of matter which oxygen and other agents produce throughout the system, is accompanied by production of nerve-force, is shown by various facts;--by the fact that nerve-force is no longer generated if oxygen be withheld or the blood prevented from circulating; by the fact that when the chemical transformation is diminished, as during sleep with its slow respiration and circulation, there is a diminution in the quantity of nerve-force; by the fact that an excessive expenditure of nerve-force involves excessive respiration and circulation, and excessive waste of tissue. to these proofs that nerve-force is evolved in greater or less quantity, according as the conditions to rapid molecular change throughout the body are well or ill fulfilled, may be added proofs that certain special molecular actions are the causes of these special re-actions. the effects of the vegeto-alkalies put beyond doubt the inference that the overthrow of molecular equilibrium by chemical affinity, when it occurs in certain parts, causes excitement in the nerves proceeding from those parts. indeed, looked at from this point of view, the two classes of nervous changes--the one initiated from without and the other from within--are seen to merge into one class. both of them may be traced to metamorphosis of tissue. the sensations of touch and pressure are doubtless consequent on accelerated changes of matter, produced by mechanical disturbance of the mingled fluids and solids composing the parts affected. there is abundant evidence that the gustatory sensation is due to the chemical actions set up by particles which find their way through the membrane covering the nerves of taste; for, as prof. graham points out, sapid substances belong to the class of crystalloids, which are able rapidly to permeate animal tissue, while the colloids which cannot pass through animal tissue are insipid. similarly with the sense of smell. substances which excite this sense are necessarily more or less volatile; and their volatility being the result of their molecular mobility, implies that they have, in a high degree, the power of getting at the olfactory nerves by penetrating their mucous investment. again, the facts which photography has familiarized us with, show that those nervous impressions called colours, are primarily due to certain changes wrought by light in the substance of the retina. and though, in the case of hearing, we cannot so clearly trace the connexion of cause and effect, yet as we see that the auditory apparatus is one fitted to intensify those vibrations constituting sound, and to convey them to a receptacle containing liquid in which nerves are immersed, it can scarcely be doubted that the sensation of sound proximately results from molecular re-arrangements caused in these nerves by the vibrations of the liquid: knowing, as we do, that the re-arrangement of molecules is in all cases aided by agitation. perhaps, however, the best proof that nerve-force, whether peripheral or central in origin, results from chemical change, lies in the fact that most of the chemical agents which powerfully affect the nervous system, affect it whether applied at the centre or at the periphery. various mineral acids are tonics--the stronger ones being usually the stronger tonics; and this which we call their acidity implies a power in them of acting on the nerves of taste, while the tingling or pain following their absorption through the skin, implies that the nerves of the skin are acted on by them. similarly with certain vegeto-alkalies which are peculiarly bitter. by their bitterness these show that they affect the extremities of the nerves, while, by their tonic properties, they show that they affect the nervous centres: the most intensely bitter among them, strychnia, being the most powerful nervous stimulant.[ ] however true it may be that this relation is not a regular one, since opium, hashish, and some other drugs, which work marked effects on the brain, are not remarkably sapid--however true it may be that there are relations between particular substances and particular parts of the nervous system; yet such instances do but qualify, without negativing, the general proposition. the truth of this proposition can scarcely be doubted when, to the facts above given, is added the fact that various condiments and aromatic drugs act as nervous stimulants; and the fact that anæsthetics, besides the general effects they produce when inhaled or swallowed, produce local effects of like kind--first stimulant and then sedative--when absorbed through the skin; and the fact that ammonia, which in consequence of its extreme molecular mobility so quickly and so violently excites the nerves beneath the skin, as well as those of the tongue and the nose, is a rapidly-acting stimulant when taken internally. whether a nerve is merely a conductor, which delivers at one of its extremities an impulse received at the other, or whether, as some now think, it is itself a generator of force which is initiated at one extremity and accumulates in its course to the other extremity, are questions which cannot yet be answered. all we know is that agencies capable of working molecular changes in nerves are capable of calling forth from them manifestations of activity. and our evidence that nerve-force is thus originated, consists not only of such facts as the above, but also of more conclusive facts established by direct experiments on nerves--experiments which show that nerve-force results when the cut end of a nerve is either mechanically irritated, or acted on by some chemical agent, or subject to the galvanic current--experiments which prove that nerve-force is generated by whatever disturbs the molecular equilibrium of nerve-substance. § . the most important of the re-actions called forth from organisms by surrounding actions, remains to be noticed. to the various forms of insensible motion thus caused, we have to add sensible motion. on the production of this mode of force more especially depends the possibility of all vital phenomena. it is, indeed, usual to regard the power of generating sensible motion as confined to one out of the two organic sub-kingdoms; or, at any rate, as possessed by but few members of the other. on looking closer into the matter, however, we see that plant-life as well as animal-life, is universally accompanied by certain manifestations of this power; and that plant-life could not otherwise continue. through the humblest, as well as through the highest, vegetal organisms, there are ever going on certain re-distributions of matter. in protophytes the microscope shows us an internal transposition of parts, which, when not immediately visible, is proved to exist by the changes of arrangement that become manifest in the course of hours and days. in the individual cells of many higher plants, an active movement among the contained granules may be witnessed. and well-developed cryptogams, in common with all phanerogams, exhibit this genesis of mechanical motion still more conspicuously in the circulation of sap. it might, indeed, be concluded _a priori_, that through plants displaying much differentiation of parts, an internal movement must be going on; since, without it, the mutual dependence of organs having unlike functions would be impossible. besides keeping up these motions of liquids internally, plants, especially of the lower orders, move their external parts in relation to each other, and also move about from place to place. there are countless such illustrations as the active locomotion of the zoospores of many _algæ_, the rhythmical bendings of the _oscillatoræ_, the rambling progression of the _diatomaceæ_. in fact many of these smallest vegetals, and many of the larger ones in their early stages, display a mechanical activity not distinguishable from that of the simplest animals. among well-organized plants, which are never locomotive in their adult states, we still not unfrequently meet with relative motions of parts. to such familiar cases as those of the sensitive plant and the venus' fly-trap, many others may be added. when its base is irritated the stamen of the berberry flower leans over and touches the pistil. if the stamens of the wild _cistus_ be gently brushed with the finger, they spread themselves: bending away from the seed-vessel. and some of the orchid-flowers, as mr. darwin has shown, shoot out masses of pollen on to the entering bee, when its trunk is thrust down in search of honey. though the power of moving is not, as we see, a characteristic of animals alone, yet in them, considered as a class, it is manifested to an extent so marked as practically to become their most distinctive trait. for it is by their immensely greater ability to generate mechanical motion, that animals are enabled to perform those actions which constitute their visible lives; and it is by their immensely greater ability to generate mechanical motion, that the higher orders of animals are most obviously distinguished from the lower orders. though, on remembering the seemingly active movements of infusoria, some will perhaps question this last-named contrast, yet, on comparing the quantities of matter propelled through given spaces in given times, they will see that the momentum evolved is far less in the _protozoa_ than in the _metazoa_. these sensible motions of animals are effected in sundry ways. in the humblest forms, and even in some of the more developed forms which inhabit the water, locomotion results from the oscillations of whip-like appendages, single or double, or from the oscillations of cilia: the contractility resides in these waving hairs that grow from the surface. in many _coelenterata_ certain elongations or tails of ectodermal or endodermal cells shorten when stimulated, and by these rudimentary contractile organs the movements are effected. in all the higher animals, however, and to a smaller degree in many of the lower, sensible motion is generated by a special tissue, under a special excitement. though it is not strictly true that such animals show no sensible motions otherwise caused, since all of them have certain ciliated membranes, and since the circulation of liquids in them is partially due to osmotic and capillary actions; yet, generally speaking, we may say that their movements are effected solely by muscles which contract solely through the agency of nerves. what special transformations of force generate these various mechanical changes, we do not, in most cases, know. those re-distributions of liquid, with the alterations of form sometimes caused by them, that result from osmose, are not, indeed, incomprehensible. certain motions of plants which, like those of the "animated oat," follow contact with water, are easily interpreted; as are also such other vegetal motions as those of the touch-me-not, the squirting cucumber, and the _carpobolus_. but we are ignorant of the mode in which molecular movement is transformed into the movement of masses, in animals. we cannot refer to known causes the rhythmical action of a medusa's disc, or that slow decrease of bulk which spreads throughout the mass of an _alcyonium_ when one of its component individuals has been irritated. nor are we any better able to say how the insensible motion transmitted through a nerve, gives rise to sensitive motion in a muscle. it is true that science has given to art several methods of changing insensible into sensible motion. by applying heat to water we vaporize it, and the movement of its expanding vapour we transfer to solid matter; but evidently the genesis of muscular movement is in no way analogous to this. the force evolved in a galvanic battery or by a dynamo, we communicate to a soft iron magnet through a wire coiled round it; and it would be possible, by placing near to each other several magnets thus excited, to obtain, through the attraction of each for its neighbours, an accumulated movement made up of their separate movements, and thus mechanically to imitate a muscular contraction. but from what we know of organic matter there is no reason to suppose that anything analogous to this takes place in it. we can, however, through one kind of molecular change, produce sensible changes of aggregation such as possibly might, when occurring in organic substance, cause sensible motion in it. i refer to change that is allotropic or isomeric. sulphur, for example, assumes different crystalline and non-crystalline forms at different temperatures, and may be made to pass backwards and forwards from one form to another, by slight variations of temperature: undergoing each time an alteration of bulk. we know that this allotropism, or rather its analogue isomerism, prevails among colloids--inorganic and organic. we also know that some of these metamorphoses among colloids are accompanied by visible re-arrangements: instance hydrated silicic acid, which, after passing from its soluble state to the state of an insoluble jelly, begins, in a few days, to contract and to give out part of its contained water. now considering that such isomeric changes of organic as well as inorganic colloids, are often rapidly produced by very slight causes--a trace of a neutral salt or a degree or two rise of temperature--it seems not impossible that some of the colloids constituting muscle may be thus changed by a nervous discharge: resuming their previous condition when the discharge ceases. and it is conceivable that by structural arrangements, minute sensible motions so caused may be accumulated into large sensible motions. § . but the truths which it is here our business especially to note, are independent of hypotheses or interpretations. it is sufficient for the ends in view, to observe that organic matter _does_ exhibit these several conspicuous reactions when acted on by incident forces. it is not requisite that we should know _how_ these re-actions originate. in the last chapter were set forth the several modes in which incident forces cause re-distributions of organic matter; and in this chapter have been set forth the several modes in which is manifested the motion accompanying this re-distribution. there we contemplated, under its several aspects, the general fact that, in consequence of its extreme instability, organic matter undergoes extensive molecular re-arrangements on very slight changes of conditions. and here we have contemplated, under its several aspects, the correlative general fact that, during these extensive molecular re-arrangements, there are evolved large amounts of energy. in the one case the components of organic matter are regarded as falling from positions of unstable equilibrium to positions of stable equilibrium; and in the other case they are regarded as giving out in their falls certain momenta--momenta that may be manifested as heat, light, electricity, nerve-force, or mechanical motion, according as the conditions determine. i will add only that these evolutions of energy are rigorously dependent on these changes of matter. it is a corollary from the primordial truth which, as we have seen, underlies all other truths, (_first principles_, §§ , ,) that whatever amount of power an organism expends in any shape, is the correlate and equivalent of a power which was taken into it from without. on the one hand, it follows from the persistence of force that each portion of mechanical or other energy which an organism exerts, implies the transformation of as much organic matter as contained this energy in a latent state. and on the other hand, it follows from the persistence of force that no such transformation of organic matter containing this latent energy can take place, without the energy being in one shape or other manifested. chapter iii^{a.} metabolism. § a. in the early forties the french chemist dumas pointed out the opposed actions of the vegetal and animal kingdoms: the one having for its chief chemical effect the decomposition of carbon-dioxide, with accompanying assimilation of its carbon and liberation of its oxygen, and the other having for its chief chemical effect the oxidation of carbon and production of carbon-dioxide. omitting those plants which contain no chlorophyll, all others de-oxidize carbon; while all animals, save the few which contain chlorophyll, re-oxidize carbon. this is not, indeed, a complete account of the general relation; since it represents animals as wholly dependent on plants, either directly or indirectly through other animals, while plants are represented as wholly independent of animals; and this last representation though mainly true, since plants can obtain direct from the inorganic world certain other constituents they need, is in some measure not true, since many with greater facility obtain these materials from the decaying bodies of animals or from their _excreta_. but after noting this qualification the broad antithesis remains as alleged. how are these transformations brought about? the carbon contained in carbon-dioxide does not at a bound become incorporated in the plant, nor does the substance appropriated by the animal from the plant become at a bound carbon-dioxide. it is through two complex sets of changes that these two ultimate results are brought about. the materials forming the tissues of plants as well as the materials contained in them, are progressively elaborated from the inorganic substances; and the resulting compounds, eaten and some of them assimilated by animals, pass through successive changes which are, on the average, of an opposite character: the two sets being constructive and destructive. to express changes of both these natures the term "metabolism" is used; and such of the metabolic changes as result in building up from simple to compound are distinguished as "anabolic," while those which result in the falling down from compound to simple are distinguished as "katabolic." these antithetical names do not indeed cover all the molecular transformations going on. many of them, known as isomeric, imply neither building up nor falling down: they imply re-arrangement only. but those which here chiefly concern us are the two opposed kinds described. a qualification is needful. these antithetic changes must be understood as characterizing plant-life and animal-life in general ways rather than in special ways--as expressing the transformations in their totalities but not in their details. for there are katabolic processes in plants, though they bear but a small ratio to the anabolic ones; and there are anabolic processes in animals, though they bear but a small ratio to the katabolic ones. from the chemico-physical aspect of these changes we pass to those distinguished as vital; for metabolic changes can be dealt with only as changes effected by that living substance called protoplasm. § b. on the evolution-hypothesis we are obliged to assume that the earliest living things--probably minute units of protoplasm smaller than any the microscope reveals to us--had the ability to appropriate directly from the inorganic world both the nitrogen and the materials for carbo-hydrates without both of which protoplasm cannot be formed; since in the absence of preceding organic matter there was no other source. the general law of evolution as well as the observed actions of _protozoa_ and _protophyta_, suggest that these primordial types simultaneously displayed animal-life and plant-life. for whereas the developed animal-type cannot form from its inorganic surroundings either nitrogenous compounds or carbo-hydrates; and whereas the developed plant-type, able to form carbo-hydrates from its inorganic surroundings, depends for the formation of its protoplasm mainly, although indirectly, on the nitrogenous compounds derived from preceding organisms, as do also most of the plants devoid of chlorophyll--the fungi; we are obliged to assume that in the beginning, along with the expending activities characterizing the animal-type, there went the accumulating activities characterizing both of the vegetal types--forms of activity by-and-by differentiated. though the successive steps in the artificial formation of organic compounds have now gone so far that substances simulating proteids, if not identical with them, have been produced, yet we have no clue to the conditions under which proteids arose; and still less have we a clue to the conditions under which inert proteids became so combined as to form active protoplasm. the essential fact to be recognized is that living matter, originated as we must assume during a long stage of progressive cooling in which the infinitely varied parts of the earth's surface were slowly passing through appropriate physical conditions, possessed from the outset the power of assimilating to itself the materials from which more living matter was formed; and that since then all living matter has arisen from its self-increasing action. but now, leaving speculation concerning these anabolic changes as they commenced in the remote past, let us contemplate them as they are carried on now--first directing our attention to those presented in the vegetal world. § c. the decomposition of carbon-dioxide (§ )--the separation of its carbon from the combined oxygen so that it may enter into one or other form of carbo-hydrate,--is not now ordinarily effected, as we must assume it once was, by the undifferentiated protoplasm; but is effected by a specialized substance, chlorophyll, imbedded in the protoplasm and operating by its instrumentality. the chlorophyll-grain is not simply immersed in protoplasm but is permeated throughout its substance by a protoplasmic network or sponge-work apparently continuous with the protoplasm around; or, according to sachs, consists of protoplasm holding chlorophyll-particles in suspension: the mechanical arrangement facilitating the chemical function. the resulting abstraction of carbon from carbon-dioxide, by the aid of certain ethereal undulations, appears to be the first step in the building up of organic compounds--the first step in the primary anabolic process. we are not here concerned with details. two subsequent sets of changes only need here to be noted--the genesis of the passive materials out of which plant-structure is built up, and the genesis of the active materials by which these are produced and the building up effected. the hydrated carbon which protoplasm, having the chlorophyll-grain as its implement, produces from carbonic acid and water, appears not to be of one kind only. the possible carbo-hydrates are almost infinite in number. multitudes of them have been artificially made, and numerous kinds are made naturally by plants. though perhaps the first step in the reduction of the carbon from its dioxide may be always the same, yet it is held probable that in different types of plants different types of carbo-hydrates forthwith arise, and give differential characters to the compounds subsequently formed by such types: sundry of the changes being katabolic rather than anabolic. of leading members in the group may be named dextrin, starch, and the various sugars characteristic of various plants, as well as the cellulose elaborated by further anabolism. considered as the kind of carbo-hydrate in which the products of activity are first stored up, to be subsequently modified for divers purposes, starch is the most important of these; and the process of storage is suggested by the structure of the starch-grain. this consists of superposed layers, implying intermittent deposits: the probability being that the variations of light and heat accompanying day and night are associated now with arrest of the deposit and now with recommencement of it. like in composition as this stored-up starch is with sugar of one or other kind, and capable of being deposited from sugar and again assuming the sugar form, this substance passes, by further metabolism, here into the cellulose which envelopes each of the multitudinous units of protoplasm, there into the spiral fibres, annuli, or fenestrated tubes which, in early stages of tissue-growth, form channels for the sap, and elsewhere into other components of the general structure. the many changes implied are effected in various ways: now by that simple re-arrangement of components known as isomeric change; now by that taking from a compound one of its elements and inserting one of another kind, which is known as substitution; and now by oxidation, as when the oxy-cellulose which constitutes wood-fibre, is produced. besides elaborating building materials, the protoplasm elaborates itself--that is, elaborates more of itself. it is chemically distinguished from the building materials by the presence of nitrogen. derived from atmospheric ammonia, or from decaying or excreted organic matter, or from the products of certain fungi and microbes at its roots, the nitrogen in one or other combination is brought into a plant by the upward current; and by some unknown process (not dependent on light, since it goes on equally well if not better in darkness) the protoplasm dissociates and appropriates this combined nitrogen and unites it with a carbo-hydrate to form one or other proteid--albumen, gluten, or some isomer; appropriating at the same time from certain of the earth-salts the requisite amount of sulphur and in some cases phosphorus. the ultimate step, as we must suppose, is the formation of living protoplasm out of these non-living proteids. a cardinal fact is that proteids admit of multitudinous transformations; and it seems not improbable that in protoplasm various isomeric proteids are mingled. if so, we must conclude that protoplasm admits of almost infinite variations in nature. of course _pari passu_ with this dual process--augmentation of protoplasm and accompanying production of carbo-hydrates--there goes extension of plant-structure and plant-life. to these essential metabolic processes have to be added certain ancillary and non-essential ones, ending in the formation of colouring matters, odours, essential oils, acrid secretions, bitter compounds and poisons: some serving to attract animals and others to repel them. sundry of these appear to be excretions--useless matters cast out, and are doubtless katabolic. the relation of these facts here sketched in rude outline to the doctrine of evolution at large should be observed. already we have seen how (§ a), in the course of terrestrial evolution, there has been an increasingly heterogeneous assemblage of increasing heterogeneous compounds, preparing the way for organic life. and here we may see that during the development of plant-life from its lowest algoid and fungoid forms up to those forms which constitute the chief vegetal world, there has been an increasing number of complex organic compounds formed; displayed at once in the diversity of them contained in the same plant and in the still greater diversity displayed in the vast aggregate of species, genera, orders, and classes of plants. § d. on passing to the metabolism characterizing animal life, which, as already indicated, is in the main a process of decomposition undoing the process of composition characterizing vegetal life, we may fitly note at the outset that it must have wide limits of variation, alike in different classes of animals and even in the same animal. if we take, on the one hand, a carnivore living on muscular tissue (for wild carnivores preying upon herbivores which can rarely become fat obtain scarcely any carbo-hydrates) and observe that its food is almost exclusively nitrogenous; and if, on the other hand, we take a graminivorous animal the food of which (save when it eats seeds) contains comparatively little nitrogenous matter; we seem obliged to suppose that the parts played in the organic processes by the proteids and the carbo-hydrates can in considerable measures replace one another. it is true that the quantity of food and the required alimentary system in the last case, are very much greater than in the first case. but this difference is mainly due to the circumstance that the food of the graminivorous animal consists chiefly of waste-matter--ligneous fibre, cellulose, chlorophyll--and that could the starch, sugar, and protoplasm be obtained without the waste-matter, the required bulks of the two kinds of food would be by no means so strongly contrasted. this becomes manifest on comparing flesh-eating and grain-eating birds--say a hawk and a pigeon. in powers of flight these do not greatly differ, nor is the size of the alimentary system conspicuously greater in the last than in the first; though probably the amount of food consumed is greater. still it seems clear that the supply of energy obtained by a pigeon from carbo-hydrates with a moderate proportion of proteids is not widely unlike that obtained by a hawk from proteids alone. even from the traits of men differently fed a like inference may be drawn. on the one hand we have the masai who, during their warrior-days, eat flesh exclusively; and on the other hand we have the hindus, feeding almost wholly on vegetable food. doubtless the quantities required in these cases differ much; but the difference between the rations of the flesh-eater and the grain-eater is not so immense as it would be were there no substitution in the physiological uses of the materials. concerning the special aspects of animal-metabolism, we have first to note those various minor transformations that are auxiliary to the general transformation by which force is obtained from food. for many of the vital activities merely subserve the elaboration of materials for activity at large, and the getting rid of waste products. from blood passing through the salivary glands is prepared in large quantity a secretion containing among other matters a nitrogenous ferment, ptyaline, which, mixed with food during mastication, furthers the change of its starch into sugar. then in the stomach come the more or less varying secretions known in combination as gastric juice. besides certain salts and hydrochloric acid, this contains another nitrogenous ferment, pepsin, which is instrumental in dissolving the proteids swallowed. to these two metabolic products aiding solution of the various ingested solids, is presently added that product of metabolism in the pancreas which, added to the chyme, effects certain other molecular changes--notably that of such amylaceous matters as are yet unaltered, into saccharine matters to be presently absorbed. and let us note the significant fact that the preparation of food-materials in the alimentary canal, again shows us that unstable nitrogenous compounds are the agents which, while themselves changing, set up changes in the carbo-hydrates and proteids around: the nitrogen plays the same part here as elsewhere. it does the like in yet another viscus. blood which passes through the spleen on its way to the liver, is exposed to the action of "a special proteid of the nature of alkali-albumin, holding iron in some way peculiarly associated with it." lastly we come to that all-important organ the liver, at once a factory and a storehouse. here several metabolisms are simultaneously carried on. there is that which until recent years was supposed to be the sole hepatic process--the formation of bile. in some liver-cells are masses of oil-globules, which seem to imply a carbo-hydrate metamorphosis. and then, of leading importance, comes the extensive production of that animal-starch known as glycogen--a substance which, in each of the cells generating it, is contained in a plexus of protoplasmic threads: again a nitrogenous body diffused through a mass which is now formed out of sugar and is now dissolved again into sugar. for it appears that this soluble form of carbo-hydrate, taken into the liver from the intestine, is there, when not immediately needed, stored up in the form of glycogen, ready to be re-dissolved and carried into the system either for immediate use or for re-deposit as glycogen at the places where it is presently to be consumed: the great deposit in the liver and the minor deposits in the muscles being, to use the simile of prof. michael foster, analogous in their functions to a central bank and branch banks. an instructive parallelism may be noted between these processes carried on in the animal organism and those carried on in the vegetal organism. for the carbo-hydrates named, easily made to assume the soluble or the insoluble form by the addition or subtraction of a molecule of water, and thus fitted sometimes for distribution and sometimes for accumulation, are similarly dealt with in the two cases. as the animal-starch, glycogen, is now stored up in the liver or elsewhere and now changed into glucose to be transferred, perhaps for consumption and perhaps for re-deposit; so the vegetal starch, made to alternate between soluble and insoluble states, is now carried to growing parts where by metabolic change it becomes cellulose or other component of tissue and now carried to some place where, changed back into starch, it is laid aside for future use; as it is in the turgid inside leaves of a cabbage, the root of a turnip, or the swollen underground stem we know as a potato: the matter which in the animal is used up in generating movement and heat, being in the plant used up in generating structures. nor is the parallelism even now exhausted; for, as by a plant starch is stored up in each seed for the subsequent use of the embryo, so in an embryo-animal glycogen is stored up in the developing muscles for subsequent use in the completion of their structures. § e. we come now to the supreme and all-pervading metabolism which has for its effects the conspicuous manifestations of life--the nervous and muscular activities. here comes up afresh a question discussed in the edition of --a question to be reconsidered in the light of recent knowledge--the question what particular metabolic changes are they by which in muscle the energy existing under the form of molecular motion is transformed into the energy manifested as molar motion? there are two views respecting the nature of this transformation. one is that the carbo-hydrate present in muscle must, by further metabolism, be raised into the form of a nitrogenous compound or compounds before it can be made to undergo that sudden decomposition which initiates muscular contraction. the other is the view set forth in § , and there reinforced by further illustrations which have occurred to me while preparing this revised edition--the view that the carbo-hydrate in muscle, everywhere in contact with unstable nitrogenous substance, is, by the shock of a small molecular change in this, made to undergo an extensive molecular change, resulting in the oxidation of its carbon and consequent liberation of much molecular motion. both of these are at present only hypotheses, in support of which respectively the probabilities have to be weighed. let us compare them and observe on which side the evidence preponderates. we are obliged to conclude that in carnivorous animals the katabolic process is congruous with the first of these views, in so far that the evolution of energy must in some way result solely from the fall of complex nitrogenous compounds into those simpler matters which make their appearance as waste; for, practically, the carnivorous animal has no carbo-hydrates out of which otherwise to evolve force. to this admission, however, it should be added that possibly out of the exclusively nitrogenous food, glycogen or sugar has to be obtained by partial decomposition before muscular action can take place. but when we pass to animals having food consisting mainly of carbo-hydrates, several difficulties stand in the way of the hypothesis that, by further compounding, proteids must be formed from the carbo-hydrates before muscular energy can be evolved. in the first place the anabolic change through which, by the addition of nitrogen, &c., a proteid is formed from a carbo-hydrate, must absorb an energy equal to a moiety of that which is given out in the subsequent katabolic change. there can be no dynamic profit on such part of the transaction as effects the composition and subsequent decomposition of the proteid, but only on such part of the transaction as effects the decomposition of the carbo-hydrate. in the second place there arises the question--whence comes the nitrogen required for the compounding of the carbo-hydrates into proteids? there is none save that contained in the serum-albumen or other proteid which the blood brings; and there can be no gain in robbing this proteid of nitrogen for the purpose of forming another proteid. hence the nitrogenizing of the surplus carbo-hydrates is not accounted for. one more difficulty remains. if the energy given out by a muscle results from the katabolic consumption of its proteids, then the quantity of nitrogenous waste matters formed should be proportionate to the quantity of work done. but experiments have proved that this is not the case. long ago it was shown that the amount of urea excreted does not increase in anything like proportion to the amount of muscular energy expended; and recently this has been again shown. on this statement a criticism has been made to the following effect:--considering that muscle will contract when deprived of oxygen and blood and must therefore contain matter from which the energy is derived; and considering that since carbonic acid is given out the required carbon and oxygen must be derived from some component of muscle; it results that the energy must be obtained by decomposition of a nitrogenous body. to this reasoning it may be objected, in the first place, that the conditions specified are abnormal, and that it is dangerous to assume that what takes place under abnormal conditions takes place also under normal ones. in presence of blood and oxygen the process may possibly, or even probably, be unlike that which arises in their absence: the muscular substance may begin consuming itself when it has not the usual materials to consume. then, in the second place, and chiefly, it may be replied that the difficulty raised in the foregoing argument is not escaped but merely obscured. if, as is alleged, the carbon and oxygen from which carbonic acid is produced, form, under the conditions stated, parts of a complex nitrogenous substance contained in muscle, then the abstraction of the carbon and oxygen must cause decomposition of this nitrogenous substance; and in that case the excretion of nitrogenous waste must be proportionate to the amount of work done, which it is not. this difficulty is evaded by supposing that the "stored complex explosive substance must be, in living muscle, of such nature" that after explosion it leaves a "nitrogenous residue available for re-combination with fresh portions of carbon and oxygen derived from the blood and thereby the re-constitution of the explosive substance." this implies that a molecule of the explosive substance consists of a complex nitrogenous molecule united with a molecule of carbo-hydrate, and that time after time it suddenly decomposes this carbo-hydrate molecule and thereupon takes up another such from the blood. that the carbon is abstracted from the carbo-hydrate molecule can scarcely be said, since the feebler affinities of the nitrogenous molecule can hardly be supposed to overcome the stronger affinities of the carbo-hydrate molecule. the carbo-hydrate molecule must therefore be incorporated bodily. what is the implication? the carbo-hydrate part of the compound is relatively stable, while the nitrogenous part is relatively unstable. hence the hypothesis implies that, time after time, the unstable nitrogenous part overthrows the stable carbo-hydrate part, without being itself overthrown. this conclusion, to say the least of it, does not appear very probable. the alternative hypothesis, indirectly supported as we saw by proofs that outside the body small amounts of change in nitrogenous compounds initiate large amounts of change in carbonaceous compounds, may in the first place be here supported by some further indirect evidences of kindred natures. a haystack prematurely put together supplies one. enough water having been left in the hay to permit chemical action, the decomposing proteids forming the dead protoplasm in each cell, set up decomposition of the carbo-hydrates with accompanying oxidation of the carbon and genesis of heat; even to the extent of producing fire. again, as shown above, this relation between these two classes of compounds is exemplified in the alimentary canal; where, alike in the saliva and in the pancreatic secretion, minute quantities of unstable nitrogenous bodies transform great quantities of stable carbo-hydrates. thus we find indirect reinforcements of the belief that the katabolic change generating muscular energy is one in which a large decomposition of a carbo-hydrate is set up by a small decomposition of a proteid.[ ] § f. a certain general trait of animal organization may fitly be named because its relevance, though still more indirect, is very significant. under one of its aspects an animal is an apparatus for the multiplication of energies--a set of appliances by means of which a minute amount of motion initiates a larger amount of motion, and this again a still larger amount. there are structures which do this mechanically and others which do it chemically. associated with the peripheral ends of the nerves of touch are certain small bodies--_corpuscula tactus_--each of which, when disturbed by something in contact with the skin, presses on the adjacent fibre more strongly than soft tissue would do, and thus multiplies the force producing sensation. while serving the further purpose of touching at a distance, the _vibrissæ_ or whiskers of a feline animal achieve a like end in a more effectual way. the external portion of each bristle acts as the long arm of a lever, and the internal portion as the short arm. the result is that a slight touch at the outer end of the bristle produces a considerable pressure of the inner end on the nerve-terminal: so intensifying the impression. in the hearing organs of various inferior types of animals, the otolites in contact with the auditory nerves, when they are struck by sound-waves, give to the nerves much stronger impressions than these would have were they simply immersed in loose tissue; and in the ears of developed creatures there exist more elaborate appliances for augmenting the effects of aerial vibrations. from this multiplication of molar actions let us pass to the multiplication of molecular actions. the retina is made up of minute rods and cones, so packed together side by side that they can be separately affected by the separate parts of the images of objects. as each of them is but / , th of an inch in diameter, the ethereal undulations falling upon it can produce an amount of change almost infinitesimal--an amount probably incapable of exciting a nerve-centre, or indeed of overcoming the molecular inertia of the nerve leading to it. but in close proximity are layers of granules into which the rods and cones send fibres, and beyond these, about / th of an inch from the retinal layer, lie ganglion-cells, in each of which a minute disturbance may readily evolve a larger disturbance; so that by multiplication, single or perhaps double, there is produced a force sufficient to excite the fibre connected with the centre of vision. such, at least, judging from the requirement and the structure, seems to me the probable interpretation of the visual process; though whether it is the accepted one i do not know. but now, carrying with us the conception made clear by the first cases and suggested by the last, we shall appreciate the extent to which this general physiological method, as we may call it, is employed. the convulsive action caused by tickling shows it conspicuously. an extremely small amount of molecular change in the nerve-endings produces an immense amount of molecular change, and resulting molar motion, in the muscles. especially is this seen in one whose spinal cord has been so injured that it no longer conveys sensations from the lower limbs to the brain; and in whom, nevertheless, tickling of the feet produces convulsive actions of the legs more violent even than result when sensation exists: clearly proving that since the minute molecular change produced by the tickling in the nerve-terminals cannot be equivalent in quantity to the amount implied by the muscular contraction, there must be a multiplication of it in those parts of the spinal cord whence issue the reflex stimuli to the muscles. returning now to the question of metabolism, we may see that the processes of multiplication above supposed to take place in muscle, are analogous in their general nature to various other physiological processes. carrying somewhat further the simile used in § and going back to the days when detonators, though used for small arms, were not used for artillery, we may compare the metabolic process in muscle to that which would take place if a pistol were fired against the touch-hole of a loaded cannon: the cap exploding the pistol and the pistol the cannon. for in the case of the muscle, the implication is that a nervous discharge works in certain unstable proteids through which the nerve-endings are distributed, a small amount of molecular change; that the shock of this causes a much larger amount of molecular change in the inter-diffused carbo-hydrate, with accompanying oxidation of its carbon; and that the heat liberated sets up a transformation, probably isomeric, in the contractile substance of the muscular fibre: an interpretation supported by cases in which small rises and falls of temperature cause alternating isomeric changes; as instance mensel's salt. ending here this exposition, somewhat too speculative and running into details inappropriate to a work of this kind, it suffices to note the most general facts concerning metabolism. regarded as a whole it includes, in the first place, those anabolic or building-up processes specially characterizing plants, during which the impacts of ethereal undulations are stored up in compound molecules of unstable kinds; and it includes, in the second place, those katabolic or tumbling-down changes specially characterizing animals, during which this accumulated molecular motion (contained in the food directly or indirectly supplied by plants), is in large measure changed into those molar motions constituting animal activities. there are multitudinous metabolic changes of minor kinds which are ancillary to these--many katabolic changes in plants and many anabolic changes in animals--but these are the essential ones.[ ] chapter iv.[ ] proximate conception of life. § . to those who accept the general doctrine of evolution, it need scarcely be pointed out that classifications are subjective conceptions, which have no absolute demarcations in nature corresponding to them. they are appliances by which we limit and arrange the matters under investigation; and so facilitate our thinking. consequently, when we attempt to define anything complex, or make a generalization of facts other than the most simple, we can scarcely ever avoid including more than we intended, or leaving out something which should be taken in. thus it happens that on seeking a definite idea of life, we have great difficulty in finding one that is neither more nor less than sufficient. let us look at a few of the most tenable definitions that have been given. while recognizing the respects in which they are defective, we shall see what requirements a more satisfactory one must fulfil. schelling said that life is the tendency to individuation. this formula, until studied, conveys little meaning. but we need only consider it as illustrated by the facts of development, or by the contrast between lower and higher forms of life, to recognize its significance; especially in respect of comprehensiveness. as before shown, however (_first principles_, § ), it is objectionable; partly on the ground that it refers not so much to the functional changes constituting life, as to the structural changes of those aggregates of matter which manifest life; and partly on the ground that it includes under the idea life, much that we usually exclude from it: for instance--crystallization. the definition of richerand,--"life is a collection of phenomena which succeed each other during a limited time in an organized body,"--is liable to the fatal criticism, that it equally applies to the decay which goes on after death. for this, too, is "a collection of phenomena which succeed each other during a limited time in an organized body." "life," according to de blainville, "is the two-fold internal movement of composition and decomposition, at once general and continuous." this conception is in some respects too narrow, and in other respects too wide. on the one hand, while it expresses what physiologists distinguish as vegetative life, it does not indicate those nervous and muscular functions which form the most conspicuous and distinctive classes of vital phenomena. on the other hand, it describes not only the integrating and disintegrating process going on in a living body, but it equally well describes those going on in a galvanic battery; which also exhibits a "two-fold internal movement of composition and decomposition, at once general and continuous." elsewhere, i have myself proposed to define life as "the co-ordination of actions."[ ] this definition has some advantages. it includes all organic changes, alike of the viscera, the limbs, and the brain. it excludes the great mass of inorganic changes; which display little or no co-ordination. by making co-ordination the specific character of vitality, it involves the truths, that an arrest of co-ordination is death, and that imperfect co-ordination is disease. moreover, it harmonizes with our ordinary ideas of life in its different grades; seeing that the organisms which we rank as low in their degrees of life, are those which display but little co-ordination of actions; and seeing that from these up to man, the recognized increase in degree of life corresponds with an increase in the extent and complexity of co-ordinations. but, like the others, this definition includes too much. it may be said of the solar system, with its regularly-recurring movements and its self-balancing perturbations, that it, also, exhibits co-ordination of actions. and however plausibly it may be argued that, in the abstract, the motions of the planets and satellites are as properly comprehended in the idea of life as the changes going on in a motionless, unsensitive seed: yet, it must be admitted that they are foreign to that idea as commonly received, and as here to be formulated. it remains to add the definition since suggested by mr. g. h. lewes--"life is a series of definite and successive changes, both of structure and composition, which take place within an individual without destroying its identity." the last fact which this statement brings into view--the persistence of a living organism as a whole, in spite of the continuous removal and replacement of its parts--is important. but otherwise it may be argued that, since changes of structure and composition, though concomitants of muscular and nervous actions, are not the muscular and nervous actions themselves, the definite excludes the more visible movements with which our idea of life is most associated; and further that, in describing vital changes as _a series_, it scarcely includes the fact that many of them, as nutrition, circulation, respiration, and secretion, in their many subdivisions, go on simultaneously. thus, however well each of these definitions expresses the phenomena of life under some of its aspects, no one of them is more than approximately true. it may turn out that to find a formula which will bear every test is impossible. meanwhile, it is possible to frame a more adequate formula than any of the foregoing. as we shall presently find, these all omit an essential peculiarity of vital changes in general--a peculiarity which, perhaps more than any other, distinguishes them from non-vital changes. before specifying this peculiarity, however, it will be well to trace our way, step by step, to as complete an idea of life as may be reached from our present stand-point; by doing which we shall both see the necessity for each limitation as it is made, and ultimately be led to feel the need for a further limitation. and here, as the best mode of determining what are the traits which distinguish vitality from non-vitality, we shall do well to compare the two most unlike kinds of vitality, and see in what they agree. manifestly, that which is essential to life must be that which is common to life of all orders. and manifestly, that which is common to all forms of life, will most readily be seen on contrasting those forms of life which have the least in common, or are the most unlike.[ ] § . choosing assimilation, then, for our example of bodily life, and reasoning for our example of that life known as intelligence; it is first to be observed, that they are both processes of change. without change, food cannot be taken into the blood nor transformed into tissue; without change, there can be no getting from premisses to conclusion. and it is this conspicuous display of changes which forms the substratum of our idea of life in general. doubtless we see innumerable changes to which no notion of vitality attaches. inorganic bodies are ever undergoing changes of temperature, changes of colour, changes of aggregation; and decaying organic bodies also. but it will be admitted that the great majority of the phenomena displayed by inanimate bodies, are statical and not dynamical; that the modifications of inanimate bodies are mostly slow and unobtrusive; that on the one hand, when we see sudden movements in inanimate bodies, we are apt to assume living agency, and on the other hand, when we see no movements in living bodies, we are apt to assume death. manifestly then, be the requisite qualifications what they may, a true idea of life must be an idea of some kind of change or changes. on further comparing assimilation and reasoning, with a view of seeing in what respect the changes displayed in both differs from non-vital changes, we find that they differ in being not simple changes; in each case there are _successive_ changes. the transformation of food into tissue involves mastication, deglutition, chymification, chylification, absorption, and those various actions gone through after the lacteal ducts have poured their contents into the blood. carrying on an argument necessitates a long chain of states of consciousness; each implying a change of the preceding state. inorganic changes, however, do not in any considerable degree exhibit this peculiarity. it is true that from meteorologic causes, inanimate objects are daily, sometimes hourly, undergoing modifications of temperature, of bulk, of hygrometric and electric condition. not only, however, do these modifications lack that conspicuousness and that rapidity of succession which vital ones possess, but vital ones form an _additional_ series. living as well as not-living bodies are affected by atmospheric influences; and beyond the changes which these produce, living bodies exhibit other changes, more numerous and more marked. so that though organic change is not rigorously distinguished from inorganic change by presenting successive phases; yet vital change so greatly exceeds other change in this respect, that we may consider it as a distinctive character. life, then, as thus roughly differentiated, may be regarded as change presenting successive phases; or otherwise, as a series of changes. and it should be observed, as a fact in harmony with this conception, that the higher the life the more conspicuous the variations. on comparing inferior with superior organisms, these last will be seen to display more rapid changes, or a more lengthened series of them, or both. on contemplating afresh our two typical phenomena, we may see that vital change is further distinguished from non-vital change, by being made up of many _simultaneous_ changes. nutrition is not simply a series of actions, but includes many actions going on together. during mastication the stomach is busy with food already swallowed, on which it is pouring out solvent fluids and expending muscular efforts. while the stomach is still active, the intestines are performing their secretive, contractile, and absorbent functions; and at the same time that one meal is being digested, the nutriment obtained from a previous meal is undergoing transformation into tissue. so too is it, in a certain sense, with mental changes. though the states of consciousness which make up an argument occur in series, yet, as each of them is complex, a number of simultaneous changes have taken place in establishing it. here as before, however, it must be admitted that the distinction between animate and inanimate is not precise. no mass of dead matter can have its temperature altered, without at the same time undergoing an alteration in bulk, and sometimes also in hygrometric state. an inorganic body cannot be compressed, without being at the same time changed in form, atomic arrangement, temperature, and electric condition. and in a vast and mobile aggregate like the sea, the simultaneous as well as the successive changes outnumber those going on in an animal. nevertheless, speaking generally, a living thing is distinguished from a dead thing by the multiplicity of the changes at any moment taking place in it. moreover, by this peculiarity, as by the previous one, not only is the vital more or less clearly marked off from the non-vital; but creatures possessing high vitality are marked off from those possessing low vitality. it needs but to contrast the many organs cooperating in a mammal, with the few in a polype, to see that the actions which are progressing together in the body of the first, as much exceed in number the actions progressing together in the body of the last, as these do those in a stone. as at present conceived, then, life consists of simultaneous and successive changes. continuance of the comparison shows that vital changes, both visceral and cerebral, differ from other changes in their _heterogeneity_. neither the simultaneous acts nor the serial acts, which together constitute the process of digestion, are alike. the states of consciousness comprised in any ratiocination are not repetitions one of another, either in composition or in modes of dependence. inorganic processes, on the other hand, even when like organic ones in the number of the simultaneous and successive changes they involve, are unlike them in the relative homogeneity of these changes. in the case of the sea, just referred to, it is observable that countless as are the actions at any moment going on, they are mostly mechanical actions that are to a great degree similar; and in this respect differ widely from the actions at any moment taking place in an organism. even where life is nearly simulated, as by the working of a steam-engine, we see that considerable as is the number of simultaneous changes, and rapid as are the successive ones, the regularity with which they soon recur in the same order and degree, renders them unlike those varied changes exhibited by a living creature. still, this peculiarity, like the foregoing ones, does not divide the two classes of changes with precision; since there are inanimate things presenting considerable heterogeneity of change: for instance, a cloud. the variations of state which this undergoes, both simultaneous and successive, are many and quick; and they differ widely from one another both in quality and quantity. at the same instant there may occur change of position, change of form, change of size, change of density, change of colour, change of temperature, change of electric state; and these several kinds of change are continuously displayed in different degrees and combinations. yet when we observe that very few inorganic objects manifest heterogeneity of change comparable to that manifested by organic objects, and further, that in ascending from low to high forms of life, we meet with an increasing variety in the kinds of changes displayed; we see that there is here a further leading distinction between vital and non-vital actions. according to this modified conception, then, life is made up of heterogeneous changes both simultaneous and successive. if, now, we look for some trait common to the nutritive and logical processes, by which they are distinguished from those inorganic processes that are most like them in the heterogeneity of the simultaneous and successive changes they comprise, we discover that they are distinguished by the _combination_ among their constituent changes. the acts which make up digestion are mutually dependent. those composing a train of reasoning are in close connection. and, generally, it is to be remarked of vital changes, that each is made possible by all, and all are affected by each. respiration, circulation, absorption, secretion, in their many sub-divisions, are bound up together. muscular contraction involves chemical change, change of temperature, and change in the excretions. active thought influences the operations of the stomach, of the heart, of the kidneys. but we miss this union among non-vital activities. life-like as may seem the action of a volcano in respect of the heterogeneity of its many simultaneous and successive changes, it is not life-like in respect of their combination. though the chemical, mechanical, thermal, and electric phenomena exhibited have some inter-dependence, yet the emissions of stones, mud, lava, flame, ashes, smoke, steam, take place irregularly in quantity, order, intervals, and mode of conjunction. even here, however, it cannot be said that inanimate things present no parallels to animate ones. a glacier may be instanced as showing nearly as much combination in its change as a plant of the lowest organization. it is ever growing and ever decaying; and the rates of its composition and decomposition preserve a tolerably constant ratio. it moves; and its motion is in immediate dependence on its thawing. it emits a torrent of water, which, in common with its motion, undergoes annual variations as plants do. during part of the year the surface melts and freezes alternately; and on these changes depend the variations in movement, and in efflux of water. thus we have growth, decay, changes of temperature, changes of consistence, changes of velocity, changes of excretion, all going on in connexion; and it may be as truly said of a glacier as of an animal, that by ceaseless integration and disintegration it gradually undergoes an entire change of substance without losing its individuality. this exceptional instance, however, will scarcely be held to obscure that broad distinction from inorganic processes which organic processes derive from the combination among their constituent changes. and the reality of this distinction becomes yet more manifest when we find that, in common with previous ones, it not only marks off the living from the not-living, but also things which live little from things which live much. for while the changes going on in a plant or a zoophyte are so imperfectly combined that they can continue after it has been divided into two or more pieces, the combination among the changes going on in a mammal is so close that no part cut off from the rest can live, and any considerable disturbance of one chief function causes a cessation of the others. hence, as we now regard it, life is a combination of heterogeneous changes, both simultaneous and successive. when we once more look for a character common to these two kinds of vital action, we perceive that the combinations of heterogeneous changes which constitute them, differ from the few combinations which they otherwise resemble, in respect of _definiteness_. the associated changes going on in a glacier, admit of indefinite variation. under a conceivable alteration of climate, its thawing and its progression may be stopped for a million years, without disabling it from again displaying these phenomena under appropriate conditions. by a geological convulsion, its motion may be arrested without an arrest of its thawing; or by an increase in the inclination of the surface it slides over, its motion may be accelerated without accelerating its rate of dissolution. other things remaining the same, a more rapid deposit of snow may cause great increase of bulk; or, conversely, the accretion may entirely cease, and yet all the other actions continue until the mass disappears. here, then, the combination has none of that definiteness which, in a plant, marks the mutual dependence of respiration, assimilation, and circulation; much less has it that definiteness seen in the mutual dependence of the chief animal functions; no one of which can be varied without varying the rest; no one of which can go on unless the rest go on. moreover, this definiteness of combination distinguishes the changes occurring in a living body from those occurring in a dead one. decomposition exhibits both simultaneous and successive changes, which are to some extent heterogeneous, and in a sense combined; but they are not combined in a definite manner. they vary according as the surrounding medium is air, water, or earth. they alter in nature with the temperature. if the local conditions are unlike, they progress differently in different parts of the mass, without mutual influence. they may end in producing gases, or adipocire, or the dry substance of which mummies consist. they may occupy a few days or thousands of years. thus, neither in their simultaneous nor in their successive changes, do dead bodies display that definiteness of combination which characterizes living ones. it is true that in some inferior creatures the cycle of successive changes admits of a certain indefiniteness--that it may be suspended for a long period by desiccation or freezing, and may afterwards go on as though there had been no breach in its continuity. but the circumstance that only a low order of life can have its changes thus modified, serves but to suggest that, like the previous characteristics, this characteristic of definiteness in its combined changes, distinguishes high vitality from low vitality, as it distinguishes low vitality from inorganic processes. hence, our formula as further amended reads thus:--life is a definite combination of heterogenous changes, both simultaneous and successive. finally, we shall still better express the facts if, instead of saying _a_ definite combination of heterogeneous changes, we say _the_ definite combination of heterogeneous changes. as it at present stands, the definition is defective both in allowing that there may be _other_ definite combinations of heterogeneous changes, and in directing attention to the heterogeneous changes rather than to the definiteness of their combination. just as it is not so much its chemical elements which constitute an organism, as it is the arrangement of them into special tissues and organs; so it is not so much its heterogeneous changes which constitute life, as it is the co-ordination of them. observe what it is that ceases when life ceases. in a dead body there are going on heterogeneous changes, both simultaneous and successive. what then has disappeared? the definite combination has disappeared. mark, too, that however heterogeneous the simultaneous and successive changes exhibited by such an inorganic object as a volcano, we much less tend to think of it as living than we do a watch or a steam-engine, which, though displaying changes that, serially contemplated, are largely homogeneous, displays them definitely combined. so dominant an element is this in our idea of life, that even when an object is motionless, yet, if its parts be definitely combined, we conclude either that it has had life, or has been made by something having life. thus, then, we conclude that life is--_the_ definite combination of heterogeneous changes, both simultaneous and successive. § . such is the conception at which we arrive without changing our stand-point. it is, however, an incomplete conception. this ultimate formula (which is to a considerable extent identical with one above given--"the co-ordination of actions;" seeing that "definite combination" is synonymous with "co-ordination," and "changes both simultaneous and successive" are comprehended under the term "actions;" but which differs from it in specifying the fact, that the actions or changes are "heterogeneous")--this ultimate formula, i say, is after all but a rude approximation. it is true that it does not fail by including the growth of a crystal; for the successive changes this implies cannot be called heterogeneous. it is true that the action of a galvanic battery is not comprised in it; since here, too, heterogeneity is not exhibited by the successive changes. it is true that by this same qualification the motions of the solar system are excluded, as are also those of a watch and a steam-engine. it is true, moreover, that while, in virtue of their heterogeneity, the actions going on in a cloud, in a volcano, in a glacier, fulfil the definition; they fall short of it in lacking definiteness of combination. it is further true that this definiteness of combination distinguishes the changes taking place in an organism during life from those which commence at death. and beyond all this it is true that, as well as serving to mark off, more or less clearly, organic actions from inorganic actions, each member of the definition serves to mark off the actions constituting high vitality from those constituting low vitality; seeing that life is high in proportion to the number of successive changes occurring between birth and death; in proportion to the number of simultaneous changes; in proportion to the heterogeneity of the changes; in proportion to the combination subsisting among the changes; and in proportion to the definiteness of their combination. nevertheless, answering though it does to so many requirements, this definition is essentially defective. _the definite combination of heterogeneous changes, both simultaneous and successive_, is a formula which fails to call up an adequate conception. and it fails from omitting the most distinctive peculiarity--the peculiarity of which we have the most familiar experience, and with which our notion of life is, more than with any other, associated. it remains now to supplement the conception by the addition of this peculiarity. chapter v. the correspondence between life and its circumstances. § . we habitually distinguish between a live object and a dead one, by observing whether a change which we make in the surrounding conditions, or one which nature makes in them, is or is not followed by some perceptible change in the object. by discovering that certain things shrink when touched, or fly away when approached, or start when a noise is made, the child first roughly discriminates between the living and the not-living; and the man when in doubt whether an animal he is looking at is dead or not, stirs it with his stick; or if it be at a distance, shouts, or throws a stone at it. vegetal and animal life are alike primarily recognized by this process. the tree that puts out leaves when the spring brings increase of temperature, the flower which opens and closes with the rising and setting of the sun, the plant that droops when the soil is dry and re-erects itself when watered, are considered alive because of these induced changes; in common with the acorn-shell which contracts when a shadow suddenly falls on it, the worm that comes to the surface when the ground is continuously shaken, and the hedgehog that rolls itself up when attacked. not only, however, do we look for some response when an external stimulus is applied to a living organism, but we expect a fitness in the response. dead as well as living things display changes under certain changes of condition: instance, a lump of carbonate of soda that effervesces when dropped into sulphuric acid; a cord that contracts when wetted; a piece of bread that turns brown when held near the fire. but in these cases, we do not see a connexion between the changes undergone and the preservation of the things that undergo them; or, to avoid any teleological implication--the changes have no apparent relations to future events which are sure or likely to take place. in vital changes, however, such relations are manifest. light being necessary to vegetal life, we see in the action of a plant which, when much shaded, grows towards the unshaded side, an appropriateness which we should not see did it grow otherwise. evidently the proceedings of a spider which rushes out when its web is gently shaken and stays within when the shaking is violent, conduce better to the obtainment of food and the avoidance of danger than were they reversed. the fact that we feel surprise when, as in the case of a bird fascinated by a snake, the conduct tends towards self-destruction, at once shows how generally we have observed an adaptation of living changes to changes in surrounding circumstances. a kindred truth, rendered so familiar by infinite repetition that we forget its significance, must be named. there is invariably, and necessarily, a conformity between the vital functions of any organism and the conditions in which it is placed--between the processes going on inside of it and the processes going on outside of it. we know that a fish cannot live long in air, or a man under water. an oak growing in the ocean and a seaweed on the top of a hill, are incredible combinations of ideas. we find that each kind of animal is limited to a certain range of climate; each kind of plant to certain zones of latitude and elevation. of the marine flora and fauna, each species is found only between such and such depths. some blind creatures flourish in dark caves; the limpet where it is alternately covered and uncovered by the tide; the red-snow alga rarely elsewhere than in the arctic regions or among alpine peaks. grouping together the cases first named, in which a particular change in the circumstances of an organism is followed by a particular change in it, and the cases last named, in which the constant actions occurring within an organism imply some constant actions occurring without it; we see that in both, the changes or processes displayed by a living body are specially related to the changes or processes in its environment. and here we have the needful supplement to our conception of life. adding this all-important characteristic, our conception of life becomes--the definite combination of heterogeneous changes, both simultaneous and successive, _in correspondence with external co-existences and sequences_. that the full significance of this addition may be seen, it will be necessary to glance at the correspondence under some of its leading aspects.[ ] § . neglecting minor requirements, the actions going on in a plant pre-suppose a surrounding medium containing at least carbonic acid and water, together with a due supply of light and a certain temperature. within the leaves carbon is being appropriated and oxygen given off; without them, is the gas from which the carbon is taken, and the imponderable agents that aid the abstraction. be the nature of the process what it may, it is clear that there are external elements prone to undergo special re-arrangements under special conditions. it is clear that the plant in sunshine presents these conditions and so effects these re-arrangements. and thus it is clear that the changes which primarily constitute the plant's life, are in correspondence with co-existences in its environment. if, again, we ask respecting the lowest protozoon how it lives; the answer is, that while on the one hand its substance is undergoing disintegration, it is on the other hand absorbing nutriment; and that it may continue to exist, the one process must keep pace with, or exceed, the other. if further we ask under what circumstances these combined changes are possible, there is the reply that the medium in which the protozoon is placed, must contain oxygen and food--oxygen in such quantity as to produce some disintegration; food in such quantity as to permit that disintegration to be made good. in other words--the two antagonistic processes taking place internally, imply the presence externally of materials having affinities that can give rise to them. leaving those lowest animal forms which simply take in through their surfaces the nutriment and oxygenated fluids coming in contact with them, we pass to those somewhat higher forms which have their tissues slightly specialized. in these we see a correspondence between certain actions in the digestive sac, and the properties of certain surrounding bodies. that a creature of this order may continue to live, it is necessary not only that there be masses of substance in the environment capable of transformation into its own tissue, but also that the introduction of these masses into its stomach, shall be followed by the secretion of a solvent fluid which will reduce them to a fit state for absorption. special outer properties must be met by special inner properties. when, from the process by which food is digested, we turn to the process by which it is seized, the same general truth faces us. the stinging and contractile power of a polype's tentacle, correspond to the sensitiveness and strength of the creatures serving it for prey. unless that external change which brings one of these creatures in contact with the tentacle, were quickly followed by those internal changes which result in the coiling and drawing up of the tentacle, the polype would die of inanition. the fundamental processes of integration and disintegration within it, would get out of correspondence with the agencies and processes without it, and the life would cease. similarly, when the creature becomes so large that its tissue cannot be efficiently supplied with nutriment by mere absorption through its lining membrane, or duly oxygenated by contact with the fluid bathing its surface, there arises a need for a distributing system by which nutriment and oxygen may be carried throughout the mass; and the functions of this system, being subsidiary to the two primary functions, form links in the correspondence between internal and external actions. the like is obviously true of all those subordinate functions, secretory and excretory, that facilitate oxidation and assimilation. ascending from visceral actions to muscular and nervous actions, we find the correspondence displayed in a manner still more obvious. every act of locomotion implies the expenditure of certain internal forces, adapted in amounts and directions to balance or out-balance certain external forces. the recognition of an object is impossible without a harmony between the changes constituting perception, and particular properties co-existing in the environment. escape from enemies implies motions within the organism, related in kind and rapidity to motions without it. destruction of prey requires a special combination of subjective actions, fitted in degree and succession to overcome a group of objective ones. and so with those countless automatic processes constituting instincts. in the highest order of vital changes the same fact is equally manifest. the empirical generalization that guides the farmer in his rotation of crops, serves to bring his actions into concord with certain of the actions going on in plants and soil. the rational deductions of the educated navigator who calculates his position at sea, form a series of mental acts by which his proceedings are conformed to surrounding circumstances. alike in the simplest inferences of the child and the most complex ones of the man of science, we find a correspondence between simultaneous and successive changes in the organism, and co-existences and sequences in its environment. § . this general formula which thus includes the lowest vegetal processes along with the highest manifestations of human intelligence, will perhaps call forth some criticisms which it is desirable here to meet. it may be thought that there are still a few inorganic actions included in the definition; as, for example, that displayed by the mis-named storm-glass. the feathery crystallization which, on a certain change of temperature, takes place in its contained solution, and which afterwards dissolves to reappear in new forms under new conditions, may be held to present simultaneous and successive changes that are to some extent heterogeneous, that occur with some definiteness of combination, and, above all, occur in apparent correspondence with external changes. in this case vegetal life is simulated to a considerable extent; but it is _merely_ simulated. the relation between the phenomena occurring in the storm-glass and in the atmosphere respectively, is not a correspondence at all, in the proper sense of the word. outside there is a thermal change; inside there is a change of atomic arrangement. outside there is another thermal change; inside there is another change of atomic arrangement. but subtle as is the dependence of each internal upon each external change, the connexion between them does not, in the abstract, differ from the connexion between the motion of a straw and the motion of the wind that disturbs it. in either case a change produces a change, and there it ends. the alteration wrought by some environing agency on this or any other inanimate object, does not tend to induce in it a secondary alteration which anticipates some secondary alteration in the environment. but in every living body there is a tendency towards secondary alterations of this nature; and it is in their production that the correspondence consists. the difference may be best expressed by symbols. let a be a change in the environment, and b some resulting change in an inorganic mass. then a having produced b, the action ceases. though the change a in the environment is followed by some consequent change _a_ in it; no parallel sequence in the inorganic mass simultaneously generates in it some change _b_ that has reference to the change _a_. but if we take a living body of the requisite organization, and let the change a impress on it some change c; then, while in the environment a is occasioning _a_, in the living body c will be occasioning _c_; of which _a_ and _c_ will show a certain concord in time, place, or intensity. and while it is _in_ the continuous production of such concords or correspondences that life consists, it is _by_ the continuous production of them that life is maintained. the further criticism to be expected concerns certain verbal imperfections in the definition, which it seems impossible to avoid. it may fairly be urged that the word _correspondence_ will not include, without straining, the various relations to be expressed by it. it may be asked:--how can the continuous _processes_ of assimilation and respiration correspond with the _co-existence_ of food and oxygen in the environment? or again:--how can the act of secreting some defensive fluid correspond with some external danger which may never occur? or again:--how can the _dynamical_ phenomena constituting perception correspond with the _statical_ phenomena of the solid body perceived? the only reply is, that we have no word sufficiently general to comprehend all forms of this relation between the organism and its medium, and yet sufficiently specific to convey an adequate idea of the relation; and that the word _correspondence_ seems the least objectionable. the fact to be expressed in all cases is that certain changes, continuous or discontinuous, in the organism, are connected after such a manner that in their amounts, or variations, or periods of occurrence, or modes of succession, they have a reference to external actions, constant or serial, actual or potential--a reference such that a definite relation among any members of the one group, implies a definite relation among certain members of the other group. § . the presentation of the phenomena under this general form, suggests that our conception of life may be reduced to its most abstract shape by regarding its elements as relations only. if a creature's rate of assimilation is increased in consequence of a decrease of temperature in the environment, it is that the relation between the food consumed and the heat produced, is so re-adjusted by multiplying both its members, that the altered relation in the environment between the quantity of heat absorbed from, and radiated to, bodies of a given temperature, is counterbalanced. if a sound or a scent wafted to it on the breeze prompts the stag to dart away from the deer-stalker, it is that there exists in its neighbourhood a relation between a certain sensible property and certain actions dangerous to the stag, while in its body there exists an adapted relation between the impression this sensible property produces, and the actions by which danger may be escaped. if inquiry has led the chemist to a law, enabling him to tell how much of any one element will combine with so much of another, it is that there has been established in him specific mental relations, which accord with specific chemical relations in the things around. seeing, then, that in all cases we may consider the external phenomena as simply in relation, and the internal phenomena also as simply in relation; our conception of life under its most abstract aspect will be--_the continuous adjustment of internal relations to external relations_.[ ] while it is simpler, this formula has the further advantage of being somewhat more comprehensive. to say that it includes not only those definite combinations of simultaneous and successive changes in an organism, which correspond to co-existences and sequences in the environment, but also those structural arrangements which _enable_ the organism to adapt its actions to actions in the environment, is going too far; for though these structural arrangements present internal relations adjusted to external relations, yet the _continuous adjustment_ of relations cannot be held to include a _fixed adjustment_ already made. life, which is made up of _dynamical_ phenomena, cannot be described in terms that shall at the same time describe the apparatus manifesting it, which presents only _statical_ phenomena. but while this antithesis serves to remind us that the distinction between the organism and its actions is as wide as that between matter and motion, it at the same time draws attention to the fact that, if the structural arrangements of the adult are not properly included in the definition, yet the developmental processes by which those arrangements were established, are included. for that process of evolution during which the organs of the embryo are fitted to their prospective functions, is the gradual or continuous adjustment of internal relations to external relations. moreover, those structural modifications of the adult organism which, under change of climate, change of occupation, change of food, bring about some re-arrangement in the organic balance, may similarly be regarded as progressive or continuous adjustments of internal relations to external relations. so that not only does the definition, as thus expressed, comprehend all those activities, bodily and mental, which constitute our ordinary idea of life; but it also comprehends both those processes of development by which the organism is brought into general fitness for such activities, and those after-processes of adaptation by which it is specially fitted to its special activities. nevertheless, so abstract a formula as this is scarcely fitted for our present purpose. reserving it for use where specially appropriate, it will be best commonly to employ its more concrete equivalent--to consider the internal relations as "definite combinations of simultaneous and successive changes;" the external relations as "co-existences and sequences;" and the connexion between them as a "correspondence." chapter vi. the degree of life varies as the degree of correspondence. § . already it has been shown respecting each other component of the foregoing definition, that the life is high in proportion as that component is conspicuous; and it is now to be remarked, that the same thing is especially true respecting this last component--the correspondence between internal and external relations. it is manifest, _a priori_, that since changes in the physical state of the environment, as also of those mechanical actions and those variations of available food which occur in it, are liable to stop the processes going on in the organism; and since the adaptive changes in the organism have the effects of directly or indirectly counter-balancing these changes in the environment; it follows that the life of the organism will be short or long, low or high, according to the extent to which changes in the environment are met by corresponding changes in the organism. allowing a margin for perturbations, the life will continue only while the correspondence continues; the completeness of the life will be proportionate to the completeness of the correspondence; and the life will be perfect only when the correspondence is perfect. not to dwell in general statements, however, let us contemplate this truth under its concrete aspects. § . in life of the lowest order we find that only the most prevalent co-existences and sequences in the environment, have any simultaneous and successive changes answering to them in the organism. a plant's vital processes display adjustment solely to the continuous co-existence of certain elements and forces surrounding its roots and leaves; and vary only with the variations produced in these elements and forces by the sun--are unaffected by the countless mechanical movements and contacts occurring around; save when accidentally arrested by these. the life of a worm is made up of actions referring to little else than the tangible properties of adjacent things. all those visible and audible changes which happen near it, and are connected with other changes that may presently destroy it, pass unrecognized--produce in it no adapted changes: its only adjustment of internal relations to external relations of this order, being seen when it escapes to the surface on feeling the vibrations produced by an approaching mole. adjusted as are the proceedings of a bird to a far greater number of co-existences and sequences in the environment, cognizable by sight, hearing, scent, and their combinations: and numerous as are the dangers it shuns and the needs it fulfils in virtue of this extensive correspondence; it exhibits no such actions as those by which a human being counterbalances variations in temperature and supply of food, consequent on the seasons. and when we see the plant eaten, the worm trodden on, the bird dead from starvation; we see alike that the death is an arrest of such correspondence as existed, that it occurred when there was some change in the environment to which the organism made no answering change, and that thus, both in shortness and simplicity, the life was incomplete in proportion as the correspondence was incomplete. progress towards more prolonged and higher life, evidently implies ability to respond to less general co-existences and sequences. each step upwards must consist in adding to the previously-adjusted relations of actions or structures which the organism exhibits, some further relation parallel to a further relation in the environment. and the greater correspondence thus established, must, other things equal, show itself both in greater complexity of life, and greater length of life: a truth which will be fully perceived on remembering the enormous mortality which prevails among lowly-organized creatures, and the gradual increase of longevity and diminution of fertility which we meet with on ascending to creatures of higher and higher developments. it must be remarked, however, that while length and complexity of life are, to a great extent, associated--while a more extended correspondence in the successive changes commonly implies increased correspondence in the simultaneous changes; yet it is not uniformly so. between the two great divisions of life--animal and vegetal--this contrast by no means holds. a tree may live a thousand years, though the simultaneous changes going on in it answer only to the few chemical affinities in the air and the earth, and though its serial changes answer only to those of day and night, of the weather and the seasons. a tortoise, which exhibits in a given time nothing like the number of internal actions adjusted to external ones that are exhibited by a dog, yet lives far longer. the tree by its massive trunk and the tortoise by its hard carapace, are saved the necessity of responding to those many surrounding mechanical actions which organisms not thus protected must respond to or die; or rather--the tree and the tortoise display in their structures, certain simple statical relations adapted to meet countless dynamical relations external to them. but notwithstanding the qualifications suggested by such cases, it needs but to compare a microscopic fungus with an oak, an animalcule with a shark, a mouse with a man, to recognize the fact that this increasing correspondence of its changes with those of the environment which characterizes progressing life, habitually shows itself at the same time in continuity and in complication. even were not the connexion between length of life and complexity of life thus conspicuous, it would still be true that the life is great in proportion as the correspondence is great. for if the lengthened existence of a tree be looked upon as tantamount to a considerable amount of life; then it must be admitted that its lengthened display of correspondence is tantamount to a considerable amount of correspondence. if, otherwise, it be held that notwithstanding its much shorter existence, a dog must rank above a tortoise in degree of life because of its superior activity; then it is implied that its life is higher because its simultaneous and successive changes are more complex and more rapid--because the correspondence is greater. and since we regard as the highest life that which, like our own, shows great complexity in the correspondences, great rapidity in the succession of them, and great length in the series of them; the equivalence between degree of life and degree of correspondence is unquestionable. § . in further elucidation of this general truth, and especially in explanation of the irregularities just referred to, it must be pointed out that as the life becomes higher the environment itself becomes more complex. though, literally, the environment means all surrounding space with the co-existences and sequences contained in it: yet, practically, it often means but a small part of this. the environment of an entozoon can scarcely be said to extend beyond the body of the animal in which the entozoon lives. that of a freshwater alga is virtually limited to the ditch inhabited by the alga. and, understanding the term in this restricted sense, we shall see that the superior organisms inhabit the more complicated environments. thus, contrasted with the life found on land, the lower life is that found in the sea; and it has the simpler environment. marine creatures are affected by fewer co-existences and sequences than terrestrial ones. being very nearly of the same specific gravity as the surrounding medium, they have to contend with less various mechanical actions. the sea-anemone fixed to a stone, and the acalephe borne along in the current, need to undergo no internal changes such as those by which the caterpillar meets the varying effects of gravitation, while creeping over and under the leaves. again, the sea is liable to none of those extreme and rapid alterations of temperature which the air suffers. night and day produce no appreciable modifications in it; and it is comparatively little affected by the seasons. thus its contained fauna show no marked correspondences similar to those by which air-breathing creatures counterbalance thermal changes. further, in respect to the supply of nutriment, the conditions are more simple. the lower tribes of animals inhabiting the water, like the plants inhabiting the air, have their food brought to them. the same current which brings oxygen to the oyster, also brings it the microscopic organisms on which it lives: the disintegrating matter and the matter to be integrated, co-exist under the simplest relation. it is otherwise with land animals. the oxygen is everywhere, but the sustenance is not everywhere: it has to be sought; and the conditions under which it is to be obtained are more or less complex. so too with that liquid by the agency of which the vital processes are carried on. to marine creatures water is ever present, and by the lowest is passively absorbed; but to most creatures living on the earth and in the air, it is made available only through those nervous changes constituting perception, and those muscular ones by which drinking is effected. similarly, after tracing upwards from the _amphibia_ the widening extent and complexity which the environment, as practically considered, assumes--after observing further how increasing heterogeneity in the flora and fauna of the globe, itself progressively complicates the environment of each species of organism--it might finally be shown that the same general truth is displayed in the history of mankind, who, in the course of their progress, have been adding to their physical environment a social environment that has been growing ever more involved. thus, speaking generally, it is clear that those relations in the environment to which relations in the organism must correspond, themselves increase in number and intricacy as the life assumes a higher form. § . to make yet more manifest the fact that the degree of life varies as the degree of correspondence, let me here point out, that those other distinctions successively noted when contrasting vital changes with non-vital changes, are all implied in this last distinction--their correspondence with external co-existences and sequences; and further, that the increasing fulfilment of those other distinctions which we found to accompany increasing life, is involved in the increasing fulfilment of this last distinction. we saw that living organisms are characterized by successive changes, and that as the life becomes higher, the successive changes become more numerous. well, the environment is full of successive changes, and the greater the correspondence, the greater must be the number of successive changes in the organism. we saw that life presents simultaneous changes, and that the more elevated it is, the more marked the multiplicity of them. well, besides countless co-existences in the environment, there are often many changes occurring in it at the same moment; and hence increased correspondence with it implies in the organism an increased display of simultaneous changes. similarly with the heterogeneity of the changes. in the environment the relations are very varied in their kinds, and hence, as the organic actions come more and more into correspondence with them, they too must become very varied in their kinds. so again is it even with definiteness of combination. as the most important surrounding changes with which each animal has to deal, are the definitely-combined changes exhibited by other animals, whether prey or enemies, it results that definiteness of combination must be a general characteristic of the internal ones which have to correspond with them. so that throughout, the correspondence of the internal relations with the external ones is the essential thing; and all the special characteristics of the internal relations, are but the collateral results of this correspondence. §§ , . before closing the chapter, it will be useful to compare the definition of life here set forth, with the definition of evolution set forth in _first principles_. living bodies being bodies which display in the highest degree the structural changes constituting evolution; and life being made up of the functional changes accompanying these structural changes; we ought to find a certain harmony between the definitions of evolution and of life. such a harmony is not wanting. the first distinction we noted between the kind of change shown in life, and other kinds of change, was its serial character. we saw that vital change is substantially unlike non-vital change, in being made up of _successive_ changes. now since organic bodies display so much more than inorganic bodies those continuous differentiations and integrations which constitute evolution; and since the re-distributions of matter thus carried so far in a comparatively short period, imply concomitant re-distributions of motion; it is clear that in a given time, organic bodies must undergo changes so comparatively numerous as to render the successiveness of their changes a marked characteristic. and it will follow _a priori_, as we found it to do _a posteriori_, that the organisms exhibiting evolution in the highest degree, exhibit the longest or the most rapid successions of changes, or both. again, it was shown that vital change is distinguished from non-vital change by being made up of many _simultaneous_ changes; and also that creatures possessing high vitality are marked off from those possessing low vitality, by the far greater number of their simultaneous changes. here, too, there is entire congruity. in _first principles_, § , we reached the conclusion that a force falling on any aggregate is divided into several forces; that when the aggregate consists of parts that are unlike, each part becomes a centre of unlike differentiations of the incident force; and that thus the multiplicity of such differentiations must increase with the multiplicity of the unlike parts. consequently organic aggregates, which as a class are distinguished from inorganic aggregates by the greater number of their unlike parts, must be also distinguished from them by the greater number of simultaneous changes they display; and, further, that the higher organic aggregates, having more numerous unlike parts than the lower, must undergo more numerous simultaneous changes. we next found that the changes occurring in living bodies are contrasted with those occurring in other bodies, as being much more _heterogeneous_; and that the changes occurring in the superior living bodies are similarly contrasted with those occurring in inferior ones. well, heterogeneity of function is the correlate of heterogeneity of structure; and heterogeneity of structure is the leading distinction between organic and inorganic aggregates, as well as between the more highly organized and the more lowly organized. by reaction, an incident force must be rendered multiform in proportion to the multiformity of the aggregate on which it falls; and hence those most multi-form aggregates which display in the highest degree the phenomena of evolution structurally considered, must also display in the highest degree the multiform actions which constitute evolution functionally considered. these heterogeneous changes, exhibited simultaneously and in succession by a living organism, prove, on further inquiry, to be distinguished by their _combination_ from certain non-vital changes which simulate them. here, too, the parallelism is maintained. it was shown in _first principles_, chap. xiv, that an essential characteristic of evolution is the integration of parts, which accompanies their differentiation--an integration shown both in the consolidation of each part, and in the union of all the parts into a whole. hence, animate bodies having greater co-ordination of parts than inanimate ones must exhibit greater co-ordination of changes; and this greater co-ordination of their changes must not only distinguish organic from inorganic aggregates, but must, for the same reason, distinguish higher organisms from lower ones, as we found that it did. once more, it was pointed out that the changes constituting life differ from other changes in the _definiteness_ of their combination, and that a distinction like in kind though less in degree, holds between the vital changes of superior creatures and those of inferior creatures. these, also, are contrasts in harmony with the contrasts disclosed by the analysis of evolution. we saw (_first principles_, §§ - ) that during evolution there is an increase of definiteness as well as an increase of heterogeneity. we saw that the integration accompanying differentiation has necessarily the effect of increasing the distinctness with which the parts are marked off from each other, and that so, out of the incoherent and indefinite there arises the coherent and definite. but a coherent whole made up of definite parts definitely combined, must exhibit more definitely combined changes than a whole made up of parts that are neither definite in themselves nor in their combination. hence, if living bodies display more than other bodies this structural definiteness, then definiteness of combination must be a characteristic of the changes constituting life, and must also distinguish the vital changes of higher organisms from those of lower organisms. finally, we discovered that all these peculiarities are subordinate to the fundamental peculiarity, that vital changes take place in correspondence with external co-existences and sequences, and that the highest life is reached, when there is some inner relation of actions fitted to meet every outer relation of actions by which the organism can be affected. but this conception of the highest life, is in harmony with the conception, before arrived at, of the limit of evolution. when treating of equilibration as exhibited in organisms (_first principles_, §§ , ), it was pointed out that the tendency is towards the establishment of a balance between inner and outer changes. it was shown that "the final structural arrangements must be such as will meet all the forces acting on the aggregate, by equivalent antagonistic forces," and that "the maintenance of such a moving equilibrium" as an organism displays, "requires the habitual genesis of internal forces corresponding in number, directions, and amounts, to the external incident forces--as many inner functions, single or combined, as there are single or combined outer actions to be met." it was shown, too, that the relations among ideas are ever in progress towards a better adjustment between mental actions and those actions in the environment to which conduct must be adjusted. so that this continuous correspondence between inner and outer relations which constitutes life, and the perfection of which is the perfection of life, answers completely to that state of organic moving equilibrium which we saw arises in the course of evolution and tends ever to become more complete. chapter vi^a. the dynamic element in life. § a. a critical comparison of the foregoing formula with the facts proves it to be deficient in more ways than one. let us first look at vital phenomena which are not covered by it. some irritant left by an insect's ovipositor, sets up on a plant the morbid growth named a gall. the processes in the gall do not correspond with any external co-existences or sequences relevant to the plant's life--show no internal relations adjusted to external relations. yet we cannot deny that the gall is alive. so, too, is it with a cancer in or upon an animal's body. the actions going on in it have no reference, direct or indirect, to actions in the environment. nevertheless we are obliged to say that they are vital; since it grows and after a time dies and decomposes. a kindred lesson meets us when from pathological evidence we turn to physiological evidence. the functions of some important organs may still be carried on for a time apart from those of the body as a whole. an excised liver, kept at a fit temperature and duly supplied with blood, secretes bile. still more striking is the independent action of the heart. if belonging to a cold-blooded animal, as a frog, the heart, when detached, continues to beat, even until its integuments have become so dry that they crackle. now though under such conditions its pulsations, which ordinarily form an essential part of the linked processes by which the correspondence between inner and outer actions is maintained, no longer form part of such processes, we must admit that the continuance of them implies a vital activity. embryological changes force the same truth upon us. what are we to say of the repeated cell-fissions by which in some types a blastula, or mulberry-mass, is formed, and in other types a blastoderm? neither these processes nor the structures immediately resulting from them, show any correspondences with co-existences and sequences in the environment; though they are first steps towards the organization which is to carry on such correspondences. even this extremely small fulfilment of the definition is absent in the cases of rudimentary organs, and especially those rudimentary organs which after being partly formed are absorbed. no adjustment can be alleged between the inner relations which these present and any outer relations. the outer relations they refer to ceased millions of years ago. yet unquestionably the changes which bring about the production and absorption of these futile structures are vital changes. take another class of exceptions. what are we to say of a laugh? no correspondence, or part of a correspondence, by which inner actions are made to balance outer actions, can be seen in it. or again, if, while working, an artisan whistles, the making of the sounds and the co-ordination of ideas controlling them, cannot be said to exhibit adjustment between certain relations of thoughts, and certain relations of things. such kinds of vital activities lie wholly outside of the definition given. but perhaps the clearest and simplest proof is yielded by contrasting voluntary and involuntary muscular actions. here is a hawk adapting its changing motions to the changing motions of a pigeon, so as eventually to strike it: the adjustment of inner relations to outer relations is manifest. here is a boy in an epileptic fit. between his struggles and the co-existences and sequences around him there is no correspondence whatever. yet his movements betray vitality just as much as do the movements of the hawk. both exhibit that principle of _activity_ which constitutes the essential element in our conception of life. § b. evidently, then, the preceding chapters recognize only the _form_ of our conception of life and ignore the _body_ of it. partly sufficing as does the definition reached to express the one, it fails entirely to express the other. life displays itself in ways which conform to the definition; but it also displays itself in many other ways. we are obliged to admit that the element which is common to the two groups of ways is the essential element. the essential element, then, is that special kind of energy seen alike in the usual classes of vital actions and in those unusual classes instanced above. otherwise presenting the contrast, we may say that due attention has been paid to the connexions among the manifestations, while no attention has been paid to that which is manifested. when it is said that life is "the definite correspondence of heterogeneous changes, both simultaneous and successive, in correspondence with external co-existences and sequences," there arises the question--changes of what? within the body there go on many changes, mechanical, chemical, thermal, no one of which is the kind of change in question; and if we combine in thought so far as we can these kinds of changes, in such wise that each maintains its character as mechanical, chemical, or thermal, we cannot get out of them the idea of life. still more clearly do we see this insufficiency when we take the more abstract definition--"the continuous adjustment of internal relations to external relations." relations between what things? is the question then to be asked. a relation of which the terms are unspecified does not connote a thought but merely the blank form of a thought. its value is comparable to that of a cheque on which no amount is written. if it be said that the terms cannot be specified because so many heterogeneous kinds of them have to be included, then there comes the reply that under cover of this inability to make a specification of terms that shall be adequately comprehensive, there is concealed the inability to conceive the required terms in any way. thus a critical testing of the definition brings us, in another way, to the conclusion reached above, that that which gives the substance to our idea of life is a certain unspecified principle of activity. the dynamic element in life is its essential element. § c. under what form are we to conceive this dynamic element? is this principle of activity inherent in organic matter, or is it something superadded? of these alternative suppositions let us begin with the last. as i have remarked, in another place, the worth of an hypothesis may be judged from its genealogy; and so judged the hypothesis of an independent vital principal does not commend itself. its history carries us back to the ghost-theory of the savage. suggested by experiences of dreams, there arises belief in a double--a second self which wanders away during sleep and has adventures but comes back on waking; which deserts the body during abnormal insensibility of one or other kind; and which is absent for a long period at death, though even then is expected eventually to return. this indwelling other-self, which can leave the body at will, is by-and-by regarded as able to enter the bodies of fellow men or of animals; or again, by implication, as liable to have its place usurped by the intruding doubles of fellow men, living or dead, which cause fits or other ills. along with these developments its quality changes. at first thought of as quite material it is gradually de-materialized, and in advanced times comes to be regarded as spirit or breath; as we see in ancient religious books, where "giving up the ghost" is shown by the emergence of a small floating figure from the mouth of a dying man. this indwelling second self, more and more conceived as the real self which uses the body for its purposes, is, with the advance of intelligence, still further divested of its definite characters; and, coming in mediæval days to be spoken of as "animal spirits," ends in later days in being called a vital principle. entirely without assignable attributes, this something occurs in thought not as an idea but as a pseud-idea (_first principles_, chap. ii). it is assumed to be representable while really unrepresentable. we need only insist on answers to certain questions to see that it is simply a name for an alleged existence which has not been conceived and cannot be conceived. . is there one kind of vital principle for all kinds of organisms, or is there a separate kind for each? to affirm the first alternative is to say that there is the same vital principle for a microbe as for a whale, for a tape-worm as for the person it inhabits, for a protococcus as for an oak; nay more--is to assert community of vital principle in the thinking man and the unthinking plant. moreover, asserting unity of the vital principle for all organisms, is reducing it to a force having the same unindividualized character as one of the physical forces. if, on the other hand, different kinds of organisms have different kinds of vital principles, these must be in some way distinguished from one another. how distinguished? manifestly by attributes. do they differ in extension? evidently; since otherwise that which animates the vast _sequoia_ can be no larger than that which animates a yeast-plant, and to carry on the life of an elephant requires a quantity of vital principle no greater than that required for a microscopic monad. do they differ otherwise than in amount? certainly; since otherwise we revert to the preceding alternative, which implies that the same quality of vital principle serves for all organisms, simple and complex: the vital principle is a uniform force like heat or electricity. hence, then, we have to suppose that every species of animal and plant has a vital principle peculiar to itself--a principle adapted to use the particular set of structures in which it is contained. but dare anyone assert this multiplication of vital principles, duplicating not only all existing plants and animals but all past ones, and amounting in the aggregate to some millions? . how are we to conceive that genesis of a vital principle which must go along with the genesis of an organism? here is a pollen-grain which, through the pistil, sends its nucleus to unite with the nucleus of the ovule; or here are the nuclei of spermatozoon and ovum, which, becoming fused, initiate a new animal: in either case failure of union being followed by decomposition of the proteid materials, while union is followed by development. whence comes that vital principle which determines the organizing process? is it created afresh for every plant and animal? or, if not, where and how did it pre-exist? take a simpler form of this problem. a protophyte or protozoon, having grown to a certain size, undergoes a series of complex changes ending in fission. in its undivided state it had a vital principle. what of its divided state? the parts severally swim away, each fully alive, each ready to grow and presently to subdivide, and so on and so on, until millions are soon formed. that is to say, there is a multiplication of vital principles as of the protozoa animated by them. a vital principle, then, both divides and grows. but growth implies incorporation of something. what does the vital principle incorporate? is it some other vital principle external to it, or some materials out of which more vital principle is formed? and how, in either case, can the vital principle be conceived as other than a material something, which in its growth and multiplication behaves just as visible matter behaves? . equally unanswerable is the question which arises in presence of life that has become latent. passing over the alleged case of the mummy wheat, the validity of which is denied, there is experimental proof that seeds may, under conditions unfavourable to germination, retain for ten, twenty, and some even for thirty years, the power to germinate when due moisture and warmth are supplied. (_cf._ kerner's _nat. hist. of plants_, i, - ). under what form has the vital principle existed during these long intervals? it is a principle of activity. in this case, then, the principle of activity becomes inactive. but how can we conceive an inactive activity? if it is a something which though inactive may be rendered active when conditions favour, we are introduced to the idea of a vital principle of which the vitality may become latent, which is absurd. what shall we say of the desiccated rotifer which for years has seemed to be nothing more than a particle of dust, but which now, when water is supplied, absorbs it, swells up, and resumes those ciliary motions by which it draws in nutriment? was the vital principle elsewhere during these years of absolute quiescence? if so, why did it come back at the right moment? was it all along present in the rotifer though asleep? how happened it then to awaken at the time when the supply of water enabled the tissues to resume their functions? how happened the physical agent to act not only on the material substance of the rotifer, but also on this something which is not a material substance but an immaterial source of activity? evidently neither alternative is thinkable. thus, the alleged vital principle exists in the minds of those who allege it only as a verbal form, not as an idea; since it is impossible to bring together in consciousness the terms required to constitute an idea. it is not even "a figment of imagination," for that implies something imaginable, but the supposed vital principle cannot even be imagined. § d. when, passing to the alternative, we propose to regard life as inherent in the substances of the organisms displaying it, we meet with difficulties different in kind but scarcely less in degree. the processes which go on in living things are incomprehensible as results of any physical actions known to us. consider one of the simplest--that presented by an ordinary vegetal cell forming part of a leaf or other plant-structure. its limiting membrane, originally made polyhedral by pressure of adjacent cells, is gradually moulded "into one of cylindrical, fibrous, or tabular shape, and strengthening its walls with pilasters, borders, ridges, hooks, bands, and panels of various kinds" (kerner, i, ): small openings into adjacent cells being either left or subsequently made. consisting of non-nitrogenous, inactive matters, these structures are formed by the inclosed protoplast. how formed? is it by the agency of the nucleus? but the nucleus, even had it characters conceivably adapting it to this function, is irregularly placed; and that it should work the same effects upon the cell-wall whether seated in the middle, at one end, or one side, is incomprehensible. is the protoplasm then the active agent? but this is arranged into a network of strands and threads utterly irregular in distribution and perpetually altering their shapes and connexions. exercise of fit directive action by the protoplasm is unimaginable. another instance:--consider the reproductive changes exhibited by the _spirogyra_. the delicate threads which, in this low type of alga, are constituted of single elongated cells joined end to end, are here and there adjacent to one another; and from a cell of one thread and a cell of another at fit distance, grow out prominences which, meeting in the interspace and forming a channel by the dissolution of their adjoined cell-walls, empty through it the endochrome of the one cell into the other: forming by fusion of the two a zygote or reproductive body. under what influence is this action initiated and guided? there is no conceivable directive agency in either cell by which, when conditions are fit, a papilla is so formed as to meet an opposite papilla. or again, contemplate the still more marvellous transformation occurring in _hydrodictyon utriculosum_. united with others to form a cylindrical network, each sausage-shaped cell of this alga contains, when fully developed, a lining chromatophore made of nucleated protoplasm with immersed chlorophyll-grains. this, when the cell is adult, divides into multitudinous zoospores, which presently join their ends in such ways as to form a network with meshes mostly hexagonal, minute in size, but like in arrangement to the network of which the parent cell formed a part. eventually escaping from the mother-cell, this network grows and presently becomes as large as the parent network. under what play of forces do these zoospores arrange themselves into this strange structure? kindred insoluble problems are presented by animal organisms of all grades. of microscopic types instance the coccospheres and rhabdospheres found in the upper strata of sea-water. each is a fragment of protoplasm less than one-thousandth of an inch in diameter, shielded by the elaborate protective structures it has formed. the elliptic coccoliths of the first, severally having a definite pattern, unite to form by overlapping an imbricated covering; and of the other the covering consists of numerous trumpet-mouthed processes radiating on all sides. to the question--how does this particle of granular protoplasm, without organs or definite structure, make for itself this complicated calcareous armour? there is no conceivable answer. like these _protozoa_, the lowest _metazoa_ do things which are quite incomprehensible. here is a sponge formed of classes of monads having among them no internuncial appliances by which in higher types cooperation is carried on--flagellate cells that produce the permeating currents of water, flattened cells forming protective membranes, and amoeboid cells lying free in the gelatinous mesoderm. these, without apparent concert, build up not only the horny network constituting the chief mass of their habitation, but also embodied spicules, having remarkable symmetrical forms. by what combined influences the needful processes are effected, it is impossible to imagine. if we turn to higher types of _metazoa_ in which, by the agency of a nervous system, many cooperations of parts are achieved in ways that are superficially comprehensible, we still meet with various actions of which the causation cannot be represented in thought. lacking other calcareous matter, a hen picks up and swallows bits of broken egg-shells; and, occasionally, a cow in calf may be seen mumbling a bone she has found--evidently scraping off with her teeth some of its mass. these proceedings have reference to constitutional needs; but how are they prompted? what generates in the cow a desire to bite a substance so unlike in character to her ordinary food? if it be replied that the blood has become poor in certain calcareous salts and that hence arises the appetite for things containing them, there remains the question--how does this deficiency so act on the nervous system as to generate this vague desire and cause the movements which satisfy it? by no effort can we figure to ourselves the implied causal processes. in brief, then, we are obliged to confess that life in its essence cannot be conceived in physico-chemical terms. the required principle of activity, which we found cannot be represented as an independent vital principle, we now find cannot be represented as a principle inherent in living matter. if, by assuming its inherence, we think the facts are accounted for, we do but cheat ourselves with pseud-ideas. § e. what then are we to say--what are we to think? simply that in this direction, as in all other directions, our explanations finally bring us face to face with the inexplicable. the ultimate reality behind this manifestation, as behind all other manifestations, transcends conception. it needs but to observe how even simple forms of existence are in their ultimate natures incomprehensible, to see that this most complex form of existence is in a sense doubly incomprehensible. for the actions of that which the ignorant contemptuously call brute matter, cannot in the last resort be understood in their genesis. were it not that familiarity blinds us, the fall of a stone would afford matter for wonder. neither newton nor anyone since his day has been able to conceive how the molecules of matter in the stone are affected not only by the molecules of matter in the adjacent part of the earth but by those forming parts of its mass , miles off which severally exercise their influence without impediment from intervening molecules; and still less has there been any conceivable interpretation of the mode in which every molecule of matter in the sun, millions of miles away, has a share in controlling the movements of the earth. what goes on in the space between a magnet and the piece of iron drawn towards it, or how on repeatedly passing a magnet along a steel needle this, by some change of molecular state as we must suppose, becomes itself a magnet and when balanced places its poles in fixed directions, we do not know. and still less can we fathom the physical process by which an ordered series of electric pulses sent through a telegraph wire may be made to excite a corresponding series of pulses in a parallel wire many miles off. turn to another class of cases. consider the action of a surface of glass struck by a cathode current and which thereupon generates an order of rays able to pass through solid matters impermeable to light. or contemplate the power possessed by uranium and other metals of emitting rays imperceptible by our eyes as light but which yet, in what appears to us absolute darkness, will, if passed through a camera, produce photographs. even the actions of one kind of matter on another are sufficiently remarkable. here is a mass of gold which, after the addition of - th part of bismuth, has only - th of the tensile strength it previously had; and here is a mass of brass, ordinarily ductile and malleable, but which, on the addition of - , th part of antimony, loses its character. more remarkable still are the influences of certain medicines. one-hundredth of a grain of nitro-glycerine is a sufficient dose. taking an average man's weight as pounds, it results that his body is appreciably affected in its state by the -millionth part of its weight of this nitrogenous compound. in presence of such powers displayed by matter of simple kinds we shall see how impossible it is even to imagine those processes going on in organic matter out of which emerges the dynamic element in life. as no separate form of proteid possesses vitality, we seem obliged to assume that the molecule of protoplasm contains many molecules of proteids, probably in various isomeric states, all capable of ready change and therefore producing great instability of the aggregate they form. as before pointed out (§ ), a proteid-molecule includes more than equivalents of several so-called elements. each of these undecomposed substances is now recognized by chemists as almost certainly consisting of several kinds of components. hence the implication is that a proteid-molecule contains thousands of units, of which the different classes have their respective rates of inconceivably rapid oscillation, while each unit, receiving and emitting ethereal undulations, affects others of its kind in its own and adjacent molecules: an immensely complex structure having immensely complex activities. and this complexity, material and dynamic, in the proteid-molecule we must regard as raised to a far higher degree in the unit of protoplasm. here as elsewhere alternative impossibilities of thought present themselves. we find it impossible to think of life as imported into the unit of protoplasm from without; and yet we find it impossible to conceive it as emerging from the cooperation of the components. § f. but now, having confessed that life as a principle of activity is unknown and unknowable--that while its phenomena are accessible to thought the implied noumenon is inaccessible--that only the manifestations come within the range of our intelligence while that which is manifested lies beyond it; we may resume the conclusions reached in the preceding chapters. our surface knowledge continues to be a knowledge valid of its kind, after recognizing the truth that it is only a surface knowledge. for the conclusions we lately reached and the definition emerging from them, concern the _order_ existing among the actions which living things exhibit; and this order remains the same whether we know or do not know the nature of that from which the actions originate. we found a distinguishing trait of life to be that its changes display a correspondence with co-existences and sequences in the environment; and this remains a distinguishing trait, though the thing which changes remains inscrutable. the statement that the continuous adjustment of internal relations to external relations constitutes life as cognizable by us, is not invalidated by the admission that the reality in which these relations inhere is incognizable. hence, then, after duly recognizing the fact that, as pointed out above, life, even phenomenally considered, is not entirely covered by the definition, since there are various abnormal manifestations of life which it does not include, we may safely accept it as covering the normal manifestations--those manifestations which here concern us. carrying with us the definition, therefore we may hereafter use it for guidance through all those regions of inquiry upon which we now enter. chapter vii. the scope of biology. § . as ordinarily conceived, the science of biology falls into two great divisions, the one dealing with animal life, called zoology, and the other dealing with vegetal life, called botany, or more properly to be called phytology. but convenient as is this division, it is not that which arises if we follow the scientific method of including in one group all the phenomena of fundamentally the same order and putting separately in another group all the phenomena of a fundamentally different order. for animals and plants are alike in having structures; and animals and plants are alike in having functions performed by these structures; and the distinction between structures and functions transcends the difference between any one structure and any other or between any one function and any other--is, indeed, an absolute distinction, like that between matter and motion. recognizing, then, the logic of the division thus indicated, we must group the parts of biology thus:-- . an account of the structural phenomena presented by organisms. this subdivides into:-- _a._ the established structural phenomena presented by individual organisms. _b._ the changing structural phenomena presented by successions of organisms. . an account of the functional phenomena which organisms present. this, too, admits of subdivision into:-- _a._ the established functional phenomena of individual organisms. _b._ the changing functional phenomena of successions of organisms. . an account of the actions of structures on functions and the re-actions of functions on structures. like the others, this is divisible into:-- _a._ the actions and re-actions as exhibited in individual organisms. _b._ the actions and re-actions as exhibited in successions of organisms. . an account of the phenomena attending the production of successions of organisms: in other words--the phenomena of genesis. of course, for purposes of exploration and teaching, the division into zoology and botany, founded on contrasts so marked and numerous, must always be retained. but here recognizing this familiar distinction only as much as convenience obliges us to do, let us now pass on to consider, more in detail, the classification of biologic phenomena above set down in its leading outlines. § . the facts of structure shown in an individual organism, are of two chief kinds. in order of conspicuousness, though not in order of time, there come first those arrangements of parts which characterize the mature organism; an account of which, originally called anatomy, is now called morphology. then come those successive modifications through which the organism passes in its progress from the germ to the developed form; an account of which is called embryology. the structural changes which any series of individual organisms exhibits, admit of similar classification. on the one hand, we have those inner and outer differences of shape, that arise between the adult members of successive generations descended from a common stock--differences which, though usually not marked between adjacent generations, become great in course of multitudinous generations. on the other hand, we have those developmental modifications, seen in the embryos, through which such modifications of the descended forms are reached. interpretation of the structures of individual organisms and successions of organisms, is aided by two subsidiary divisions of biologic inquiry, named comparative anatomy (properly comparative morphology) and comparative embryology. these cannot be regarded as in themselves parts of biology; since the facts embraced under them are not substantive phenomena, but are simply incidental to substantive phenomena. all the truths of structural biology are comprehended under the two foregoing subdivisions; and the comparison of these truths as presented in different classes of organisms, is simply a _method_ of interpreting them. nevertheless, though comparative morphology and comparative embryology do not disclose additional concrete facts, they lead to the establishment of certain abstract facts. by them it is made manifest that underneath the superficial differences of groups and classes and types of organisms, there are hidden fundamental similarities; and that the courses of development in such groups and classes and types, though in many respects divergent, are in some essential respects, coincident. the wide truths thus disclosed, come under the heads of general morphology and general embryology. by contrasting organisms there is also achieved that grouping of the like and separation of the unlike, called classification. first by observation of external characters; second by observation of internal characters; and third by observation of the phases of development; it is ascertained what organisms are most similar in all respects; what organisms otherwise unlike are like in important traits; what organisms though apparently unallied have common primordial characters. whence there results such an arrangement of organisms, that if certain structural attributes of any one be given, its other structural attributes may be _empirically_ predicted; and which prepares the way for that interpretation of their relations and genesis, which forms an important part of _rational_ biology. § . the second main division of biology, above described as embracing the functional phenomena of organisms, is that which is in part signified by physiology: the remainder being distinguishable as objective psychology. both of these fall into subdivisions that may best be treated separately. that part of physiology which is concerned with the molecular changes going on in organisms, is known as organic chemistry. an account of the modes in which the force generated in organisms by chemical change, is transformed into other forces, and made to work the various organs that carry on the functions of life, comes under the head of organic physics. psychology, which is mainly concerned with the adjustment of vital actions to actions in the environment (in contrast with physiology, which is mainly concerned with vital actions apart from actions in the environment) consists of two quite distinct portions. objective psychology deals with those functions of the nervo-muscular apparatus by which such organisms as possess it are enabled to adjust inner to outer relations; and includes also the study of the same functions as externally manifested in conduct. subjective psychology deals with the sensations, perceptions, ideas, emotions, and volitions that are the direct or indirect concomitants of this visible adjustment of inner to outer relations. consciousness under its different modes and forms, being a subject-matter radically distinct in nature from the subject-matter of biology in general; and the method of self-analysis, by which alone the laws of dependence among changes of consciousness can be found, being a method unparalleled by anything in the rest of biology; we are obliged to regard subjective psychology as a separate study. and since it would be very inconvenient wholly to dissociate objective psychology from subjective psychology, we are practically compelled to deal with the two as forming an independent science. obviously, the functional phenomena presented in successions of organisms, similarly divide into physiological and psychological. under the physiological come the modifications of bodily actions that arise in the course of generations, as concomitants of structural modifications; and these may be modifications, qualitative or quantitative, in the molecular changes classed as chemical, or in the organic actions classed as physical, or in both. under the psychological come the qualitative and quantitative modifications of instincts, feelings, conceptions, and mental processes in general, which occur in creatures having more or less intelligence, when certain of their conditions are changed. this, like the preceding department of psychology, has in the abstract two different aspects--the objective and the subjective. practically, however, the objective, which deals with these mental modifications as exhibited in the changing habits and abilities of successive generations of creatures, is the only one admitting of investigation; since the corresponding alterations in consciousness cannot be immediately known to any but the subjects of them. evidently, convenience requires us to join this part of psychology along with the other parts as components of a distinct sub-science. light is thrown on functions, as well as on structures, by comparing organisms of different kinds. comparative physiology and comparative psychology, are the names given to those collections of facts respecting the homologies and analogies, bodily and mental, disclosed by this kind of inquiry. these classified observations concerning likenesses and differences of functions, are helpers to interpret functions in their essential natures and relations. hence comparative physiology and comparative psychology are names of methods rather than names of true subdivisions of biology. here, however, as before, comparison of special truths, besides facilitating their interpretation, brings to light certain general truths. contrasting functions bodily and mental as exhibited in various kinds of organisms, shows that there exists, more or less extensively, a community of processes and methods. hence result two groups of propositions constituting general physiology and general psychology. § . in these divisions and subdivisions of the first two great departments of biology, facts of structure are considered separately from facts of function, so far as separate treatment of them is possible. the third great department of biology deals with them in their necessary connexions. it comprehends the determination of functions by structures, and the determination of structures by functions. as displayed in individual organisms, the effects of structures on functions are to be studied, not only in the broad fact that the general kind of life an organism leads is necessitated by the main characters of its organization, but in the more special and less conspicuous fact, that between members of the same species, minor differences of structure lead to minor differences of power to perform certain actions, and of tendencies to perform such actions. conversely, under the reactions of functions on structures in individual organisms, come the facts showing that functions, when fulfilled to their normal extents, maintain integrity of structure in their respective organs; and that within certain limits increases of functions are followed by such structural changes in their respective organs, as enable them to discharge better their extra functions. inquiry into the influence of structure on function as seen in successions of organisms, introduces us to such phenomena as mr. darwin's _origin of species_ deals with. in this category come all proofs of the general truth, that when an individual is enabled by a certain structural peculiarity to perform better than others of its species some advantageous action; and when it bequeaths more or less of its structural peculiarity to descendants, among whom those which have it most markedly are best able to thrive and propagate; there arises a visibly modified type of structure, having a more or less distinct function. in the correlative class of facts (by some asserted and by others denied), which come under the category of reactions of function on structure as exhibited in successions of organisms, are to be placed all those modifications of structure which arise in races, when changes of conditions entail changes in the balance of their functions--when altered function externally necessitated, produces altered structure, and continues doing this through successive generations. § . the fourth great division of biology, comprehending the phenomena of genesis, may be conveniently separated into three subdivisions. under the first, comes a description of all the special modes whereby the multiplication of organisms is carried on; which modes range themselves under the two chief heads of sexual and asexual. an account of sexual multiplication includes the various processes by which germs and ova are fertilized, and by which, after fertilization, they are furnished with the materials, and maintained in the conditions, needful for their development. an account of asexual multiplication includes the various processes by which, from the same fertilized germ or ovum, there are produced many organisms partially or totally independent of one another. the second of these subdivisions deals with the phenomena of genesis in the abstract. it takes for its subject-matter such general questions as--what is the end subserved by the union of sperm-cell and germ-cell? why cannot all multiplication be carried on after the asexual method? what are the laws of hereditary transmission? what are the causes of variation? the third subdivision is devoted to still more abstract aspects of the subject. recognizing the general facts of multiplication, without reference to their modes or immediate causes, it concerns itself simply with the different rates of multiplication in different kinds of organisms and different individuals of the same kind. generalizing the numerous contrasts and variations of fertility, it seeks a rationale of them in their relations to other organic phenomena. § . such appears to be the natural arrangement of divisions and subdivisions which biology presents. it is, however, a classification of the parts of the science when fully developed; rather than a classification of them as they now stand. some of the subdivisions above named have no recognized existence, and some of the others are in quite rudimentary states. it is impossible now to fill in, even in the roughest way, more than a part of the outlines here sketched. our course of inquiry being thus in great measure determined by the present state of knowledge, we are compelled to follow an order widely different from this ideal one. it will be necessary first to give an account of those empirical generalizations which naturalists and physiologists have established: appending to those which admit of it, such deductive interpretations as _first principles_ furnishes us with. having done this, we shall be the better prepared for dealing with the leading truths of biology in connexion with the doctrine of evolution. part ii. the inductions of biology. chapter i. growth. § . perhaps the widest and most familiar induction of biology, is that organisms grow. while, however, this is a characteristic so uniformly and markedly displayed by plants and animals, as to be carelessly thought peculiar to them, it is really not so. under appropriate conditions, increase of size takes place in inorganic aggregates, as well as in organic aggregates. crystals grow; and often far more rapidly than living bodies. where the requisite materials are supplied in the requisite forms, growth may be witnessed in non-crystalline masses: instance the fungous-like accumulation of carbon that takes place on the wick of an unsnuffed candle. on an immensely larger scale, we have growth in geologic formations: the slow accumulation of deposited sediment into a stratum, is not distinguishable from growth in its widest acceptation. and if we go back to the genesis of celestial bodies, assuming them to have arisen by evolution, these, too, must have gradually passed into their concrete shapes through processes of growth. growth is, indeed, as being an integration of matter, the primary trait of evolution; and if evolution of one kind or other is universal, growth is universal--universal, that is, in the sense that all aggregates display it in some way at some period. the essential community of nature between organic growth and inorganic growth, is, however, most clearly seen on observing that they both result in the same way. the segregation of different kinds of detritus from each other, as well as from the water carrying them, and their aggregation into distinct strata, is but an instance of a universal tendency towards the union of like units and the parting of unlike units (_first principles_, § ). the deposit of a crystal from a solution is a differentiation of the previously mixed molecules; and an integration of one class of molecules into a solid body, and the other class into a liquid solvent. is not the growth of an organism an essentially similar process? around a plant there exist certain elements like the elements which form its substance; and its increase of size is effected by continually integrating these surrounding like elements with itself. nor does the animal fundamentally differ in this respect from the plant or the crystal. its food is a portion of the environing matter that contains some compound atoms like some of the compound atoms constituting its tissues; and either through simple imbibition or through digestion, the animal eventually integrates with itself, units like those of which it is built up, and leaves behind the unlike units. to prevent misconception, it may be well to point out that growth, as here defined, must be distinguished from certain apparent and real augmentations of bulk which simulate it. thus, the long, white potato-shoots thrown out in the dark, are produced at the expense of the substances which the tuber contains: they illustrate not the accumulation of organic matter, but simply its re-composition and re-arrangement. certain animal-embryos, again, during their early stages, increase considerably in size without assimilating any solids from the environment; and they do this by absorbing the surrounding water. even in the highest organisms, as in children, there appears sometimes to occur a rapid gain in dimensions which does not truly measure the added quantity of organic matter; but is in part due to changes analogous to those just named. alterations of this kind must not be confounded with that growth, properly so called, of which we have here to treat. the next general fact to be noted respecting organic growth, is, that it has limits. here there appears to be a distinction between organic and inorganic growth; but this distinction is by no means definite. though that aggregation of inanimate matter which simple attraction produces, may go on without end; yet there appears to be an end to that more definite kind of aggregation which results from polar attraction. different elements and compounds habitually form crystals more or less unlike in their sizes; and each seems to have a size that is not usually exceeded without a tendency arising to form new crystals rather than to increase the old. on looking at the organic kingdom as a whole, we see that the limits between which growth ranges are very wide apart. at the one extreme we have monads so minute as to be rendered but imperfectly visible by microscopes of the highest power; and at the other extreme we have trees of to feet high and animals of feet long. it is true that though in one sense this contrast may be legitimately drawn, yet in another sense it may not; since these largest organisms arise by the combination of units which are individually like the smallest. a single plant of the genus _protococcus_, is of the same essential structure as one of the many cells united to form the thallus of some higher alga, or the leaf of a phænogam. each separate shoot of a phænogam is usually the bearer of many leaves. and a tree is an assemblage of numerous united shoots. one of these great teleophytes is thus an aggregate of aggregates of aggregates of units, which severally resemble protophytes in their sizes and structures; and a like building up is traceable throughout a considerable part of the animal kingdom. even, however, when we bear in mind this qualification, and make our comparisons between organisms of the same degree of composition, we still find the limit of growth to have a great range. the smallest branched flowering plant is extremely insignificant by the side of a forest tree; and there is an enormous difference in bulk between the least and the greatest mammal. but on comparing members of the same species, we discover the limit of growth to be much less variable. among the _protozoa_ and _protophyta_, each kind has a tolerably constant adult size; and among the most complex organisms the differences between those of the same kind which have reached maturity, are usually not very great. the compound plants do, indeed, sometimes present marked contrasts between stunted and well-grown individuals; but the higher animals diverge but inconsiderably from the average standards of their species. on surveying the facts with a view of empirically generalizing the causes of these differences, we are soon made aware that by variously combining and conflicting with one another, these causes produce great irregularities of result. it becomes manifest that no one of them can be traced to its consequences, unqualified by the rest. hence the several statements contained in the following paragraphs must be taken as subject to mutual modification. let us consider first the connexion between degree of growth and complexity of structure. this connexion, being involved with many others, becomes apparent only on so averaging the comparisons as to eliminate differences among the rest. nor does it hold at all where the conditions are radically dissimilar, as between plants and animals. but bearing in mind these qualifications, we shall see that organization has a determining influence on increase of mass. of plants the lowest, classed as thallophytes, usually attain no considerable size. algæ, fungi, and the lichens formed by association of them count among their numbers but few bulky species: the largest, such as certain algæ found in antarctic seas, not serving greatly to raise the average; and these gigantic seaweeds possess a considerable complexity of histological organization very markedly exceeding that of their smaller allies. though among bryophytes and pteridophytes there are some, as the tree-ferns, which attain a considerable height, the majority are but of humble growth. the monocotyledons, including at one extreme small grasses and at the other tall palms, show us an average and a maximum greater than that reached by the pteridophytes. and the monocotyledons are exceeded by the dicotyledons; among which are found the monarchs of the vegetal kingdom. passing to animals, we meet the fact that the size attained by _vertebrata_ is usually much greater than the size attained by _invertebrata_. of invertebrate animals the smallest, classed as _protozoa_, are also the simplest; and the largest, belonging to the _annulosa_ and _mollusca_, are among the most complex of their respective types. of vertebrate animals we see that the greatest are mammals, and that though, in past epochs, there were reptiles of vast bulks, their bulks did not equal that of the whale: the great dinosaurs, though as long, being nothing like as massive. between reptiles and birds, and between land-vertebrates and water-vertebrates, the relation does not hold: the conditions of existence being in these cases widely different. but among fishes as a class, and among reptiles as a class, it is observable that, speaking generally, the larger species are framed on the higher types. the critical reader, who has mentally checked these statements in passing them, has doubtless already seen that this relation is not a dependence of organization on growth but a dependence of growth on organization. the majority of dicotyledons are smaller than some monocotyledons; many monocotyledons are exceeded in size by certain pteridophytes; and even among thallophytes, the least developed among compound plants, there are kinds of a size which many plants of the highest order do not reach. similarly among animals. there are plenty of crustaceans less than _actiniæ_; numerous reptiles are smaller than some fish; the majority of mammals are inferior in bulk to the largest reptiles; and in the contrast between a mouse and a well-grown _medusa_, we see a creature that is elevated in type of structure exceeded in mass by one that is extremely low. clearly then, it cannot be held that high organization is habitually accompanied by great size. the proposition here illustrated is the converse one, that great size is habitually accompanied by high organization. the conspicuous facts that the largest species of both animals and vegetals belong to the highest classes, and that throughout their various sub-classes the higher usually contain the more bulky forms, show this connexion as clearly as we can expect it to be shown, amid so many modifying causes and conditions. the relation between growth and supply of available nutriment, is too familiar a relation to need proving. there are, however, some aspects of it that must be contemplated before its implications can be fully appreciated. among plants, which are all constantly in contact with the gaseous, liquid, and solid matters to be incorporated with their tissues, and which, in the same locality, receive not very unlike amounts of light and heat, differences in the supplies of available nutriment have but a subordinate connexion with differences of growth. though in a cluster of herbs springing up from the seeds let fall by a parent, the greater sizes of some than of others is doubtless due to better nutrition, consequent on accidental advantages; yet no such interpretation can be given of the contrast in size between these herbs and an adjacent tree. other conditions here come into play: one of the most important being, an absence in the one case, and presence in the other, of an ability to secrete such a quantity of ligneous fibre as will produce a stem capable of supporting a large growth. among animals, however, which (excepting some _entozoa_) differ from plants in this, that instead of bathing their surfaces the matters they subsist on are dispersed, and have to be obtained, the relation between available food and growth is shown with more regularity. the _protozoa_, living on microscopic fragments of organic matter contained in the surrounding water, are unable, during their brief lives, to accumulate any considerable quantity of nutriment. _polyzoa_, having for food these scarcely visible members of the animal kingdom, are, though large compared with their prey, small as measured by other standards; even when aggregated into groups of many individuals, which severally catch food for the common weal, they are often so inconspicuous as readily to be passed over by the unobservant. and if from this point upwards we survey the successive grades of animals, it becomes manifest that, in proportion as the size is great, the masses of nutriment are either large, or, what is practically the same thing, are so abundant and so grouped that large quantities may be readily taken in. though, for example, the greatest of mammals, the arctic whale, feeds on such comparatively small creatures as the acalephes and molluscs floating in the seas it inhabits, its method of gulping in whole shoals of them and filtering away the accompanying water, enables it to secure great quantities of food. we may then with safety say that, other things equal, the growth of an animal depends on the abundance and sizes of the masses of nutriment which its powers enable it to appropriate. perhaps it may be needful to add that, in interpreting this statement, the proportion of competitors must be taken into account. clearly, not the absolute, but the relative, abundance of fit food is the point; and this relative abundance very much depends on the number of individuals competing for the food. thus all who have had experience in fishing in highland lochs, know that where the trout are numerous they are small, and that where they are comparatively large they are comparatively few. what is the relation between growth and expenditure of energy? is a question which next presents itself. though there is reason to believe such a relation exists, it is not very readily traced: involved as it is with so many other relations. some contrasts, however, may be pointed out that appear to give evidence of it. passing over the vegetal kingdom, throughout which the expenditure of force is too small to allow of such a relation being visible, let us seek in the animal kingdom, some case where classes otherwise allied, are contrasted in their locomotive activities. let us compare birds on the one hand, with reptiles and mammals on the other. it is an accepted doctrine that birds are organized on a type closely allied to the reptilian type, but superior to it; and though in some respects the organization of birds is inferior to that of mammals, yet in other respects, as in the greater heterogeneity and integration of the skeleton, the more complex development of the respiratory system, and the higher temperature of the blood, it may be held that birds stand above mammals. hence were growth dependent only on organization, we might infer that the limit of growth among birds should not be much short of that among mammals; and that the bird-type should admit of a larger growth than the reptile-type. again, we see no manifest disadvantages under which birds labour in obtaining food, but from which reptiles and mammals are free. on the contrary, birds are able to get at food that is fixed beyond the reach of reptiles and mammals; and can catch food that is too swift of movement to be ordinarily caught by reptiles and mammals. nevertheless, the limit of growth in birds falls far below that reached by reptiles and mammals. with what other contrast between these classes, is this contrast connected? may we not suspect that it is connected (partially though not wholly) with the contrast between their amounts of locomotive exertion? whereas mammals (excepting bats, which are small), are during all their movements supported by solid surfaces or dense liquids; and whereas reptiles (excepting the ancient pterodactyles, which were not very large), are similarly restricted in their spheres of movement; the majority of birds move more or less habitually through a rare medium, in which they cannot support themselves without relatively great efforts. and this general fact may be joined with the special fact, that those members of the class _aves_, as the _dinornis_ and _epiornis_, which approached in size to the larger _mammalia_ and _reptilia_, were creatures incapable of flight--creatures which did not expend this excess of force in locomotion. but as implied above, and as will presently be shown, another factor of importance comes into play; so that perhaps the safest evidence that there is an antagonism between the increase of bulk and the quantity of motion evolved is that supplied by the general experience, that human beings and domestic animals, when overworked while growing, are prevented from attaining the ordinary dimensions. one other general truth concerning degrees of growth, must be set down. it is a rule, having exceptions of no great importance, that large organisms commence their separate existences as masses of organic matter more or less considerable in size, and commonly with organizations more or less advanced; and that throughout each organic sub-kingdom, there is a certain general, though irregular, relation between the initial and the final bulks. vegetals exhibit this relation less manifestly than animals. yet though, among the plants that begin life as minute spores, there are some which, by the aid of an intermediate form, grow to large sizes, the immense majority of them remain small. while, conversely, the great monocotyledons and dicotyledons, when thrown off from their parents, have already the formed organs of young plants, to which are attached stores of highly nutritive matter. that is to say, where the young plant consists merely of a centre of development, the ultimate growth is commonly insignificant; but where the growth is to become great, there exists to start with, a developed embryo and a stock of assimilable matter. throughout the animal kingdom this relation is tolerably manifest though by no means uniform. save among classes that escape the ordinary requirements of animal life, small germs or eggs do not in most cases give rise to bulky creatures. where great bulk is to be reached, the young proceeds from an egg of considerable bulk, or is born of considerable bulk ready-organized and partially active. in the class fishes, or in such of them as are subject to similar conditions of life, some proportion usually obtains between the sizes of the ova and the sizes of the adult individuals; though in the cases of the sturgeon and the tunny there are exceptions, probably determined by the circumstances of oviposition and those of juvenile life. reptiles have eggs that are smaller in number, and relatively greater in mass, than those of fishes; and throughout this class, too, there is a general congruity between the bulk of the egg and the bulk of the adult creature. as a group, birds show us further limitations in the numbers of their eggs as well as farther increase in their relative sizes; and from the minute eggs of the humming-bird up to the immense ones of the _epiornis_, holding several quarts, we see that, speaking generally, the greater the eggs the greater the birds., finally, among mammals (omitting the marsupials) the young are born, not only of comparatively large sizes, but with advanced organizations; and throughout this sub-division of the _vertebrata_, as throughout the others, there is a manifest connexion between the sizes at birth and the sizes at maturity. as having a kindred meaning, there must finally be noted the fact that the young of these highest animals, besides starting in life with bodies of considerable sizes, almost fully organized, are, during subsequent periods of greater or less length, supplied with nutriment--in birds by feeding and in mammals by suckling and afterwards by feeding. so that beyond the mass and organization directly bequeathed, a bird or mammal obtains a further large mass at but little cost to itself. were exhaustive treatment of the topic intended, it would be needful to give a paragraph to each of the incidental circumstances by which growth may be aided or restricted:--such facts as that an entozoon is limited by the size of the creature, or even the organ, in which it thrives; that an epizoon, though getting abundant nutriment without appreciable exertion, is restricted to that small bulk at which it escapes ready detection by the animal it infests; that sometimes, as in the weazel, smallness is a condition to successful pursuit of the animals preyed upon; and that in some cases, the advantage of resembling certain other creatures, and so deceiving enemies or prey, becomes an indirect cause of restricted size. but the present purpose is simply to set down those most general relations between growth and other organic traits, which induction leads us to. having done this, let us go on to inquire whether these general relations can be deductively established. § . that there must exist a certain dependence of growth on organization, may be shown _a priori_. when we consider the phenomena of life, either by themselves or in their relations to surrounding phenomena, we see that, other things equal, the larger the aggregate the greater is the needful complexity of structure. in plants, even of the highest type, there is a comparatively small mutual dependence of parts: a gathered flower-bud will unfold and flourish for days if its stem be immersed in water; and a shoot cut off from its parent-tree and stuck in the ground will grow. the respective parts having vital activities that are not widely unlike, it is possible for great bulk to be reached without that structural complexity required for combining the actions of parts. even here, however, we see that for the attainment of great bulk there requires such a degree of organization as shall co-ordinate the functions of roots and branches--we see that such a size as is reached by trees, is not possible without a vascular system enabling the remote organs to utilize each other's products. and we see that such a co-existence of large growth with comparatively low organization as occurs in some of the marine _algæ_, occurs where the conditions of existence do not necessitate any considerable mutual dependence of parts--where the near approach of the plant to its medium in specific gravity precludes the need of a well-developed stem, and where all the materials of growth being derived from the water by each portion of the thallus, there requires no apparatus for transferring the crude food materials from part to part. among animals which, with but few exceptions, are, by the conditions of their existence, required to absorb nutriment through one specialized part of the body, it is clear that there must be a means whereby other parts of the body, to be supported by this nutriment, must have it conveyed to them. it is clear that for an equally efficient maintenance of their nutrition, the parts of a large mass must have a more elaborate propelling and conducting apparatus; and that in proportion as these parts undergo greater waste, a yet higher development of the vascular system is necessitated. similarly with the prerequisites to those mechanical motions which animals are required to perform. the parts of a mass cannot be made to move, and have their movements so co-ordinated as to produce locomotive and other actions, without certain structural arrangements; and, other things equal, a given amount of such activity requires more involved structural arrangements in a large mass than in a small one. there must at least be a co-ordinating apparatus presenting greater contrasts in its central and peripheral parts. the qualified dependence of growth on organization, is equally implied when we study it in connexion with that adjustment of inner to outer relations which constitutes life as phenomenally known to us. in plants this is less striking than in animals, because the adjustment of inner to outer relations does not involve conspicuous motions. still, it is visible in the fact that the condition on which alone a plant can grow to a great size, is, that it shall, by the development of a massive trunk, present inner relations of forces fitted to counterbalance those outer relations of forces which tend continually, and others which tend occasionally, to overthrow it; and this formation of a core of regularly-arranged woody fibres is an advance in organization. throughout the animal kingdom this connexion of phenomena is manifest. to obtain materials for growth; to avoid injuries which interfere with growth; and to escape those enemies which bring growth to a sudden end; implies in the organism the means of fitting its movements to meet numerous external co-existences and sequences--implies such various structural arrangements as shall make possible these variously-adapted actions. it cannot be questioned that, everything else remaining constant, a more complex animal, capable of adjusting its conduct to a greater number of surrounding contingencies, will be the better able to secure food and evade damage, and so to increase bulk. and evidently, without any qualification, we may say that a large animal, living under such complex conditions of existence as everywhere obtain, is not possible without comparatively high organization. while, then, this relation is traversed and obscured by sundry other relations, it cannot but exist. deductively we see that it must be modified, as inductively we saw that it is modified, by the circumstances amid which each kind of organism is placed, but that it is always a factor in determining the result. § . that growth is, _cæteris paribus_, dependent on the supply of assimilable matter, is a proposition so continually illustrated by special experience, as well as so obvious from general experience, that it would scarcely need stating, were it not requisite to notice the qualifications with which it must be taken. the materials which each organism requires for building itself up, are not of one kind but of several kinds. as a vehicle for transferring matter through their structures, all organisms require water as well as solid constituents; and however abundant the solid constituents there can be no growth in the absence of water. among the solids supplied, there must be a proportion ranging within certain limits. a plant round which carbonic acid, water, and ammonia exist in the right quantities, may yet be arrested in its growth by a deficiency of potassium. the total absence of lime from its food may stop the formation of a mammal's skeleton: thus dwarfing, if not eventually destroying, the mammal; and this no matter what quantities of other needful colloids and crystalloids are furnished. again, the truth that, other things equal, growth varies according to the supply of nutriment, has to be qualified by the condition that the supply shall not exceed the ability to appropriate it. in the vegetal kingdom, the assimilating surface being external and admitting of rapid expansion by the formation of new roots, shoots, and leaves, the effect of this limitation is not conspicuous. by artificially supplying plants with those materials which they have usually the most difficulty in obtaining, we can greatly facilitate their growth; and so can produce striking differences of size in the same species. even here, however, the effect is confined within the limits of the ability to appropriate; since in the absence of that solar light and heat by the help of which the chief appropriation is carried on, the additional materials for growth are useless. in the animal kingdom this restriction is rigorous. the absorbent surface being, in the great majority of cases, internal; having a comparatively small area, which cannot be greatly enlarged without reconstruction of the whole body; and being in connexion with a vascular system which also must be re-constructed before any considerable increase of nutriment can be made available; it is clear that beyond a certain point, very soon reached, increase of nutriment will not cause increase of growth. on the contrary, if the quantity of food taken in is greatly beyond the digestive and absorbent power, the excess, becoming an obstacle to the regular working of the organism, may retard growth rather than advance it. while then it is certain, _a priori_, that there cannot be growth in the absence of such substances as those of which an organism consists; and while it is equally certain that the amount of growth must primarily be governed by the supply of these substances; it is not less certain that extra supply will not produce extra growth, beyond a point very soon reached. deduction shows to be necessary, as induction makes familiar, the truths that the value of food for purposes of growth depends not on the quantity of the various organizable materials it contains, but on the quantity of the material most needed; that given a right proportion of materials, the pre-existing structure of the organism limits their availability; and that the higher the structure, the sooner is this limit reached. § . but why should the growth of every organism be finally arrested? though the rate of increase may, in each case, be necessarily restricted within a narrow range of variation--though the increment that is possible in a given time, cannot exceed a certain amount; yet why should the increments decrease and finally become insensible? why should not all organisms, when supplied with sufficient materials, continue to grow as long as they live? to find an answer to this question we must revert to the nature and functions of organic matter. in the first three chapters of part i, it was shown that plants and animals mainly consist of substances in states of unstable equilibrium--substances which have been raised to this unstable equilibrium by the expenditure of the forces we know as solar radiations, and which give out these forces in other forms on falling into states of stable equilibrium. leaving out the water, which serves as a vehicle for these materials and a medium for their changes; and excluding those mineral matters that play either passive or subsidiary parts; organisms are built up of compounds which are stores of force. thus complex colloids and crystalloids which, as united together, form organized bodies, are the same colloids and crystalloids which give out, on their decomposition, the forces expended by organized bodies. thus these nitrogenous and carbonaceous substances, being at once the materials for organic growth and the sources of organic energy, it results that as much of them as is used up for the genesis of energy is taken away from the means of growth, and as much as is economized by diminishing the genesis of energy, is available for growth. given that limited quantity of nutritive matter which the pre-existing structure of an organism enables it to absorb; and it is a necessary corollary from the persistence of force, that the matter accumulated as growth cannot exceed that surplus which remains undecomposed after the production of the required amounts of sensible and insensible motion. this, which would be rigorously true under all conditions if exactly the same substances were used in exactly the same proportions for the production of force and for the formation of tissue, requires, however, to be taken with the qualification that some of the force-evolving substances are not constituents of tissue; and that thus there may be a genesis of force which is not at the expense of potential growth. but since organisms (or at least animal organisms, with which we are here chiefly concerned) have a certain power of selective absorption, which, partially in an individual and more completely in a race, adapts the proportions of the substances absorbed to the needs of the system; then if a certain habitual expenditure of force leads to a certain habitual absorption of force-evolving matters that are not available for growth; and if, were there less need for such matters, the ability to absorb matters available for growth would be increased to an equivalent extent; it follows that the antagonism described does, in the long run, hold even without this qualification. hence, growth is substantially equivalent to the absorbed nutriment, minus the nutriment used up in action. this, however, is no answer to the question--why has individual growth a limit?--why do the increments of growth bear decreasing ratios to the mass and finally come to an end? the question is involved. there are more causes than one why the excess of absorbed nutriment over expended nutriment must, other things equal, become less as the size of the animal becomes greater. in similarly-shaped bodies the masses, and therefore the weights, vary as the cubes of the dimensions; whereas the powers of bearing the stresses imposed by the weights vary as the squares of the dimensions. suppose a creature which a year ago was one foot high, has now become two feet high, while it is unchanged in proportions and structure; what are the necessary concomitant changes? it is eight times as heavy; that is to say, it has to resist eight times the strain which gravitation puts upon certain of its parts; and when there occurs sudden arrest of motion or sudden genesis of motion, eight times the strain is put upon the muscles employed. meanwhile the muscles and bones have severally increased their abilities to bear strains in proportion to the areas of their transverse sections, and hence have severally only four times the tenacity they had. this relative decrease in the power of bearing stress does not imply a relative decrease in the power of generating energy and moving the body; for in the case supposed the muscles have not only increased four times in their transverse sections but have become twice as long, and will therefore generate an amount of energy proportionate to their bulk. the implication is simply that each muscle has only half the power to withstand those shocks and strains which the creature's movements entail; and that consequently the creature must be either less able to bear these, or must have muscles and bones having relatively greater transverse dimensions: the result being that greater cost of nutrition is inevitably caused and therefore a correlative tendency to limit growth. this necessity will be seen still more clearly if we leave out the motor apparatus, and consider only the forces required and the means of supplying them. for since, in similar bodies, the areas vary as the squares of the dimensions, and the masses vary as the cubes; it follows that the absorbing surface has become four times as great, while the weight to be moved by the matter absorbed has become eight times as great. if then, a year ago, the absorbing surface could take up twice as much nutriment as was needed for expenditure, thus leaving one-half for growth, it is now able only just to meet expenditure, and can provide nothing for growth. however great the excess of assimilation over waste may be during the early life of an active organism, we see that because a series of numbers increasing as the cubes, overtakes a series increasing as the squares, even though starting from a much smaller number, there must be reached, if the organism lives long enough, a point at which the surplus assimilation is brought down to nothing--a point at which expenditure balances nutrition--a state of moving equilibrium. the only way in which the difficulty can be met is by gradual re-organization of the alimentary system; and, in the first place, this entails direct cost upon the organism, and, in the second place, indirect cost from the carrying of greater weight: both tending towards limitation. there are two other varying relations between degrees of growth and amounts of expended force; one of which conspires with the last, while the other conflicts with it. consider, in the first place, the cost at which nutriment is distributed through the body and effete matters removed from it. each increment of growth being added at the periphery of the organism, the force expended in the transfer of matter must increase in a rapid progression--a progression more rapid than that of the mass. but as the dynamic expense of distribution is small compared with the dynamic value of the materials distributed, this item in the calculation is unimportant. now consider, in the second place, the changing proportion between production and loss of heat. in similar organisms the quantities of heat generated by similar actions going on throughout their substance, must increase as the masses, or as the cubes of the dimensions. meanwhile, the surfaces from which loss of heat takes place, increase only as the squares of the dimensions. though the loss of heat does not therefore increase only as the squares of the dimensions, it certainly increases at a smaller rate than the cubes. and to the extent that augmentation of mass results in a greater retention of heat, it effects an economization of force. this advantage is not, however, so important as at first appears. organic heat is a concomitant of organic action, and is so abundantly produced during action that the loss of it is then usually of no consequence: indeed the loss is often not rapid enough to keep the supply from rising to an inconvenient excess. it is chiefly in respect of that maintenance of heat which is needful during quiescence, that large organisms have an advantage over small ones in this relatively diminished loss. thus these two subsidiary relations between degrees of growth and amounts of expended force, being in antagonism, we may conclude that their differential result does not greatly modify the result of the chief relation. comparisons of these deductions with the facts appear in some cases to verify them and in other cases not to do so. throughout the vegetal kingdom, there are no distinct limits to growth except those which death entails. passing over a large proportion of plants which never exceed a comparatively small size, because they wholly or partially die down at the end of the year, and looking only at trees that annually send forth new shoots, even when their trunks are hollowed by decay; we may ask--how does growth happen here to be unlimited? the answer is, that plants are only accumulators: they are in no very appreciable degree expenders. as they do not undergo waste there is no reason why their growth should be arrested by the equilibration of assimilation and waste. again, among animals there are sufficient reasons why the correspondence cannot be more than approximate. besides the fact above noted, that there are other varying relations which complicate the chief one. we must bear in mind that the bodies compared are not truly similar: the proportions of trunk to limbs and trunk to head, vary considerably. the comparison is still more seriously vitiated by the inconstant ratio between the constituents of which the body is composed. in the flesh of adult mammalia, water forms from to per cent., organic substance from to per cent., and inorganic substance from to per cent.; whereas in the foetal state, the water amounts to per cent., and the solid organic constituents to only per cent. clearly this change from a state in which the force-evolving matter forms one-tenth of the whole, to a state in which it forms two and a half tenths, must greatly interfere with the parallelism between the actual and the theoretical progression. yet another difficulty may come under notice. the crocodile is said to grow as long as it lives; and there appears reason to think that some predaceous fishes, such as the pike, do the same. that these animals of comparatively high organization have no definite limits of growth, is, however, an exceptional fact due to the exceptional non-fulfilment of those conditions which entail limitation. what kind of life does a crocodile lead? it is a cold-blooded, or almost cold-blooded, creature; that is, it expends very little for the maintenance of heat. it is habitually inert: not usually chasing prey but lying in wait for it; and undergoes considerable exertion only during its occasional brief contests with prey. such other exertion as is, at intervals, needful for moving from place to place, is rendered small by the small difference between the animal's specific gravity and that of water. thus the crocodile expends in muscular action an amount of force that is insignificant compared with the force commonly expended by land-animals. hence its habitual assimilation is diminished much less than usual by habitual waste; and beginning with an excessive disproportion between the two, it is quite possible for the one never quite to lose its advance over the other while life continues. on looking closer into such cases as this and that of the pike, which is similarly cold-blooded, similarly lies in wait, and is similarly able to obtain larger and larger kinds of prey as it increases in size; we discover a further reason for this absence of a definite limit. to overcome gravitative force the creature has not to expend a muscular power that is large at the outset, and increases as the cubes of its dimensions: its dense medium supports it. the exceptional continuance of growth observed in creatures so circumstanced, is therefore perfectly explicable. § a. if we go back upon the conclusions set forth in the preceding section, we find that from some of them may be drawn instructive corollaries respecting the limiting sizes of creatures inhabiting different media. more especially i refer to those varying proportions between mass and stress from which, as we have seen, there results, along with increasing size, a diminishing power of mechanical self-support: a relation illustrated in its simplest form by the contrast between a dew-drop, which can retain its spheroidal form, and the spread-out mass of water which results when many dew-drops run together. the largest bird that flies (the argument excludes birds which do not fly) is the condor, which reaches a weight of from to lbs. why does there not exist a bird of the size of an elephant? supposing its habits to be carnivorous, it would have many advantages in obtaining prey: mammals would be at its mercy. evidently the reason is one which has been pointed out--the reason that while the weight to be raised and kept in the air by a bird increases as the cubes of its dimensions, the ability of its bones and muscles to resist the strains which flight necessitates, increases only as the squares of the dimensions. though, could the muscles withstand any tensile strain they were subject to, the power like the weight might increase with the cubes, yet since the texture of muscle is such that beyond a certain strain it tears, it results that there is soon reached a size at which flight becomes impossible: the structures must give way. in a preceding paragraph the limit to the size of flying creatures was ascribed to the greater physiological cost of the energy required; but it seems probable that the mechanical obstacle here pointed out has a larger share in determining the limit. in a kindred manner there results a limitation of growth in a land-animal, which does not exist for an animal living in the water. if, after comparing the agile movements of a dog with those of a cow, the great weight of which obviously prevents agility; or if, after observing the swaying flesh of an elephant as it walks along, we consider what would happen could there be formed a land-animal equal in mass to the whale (the long dinosaurs were not proportionately massive) it needs no argument to show that such a creature could not stand, much less move about. but in the water the strain put upon its structures by the weights of its various parts is almost if not quite taken away. probably limitation in the quantity of food obtainable becomes now the chief, if not the sole, restraint. and here we may note, before leaving the topic, something like a converse influence which comes into play among creatures inhabiting the water. up to the point at which muscles tear from over-strain, larger and smaller creatures otherwise alike, remain upon a par in respect of the relative amounts of energy they can evolve. had they to encounter no resistance from their medium, the implication would be that neither would have an advantage over the other in respect of speed. but resistance of the medium comes into play; and this, other things equal, gives to the larger creature an advantage. it has been found, experimentally, that the forces to be overcome by vessels moving through the water, built as they are with immersed hinder parts which taper as fish taper, are mainly due to what is called "skin-friction." now in two fish unlike in size but otherwise similar skin-friction bears to the energy that can be generated, a smaller proportion in the larger than in the smaller; and the larger can therefore acquire a greater velocity. hence the reason why large fish, such as the shark, become possible. in a habitat where there is no ambush (save in exceptional cases like that of the _lophius_ or angler) everything depends on speed; and if, other things equal, a larger fish had no mechanical advantage over a smaller, a larger fish could not exist--could not catch the requisite amount of prey. § . obviously this antagonism between accumulation and expenditure, must be a leading cause of the contrasts in size between allied organisms that are in many respects similarly conditioned. the life followed by each kind of animal is one involving a certain average amount of exertion for the obtainment of a given amount of nutriment--an exertion, part of which goes to the gathering or catching of food, part to the tearing and mastication of it, and part to the after-processes requisite for separating the nutritive molecules--an exertion which therefore varies according as the food is abundant or scarce, fixed or moving, according as it is mechanically easy or difficult to deal with when secured, and according as it is, or is not, readily soluble. hence, while among animals of the same species having the same mode of life, there will be a tolerably constant ratio between accumulation and expenditure, and therefore a tolerably constant limit of growth, there is every reason to expect that different species, following different modes of life, will have unlike ratios between accumulation and expenditure, and therefore unlike limits of growth. though the facts as inductively established, show a general harmony with this deduction, we cannot usually trace it in any specific way; since the conflicting and conspiring factors which affect growth are so numerous. § . one of the chief causes, if not the chief cause, of the differences between the sizes of organisms, has yet to be considered. we are introduced to it by pushing the above inquiry a little further. small animals have been shown to possess an advantage over large ones in the greater ratio which, other things equal, assimilation bears to expenditure; and we have seen that hence small animals in becoming large ones, gradually lose that surplus of assimilative power which they had, and eventually cannot assimilate more than is required to balance waste. but how come these animals while young and small to have surplus assimilative powers? have all animals equal surpluses of assimilative powers? and if not, how far do differences between the surpluses determine differences between the limits of growth? we shall find, in the answers to these questions, the interpretation of many marked contrasts in growth that are not due to any of the causes above assigned. for example, an ox immensely exceeds a sheep in mass. yet the two live from generation to generation in the same fields, eat the same grass, obtain these aliments with the same small expenditure of energy, and differ scarcely at all in their degrees of organization. whence arises, then, their striking unlikeness of bulk? we noted when studying the phenomena of growth inductively, that organisms of the larger and higher types commence their separate existences as masses of organic matter having tolerable magnitudes. speaking generally, we saw that throughout each organic sub-kingdom the acquirement of great bulk occurs only where the incipient bulk and organization are considerable; and that they are the more considerable in proportion to the complexity of the life which the organism is to lead. the deductive interpretation of this induction may best be commenced by an analogy. a street orange-vendor makes but a trifling profit on each transaction; and unless more than ordinarily fortunate, he is unable to realize during the day a larger amount than will meet his wants; leaving him to start on the morrow in the same condition as before. the trade of the huxter in ounces of tea and half-pounds of sugar, is one similarly entailing much labour for small returns. beginning with a capital of a few pounds, he cannot have a shop large enough, or goods sufficiently abundant and various, to permit an extensive business. he must be content with the half-pence and pence which he makes by little sales to poor people; and if, avoiding bad debts, he is able by strict economy to accumulate anything, it can be but a trifle. a large retail trader is obliged to lay out much money in fitting up an adequate establishment; he must invest a still greater sum in stock; and he must have a further floating capital to meet the charges that fall due before his returns come in. setting out, however, with means enough for these purposes, he is able to make many and large sales; and so to get greater and more numerous increments of profit. similarly, to get returns in thousands merchants and manufacturers must make their investments in tens of thousands. in brief, the rate at which a man's wealth accumulates is measured by the surplus of income over expenditure; and this, save in exceptionably favourable cases, is determined by the capital with which he begins business. now applying the analogy, we may trace in the transactions of an organism, the same three ultimate elements. there is the expenditure required for the obtainment and digestion of food; there is the gross return in the shape of nutriment assimilated or fit for assimilation; and there is the difference between this gross return of nutriment and the nutriment that was used up in the labour of securing it--a difference which may be a profit or a loss. clearly, however, a surplus implies that the force expended is less than the force latent in the assimilated food. clearly, too, the increment of growth is limited to the amount of this surplus of income over expenditure; so that large growth implies both that the excess of nutrition over waste shall be relatively considerable, and that the waste and nutrition shall be on extensive scales. and clearly, the ability of an organism to expend largely and assimilate largely, so as to make a large surplus, presupposes a large physiological capital in the shape of organic matter more or less developed in its structural arrangements. throughout the vegetal kingdom, the illustrations of this truth are not conspicuous and regular: the obvious reason being that since plants are accumulators and in so small a degree expenders, the premises of the above argument are but very partially fulfilled. the food of plants (excepting fungi and certain parasites) being in great measure the same for all, and bathing all so that it can be absorbed without effort, their vital processes result almost entirely in profit. once fairly rooted in a fit place, a plant may thus from the outset add a very large proportion of its entire returns to capital; and may soon be able to carry on its processes on a large scale, though it does not at first do so. when, however, plants are expenders, namely, during their germination and first stages of growth, their degrees of growth _are_ determined by their amounts of vital capital. it is because the young tree commences life with a ready-formed embryo and store of food sufficient to last for some time, that it is enabled to strike root and lift its head above the surrounding herbage. throughout the animal kingdom, however, the necessity of this relation is everywhere obvious. the small carnivore preying on small herbivores, can increase in size only by small increments: its organization unfitting it to digest larger creatures, even if it can kill them, it cannot profit by amounts of nutriment exceeding a narrow limit; and its possible increments of growth being small to set out with, and rapidly decreasing, must come to an end before any considerable size is attained. manifestly the young lion, born of tolerable bulk, suckled until much bigger, and fed until half-grown, is enabled by the power and organization which he thus gets _gratis_, to catch and kill animals big enough to give him the supply of nutriment needed to meet his large expenditure and yet leave a large surplus for growth. thus, then, is explained the above-named contrast between the ox and the sheep. a calf and a lamb commence their physiological transactions on widely different scales; their first increments of growth are similarly contrasted in their amounts; and the two diminishing series of such increments end at similarly-contrasted limits. § . such are the several conditions by which the phenomena of growth are determined. conspiring and conflicting in endless unlike ways and degrees, they in every case qualify more or less differently each other's effects. hence it happens that we are obliged to state each generalization as true on the average, or to make the proviso--other things equal. understood in this qualified form, our conclusions are these. first, that growth being an integration with the organism of such environing matters as are of like natures with the matters composing the organism, its growth is dependent on the available supply of them. second, that the available supply of assimilable matter being the same, and other conditions not dissimilar, the degree of growth varies according to the surplus of nutrition over expenditure--a generalization which is illustrated in some of the broader contrasts between different divisions of organisms. third, that in the same organism the surplus of nutrition over expenditure differs at different stages; and that growth is unlimited or has a definite limit, according as the surplus does or does not rapidly decrease. this proposition we found exemplified by the almost unceasing growth of organisms that expend relatively little energy; and by the definitely limited growth of organisms that expend much energy. fourth, that among organisms which are large expenders of force, the size ultimately attained is, other things equal, determined by the initial size: in proof of which conclusion we have abundant facts, as well as the _a priori_ necessity that the sum-totals of analogous diminishing series, must depend upon the amounts of their initial terms. fifth, that where the likeness of other circumstances permits a comparison, the possible extent of growth depends on the degree of organization; an inference testified to by the larger forms among the various divisions and sub-divisions of organisms. chapter ii. development.[ ] § . certain general aspects of development may be studied apart from any examination of internal structures. these fundamental contrasts between the modes of arrangement of parts, originating, as they do, the leading external distinctions among the various forms of organization, will be best dealt with at the outset. if all organisms have arisen by evolution, it is of course not to be expected that such several modes of development can be absolutely demarcated: we are sure to find them united by transitional modes. but premising that a classification of modes can but approximately represent the facts, we shall find our general conceptions of development aided by one. development is primarily _central_. all organic forms of which the entire history is known, set out with a symmetrical arrangement of parts round a centre. in organisms of the lowest grade no other mode of arrangement is ever definitely established; and in the highest organisms central development, though subordinate to another mode of development, continues to be habitually shown in the changes of minute structure. let us glance at these propositions in the concrete. practically every plant and every animal in its earliest stage is a portion of protoplasm, in the great majority of cases approximately spherical but sometimes elongated, containing a rounded body consisting of specially modified protoplasm, which is called a nucleus; and the first changes that occur in the germ thus constituted, are changes that take place in this nucleus, followed by changes round the centres produced by division of this original centre. from this type of structure, the simplest organisms do not depart; or depart in no definite or conspicuous ways. among plants, many of the simplest _algæ_ and _fungi_ permanently maintain such a central distribution; while among animals it is permanently maintained by creatures like the _gregarina_, and in a different manner by the _amoeba_, _actinophrys_, and their allies: the irregularities which are many and great do not destroy this general relation of parts. in larger organisms, made up chiefly of units that are analogous to these simplest organisms, the formation of units ever continues to take place round nuclei; though usually the nuclei soon cease to be centrally placed. central development may be distinguished into _unicentral_ and _multicentral_; according as the product of the original germ develops more or less symmetrically round one centre, or develops without subordination to one centre--develops, that is, in subordination to many centres. unicentral development, as displayed not in the formation of single cells but in the formation of aggregates, is not common. the animal kingdom shows it only in some of the small group of colonial _radiolaria_. it is feebly represented in the vegetal kingdom by a few members of the _volvocineæ_. on the other hand, multicentral development, or development round insubordinate centres, is variously exemplified in both divisions of the organic world. it is exemplified in two distinct ways, according as the insubordination among the centres of development is partial or total. we may most conveniently consider it under the heads hence arising. total insubordination among the centres of development, is shown where the units or cells, as fast as they are severally formed, part company and lead independent lives. this, in the vegetal kingdom, habitually occurs among the _protophyta_, and in the animal kingdom, among the _protozoa_. partial insubordination is seen in those somewhat advanced organisms, that consist of units which, though they have not separated, have so little mutual dependence that the aggregate they form is irregular. among plants, the thallophytes very generally exemplify this mode of development. lichens, spreading with flat or corrugated edges in this or that direction as the conditions determine, have no manifest co-ordination of parts. in the _algæ_ the nostocs and various other forms similarly show us an unsymmetrical structure. of _fungi_ we may say that creeping kinds display no further dependence of one part on another than is implied by their cohesion. and even in such better-organized plants as the _marchantia_, the general arrangement shows no reference to a directive centre. among animals many of the sponges in their adult forms may be cited as devoid of that co-ordination implied by symmetry: the units composing them, though they have some subordination to local centres, have no subordination to a general centre. to distinguish that kind of development in which the whole product of a germ coheres in one mass, from that kind of development in which it does not, professor huxley has introduced the words "_continuous_" and "_discontinuous_;" and these seem the best fitted for the purpose. multicentral development, then, is divisible into continuous and discontinuous. from central development we pass insensibly to that higher kind of development for which _axial_ seems the most appropriate name. a tendency towards this is vaguely manifested almost everywhere. the great majority even of _protophyta_ and _protozoa_ have different longitudinal and transverse dimensions--have an obscure if not a distinct axial structure. the originally spheroidal and polyhedral units out of which higher organisms are mainly built, usually pass into shapes that are subordinated to lines rather than to points. and in the higher organisms, considered as wholes, an arrangement of parts in relation to an axis is distinct and nearly universal. we see it in the superior orders of thallophytes; and in all the cormophytic plants. with few exceptions the _coelenterata_ clearly exhibit it; it is traceable, though less conspicuously, throughout the _mollusca_; and the _annelida_, _arthropoda_, and _vertebrata_ uniformly show it with perfect definiteness. this kind of development, like the first kind, is of two orders. the whole germ-product may arrange itself round a single axis, or it may arrange itself round many axes: the structure may be _uniaxial_ or _multiaxial_. each division of the organic kingdom furnishes examples of both these orders. in such _fungi_ as exhibit axial development at all, we commonly see development round a single axis. some of the _algæ_, as the common tangle, show us this arrangement. and of the higher plants, many monocotyledons and small dicotyledons are uniaxial. of animals, the advanced are without exception in this category. there is no known vertebrate in which the whole of the germ-product is not subordinated to a single axis. in the _arthropoda_, the like is universal; as it is also in the superior orders of _mollusca_. multiaxial development occurs in most of the plants we are familiar with--every branch of a shrub or tree being an independent axis. but while in the vegetal kingdom multiaxial development prevails among the highest types, in the animal kingdom it prevails only among the lowest types. it is extremely general, if not universal, among the _coelenterata_; it is characteristic of the _polyzoa_; the compound ascidians exhibit it; and it is seen, though under another form, in certain of the inferior annelids. development that is axial, like development that is central, may be either continuous or discontinuous: the parts having different axes may continue united, or they may separate. instances of each alternative are supplied by both plants and animals. continuous multiaxial development is that which plants usually display, and need not be illustrated further than by reference to every garden. as cases of it in animals may be named all the compound _hydrozoa_ and _actinozoa_; and such ascidian forms as the _botryllidæ_. of multiaxial development that is discontinuous, a familiar instance among plants exists in the common strawberry. this sends out over the neighbouring surface, long slender shoots, bearing at their extremities buds that presently strike roots and become new individuals; and these by and by lose their connexions with the original axis. other plants there are that produce certain specialized buds called bulbils, which separating themselves and falling to the ground, grow into independent plants. among animals the fresh-water polype very clearly shows this mode of development: the young polypes, budding out from its surface, severally arrange their parts around distinct axes, and eventually detaching themselves, lead separate lives, and produce other polypes after the same fashion. by some of the lower _annelida_, this multiplication of axes from an original axis, is carried on after a different manner: the string of segments spontaneously divides; and after further growth, division recurs in one or both of the halves. moreover in the _syllis ramosa_, there occurs lateral branching also. grouping together its several modes as above delineated, we see that { unicentral { central { or { continuous { { multicentral { or { { discontinuous development is { or { { { uniaxial { axial { or { continuous { multiaxial { or { discontinuous any one well acquainted with the facts, may readily raise objections to this arrangement. he may name forms which do not obviously come under any of these heads. he may point to plants that are for a time multicentral but afterwards develop axially. and from lower types of animals he may choose many in which the continuous and discontinuous modes are both displayed. but, as already hinted, an arrangement free from such anomalies must be impossible, if the various kinds of organization have arisen by evolution. the one above sketched out is to be regarded as a rough grouping of the facts, which helps us to a conception of them in their totality; and, so regarded, it will be of service when we come to treat of individuality and reproduction. § . from these most general external aspects of organic development, let us now turn to its internal and more special aspects. when treating of evolution as a universal process of things, a rude outline of the course of structural changes in organisms was given (_first principles_, §§ , , ). here it will be proper to describe these changes more fully. the bud of any common flowering plant in its earliest stage, consists of a small hemispherical or sub-conical projection. while it increases most rapidly at the apex, this presently develops on one side of its base, a smaller projection of like general shape with itself. here is the rudiment of a leaf, which presently spreads more or less round the base of the central hemisphere or main axis. at the same time that the central hemisphere rises higher, this lateral prominence, also increasing, gives rise to subordinate prominences or lobes. these are the rudiments of stipules, where the leaves are stipulated. meanwhile, towards the other side of the main axis and somewhat higher up, another lateral prominence arising marks the origin of a second leaf. by the time that the first leaf has produced another pair of lobes, and the second leaf has produced its primary pair, the central hemisphere, still increasing at its apex, exhibits the rudiment of a third leaf. similarly throughout. while the germ of each succeeding leaf thus arises, the germs of the previous leaves, in the order of their priority, are changing their rude nodulated shapes into flattened-out expansions; which slowly put on those sharp outlines they show when unfolded. thus from that extremely indefinite figure, a rounded lump, giving off from time to time lateral lumps, which severally becoming symmetrically lobed gradually assume specific and involved forms, we pass little by little to that comparatively complex thing--a leaf-bearing shoot. internally, a bud undergoes analogous changes; as witness this account:--"the general mass of thin-walled parenchymatous cells which occupies the apical region, and forms the _growing point_ of the shoot, is covered by a single external layer of similar cells, which increase in number by the formation of new walls in one direction only, perpendicular to the surface of the shoot, and thus give rise only to the _epidermis_ or single layer of cells covering the whole surface of the shoot. meanwhile the general mass below grows as a whole, its constituent cells dividing in all directions. of the new cells so formed, those removed by these processes of growth and division from the actual apex, begin, at a greater or less distance from it, to show signs of the differentiation which will ultimately lead to the formation of the various tissues enclosed by the epidermis of the shoot. first the pith, then the vascular bundles, and then the cortex of the shoot, begin to take on their special characters." similarly with secondary structures, as the lateral buds whence leaves arise. in the, at first, unorganized mass of cells constituting the rudimentary leaf, there are formed vascular bundles which eventually become the veins of the leaf; and _pari passu_ with these are formed the other tissues of the leaf. nor do we fail to find an essentially parallel set of changes, when we trace the histories of the individual cells. while the tissues they compose are separating, the cells are growing step by step more unlike. some become flat, some polyhedral, some cylindrical, some prismatic, some spindle-shaped. these develop spiral thickenings in their interiors; and those, reticulate thickenings. here a number of cells unite together to form a tube: and there they become almost solid by the internal deposition of woody or other substance. through such changes, too numerous and involved to be here detailed, the originally uniform cells go on diverging and rediverging until there are produced various forms that seem to have very little in common. the arm of a man makes its first appearance in as simple a way as does the shoot of a plant. according to bischoff, it buds-out from the side of the embryo as a little tongue-shaped projection, presenting no differences of parts; and it might serve for the rudiment of some one of the various other organs that also arise as buds. continuing to lengthen, it presently becomes somewhat enlarged at its end; and is then described as a pedicle bearing a flattened, round-edged lump. this lump is the representative of the future hand, and the pedicle of the future arm. by and by, at the edges of this flattened lump, there appear four clefts, dividing from each other the buds of the future fingers; and the hand as a whole grows a little more distinguishable from the arm. up to this time the pedicle has remained one continuous piece, but it now begins to show a bend at its centre, which indicates the division into arm and forearm. the distinctions thus rudely indicated gradually increase: the fingers elongate and become jointed, and the proportions of all the parts, originally very unlike those of the complete limb, slowly approximate to them. during its bud-like stage, the rudimentary arm consists only of partially-differentiated tissues. by the diverse changes these gradually undergo they are transformed into bones, muscles, blood-vessels, and nerves. the extreme softness and delicacy of these primary tissues, renders it difficult to trace the initial stages of the differentiations. in consequence of the colour of their contents, the blood-vessels are the first parts to become distinct. afterwards the cartilaginous parts, which are the bases of the future bones, become marked out by the denser aggregation of their constituent cells, and by the production between these of a hyaline substance which unites them into a translucent mass. when first perceptible, the muscles are gelatinous, pale, yellowish, transparent, and indistinguishable from their tendons. the various other tissues of which the arm consists, beginning with very faintly-marked differences, become day by day more definite in their qualitative appearances. in like manner the units composing these tissues severally assume increasingly-specific characters. the fibres of muscle, at first made visible in the midst of their gelatinous matrix only by immersion in alcohol, grow more numerous and distinct; and by and by they begin to exhibit transverse stripes. the bone-cells put on by degrees their curious structure of branching canals. and so in their respective ways with the units of skin and the rest. thus in each of the organic sub-kingdoms, we see this change from an incoherent, indefinite homogeneity to a coherent, definite heterogeneity, illustrated in a quadruple way. the originally-like units called cells, become unlike in various ways, and in ways more numerous and marked as the development goes on. the several tissues which these several classes of cells form by aggregation, grow little by little distinct from each other; and little by little put on those structural complexities that arise from differentiations among their component units. in the shoot, as in the limb, the external form, originally very simple, and having much in common with simple forms in general, gradually acquires an increasing complexity, and an increasing unlikeness to other forms. meanwhile, the remaining parts of the organism to which the shoot or limb belongs, having been severally assuming structures divergent from one another and from that of this particular shoot or limb, there has arisen a greater heterogeneity in the organism as a whole. § . one of the most remarkable inductions of embryology comes next in order. and here we find illustrated the general truth that in mental evolution as in bodily evolution the progress is from the indefinite and inexact to the definite and exact. for the first statement of this induction was but an adumbration of the correct statement. as a result of his examinations von baer alleged that in its earliest stage every organism has the greatest number of characters in common with all other organisms in their earliest stages; that at a stage somewhat later its structure is like the structures displayed at corresponding phases by a less extensive assemblage of organisms; that at each subsequent stage traits are acquired which successively distinguish the developing embryo from groups of embryos that it previously resembled--thus step by step diminishing the group of embryos which it still resembles; and that thus the class of similar forms is finally narrowed to the species of which it is a member. this abstract proposition will perhaps not be fully comprehended by the general reader. it will be best to re-state it in a concrete shape. supposing the germs of all kinds of organisms to be simultaneously developing, we may say that all members of the vast multitude take their first steps in the same direction; that at the second step one-half of this vast multitude diverges from the other half, and thereafter follows a different course of development; that the immense assemblage contained in either of these divisions very soon again shows a tendency to take two or more routes of development; that each of the two or more minor assemblages thus resulting, shows for a time but small divergences among its members, but presently again divides into groups which separate ever more widely as they progress; and so on until each organism, when nearly complete, is accompanied in its further modifications only by organisms of the same species; and last of all, assumes the peculiarities which distinguish it as an individual--diverges to a slight extent to the organisms it is most like. but, as above said, this statement is only an adumbration. the order of nature is habitually more complex than our generalizations represent it as being--refuses to be fully expressed in simple formulæ; and we are obliged to limit them by various qualifications. it is thus here. since von baer's day the careful observations of numerous observers have shown his allegation to be but approximately true. hereafter, when discussing the embryological evidence of evolution, the causes of deviations will be discussed. for the present it suffices to recognize as unquestionable the fact that whereas the germs of organisms are extremely similar, they gradually diverge widely, in modes now regular and now irregular, until in place of a multitude of forms practically alike we finally have a multitude of forms most of which are extremely unlike. thus, in conformity with the law of evolution, not only do the parts of each organism advance from indefinite homogeneity to definite heterogeneity, but the assemblage of all organisms does the same: a truth already indicated in _first principles_. § . this comparison between the course of development, in any creature, and the course of development in all other creatures--this arrival at the conclusion that the course of development in each, at first the same as in all others, becomes stage by stage differentiated from the courses in all others, brings us within view of an allied conclusion. if we contemplate the successive stages passed through by any higher organism, and observe the relation between it and its environment at each of these stages; we shall see that this relation is modified in a way analogous to that in which the relation between the organism and its environment is modified, as we advance from the lowest to the highest grades. along with the progressing differentiation of each organism from others, we find a progressing differentiation of it from its environment; like that progressing differentiation from the environment which we meet with in the ascending forms of life. let us first glance at the way in which the ascending forms of life exhibit this progressing differentiation from the environment. in the first place, it is illustrated in _structure_. advance from the homogeneous to the heterogeneous, itself involves an increasing distinction from the inorganic world. passing over the _protozoa_, of which the simplest probably disappeared during the earliest stages of organic evolution, and limiting our comparison to the _metazoa_, we see that low types of these, as the _coelenterata_, are relatively simple in their organization; and the ascent to organisms of greater and greater complexity of structure, is an ascent to organisms which are in that respect more strongly contrasted with the structureless environment. in _form_, again, we see the same truth. an ordinary characteristic of inorganic matter is its indefiniteness of form; and this is also a characteristic of the lower organisms, as compared with the higher. speaking generally, plants are less definite than animals, both in shape and size--admit of greater modifications from variations of position and nutrition. among animals, the simplest rhizopods may almost be called amorphous: the form is never specific, and is constantly changing. of the organisms resulting from the aggregation of such creatures, we see that while some, as the _foraminifera_, assume a certain definiteness of form, in their shells at least, others, as the sponges, are very irregular. the zoophytes and the _polyzoa_ are compound organisms, most of which have a mode of growth not more determinate than that of plants. but among the higher animals, we find not only that the mature shape of each species is very definite, but that the individuals of each species differ little in size. a parallel increase of contrast is seen in _chemical composition_. with but few exceptions, and those only partial ones, the lowest animal and vegetal forms are inhabitants of the water; and water is almost their sole constituent. desiccated _protophyta_ and _protozoa_ shrink into mere dust; and among the acalephes we find but a few grains of solid matter to a pound of water. the higher aquatic plants, in common with the higher aquatic animals, possessing as they do increased tenacity of substance, also contain a greater proportion of the organic elements; further they show us a greater variety of composition in their different parts; and thus in both ways are chemically more unlike their medium. and when we pass to the superior classes of organisms--land-plants and land-animals--we see that, chemically considered, they have little in common either with the earth on which they stand or the air which surrounds them. in _specific gravity_ too, we may note a like truth. the simplest forms, in common with the spores and gemmules of higher ones, are as nearly as may be of the same specific gravity as the water in which they float; and though it cannot be said that among aquatic creatures, superior specific gravity is a standard of general superiority, yet we may fairly say that the higher orders of them, when divested of the appliances by which their specific gravity is regulated, differ more from water in their relative weights than do the lowest. in terrestrial organisms, the contrast becomes marked. trees and plants, in common with insects, reptiles, mammals, birds, are all of a specific gravity considerably less than that of the earth and immensely greater than that of the air. yet further, we see the law fulfilled in respect of _temperature_. plants generate but extremely small quantities of heat, which are to be detected only by delicate experiments; and practically they may be considered as having the same temperature as their environment. the temperature of aquatic animals is very little above that of the surrounding water: that of the invertebrata being mostly less than a degree above it, and that of fishes not exceeding it by more than two or three degrees; save in the case of some large red-blooded fishes, as the tunny, which exceed it in temperature by nearly ten degrees. among insects the range is from two to ten degrees above that of the air: the excess varying according to their activity. the heat of reptiles is from four to fifteen degrees more than the heat of their medium. while mammals and birds maintain a heat which continues almost unaffected by external variations, and is often greater than that of the air by seventy, eighty, ninety, and even a hundred degrees. once more, in greater _self-mobility_ a progressive differentiation is traceable. the chief characteristic by which we distinguish dead matter is its inertness: some form of independent motion is our most familiar proof of life. passing over the indefinite border-land between the animal and vegetal kingdoms, we may roughly class plants as organisms which, while they exhibit that kind of motion implied in growth, are not only devoid of locomotive power, but with some unimportant exceptions are devoid of the power of moving their parts in relation to each other; and thus are less differentiated from the inorganic world than animals. though in those microscopic _protophyta_ and _protozoa_ inhabiting the water we see locomotion produced by ciliary action; yet this locomotion, while rapid relatively to the sizes of their bodies, is absolutely slow. of the _coelenterata_ a great part are either permanently rooted or habitually stationary; and so have scarcely any self-mobility but that implied in the relative movements of parts; while the rest, of which the common jelly-fish serves as a sample, have mostly but little ability to move themselves through the water. among the higher aquatic _invertebrata_,--cuttlefishes and lobsters, for instance,--there is a very considerable power of locomotion; and the aquatic _vertebrata_ are, considered as a class, much more active in their movements than the other inhabitants of the water. but it is only when we come to air-breathing creatures that we find the vital characteristics of self-mobility manifested in the highest degree. flying insects, mammals, birds, travel with velocities far exceeding those attained by any of the lower classes of animals. thus, on contemplating the various grades of organisms in their ascending order, we find them more and more distinguished from their inanimate media, in _structure_, in _form_, in _chemical composition_, in _specific gravity_, in _temperature_, in _self-mobility_. it is true that this generalization does not hold with complete regularity. organisms which are in some respects the most strongly contrasted with the environing inorganic world, are in other respects less contrasted than inferior organisms. as a class, mammals are higher than birds; and yet they are of lower temperature and have smaller powers of locomotion. the stationary oyster is of higher organization than the free-swimming medusa; and the cold-blooded and less heterogeneous fish is quicker in its movements than the warm-blooded and more heterogeneous sloth. but the admission that the several aspects under which this increasing contrast shows itself, bear variable ratios to each other, does not conflict with the general truth that as we ascend in the hierarchy of organisms, we meet with not only an increasing differentiation of parts but also an increasing differentiation from the surrounding medium in sundry other physical attributes. it would seem that this trait has some necessary connexion with superior vital manifestations. one of those lowly gelatinous forms, so transparent and colourless as to be with difficulty distinguished from the water it floats in, is not more like its medium in chemical, mechanical, optical, thermal, and other properties, than it is in the passivity with which it submits to all the influences and actions brought to bear upon it; while the mammal does not more widely differ from inanimate things in these properties, than it does in the activity with which it meets surrounding changes by compensating changes in itself. and between these extremes, these two kinds of contrast vary together. so that in proportion as an organism is physically like its environment it remains a passive partaker of the changes going on in its environment; while in proportion as it is endowed with powers of counteracting such changes, it exhibits greater unlikeness to its environment.[ ] if now, from this same point of view, we consider the relation borne to its environment by any superior organism in its successive stages, we find an analogous series of contrasts. of course in respect of degrees of _structure_ the parallelism is complete. the difference, at first small, between the little-structured germ and the little-structured inorganic world, necessarily becomes greater, step by step, as the differentiations of the germ become more numerous and definite. how of _form_ the like holds is equally manifest. the sphere, which is the point of departure common to all organisms, is the most generalized of figures; and one that is, under various circumstances, assumed by inorganic matter. but as it develops it loses all likeness to inorganic objects in the environment; and eventually becomes distinct even from nearly all organic objects in its environment. in _specific gravity_ the alteration, though not very marked, is still in the same direction. development being habitually accompanied by a relative decrease in the quantity of water and an increase in the quantity of constituents that are heavier than water, there results a small augmentation of relative weight. in power of maintaining a _temperature_ above that of surrounding things, the differentiation from the environment that accompanies development is marked. all ova are absolutely dependent for their heat on external sources. the mammalian young one is, during its uterine life, dependent on the maternal heat; and at birth has but a partial power of making good the loss by radiation. but as it advances in development it gains an ability to maintain a constant temperature above that of surrounding things: so becoming markedly unlike them. lastly, in _self-mobility_ this increasing contrast is no less decided. save in a few aberrant tribes, chiefly parasitic, we find the general fact to be that the locomotive power, totally absent or very small at the outset, increases with the advance towards maturity. the more highly developed the organism becomes, the stronger grows the contrast between its activity and the inertness of the objects amid which it moves. thus we may say that the development of an individual organism, is at the same time a differentiation of its parts from each other, and a differentiation of the consolidated whole from the environment; and that in the last as in the first respect, there is a general analogy between the progression of an individual organism and the progression from the lowest orders of organisms to the highest orders. it may be remarked that some kinship seems to exist between these generalizations and the doctrine of schelling, that life is the tendency to individuation. for evidently, in becoming more distinct from one another and from their environment, organisms acquire more marked individualities. as far as i can gather from outlines of his philosophy, however, schelling entertained this conception in a general and transcendental sense, rather than in a special and scientific one. § . deductive interpretations of these general facts of development, in so far as they are possible, must be postponed until we arrive at the fourth and fifth divisions of this work. there are, however, one or two general aspects of these inductions which may be here conveniently dealt with deductively. grant that each organism is at the outset relatively homogeneous and that when complete it is relatively heterogeneous, and it necessarily follows that development is a change from the homogeneous to the heterogeneous--a change during which there must be gone through all the gradations of heterogeneity that lie between these extremes. if, again, there is at first indefiniteness and at last definiteness, the transition cannot but be from the one to the other of these through all intermediate degrees of definiteness. further, if the parts, originally incoherent or uncombined, eventually become relatively coherent or combined, there must be a continuous increase of coherence or combination. hence the general truth that development is a change from incoherent, indefinite homogeneity, to coherent, definite heterogeneity, becomes a self-evident one when observation has shown us the state in which organisms begin and the state in which they end. just in the same way that the growth of an entire organism is carried on by abstracting from the environment substances like those composing the organism; so the production of each organ within the organism is carried on by abstracting from the substances contained in the organism, those required by this particular organ. each organ at the expense of the organism as a whole, integrates with itself certain kinds and proportions of the matters circulating around it; in the same way that the organism as a whole, integrates with itself certain kinds and proportions of matters at the expense of the environment as a whole. so that the organs are qualitatively differentiated from each other, in a way analogous to that by which the entire organism is qualitatively differentiated from things around it. evidently this selective assimilation illustrates the general truth, set forth and illustrated in _first principles_, that like units tend to segregate. it illustrates, moreover, the further aspect of this general truth, that the pre-existence of a mass of certain units produces a tendency for diffused units of the same kind to aggregate with this mass rather than elsewhere. it has been shown of particular salts, a and b, co-existing in a solution not sufficiently concentrated to crystallize, that if a crystal of the salt a be put into the solution, it will increase by uniting with itself the dissolved atoms of the salt a; and that similarly, though there otherwise takes place no deposition of the salt b, yet if a crystal of the salt b is placed in the solution, it will exercise a coercive force on the diffused atoms of this salt, and grow at their expense. probably much organic assimilation occurs in the same way. particular parts of the organism are composed of special units or have the function of secreting special units, which are ever present in them in large quantities. the fluids circulating through the body contain special units of this same order. and these diffused units are continually being deposited along with the groups of like units that already exist. how purely physical are the causes of this selective assimilation, is, indeed, shown by the fact that abnormal constituents of the blood are segregated in the same way. the chalky deposits of gout beginning at certain points, collect more and more around those points. and similarly in numerous pustular diseases. where the component units of an organ, or some of them, do not exist as such in the circulating fluids, but are formed out of elements or compounds that exist separately in the circulating fluids, the process of differential assimilation must be of a more complex kind. still, however, it seems not impossible that it is carried on in an analogous way. if there be an aggregate of compound atoms, each of which contains the constituents a, b, c; and if round this aggregate the constituents a and b and c are diffused in uncombined states; it may be suspected that the coercive force of these aggregated compound atoms a, b, c, may not only bring into union with themselves adjacent compound atoms a, b, c, but may cause the adjacent constituents a and b and c to unite into such compound atoms, and then aggregate with the mass. chapter ii^a. structure.[ ] § a. as, in the course of evolution, we rise from the smallest to the largest aggregates by a process of integration, so do we rise by a process of differentiation from the simplest to the most complex aggregates. the initial types of life are at once extremely small and almost structureless. passing over those which swarm in the air, the water, and the soil, and are now some of them found to be causes of diseases, we may set out with those ordinarily called _protozoa_ and _protophyta_: the lowest of which, however, are either at once plants and animals, or are now one and now the other. that the first living things were minute portions of simple protoplasm is implied by the general theory of evolution; but we have no evidence that such portions exist now. even admitting that there are protoplasts (using this word to include plant and animal types) which are without nuclei, still they are not homogeneous--they are granular. whether a nucleus is always present is a question still undecided; but in any case the types from which it is absent are extremely exceptional. thus the most general structural traits of protoplasts are--the possession of an internal part, morphologically central though often not centrally situated, a general mass of protoplasm surrounding it, and an inclosing differentiated portion in contact with the environment. these essential elements are severally subject to various complications. in some simple types the limiting layer or cortical substance can scarcely be said to exist as a separate element. the exoplasm, distinguished from the endoplasm by absence or paucity of granules, is continually changing places with it by the sending out of pseudopodia which are presently drawn back into the general mass: the inner and outer, being unsettled in position, are not permanently differentiated. then we have types, exemplified by _lithamoeba_, constituted of protoplasm covered by a distinct pellicle, which in sundry groups becomes an outer shell of various structure: now jelly-like, now of cellulose, now siliceous or calcareous. while here this envelope has a single opening, there it is perforated all over--a fenestrated shell. in some cases an external layer is formed of agglutinated sand-particles; in others of imbricated plates, as in coccospheres; and in many others radiating spicules stand out on all sides. throughout sundry classes the exoplasm develops cilia, by the wavings of which the creatures are propelled through the water--cilia which may be either general or local. and then this cortical layer, instead of being spherical or spheroidal, may become plano-spiral, cyclical, crosier-shaped, and often many-chambered; whence there is a transition to colonies. meanwhile the inclosed protoplasm, at first little more than a network or foamwork containing granules and made irregular by objects drawn in as nutriment, becomes variously complicated. in some low types its continuity is broken by motionless, vacant spaces, but in higher types there are contractile vacuoles slowly pulsing, and, as we may suppose, moving the contained liquid hither and thither; while there are types having many passive vacuoles along with a few active ones. in some varieties the protruded parts, or pseudopodia, into which the protoplasm continually shapes itself, are comparatively short and club-shaped; in others they are long and fine filaments which anastomose, so forming a network running here and there into little pools of protoplasm. then there are kinds in which the protoplasm streams up and down the protruding spicules: sometimes inside of them, sometimes outside. always, too, there is included in the protoplasm a small body known as a centrosome. lastly, we have the innermost element, considered the essential element--the nucleus. according to prof. lankester, it is absent from _archerina_, and there are types in which it is made visible only by the aid of special reagents. ordinarily it is marked off from the surrounding protoplasm by a delicate membrane, just as the protoplasm itself is marked off by the exoplasm from the environment. most commonly there is a single nucleus, but occasionally there are many, and sometimes there is a chief one with minor ones. moreover, within the nucleus itself there have of late years been discovered remarkable structural elements which undergo complicated changes. these brief statements indicate only the most general traits of an immense variety of structures--so immense a variety that prof. lankester, in distinguishing the classes, sub-classes, orders, and genera in the briefest way, occupies quarto pages of small type. and to give a corresponding account of _protophyta_ would require probably something like equal space. thus these living things, so minute that unaided vision fails to disclose them, constitute a world exhibiting varieties of structure which it requires the devotion of a life to become fully acquainted with. § b. if higher forms of life have arisen from lower forms by evolution, the implication is that there must once have existed, if there do not still exist, transitional forms; and there follows the comment that there _do_ still exist transitional forms. both in the plant-world and in the animal-world there are types in which we see little more than simple assemblages of _protophyta_ or of _protozoa_--types in which the units, though coherent, are not differentiated but constitute a uniform mass. in treating of structure we are not here concerned with these unstructured types, but may pass on to those aggregates of protoplasts which show us differentiated parts--_metaphyta_ and _metazoa_: economizing space by limiting our attention chiefly to the last. when, half a century ago, some currency was given to the statement that all kinds of organisms, plant and animal, which our unaided eyes disclose, are severally composed of myriads of living units, some of them partially, if not completely, independent, and that thus a man is a vast nation of minute individuals of which some are relatively passive and others relatively active, the statement met, here with incredulity and there with a shudder. but what was then thought a preposterous assertion has now come to be an accepted truth. along with gradual establishment of this truth has gone gradual modification in the form under which it was originally asserted. if some inhabitant of another sphere were to describe one of our towns as composed exclusively of houses, saying nothing of the contained beings who had built them and lived in them, we should say that he had made a profound error in recognizing only the inanimate elements of the town and disregarding the animate elements. early histologists made an analogous error. plants and animals were found to consist of minute members, each of which appeared to be simply a wall inclosing a cavity--a cell. but further investigation proved that the content of the cell, presently distinguished as protoplasm, is its essential living part, and that the cell-wall, when present, is produced by it. thus the unit of composition is a protoplast, usually enclosed, with its contained nucleus and centrosome. § c. as above implied, the individualities of the units are not wholly lost in the individuality of the aggregate, but continue, some of them, to be displayed in various degrees: the great majority of them losing their individualities more and more as the type of the aggregate becomes higher. in a slightly organized metazoon like the sponge, the subordination is but small. only those members of the aggregate which, flattened and united together, form the outer layer and those which become metamorphosed into spicules, have entirely lost their original activities. of the rest nearly all, lining the channels which permeate the mass, and driving onwards the contained sea-water by the motions of their whip-like appendages, substantially retain their separate lives; and beyond these there exist in the gelatinous substance lying between the inner and outer layers, which is regarded as homologous with a mesoderm, amoeba-form protoplasts which move about from place to place. relations between the aggregate and the units which are in this case permanent, are in other cases temporary: characterizing early stages of embryonic development. for example, drawings of echinoderm larvæ at an early stage, show us the potential independence of all the cells forming the blastosphere; for in the course of further development some of these resume the primitive amoeboid state, migrate through the internal space, and presently unite to form certain parts of the growing structures. but with the progress of organization independence of this kind diminishes. converse facts are presented after development has been completed; for with the commencement of reproduction we everywhere see more or less resumption of individual life among the units, or some of them. it is a trait of transitional types between _protozoa_ and _metazoa_ to lead an aggregate life as a plasmodium, and then for this to break up into its members, which for a time lead individual lives as generative agents; and sundry low kinds of plants possessing small amounts of structure, have generative elements--zoospores and spermatozoids--which show us a return to unit life. nor, indeed, are we shown this only in the lowest plants; for it has recently been found that in certain of the higher plants--even in phænogams--spermatozoids are produced. that is to say, the units resume active lives at places where the controlling influence of the aggregate is failing; for, as we shall hereafter see, places at which generation commences answer to this description. these different kinds of evidence jointly imply that the individual lives of the units are subordinate to the general life in proportion as this is high. where the organism is very inferior in type the unit-life remains permanently conspicuous. in some superior types there is a display of unit-life during embryonic stages in which the co-ordinating action of the aggregate is but incipient. with the advance of development the unit-life diminishes; but still, in plants, recommences where the disintegrating process which initiates generation shows the coercive power of the organization to have become small. even in the highest types, however, and even when they are fully developed, unit-life does not wholly disappear: it is clearly shown in ourselves. i do not refer simply to the fact that, as throughout the animal kingdom at large and a considerable part of the vegetal kingdom, the male generative elements are units which have resumed the primitive independent life, but i refer to a much more general fact. in that part of the organism which, being fundamentally an aqueous medium, is in so far like the aqueous medium in which ordinary protozoon life is carried on, we find an essentially protozoon life. i refer of course to the blood. whether the tendency of the red corpuscles (which are originally developed from amoeba-like cells) to aggregate into _rouleaux_ is to be taken as showing life in them, may be left an open question. it suffices that the white corpuscles or leucocytes, retaining the primitive amoeboid character, exhibit individual activities: send out prolongations like pseudopodia, take in organic particles as food, and are independently locomotive. though far less numerous than the red corpuscles, yet, as ten thousand are contained in a cubic millimetre of blood--a mass less than a pin's head--it results that the human body is pervaded throughout all its blood-vessels by billions of these separately living units. in the lymph, too, which also fulfils the requirements of liquidity, these amoeboid units are found. then we have the curious transitional stage in which units partially imbedded and partially free display a partial unit-life. these are the ciliated epithelium-cells, lining the air-passages and covering sundry of the mucous membranes which have more remote connexions with the environment, and covering also the lining membranes of certain main canals and chambers in the nervous system. the inner parts of these unite with their fellows to form an epithelium, and the outer parts of them, immersed either in liquid or semi-liquid (mucus), bear cilia that are in constant motion and "produce a current of fluid over the surface they cover:" thus simulating in their positions and actions the cells lining the passages ramifying through a sponge. the partially independent lives of these units is further seen in the fact that after being detached they swim about in water for a time by the aid of their cilia. § d. but in the _metazoa_ and _metaphyta_ at large, the associated units are, with the exceptions just indicated, completely subordinated. the unit-life is so far lost in the aggregate life that neither locomotion nor the relative motion of parts remains; and neither in shape nor composition is there resemblance to protozoa. though in many cases the internal protoplasm continues to carry on vital processes subserving the needs of the aggregate, in others vital processes of an independent kind appear to cease. it will naturally be supposed that after recognizing this fundamental trait common to all types of organisms above the _protozoa_ and _protophyta_, the next step in an account of structure must be a description of their organs, variously formed and combined--if not in detail yet in their general characters. this, however, is an error. there are certain truths of structure higher in generality than any which can be alleged of organs. we shall see this if we compare organs with one another. here is a finger stiffened by its small bones and yet made flexible by the uniting joints. there is a femur which helps its fellow to support the weight of the body; and there again is a rib which, along with others, forms a protective box for certain of the viscera. dissection reveals a set of muscles serving to straighten and bend the fingers, certain other muscles that move the legs, and some inconspicuous muscles which, contracting every two or three seconds, slightly raise the ribs and aid in inflating the lungs. that is to say, fingers, legs, and chest possess certain structures in common. there is in each case a dense substance capable of resisting stress and a contractile substance capable of moving the dense substance to which it is attached. hence, then, we have first to give an account of these and other chief elements which, variously joined together, form the different organs: we have to observe the general characters of _tissues_. on going back to the time when the organism begins with a single cell, then becomes a spherical cluster of cells, and then exhibits differences in the modes of aggregation of these cells, the first conspicuous rise of structure (limiting ourselves to animals) is the formation of three layers. of these the first is, at the outset and always, the superficial layer in direct contact with the environment. the second, being originally a part of the first, is also in primitive types in contact with the environment, but, being presently introverted, forms the rudiment of the food-cavity; or, otherwise arising in higher types, is in contact with the yelk or food provided by the parent. and the third, presently formed between these two, consists at the outset of cells derived from them imbedded in an intercellular substance of jelly-like consistence. hence originate the great groups classed as epithelium-tissue, connective tissue (including osseous tissue), muscular tissue, nervous tissue. these severally contain sub-kinds, each of which is a complex of differentiated cells. being brief, and therefore fitted for the present purposes, the sub-classification given by prof. r. hertwig may here be quoted;-- "the physiological character of epithelia is given in the fact that they cover the surfaces of the body, their morphological character in that they consist of closely compressed cells united only by a cementing substance. "according to their further functional character epithelia are divided into glandular epithelia (unicellular and multicellular glands), sensory, germinal, and pavement epithelia. "according to the structure are distinguished one-layered (cubical, cylindrical, pavement epithelia) and many-layered epithelia, ciliated and flagellated epithelia, epithelia with or without cuticle. "the physiological character of the connective tissues rests upon the fact that they fill up spaces between other tissues in the interior of the body. "the morphological character depends upon the presence of the intercellular substance. "according to the quantity and the structure of the intercellular substance the connective substances are divided into ( ) cellular (with little intercellular substance); ( ) homogeneous; ( ) fibrillar connective tissue; ( ) cartilage; ( ) bone. "the physiological character of muscular tissue is contained in the increased capacity for contraction. "the morphological character is found in the fact that the cells have secreted muscle-substance. "according to the nature of the muscle-substance are distinguished smooth and cross-striated muscle-fibres. "according to the character and derivation of the cells (muscle-corpuscles) the musculature is divided into epithelial (epithelial muscle-cells, primary bundles) and connective-tissue muscle cells (contractile fibre-cells). "the physiological character of nervous tissue rests upon the transmission of sensory stimuli and voluntary impulses, and upon the co-ordination of these into unified psychic activity. "the conduction takes place by means of nerve-fibres (non-medullated and medullated fibrils and bundles of fibrils); the co-ordination of stimuli by means of ganglion-cells (bipolar, multipolar ganglion-cells)." (_general principles of zoology_, pp. - .) but now concerning cells out of which, variously modified, obscured, and sometimes obliterated, tissues are formed, we have to note a fact of much significance. along with the cell-doctrine as at first held, when attention was given to the cell itself rather than to its contents, there went the belief that each of these morphological units is structurally separate from its neighbours. but since establishment of the modern view that the essential element is the contained protoplasm, histologists have discovered that there are protoplasmic connexions between the contents of adjacent cells. though cursorily observed at earlier dates, it was not until some twenty years ago that in plant-tissues these were clearly shown to pass through openings in the cell-walls. it is said that in some cases the openings are made, and the junctions established, by a secondary process; but the implication is that usually these living links are left between multiplying protoplasts; so that from the outset the protoplasm pervading the whole plant maintains its continuity. more recently sundry zoologists have alleged that a like continuity exists in animals. especially has this been maintained by mr. adam sedgwick. numerous observations made on developing ova of fishes have led him to assert that in no case do the multiplying cells so-called--blastomeres and their progeny--become entirely separate. their fission is in all cases incomplete. a like continuity has been found in the embryos of many arthropods, and more recently in the segmenting eggs and blastulæ of echinoderms. the _syncytium_ thus formed is held by mr. sedgwick to be maintained in adult life, and in this belief he is in agreement with sundry others. bridges of protoplasm have been seen between epithelium-cells, and it is maintained that cartilage-cells, connective tissue cells, the cells forming muscle-fibres, as well as nerve-cells, have protoplasmic unions. nay, some even assert that an ovum preserves a protoplasmic connexion with the matrix in which it develops. a corollary of great significance may here be drawn. it has been observed that within a vegetal cell the strands of protoplasm stretched in this or that direction contain moving granules, showing that the strands carry currents. it has also been observed that when the fission of a protozoon is so nearly complete that its two halves remain connected only by a thread, currents of protoplasm move through this thread, now one way now the other. the inference fairly to be drawn is that such currents pass also through the strands which unite the protoplasts forming a tissue. what must happen? so long as adjacent cells with their contents are subject to equal pressures no tendency to redistribution of the protoplasm exists, and there may then occur the action sometimes observed inside the strands within a cell: currents with their contained granules moving in opposite directions. but if the cells forming a portion of tissue are subject to greater pressure than the cells around, their contained protoplasm must be forced through the connecting threads into these surrounding cells. every change of pressure at every point must cause movements and counter-movements of this kind. now in the _metazoa_ at large, or at least in all exhibiting relative motions of parts, and especially in all which are capable of rapid locomotion, such changes of pressure are everywhere and always taking place. the contraction of a muscle, besides compressing its components, compresses neighbouring tissues; and every instant contractions and relaxations of muscles go on throughout the limbs and body during active exertion. moreover, each attitude--standing, sitting, lying down, turning over--entails a different set of pressures, both of the parts on one another and on the ground; and those partial arrests of motion which result from sitting down the feet alternately when running, send jolts or waves of varying pressure through the body. the vital actions, too, have kindred effects. an inspiration alters the stress on the tissues throughout a considerable part of the trunk, and a heart-beat propels, down to the smallest arteries, waves which slightly strain the tissues at large. the component cells, thus subject to mechanical disturbances, small and great, perpetual and occasional, are ever having protoplasm forced into them and forced out of them. there are gurgitations and regurgitations which, if they do not constitute a circulation properly so called, at least imply an unceasing redistribution. and the implication is that in the course of days, weeks, months, years, each portion of protoplasm visits every part of the body. without here stating specifically the bearings of these inferences upon the problems of heredity, it will be manifest that certain difficulties they present are in a considerable degree diminished. § e. returning from this parenthetical discussion to the subject of structure, we have to observe that besides facts presented by tissues and facts presented by organs, there are certain facts, less general than the one and more general than the other, which must now be noted. in the order of decreasing generality an account of organs should be preceded by an account of systems of organs. some of these, as the muscular system and the osseous system, are co-extensive with tissues, but others of them are not. the nervous system, for example, contains more than one kind of tissue and is constituted of many different structures: besides afferent and efferent nerves there are the ganglia immediately controlling the viscera, and there are the spinal and cerebral masses, the last of which is divisible into numerous unlike parts. then we have the vascular system made up of the heart, arteries, veins, and capillaries. the lymphatic system, too, with its scattered glands and ramifying channels has to be named. and then, not forgetting the respiratory system with its ancillary appliances, we have the highly heterogeneous alimentary system; including a great number of variously-constructed organs which work together. on contemplating these systems we see their common character to be that while as wholes they cooperate for the carrying on of the total life, each of them consists of cooperative parts: there is cooperation within cooperation. there is another general aspect under which structures must be contemplated. they are divisible into the universal and the particular--those which are everywhere present and those which occupy special places. the blood which a scratch brings out shows us that the vascular system sends branches into each spot. the sensation accompanying a scratch proves that the nervous system, too, has there some of its ultimate fibrils. unobtrusive, and yet to be found at every point, are the ducts of the lymphatic system. and in all parts exists the connective tissue--an inert tough substance which, running through interspaces, wraps up and binds together the other tissues. as is implied by this description, these structures stand in contrast with local structures. here is a bone, there is a muscle, in this place a gland, in that a sense-organ. each has a limited extent and a particular duty. but through every one of them ramify branches of these universal structures. every one of them has its arteries and veins and capillaries, its nerves, its lymphatics, its connective tissue. recognition of this truth introduces what little has here to be said concerning organs; for of course in a work limited to principles no detailed account of these can be entered upon. this remainder truth is that, different as they may be in the rest of their structures, all organs are alike in certain of their structures. all are furnished with these appliances for nutrition, depuration and excitation: they have all to be sustained, all to be stimulated, all to be kept clean. it has finally to be remarked that the general structures which pervade all the special structures at the same time pervade one another. the universal nervous system has everywhere ramifying through it the universal vascular system which feeds it; and the universal vascular system is followed throughout all its ramifications by special nerves which control it. the lymphatics forming a drainage-system run throughout the other systems; and in each of these universal systems is present the connective tissue holding their parts in position. § f. so vast and varied a subject as organic structure, even though the treatment of it is limited to the enunciation of principles, cannot, of course, be dealt with in the space here assigned. next to nothing has been said about plant-structures, and in setting forth the leading traits of animal-structures the illustrations given have been mostly taken from highly-developed creatures. in large measure adumbration rather than exposition is the descriptive word to be applied. nevertheless the reader may carry away certain truths which, exemplified in a few cases, are exemplified more or less fully in all cases. there is the fundamental fact that the plants and animals with which we are familiar--_metaphyta_ and _metazoa_--are formed by the aggregation of units homologous with _protozoa_. these units, often conspicuously showing their homology in early embryonic stages, continue some of them to show it throughout the lives of the highest type of _metazoa_, which contain billions of units carrying on a protozoon life. of the protoplasts not thus active the great mass, comparatively little transformed in low organisms, become more and more transformed as the ascent to high organisms goes on; so that, undergoing numerous kinds of metamorphoses, they lose all likeness to their free homologues, both in shape and composition. the cell-contained protoplasts thus variously changed are fused together into tissues in which their individualities are practically lost; but they nevertheless remain connected throughout by permeable strands of protoplasm. arising by complication of the outer and inner layers of the embryo and growing more unlike as their units become more obscured, these tissues are formed into systems, which develop into sets of organs. some of the resulting structures are localized and special but others are everywhere interfused. while the first named of these facts are displayed in every _metazoon_, and while the last named are visible only in _metazoa_ of considerably developed structures, a gradual transition is shown in intermediate kinds of _metazoa_. of this transition it remains to say that it is effected by the progressive development of auxiliary appliances. for example, the primitive foot-cavity is a sac with one opening only; then comes a second opening through which the waste-matter of the food is expelled. the alimentary canal between these openings is at first practically uniform; afterwards in a certain part of its wall arise numerous bile-cells; these accumulating form a hollow prominence; and this, enlarging, becomes in higher types a liver, while the hollow becomes its duct. in other gradual ways are formed other appended glands. meanwhile the canal itself has its parts differentiated: one being limited to swallowing, another to triturating, another to adding various solvents, another to absorbing the prepared nutriment, another to ejecting the residue. take again the visual organ. the earliest form of it is a mere pigment-speck below the surface. from this (saying nothing here of multiple eyes) we rise by successive complications to a retina formed of multitudinous sensory elements, lenses for throwing images upon it, a curtain for shutting out more or less light, muscles for moving the apparatus about, others for adjusting its focus; and, finally, added to these, either a nictitating membrane or eyelids for perpetually wiping its surface, and a set of eyelashes giving notice when a foreign body is dangerously near. this process of elaborating organs so as to meet additional requirements by additional parts, is the process pursued throughout the body at large. of plant-structures, concerning which so little has been said, it may here be remarked that their relative simplicity is due to the simplicity of their relations to food. the food of plants is universally distributed, while that of animals is dispersed. the immediate consequences are that in the one case motion and locomotion are superfluous, while in the other case they are necessary: the differences in the degrees of structure being consequences. recognizing the locomotive powers of minute _algæ_ and the motions of such other _algæ_ as _oscillatoria_, as well as those movements of leaves and fructifying organs seen in some phænogams, we may say, generally, that plants are motionless; but that they can nevertheless carry on their lives because they are bathed by the required nutriment in the air and in the soil. contrariwise, the nutriment animals require is distributed through space in portions: in some cases near one another and in other cases wide apart. hence motion and locomotion are necessitated; and the implication is that animals must have organs which render them possible. in the first place there must be either limbs or such structures as those which in fish, snakes, and worms move the body along. in the second place, since action implies waste, there must be a set of channels to bring repairing materials to the moving parts. in the third place there must be an alimentary system for taking in and preparing these materials. in the fourth place there must be organs for separating and excreting waste-products. all these appliances must be more highly developed in proportion as the required activity is greater. then there must be an apparatus for directing the motions and locomotions--a nervous system; and as fast as these become rapid and complex the nervous system must be largely developed, ending in great nervous centres--seats of intelligence by which the activities at large are regulated. lastly, underlying all the structural contrasts between plants and animals thus originating, there is the chemical contrast; since the necessity for that highly nitrogenous matter of which animals are formed, is entailed by the necessity for rapidly evolving the energy producing motion. so that, strange as it seems, those chemical, physical, and mental characters of animals which so profoundly distinguish them from plants, are all remote results of the circumstance that their food is dispersed instead of being everywhere present. chapter iii. function. § . does structure originate function, or does function originate structure? is a question about which there has been disagreement. using the word function in its widest signification, as the totality of all vital actions, the question amounts to this--does life produce organization, or does organization produce life? to answer this question is not easy, since we habitually find the two so associated that neither seems possible without the other; and they appear uniformly to increase and decrease together. if it be said that the arrangement of organic substances in particular forms, cannot be the ultimate cause of vital changes, which must depend on the properties of such substances; it may be replied that, in the absence of structural arrangements, the forces evolved cannot be so directed and combined as to secure that correspondence between inner and outer actions which constitutes life. again, to the allegation that the vital activity of every germ whence an organism arises, is obviously antecedent to the development of its structures, there is the answer that such germ is not absolutely structureless. but in truth this question is not determinable by any evidence now accessible to us. the very simplest forms of life known (even the non-nucleated, if there are any) consist of granulated protoplasm; and granulation implies structure. moreover since each kind of protozoon, even the lowest, has its specific mode of development and specific activity--even down to bacteria, some kinds of which, otherwise indistinguishable, are distinguishable by their different reactions on their media--we are obliged to conclude that there must be constitutional differences between the protoplasms they consist of, and this implies structural differences. it seems that structure and function must have advanced _pari passu_: some difference of function, primarily determined by some difference of relation to the environment, initiating a slight difference of structure, and this again leading to a more pronounced difference of function; and so on through continuous actions and reactions. § . function falls into divisions of several kinds according to our point of view. let us take these divisions in the order of their simplicity. under function in its widest sense, are included both the statical and the dynamical distributions of force which an organism opposes to the forces brought to bear on it. in a tree the woody core of trunk and branches, and in an animal the skeleton, internal or external, may be regarded as passively resisting the gravity and momentum which tend habitually or occasionally to derange the requisite relations between the organism and its environment; and since they resist these forces simply by their cohesion, their functions may be classed as _statical_. conversely, the leaves and sap-vessels in a tree, and those organs which in an animal similarly carry on nutrition and circulation, as well as those which generate and direct muscular motion, must be considered as _dynamical_ in their actions. from another point of view function is divisible into the _accumulation of energy_ (latent in food); the _expenditure of energy_ (latent in the tissues and certain matters absorbed by them); and the _transfer of energy_ (latent in the prepared nutriment or blood) from the parts which accumulate to the parts which expend. in plants we see little beyond the first of these: expenditure being comparatively slight, and transfer required mainly to facilitate accumulation. in animals the function of _accumulation_ comprehends those processes by which the materials containing latent energy are taken in, digested, and separated from other materials; the function of _transfer_ comprehends those processes by which these materials, and such others as are needful to liberate the energies they contain, are conveyed throughout the organism; and the function of _expenditure_ comprehends those processes by which the energy is liberated from these materials and transformed into properly co-ordinated motions. each of these three most general divisions includes several more special divisions. the accumulation of energy may be separated into _alimentation_ and _aeration_; of which the first is again separable into the various acts gone through between prehension of food and the transformation of part of it into blood. by the transfer of energy is to be understood what we call _circulation_; if the meaning of circulation be extended to embrace the duties of both the vascular system and the lymphatics. under the head of expenditure of energy come _nervous actions_ and _muscular actions_: though not absolutely co-extensive with expenditure these are almost so. lastly, there are the subsidiary functions which do not properly fall within any of these general functions, but subserve them by removing the obstacles to their performance: those, namely, of _excretion_ and _exhalation_, whereby waste products are got rid of. again, disregarding their purposes and considering them analytically, the general physiologist may consider functions in their widest sense as the correlatives of tissues--the actions of epidermic tissue, cartilaginous tissue, elastic tissue, connective tissue, osseous tissue, muscular tissue, nervous tissue, glandular tissue. once more, physiology in its concrete interpretations recognizes special functions as the ends of special organs--regards the teeth as having the office of mastication; the heart as an apparatus to propel blood; this gland as fitted to produce one requisite secretion and that to produce another; each muscle as the agent of a particular motion; each nerve as the vehicle of a special sensation or a special motor impulse. it is clear that dealing with biology only in its larger aspects, specialities of function do not concern us; except in so far as they serve to illustrate, or to qualify, its generalities. § . the first induction to be here set down is a familiar and obvious one; the induction, namely, that complexity of function is the correlative of complexity of structure. the leading aspects of this truth must be briefly noted. where there are no distinctions of structure there are no distinctions of function. a rhizopod will serve as an illustration. from the outside of this creature, which has not even a limiting membrane, there are protruded numerous processes. originating from any point of the surface, each of these may contract again and disappear, or it may touch some fragment of nutriment which it draws with it, when contracting, into the general mass--thus serving as hand and mouth; or it may come in contact with its fellow-processes at a distance from the body and become confluent with them; or it may attach itself to an adjacent fixed object, and help by its contraction to draw the body into a new position. in brief, this speck of animated jelly is at once all stomach, all skin, all mouth, all limb, and doubtless, too, all lung. in organisms having a fixed distribution of parts there is a concomitant fixed distribution of actions. among plants we see that when, instead of a uniform tissue like that of many _algæ_, everywhere devoted to the same process of assimilation, there arise, as in the higher plants, root and stem and leaves, there arise correspondingly unlike processes. still more conspicuously among animals do there result varieties of function when the originally homogeneous mass is replaced by heterogeneous organs; since, both singly and by their combinations, modified parts generate modified changes. up to the highest organic types this dependence continues manifest; and it may be traced not only under this most general form, but also under the more special form that in animals having one set of functions developed to more than usual heterogeneity there is a correspondingly heterogeneous apparatus devoted to them. thus among birds, which have more varied locomotive powers than mammals, the limbs are more widely differentiated; while the higher mammals, which rise to more numerous and more involved adjustments of inner to outer relations than birds, have more complex nervous systems. § . it is a generalization almost equally obvious with the last, that functions, like structures, arise by progressive differentiations. just as an organ is first an indefinite rudiment, having nothing but some most general characteristic in common with the form it is ultimately to take; so a function begins as a kind of action that is like the kind of action it will eventually become, only in a very vague way. and in functional development, as in structural development, the leading trait thus early manifested is followed successively by traits of less and less importance. this holds equally throughout the ascending grades of organisms and throughout the stages of each organism. let us look at cases: confining our attention to animals, in which functional development is better displayed than in plants. the first differentiation established separates the two fundamentally-opposed functions above named--the accumulation of energy and the expenditure of energy. passing over the _protozoa_ (among which, however, such tribes as present fixed distributions of parts show us substantially the same thing), and commencing with the lowest _coelenterata_, where definite tissues make their appearance, we observe that the only large functional distinction is between the endoderm, which absorbs nutriment, and the ectoderm which, by its own contractions and those of the tentacles it bears, produces motion: the contractility being however to some extent shared by the endoderm. that the functions of accumulation and expenditure are here very incompletely distinguished, may be admitted without affecting the position that this is the first specialization which begins to appear. these two most general and most radically-opposed functions become in the _polyzoa_, much more clearly marked-off from each other: at the same time that each of them becomes partially divided into subordinate functions. the endoderm and ectoderm are no longer merely the inner and outer walls of the same simple sac into which the food is drawn: but the endoderm forms a true alimentary canal, separated from the ectoderm by a peri-visceral cavity, containing the nutritive matters absorbed from the food. that is to say, the function of accumulating force is exercised by a part distinctly divided from the part mainly occupied in expending force: the structure between them, full of absorbed nutriment, effecting in a vague way that transfer of force which, at a higher stage of evolution, becomes a third leading function. meanwhile, the endoderm no longer discharges the accumulative function in the same way throughout its whole extent; but its different portions, oesophagus, stomach and intestine, perform different portions of this function. and instead of a contractility uniformly diffused through the ectoderm, there have arisen in the intermediate mesoderm some parts which have the office of contracting (muscles), and some parts which have the office of making them contract (nerves and ganglia). as we pass upwards, the transfer of force, hitherto effected quite incidentally, comes to have a special organ. in the ascidian, circulation is produced by a muscular tube, open at both ends, which, by a wave of contraction passing along it, sends out at one end the nutrient fluid drawn in at the other; and which, having thus propelled the fluid for a time in one direction, reverses its movement and propels it in the opposite direction. by such means does this rudimentary heart generate alternating currents in the nutriment occupying the peri-visceral cavity. how the function of transferring energy, thus vaguely indicated in these inferior forms, comes afterwards to be the definitely-separated office of a complicated apparatus made up of many parts, each of which has a particular portion of the general duty, need not be described. it is sufficiently manifest that this general function becomes more clearly marked-off from the others, at the same time that it becomes itself parted into subordinate functions. in a developing embryo, the functions or more strictly the structures which are to perform them, arise in the same general order. a like primary distinction very early appears between the endoderm and the ectoderm--the part which has the office of accumulating energy, and the part out of which grow those organs that are the great expenders of energy. between these two there presently arises the mesoderm in which becomes visible the rudiment of that vascular system, which has to fulfil the intermediate duty of transferring energy. of these three general functions, that of accumulating energy is carried on from the outset: the endoderm, even while yet incompletely differentiated from the ectoderm, absorbs nutritive matters from the subjacent yelk. the transfer of energy is also to some extent effected by the rudimentary vascular system, as soon as its central cavity and attached vessels are sketched out. but the expenditure of energy (in the higher animals at least) is not appreciably displayed by those ectodermic and mesodermic structures that are afterwards to be mainly devoted to it: there is no sphere for the actions of these parts. similarly with the chief subdivisions of these fundamental functions. the distinction first established separates the office of transforming other energy into mechanical motion, from the office of liberating the energy to be so transformed. while in the layer between endoderm and ectoderm are arising the rudiments of the muscular system, there is marked out in the ectoderm the rudiment of the nervous system. this indication of structures which are to share between them the general duty of expending energy, is soon followed by changes that foreshadow further specializations of this general duty. in the incipient nervous system there begins to arise that contrast between the cerebral mass and the spinal cord, which, in the main, answers to the division of nervous actions into directive and executive; and, at the same time, the appearance of vertebral laminæ foreshadows the separation of the osseous system, which has to resist the strains of muscular action, from the muscular system, which, in generating motion, entails these strains. simultaneously there have been going on similar actual and potential specializations in the functions of accumulating energy and transferring energy. and throughout all subsequent phases the method is substantially the same. this progress from general, indefinite, and simple kinds of action to special, definite, and complex kinds of action, has been aptly termed by milne-edwards, "the physiological division of labour." perhaps no metaphor can more truly express the nature of this advance from vital activity in its lowest forms to vital activity in its highest forms. and probably the general reader cannot in any other way obtain so clear a conception of functional development in organisms, as he can by tracing out functional development in societies: noting how there first comes a distinction between the governing class and the governed class; how while in the governing class there slowly grow up such differences of duty as the civil, military, and ecclesiastical, there arise in the governed class fundamental industrial differences like those between agriculturists and artizans; and how there is a continual multiplication of such specialized occupations and specialized shares of each occupation. § . fully to understand this change from homogeneity of function to heterogeneity of function, which accompanies the change from homogeneity of structure to heterogeneity of structure, it is needful to contemplate it under a converse aspect. standing alone, the above exposition conveys an idea that is both inadequate and erroneous. the divisions and subdivisions of function, becoming definite as they become multiplied, do not lead to a more and more complete independence of functions; as they would do were the process nothing beyond that just described; but by a simultaneous process they are rendered more mutually dependent. while in one respect they are separating from each other, they are in another respect combining with each other. at the same time that they are being differentiated they are also being integrated. some illustrations will make this plain. in animals which display little beyond the primary differentiation of functions, the activity of that part which absorbs nutriment or accumulates energy, is not immediately bound up with the activity of that part which, in producing motion, expends energy. in the higher animals, however, the performance of the alimentary functions depends on the performance of various muscular and nervous functions. mastication and swallowing are nervo-muscular acts; the rhythmical contractions of the stomach and the allied vermicular motions of the intestines, result from the reflex stimulation of certain muscular coats caused by food; the secretion of the several digestive fluids by their respective glands, is due to nervous excitation of them; and digestion, besides requiring these special aids, is not properly performed in the absence of a continuous discharge of energy from the great nervous centres. again, the function of transferring nutriment or latent energy, from part to part, though at first not closely connected with the other functions, eventually becomes so. the short contractile tube which propels backwards and forwards the blood contained in the peri-visceral cavity of an ascidian, is neither structurally nor functionally much entangled with the creature's other organs. but on passing upwards through higher types, in which this simple tube is replaced by a system of branched tubes, that deliver their contents through their open ends into the tissues at distant parts; and on coming to those advanced types which have closed arterial and venous systems, ramifying minutely in every corner of every organ; we find that the vascular apparatus, while it has become structurally interwoven with the whole body, has become unable properly to fulfil its office without the help of offices that are quite separated from its own. the heart, though mainly automatic in its actions, is controlled by the nervous system, which takes a share in regulating the contractions both of the heart and the arteries. on the due discharge of the respiratory function, too, the function of circulation is directly dependent: if the aeration of the blood is impeded the vascular activity is lowered; and arrest of the one very soon causes stoppage of the other. similarly with the duties of the nervo-muscular system. animals of low organization, in which the differentiation and integration of the vital actions have not been carried far, will move about for a considerable time after being eviscerated, or deprived of those appliances by which energy is accumulated and transferred. but animals of high organization are instantly killed by the removal of these appliances, and even by the injury of minor parts of them: a dog's movements are suddenly brought to an end, by cutting one of the main canals along which the materials that evolve movements are conveyed. thus while in well-developed creatures the distinction of functions is very marked, the combination of functions is very close. from instant to instant the aeration of blood implies that certain respiratory muscles are being made to contract by nervous impulses passing along certain nerves; and that the heart is duly propelling the blood to be aerated. from instant to instant digestion proceeds only on condition that there is a supply of aerated blood, and a due current of nervous energy through the digestive organs. that the heart of a mammal may act, its muscle substance must be continuously fed with an abundant supply of arterial blood. it is not easy to find an adequate expression for this double re-distribution of functions. it is not easy to realize a transformation through which the functions thus become in one sense separated and in another sense combined, or even interfused. here, however, as before, an analogy drawn from social organization helps us. if we observe how the increasing division of labour in societies is accompanied by a closer co-operation; and how the agencies of different social actions, while becoming in one respect more distinct, become in another respect more minutely ramified through one another; we shall understand better the increasing physiological co-operation that accompanies increasing physiological division of labour. note, for example, that while local divisions and classes of the community have been growing unlike in their several occupations, the carrying on of their several occupations has been growing dependent on the due activity of that vast organization by which sustenance is collected and diffused. during the early stages of social development, every small group of people, and often every family, obtained separately its own necessaries; but now, for each necessary, and for each superfluity, there exists a combined body of wholesale and retail distributors, which brings its branched channels of supply within reach of all. while each citizen is pursuing a business that does not immediately aim at the satisfaction of his personal wants, his personal wants are satisfied by a general agency which brings from all places commodities for him and his fellow-citizens--an agency which could not cease its special duties for a few days, without bringing to an end his own special duties and those of most others. consider, again, how each of these differentiated functions is everywhere pervaded by certain other differentiated functions. merchants, manufacturers, wholesale distributors of their several species, together with lawyers, bankers, &c., all employ clerks. in clerks we have a specialized class dispersed through various other classes; and having its function fused with the different functions of these various other classes. similarly commercial travellers, though having in one sense a separate occupation, have in another sense an occupation forming part of each of the many occupations which it aids. as it is here with the sociological division of labour, so is it with the physiological division of labour above described. just as we see in an advanced community, that while the magisterial, the clerical, the medical, the legal, the manufacturing, and the commercial activities, have grown distinct, they have yet their agencies mingled together in every locality; so in a developed organism, we see that while the general functions of circulation, secretion, absorption, excretion, contraction, excitation, &c., have become differentiated, yet through the ramifications of the systems apportioned to them, they are closely combined with one another in every organ. § . the physiological division of labour is usually not carried so far as wholly to destroy the primary physiological community of labour. as in societies the adaptation of special classes to special duties, does not entirely disable these classes from performing one another's duties on an emergency; so in organisms, tissues and structures that have become fitted to the particular offices they have ordinarily to discharge, often remain partially able to discharge other offices. it has been pointed out by dr. carpenter, that "in cases where the different functions are highly specialized, the general structure retains, more or less, the primitive community of function which originally characterized it." a few instances will bring home this generalization. the roots and leaves of plants are widely differentiated in their functions: by the roots, water and mineral substances are absorbed; while the leaves take in, and decompose, carbonic acid. nevertheless, by many botanists it is held that some leaves, or parts of them, can absorb water; and in what are popularly called "air-plants," or at any rate in some kinds of them, the absorption of water is mainly and in some cases wholly carried on by them and by the stems. conversely, the underground parts can partially assume the functions of leaves. the exposed tuber of a potato develops chlorophyll on its surface, and in other cases, as in that of the turnip, roots, properly so called, do the like. in trees the trunks, which have in great measure ceased to produce buds, recommence producing them if the branches are cut off; sometimes aerial branches send down roots to the earth; and under some circumstances the roots, though not in the habit of developing leaf-bearing organs, send up numerous suckers. when the excretion of bile is arrested, part goes to the skin and some to the kidneys, which presently suffer under their new task. various examples of vicarious functions may be found among animals. the excretion of carbonic acid and absorption of oxygen are mainly performed by the lungs, in creatures which have lungs; but in such creatures there continues a certain amount of cutaneous respiration, and in soft-skinned batrachians like the frog, this cutaneous respiration is important. again, when the kidneys are not discharging their duties a notable quantity of urea is got rid of by perspiration. other instances are supplied by the higher functions. in man the limbs, which among lower vertebrates are almost wholly organs of locomotion, are specialized into organs of locomotion and organs of manipulation. nevertheless, the human arms and legs do, when needful, fulfil, to some extent, each other's offices. not only in childhood and old age are the arms used for purposes of support, but on occasions of emergency, as when mountaineering, they are used by men in full vigour. and that legs are to a considerable degree capable of performing the duties of arms, is proved by the great amount of manipulatory skill reached by them when the arms are absent. among the perceptions, too, there are examples of partial substitution. the deaf dr. kitto described himself as having become excessively sensitive to vibrations propagated through the body; and as so having gained the power of perceiving, through his general sensations, those neighbouring concussions of which the ears ordinarily give notice. blind people make hearing perform, in part, the office of vision. instead of identifying the positions and sizes of neighbouring objects by the reflection of light from their surfaces, they do this in a rude way by the reflection of sound from their surfaces. we see, as we might expect to see, that this power of performing more general functions, is great in proportion as the organs have been but little adapted to their special functions. those parts of plants which show so considerable an ability to discharge each others' offices, are not widely unlike in their minute structures. and the tissues which in animals are to some extent mutually vicarious, are tissues in which the original cellular composition is still conspicuous. but we do not find evidence that the muscular, nervous, or osseous tissues are able in any degree to perform those processes which the less differentiated tissues perform. nor have we any proof that nerve can partially fulfil the duty of muscle, or muscle that of nerve. we must say, therefore, that the ability to resume the primordial community of function, varies inversely as the established specialization of function; and that it disappears when the specialization of function becomes great. § . something approaching to _a priori_ reasons may be given for the conclusions thus reached _a posteriori_. they must be accepted for as much as they seem worth. it may be argued that on the hypothesis of evolution, life necessarily comes before organization. on this hypothesis, organic matter in a state of homogeneous aggregation must precede organic matter in a state of heterogeneous aggregation. but since the passing from a structureless state to a structured state, is itself a vital process, it follows that vital activity must have existed while there was yet no structure: structure could not else arise. that function takes precedence of structure, seems also implied in the definition of life. if life is shown by inner actions so adjusted as to balance outer actions--if the implied energy is the _substance_ of life while the adjustment of the actions constitutes its _form_; then may we not say that the actions to be formed must come before that which forms them--that the continuous change which is the basis of function, must come before the structure which brings function into shape? or again, since in all phases of life up to the highest, every advance is the effecting of some better adjustment of inner to outer actions; and since the accompanying new complexity of structure is simply a means of making possible this better adjustment; it follows that the achievement of function is, throughout, that for which structure arises. not only is this manifestly true where the modification of structure results by reaction from modification of function; but it is also true where a modification of structure otherwise produced, apparently initiates a modification of function. for it is only when such so-called spontaneous modification of structure subserves some advantageous action, that it is permanently established. if it is a structural modification that happens to facilitate the vital activities, "natural selection" retains and increases it; but if not, it disappears. the connexion which we noted between heterogeneity of structure and heterogeneity of function--a connexion made so familiar by experience as to appear scarcely worth specifying--is clearly a necessary one. it follows from the general truth that in proportion to the heterogeneity of any aggregate, is the heterogeneity it will produce in any incident force (_first principles_, § ). the energy continually liberated in the organism by decomposition, is here the incident force; the functions are the variously modified forms produced in its divisions by the organs they pass through; and the more multiform the organs the more multiform must be the differentiations of the force passing through them. it follows obviously from this, that if structure progresses from the homogeneous, indefinite, and incoherent, to the heterogeneous, definite, and coherent, so too must function. if the number of different parts in an aggregate must determine the number of differentiations produced in the energies passing through it--if the distinctness of these parts from one another, must involve distinctness in their reactions, and therefore distinctness between the divisions of the differentiated energy; there cannot but be a complete parallelism between the development of structure and the development of function. if structure advances from the simple and general to the complex and special, function must do the same. chapter iv. waste and repair. § . throughout the vegetal kingdom, the processes of waste and repair are comparatively insignificant in their amounts. though all parts of plants save the leaves, or other parts which are green, give out carbonic acid; yet this carbonic acid, assuming it to indicate consumption of tissue, or rather of the protoplasm contained in the tissue, indicates but a small consumption. of course if there is little waste there can be but little repair--that is, little of the interstitial repair which restores the integrity of parts worn by functional activity. nor, indeed, is there displayed by plants in any considerable degree, if at all, that other species of repair which consists in the restoration of lost or injured organs. torn leaves and the shoots that are shortened by the pruner, do not reproduce their missing parts; and though when the branch of a tree is cut off close to the trunk, the place is in course of years covered over, it is not by any reparative action in the wounded surface but by the lateral growth of the adjacent bark. hence, without saying that waste and repair do not go on at all in plants, we may fitly pass them over as of no importance. there are but slight indications of waste in those lower orders of animals which, by their comparative inactivity, show themselves least removed from vegetal life. actiniæ kept in an aquarium, do not appreciably diminish in bulk from prolonged abstinence. even fish, though much more active than most other aquatic creatures, appear to undergo but little loss of substance when kept unfed during considerable periods. reptiles, too, maintaining no great temperature, and passing their lives mostly in a state of torpor, suffer but little diminution of mass by waste. when, however, we turn to those higher orders of animals which are active and hot-blooded, we see that waste is rapid: producing, when unchecked, a notable decrease in bulk and weight, ending very shortly in death. besides finding that waste is inconsiderable in creatures which produce but little insensible and sensible motion, and that it becomes conspicuous in creatures which produce much insensible and sensible motion; we find that in the same creatures there is most waste when most motion is generated. this is clearly proved by hybernating animals. "valentin found that the waking marmot excreted in the average times more carbonic acid, and inhaled times more oxygen than the same animal in the most complete state of hybernation. the stages between waking and most profound hybernation yielded intermediate figures. a waking hedgehog yielded about . times more carbonic acid, and consumed . times more oxygen than one in the state of hybernation."[ ] if we take these quantities of absorbed oxygen and excreted carbonic acid, as indicating something like the relative amounts of consumed organic substance, we see that there is a striking contrast between the waste accompanying the ordinary state of activity, and the waste accompanying complete quiescence and reduced temperature. this difference is still more definitely shown by the fact, that the mean daily loss from starvation in rabbits and guinea-pigs, bears to that from hybernation, the proportion of . : . among men and domestic animals, the relation between degree of waste and amount of expended energy, though one respecting which there is little doubt, is less distinctly demonstrable; since waste is not allowed to go on uninterfered with. we have, however, in the lingering lives of invalids who are able to take scarcely any nutriment but are kept warm and still, an illustration of the extent to which waste diminishes as the expenditure of energy declines. besides the connexion between the waste of the organism as a whole and the production of sensible and insensible motion by the organism as a whole, there is a traceable connexion between the waste of special parts and the activities of such special parts. experiments have shown that "the starving pigeon daily consumes in the average times more muscular substance that the marmot in the state of torpor, and only times more fat, times more of the tissue of the alimentary canal, . times more liver, times more lung, times more skin." that is to say, in the hybernating animal the parts least consumed are the almost totally quiescent motor-organs, and the part most consumed is the hydro-carbonaceous deposit serving as a store of energy; whereas in the pigeon, similarly unsupplied with food but awake and active, the greatest loss takes place in the motor-organs. the relation between special activity and special waste, is illustrated, too, in the daily experiences of all: not indeed in the amount of decrease of the active parts in bulk or weight, for this we have no means of ascertaining; but in the diminished ability of such parts to perform their functions. that legs exerted for many hours in walking and arms long strained in rowing, lose their powers--that eyes become enfeebled by reading or writing without intermission--that concentrated attention, unbroken by rest, so prostrates the brain as to incapacitate it for thinking; are familiar truths. and though we have no direct evidence to this effect, there is little danger in concluding that muscles exercised until they ache or become stiff, and nerves of sense rendered weary or obtuse by work, are organs so much wasted by action as to be partially incompetent. repair is everywhere and always making up for waste. though the two processes vary in their relative rates both are constantly going on. though during the active, waking state of an animal waste is in excess of repair, yet repair is in progress; and though during sleep repair is in excess of waste, yet some waste is necessitated by the carrying on of certain never-ceasing functions. the organs of these never-ceasing functions furnish, indeed, the most conclusive proofs of the simultaneity of repair and waste. day and night the heart never stops beating, but only varies in the rapidity and vigour of its beats; and hence the loss of substance which its contractions from moment to moment entail, must from moment to moment be made good. day and night the lungs dilate and collapse; and the muscles which make them do this must therefore be kept in a state of integrity by a repair which keeps pace with waste, or which alternately falls behind and gets in advance of it to a very slight extent. on a survey of the facts we see, as we might expect to see, that the progress of repair is most rapid when activity is most reduced. assuming that the organs which absorb and circulate nutriment are in proper order, the restoration of the body to a state of integrity, after the disintegration consequent on expenditure of energy, is proportionate to the diminution in expenditure of energy. thus we all know that those who are in health, feel the greatest return of vigour after profound sleep--after complete cessation of motion. we know that a night during which the quiescence, bodily and mental, has been less decided, is usually not followed by that spontaneous overflow of energy which indicates a high state of efficiency throughout the organism. we know, again, that long-continued recumbency, even with wakefulness (providing the wakefulness is not the result of disorder), is followed by a certain renewal of strength; though a renewal less than that which would have followed the greater inactivity of slumber. we know, too, that when exhausted by labour, sitting brings a partial return of vigour. and we also know that after the violent exertion of running, a lapse into the less violent exertion of walking, results in a gradual disappearance of that prostration which the running produced. this series of illustrations conclusively proves that the rebuilding of the organism is ever making up for the pulling down of it caused by action; and that the effect of this rebuilding becomes more manifest, in proportion as the pulling down is less rapid. from each digested meal there is every few hours absorbed into the mass of prepared nutriment circulating through the body, a fresh supply of the needful organic compounds; and from the blood, thus occasionally re-enriched, the organs through which it passes are ever taking up materials to replace the materials used up in the discharge of functions. during activity the reintegration falls in arrear of the disintegration; until, as a consequence, there presently comes a general state of functional languor; ending, at length, in a quiescence which permits the reintegration to exceed the disintegration, and restore the parts to their state of integrity. here, as wherever there are antagonistic actions, we see rhythmical divergences on opposite sides of the medium state--changes which equilibrate each other by their alternate excesses. (_first principles_, §§ , .) illustrations are not wanting of special repair that is similarly ever in progress, and similarly has intervals during which it falls below waste and rises above it. every one knows that a muscle, or a set of muscles, continuously strained, as by holding out a weight at arm's length, soon loses its power; and that it recovers its power more or less fully after a short rest. the several organs of the special sensations yield us like experiences. strong tastes, powerful odours, loud sounds, temporarily unfit the nerves impressed by them for appreciating faint tastes, odours, or sounds; but these incapacities are remedied by brief intervals of repose. vision still better illustrates this simultaneity of waste and repair. looking at the sun so affects the eyes that, for a short time, they cannot perceive the things around with the usual clearness. after gazing at a bright light of a particular colour, we see, on turning the eyes to adjacent objects, an image of the complementary colour; showing that the retina has, for the moment, lost the power to feel small amounts of those rays which have strongly affected it. such inabilities disappear in a few seconds or a few minutes, according to circumstances. and here, indeed, we are introduced to a conclusive proof that special repair is ever neutralizing special waste. for the rapidity with which the eyes recover their sensitiveness, varies with the reparative power of the individual. in youth the visual apparatus is so quickly restored to its state of integrity, that many of these _photogenes_, as they are called, cannot be perceived. when sitting on the far side of a room, and gazing out of the window against a light sky, a person who is debilitated by disease or advancing years, perceives, on transferring the gaze to the adjacent wall, a momentary negative image of the window--the sash-bars appearing light and the squares dark; but a young and healthy person has no such experience. with a rich blood and vigorous circulation, the repair of the visual nerves after impressions of moderate intensity, is nearly instantaneous. function carried to excess may produce waste so great that repair cannot make up for it during the ordinary daily periods of rest; and there may result incapacities of the over-taxed organs, lasting for considerable periods. we know that eyes strained by long-continued minute work lose their power for months or years: perhaps suffering an injury from which they never wholly recover. brains, too, are often so unduly worked that permanent relaxation fails to restore them to vigour. even of the motor organs the like holds. the most frequent cause of what is called "wasting palsy," or atrophy of the muscles, is habitual excess of exertion: the proof being that the disease occurs most frequently among those engaged in laborious handicrafts, and usually attacks first the muscles which have been most worked. there has yet to be noticed another kind of repair--that, namely, by which injured or lost parts are restored. among the _hydrozoa_ it is common for any portion of the body to reproduce the rest; even though the rest to be so reproduced is the greater part of the whole. in the more highly-organized _actinozoa_ the half of an individual will grow into a complete individual. some of the lower annelids, as the _nais_, may be cut into thirty or forty pieces and each piece will eventually become a perfect animal. as we ascend to higher forms we find this reparative power much diminished, though still considerable. the reproduction of a lost claw by a lobster or crab, is a familiar instance. some of the inferior _vertebrata_ also, as lizards, can develop new limbs or new tails, in place of those which have been cut off; and can even do this several times over, though with decreasing completeness. the highest animals, however, thus repair themselves to but a very small extent. mammals and birds do it only in the healing of wounds; and very often but imperfectly even in this. for in muscular and glandular organs the tissues destroyed are not properly reproduced, but are replaced by tissue of an irregular kind which serves to hold the parts together. so that the power of reproducing lost parts is greatest where the organization is lowest; and almost disappears where the organization is highest. and though we cannot say that in the intermediate stages there is a constant inverse relation between reparative power and degree of organization; yet we may say that there is some approach to such a relation. § . there is an obvious and complete harmony between the first of the above inductions and the deduction which follows immediately from first principles. we have already seen (§ ) "that whatever amount of power an organism expends in any shape, is the correlate and equivalent of a power that was taken into it from without." motion, sensible or insensible, generated by an organism, is insensible motion which was absorbed in producing certain chemical compounds appropriated by the organism under the form of food. as much energy as was required to raise the elements of these complex atoms to their state of unstable equilibrium, is given out in their falls to a state of stable equilibrium; and having fallen to a state of stable equilibrium they can give out no further energy, but have to be got rid of as inert and useless. it is an inevitable corollary "from the persistence of force, that each portion of mechanical or other energy which an organism exerts, implies the transformation of as much organic matter as contained this energy in a latent state;" and that this organic matter in yielding up its latent energy, loses its value for the purposes of life, and becomes waste matter needing to be excreted. the loss of these complex unstable substances must hence be proportionate to the quantity of expended force. here, then, is the rationale of certain general facts lately indicated. plants do not waste to any considerable degree, for the obvious reason that the sensible and insensible motions they generate are inconsiderable. between the small waste, small activity, and low temperature of the inferior animals, the relation is similarly one admitting of _a priori_ establishment. conversely, the rapid waste of energetic, hot-blooded animals might be foreseen with equal certainty. and not less manifestly necessary is the variation in waste which, in the same organism, attends the variation in the heat and mechanical motion produced. between the activity of a special part and the waste of that part, a like relation may be deductively inferred; though it cannot be inferred that this relation is equally definite. were the activity of every organ quite independent of the activities of other organs, we might expect to trace out this relation distinctly; but since increased activity in any organ or group of organs, as the muscles, necessarily entails increased activity in other organs, as in the heart, lungs, and nervous system, it is clear that special waste and general waste are too much entangled to admit of a definite relation being established between special waste and special activity. we may fairly say, however, that this relation is quite as manifest as we can reasonably anticipate. § . deductive interpretation of the phenomena of repair, is by no means so easy. the tendency displayed by an animal organism, as well as by each of its organs, to return to a state of integrity by the assimilation of new matter, when it has undergone the waste consequent on activity, is a tendency which is not manifestly deducible from first principles; though it appears to be in harmony with them. if in the blood there existed ready-formed units exactly like in kind to those of which each organ consists, the sorting of these units, ending in the union of each kind with already existing groups of the same kind, would be merely a good example of segregation (_first principles_, § ). it would be analogous to the process by which, from a mixed solution of salts, there are, after an interval, deposited separate masses of these salts in the shape of different crystals. but as already said (§ ), though the selective assimilation by which the repair of organs is effected, may result in part from an action of this kind, the facts cannot be thus wholly accounted for; since organs are in part made up of units which do not exist as such in the circulating fluids. we must suppose that, as suggested in § , groups of compound units have a certain power of moulding adjacent fit materials into units of their own form. let us see whether there is not reason to think such a power exists. "the poison of small-pox or of scarlatina," remarks mr. (now sir james) paget, "being once added to the blood, presently affects the composition of the whole: the disease pursues its course, and, if recovery ensue, the blood will seem to have returned to its previous condition: yet it is not as it was before; for now the same poison may be added to it with impunity." ... "the change once effected, may be maintained through life. and herein seems to be a proof of the assimilative force in the blood: for there seems no other mode of explaining these cases than by admitting that the altered particles have the power of assimilating to themselves all those by which they are being replaced: in other words, all the blood that is formed after such a disease deviates from the natural composition, so far as to acquire the peculiarity engendered by the disease: it is formed according to the altered model." now if the compound molecules of the blood, or of an organism considered in the aggregate, have the power of moulding into their own type the matters which they absorb as nutriment; and if they have the power when their type has been changed by disease, of moulding materials afterwards received into the modified type; may we not reasonably suspect that the more or less specialized molecules of each organ have, in like manner, the power of moulding the materials which the blood brings to them into similarly specialized molecules? the one conclusion seems to be a corollary from the other. such a power cannot be claimed for the component units of the blood without being conceded to the component units of every tissue. indeed the assertion of this power is little more than an assertion of the fact that organs composed of specialized units _are_ capable of resuming their structural integrity after they have been wasted by function. for if they do this, they must do it by forming from the materials brought to them, certain specialized units like in kind to those of which they are composed; and to say that they do this, is to say that their component units have the power of moulding fit materials into other units of the same order. § . what must we say of the ability an organism has to re-complete itself when one of its parts has been cut off? is it of the same order as the ability of an injured crystal to re-complete itself. in either case new matter is so deposited as to restore the original outline. and if in the case of the crystal we say that the whole aggregate exerts over its parts a force which constrains the newly-integrated molecules to take a certain definite form, we seem obliged, in the case of the organism, to assume an analogous force. if when the leg of a lizard has been amputated there presently buds out the germ of a new one, which, passing through phases of development like those of the original leg, eventually assumes a like shape and structure, we assert only what we see, when we assert that the entire organism, or the adjacent part of it, exercises such power over the forming limb as makes it a repetition of its predecessor. if a leg is reproduced, where there was a leg, and a tail where there was a tail, there seems no alternative but to conclude that the forces around it control the formative processes going on in each part. and on contemplating these facts in connexion with various kindred ones, there is suggested the hypothesis, that the form of each species of organism is determined by a peculiarity in the constitution of its units--that these have a special structure in which they tend to arrange themselves; just as have the simpler units of inorganic matter. let us glance at the evidences which more especially thrust this conclusion upon us. a fragment of a begonia-leaf imbedded in fit soil and kept at an appropriate temperature, will develop a young begonia; and so small is the fragment which is thus capable of originating a complete plant, that something like a hundred plants may be produced from a single leaf. the friend to whom i owe this observation, tells me that various succulent plants have like powers of multiplication. illustrating a similar power among animals, we have the often-cited experiments of trembley on the common polype. each of the four pieces into which one of these creatures was cut, grew into a perfect individual. in each of these, again, bisection and tri-section were followed by like results. and so with their segments, similarly produced, until as many as fifty polypes had resulted from the original one. bodies when cut off regenerated heads; heads regenerated bodies; and when a polype had been divided into as many pieces as was practicable, nearly every piece survived and became a complete animal. what, now, is the implication? we cannot say that in each portion of a begonia-leaf, and in every fragment of a hydra's body, there exists a ready-formed model of the entire organism. even were there warrant for the doctrine that the germ of every organism contains the perfect organism in miniature, it still could not be contended that each considerable part of the perfect organism resulting from such a germ, contains another such miniature. indeed the one hypothesis negatives the other. the implication seems, therefore, to be that the living particles composing one of these fragments, have an innate tendency to arrange themselves into the shape of the organism to which they belong. we must infer that the active units composing a plant or animal of any species have an intrinsic aptitude to aggregate into the form of that species. it seems difficult to conceive that this can be so; but we see that it _is_ so. groups of units taken from an organism (providing they are of a certain bulk and not much differentiated into special structures) _have_ this power of re-arranging themselves. manifestly, too, if we are thus to interpret the reproduction of an organism from one of its amorphous fragments, we must thus interpret the reproduction of any minor portion of an organism by the remainder. when in place of its lost claw a lobster puts forth a cellular mass which, while increasing in bulk, assumes the form and structure of the original claw, we cannot avoid ascribing this result to a play of forces like that which moulds the materials contained in a piece of begonia-leaf into the shape of a young begonia. § . as we shall have frequent occasion hereafter to refer to these units which possess the property of arranging themselves into the special structures of the organisms to which they belong; it will be well here to ask by what name they may be most fitly called. on the one hand, it cannot be in those chemical compounds characterizing organic bodies that this specific property dwells. it cannot be that the molecules of albumin, or fibrin, or gelatine, or other proteid, possess this power of aggregating into these specific shapes; for in such case there would be nothing to account for the unlikenesses of different organisms. if the proclivities of proteid molecules determined the forms of the organisms built up of them or by them, the occurrence of such endlessly varied forms would be inexplicable. hence what we may call the _chemical units_ are clearly not the possessors of this property. on the other hand, this property cannot reside in what may be roughly distinguished as the _morphological units_. the germ of every organism is a minute portion of encased protoplasm commonly called a cell. it is by multiplication of cells that all the early developmental changes are effected. the various tissues which successively arise in the unfolding organism, are primarily cellular; and in many of them the formation of cells continues to be, throughout life, the process by which repair is carried on. but though cells are so generally the ultimate visible components of organisms, that they may with some show of reason be called the morphological units; yet we cannot say that this tendency to aggregate into special forms dwells in them. in many cases a fibrous tissue arises out of a nucleated blastema, without cell-formation; and in such cases cells cannot be regarded as units possessing the structural proclivity. but the conclusive proof that the morphological units are not the building factors in an organism composed of them, is yielded by their independent homologues the so-called unicellular organisms. for each of these displays the power to assume its specific structure. clearly, if the ability of a multicellular organism to assume its specific structure resulted from the cooperation of its component cells, then a single cell, or the independent homologue of a single cell, having no other to cooperate with, could exhibit no structural traits. not only, however, do single-celled organisms exhibit structural traits, but these, even among the simplest, are so distinct as to originate classification into orders, genera, and species; and they are so constant as to remain the same from generation to generation. if, then, this organic polarity (as we might figuratively call this proclivity towards a specific structural arrangement) can be possessed neither by the chemical units nor the morphological units, we must conceive it as possessed by certain intermediate units, which we may term _physiological_. there seems no alternative but to suppose that the chemical units combine into units immensely more complex than themselves, complex as they are; and that in each organism the physiological units produced by this further compounding of highly compound molecules, have a more or less distinctive character. we must conclude that in each case some difference of composition in the units, or of arrangement in their components, leading to some difference in their mutual play of forces, produces a difference in the form which the aggregate of them assumes. the facts contained in this chapter form but a small part of the evidence which thrusts this assumption upon us. we shall hereafter find various reasons for inferring that such physiological units exist, and that to their specific properties, more or less unlike in each plant and animal, various organic phenomena are due. chapter v. adaptation. § . in plants waste and repair being scarcely appreciable, there are not likely to arise appreciable changes in the proportions of already-formed parts. the only divergences from the average structures of a species, which we may expect particular conditions to produce, are those producible by the action of these conditions on parts in course of formation; and such divergences we do find. we know that a tree which, standing alone in an exposed position, has a short and thick stem, has a tall and slender stem when it grows in a wood; and that also its branches then take a different inclination. we know that potato-sprouts which, on reaching the light, develop into foliage, will, in the absence of light, grow to a length of several feet without foliage. and every in-door plant furnishes proof that shoots and leaves, by habitually turning themselves to the light, exhibit a certain adaptation--an adaptation due, as we must suppose; to the special effects of the special conditions on the still growing parts. in animals, however, besides analogous structural changes wrought during the period of growth, by subjection to circumstances unlike the ordinary circumstances, there are structural changes similarly wrought after maturity has been reached. organs that have arrived at their full sizes possess a certain modifiability; so that while the organism as a whole retains pretty nearly the same bulk, the proportions of its parts may be considerably varied. their variations, here treated of under the title adaptation, depend on specialities of individual action. in the last chapter we saw that the actions of organisms entail re-actions on them; and that specialities of action entail specialities of re-action. here it remains to be pointed out that these special actions and re-actions do not end with temporary changes, but work permanent changes. if, in an adult animal, the waste and repair in all parts were exactly balanced--if each organ daily gained by nutrition exactly as much as it lost daily by the discharge of its function--if excess of function were followed only by such excess of nutrition as balanced the extra waste; it is clear that there would occur no change in the relative sizes of organs. but there is no such exact balance. if the excess of function, and consequent excess of waste, is moderate, it is not simply compensated by repair but more than compensated--there is a certain increase of bulk. this is true to some degree of the organism as a whole, when the organism is framed for activity. a considerable waste giving considerable power of assimilation, is more favourable to accumulation of tissue than is quiescence with its comparatively feeble assimilation: whence results a certain adaptation of the whole organism to its requirements. but it is more especially true of the parts of an organism in relation to one another. the illustrations fall into several groups. the growth of muscles exercised to an unusual degree is a matter of common observation. in the often-cited blacksmith's arm, the dancer's legs and the jockey's crural adductors, we have marked examples of a modifiability which almost every one has to some extent experienced. it is needless to multiply proofs. the occurrence of changes in the structure of the skin, where the skin is exposed to unusual stress of function, is also familiar. that thickening of the epidermis on a labourer's palm results from continual pressure and friction, is certain. those who have not before exerted their hands, find that such an exercise as rowing soon begins to produce a like thickening. this relation of cause and effect is still better shown by the marked indurations at the ends of a violinist's fingers. even in mucous membrane, which ordinarily is not subject to mechanical forces of any intensity, similar modifications are possible: witness the callosity of the gums which arises in those who have lost their teeth, and have to masticate without teeth. the vascular system furnishes good instances of the increased growth that follows increased function. when, because of some permanent obstruction to the circulation, the heart has to exert a greater contractile force on the mass of blood which it propels at each pulsation, and when there results the laboured action known as palpitation, there usually occurs dilatation, or hypertrophy, or a mixture of the two: the dilatation, which is a yielding of the heart's structure under the increased strain, implying a failure to meet the emergency; but the hypertrophy, which consists in a thickening of the heart's muscular walls, being an adaptation of it to the additional effort required. again, when an aneurism in some considerable artery has been obliterated, either artifically or by a natural inflammatory process; and when this artery has consequently ceased to be a channel for the blood; some of the adjacent arteries which anastomose with it become enlarged, so as to carry the needful quantity of blood to the parts supplied. though we have no direct proof of analogous modifications in nervous structures, yet indirect proof is given by the greater efficiency that follows greater activity. this is manifested alike in the senses and the intellect. the palate may be cultivated into extreme sensitiveness, as in professional tea-tasters. an orchestral conductor gains, by continual practice, an unusually great ability to discriminate differences of sound. in the finger-reading of the blind we have evidence that the sense of touch may be brought by exercise to a far higher capability than is ordinary.[ ] the increase of power which habitual exertion gives to mental faculties needs no illustration: every person of education has personal experience of it. even from the osseous structures evidence may be drawn. the bones of men accustomed to great muscular action are more massive, and have more strongly marked processes for the attachment of muscles, than the bones of men who lead sedentary lives; and a like contrast holds between the bones of wild and tame animals of the same species. adaptations of another order, in which there is a qualitative rather than a quantitative modification, arise after certain accidents to which the skeleton is liable. when the hip-joint has been dislocated, and long delay has made it impossible to restore the parts to their proper places, the head of the thigh-bone, imbedded in the surrounding muscles, becomes fixed in its new position by attachments of fibrous tissue, which afford support enough to permit a halting walk. but the most remarkable modification of this order occurs in united ends of fractured bones. "false joints" are often formed--joints which rudely simulate the hinge structure or the ball-and-socket structure, according as the muscles tend to produce a motion of flexion and extension or a motion of rotation. in the one case, according to rokitansky, the two ends of the broken bone become smooth and covered with periosteum and fibrous tissue, and are attached by ligaments that allow a certain backward and forward motion; and in the other case the ends, similarly clothed with the appropriate membranes, become the one convex and the other concave, are inclosed in a capsule, and are even occasionally supplied with synovial fluid! the general truth that extra function is followed by extra growth, must be supplemented by the equally general truth, that beyond a limit, usually soon reached, very little, if any, further modification can be produced. the experiences which we colligate into the one induction thrust the other upon us. after a time no training makes the pugilist or the athlete any stronger. the adult gymnast at last acquires the power to perform certain difficult feats; but certain more difficult feats no additional practice enables him to perform. years of discipline give the singer a particular loudness and range of voice, beyond which further discipline does not give greater loudness or wider range: on the contrary, increased vocal exercise, causing a waste in excess of repair, is often followed by decrease of power. in the exaltation of the perceptions we see similar limits. the culture which raises the susceptibility of the ear to the intervals and harmonies of notes, will not turn a bad ear into a good one. lifelong effort fails to make this artist a correct draftsman or that a fine colourist: each does better than he did at first, but each falls short of the power attained by some other artists. nor is this truth less clearly illustrated among the more complex mental powers. a man may have a mathematical faculty, a poetical faculty, or an oratorical faculty, which special education improves to a certain extent. but unless he is unusually endowed in one of those directions, no amount of education will make him a first-rate mathematician, a first-rate poet, or a first-rate orator. thus the general fact appears to be that while in each individual certain changes in the proportions of parts may be caused by variations of functions, the congenital structure of each individual puts a limit to the modifiability of every part. nor is this true of individuals only: it holds, in a sense, of species. leaving open the question whether, in indefinite times, indefinite modifications may not be produced by inheritance of functionally wrought adaptations; experience proves that within assigned times, the changes wrought in races of organisms by changes of conditions fall within narrow limits. though by discipline, aided by selective breeding, one variety of horse has had its locomotive power increased considerably beyond the locomotive powers of other varieties; yet further increase takes place, if at all, at an inappreciable rate. the different kinds of dogs, too, in which different forms and capacities have been established, do not now show aptitudes for diverging in the same directions at considerable rates. in domestic animals generally, certain accessions of intelligence have been produced by culture; but accessions beyond these are inconspicuous. it seems that in each species of organism there is a margin for functional oscillations on all sides of a mean state, and a consequent margin for structural variations; that it is possible rapidly to push functional and structural changes towards the extreme of this margin in any direction, both in an individual and in a race; but that to push these changes further in any direction, and so to alter the organism as to bring its mean state up to the extreme of the margin in that direction, is a comparatively slow process.[ ] we also have to note that the limited increase of size produced in any organ by a limited increase of its function, is not maintained unless the increase of function is permanent. a mature man or other animal, led by circumstances into exerting particular members in unusual degrees, and acquiring extra sizes in these members, begins to lose such extra sizes on ceasing to exert the members; and eventually lapses more or less nearly into the original state. legs strengthened by a pedestrian tour, become relatively weak again after a prolonged return to sedentary life. the acquired ability to perform feats of skill disappears in course of time, if the performance of them be given up. for comparative failure in executing a piece of music, in playing a game at chess, or in anything requiring special culture, the being out of practice is a reason which every one recognizes as valid. it is observable, too, that the rapidity and completeness with which an artificial power is lost, is proportionate to the shortness of the cultivation which evoked it. one who has for many years persevered in habits which exercise special muscles or special faculties of mind, retains the extra capacity produced, to a very considerable degree, even after a long period of desistance; but one who has persevered in such habits for but a short time has, at the end of a like period, scarcely any of the facility he had gained. here too, as before, successions of organisms present an analogous fact. a species in which domestication continued through many generations, has organized certain peculiarities; and which afterwards, escaping domestic discipline, returns to something like its original habits; soon loses, in great measure, such peculiarities. though it is not true, as alleged, that it resumes completely the structure it had before domestication, yet it approximates to that structure. the dingo, or wild dog of australia, is one of the instances given of this; and the wild horse of south america is another. mankind, too, supplies us with instances. in the australian bush and in the backwoods of america, the anglo-saxon race, in which civilization has developed the higher feelings to a considerable degree, rapidly lapses into comparative barbarism: adopting the moral code, and sometimes the habits, of savages. § . it is important to reach, if possible, some rationale of these general truths--especially of the last two. a right understanding of these laws of organic modification underlies a right understanding of the great question of species. while, as before hinted (§ ), the action of structure on function is one of the factors in that process of differentiation by which unlike forms of plants and animals are produced, the reaction of function on structure is another factor. hence, it is well worth while inquiring how far these inductions are deductively interpretable. the first of them is the most difficult to deal with. why an organ exerted somewhat beyond its wont should presently grow, and thus meet increase of demand by increase of supply, is not obvious. we know, indeed, (_first principles_, §§ , ,) that of necessity, the rhythmical changes produced by antagonistic organic actions cannot any of them be carried to an excess in one direction, without there being produced an equivalent excess in the opposite direction. it is a corollary from the persistence of force, that any deviation effected by a disturbing cause, acting on some member of a moving equilibrium, must (unless it altogether destroys the moving equilibrium) be eventually followed by a compensating deviation. hence, that excess of repair should succeed excess of waste, is to be expected. but how happens the mean state of the organ to be changed? if daily extra waste naturally brings about daily extra repair only to an equivalent extent, the mean state of the organ should remain constant. how then comes the organ to augment in size and power? such answer to this question as we may hope to find, must be looked for in the effects wrought on the organism as a whole by increased function in one of its parts. for since the discharge of its function by any part is possible only on condition that those various other functions on which its own is immediately dependent are also discharged, it follows that excess in its function presupposes some excess in their functions. additional work given to a muscle implies additional work given to the branch arteries which bring it blood, and additional work, smaller in proportion, to the arteries from which these branch arteries come. similarly, the smaller and larger veins which take away the blood, as well as those structures which deal with effete products, must have more to do. and yet further, on the nervous centres which excite the muscle a certain extra duty must fall. but excess of waste will entail excess of repair, in these parts as well as in the muscle. the several appliances by which the nutrition and excitation of an organ are carried on, must also be influenced by this rhythm of action and reaction; and therefore, after losing more than usual by the destructive process they must gain more than usual by the constructive process. but temporarily-increased efficiency in these appliances by which blood and nervous force are brought to an organ, will cause extra assimilation in the organ, beyond that required to balance its extra expenditure. regarding the functions as constituting a moving equilibrium, we may say that divergence of any function in the direction of increase, causes the functions with which it is bound up to diverge in the same direction; that these, again, cause the functions which they are bound up with, also to diverge in the same direction; and that these divergences of the connected functions allow the specially-affected function to be carried further in this direction than it could otherwise be--further than the perturbing force could carry it if it had a fixed basis. it must be admitted that this is but a vague explanation. among actions so involved as these, we can scarcely expect to do more than dimly discern a harmony with first principles. that the facts are to be interpreted in some such way, may, however, be inferred from the circumstance that an extra supply of blood continues for some time to be sent to an organ that has been unusually exercised; and that when unusual exercise is long continued a permanent increase of vascularity results. § . answers to the questions--why do these adaptive modifications in an individual animal soon reach a limit? and why, in the descendants of such animal, similarly conditioned, is this limit very slowly extended?--are to be found in the same direction as was the answer to the last question. and here the connexion of cause and consequence is more manifest. since the function of any organ is dependent on the functions of the organs which supply it with materials and stimuli; and since the functions of these subsidiary organs are dependent on the functions of organs which supply them with materials and stimuli; it follows that before any great extra power of discharging its function can be gained by a specially-exercised organ, a considerable extra power must be gained by a series of immediately-subservient organs, and some extra power by a secondary series of remotely-subservient organs. thus there are required numerous and wide-spread modifications. before the artery which feeds a hard-worked muscle can permanently furnish a large additional quantity of blood, it must increase in diameter; and that its increase of diameter may be of use, the main artery from which it diverges must also be so far modified as to bring this additional quantity of blood to the branch artery. similarly with the veins; similarly with the structures which remove waste-products; similarly with the nerves. and when we ask what these subsidiary changes imply, we are forced to conclude that there must be an analogous group of more numerous changes ramifying throughout the system. the growth of the arteries primarily and secondarily implicated, cannot go to any extent without growth in the minor blood-vessels on which their nutrition depends; while their greater contractile power involves enlargement of the nerves which excite them, and some modification of that part of the spinal cord whence these nerves proceed. thus, without tracing the like remote alterations implied by extra growth of the veins, lymphatics, glandular organs, and other agencies, it is manifest that a large amount of rebuilding must be done throughout the organism, before any organ of importance can be permanently increased in size and power to a great extent. hence, though such extra growth in any part as does not necessitate considerable changes throughout the rest of the organism, may rapidly take place; a further growth in this part, requiring a re-modelling of numerous parts remotely and slightly affected, must take place but slowly. we have before found our conceptions of vital processes made clearer by studying analogous social processes. in societies there is a mutual dependence of functions, essentially like that which exists in organisms; and there is also an essentially like reaction of functions on structures. from the laws of adaptive modification in societies, we may therefore hope to get a clue to the laws of adaptive modification in organisms. let us suppose, then, that a society has arrived at a state of equilibrium analogous to that of a mature animal--a state not like our own, in which growth and structural development are rapidly going on, but a state of settled balance among the functional powers of the various classes and industrial bodies, and a consequent fixity in the relative sizes of such classes and bodies. further, let us suppose that in a society thus balanced there occurs something which throws an unusual demand on one industry--say an unusual demand for ships (which we will assume to be built of iron) in consequence of a competing mercantile nation having been prostrated by famine or pestilence. the immediate result of this additional demand for iron ships is the employment of more workmen, and the purchase of more iron, by the ship-builders; and when, presently, the demand continuing, the ship-builders find their premises and machinery insufficient, they enlarge them. if the extra requirement persists, the high interest and high wages bring such extra capital and labour into the business as are needed for new ship-building establishments. but such extra capital and labour do not come quickly; since, in a balanced community, not increasing in population and wealth, labour and capital have to be drawn from other industries, where they are already yielding the ordinary returns. let us now go a step further. suppose that this iron-ship-building industry, having enlarged as much as the available capital and labour permit, is still unequal to the demand; what limits its immediate further growth? the lack of iron. by the hypothesis, the iron-producing industry, like all the other industries throughout the community, yields only as much iron as is habitually required for all the purposes to which iron is applied: ship-building being only one. if, then, extra iron is required for ship-building, the first effect is to withdraw part of the iron habitually consumed for other purposes, and to raise the price of iron. presently, the iron-makers feel this change and their stocks dwindle. as, however, the quantity of iron required for ship-building forms but a small part of the total quantity required for all purposes, the extra demand on the iron-makers can be nothing like so great in proportion as is the extra demand on the ship-builders. whence it follows that there will be much less tendency to an immediate enlargement of the iron-producing industry; since the extra quantity will for some time be obtained by working extra hours. nevertheless if, as fast as more iron can be thus supplied, the ship-building industry goes on growing--if, consequently, the iron-makers experience a permanently-increased demand, and out of their greater profits get higher interest on capital, as well as pay higher wages; there will eventually be an abstraction of capital and labour from other industries to enlarge the iron-producing industry: new blast-furnaces, new rolling-mills, new cottages for workmen, will be erected. but obviously, the inertia of capital and labour to be overcome before the iron-producing industry can grow by a decrease of certain other industries, will prevent its growth from taking place until long after the increased ship-building industry has demanded it; and meanwhile, the growth of the ship-building industry must be limited by the deficiency of iron. a remoter restraint of the same nature meets us if we go a step further--a restraint which can be overcome only in a still longer time. for the manufacture of iron depends on the supply of coal. the production of coal being previously in equilibrium with the consumption; and the consumption of coal for the manufacture of iron being but a small part of the total consumption; it follows that a considerable extension of the iron manufacture, when it at length takes place, will cause but a comparatively small additional demand on the coal-owners and coal-miners--a demand which will not, for a long period, suffice to cause enlargement of the coal-trade, by drawing capital and labour from other investments and occupations. and until the permanent extra demand for coal has become great enough to draw from other investments and occupations sufficient capital and labour to sink new mines, the increasing production of iron must be restricted by the scarcity of coal, and the multiplication of ship-yards and ship-builders must be checked by the want of iron. thus, in a community which has reached a state of moving equilibrium, though any one industry directly affected by an additional demand may rapidly undergo a small extra growth, yet a growth beyond this, requiring as it does the building-up of subservient industries, less directly and strongly affected, as well as the partial unbuilding of other industries, can take place only with comparative slowness. and a still further growth, requiring structural modifications of industries still more distantly affected, must take place still more slowly. on returning from this analogy, we see more clearly the truth that any considerable member of an animal organism, cannot be greatly enlarged without some general reorganization. besides a building up of the primary, secondary, and tertiary groups of the subservient parts, there must be an unbuilding of sundry non-subservient parts; or, at any rate, there must be permanently established a lower nutrition of such non-subservient parts. for it must be remembered that in a mature animal, or one which has reached a balance between assimilation and expenditure, there cannot (supposing general conditions to remain constant) be an increase in the nutrition of some organs without a decrease in the nutrition of others; and an organic establishment of the increase implies an organic establishment of the decrease--implies more or less change in the processes and structures throughout the entire system. and here, indeed, is disclosed one reason why growing animals undergo adaptations so much more readily than adult ones. for while there is surplus nutrition, it is possible for specially-exercised parts to be specially enlarged without any positive deduction from other parts. there is required only that negative deduction implied in the diminished growth of other parts. § . pursuing the argument further, we reach an explanation of the third general truth; namely that organisms, and species of organisms, which, under new conditions, have undergone adaptive modifications, soon return to something like their original structures when restored to their original conditions. seeing, as we have done, how excess of action and excess of nutrition in any part of an organism, must affect action and nutrition in subservient parts, and these again in other parts, until the re-action has divided and subdivided itself throughout the organism, affecting in decreasing degrees the more and more numerous parts more and more remotely implicated; we see that the consequent changes in the parts remotely implicated, constituting the great mass of the organism, must be extremely slow. hence, if the need for the adaptive modification ceases before the great mass of the organism has been much altered in its structure by these ramified but minute reactions, we shall have a condition in which the specially-modified part is not in equilibrium with the rest. all the remotely-affected organs, as yet but little changed, will, in the absence of the perturbing cause, resume very nearly their previous actions. the parts that depend on them will consequently by and by do the same. until at length, by a reversal of the adaptive process, the organ at first affected will be brought back almost to its original state. reconsidering the above-drawn analogy between an organism and a society, will enable us better to recognize this necessity. if, in the case supposed, the extra demand for iron ships, after causing the erection of some additional ship-yards and the drawing of iron from other manufactures, were to cease; the old dimensions of the ship-building trade would be quickly returned to: discharged workmen would seek fresh occupations, and the new yards would be devoted to other uses. but if the increased need for ships lasted long enough, and became great enough, to cause a flow of capital and labour from other industries into the iron-manufacture, a falling off in the demand for ships, would much less rapidly entail a dwindling of the ship-building industry. for iron being now produced in greater quantity, a diminished consumption of it for ships would cause a fall in its price, and a consequent fall in the cost of ships: thus enabling the ship-builders to meet the competition which we may suppose led to a decrease in the orders they received. and since, when new blast-furnaces and rolling-mills, &c., had been built with capital drawn from other industries, its transference back into other industries would involve great loss; the owners, rather than transfer it, would accept unusually low interest, and an excess of iron would continue to be produced; resulting in an undue cheapness of ships, and a maintenance of the ship-building industry at a size beyond the need. eventually, however, if the number of ships required still diminished, the production of iron in excess would become very unremunerative: some of the blast-furnaces would be blown out; and as much of the capital and labour as remained available would be re-distributed among other occupations. without repeating the steps of the argument, it will be clear that were the enlargement of the ship-building industry great enough, and did it last long enough to cause an increase in the number of coal-mines, the ship-building industry would be still better able to maintain itself under adverse circumstances; but that it would, though at a more distant period, end by sinking down to the needful dimensions. thus our conclusions are:--first, that if the extra growth caused by extra activity in a particular industry has lasted long enough only to remodel the proximately-affected industries; it will dwindle away again after a moderate period, if the need for it disappears. second, that a long period must be required before the re-actions produced by an enlarged industry can cause a re-construction of the whole society, and before the countless re-distributions of capital and labour can again reach a state of equilibrium. and third, that only when such a new state of equilibrium is eventually reached, can the adaptive modification become a permanent one. how, in animal organisms the like argument holds, need not be pointed out. the reader will readily follow the parallel. that organic types should be comparatively stable, might be anticipated on the hypothesis of evolution. the structure of any organism being a product of the almost infinite series of actions and reactions to which ancestral organisms have been exposed; any unusual actions and reactions brought to bear on an individual, can have but an infinitesimal effect in permanently changing the structure of the organism as a whole. the new set of forces, compounded with all the antecedent sets of forces, can but inappreciably modify that moving equilibrium of functions which all these antecedent sets of forces have established. though there may result a considerable perturbation of certain functions--a considerable divergence from their ordinary rhythms--yet the general centre of equilibrium cannot be sensibly changed. on the removal of the perturbing cause the previous balance will be quickly restored: the effect of the new forces being almost obliterated by the enormous aggregate of forces which the previous balance expresses. § . as thus understood, the phenomena of adaptation fall into harmony with first principles. the inference that organic types are fixed, because the deviations from them which can be produced within assignable periods are relatively small, and because, when a force producing deviation ceases, there is a return to something like the original state; proves to be an invalid inference. without assuming fixity of species, we find good reasons for anticipating that kind and degree of stability which is observed. we find grounds for concluding, _a priori_, that an adaptive change of structure will soon reach a point beyond which further adaptation will be slow; for concluding that when the modifying cause has been but a short time in action, the modification generated will be evanescent; for concluding that a modifying cause acting even for many generations, will do but little towards permanently altering the organic equilibrium of a race; and for concluding that on the cessations of such cause, its effects will become unapparent in the course of a few generations. chapter vi. individuality. § . what is an individual? is a question which many readers will think it easy to answer. yet it is a question that has led to much controversy among zoologists and botanists, and no quite satisfactory reply to it seems possible. as applied to a man, or to any one of the higher animals, which are all sharply-defined and independent, the word individual has a clear meaning: though even here, when we turn from average cases to exceptional cases--as a calf with two heads and two pairs of fore-limbs--we find ourselves in doubt whether to predicate one individuality or two. but when we extend our range of observation to the organic world at large, we find that difficulties allied to this exceptional one meets us everywhere under every variety of form. each uniaxial plant may perhaps fairly be regarded as a distinct individual; though there are botanists who do not make even this admission. what, however, are we to say of a multiaxial plant? it is, indeed, usual to speak of a tree with its many branches and shoots as singular; but strong reasons may be urged for considering it as plural. every one of its axes has a more or less independent life, and when cut off and planted may grow into the likeness of its parent; or, by grafting and budding, parts of this tree may be developed upon another tree, and there manifest their specific peculiarities. shall we regard all the growing axes thus resulting from slips and grafts and buds, as parts of one individual or as distinct individuals? if a strawberry-plant sends out runners carrying buds at their ends, which strike root and grow into independent plants that separate from the original one by decay of the runners, must we not say that they possess separate individualities; and yet if we do this, are we not at a loss to say when their separate individualities were established, unless we admit that each bud was from the beginning an individual? commenting on such perplexities schleiden says--"much has been written and disputed concerning the conception of the individual, without, however, elucidating the subject, principally owing to the misconception that still exists as to the origin of the conception. now the individual is no conception, but the mere subjective comprehension of an actual object, presented to us under some given specific conception, and on this latter it alone depends whether the object is or is not an individual. under the specific conception of the solar system, ours is an individual: in relation to the specific conception of a planetary body, it is an aggregate of many individuals." ... "i think, however, that looking at the indubitable facts already mentioned, and the relations treated of in the course of these considerations, it will appear most advantageous and most useful, in a scientific point of view, to consider the vegetable cell as the general type of the plant (simple plant of the first order). under this conception, _protococcus_ and other plants consisting of only one cell, and the spore and pollen-granule, will appear as individuals. such individuals may, however, again, with a partial renunciation of their individual independence, combine under definite laws into definite forms (somewhat as the individual animals do in the globe of the _volvox globator_[ ]). these again appear empirically as individual beings, under a conception of a species (simple plants of the second order) derived from the form of the normal connexion of the elementary individuals. but we cannot stop here, since nature herself combines these individuals, under a definite form, into larger associations, whence we draw the third conception of the plant, from a connexion, as it were, of the second power (compound plants--plants of the third order). the simple plant proceeding from the combination of the elementary individuals is then termed a bud (_gemma_), in the composition of plants of the third order." the animal kingdom presents still greater difficulties. when, from sundry points on the body of a common polype, there bud out young polypes which, after acquiring mouths and tentacles and closing up the communications between their stomachs and the stomach of the parent, finally separate from the parent; we may with propriety regard them as distinct individuals. but when in the allied compound _hydrozoa_, we find that these young polypes continue permanently connected with the parent; and when by this continuous budding-out there is presently produced a tree-like aggregation, having a common alimentary canal into which the digestive cavity of each polype opens; it is no longer so clear that these little sacs, furnished with mouths and tentacles, are severally to be regarded as distinct individuals. we cannot deny a certain individuality to the polypedom. and on discovering that some of the buds, instead of unfolding in the same manner as the rest, are transformed into capsules in which eggs are developed--on discovering that certain of the incipient polypes thus become wholly dependent on the aggregate for their nutrition, and discharge functions which have nothing to do with their own maintenance, we have still clearer proof that the individualities of the members are partially merged in the individuality of the group. other organisms belonging to the same order, display still more decidedly this transition from simple individualities to a complex individuality. in the _diphyes_ there is a special modification of one or more members of the polypedom into a swimming apparatus which, by its rhythmical contractions, propels itself through the water, drawing the polypedom after it. and in the more differentiated _physalia_ various organs result from the metamorphosis of parts which are the homologues of individual polypes. in this last instance, the individuality of the aggregate is so predominant that the individualities of its members are practically lost. this combination of individualities in such way as to produce a composite individual, meets us in other forms among the ascidians. while in some of these, as in the _clavelina_ and in the _botryllidæ_, the animals associated are but little subordinated to the community they form, in others they are so combined as to form a compound individual. the pelagic ascidian _doliolum_ is an example. "here we find a large individual which swims by contractions of circular muscular bands, carries a train of smaller individuals attached to a long dorsal process of the test. these are arranged in three rows: those constituting the lateral row have wide mouths and no sexual organs or organs of locomotion--they subserve the nutrition of the colony, a truth which is illustrated by the fact that as soon as they are properly developed the large individual (the mother) loses her alimentary canal;" while from the median row are eventually derived the sexual zoids. on the hypothesis of evolution, perplexities of this nature are just such as we might anticipate. if life in general commenced with minute and simple forms, like those out of which all organisms, however complex, now originate; and if the transitions from these primordial units to organisms made up of groups of such units, and to higher organisms made up of groups of such groups took place by degrees; it is clear that individualities of the first and simplest order would merge gradually in those of a larger and more complex order, and these again in others of an order having still greater bulk and organization. hence it would be impossible to say where the lower individualities ceased and the higher individualities commenced. § . to meet these difficulties, it has been proposed that the whole product of a single fertilized germ shall be regarded as a single individual; whether such whole product be organized into one mass, or whether it be organized into many masses that are partially or completely separate. it is urged that whether the development of the fertilized germ be continuous or discontinuous (§ ) is a matter of secondary importance; that the totality of living tissue to which the fertilized germ gives rise in any one case, is the equivalent of the totality to which it gives rise in any other case; and that we must recognize this equivalence, whether such totality of living tissue takes a concrete or a discrete arrangement. in pursuance of this view, a zoological individual is constituted either by any such single animal as a mammal or bird, which may properly claim the title of a _zoon_, or by any such group of animals as the numerous _medusæ_ that have been developed from the same egg, which are to be severally distinguished as _zooids_. admitting it to be very desirable that there should be words for expressing these relations and this equivalence, it may be objected that to apply the word individual to a number of separate living bodies, is inconvenient: conflicting so much, as it does, with the ordinary conception which this word suggests. it seems a questionable use of language to say that the countless masses of _anacharis alsinastrum_ (now _eloidea canadensis_) which, within these few years, have grown up in our rivers, canals, and ponds, are all parts of one individual: and yet as this plant does not seed in england, these countless masses, having arisen by discontinuous development, must be so regarded if we accept the above definition. it may be contended, too, that while it does violence to our established way of thinking, this mode of interpreting the facts is not without its difficulties. something seems to be gained by restricting the application of the title individual, to organisms which, being in all respects fully developed, possess the power of producing their kind after the ordinary sexual method, and denying this title to those incomplete organisms which have not this power. but the definition does not really establish this distinction for us. on the one hand, we have cases in which, as in the working bee, the whole of the germ-product is aggregated into a single organism; and yet, though an individual according to the definition, this organism has no power of reproducing its kind. on the other hand, we have cases like that of the perfect _aphis_, where the organism is but an infinitesimal part of the germ product, and yet has that completeness required for sexual reproduction. further, it might be urged with some show of reason, that if the conception of individuality involves the conception of completeness, then, an organism which possesses an independent power of reproducing itself, being more complete than an organism in which this power is dependent on the aid of another organism, is more individual. § . there is, indeed, as already implied, no definition of individuality that is unobjectionable. all we can do is to make the best practicable compromise. as applied either to an animate or an inanimate object, the word individual ordinarily connotes union among the parts of the object and separateness from other objects. this fundamental element in the conception of individuality, we cannot with propriety ignore in the biological application of the word. that which we call an individual plant or animal must, therefore, be some concrete whole and not a discrete whole. if, however, we say that each concrete living whole is to be regarded as an individual, we are still met by the question--what constitutes a concrete living whole? a young organism arising by internal or external gemmation from a parent organism, passes gradually from a state in which it is an indistinguishable part of the parent organism to a state in which it is a separate organism of like structure with the parent. at what stage does it become an individual? and if its individuality be conceded only when it completely separates from the parent, must we deny individuality to all organisms thus produced which permanently retain their connexions with their parents? or again, what must we say of the _hectocotylus_, which is an arm of the cuttle-fish that undergoes a special development and then, detaching itself, lives independently for a considerable period? and what must we say of the larval nemertine worm the pilidium of which with its nervous system is left to move about awhile after the developing worm has dropped out of it? to answer such questions we must revert to the definition of life. the distinction between individual in its biological sense, and individual in its more general sense, must consist in the manifestation of life, properly so called. life we have seen to be, "the definite combination of heterogeneous change, both simultaneous and successive, in correspondence with external co-existences and sequences." hence, a biological individual is any concrete whole having a structure which enables it, when placed in appropriate conditions, to continuously adjust its internal relations to external relations, so as to maintain the equilibrium of its functions. in pursuance of this conception, we must consider as individuals all those wholly or partially independent organized masses which arise by multicentral and multiaxial development that is either continuous or discontinuous (§ ). we must accord the title to each separate aphis, each polype of a polypedom, each bud or shoot of a lowering plant, whether it detaches itself as a bulbil or remains attached as a branch. by thus interpreting the facts we do not, indeed, avoid all anomalies. while, among flowering plants, the power of independent growth and development is usually possessed only by shoots or axes; yet, in some cases, as in that of the begonia-leaf awhile since mentioned, the appendage of an axis, or even a small fragment of such appendage, is capable of initiating and carrying on the functions of life; and in other cases, as shown by m. naudin in the _drosera intermedia_, young plants are occasionally developed from the surfaces of leaves. nor among forms like the compound _hydrozoa_, does the definition enable us to decide where the line is to be drawn between the individuality of the group and the individualities of the members: merging into each other, as these do, in different degrees. but, as before said, such difficulties must necessarily present themselves if organic forms have arisen by insensible gradations. we must be content with a course which commits us to the smallest number of incongruities; and this course is, to consider as an individual any organized mass which is capable of independently carrying on that continuous adjustment of inner to outer relations which constitutes life. chapter vi^a. cell-life and cell-multiplication. § a. the progress of science is simultaneously towards simplification and towards complication. analysis simplifies its conceptions by resolving phenomena into their factors, and by then showing how each simple mode of action may be traced under multitudinous forms; while, at the same time, synthesis shows how each factor, by cooperation with various other factors in countless modes and degrees, produces different results innumerable in their amounts and varieties. of course this truth holds alike of processes and of products. observation and the grouping into classes make it clear that through multitudinous things superficially unlike there run the same cardinal traits of structure; while, along with these major unities, examination discloses innumerable minor diversities. a concomitant truth, or the same truth under another aspect, is that nature everywhere presents us with complexities within complexities, which go on revealing themselves as we investigate smaller and smaller objects. in a preceding chapter (§§ a, b) it was pointed out that each primitive organism, in common with each of the units out of which the higher and larger organisms are built, was found a generation ago to consist of nucleus, protoplasm, and cell-wall. this general conception of a cell remained for a time the outcome of inquiry; but with the advance of microscopy it became manifest that within these minute structures processes and products of an astonishing nature are to be seen. these we have now to contemplate. in the passages just referred to it was said that the external layer or cell-wall is a non-essential, inanimate part produced by the animate contents. itself a product of protoplasmic action, it takes no part in protoplasmic changes, and may therefore here be ignored. § b. one of the complexities within complexities was disclosed when it was found that the protoplasm itself has a complicated structure. different observers have described it as constituted by a network or reticulum, a sponge-work, a foam-work. of these the first may be rejected; since it implies a structure lying in one plane. if we accept the second we have to conceive the threads of protoplasm, corresponding to the fibres of the sponge, as leaving interstices filled either with liquid or solid. they cannot be filled with a continuous solid, since all motion of the protoplasm would be negatived; and that their content is not liquid seems shown by the fact that its parts move about under the form of granules or microsomes. but the conception of moving granules implies the conception of immersion in a liquid or semi-liquid substance in which they move--not a sponge-work of threads but a foam-work, consisting everywhere of septa interposed among the granules. this is the hypothesis which sundry microscopists espouse, and which seems mechanically the most feasible: the only one which consists with the "streaming" of protoplasm. ordinarily the name protoplasm is applied to the aggregate mass--the semi-liquid, hyaline substance and the granules or microsomes it contains. what these granules or microsomes are--whether, as some have contended, they are the essential living elements of the protoplasm, or whether, as is otherwise held, they are nutritive particles, is at present undecided. but the fact, alleged by sundry observers, that the microsomes often form rows, held together by intervening substance, seems to imply that these minute bodies are not inert. leaving aside unsettled questions, however, one fact of significance is manifest--an immense multiplication of surfaces over which inter-action may take place. anyone who drops into dilute sulphuric acid a small nail and then drops a pinch of iron filings, will be shown, by the rapid disappearance of the last and the long continuance of the first, how greatly the increasing of surfaces by multiplication of fragments facilitates change. the effect of subdivision in producing a large area in a small space, is shown in the lungs, where the air-cells on the sides of which the blood-vessels ramify, are less than / th of an inch in diameter, while they number , , . in the composition of every tissue we see the same principle. the living part, or protoplasm, is divided into innumerable protoplasts, among which are distributed the materials and agencies producing changes. and now we find this principle carried still deeper in the structure of the protoplasm itself. each microscopic portion of it is minutely divided in such ways that its threads or septa have multitudinous contacts with those included portions of matter which take part in its activities. concerning the protoplasm contained in each cell, named by some cytoplasm, it remains to say that it always includes a small body called the centrosome, which appears to have a directive function. usually the centrosome lies outside the nucleus, but is alleged to be sometimes within it. during what is called the "resting stage," or what might more properly be called the growing stage (for clearly the occasional divisions imply that in the intervals between them there has been increase) the centrosome remains quiescent, save in the respect that it exercises some coercive influence on the protoplasm around. this results in the radially-arranged lines constituting an "aster." what is the nature of the coercion exercised by the centrosome--a body hardly distinguishable in size from the microsomes or granules of protoplasm around--is not known. it can scarcely be a repelling force; since, in a substance of liquid or semi-liquid kind, this could not produce approximately straight lines. that it is an attractive force seems more probable; and the nature of the attraction would be comprehensible did the centrosome augment in bulk with rapidity. for if integration were in progress, the drawing in of materials might well produce converging lines. but this seems scarcely a tenable interpretation; since, during the so-called "resting stage," this star-like structure exists--exists, that is, while no active growth of the centrosome is going on. respecting this small body we have further to note that, like the cell as a whole, it multiplies by fission, and that the bisection of it terminates the resting or growing stage and initiates those complicated processes by which two cells are produced out of one: the first step following the fission being the movement of the halves, with their respective completed asters, to the opposite sides of the nucleus. § c. with the hypothesis, now general, that the nucleus or kernel of a cell is its essential part, there has not unnaturally grown up the dogma that it is always present; but there is reason to think that the evidence is somewhat strained to justify this dogma. in the first place, beyond the cases in which the nucleus, though ordinarily invisible, is said to have been rendered visible by a re-agent, there are cases, as in the already-named _archerina_, where no re-agent makes one visible. in the second place, there is the admitted fact that some nuclei are diffused; as in _trachelocerca_ and some other infusoria. in them the numerous scattered granules are supposed to constitute a nucleus: an interpretation obviously biassed by the desire to save the generalization. in the third place, the nucleus is frequently multiple in cells of low types; as in some families of algæ and predominantly among fungi. once more, the so-called nucleus is occasionally a branching structure scarcely to be called a "kernel." the facts as thus grouped suggest that the nucleus has arisen in conformity with the law of evolution--that the primitive protoplast, though not homogeneous in the full sense, was homogeneous in the sense of being a uniformly granular protoplasm; and that the protoplasts with diffused nuclei, together with those which are multi-nucleate, and those which have nuclei of a branching form, represent stages in that process by which the relatively homogeneous protoplast passed into the relatively heterogeneous one now almost universal. concerning the structure and composition of the developed nucleus, the primary fact to be named is that, like the surrounding granular cytoplasm, it is formed of two distinct elements. it has a groundwork or matrix not differing much from that of the cytoplasm, and at some periods continuous with it; and immersed in this it has a special matter named chromatin, distinguished from its matrix by becoming dyed more or less deeply when exposed to fit re-agents. during the "resting stage," or period of growth and activity which comes between periods of division, the chromatin is dispersed throughout the ground-substance, either in discrete portions or in such way as to form an irregular network or sponge-work, various in appearance. when the time for fission is approaching this dispersed chromatin begins to gather itself together: reaching its eventual concentration through several stages. by its concentration are produced the chromosomes, constant in number in each species of plant or animal. it is alleged that the substance of the chromosomes is not continuous, but consists of separate elements or granules, which have been named chromomeres; and it is also alleged that, whether in the dispersed or integrated form, each chromosome retains its individuality--that the chromomeres composing it, now spreading out into a network and now uniting into a worm-like body, form a group which never loses its identity. be this as it may, however, the essential fact is that during the growth-period the chromatin substance is widely distributed, and concentration of it is one of the chief steps towards a division of the nucleus and presently of the cell. during this process of mitosis or karyokinesis, the dispersed chromatin having passed through the coil-stage, reaches presently the star-stage, in which the chromosomes are arranged symmetrically about the equatorial plane of the nucleus. meanwhile in each of them there has been a preparation for splitting longitudinally in such way that the halves when separated contain (or are assumed to contain) equal numbers of the granules or chromomeres, which some think are the ultimate morphological units of the chromosomes. a simultaneous change has occurred: there has been in course of formation a structure known as the _amphiaster_. the two centrosomes which, as before said, place themselves on opposite sides of the nucleus, become the terminal poles of a spindle-shaped arrangement of fibres, arising mainly from the groundwork of the nucleus, now continuous with the groundwork of the cytoplasm. a conception of this structure may be formed by supposing that the radiating fibres of the respective asters, meeting one another and uniting in the intermediate space, thereafter exercise a tractive force; since it is clear that, while the central fibres of the bundle will form straight lines, the outer ones, pulling against one another not in straight lines, will form curved lines, becoming more pronounced in their curvatures as the distance from the axis increases. that a tractive force is at work seems inferable from the results. for the separated halves of the split chromosomes, which now form clusters on the two sides of the equatorial plane, gradually part company, and are apparently drawn as clusters towards the opposing centrosomes. as this change progresses the original nucleus loses its individuality. the new chromosomes, halves of the previous chromosomes, concentrate to found two new nuclei; and, by something like a reversal of the stages above described, the chromatin becomes dispersed throughout the substance of each new nucleus. while this is going on the cell itself, undergoing constriction round its equator, divides into two. many parts of this complex process are still imperfectly understood, and various opinions concerning them are current. but the essential facts are that this peculiar substance, the chromatin, at other times existing dispersed, is, when division is approaching, gathered together and dealt with in such manner as apparently to insure equal quantities being bequeathed by the mother-cell to the two daughter-cells. § d. what is the physiological interpretation of these structures and changes? what function does the nucleus discharge; and, more especially, what is the function discharged by the chromatin? there have been to these questions sundry speculative answers. the theory espoused by some, that the nucleus is the regulative organ of the cell, is met by difficulties. one of them is that, as pointed out in the chapter on "structure," the nucleus, though morphologically central, is not central geometrically considered; and that its position, often near to some parts of the periphery and remote from others, almost of itself negatives the conclusion that its function is directive in the ordinary sense of the word. it could not well control the cytoplasm in the same ways in all directions and at different distances. a further difficulty is that the cytoplasm when deprived of its nucleus can perform for some time various of its actions, though it eventually dies without reproducing itself. for the hypothesis that the nucleus is a vehicle for transmitting hereditary characters, the evidence seems strong. when it was shown that the head of a spermatozoon is simply a detached nucleus, and that its fusion with the nucleus of an ovum is the essential process initiating the development of a new organism, the legitimate inference appeared to be that these two nuclei convey respectively the paternal and maternal traits which are mingled in the offspring. and when there came to be discerned the karyokinesis by which the chromatin is, during cell-fission, exactly halved between the nuclei of the daughter-cells, the conclusion was drawn that the chromatin is more especially the agent of inheritance. but though, taken by themselves, the phenomena of fertilization seem to warrant this inference, the inference does not seem congruous with the phenomena of ordinary cell-multiplication--phenomena which have nothing to do with fertilization and the transmission of hereditary characters. no explanation is yielded of the fact that ordinary cell-multiplication exhibits an elaborate process for exact halving of the chromatin. why should this substance be so carefully portioned out among the cells of tissues which are not even remotely concerned with propagation of the species? if it be said that the end achieved is the conveyance of paternal and maternal qualities in equal degrees to every tissue; then the reply is that they do not seem to be conveyed in equal degrees. in the offspring there is not a uniform diffusion of the two sets of traits throughout all parts, but an irregular mixture of traits of the one with traits of the other. in presence of these two suggested hypotheses and these respective difficulties, may we not suspect that the action of the chromatin is one which in a way fulfils both functions? let us consider what action may do this. § e. the chemical composition of chromatin is highly complex, and its complexity, apart from other traits, implies relative instability. this is further implied by the special natures of its components. various analyses have shown that it consists of an organic acid (which has been called nucleic acid) rich in phosphorus, combined with an albuminous substance: probably a combination of various proteids. and the evidence, as summarised by wilson, seems to show that where the proportion of phosphorized acid is high the activity of the substance is great, as in the heads of spermatozoa; while, conversely, where the quantity of phosphorus is relatively small, the substance approximates in character to the cytoplasm. now (like sulphur, present in the albuminoid base), phosphorus is an element which, besides having several allotropic forms, has a great affinity for oxygen; and an organic compound into which it enters, beyond the instability otherwise caused, has a special instability caused by its presence. the tendency to undergo change will therefore be great when the proportion of the phosphorized component is great. hence the statement that "the chemical differences between chromatin and cytoplasm, striking and constant as they are, are differences of degree only;" and the conclusion that the activity of the chromatin is specially associated with the phosphorus.[ ] what, now, are the implications? molecular agitation results from decomposition of each phosphorized molecule: shocks are continually propagated around. from the chromatin, units of which are thus ever falling into stabler states, there are ever being diffused waves of molecular motion, setting up molecular changes in the cytoplasm. the chromatin stands towards the other contents of the cell in the same relation that a nerve-element stands to any element of an organism which it excites: an interpretation congruous with the fact that the chromatin is as near to as, and indeed nearer than, a nerve-ending to any minute structure stimulated by it. several confirmatory facts may be named. during the intervals between cell-fissions, when growth and the usual cell-activities are being carried on, the chromatin is dispersed throughout the nucleus into an irregular network: thus greatly increasing the surface of contact between its substance and the substances in which it is imbedded. as has been remarked, this wide distribution furthers metabolism--a metabolism which in this case has, as we infer, the function of generating, not special matters but special motions. moreover, just as the wave of disturbance a nerve carries produces an effect which is determined, not by anything which is peculiar in itself, but by the peculiar nature of the organ to which it is carried--muscular, glandular or other; so here, the waves diffused from the chromatin do not determine the kinds of changes in the cytoplasm, but simply excite it: its particular activities, whether of movement, absorption, or structural excretion, being determined by its constitution. and then, further, we observe a parallelism between the metabolic changes in the two cases; for, on the one hand, "diminished staining capacity of the chromatin [implying a decreased amount of phosphorus, which gives the staining capacity] occurs during a period of intense constructive activity in the cytoplasm;" and, on the other hand, in high organisms having nervous systems, the intensity of nervous action is measured by the excretion of phosphates--by the using up of the phosphorus contained in nerve-cells. for thus interpreting the respective functions of chromatin and cytoplasm, yet a further reason may be given. one of the earliest general steps in the evolution of the _metazoa_, is the differentiation of parts which act from parts which make them act. the _hydrozoa_ show us this. in the hydroid stage there are no specialized contractile organs: these are but incipient: individual ectoderm cells have muscular processes. nor is there any "special aggregation of nerve-cells." if any stimulating units exist they are scattered. but in the _medusa_-stage nerve-matter is collected into a ring round the edge of the umbrella. that is to say, in the undeveloped form such motor action as occurs is not effected by a specialized part which excites another part; but in the developed form a differentiation of the two has taken place. all higher types exhibit this differentiation. be it muscle or gland or other operating organ, the cause of its activity lies not in itself but in a nervous agent, local or central, with which it is connected. hence, then, there is congruity between the above interpretation and certain general truths displayed by animal organization at large. we may infer that in a way parallel to that just indicated, cell-evolution was, under one of its aspects, a change from a stage in which the exciting substance and the substance excited were mingled with approximate uniformity, to a stage in which the exciting substance was gathered together into the nucleus and finally into the chromosomes: leaving behind the substance excited, now distinguished as cytoplasm. § f. some further general aspects of the phenomena appear to be in harmony with this interpretation. let us glance at them. in chapters iii and iiia of the first part, reasons were given for concluding that in the animal organism nitrogenous substances play the part of decomposing agents to the carbo-hydrates--that the molecular disturbance set up by the collapse of a proteid molecule destroys the equilibrium of sundry adjacent carbo-hydrate molecules, and causes that evolution of energy which accompanies their fall into molecules of simpler compounds. here, if the foregoing argument is valid, we may conclude that this highly complex phosphorized compound which chromatin contains, plays the same part to the adjacent nitrogenous compounds as these play to the carbo-hydrates. if so, we see arising a stage earlier that "general physiological method" illustrated in § f. it was there pointed out that in animal organisms the various structures are so arranged that evolution of a small amount of energy in one, sets up evolution of a larger amount of energy in another; and often this multiplied energy undergoes a second multiplication of like kind. if this view is tenable, we may now suspect that this method displayed in the structures of the _metazoa_ was initiated in the structures of the _protozoa_, and consequently characterizes those homologues of them which compose the _metazoa_. when contemplated from the suggested point of view, karyokinesis appears to be not wholly incomprehensible. for if the chromatin yields the energy which initiates changes throughout the rest of the cell, we may see why there eventually arises a process for exact halving of the chromatin in a mother-cell between two daughter-cells. to make clear the reason, let us suppose the portioning out of the chromatin leaves one of the two with a sensibly smaller amount than the other. what must result? its source of activity being relatively less, its rate of growth and its energy of action will be less. if a protozoon, the weaker progeny arising by division of it will originate an inferior stirp, unable to compete successfully with that arising from the sister-cell endowed with a larger portion of chromatin. by continual elimination of the varieties which produce unequal halving, necessarily at a disadvantage if a moiety of their members tend continually to disappear, there will be established a variety in which the halving is exact: the character of this variety being such that all its members aid the permanent multiplication of the species. if, again, the case is that of a metazoon, there will be the same eventual result. an animal or plant in which the chromatin is unequally divided among the cells, must have tissues of uncertain formation. assume that an organ has, by survival of the fittest, been adjusted in the proportions and qualities of its parts to a given function. if the multiplying protoplasts, instead of taking equal portions of chromatin, have some of them smaller portions, the parts of the organ formed of these, developing less rapidly and having inferior energies, will throw the organ out of adjustment, and the individual will suffer in the struggle for life. that is to say, irregular division of the chromatin will introduce a deranging factor and natural selection will weed out individuals in which it occurs. of course no interpretation is thus yielded of the special process known as karyokinesis. probably other modes of equal division might have arisen. here the argument implies merely that the tendency of evolution is to establish _some_ mode. in verification of the view that equal division arises from the cause named, it is pointed out to me that amitosis, which is a negation of mitosis or karyokinesis, occurs in transitory tissues or diseased tissues or where degeneracy is going on. but how does all this consist with the conclusion that the chromatin conveys hereditary traits--that it is the vehicle in which the constitutional structure, primarily of the species and secondarily of recent ancestors and parents, is represented? to this question there seems to be no definite answer. we may say only that this second function is not necessarily in conflict with the first. while the unstable units of chromatin, ever undergoing changes, diffuse energy around, they may also be units which, under the conditions furnished by fertilization, gravitate towards the organization of the species. possibly it may be that the complex combination of proteids, common to chromatin and cytoplasm, is that part in which the constitutional characters inhere; while the phosphorized component, falling from its unstable union and decomposing, evolves the energy which, ordinarily the cause of changes, now excites the more active changes following fertilization. this suggestion harmonizes with the fact that the fertilizing substance which in animals constitutes the head of the spermatozoon, and in plants that of the spermatozoid or antherozoid, is distinguished from the other agents concerned by having the highest proportion of the phosphorized element; and it also harmonizes with the fact that the extremely active changes set up by fertilization are accompanied by decrease of this phosphorized element. speculation aside, however, we may say that the two functions of the chromatin do not exclude one another, but that the general activity which originates from it may be but a lower phase of that special activity caused by fertilization.[ ] § g. here we come unawares to the remaining topic embraced under the title cell-life and cell-multiplication. we pass naturally from asexual multiplication of cells to sexual multiplication--from cell-reproduction to cell-generation. the phenomena are so numerous and so varied that a large part of them must be passed over. conjugation among the _protophyta_ and _protozoa_, beginning with cases in which there is a mingling of the contents of two cells in no visible respect different from one another, and developing into a great variety of processes in which they differ, must be left aside, and attention limited to the terminal process of fertilization as displayed in higher types of organisms. before fertilization there occurs in the ovum an incidental process of a strange kind--"strange" because it is a collateral change taking no part in subsequent changes. i refer to the production and extrusion of the "polar bodies." it is recognized that the formation of each is analogous to cell-formation in general; though process and product are both dwarfed. apart from any ascribed meaning, the fact itself is clear. there is an abortive cell-formation. abortiveness is seen firstly in the diminutive size of the separated body or cell, and secondly in the deficient number of its chromosomes: a corresponding deficiency being displayed in the group of chromosomes remaining in the egg--remaining, that is (on the hypothesis here to be suggested), in the sister-cell, supposing the polar body to be an aborted cell. it is currently assumed that the end to be achieved by thus extruding part of the chromosomes, is to reduce the remainder to half the number characterizing the species; so that when, to this group in the germ-cell, the sperm-cell brings a similarly-reduced group, union of the two shall bring the chromosomes to the normal number. i venture to suggest another interpretation. in doing this, however, i must forestall a conclusion contained in the next chapter; namely, the conclusion that gamogenesis begins when agamogenesis is being arrested by unfavourable conditions, and that the failing agamogenesis initiates the gamogenesis. of numerous illustrations to be presently given i will, to make clear the conception, name only one--the formation of fructifying organs in plants at times when, and in places where, shoots are falling off in vigour and leaves in size. here the successive foliar organs, decreasingly fitted alike in quality and dimensions for carrying on their normal lives, show us an approaching cessation of asexual multiplication, ending in the aborted individuals we call stamens; and the fact that sudden increase of nutrition while gamogenesis is being thus initiated, causes resumption of agamogenesis, shows that the gamogenesis is consequent upon the failing agamogenesis. see then the parallel. on going back from multicellular organisms to unicellular organisms (or those homologues of them which form the reproductive agents in multicellular organisms), we find the same law hold. the polar bodies are aborted cells, indicating that asexual multiplication can no longer go on, and that the conditions leading to sexual multiplication have arisen. if this be so, decrease in the chromatin becomes an initial cause of the change instead of an accompanying incident; and we need no longer assume that a quantity of precious matter is lost, not by passive incapacity, but by active expulsion. another anomaly disappears. if from the germ-cell there takes place this extrusion of superfluous chromatin, the implication would seem to be that a parallel extrusion takes place from the sperm-cell. but this is not true. in the sperm-cell there occurs just that failure in the production of chromatin which, according to the hypothesis above sketched out, is to be expected; for, in the process of cell-multiplication, the cells which become spermatozoa are _left_ with half the number of chromosomes possessed by preceding cells: there is actually that impoverishment and declining vigour here suggested as the antecedent of fertilization. it needs only to imagine the ovum and the polar body to be alike in size, to see the parallelism; and to see that obscuration of it arises from the accumulation of cytoplasm in the ovum. a test fact remains. sometimes the first polar body extruded undergoes fission while the second is being formed. this can have nothing to do with reducing the number of chromosomes in the ovum. unquestionably, however, this change is included with the preceding changes in one transaction, effected by one influence. if, then, it is irrelevant to the decrease of chromosomes, so must the preceding changes be irrelevant: the hypothesis lapses. contrariwise this fact supports the view suggested above. that extrusion of a polar body is a process of cell-fission is congruous with the fact that another fission occurs after extrusion. and that this occurs irregularly shows that the vital activities, seen in cell-growth and cell-multiplication, now succeed in producing further fission of the dwarfed cell and now fail: the energies causing asexual multiplication are exhausted and there arises the state which initiates sexual multiplication. maturation of the ovum having been completed, entrance of the spermatozoon, sometimes through the limiting membrane and sometimes through a micropyle or opening in it, takes place. this instantly initiates a series of complicated changes: not many seconds passing before there begins the formation of an aster around one end of the spermatozoon-head. the growth of this aster, apparently by linear rangings of the granules composing the reticulum of the germ-cell, progresses rapidly; while the whole structure hence arising moves inward. soon there takes place the fusion of this sperm-nucleus with the germ-nucleus to form the cleavage-nucleus, which, after a pause, begins to divide and subdivide in the same manner as cells at large: so presently forming a cluster of cells out of which arise the layers originating the embyro. the details of this process do not concern us. it suffices to indicate thus briefly its general nature. and now ending thus the account of genesis under its histological aspect, we pass to the account of genesis under its wider and more significant aspects. chapter vii. genesis. § . having, in the last chapter but one, concluded what constitutes an individual, and having, in the last chapter, contemplated the histological process which initiates a new individual, we are in a position to deal with the multiplication of individuals. for this, the title genesis is here chosen as being the most comprehensive title--the least specialized in its meaning. by some biologists generation has been used to signify one method of multiplication, and reproduction to signify another method; and each of these words has been thus rendered in some degree unfit to signify multiplication in general. here the reader is indirectly introduced to the fact that the production of new organisms is carried on in fundamentally unlike ways. up to quite recent times it was believed, even by naturalists, that all the various processes of multiplication observable in different kinds of organisms, have one essential character in common: it was supposed that in every species the successive generations are alike. it has now been proved, however, that in many plants and in numerous animals, the successive generations are not alike; that from one generation there proceeds another whose members differ more or less in structure from their parents; that these produce others like themselves, or like their parents, or like neither; but that eventually, the original form re-appears. instead of there being, as in the cases most familiar to us, a constant recurrence of the same form, there is a cyclical recurrence of the same form. these two distinct processes of multiplication, may be aptly termed _homogenesis_ and _heterogenesis_.[ ] under these heads let us consider them. there are two kinds of homogenesis, the simplest of them, probably once universal but now exceptional, being that in which there is no other form of multiplication than one resulting from perpetual spontaneous fission. the rise of distinct sexes was doubtless a step in evolution, and before it took place the formation of new individuals could have arisen only by division of the old, either into two or into many. at present this process survives, so far as appears, among _bacteria_, certain _algæ_, and sundry _protozoa_; though it is possible that a rarely-occurring conjugation has in these cases not yet been observed. it is a probable conclusion, however, that in the _bacteria_ at any rate, the once universal mode of multiplication still survives as an exceptional mode. but now passing over these cases, we have to note that the kind of genesis (once supposed to be the sole kind), in which the successive generations are alike, is sexual genesis, or, as it has been otherwise called--_gamogenesis_. in every species which multiplies by this kind of homogenesis, each generation consists of males and females; and from the fertilized germs they produce the next generation of similar males and females arises: the only needful qualification of this statement being that in many _protophyta_ and _protozoa_ the conjugating cells or protoplasts are not distinguishable in character. this mode of propagation has the further trait, that each fertilized germ usually gives rise to but one individual--the product of development is organized round one axis and not round several axes, homogenesis in contrast with heterogenesis as exhibited in species which display distinct sexuality, has also the characteristic that each new individual begins as an egg detached from the maternal tissues, instead of being a portion of protoplasm continuous with them, and that its development proceeds independently. this development may be carried on either internally or externally; whence results the division into the oviparous and the viviparous. the oviparous kind is that in which the fertilized germ is extruded from the parent before it has undergone any considerable development. the viviparous kind is that in which development is considerably advanced, or almost completed, before extrusion takes place. this distinction is, however, not a sharply-defined one: there are transitions between the oviparous and the viviparous processes. in ovo-viviparous genesis there is an internal incubation; and though the young are in this case finally extruded from the parent in the shape of eggs, they do not leave the parent's body until after they have assumed something like the parental form. looking around, we find that homogenesis is universal among the _vertebrata_. every vertebrate animal arises from a fertilized germ, and unites into its single individuality the whole product of this fertilized germ. in the mammals or highest _vertebrata_, this homogenesis is in every case viviparous; in birds it is uniformly oviparous; and in reptiles and fishes it is always essentially oviparous, though there are cases of the kind above referred to, in which viviparity is simulated. passing to the _invertebrata_, we find oviparous homogenesis universal among the _arachnida_ (except the scorpions, which are ovo-viviparous); universal among the higher _crustacea_, but not among the lower; extremely general, though not universal, among insects; and universal among the higher _mollusca_ though not among the lower. along with extreme inferiority among animals, we find homogenesis to be the exception rather than the rule; and in the vegetal kingdom there appear to be no cases, except among the _algæ_ and a few aberrant parasites like the _rafflesiaceæ_, in which the centre or axis which arises from a fertilized germ becomes the immediate producer of fertilized germs. in propagation characterized by unlikeness of the successive generations, there is asexual genesis with occasionally-recurring sexual genesis; in other words--_agamogenesis_ interrupted more or less frequently by _gamogenesis_. if we set out with a generation of perfect males and females, then, from their ova arise individuals which are neither males nor females, but which produce the next generation from buds. by this method of multiplication many individuals originate from a single fertilized germ. the product of development is organized round more than one centre or axis. the simplest form of heterogenesis is that seen in most uniaxial plants. if, as we find ourselves obliged to do, we regard each separate shoot or axis of growth as a distinct individual, homogenesis is seen in those which have absolutely terminal flowers; but in all other uniaxial plants, the successive individuals are not represented by the series a, a, a, a, &c., but they are represented by the series a, b, a, b, a, b, &c. for in the majority of plants which were classed as uniaxial (§ ), and which may be conveniently so distinguished from other plants, the axis which shoots up from the seed, and substantially constitutes the plant, does not itself flower but gives lateral origin to flowering axes. though in ordinary uniaxial plants the fructifying apparatus _appears_ to be at the end of the primary, vertical axis; yet dissection shows that, morphologically considered, each fructifying axis is an offspring from the primary axis. there arises from the seed a sexless individual, from which spring by gemmation individuals having reproductive organs; and from these there result fertilized germs or seeds that give rise to sexless individuals. that is to say, gamogenesis and agamogenesis alternate: the peculiarity being that the sexual individuals arise from the sexless ones by continuous development. the _salpæ_ show us an allied form of heterogenesis in the animal kingdom. individuals developed from fertilized ova, instead of themselves producing fertilized ova, produce, by gemmation, strings of individuals from which fertilized ova again originate. in multiaxial plants, we have a succession of generations represented by the series a, b, b, b, &c., a, b, b, b, &c. supposing a to be a flowering axis or sexual individual, then, from any fertilized germ it casts off, there grows up a sexless individual, b; from this there bud-out other sexless individuals, b, and so on for generations more or less numerous, until at length, from some of these sexless individuals, there bud-out seed-bearing individuals of the original form a. branched herbs, shrubs, and trees, exhibit this form of heterogenesis: the successive generations of sexless individuals thus produced being, in most cases, continuously developed, or aggregated into a compound individual, but being in some cases discontinuously developed. among animals a kind of heterogenesis represented by the same succession of letters, occurs in such compound polypes as the _sertularia_, and in those of the _hydrozoa_ which assume alternately the polypoid form and the form of the _medusa_. the chief differences presented by these groups arise from the fact that the successive generations of sexless individuals produced by budding, are in some cases continuously developed, and in others discontinuously developed; and from the fact that, in some cases, the sexual individuals give off their fertilized germs while still growing on the parent-polypedom, but in other cases not until after leaving the parent-polypedom and undergoing further development. where, as in all the foregoing kinds of agamogenesis, the new individuals bud out, not from any specialized reproductive organs but from unspecialized parts of the parent, the process has been named, by prof. owen, _metagenesis_. in most instances the individuals thus produced grow from the outsides of the parents--the metagenesis is external. but there is also a kind of metagenesis which we may distinguish as internal. certain _entozoa_ of the genus _distoma_ exhibit it. from the egg of a _distoma_ there results a rudely-formed creature known as a sporocyst and from this a redia. gradually, as this divides and buds, the greater part of the inner substance is transformed into young animals called _cercariæ_ (which are the larvæ of _distomata_); until at length it becomes little more than a living sac full of living offspring. in the _distoma pacifica_, the brood of young animals thus arising by internal gemmation are not _cercariæ_, but are like their parent: themselves becoming the producers of _cercariæ_, after the same manner, at a subsequent period. so that now the succession of forms is represented by the series a, b, a, b, &c., now by the series a, b, b, a, b, b, &c., and now by a, b, b, c, a. both cases, however, exemplify internal metagenesis in contrast with the several kinds of external metagenesis described above. that agamogenesis which is carried on in a reproductive organ--either an ovarium or the homologue of one--has been called, by prof. owen, _parthenogenesis_. it is the process familiarly exemplified in the _aphides_. here, from the fertilized eggs laid by perfect females there grow up imperfect females, in the ovaria of which are developed ova that though unfertilized, rapidly assume the organization of other imperfect females, and are born viviparously. from this second generation of imperfect females, there by-and-by arises, in the same manner, a third generation of the same kind; and so on for many generations: the series being thus symbolized by the letters a, b, b, b, b, b, &c., a. respecting this kind of heterogenesis it should be added that, in animals as in plants, the number of generations of sexless individuals produced before the re-appearance of sexual ones, is indefinite; both in the sense that in the same species it may go on to a greater or less extent according to circumstances, and in the sense that among the generations of individuals proceeding from the same fertilized germ, a recurrence of sexual individuals takes place earlier in some of the diverging lines of multiplication than in others. in trees we see that on some branches flower-bearing axes arise while other branches are still producing only leaf-bearing axes; and in the successive generations of _aphides_ a parallel fact has been observed. lastly has to be set down that kind of heterogenesis in which, along with gamogenesis, there occurs a form of agamogenesis exactly like it, save in the absence of fecundation. this is called true parthenogenesis--reproduction carried on by virgin mothers which are in all respects like other mothers. among silk-worm-moths this parthenogenesis is exceptional rather than ordinary. usually the eggs of these insects are fertilized; but if they are not they are still laid, and some of them produce larvæ. in certain _lepidoptera_, however, of the groups _psychidæ_ and _tineidæ_, parthenogenesis appears to be a normal process--indeed, so far as is known, the only process; for of some species the males have never been found. a general conception of the relations among the different modes of genesis, thus briefly described, will be best given by the following tabular statement. genesis is { oviparous { or homogenesis, which is usually gamogenesis { ovo-viviparous { or { viviparous or { gamogenesis { alternating heterogenesis, which is { with { parthenogenesis { agamogenesis { or { internal { metagenesis { or { external this, like all other classifications of such phenomena, presents anomalies. it may be justly objected that the processes here grouped under the head agamogenesis, are the same as those before grouped under the head of discontinuous development (§ ): thus making development and genesis partially coincident. doubtless it seems awkward that what are from one point of view considered as structural changes are from another point of view considered as modes of multiplication.[ ] there is, however, nothing for us but a choice of imperfections. we cannot by any logical dichotomies accurately express relations which, in nature, graduate into one another insensibly. neither the above, nor any other scheme, can do more than give an approximate idea of the truth. § . genesis under every form is a process of negative or positive disintegration; and is thus essentially opposed to that process of integration which is the primary process in individual evolution. negative disintegration occurs in those cases where, as among the compound _hydrozoa_, there is a continuous development of new individuals by budding from the bodies of older individuals; and where the older individuals are thus prevented from growing to a greater size, or reaching a higher degree of integration. positive disintegration occurs in those forms of agamogenesis where the production of new individuals is discontinuous, as well as in all cases of gamogenesis. the degrees of disintegration are various. at the one extreme the parent organism is completely broken up, or dissolved into new individuals; and at the other extreme each new individual forms but a small deduction from the parent organism. _protozoa_ and _protophyta_ show us that form of disintegration called spontaneous fission: two or more individuals being produced by the splitting-up of the original one. the _volvox_ and the _hydrodictyon_ are plants which, having developed broods within themselves, give them exit by bursting; and among animals the one lately referred to which arises from the _distoma_ egg, entirely loses its individuality in the individualities of the numerous _distoma_-larvæ with which it becomes filled. speaking generally, the degree of disintegration becomes less marked as we approach the higher organic forms. plants of superior types throw off from themselves, whether by gamogenesis or agamogenesis, parts that are relatively small; and among superior animals there is no case in which the parent individuality is habitually lost in the production of new individuals. to the last, however, there is of necessity a greater or less disintegration. the seeds and pollen-grains of a flowering plant are disintegrated portions of tissue; as are also the ova and spermatozoa of animals. and whether the fertilized germs carry away from their parents small or large quantities of nutriment, these quantities in all cases involve further negative or positive disintegrations of the parents. except in spore-producing plants, new individuals which result from agamogenesis usually do not separate from the parent-individuals until they have undergone considerable development, if not complete development. the agamogenetic offspring of those lowest organisms which develop centrally, do not, of course, pass beyond central structure; but the agamogenetic offspring of organisms which develop axially, commonly assume an axial structure before they become independent. the vegetal kingdom shows us this in the advanced organization of detached bulbils, and of buds that root themselves before separating. of animals, the _hydrozoa_, the _trematoda_, and the _salpæ_, present us with different kinds of agamogenesis, in all of which the new individuals are organized to a considerable extent before being cast off. this rule is not without exceptions, however. the statoblasts of the _plumatella_ (which play the part of winter eggs), developed in an unspecialized part of the body, furnish a case of metagenesis in which centres of development, instead of axes, are detached; and in the above-described parthenogenesis of moths and bees, such centres are detached from an ovarium. when produced by gamogenesis, the new individuals become (in a morphological sense) independent of the parents while still in the shape of centres of development, rather than axes of development; and this even where the reverse is apparently the case. the fertilized germs of those inferior plants which are central, or multicentral, in their development, are of course thrown off as centres; and the same is usually the case even in those which are uniaxial or multiaxial. in the higher plants, of the two elements that go to the formation of the fertilized germ, the pollen-cell is absolutely separated from the parent-plant under the shape of a centre, and the egg-cell, though not absolutely separated from the parent, is still no longer subordinate to the organizing forces of the parent. so that when, after the egg-cell has been fertilized by matter from the pollen-tube, the development commences, it proceeds without parental control: the new individual, though remaining physically united with the old individual, becomes structurally and functionally separate: the old individual doing no more than supply materials. throughout the animal kingdom, the new individuals produced by gamogenesis are obviously separated in the shape of centres of development wherever the reproduction is oviparous: the only conspicuous variation being in the quantity of nutritive matter bequeathed by the parent at the time of separation. and though, where the reproduction is viviparous, the process appears to be different, and in one sense is so, yet, intrinsically, it is the same. for in these cases the new individual really detaches itself from the parent while still only a centre of development; but instead of being finally cast off in this state it is re-attached, and supplied with nutriment until it assumes a more or less complete axial structure. § . as we have lately seen, the essential act in gamogenesis is the union of two cell-nuclei, produced in the great majority of cases by different parent organisms. nearly always the containing cells, often called _gametes_, are unlike: the sperm-cell being the male product, and the germ-cell the female. but among some _protozoa_ and many of the lower _algæ_ and _fungi_, the uniting cells show no differentiation. sexuality is only nascent. there are very many modes and modifications of modes in which these cells are produced; very many modes and modifications of modes by which they are brought into contact; and very many modes and modifications of modes by which the resulting fertilized germs have secured to them the fit conditions for their development. but passing over these divergent and re-divergent kinds of sexual multiplication, which it would take too much space here to specify, the one universal trait is this coalescence of a detached portion of one organism with a more or less detached portion of another. such simple _algæ_ as the _desmidieæ_, which are sometimes called unicellular plants, show us a coalescence, not of detached portions of two organisms, but of two entire organisms: the entire contents of the individuals uniting to form the germ-mass. where, as among the _confervoideæ_, we have aggregated cells whose individualities are scarcely at all subordinate to that of the aggregate, the gamogenetic act is often effected by the union "of separate motile protoplasmic masses produced by the division of the contents of any cell of the aggregate. these free-swimming masses of protoplasm, which are quite similar to (but generally smaller than) the agamogenetic 'zoospores' of the same plants, and to the free-swimming individuals of many _protophyta_, are apparently the primitive type of gametes (conjugating cells); but it is noteworthy that such a gamete nearly always unites with one derived from another cell or from another individual. the same fact holds with regard to the gametes of the protophytes themselves, which are formed in the same way from the single cell of the mother individual. in the higher types of _confervoideæ_, and in _vaucheria_, we find these equivalent, free-swimming, gametes replaced by sexually differentiated sperm- and germ-cells, in some cases arising in different organs set apart for their production, and essentially representing those found in the higher plants. transitional forms, intermediate between these and the cases where equivalent gametes are formed from any cell of the plant are also known." recent investigations concerning the conjugation of _protozoa_ have shown that there is not, as was at one time thought, a fusion of two individualities, but a fusion of parts of their nuclei. the macro-nucleus having disappeared, and the micro-nucleus having broken up into portions, each individual receives from the other one of these portions, which becomes fused with its own nuclear matter. so that even in these humble forms, where there is no differentiation of sexes, the union is not between elements that have arisen in the same individual but between those which have arisen in different individuals: the parts being in this case alike. the marvellous phenomena initiated by the meeting of sperm-cell and germ-cell, or rather of their nuclei, naturally suggest the conception of some quite special and peculiar properties possessed by these cells. it seems obvious that this mysterious power which they display of originating a new and complex organism, distinguishes them in the broadest way from portions of organic substance in general. nevertheless, the more we study the evidence the more are we led towards the conclusion that these cells are not fundamentally different from other cells. the first fact which points to this conclusion is the fact recently dwelt upon (§ ), that in many plants and inferior animals, a small fragment of tissue which is but little differentiated, is capable of developing into an organism like that from which it was taken. this implies that the component units of tissues have inherent powers of arranging themselves into the forms of the organisms which originated them. and if in these component units, which we distinguished as physiological, such powers exist,--if, under fit conditions, and when not much specialized, they manifest such powers in a way as marked as that in which the contents of sperm-cells and germ-cells manifest them; then, it becomes clear that the properties of sperm-cells and germ-cells are not so peculiar as we are apt to assume. again, the organs emitting sperm-cells and germ-cells have none of the specialities of structure which might be looked for, did sperm-cells and germ-cells need endowing with properties unlike those of all other organic agents. on the contrary, these reproductive centres proceed from tissues characterized by their low organization. in plants, for example, it is not appendages that have acquired considerable structure which produce the fructifying particles: these arise at the extremities of the axes where the degree of structure is the least. the cells out of which come the egg and the pollen-grains, are formed from undifferentiated tissue in the interior of the ovule and of the stamen. among many inferior animals devoid of special reproductive organs, such as the _hydra_, the ova and spermatozoa originate from the interstitial cells of the ectoderm, which lie among the bases of the functional cells--have not been differentiated for function; and in the _medusæ_, according to weismann, they arise in the homologous layer, save where the medusoid form remains attached, and then they arise in the endoderm and migrate to the ectoderm: lack of specialization being in all cases implied. then in the higher animals these same generative agents appear to be merely modified epithelium-cells--cells not remarkable for their complexity of structure but rather for their simplicity. if, by way of demurrer to this view, it be asked why other epithelium-cells do not exhibit like properties; there are two replies. the first is that other epithelium-cells are usually so far changed to fit them to their special functions that they are unfitted for assuming the reproductive function. the second is that in some cases, where they are but little specialized, they _do_ exhibit the like properties: not, indeed, by uniting with other cells to produce new germs but by producing new germs without such union. i learn from dr. hooker that the _begonia phyllomaniaca_ habitually develops young plants from the scales of its stem and leaves--nay, that many young plants are developed by a single scale. the epidermal cells composing one of these scales swell, here and there, into large globular cells; form chlorophyll in their interiors; shoot out rudimentary axes; and then, by spontaneous constrictions, cut themselves off; drop to the ground; and grow into begonias. moreover, in a succulent english plant, the _malaxis paludosa_, a like process occurs: the self-detached cells being, in this case, produced by the surfaces of the leaves.[ ] thus, there is no warrant for the assumption that sperm-cells and germ-cells possess powers fundamentally unlike those of other cells. the inference to which the facts point, is, that they differ from the rest mainly in not having undergone functional adaptations. they are cells which have departed but little from the original and most general type: such specializations as some of them exhibit in the shape of locomotive appliances, being interpretable as extrinsic modifications which have reference to nothing beyond certain mechanical requirements. sundry facts tend likewise to show that there does not exist the profound distinction we are apt to assume between the male and female reproductive elements. in the common polype sperm-cells and germ-cells are developed in the same layer of indifferent tissue; and in _tethya_, one of the sponges, prof. huxley has observed that they occur mingled together in the general parenchyma. the pollen-grains and embryo-cells of plants arise in adjacent parts of the meristematic tissue of the flower-bud; and from the description of a monstrosity in the passion-flower, recently given by mr. salter to the linnæan society, it appears both that ovules may, in their general structure, graduate into anthers, and that they may produce pollen in their interiors. moreover, among the lower _algæ_, which show the beginning of sexual differentiation, the smaller gametes, which we must regard as incipient sperm-cells, are sometimes able to fuse _inter se_, and give rise to a zygote which will produce a new plant. all which evidence is in perfect harmony with the foregoing conclusion; since, if sperm-cells and germ-cells have natures not essentially unlike those of unspecialized cells in general, their natures cannot be essentially unlike each other. the next general fact to be noted is that these cells whose union constitutes the essential act of gamogenesis, are cells in which the developmental changes have come to a close--cells which are incapable of further evolution. though they are not, as many cells are, unfitted for growth and metamorphosis by being highly specialized, yet they have lost the power of growth and metamorphosis. they have severally reached a state of equilibrium. and while the internal balance of forces prevents a continuance of constructive changes, it is readily overthrown by external destructive forces. for it almost uniformly happens that sperm-cells and germ-cells which are not brought in contact disappear. in a plant, the egg-cell, if not fertilized, is absorbed or dissipated, while the ovule aborts; and the unimpregnated ovum eventually decomposes: save, indeed, in those types in which parthenogenesis is a part of the normal cycle. such being the characters of these cells, and such being their fates if kept apart, we have now to observe what happens when they are united. in plants the extremity of the elongated pollen-cell applies itself to the surface of the embryo-sac, and one of its nuclei having, with some protoplasm, passed into the egg-cell, there becomes fused with the nucleus of the egg-cell. similarly in animals, the spermatozoon passes through the limiting membrane of the ovum, and a mixture takes place between the substance of its nucleus and the substance of the nucleus of the ovum. but the important fact which it chiefly concerns us to notice, is that on the union of these reproductive elements there begins, either at once or on the return of favourable conditions, a new series of developmental changes. the state of equilibrium at which each had arrived is destroyed by their mutual influence, and the constructive changes, which had come to a close, recommence. a process of cell-multiplication is set up; and the resulting cells presently begin to aggregate into the rudiment of a new organism. thus, passing over the variable concomitants of gamogenesis, and confining our attention to what is constant in it, we see:--that there is habitually, if not universally, a fusion of two portions of organic substance which are either themselves distinct individuals, or are thrown off by distinct individuals; that these portions of organic substance, which are severally distinguished by their low degree of specialization, have arrived at states of structural quiescence or equilibrium; that if they are not united this equilibrium ends in dissolution; but that by the mixture of them this equilibrium is destroyed and a new evolution initiated. § . what are the conditions under which genesis takes place? how does it happen that some organisms multiply by homogenesis and others by heterogenesis? why is it that where agamogenesis prevails it is usually from time to time interrupted by gamogenesis? a survey of the facts discloses certain correlations which, if not universal, are too general to be without significance. where multiplication is carried on by heterogenesis we find, in numerous cases, that agamogenesis continues as long as the forces which result in growth are greatly in excess of the antagonist forces. conversely, we find that the recurrence of gamogenesis takes place when the conditions are no longer so favourable to growth. in like manner where there is homogenetic multiplication, new individuals are usually not formed while the preceding individuals are still rapidly growing--that is, while the forces producing growth exceed the opposing forces to a great extent; but the formation of new individuals begins when nutrition is nearly equalled by expenditure. a few out of the many facts which seem to warrant these inductions must suffice. the relation in plants between fructification and innutrition (or rather, between fructification and such diminished nutrition as makes growth relatively slow) was long ago asserted by a german biologist--wolff, i am told. since meeting with this assertion i have examined into the facts for myself. the result has been a conviction, strengthened by every inquiry, that some such relation exists. uniaxial plants begin to produce their lateral, flowering axes, only after the main axis has developed the great mass of its leaves, and is showing its diminished nutrition by smaller leaves, or shorter internodes, or both. in multiaxial plants two, three, or more generations of leaf-bearing axes, or sexless individuals, are produced before any seed-bearing individuals show themselves. when, after this first stage of rapid growth and agamogenetic multiplication, some gamogenetic individuals arise, they do so where the nutrition is least;--not on the main axis, or on secondary axes, or even on tertiary axes, but on axes that are the most removed from the channels which supply nutriment. again, a flowering axis is commonly less bulky than the others: either much shorter or, if long, much thinner. and further, it is an axis of which the terminal internodes are undeveloped: the foliar organs, which instead of becoming leaves become sepals, and petals, and stamens, follow each other in close succession, instead of being separated by portions of the still-growing axis. another group of evidences meets us when we observe the variations of fruit-bearing which accompany variations of nutrition in the plant regarded as a whole. besides finding, as above, that gamogenesis commences only when growth has been checked by extension of the remoter parts to some distance from the roots, we find that gamogenesis is induced at an earlier stage than usual by checking the nutrition. trees are made to fruit while still quite small by cutting their roots or putting them into pots; and luxuriant branches which have had the flow of sap into them diminished, by what gardeners call "ringing," begin to produce flower-shoots instead of leaf-shoots. moreover, it is to be remarked that trees which, by flowering early in the year, seem to show a direct relation between gamogenesis and increasing nutrition, really do the reverse; for in such trees the flower-buds are formed in the autumn. that structure which determines these buds into sexual individuals is given when the nutrition is declining. conversely, very high nutrition in plants prevents, or arrests, gamogenesis. it is notorious that unusual richness of soil, or too large a quantity of manure, results in a continuous production of leaf-bearing or sexless shoots; and a like result happens when the cutting down of a tree, or of a large part of it, is followed by the sending out of new shoots: these, supplied with excess of sap, are luxuriant and sexless. besides being prevented from producing sexual individuals by excessive nutrition, plants are, by excessive nutrition, made to change the sexual individuals they were about to produce, into sexless ones. this arrest of gamogenesis may be seen in various stages. the familiar instance of flowers made barren by the transformation of their stamens into petals, shows us the lowest degree of this reversed metamorphosis. where the petals and stamens are partially changed into green leaves, the return towards the agamogenetic structure is more marked; and it is still more marked when, as occasionally happens in luxuriantly-growing plants, new flowering axes, and even leaf-bearing axes, grow out of the centres of flowers.[ ] the anatomical structure of the sexual axis affords corroborative evidence: giving the impression, as it does, of an aborted sexless axis. besides lacking those internodes which the leaf-bearing axis commonly possesses, the flowering axis differs by the absence of rudimentary lateral axes. in a leaf-bearing shoot the axil of every leaf usually contains a small bud, which may or may not develop into a lateral shoot; but though the petals of a flower are homologous with leaves, they do not bear homologous buds at their bases. ordinarily, too, the foliar appendages of sexual axes are much smaller than those of sexless ones--the stamens and pistils especially, which are the last formed, being extremely dwarfed; and it may be that the absence of chlorophyll from the parts of fructification is a fact of like meaning. moreover, the formation of the seed-vessel appears to be a direct consequence of arrested nutrition. if a gloved-finger be taken to represent a growing shoot, (the finger standing for the pith of the shoot and the glove for the peripheral layers of meristem and young tissue, in which the process of growth takes place); and if it be supposed that there is a diminished supply of material for growth; then, it seems a fair inference that growth will first cease at the apex of the axis, represented by the end of the glove-finger; and supposing growth to continue in those parts of the peripheral layers of young tissue that are nearer to the supply of nutriment, their further longitudinal extension will lead to the formation of a cavity at the extremity of the shoot, like that which results in a glove-finger when the finger is partially withdrawn and the glove sticks to its end. whence it seems, both that this introversion of the apical meristem may be considered as due to failing nutrition, and that the ovules growing from its introverted surface (which would have been its outer surface but for the defective nutrition) are extremely aborted homologues of external appendages: both they and the pollen-grains being either morphologically or literally quite terminal, and the last showing by their dehiscence the exhaustion of the organizing power.[ ] those kinds of animals which multiply by heterogenesis, present us with a parallel relation between the recurrence of gamogenesis and the recurrence of conditions checking rapid growth: at least, this is shown where experiments have thrown light on the connexion of cause and effect; namely, among the _aphides_. these creatures, hatched from eggs in the spring, multiply by agamogenesis, which in this case is parthenogenesis, throughout the summer. when the weather becomes cold and plants no longer afford abundant sap, perfect males and females are produced; and from gamogenesis result fertilized ova. but beyond this evidence we have much more conclusive evidence. for it has been shown, both that the rapidity of the agamogenesis is proportionate to the warmth and nutrition, and that if the temperature and supply of food be artificially maintained, the agamogenesis continues through the winter. nay more--it not only, under these conditions, continues through one winter, but it has been known to continue for four successive years: some forty or fifty sexless generations being thus produced. and those who have investigated the matter see no reason to doubt the indefinite continuance of this agamogenetic multiplication, so long as the external requirements are duly met. evidence of another kind, complicated by special influences, is furnished by the heterogenesis of the _daphnia_--a small crustacean commonly known as the water-flea, which inhabits ponds and ditches. from the nature of its habitat this little creature is exposed to very variable conditions. besides being frozen in winter, the small bodies of water in which it lives are often unduly heated by the summer sun, or dried up by continued drought. the circumstances favourable to the _daphnia's_ life and growth, being thus liable to interruptions which, in our climate, have a regular irregularity of recurrence; we may, in conformity with the hypothesis, expect to find both that the gamogenesis recurs along with declining physical prosperity and that its recurrence is very variable. i use the expression "declining physical prosperity" advisedly; since "declining nutrition," as measured by supply of food, does not cover all the conditions. this is shown by the experiments of weismann (abstracted for me by mr. cunningham) who found that in various _daphnideæ_ which bring forth resting eggs, sexual and asexual reproduction go on simultaneously, as well as separately, in the spring and summer: these variable results being adapted to variable conditions. for not only are these creatures liable to die from lack of food, from the winter's cold, and from the drying up of their ditches, &c., as well as from the over-heating of them, but during this period of over-heating they are liable to die from that deoxygenation of the water which heat causes. manifestly the favourable and unfavourable conditions recurring in combinations that are rarely twice alike, cannot be met by any regularly recurring form of heterogenesis; and it is interesting to see how survival of the fittest has established a mixed form. in the spring, as well as in the autumn, there is in some cases a formation of resting or winter eggs; and evidently these provide against the killing off of the whole population by summer drought. meanwhile, by ordinary males and females there is a production of summer eggs adapted to meet the incident of drying up by drought and subsequent re-supply of water. and all along successive generations of parthenogenetic females effect a rapid multiplication as long as conditions permit. since life and growth are impeded or arrested not by lack of food only, but by other unfavourable conditions, we may understand how change in one or more of these may set up one or other form of genesis, and how the mixture of them may cause a mixed mode of multiplication which, originally initiated by external causes, becomes by inheritance and selection a trait of the species.[ ] and then in proof that external causes initiate these peculiarities, we have the fact that in certain _daphnideæ_ "which live in places where existence and parthenogenesis are possible throughout the year, the sexual period has disappeared:" there are no males. passing now to animals which multiply by homogenesis--animals in which the whole product of a fertilized germ aggregates round a single centre or axis instead of round many centres or axes--we see, as before, that so long as the conditions allow rapid increase in the mass of this germ-product, the formation of new individuals by gamogenesis does not take place. only when growth is declining in relative rate, do perfect sperm-cells and germ-cells begin to appear; and the fullest activity of the reproductive function arises as growth ceases: speaking generally, at least; for though this relation is tolerably definite in the highest orders of animals which multiply by gamogenesis, it is less definite in the lower orders. this admission does not militate against the hypothesis, as it seems to do; for the indefiniteness of the relation occurs where the limit of growth is comparatively indefinite. we saw (§ ) that among active, hot-blooded creatures, such as mammals and birds, the inevitable balancing of assimilation by expenditure establishes, for each species, an almost uniform adult size; and among creatures of these kinds (birds especially, in which this restrictive effect of expenditure is most conspicuous), the connexion between cessation of growth and commencement of reproduction is distinct. but we also saw (§ ) that where, as in the crocodile and the pike, the conditions and habits of life are such that expenditure does not overtake assimilation as size increases, there is no precise limit of growth; and in creatures thus circumstanced we may naturally look for a comparatively indeterminate relation between declining growth and commencing reproduction.[ ] there is, indeed, among fishes, at least one case which appears very anomalous. the male parr, or young of the male salmon, a fish of four or five inches in length, is said to produce milt. having, at this early stage of its growth, not one-hundredth of the weight of a full-grown salmon, how does its production of milt consist with the alleged general law? the answer must be in great measure hypothetical. if the salmon is (as it appears to be in its young state) a species of fresh-water trout that has contracted the habit of annually migrating to the sea, where it finds a food on which it thrives--if the original size of this species was not much greater than that of the parr (which is nearly as large as some varieties of trout)--and if the limit of growth in the trout tribe is very indefinite, as we know it to be; then we may reasonably infer that the parr has nearly the adult form and size which this species of trout had before it acquired its migratory habit; and that this production of milt is, in such case, a concomitant of the incipient decline of growth naturally arising in the species when living under the conditions of the ancestral species. should this be so, the immense subsequent growth of the parr into the salmon, consequent on a suddenly-increased facility in obtaining food, removes to a great distance the limit at which assimilation is balanced by expenditure; and has the effect, analogous to that produced in plants, of arresting the incipient reproductive process. a confirmation of this view may be drawn from the fact that when the parr, after its first migration to the sea, returns to fresh water, having increased in a few months from a couple of ounces to five or six pounds, it no longer shows any fitness for propagation: the grilse, or immature salmon, does not produce milt or spawn. we conclude, then, that the products of a fertilized germ go on accumulating by simple growth, so long as the forces whence growth results are greatly in excess of the antagonist forces; but that when diminution of the one set of forces or increase of the other, causes a considerable decline in this excess and an approach towards equilibrium, fertilized germs are again produced. whether the germ-product be organized round one axis or round the many axes that arise by agamogenesis, matters not. whether, as in the higher animals, this approach to equilibrium results from that disproportionate increase of expenditure entailed by increase of size; or whether, as in most plants and many inferior animals, it results from absolute or relative decline of nutrition; matters not. in any case the recurrence of gamogenesis is associated with a decrease in the excess of tissue-producing power. we cannot say, indeed, that this decrease always results in gamogenesis: some organisms multiply for an indefinite period by agamogenesis only. the weeping willow, which has been propagated throughout europe, does not seed in europe; and yet, as the weeping willow, by its large size and the multiplication of generation upon generation of lateral axes, presents the same causes of local innutrition as other trees, we cannot ascribe the absence of sexual axes to the continued predominance of nutrition. among animals, too, the anomalous case of the _tineidæ_, a group of moths in which parthenogenetic multiplication goes on for generation after generation, seems to imply that gamogenesis does not necessarily result from an approximate balance of assimilation by expenditure. what we must say is that an approach towards equilibrium between the forces which cause growth and the forces which oppose growth, is the chief condition to the recurrence of gamogenesis; but that there appear to be other conditions, in the absence of which approach to equilibrium is not followed by gamogenesis. § . the above induction is an approximate answer to the question--_when_ does gamogenesis recur? but not to the question which was propounded--_why_ does gamogenesis recur?--_why_ cannot multiplication be carried on in all cases, as it is in many cases, by agamogenesis? as already said, biologic science is not yet advanced enough to reply. meanwhile, the evidence above brought together suggests a certain hypothetical answer. seeing, on the one hand, that gamogenesis recurs only in individuals which are approaching a state of organic equilibrium; and seeing, on the other hand, that the sperm-cells and germ-cells thrown off by such individuals are cells in which developmental changes have ended in quiescence, but in which, after their union, there arises a process of active cell-formation; we may suspect that the approach towards a state of general equilibrium in such gamogenetic individuals, is accompanied by an approach towards molecular equilibrium in them; and that the need for this union of sperm-cell and germ-cell is the need for overthrowing this equilibrium, and re-establishing active molecular change in the detached germ--a result probably effected by mixing the slightly different physiological units of slightly different individuals. the several arguments which support this view, cannot be satisfactorily set forth until after the topics of heredity and variation have been dealt with. leaving it for the present, i propose hereafter to re-consider it in connexion with sundry others raised by the phenomena of genesis. but before ending the chapter, it may be well to note the relations between these different modes of multiplication, and the conditions of existence under which they are respectively habitual. while the explanation of the teleologist is untrue, it is often an obverse to the truth; for though, on the hypothesis of evolution, it is clear that things are not arranged thus or thus for the securing of special ends, it is also clear that arrangements which _do_ secure these special ends tend to establish themselves--are established by their fulfilment of these ends. besides insuring a structural fitness between each kind of organism and its circumstances, the working of "natural selection" also insures a fitness between the mode and rate of multiplication of each kind of organism and its circumstances. we may, therefore, without any teleological implication, consider the fitness of homogenesis and heterogenesis to the needs of the different classes of organisms which exhibit them. heterogenesis prevails among organisms of which the food, though abundant compared with their expenditure, is dispersed in such a way that it cannot be appropriated in a wholesale manner. _protophyta_, subsisting on diffused gases and decaying organic matter in a state of minute subdivision, and _protozoa_, to which food comes in the shape of extremely small floating particles, are enabled, by their rapid agamogenetic multiplication, to obtain materials for growth better than they would do did they not thus continually divide and disperse in pursuit of it. the higher plants, having for nutriment the carbonic acid of the air and certain mineral components of the soil, show us modes of multiplication adapted to the fullest utilization of these substances. a herb with but little power of forming the woody fibre requisite to make a stem that can support wide-spreading branches, after producing a few sexless axes produces sexual ones; and maintains its race better, by the consequent early dispersion of seeds, than by a further production of sexless axes. but a tree, able to lift its successive generations of sexless axes high into the air, where each gets carbonic acid and light almost as freely as if it grew by itself, may with advantage go on budding-out sexless axes year after year; since it thereby increases its subsequent power of budding-out sexual axes. meanwhile it may advantageously transform into seed-bearers those axes which, in consequence of their less direct access to materials absorbed by the roots, are failing in their nutrition; for it thus throws off from a point at which sustenance is deficient, a migrating group of germs that may find sustenance elsewhere. the heterogenesis displayed by animals of the coelenterate type has evidently a like utility. a polype, feeding on minute annelids and crustaceans which, flitting through the water, come in contact with its tentacles, and limited to that quantity of prey which chance brings within its grasp, buds out young polypes which, either as a colony or as dispersed individuals, spread their tentacles through a larger space of water than the parent alone can; and by producing them, the parent better insures the continuance of its species than it would do if it went on slowly growing until its nutrition was nearly balanced by its waste, and then multiplied by gamogenesis. similarly with the _aphis_. living on sap sucked from tender shoots and leaves, and able thus to take in but a very small quantity in a given time, this creature's race is more likely to be preserved by a rapid asexual propagation of small individuals, which disperse themselves over a wide area of nutrition, than it would be did the individual growth continue so as to produce large individuals multiplying sexually. and then when autumnal cold and diminishing supply of sap put a check to growth, the recurrence of gamogenesis, or production of fertilized ova which remain dormant through the winter, is more favourable to the preservation of the race than would be a further continuance of agamogenesis. on the other hand, among the higher animals living on food which, though dispersed, is more or less aggregated into large masses, this alternation of gamic and agamic reproduction ceases to be useful. the development of the germ-product into a single organism of considerable bulk, is in many cases a condition without which these large masses of nutriment could not be appropriated; and here the formation of many individuals instead of one would be fatal. but we still see the beneficial results of the general law--the postponement of gamogenesis until the rate of growth begins to decline. for so long as the rate of growth continues rapid, there is proof that the organism gets food with facility--that expenditure does not seriously check accumulation; and that the size reached is as yet not disadvantageous: or rather, indeed, that it is advantageous. but when the rate of growth is much decreased by the increase of expenditure--when the excess of assimilative power is diminishing so fast as to indicate its approaching disappearance--it becomes needful, for the maintenance of the species, that this excess shall be turned to the production of new individuals; since, did growth continue until there was a complete balancing of assimilation and expenditure, the production of new individuals would be either impossible or fatal to the parent. and it is clear that "natural selection" will continually tend to determine the period at which gamogenesis commences, in such a way as most favours the maintenance of the race. here, too, may fitly be pointed out the fact that, by "natural selection," there will in every case be produced the most advantageous proportion of males and females. if the conditions of life render numerical inequality of the sexes beneficial to the species, in respect either of the number of the offspring or the character of the offspring; then, those varieties of the species which approach more than other varieties towards this beneficial degree of inequality, will be apt to supplant other varieties. and conversely, where equality in the number of males and females is beneficial, the equilibrium will be maintained by the dying out of such varieties as produce offspring among which the sexes are not balanced. note.--such alterations of statement in this chapter as have been made necessary by the advance of biological knowledge since have not, i think, tended to invalidate its main theses, but have tended to verify them. some explanations to be here added may remove remaining difficulties. certain types, which are transitional between _protozoa_ and _metazoa_, exhibit under its simplest form the relation between self-maintenance and race-maintenance--the integration primarily effecting the one and the disintegration primarily effecting the other. among the _mycetozoa_ a number of amoeba-like individuals aggregate into what is called a plasmodium; and while, in some orders, they become fused into a mass of protoplasm through which their nuclei are dispersed, in other orders (_sorophora_) they retain their individualities and simply form a coherent aggregate. these last, presumably the earliest in order of evolution, remain united so long as the plasmodium, having a small power of locomotion, furthers the general nutrition; but when this is impeded by drought or cold, there arise spores. each spore contains an amoeboid individual; and this, escaping when favourable conditions return, establishes by fission and by union with others like itself a new colony or plasmodium. reduced to its lowest terms, we here see the antagonism between that growth of the coherent mass of units which accompanies its physical prosperity, and that incoherence and dispersion of the units which follows unfavourable conditions and arrest of growth, and which presently initiates new plasmodia. this antagonism, seen in these incipient _metazoa_ which show us none of that organization characterizing the _metazoa_ in general, is everywhere in more or less disguised forms exhibited by them--must necessarily be so if growth of the individual is a process of integration while formation of new individuals is a process of disintegration. and, primarily, it is an implication that whatever furthers the one impedes the other. but now while recognizing the truth that nutrition and innutrition (using these words to cover not supply of nutriment only but the presence of other influences favourable or unfavourable to the vital processes) primarily determine the alternations of these; we have also to recognize the truth that from the beginning survival of the fittest has been shaping the forms and effects of their antagonism. by inheritance a physiological habit which modifies the form of the antagonism in a way favourable to the species, will become established. especially will this be the case where the lives of the individuals have become relatively definite and where special organs have been evolved for casting off reproductive centres. the resulting physiological rhythm may in such cases become so pronounced as greatly to obscure the primitive relation. among plants we see this in the fact that those which have been transferred from one habitat to another having widely different seasons, long continue their original time of flowering, though it is inappropriate to the new circumstances--the reproductive periodicity has become organic. similarly in each species of higher animal, development of the reproductive organs and maturation of reproductive cells take place at a settled age, whether the conditions have been favourable or unfavourable to physical prosperity. the established constitutional tendency, adapted to the needs of the species, over-rides the constitutional needs of the individual. even here, however, the primitive antagonism, though greatly obscured, occasionally shows itself. instance the fact that in plants where gamogenesis is commencing a sudden access of nutrition will cause resumption of agamogenesis; and i suspect that an illustration may be found among human beings in the earlier establishment of the reproductive function among the ill-fed poor than among the well-fed rich. one other qualification has to be added. in plants and animals which have become so definitely constituted that at an approximately fixed stage, the proclivity towards the production of new individuals becomes pronounced, it naturally happens that good nutrition aids it. surplus nutriment being turned into the reproductive channel, the reproduction is efficient in proportion as the surplus is great. hence the fact that in fruit trees which have reached the flowering stage, manuring has the effect that though it does not increase the quantity of blossoms it increases the quantity of fruit; and hence the fact that well-fed and easy-living races of men are prolific. chapter viii. heredity. § . already, in the last two chapters, the law of hereditary transmission has been tacitly assumed; as, indeed, it unavoidably is in all such discussions. understood in its entirety, the law is that each plant or animal, if it reproduces, gives origin to others like itself: the likeness consisting, not so much in the repetition of individual traits as in the assumption of the same general structure. this truth has been rendered so familiar by daily illustration as almost to have lost its significance. that wheat produces wheat--that existing oxen have descended from ancestral oxen--that every unfolding organism eventually takes the form of the class, order, genus, and species from which it sprang; is a fact which, by force of repetition, has acquired in our minds almost the aspect of a necessity. it is in this, however, that heredity is principally displayed: the manifestations of it commonly referred to being quite subordinate. and, as thus understood, heredity is universal. the various instances of heterogenesis lately contemplated seem, indeed, to be at variance with this assertion. but they are not really so. though the recurrence of like forms is, in these instances, not direct but cyclical, still, the like forms do recur; and, when taken together, the group of forms produced during one of the cycles is as much like the groups produced in preceding cycles, as the single individual arising by homogenesis is like ancestral individuals. while, however, the general truth that organisms of a given type uniformly descend from organisms of the same type, is so well established by infinite illustrations as to have assumed the character of an axiom; it is not universally admitted that non-typical peculiarities are inherited. many entertain a vague belief that the law of heredity applies only to main characters of structure and not to details; or, at any rate, that though it applies to such details as constitute differences of species, it does not apply to smaller details. the circumstance that the tendency to repetition is in a slight degree qualified by the tendency to variation (which, as we shall hereafter see, is but an indirect result of the tendency to repetition), leads some to doubt whether heredity is unlimited. a careful weighing of the evidence, however, and a due allowance for the influences by which the minuter manifestations of heredity are obscured, may remove this scepticism. first in order of importance comes the fact that not only are there uniformly transmitted from an organism to its offspring, those traits of structure which distinguish the class, order, genus, and species; but also those which distinguish the variety. we have numerous cases, among both plants and animals, where, by natural or artificial conditions, there have been produced divergent modifications of the same species; and abundant proof exists that the members of any one sub-species habitually transmit their distinctive peculiarities to their descendants. agriculturists and gardeners can furnish unquestionable illustrations. several varieties of wheat are known, of which each reproduces itself. since the potato was introduced into england there have been formed from it a number of sub-species; some of them differing greatly in their forms, sizes, qualities, and periods of ripening. of peas, also, the like may be said. and the case of the cabbage-tribe is often cited as showing the permanent establishment of races which have diverged widely from a common stock. among fruits and flowers the multiplication of kinds, and the continuance of each kind with certainty by agamogenesis, and to some extent by gamogenesis, might be exemplified without end. from all sides evidence may be gathered showing a like persistence of varieties among animals. we have our distinct breeds of sheep, our distinct breeds of cattle, our distinct breeds of horses: each breed maintaining its characteristics. the many sorts of dogs which, if we accept the physiological test, we must consider as all of one species, show us in a marked manner the hereditary transmission of small differences--each sort, when kept pure, reproducing itself not only in size, form, colour, and quality of hair, but also in disposition and speciality of intelligence. poultry, too, have their permanently-established races. and the isle of man sends us a tail-less kind of cat. even in the absence of other evidence, that which ethnology furnishes would suffice. grant them to be derived from one stock, and the varieties of man yield proof upon proof that non-specific traits of structure are bequeathed from generation to generation. or grant only their derivation from several stocks, and we still have, between races descended from a common stock, distinctions which prove the inheritance of minor peculiarities. besides seeing the negroes continue to produce negroes, copper-coloured men to produce men of a copper colour, and the fair-skinned races to perpetuate their fair skins--besides seeing that the broad-faced and flat-nosed calmuck begets children with broad faces and flat noses, while the jew bequeaths to his offspring the features which have so long characterized jews; we see that those small unlikenesses which distinguish more nearly-allied varieties of men, are maintained from generation to generation. in germany, the ordinary shape of skull is appreciably different from that common in britain: near akin though the germans are to the british. the average italian face continues to be unlike the faces of northern nations. the french character is now, as it was centuries ago, contrasted in sundry respects with the characters of neighbouring peoples. nay, even between races so closely allied as the scotch celts, the welsh celts, and the irish celts, appreciable differences of form and nature have become established. the fact that sub-species and sub-sub-species thus exemplify the general law of inheritance which shows itself in the perpetuation of ordinal, generic, and species peculiarities, is strong reason for the belief that this general lay is unlimited in its application. this has the support of still more special evidences. they are divisible into two classes. in the one come cases where congenital peculiarities, not traceable to any obvious causes, are bequeathed to descendants. in the other come cases where the peculiarities thus bequeathed are not congenital, but have resulted from changes of functions during the lives of the individuals bequeathing them. we will consider first the cases that come in the first class. § . note at the outset the character of the chief testimony. excluding those inductions that have been so fully verified as to rank with exact science, there are no inductions so trustworthy as those which have undergone the mercantile test. when we have thousands of men whose profit or loss depends on the truth of their inferences from perpetually-repeated observations; and when we find that their inferences, handed down from generation to generation, have generated an unshakable conviction; we may accept it without hesitation. in breeders of animals we have such a class, led by such experiences, and entertaining such a conviction--the conviction that minor peculiarities of organization are inherited as well as major peculiarities. hence the immense prices given for successful racers, bulls of superior forms, sheep that have certain desired peculiarities. hence the careful record of pedigrees of high-bred horses and sporting dogs. hence the care taken to avoid intermixture with inferior stocks. as quoted by mr. darwin, youatt says the principle of selection "enables the agriculturist not only to modify the character of his flock but to change it altogether." lord somerville, speaking of what breeders have done for sheep, says:--"it would seem that they have chalked upon a wall a form perfect in itself and then given it existence." that most skilful breeder, sir john sebright, used to say, with respect to pigeons, that "he would produce any given feather in three years, but it would take him six years to obtain head and beak." in all which statements the tacit assertion is, that individual traits are bequeathed from generation to generation, and may be so perpetuated and increased as to become permanent distinctions. of special instances there are many besides that of the often-cited otto-breed of sheep, descended from a single short-legged lamb, and that of the six-fingered gratio kelleia, who transmitted his peculiarity, in different degrees, to several of his children and to some of his grandchildren. in a paper contributed to the _edinburgh new philosophical journal_ for july, , dr. (now sir john) struthers gives cases of hereditary digital variations. esther p----, who had six fingers on one hand, bequeathed this malformation along some lines of her descendants for two, three, and four generations. a---- s---- inherited an extra digit on each hand and each foot from his father; and c---- g----, who also had six fingers and six toes, had an aunt and a grandmother similarly formed. a collection of evidence published by mr. sedgwick in the _medico-chirurgical review_ for april and for july, , in two articles on "the influence of sex in limiting hereditary transmission," includes the following cases:--augustin duforet, a pastry-cook of douai, who had but two instead of three phalanges to all his fingers and toes, inherited this malformation from his grandfather and father, and had it in common with an uncle and numerous cousins. an account has been given by dr. lepine, of a man with only three fingers on each hand and four toes on each foot, and whose grandfather and son exhibited the like anomaly. béchet describes victoire barré as a woman who, like her father and sister, had but one developed finger on each hand and but two toes on each foot, and whose monstrosity re-appeared in two daughters. and there is a case where the absence of two distal phalanges on the hands was traced for two generations. the various recorded instances in which there has been transmission from one generation to another, of webbed-fingers, of webbed-toes, of hare-lip, of congenital luxation of the thigh, of absent patellæ, of club-foot, &c., would occupy more space than can here be spared. defects in the organs of sense are also not unfrequently inherited. four sisters, their mother, and grandmother, are described by duval as similarly affected by cataract. prosper lucas details an example of amaurosis affecting the females of a family for three generations. duval, graffe, dufon, and others testify to like cases coming under their observation.[ ] deafness, too, is occasionally transmitted from parent to child. there are deaf-mutes whose imperfections have been derived from ancestors; and malformations of the external ears have also been perpetuated in offspring. of transmitted peculiarities of the skin and its appendages, many cases have been noted. one is that of a family remarkable for enormous black eyebrows; another that of a family in which every member had a lock of hair of a lighter colour than the rest on the top of the head; and there are also instances of congenital baldness being hereditary. from one of our leading sculptors i learn that his wife has a flat mole under the foot near the little toe, and one of her sons has the same. entire absence of teeth, absence of particular teeth, and anomalous arrangements of teeth, are recorded as traits that have descended to children. and we have evidence that soundness and unsoundness of teeth are transmissible. the inheritance of tendencies to such diseases as gout, consumption, and insanity is universally admitted. among the less-common diseases of which the descent has been observed, are ichthyosis, leprosy, pityriasis, sebaceous tumours, plica polonica, dipsomania, somnambulism, catalepsy, epilepsy, asthma, apoplexy, elephantiasis. general nervousness displayed by parents almost always re-appears in their children. even a bias towards suicide appears to be sometimes hereditary. § . to prove the transmission of those structural peculiarities which have resulted from functional peculiarities, is, for several reasons, comparatively difficult. changes produced in the sizes of parts by changes in their amounts of action, are mostly unobtrusive. a muscle which has increased in bulk is usually so obscured by natural or artificial clothing, that unless the alteration is extreme it passes without remark. such nervous developments as are possible in the course of a single life, cannot be seen externally. visceral modifications of a normal kind are observable but obscurely, or not at all. and if the changes of structure worked in individuals by changes in their habits are thus difficult to trace, still more difficult to trace must be the transmission of them: further hidden, as this is, by the influences of other individuals who are often otherwise modified by other habits. moreover, such specialities of structure as are due to specialities of function, are usually entangled with specialities of structure which are, or may be, due to selection, natural or artificial. in most cases it is impossible to say that a structural peculiarity which seems to have arisen in offspring from a functional peculiarity in a parent, is wholly independent of some congenital peculiarity of structure in the parent, whence this functional peculiarity arose. we are restricted to cases with which natural or artificial selection can have had nothing to do, and such cases are difficult to find. some, however, may be noted. a species of plant that has been transferred from one soil or climate to another, frequently undergoes what botanists call "change of habit"--a change which, without affecting its specific characters, is yet conspicuous. in its new locality the species is distinguished by leaves that are much larger or much smaller, or differently shaped, or more fleshy; or instead of being as before comparatively smooth, it becomes hairy; or its stem becomes woody instead of being herbaceous; or its branches, no longer growing upwards, assume a drooping character. now these "changes of habit" are clearly determined by functional changes. occurring, as they do, in many individuals which have undergone the same transportation, they cannot be classed as "spontaneous variations." they are modifications of structure consequent on modifications of function that have been produced by modifications in the actions of external forces. and as these modifications re-appear in succeeding generations, we have, in them, examples of functionally-established variations that are hereditarily transmitted. evidence of analogous changes in animals is difficult to disentangle. only among domesticated kinds have we any opportunity of tracing the results of altered habits; and here, in nearly all cases, artificial selection has obscured them. still, there are some facts which seem to the point. mr. darwin, while ascribing almost wholly to "natural selection" the production of those modifications which eventuate in differences of species, nevertheless admits the effects of use and disuse. he says--"i find in the domestic duck that the bones of the wing weigh less and the bones of the leg more, in proportion to the whole skeleton, than do the same bones in the wild duck; and i presume that this change may be safely attributed to the domestic duck flying much less, and walking more, than its wild parent. the great and inherited development of the udders in cows and goats in countries where they are habitually milked, in comparison with the state of these organs in other countries, is another instance of the effect of use. not a single domestic animal can be named which has not in some country drooping ears; and the view suggested by some authors, that the drooping is due to the disuse of the muscles of the ear, from the animals not being much alarmed by danger, seems probable." again--"the eyes of moles and of some burrowing rodents are rudimentary in size, and in some cases are quite covered up by skin and fur. this state of the eyes is probably due to gradual reduction from disuse, but aided perhaps by natural selection." ... "it is well known that several animals belonging to the most different classes, which inhabit the caves of styria and of kentucky, are blind. in some of the crabs the footstalk of the eye remains, though the eye is gone; the stand for the telescope is there, though the telescope with its glasses has been lost. as it is difficult to imagine that eyes, though useless, could be in any way injurious to animals living in darkness, i attribute their loss wholly to disuse."[ ] the direct inheritance of an acquired peculiarity is sometimes observable. mr. lewes gives a case. he "had a puppy taken from its mother at six weeks old, who, although never taught 'to beg' (an accomplishment his mother had been taught), spontaneously took to begging for everything he wanted when about seven or eight months old: he would beg for food, beg to be let out of the room, and one day was found opposite a rabbit hutch begging for rabbits." instances are on record, too, of sporting dogs which spontaneously adopted in the field, certain modes of behaviour which their parents had learnt. but the best examples of inherited modifications produced by modifications of function, occur in mankind. to no other cause can be ascribed the rapid metamorphoses undergone by the british races when placed in new conditions. in the united states the descendants of the immigrant irish lose their celtic aspect, and become americanized. this cannot be ascribed to mixture, since the feeling with which irish are regarded by americans prevents any considerable amount of intermarriage. equally marked is the case of the immigrant germans who, though they keep very much apart, rapidly assume the prevailing type. to say that "spontaneous variation" increased by natural selection, can have produced this effect, is going too far. peoples so numerous cannot have been supplanted in the course of two or three generations by varieties springing from them. hence the implication is that physical and social conditions have wrought modifications of function and structure, which offspring have inherited and increased. similarly with special cases. in the _cyclopædia of practical medicine_, vol. ii., p. , dr. brown states that he "has in many instances observed in the case of individuals whose complexion and general appearance has been modified by residence in hot climates, that children born to them subsequently to such residence, have resembled them rather in their acquired than primary mien." some visible modifications of organs caused by changes in their functions, may be noted. that large hands are inherited by those whose ancestors led laborious lives, and that those descended from ancestors unused to manual labour commonly have small hands, are established opinions. it seems very unlikely that in the absence of any such connexion, the size of the hand should have come to be generally regarded as some index of extraction. that there exists a like relation between habitual use of the feet and largeness of the feet, we have strong evidence in the customs of the chinese. the torturing practice of artificially arresting the growth of the feet, could never have become established among the ladies of china, had they not seen that a small foot was significant of superior rank--that is of a luxurious life--that is of a life without bodily labour. there is evidence, too, that modifications of the eyes, caused by particular uses of the eyes, are inherited. short sight appears to be uncommon among peasants; but it is frequent among classes who use their eyes much for reading and writing, and is often congenital. still more marked is this relation in germany. there, the educated are notoriously studious, and judging from the numbers of young germans who wear spectacles, there is reason to think that congenital myopia is very frequent among them. some of the best illustrations of functional heredity, are furnished by mental characteristics. certain powers which mankind have gained in the course of civilization cannot, i think, be accounted for without admitting the inheritance of acquired modifications. the musical faculty is one of these. to say that "natural selection" has developed it by preserving the most musically endowed, seems an inadequate explanation. even now that the development and prevalence of the faculty have made music an occupation by which the most musical can get sustenance and bring up families; it is very questionable whether, taking the musical career as a whole, it has any advantage over other careers in the struggle for existence and multiplication. still more if we look back to those early stages through which the faculty must have passed before definite perception of melody was arrived at, we fail to see how those possessing the rudimentary faculty in a somewhat greater degree than the rest, would thereby be enabled the better to maintain themselves and their children. there is no explanation but that the habitual association of certain cadences of speech with certain emotions, has slowly established in the race an organized and inherited connection between such cadences and such emotions; that the combination of such cadences, more or less idealized, which constitutes melody, has all along had a meaning in the average mind, only because of the meaning which cadences had acquired in the average mind; and that by the continual hearing and practice of melody there has been gained and transmitted an increasing musical sensibility. confirmation of this view may be drawn from individual cases. grant that among a people endowed with musical faculty to a certain degree, spontaneous variation will occasionally produce men possessing it in a higher degree; it cannot be granted that spontaneous variation accounts for the frequent production, by such highly-endowed men, of men still more highly endowed. on the average, the children of marriages with others not similarly endowed, will be less distinguished rather than more distinguished. the most that can be expected is that this unusual amount of faculty shall re-appear in the next generation undiminished. how then shall we explain cases like those of bach, mozart, and beethoven, all of them sons of men having unusual musical powers who were constantly exercising those powers, and who greatly excelled their fathers in their musical powers? what shall we say to the facts that haydn was the son of an organist, that hummel was born to a music master, and that weber's father was a distinguished violinist? the occurrence of so many cases in one nation within a short period of time, cannot rationally be ascribed to the coincidence of "spontaneous variations." it can be ascribed to nothing but inherited developments of structure caused by augmentations of function. but the clearest proof that structural alterations caused by alterations of function are inherited, occurs when the alterations are morbid. i had originally named in this place the results of m. brown-sequard's experiments on guinea-pigs, showing that those which had been artificially made epileptic had offspring which were epileptic; and i name them again though his inference is by many rejected. for, as exemplified a few pages back, strong evidence is often disregarded for trivial reasons by those who dislike the conclusion drawn. just naming this evidence and its possible invalidity, let me pass to some results of experiences recently set forth by dr. savage, president of the neurological society. in an essay on "heredity and neurosis" published in _brain_, parts lxxvii, lxxviii, , he says:--"we recognise the transmission of a tendency to develop gout, and we recognise that the disease produced by the individual himself differs little from that which may have been inherited." [that is, acquired gout may be transmitted as constitutional gout.] "i have seen several patients whose history i have been able to examine carefully, in whom mental tricks have been transmitted from one generation to another." in the "musical prodigies" descending from musical parents, "there seemed to be a transmission of a greatly increased aptitude or tendency which is all one is contending for." "though there is, in my opinion, power to transmit acquired peculiarities, yet the tendency is to transmit a predisposition." (pp. - .) and an authority on nervous diseases who is second to none--dr. hughlings jackson--takes the same view. the liability to consumption shown by children of consumptive parents, which no one doubts, shows us the same thing. it is admitted that consumption may be produced by conditions very unfavourable to life; and unless it is held that the disease so produced differs from the disease when inherited, the conclusion must be that here, too, there is a transmission of functionally-produced organic changes. this holds true whether the production of tubercle is due to innate defect or whether it is due to the invasion of a bacillus. for in this last case the consumptive diathesis must be regarded as a state of body more than usually liable to invasion by the bacillus, and this is the same when acquired as when transmitted. § . two modified manifestations of heredity remain to be noticed. the one is the re-appearance in offspring of traits not borne by the parents, but borne by the grandparents or by remoter ancestors. the other is the limitation of heredity by sex--the restriction of transmitted peculiarities to offspring of the same sex as the parent possessing them. atavism, which is the name given to the recurrence of ancestral traits, is proved by many and varied facts. in the picture-galleries of old families, and on the monumental brasses in the adjacent churches, are often seen types of feature which are still, from time to time, repeated in members of these families. it is a matter of common remark that some constitutional diseases, such as gout and insanity, after missing a generation, will show themselves in the next. dr. struthers, in his above-quoted paper "on variation in the number of fingers and toes, and in the phalanges in man," gives cases of malformations common to grandparent and grandchild, but of which the parent had no trace. m. girou (as quoted by mr. sedgwick) says--"one is often surprised to see lambs black, or spotted with black, born of ewes and rams with white wool, but if one takes the trouble to go back to the origin of this phenomena, it is found in the ancestors." instances still more remarkable, in which the remoteness of the ancestors copied is very great, are given by mr. darwin. he points out that in crosses between varieties of the pigeon, there will sometimes re-appear the plumage of the original rock-pigeon, from which these varieties descended; and he thinks the faint zebra-like markings occasionally traceable in horses have probably a like meaning. the other modified manifestation of heredity above referred to is the limitation of heredity by sex. in mr. sedgwick's essays, already named, will be found evidence implying that there exists some such tendency to limitation, which does or does not show itself distinctly according to the nature of the organic modification to be conveyed. on joining to the evidence he gives certain bodies of allied evidence we shall, i think, find the inconsistences comprehensible. beyond the familiar facts that in ourselves, along with the essential organs of sex there go minor structures and traits distinctive of sex, such as the beard and the voice in man, we have numerous cases in which, along with different sex-organs there go general differences, sometimes immense and often conspicuous. we have those in which (as in sundry parasites) the male is extremely small compared with the female; we have those in which the male is winged and the female wingless; we have those, as among birds, in which the plumage of males contrasts strongly with that of females; and among butterflies we have kindred instances in which the wings of the two sexes are wholly unlike--some, indeed, in which there is not simply dimorphism but polymorphism: two kinds of females both differing from the male. how shall we range these facts with the ordinary facts of inheritance? without difficulty if heredity results from the proclivity which the component units contained in a germ-cell or a sperm-cell have to arrange themselves into a structure like that of the structure from which they were derived. for the obvious corollary is that where there is gamogenesis there will result partly concurring and partly conflicting proclivities. in the fertilized germ we have two groups of physiological units, slightly different in their structures. these slightly-different units severally multiply at the expense of the nutriment supplied to the unfolding germ--each kind moulding this nutriment into units of its own type. throughout the process of development the two kinds of units, mainly agreeing in their proclivities and in the form which they tend to build themselves into, but having minor differences, work in unison to produce an organism of the species from which they were derived, but work in antagonism to produce copies of their respective parent-organisms. and hence ultimately results an organism in which traits of the one are mixed with traits of the other; and in which, according to the predominance of one or other group of units, one or other sex with all its concomitants is produced. if so, it becomes comprehensible that with the predominance of either group, and the production of the same sex as that of the parent whence it was derived, there will go the repetition not only of the minor sex-traits of that parent but also of any peculiarities he or she possessed, such as monstrosities. since the two groups are nearly balanced, and since inheritance is never an average of the two parents but a mixture of traits of the one with traits of the other, it is not difficult to see why there should be some irregularity in the transmission of these monstrosities and constitutional tendencies, though they are most frequently transmitted only to those of the same sex.[ ] § . unawares in the last paragraph there has been taken for granted the truth of that suggestion concerning heredity ventured in § . anything like a positive explanation is not to be expected in the present stage of biology, if at all. we can look for nothing beyond a simplification of the problem; and a reduction of it to the same category with certain other problems which also admit of hypothetical solutions only. if an hypothesis which sundry widespread phenomena have already thrust upon us, can be shown to render the phenomena of heredity more intelligible than they at present seem, we shall have reason to entertain it. the applicability of any method of interpretation to two different but allied classes of facts, is evidence of its truth. the power which many animals display of reproducing lost parts, we saw to be inexplicable except on the assumption that the units of which any organism is built have a tendency to arrange themselves into the shape of that organism (§ ). this power is sufficiently remarkable in cases where a lost limb or tail is replaced, but it is still more remarkable in cases where, as among some annelids, the pieces into which an individual is cut severally complete themselves by developing heads and tails, or in cases like that of the _holothuria_, which having, when alarmed, ejected its viscera, reproduces them. such facts compel us to admit that the components of an organism have a proclivity towards a special structure--that the adult organism when mutilated exhibits that same proclivity which is exhibited by the young organism in the course of its normal development. as before said, we may, for want of a better name, figuratively call this power organic polarity: meaning by this phrase nothing more than the observed tendency towards a special arrangement. and such facts as those presented by the fragments of a _hydra_, and by fragments of leaves from which complete plants are produced, oblige us to recognize this proclivity as existing throughout the tissues in general--nay, in the case of the _begonia phyllomaniaca_, obliges us to recognize this proclivity as existing in the physiological units contained in each undifferentiated cell. quite in harmony with this conclusion, are certain implications since noticed, respecting the characters of sperm-cells and germ-cells. we saw sundry reasons for rejecting the supposition that these are highly-specialized cells and for accepting the opposite supposition, that they are cells differing from others rather in being unspecialized. and here the assumption to which we seem driven by the _ensemble_ of the evidence, is, that sperm-cells and germ-cells are essentially nothing more than vehicles in which are contained small groups of the physiological units in a fit state for obeying their proclivity towards the structural arrangement of the species they belong to. if the likeness of offspring to parents is thus determined, it becomes manifest, _à priori_, that besides the transmission of generic and specific peculiarities, there will be a transmission of those individual peculiarities which, arising without assignable causes, are classed as "spontaneous." for if the assumption of a special arrangement of parts by an organism, is due to the proclivity of its physiological units towards that arrangement; then the assumption of an arrangement of parts slightly different from that of the species, implies physiological units slightly unlike those of the species; and these slightly-unlike physiological units, communicated through the medium of sperm-cell or germ-cell, will tend, in the offspring, to build themselves into a structure similarly diverging from the average of the species. but it is not equally manifest that, on this hypothesis, alterations of structure caused by alterations of function must be transmitted to offspring. it is not obvious that change in the form of a part, caused by changed action, involves such change in the physiological units throughout the organism that these, when groups of them are thrown off in the shape of reproductive centres, will unfold into organisms that have this part similarly changed in form. indeed, when treating of adaptation (§ ), we saw that an organ modified by increase or decrease of function, can but slowly re-act on the system at large, so as to bring about those correlative changes required to produce a new equilibrium; and yet only when such new equilibrium has been established, can we expect it to be _fully_ expressed in the modified physiological units of which the organism is built--only then can we count on a complete transfer of the modification to descendants. nevertheless, that changes of structure caused by changes of action must also be transmitted, however obscurely, appears to be a deduction from first principles--or if not a specific deduction, still, a general implication. for if an organism a, has, by any peculiar habit or condition of life, been modified into the form a', it follows that all the functions of a', reproductive function included, must be in some degree different from the functions of a. an organism being a combination of rhythmically-acting parts in moving equilibrium, the action and structure of any one part cannot be altered without causing alterations of action and structure in all the rest; just as no member of the solar system could be modified in motion or mass, without producing rearrangements throughout the whole solar system. and if the organism a, when changed to a', must be changed in all its functions; then the offspring of a' cannot be the same as they would have been had it retained the form a. that the change in the offspring must, other things equal, be in the same direction as the change in the parent, appears implied by the fact that the change propagated throughout the parental system is a change towards a new state of equilibrium--a change tending to bring the actions of all organs, reproductive included, into harmony with these new actions. or, bringing the question to its ultimate and simplest form, we may say that as, on the one hand, physiological units will, because of their special polarities, build themselves into an organism of a special structure; so, on the other hand, if the structure of this organism is modified by modified function, it will impress some corresponding modification on the structures and polarities of its units. the units and the aggregate must act and re-act on each other. if nothing prevents, the units will mould the aggregate into a form in equilibrium with their pre-existing polarities. if, contrariwise, the aggregate is made by incident actions to take a new form, its forces must tend to re-mould the units into harmony with this new form. and to say that the physiological units are in any degree so re-moulded as to bring their polar forces towards equilibrium with the forces of the modified aggregate, is to say that when separated in the shape of reproductive centres, these units will tend to build themselves up into an aggregate modified in the same direction. note.--a large amount of additional evidence supporting the belief that functionally produced modifications are inherited, will be found in appendix b. chapter ix. variation. § . equally conspicuous with the truth that every organism bears a general likeness to its parents, is the truth that no organism is exactly like either parent. though similar to both in generic and specific traits, and usually, too, in those traits which distinguish the variety, it diverges in numerous traits of minor importance. no two plants are indistinguishable; and no two animals are without differences. variation is co-extensive with heredity. the degrees of variation have a wide range. there are deviations so small as to be not easily detected; and there are deviations great enough to be called monstrosities. in plants we may pass from cases of slight alteration in the shape of a leaf, to cases where, instead of a flower with its calyx above the seed-vessel, there is produced a flower with its calyx below the seed-vessel; and while in one animal there arises a scarcely noticeable unlikeness in the length or colour of the hair, in another an organ is absent or a supernumerary organ appears. though small variations are by far the most general, yet variations of considerable magnitude are not uncommon; and even those variations constituted by additions or suppressions of parts, are not so rare as to be excluded from the list of causes by which organic forms are changed. cattle without horns are frequent. of sheep there are horned breeds and breeds that have lost their horns. at one time there existed in scotland a race of pigs with solid feet instead of cleft feet. in pigeons, according to mr. darwin, "the number of the caudal and sacral vertebræ vary; as does the number of the ribs, together with their relative breadth and the presence of processes." that variations, both small and large, which arise without any specific assignable cause, tend to become hereditary, was shown in the last chapter. indeed the evidence which proves heredity in its smaller manifestations is the same evidence which proves variation; since it is only when there occur variations that the inheritance of anything beyond the structural peculiarities of the species can be proved. it remains here, however, to be observed that the transmission of variations is itself variable; and that it varies both in the direction of decrease and in the direction of increase. an individual trait of one parent may be so counteracted by the influence of the other parent, that it may not appear in the offspring; or, not being so counteracted, the offspring may possess it, perhaps in an equal degree or perhaps in a less degree; or the offspring may exhibit the trait in even a still higher degree. among illustrations of this, one must suffice. i quote it from the essay by sir j. struthers referred to in the last chapter. "the great-great-grandmother, esther p---- (who married a---- l----), had a sixth little finger on one hand. of their eighteen children (twelve daughters and six sons), only one (charles) is known to have had digital variety. we have the history of the descendants of three of the sons, andrew, charles, and james. "( .) andrew l---- had two sons, thomas and andrew; and thomas had two sons all without digital variety. here we have three successive generations without the variety possessed by the great-grandmother showing itself. "( .) james l----, who was normal, had two sons and seven daughters, also normal. one of the daughters became mrs. j---- (one of the informants), and had three daughters and five sons, all normal except one of the sons, james j----, now æt. , who had six fingers on each hand.... "in this branch of the descendants of esther, we see it passing over two generations and reappearing in one member of the third generation, and now on both hands. "( .) charles l----, the only child of esther who had digital variety, had six fingers on each hand. he had three sons, james, thomas, and john, all of whom were born with six fingers on each hand, while john has also a sixth toe on one foot. he had also five other sons and four daughters, all of whom were normal. "(a.) of the normal children of this, the third generation, the five sons had twelve sons and twelve daughters, and the four daughters have had four sons and four daughters, being the fourth generation, all of whom were normal. a fifth generation in this sub-group consists as yet of only two boys and two girls who are also normal. "in this sub-branch, we see the variety of the first generation present in the second, passing over the third and fourth, and also the fifth as far as it has yet gone. "(b.) james had three sons and two daughters, who are normal. "(c.) thomas had four sons and five daughters, who are normal; and has two grandsons, also normal. "in this sub-branch of the descent, we see the variety of the first generation, showing itself in the second and third, and passing over the fourth, and (as far as it yet exists) the fifth generation. "(d.) john l---- (one of the informants) had six fingers, the additional finger being attached on the outer side, as in the case of his brothers james and thomas. all of them had the additional digits removed. john has also a sixth toe on one foot, situated on the outer side. the fifth and sixth toes have a common proximal phalange, and a common integument invests the middle and distal phalanges, each having a separate nail. "john l---- has a son who is normal, and a daughter, jane, who was born with six fingers on each hand and six toes on each foot. the sixth fingers were removed. the sixth toes are not wrapped with the fifth as in her father's case, but are distinct from them. the son has a son and daughter, who, like himself, are normal. "in this, the most interesting sub-branch of the descent, we see digital increase, which appeared in the first generation on one limb, appearing in the second on two limbs, the hands; in the third on three limbs, the hands and one foot; in the fourth on all the four limbs. there is as yet no fifth generation in uninterrupted transmission of the variety. the variety does not yet occur in any member of the fifth generation of esther's descendants, which consists, as yet, only of three boys and one girl, whose parents were normal, and of two boys and two girls, whose grandparents were normal. it is not known whether in the case of the great-great-grandmother, esther p----, the variety was original or inherited."[ ] § . where there is great uniformity among the members of a species, the divergences of offspring from the average type are usually small; but where, among the members of a species, considerable unlikenesses have once been established, unlikenesses among the offspring are frequent and great. wild plants growing in their natural habitats are uniform over large areas, and maintain from generation to generation like structures; but when cultivation has caused appreciable differences among the members of any species of plant, extensive and numerous deviations are apt to arise. similarly, between wild and domesticated animals of the same species, we see the contrast that though the homogeneous wild race maintains its type with great persistence, the comparatively heterogeneous domestic race frequently produces individuals more unlike the average type than the parents are. though unlikeness among progenitors is one antecedent of variation, it is by no means the sole antecedent. were it so, the young ones successively born to the same parents would be alike. if any peculiarity in a new organism were a direct resultant of the structural differences between the two organisms which produced it; then all subsequent new organisms produced by these two would show the same peculiarity. but we know that the successive offspring have different peculiarities: no two of them are ever exactly alike. one cause of such structural variation in progeny, is functional variation in parents. proof of this is given by the fact that, among progeny of the same parents, there is more difference between those begotten under different constitutional states than between those begotten under the same constitutional state. it is notorious that twins are more nearly alike than children borne in succession. the functional conditions of the parents being the same for twins, but not the same for their brothers and sisters (all other antecedents being constant), we have no choice but to admit that variations in the functional conditions of the parents, are the antecedents of those greater unlikenesses which their brothers and sisters exhibit. some other antecedent remains, however. the parents being the same, and their constitutional states the same, variation, more or less marked, still manifests itself. plants grown from seeds out of one pod, or animals produced at one birth, are not alike. sometimes they differ considerably. in a litter of pigs or of kittens, we rarely see uniformity of markings; and occasionally there are important structural contrasts. i have myself recently been shown a litter of newfoundland puppies, some of which had four digits to their feet, while in others there was present, on each hind-foot, what is called the "dew-claw"--a rudimentary fifth digit. thus, induction points to three causes of variation, all in action together. we have heterogeneity among progenitors, which, did it act uniformly and alone in generating, by composition of forces, new deviations, would impress such new deviations to the same extent on all offspring of the same parents; which it does not. we have functional variation in the parents, which, acting either alone or in combination with the preceding cause, would entail the same structural variations on all young ones simultaneously produced; which it does not. consequently there is some third cause of variation, yet to be found, which acts along with the structural and functional variations of ancestors and parents. § . already, in the last section, there has been implied some relation between variation and the action of external conditions. the above-cited contrast between the uniformity of a wild species and the multiformity of the same species when cultivated or domesticated, thrusts this truth upon us. respecting the variations of plants, mr. darwin remarks that "'sports' are extremely rare under nature, but far from rare under cultivation." others who have studied the matter assert that if a species of plant which, up to a certain time, has maintained great uniformity, once has its constitution thoroughly disturbed, it will go on varying indefinitely. though, in consequence of the remoteness of the periods at which they were domesticated, there is a lack of positive proof that our extremely variable domestic animals have become variable under the changed conditions implied by domestication, having been previously constant; yet competent judges do not doubt that this has been the case. now the constitutional disturbance which precedes variation, can be nothing else than an overthrowing of the pre-established equilibrium of functions. transferring a plant from forest lands to a ploughed field or a manured garden, is altering the balance of forces to which it has been hitherto subject, by supplying it with different proportions of the assimilable matters it requires, and taking away some of the positive impediments to its growth which competing wild plants before offered. an animal taken from woods or plains, where it lived on wild food of its own procuring, and placed under restraint while artificially supplied with food not quite like what it had before, is an animal subject to new outer actions to which its inner actions must be adjusted. from the general law of equilibration we found it to follow that "the maintenance of such a moving equilibrium" as an organism displays, "requires the habitual genesis of internal forces corresponding in number, directions, and amounts, to the external incident forces--as many inner functions, single or combined, as there are single or combined outer actions to be met" (_first principles_, § ); and more recently (§ ), we have seen that life itself is "the definite combination of heterogeneous changes, both simultaneous and successive, in correspondence with external co-existences and sequences." necessarily, therefore, an organism exposed to a permanent change in the arrangement of outer forces must undergo a permanent change in the arrangement of inner forces. the old equilibrium has been destroyed; and a new equilibrium must be established. there must be functional perturbations, ending in a re-adjusted balance of functions. if, then, change of conditions is the only known cause by which the original homogeneity of a species is destroyed; and if change of conditions can affect an organism only by altering its functions; it follows that alteration of functions is the only known internal cause to which the commencement of variation can be ascribed. that such minor functional changes as parents undergo from year to year are influential on the offspring, we have seen is proved by the greater unlikeness that exists between children born to the same parents at different times, than exists between twins. and here we seem forced to conclude that the larger functional variations produced by greater external changes, are the initiators of those structural variations which, when once commenced in a species, lead by their combinations and antagonisms to multiform results. whether they are or are not the direct initiators, they must still be the indirect initiators. § a. in the foregoing sentence those pronounced structural variations from which may presently arise new varieties and eventually species, are ascribed to "the larger functional variations produced by greater external changes"; and this limitation is a needful one, since there is a constant cause of minor variations of a wholly different kind. there are the variations arising from differences in the conditions to which the germ is subject, both before detachment from the parent and after. at first sight it seems that plants grown from seeds out of the same seed-vessel and animals belonging to the same litter, ought, in the absence of any differences of ancestral antecedents, to be entirely alike. but this is not so. inevitably they are subject from the very outset to slightly different sets of agencies. the seeds in a seed-vessel do not stand in exactly the same relations to the sources of nutriment: some are nearer than others. they are somewhat differently exposed to the heat and light penetrating their envelope; and some are more impeded in their growth by neighbours than others are. similarly with young animals belonging to the same litter. their uterine lives are made to some extent unlike by unlike connexions with the blood-supply, by mutual interferences not all the same, and even by different relations to the disturbances caused by the mother's movements. so, too, is it after separation from the parent plant or animal. even the biblical parable reminds us that seeds fall into places here favourable and there unfavourable in various degrees. in respect of soil, in respect of space for growth, in respect of shares of light, none of them are circumstanced in quite the same ways. with animals the like holds. in a litter of pigs some, weaker than others, do not succeed as often in getting possession of teats. and then in both cases the differences thus initiated become increasingly pronounced. among young plants the smaller, outgrown by their better-placed neighbours, are continually more shaded and more left behind; and among the litter the weakly ones, continually thrust aside by the stronger, become relatively more weakly from deficient nutrition. differentiations thus arising, both before and after separation from parents, though primarily differences of growth, entail structural differences; for it is a general law of nutrition that when there is deficiency of food the non-essential organs suffer more than the essential ones, and the unlikenesses of proportion hence arising constitute unlikenesses of structure. it may be concluded, however, that variations generated in this manner usually have no permanent results. in the first place, the individuals which, primarily in growth and secondarily in smaller developments of less-important organs, are by implication inferior, are likely to be eliminated from the species. in the second place, differences of structure produced in the way shown do not express differences of constitution--are not the effects of somewhat divergent physiological units; and consequently are not likely to be repeated in posterity. § . we have still, therefore, to explain those variations which have no manifest causes of the kinds thus far considered. these are the variations termed "spontaneous." not that those who apply to them this word, or some equivalent, mean to imply that they are uncaused. mr. darwin expressly guards himself against such an interpretation. he says:--"i have hitherto sometimes spoken as if the variations--so common and multiform in organic beings under domestication, and in a lesser degree in those in a state of nature--had been due to chance. this, of course, is a wholly incorrect expression, but it serves to acknowledge plainly our ignorance of the cause of each particular variation." not only, however, do i hold, in common with mr. darwin, that there must be some cause for these apparently-spontaneous variations, but it seems to me that a definite cause is assignable. i think it may be shown that unlikenesses must necessarily arise even between the new individuals simultaneously produced by the same parents. instead of the occurrence of such variations being inexplicable, the absence of them would be inexplicable. in any series of dependent changes a small initial difference often works a marked difference in the results. the mode in which a particular breaker bursts on the beach, may determine whether the seed of some foreign plant which it bears is or is not stranded--may cause the presence or absence of this plant from the flora of the land; and may so affect, for millions of years, in countless ways, the living creatures throughout the land. a single touch, by introducing into the body some morbid matter, may set up an immensely involved set of functional disturbances and structural alterations. the whole tenor of a life may be changed by a word of advice; or a glance may determine an action which alters thoughts, feelings, and deeds throughout a long series of years. in those still more involved combinations of changes which societies exhibit, this truth is still more conspicuous. a hair's-breadth difference in the direction of some soldier's musket at the battle of arcola, by killing napoleon, might have changed events throughout europe; and though the type of social organization in each european country would have been now very much what it is, yet in countless details it would have been different. illustrations like these, with which pages might be filled, prepare us for the conclusion that organisms produced by the same parents at the same time, must be more or less differentiated, both by insensible initial differences and by slight differences in the conditions to which they are subject during their evolution. we need not, however, rest with assuming such initial differences: the necessity of them is demonstrable. the individual germ-cells which, in succession or simultaneously, are separated from the same parent, can never be exactly alike; nor can the sperm-cells which fertilize them. when treating of the instability of the homogeneous (_first principles_, § ), we saw that no two parts of any aggregate can be similarly conditioned with respect to incident forces; and that being subject to forces that are more or less unlike, they must become more or less unlike. hence, no two ova in an ovarium or ovules in a seed-vessel--no two spermatozoa or pollen-cells, can be identical. whether or not there arise other contrasts, there are certain to arise quantitative contrasts; since the process of nutrition cannot be absolutely alike for all. the reproductive centres must begin to differentiate from the very outset. such being the necessities of the case, what will happen on any successive or simultaneous fertilizations? inevitably unlikenesses between the respective parental influences must result. quantitative differences among the sperm-cells and among the germ-cells, will insure this. grant that the number of physiological units contained in any one reproductive cell, can rarely if ever be exactly equal to the number contained in any other, ripened at the same time or at a different time; and it follows that among the fertilized germs produced by the same parents, the physiological units derived from them respectively will bear a different numerical ratio to each other in every case. if the parents are constitutionally quite alike, the variation in the ratio between the units they severally bequeath, cannot cause unlikenesses among the offspring. but if otherwise, no two of the offspring can be alike. in every case the small initial difference in the proportions of the slightly-unlike units, will lead, during evolution, to a continual multiplication of differences. the insensible divergence at the outset will generate sensible divergences at the conclusion. possibly some may hence infer that though, in such case, the offspring must differ somewhat from each other and from both parents, yet that in every one of them there must result a homogeneous mixture of the traits of the two parents. a little consideration shows that the reverse is inferable. if, throughout the process of development, the physiological units derived from each parent preserved the same ratio in all parts of the growing organism, each organ would show as much as every other, the influence of either parent. but no such uniform distribution is possible. it has been shown (_first principles_, § ), that in any aggregate of mixed units segregation must inevitably go on. incident forces will tend ever to cause separation of the two orders of units from each other--will tend to integrate groups of the one order in one place and groups of the other order in another place. hence there must arise not a homogeneous mean between the two parents, but a mixture of organs, some of which mainly follow the one and some the other. and this is the kind of mixture which observation shows us. still it may be fairly objected that however the attributes of the two parents are variously mingled in their offspring, they must in all of them fall between the extremes displayed in the parents. in no characteristic could one of the young exceed both parents, were there no cause of "spontaneous variation" but the one alleged. evidently, then, there is a cause yet unfound. § . thus far we have contemplated the process under its simplest aspect. while we have assumed the two parents to be somewhat unlike, we have assumed that each parent has a homogeneous constitution--is built up of physiological units which are exactly alike. but in no case can such a homogeneity exist. each parent had parents who were more or less contrasted--each parent inherited at least two orders of physiological units not quite identical. here then we have a further cause of variation. the sperm-cells or germ-cells which any organism produces, will differ from each other not quantitatively only but qualitatively. of the slightly-unlike physiological units bequeathed to it, the reproductive cells it casts off cannot habitually contain the same proportions; and we may expect the proportions to vary not slightly but greatly. just as, during the evolution of an organism, the physiological units derived from the two parents tend to segregate, and produce likeness to the male parent in this part and to the female parent in that; so, during the formation of reproductive cells, there will arise in one a predominance of the physiological units derived from the father, and in another a predominance of the physiological units derived from the mother. thus, then, every fertilized germ, besides containing different _amounts_ of the two parental influences, will contain different _kinds_ of influences--this having received a marked impress from one grandparent, and that from another. without further exposition the reader will see how this cause of complication, running back through each line of ancestry, must produce in every germ numerous minute differences among the units. here, then, we have a clue to the multiplied variations, and sometimes extreme variations, that arise in races which have once begun to vary. amid countless different combinations of units derived from parents, and through them from ancestors, immediate and remote--amid the various conflicts in their slightly-different organic polarities, opposing and conspiring with one another in all ways and degrees; there will from time to time arise special proportions causing special deviations. from the general law of probabilities it may be concluded that while these involved influences, derived from many progenitors, must, on the average of cases, obscure and partially neutralize one another; there must occasionally result such combinations of them as will produce considerable divergences from average structures; and, at rare intervals, such combinations as will produce very marked divergences. there is thus a correspondence between the inferable results and the results as habitually witnessed. § . still there remains a difficulty. it may be said that admitting functional change to be the initiator of variation--granting that the physiological units of an organism long subject to new conditions, will tend to become modified in such way as to cause change of structure in offspring; yet there will still be no cause of the supposed heterogeneity among the physiological units of different individuals. there seems validity in the objection, that as all the members of a species whose circumstances have been altered will be affected in the same manner, the results, when they begin to show themselves in descendants, will show themselves in the same manner: not multiform variations will arise, but deviations all in one direction. the reply is simple. the members of a species thus circumstanced will _not_ be similarly affected. in the absence of absolute uniformity among them, the functional changes caused in them will be more or less dissimilar. just as men of slightly-unlike dispositions behave in quite opposite ways under the same circumstances; or just as men of slightly-unlike constitutions get diverse disorders from the same cause, and are diversely acted on by the same medicine; so, the insensibly-differentiated members of a species whose conditions have been changed, may at once begin to undergo various kinds of functional changes. as we have already seen, small initial contrasts may lead to large terminal contrasts. the intenser cold of the climate into which a species has migrated, may cause in one individual increased consumption of food to balance the greater loss of heat; while in another individual the requirement may be met by a thicker growth of fur. or, when meeting with the new foods which a new region furnishes, accident may determine one member of the species to begin with one kind and another member with another kind; and hence may arise established habits in these respective members and their descendants. now when the functional divergences thus set up in sundry families of a species have lasted long enough to affect their constitutions, and to modify somewhat the physiological units thrown off in their reproductive cells, the divergences produced by these in offspring will be of divers kinds. and the original homogeneity of constitution having been thus destroyed, variation may go on with increasing facility. there will result a heterogeneous mixture of modifications of structure caused by modifications of function; and of still more numerous correlated modifications, indirectly so caused. by natural selection of the most divergent forms, the unlikenesses of parents will be rendered more marked, and the limits of variation wider. until at length the divergences of constitutions and modes of life, become great enough to lead to segregation of the varieties. § . that variations must occur, and that they must ever tend, both directly and indirectly, towards adaptive modifications, are conclusions deducible from first principles; apart from any detailed interpretations like the above. that the state of homogeneity is an unstable state we have found to be a universal truth. each species must pass from the uniform into the more or less multiform, unless the incidence of external forces is exactly the same for all its members, which it never can be. through the process of differentiation and integration, which of necessity brings together, or keeps together, like individuals, and separates unlike ones from them, there must nevertheless be maintained a tolerably uniform species, so long as there continues a tolerably uniform set of conditions in which it may exist. but if the conditions change, either absolutely by some disturbance of the habitat or relatively by spread of the species into other habitats, then the divergent individuals that result must be segregated by the divergent sets of conditions into distinct varieties (_first principles_, § ). when, instead of contemplating a species in the aggregate, we confine our attention to a single member and its descendants, we see it to be a corollary from the general law of equilibration that the moving equilibrium constituted by the vital actions in each member of this family, must remain constant so long as the external actions to which they correspond remain constant; and that if the external actions are changed, the disturbed balance of internal changes, if not overthrown, cannot cease undergoing modification until the internal changes are again in equilibrium with the external actions: corresponding structural alterations having arisen. on passing from these derivative laws to the ultimate law, we see that variation is necessitated by the persistence of force. the members of a species inhabiting any area cannot be subject to like sets of forces over the whole of that area. and if, in different parts of the area, different kinds or amounts or combinations of forces act on them, they cannot but become different in themselves and in their progeny. to say otherwise, is to say that differences in the forces will not produce differences in the effects; which is to deny the persistence of force. chapter x. genesis, heredity, and variation. § . a question raised, and hypothetically answered, in §§ and , was there postponed until we had dealt with the topics of heredity and variation. let us now resume the consideration of this question, in connexion with sundry others which the facts suggest. after contemplating the several methods by which the multiplication of organisms is carried on--after ranging them under the two heads of homogenesis, in which the successive generations are similarly produced, and heterogenesis, in which they are dissimilarly produced--after observing that homogenesis is nearly always sexual genesis, while heterogenesis is asexual genesis with occasionally-recurring sexual genesis; we came to the questions--why is it that some organisms multiply in the one way and some in the other? and why is it that where agamogenesis prevails it is usually, from time to time, interrupted by gamogenesis? in seeking answers to these questions, we inquired whether there are common to both homogenesis and heterogenesis, any conditions under which alone sperm-cells and germ-cells arise and are united for the production of new organisms; and we reached the conclusion that, in all cases, they arise only when there is an approach to equilibrium between the forces which produce growth and the forces which oppose growth. this answer to the question--_when_ does gamogenesis recur? still left unanswered the question--_why_ does gamogenesis recur? and to this the reply suggested was, that the approach towards general equilibrium in organisms, "is accompanied by an approach towards molecular equilibrium in them; and that the need for this union of sperm-cell with germ-cell is the need for overthrowing this equilibrium, and re-establishing active molecular change in the detached germ--a result probably effected by mixing the slightly-different physiological units of slightly-different individuals." this is the hypothesis which we have now to consider. let us first look at the evidences which certain inorganic phenomena furnish. the molecules of any aggregate which have not a balanced arrangement, inevitably tend towards a balanced arrangement. as before mentioned (_first principles_, § ), amorphous wrought iron, when subject to continuous jar, begins to arrange itself into crystals--its atoms assume a condition of polar equilibrium. the particles of unannealed glass, which are so unstably arranged that slight disturbing forces make them separate into small groups, take advantage of that greater freedom of movement given by a raised temperature, to adjust themselves into a state of relative rest. during any such re-arrangement the aggregate exercises a coercive force over its units. just as in a growing crystal the atoms successively assimilated from the solution, are made by the already crystallized atoms to take a certain form, and even to re-complete that form when it is broken; so in any mass of unstably-arranged atoms which passes into a stable arrangement, each atom conforms to the forces exercised on it by all the other atoms. this is a corollary from the general law of equilibration. we saw (_first principles_, § ) that every change is towards equilibrium; and that change can never cease until equilibrium is reached. organisms, above all other aggregates, conspicuously display this progressive equilibration; because their units are of such kinds, and so conditioned, as to admit of easy re-arrangement. those extremely active changes which go on during the early stages of evolution, imply an immense excess of the molecular forces over those antagonist forces which the aggregate exercises on the molecules. while this excess continues, it is expended in growth, development, and function: expenditure for any of these purposes being proof that part of the force constituting molecular tensions remains unbalanced. eventually, however, this excess diminishes. either, as in organisms which do not expend much energy, decrease of assimilation leads to its decline; or, as in organisms which expend much energy, it is counterbalanced by the rapidly-increasing reactions of the aggregate (§ ). the cessation of growth when followed, as in some organisms, by death, implies the arrival at an equilibrium between the molecular forces and those forces which the aggregate opposes to them. when, as in other organisms, growth ends in the establishment of a moving equilibrium, there is implied such a decreased preponderance of the molecular forces, as leaves no surplus beyond that which is used up in functions. the declining functional activity characteristic of advancing life, expresses a further decline in this surplus. and when all vital movements come to an end, the implication is that the actions of the units on the aggregate and the reactions of the aggregate on the units are completely balanced. hence, while a state of rapid growth indicates such a play of forces among the units of an aggregate as will produce active re-distribution, the diminution and arrest of growth shows that the units have fallen into such relative positions that re-distribution is no longer so facile. when, therefore, we see that gamogenesis recurs only when growth is decreasing, or has come to an end, we must say that it recurs only when the organic units are approximating to equilibrium--only when their mutual restraints prevent them from readily changing their arrangements in obedience to incident forces. that units of like forms can be built up into a more stable aggregate than units of slightly unlike forms, is tolerably manifest _à priori_. and we have facts which prove that mixing allied but somewhat different units, _does_ lead to comparative instability. most metallic alloys exemplify this truth. common solder, which is a mixture of lead and tin, melts at a much lower temperature than either lead or tin. the compound of lead, tin, and bismuth, called "fusible metal," becomes fluid at the temperature of boiling water; while the temperatures at which lead, tin, and bismuth become fluid are, respectively, °, °, and ° f. still more remarkable is the illustration furnished by potassium and sodium. these metals are very near akin in all respects--in their specific gravities, their atomic weights, their chemical affinities, and the properties of their compounds. that is to say, all the evidences unite to show that their units, though not identical, have a close resemblance. what now happens when they are mixed? potassium alone melts at °, sodium alone melts at °, but the alloy of potassium and sodium is liquid at the ordinary temperature of the air. observe the meaning of these facts, expressed in general terms. the maintenance of a solid form by any group of units implies among them an arrangement so stable that it is not overthrown by the incident forces. whereas the assumption of a liquid form implies that the incident forces suffice to destroy the arrangement of the units. in the one case the thermal undulations fail to dislocate the parts; while in the other case the parts are so dislocated by the thermal undulations that they fall into total disorder--a disorder admitting of easy re-arrangement into any other order. for the liquid state is a state in which the units become so far free from mutual restraints, that incident forces can change their relative positions very readily. thus we have reason to conclude that an aggregate of units which, though in the main similar to one another, have minor differences, must be more unstable than an aggregate of homogeneous units. the one will yield to disturbing forces which the other successfully resists. now though the colloidal molecules of which organisms are mainly built, are themselves highly composite; and though the physiological units compounded out of these colloidal molecules must have structures far more involved; yet it must happen with such units, as with simple units, that those which have exactly like forms will admit of arrangement into a more stable aggregate than those which have slightly-unlike forms. among units of this order, as among units of a simpler order, imperfect similarity must entail imperfect balance in anything formed of them, and consequent diminished ability to withstand disturbing forces. hence, given two organisms which, by diminished nutrition or increased expenditure, are being arrested in their growths--given in each an approaching equilibrium between the forces of the units and the forces of the aggregate--given, that is, such a comparatively balanced state among the units that re-arrangement of them by incident forces is no longer so easy; and it will follow that by uniting a group of units from the one organism with a group of slightly-different units from the other, the tendency towards equilibrium will be diminished, and the mixed units will be rendered more modifiable in their arrangements by the forces acting on them: they will be so far freed as to become again capable of that re-distribution which constitutes evolution. and now let us test this hypothesis by seeing what power it gives us of interpreting established inductions. § . the majority of plants being hermaphrodites, it has, until quite recently, been supposed that the ovules of each flower are fertilized by pollen from the anthers of the same flower. mr. darwin, however, has shown that the arrangements are generally such as to prevent this. either the ovules and the pollen are not ripe simultaneously, or obstacles prevent access of the one to the other. at the same time he has shown that there exist arrangements, often of a remarkable kind, which facilitate the transfer of pollen by insects from the stamens of one flower to the pistil of another. similarly, it has been found that among the lower animals, hermaphrodism does not usually involve the production of fertile ova by the union of sperm-cells and germ-cells developed in the same individual; but that the reproductive centres of one individual are united with those of another to produce fertile ova. either, as in _pyrosoma_, _perophora_, and in many higher molluscs, the ova and spermatozoa are matured at different times; or, as in annelids, they are prevented by their relative positions from coming in contact. remembering the fact that among the higher classes of organisms, fertilization is always effected by combining the sperm-cell of one individual with the germ-cell of another; and joining with it the above fact that among hermaphrodite organisms, the germ-cells developed in any individual are usually not fertilized by sperm-cells developed in the same individual; we see reason for thinking that the essential thing in fertilization, is the union of specially-fitted portions of _different_ organisms. if fertilization depended on the peculiar properties of sperm-cell and germ-cell, as such; then, in hermaphrodite organisms, it would be a matter of indifference whether the united sperm-cells and germ-cells were those of the same individual or those of different individuals. but the circumstance that there exist in such organisms elaborate appliances for mutual fertilization, shows that unlikeness of derivation in the united reproductive centres, is the desideratum. now this is just what the foregoing hypothesis implies. if, as was concluded, fertilization has for its object the disturbance of that approaching equilibrium existing among the physiological units separated from an adult organism; and if, as we saw reason to think, this object is effected by mixture with the slightly-different physiological units of another organism; then, we at the same time see that this object will not be effected by mixture with physiological units belonging to the same organism. thus, the hypothesis leads us to expect such provisions as we find. § . but here a difficulty presents itself. these propositions seem to involve the conclusion that self-fertilization is impossible. it apparently follows from them, that a group of physiological units from one part of an organism ought to have no power of altering the state of approaching balance in a group from another part of it. yet self-fertilization does occur. though the ovules of one plant are generally fertilized by pollen from another plant of the same kind, yet they may be, some of them, fertilized by pollen of the same plant; and, indeed, there are plants in which self-fertilization is the rule: even provision being in some cases made to prevent fertilization by pollen from other individuals. and though, among hermaphrodite animals, self-fertilization is usually negatived by structural or functional arrangements, yet in certain _entozoa_ there appear to be special provisions by which the sperm-cells and the germ-cells of the same individual may be united, when not previously united with those of another individual. nay, it has even been shown that in certain ascidians the contents of oviduct and spermiduct of the same individual produce, when united, fertile ova whence evolve perfect individuals. certainly, at first sight, these facts do not consist with the above supposition. nevertheless there is something like a solution. in the last chapter, when considering the variations caused in offspring from uniting elements representing unlike parental constitutions, it was pointed out that in an unfolding organism, composed of slightly-different physiological units derived from slightly-different parents, there cannot be maintained an even distribution of the two orders of units. we saw that the instability of the homogeneous negatives the uniform blending of them; and that, by the process of differentiation and integration, they must be more or less separated; so that in one part of the body the influence of one parent will predominate, and in another part of the body the influence of the other parent: an inference which harmonizes with daily observation. we also saw that the sperm-cells or germ-cells produced by such an organism must, in virtue of these same laws, be more or less unlike one another. it was shown that through segregation, some of the sperm-cells or germ-cells will get an excess of the physiological units derived from one side, and some of them an excess of those derived from the other side: a cause which accounts for the unlikenesses among offspring simultaneously produced. now from this segregation of the different orders of physiological units, inherited from different parents and lines of ancestry, there arises the possibility of self-fertilization in hermaphrodite organisms. if the physiological units contained in the sperm-cells and germ-cells of the same flower, are not quite homogeneous--if in some of the ovules the physiological units derived from the one parent greatly predominate, and in some of the ovules those derived from the other parent; and if the like is true of the pollen-cells; then, some of the ovules may be nearly as much contrasted with some of the pollen-cells in the characters of their contained units, as were the ovules and pollen-cells of the parents from which the plant proceeded. between part of the sperm-cells and part of the germ-cells, the community of nature will be such that fertilization will not result from their union; but between some of them, the differences of constitution will be such that their union will produce the requisite molecular instability. the facts, so far as they are known, seem in harmony with this deduction. self-fertilization in flowers, when it takes place, is not so efficient as mutual fertilization. though some of the ovules produce seeds, yet more of them than usual are abortive. from which, indeed, results the establishment of varieties that have structures favourable to mutual fertilization; since, being more prolific, these have, other things equal, greater chances in the "struggle for existence." further evidence is at hand supporting this interpretation. there is reason to believe that self-fertilization, which at the best is comparatively inefficient, loses all efficiency in course of time. after giving an account of the provisions for an occasional, or a frequent, or a constant crossing between flowers; and after quoting prof. huxley to the effect that among hermaphrodite animals, there is no case in which "the occasional influence of a distinct individual can be shown to be physically impossible;" mr. darwin writes--"from these several considerations and from the many special facts which i have collected, but which i am not here able to give, i am strongly inclined to suspect that, both in the vegetable and animal kingdoms, an occasional intercross with a distinct individual is a law of nature ... in none, as i suspect, can self-fertilization go on for perpetuity." this conclusion, based wholly on observed facts, is just the conclusion to which the foregoing argument points. that necessary action and the re-action between the parts of an organism and the organism as a whole--that power of an aggregate to re-mould the units, which is the correlative of the power of the units to build up into such an aggregate; implies that any differences existing among the units inherited by an organism, must gradually diminish. being subject in common to the total forces of the organism, they will in common be modified towards congruity with these forces, and therefore towards likeness with one another. if, then, in a self-fertilizing organism and its self-fertilizing descendants, such contrasts as originally existed among the physiological units are progressively obliterated--if, consequently, there can no longer be a segregation of different physiological units in different sperm-cells and germ-cells; self-fertilization will become impossible. step by step the fertility will diminish, and the series will finally die out. and now observe, in confirmation of this view, that self-fertilization is limited to organisms in which an approximate equilibrium among the organic forces is not long maintained. while growth is actively going on, and the physiological units are subject to a continually-changing distribution of forces, no decided assimilation of the units can be expected: like forces acting on the unlike units will tend to segregate them, so long as continuance of evolution permits further segregation; and only when further segregation cannot go on, will the like forces tend to assimilate the units. hence, where there is no prolonged maintenance of an approximate organic balance, self-fertilization may be possible for some generations; but it will be impossible in organisms distinguished by a sustained moving equilibrium. § . the interpretation which it affords of sundry phenomena familiar to breeders of animals, adds probability to the hypothesis. mr. darwin has collected a large "body of facts, showing, in accordance with the almost universal belief of breeders, that with animals and plants a cross between different varieties, or between individuals of the same variety but of another strain, gives vigour and fertility to the offspring; and on the other hand, that _close_ interbreeding diminishes vigour and fertility,"--a conclusion harmonizing with the current belief respecting family-intermarriages in the human race. have we not here a solution of these facts? relations must, on the average of cases, be individuals whose physiological units are more nearly alike than usual. animals of different varieties must be those whose physiological units are more unlike than usual. in the one case, the unlikeness of the units may frequently be insufficient to produce fertilization; or, if sufficient to produce fertilization, not sufficient to produce that active molecular change required for vigorous development. in the other case, both fertilization and vigorous development will be made probable. nor are we without a cause for the irregular manifestations of these general tendencies. the mixed physiological units composing any organism being, as we have seen, more or less segregated in the reproductive centres it throws off; there may arise various results according to the degrees of difference among the units, and the degrees in which the units are segregated. of two cousins who have married, the common grandparents may have had either similar or dissimilar constitutions; and if their constitutions were dissimilar, the probability that their married grandchildren will have offspring will be greater than if their constitutions were similar. or the brothers and sisters from whom these cousins descended, instead of severally inheriting the constitutions of their parents in tolerably equal degrees, may have severally inherited them in very different degrees: in which last case, intermarriages among the cousins will be less likely to prove infertile. or the brothers and sisters from whom these cousins descended, may severally have married persons very like, or very unlike, themselves; and from this cause there may have resulted, either an undue likeness, or a due unlikeness, between the married cousins.[ ] these several causes, conspiring and conflicting in endless ways and degrees, will work multiform effects. moreover, differences of segregation will make the reproductive centres produced by the same nearly-related organisms, vary considerably in their amounts of unlikeness; and therefore, supposing their amounts of unlikeness great enough to cause fertilization, this fertilization will be effective in various degrees. hence it may happen that among offspring of nearly-related parents, there may be some in which the want of vigour is not marked, and others in which there is decided want of vigour. so that we are alike shown why in-and-in breeding tends to diminish both fertility and vigour: and why the effect cannot be a uniform effect, but only an average effect. § . while, if the foregoing arguments are valid, gamogenesis has for its main result the initiation of a new development by the overthrow of that approximate equilibrium arrived at among the molecules of the parent-organisms, a further result appears to be subserved by it. those inferior organisms which habitually multiply by agamogenesis, have conditions of life that are simple and uniform; while those organisms which have highly-complex and variable conditions of life, habitually multiply by gamogenesis. now if a species has complex and variable conditions of life, its members must be severally exposed to sets of conditions that are slightly different: the aggregates of incident forces cannot be alike for all the scattered individuals. hence, as functional deviation must ever be inducing structural deviation, each individual throughout the area occupied tends to become fitted for the particular habits which its particular conditions necessitate; and in so far, _un_fitted for the average habits proper to the species. but these undue specializations are continually checked by gamogenesis. as mr. darwin remarks, "intercrossing plays a very important part in nature in keeping the individuals of the same species, or of the variety, true and uniform in character:" the idiosyncratic divergences obliterate one another. gamogenesis, then, is a means of turning to positive advantage the individual differentiations which, in its absence, would result in positive disadvantage. were it not that individuals are ever being made unlike one another by their unlike conditions, there would not arise in them those contrasts of molecular constitution, which we have seen to be needful for producing the fertilized germs of new individuals. and were not these individual differentiations ever being mutually cancelled, they would end in a fatal narrowness of adaptation. this truth will be most clearly seen if we reduce it to its purely abstract form, thus:--suppose a quite homogeneous species, placed in quite homogeneous conditions; and suppose the constitutions of all its members in complete concord with their absolutely-uniform and constant conditions; what must happen? the species, individually and collectively, is in a state of perfect moving equilibrium. all disturbing forces have been eliminated. there remains no force which can, in any way, change the state of this moving equilibrium; either in the species as a whole or in its members. but we have seen (_first principles_, § ) that a moving equilibrium is but a transition towards complete equilibration, or death. the absence of differential or un-equilibrated forces among the members of a species, is the absence of all forces which can cause changes in the conditions of its members--is the absence of all forces which can initiate new organisms. to say, as above, that complete molecular homogeneity existing among the members of a species, must render impossible that mutual molecular disturbance which constitutes fertilization, is but another way of saying that the actions and re-actions of each organism, being in perfect balance with the actions and re-actions of the environment upon it, there remains in each organism no force by which it differs from any other--no force which any other does not meet with an equal force--no force which can set up a new evolution among the units of any other. and so we reach the remarkable conclusion that the life of a species, like the life of an individual, is maintained by the unequal and ever-varying actions of incident forces on its different parts.[ ] an individual homogeneous throughout, and having its substance everywhere continuously subject to like actions, could undergo none of those changes which life consists of; and similarly, an absolutely-uniform species, having all its members exposed to identical influences, would be deprived of that initiator of change which maintains its existence as a species. just as, in each organism, incident forces constantly produce divergences from the mean state in various directions, which are constantly balanced by opposite divergences indirectly produced by other incident forces; and just as the combination of rhythmical functions thus maintained, constitutes the life of the organism; so, in a species, there is, through gamogenesis, a perpetual neutralization of those contrary deviations from the mean state which are caused in its different parts by different sets of incident forces; and it is similarly by the rhythmical production and compensation of these contrary deviations, that the species continues to live. the moving equilibrium in a species, like the moving equilibrium in an individual, would rapidly end in complete equilibration, or death, were not its continually-dissipated forces continually re-supplied from without. besides owing to the external world those energies which, from moment to moment, keep up the lives of its individual members, every species owes to certain more indirect actions of the external world, those energies which enable it to perpetuate itself in successive generations. § . what evidence still remains may be conveniently woven up along with a recapitulation of the argument pursued through the last three chapters. let us contemplate the facts in their synthetic order. that compounding and re-compounding through which we pass from the simplest inorganic substances to the most complex organic substances, has several concomitants. each successive stage of composition presents us with molecules that are severally larger or more integrated, that are severally more heterogeneous, that are severally more unstable, and that are more numerous in their kinds (_first principles_, § ). and when we come to the substances of which living bodies are formed, we find ourselves among innumerable divergent groups and sub-groups of compounds, the units of which are large, heterogeneous, and unstable, in high degrees. there is no reason to assume that this process ends with the formation of those complex colloids which constitute organic matter. a more probable assumption is that out of the complex colloidal molecules there are evolved, by a still further integration, molecules which are still more heterogeneous, and of kinds which are still more multitudinous. what must be their properties? already the colloidal molecules are extremely unstable--capable of being variously modified in their characters by very slight incident forces; and already the complexity of their polarities prevents them from readily falling into such positions of equilibrium as results in crystallization. now the organic molecules composed of these colloidal molecules, must be similarly characterized in far higher degrees. far more numerous must be the minute changes that can be wrought in them by minute external forces; far more free must they remain for a long time to obey forces tending to re-distribute them; and far greater must be the number of their kinds. setting out with these physiological units, the existence of which various organic phenomena compel us to recognize, and the production of which the general law of evolution thus leads us to anticipate; we get an insight into the phenomena of genesis, heredity, and variation. if each organism is built of certain of these highly-plastic units peculiar to its species--units which slowly work towards an equilibrium of their complex proclivities, in producing an aggregate of the specific structure, and which are at the same time slowly modifiable by the re-actions of this aggregate--we see why the multiplication of organisms proceeds in the several ways, and with the various results, which naturalists have observed. heredity, as shown not only in the repetition of the specific structure but in the repetition of ancestral deviations from it, becomes a matter of course; and it falls into unison with the fact that, in various inferior organisms, lost parts can be replaced, and that, in still lower organisms, a fragment can develop into a whole. while an aggregate of physiological units continues to grow by the assimilation of matter which it moulds into other units of like type; and while it continues to undergo changes of structure; no equilibrium can be arrived at between the whole and its parts. under these conditions, then, an un-differentiated portion of the aggregate--a group of physiological units not bound up into a specialized tissue--will be able to arrange itself into the structure peculiar to the species; and will so arrange itself, if freed from controlling forces and placed in fit conditions of nutrition and temperature. hence the continuance of agamogenesis in little-differentiated organisms, so long as assimilation continues to be greatly in excess of expenditure. but let growth be checked and development approach its completion--let the units of the aggregate be severally exposed to an almost constant distribution of forces; and they must begin to equilibrate themselves. arranged, as they will gradually be, into comparatively stable attitudes in relation to one another, their mobility will diminish; and groups of them, partially or wholly detached, will no longer readily re-arrange themselves into the specific form. agamogenesis will be no longer possible; or, if possible, will be no longer easy. when we remember that the force which keeps the earth in its orbit is the gravitation of each particle in the earth towards every one of the group of particles existing , , of miles off; we cannot reasonably doubt that each unit in an organism acts on all the other units, and is reacted on by them: not by gravitation only but chiefly by other energies. when, too, we learn that glass has its molecular constitution changed by light, and that substances so rigid and stable as metals have their atoms re-arranged by forces radiated in the dark from adjacent objects;[ ] we are obliged to conclude that the excessively-unstable units of which organisms are built, must be sensitive in a transcendant degree to all the forces pervading the organisms composed of them--must be tending ever to re-adjust, not only their relative attitudes but their molecular structures, into equilibrium with these forces. hence, if aggregates of the same species are differently conditioned, and re-act differently on their component units, their component units will be rendered somewhat different; and they will become the more different the more widely the re-actions of the aggregates upon them differ, and the greater the number of generations through which these different re-actions of the aggregates upon them are continued. if, then, unlikenesses of function among individuals of the same species, produce unlikenesses between the physiological units of one individual and those of another, it becomes comprehensible that when groups of units derived from two individuals are united, the group formed will be more unstable than either of the groups was before their union. the mixed units will be less able to resist those re-distributing forces which cause evolution; and may thus have restored to them the capacity for development which they had lost. this view harmonizes with the conclusion, which we saw reason to draw, that fertilization does not depend on any intrinsic peculiarities of sperm-cells and germ-cells, but depends on their derivation from different individuals. it explains the facts that nearly-related individuals are less likely to have offspring than others, and that their offspring, when they have them, are frequently feeble. and it gives us a key to the converse fact that the crossing of varieties results in unusual vigour. bearing in mind that the slightly-different orders of physiological units which an organism inherits from its parents, are subject to the same set of forces, and that when the organism is fully developed this set of forces, becoming constant, tends slowly to re-mould the two orders of units into the same form; we see how it happens that self-fertilization becomes impossible in the higher organisms, while it remains possible in the lower organisms. in long-lived creatures which have tolerably-definite limits of growth, this assimilation of the somewhat-unlike physiological units is liable to go on to an appreciable extent; whereas in organisms which do not continuously subject their component units to constant forces, there will be much less of this assimilation. and where the assimilation is not considerable, the segregation of mixed units may cause the sperm-cells and germ-cells developed in the same individual, to be sufficiently different to produce, by their union, fertile germs; and several generations of self-fertilizing descendants may succeed one another, before the two orders of units have had their unlikenesses so far diminished that they will no longer do this. the same principles explain for us the variable results of union between nearly-related organisms. according to the contrasts among the physiological units they inherit from parents and ancestors; according to the unlike proportions of the contrasted units which they severally inherit; and according to the degrees of segregation of such units in different sperm-cells and germ-cells; it may happen that two kindred individuals will produce the ordinary number of offspring or will produce none; or will at one time be fertile and at another not; or will at one time have offspring of tolerable strength and at another time feeble offspring. to the like causes are also ascribable the phenomena of variation. these are unobtrusive while the tolerably-uniform conditions of a species maintain tolerable uniformity among the physiological units of its members; but they become obtrusive when differences of conditions, entailing considerable functional differences, have entailed decided differences among the physiological units, and when the different physiological units, differently mingled in every individual, come to be variously segregated and variously combined. did space permit, it might be shown that this hypothesis is a key to many further facts--to the fact that mixed races are comparatively plastic under new conditions; to the fact that pure races show predominant influences in the offspring when crossed with mixed races; to the fact that while mixed breeds are often of larger growth, pure breeds are the more hardy--have functions less-easily thrown out of balance. but without further argument it will, i think, be admitted that the power of this hypothesis to explain so many phenomena, and to bring under a common bond phenomena which seem so little allied, is strong evidence of its truth. and such evidence gains greatly in strength on observing that this hypothesis brings the facts of genesis, heredity, and variation into harmony with first principles. we see that these plastic physiological units, which we find ourselves obliged to assume, are just such more integrated, more heterogeneous, more unstable, and more multiform molecules, as would result from continuance of the steps through which organic matter is reached. we see that the differentiations of them assumed to occur in differently-conditioned aggregates, and the equilibrations of them assumed to occur in aggregates which maintain constant conditions, are but corollaries from those universal principles implied by the persistence of force. we see that the maintenance of life in the successive generations of a species, becomes a consequence of the continual incidence of new forces on the species, to replace the forces that are ever being rhythmically equilibrated in the propagation of the species. and we thus see that these apparently-exceptional phenomena displayed in the multiplication of organic beings, fall into their places as results of the general laws of evolution. we have, therefore, weighty reasons for entertaining the hypothesis which affords us this interpretation. chapter x^a. genesis, heredity, and variation _concluded_. § a. since the foregoing four chapters were written, thirty-four years ago, the topics with which they deal have been widely discussed and many views propounded. ancient hypotheses have been abandoned, and other hypotheses, referring tacitly or avowedly to the cell-doctrine, have been set forth. before proceeding it will be well to describe the chief among these. most if not all of them proceed on the assumption, shown in § to be needful, that the structural characters of organisms are determined by the special natures of units which are intermediate between the chemical units and the morphological units--between the invisible molecules of proteid-substances and the visible tissue-components called cells. four years after the first edition of this volume was published, appeared mr. darwin's work, _the variation of animals and plants under domestication_; and in this he set forth his doctrine of pangenesis. referring to the doctrine of physiological units which the preceding chapters work out, he at first expressed a doubt whether his own was or was not the same, but finally concluded that it was different. he was right in so concluding. throughout my argument the implication everywhere is that the physiological units are all of one kind; whereas mr. darwin regards his component units, or "gemmules," as being of innumerable unlike kinds. he supposes that every cell of every tissue gives off gemmules special to itself, and capable of developing into similar cells. we may here, in passing, note that this view implies a fundamental distinction between unicellular organisms and the component cells of multicellular organisms, which are otherwise homologous with them. for while in their essential structures, their essential internal changes, and their essential processes of division, the _protozoa_ and the component units of the _metazoa_ are alike, the doctrine of pangenesis implies that though the units when separate do not give off invisible gemmules the grouped units do. much more recently have been enunciated the hypotheses of prof. weismann, differing from the foregoing hypotheses in two respects. in the first place it is assumed that the fragment of matter out of which each organism arises consists of two portions--one of them, the germ-plasm, reserved within the generative organ of the incipient individual, representing in its components the structure of the species, and gives origin to the germs of future individuals; and the other of them, similarly representative of the specific structure, giving origin to the rest of the body, or soma, but contains in its components none of those latent powers possessed by those of the germ-plasm. in the second place the germ-plasm, in common with the soma-plasm, consists of multitudinous kinds of units portioned out to originate the various organs. of these there are groups, sub-groups, and sub-sub-groups. the largest of them, called "idants," are supposed each to contain a number of "ids"; within each id there are numerous "determinants"; and each determinant is made up of many "biophors"--the smallest elements possessing vitality. passing over details, the essential assumption is that there exists a separate determinant for each part of the organism capable of independent variation; and prof. weismann infers that while there may be but one for the blood and but one for a considerable area of skin (as a stripe of the zebra) there must be a determinant for each scale on a butterfly's wing: the number on the four wings being over two hundred thousand. and then each cluster of biophors composing a determinant has to find its way to the place where there is to be formed the part it represents. here it is needless to specify the modifications of these hypotheses espoused by various biologists--all of them assuming that the structural traits of each species are expressed in certain units intermediate between morphological units and chemical units. § b. a true theory of heredity must be one which recognizes the relevant phenomena displayed by all classes of organism. we cannot assume two kinds of heredity, one for plants and another for animals. hence a theory of heredity may be first tested by observing whether it is equally applicable to both kingdoms of living things. genesis, heredity, and variation, as seen in plants, are simpler and more accessible than as seen in animals. let us then note what these imply. already in § i have illustrated the power which some plants possess of developing new individuals from mere fragments of leaves and even from detached scales. striking as are the facts there instanced, they are scarcely more significant than some which are familiar. the formation of cauline buds, presently growing into shoots, shows us a kind of inheritance which a true theory must explain. as described by kerner, such buds arise in pimpernel, toad-flax, etc., below the seed-leaves, even while yet there are no axils in which buds usually grow; and in many plants they arise from intermediate places on the stem: that is, without definite relations to pre-existing structures. how fortuitous is their origin is shown when a branch is induced to bud by keeping it wrapped round with a wet cloth. even still better proved is the absence of any relation between cauline buds and normal germs by the frequent growth of them out of "callus"--the tissue which spreads over wounds and the cut ends of branches. it is not easy to reconcile these facts with mr. darwin's hypothesis of gemmules. we have to assume that where a cauline bud emerges there are present in due proportions gemmules of all the parts which will presently arise from it--leaves, stipules, bracts, petals, stamens, anthers, etc. we have to assume this though, at the time the bud originates, sundry of these organs, as the parts of flowers, do not exist on the plant or tree. and we have to assume that the gemmules of such parts are duly provided in a portion of adventitious callus, far away from the normal places of fructification. moreover, the resulting shoot may or may not produce all the parts which the gemmules represent; and when, perhaps after years, flowers are produced on its side shoots, there must exist at each point the needful proportion of the required gemmules; though there have been no cells continually giving them off. still less does the hypothesis of prof. weismann harmonize with the evidence as plants display it. plant-embryogeny yields no sign of separation between germ-plasm and soma-plasm; and, indeed, the absence of such separation is admitted. after instancing cases among certain of the lower animals, in which no differentiation of the two arises in the first generation resulting from a fertilized ovum, prof. weismann continues:-- "the same is true as regards the higher plants, in which the first shoot arising from the seed never contains germ-cells, or even cells which subsequently become differentiated into germ cells. in all these last-mentioned cases the germ-cells are not present in the first person arising by embryogeny as special cells, but are only formed in much later cell-generations from the offspring of certain cells of which this first person was composed." (_germ-plasm_, p. .) how this admission consists with the general theory it is difficult to understand. the units of the soma-plasm are here recognized as having the same generative powers as the units of the germ-plasm. in so far as one organic kingdom and a considerable part of the other are concerned the doctrine is relinquished. relinquishment is, indeed, necessitated even by the ordinary facts, and still more by the facts just instanced. defence of it involves the assertion that where buds arise, normal or cauline, there exist in due proportion the various ids with their contained determinants--that these are diffused throughout the growing part of the soma; and this implies that the somatic tissue does not differ in generative power from the germ-plasm. the hypothesis of physiological units, then, remains outstanding. for cauline buds imply that throughout the plant-tissue, where not unduly differentiated, the local physiological units have a power of arranging themselves into the structure of the species. but this hypothesis, too, as it now stands, is inadequate. under the form thus far given to it, it fails to explain some accompanying facts. for if the branch just instanced as producing a cauline bud be cut off and its end stuck in the ground, or if it be bent down and a portion of it covered with earth, there will grow from it rootlets and presently roots. the same portion of tissue which otherwise would have produced a shoot with all its appendages, constituting an individual, now produces only a special part of an individual. § c. certain kindred facts of animal development may now be considered. similar insufficiencies are disclosed. the often-cited reproduction of a crab's lost claw or a lizard's tail, mr. darwin thought explicable by his hypothesis of diffused gemmules, representing all organs or their component cells. but though, after simple amputation, regrowth of the proximate part of the tail is conceivable as hence resulting, it is not easy to understand how the remoter part, the components of which are now absent from the organism, can arise afresh from gemmules no longer originated in due proportion. prof. weismann's hypothesis, again, implies that there must exist at the place of separation, a ready-provided supply of determinants, previously latent, able to reproduce the missing tail in all its details--nay, even to do this several times over: a strong supposition! the hypothesis of physiological units, as set forth in preceding chapters, appears less incompetent: reproduction of the lost part would seem to be a normal result of the proclivity towards the form of the entire organism. but now what are we to say when, instead of being cut off transversely, the tail is divided longitudinally and each half becomes a complete tail? what are we to say when, if these two tails are similarly dealt with, the halves again complete themselves; and so until as many as sixteen tails have been formed? here the hypothesis of physiological units appears to fail utterly; for the tendency it implies is to complete the specific form, by reproducing a single tail only. various annulose animals display anomalies of development difficult to explain on any hypothesis. we have creatures like the fresh-water _nais_ which, though it has advanced structures, including a vascular system, branchiæ, and a nervous cord ending with cephalic ganglia, nevertheless shows us an ability like that of the _hydra_ to reproduce the whole from a small part: nearly forty pieces into which a _nais_ was cut having severally grown into complete animals. again we have, in the order _polychætæ_, types like _myrianida_, in which by longitudinal budding a string of individuals, sometimes numbering even thirty, severally develop certain segments into heads, while increasing their segments in number. in yet other types there occurs not longitudinal gemmation only, but lateral gemmation: a segment will send out sideways a bud which presently becomes a complete worm. once more, _syllis ramosa_ is a species in which the individual worms growing from lateral buds, while remaining attached to the parent, themselves give origin to buds; and so produce a branched aggregate of worms. how shall we explain the reparative and reproductive powers thus exemplified? it seems undeniable that each portion has an ability to produce, according to circumstances, the whole creature or a missing part of the creature. when we read of sir j. dalyell that he "cut a _dasychone_ into three pieces; the hindermost produced a head, the anterior piece developed an anus, and the middle portion formed both a head and a tail" we are not furnished with an explanation by the hypothesis of gemmules or by the hypothesis of determinants; for we cannot arbitrarily assume that wherever a missing organ has to be produced there exists the needful supply of gemmules or of determinants representing that organ. the hypothesis that physiological units have everywhere a proclivity towards the organic form of the species, appears more congruous with the facts; but even this does not cover the cases in which a new worm grows from a lateral bud. the tendency to complete the individual structure might be expected rather to restrain this breaking of the lines of complete structure. still less explicable in any way thus far proposed are certain remedial actions seen in animals. an example of them was furnished in § , where "false joints" were described--joints formed at places where the ends of a broken bone, failing to unite, remain moveable one upon the other. according to the character of the habitual motions there results a rudely formed hinge-joint or a ball-and-socket joint, either having the various constituent parts--periosteum, fibrous tissue, capsule, ligaments. now mr. darwin's hypothesis, contemplating only normal structures, fails to account for this formation of an abnormal structure. neither can we ascribe this local development to determinants: there were no appropriate ones in the germ-plasm, since no such structure was provided for. nor does the hypothesis of physiological units, as presented in preceding chapters, yield an interpretation. these could have no other tendency than to restore the normal form of the limb, and might be expected to oppose the genesis of these new parts. thus we have to seek, if not another hypothesis, then some such qualification of an existing hypothesis as will harmonize it with various exceptional phenomena. § d. in part ii of the _principles of sociology_, published in , will be found elaborated in detail that analogy between individual organization and social organization which was briefly sketched out in an essay on "the social organism" published in . in §§ - a parallel is drawn between the developments of the sustaining systems of the two; and it is pointed out how, in the one case as in the other, the components--here organic units and there citizens--have their activities and arrangements mainly settled by local conditions. one leading example is that the parts constituting the alimentary canal, while jointly fitted to the nature of the food, are severally adapted to the successive stages at which the food arrives in its progress; and that in an analogous way the industries carried on by peoples forming different parts of a society, are primarily determined by the natures of things around--agriculture, pastoral and arable, special manufactures and minings, ship-building and fishing: the respective groups falling into fit combinations and becoming partially modified to suit their work. the implication is that while the organization of a society as a whole depends on the characters of its units, in such way that by some types of men despotisms are always evolved while by other types there are evolved forms of government partially free--forms which repeat themselves in colonies--there is, on the other hand, in every case a local power of developing appropriate structures. and it might have been pointed out that similarly in types of creatures not showing much consolidation, as the _annelida_, many of the component divisions, largely independent in their vitalities, are but little affected in their structures by the entire aggregate. my purpose at that time being the elucidation of sociological truths, it did not concern me to carry further the biological half of this comparison. otherwise there might have been named the case in which a supernumerary finger, beginning to bud out, completes itself as a local organ with bones, muscles, skin, nail, etc., in defiance of central control: even repeating itself when cut off. there might also have been instanced the above-named formation of a false joint with its appurtenances. for the implication in both cases is that a local group of units, determined by circumstances towards a certain structure, coerces its individual units into that structure. now let us contemplate the essential fact in the analogy. the men in an australian mining-camp, as m. pierre leroy beaulieu points out, fall into anglo-saxon usages different from those which would characterize a french mining-camp. emigrants to a far west settlement in america quickly establish post-office, bank, hotel, newspaper, and other urban institutions. we are thus shown that along with certain traits leading to a general type of social organization, there go traits which independently produce fit local organizations. individuals are led into occupations and official posts, often quite new to them, by the wants of those around--are now influenced and now coerced into social arrangements which, as shown perhaps by gambling saloons, by shootings at sight, and by lynchings, are scarcely at all affected by the central government. now the physiological units in each species appear to have a similar combination of capacities. besides their general proclivity towards the specific organization, they show us abilities to organize themselves locally; and these abilities are in some cases displayed in defiance of the general control, as in the supernumerary finger or the false joint. apparently each physiological unit, while having in a manner the whole organism as the structure which, along with the rest, it tends to form, has also an aptitude to take part in forming any local structure, and to assume its place in that structure under the influence of adjacent physiological units. a familiar fact supports this conclusion. everyone has at hand, not figuratively but literally, an illustration. let him compare the veins on the backs of his two hands, either with one another or with the veins on another person's hands, and he will see that the branchings and inosculations do not correspond: there is no fixed pattern. but on progressing inwards from the extremities, the distribution of the veins becomes settled--there is a pattern-arrangement common to all persons. these facts imply a predominating control by adjacent parts where control by the aggregate is less easy. a constant combination of forces which, towards the centre, produces a typical structure, fails to do this at the periphery where, during development, the play of forces is less settled. this peripheral vascular structure, not having become fixed because one arrangement is as good as another, is in each determined by the immediately surrounding influences. § e. and now let us contemplate the verifications which recent experiments have furnished--experiments made by prof. g. born of breslau, confirming results earlier reached by vulpian and adding more striking results of kindred nature. they leave no longer doubtful the large share taken by local organizing power as distinguished from central organizing power. the independent vitality shown by separated portions of ventral skin from frog-larvæ may be named as the first illustration. with their attached yolk-cells these lived for days, and underwent such transformations as proved some structural proclivity, though of course the product was amorphous. detached portions of tails of larvæ went on developing their component parts in much the same ways as they would have done if remaining attached. more striking still was the evidence furnished by experiments in grafting. these proved that the undifferentiated rudiment of an organ will, when cut off and joined to a non-homologous place in another individual, develop itself as it would have done if left in its original place. in brief, then, we may say that each part is in chief measure autogenous. these strange facts presented by small aggregates of organic matter, which are the seats of extremely complex forces, will seem less incomprehensible if we observe what has taken place in a vast aggregate of inorganic matter which is the seat of very simple forces--the solar system. transcendently different as this is in all other respects, it is analogous in the respect that, as factors of local structures, local influences predominate over the influences of the aggregate. for while the members of the solar system, considered as a whole, are subordinate to the totality of its forces, the arrangements in each part of it are produced almost wholly by the play of forces in that part. though the sun affects the motions of the moon, and though during the evolution of the earth-and-moon system the sun exercised an influence, yet the relations of our world and its satellite in respect of masses and motions were in the main locally determined. still more clearly was it thus with jupiter and his satellites or saturn with his rings and satellites. remembering that the ultimate units of matter of which the solar system is composed are of the same kinds, and that they act on one another in conformity with the same laws, we see that, remote as the case is from the one we are considering in all other respects, it is similar in the respect that during organization the energies in each locality work effects which are almost independent of the effects worked by the general energies. in this vast aggregate, as in the minute aggregates now in question, the parts are practically autogenous. having thus seen that in a way we have not hitherto recognized the same general principles pervade inorganic and organic evolution, let us revert to the case of super-organic evolution from which a parallel was drawn above. as analogous to the germinal mass of units out of which a new organism is to evolve, let us take an assemblage of colonists not yet socially organized but placed in a fertile region--men derived from a society (or rather a succession of societies) of long-established type, who have in their adapted natures the proclivity towards that type. in passing from its wholly unorganized state to an organized state, what will be the first step? clearly this assemblage, though it may have within the constitutions of its units the potentialities of a specific structure, will not develop forthwith the details of that structure. the inherited natures of its units will first show themselves by separating into large groups devoted to strongly-distinguished occupations. the great mass, dispersing over promising lands, will make preparations for farming. another considerable portion, prompted by the general needs, will begin to form a cluster of habitations and a trading centre. yet a third group, recognizing the demand for wood, alike for agricultural and building purposes, will betake themselves to the adjacent forests. but in no case will the primary assemblage, before these separations, settle the arrangements and actions of each group: it will leave each group to settle them for itself. so, too, after these divisions have arisen. the agricultural division will not as a whole prescribe the doings of its members. spontaneous segregation will occur: some going to a pastoral region and some to a tract which promises good crops. nor within each of these bodies will the organization be dictated by the whole. the pastoral group will separate itself into clusters who tend sheep on the hills and clusters who feed cattle on the plains. meanwhile those who have gravitated towards urban occupations will some of them make bricks or quarry stone, while others fall into classes who build walls, classes who prepare fittings, classes who supply furniture. then along with completion of the houses will go occupation of them by men who bake bread, who make clothing, who sell liquors, and so on. thus each great group will go on organizing itself irrespective of the rest; the sub-groups within each will do the same; and so will the sub-sub-groups. quite independently of the people on the hills and the plains and in the town, those in the forest will divide spontaneously into parties who cut down trees, parties who trim and saw them, parties who carry away the timbers; while every party forms for itself an organization of "butty" or "boss," and those who work under him. similarly with the ultimate divisions--the separate families: the arrangements and apportionments of duties in each are internally determined. mark the fact which here chiefly concerns us. this formation of a heterogeneous aggregate with its variously adapted parts, which while influenced by the whole are mainly self-formed, goes on among units of essentially the same natures, inherited from units who belonged to similar societies. and now, carrying this conception with us, we may dimly perceive how, in a developing embryo, there may take place the formation, first of the great divisions--the primary layers--then of the outlines of systems, then of component organs, and so on continually with the minor structures contained in major structures; and how each of these progressively smaller divisions develops its own organization, irrespective of the changes going on throughout the rest of the embryo. so that though all parts are composed of physiological units of the same nature, yet everywhere, in virtue of local conditions and the influence of its neighbours, each unit joins in forming the particular structure appropriate to the place. thus conceiving the matter, we may in a vague way understand the strange facts of autogenous development disclosed by the above named experiments. § f. "but how immeasurably complex must be the physiological units which can behave thus!" will be remarked by the reader. "to be able to play all parts, alike as members of the whole and as members of this or that organ, they must have an unimaginable variety of potentialities in their natures. each must, indeed, be almost a microcosm within a microcosm." doubtless this is true. still we have a _consensus_ of proofs that the component units of organisms have constitutions of extremely involved kinds. contemplate the facts and their implications. ( ) here is some large division of the animal kingdom--say the _vertebrata_. the component units of all its members have certain fundamental traits in common: all of them have proclivities towards formation of a vertebral column. leaving behind the great assemblage of fishes with its multitudinous types, each having special units of composition, we pass to the _amphibia_, in the units of which there exist certain traits superposed upon the traits they have in common with those of fishes. through unknown links we ascend to incipient mammalian types and then to developed mammalian types, the units of which must have further superposed traits. additional traits distinguish the units of each mammalian order; and, again, those of every genus included in it; while others severally characterize the units of each species. similarly with the varieties in each species, and the stirps in each variety. now the ability of any component unit to carry within itself the traits of the sub-kingdom, class, order, genus, species, variety, and at the same time to bear the traits of immediate ancestors, can exist only in a something having multitudinous proximate elements arranged in innumerable ways. ( ) again, these units must be at once in some respects fixed and in other respects plastic. while their fundamental traits, expressing the structure of the type, must be unchangeable, their superficial traits must admit of modification without much difficulty; and the modified traits, expressing variations in the parents and immediate ancestors, though unstable, must be considered as capable of becoming stable in course of time. ( ) once more we have to think of these physiological units (or constitutional units as i would now re-name them) as having such natures that while a minute modification, representing some small change of local structure, is inoperative on the proclivities of the units throughout the rest of the system, it becomes operative in the units which fall into the locality where that change occurs. but unimaginable as all this is, the facts may nevertheless in some way answer to it. as before remarked, progressing science reveals complexity within complexity--tissues made up of cells, cells containing nuclei and cytoplasm, cytoplasm formed of a protoplasmic matrix containing granules; and if now we conclude that the unit of protoplasm is itself an inconceivably elaborate structure, we do but recognize the complexity as going still deeper. further, if we must assume that these component units are in every part of the body acting on one another by extremely complicated sets of forces (ethereal undulations emanating from each of the constituent molecules) determining their relative positions and actions, we are warranted by the discoveries which every day disclose more of the marvellous properties of matter. when to such examples as were given in § e we add the example yielded by recent experiments, showing that even a piece of bread, after subjection to pressure, exhibits diamagnetic properties unlike those it previously exhibited, we cannot doubt that these complex units composing living bodies are all of them seats of energies diffused around, enabling them to act and re-act so as to modify one another's states and positions. we are shown, too, that whatever be the natures of the complex forces emanating from each, it will, as a matter of course, happen that the power of each will be relatively great in its own neighbourhood and become gradually smaller in parts increasingly remote: making more comprehensible the autogenous character of each local structure. whatever be their supposed natures we are compelled to ascribe extreme complexity to these unknown somethings which have the power of organizing themselves into a structure of this or that species. if gemmules be alleged, then the ability of every organ and part of an organ to vary, implies that the gemmules it gives off are severally capable of receiving minute modifications of their ordinary structures: they must have many parts admitting of innumerable relations. supposing determinants be assumed, then in addition to the complexity which each must have to express in itself the structure of the part evolved from it, it must have the further complexity implied by every superposed modification which causes a variation of that part. and, as we have just seen, the hypothesis of physiological units does not relieve us from the need for kindred suppositions. one more assumption seems necessary if we are to imagine how changes of structure caused by changes of function can be transmitted. reverting to § d, where an unceasing circulation of protoplasm throughout an organism was inferred, we must conceive that the complex forces of which each constitutional unit is the centre, and by which it acts on other units while it is acted on by them, tend continually to remould each unit into congruity with the structures around: superposing on it modifications answering to the modifications which have arisen in those structures. whence is to be drawn the corollary that in course of time all the circulating units,--physiological, or constitutional if we prefer so to call them--visiting all parts of the organism, are severally made bearers of traits expressing local modifications; and that those units which are eventually gathered into sperm-cells and germ-cells also bear these superposed traits. if against all this it be urged that such a combination of structures and forces and processes is inconceivably involved, then the reply is that so astonishing a transformation as that which an unfolding organism displays cannot possibly be effected by simple agencies. § g. but now let it be confessed that none of these hypotheses serves to render the phenomena really intelligible; and that probably no hypothesis which can be framed will do this. many problems beyond those which embryology presents have to be solved; and no solution is furnished. what are we to say of the familiar fact that certain small organs which, with the approach to maturity, become active, entail changes of structure in remote parts--that after the testes have undergone certain final developments, the hairs on the chin grow and the voice deepens? it has been contended that certain concomitant modifications in the fluids throughout the body may produce correlated sexual traits; and there is proof that in many of the lower animals the period of sexual activity is accompanied by a special bodily state--sometimes such that the flesh becomes unwholesome and even poisonous. but a change of this kind can hardly account for a structural change in the vocal organs in man. no hypothesis of gemmules or determinants or physiological units enables us to understand how removal of the testes prevents those developments of the larynx and vocal cords which take place if they remain. the inadequacy of our explanations we at once see in presence of a structure like a peacock's tail-feather. mr. darwin's hypothesis is that all parts of every organ are continually giving off gemmules, which are consequently everywhere present in their due proportions. but a completed feather is an inanimate product and, once formed, can add to the circulating fluids no gemmules representing all its parts. if we follow prof. weismann we are led into an astounding supposition. he admits that every variable part must have a special determinant, and that this results in the assumption of over two hundred thousand for the four wings of a butterfly. let us ask what must happen in the case of a peacock's feather. on looking at the eye near its end, we see that the minute processes on the edge of each lateral thread must have been in some way exactly adjusted, in colour and position, so as to fall into line with the processes on adjacent threads: otherwise the symmetrical arrangement of coloured rings would be impossible. each of these processes, then, being an independent variable, must have had its particular determinant. now there are about threads on the shaft of a large feather, and each of them bears on the average , processes, making for the whole feather , of these processes. for one feather alone there must have been , determinants, and for the whole tail many millions. and these, along with the determinants for the detailed parts of all the other feathers, and for the variable components of all organs forming the body at large, must have been contained in the microscopic head of a spermatozoon! hardly a credible supposition. nor is it easy to see how we are helped by the hypothesis of constitutional units. take the feather in its budding state and ask how the group of such units, alike in structure and perpetually multiplying while the unfolding goes on, can be supposed by their mutual actions so to affect one another as eventually to produce the symmetrically-adjusted processes which constitute the terminal eye. imagination, whatever licence may be given, utterly fails us. at last then we are obliged to admit that the actual organizing process transcends conception. it is not enough to say that we cannot know it; we must say that we cannot even conceive it. and this is just the conclusion which might have been drawn before contemplating the facts. for if, as we saw in the chapter on "the dynamic element in life," it is impossible for us to understand the nature of this element--if even the ordinary manifestations of it which a living body yields from moment to moment are at bottom incomprehensible; then, still more incomprehensible must be that astonishing manifestation of it which we have in the initiation and unfolding of a new organism. thus all we can do is to find some way of symbolizing the process so as to enable us most conveniently to generalize its phenomena; and the only reason for adopting the hypothesis of physiological units or constitutional units is that it best serves this purpose. chapter xi. classification. § . that orderly arrangement of objects called classification has two purposes, which, though not absolutely distinct, are distinct in great part. it may be employed to facilitate identification, or it may be employed to organize our knowledge. if a librarian places his books in the alphabetical succession of the author's names, he places them in such way that any particular book may easily be found, but not in such way that books of a given nature stand together. when, otherwise, he makes a distribution of books according to their subjects, he neglects various superficial similarities and distinctions, and groups them according to certain primary and secondary and tertiary attributes, which severally imply many other attributes--groups them so that any one volume being inspected, the general characters of all the neighbouring volumes may be inferred. he puts together in one great division all works on history; in another all biographical works; in another all works that treat of science; in another voyages and travels; and so on. each of his great groups he separates into sub-groups; as when he puts different kinds of literature under the heads of fiction, poetry, and the drama. in some cases he makes sub-sub-groups; as when, having divided his scientific treatises into abstract and concrete, putting in the one logic and mathematics and in the other physics, astronomy, geology, chemistry, physiology, &c.; he goes on to sub-divide his books on physics, into those which treat of mechanical motion, those which treat of heat, those which treat of light, of electricity, of magnetism. between these two modes of classification note the essential distinctions. arrangement according to any single conspicuous attribute is comparatively easy, and is the first that suggests itself: a child may place books in the order of their sizes, or according to the styles of their bindings. but arrangement according to combinations of attributes which, though fundamental, are not conspicuous, requires analysis; and does not suggest itself till analysis has made some progress. even when aided by the information which the author gives on his title page, it requires considerable knowledge to classify rightly an essay on polarization; and in the absence of a title page it requires much more knowledge. again, classification by a single attribute, which the objects possess in different degrees, may be more or less serial, or linear. books may be put in the order of their dates, in single file; or if they are grouped as works in one volume, works in two volumes, works in three volumes, &c., the groups may be placed in an ascending succession. but groups severally formed of things distinguished by some common attribute which implies many other attributes, do not admit of serial arrangement. you cannot rationally say either that historical works should come before biographical works, or biographical works before historical works; nor of the sub-divisions of creative literature, into fiction, poetry, and the drama, can you give a good reason why any one should take precedence of the others. hence this grouping of the like and separation of the unlike which constitutes classification, can reach its complete form only by slow steps. i have shown (_essays_, vol. ii., pp. - ) that, other things equal, the relations among phenomena are recognized in the order of their conspicuousness; and that, other things equal, they are recognized in the order of their simplicity. the first classifications are sure, therefore, to be groupings of objects which resemble one another in external or easily-perceived attributes, and attributes that are not of complex characters. those likenesses among things which are due to their possession in common of simple obvious properties, may or may not coexist with further likenesses among them. when geometrical figures are classed as curvilinear and rectilinear, or when the rectilinear are divided into trilateral, quadrilateral, &c., the distinctions made connote various other distinctions with which they are necessarily bound up; but if liquids be classed according to their visible characters--if water, alcohol, sulphuret of carbon, &c., be grouped as colourless and transparent, we have things placed together which are unlike in their essential natures. thus, where the objects classed have numerous attributes, the probabilities are that the early classifications, based on simple and manifest attributes, unite under the same head many objects that have no resemblance in the majority of their attributes. as the knowledge of objects increases, it becomes possible to make groups of which the members have more numerous properties in common; and to ascertain what property, or combination of properties, is most characteristic of each group. and the classification eventually arrived at is of such kind that the objects in each group have more attributes in common with one another than they have in common with any excluded objects; one in which the groups of such groups are integrated on the same principle; and one in which the degrees of differentiation and integration are proportioned to the degrees of intrinsic unlikeness and likeness. and this ultimate classification, while it serves to identify the things completely, serves also to express the greatest amount of knowledge concerning the things--enables us to predicate the greatest number of facts about each thing; and by so doing implies the most precise correspondence between our conceptions and the realities. § . biological classifications illustrate well these phases through which classifications in general pass. in early attempts to arrange organisms in some systematic manner, we see at first a guidance by conspicuous and simple characters, and a tendency towards arrangement in linear order. in successively later attempts, we see more regard paid to combinations of characters which are essential but often inconspicuous, and an abandonment of a linear arrangement for an arrangement in divergent groups and re-divergent sub-groups. in the popular mind, plants are still classed under the heads of trees, shrubs, and herbs; and this serial classing according to the single attribute of magnitude, swayed the earliest observers. they would have thought it absurd to call a bamboo, thirty feet high, a kind of grass; and would have been incredulous if told that the hart's-tongue should be placed in the same great division with the tree-ferns. the zoological classifications current before natural history became a science, had divisions similarly superficial and simple. beasts, birds, fishes, and creeping-things are names of groups marked off from one another by conspicuous differences of appearance and modes of life--creatures that walk and run, creatures that fly, creatures that live in the water, creatures that crawl. and these groups were thought of in the order of their importance. the first arrangements made by naturalists were based either on single characters or on very simple combinations of characters; as that of clusius, and afterwards the more scientific system of cesalpino, recognizing the importance of inconspicuous structures. describing plant-classifications, lindley says:--"rivinus invented, in , a system depending upon the formation of the corolla; kamel, in , upon the fruit alone; magnol, in , on the calyx and corolla; and finally, linnæus, in , on variations in the stamens and pistil." in this last system, which has been for so long current as a means of identification (regarded by its author as transitional), simple external attributes are still depended on; and an arrangement, in great measure serial, is based on the degrees in which these attributes are possessed. in , some thirty years before the time of linnæus, our countryman ray had sketched the outlines of a more advanced system. he said that-- plants are either flowerless, or flowering; and these are dicotyledones, or monocotyledones. among the minor groups which he placed under these general heads, "were fungi, mosses, ferns, composites, cichoraceæ, umbellifers, papilionaceous plants, conifers, labiates, &c., under other names, but with limits not very different from those now assigned to them." being much in advance of his age, ray's ideas remained dormant until the time of jussieu; by whom they were developed into what has become known as the natural system: a system subsequently improved by de candolle. passing through various modifications in the hands of successive botanists, the natural system is now represented by the following form, which is based upon the table of contents prefixed to vol. ii. of prof. oliver's translation of the _natural history of plants_, by prof. kerner. his first division, myxothallophyta (= myxomycetes), i have ventured to omit. the territory it occupies is in dispute between zoologists and botanists, and as i have included the group in the zoological classification, agreeing that its traits are more animal than vegetal, i cannot also include it in the botanical classification. here, linear arrangement has disappeared: there is a breaking up into groups and sub-groups and sub-sub-groups, which do not admit of being placed in serial order, but only in divergent and re-divergent order. were there space to exhibit the way in which the alliances are subdivided into orders, and these into genera, and these into species, the same principle of co-ordination would be still further manifested. phyla. classes. alliances. sub-phyla. sub-classes. thallophyta i. schizophyta . cyanophyceæ. blue-green algæ. . schizomycetes. ii. dinoflagellata peridineæ . iii. bacillariales . iv. gamophyceæ i. chlorophyceæ . protococcoideæ. . siphoneæ. . confervoideæ. . conjugatæ. . charales. . phæophyceæ. . dictyotales. . florideæ, red seaweeds. v. fungi i. phycomycetes . oomycetes. . zygomycetes. ii. mesomycetes . . iii. mycomycetes . . additional group of fungi, lichenes. archegoniatÆ i. bryophyta . hepaticæ, liverworts. . musci, mosses. ii. pteridophyta vas. cryptogams . filices, ferns. . hydropterides, rhizocarps. . equisetales, horse-tails. . lycopodiales, club-mosses. phanerogamia (flowering plants.) gymnospermÆ i. cycadales, cycads . ii. coniferæ . iii. gnetales . angiospermÆ i. monocotyledons . liliifloræ. . scitamineæ. . gynandræ. . fluviales. . spadicifloræ. . glumifloræ. ii. dicotyledons i. monochlamydæ . centrospermæ. . protiales. . daphnales. . santalales. . rafflesiales. . asarales. . euphorbiales. . podostemales. . viridifloræ. . amentales. . balanophorales. ii. monopetalæ . caprifoliales. . asterales. . campanales. . ericales. . vaccinales. . primulales. . tubifloræ. iii. polypetalæ . ranales. . parietales. . malvales. . discifloræ. . crateranthæ. . myrtales. . melastomales. . lythrales. . hygrobiæ. . passifloræ. . pepones. . cactales. . ficoidales. . umbellales. on studying the definitions of these primary, secondary, and tertiary classes, it will be found that the largest are marked off from one another by some attribute which connotes sundry other attributes; that each of the smaller classes comprehended in one of these largest classes, is marked off in a similar way from the other smaller classes bound up with it; and that so, each successively smaller class has an increased number of co-existing attributes. § . zoological classification has had a parallel history. the first attempt which we need notice, to arrange animals in such a way as to display their affinities, is that of linnæus. he grouped them thus:[ ]-- cl. . mammalia. _ord._ primates, bruta, feræ, glires, pecora, belluæ, cete. cl. . aves. _ord._ accipitres, picæ, anseres, grallæ, gallinæ, passeres. cl. . amphibia. _ord._ reptiles, serpentes, nantes. cl. . pisces. _ord._ apodes, jugulares, thoracici, abdominales. cl. . insecta. _ord._ coleoptera, hemiptera, lepidoptera, neuroptera, diptera, aptera. cl. . vermes. _ord._ intestina, mollusca, testacea, lithophyta, zoophyta. this arrangement of classes is obviously based on apparent gradations of rank; and the placing of the orders similarly betrays an endeavour to make successions, beginning with the most superior forms and ending with the most inferior forms. while the general and vague idea of perfection determines the leading character of the classification, its detailed groupings are determined by the most conspicuous external attributes. not only linnæus but his opponents, who proposed other systems, were "under the impression that animals were to be arranged together into classes, orders, genera, and species, according to their more or less close external resemblance." this conception survived until the time of cuvier. "naturalists," says agassiz, "were bent upon establishing one continual uniform series to embrace all animals, between the links of which it was supposed there were no unequal intervals. the watchword of their school was: _natura non facit saltum_. they called their system _la chaine des êtres_." the classification of cuvier, based on internal organization instead of external appearance, was a great advance. he asserted that there are four principal forms, or four general plans, on which animals are constructed; and, in pursuance of this assertion, he drew out the following scheme. first branch. animalia vertebrata. cl. . mammalia. cl. . birds. cl. . reptilia. cl. . fishes. second branch. animalia mollusca. cl. . cephalapoda. cl. . pteropoda. cl. . gasteropoda. cl. . acephala. cl. . brachiopoda. cl. . cirrhopoda. third branch. animalia articulata. cl. . annelides. cl. . crustacea. cl. . arachnides. cl. . insects. fourth branch. animalia radiata. cl. . echinoderms. cl. . intestinal worms. cl. . acalephæ. cl. . polypi. cl. . infusoria. but though cuvier emancipated himself from the conception of a serial progression throughout the animal kingdom, sundry of his contemporaries and successors remained fettered by the old error. less regardful of the differently-combined sets of attributes distinguishing the different sub-kingdoms, and swayed by the belief in a progressive development which was erroneously supposed to imply a linear arrangement of animals, they persisted in thrusting organic forms into a quite unnatural order. the following classification of lamarck illustrates this. invertebrata. i. apathetic animals. } } cl. . infusoria. } do not feel, and move only by their cl. . polypi. } excited irritability. no brain, no cl. . radiaria. } elongated medullary mass; no senses; cl. . tunicata. } forms varied; rarely articulations. cl. . vermes. } ii. sensitive animals. } feel, but obtain from their sensations } only perceptions of objects, a cl. . insects. } sort of simple ideas, which they are cl. . arachnids. } unable to combine to obtain complex cl. . crustacea. } ones. no vertebral column; a brain cl. . annelids. } and mostly an elongated medullary cl. . cirripeds. } mass; some distinct senses; muscles cl. . conchifera. } attached under the skin; form symmetrical, cl. . mollusks. } the parts being in pairs. vertebrata. { feel; acquire preservable ideas; iii. intelligent animals. { perform with them operations by which { they obtain others; are intelligent in cl. . fishes. { different degrees. a vertebral column; cl. . reptiles. { a brain and a spinal marrow; distinct cl. . birds. { senses; the muscles attached to the cl. . mammalia. { internal skeleton; form symmetrical, { the parts being in pairs. passing over sundry classifications in which the serial arrangement dictated by the notion of ascending complexity, is variously modified by the recognition of conspicuous anatomical facts, we come to classifications which recognize another order of facts--those of development. the embryological inquiries of von baer led him to arrange animals as follows:-- i. peripheric type. (radiata.) _evolutio radiata._ the development proceeds from a centre, producing identical parts in a radiating order. ii. massive type. (mollusca.) _evolutio contorta._ the development produces identical parts curved around a conical or other space. iii. longitudinal type. (articulata.) _evolutio gemina._ the development produces identical parts arising on both sides of an axis, and closing up along a line opposite the axis. iv. doubly symmetrical type. (vertebrata.) _evolutio bigemina._ the development produces identical parts arising on both sides of an axis, growing upwards and downwards, and shutting up along two lines, so that the inner layer of the germ is inclosed below, and the upper layer above. the embryos of these animals have a dorsal cord, dorsal plates, and ventral plates, a nervous tube and branchial fissures. recognizing these fundamental differences in the modes of development, as answering to fundamental divisions in the animal kingdom, von baer shows (among the _vertebrata_ at least) how the minor differences which arise at successively later embryonic stages, correspond with the minor divisions. like the modern classification of plants, the modern classification of animals shows us the assumed linear order completely broken up. in his lectures at the royal institution, in , prof. huxley expressed the relations existing among the several great groups of the animal kingdom, by placing them at the ends of four or five radii, diverging from a centre. the diagram i cannot obtain; but in the published reports of his lectures at the school of mines the groups were arranged as on the following page. what remnant there may seem to be of linear succession in some of the sub-groups contained in it, is merely an accident of typographical convenience. each of them is to be regarded simply as a cluster. and if prof. huxley had further developed the arrangement, by dispersing the sub-groups and sub-sub-groups on the same principle, there would result an arrangement perhaps not much unlike that shown on the page succeeding this. vertebrata (_abranchiata_) mammalia aves reptilia (_branchiata_) amphibia pisces. mollusca annulosa cephalopoda heteropoda } _articulata._ gasteropoda-dioecia } insecta arachnida } myriapoda crustacea { pulmonata gasteropoda-monoecia { pteropoda _annuloida._ lamellibranchiata annellata scoleidæ echinodermata trematoda rotifera tæniadæ turbellaria nematoidea coelenterata hydrozoa actinozoa. protozoa infusoria spongiadæ gregarinidæ _noctilucidæ_ foraminifera _thallassicollidæ_ in the woodcut, the dots represent orders, the names of which it is impracticable to insert. if it be supposed that when magnified, each of these dots resolves itself into a cluster of clusters, representing genera and species, an approximate idea will be formed of the relations among the successively-subordinate groups constituting the animal kingdom. besides the subordination of groups and their general distribution, some other facts are indicated. by the distances of the great divisions from the general centre, are rudely symbolized their respective degrees of divergence from the form of simple, undifferentiated organic matter; which we may regard as their common source. within each group, the remoteness from the local centre represents, in a rough way, the degree of departure from the general plan of the group. and the distribution of the sub-groups within each group, is in most cases such that those which come nearest to neighbouring groups, are those which show the nearest resemblances to them--in their analogies though not in their homologies. no such scheme, however, can give a correct conception. even supposing the above diagram expressed the relations of animals to one another as truly as they can be expressed on a plane surface (which of course it does not), it would still be inadequate. such relations cannot be represented in space of two dimensions, but only in space of three dimensions. _mammalia_ _aves_ _reptilia_ vertebrata \ _amphibia_\ _pisces_ \ \ _arachnida_ \ _insecta_ \ _crustacea_ \ \ articulata \ | \ | _myriapoda_ \ | \ annulosa \ | \ |_annelida_ \ _scolecida_ \ | \ annuloida \ | / \ _echinodermata_ \ | / _pteropoda_ _cephalopoda_\ | | _gasteropoda dioecia_ | | _gasteropoda monoecia_ _pulmonata_ | | \ | | mollusca------------- \ | | \ \ | | _lamellibranchiata_ \ \ | | \ \ | | _brachiopoda_ \ \ | | \ \ _gregarinida_ molluscoida------------ _rhizopoda_ \ _ascidioida_ _polyzoa_ / \ / protozoa / _spongida_ _infusoria_ _hydrozoa_ / / coelenterata _actinozoa_ § a. two motives have prompted me to include in its original form the foregoing sketch: the one being that in conformity with the course previously pursued, of giving the successive forms of classifications, it seems desirable to give this form which was approved thirty-odd years ago; and the other being that the explanatory comments remain now as applicable as they were then. replacement of the diagram by one expressing the relations of classes as they are now conceived, is by no means an easy task; for the conceptions formed of them are unsettled. concerning the present attitude of zoologists, prof. macbride writes:-- "they all recognize a certain number of phyla. each phylum includes a group of animals about whose relation to each other no one entertains a doubt. each zoologist, however, has his own idea as to the relationship which the various phyla bear to each other. "the phyla recognized at present are:-- ( ) protozoa. ( ) porifera (sponges). ( ) coelenterata. ( ) echinodermata. { cestodes. ( ) platyhelminthes { trematodes. { turbellaria. ( ) nemertea. ( ) nematoda. ( ) acanthocephala (echinorhyncus). ( ) chætognatha (sagitta). ( ) rotifera. ( ) annelida (includes leeches and gephyrea, chætifera). ( ) gephyrea, achæta. { tracheata (peripatus, myriapods, insects). ( ) arthropods { arachnids. { crustacea. { pycnogonida. ( ) mollusca. ( ) polyzoa (including phoronis). ( ) brachiopoda. ( ) chordata (includes balanoglossus and tunicates. some continental zoologists do not admit balanoglossus)." [this last phylum of course includes the _vertebrata_.] though under present conditions, as above implied, it would be absurd to attempt a definite scheme of relationships, yet it has seemed to me that the adumbration of a scheme, presenting in a vague way such relationships as are generally agreed upon and leaving others indeterminate, may be ventured; and that a general impression hence resulting may be useful. on the adjacent page i have tried to make a tentative arrangement of this kind. at the bottom of the table i have placed together, under the name "compound _protozoa_," those kinds of aggregated _protozoa_ which show no differentiations among the members of groups, and are thus distinguished from _metazoa_; and i have further marked the distinction by their position, which implies that from them no evolution of higher types has taken place. respecting the naming of the sub-kingdoms, phyla, classes, orders, &c., i have not maintained entire consistency. the relative values of groups cannot be typographically expressed in a small space with a limited variety of letters. the sizes of the letters mark the classificatory ranks, and by the thickness i have rudely indicated their zoological importance. in fixing the order of subordination of groups i have been aided by the table of contents prefixed to mr. adam sedgwick's _student's text book of zoology_ and have also made use of prof. ray lankester's classifications of several sub-kingdoms. _placental_-----+ | _aves_ _mammalia_----+ | _arachnida_ | | | _implacental_---+ | _insecta_ | _crustacea_ | | | | | vertebrata | | | | | | | | | | _reptilia_ _chilopoda_ | | _amphibia_ arthropoda | | | | _diplopoda_ _pisces_ | | _chætopoda_ _cephalochorda_ | | chordata annelida _urochorda_ | | _echiuroidea_ _(tunicata)_ | | _hirudinea_ +---------+ _archiannelida_ | | brachiopoda | | rotifera _dibranchiata_ | | | | _cephalopoda_ | | +----+ _tetrabranchiata_ | | | _crinoidea_ mollusca ---------------+ | | | +------ echinodermata _scaphopoda_ | | | | | _asteroidea_ _solenogastres_ | | | | | _echinoidea_ _gasteropoda_ | | | | | _holothuroidea_ _lammellibranchiata_----+ | | | | _enteropneusta_ | | | | | | | | | | _acanthocephala_ polyzoa --------------------+ | | | | | +--- nemathelminthes | | | | | | | _nematomorpha_ | | | | | | | _nematoda_ | | | | | | | _zoantharia_ ctenophora _rugosa_ _acalephos_ _alcyonaria_ coelenterata _hydromedusae_ actinozoa hydrozoa | | | |-------- nemertea | | | | +----------+ _turbellaria_ ----------------| | protozoa platyhelminthes _corticata_ _trematoda_ _gymnomyxa_ _cestoda_ | | | | myxomycetes | _triaronia_ | _vobrocina_ _calcarea_ _foraminifera_ | porifera compound protozoa _demospongiae_ _radiolaria_ let me again emphasize the fact that the relationships of these diverging and re-diverging groups cannot be expressed on a flat surface. if we imagine a laurel-bush to be squashed flat by a horizontal plane descending upon it, we shall see that sundry of the upper branches and twigs which were previously close together will become remote, and that the relative positions of parts can remain partially true only with the minor branches. the reader must therefore expect to find some of the zoological divisions which in the order of nature are near one another, shown in the table as quite distant. § . while the classifications of botanists and zoologists have become more and more natural in their arrangements, there has grown up a certain artificiality in their abstract nomenclature. when aggregating the smallest groups into larger groups and these into groups still larger, they have adopted certain general terms expressive of the successively more comprehensive divisions; and the habitual use of these terms, needful for purposes of convenience, has led to the tacit assumption that they answer to actualities in nature. it has been taken for granted that species, genera, orders, and classes, are assemblages of definite values--that every genus is the equivalent of every other genus in respect of its degree of distinctness; and that orders are separated by lines of demarcation which are as broad in one place as another. though this conviction is not a formulated one, the disputes continually occurring among naturalists on the questions, whether such and such organisms are specifically or generically distinct, and whether this or that peculiarity is or is not of ordinal importance, imply that the conviction is entertained even where not avowed. yet that differences of opinion like these arise and remain unsettled, except when they end in the establishment of sub-species, sub-genera, sub-orders, and sub-classes, sufficiently shows that the conviction is ill-based. and this is equally shown by the impossibility of obtaining any definition of the degree of difference which warrants each further elevation in the hierarchy of classes. it is, indeed, a wholly gratuitous assumption that organisms admit of being placed in groups of equivalent values; and that these may be united into larger groups which are also of equivalent values; and so on. there is no _à priori_ reason for expecting this; and there is no _à posteriori_ evidence implying it, save that which begs the question--that which asserts one distinction to be generic and another to be ordinal, because it is assumed that such distinctions must be either generic or ordinal. the endeavour to thrust plants and animals into these definite partitions is of the same nature as the endeavour to thrust them into linear series. not that it does violence to the facts in anything like the same degree; but still, it does violence to the facts. doubtless the making of divisions and sub-divisions, is extremely useful; or rather, it is necessary. doubtless, too, in reducing the facts to something like order they must be partially distorted. so long as the distorted form is not mistaken for the actual form, no harm results. but it is needful for us to remember that while our successively subordinate groups have a certain general correspondence with the realities, they tacitly ascribe to the realities a regularity which does not exist. § . a general truth of much significance is exhibited in these classifications. on observing the natures of the attributes which are common to the members of any group of the first, second, third, or fourth rank, we see that groups of the widest generality are based on characters of the greatest importance, physiologically considered; and that the characters of the successively-subordinate groups, are characters of successively-subordinate importance. the structural peculiarity in which all members of one sub-kingdom differ from all members of another sub-kingdom, is a peculiarity that affects the vital actions more profoundly than does the structural peculiarity which distinguishes all members of one class from all members of another class. let us look at a few cases. we saw (§ ), that the broadest division among the functions is the division into "the _accumulation of energy_ (latent in food); the _expenditure of energy_ (latent in the tissues and certain matters absorbed by them); and the _transfer of energy_ (latent in the prepared nutriment or blood) from the parts which accumulate to the parts which expend." now in the lowest animals, united under the general name _protozoa_, there is either no separation of the parts performing these functions or very indistinct separation: in the _rhizopoda_, all parts are alike accumulators of energy, expenders of energy and transferers of energy; and though in the higher members of the group, the _infusoria_, there are some specializations corresponding to these functions, yet there are no distinct tissues appropriated to them. similarly when we pass from simple types to compound types--from _protozoa_ to _metazoa_. the animals known as _coelenterata_ are characterized in common by the possession of a part which accumulates energy more or less marked off from the part which does not accumulate energy, but only expends it; and the _hydrozoa_ and _actinozoa_, which are sub-divisions of the _coelenterata_, are contrasted in this, that in the second these parts are much more differentiated from one another, as well as more complicated. besides a completer differentiation of the organs respectively devoted to the accumulation of energy and the expenditure of energy, animals next above the _coelenterata_ possess rude appliances for the transfer of energy: the peri-visceral sac, or closed cavity between the intestine and the walls of the body, serves as a reservoir of absorbed nutriment, from which the surrounding tissues take up the materials they need. and then out of this sac originates a more efficient appliance for the transfer of energy: the more highly-organized animals, belonging to whichever sub-kingdom, all of them possess definitely-constructed channels for distributing the matters containing energy. in all of them, too, the function of expenditure is divided between a directive apparatus and an executive apparatus--a nervous system and a muscular system. but these higher sub-kingdoms are clearly separated from one another by differences in the relative positions of their component sets of organs. the habitual attitudes of annulose and molluscous creatures, is such that the neural centres are below the alimentary canal and the hæmal centres above. and while by these traits the annulose and molluscous types are separated from the vertebrate, they are separated from each other by this, that in the one the body is "composed of successive segments, usually provided with limbs," but in the other, the body is not segmented, "and no true articulated limbs are ever developed." the sub-kingdoms being thus distinguished from one another, by the presence or absence of specialized parts devoted to fundamental functions, or else by differences in the distributions of such parts, we find, on descending to the classes, that these are distinguished from one another, either by modifications in the structures of fundamental parts, or by the presence or absence of subsidiary parts, or by both. fishes and _amphibia_ are unlike higher vertebrates in possessing branchiæ, either throughout life or early in life. and every higher vertebrate, besides having lungs, is characterized by having, during development, an amnion and an allantois. mammals, again, are marked off from birds and reptiles by the presence of mammæ, as well as by the form of the occipital condyles. among mammals, the next division is based on the presence or absence of a placenta. and divisions of the _placentalia_ are mainly determined by the characters of the organs of external action. thus, without multiplying illustrations and without descending to genera and species, we see that, speaking generally, the successively smaller groups are distinguished from one another by traits of successively less importance, physiologically considered. the attributes possessed in common by the largest assemblages of organisms, are few in number but all-essential in kind. each secondary assemblage, included in one of the primary assemblages, is characterized by further common attributes that influence the functions less profoundly. and so on with each lower grade. § . what interpretation is to be put on these truths of classification? we find that organic forms admit of an arrangement everywhere indicating the fact, that along with certain attributes, certain other attributes, which are not directly connected with them, always exist. how are we to account for this fact? and how are we to account for the fact that the attributes possessed in common by the largest assemblages of forms, are the most vitally-important attributes? no one can believe that combinations of this kind have arisen fortuitously. even supposing fortuitous combinations of attributes might produce organisms that would work, we should still be without a clue to this special mode of combination. the chances would be infinity to one against organisms which possessed in common certain fundamental attributes, having also in common numerous non-essential attributes. nor, again, can any one allege that such combinations are necessary, in the sense that all other combinations are impracticable. there is not, in the nature of things, a reason why creatures covered with feathers should always have beaks: jaws carrying teeth would, in many cases, have served them equally well or better. the most general characteristic of an entire sub-kingdom, equal in extent to the _vertebrata_, might have been the possession of nictitating membranes; while the internal organizations throughout this sub-kingdom might have been on many different plans. if, as an alternative, this peculiar subordination of traits which organic forms display be ascribed to design, other difficulties suggest themselves. to suppose that a certain plan of organization was fixed on by a creator for each vast and varied group, the members of which were to have many different modes of life, and that he bound himself to adhere rigidly to this plan, even in the most aberrant forms of the group where some other plan would have been more appropriate, is to ascribe a very strange motive. when we discover that the possession of seven cervical vertebræ is a general characteristic of mammals, whether the neck be immensely long as in the giraffe, or quite rudimentary as in the whale, shall we say that though, for the whale's neck, one vertebra would have been equally good, and though, for the giraffe's neck, a dozen would probably have been better than seven, yet seven was the number adhered to in both cases, because seven was fixed upon for the mammalian type? and then, when it turns out that this possession of seven cervical vertebræ is not an absolutely-universal characteristic of mammals (there is one which has eight), shall we conclude that while, in a host of cases, there was a needless adherence to a plan for the sake of consistency, there was yet, in some cases, an inconsistent abandonment of the plan? i think we may properly refuse to draw any such conclusion. what, then, is the meaning of these peculiar relations of organic forms? the answer to this question must be postponed. having here contemplated the problem as presented in these wide inductions which naturalists have reached; and having seen what proposed solutions of it are inadmissible; we shall see, in the next division of this work, what is the only possible solution. chapter xii. distribution. § . there is a distribution of organisms in space, and there is a distribution of organisms in time. looking first at their distribution in space, we observe in it two different classes of facts. on the one hand, the plants and animals of each species have their habitats limited by external conditions: they are necessarily restricted to spaces in which their vital actions can be performed. on the other hand, the existence of certain conditions does not determine the presence of organisms that are fit for them. there are many spaces perfectly adapted for life of a high order in which only life of a much lower order is found. while, in the inevitable restriction of organisms to environments with which their natures correspond we find a _negative_ cause of distribution, there remains to be found that _positive_ cause whence results the presence of organisms in some places appropriate to them and their absence from other places equally appropriate or more appropriate. let us consider the phenomena as thus classed. § . facts which illustrate the limiting influence of surrounding conditions are abundant, and familiar to all readers. it will be needful, however, here to cite a few typical ones of each order. the confinement of different kinds of plants and different kinds of animals, to the media for which they are severally adapted, is the broadest fact of distribution. we have extensive groups of plants that are respectively sub-aerial and sub-aqueous; and of the sub-aqueous some are exclusively marine, while others exist only in rivers and lakes. among animals we similarly find some classes confined to the air and others to the water; and of the water-breathers some are restricted to salt water and others to fresh water. less conspicuous is the fact that within each of these contrasted media there are further widespread limitations. in the sea, certain organisms exist only between certain depths, and others only between other depths--the limpet and the mussel within the littoral zone, and numerous kinds at the bottom of the ocean; and on the land, there are floras and faunas peculiar to low regions and others peculiar to high regions. next we have the familiar geographical limitations made by climate. there are temperatures which restrict each kind of organism between certain isothermal lines, and hygrometric states which prevent the spread of each kind of organism beyond areas having a certain humidity or a certain dryness. besides such general limitations we find much more special limitations. some minute vegetal forms occur only in snow. hot springs have their peculiar _infusoria_. the habitats of certain fungi are mines or other dark places. and there are creatures unknown beyond the water contained in particular caves. after these limits to distribution imposed by physical conditions, come limits imposed by the presence or absence of other organisms. obviously, graminivorous animals are confined within tracts which produce plants fit for them to feed on. the great carnivores cannot exist out of regions where there are creatures large enough and numerous enough to serve for prey. the needs of the sloth limit it to certain forest-covered spaces; and there can be no insectivorous bats where there are no night-flying insects. to these dependences of the relatively-superior organisms on the relatively-inferior organisms which they consume, must be added certain reciprocal dependences of the inferior on the superior. mr. darwin's inquiries have shown how generally the fertilization of plants is due to the agency of insects, and how certain plants, being fertilizable only by insects of certain structures, are limited to regions inhabited by insects of such structures. conversely, the spread of organisms is often bounded by the presence of particular organisms beyond the bounds--either competing organisms or organisms directly inimical. a plant fit for some territory adjacent to its own, fails to overrun it because the territory is pre-occupied by some plant which is its superior, either in fertility or power of resisting destructive agencies; or else fails because there lives in the territory some mammal which browses on its foliage or bird which devours nearly all its seeds. similarly, an area in which animals of a particular species might thrive, is not colonized by them because they are not fleet enough to escape some beast of prey inhabiting this area, or because the area is infested by some insect which destroys them, as the tsetse destroys the cattle in parts of africa. yet another more special series of limitations accompanies parasitism. there are parasitic plants that flourish only on trees of some few species, and others that have particular animals for their habitats--as the fungus which is fatal to the silk-worm, or that which so strangely grows out of a new zealand caterpillar. of animal-parasites various kinds lead lives involving specialities of distribution. we have kinds which use other creatures for purposes of locomotion, as the _chelonobia_ uses the turtle, and as a certain _actinia_ uses the shell inhabited by a hermit-crab. we have the parasitism in which one creature habitually accompanies another to share its prey, like the annelid which takes up its abode in a hermit-crab's shell, and snatches from the hermit-crab the morsels of food it is eating. we have again the commoner parasitism of the _epizoa_--animals which attach themselves to the surfaces of other animals, and feed on their juices or on their secretions. and once more, we have the equally common parasitism of the _entozoa_--creatures which live within other creatures. besides being restricted to the bodies of the organisms it infests, each species has usually still narrower limits of distribution; in some cases the infested organisms furnish fit habitats for the parasites only in certain regions, and in other cases only when in certain constitutional states. there are more indirect modes in which the distributions of organisms affect one another. plants of some kinds are eaten by animals only in the absence of kinds that are preferred to them; and hence the prosperity of such plants partly depends on the presence of the preferred plants. mr. bates has shown that some south american butterflies thrive in regions where insectivorous birds would destroy them, did they not closely resemble butterflies of another genus which are disliked by those birds. and mr. darwin gives cases of dependence still more remote and involved. such are the chief negative causes of distribution--the inorganic and organic agencies that set bounds to the spaces which organisms of each species inhabit. fully to understand their actions we must contemplate them as working not separately but in concert. we have to regard the physical influences, varying from year to year, as now producing an extension or restriction of the habitat in this direction and now in that, and as producing secondary extensions and restrictions by their effects on other kinds of organisms. we have to regard the distribution of each species as affected not only by causes which favour multiplication of prey or of enemies within its own area, but also by causes which produce such results in neighbouring areas. we have to conceive the forces by which the limit is maintained, as including all meteorologic influences, united with the influences, direct or remote, of numerous co-existing species. one general truth, indicated by sundry of the above illustrations, calls for special notice--the truth that all kinds of organisms intrude on one another's spheres of existence. of the ways in which they do this the commonest is invasion of territory. that tendency which we see in the human races, to overrun and occupy one another's lands, as well as the lands inhabited by inferior creatures, is a tendency exhibited by all classes of organisms in various ways. among them, as among mankind, there are permanent conquests, temporary occupations, and occasional raids. every spring an inroad is made into the area which our own birds occupy, by birds from the south; and every winter the fieldfares of the north come to share the hips and haws of our hedges, and thus entail on our native birds some mortality. besides these regularly-recurring incursions there are irregular ones; as of locusts into countries not usually visited by them, or of certain rodents which from time to time swarm into areas adjacent to their own. every now and then an incursion ends in permanent settlement--perhaps in conquest over indigenous species. within these few years an american water-weed has taken possession of our ponds and rivers, and to some extent supplanted native water-weeds. of animals may be named a small kind of red ant, having habits allied to those of tropical ants, which has of late overrun many houses in london. the rat, which must have taken to infesting ships within these few centuries, furnishes a good illustration of the readiness of animals to occupy new places that are available. and the way in which vessels visiting india are cleared of the european cockroach by the kindred _blatta orientalis_, shows us how these successful invasions last only until there come more powerful invaders. animals encroach on one another's spheres of existence in further ways than by trespassing on one another's areas: they adopt one another's modes of life. there are cases in which this usurpation of habits is slight and temporary; and there are cases where it is marked and permanent. grey crows often join gulls in picking up food between tide-marks; and gulls may occasionally be seen many miles inland, feeding in ploughed fields and on moors. mr. darwin has watched a fly-catcher catching fish. he says that the greater titmouse sometimes adopts the practices of the shrike, and sometimes of the nuthatch, and that some south american woodpeckers are frugivorous while others chase insects on the wing. of habitual intrusions on the occupations of other creatures, one case is furnished by the sea-eagle, which, besides hunting the surface of the land for prey, like the rest of the hawk-tribe, often swoops down upon fish. and mr. darwin names a species of petrel that has taken to diving, and has a considerably modified organization. the last cases introduce a still more remarkable class of facts of kindred meaning. this intrusion of organisms on one another's modes of life goes to the extent of intruding on one another's media. the great mass of flowering plants are terrestrial, and (aside from other needs) are required to be so by their process of fructification. but there are some which live in the water, and protrude their flowers above the surface. nay, there is a still more striking instance. at the sea-side may be found an alga a hundred yards inland, and a phænogam rooted in salt water. among animals these interchanges of media are numerous. nearly all coleopterous insects are terrestrial; but the water-beetle, which like the rest of its order is an air-breather, has aquatic habits. water appears to be an extremely unfit medium for a fly; and yet mr. [now sir john] lubbock has discovered more than one species of fly living beneath the surface of the water and coming up occasionally for air. birds, as a class, are specially fitted for an aerial existence; but certain tribes of them have taken to an aquatic existence--swimming on the surface of the water and making continual incursions beneath it, and some kinds have wholly lost the power of flight. among mammals, too, which have limbs and lungs implying an organization for terrestrial life, may be named kinds living more or less in the water and are more or less adapted to it. we have water-rats and otters which unite the two kinds of life, and show but little modification; hippopotami passing the greater part of their time in the water, and somewhat more fitted to it; seals living almost exclusively in the sea, and having the mammalian form greatly obscured; whales wholly confined to the sea, and having so little the aspect of mammals as to be mistaken for fish. conversely, sundry inhabitants of the water make excursions on the land. eels migrate at night from one pool to another. there are fish with specially-modified gills and fin-rays serving as stilts, which, when the rivers they inhabit are partially dried-up, travel in search of better quarters. and while some kinds of crabs do not make land-excursions beyond high-water mark, other kinds pursue lives almost wholly terrestrial. guided by these two classes of facts, we must regard the bounds to each species' sphere of existence as determined by the balancing of two antagonist sets of forces. the tendency which every species has to intrude on other areas, other modes of life, and other media, is restrained by the direct and indirect resistance of conditions, organic and inorganic. and these expansive and repressive energies, varying continually in their respective intensities, rhythmically equilibrate each other--maintain a limit that perpetually oscillates from side to side of a certain mean. § . as implied at the outset, the character of a region, when unfavourable to any species, sufficiently accounts for the absence of this species; and thus its absence is not inconsistent with the hypothesis that each species was originally placed in the regions most favourable to it. but the absence of a species from regions that _are_ favourable to it cannot be thus accounted for. were plants and animals localized wholly with reference to the fitness of their constitutions to surrounding conditions, we might expect floras to be similar, and faunas to be similar, where the conditions are similar; and we might expect dissimilarities among floras and among faunas, proportionate to the dissimilarities of their conditions. but we do not find such anticipations verified. mr. darwin says that "in the southern hemisphere, if we compare large tracts of land in australia, south africa, and western south america, between latitudes ° and °, we shall find parts extremely similar in all their conditions, yet it would not be possible to point out three faunas and floras more utterly dissimilar. or again we may compare the productions of south america south of lat. ° with those north of °, which consequently inhabit a considerably different climate, and they will be found incomparably more closely related to each other, than they are to the productions of australia or africa under nearly the same climate." still more striking are the contrasts which mr. darwin points out between adjacent areas that are totally cut off from each other. "no two marine faunas are more distinct, with hardly a fish, shell, or crab in common, than those of the eastern and western shores of south and central america; yet these great faunas are separated only by the narrow, but impassable, isthmus of panama." on opposite sides of high mountain-chains, also, there are marked differences in the organic forms--differences not so marked as where the barriers are absolutely impassable, but much more marked than are necessitated by unlikenesses of physical conditions. not less suggestive is the converse fact that wide geographical areas which offer decided geologic and meteorologic contrasts, are peopled by nearly-allied groups of organisms, if there are no barriers to migration. "the naturalist in travelling, for instance, from north to south never fails to be struck by the manner in which successive groups of beings, specifically distinct, yet clearly related, replace each other. he hears from closely allied, yet distinct kinds of birds, notes nearly similar, and sees their nests similarly constructed, but not quite alike, with eggs coloured in nearly the same manner. the plains near the straits of magellan are inhabited by one species of rhea (american ostrich), and northward the plains of la plata by another species of the same genus; and not by a true ostrich or emu, like those found in africa and australia under the same latitude. on these same plains of la plata, we see the agouti and bizcacha, animals having nearly the same habits as our hares and rabbits and belonging to the same order of rodents, but they plainly display an american type of structure. we ascend the lofty peaks of the cordillera and we find an alpine species of bizcacha; we look to the waters, and we do not find the beaver or musk-rat, but the coypu and capybara, rodents of the american type. innumerable other instances could be given. if we look to the islands off the american shore, however much they may differ in geological structure, the inhabitants, though they may be all peculiar species, are essentially american." what is the generalization implied by these two groups of facts? on the one hand, we have similarly-conditioned, and sometimes nearly-adjacent, areas, occupied by quite different faunas. on the one hand, we have areas remote from one another in latitude, and contrasted in soil as well as climate, occupied by closely-allied faunas. clearly then, as like organisms are not universally, or even generally, found in like habitats, nor very unlike organisms in very unlike habitats, there is no manifest pre-determined adaptation of the organisms to the habitats. the organisms do no occur in such and such places solely because they are either specially fit for those places, or more fit for them than all other organisms. the induction under which these facts come, and which unites them with various other facts, is a totally-different one. when we see that the similar areas peopled by dissimilar forms, are those between which there are impassable barriers; while the dissimilar areas peopled by similar forms, are those between which there are no such barriers; we are at once reminded of the general truth exemplified in the last section--the truth that each species of organism tends ever to expand its sphere of existence--to intrude on other areas, other modes of life, other media. and we are shown that through these perpetually-recurring attempts to thrust itself into every accessible habitat, each species spreads until it reaches limits which are for the time insurmountable. § . we pass now to the distribution of organic forms in time. geological inquiry has established the truth that during a past of immeasurable duration, plants and animals have existed on the earth. in all countries their buried remains are found in greater or less abundance. from comparatively small areas multitudinous different types have been exhumed. every exploration of new areas, and every closer inspection of areas already explored, brings more types to light. and beyond question, an exhaustive examination of all exposed strata, and of all strata now covered by the sea, would disclose types immensely out-numbering those at present known. further, geologists agree that even had we before us every kind of fossil which exists, we should still have nothing like a complete index to the past inhabitants of our globe. many sedimentary deposits have been so altered by the heat of adjacent molten matter, as greatly to obscure the organic remains contained in them. the extensive formations once called "transition," and now re-named "metamorphic," are acknowledged to be formations of sedimentary origin, from which all traces of such fossils as they probably included have been obliterated by igneous action. and the accepted conclusion is that igneous rock has everywhere resulted from the melting-up of beds of detritus originally deposited by water. how long the reactions of the earth's molten nucleus on its cooling crust, have been thus destroying the records of life, it is impossible to say; but there are strong reasons for believing that the records which remain bear but a small ratio to the records which have been destroyed. thus we have but extremely imperfect data for conclusions respecting the distribution of organic forms in time. some few generalizations, however, may be regarded as established. one is that the plants and animals now existing mostly differ from the plants and animals which have existed. though there are species common to our present fauna and to past faunas, yet the _facies_ of our present fauna differs, more or less, from the _facies_ of each past fauna. on carrying out the comparison, we find that past faunas differ from one another, and that the differences between them are proportionate to their degrees of remoteness from one another in time, as measured by their relative positions in the sedimentary series. so that if we take the assemblage of organic forms living now, and compare it with the successive assemblages of organic forms which have lived in successive geologic epochs, we find that the farther we go back into the past, the greater does the unlikeness become. the number of species and genera common to the compared assemblages, becomes smaller and smaller; and the assemblages differ more and more in their general characters. though a species of brachiopod now extant is almost identical with a species found in silurian strata, though between the silurian fauna and our own there are sundry common genera of molluscs, yet it is undeniable that there is a proportion between lapse of time and divergence of organic forms. this divergence is comparatively slow and continuous where there is continuity in the geological formations, but is sudden, and comparatively wide, wherever there occurs a great break in the succession of strata. the contrasts which thus arise, gradually or all at once, in formations that are continuous or discontinuous, are of two kinds. faunas of different eras are distinguished partly by the absence from the one of type's present in the other, and partly by the unlikenesses between the types common to both. such contrasts between faunas as are due to the appearance or disappearance of types, are of secondary significance: they possibly, or probably, do not imply anything more than migrations or extinctions. the most significant contrasts are those between successive groups of organisms of the same type. and among such, as above said, the differences are, speaking generally, small and continuous where a series of conformable strata gives proof of continued existence of the type in the locality; while they are comparatively large and abrupt where the adjacent formations are shown to have been separated by long intervals. another general fact, referred to by mr. darwin as one which palæontology has made tolerably certain, is that forms and groups of forms which have once disappeared from the earth, do not reappear. passing over the few species which have continued throughout the whole period geologically recorded, it may be said that each species after arising, spreading for an era, and continuing abundant for an era, eventually declines and becomes extinct; and that similarly, each genus during a longer period increases in the number of its species, and during a longer period dwindles and at last dies out. after making its exit neither species nor genus ever re-enters. the like is true even of those larger groups called orders. four types of reptiles which were once abundant have not been found in modern formations, and do not at present exist. though nothing less than an exhaustive examination of all strata, can prove conclusively that a type of organization when once lost is never reproduced, yet so many facts point to this inference that its truth can scarcely be doubted. to frame a conception of the total amount and general direction of the change in organic forms during the time measured by our sedimentary series, is at present impossible--the data are insufficient. the immense contrast between the few and low forms of the earliest-known fauna, and the many and high forms of our existing fauna, has been commonly supposed to prove, not only great change but great progress. nevertheless, this appearance of progress may be, and probably is, mainly illusive. wider knowledge has shown that remains of comparatively well-organized creatures really existed in strata long supposed to be devoid of them, and that where they are absent, the nature of the strata often explains their absence, without assuming that they did not exist when these strata were formed. it is a tenable hypothesis that the successively-higher types fossilized in our successively-later deposits, indicate nothing more than successive migrations from pre-existing continents to continents that were step by step emerging from the ocean--migrations which necessarily began with the inferior orders of organisms, and included the successively-superior orders as the new lands became more accessible to them and better fitted for them.[ ] while the evidence usually supposed to prove progression is thus untrustworthy, there is trustworthy evidence that there has been, in many cases, little or no progression. though the orders which have existed from palæozoic and mesozoic times down to the present day, are almost universally changed, yet a comparison of ancient and modern members of these orders shows that the total amount of change is not relatively great, and that it is not manifestly towards a higher organization. though nearly all the living forms which have prototypes in early formations differ from these prototypes specially, and in most cases generically, yet ordinal peculiarities are, in numerous cases, maintained from the earliest times geologically recorded, down to our own time; and we have no visible evidence of superiority in the existing genera of these orders. in his lecture "on the persistent types of animal life," prof. huxley enumerated many cases. on the authority of dr. hooker he stated "that there are carboniferous plants which appear to be generically identical with some now living: that the cone of the oolitic _araucaria_ is hardly distinguishable from that of an existing species; that a true _pinus_ appears in the purbecks and a _juglans_ in the chalk." among animals he named palæozoic and mesozoic corals which are very like certain extant corals; genera of silurian molluscs that answer to existing genera; insects and arachnids in the coal-formations that are not more than generically distinct from some of our own insects and arachnids. he instanced "the devonian and carboniferous _pleuracanthus_, which differs no more from existing sharks than these do from one another;" early mesozoic reptiles "identical in the essential characters of their organization with those now living;" and triassic mammals which did not differ "nearly so much from some of those which now live, as these differ from one another." continuing the argument in his "anniversary address to the geological society" in , prof. huxley gave many cases in which the changes that have taken place, are not changes towards a more specialized or higher organization--asking "in what sense are the liassic chelonia inferior to those which now exist? how are the cretaceous ichthyosauria, plesiosauria, or pterosauria less embryonic or more differentiated species than those of the lias?" while, however, contending that in most instances "positive evidence fails to demonstrate any sort of progressive modification towards a less embryonic or less generalized type in a great many groups of animals of long-continued geological existence," prof. huxley added that there are other groups, "co-existing with them under the same conditions, in which more or less distinct indications of such a process seem to be traceable." and in illustration of this, he named that better development of the vertebræ which characterizes some of the more modern fishes and reptiles, when compared with ancient fishes and reptiles of the same orders; and the "regularity and evenness of the dentition of the _anoplotherium_ as contrasting with that of existing artiodactyles."[ ] the facts thus summed up do not show that higher forms have not arisen in the course of geologic time, any more than the facts commonly cited prove that higher forms have arisen; nor are they regarded by professor huxley as showing this. were those which have survived from palæozoic and mesozoic days down to our own day, the only types; and did the modifications, rarely of more than generic value, which these types have undergone, give no better evidences of increased complexity than are actually given by them; then it would be inferable that there has been no appreciable advance. but there now exist, and have existed during the more recent geologic epochs, various types which are not known to have existed in earlier epochs--some of them widely unlike these persistent types and some of them nearly allied to these persistent types. as yet, we know nothing about the origins of these new types. but it is possible that causes like those which have produced generic differences in the persistent types, have, in some or many cases, produced modifications great enough to constitute ordinal differences. if structural contrasts not exceeding certain moderate limits are held to mark only generic distinctions; and if organisms displaying larger contrasts are regarded as ordinally or typically distinct; it is obvious that the persistence of a given type through a long geologic period without apparently undergoing deviations of more than generic value, by no means disproves the occurrence of far greater deviations in other cases; since the forms resulting from such far greater deviations, being regarded as typically distinct forms, will not be taken as evidence of great change in an original type. that which prof. huxley's argument proves, and that only which he considers it to prove, is that organisms have no innate tendencies to assume higher forms; and that "any admissible hypothesis of progressive modification, must be compatible with persistence without progression through indefinite periods." one very significant fact must be added concerning the relation between distribution in time and distribution in space. i quote it from mr. darwin:--"mr. clift many years ago showed that the fossil mammals from the australian caves were closely allied to the living marsupials of that continent. in south america a similar relationship is manifest, even to an uneducated eye, in the gigantic pieces of armour like those of the armadillo, found in several parts of la plata; and professor owen has shown in the most striking manner that most of the fossil mammals, buried there in such numbers, are related to the south american types. this relationship is even more clearly seen in the wonderland collection of fossil bones made by mm. lund and clausen in the caves of brazil. i was so much impressed with these facts that i strongly insisted, in and , on this 'law of the succession of types,'--on 'this wonderful relationship in the same continent between the dead and the living.' professor owen has subsequently extended the same generalization to the mammals of the old world. we see the same law in this author's restorations of the extinct and gigantic birds of new zealand. we see it also in the birds of the caves of brazil. mr. woodward has shown that the same law holds good with sea-shells, but from the wide distribution of most genera of molluscs, it is not well displayed by them. other cases could be added, as the relation between the extinct and living landshells of madeira, and between the extinct and living brackish-water shells of the aralo-caspian sea." the general results, then, are these. our knowledge of distribution in time, being derived wholly from the evidence afforded by fossils, is limited to that geologic time of which some records remain--cannot extend to those remoter times the records of which have been obliterated. from these remaining records, which probably form but a small fraction of the whole, the general facts deducible are these:--that such organic types as have lived through successive epochs, have almost universally undergone modifications of specific and generic values--modifications which have commonly been great in proportion as the period has been long. that besides the types which have persisted from ancient eras down to our own era, other types have from time to time made their appearance in the ascending series of strata--types of which some are lower and some higher than the types previously recorded; but whence these new types came, and whether any of them arose by divergence from the previously-recorded types, the evidence does not yet enable us to say. that in the course of long geologic epochs nearly all species, most genera, and a few orders, have become extinct; and that a species, genus, or order, which has once disappeared from the earth never reappears. and, lastly, that the fauna now occupying each separate area of the earth's surface is very nearly allied to the fauna which existed on that area during recent geologic times. § . omitting sundry minor generalizations, the exposition of which would involve too much detail, what is to be said of these major generalizations? the distribution in space cannot be said to imply that organisms have been designed for their particular habitats and placed in them; since, besides the habitat in which each kind of organism is found there are commonly other habitats, as good or better for it, from which it is absent--habitats to which it is so much better fitted than organisms now occupying them, that it extrudes these organisms when allowed the opportunity. neither can we suppose that the purpose has been to establish varieties of floras and faunas; since, if so, why are the floras and faunas but little divergent in widely-sundered areas between which migration is possible, while they are markedly divergent in adjacent areas between which migration is impossible? passing to distributions in time, there arise the questions--why during nearly the whole of that vast period geologically recorded have there existed none of those highest organic forms which have now overrun the earth?--how is it that we find no traces of a creature endowed with large capacities for knowledge and happiness? the answer that the earth was not, in remote times, a fit habitation for such a creature, besides being unwarranted by the evidence, suggests the equally awkward question--why during untold millions of years did the earth remain fit only for inferior creatures? what, again, is the meaning of extinction of types? to conclude that the saurian type was replaced by other types at the beginning of the tertiary period, because it was not adapted to the conditions which then arose, is to conclude that it could not be modified into fitness for the conditions; and this conclusion is at variance with the hypothesis that creative skill is shown in the multiform adaptations of one type to many ends. what interpretations may rationally be put on these and other general facts of distribution in space and time, will be seen in the next division of this work. part iii. the evolution of life. chapter i. preliminary. § . in the foregoing part, we have contemplated the most important of the generalizations to which biologists have been led by observation of organisms; as well as some others which contemplation of the facts has suggested to me. these inductions of biology have also been severally glanced at on their deductive sides; for the purpose of noting the harmony existing between them and those primordial truths set forth in _first principles_. having thus studied the leading phenomena of life separately, we are prepared for studying them as an aggregate, with the view of arriving at the most general interpretation of them. there is an _ensemble_ of vital phenomena presented by each organism in the course of its growth, development, and decay; and there is an _ensemble_ of vital phenomena presented by the organic world as a whole. neither of these can be properly dealt with apart from the other. but the last of them may be separately treated more conveniently than the first. what interpretation we put on the facts of structure and function in each living body, depends entirely on our conception of the mode in which living bodies in general have originated. to form some conclusion respecting this mode--a provisional if not a permanent conclusion--must therefore be our first step. we have to choose between two hypotheses--the hypothesis of special creation and the hypothesis of evolution. either the multitudinous kinds of organisms which now exist, and the far more multitudinous kinds which have existed during past geologic eras, have been from time to time separately made; or they have arisen by insensible steps, through actions such as we see habitually going on. both hypotheses imply a cause. the last, certainly as much as the first, recognizes this cause as inscrutable. the point at issue is, how this inscrutable cause has worked in the production of living forms. this point, if it is to be decided at all, is to be decided only by examination of evidence. let us inquire which of these antagonist hypotheses is most congruous with established facts. chapter ii. general aspects of the special-creation-hypothesis.[ ] § . early ideas are not usually true ideas. undeveloped intellect, be it that of an individual or that of the race, forms conclusions which require to be revised and re-revised, before they reach a tolerable correspondence with realities. were it otherwise there would be no discovery, no increase of intelligence. what we call the progress of knowledge, is the bringing of thoughts into harmony with things; and it implies that the first thoughts are either wholly out of harmony with things, or in very incomplete harmony with them. if illustrations be needed the history of every science furnishes them. the primitive notions of mankind as to the structure of the heavens were wrong; and the notions which replaced them were successively less wrong. the original belief respecting the form of the earth was wrong; and this wrong belief survived through the first civilizations. the earliest ideas that have come down to us concerning the natures of the elements were wrong; and only in quite recent times has the composition of matter in its various forms been better understood. the interpretations of mechanical facts, of meteorological facts, of physiological facts, were at first wrong. in all these cases men set out with beliefs which, if not absolutely false, contained but small amounts of truth disguised by immense amounts of error. hence the hypothesis that living beings resulted from special creations, being a primitive hypothesis, is probably an untrue hypothesis. it would be strange if, while early men failed to reach the truth in so many cases where it is comparatively conspicuous, they reached it in a case where it is comparatively hidden. § . besides the improbability given to the belief in special creations, by its association with mistaken beliefs in general, a further improbability is given to it by its association with a special class of mistaken beliefs. it belongs to a family of beliefs which have one after another been destroyed by advancing knowledge; and is, indeed, almost the only member of the family surviving among educated people. we all know that the savage thinks of each striking phenomenon, or group of phenomena, as caused by some separate personal agent; that out of this conception there grows up a polytheistic conception, in which these minor personalities are variously generalized into deities presiding over different divisions of nature; and that these are eventually further generalized. this progressive consolidation of causal agencies may be traced in the creeds of all races, and is far from complete in the creed of the most advanced races. the unlettered rustics who till our fields, do not let the consciousness of a supreme power wholly absorb the aboriginal conceptions of good and evil spirits, and of charms or secret potencies dwelling in particular objects. the earliest mode of thinking changes only as fast as the constant relations among phenomena are established. scarcely less familiar is the truth, that while accumulating knowledge makes these conceptions of personal causal agents gradually more vague, as it merges them into general causes, it also destroys the habit of thinking of them as working after the methods of personal agents. we do not now, like kepler, assume guiding spirits to keep the planets in their orbits. it is no longer the universal belief that the sea was once for all mechanically parted from the dry land; or that the mountains were placed where we see them by a sudden creative act. all but a narrow class have ceased to suppose sunshine and storm to be sent in some arbitrary succession. the majority of educated people have given up thinking of epidemics of punishments inflicted by an angry deity. nor do even the common people regard a madman as one possessed by a demon. that is to say, we everywhere see fading away the anthropomorphic conception of cause. in one case after another, is abandoned the ascription of phenomena to a will analogous to the human will, working by methods analogous to human methods. if, then, of this once-numerous family of beliefs the immense majority have become extinct, we may not unreasonably expect that the few remaining members of the family will become extinct. one of these is the belief we are here considering--the belief that each species of organism was specially created. many who in all else have abandoned the aboriginal theory of things, still hold this remnant of the aboriginal theory. ask any well-informed man whether he accepts the cosmogony of the indians, or the greeks, or the hebrews, and he will regard the question as next to an insult. yet one element common to these cosmogonies he very likely retains: not bearing in mind its origin. for whence did he get the doctrine of special creations? catechise him, and he is forced to confess that it was put into his mind in childhood, as one portion of a story which, as a whole, he has long since rejected. why this fragment is likely to be right while all the rest is wrong, he is unable to say. may we not then expect that the relinquishment of all other parts of this story, will by and by be followed by the relinquishment of this remaining part of it? § . the belief which we find thus questionable, both as being a primitive belief and as being a belief belonging to an almost-extinct family, is a belief not countenanced by a single fact. no one ever saw a special creation; no one ever found proof of an indirect kind that a special creation had taken place. it is significant, as dr. hooker remarks, that naturalists who suppose new species to be miraculously originated, habitually suppose the origination to occur in some region remote from human observation. wherever the order of organic nature is exposed to the view of zoologists and botanists, it expels this conception; and the conception survives only in connexion with imagined places, where the order of organic nature is unknown. besides being absolutely without evidence to give it external support, this hypothesis of special creations cannot support itself internally--cannot be framed into a coherent thought. it is one of those illegitimate symbolic conceptions which are mistaken for legitimate symbolic conceptions (_first principles_, § ), because they remain untested. immediately an attempt is made to elaborate the idea into anything like a definite shape, it proves to be a pseud-idea, admitting of no definite shape. is it supposed that a new organism, when specially created, is created out of nothing? if so, there is a supposed creation of matter; and the creation of matter is inconceivable--implies the establishment of a relation in thought between nothing and something--a relation of which one term is absent--an impossible relation. is it supposed that the matter of which the new organism consists is not created for the occasion, but is taken out of its pre-existing forms and arranged into a new form? if so, we are met by the question--how is the re-arrangement effected? of the myriad atoms going to the composition of the new organism, all of them previously dispersed through the neighbouring air and earth, does each, suddenly disengaging itself from its combinations, rush to meet the rest, unite with them into the appropriate chemical compounds, and then fall with certain others into its appointed place in the aggregate of complex tissues and organs? surely thus to assume a myriad supernatural impulses, differing in their directions and amounts, given to as many different atoms, is a multiplication of mysteries rather than the solution of a mystery. for every one of these impulses, not being the result of a force locally existing in some other form, implies the creation of force; and the creation of force is just as inconceivable as the creation of matter. it is thus with all attempted ways of representing the process. the old hebrew idea that god takes clay and moulds a new creature, as a potter moulds a vessel, is probably too grossly anthropomorphic to be accepted by any modern defender of the special-creation doctrine. but having abandoned this crude belief, what belief is he prepared to substitute? if a new organism is not thus produced, then in what way is one produced? or rather--in what way does he conceive a new organism to be produced? we will not ask for the ascertained mode, but will be content with a mode which can be consistently imagined. no such mode, however, is assignable. those who entertain the proposition that each kind of organism results from a divine interposition, do so because they refrain from translating words into thoughts. they do not really believe, but rather _believe they believe_. for belief, properly so called, implies a mental representation of the thing believed, and no such mental representation is here possible. § . if we imagine mankind to be contemplated by some being as short-lived as an ephemeron, but possessing intelligence like our own--if we imagine such a being studying men and women, during his few hours of life, and speculating as to the mode in which they came into existence; it is manifest that, reasoning in the usual way, he would suppose each man and woman to have been separately created. no appreciable changes of structure occurring in any of them during the time over which his observations extended, this being would probably infer that no changes of structure were taking place, or had taken place; and that from the outset each man and woman had possessed all the characters then visible--had been originally formed with them. the application is obvious. a human life is ephemeral compared with the life of a species; and even the period over which the records of all human lives extend, is ephemeral compared with the life of a species. there is thus a parallel contrast between the immensely-long series of changes which have occurred during the life of a species, and that small portion of the series open to our view. and there is no reason to suppose that the first conclusion drawn by mankind from this small part of the series visible to them, is any nearer the truth than would be the conclusion of the supposed ephemeral being respecting men and women. this analogy, suggesting as it does how the hypothesis of special creations is merely a formula for our ignorance, raises the question--what reason have we to assume special creations of species but not of individuals; unless it be that in the case of individuals we directly know the process to be otherwise, but in the case of species do not directly know it to be otherwise? have we any ground for concluding that species were specially created, except the ground that we have no immediate knowledge of their origin? and does our ignorance of the manner in which they arose warrant us in asserting that they arose by special creation? another question is suggested by this analogy. those who, in the absence of immediate evidence of the way in which species arose, assert that they arose not in a natural way allied to that in which individuals arise, but in a supernatural way, think that by this supposition they honour the unknown cause of things; and they oppose any antagonist doctrine as amounting to an exclusion of divine power from the world. but if divine power is demonstrated by the separate creation of each species, would it not have been still better demonstrated by the separate creation of each individual? why should there exist this process of natural genesis? why should not omnipotence have been proved by the supernatural production of plants and animals everywhere throughout the world from hour to hour? is it replied that the creator was able to make individuals arise from one another in a natural succession, but not to make species thus arise? this is to assign a limit to power instead of magnifying it. either it was possible or not possible to create species and individuals after the same general method. to say that it was not possible is suicidal in those who use this argument; and if it was possible, it is required to say what end is served by the special creation of species which would not have been better served by the special creation of individuals. again, what is to be thought of the fact that the immense majority of these supposed special creations took place before mankind existed? those who think that divine power is demonstrated by special creations, have to answer the question--to whom demonstrated? tacitly or avowedly, they regard the demonstrations as being for the benefit of mankind. but if so, to what purpose were the millions of these demonstrations which took place on the earth when there were no intelligent beings to contemplate them? did the unknowable thus demonstrate his power to himself? few will have the hardihood to say that any such demonstration was needful. there is no choice but to regard them, either as superfluous exercises of power, which is a derogatory supposition, or as exercises of power that were necessary because species could not be otherwise produced, which is also a derogatory supposition. § a. other implications concerning the divine character must be recognized by those who contend that each species arose by divine fiat. it is hardly supposable that infinite power is exercised in trivial actions effecting trivial changes. yet the organic world in its hundreds of thousands of species shows in each sub-division multitudinous forms which, though unlike enough to be classed as specifically distinct, diverge from one another only in small details which have no significance in relation to the life led. sometimes the number of specific distinctions is so great that did they result from human agency we should call them whimsical. for example, in lake baikal are found species of an amphipod, _gammarus_; and the multiplicity becomes startling on learning that this number exceeds the number of all other species of the genus: various as are the conditions to which, throughout the rest of the world, the genus is subject. still stranger seems the superfluous exercise of power on examining the carpet of living forms at the bottom of the ocean. not dwelling on the immense variety of creatures unlike in type which live miles below the surface in absolute darkness, it will suffice to instance the _polyzoa_ alone: low types of animals so small that a thousand of them would not cover a square inch, and on which, nevertheless, there has been, according to the view we are considering, an exercise of creative skill such that by small variations of structure more than species have been produced! kindred illustrations are furnished by the fauna of caverns. are we to suppose that numerous blind creatures--crustaceans, myriapods, spiders, insects, fishes--were specially made sightless to fit them for the mammoth cave? or what shall we say of the _proteus_, a low amphibian with rudimentary eyes, which inhabits certain caves in carniola, carinthia and dalmatia and is not found elsewhere. must we conclude that god went out of his way to devise an animal for these places? more puzzling still is a problem presented to the special-creationist by a batrachian inhabiting central australia. in a region once peopled by numerous animals but now made unfit by continuous droughts, there exists a frog which, when the pools are drying up, fills itself with water and burrowing in the mud hibernates until the next rains; which may come in a year or may be delayed for two years. what is to be thought of this creature? were its structure and the accompanying instinct divinely planned to fit it to this particular habitat? many such questions might be asked which, if answered as the current theory necessitates, imply a divine nature hardly like that otherwise assumed. § . those who espouse the aboriginal hypothesis entangle themselves in yet other theological difficulties. this assumption that each kind of organism was specially designed, carries with it the implication that the designer intended everything which results from the design. there is no escape from the admission that if organisms were severally constructed with a view to their respective ends, then the character of the constructor is indicated both by the ends themselves, and the perfection or imperfection with which the organisms are fitted to them. observe the consequences. without dwelling on the question recently raised, why during untold millions of years there existed on the earth no beings endowed with capacities for wide thought and high feeling, we may content ourselves with asking why, at present, the earth is largely peopled by creatures which inflict on one another so much suffering? omitting the human race, whose defects and miseries the current theology professes to account for, and limiting ourselves to the lower creation, what must we think of the countless different pain-inflicting appliances and instincts with which animals are endowed? not only now, and not only ever since men have lived, has the earth been a scene of warfare among all sentient creatures; but palæontology shows us that from the earliest eras geologically recorded, there has been going on this universal carnage. fossil structures, in common with the structures of existing animals, show us elaborate weapons for destroying other animals. we have unmistakable proof that throughout all past time, there has been a ceaseless devouring of the weak by the strong. how is this to be explained? how happens it that animals were so designed as to render this bloodshed necessary? how happens it that in almost every species the number of individuals annually born is such that the majority die by starvation or by violence before arriving at maturity? whoever contends that each kind of animal was specially designed, must assert either that there was a deliberate intention on the part of the creator to produce these results, or that there was an inability to prevent them. which alternative does he prefer?--to cast an imputation on the divine character or to assert a limitation of the divine power? it is useless for him to plead that the destruction of the less powerful by the more powerful, is a means of preventing the miseries of decrepitude and incapacity, and therefore works beneficently. for even were the chief mortality among the aged instead of among the young, there would still arise the unanswerable question--why were not animals constructed in such ways as to avoid these evils? why were not their rates of multiplication, their degrees of intelligence, and their propensities, so adjusted that these sufferings might be escaped? and if decline of vigour was a necessary accompaniment of age, why was it not provided that the organic actions should end in sudden death, whenever they fell below the level required for pleasurable existence? will any one who contends that organisms were specially designed, assert that they could not have been so designed as to prevent suffering? and if he admits that they could have been made so as to prevent suffering, will he assert that the creator preferred making them in such ways as to inflict suffering? even as thus presented the difficulty is sufficiently great; but it appears immensely greater when we examine the facts more closely. so long as we contemplate only the preying of the superior on the inferior, some good appears to be extracted from the evil--a certain amount of life of a higher order, is supported by sacrificing a great deal of life of a lower order. so long, too, as we leave out all mortality but that which, by carrying off the least perfect members of each species, leaves the most perfect members to survive and multiply; we see some compensating benefit reached through the suffering inflicted. but what shall we say on finding innumerable cases in which the suffering inflicted brings no compensating benefit? what shall we say when we see the inferior destroying the superior? what shall we say on finding elaborate appliances for furthering the multiplication of organisms incapable of feeling, at the expense of misery to organisms capable of happiness? of the animal kingdom as a whole, more than half the species are parasites. "the number of these parasites," says prof. owen, "may be conceived when it is stated that almost every known animal has its peculiar species, and generally more than one, sometimes as many as, or even more kinds than, infest the human body." this parasitism begins among the most minute creatures and pervades the entire animal kingdom from the lowest to the highest. even _protozoa_, made visible to us only by the microscope, are infested, as is _paramoecium_ by broods of _sphærophrya_; while in large and complex animals parasites are everywhere present in great variety. more than this is true. there are parasites upon parasites--an arrangement such that those which are torturing the creatures they inhabit are themselves tortured by indwelling creatures still smaller: looking like an ingenious accumulation of pains upon pains. but passing over the evils thus inflicted on animals of inferior dignity, let us limit ourselves to the case of man. the _bothriocephalus latus_ and the _tænia solium_, are two kinds of tape-worm, which flourish in the human intestines; producing great constitutional disturbances, sometimes ending in insanity; and from the germs of the _tænia_, when carried into other parts of the body, arise certain partially-developed forms known as _cysticerci_, _echinococci_, and _coenuri_, which cause disorganization more or less extensive in the brain, the lungs, the liver, the heart, the eye, &c., often ending fatally after long-continued suffering. five other parasites, belonging to a different class, are found in the viscera of man--the _trichocephalus_, the _oxyuris_, the _strongylus_ (two species), the _ancylostomum_ and the _ascaris_; which, beyond that defect of nutrition which they necessarily cause, sometimes induce certain irritations that lead to complete demoralization. of another class of _entozoa_, belonging to the subdivision _trematoda_, there are five kinds found in different organs of the human body--the liver and gall-duct, the portal vein, the intestine, the bladder, the eye. then we have the _trichina spiralis_, which passes through one phase of its existence imbedded in the muscles and through another phase of its existence in the intestine; and which, by the induced disease _trichinosis_, has lately committed such ravages in germany as to cause a panic. to these we must add the guinea-worm, which in some part of africa and india makes men miserable by burrowing in their legs; and the more terrible african parasite the _bilharzia_, which affects per cent. of the natives on the east coast with bleeding of the bladder. from _entozoa_, let us pass to _epizoa_. there are two kinds of _acari_, one of them inhabiting the follicles of the skin and the other producing the itch. there are creatures that bury themselves beneath the skin and lay their eggs there; and there are three species of lice which infest the surface of the body. nor is this all. besides animal parasites there are sundry vegetal parasites, which grow and multiply at our cost. the _sarcina ventriculi_ inhabits the stomach, and produces gastric disturbance. the _leptothrix buccalis_ is extremely general in the mouth, and may have something to do with the decay of teeth. and besides these there are microscopic fungi which produce ringworm, porrigo, pityriasis, thrush, &c. thus the human body is the habitat of parasites, internal and external, animal and vegetal, numbering, if all are set down, between two and three dozen species; sundry of which are peculiar to man, and many of which produce great suffering and not unfrequently death. what interpretation is to be put on these facts by those who espouse the hypothesis of special creations? according to this hypothesis, all these parasites were designed for their respective modes of life. they were endowed with constitutions fitting them to live by absorbing nutriment from the human body; they were furnished with appliances, often of a formidable kind, enabling them to root themselves in and upon the human body; and they were made prolific in an almost incredible degree, that their germs might have a sufficient number of chances of finding their way into the human body. in short, elaborate contrivances were combined to insure the continuance of their respective races; and to make it impossible for the successive generations of men to avoid being preyed on by them. what shall we say to this arrangement? shall we say that "the head and crown of things," was provided as a habitat for these parasites? shall we say that these degraded creatures, incapable of thought or enjoyment, were created that they might cause human misery? one or other of these alternatives must be chosen by those who contend that every kind of organism was separately devised by the creator. which do they prefer? with the conception of two antagonist powers, which severally work good and evil in the world, the facts are congruous enough. but with the conception of a supreme beneficence, this gratuitous infliction of pain is absolutely incompatible. § . see then the results of our examination. the belief in special creations of organisms arose among men during the era of profoundest darkness; and it belongs to a family of beliefs which have nearly all died out as enlightenment has increased. it is without a solitary established fact on which to stand; and when the attempt is made to put it into definite shape in the mind, it turns out to be only a pseud-idea. this mere verbal hypothesis, which men idly accept as a real or thinkable hypothesis, is of the same nature as would be one, based on a day's observation of human life, that each man and woman was specially created--an hypothesis not suggested by evidence but by lack of evidence--an hypothesis which formulates ignorance into a semblance of knowledge. further, we see that this hypothesis, failing to satisfy men's intellectual need of an interpretation, fails also to satisfy their moral sentiment. it is quite inconsistent with those conceptions of the divine nature which they profess to entertain. if infinite power was to be demonstrated, then, either by the special creation of every individual, or by the production of species by some method of natural genesis, it would be better demonstrated than by the use of two methods, as assumed by the hypothesis. and if infinite goodness was to be demonstrated, then, not only do the provisions of organic structure, if they are specially devised, fail to demonstrate it, but there is an enormous mass of them which imply malevolence rather than benevolence. thus the hypothesis of special creations turns out to be worthless by its derivation; worthless in its intrinsic incoherence; worthless as absolutely without evidence; worthless as not supplying an intellectual need; worthless as not satisfying a moral want. we must therefore consider it as counting for nothing, in opposition to any other hypothesis respecting the origin of organic beings. chapter iii. general aspects of the evolution-hypothesis. § . just as the supposition that races of organisms have been specially created, is discredited by its origin; so, conversely, the supposition that races of organisms have been evolved, is credited by its origin. instead of being a conception suggested and accepted when mankind were profoundly ignorant, it is a conception born in times of comparative enlightenment. moreover, the belief that plants and animals have arisen in pursuance of uniform laws, instead of through breaches of uniform laws, is a belief which has come into existence in the most-instructed class, living in these better-instructed times. not among those who have disregarded the order of nature, has this idea made its appearance; but among those who have familiarized themselves with the order of nature. thus the derivation of this modern hypothesis is as favourable as that of the ancient hypothesis is unfavourable. § . a kindred antithesis exists between the two families of beliefs, to which the beliefs we are comparing severally belong. while the one family has been dying out the other family has been multiplying. as fast as men have ceased to regard different classes of phenomena as caused by special personal agents, acting irregularly; so fast have they come to regard these different classes of phenomena as caused by a general agency acting uniformly--the two changes being correlatives. and as, on the one hand, the hypothesis that each species resulted from a supernatural act, having lost nearly all its kindred hypotheses, may be expected soon to die; so, on the other hand, the hypothesis that each species resulted from the action of natural causes, being one of an increasing family of hypotheses, may be expected to survive. still greater will the probability of its survival and establishment appear, when we observe that it is one of a particular genus of hypotheses which has been rapidly extending. the interpretation of phenomena as results of evolution, has been independently showing itself in various fields of inquiry, quite remote from one another. the supposition that the solar system has been evolved out of diffused matter, is a supposition wholly astronomical in its origin and application. geologists, without being led thereto by astronomical considerations, have been step by step advancing towards the conviction that the earth has reached its present varied structure by modification upon modification. the inquiries of biologists have proved the falsity of the once general belief, that the germ of each organism is a minute repetition of the mature organism, differing from it only in bulk; and they have shown, contrariwise, that every organism advances from simplicity to complexity through insensible changes. among philosophical politicians, there has been spreading the perception that the progress of society is an evolution: the truth that "constitutions are not made but grow," is seen to be a part of the more general truth that societies are not made but grow. it is now universally admitted by philologists that languages, instead of being artificially or supernaturally formed, have been developed. and the histories of religion, of science, of the fine arts, of the industrial arts, show that these have passed through stages as unobtrusive as those through which the mind of a child passes on its way to maturity. if, then, the recognition of evolution as the law of many diverse orders of phenomena, has been spreading; may we not say that there thence arises the probability that evolution will presently be recognized as the law of the phenomena we are considering? each further advance of knowledge confirms the belief in the unity of nature; and the discovery that evolution has gone on, or is going on, in so many departments of nature, becomes a reason for believing that there is no department of nature in which it does not go on. § . the hypotheses of special creation and evolution, are no less contrasted in respect of their legitimacy as hypotheses. while, as we have seen, the one belongs to that order of symbolic conceptions which are proved to be illusive by the impossibility of realizing them in thought; the other is one of those symbolic conceptions which are more or less fully realizable in thought. the production of all organic forms by the accumulation of modifications and of divergences by the continual addition of differences to differences, is mentally representable in outline, if not in detail. various orders of our experiences enable us to conceive the process. let us look at one of the simplest. there is no apparent similarity between a straight line and a circle. the one is a curve; the other is defined as without curvature. the one encloses a space; the other will not enclose a space though produced for ever. the one is finite; the other may be infinite. yet, opposite as the two are in their characters, they may be connected together by a series of lines no one of which differs from the adjacent ones in any appreciable degree. thus, if a cone be cut by a plane at right angles to its axis we get a circle. if, instead of being perfectly at right angles, the plane subtends with the axis an angle of ° ', we have an ellipse which no human eye, even when aided by an accurate pair of compasses, can distinguish from a circle. decreasing the angle minute by minute, this closed curve becomes perceptibly eccentric, then manifestly so, and by and by acquires so immensely elongated a form so as to bear no recognizable resemblance to a circle. by continuing this process the ellipse changes insensibly into a parabola. on still further diminishing the angle, the parabola becomes an hyperbola. and finally, if the cone be made gradually more obtuse, the hyperbola passes into a straight line as the angle of the cone approaches °. here then we have five different species of line--circle, ellipse, parabola, hyperbola, and straight line--each having its peculiar properties and its separate equation, and the first and last of which are quite opposite in nature, connected together as members of one series, all producible by a single process of insensible modification. but the experiences which most clearly illustrate the process of general evolution, are our experiences of special evolution, repeated in every plant and animal. each organism exhibits, within a short time, a series of changes which, when supposed to occupy a period indefinitely great, and to go on in various ways instead of one way, give us a tolerably clear conception of organic evolution at large. in an individual development, we see brought into a comparatively infinitesimal time, a series of metamorphoses equally great with each of those which the hypothesis of evolution assumes to have taken place during immeasurable geologic epochs. a tree differs from a seed in every respect--in bulk, in structure, in colour, in form, in chemical composition. yet is the one changed in the course of a few years into the other: changed so gradually, that at no moment can it be said--now the seed ceases to be and the tree exists. what can be more widely contrasted than a newly-born child and the small, semi-transparent, gelatinous spherule constituting the human ovum? the infant is so complex in structure that a cyclopædia is needed to describe its constituent parts. the germinal vesicle is so simple that it may be defined in a line. nevertheless, nine months suffice to develop the one out of the other; and that, too, by a series of modifications so small, that were the embryo examined at successive minutes, even a microscope would not disclose any sensible changes. aided by such facts, the conception of general evolution may be rendered as definite a conception as any of our complex conceptions can be rendered. if, instead of the successive minutes of a child's foetal life, we take the lives of successive generations of creatures--if we regard the successive generations as differing from one another no more than the foetus differs in successive minutes; our imaginations must indeed be feeble if we fail to realize in thought, the evolution of the most complex organism out of the simplest. if a single cell, under appropriate conditions, becomes a man in the space of a few years; there can surely be no difficulty in understanding how, under appropriate conditions, a cell may, in the course of untold millions of years, give origin to the human race. doubtless many minds are so unfurnished with those experiences of nature out of which this conception is built, that they find difficulty in forming it. looking at things rather in their statical than in their dynamical aspects, they never realize the fact that, by small increments of modification, any amount of modification may in time be generated. the surprise they feel on finding one whom they last saw as a boy, grown into a man, becomes incredulity when the degree of change is greater. to such, the hypothesis that by any series of changes a protozoon can give origin to a mammal, seems grotesque--as grotesque as galileo's assertion of the earth's movement seemed to his persecutors; or as grotesque as the assertion of the earth's sphericity seems now to the new zealanders. but those who accept a literally-unthinkable proposition as quite satisfactory, may not unnaturally be expected to make a converse mistake. § . the hypothesis of evolution is contrasted with the hypothesis of special creations, in a further respect. it is not simply legitimate instead of illegitimate, because representable in thought instead of unrepresentable; but it has the support of some evidence, instead of being absolutely unsupported by evidence. though the facts at present assignable in _direct_ proof that by progressive modifications, races of organisms which are apparently distinct from antecedent races have descended from them, are not sufficient; yet there are numerous facts of the order required. beyond all question unlikenesses of structure gradually arise among the members of successive generations. we find that there is going on a modifying process of the kind alleged as the source of specific differences: a process which, though slow, does, in time, produce conspicuous changes--a process which, to all appearance, would produce in millions of years, any amount of change. in the chapters on "heredity" and "variation," contained in the preceding part, many such facts were given, and more might be added. although little attention has been paid to the matter until recent times, the evidence already collected shows that there take place in successive generations, alterations of structure quite as marked as those which, in successive short intervals, arise in a developing embryo--nay, often much more marked; since, besides differences due to changes in the relative sizes or parts, there sometimes arise differences due to additions and suppressions of parts. the structural modification proved to have taken place since organisms have been observed, is not less than the hypothesis demands--bears as great a ratio to this brief period, as the total amount of structural change seen in the evolution of a complex organism out of a simple germ, bears to that vast period during which living forms have existed on the earth. we have, indeed, much the same kind and quantity of direct evidence that all organic beings have arisen through the actions of natural causes, which we have that all the structural complexities of the earth's crust have arisen through the actions of natural causes. between the known modifications undergone by organisms, and the totality of modifications displayed in their structures, there is no greater disproportion than between the observed geological changes, and the totality of geological changes supposed to have been similarly caused. here and there are sedimentary deposits now slowly taking place. at this place a shore has been greatly encroached on by the sea during recorded times; and at another place an estuary has become shallower within some generations. in one region an upheaval is going on at the rate of a few feet in a century; while in another region occasional earthquakes cause slight variations of level. appreciable amounts of denudation by water are visible in some localities; and in other localities glaciers are detected in the act of grinding down the rocky surfaces over which they glide. but these changes are infinitesimal compared with the aggregate of changes to which the earth's crust testifies, even in its still extant systems of strata. if, then, the small changes now being wrought on the earth's crust by natural agencies, yield warrant for concluding that by such agencies acting through vast epochs, all the structural complexities of the earth's crust have been produced; do not the small known modifications produced in races of organisms by natural agencies, yield warrant for concluding that by natural agencies have been produced all those structural complexities which we see in them? the hypothesis of evolution then, has direct support from facts which, though small in amount, are of the kind required; and the ratio which these facts bear to the generalization based on them, seems as great as is the ratio between facts and generalization which, in another case, produces conviction. § . let us put ourselves for a moment in the position of those who, from their experiences of human modes of action, draw differences respecting the mode of action of that ultimate power manifested to us through phenomena. we shall find the supposition that each kind of organism was separately designed and put together, to be much less consistent with their professed conception of this ultimate power, than is the supposition that all kinds of organisms have resulted from one unbroken process. irregularity of method is a mark of weakness. uniformity of method is a mark of strength. continual interposition to alter a pre-arranged set of actions, implies defective arrangement in those actions. the maintenance of those actions, and the working out by them of the highest results, implies completeness of arrangement. if human workmen, whose machines as at first constructed require perpetual adjustment, show their increasing skill by making their machines self-adjusting; then, those who figure to themselves the production of the world and its inhabitants by a "great artificer," must admit that the achievement of this end by a persistent process, adapted to all contingencies, implies greater skill than its achievement by the process of meeting the contingencies as they severally arise. so, too, it is with the contrast under its moral aspect. we saw that to the hypothesis of special creations, a difficulty is presented by the absence of high forms of life during immeasurable epochs of the earth's existence. but to the hypothesis of evolution, absence of them is no such obstacle. suppose evolution, and this question is necessarily excluded. suppose special creations, and this question can have no satisfactory answer. still more marked is the contrast between the two hypotheses, in presence of that vast amount of suffering entailed on all orders of sentient beings by their imperfect adaptations to their conditions of life, and the further vast amount of suffering entailed on them by enemies and by parasites. we saw that if organisms were severally designed for their respective places in nature, the inevitable conclusion is that these innumerable kinds of inferior organisms which prey on superior organisms, were intended to inflict all the pain and mortality which results. but the hypothesis of evolution involves us in no such dilemma. slowly, but surely, evolution brings about an increasing amount of happiness. in all forms of organization there is a progressive adaptation, and a survival of the most adapted. if, in the uniform working out of the process, there are evolved organisms of low types which prey on those of higher types, the evils inflicted form but a deduction from the average benefits. the universal multiplication of the most adapted must cause the spread of those superior organisms which, in one way or other, escape the invasions of the inferior; and so tends to produce a type less liable to the invasions of the inferior. thus the evils accompanying evolution are ever being self-eliminated. though there may arise the question--why could they not have been avoided? there does not arise the question--why were they deliberately inflicted? whatever may be thought of them, it is clear that they do not imply gratuitous malevolence. § . in all respects, then, the hypothesis of evolution contrasts favourably with the hypothesis of special creation. it has arisen in comparatively-instructed times and in the most cultivated class. it is one of those beliefs in the uniform concurrence of phenomena, which are gradually supplanting beliefs in their irregular and arbitrary concurrence; and it belongs to a genus of these beliefs which has of late been rapidly spreading. it is a definitely-conceivable hypothesis; being simply an extension to the organic world at large, of a conception framed from our experiences of individual organisms; just as the hypothesis of universal gravitation was an extension of the conception which our experiences of terrestrial gravitation had produced. this definitely-conceivable hypothesis, besides the support of numerous analogies, has the support of direct evidence. we have proof that there is going on a process of the kind alleged; and though the results of this process, as actually witnessed, are minute in comparison with the totality of results ascribed to it, yet they bear to such totality a ratio as great as that by which an analogous hypothesis is justified. lastly, that sentiment which the doctrine of special creations is thought necessary to satisfy, is much better satisfied by the doctrine of evolution; since this doctrine raises no contradictory implications respecting the unknown cause, such as are raised by the antagonist doctrine. and now, having observed how, under its most general aspects, the hypothesis of organic evolution commends itself to us by its derivation, by its coherence, by its analogies, by its direct evidence, by its implications; let us go on to consider the several orders of facts which yield indirect support to it. we will begin by noting the harmonies between it and sundry of the inductions set forth in part ii. chapter iv. the arguments from classification. § . in § , we saw that the relations which exist among the species, genera, orders, and classes of organisms, are not interpretable as results of any such causes as have usually been assigned. we will here consider whether they are interpretable as the results of evolution. let us first contemplate some familiar facts. the norwegians, swedes, danes, germans, dutch, and anglo-saxons, form together a group of scandinavian races, which are but slightly divergent in their characters. welsh, irish, and highlanders, though they have differences, have not such differences as hide a decided community of nature: they are classed together as celts. between the scandinavian race as a whole and the celtic race as a whole, there is a distinction greater than that between the sub-divisions which make up the one or the other. similarly, the several peoples inhabiting southern europe are more nearly allied to one another, than the aggregate they form is allied to the aggregates of northern peoples. if, again, we compare these european varieties of man, taken as a group, with that group of eastern varieties which had a common origin with it, we see a stronger contrast than between the groups of european varieties themselves. and once more, ethnologists find differences of still higher importance between the aryan stock as a whole and the mongolian stock as a whole, or the negro stock as a whole. though these contrasts are partially obscured by intermixtures, they are not so much obscured as to hide the truths that the most-nearly-allied varieties of man are those which diverged from one another at comparatively-recent periods; that each group of nearly-allied varieties is more strongly contrasted with other such groups that had a common origin with it at a remoter period; and so on until we come to the largest groups, which are the most strongly contrasted, and of whose divergence no trace is extant. the relations existing among the classes and sub-classes of languages, have been briefly referred to by mr. darwin in illustration of his argument. we know that languages have arisen by evolution. let us then see what grouping of them evolution has produced. on comparing the dialects of adjacent counties in england, we find that their differences are so small as scarcely to distinguish them. between the dialects of the northern counties taken together, and those of the southern counties taken together, the contrast is stronger. these clusters of dialects, together with those of scotland and ireland, are nevertheless so similar that we regard them as one language. the several languages of scandinavian europe, including english, are much more unlike one another than are the several dialects which each of them includes; in correspondence with the fact that they diverged from one another at earlier periods than did their respective dialects. the scandinavian languages have nevertheless a certain community of character, distinguishing them as a group from the languages of southern europe; between which there are general and special affinities that similarly unite them into a group formed of sub-groups containing sub-sub-groups. and this wider divergence between the order of languages spoken in northern europe and the order of languages spoken in southern europe, answers to the longer time that has elapsed since their differentiation commenced. further, these two orders of modern european languages, as well as latin and greek and certain extinct and spoken languages of the east, are shown to have traits in common which unite them into one great class known as aryan languages; radically distinguished from the classes of languages spoken by the other main divisions of the human race. § . now this kind of subordination of groups which we see arises in the course of continuous descent, multiplication, and divergence, is just the kind of subordination of groups which plants and animals exhibit: it is just the kind of subordination which has thrust itself on the attention of naturalists in spite of pre-conceptions. the original idea was that of arrangement in linear order. we saw that even after a considerable acquaintance with the structures of organisms had been acquired, naturalists continued their efforts to reconcile the facts with the notion of a uni-serial succession. the accumulation of evidence necessitated the breaking up of the imagined chain into groups and sub-groups. gradually there arose the conviction that these groups do not admit of being placed in a line. and the conception finally arrived at, is that of certain great sub-kingdoms, very widely divergent, each made up of classes much less divergent, severally containing orders still less divergent; and so on with genera and species. hence this "grand fact in natural history of the subordination of group under group, which from its familiarity does not always sufficiently strike us," is perfectly in harmony with the hypothesis of evolution. the extreme significance of this kind of relation among organic forms is dwelt on by mr. darwin, who shows how an ordinary genealogical tree represents, on a small scale, a system of grouping analogous to that which exists among organisms in general, and which is explained on the supposition of a genealogical tree by which all organisms are affiliated. if, wherever we can trace direct descent, multiplication, and divergence, this formation of groups within groups takes place; there results a strong presumption that the groups within groups which constitute the animal and vegetal kingdoms, have arisen by direct descent, multiplication, and divergence--that is, by evolution. § . strong confirmation of this inference is yielded by the fact, that the more marked differences which divide groups are, in both cases, distinguished from the less marked differences which divide sub-groups, by this, that they are not simply greater in _degree_, but they are more radical in _kind_. objects, as the stars, may present themselves in small clusters, which are again more or less aggravated into clusters of clusters, in such manner that the individuals of each simple cluster are much closer together than are the simple clusters gathered into a compound cluster: in which case, the trait that unites groups of groups differs from the trait that unites groups, not in _nature_ but only in _amount_. but this is not so either with the groups and sub-groups which we know have resulted from evolution, or with those which we here infer have resulted from evolution. in both cases the highest or most general classes, are marked off from one another by fundamental differences that have no common measure with the differences that mark off small classes. observe the parallelism. we saw that each sub-kingdom of animals is distinguished from other sub-kingdoms, by some unlikeness in its main plan of organization; such as the presence or absence of a peri-visceral cavity. contrariwise, the members of the smallest groups are united together, and separated from the members of other small groups, by modifications which do not affect the relations of essential parts. that this is just the kind of arrangement which results from evolution, the case of languages will show. on comparing the dialects spoken in different parts of england, we find scarcely any difference but those of pronunciation: the structures of the sentences are almost uniform. between english and the allied modern languages there are divergences of structure: there are some unlikenesses of idiom; some unlikenesses in the ways of modifying the meanings of verbs; and considerable unlikenesses in the uses of genders. but these unlikenesses are not sufficient to hide a general community of organization. a greater contrast of structure exists between these modern languages of western europe, and the classic languages. differentiation into abstract and concrete elements, which is shown by the substitution of auxiliary words for inflections, has produced a higher specialization, distinguishing these languages as a group from the older languages. nevertheless, both the ancient and modern languages of europe, together with some eastern languages derived from the same original, have, under all their differences of organization, a fundamental likeness; since in all of them words are formed by such a coalescence and integration of roots as destroys the independent meanings of the roots. these aryan languages, and others which have the _amalgamate_ character, are united by it into a class distinguished from the _aptotic_ and _agglutinate_ languages; in which the roots are either not united at all, or so incompletely united that one of them still retains its independent meaning. and philologists find that these radical traits which severally determine the grammatical forms, or modes of combining ideas, characterize the primary divisions among languages. so that among languages, where we know that evolution has been going on, the greatest groups are marked off from one another by the strongest structural contrasts; and as the like holds among groups of organisms, there results a further reason for inferring that these have been evolved. § . there is yet another parallelism of like meaning. we saw (§ ) that the successively-subordinate groups--classes, orders, genera, and species--into which zoologists and botanists segregate animals and plants, have not, in reality, those definite values conventionally given to them. there are well-marked species, and species so imperfectly marked that some systematists regard them as varieties. between genera strong contrasts exist in many cases, and in other cases contrasts so much less decided as to leave it doubtful whether they imply generic distinctions. so, too, is it with orders and classes: in some of which there have been introduced sub-divisions, having no equivalents in others. even of the sub-kingdoms the same truth holds. the contrast between the _coelenterata_ and the _mollusca_, is far less than that between the _coelenterata_ and the _vertebrata_. now just this same indefiniteness of value, or incompleteness of equivalence, is observable in those simple and compound and re-compound groups which we see arising by evolution. in every case the endeavour to arrange the divergent products of evolution, is met by a difficulty like that which would meet the endeavour to classify the branches of a tree, into branches of the first, second, third, fourth, &c., orders--the difficulty, namely, that branches of intermediate degrees of composition exist. the illustration furnished by languages will serve us once more. some dialects of english are but little contrasted; others are strongly contrasted. the alliances of the several scandinavian tongues with one another are different in degree. dutch is much less distinct from german than swedish is; while between danish and swedish there is so close a kinship that they might almost be regarded as widely-divergent dialects. similarly on comparing the larger divisions, we see that the various languages of the aryan stock have deviated from their original to very unlike distances. the general conclusion is manifest. while the kinds of human speech fall into groups, and sub-groups, and sub-sub-groups; yet the groups are not equal to one another in value, nor have the sub-groups equal values, nor the sub-sub-groups. if, then, when classified, organisms fall into assemblages such that those of the same grade are but indefinitely equivalent; and if, where evolution is known to have taken place, there have arisen assemblages between which the equivalence is similarly indefinite; there is additional reason for inferring that organisms are products of evolution. § . a fact of much significance remains. if groups of organic forms have arisen by divergence and re-divergence; and if, while the groups have been developing from simple groups into compound groups, each group and sub-group has been giving origin to more complex forms of its own type; then it is inferable that there once existed greater structural likenesses between the members of allied groups than exists now. this, speaking generally, proves to be so. between the sub-kingdoms the gaps are extremely wide; but such distant kinships as may be discerned, bear out anticipation. thus in the formation of the germinal layers there is a general agreement among them; and there is a further agreement among sundry of them in the formation of a gastrula. this simplest and earliest likeness, significant of primitive kinship, is in most cases soon obscured by divergent modes of development; but sundry sub-kingdoms continue to show relationships by the likenesses of their larval forms; as we see in the trochophores of the _polyzoa_, _annelida_, and _mollusca_--sub-kingdoms the members of which by their later structural changes are rendered widely unlike. more decided approximations exist between the lower members of classes. in tracing down the _crustacea_ and the _arachnida_ from their more complex to their simpler forms, zoologists meet with difficulties: respecting some of these simpler forms, it becomes a question which class they belong to. the _lepidosiren_, about which there have been disputes whether it is a fish or an amphibian, is inferior, in the organization of its skeleton, to the great majority of both fishes and amphibia. widely as they differ from them, the lower mammals have some characters in common with birds, which the higher mammals do not possess. now since this kind of relationship of groups is not accounted for by any other hypothesis, while the hypothesis of evolution gives us a clue to it; we must include it among the supports of this hypothesis which the facts of classification furnish. § . what shall we say of these leading truths when taken together? that naturalists have been gradually compelled to arrange organisms in groups within groups, and that this is the arrangement which we see arises by descent, alike in individual families and among races of men, is a striking circumstance. that while the smallest groups are the most nearly related, there exist between the great sub-kingdoms, structural contrasts of the profoundest kind, cannot but impress us as remarkable, when we see that where it is known to take place evolution actually produces these feebly-distinguished small groups, and these strongly-distinguished great groups. the impression made by these two parallelisms, which add meaning to each other, is deepened by the third parallelism, which enforces the meaning of both--the parallelism, namely, that as, between the species, genera, orders, classes, &c., which naturalists have formed, there are transitional types; so between the groups, sub-groups, and sub-sub-groups, which we know to have been evolved, types of intermediate values exist. and these three correspondences between the known results of evolution and the results here ascribed to evolution, have further weight given to them by the fact, that the kinship of groups through their lowest members is just the kinship which the hypothesis of evolution implies. even in the absence of these specific agreements, the broad fact of unity amid multiformity, which organisms so strikingly display, is strongly suggestive of evolution. freeing ourselves from pre-conceptions, we shall see good reason to think with mr. darwin, "that propinquity of descent--the only known cause of the similarity of organic beings--is the bond, hidden as it is by various degrees of modification, which is partly revealed to us by our classifications." when we consider that this only known cause of similarity, joined with the only known cause of divergence (the influence of conditions), gives us a key to these likenesses obscured by unlikenesses; we shall see that were there none of those remarkable harmonies above pointed out, the truths of classification would still yield strong support to our conclusion. chapter v. the arguments from embryology. § a. already i have emphasized the truth that nature is always more complex than we suppose (§ a)--that there are complexities within complexities. here we find illustrated this truth under another aspect. when seeking to formulate the arguments from embryology, we are shown that the facts as presented in nature are not to be expressed in the simple generalizations we at first make. while we recognize this truth we must also recognize the truth that only by enunciation and acceptance of imperfect generalizations can we progress to perfect ones. the order of evolution is conformed to by ideas as by other things. the advance is, and must be, from the indefinite to the definite. it is impossible to express the totality of any natural phenomenon in a single proposition. to the primary statement expressing that which is most dominant have to be added secondary statements qualifying it. we see this even in so simple a case as the flight of a projectile. the young artillery officer is first taught that a cannon-shot describes a curve treated as a parabola, though literally part of an extremely eccentric ellipse not distinguishable from a parabola. presently he learns that atmospheric resistance, causing a continual decrease of velocity, entails a deviation from that theoretical path which is calculated on the supposition that the velocity is uniform; and this incorrectness he has to allow for. then, further, there comes the lateral deviation due to wind, which may be appreciable if the wind is strong and the range great. to introduce him all at once to the correct conception thus finally reached would be impossible: it has to be reached through successive qualifications. and that which holds even in this simple case necessarily holds more conspicuously in complex cases. the title of the chapter suggests a metaphor, which is, indeed, something more than a metaphor. there is an embryology of conceptions. that this statement is not wholly a figure of speech, we shall see on considering that cerebral organization is a part of organization at large; and that the evolving nervous plexus which is the correlative of an evolving conception, must conform to the general law of change conformed to in the evolution of the whole nervous structure as well as in the evolution of the whole bodily structure. as the body has at first a rude form, very remotely suggesting that which is presently developed by the superposing of modifications on modifications; so the brain as a whole and its contained ideas together make up an inner world answering with extreme indefiniteness to that outer world to which it is brought by successive approximations into tolerable correspondence; and so any nervous plexus and its associated hypothesis, which refer to some external group of phenomena under investigation, have to reach their final developments by successive corrections. this being the course of discovery must also be the course of exposition. in pursuance of this course we may therefore fitly contemplate that early _formula_ of embryological development which we owe to von baer. § . already in § , where the generalization of von baer respecting the relations of embryos was set forth, there was given the warning, above repeated with greater distinctness, that it is only an adumbration. in the words of his translator, he "found that in its earliest stage, every organism has the greatest number of characters in common with all other organisms in their earliest stages; that at a stage somewhat later, its structure is like the structures displayed at corresponding phases by a less extensive multitude of organisms; that at each subsequent stage, traits are acquired which successively distinguished the developing embryo from groups of embryos that it previously resembled--thus step by step diminishing the class of embryos which it still resembles; and that thus the class of similar forms is finally narrowed to the species of which it is a member." assuming for a moment that this generalization is true as it stands, or rather, assuming that the qualifications needed are not such as destroy its correspondence with the average facts, we shall see that it has profound significance. for if we follow out in thought the implications--if we conceive the germs of all kinds of organisms simultaneously developing, and imagine that after taking their first step together, at the second step one half of the vast multitude diverges from the other half; if, at the next step, we mentally watch the parts of each great assemblage beginning to take two or more routes of development; if we represent to ourselves such bifurcations going on, stage after stage, in all the branches; we shall see that there must result an aggregate analogous, in its arrangement of parts, to a tree. if this vast genealogical tree be contemplated as a whole, made up of trunk, main branches, secondary branches, and so on as far as the terminal twigs; it will be perceived that all the various kinds of organisms represented by these terminal twigs, forming the periphery of the tree, will stand related to one another in small groups, which are united into groups of groups, and so on. the embryological tree, expressing the developmental relations of organisms, will be similar to the tree which symbolizes their classificatory relations. that subordination of classes, orders, genera, and species, to which naturalists have been gradually led, is just that subordination which results from the divergence and re-divergence of embryos, as they all unfold. on the hypothesis of evolution this parallelism has a meaning--indicates that primordial kinship of all organisms, and that progressive differentiation of them, which the hypothesis alleges. but on any other hypothesis the parallelism is meaningless; or rather, it raises a difficulty; since it implies either an effect without a cause or a design without a purpose. § . this conception of a tree, symbolizing the relationships of types and a species derived from the same root, has a concomitant conception. the implication is that each organism, setting out from the simple nucleated cell, must in the course of its development follow the line of the trunk, some main branch, some sub-branch, some sub-sub-branch, &c., of this embryological tree; and so on till it reaches that ultimate twig representing the species of which it is a member. it must in a general way go through the particular line of forms which preceded it in all past times: there must be what has been aptly called a "recapitulation" of the successive ancestral structures. this, at least, is the conclusion necessitated by the generalization we are considering under its original crude form. von baer lived in the days when the development hypothesis was mentioned only to be ridiculed, and he joined in the ridicule. what he conceived to be the meaning of these groupings of organisms and these relations among their embryological histories, is not obvious. the only alternative to the hypothesis of evolution is the hypothesis of special creation; and as he did not accept the one it is inferable that he accepted the other. but if he did this he must in the first place have found no answer to the inquiry why organisms specially created should have the embryological kinships he described. and in the second place, after discovering that his alleged law was traversed by many and various nonconformities, he would have been without any explanation of these. observe the positions which were open to him and the reasons which show them to be untenable. if it be said that the conditions of the case necessitated the derivation of all organisms from simple germs, and therefore necessitated a morphological unity in their primitive states; there arises the obvious answer, that the morphological unity thus implied, is not the only morphological unity to be accounted for. were this the only unity, the various kinds of organisms, setting out from a common primordial form, should all begin from the first to diverge individually, as so many radii from a centre; which they do not. if, otherwise, it be said that organisms were framed upon certain types, and that those of the same type continue developing together in the same direction, until it is time for them to begin putting on their specialities of structure; then the answer is, that when they do finally diverge they ought severally to develop in direct lines towards their final forms. no reason can be assigned why, having parted company, some should progress towards their final forms by irregular or circuitous routes. on the hypothesis of design such deviations are inexplicable. the hypothesis of evolution, however, while it pre-supposes those kinships among embryos in their early forms which are found to exist, also leads us to expect nonconformities in their courses of development. if, as any rational theory of evolution implies, the progressive differentiations of types from one another during past times, have resulted from the direct and indirect effects of external conditions--if races of organisms have become different, either by immediate adaptations to unlike habits of life, or by the mediate adaptations resulting from preservation of the individuals most fitted for such habits of life, or by both; and if most embryonic changes are significant of changes that were undergone by ancestral races; then these irregularities must be anticipated. for the successive changes in modes of life pursued by successive ancestral races, can have had no regularity of sequence. in some cases they must have been more numerous than in others; in some cases they must have been greater in degree than in others; in some cases they must have been to simpler modes, in some cases to more complex modes, and in some cases to modes neither higher nor lower. of two cognate races which diverged in the remote past, the one may have had descendants that have remained tolerably constant in their habits, while the other may have had descendants that have passed through widely-aberrant modes of life; and yet some of these last may have eventually taken to modes of life like those of the other races derived from the same stock. and if the metamorphoses of embryos indicate, in a general way, the changes of structure undergone by ancestors; then, the later embryologic changes of such two allied races will be somewhat different, though they may end in very similar forms. an illustration will make this clear. mr. darwin says: "petrels are the most aërial and oceanic of birds, but in the quiet sounds of tierra del fuego, the _puffinuria berardi_, in its general habits, in its astonishing power of diving, its manner of swimming, and of flying when unwillingly it takes flight, would be mistaken by any one for an auk or grebe; nevertheless, it is essentially a petrel, but with many parts of its organization profoundly modified." now if we suppose these grebe-like habits to be continued through a long epoch, the petrel-form to be still more obscured, and the approximation to the grebe-form still closer; it is manifest that while the chicks of the grebe and the _puffinuria_ will, during their early stages of development, display that likeness involved by their common derivation from some early type of bird, the chick of the _puffinuria_ will eventually begin to show deviations, representative of the ancestral petrel-structure, and will afterwards begin to lose these distinctions and assume the grebe-structure. hence, remembering the perpetual intrusions of organisms on one another's modes of life, often widely different; and remembering that these intrusions have been going on from the beginning; we shall be prepared to find that the general law of embryonic parallelism is qualified by irregularities which are mostly small, in many cases considerable, and occasionally great. the hypothesis of evolution accounts for these: it does more--it implies the necessity of them. § . the substitutions of organs and the suppressions of organs, are among those secondary embryological phenomena which harmonize with the belief in evolution but cannot be reconciled with any other belief. some embryos, during early stages of development, possess organs that afterwards dwindle away, as there arise other organs to discharge the same functions. and in other embryos organs make their appearance, grow to certain points, have no functions to discharge, and disappear by absorption. we have a remarkable instance of substitution in the temporary appliances for respiration, which some embryos exhibit. during the first phase of its development, the mammalian embryo possesses a system of blood-vessels distributed over what is called the _area vasculosa_--a system of vessels homologous with one which, among fishes, serves for aërating the blood until the permanent respiratory organs come into play. now since this system of blood-vessels, not being in proximity to an oxygenated medium, cannot be serviceable to the mammalian embryo during development of the lungs, as it is serviceable in the embryo-fish during development of the gills, this needless formation of it is unaccountable as a result of design. but it is quite congruous with the supposition that the mammalian type arose out of lower vertebrate types. for in such case the mammalian embryo, passing through states representing in a general way those which its remote ancestors had in common with the lower _vertebrata_, develops this system of vessels in like manner with them. an instance more significant still is furnished by certain _amphibia_. one of the facts early made familiar to the natural-history student is that the tadpole breathes by external branchiæ, and that these, needful during its aquatic life, dwindle away as fast as it develops the lungs fitting it for terrestrial life. but in one of the higher _amphibia_, the viviparous salamander, these transformations ordinarily undergone during the free life of the larva, are undergone by the embryo in the egg. the branchiæ are developed though there is no use for them: lungs being substituted as breathing appliances before the creature is born. even more striking than the substitutions of organs are the suppressions of organs. mr. darwin names some cases as "extremely curious; for instance, the presence of teeth in foetal whales, which when grown up have not a tooth in their heads;... it has even been stated on good authority that rudiments of teeth can be detected in the beaks of certain embryonic birds." irreconcilable with any teleological theory, these facts do not even harmonize with the theory of fixed types which are maintained by the development of all the typical parts, even where not wanted; seeing that the disappearance of these incipient organs during foetal life spoils the typical resemblance. but while to other hypotheses these facts are stumbling-blocks, they yield strong support to the hypothesis of evolution. allied to these cases, are the cases of what has been called retrograde development. many parasitic creatures and creatures which, after leading active lives for a time, become fixed, lose, in their adult states, the limbs and senses they had when young. it may be alleged, however, that these creatures could not secure the habitats needful for them, without possessing, during their larval stages, eyes and swimming appendages which eventually become useless; that though, by losing these, their organization retrogresses in one direction, it progresses in another direction; and that, therefore, they do not exhibit the needless development of a higher type on the way to a lower type. nevertheless there are instances of a descent in organization, following an apparently-superfluous ascent. mr. darwin says that in some genera of cirripedes, "the larvæ become developed either into hermaphrodites having the ordinary structure, or into what i have called complemental males, and in the latter, the development has assuredly been retrograde; for the male is a mere sack, which lives for a short time, and is destitute of mouth, stomach, or other organ of importance, excepting for reproduction." § a. but now let us contemplate more closely the energies at work in the unfolding embryo, or rather the energies which the facts appear to imply. whatever natures we ascribe to the hypothetical units proper to each kind of organism, we must conclude that from the beginning of embryonic development, they have a proclivity towards the structure of that organism. because of their phylogenetic origin, they must tend towards the form of the primitive type; but the superposed modifications, conflicting with their initial tendency, must cause a swerving towards each successively higher type. to take an illustration:--if in the germ-plasm out of which will come a vertebrate animal there is a proclivity towards the primitive piscine form, there must, if the germ-plasm is derived from a mammal, be also from the outset a proclivity towards the mammalian form. while the initial type tends continually to establish itself the terminal type tends also to establish itself. the intermediate structures must be influenced by their conflict, as well as by the conflict of each with the proclivities towards the amphibian and reptilian types. this complication of tendencies is increased by the intervention of several other factors. there is the factor of economy. an embryo in which the transformations have absorbed the smallest amount of energy and wasted the smallest amount of matter, will have an advantage over embryos the transformations of which have cost more in energy and matter: the young animal will set out with a greater surplus of vitality, and will be more likely than others to live and propagate. again, in the embryos of its descendants, inheriting the tendency to economical transformation, those which evolve at the least cost will thrive more than the rest and be more likely to have posterity. thus will result a continual shortening of the processes. we can see alike that this must take place and that it does take place. if the whole series of phylogenetic changes had to be repeated--if the embryo mammal had to become a complete fish, and then a complete amphibian, and then a complete reptile, there would be an immense amount of superfluous building up and pulling down, entailing great waste of time and of materials. evidently these abridgments which economy entails, necessitate that unfolding embryos bear but rude resemblances to lower types ancestrally passed through--vaguely represent their dominant traits only. from this principle of economy arise several derivative principles, which may be best dealt with separately. § b. in some cases the substitution of an abridged for an unabridged course of evolution causes the entire disappearance of certain intermediate forms. structural arrangements once passed through during the unfolding are dropped out of the series. in the evolution of these embryos with which there is not laid up a large amount of food-yolk there occurs at the outset a striking omission of this kind. when, by successive fissions, the fertilized cell has given rise to a cluster of cells constituting a hollow sphere, known as a _blastula_, the next change under its original form is the introversion of one side, so as to produce two layers in place of one. an idea of the change may be obtained by taking an india-rubber ball (having a hole through which the air may escape) and thrusting in one side until its anterior surface touches the interior surface of the other side. if the cup-shaped structure resulting be supposed to have its wide opening gradually narrowed, until it becomes the mouth of an internal chamber, it will represent what is known as a _gastrula_--a double layer of cells, of which the outer is called epiblast and the inner hypoblast (answering to ectoderm and endoderm) inclosing a cavity known as the _archenteron_, or primitive digestive sac. but now in place of this original mode of forming the _gastrula_, there occurs a mode known as delamination. throughout its whole extent the single layer splits so as to become a double layer--one sphere of cells inclosing the other; and after this direct formation of the double layer there is a direct formation of an opening through it into the internal cavity. there is thus a shortening of the primitive process: a number of changes are left out. often a kindred passing over of stages at later periods of development may be observed. in certain of the _mollusca_, as the _patella chiton_, the egg gives origin to a free-swimming larva known as a trochosphere, from which presently comes the ordinary molluscous organization. in the highest division of the molluscs, however, the cephalopods, no trochosphere is formed. the nutritive matter laid up in the egg is used in building up the young animal without any indication of an ancestral larva. § c. among principles derived from the principle of economy is the principle of pre-adaptation--a name which we may appropriately coin to indicate an adaptation made in advance of the time at which it could have arisen in the course of phylogenetic history. how pre-adaptation may result from economy will be shown by an illustration which human methods of construction furnish. let us assume that building houses of a certain type has become an established habit, and that, as a part of each house, there is a staircase of given size. and suppose that in consequence of changed conditions--say the walling in of the town, limiting the internal space and increasing ground-rents--it becomes the policy to build houses of many stories, let out in flats to different tenants. for the increased passing up and down, a staircase wider at its lower part will be required. if now the builder, when putting up the ground floor, follows the old dimensions, then after all the stories are built, the lower part of the staircase, if it is to yield equal facilities for passage, must be reconstructed. instead of a staircase adapted to those few stories which the original type of house had, economy will dictate a pre-adaptation of the staircase to the additional stories. on carrying this idea with us, we shall see that if from some type of organism there is evolved a type in which enlargement of a certain part is needed to meet increased functions, the greater size of this part will begin to show itself during early stages of unfolding. that unbuilding and rebuilding which would be needful were it laid down of its original size, will be made needless if from the beginning it is laid down of a larger size. hence, in successive generations, the greater prosperity and multiplication of individuals in which this part is at the outset somewhat larger than usual, must eventually establish a marked excess in its development at an early stage. the facts agree with this inference. referring to the contrasts between embryos, mr. adam sedgwick says that "a species is distinct and distinguishable from its allies from the very earliest stages." whereas, according to the law of von baer, "animals so closely allied as the fowl and duck would be indistinguishable in the early stages of development," "yet i can distinguish a fowl and a duck embryo on the second day by the inspection of a single transverse section through the trunk." this experience harmonizes with the statement of the late prof. agassiz, that in some cases traits characterizing the species appear at an earlier period than traits characterizing the genus. similar in their implications are the facts recently published by dr. e. mehnert, concerning the feet of pentadactyle vertebrates. a leading example is furnished by the foot in the struthious birds. out of the original five digits the two which eventually become large while the others disappear, soon give sign of their future predominance: their early sizes being in excess of those required for the usual functional requirements in birds, and preparing the way for their special requirements in the struthious birds. dr. mehnert shows that a like lesson is given by the relative developments of legs and wings in these birds. ordinarily in vertebrates the fore limbs grow more rapidly than the hind limbs; but in the ostrich, in which the hind limbs or legs have to become so large while the wings are but little wanted, the leg development goes in advance of the wing-development in early embryonic stages: there is a pre-adaptation. much more striking are examples furnished by creatures whose modes of existence require that they shall have enormous fertility--require that the generative system shall be very large. ordinarily the organs devoted to maintenance of the race develop later than the organs devoted to maintenance of the individual. but this order is inverted in certain _entozoa_. to these creatures, imbedded in nutritive matters, self-maintenance cost nothing, and the structures devoted to it are relatively of less importance than the structures devoted to race-maintenance, which, to make up for the small chance any one germ has of getting into a fit habitat, have to produce immense numbers of germs. here the rudiments of the generative systems are the first to become visible--here, in virtue of the principle of pre-adaptation, a structure belonging to the terminal form asserts itself so early in the developmental process as almost to obliterate the structure of the initial form. it may be that in some cases where the growth of certain organs goes in advance of the normal order, the element of time comes into play--the greater time required for construction. to elucidate this let us revert to our simile. suppose that the staircase above instanced, or at any rate its lower part, is required to be of marble with balusters finely carved. if this piece of work is not promptly commenced and pushed on fast, it will not be completed when the rest of the house is ready: workmen and tools will still block it up at a time when it should be available. similarly among the parts of an unfolding embryo, those in which there is a great deal of constructive work must early take such shape as will allow of this. now of all the tissues the nervous tissue is that which takes longest to repair when injured; and it seems a not improbable inference that it is a tissue which is slower in its histological development than others. if this be so, we may see why, in the embryos of the higher vertebrates, the central nervous system quickly grows large in comparison to the other systems--why by pre-adaptation the brain of a chick develops in advance of other organs so much more than the brain of a fish. § d. yet another complication has to be noted. from the principle of economy, it seems inferable that decrease and disappearance of organs which were useful in ancestral types but have ceased to be useful, should take place uniformly; but they do not. in the words of mr. adam sedgwick, "some ancestral organs persist in the embryo in a functionless rudimentary (vestigial) condition and at the same time without any reference to adult structures, while other ancestral organs have disappeared without leaving a trace."[ ] this anomaly is rendered more striking when joined with the fact that some of the structures which remain conspicuous are relatively ancient, while some which have been obliterated are relatively modern--_e. g._, "gill slits [which date back to the fish-ancestor], have been retained in embryology, whereas other organs which have much more recently disappeared, _e. g._ teeth of birds, fore-limbs of snakes [dating back to the reptile ancestor], have been entirely lost."[ ] mr. sedgwick ascribes these anomalies to the difference between larval development and embryonic development, and expresses his general belief thus:-- "the conclusion here reached is that, whereas larval development must retain traces (it may be very faint) of ancestral stages of structure because they are built out of ancestral stages, embryonic development need not necessarily do so, and very often does not; that embryonic development in so far as it is a record at all, is a record of structural features of previous larval stages. characters which disappear during free life disappear also in the embryo, but characters which though lost by the adult are retained in the larva may ultimately be absorbed into the embryonic phase and leave their traces in embryonic development."[ ] to set forth the evidence justifying this view would encumber too much the general argument. towards elucidation of such irregularities let me name two factors which should i think be taken into account. abridgment of embryonic stages cannot go on uniformly with all disused organs. where an organ is of such size that progressive diminution of it will appreciably profit the young animal, by leaving it a larger surplus of unused material, we may expect progressive diminution to occur. contrariwise, if the organ is relatively so small that each decrease will not, by sensibly increasing the reserve of nutriment, give the young animal an advantage over others, decrease must not be looked for: there may be a survival of it even though of very ancient origin. again, the reduction of a superfluous part can take place only on condition that the economy resulting from each descending variation of it, is of greater importance than are the effects of variations simultaneously occurring in other parts. if by increase or decrease of any other parts of the embryo, survival of the animal is furthered in a greater degree than by decrease of this superfluous part, then such decrease is unlikely; since it is illegitimate to count upon the repeated concurrence of favourable variations in two or more parts which are independent. so that if changes of an advantageous kind are going on elsewhere in the embryo a useless part may remain long undiminished. yet another cause operates, and perhaps cooperates. embryonic survival of an organ which has become functionless, may readily happen if, during subsequent stages of development, parts of it are utilized as parts of other organs. in the words of mr. j. t. cunningham:-- "it seems to be a general fact that a structure which in metamorphosis disappears completely may easily be omitted altogether in embryonic development, while one which is modified into something else continues to pass more or less through its original larval condition." (_science progress_, july, , p. .) one more factor of considerable importance should be taken into account. a disused organ which entails evil because construction of it involves needless cost, may entail further evil by being in the way. this, it seems to me, is the reason why the fore-limbs of snakes have disappeared from their embryos. when the long-bodied lizard out of which the ophidian type evolved, crept through stiff herbage, and moved its head from side to side to find openings, there resulted alternate bends of its body, which were the beginnings of lateral undulations; and we may easily see that in proportion as it thus progressed by insinuating itself through interstices, the fore-limbs, less and less used for walking, would be more and more in the way; and the lengthening of the body, increasing the undulatory motion and decreasing the use of the fore-limbs, would eventually make them absolute impediments. hence besides the benefit in economy of construction gained by embryos in which the fore-limbs were in early stages a little less developed than usual, they would gain an advantage by having, when mature, smaller fore-limbs than usual, leading to greater facility of locomotion. there would be a double set of influences causing, through selection, a comparatively rapid decrease of these appendages. and we may i think see also, on contemplating the kind of movement, that the fore-limbs would be more in the way than the hind limbs, which would consequently dwindle with such smaller rapidity as to make continuance of the rudiments of them comprehensible. § - . so that while the embryonic law enunciated by von baer is in harmony with the hypothesis of evolution, and is, indeed, a law which this hypothesis implies, the nonconformities to the law are also interpretable by this hypothesis. parallelism between the courses of development in species allied by remote ancestry, is liable to be variously modified in correspondence with the later ancestral forms passed through after divergence of such species. the substitution of a direct for an indirect process of formation, which we have reason to believe will show itself, must obscure the embryonic history. and the principle of economy which leads to this substitution produces effects that are very irregular and uncertain in consequence of the endlessly varied conditions. thus several causes conspire to produce deviations from the general law. let it be remarked, finally, that the ability to trace out embryologic kinships and the inability to do this, occur just where, according to the hypothesis of evolution, they should occur. we saw in § a that zoologists are agreed in grouping animals into some phyla--_mollusca_, _arthropoda_, _echinodermata_, &c.--each of which includes a number of classes severally sub-divided into orders, genera, species. all the members of each phylum are so related embryologically, that the existence of a common ancestor of them in the remote past is considered certain. but when it comes to the relations among the archaic ancestors, opinion is unsettled. whether, for instance, the primitive _chordata_, out of which the _vertebrata_ emerged, have molluscan affinities or annelidan affinities, is still a matter in dispute. with regard to the origins of various other types no settled conclusions are held. now it is clear that on tracing down each branch of the great genealogical tree, kinships would be much more manifest among the recently-differentiated forms than among those forms which diverged from one another in the earliest stages of organic life, and had separated widely before any of the types we now know had come into existence. chapter vi. the arguments from morphology. § . leaving out of consideration those parallelisms among their modes of development which characterize organisms belonging to each group, that community of plan which exists among them when mature is extremely remarkable and extremely suggestive. as before shown (§ ), neither the supposition that these combinations of attributes which unite classes are fortuitous, nor the supposition that no other combinations were practicable, nor the supposition of adherence to pre-determined typical plans, suffices to explain the facts. an instance will best prepare the reader for seeing the true meaning of these fundamental likenesses. under the immensely-varied forms of insects, greatly elongated like the dragon-fly or contracted in shape like the lady-bird, winged like the butterfly or wingless like the flea, we find this character in common--there are primarily seventeen segments.[ ] these segments may be distinctly marked or they may be so fused as to make it difficult to find the divisions between them, but they always exist. what now can be the meaning of this community of structure throughout the hundred thousand kinds of insects filling the air, burrowing in the earth, swimming in the water? why under the down-covered body of a moth and under the hard wing-cases of a beetle, should there be discovered the same number of divisions? why should there be no more somites in the stick-insect, or other phasmid a foot long, than there are in a small creature like the louse? why should the inert _aphis_ and the swift-flying emperor-butterfly be constructed on the same fundamental plan? it cannot be by chance that there exist equal numbers of segments in all these multitudes of species. there is no reason to think it was _necessary_, in the sense that no other number would have made a possible organism. and to say that it is the result of _design_--to say that the creator followed this pattern throughout, merely for the purpose of maintaining the pattern--is to assign an absurd motive. no rational interpretation of these and countless like morphological facts, can be given except by the hypothesis of evolution; and from the hypothesis of evolution they are corollaries. if organic forms have arisen from common stocks by perpetual divergences and re-divergences--if they have continued to inherit, more or less clearly, the characters of ancestral races; then there will naturally result these communities of fundamental structure among creatures which have severally become modified in multitudinous ways and degrees, in adaptation to their respective modes of life. to this let it be added that while the belief in an intentional adhesion to a pre-determined pattern throughout a whole group, is negatived by the occurrence of occasional deviations from the pattern; such deviations are reconcilable with the belief in evolution. as pointed out in the last chapter, ancestral traits will be obscured more or less according as the superposed modifications of structure, have or have not been furthered by the conditions of life and development to which the type has been subjected. § . besides these wide-embracing and often deeply-hidden homologies, which hold together different animals, there are the scarcely-less significant homologies between different organs of the same animal. these, like the others, are obstacles to the supernatural interpretations and supports of the natural interpretation. one of the most familiar and instructive examples is furnished by the vertebral column. snakes, which move sinuously through and over plants and stones, obviously need a segmentation of the bony axis from end to end; and inasmuch as flexibility is required throughout the whole length of the body, there is advantage in the comparative uniformity of this segmentation. the movements would be impeded if, instead of a chain of vertebræ varying but little in their lengths, there existed in the middle of the series some long bony mass that would not bend. but in the higher _vertebrata_, the mechanical actions and reactions demand that while some parts of the vertebral column shall be flexible, other parts shall be inflexible. inflexibility is specially requisite in that part of it called the sacrum; which, in mammals and birds, forms a fulcrum exposed to the greatest strains the skeleton has to bear. now in both mammals and birds, this rigid portion of the vertebral column is not made of one long segment or vertebra, but of several segments fused together. in man there are five of these confluent sacral vertebræ; and in the ostrich tribe they number from seventeen to twenty. why is this? why, if the skeleton of each species was separately contrived, was this bony mass made by soldering together a number of vertebræ like those forming the rest of the column, instead of being made out of one single piece? and why, if typical uniformity was to be maintained, does the number of sacral vertebræ vary within the same order of birds? why, too, should the development of the sacrum be by the round-about process of first forming its separate constituent vertebræ, and then destroying their separateness? in the embryo of a mammal or bird, the central element of the vertebral column is, at the outset, continuous. the segments that are to become vertebræ, arise gradually in the adjacent mesoderm, and enwrap this originally-homogeneous axis or notochord. equally in those parts of the spine which are to remain flexible, and in those parts which are to grow rigid, these segments are formed; and that part of the spine which is to compose the sacrum, having acquired this segmental structure, loses it again by coalescence of the segments. to what end is this construction and re-construction? if, originally, the spine in vertebrate animals consisted from head to tail of separate moveable segments, as it does still in fishes and some reptiles--if, in the evolution of the higher _vertebrata_, certain of these moveable segments were rendered less moveable with respect to one another, by the mechanical conditions they were exposed to, and at length became relatively immovable; it is comprehensible why the sacrum formed out of them, should continue ever after to show its originally-segmented structure. but on any other hypothesis this segmented structure is inexplicable. "we see the same law in comparing the wonderfully complex jaws and legs in crustaceans," says mr. darwin: referring to the fact that those numerous lateral appendages which, in the lower crustaceans, most of them serve as legs, and have like shapes, are, in the higher crustaceans, some of them represented by enormously-developed claws, and others by variously-modified foot-jaws. "it is familiar to almost every one," he continues, "that in a flower the relative position of the sepals, petals, stamens, and pistils, as well as their intimate structure, are intelligible on the view that they consist of metamorphosed leaves arranged in a spire. in monstrous plants we often get direct evidence of the possibility of one organ being transformed into another; and we can actually see in embryonic crustaceans and in many other animals, and in flowers, that organs, which when mature become extremely different, are at an early stage of growth exactly alike." ... "why should one crustacean, which has an extremely complex mouth formed of many parts consequently always have fewer legs; or conversely, those with many legs have simpler mouths? why should the sepals, petals, stamens, and pistils in any individual flower, though fitted for such widely-different purposes, be all constructed on the same pattern?" to these and countless similar questions, the theory of evolution furnishes the only rational answer. in the course of that change from homogeneity to heterogeneity of structure displayed in evolution under every form, it will necessarily happen that from organisms made up of numerous like parts, there will arise organisms made up of parts more and more unlike: which unlike parts will nevertheless continue to bear traces of their primitive likeness. § . one more striking morphological fact, near akin to some of the facts dwelt on in the last chapter, must be here set down--the frequent occurrence, in adult animals and plants, of rudimentary and useless organs, which are homologous with organs that are developed and useful in allied animals and plants. in the last chapter we saw that during the development of embryos, there often arise organs which disappear on being replaced by other organs discharging the same functions in better ways; and that in some cases, organs develop to certain points and are then re-absorbed without performing any functions. very generally, however, the partially-developed organs are retained throughout life. the osteology of the higher _vertebrata_ supplies abundant examples. vertebral processes which, in one tribe, are fully formed and ossified from independent centres, are, in other tribes, mere tubercles not having independent centres of ossification. while in the tail of this animal the vertebræ are severally composed of centrum and appendages, in the tail of that animal they are simple osseous masses without any appendages; and in another animal they have lost their individualities by coalescence with neighbouring vertebræ into a rudimentary tail. from the structures of the limbs analogous facts are cited by comparative anatomists. the undeveloped state of certain metacarpal bones, characterizes whole groups of mammals. in one case we find the normal number of digits; and, in another case, a smaller number with an atrophied digit to make out the complement. here is a digit with its full number of phalanges; and there a digit of which one phalange has been arrested in its growth. still more remarkable are the instances of entire limbs being rudimentary; as in certain snakes, which have hind legs hidden beneath the integument. so, too, is it with dermal appendages. some of the smooth-skinned amphibia have scales buried in the skin. the seal, which is a mammal considerably modified in adaptation to an aquatic life, and which uses its feet mainly as paddles, has toes that still bear external nails; but the manatee, which is a much more transformed mammal, has nailless paddles which, when the skin is removed, are said, by humboldt, to display rudimentary nails at the ends of the imbedded digits. nearly all birds are covered with developed feathers, severally composed of a shaft bearing fibres, each of which, again, bears a fringe of down. but in some birds, as in the ostrich, various stages of arrested development of the feathers may be traced: between the unusually-elaborated feathers of the tail, and those about the beak which are reduced to simple hairs, there are transitions. nor is this the extreme case. in the _apteryx_ we see the whole of the feathers reduced to a hair-like form. again, the hair which commonly covers the body in mammals is, over the greater part of the human body almost rudimentary, and is in some parts reduced to mere down--down which nevertheless proves itself to be homologous with the hair of mammals in general, by occasionally developing into the original form. numerous cases of aborted organs are given by mr. darwin, of which a few may be here added. "nothing can be plainer," he remarks, "than that wings are formed for flight, yet in how many insects do we see wings so reduced in size as to be utterly incapable of flight, and not rarely lying under wing-cases, firmly soldered together?" ... "in plants with separated sexes, the male flowers often have a rudiment of a pistil; and kölreuter found that by crossing such male plants with an hermaphrodite species, the rudiment of the pistil in the hybrid offspring was much increased in size; and this shows that the rudiment and the perfect pistil are essentially alike in nature." and then, to complete the proof that these undeveloped parts are marks of descent from races in which they were developed, there are not a few direct experiences of this relation. "we have plenty of cases of rudimentary organs in our domestic productions--as the stump of a tail in tailless breeds--the vestige of an ear in earless breeds--the re-appearance of minute dangling horns in hornless breeds of cattle." (_origin of species_, , pp. , .) here, as before, the teleological doctrine fails utterly; for these rudimentary organs are useless, and occasionally even detrimental; as is the _appendix vermiformis_, in man--a part of the cæcum which is of no value for the purpose of absorption but which, by detaining small foreign bodies, often causes severe inflammation and death. the doctrine of typical plans is equally out of court; for while, in some members of a group, rudimentary organs completing the general type are traceable, in other members of the same group such organs are unrepresented. there remains only the doctrine of evolution; and to this, these rudimentary organs offer no difficulties. on the contrary, they are among its most striking evidences. § . the general truths of morphology thus coincide in their implications. unity of type, maintained under extreme dissimilarities of form and mode of life, is explicable as resulting from descent with modification; but is otherwise inexplicable. the likenesses disguised by unlikenesses, which the comparative anatomist discovers between various organs in the same organism, are worse than meaningless if it be supposed that organisms were severally framed as we now see them; but they fit in quite harmoniously with the belief that each kind of organism is a product of accumulated modifications upon modifications. and the presence, in all kinds of animals and plants, of functionally-useless parts corresponding to parts that are functionally-useful in allied animals and plants, while it is totally incongruous with the belief in a construction of each organism by miraculous interposition, is just what we are led to expect by the belief that organisms have arisen by progression. chapter vii. the arguments from distribution. § . in §§ and , we contemplated the phenomena of distribution in space. the general conclusions reached, in great part based on the evidence brought together by mr. darwin, were that, "on the one hand, we have similarly-conditioned, and sometimes nearly-adjacent, areas, occupied by quite different faunas. on the other hand, we have areas remote from each other in latitude, and contrasted in soil as well as climate, which are occupied by closely-allied faunas." whence it was inferred that "as like organisms are not universally, or even generally, found in like habitats; nor very unlike organisms, in very unlike habitats; there is no manifest pre-determined adaptation of the organisms to the habitats." in other words, the facts of distribution in space do not conform to the hypothesis of design. at the same time we saw that "the similar areas peopled by dissimilar forms, are those between which there are impassable barriers; while the dissimilar areas peopled by similar forms, are those between which there are no such barriers;" and these generalizations appeared to harmonize with the abundantly-illustrated truth, "that each species of organism tends ever to expand its sphere of existence--to intrude on other areas, other modes of life, other media." by way of showing still more clearly the effects of competition among races of organisms, let me here add some recently-published instances of the usurpations of areas, and changes of distribution hence resulting. in the _natural history review_ for january, , dr. hooker quotes as follows from some new zealand naturalists:--"you would be surprised at the rapid spread of european and other foreign plants in this country. all along the sides of the main lines of road through the plains, a _polygonum_ (_aviculare_), called 'cow grass,' grows most luxuriantly, the roots sometimes two feet in depth, and the plants spreading over an area from four to five feet in diameter. the dock (_rumex obtusifolius_ or _r. crispus_) is to be found in every river bed, extending into the valleys of the mountain rivers, until these become mere torrents. the sow-thistle is spread all over the country, growing luxuriantly nearly up to feet. the water-cress increases in our still rivers to such an extent, as to threaten to choke them altogether ... i have measured stems twelve feet long and three-quarters of an inch in diameter. in some of the mountain districts, where the soil is loose, the white clover is completely displacing the native grasses, forming a close sward.... in fact, the young native vegetation appears to shrink from competition with these more vigorous intruders." "the native (maori) saying is 'as the white man's rat has driven away the native rat, so the european fly drives away our own, and the clover kills our fern, so will the maoris disappear before the white man himself.'" given this universal tendency of the superior to overrun the habitats of the inferior,[ ] let us consider what, on the hypothesis of evolution, will be the effects on the geographical relationships of species. § . a race of organisms cannot expand its sphere of existence without subjecting itself to new external conditions. those of its members which spread over adjacent areas, inevitably come in contact with circumstances partially different from their previous circumstances; and such of them as adopt the habits of other organisms, necessarily experience re-actions more or less contrasted with the re-actions before experienced. now if changes of organic structure are caused, directly or indirectly, by changes in the incidence of forces; there must result unlikenesses of structure between the divisions of a race which colonizes new habitats. hence, in the absence of obstacles to migration, we may anticipate manifest kinships between the animals and plants of one area, and those of areas adjoining it. this inference corresponds with an induction before set down (§ ). in addition to illustrations of it already quoted from mr. darwin, his pages furnish others. one is that species which inhabit islands are allied to species which inhabit neighbouring main lands; and another is that the faunas of clustered islands show marked similarities. "thus the several islands of the galapagos archipelago are tenanted," says mr. darwin, "in a quite marvellous manner, by very closely related species; so that the inhabitants of each separate island, though mostly distinct, are related in an incomparably closer degree to each other than to the inhabitants of any other part of the world." mr. wallace has traced "variation as specially influenced by locality" among the _papilionidæ_ inhabiting the east indian archipelago: showing how "the species and varieties of celebes possess a striking character in the form of the anterior wings, different from that of the allied species and varieties of all the surrounding islands;" and how "tailed species in india and the western islands lose their tails as they spread eastward through the archipelago." during his travels on the upper amazons, mr. bates found that "the greater part of the species of _ithomiæ_ changed from one locality to another, not further removed than to miles;" that "many of these local species have the appearance of being geographical varieties;" and that in some species "most of the local varieties are connected with their parent form by individuals exhibiting all the shades of variation." further general relationships are to be inferred. if races of organisms, ever being thrust by pressure of population into new habitats, undergo modifications of structure as they diverge more and more widely in space, it follows that, speaking generally, the widest divergences in space will indicate the longest periods during which the descendants from a common stock have been subject to modifying conditions; and hence that, among organisms of the same group, the smaller contrasts of structure will be limited to the smaller areas. this we find: "varieties being," as dr. hooker says in his _flora of tasmania_, "more restricted in locality than species, and these again than genera." again, if races of organisms spread, and as they spread are altered by changing incident forces; it follows that where the incident forces vary greatly within given areas, the alterations will be more numerous than in equal areas which are less-variously conditioned. this, too, proves to be the fact. dr. hooker points out that the relatively uniform regions have the fewest species; while in the most multiform regions the species are the most numerous. § . let us consider next, how the hypothesis of evolution corresponds with the facts of distribution, not over different areas but through different media. if all forms of organisms have descended from some primordial form, it follows that since this primordial form must have inhabited some one medium out of the several media now inhabited, the peopling of other media by its descendants implies migration from one medium to others--implies adaptations to media quite unlike the original medium. to speak specifically--water being the medium in which the lowest living forms exist, the implication is that the earth and the air have been colonized from the water. great difficulties appear to stand in the way of this assumption. ridiculing those who alleged the uniserial development of organic forms, who, indeed, laid themselves open to ridicule by their many untenable propositions, von baer writes--"a fish, swimming towards the shore desires to take a walk, but finds his fins useless. they diminish in breadth for want of use, and at the same time elongate. this goes on with children and grandchildren for a few millions of years, and at last who can be astonished that the fins become feet? it is still more natural that the fish in the meadow, finding no water, should gape after air, thereby, in a like period of time developing lungs; the only difficulty being that in the meanwhile, a few generations must manage without breathing at all." though, as thus presented, the belief in a transition looks laughable; and though such derivation of terrestrial vertebrates by direct modification of piscine vertebrates, is untenable; yet we must not conclude that no migrations of the kind alleged can have taken place. the adage that "truth is stranger than fiction," applies quite as much to nature in general as to human life. besides the fact that certain fish actually do "take a walk" without any obvious reason; and besides the fact that sundry kinds of fish ramble about on land when prompted by the drying-up of the waters they inhabit; there is the still more astounding fact that one kind of fish climbs trees. few things seem more manifestly impossible, than that a water-breathing creature without efficient limbs, should ascend eight or ten feet up the trunk of a palm; and yet the _anabas scandens_ does as much. to previous testimonies on this point capt. mitchell has recently added others. such remarkable cases of temporary changes of media, will prepare us for conceiving how, under special conditions, permanent changes of media may have taken place; and for considering how the doctrine of evolution is elucidated by them. inhabitants of the sea, of rivers, and of lakes, are many of them left from time to time partially or completely without water; and those which show the power to change their media temporarily or permanently, are in very many cases of the kinds most liable to be thus deserted by their medium. let us consider what the sea-shore shows us. twice a day the rise and the fall of the tide covers and uncovers plants and animals, fixed and moving; and through the alternation of spring and neap tides, it results that the exposure of the organisms living low down on the beach, varies both in frequency and duration: while some of them are left dry only once a fortnight for a very short time, others, a little higher up, are left dry during two or three hours at several ebb tides every fortnight. then by small gradations we come to such as, living at the top of the beach, are bathed by salt-water only at long intervals; and still higher to some which are but occasionally splashed in stormy weather. what, now, do we find among the organisms thus subject to various regular and irregular alterations of media? besides many plants and many fixed animals, we find moving animals of numerous kinds; some of which are confined to the lower zones of this littoral region, but others of which wander over the whole of it. omitting the humbler types, it will suffice to observe that each of the two great sub-kingdoms, _mollusca_ and _arthropoda_, supplies examples of creatures having a wide excursiveness within this region. we have gasteropods which, when the tide is down, habitually creep snail-like over sand and sea-weed, even up as far as high-water mark. we have several kinds of crustaceans, of which the crab is the most conspicuous, running about on the wet beach, and sometimes rambling beyond the reach of the water. and then note the striking fact that each of the forms thus habituated to changes of media, is allied to forms which are mainly or wholly terrestrial. on the west coast of ireland marine gasteropods are found on the rocks three hundred feet above the sea, where they are only at long intervals wetted by the spray; and though between gasteropods of this class and land-gasteropods the differences are considerable, yet the land-gasteropods are more closely allied to them than to any other _mollusca_. similarly, the two highest orders of crustaceans have their species which live occasionally, or almost entirely, out of the water: there is a kind of lobster in the mauritius which climbs trees; and there is the land-crab of the west indies, which deserts the sea when it reaches maturity and re-visits it only to spawn. seeing, thus, how there are many kinds of marine creatures whose habitats expose them to frequent changes of media; how some of the higher kinds so circumstanced, show a considerable adaptation to both media; and how these amphibious kinds are allied to kinds that are mainly or wholly terrestrial; we shall see that the migrations from one medium to another, which evolution pre-supposes, are by no means impracticable. with such evidence before us, the assumption that the distribution of the _vertebrata_ through media so different as air and water, may have been gradually effected in some analogous manner, would not be altogether unwarranted even had we no clue to the process. we shall find, however, a tolerably distinct clue. though rivers, and lakes, and pools, have no sensible tidal variations, they have their rises and falls, regular and irregular, moderate and extreme. especially in tropical climates, we see them annually full for a certain number of months, and then dwindling away and drying up. the drying up may reach various degrees and last for various periods. it may go to the extent only of producing a liquid mud, or it may reduce the mud to a hardened, fissured solid. it may last for a few days or for months. that is to say, aquatic forms which are in one place annually subject to a slight want of water for a short time, are elsewhere subject to greater wants for longer times: we have gradations of transition, analogous to those which the tides furnish. now it is well known that creatures inhabiting such waters have, in various degrees, powers of meeting these contingencies. the contained fish either bury themselves in the mud when the dry season comes, or ramble in search of other waters. this is proved by evidence from india, guiana, siam, ceylon; and some of these fish, as the _anabas scandens_, are known to survive for days out of the water. but the facts of greatest significance are furnished by an allied class of _vertebrata_, almost peculiar to habitats of this kind. the _amphibia_ are not, like fish, usually found in waters that are never partially or wholly dried up; but they nearly all inhabit waters which, at certain seasons, evaporate, in great measure or completely--waters in which most kinds of fish cannot exist. and what are the leading structural traits of these _amphibia_? they have two respiratory systems--pulmonic and branchial--variously developed in different orders; and they have two or four limbs, also variously developed. further, the class _amphibia_ consists of two groups, in one of which this duality of the respiratory system is permanent, and the development of the limbs always incomplete; and in the other of which the branchiæ disappear as the lungs and limbs become fully developed. the lowest group, the _perennibranchiata_, have internal organs for aerating the blood which approach in various degrees to lungs, until "in the _siren_, the pulmonic respiration is more extensive and important than the branchial;" and to these creatures, having a habitat partially aërial and partially aquatic, there are at the same time supplied, in the shallow water covering soft mud, the mechanical conditions which render swimming difficult and rudimentary limbs useful. in the higher group, the _caducibranchiata_, we find still more suggestive transformations. having at first a structure resembling that which is permanent in the perennibranchiate amphibian, the larva of the caducibranchiate amphibian pursues for a time a similar life; but, eventually, while the branchial appendages dwindle the lungs grow: the respiration of air, originally supplementary to the respiration of water, predominates over it more and more, till it replaces it entirely; and an additional pair of legs is produced. this having been done, the creature either becomes, like the _triton_, one which quits the water only occasionally; or, like the frog, one which pursues a life mainly terrestrial, and returns to the water now and then. finally, if we ask under what conditions this metamorphosis of a water-breather into an air-breather completes itself, the answer is--it completes itself at the time when the shallow pools inhabited by the larvæ are being dried up, or in danger of being dried up, by the summer's sun.[ ] see, then, how significant are the facts when thus brought together. there are particular habitats in which animals are subject to changes of media. in such habitats exist animals having, in various degrees, the power to live in both media, consequent on various phases of transitional organization. near akin to these animals there are some that, after passing their early lives in the water, acquire more completely the structures fitting them to live on land, to which they then migrate. lastly, we have closely-allied creatures, like the surinam toad and the terrestrial salamander, which, though they belong by their structures to the class _amphibia_, are not amphibious in their habits--creatures the larvæ of which do not pass their early lives in the water, and yet go through these same metamorphoses! must we then think, like von baer, that the distribution of kindred organisms through different media presents an insurmountable difficulty? on the contrary, with facts like these before us, the evolution-hypothesis supplies possible interpretations of many phenomena that are else unaccountable. after seeing the ways in which such changes of media are in some cases gradually imposed by physical conditions, and in other cases voluntarily commenced and slowly increased in the search after food; we shall begin to understand how, in the course of evolution, there have arisen strange obscurations of one type by the externals of another type. when we see land-birds occasionally feeding by the water-side, and then learn that one of them, the water-ouzel, an "anomalous member of the strictly terrestrial thrush family, wholly subsists by diving--grasping the stones with its feet and using its wings under water"--we are enabled to comprehend how, under pressure of population, aquatic habits may be acquired by creatures organized for aërial life; and how there may eventually arise an ornithic type in which the traits of the bird are very much disguised. on finding among mammals some that, in search of prey or shelter, have taken to the water in various degrees, we shall cease to be perplexed on discovering the mammalian structure hidden under a fish-like form, as it is in the _cetacea_ and the _sirenia_: especially on finding that in the sea-lion and the seals there are transitional forms. grant that there has ever been going on that re-distribution of organisms which we see still resulting from their intrusions on one another's areas, media, and modes of life; and we have an explanation of those multitudinous cases in which homologies of structure are complicated with analogies. and while it accounts for the occurrence in one medium of organic types fundamentally organized for another medium, the doctrine of evolution accounts also for the accompanying unfitnesses. either the seal has descended from some mammal which little by little became aquatic in its habits, in which case the structure of its hind limbs has a meaning; or else it was specially framed for its present habitat, in which case the structure of its hind limbs is incomprehensible. § . the facts respecting distribution in time, which have more than any others been cited both in proof and in disproof of evolution, are too fragmentary to be conclusive either way. were the geological record complete, or did it, as both uniformitarians and progressionists have commonly assumed, give us traces of the earliest organic forms; the evidence hence derived, for or against, would have had more weight than any other evidence. as it is, all we can do is to see whether such fragmentary evidence as remains, is congruous with the hypothesis. palæontology has shown that there is a "general relation between lapse of time and divergence of organic forms" (§ ); and that "this divergence is comparatively slow and continuous where there is continuity in the geological formations, but is sudden and comparatively wide wherever there occurs a great break in the succession of strata." now this is obviously what we should expect. the hypothesis implies structural changes that are not sudden but gradual. hence, where conformable strata indicate a continuous record, we may anticipate successions of forms only slightly different from one another; while we may rationally look for marked contrasts between the groups of forms fossilized in adjacent strata, where there is evidence of a great blank in the record. the permanent disappearances of species, of genera, and of orders, which we saw to be a fact tolerably-well established, is also a fact for which the belief in evolution prepares us. if later organic forms have in all cases descended from earlier organic forms, and have diverged during their descent, both from their prototypes and from one another; then it follows that such of them as become extinct at any epoch, will never re-appear at a subsequent epoch; since there can never again arise a concurrence and succession of conditions such as those under which each type was evolved. though comparisons of ancient and modern organic forms, prove that many types have persisted through enormous periods of time, without undergoing great changes; it was shown that such comparisons do not disprove the occurrence in other organic forms, of changes great enough to produce what are called different types. the result of inductive inquiry we saw to be, that while a few modern higher types yield signs of having been developed from ancient lower types; and that while there are many modern types which _may_ have been thus developed, though we are without evidence that they have been so; yet that "any admissible hypothesis of progressive modification must be compatible with persistence without progression through indefinite periods." now these results are quite congruous with the hypothesis of evolution. as rationally interpreted, evolution must in all cases be understood to result, directly or indirectly, from the incidence of forces. if there are no changes of conditions entailing organic changes, organic changes are not to be expected. only in organisms which fall under conditions leading to additional modifications answering to additional needs, will there be that increased heterogeneity which characterizes higher forms. hence, though the facts of palæontology cannot be held conclusive proof of evolution, yet they are congruous with it; and some of them yield it strong support. § . one general truth respecting distribution in time, is profoundly significant. if, instead of contemplating the relations among past forms of life taken by themselves, we contemplate the relations between them and the forms now existing, we find a connexion which is in harmony with the belief in evolution but irreconcilable with any other belief. note, first, how full of meaning is the close kinship existing between the aggregate of organisms now living, and the aggregate of organisms which lived in the most recent geologic times. in the last-formed strata, nearly all the imbedded remains are those of species which still flourish. strata a little older contain a few fossils of species now extinct, though, usually, species greatly resembling extant ones. of the remains found in strata of still earlier date, the extinct species form a larger percentage; and the differences between them and the allied species now living are more marked. that is to say, the gradual change of organic types in time, which we before saw is indicated by the geological record, is equally indicated by the relation between existing organic types and organic types of the epochs preceding our own. the evidence completely accords with the belief in a descent of present life from past life. doubtless such a kinship is not incongruous with the doctrine of special creations. it may be argued that the introduction, from time to time, of new species better fitted to the somewhat changed conditions of the earth's surface, would result in an apparent alliance between our living flora and fauna, and the floras and faunas that lately lived. no one can deny it. but on passing from the most general aspect of the alliance to its more special aspects, we shall find this interpretation completely negatived. for besides a close kinship between the aggregate of surviving forms and the aggregate of forms which have died out in recent geologic times; there is a peculiar connexion of like nature between present and past forms in each great geographical region. the instructive fact, before cited from mr. darwin, is the "wonderful relationship in the same continent between the dead and the living." this relationship is not explained by the supposition that new species have been at intervals supernaturally placed in each habitat, as the habitat became modified; since, as we saw, species are by no means uniformly found in the habitats to which they are best adapted. it cannot be said that the marsupials imbedded in recent australian strata, having become extinct because of unfitness to some new external condition, the existing marsupials were then specially created to fit the modified environment; since sundry animals found elsewhere are so much more in harmony with these new australian conditions that, when taken to australia, they rapidly extrude the marsupials. while, therefore, the similarity between the existing australian fauna and the fauna which immediately preceded it over the same area, is just that which the belief in evolution leads us to expect; it is a similarity which cannot be otherwise accounted for. and so is it with parallel relations in new england, in south america, and in europe. § . given, then, that pressure which species exercise on one another, in consequence of the universal overfilling of their respective habitats--given the resulting tendency to thrust themselves into one another's areas, and media, and modes of life, along such lines of least resistance as from time to time are found--given besides the changes in modes of life, hence arising, those other changes which physical alterations of habitats necessitate--given the structural modifications directly or indirectly produced in organisms by modified conditions; and the facts of distribution in space and time are accounted for. that divergence and re-divergence of organic forms, which we saw to be shadowed forth by the truths of classification and the truths of embryology, we see to be also shadowed forth by the truths of distribution. if that aptitude to multiply, to spread, to separate, and to differentiate, which the human races have in all times shown, be a tendency common to races in general, as we have ample reason to assume; then there will result those kinds of spacial relations and chronological relations among the species, and genera, and orders, peopling the earth's surface, which we find exist. the remarkable identities of type discovered between organisms inhabiting one medium, and strangely modified organisms inhabiting another medium, are at the same time rendered comprehensible. and the appearances and disappearances of species which the geological record shows us, as well as the connexions between successive groups of species from early eras down to our own, cease to be inexplicable. chapter viii. how is organic evolution caused? § . already it has been necessary to speak of the causes of organic evolution in general terms; and now we are prepared for considering them specifically. the task before us is to affiliate the leading facts of organic evolution, on those same first principles conformed to by evolution at large. before attempting this, however, it will be instructive to glance at the causes of organic evolution which have been from time to time alleged. § . the theory that plants and animals of all kinds were gradually evolved, seems to have been at first accompanied only by the vaguest conception of cause--or rather, by no conception of cause properly so called, but only by the blank form of a conception. one of the earliest who in modern times ( ) contended that organisms are indefinitely modifiable, and that through their modifications they have become adapted to various modes of existence, was de maillet. but though de maillet supposed all living beings to have arisen by a natural, continuous process, he does not appear to have had any definite idea of that which determines this process. in , in his _zoonomia_, dr. erasmus darwin gave reasons (sundry of them valid ones) for believing that organized beings of every kind, have descended from one, or a few, primordial germs; and along with some observable causes of modification, which he points out as aiding the developmental process, he apparently ascribes it, in part, to a tendency given to such germ or germs when created. he suggests the possibility "that all warm-blooded animals have arisen from one living filament, which the great first cause endued with animality, with the power of acquiring new parts, attended with new propensities, directed by irritations, sensations, volitions, and associations; and thus possessing the faculty of continuing to improve by its own inherent activity." in this passage we see the idea to be, that evolution is pre-determined by some intrinsic proclivity. "it is curious," says mr. charles darwin, "how largely my grandfather, dr. erasmus darwin, anticipated the erroneous grounds of opinion, and the views of lamarck." one of the anticipations was this ascription of development to some inherent tendency. to the "plan général de la nature, et sa marche uniforme dans ses opérations," lamarck attributes "la progression évidente qui existe dans la composition de l'organisation des animaux;" and "la _gradation_ régulière qu'ils devroient offrir dans la composition de leur organisation," he thinks is rendered irregular by secondary causes. essentially the same in kind, though somewhat different in form, is the conception put forth in the _vestiges of creation_; the author of which contends "that the several series of animated beings, from the simplest and oldest up to the highest and most recent, are, under the providence of god, the results, _first_, of an impulse which has been imparted to the forms of life, advancing them, in definite times, by generation, through grades of organization terminating in the highest dicotyledons and vertebrata;" and that the progression resulting from these impulses, is modified by certain other causes. the broad contrasts between lower and higher forms of life, are regarded by him as implying an innate aptitude to give birth to forms of more perfect structures. the last to re-enunciate this doctrine has been prof. owen; who asserts "the axiom of the continuous operation of creative power, or of the ordained becoming of living things." though these words do not suggest a very definite idea, yet they indicate the belief that organic progress is a result of some in-dwelling tendency to develop, supernaturally impressed on living matter at the outset--some ever-acting constructive force which, independently of other forces, moulds organisms into higher and higher forms. in whatever way it is formulated, or by whatever language it is obscured, this ascription of organic evolution to some aptitude naturally possessed by organisms, or miraculously imposed on them, is unphilosophical. it is one of those explanations which explain nothing--a shaping of ignorance into the semblance of knowledge. the cause assigned is not a true cause--not a cause assimilable to known causes--not a cause that can be anywhere shown to produce analogous effects. it is a cause unrepresentable in thought: one of those illegitimate symbolic conceptions which cannot by any mental process be elaborated into a real conception. in brief, this assumption of a persistent formative power inherent in organisms, and making them unfold into higher types, is an assumption no more tenable than the assumption of special creations: of which, indeed, it is but a modification; differing only by the fusion of separate unknown processes into a continuous unknown process. § . besides this intrinsic tendency to progress which dr. darwin ascribes to animals, he says they have a capacity for being modified by processes which their own desires initiate. he speaks of powers as "excited into action by the necessities of the creatures which possess them, and on which their existence depends;" and more specifically he says that "from their first rudiment or primordium, to the termination of their lives, all animals undergo perpetual transformations; which are in part produced by their own exertions, in consequence of their desires and aversions, of their pleasures and their pains, or of irritations, or of associations; and many of these acquired forms or properties are transmitted to their posterity." while it embodies a belief for which much may be said, this passage involves the assumption that desires and aversions, existing before experiences of the actions to which they are related, were the originators of the actions, and therefore of the structural modifications caused by them. in his _philosophie zoologique_, lamarck much more specifically asserts "le _sentiment intérieur_," to be in all creatures that have developed nervous systems, an independent cause of those changes of form which are due to the exercise of organs: distinguishing it from that simple _irritability_ possessed by inferior animals, which cannot produce what we call a desire or emotion; and holding that these last, along with all "qui manquent de système nerveux, ne vivent qu'à l'aide des excitations qu'ils reçoivent de l'extérieur." afterwards he says--"je reconnus que la nature, obligée d'abord d'emprunter des milieux environnants la _puissance excitatrice_ des mouvements vitaux et des actions des animaux imparfaits, sut, en composant de plus en plus l'organisation animale, transporter cette puissance dans l'intérieur même de ces êtres, et qu'à la fin, elle parvint à mettre cette même puissance à la disposition de l'individu." and still more definitely he contends that if one considers "la _progression_ qui se montre dans la composition de l'organisation," ... "alors on eût pu apercevoir comment les _besoins_, d'abord réduits à nullité, et dont le nombre ensuite s'est accru graduellement, ont amené le penchant aux actions propres à y satisfaire: comment les actions devenues habituelles et énergiques, ont occasionné le développement des organes qui les exécutent." now though this conception of lamarck is more precisely stated, and worked out with much greater elaboration and wider knowledge of the facts, it is essentially the same as that of dr. darwin; and along with the truth it contains, contains also the same error more distinctly pronounced. merely noting that desires or wants, acting directly only on the nervo-muscular system, can have no immediate influence on very many organs, as the viscera, or such external appendages as hair and feathers; and observing, further, that even some parts which belong to the apparatus of external action, such as the bones of the skull, cannot be made to grow by increase of function called forth by desire; it will suffice to point out that the difficulty is not solved, but simply slurred over, when needs or wants are introduced as independent causes of evolution. true though it is, as dr. darwin and lamarck contend, that desires, by leading to increased actions of motor organs, may induce further developments of such organs; and true, as it probably is, that the modifications hence arising are transmissible to offspring; yet there remains the unanswered question--whence do these desires originate? the transference of the exciting power from the exterior to the interior, as described by lamarck, begs the question. how comes there a wish to perform an action not before performed? until some beneficial result has been felt from going through certain movements, what can suggest the execution of such movements? every desire consists primarily of a mental representation of that which is desired, and secondarily excites a mental representation of the actions by which it is attained; and any such mental representations of the end and the means, imply antecedent experience of the end and antecedent use of the means. to assume that in the course of evolution there from time to time arise new kinds of actions dictated by new desires, is simply to remove the difficulty a step back. § . changes of external conditions are named, by dr. darwin, as causes of modifications in organisms. assigning as evidence of original kinship, that marked similarity of type which exists among animals, he regards their deviations from one another, as caused by differences in their modes of life: such deviations being directly adaptive. after enumerating various appliances for procuring food, he says they all "seem to have been gradually produced during many generations by the perpetual endeavour of the creatures to supply the want of food, and to have been delivered to their posterity with constant improvement of them for the purposes required." and the creatures possessing these various appliances are considered as having been rendered unlike by seeking for food in unlike ways. as illustrating the alterations wrought by changed circumstances, he names the acquired characters of domestic animals. lamarck has elaborated the same view in detail: using for the purpose, with great ingenuity, his extensive knowledge of the animal kingdom. from a passage in the _avertissement_ it would at first sight seem that he looks upon direct adaptation to new conditions as the chief cause of evolution. he says--"je regardai comme certain que le _mouvement des fluides_ dans l'intérieur des animaux, mouvement qui c'est progressivement accéléré avec la composition plus grande de l'organisation; et que _l'influence des circonstances_ nouvelles, à mesure que les animaux s'y exposèrent en se répandant dans tous les lieux habitables, furent les deux causes générales qui ont amené les différents animaux à l'état où nous les voyons actuellement." but elsewhere the view he expresses appears decidedly different from this. he asserts that "dans sa marche, la nature a commencé, et recommence encore tous les jours, par former les corps organisés les plus simples;" and that "les premières ébauches de l'animal et du végétal étant formées dans les lieux et les circonstances convenables, les facultés d'une vie commençante et d'un mouvement organique établi, ont nécessairement développé peu à peu les organes, et qu'avec le temps elles les ont diversifies ainsi que les parties." and then, further on, he puts in italics this proposition:--"_la progression dans la composition de l'organisation subit, çà et là, dans la série générale des animaux, des anomalies opérées par l'influence des circonstances d'habitation, et par celle des habitudes contractées._" these, and sundry other passages, joined with his general scheme of classification, make it clear that lamarck conceived adaptive modification to be, not the cause of progression, but the cause of irregularities in progression. the inherent tendency which organisms have to develop into more perfect forms, would, according to him, result in a uniform series of forms; but varieties in their conditions work divergences of structure, which break up the series into groups: groups which he nevertheless places in uni-serial order, and regards as still substantially composing an ascending succession. § . these speculations, crude as they may be considered, show much sagacity in their respective authors, and have done good service. without embodying the truth in definite shapes, they contain adumbrations of it. not directly, but by successive approximations, do mankind reach correct conclusions; and those who first think in the right direction, loose as may be their reasonings, and wide of the mark as their inferences may be, yield indispensable aid by framing provisional conceptions and giving a bent to inquiry. contrasted with the dogmas of his age, the idea of de maillet was a great advance. before it can be ascertained how organized beings have been gradually evolved, there must be reached the conviction that they _have_ been gradually evolved; and this conviction he reached. his wild notions about the way in which natural causes acted in the production of plants and animals, must not make us forget the merit of his intuition that animals and plants _were_ produced by natural causes. in dr. darwin's brief exposition, the belief in a progressive genesis of organisms is joined with an interpretation having considerable definiteness and coherence. in the space of ten pages he not only indicates several of the leading classes of facts which support the hypothesis of development, but he does something towards suggesting the process of development. his reasonings show an unconscious mingling of the belief in a supernaturally-impressed tendency to develop, with the belief in a development arising from the changing incidence of conditions. probably had he pursued the inquiry further, this last belief would have grown at the expense of the first. lamarck, in elaborating this general conception, has given greater precision both to its truth and to its error. asserting the same imaginary factors and the same real factors, he has traced out their supposed actions in detail; and has, in consequence, committed himself to a greater number of untenable positions. but while, in trying to reconcile the facts with a theory which is only an adumbration of the truth, he laid himself open to the criticisms of his contemporaries; he proved himself profounder than his contemporaries by seeing that natural genesis, however caused, has been going on. if they were wise in not indorsing a theory which fails to account for a great part of the facts; they were unwise in ignoring that degree of congruity with the facts, which shows the theory to contain some fundamental verity. leaving out, however, the imaginary factors of evolution which these speculations allege, and looking only at the one actual factor which dr. darwin and lamarck assign as accounting for some of the phenomena; it is manifest, from our present stand-point, that this, so far as it is a cause of evolution, is a proximate cause and not an ultimate cause. to say that functionally-produced adaptation to conditions originates either evolution in general, or the irregularities of evolution, is to raise the further question--why is there a functionally-produced adaptation to conditions?--why do use and disuse generate appropriate changes of structure? neither this nor any other interpretation of biologic evolution which rests simply on the basis of biologic induction, is an ultimate interpretation. the biologic induction must itself be interpreted. only when the process of evolution of organisms is affiliated on the process of evolution in general, can it be truly said to be explained. the thing required is to show that its various results are corollaries from first principles. we have to reconcile the facts with the universal laws of the re-distribution of matter and motion. chapter ix. external factors. § . when illustrating the rhythm of motion (_first principles_, § ) it was pointed out that besides the daily and annual alternations in the quantities of light and heat which any portion of the earth's surface receives from the sun, there are alternations which require immensely-greater periods to complete. reference was made to the fact that "every planet, during a certain long period, presents more of its northern than of its southern hemisphere to the sun at the time of its nearest approach to him; and then again, during a like period, presents more of its southern hemisphere than of its northern--a recurring coincidence which, though it causes in some planets no sensible alterations of climate, involves, in the case of the earth, an epoch of , years during which each hemisphere goes through a cycle of temperate seasons, and seasons that are extreme in their heat and cold." further, we saw that there is a variation of this variation. the slow rhythm of temperate and intemperate climates, which takes , years to complete itself, undergoes exaggeration and mitigation during epochs that are far longer. the earth's orbit slowly alters in form: now approximating to a circle, and now becoming more eccentric. during the period in which the earth's orbit has least eccentricity, the temperate and intemperate climates which repeat their cycle in , years, are severally less temperate and less intemperate, than when, some one or two millions of years later, the earth's orbit has reached its extreme of eccentricity. thus, besides those daily variations in the quantities of light and heat received by organisms, and responded to by variations in their functions; and besides the annual variations in the quantities of light and heat which organisms receive, and similarly respond to by variations in their functions; there are variations that severally complete themselves in , years and in some millions of years--variations to which there must also be responses in the changed functions of organisms. the whole vegetal and animal kingdoms, are subject to quadruply-compounded rhythms in the incidence of the forces on which life primarily depends--rhythms so involved in their slow working round that at no time during one of these vast epochs, can the incidence of these various forces be exactly the same as at any other time. to the direct effects so produced on organisms, have to be added much more important indirect effects. changes of distribution must result. certain redistributions are occasioned even by the annual variations in the quantities of the solar rays received by each part of the earth's surface. the migrations of birds thus caused are familiar. so, too, are the migrations of certain fishes: in some cases from one part of the sea to another; in some cases from salt water to fresh water; and in some cases from fresh water to salt water. now just as the yearly changes in the amounts of light and heat falling on each locality, yearly extend and restrict the habitats of many organisms which are able to move about with some rapidity; so must the alterations of temperate and intemperate climates produce extensions and restrictions of habitats. these, though slow, must be universal--must affect the habitats of stationary organisms as well as those of locomotive ones. for if, during an astronomic era, there is going on at any limit to a plant's habitat, a diminution of the winter's cold or summer's heat, which had before stopped its spread at that limit; then, though the individual plants are fixed, yet the species will move: the seeds of plants living at the limit, will produce individuals which survive beyond the limit. the gradual spread so effected, having gone on for some ten thousand years, the opposite change of climate will begin to cause retreat. the tide of each species will, during one half of a long epoch, slowly flow into new regions, and then will slowly ebb away from them. further, this rise and fall in the tide of each species will, during far longer intervals, undergo increasing rises and falls and then decreasing rises and falls. there will be an alteration of spring tides and neap tides, answering to the changing eccentricity of the earth's orbit. these astronomical rhythms, therefore, entail on organisms unceasing changes in the incidence of forces in two ways. they directly subject them to variations of solar influences, in such a manner that each generation is somewhat differently affected in its functions; and they indirectly bring about complicated alterations in the environing agencies, by carrying each species into the presence of new physical conditions, new soil and surface. § . the power of geological actions to modify everywhere the circumstances in which plants and animals are placed, is conspicuous. in each locality denudation slowly uncovers different deposits, and slowly changes the exposed areas of deposits already uncovered. simultaneously, the alluvial beds in course of formation, are qualitatively affected by these progressive changes in the natures and proportions of the strata denuded. the inclinations of surfaces and their directions with respect to the sun, are at the same time modified; and the organisms existing on them are thus having their thermal conditions continually altered, as well as their drainage. igneous action, too, complicates these gradual modifications. a flat region cannot be step by step thrust up into a protuberance without unlike climatic changes being produced in its several parts, by their exposures to different aspects. extrusions of trap, wherever they take place, revolutionize the localities; both over the areas covered and over the areas on to which their detritus is carried. and where volcanoes are formed, the ashes they occasionally send out modify the character of the soil throughout large surrounding tracts. in like manner alterations in the earth's crust cause the ocean to be ever subjecting the organisms it contains to new combinations of conditions. here the water is being deepened by subsidence, and there shallowed by upheaval. while the falling upon it of sediment brought down by neighbouring large rivers, is raising the sea-bottom in one place, in another the habitual rush of the tide is carrying away the sediment deposited in past times. the mineral character of the submerged surface on which sea-weeds grow and molluscs crawl, is everywhere occasionally changed; now by the bringing away from an adjacent shore some previously untouched strata; and now by the accumulation of organic remains, such as the shells of pteropods or of foraminifera. a further series of alterations in the circumstances of marine organisms, is entailed by changes in the movements of the water. each modification in the outlines of neighbouring shores makes the tidal streams vary their directions or velocities or both. and the local temperature is from time to time raised or lowered, because some far-distant change of form in the earth's crust has wrought a divergence in those circulating currents of warm and cold water which pervade the ocean. these geologically-caused changes in the physical characters of each environment, occur in ever-new combinations, and with ever-increasing complexity. as already shown (_first principles_, § ), it follows from the law of the multiplication of effects, that during long periods each tract of the earth's surface increases in heterogeneity of both form and substance. so that plants and animals of all kinds are, in the course of generations, subjected by alterations in the crust of the earth, to sets of incident forces differing from previous sets, both by changes in the proportions of the factors and, occasionally, by the addition of new factors. § . variations in the astronomical conditions joined with variations in the geological conditions, bring about variations in the meteorological conditions. those slow alternations of elevation and subsidence which take place over immense areas, here producing a continent where once there was a fathomless ocean, and there causing wide seas to spread where in a long past epoch there stood snow-capped mountains, gradually work great atmospheric changes. while the highest parts of an emerging surface of the earth's crust exist as a cluster of islands, the plants and animals which in course of time migrate to them have climates that are peculiar to small tracts of land surrounded by large tracts of water. as, by successive upheavals, greater areas are exposed, there begin to arise sensible contrasts between the states of their peripheral parts and their central parts. the breezes which daily moderate the extremes of temperature near the shores, cease to affect the interiors; and the interiors, less qualified too in their heat and cold by such ocean-currents as approach the coast, acquire more decidedly the characters due to their latitudes. along with the further elevations which unite the members of the archipelago into a continent, there come new meteorologic changes, as well as exacerbations of the old. the winds, which were comparatively uniform in their directions and periods when only islands existed, grow involved in their distribution, and widely-different in different parts of the continent. the quantities of rain which they discharge and of moisture which they absorb, vary everywhere according to the proximity to the sea and to surfaces of land having special characters. other complications result from variations of height above the sea: elevation producing a decrease of heat and consequently an increase in the precipitation of water--a precipitation which takes the shape of snow where the elevation is very great, and of rain where it is not so great. the gatherings of clouds and descents of showers around mountain tops, are familiar to every tourist. inquiries in the neighbouring valleys prove that within distances of a mile or two the recurring storms differ in their frequency and violence. nay, even a few yards off, the meteorological conditions vary in such regions: as witness the way in which the condensing vapour keeps eddying round on one side of some high crag, while the other side is clear; or the way in which the snowline runs irregularly to different heights, in all the hollows and ravines of each mountain side. as climatic variations thus geologically produced, are compounded with those which result from slow astronomical changes; and as no correspondence exists between the geologic and the astronomic rhythms; it results that the same plexus of actions never recurs. hence the incident forces to which the organisms of every locality are exposed by atmospheric agencies, are ever passing into unparalleled combinations; and these are on the average ever becoming more complex. § . besides changes in the incidence of inorganic forces, there are equally continuous, and still more involved, changes in the incidence of forces which organisms exercise on one another. as before pointed out (§ ), the plants and animals inhabiting each locality are held together in so entangled a web of relations, that any considerable modification which one species undergoes, acts indirectly on many other species, and eventually changes, in some degree, the circumstances of nearly all the rest. if an increase of heat, or modification of soil, or decrease of humidity, causes a particular kind of plant either to thrive or to dwindle, an unfavourable or favourable effect is wrought on all such competing kinds of plants as are not immediately influenced in the same way. the animals which eat the seeds or browse on the leaves, either of the plant primarily affected or those of its competitors, are severally altered in their states of nutrition and in their numbers; and this change presently tells on various predatory animals and parasites. and since each of these secondary and tertiary changes becomes itself a centre of others, the increase or decrease of each species produces waves of influence which spread and reverberate and re-reverberate throughout the whole flora and fauna of the locality. more marked and multiplied still, are the ultimate effects of those causes which make possible the colonization of neighbouring areas. each intruding plant or animal, besides the new inorganic conditions to which it is subject, is subject to organic conditions different from those to which it has been accustomed. it has to compete with some organisms unlike those of its preceding habitat. it must preserve itself from enemies not before encountered. or it may meet with a species over which it has some advantage greater than any it had over the species it was previously in contact with. even where migration does not bring it face to face with new competitors or new enemies or new prey, it inevitably experiences new proportions among these. further, an expanding species is almost certain to invade more than one adjacent region. spreading both north and south, or east and west, it will come among the plants and animals, here of a level district and there of a hilly one--here of an inland tract and there of a tract bordered by the sea. and while different groups of its members will thus expose themselves to the actions and reactions of different floras and faunas, these different floras and faunas will simultaneously have their organic conditions changed by the intruders. this process becomes gradually more active and more complicated. though, in particular cases, a plant or animal may fall into simpler relations with the living things around than those it was before placed in, yet it is manifest that, on the average, the organic environments of organisms have been advancing in heterogeneity. as the number of species with which each species is directly or indirectly implicated, multiplies, each species is oftener subject to changes in the organic actions which influence it. these more frequent changes severally grow more involved. and the corresponding reactions affect larger floras and faunas, in ways increasingly complex and varied. § . when the astronomic, geologic, meteorologic, and organic agencies which are at work on each species of plant and animal are contemplated as becoming severally more complicated in themselves, and as co-operating in ways that are always partially new; it will be seen that throughout all time there has been an exposure of organisms to endless successions of modifying causes which gradually acquire an intricacy scarcely conceivable. every kind of plant and animal may be regarded as for ever passing into a new environment--as perpetually having its relations to external circumstances altered, either by their changes with respect to it when it remains stationary, or by its changes with respect to them when it migrates, or by both. yet a further cause of progressive alteration and complication in the incident forces, exists. all other things continuing the same, every additional faculty by which an organism is brought into relation with external objects, as well as every improvement in such faculty, becomes a means of subjecting the organism to a greater number and variety of external stimuli, and to new combinations of external stimuli. so that each advance in complexity of organization, itself becomes an added source of complexity in the incidence of external forces. once more, every increase in the locomotive powers of animals, increases both the multiplicity and the multiformity of the actions of things upon them, and of their reactions upon things. doubling a creature's activity quadruples the area that comes within the range of its excursions; thus augmenting in number and heterogeneity, the external agencies which act on it during any given interval. by compounding the actions of these several orders of factors, there is produced a geometric progression of changes, increasing with immense rapidity. and there goes on an equally rapid increase in the frequency with which the combinations of the actions are altered, and the intricacies of their co-operations enhanced. chapter x. internal factors. § . we saw at the outset (§§ - ), that organic matter is built up of molecules so unstable, that the slightest variation in their conditions destroys their equilibrium, and causes them either to assume altered structures or to decompose. but a substance which is beyond all others changeable by the actions and reactions of the forces liberated from instant to instant within its own mass, must be a substance which is beyond all others changeable by the forces acting on it from without. if their composition fits organic aggregates for undergoing with special facility and rapidity those re-distributions of matter and motion whence result individual organization and life; then their composition must make them similarly apt to undergo those permanent re-distributions of matter and motion which are expressed by changes of structure, in correspondence with permanent re-distributions of matter and motion in their environments. in _first principles_, when considering the phenomena of evolution at large, the leading characters and causes of those changes which constitute organic evolution were briefly traced. under each of the derivative laws of force to which the passage from an incoherent, indefinite homogeneity to a coherent, definite heterogeneity, conforms, were given illustrations drawn from the metamorphoses of living bodies. here it will be needful to contemplate the several resulting processes as going on at once, in both individuals and species. § . our postulate being that organic evolution in general commenced with homogeneous organic matter, we have first to remember that the state of homogeneity is an unstable state (_first principles_, § ). in any aggregate "the relations of outside and inside, and of comparative nearness to neighbouring sources of influence, imply the reception of influences that are unlike in quantity, or quality, or both; and it follows that unlike changes will be produced in the parts thus dissimilarly acted upon." further, "if any given whole, instead of being absolutely uniform throughout, consists of parts distinguishable from one another--if each of these parts, while somewhat unlike other parts, is uniform within itself; then, each of them being in unstable equilibrium, it follows that while the changes set up within it must render it multiform, they must at the same time render the whole more multiform than before;" and hence, "whether that state with which we commence be or be not one of perfect homogeneity, the process must equally be towards a relative heterogeneity." this loss of homogeneity which the special instability of organic aggregates fits them to display more promptly and variously than any other aggregates, must be shown in more numerous ways in proportion as the incident forces are more numerous. every differentiation of structure being a result of some difference in the relations of the parts to the agencies acting on them, it follows that the more multiplied and more unlike the agencies, the more varied must be the differentiations wrought. hence the change from a state of homogeneity to a state of heterogeneity, will be marked in proportion as the environing actions to which the organism is supposes it is only are complex. this transition from a uniform to a multiform state, must continue through successive individuals. given a series of organisms, each of which is developed from a portion of a preceding organism, and the question is whether, after exposure of the series for a million years to changed incident forces, one of its members will be the same as though the incident forces had only just changed. to say that it will, is implicitly to deny the persistence of force. in relation to any cause of divergence, the whole series of such organisms may be considered as fused together into a continuously-existing organism; and when so considered, it becomes manifest that a continuously-acting cause will go on working a continuously-increasing effect, until some counteracting cause prevents any further effect. but now if any primordial organic aggregate must, in itself and through its descendants, gravitate from uniformity to multiformity, in obedience to the more or less multiform forces acting on it; what must happen if these multiform forces are themselves undergoing slow variations and complications? clearly the process, ever-advancing towards a temporary limit but ever having its limit removed, must go on unceasingly. on those structural changes wrought in the once homogeneous aggregate by an original set of incident forces, will be superposed further changes wrought by a modified set of incident forces; and so on throughout all time. omitting for the present those circumstances which check and qualify its consequences, the instability of the homogeneous must be recognized as an ever-acting cause of organic evolution, as of all other evolution. while it follows that every organism, considered as an individual and as one of a series, tends thus to pass into a more heterogeneous state; it also follows that every species, considered as an aggregate of individuals, tends to do the like. throughout the area it inhabits, the conditions can never be absolutely uniform: its members must, in different parts of the area, be exposed to different sets of incident forces. still more decided must this difference of exposure be when its members spread into other habitats. those expansive and repressive energies which set to each species a limit that perpetually oscillates from side to side of a certain mean, are, as we lately saw, frequently changed by new combinations of the external factors--astronomic, geologic, meteorologic, and organic. hence there from time to time arise lines of diminished resistance, along which the species flows into new localities. such portions of the species as thus migrate, are subject to circumstances unlike its previous average circumstances. and from multiformity of the circumstances, must come multiformity of the species. thus the law of the instability of the homogeneous has here a three-fold corollary. as interpreted in connexion with the ever-progressing, ever-complicating changes in external factors, it involves the conclusion that there is a prevailing tendency towards greater heterogeneity in all kinds of organisms, considered both individually and in successive generations; as well as in each assemblage of organisms constituting a species; and, by consequence, in each genus, order, and class. § . when considering the causes of evolution in general, we further saw (_first principles_, § ), that the multiplication of effects aids continually to increase that heterogeneity into which homogeneity inevitably lapses. it was pointed out that since "the several parts of an aggregate are differently modified by any incident force;" and since "by the reactions of the differently modified parts the incident force itself must be divided into differently modified parts;" it follows that "each differentiated division of the aggregate thus becomes a centre from which a differentiated division of the original force is again diffused. and since unlike forces must produce unlike results, each of these differentiated forces must produce, throughout the aggregate, a further series of differentiations." to this it was added that, in proportion as the heterogeneity increases, the complications arising from this multiplication of effects grow more marked; because the more strongly contrasted the parts of an aggregate become, the more different must be their reactions on incident forces, and the more unlike must be the secondary effects which these initiate; and because every increase in the number of unlike parts adds to the number of such differentiated incident forces, and such secondary effects. how this multiplication of effects conspires, with the instability of the homogeneous, to work an increasing multiformity of structure in an organism, was shown at the time; and the foregoing pages contain further incidental illustrations. in § it was pointed out that a change in one function must produce ever-complicating perturbations in other functions; and that, eventually, all parts of the organism must be modified in their states. suppose that the head of a bison becomes much heavier, what must be the indirect results? the muscles of the neck are put to greater exertions; and its vertebræ have to bear additional tensions and pressures, caused both by the increased weight of the head, and by the stronger contractions of the muscles that support and move it. these muscles also affect their special attachments: several of the dorsal spines suffer augmented strains; and the vertebræ to which they are fixed are more severely taxed. further, this heavier head and the more massive neck it necessitates, require a stronger fulcrum: the whole thoracic arch, and the fore-limbs which support it, are subject to greater continuous stress and more violent occasional shocks. and the required strengthening of the fore-quarters cannot take place without the centre of gravity being changed, and the hind limbs being differently reacted upon during locomotion. any one who compares the outline of the bison with that of its congener, the ox, will see how profoundly a heavier head affects the entire osseous and muscular systems. besides this multiplication of mechanical effects, there is a multiplication of physiological effects. the vascular apparatus is modified throughout its whole structure by each considerable modification in the proportions of the body. increase in the size of any organ implies a quantitative, and often a qualitative, reaction on the blood; and thus alters the nutrition of all other organs. such physiological correlations are exemplified in the many differences which accompany difference of sex. that the minor sexual peculiarities are brought about by the physiological actions and reactions, is shown both by the fact that they are commonly but faintly marked until the fundamentally distinctive organs are developed, and that when the development of these is prevented, the minor sexual peculiarities do not arise. no further proof is, i think, needed, that in any individual organism or its descendants, a new external action must, besides the primary internal change which it works, work many secondary changes, as well as tertiary changes still more multiplied. that tendency towards greater heterogeneity which is given to an organism by disturbing its environment, is helped by the tendency which every modification has to produce other modifications--modifications which must become more numerous in proportion as the organism becomes more complex. lastly, among the indirect and involved manifestations of this tendency, we must not omit the innumerable small irregularities of structure which result from the crossing of dissimilarly-modified individuals. it was shown (§§ , ) that what are called "spontaneous variations," are interpretable as results of miscellaneously compounding the changes wrought in different lines of ancestors by different conditions of life. these still more complex and multitudinous effects so produced, are further illustrations of the multiplication of effects. equally in the aggregate of individuals constituting a species, does multiplication of effects become the continual cause of increasing multiformity. the lapse of a species into divergent varieties, initiates fresh combinations of forces tending to work further divergences. the new varieties compete with the parent species in new ways; and so add new elements to its circumstances. they modify somewhat the conditions of other species existing in their habitat, or in the habitat they have invaded; and the modifications wrought in such other species become additional sources of influence. the flora and fauna of every region are united by their entangled relations into a whole, of which no part can be affected without affecting the rest. hence, each differentiation in a local assemblage of species, becomes the cause of further differentiations. § . one of the universal principles to which we saw that the re-distribution of matter and motion conforms, is that in any aggregate made up of mixed units, incident forces produce segregation--separate unlike units and bring together like units; and it was shown that the increasing integration and definiteness which characterizes each part of an evolving organic aggregate, as of every other aggregate, results from this (_first principles_, § ). it remains here to say that while the actions and reactions between organisms and their changing environments, add to the heterogeneity of organic structures, they also give to the heterogeneity this growing distinctness. at first sight the reverse might be inferred. it might be argued that any new set of effects wrought in an organism by some new set of external forces, must tend more or less to obliterate the effects previously wrought--must produce confusion or indefiniteness. a little consideration, however, will dissipate this impression. doubtless the condition under which alone increasing definiteness of structure can be acquired by any part of an organism, either in an individual or in successive generations, is that such part shall be exposed to some set of tolerably-constant forces; and doubtless, continual change of circumstances interferes with this. but the interference can never be considerable. for the pre-existing structure of an organism prevents it from living under any new conditions except such as are congruous with the fundamental characters of its organization--such as subject its essential organs to actions substantially the same as before. great changes must kill it. hence, it can continuously expose itself and its descendants, only to those moderate changes which do not destroy the general harmony between the aggregate of incident forces and the aggregate of its functions. that is, it must remain under influences calculated to make greater the definiteness of the chief differentiations already produced. if, for example, we set out with an animal in which a rudimentary vertebral column with its attached muscular system has been established; it is clear that the mechanical arrangements have become thereby so far determined, that subsequent modifications are extremely likely, if not certain, to be consistent with the production of movement by the actions of muscles on a flexible central axis. hence, there will continue a general similarity in the play of forces to which the flexible central axis is subject; and so, notwithstanding the metamorphoses which the vertebrate type undergoes, there will be a maintenance of conditions favourable to increasing definiteness and integration of the vertebral column. moreover, this maintenance of such conditions becomes secure in proportion as organization advances. each further complexity of structure, implying some further complexity in the relations between an organism and its environment, must tend to specialize the actions and reactions between it and its environment--must tend to increase the stringency with which it is restrained within such environments as admit of those special actions and reactions for which its structure fits it; that is, must further guarantee the continuance of those actions and reactions to which its essential organs respond, and therefore the continuance of the segregating process. how in each species, considered as an aggregate of individuals, there must arise stronger and stronger contrasts among those divergent varieties which result from the instability of the homogeneous and the multiplication of effects, need only be briefly indicated. it has already been shown (_first principles_, § ), that in conformity to the universal law that mixed units are segregated by like incident forces, there are produced increasingly-definite distinctions among varieties, wherever there occur definitely-distinguished sets of conditions to which the varieties are respectively subject. § . probably in the minds of some, the reading of this chapter has been accompanied by a running commentary, to the effect that the argument proves too much. the apparent implication is, that the passage from an indefinite, incoherent homogeneity to a definite, coherent heterogeneity in organic aggregates, must have been going on universally; whereas we find that in many cases there has been persistence without progression. this apparent implication, however, is not a real one. for though every environment on the earth's surface undergoes changes; and though usually the organisms which each environment contains, cannot escape certain resulting new influences; yet occasionally such new influences are escaped, by the survival of species in the unchanged parts of their habitats, or by their spread into neighbouring habitats which the change has rendered like their original habitats, or by both. any alteration in the temperature of a climate or its degree of humidity, is unlikely to affect simultaneously the whole area occupied by a species; and further, it can scarcely fail to happen that the addition or subtraction of heat or moisture, will give to a part of some adjacent area, a climate like that to which the species has been habituated. if, again, the circumstances of a species are modified by the intrusion of some foreign kind of plant or animal, it follows that since the intruders will probably not spread throughout its whole habitat, the species will, in one or more localities, remain unaffected by them. especially among marine creatures, must there frequently occur cases in which modifying causes are continually eluded. comparatively uniform as are the physical conditions to which the sea exposes its inhabitants, it becomes possible for such of them as live on widely-diffused food, to be widely distributed; and wide distribution generally prevents the members of a species from being all subject to the same cause. our commonest cirriped, for instance, subsisting on minute creatures everywhere dispersed through the water; needing only to have some firm surface on which to build up its shell; and in scarcely any danger from surrounding animals; is able to exist on shores so widely remote from one another, that nearly every change in the incident forces must fall within narrower areas than that which the species occupies. nearly always, therefore, a portion of the species will survive unmodified. its easily-transported germs will take possession of such new habitats as have been rendered fitter by the change that has unfitted some parts of its original habitat. hence, on successive occasions, while some parts of the species are slightly transformed, another part may continually escape transformation by migrating hither and thither, where the simple conditions needed for its existence recur in nearly the same combinations as before. and it will so become possible for it to survive, with insignificant structural changes, throughout long geologic periods. § . the results to which we find ourselves led, are these. in subordination to the different amounts and kinds of forces to which its different parts are exposed, every individual organic aggregate, like all other aggregates, tends to pass from its original indistinct simplicity towards a more distinct complexity. unless we deny the persistence of force, we must admit that the lapse of an organism's structure from an indefinitely homogeneous to a definitely heterogeneous state, must be cumulative in successive generations, if the forces causing it continue to act. and for the like reasons, the increasing assemblage of individuals arising from a common stock, is also liable to lose its original uniformity; and, in successive generations, to grow more pronounced in its multiformity. these changes, which would go to but a comparatively small extent were organisms exposed to constant external conditions, are kept up by the continual changes in external conditions, produced by astronomic, geologic, meteorologic, and organic agencies: the average result being, that on previous complications wrought by previous incident forces, new complications are continually superposed by new incident forces. and hence simultaneously arises increasing heterogeneity in the structures of individuals, in the structures of species, and in the structures of the earth's flora and fauna. but while, in very many or in most cases, the ever-changing incidence of forces is ever adding to the complexity of organisms, and to the complexity of the organic world as a whole; it does this only where its action cannot be eluded. and since, by migration, it is possible for a species to keep itself under conditions that are tolerably constant, there must be a proportion of cases in which greater heterogeneity of structure is not to be expected. to show, however, that there must arise a certain average tendency to the production of greater heterogeneity is not sufficient. aggregates might be rendered more heterogeneous by changing incident forces, without having given to them that kind of heterogeneity required for carrying on life. hence it remains now to inquire how the production and maintenance of this kind of heterogeneity is insured. chapter xi. direct equilibration. § . every change is towards a balance of forces; and of necessity can never cease until a balance of forces is reached. when treating of equilibration under its general aspects (_first principles_, part ii., chap. xxii.), we saw that every aggregate having compound movements tends continually towards a moving equilibrium; since any unequilibrated force to which such an aggregate is subject, if not of a kind to overthrow it altogether, must continue modifying its state until an equilibrium is brought about. and we saw that the structure simultaneously reached must be "one presenting an arrangement of forces that counterbalance all the forces to which the aggregate is subject;" since, "so long as there remains a residual force in any direction--be it excess of a force exercised by an aggregate on its environment, or of a force exercised by its environment on the aggregate, equilibrium does not exist; and therefore the re-distribution of matter must continue." it is essential that this truth should here be fully comprehended; and to the end of insuring clear comprehension of it, some re-illustration is desirable. the case of the solar system will best serve our purpose. an assemblage of bodies, each of which has its simple and compound motions that severally alternate between two extremes, and the whole of which has its involved perturbations, that now increase and now decrease, is here presented to us. suppose a new factor were brought to bear on this moving equilibrium--say by the arrival of some wandering mass, or by an additional momentum given to one of the existing masses--what would be the result? if the strange body or the extra energy were very large, it might so derange the entire system as to cause its collapse. but what if the incident energy, falling on the system from without, proved insufficient to overthrow it? there would then arise a set of perturbations which would, in the course of an enormous period, slowly work round into a modified moving equilibrium. the effects primarily impressed on the adjacent masses, and in a smaller degree on the remoter masses, would presently become complicated with the secondary effects impressed by the disturbed masses on one another; and these again with tertiary effects. waves of perturbation would continue to be propagated throughout the entire system; until, around a new centre of gravity, there had been established a set of planetary motions different from the preceding ones. the new energy must gradually be used up in overcoming the energies resisting the divergence it generates; which antagonizing energies, when no longer opposed, set up a counter-action, ending in a compensating divergence in the opposite direction, followed by a re-compensating divergence, and so on. now though instead of being, like the solar system, in a state of _independent_ moving equilibrium, an organism is in a state of _dependent_ moving equilibrium (_first principles_, § ); yet this does not prevent the manifestation of the same law. every animal daily obtains from without, a supply of energy to replace the energy it expends; but this continual giving to its parts a new momentum, to make up for the momentum continually lost, does not interfere with the carrying on of actions and reactions like those just described. here, as before, we have a definitely-arranged aggregate of parts, called organs, having their definitely-established actions and reactions, called functions. these rhythmical actions or functions, and the various compound rhythms resulting from their combinations, are so adjusted as to balance the actions to which the organism is subject: there is a constant or periodic genesis of energies which, in their kinds, amounts, and directions, suffice to antagonize the energies the organism has constantly or periodically to bear. if, then, there exists this moving equilibrium among a set of internal actions, exposed to a set of external actions, what must result if any of the external actions are changed? of course there is no longer an equilibrium. some energy which the organism habitually generates, is too great or too small to balance some incident energy; and there arises a residual energy exerted by the environment on the organism, or by the organism on the environment. this residual or unbalanced energy, of necessity expends itself in producing some change of state in the organism. acting directly on some organ and modifying its function, it indirectly modifies dependent functions and remotely influences all the functions. as we have already seen (§§ , ), if this new energy is permanent, its effects must be gradually diffused throughout the entire system; until it has come to be equilibrated in producing those structural rearrangements whence result a counter-balancing energy. the bearing of this general truth on the question we are now dealing with is obvious. those modifications upon modifications, which the unceasing mutations of their environments have been all along generating in organisms, have been in each case modifications involved by the establishment of a new balance with the new combination of actions. in every species throughout all geologic time, there has been perpetually going on a rectification of the equilibrium, which has been perpetually disturbed by the alteration of its circumstances; and every further heterogeneity has been the addition of a structural change entailed by a new equilibration, to the structural changes entailed by previous equilibrations. there can be no other ultimate interpretation of the matter, since change can have no other goal. this equilibration between the functions of an organism and the actions in its environment, may be either direct or indirect. the new incident force may either immediately call forth some counteracting force, and its concomitant structural change; or it may be eventually balanced by some otherwise-produced change of function and structure. these two processes of equilibration are quite distinct, and must be separately dealt with. we will devote this chapter to the first of them. § . direct equilibration is that process currently known as _adaptation_. we have already seen (part ii., chap, v.), that individual organisms become modified when placed in new conditions of life--so modified as to re-adjust the powers to the requirements; and though there is great difficulty in disentangling the evidence, we found reason for thinking (§ ) that structural changes thus caused by functional changes are inherited. in the last chapter, it was argued that if, instead of the succession of individuals constituting a species, there were a continuously-existing individual, any functional and structural divergence produced by a new incident action, would increase until the new incident action was counterpoised; and that the replacing of a continuously-existing individual by a succession of individuals, each formed out of the modified substance of its predecessor, will not prevent the like effect from being produced. here we further find that this limit towards which any such organic change advances, in the species as in the individual, is a new moving equilibrium adjusted to the new arrangement of external forces. but now what are the conditions under which alone, direct equilibration can occur? are all the modifications that serve to re-fit organisms to their environments, directly adaptive modifications? and if otherwise, which are the directly adaptive and which are not? how are we to distinguish between them? there can be no direct equilibration with an external agency which, if it acts at all, acts fatally; since the organism to be adapted disappears. conversely, some inaccessible benefit which a small modification in the organism would make accessible, cannot by its action tend to produce this modification: the modification and the benefit do not stand in dynamic relation. the only new incident forces which can work the changes of function and structure required to bring any animal or plant into equilibrium with them, are such incident forces as operate on this animal or plant, either continuously or frequently. they must be capable of appreciably changing that set of complex rhythmical actions and reactions constituting the life of the organism; and yet must not usually produce perturbations that are fatal. let us see what are the limits to direct equilibration hence arising. § . in plants, organs engaged in nutrition, and exposed to variations in the amounts and proportions of matters and forces utilized in nutrition, may be expected to undergo corresponding variations. we find evidence that they do this. the "changes of habit" which are common in plants, when taken to places unlike in climate or soil to those before inhabited by them, are changes of parts in which the modified external actions directly produce modified internal actions. the characters of the stem and shoots as woody or succulent, erect or procumbent; of the leaves in respect of their sizes, thicknesses, and textures; of the roots in their degrees of development and modes of growth; are obviously in immediate relation to the characters of the environment. a permanent difference in the quantity of light or heat affects, day after day, the processes going on in the leaves. habitual rain or drought alters all the assimilative actions, and appreciably influences the organs that carry them on. some particular substance, by its presence in the soil, gives new qualities to some of the tissues; causing greater rigidity or flexibility, and so affecting the general aspect. here then we have changes towards modified sets of functions and structures, in equilibrium with modified sets of external forces. but now let us turn to other classes of organs possessed by plants--organs which are not at once affected in their actions by variations of incident forces. take first the organs of defence. many plants are shielded against animals that would else devour them, by formidable thorns; and others, like the nettle, by stinging hairs. these must be counted among the appliances by which equilibrium is maintained between the actions in the organism and the actions in its environment; seeing that were these defences absent, the destruction by herbivorous animals would be so much increased, that the number of young plants annually produced would not suffice, as it now does, to balance the mortality, and the species would disappear. but these defensive appliances, though they aid in maintaining the balance between inner and outer actions, cannot have been directly called forth by the outer actions which they serve to neutralize; for these outer actions do not continuously affect the functions of the plant even in a general way, still less in the special way required. suppose a species of nettle bare of poison-hairs, to be habitually eaten by some mammal intruding on its habitat. the actions of this mammal would have no direct tendency to develop poison-hairs in the plant; since the individuals devoured could not bequeath changes of structure, even were the actions of a kind to produce fit ones; and since the individuals which perpetuated themselves would be those on which the new incident force had not fallen. organs of another class, similarly circumstanced, are those of reproduction. like the organs of defence these are not, during the life of the individual plant, variably exercised by variable external actions; and therefore do not fulfil those conditions under which structural changes may be directly caused by changes in the environment. the generative apparatus contained in every flower acts only once during its existence; and even then, the parts subserve their ends in a passive rather than an active way. functionally-produced modifications are therefore out of the question. if a plant's anthers are so placed that the insect which most commonly frequents its flowers, must come in contact with the pollen, and fertilize with it other flowers of the same species; and if this insect, dwindling away or disappearing from the locality, leaves behind no insects having such shapes and habits as cause them to do the same thing efficiently, but only some which do it inefficiently; it is clear that this change of its conditions has no immediate tendency to work in the plant any such structural change as shall bring about a new balance with its conditions. for the anthers, which, even when they discharge their functions, do it simply by standing in the way of the insect, are, under the supposed circumstances, left untouched by the insect; and this remaining untouched cannot have the effect of so modifying the stamens as to bring the anthers into a position to be touched by some other insect. only those individuals whose parts of fructification so far differed from the average form that some other insect could serve them as pollen-carrier, would have good chances of perpetuating themselves. and on their progeny, inheriting the deviation, there would act no external force directly tending to make the deviation greater; since the new circumstances to which re-adaptation is required, are such as do not in the least alter the equilibrium of functions constituting the life of the individual plant. § . among animals, adaptation by direct equilibration is similarly traceable wherever, during the life of the individual, an external change generates some constant or repeated change of function. this is conspicuously the case with such parts of an animal as are immediately exposed to diffused influences, like those of climate, and with such parts of an animal as are occupied in its mechanical actions on the environment. of the one class of cases, the darkening of the skin which follows exposure to one or other extreme of temperature, may be taken as an instance; and with the other class of cases we are made familiar by the increase and decrease which use and disuse cause in the organs of motion. it is needless here to exemplify these: they were treated of in the second part of this work. but in animals, as in plants, there are many indispensable offices fulfilled by parts between which and the external conditions they respond to, there is no such action and reaction as can directly produce an equilibrium. this is especially manifest with dermal appendages. some ground exists for the conclusion that the greater or less development of hairs, is in part immediately due to increase or decrease of demand on the passive function, as forming a non-conducting coat; but be this as it may, it is impossible that there can exist any such cause for those immense developments of hairs which we see in the quills of the porcupine, or those complex developments of them known as feathers. such an enamelled armour as is worn by _lepidosteus_, is inexplicable as a direct result of any functionally-worked change. for purposes of defence, such an armour is as needful, or more needful, for hosts of other fishes; and did it result from any direct reaction of the organism against any offensive actions it was subject to, there seems no reason why other fishes should not have developed similar protective coverings. of sundry reproductive appliances the like may be said. the secretion of an egg-shell round the substance of an egg, in the oviduct of a bird, is quite inexplicable as a consequence of some functionally-wrought modification of structure, immediately caused by some modification of external conditions. the end fulfilled by the egg-shell, is that of protecting the contained mass against certain slight pressures and collisions, to which it is liable during incubation. how, by any process of direct equilibration, could it come to have the required thickness? or, indeed, how could it come to exist at all? suppose this protective envelope to be too weak, so that some of the eggs a bird lays are broken or cracked. in the first place, the breakages or crackings are actions which cannot react on the maternal organism in such ways as to cause the secretion of thicker shells for the future: to suppose that they can, is to suppose that the bird understands the cause of the evil, and that the secretion of thicker shells can be effected by its will. in the second place, such developing chicks as are contained in the shells which crack or break, are almost certain to die; and cannot, therefore, acquire appropriately-modified constitutions: even supposing any relation could exist between the impression received and the change required. meanwhile, such eggs as escape breakage are not influenced at all by the requirement; and hence, on the birds developed from them, there cannot have acted any force tending to work the needful adjustment of functions. in no way, therefore, can a direct equilibration between constitution and conditions be here produced. even in organs that can be modified by certain incident actions into correspondence with such incident actions, there are some re-adjustments which cannot be effected by direct balancing. it is thus with the bones. the majority of the bones have to resist muscular strains; and variations in the muscular strains call forth, by reaction, variations in the strengths of the bones. here there is direct equilibration. but though the greater massiveness acquired by bones subject to greater strains, may be ascribed to counter-acting forces evoked by forces brought into action; it is impossible that the acquirement of greater lengths by bones can be thus accounted for. it has been supposed that the elongation of the metatarsals in wading birds, has resulted from direct adaptation to conditions of life. to justify this supposition, however, it must be shown that the mechanical actions and reactions in the legs of a wading bird, differ from those in the legs of other birds; and that the differential actions are equilibrated by the extra lengths. there is not the slightest evidence of this. the metatarsals of a bird have to bear no appreciable strains but those due to the superincumbent weight. standing in the water does not appreciably alter such strains; and even if it did, an increase in the lengths of these bones would not fit them any better to meet the altered strains. § . the conclusion at which we arrive is, then, that there go on in all organisms, certain changes of function and structure that are directly consequent on changes in the incident forces--inner changes by which the outer changes are balanced, and the equilibrium restored. such re-equilibrations, which are often conspicuously exhibited in individuals, we have reason to believe continue in successive generations; until they are completed by the arrival at structures fitted to the modified conditions. but, at the same time, we see that the modified conditions to which organisms may be adapted by direct equilibration, are conditions of certain classes only. that a new external action may be met by a new internal action, it is needful that it shall either continuously or frequently be borne by the individuals of the species, without killing or seriously injuring them; and shall act in such way as to affect their functions. and we find that many of the environing agencies--evil or good--to which organisms have to be adjusted, are not of these kinds: being agencies which either do not immediately affect the functions at all, or else affect them in ways that prove fatal. hence there must be at work some other process which equilibrates the actions of organisms with the actions they are exposed to. plants and animals that continue to exist, are necessarily plants and animals whose powers balance the powers acting on them; and as their environments change, the changes which plants and animals undergo must necessarily be changes towards re-establishment of the balance. besides direct equilibration, there must therefore be an indirect equilibration. how this goes on we have now to inquire. chapter xii. indirect equilibration. § . besides those perturbations produced in any organism by special disturbing forces, there are ever going on many others--the reverberating effects of disturbing forces previously experienced by the individual, or by ancestors; and the multiplied deviations of function so caused imply multiplied deviations of structure. in § there was re-illustrated the truth, set forth at length when treating of adaptation (§ ), that an organism in a state of moving equilibrium, cannot have extra function thrown on any organ, and extra growth produced in such organ, without correlative changes being entailed throughout all other functions, and eventually throughout all other organs. and when treating of variation (§ ), we saw that individuals which have been made, by their different circumstances, to deviate functionally and structurally from the average type in different directions, will bequeath to their joint offspring, compound perturbations of function and compound deviations of structure, endlessly varied in their kinds and amounts. now if the individuals of a species are thus necessarily made unlike in countless ways and degrees--if in one individual the amount of energy in a particular direction is greater than in any other individual, or if here a peculiar combination gives a resulting action which is not found elsewhere; then, among all the individuals, some will be less liable than others to have their equilibria overthrown by a particular incident force previously unexperienced. unless the change in the environment is so violent as to be universally fatal to the species, it must affect more or less differently the slightly-different moving equilibria which the members of the species present. inevitably some will be more stable than others when exposed to this new or altered factor. that is to say, those individuals whose functions are most out of equilibrium with the modified aggregate of external forces, will be those to die; and those will survive whose functions happen to be most nearly in equilibrium with the modified aggregate of external forces. but this survival of the fittest[ ] implies multiplication of the fittest. out of the fittest thus multiplied there will, as before, be an overthrowing of the moving equilibrium wherever it presents the least opposing force to the new incident force. and by the continual destruction of the individuals least capable of maintaining their equilibria in presence of this new incident force, there must eventually be reached an altered type completely in equilibrium with the altered conditions. § . this survival of the fittest, which i have here sought to express in mechanical terms, is that which mr. darwin has called "natural selection, or the preservation of favoured races in the struggle for life." that there goes on a process of this kind throughout the organic world, mr. darwin's great work on the _origin of species_ has shown to the satisfaction of nearly all naturalists. indeed, when once enunciated, the truth of his hypothesis is so obvious as scarcely to need proof. though evidence may be required to show that natural selection accounts for everything ascribed to it, yet no evidence is required to show that natural selection has always been going on, is going on now, and must ever continue to go on. recognizing this as an _à priori_ certainty, let us contemplate it under its two distinct aspects. that organisms which live, thereby prove themselves fit for living, in so far as they have been tried, while organisms which die, thereby prove themselves in some respects unfitted for living, are facts no less manifest than is the fact that this self-purification of a species must tend ever to insure adaptation between it and its environment. this adaptation may be either so _maintained_ or so _produced_. doubtless many who have looked at nature with philosophic eyes, have observed that death of the worst and multiplication of the best, tends towards maintenance of a constitution in harmony with surrounding circumstances. that the average vigour of any race would be diminished did the diseased and feeble habitually survive and propagate; and that the destruction of such, through failure to fulfil some of the conditions to life, leaves behind those which are able to fulfil the conditions to life, and thus keeps up the average fitness to the conditions of life; are almost self-evident truths. but to recognize "natural selection" as a means of preserving an already-established balance between the powers of a species and the forces to which it is subject, is to recognize only its simplest and most general mode of action. it is the more special mode of action with which we are here concerned. this more special mode of action, mr. darwin has been the first to recognize as an all-important factor, though, besides his co-discoverer mr. a. r. wallace, some others have perceived that such a factor is at work. to him we owe due appreciation of the fact that natural selection is capable of _producing_ fitness between organisms and their circumstances. he has worked up an enormous mass of evidence showing that this "preservation of favoured races in the struggle for life," is an ever-acting cause of divergence among organic forms. he has traced out the involved results of the process with marvellous subtlety. he has shown how hosts of otherwise inexplicable facts, are accounted for by it. in brief, he has proved that the cause he alleges is a true cause; that it is a cause which we see habitually in action; and that the results to be inferred from it are in harmony with the phenomena which the organic creation presents, both as a whole and in its details. let us glance at a few of the more important interpretations which the hypothesis furnishes. a soil possessing some ingredient in unusual quantity, may supply to a plant an excess of the matter required for certain of its tissues; and may cause all the parts formed of such tissues to be abnormally developed. suppose that among these are the hairs clothing its surfaces, including those which grow on its seeds. thus furnished with somewhat longer fibres, its seeds, when shed, are carried a little further by the wind before they fall to the ground. the plants growing from them, being rather more widely dispersed than those produced by other individuals of the same species, will be less liable to smother one another; and a greater number may therefore reach maturity and fructify. supposing the next generation subject to the same peculiarity of nutrition, some of the seeds borne by its members will not simply inherit this increased development of hairs, but will carry it further; and these, still more advantaged in the same way as before, will, on the average, have still more numerous chances of continuing the race. thus, by the survival, generation after generation, of those possessing these longer hairs, and the inheritance of successive increments of growth in the hairs, there may result a seed deviating greatly from the original. other individuals of the same species, subject to the different physical conditions of other localities, may develop somewhat thicker or harder coatings to their seeds: so rendering their seeds less digestible by the birds which devour them. such thicker-coated seeds, by escaping undigested more frequently than thinner-coated ones, will have additional chances of growing and leaving offspring; and this process, acting in a cumulative manner season after season, will produce a seed diverging in another direction from the ancestral type. again, elsewhere, some modification in the physiologic actions of the plant may lead to an unusual secretion of an essential oil in the seeds; rendering them unpalatable to creatures which would otherwise feed on them: so giving an advantage to the variety in its rate of multiplication. this incidental peculiarity, proving a preservative, will, as before, be increased by natural selection until it constitutes another divergence. now in such cases, we see that plants may become better adapted, or re-adapted, to the aggregate of surrounding agencies, not through any _direct_ action of such agencies on them, but through their _indirect_ action--through the destruction by them of the individuals least congruous with them, and the survival of those most congruous with them. all these slight variations of function and structure, arising among the members of a species, serve as so many experiments; the great majority of which fail, but a few of which succeed. just as each plant bears a multitude of seeds, out of which some two or three happen to fulfil all the conditions required for reaching maturity and continuing the race; so each species is ever producing numerous slightly-modified forms, deviating in all directions from the average, out of which most fit the surrounding conditions no better than their parents, or not so well, but some few of which fit the conditions better; and, doing so, are enabled the better to preserve themselves, and to produce offspring similarly capable of preserving themselves. among animals the like process results in the like development of various structures which cannot have been affected by the performance of functions--their functions being purely passive. the thick shell of a mollusk cannot have arisen from direct reactions of the organism against the external actions to which it is exposed; but it is quite explicable as an effect of the survival, generation after generation, of individuals whose thicker coverings protected them against enemies. similarly with such dermal structure as that of the tortoise. though we have evidence that the skin, where it is continually exposed to pressure and friction, may thicken, and so re-establish the equilibrium by opposing a greater inner force to a greater outer force; yet we have no evidence that a coat of armour like that of the tortoise can be so produced. nor, indeed, are the conditions under which alone its production in such a manner could be accounted for, fulfilled; since the surface of the tortoise is not exposed to greater pressure and friction than the surfaces of other creatures. this massive carapace, and the strangely-adapted osseous frame-work which supports it, are inexplicable as results of evolution, unless through the process of natural selection. so, too, is it with the formation of odoriferous glands in some mammals, or the growth of such excrescences as those of the camel. thus, in short, is it with all those organs of animals which do not play active parts. besides giving us explanations of structural characters that are otherwise unaccountable, mr. darwin shows how natural selection explains peculiar relations between individuals in certain species. such facts as the dimorphism of the primrose and other flowers, he proves to be in harmony with his hypothesis, though stumbling-blocks to all other hypotheses. the various differences which accompany difference of sex, sometimes slight, sometimes very great, are similarly accounted for. as before suggested (§ ), natural selection appears capable of producing and maintaining the right proportion of the sexes in each species; and it requires but to contemplate the bearings of the argument, to see that the formation of different sexes may itself have been determined in the same way. to convey here an adequate idea of mr. darwin's doctrine, throughout the immense range of its applications, is of course impossible. the few illustrations just given, are intended simply to remind the reader what mr. darwin's hypothesis is, and what are the else insoluble problems which it solves for us. § . but now, though it seems to me that we are thus supplied with a key to phenomena which are multitudinous and varied beyond all conception; it also seems to me that there is a moiety of the phenomena which this key will not unlock. mr. darwin himself recognizes use and disuse of parts, as causes of modifications in organisms; and does this, indeed, to a greater extent than do some who accept his general conclusion. but i conceive that he does not recognize them to a sufficient extent. while he shows that the inheritance of changes of structure caused by changes of function, is utterly insufficient to explain a great mass--probably the greater mass--of morphological phenomena; i think he leaves unconsidered a mass of morphological phenomena which are explicable as results of functionally-produced modifications, and are not explicable as results of natural selection. by induction, as well as by inference from the hypothesis of natural selection, we know that there exists a balance among the powers of organs which habitually act together--such proportions among them that no one has any considerable excess of efficiency. we see, for example, that throughout the vascular system there is maintained an equilibrium of the component parts: in some cases, under continued excess of exertion, the heart gives way, and we have enlargement; in other cases the large arteries give way, and we have aneurisms; in other cases the minute blood-vessels give way--now bursting, now becoming chronically congested. that is to say, in the average constitution, no superfluous strength is possessed by any of the appliances for circulating the blood. take, again, a set of motor organs. great strain here causes the fibres of a muscle to tear. there the muscle does not yield but the tendon snaps. elsewhere neither muscle nor tendon is damaged, but the bone breaks. joining with these instances the general fact that, under the same adverse conditions, different individuals show their slight differences of constitution by going wrong some in one way and some in another; and that even in the same individual, similar adverse conditions will now affect one viscus and now another; it becomes manifest that though there cannot be maintained an accurate balance among the powers of the organs composing an organism, yet their excesses and deficiencies of power are extremely slight. that they must be extremely slight, is, as before said, a deduction from the hypothesis of natural selection. mr. darwin himself argues "that natural selection is continually trying to economise in every part of the organization. if under changed conditions of life a structure before useful becomes less useful, any diminution, however slight, in its development, will be seized on by natural selection, for it will profit the individual not to have its nutriment wasted in building up an useless structure." in other words, if any muscle has more fibres than are required, or if a bone is stronger than needful, no advantage results but rather a disadvantage--a disadvantage which will decrease the chances of survival. hence it follows that among any organs which habitually act in concert, an increase of one can be of no service unless there is a concomitant increase of the rest. the co-operative parts must vary together; otherwise variation will be detrimental. a stronger muscle must have a stronger bone to resist its contractions; must have stronger correlated muscles and ligaments to secure the neighbouring articulations; must have larger blood-vessels to bring it supplies; must have a more massive nerve to transmit stimulus, and some extra development of a nervous centre to supply the extra stimulus. the question arises, then,--do variations of the appropriate kinds occur simultaneously in all these co-operative parts? have we any reason to think that the parts spontaneously increase or decrease together? the assumption that they do seems to me untenable; and its untenability will, i think, become conspicuous if we take a case, and observe how extremely numerous and involved are the variations which must be supposed to occur together. in illustration of another point, we have already considered the modifications required to accompany increased weight of the head (§ ). instead of the bison, the moose deer, or the extinct irish elk, will here best serve our purpose. in this last species the male has enormously-developed horns, used for purposes of offence and defence. these horns, weighing upwards of a hundred-weight, are carried at great mechanical disadvantage: supported as they are, along with the massive skull which bears them, at the extremity of the outstretched neck. further, that these heavy horns may be of use in fighting, the supporting bones and muscles must be strong enough, not simply to carry them, but to put them in motion with the rapidity needed for giving blows. let us, then, ask how, by natural selection, this complex apparatus of bones and muscles can have been developed, _pari passu_ with the horns? if we suppose the horns to have been originally of like size with those borne by other kinds of deer; and if we suppose that in some individual they became larger by spontaneous variation; what would be the concomitant changes required to render their greater size useful? other things equal, the blow given by a larger horn would be a blow given by a heavier mass moving at a smaller velocity: the momentum would be the same as before; and the area of contact with the body struck being somewhat increased, while the velocity was decreased, the injury done would be less. that horns may become better weapons, the whole apparatus concerned in moving them must be so strengthened as to impress more force on them, and to bear the more violent reactions of the blows given. the bones of the skull on which the horns are seated must be thickened; otherwise they will break. the vertebræ of the neck must be further developed; and unless the ligaments which hold together these vertebræ, and the muscles which move them, are also enlarged, nothing will be gained. again the upper dorsal vertebræ and their spines must be strengthened, that they may withstand the stronger contractions of the neck-muscles; and like changes must be made on the scapular arch. still more must there be required a simultaneous development of the bones and muscles of the fore-legs; since these extra growths in the horns, in the skull, in the neck, in the shoulders, add to the burden they have to bear; and without they are strengthened the creature will not only suffer from loss of speed but will fail in fight. hence, to make larger horns of use, additional sizes must be acquired by numerous bones, muscles, and ligaments, as well as by the blood-vessels and nerves on which their actions depend. on calling to mind how the spraining of a single small muscle in the foot incapacitates for walking, or how permanent weakness in a knee-ligament will diminish the power of the leg, it will be seen that unless all these many changes are simultaneously made, they may as well be none of them made--or rather, they would better be none of them made; since the enlargements of some parts, by putting greater strains on connected parts, would render them relatively weaker if they remained unenlarged. can we with any propriety assume that these many enlargements duly proportioned will be simultaneously effected by spontaneous variations? i think not. it would be a strong supposition that the vertebræ and muscles of the neck suddenly became bigger at the same time as the horns. it would be a still stronger supposition that the upper dorsal vertebræ not only at the same time became more massive, but appropriately altered their proportions, by the development of their immense neural spines. and it would be an assumption still more straining our powers of belief, that along with heavier horns there should spontaneously take place the required strengthenings in the bones, muscles, arteries, and nerves of the scapular and the fore-legs. besides the multiplicity of directly-coöperative organs, the multiplicity of organs which do not coöperate, save in the degree implied by their combination in the same organism, seems to me a further hindrance to the development of special structures by natural selection alone. where the life is simple, or where circumstances render some one function supremely important, survival of the fittest may readily bring about the appropriate structural change, without aid from the transmission of functionally-caused modifications. but in proportion as the life grows complex--in proportion as a healthy existence cannot be secured by a large endowment of some one power, but demands many powers; in the same proportion do there arise obstacles to the increase of any particular power by "the preservation of favoured races in the struggle for life." as fast as the faculties are multiplied, so fast does it become possible for the several members of a species to have various kinds of superiorities over one another. while one saves its life by higher speed, another does the like by clearer vision, another by keener scent, another by quicker hearing, another by greater strength, another by unusual power of enduring cold or hunger, another by special sagacity, another by special timidity, another by special courage; and others by other bodily and mental attributes. conditions being alike, each of these life-saving attributes is likely to be transmitted to posterity. but we may not assume that it will be increased in subsequent generations by natural selection. increase of it can result only if individuals possessing average endowments of it are more frequently killed off than individuals highly endowed with it; and this can happen only when the attribute is one of greater importance, for the time being, than most of the other attributes. if those members of the species which have but ordinary shares of it, nevertheless survive by virtue of other superiorities which they severally possess; then it is not easy to see how this particular attribute can be developed by natural selection in subsequent generations. the probability seems rather to be that, by gamogenesis, this extra endowment will, on the average, be diminished in posterity--just serving in the long run to make up for the deficient endowments of those whose special powers lie in other directions; and so to keep up the normal structure of the species. as fast as the number of bodily and mental faculties increases, and as fast as maintenance of life comes to depend less on the amount of any one and more on the combined actions of all; so fast does the production of specialities of character by natural selection alone, become difficult. particularly does this seem to be so with a species so multitudinous in its powers as mankind; and above all does it seem to be so with such of the human powers as have but minor shares in aiding the struggle for life--the æsthetic faculties, for example. it by no means follows, however, that in cases of this kind, and cases of the preceding kind, natural selection plays no part. wherever it is not the chief agent in working organic changes, it is still, very generally, a secondary agent. the survival of the fittest must nearly always further the production of modifications which produce fitness, whether they be incidental modifications, or modifications caused by direct adaptation. evidently, those individuals whose constitutions have facilitated the production in them of any structural change consequent on any functional change demanded by some new external condition, will be the individuals most likely to live and to leave descendants. there must be a natural selection of functionally-acquired peculiarities, as well as of spontaneously-acquired peculiarities; and hence such structural changes in a species as result from changes of habit necessitated by changed circumstances, natural selection will render more rapid than they would otherwise be. there are, however, some modifications in the sizes and forms of parts, which cannot have been aided by natural selection; but which must have resulted wholly from the inheritance of functionally-caused alterations. the dwindling of organs of which the undue sizes entail no appreciable evils, furnishes the best evidence of this. take, for an example, that diminution of the jaws and teeth which characterizes the civilized races, as contrasted with the savage races.[ ] how can the civilized races have been benefited in the struggle for life, by the slight decrease in these comparatively-small bones? no functional superiority possessed by a small jaw over a large jaw in civilized life, can be named as having caused the more frequent survival of small-jawed individuals. the only advantage accompanying smallness of jaw, is the advantage of economized nutrition; and this cannot be great enough to further the preservation of those distinguished by it. the decrease of weight in the jaw and co-operative parts, which has arisen in the course of thousands of years, does not amount to more than a few ounces. this decrease has to be divided among the many generations which have lived and died in the interval. let us admit that the weight of these parts diminished to the extent of an ounce in a single generation (which is a large admission); it still cannot be contended that the having to carry an ounce less in weight, and to keep in repair an ounce less of tissue, could sensibly affect any man's fate. and if it never did this--nay, if it did not cause a _frequent_ survival of small-jawed individuals where large-jawed individuals died; natural selection could neither cause nor aid diminution of the jaw and its appendages. here, therefore, the decreased action which has accompanied the growth of civilized habits (the use of tools and the disuse of coarse food), must have been the sole cause at work. through direct equilibration, diminished external stress on these parts has resulted in diminution of the internal forces by which this stress is met. from generation to generation, this lessening of the parts consequent on functional decline has been inherited. and since the survival of individuals must always have been determined by more important structural traits, this trait can have neither been facilitated nor retarded by natural selection. § . returning from these extensive classes of facts for which mr. darwin's hypothesis does not account, to the still more extensive classes of facts for which it does account, and which are unaccountable on any other hypothesis; let us consider in what way this hypothesis is expressible in terms of the general doctrine of evolution. already it has been pointed out that the evolving of modified types by "natural selection or the preservation of favoured races in the struggle for life," must be a process of equilibration; since it results in the production of organisms which are in equilibrium with their environments. at the outset of this chapter, something was done towards showing how this continual survival of the fittest may be understood as the progressive establishment of a balance between inner and outer forces. here, however, we must consider the matter more closely. on previous occasions we have contemplated the assemblage of individuals composing a species, as an aggregate in a state of moving equilibrium. we have seen that its powers of multiplication give it an expansive energy which is antagonized by other energies; and that through the rhythmical variations in these two sets of energies there is maintained an oscillating limit to its habitat, and an oscillating limit to its numbers. on another occasion (§ ) it was shown that the aggregate of individuals constituting a species, has a kind of general life which, "like the life of an individual, is maintained by the unequal and ever-varying actions of incident forces on its different parts." we saw that "just as, in each organism, incident forces constantly produce divergences from the mean state in various directions, which are constantly balanced by opposite divergences indirectly produced by other incident forces; and just as the combination of rhythmical functions thus maintained, constitutes the life of the organism; so, in a species there is, through gamogenesis, a perpetual neutralization of those contrary deviations from the mean state, which are caused in its different parts by different sets of incident forces; and it is similarly by the rhythmical production and compensation of these contrary deviations that the species continues to live." hence, to understand how a species is affected by causes which destroy some of its units and favour the multiplication of others, we must consider it as a whole whose parts are held together by complex forces that are ever re-balancing themselves--a whole whose moving equilibrium is continually disturbed and continually rectified. thus much premised, let us next call to mind how moving equilibria in general are changed. in the first place, a new incident force falling on any part of an aggregate with balanced motions, produces a new motion in the direction of least resistance. in the second place, the new incident force is gradually used up in overcoming the opposing forces, and when it is all expended the opposing forces produce a recoil--a reverse deviation which counter-balances the original deviation. consequently, to consider whether the moving equilibrium of a species is modified in the same way as moving equilibria in general, is to consider whether, when exposed to a new force, a species yields in the direction of least resistance; and whether, by its thus yielding, there is generated in the species a compensating change in the opposite direction. we shall find that it does both these things. for what, expressed in mechanical terms, is the effect wrought on a species by some previously-unknown enemy, that kills such of its members as fail in defending themselves? the disappearance of those individuals which meet the destroying forces by the smallest preserving forces, is tantamount to the yielding of the species as a whole at the places where the resistances are the least. or if by some general influence, such as alteration of climate, the members of a species are subject to increase of external actions which are ever tending to overthrow their equilibria, and which they are ever counter-balancing by certain physiological actions, which are the first to die? those least able to generate the internal energies which antagonize these external energies. if the change be an increase of the winter's cold, then such members of the species as have unusual powers of getting food or of digesting food, or such as are by their constitutional aptitude for making fat, furnished with reserve stores of force, available in times of scarcity, or such as have the thickest coats and so lose least heat by radiation, survive; and their survival implies that in each of them the moving equilibrium of functions presents such an adjustment of internal forces, as prevents overthrow by the modified aggregate of external forces. conversely, the members which die are, other things equal, those deficient in the power of meeting the new action by an equivalent counter-action. thus, in all cases, a species considered as an aggregate in a state of moving equilibrium, has its state changed by the yielding of its fluctuating mass wherever this mass is weakest in relation to the special forces acting on it. the conclusion is, indeed, a truism. but now what must follow from the destruction of the least-resisting individuals and survival of the most-resisting individuals? on the moving equilibrium of the species as a whole, existing from generation to generation, the effect of this deviation from the mean state is to produce a compensating deviation. for if all such as are deficient of power in a certain direction are destroyed, what must be the effect on posterity? had they lived and left offspring, the next generation would have had the same average powers as preceding generations: there would have been a like proportion of individuals less endowed with the needful power, and individuals more endowed with it. but the more-endowed individuals being alone left to continue the race, there must result a new generation characterized by a larger average endowment of this power. that is to say, on the moving equilibrium of a species, an action producing change in a given direction is followed, in the next generation, by a reaction producing an opposite change. observe, too, that these effects correspond in their degrees of violence. if the alteration of some external factor is so great that it leaves alive only the few individuals possessing extreme endowments of the power required to antagonize it; then, in succeeding generations, there is a rapid multiplication of individuals similarly possessing extreme endowments of this power--the force impressed calls out an equivalent conflicting force. moreover, the change is temporary where the cause is temporary, and permanent where the cause is permanent. all that are deficient in the needful attribute having been killed off, and the survivors having the needful attribute in a comparatively high degree, there will descend from them, not only some possessing equal amounts of this attribute with themselves, but also some possessing less amounts of it. if the destructive agency has not continued in action, such less-endowed individuals will multiply; and the species, after sundry oscillations, will return to its previous mean state. but if this agency be a persistent one, such less endowed individuals will be continually killed off, and eventually none but highly-endowed individuals will be produced--a new moving equilibrium, adapted to the new environing conditions, will result. it may be objected that this mode of expressing the facts does not include the cases in which a species becomes modified in relation to surrounding agencies of a passive kind--cases like that of a plant which acquires hooked seed-vessels, by which it lays hold of the skins of passing animals, and makes them the distributors of its seeds--cases in which the outer agency has no direct tendency at first to affect the species, but in which the species so alters itself as to take advantage of the outer agency. to cases of this kind, however, the same mode of interpretation applies on simply changing the terms. while, in the aggregate of influences amid which a species exists, there are some which tend to overthrow the moving equilibria of its members, there are others which facilitate the maintenance of their moving equilibria, and some which are capable of giving their moving equilibria increased stability: instance the spread into their habitat of some new kind of prey, which is abundant at seasons when other prey is scarce. now what is the process by which the moving equilibrium in any species becomes adapted to some additional external factor furthering its maintenance? instead of an increased resistance to be met and counterbalanced, there is here a diminished resistance; and the diminished resistance is equilibrated in the same way as the increased resistance. as, in the one case, there is a more frequent survival of individuals whose peculiarities enable them to resist the new adverse factor; so, in the other case, there is a more frequent survival of individuals whose peculiarities enable them to take advantage of the new favourable factor. in each member of the species, the balance of functions and correlated arrangement of structures, differ slightly from those existing in other members. to say that among all its members, one is better fitted than the rest to benefit by some before-unused agency in the environment, is to say that its moving equilibrium is, in so far, more stably adjusted to the sum of surrounding influences. and if, consequently, this individual maintains its moving equilibrium when others fail, and has offspring which do the like--that is, if individuals thus characterized multiply and supplant the rest; there is, as before, a process which effects equilibration between the organism and its environment, not immediately but mediately, through the continuous intercourse between the species as a whole and the environment. § . thus we see that indirect equilibration does whatever direct equilibration cannot do. all these processes by which organisms are re-fitted to their ever-changing environments, must be equilibrations of one kind or other. as authority for this conclusion, we have not simply the universal truth that change of every order is towards equilibrium; but we have also the truth that life itself is a moving equilibrium between inner and outer actions--a continuous adjustment of internal relations to external relations; or the maintenance of a balance between the forces to which an organism is subject and the forces which it evolves. hence all changes which enable a species to live under altered conditions, are changes towards equilibrium with the altered conditions; and therefore those which do not come within the class of direct equilibrations, must come within the class of indirect equilibrations. and now we reach an interpretation of natural selection regarded as a part of evolution at large. as understood in _first principles_, evolution is a continuous redistribution of matter and motion; and a process of evolution which is not expressible in terms of matter and motion has not been reduced to its ultimate form. the conception of natural selection is manifestly one not known to physical science: its terms are not of a kind physical science can take cognisance of. but here we have found in what manner it may be brought within the realm of physical science. rejecting metaphor we see that the process called natural selection is literally a survival of the fittest; and the outcome of the above argument is that survival of the fittest is a maintenance of the moving equilibrium of the functions in presence of outer actions: implying the possession of an equilibrium which is relatively stable in contrast with the unstable equilibria of those which do not survive. chapter xiii. the co-operation of the factors. § . thus the phenomena of organic evolution may be interpreted in the same way as the phenomena of all other evolution. fully to see this, it will be needful for us to contemplate in their _ensemble_, the several processes separately described in the four preceding chapters. if the forces acting on any aggregate remain the same, the changes produced by them will presently reach a limit, at which the outer forces are balanced by the inner forces; and thereafter no further metamorphosis will take place. hence, that there may be continuous changes of structure in organisms, there must be continuous changes in the incident forces. this condition to the evolution of animal and vegetal forms, we find to be fully satisfied. the astronomic, geologic, and meteorologic changes that have been slowly but incessantly going on, and have been increasing in the complexity of their combinations, have been perpetually altering the circumstances of organisms; and organisms, becoming more numerous in their kinds and higher in their kinds, have been perpetually altering one another's circumstances. thus, for those progressive modifications upon modifications which organic evolution implies, we find a sufficient cause. the increasing inner changes for which we thus find a cause in the perpetual outer changes, conform, so far as we can trace them, to the universal law of the instability of the homogeneous. in organisms, as in all other things, the exposure of different parts to different kinds and amounts of incident forces, has necessitated their differentiation; and, for the like reason, aggregates of individuals have been lapsing into varieties, and species, and genera, and orders. further, in each type of organism, as in the aggregate of types, the multiplication of effects has continually aided this transition from a more homogeneous to a more heterogeneous state. and yet again, that increasing segregation and concomitant increasing definiteness, associated with the growing heterogeneity of organisms, has been aided by the continual destruction of those which expose themselves to aggregates of external actions markedly incongruous with the aggregates of their internal actions, and the survival of those subject only to comparatively small incongruities. finally, we have found that each change of structure, superposed on preceding changes, has been a re-equilibration necessitated by the disturbance of a preceding equilibrium. the maintenance of life being the maintenance of a balanced combination of functions, it follows that individuals and species that have continued to live, are individuals and species in which the balance of functions has not been overthrown. hence survival through successive changes of conditions, implies successive adjustments of the balance to the new conditions. the actions that are here specified in succession, are in reality simultaneous; and they must be so conceived before organic evolution can be rightly understood. some aid towards so conceiving them will be given by the annexed table, representing the co-operation of the factors. § . respecting this co-operation, it remains only to point out the respective shares of the factors in producing the total result; and the way in which the proportions of their respective shares vary as evolution progresses. astronomic } changes } } alter the } geologic } incidence } changes } of inorganic } } forces. } meteorologic } } changes } } } } } on each species: affecting } | } | } | } | } | enemies } } | competitors } } | } varying in } } | co-operators } number } } | prey } } alter the } | } incidence } | enemies } } of organic } | competitors } } forces. } | } varying in } | co-operators } kind } | prey } | | ----------------------------------------------------------------- | | { which, partially in the first | { generation, and completely in | { the course of generations, are | { directly equilibrated with the | { changed agencies. | { immediately { | { through their { which have their direct | { functions; { equilibration with the changed | { { agencies, aided by indirect | { { equilibration, through the more | { { frequent survival of those in | { { which the direct equilibration | { { is most rapid. | { | { its individuals, { { positively--by aiding the | { { { multiplication of those whose | { { { moving equilibria happen to be | { { { most congruous with the | { { mediately { changed agencies: thus, in the | { { through the { course of generations, indirectly | { { aggregate of { equilibrating certain individuals | { { individuals; { with them. --{ { { { negatively--by killing those { { whose moving equilibria are { { most incongruous with the { { changed agencies: thus, in { { the course of generations, { { indirectly equilibrating each { { of its surviving individuals { { with them. { { { by acting on it in some parts of the habitat { { more than in others; and thus differentiating { its aggregate { the species into local varieties. { of individuals, { { { and thus causing { { differentiations of { { the species into { by acting differently { varieties, irrespective { on slightly-unlike { of locality. { individuals in the { { same locality; { and thus causing { modification of the { species as a whole, { by abstracting a { certain class of { its units. at first, changes in the amounts and combinations of inorganic forces, astronomic, geologic, and meteorologic, were the only causes of the successive modifications; and these changes have continued to be causes. but as, through the diffusion of organisms and consequent differential actions of inorganic forces, there arose unlikenesses among them, producing varieties, species, genera, orders, classes, the actions of organisms on one another became new sources of organic modifications. and as fast as types have multiplied and become more complex, so fast have the mutual actions of organisms come to be more influential factors in their respective evolutions: eventually becoming the chief factors. passing from the external causes of change to the internal processes of change entailed by them, we see that these, too, have varied in their proportions: that which was originally the most important and almost the sole process, becoming gradually less important, if not at last the least important. always there must have been, and always there must continue to be, a survival of the fittest; natural selection must have been in operation at the outset, and can never cease to operate. while yet organisms had small abilities to coordinate their actions, and adjust them to environing actions, natural selection worked almost alone in moulding and remoulding organisms into fitness for their changing environments; and natural selection has remained almost the sole agency by which plants and inferior orders of animals have been modified and developed. the equilibration of organisms that are almost passive, is necessarily effected indirectly, by the action of incident forces on the species as a whole. but along with the evolution of organisms having some activity, there grows up a kind of equilibration which is in part direct. in proportion as the activity increases direct equilibration plays a more important part. until, when the nervo-muscular apparatus becomes greatly developed, and the power of varying the actions to fit the varying requirements becomes considerable, the share taken by direct equilibration rises into co-ordinate importance or greater importance. as fast as essential faculties multiply, and as fast as the number of organs which co-operate in any given function increases, indirect equilibration through natural selection becomes less and less capable of producing specific adaptations; and remains capable only of maintaining the general fitness of constitution to conditions. the production of adaptations by direct equilibration then takes the first place: indirect equilibration serving to facilitate it. until at length, among the civilized human races, the equilibration becomes mainly direct: the action of natural selection being limited to the destruction of those who are constitutionally too feeble to live, even with external aid. as the preservation of incapables is secured by our social arrangements; and as very few save incarcerated criminals are prevented by their inferiorities from leaving the average number of offspring; it results that survival of the fittest can scarcely at all act in such way as to produce specialities of nature, either bodily or mental. here the specialities of nature, chiefly mental, which we see produced, and which are so rapidly produced that a few centuries show a considerable change, must be ascribed almost wholly to direct equilibration.[ ] chapter xiv. the convergence of the evidences. § . of the three classes of evidences that have been assigned in proof of evolution, the _à priori_, which we took first, were partly negative, partly positive. on considering the "general aspects of the special-creation hypothesis," we discovered it to be worthless. discredited by its origin, and wholly without any basis of observed fact, we found that it was not even a thinkable hypothesis; and, while thus intellectually illusive, it turned out to have moral implications irreconcilable with the professed beliefs of those who hold it. contrariwise, the "general aspects of the evolution-hypothesis" begot the stronger faith in it the more nearly they were considered. by its lineage and its kindred, it was found to be as closely allied with the proved truths of modern science, as is the antagonist hypothesis with the proved errors of ancient ignorance. we saw that instead of being a mere pseud-idea, it admits of elaboration into a definite conception: so showing its legitimacy as an hypothesis. instead of positing a purely fictitious process, the process which it alleges proves to be one actually going on around us. to which add that, morally considered, this hypothesis presents no radical incongruities. thus, even were we without further means of judging, there could be no rational hesitation which of the two views should be entertained. § . further means of judging, however, we found to be afforded by bringing the two hypotheses face to face with the general truths established by naturalists. these inductive evidences were dealt with in four chapters. "the arguments from classification" were these. organisms fall into groups within groups; and this is the arrangement which we see results from evolution, where it is known to take place. of these groups within groups, the great or primary ones are the most unlike, the sub-groups are less unlike, the sub-sub-groups still less unlike, and so on; and this, too, is a characteristic of groups demonstrably produced by evolution. moreover, indefiniteness of equivalence among the groups is common to those which we know have been evolved, and those here supposed to have been evolved. and then there is the further significant fact, that divergent groups are allied through their lowest rather than their highest members. of "the arguments from embryology," the first is that when developing embryos are traced from their common starting point, and their divergences and re-divergences symbolized by a genealogical tree, there is manifest a general parallelism between the arrangement of its primary, secondary, and tertiary branches, and the arrangement of the divisions and sub-divisions of our classifications. nor do the minor deviations from this general parallelism, which look like difficulties, fail, on closer observation, to furnish additional evidence; since those traits of a common ancestry which embryology reveals, are, if modifications have resulted from changed conditions, liable to be disguised in different ways and degrees in different lines of descendants. we next considered "the arguments from morphology." apart from those kinships among organisms disclosed by their developmental changes, the kinships which their adult forms show are profoundly significant. the unities of type found under such different externals, are inexplicable except as results of community of descent with non-community of modification. again, each organism analyzed apart, shows, in the likenesses obscured by unlikenesses of its component parts, a peculiarity which can be ascribed only to the formation of a more heterogeneous organism out of a more homogeneous one. and once more, the existence of rudimentary organs, homologous with organs that are developed in allied animals or plants, while it admits of no other rational interpretation, is satisfactorily interpreted by the hypothesis of evolution. last of the inductive evidences, came "the arguments from distribution." while the facts of distribution in space are unaccountable as results of designed adaptation of organisms to their habitats, they are accountable as results of the competition of species, and the spread of the more fit into the habitats of the less fit, followed by the changes which new conditions induce. though the facts of distribution in time are so fragmentary that no positive conclusion can be drawn, yet all of them are reconcilable with the hypothesis of evolution, and some of them yield it strong support: especially the near relationship existing between the living and extinct types in each great geographical area. thus of these four groups, each furnished several arguments which point to the same conclusion; and the conclusion pointed to by the arguments of any one group, is that pointed to by the arguments of every other group. this coincidence of coincidences would give to the induction a very high degree of probability, even were it not enforced by deduction. but the conclusion deductively reached, is in harmony with the inductive conclusion. § . passing from the evidence that evolution has taken place, to the question--how has it taken place? we find in known agencies and known processes, adequate causes of its phenomena. in astronomic, geologic, and meteorologic changes, ever in progress, ever combining in new and more involved ways, we have a set of inorganic factors to which all organisms are exposed; and in the varying and complicating actions of organisms on one another, we have a set of organic factors that alter with increasing rapidity. thus, speaking generally, all members of the earth's flora and fauna experience perpetual re-arrangements of external forces. each organic aggregate, whether considered individually or as a continuously-existing species, is modified afresh by each fresh distribution of external forces. to its pre-existing differentiations new differentiations are added; and thus that lapse to a more heterogeneous state, which would have a fixed limit were the circumstances fixed, has its limit perpetually removed by the perpetual change of the circumstances. these modifications upon modifications which result in evolution structurally considered, are the accompaniments of those functional alterations continually required to re-equilibrate inner with outer actions. that moving equilibrium of inner actions corresponding with outer actions, which constitutes the life of an organism, must either be overthrown by a change in the outer actions, or must undergo perturbations that cannot end until there is a re-adjusted balance of functions and correlative adaptation of structures. but where the external changes are either such as are fatal when experienced by the individuals, or such as act on the individuals in ways that do not affect the equilibrium of their functions; then the re-adjustment results through the effects produced on the species as a whole--there is indirect equilibration. by the preservation in successive generations of those whose moving equilibria are least at variance with the requirements, there is produced a changed equilibrium completely in harmony with the requirements. § . even were this the whole of the evidence assignable for the belief that organisms have been gradually evolved, it would have a warrant higher than that of many beliefs which are regarded as established. but the evidence is far from exhausted. at the outset it was remarked that the phenomena presented by the organic world as a whole, cannot be properly dealt with apart from the phenomena presented by each organism, in the course of its growth, development, and decay. the interpretation of either implies interpretation of the other; since the two are in reality parts of one process. hence, the validity of any hypothesis respecting the one class of phenomena, may be tested by its congruity with phenomena of the other class. we are now about to pass to the more special phenomena of development, as displayed in the structures and functions of individual organisms. if the hypothesis that plants and animals have been progressively evolved be true, it must furnish us with keys to these phenomena. we shall find that it does this; and by doing it gives numberless additional vouchers for its truth. chapter xiv^a. recent criticisms and hypotheses. § a. since the first edition of this work was published, and more especially since the death of mr. darwin, an active discussion of the evolution hypothesis has led to some significant results. that organic evolution has been going on from the dawn of life down to the present time, is now a belief almost universally accepted by zoologists and botanists--"almost universally," i say, because the surviving influence of cuvier prevents acceptance of it by some of them in france. omitting the ideas of these, all biological interpretations, speculations, and investigations, tacitly assume that organisms of every kind in every era and in every region have come into existence by the process of descent with modification. but while concerning the fact of evolution there is agreement, concerning its causes there is disagreement. the ideas of naturalists have, in this respect, undergone a differentiation increasingly pronounced; which has ended in the production of two diametrically opposed beliefs. the cause which mr. darwin first made conspicuous has come to be regarded by some as the sole cause; while, on the part of others there has been a growing recognition of the cause which he at first disregarded but afterwards admitted. prof. weismann and his supporters contend that natural selection suffices to explain everything. contrariwise, among many who recognize the inheritance of functionally-produced changes, there are a few, like the rev. prof. henslow, who regard it as the sole factor. the foregoing chapters imply that the beliefs of neither extreme are here adopted. agreeing with mr. darwin that both factors have been operative, i hold that the inheritance of functionally-caused alterations has played a larger part than he admitted even at the close of his life; and that, coming more to the front as evolution has advanced, it has played the chief part in producing the highest types. i am not now about to discuss afresh these questions, but to deal with certain further questions. for while there has been taking place in the biological world the major differentiation above indicated, there have been taking place certain minor differentiations--there have been arising special views respecting the process of organic evolution. concerning each of these it is needful to say something. § b. among the implied controversies the most conspicuous one has concerned the alleged process called by prof. weismann _panmixia_--a process which dr. romanes had foreshadowed under the name of "the cessation of selection." dr. romanes says:--"at that time it appeared to me, as it now appears to weismann, entirely to supersede the necessity of supposing that the effect of disuse is ever inherited in any degree at all."[ ] the alleged mode of action is exemplified by prof. weismann as follows:-- "a goose or a duck must possess strong powers of flight in the natural state, but such powers are no longer necessary for obtaining food when it is brought into the poultry-yard, so that a rigid selection of individuals with well-developed wings, at once ceases among its descendants. hence in the course of generations, a deterioration of the organs of flight must necessarily ensue, and the other members and organs of the bird will be similarly affected."[ ] here, and throughout the arguments of those who accept the hypothesis of panmixia, there is an unwarranted assumption--nay, an assumption at variance with the doctrine in support of which it is made. it is contended that in such cases as the one given there will, apart from any effects of disuse, be decrease in the disused organs because, not being kept by natural selection up to the level of strength previously needed, they will vary in the direction of decrease; and that variations in the direction of decrease, occurring in some individuals, will, by interbreeding, produce an average decrease throughout the species. but why will the disused organs vary in the direction of decrease more than in the direction of increase? the hypothesis of natural selection postulates indeterminate variations--deviations no more in one direction than in the opposite direction: implying that increases and decreases of size will occur to equal extents and with equal frequencies. with any other assumption the hypothesis lapses; for if the variations in one direction exceed those in another the question arises--what makes them do this? and whatever makes them do this becomes the essential cause of the modification: the selection of favourable variations is tacitly admitted to be an insufficient explanation. but if the hypothesis of natural selection itself implies the occurrence of equal variations on all sides of the mean, how can panmixia produce decrease? _plus_ deviations will cancel _minus_ deviations, and the organ will remain where it was.[ ] "but you have forgotten the tendency to economy of growth," will be the reply--"you have forgotten that in mr. darwin's words 'natural selection is continually trying to economize in every part of the organization;' and that this is a constant cause favouring _minus_ variations." i have not forgotten it; but have remembered it as showing how, to support the hypothesis of panmixia, there is invoked the aid of that very hypothesis which it is to replace. for this principle of economy is but another aspect of the principle of functionally-produced modifications. nearly forty years ago i contended that "the different parts of ... an individual organism compete for nutriment; and severally obtain more or less of it according as they are discharging more or less duty:"[ ] the implication being that as all other organs are demanding blood, decrease of duty in any one, entailing decreased supply of blood, brings about decreased size. in other words, the alleged economy is nothing else than the abstraction, by active parts, of nutriment from an inactive part; and is merely another name for functionally-produced decrease. so that if the variations are supposed to take place predominantly in the direction of decrease, it can only be by silently assuming the cause which is overtly denied. but now we come to the strange fact that the particular case in which panmixia is assigned in disproof of alleged inheritance of functionally-produced modifications, is a case in which it would be inapplicable even were its assumption legitimate--the case of disused organs in domestic animals. for since nutrition is here abundant, the principle of economy under the form alleged does not come into play. contrariwise, there even occurs a partial re-development of rudimentary organs: instances named by mr. darwin being the supplementary mammæ in cows, fifth toes on the hind feet of dogs, spurs and comb in hens, and canine teeth in mares. now clearly, if organs disused for innumerable generations may thus vary in the direction of increase, it must, _a fortiori_, be so with recently disused organs, and there disappears all plea (even the illegitimate plea) for assuming that in the wing of a wild duck which has become domesticated, the _minus_ variations will exceed the _plus_ variations: the hypothesis of panmixia loses its postulate. if it be said that mr. darwin's argument is based on the changed ratio between the weights of leg-bones and wing-bones, and that this changed ratio may result not from decrease of the wing-bones but from increase of the leg-bones, then there comes a fatal reply. such, increase cannot be ascribed to selection of varieties, since there is no selective breeding to obtain larger legs, and as it is not pretended that panmixia accounts for increase the case is lost: there remains no cause for such increase save increase of function. § c. the doctrine of determinate evolution or definitely-directed evolution, which appears to be in one form or other entertained by sundry naturalists, has been set forth by the late prof. eimer under the title "orthogenesis." a distinct statement of his conception is not easily made for the reason that, as i think, the conception itself is indistinct. here are some extracts from a translation of his paper published at chicago. out of these the reader may form a notion of the theory: "orthogenesis shows that organisms develop in definite directions without the least regard for utility through purely physiological causes as the result of _organic growth_, as i term the process." "i am concerned in this paper with definitely directed evolution as the cause of transmutation, and not with the effects of the use and activity of organs which with lamarck i adopted as the second main explanatory cause thereof." "the causes of definitely directed evolution are contained, according to my view, in the effects produced by outward circumstances and influences such as climate and nutrition upon the constitution of a given organism." "at variance with all the facts of definitely directed evolution ... is also the contention of my opponent [weismann] ... that the variations demonstrably oscillate to and fro in the most diverse directions about a given zero-point. there is no oscillation in the direction of development, but simply an advance forwards in a straight line with occasional lateral divergences whereby the forkings of the ancestral tree are produced."[ ] these sentences contain one of those explanations which explain nothing; for we are not enabled to see how the "outward circumstances and influences" produce the effects ascribed to them. we are not shown in what way they cause organic evolution in general, still less in what way they cause the infinitely-varied forms in which organic evolution results. the assertion that evolution takes definitely-directed lines is accompanied by no indication of the reasons why particular lines are followed rather than others. in short, we are simply taken a step back, and for further interpretation referred to a cause said to be adequate, but the operations of which we are to imagine as best we may. this is a re-introduction of supernaturalism under a disguise. it may pair off with the conception made popular by the _vestiges of the natural history of creation_, in which it was contended that there exists a persistent tendency towards the birth of a higher form of creature; or it may be bracketed with the idea entertained by the late prof. owen, who alleged an "ordained becoming" of living things. § d. an objection to the darwinian doctrine which has risen into prominence, is that natural selection does not explain that which it professes to explain. in the words of mr. j. t. cunningham:-- "everybody knows that the theory of natural selection was put forward by darwin as a theory of the origin of species, and yet it is only a theory of the origin of adaptations. the question is: are the differences between species differences of adaptation? if so, then the origin of species and the origin of adaptations are equivalent terms. but there is scarcely a single instance in which a specific character has been shown to be useful, to be adaptive."[ ] to illustrate this last statement mr. cunningham names the plaice, flounder, and dab as three flat fishes in which, along with the adaptive characters related to the mode of life common to them all, each has specific characters which are not adaptive. no evidence is forthcoming that these in any way conduce to the welfare of the species. two propositions are here involved which should be separately dealt with. the first is that the adaptive modifications which survival of the fittest is able to produce, do not become specific traits: they are traits separate in kind from those which mark off groups proved to be specifically distinct by their inability to breed together. such evidence as we at present have seems to warrant this statement. out of the many varieties of dogs most, if not all, have been rendered distinct by adaptive modifications, mostly produced by selection. but, notwithstanding the immense divergences of structure so produced, the varieties inter-breed. to this, however, it may be replied that sufficient time has not elapsed--that the process by which a structural adaptation so reacts on the constitution as to make it a distinct one, possibly, or probably, takes many thousands of years. let us accept for the moment lord kelvin's low estimate of the geologic time during which life has existed--one hundred million years. suppose we divide that time into as many parts as there are hours occupied in the development of a human foetus. and suppose that during these hundred million years there has been going on with some uniformity the evolution of the various organic types now existing. then the amount of change undergone by the foetus in an hour, will be equivalent to the amount of change undergone by an evolving organic form in fifteen thousand years. that is to say, during general evolution it may have taken fifteen thousand years to establish, as distinct, two species differing from one another no more than the foetus differs from itself after the lapse of an hour. hence, though we lack proof that adaptive modifications become specific traits, it is quite possible that they are in course of becoming specific traits. the converse proposition, that the traits by which species are ordinarily distinguished are non-adaptive traits is well sustained; and the statement that, if not themselves useful they are correlated with those which are useful, is, to say the least, unproved. for the instances given by mr. darwin of correlated traits are not those between adaptive traits and the traits regarded as specific, but between traits none of which are specific; as between skull and limbs in swine, tusks and bristles in swine, horns and wool in sheep, beak and feet in pigeons. if we seek a clue in those processes by which correlations are brought about--the physiological actions and reactions--we may at once see that any organic modification, be it adaptive or not, must entail secondary modifications throughout the rest of the organism, most of them insensible but some of them sensible. the competition for blood among organs, referred to above, necessitates that, other things remaining the same, the extra growth of any one tells on all others, in variable degrees according to conditions, and may cause appreciable diminutions of some. this is not all. while the quantity of blood supplied to other organs is affected, its quality also is in some cases affected. each organ, or at any rate each class of organs, has special nutrition--abstracts from the blood a proportion of ingredients different from that abstracted by other organs or classes of organs. hence may result a deficiency or a surplus of some element: instance the change in the blood which must be caused by growth of a stag's antlers. now if such effects are always produced, and if, further, a change of general nutrition caused by a new food or by a difference of ability to utilize certain components of food, similarly operates (instance the above named correlation between horns and wool), then every modification must entail throughout the organism multitudinous alterations of structure. such alterations will ordinarily be neither in themselves useful nor necessarily correlated with those which are useful; since they must arise as concomitants of any change, whether adaptive or not. there will consequently arise the innumerable minute differences presented by allied species in addition to the differences called specific. on joining with recognition of this general process a recognition of the tendency towards localization of deposit, one possible origin of specific marks is suggested. when in an organism the circulating fluids contain useless matter, normal or abnormal, the excretion of it, once determined towards a certain place, continues at that place. trees furnish examples in the casting out of gums and resins. animal life yields evidence in gouty concretions and such morbid products as tubercle. a place of enfeebled nutrition is commonly chosen--not unfrequently a place where a local injury has occurred. now if we extend this principle, well recognized in pathological processes, to physiological processes, we may infer that where an adaptive modification has so reacted on the blood as to leave some matter to be got rid of, the deposit of this, initiated at some place of least resistance, may produce a local structure which eventually becomes a species-mark. a relevant inquiry suggests itself--what proportion of species-marks are formed out of inanimate tissue or tissue of low vitality--tissue which, like hair, feathers, horns, teeth, is composed of by-products unfit for carrying on vital actions. § e. in the days when, not having been better instructed by mr. darwin, i believed that all changes of structure in organisms result from changes of function, i held that the cause of such changes of function is migration. assuming as the antecedent of migration a great geologic change, such as upheaval of the east indian archipelago step by step into a continent, it was argued, in an essay i then wrote, that, subjected primarily to new influences in its original habitat, each kind of plant and animal would secondarily be subjected to the altered conditions consequent on spreading over the upheaved regions. "each species being distributed over an area of some extent, and tending continually to colonize the new area exposed, its different members would be subject to different sets of changes. plants and animals spreading towards the equator would not be affected in the same way with others spreading from it. those spreading towards the new shores would undergo changes unlike the changes undergone by those spreading into the mountains. thus, each original race of organisms would become the root from which diverged several races differing more or less from it and from one another." it was further argued that, beyond modifications caused by change of physical conditions and food, others would be caused by contact of the flora and fauna of each island with the floras and faunas of other islands: bringing experience of animals and plants before unknown.[ ] while this conception was wrong in so far as it ascribed the production of new species entirely to inheritance of functionally-wrought alterations (thus failing to recognize natural selection, which was not yet enunciated), it was right in so far as it ascribed organic changes to changes of conditions. and it was, i think, also right in so far as it implied that isolation is a condition precedent to such changes. apparently it did not occur to me as needful to specify this isolation as making possible the differentiation of species; since it goes without saying that members of a species spreading east, west, north, south, and forming groups hundreds of miles apart, must, while breeding with those of the same group be prevented from breeding with those of other groups--prevented from having their locally-caused modifications mutually cancelled. the importance of isolation has of late been emphasized by dr. romanes and others, who, to that isolation consequent on geographical diffusion, have added that isolation which results from difference of station in the same habitat, and also that due to differences in the breeding periods arising in members of the same species. doubtless in whatever way effected, the isolation of a group subject to new conditions and in course of being changed, is requisite as a means to permanent differentiation. doubtless also, as contended by mr. gulick and dr. romanes, there is a difference between the case in which an entire species being subject to the same conditions is throughout modified in character, thus illustrating what mr. gulick calls "monotypic evolution," and the case in which different parts of the species, leading different lives, will, if they are by any means prevented from inter-breeding with other parts, form divergent varieties: thus illustrating "polytypic evolution." § f. beyond geographical and topographical isolation, there is an isolation of another kind regarded by some as having had an important share in organic evolution. foreshadowed by mr. belt, subsequently enunciated by mr. catchpool, fully thought out by mr. gulick, and more recently elaborated by dr. romanes, "physiological selection" is held to account for the genesis of marked varieties side by side with their parents. it is contended that without the kind of isolation implied by it, variations will be swamped by inter-crossing, and divergence prevented; but that by the aid of this kind of isolation, a uniform species may be differentiated into two or more species, though its members continue to live in the same area. facts are assigned to show that slightly unlike varieties may become unable to inter-breed either with the parent-species or with one another. this mutual inferiority is not of the kind we might expect. we might reasonably suppose that when varieties had diverged widely, crossing would be impracticable, because their constitutions had become so far unlike as to form an unworkable mixture. but there seems evidence that the infertility arises long before such a cause could operate, and that instead of failure to produce a workable constitution, there is failure to produce any constitution at all--failure to fertilize. some change in the sexual system is suggested as accounting for this. that a minute difference in the reproductive elements may suffice, plants prove by the fact that when two members of slightly-divergent varieties are fertilized by each other's pollen, the fertility is less than if each were fertilized by the pollen of its own variety; and where the two kinds of pollen are both used, that derived from members of the same variety is prepotent in its effect over that derived from members of the other variety. the writers above named contend that variations must occur in the reproductive organs as well as in other organs; that such variations may produce relative infertility in particular directions; and that such relative infertility may be the first step towards prevention of crossing and establishment of isolation: so making possible the accumulation of such differences as mark off new species. without doubt we have here a legitimate supposition and a legitimate inference. necessarily there must happen variations of the kind alleged, and considering how sensitive the reproductive system is to occult influences (witness among ourselves the frequent infertility of healthy people while feeble unhealthy ones are fertile), it is reasonable to infer that minute and obscure alterations of this kind may make slightly-different varieties unable to inter-breed. granting that there goes on this "physiological selection," we must recognize it as one among the causes by which isolation is produced, and the differentiating influence of natural selection in the same locality made possible. § g. the foregoing criticisms and hypotheses do not, however, affect in any essential way the pre-existing conceptions. if, as in the foregoing chapters, we interpret the facts in terms of that redistribution of matter and motion constituting evolution at large, we shall see that the general theory, as previously held, remains outstanding. it is indisputable that to maintain its life an organism must maintain the moving equilibrium of its functions in presence of environing actions. this is a truism: overthrow of the equilibrium is death. it is a corollary that when the environment is changed, the equilibrium of functions is disturbed, and there must follow one of two results--either the equilibrium is overthrown or it is re-adjusted: there is a re-equilibration. only two possible ways of effecting the re-adjustment exist--the direct and the indirect. in the one case the changed outer action so alters the moving equilibrium as to call forth an equivalent reaction which balances it. if re-equilibration is not thus effected in the individual it is effected in the succession of individuals. either the species altogether disappears, or else there disappear, generation after generation, those members of it the equilibria of whose functions are least congruous with the changed actions in the environment; and this is the survival of the fittest or natural selection. if now we persist in thus contemplating the problem as a statico-dynamical one, we shall see that much of the discussion commonly carried on is beside the question. the centre around which the collision of arguments has taken place, is the question of the formation of species. but here we see that this question is a secondary and, in a sense, irrelevant one. we are concerned with the production of evolving and diverging organic forms; and whether these are or are not marked off by so-called specific traits, and whether they will or will not breed together, matters little to the general argument. if two divisions of a species, falling into unlike conditions and becoming re-equilibrated with them, eventually acquire the differences of nature called specific, this is but a collateral result. the _essential_ result is the formation of divergent organic forms. the biologic atmosphere, so to speak, has been vitiated by the conceptions of past naturalists, with whom the identification and classification of species was the be-all and end-all of their science, and who regarded the traits which enabled them to mark off their specimens from one another, as the traits of cardinal importance in nature. but after ignoring these technical ideas it becomes manifest that the distinctions, morphological or physiological, taken as tests of species, are merely incidental phenomena. moreover, on continuing thus to look at the facts, we shall better understand the relation between adaptive and specific characters, and between specific characters and those many small differences which always accompany them. for during re-equilibration there must, beyond those changes of structure required to balance outer actions by inner actions, be numerous minor changes. in any complex moving equilibrium alterations of larger elements inevitably cause alterations of elements immediately dependent on them, and these again of others: the effects reverberate and re-reverberate throughout the entire aggregate of actions down to the most minute. of resulting structural changes a few will be conspicuous, more will be less conspicuous, and so on continuously multiplying in number and decreasing in amount. here seems a fit place for remarking that there are certain processes which do not enter into these re-equilibrations but in a sense interfere with them. one example must suffice. among dogs may be observed the trick of rolling on some mass having a strong animal smell: commonly a decaying carcase. this trick has probably been derived from the trick of rolling on the body of an animal caught and killed, and so gaining a tempting odour. a male dog which first did this, and left a trail apt to be mistaken for that of prey, would be more easily found by a female, and would be more likely than others to leave posterity. now such a trick could have no relation to better maintenance of the moving equilibrium, and might very well arise in a dog having no superiority. if it arose in one of the worst it would be eliminated from the species, but if it arose in one of medium constitution, fairly capable of self-preservation, it would tend to produce survival of certain of the less fit rather than the fittest. probably there are many such minor traits which are in a sense accidental, and are neither adaptive nor specific in the ordinary sense. § h. but now let it be confessed that though all phenomena of organic evolution must fall within the lines above indicated, there remain many unsolved problems. take as an instance the descent of the testes in the _mammalia_. neither direct nor indirect equilibration accounts for this. we cannot consider it an adaptive change, since there seems no way in which the production of sperm-cells, internally carried on in a bird, is made external by adjustment to the changed requirements of mammalian life. nor can we ascribe it to survival of the fittest; for it is incredible that any mammal was ever advantaged in the struggle for life by this changed position of these organs. contrariwise, the removal of them from a place of safety to a place of danger, would seem to be negatived by natural selection. nor can we regard the transposition as a concomitant of re-equilibration; since it can hardly be due to some change in the general physiological balance. an example of another order is furnished by the mason-wasp. several instincts, capacities, peculiarities, which are in a sense independent though they cooperate to the same end, are here displayed. there is the instinct to build a cell of grains of sand, and the ability to do this, which though in a sense separate may be regarded as an accompaniment; and there is the secretion of a cement--a physiological process not directly connected with the psychological process. after oviposition there comes into play the instinct to seek, carry home, and pack into the cell, the small caterpillars, spiders, &c., which are to serve as food for the larva; and then there is the instinct to sting each of them at a spot where the injected hypnotic poison keeps the creature insensible though alive till it is wanted. these cannot be regarded as parts of a whole developed in simultaneous coordination. there is no direct connexion between the building instinct and the hypnotizing instinct; still less between these instincts and the associated appliances. what were the early stages they passed through imagination fails to suggest. their usefulness depends on their combination; and this combination would seem to have been useless until they had all reached something like their present completeness. nor can we in this case ascribe anything to the influence of teaching by imitation, supposed to explain the doings of social insects; for the mason-wasp is solitary. thus the process of organic evolution is far from being fully understood. we can only suppose that as there are devised by human beings many puzzles apparently unanswerable till the answer is given, and many necromantic tricks which seem impossible till the mode of performance is shown; so there are apparently incomprehensible results which are really achieved by natural processes. or, otherwise, we must conclude that since life itself proves to be in its ultimate nature inconceivable, there is probably an inconceivable element in its ultimate workings. end of vol. i. appendices. appendix a. the general law of animal fertility. [_in the_ westminster review _for april, , i published an essay under the title "a theory of population deduced from the general law of animal fertility." that essay was the germ of part vi of this work, "the laws of multiplication," in which its essential theses are fully developed. when developing them, i omitted some portions of the original essay--one which was not directly relevant, and another which contained a speculation open to criticism. as indicated in § f, i find that this speculation has an unexpected congruity with recent results of inquiry. i therefore decide to reproduce it here along with the definition of life propounded in that essay, which, though subsequently replaced by the definition elaborated in part i, contains an element of truth._] * * * * * some clear idea of the nature of life itself, must, indeed, form a needful preliminary. we may be sure that a search for the influences determining the maintenance and multiplication of living organisms, cannot be successfully carried out unless we understand what is the peculiar property of a living organism--what is the widest generalization of the phenomena that indicate life. by way of preparation, therefore, for the theory of population presently to be developed, we propose devoting a brief space to this prior question. * * * * * employing the term, then, in its usual sense, as applicable only to organisms, life may be defined as--_the co-ordination of actions_. the growth of a crystal, which is the highest inorganic process we are acquainted with, involves but one action--that of accretion. the growth of a cell, which is the lowest organic process, involves two actions--accretion and disintegration--repair and waste--assimilation and oxidation. wholly deprive a cell of oxygen, and it becomes inert--ceases to manifest vital phenomena; or, as we say, dies. give it no matter to assimilate, and it wastes away and disappears, from continual oxidation. evidently, then, it is in the balance of these two actions that the life consists. it is not in the assimilation alone; for the crystal assimilates: neither is it in the oxidation alone; for oxidation is common to inorganic matter: but it is in the joint maintenance of these--the _co-ordination_ of them. so long as the two go on together, life continues: suspend either of them, and the result is--death. the attribute which thus distinguishes the lowest organic from the highest inorganic bodies, similarly distinguishes the higher organisms from the lower ones. it is in the greater complexity of the co-ordination--that is, in the greater number and variety of the co-ordinated actions--that every advance in the scale of being essentially consists. and whether we regard the numerous vital processes carried on in a creature of complex structure as so many additional processes, or whether, more philosophically, we regard them as subdivisions of the two fundamental ones--oxidation and accretion--the co-ordination of them is still the life. thus turning to what is physiologically classified as the _vegetative system_, we see that stomach, lungs, heart, liver, skin, and the rest, must work in concert. if one of them does too much or too little--that is, if the co-ordination be imperfect--the life is disturbed; and if one of them ceases to act--that is, if the co-ordination be destroyed--the life is destroyed. so likewise is it with the _animal system_, which indirectly assists in co-ordinating the actions of the viscera by supplying food and oxygen. its component parts, the limbs, senses, and instruments of attack or defence must perform their several offices in proper sequence; and further, must conjointly minister to the periodic demands of the viscera, that these may in turn supply blood. how completely the several attributes of animal life come within the definition, we shall best see on going through them _seriatim_. thus _strength_ results from the co-ordination of actions; for it is produced by the simultaneous contraction of many muscles and many fibres of each muscle; and the strength is great in proportion to the number of these acting together--that is, in proportion to the co-ordination. _swiftness_ also, depending partly on strength, but requiring also the rapid alternation of movements, equally comes under the expression; seeing that, other things equal, the more quickly sequent actions can be made to follow each other, the more completely are they co-ordinated. so, too, is it with _agility_; the power of a chamois to spring with safety from crag to crag implies accurate co-ordination in the movements of many different muscles, and a due subordination of them all to the perceptions. the definition similarly includes _instinct_, which consists in the uniform succession of certain actions or series of actions after certain sensations or groups of sensations; and that which surprises us in instinct is the accuracy with which these compound actions respond to these compound sensations; that is--the completeness of their co-ordination. thus, likewise, is it with _intelligence_, even in its highest manifestations. that which we call rationality is the power to combine, or co-ordinate a great number and a great variety of complex actions for the achievement of a desired result. the husbandman has in the course of years, by drainage and manuring, to bring his ground into a fertile state; in the autumn he must plough, harrow, and sow, for his next year's crop; must subsequently hoe and weed, keep out cattle, and scare away birds; when harvest comes, must adapt the mode and time of getting in his produce to the weather and the labour market; he must afterwards decide when, and where, and how to sell to the best advantage; and must do all this that he may get food and clothing for his family. by properly coordinating these various processes (each of which involves many others)--by choosing right modes, right times, right quantities, right qualities, and performing his acts in right order, he attains his end. but if he have done too little of this, or too much of that; or have done one thing when he should have done another--if his proceedings have been badly co-ordinated--that is, if he have lacked intelligence--he fails. we find, then, that _the co-ordination of actions_ is a definition of life, which includes alike its highest and its lowest manifestations; and not only so, but expresses likewise the degree of life, seeing that the life is high in proportion as the co-ordination is great. proceeding upwards, from the simplest organic cell in which there are but two interdependent actions, on through the group in which many such cells are acting in concert, on through the higher group in which some of these cells assume mainly the respiratory and others the assimilative function--proceeding still higher to organisms in which these two functions are subdivided into many others, and in which some cells begin to act together as contractile fibres; next to organisms in which the visceral division of labour is carried yet further, and in which many contractile fibres act together as muscles--ascending again to creatures that combine the movements of several limbs and many bones and muscles in one action; and further, to creatures in which complex impressions are followed by the complex acts we term instinctive--and arriving finally at man, in whom not only are the separate acts complex, but who achieves his ends by combining together an immense number and variety of acts often extending through years--we see that the progress is uniformly towards greater co-ordination of actions. moreover, this co-ordination of actions unconsciously constitutes the essence of our common notion of life; for we shall find, on inquiry, that when we infer the death of an animal, which does not move on being touched, we infer it because we miss the usual co-ordination of a sensation and a motion: and we shall also find, that the test by which we habitually rank creatures high or low in the scale of vitality is the degree of co-ordination their actions exhibit. * * * * * there remains but to notice the objection which possibly may be raised, that the co-ordination of actions is not life, but the ability to maintain life. lack of space forbids going into this at length. it must suffice to say, that life and the ability to maintain life will be found the same. we perpetually expend the vitality we have that we may continue our vitality. our power to breathe a minute hence depends upon our breathing now. we must digest during this week that we may have strength to digest next. that we may get more food, we must use the force which the food we have eaten gives us. everywhere vigorous life is the strength, activity, and sagacity whereby life is maintained; and equally in descending the scale of being, or in watching the decline of an invalid, we see that the ebbing away of life is the ebbing away of the ability to preserve life.[ ] [only on now coming to re-read the definition of life enunciated at the commencement of this essay with the arguments used in justification of it, does it occur to me that its essential thought ought to have been incorporated in the definition of life given in part i. the idea of co-ordination is there implied in the idea of correspondence, but the idea of co-ordination is so cardinal a one that it should be expressed not by implication but overtly. it is too late to make the required amendment in the proper place, for the first part of this work is already stereotyped and printed. being unable to do better i make the amendment here. the formula as completed will run:--the definite combination of heterogeneous changes, both simultaneous and successful, _co-ordinated into_ correspondence with external co-existences and sequences.] * * * * * ending here this preliminary dissertation, let us now proceed to our special subject. § . on contemplating its general circumstances, we perceive that any race of organisms is subject to two sets of conflicting influences. on the one hand by natural death, by enemies, by lack of food, by atmospheric changes, &c., it is constantly being destroyed. on the other hand, partly by the strength, swiftness and sagacity of its members, and partly by their fertility, it is constantly being maintained. these conflicting sets of influences may be conveniently generalized as--the forces destructive of race, and the forces preservative of race. § . whilst any race continues to exist, the forces destructive of it and the forces preservative of it must perpetually tend towards equilibrium. if the forces destructive of it decrease, the race must gradually become more numerous, until, either from lack of food or from increase of enemies, the destroying forces again balance the preserving forces. if, reversely, the forces destructive of it increase, then the race must diminish, until, either from its food becoming relatively more abundant, or from its enemies dying of hunger, the destroying forces sink to the level of the preserving forces. should the destroying forces be of a kind that cannot be thus met (as great change of climate), the race, by becoming extinct, is removed out of the category. hence this is necessarily the _law of maintenance_ of all races; seeing that when they cease to conform to it they cease to be. now the forces preservative of race are two--ability in each member of the race to preserve itself, and ability to produce other members--power to maintain individual life, and power to propagate the species. these must vary inversely. when, from lowness of organization, the ability to contend with external dangers is small, there must be great fertility to compensate for the consequent mortality; otherwise the race must die out. when, on the contrary, high endowments give much capacity of self-preservation, there needs a correspondingly low degree of fertility. given the dangers to be met as a constant quantity; then, as the ability of any species to meet them must be a constant quantity too, and as this is made up of the two factors--power to maintain individual life and power to multiply--these cannot do other than vary inversely. § . to show that observed phenomena harmonise with this _à priori_ principle seems scarcely needful but, though axiomatic in its character, and therefore incapable of being rendered more certain, yet illustrations of the conformity to it which nature everywhere exhibits, will facilitate the general apprehension of it. in the vegetable kingdom we find that the species consisting of simple cells, exhibit the highest reproductive power. the yeast fungus, which in a few hours propagates itself throughout a large mass of wort, offers a familiar example of the extreme rapidity with which these lowly organisms multiply. in the _protococcus nivalis_, a microscopic plant which in the course of a night reddens many square miles of snow, we have a like example; as also in the minute _algæ_, which colour the waters of stagnant pools. the sudden appearance of green films on damp decaying surfaces, the spread of mould over stale food, and the rapid destruction of crops by mildew, afford further instances. if we ascend a step to plants of appreciable size, we still find that in proportion as the organization is low the fertility is great. thus of the common puff-ball, which is little more than a mere aggregation of cells, fries says, "in a single individual of _reticularia maxima_, i have counted (calculated?) , , sporules." from this point upwards, increase of bulk and greater complexity of structure are still accompanied by diminished reproductive power; instance the _macrocystis pyrifera_, a gigantic sea-weed, which sometimes attains a length of feet, of which carpenter remarks, "this development of the nutritive surface takes place at the expense of the fructifying apparatus, which is here quite subordinate."[ ] and when we arrive at the highly-organized exogenous trees, we find that not only are they many years before beginning to bear with any abundance, but that even then they produce, at the outside, but a few thousand seeds in a twelvemonth. during its centuries of existence, an oak does not develop as many acorns as a fungus does spores in a single night. still more clearly is this truth illustrated throughout the animal kingdom. though not so great as the fertility of the protophyta, which, as prof. henslow says, in some cases passes comprehension, the fertility of the protozoa is yet almost beyond belief. in the polygastric animalcules spontaneous fission takes place so rapidly that "it has been calculated by prof. ehrenberg that no fewer than millions might be produced in a month from a single _paramecium_;"[ ] and even this astonishing rate of increase is far exceeded in another species, one individual of which, "only to be perceived by means of a high magnifying power, is calculated to generate billions in four days."[ ] amongst the larger organisms exhibiting this lowest mode of reproduction under a modified form--that of gemmation--we see that, though not nearly so rapid as in the infusoria, the rate of multiplication is still extremely high. this fact is well illustrated by the polypes; and in the apparent suddenness with which whole districts are blighted by the aphis (multiplying by internal gemmation), we have a familiar instance of the startling results which the parthenogenetic process can achieve. where reproduction becomes occasional instead of continuous, as it does amongst higher creatures, the fertility equally bears an inverse ratio to the development. "the queen ant of the african _termites_ lays , eggs in twenty-four hours; and the common hairworm (_gordius_) as many as , , in less than one day."[ ] amongst the _vertebrata_ the lowest are still the most prolific. "it has been calculated," says carpenter, "that above a million of eggs are produced at once by a single codfish."[ ] in the strong and sagacious shark comparatively few are found. still less fertile are the higher reptiles. and amongst the mammalia, beginning with small rodents, which quickly reach maturity, produce large litters, and several litters in the year; advancing step by step to the higher mammals, some of which are long in attaining the reproductive age, others of which produce but one litter in a year, others but one young one at a time, others who unite these peculiarities; and ending with the elephant and man, the least prolific of all, we find that throughout this class, as throughout the rest, ability to multiply decreases as ability to maintain individual life increases. § . the _à priori_ principle thus exemplified has an obverse of a like axiomatic character. we have seen that for the continuance of any race of organisms it is needful that the power of self-preservation and the power of reproduction should vary inversely. we shall now see that, quite irrespective of such an end to be subserved, these powers could not do otherwise than vary inversely. in the nature of things species can subsist only by conforming to this law; and equally in the nature of things they cannot help conforming to it. reproduction, under all its forms, may be described as the separation of portions of a parent plant or animal for the purpose of forming other plants or animals. whether it be by spontaneous fission, by gemmation, or by gemmules; whether the detached products be bulbels, spores or seeds, ovisacs, ova or spermatozoa; or however the process of multiplication be modified, it essentially consists in the throwing off of parts of adult organisms for the purpose of making new organisms. on the other hand, self preservation is fundamentally a maintenance of the organism in undiminished bulk. amongst the lowest forms of life, aggregation of tissue is the only mode in which the self-preserving power is shown. even in the highest, sustaining the body in its integrity is that in which self-preservation most truly consists--is the end which the widest intelligence is indirectly made to subserve. whilst, on the one side, it cannot be denied that the increase of tissue constituting growth is self-preservation both in essence and in result; neither can it, on the other side, be denied that a diminution of tissue, either from injury, disease, or old age, is in both essence and result the reverse. hence the maintenance of the individual and the propagation of the race being respectively aggregative and separative, _necessarily_ vary inversely. every generative product is a deduction from the parental life; and, as already pointed out, to diminish life is to diminish the ability to preserve life. the portion thrown off is organised matter; vital force has been expended in the organisation of it, and in the assimilation of its component elements; which vital force, had no such portion been made and thrown off, _would have been available for the preservation of the parent_. neither of these forces, therefore, can increase, save at the expense of the other. the one draws in and incorporates new material; the other throws off material previously incorporated. the one adds to; the other takes from. using a convenient expression for describing the facts (though one that must not be construed into an hypothesis), we may say that the force which builds up and repairs the individual is an attractive force, whilst that which throws off germs is a repulsive force. but whatever may turn out to be the true nature of the two processes, it is clear that they are mutually destructive; or, stating the proposition in its briefest form--individuation and reproduction are antagonistic. again, illustrating the abstract by reference to the concrete, let us now trace throughout the organic world the various phases of this antagonism. § . all the lowest animal and vegetable forms--_protozoa_ and _protophyta_--consist essentially of a single cell containing fluid, and having usually a solid nucleus. this is true of the infusoria, the simplest entozoa, and the microscopic algæ and fungi. the organisms so constituted uniformly multiply by spontaneous fission. the nucleus, originally spherical, becomes elongated, then constricted across its smallest diameter, and ultimately separates, when "its divisions," says prof. owen, describing the process in the infusoria, "seem to repel each other to positions equidistant from each other, and from the pole or end of the body to which they are nearest. the influence of these distinct centres of assimilation is to divert the flow of the plasmatic fluid from a common course through the body of the polygastrian to two special courses about those centres. so much of the primary developmental process is renewed, as leads to the insulation of the sphere of the influence of each assimilative centre from that of the other by the progressive formation of a double party wall of integument, attended by progressive separation of one party wall from the other, and by concomitant constriction of the body of the polygastrian, until the vibratile action of the superficial cilia of each separating moiety severs the narrowed neck of union, and they become two distinct individuals."[ ] similar in its general view is dr. carpenter's description of the multiplication of vegetable cells, which he says divide, "in virtue, it may be surmised, of a sort of mutual repulsion between the two halves of the endochrome (coloured cell-contents) which leads to their spontaneous separation."[ ] under a modified form of this process, the cell-contents, instead of undergoing bisection, divide into numerous parts, each of which ultimately becomes a separate individual. in some of the algæ "a whole brood of young cells may thus be at once generated in the cavity of the parent-cell, which subsequently bursts and sets them free."[ ] the _achlya prolifera_ multiplies after this fashion. amongst the fungi, too, the same mode of increase is exemplified by the _protococcus nivalis_. and "it would appear that certain infusoria, especially the _kolpodinæ_, propagate by the breaking-up of their own mass into reproductive particles."[ ] now in this fissiparous mode of multiplication, which "is amazingly productive, and indeed surpasses in fertility any other with which we are acquainted,"[ ] we see most clearly the antagonism between individuation and reproduction. we see that the reproductive process involves destruction of the individual; for in becoming two, the parent fungus or polygastrian must be held to lose its own proper existence; and when it breaks up into a numerous progeny, does so still more completely. moreover, this rapid mode of multiplication not only destroys the individuals in whom it takes place, but also involves that their individualities, whilst they continue, shall be of the lowest kind. for assume a protozoon to be growing by imbibition at a given rate, and it follows that the oftener it divides the smaller must be the size it attains to; that is, the smaller the development of its individuality. and a further manifestation of the same truth is seen in the fact that the more frequent the spontaneous fission the shorter the existence of each individual. so that alike by preventing anything beyond a microscopic bulk being attained, by preventing the continuance of this in its integrity beyond a few hours, and by being fatal when it occurs, this most active mode of reproduction shows the strongest antagonism to individual life. § . whether or not we regard reproduction as resulting from a repulsive force (and, as seen above, both owen and carpenter lean to some such view), and whether or not we consider the formation of the individual as due to the reverse of this--an attractive force--we cannot, on studying the phenomena, help admitting that two opposite activities thus generalized are at work; we cannot help admitting that the aggregative and separative tendencies do in each case determine the respective developments of the individual and the race. on ascending one degree in the scale of organic life, we shall find this truth clearly exemplified. for if these single-celled organisms which multiply so rapidly be supposed to lose some of their separative tendency, what must be the result? they now not only divide frequently, but the divided portions fly apart. how, then, will a diminution of this separative tendency first show itself? may we not expect that it will show itself in the divided portions _not_ flying apart, but remaining near each other, and forming a group? this we find in nature to be the first step in advance. the lowest compound organisms are "_simple aggregations of vesicles without any definite arrangement, sometimes united, but capable of existing separately_."[ ] in these cases, "every component cell of the aggregate mass that springs from a single germ, being capable of existing independently of the rest, may be regarded as a distinct individual."[ ] the several stages of this aggregation are very clearly seen in both the animal and vegetable kingdoms. in the _hæmatococcus binalis_, the plant producing the reddish slime seen on damp surfaces, not only does each of the cells retain its original sphericity, but each is separated from its neighbour by a wide interval filled with mucus; so that it is only as being diffused through a mass of mucus common to them all, that these cells can be held to constitute one individual. we find, too, that "the component cells, even in the highest algæ, are generally separated from each other by a large quantity of mucilaginous intercellular substance."[ ] and, again, the tissue of the simpler lichens, "in consequence of the very slight adhesion of its component cells, is said to be pulverulent."[ ] similarly the protozoa, by their feeble union, constitute the organisms next above them. amongst the polygastrica there are many cases "in which the individuals produced by fission or gemmation do not become completely detached from each other."[ ] the _ophrydium_, for instance, "exists under the form of a motionless jelly-like mass ... made up of millions of distinct and similar individuals imbedded in a gelatinous connecting substance;"[ ] and again, the _uvella_, or "grape monad," consists of a cluster "which strongly resembles a transparent mulberry rolling itself across the field of view by the ciliary action of its component individuals."[ ] the parenchyma of the sponge, too, is made up of cells "each of which has the character of a distinct animalcule, having a certain power of spontaneous motion, obtaining and assimilating its own food, and altogether living _by_ and _for_ itself;" and so small is the cohesion of these individual cells, that the tissue they constitute "drains away when the mass is removed from the water, like white of egg."[ ] of course in proportion as the aggregate tendency leading to the formation of these groups of monads is strong, we may expect that, other things equal, the groups will be large. proceeding upwards from the yeast fungus, whose cells hold together in groups of four, five, and six,[ ] there must be found in each species of these composite organisms a size of group determined by the strength of the aggregative tendency in that species. hence we may expect that, when this limit is passed, the group no longer remains united, but divides. such we find to be the fact. these groups of cells undergo the same process that the cells themselves do. they increase up to a certain point, and then multiply either by simple spontaneous fission or by that modification of it called gemmation. the _volvox globator_, which is made up of a number of monads associated together in the form of a hollow sphere, develops within itself a number of smaller spheres similarly constituted; and after these, swimming freely in its interior, have reached a certain size, the parent group of animalcules bursts and sets the interior groups free. and here we may observe how this compound individuality of the volvox is destroyed in the act of reproduction as the simple individuality of the monad is. again, in the higher forms of grouped cells, where something like organisation begins to show itself, the aggregations are not only larger, but the separative process, now carried on by the method of gemmation, no longer wholly destroys the individual. and in fact, this gemmation may be regarded as the form which spontaneous fission must assume in ceasing to be fatal; seeing that gemmation essentially consists in the separation, not into halves, but into a larger part and a smaller part; the larger part continuing to represent the original individual. thus in the common _hydra_ or fresh-water polype, "little bud-like processes are developed from the external surface, which are soon observed to resemble the parent in character, possessing a digestive sac, mouth, and tentacula; for a long time, however, their cavity is connected with that of the parent; but at last the communication is cut off, and the young polype quits its attachment, and goes in quest of its own maintenance."[ ] § . progress from these forms of organisation to still higher forms is similarly characterized by increase of the aggregative tendency or diminution of the separative, and similarly exhibits the necessary antagonism between the development of the individual and the increase of the race. that process of grouping which constitutes the first step towards the production of complex organisms, we shall now find repeated in the formation of series of groups. just as a diminution of the separative tendency is shown in the aggregation of divided monads, so is a further diminution of it shown in the aggregation of the divided groups of monads. the first instance that occurs is afforded by the compound polypes. "some of the simpler forms of the composite _hydroida_," says carpenter, "may be likened to a _hydra_, whose gemmæ, instead of becoming detached, remain permanently connected with the parent; and as these in their turn may develop gemmæ from their own bodies, a structure of more or less arborescent character may be produced."[ ] a similar species of combination is observable amongst the _bryozoa_, and the compound _tunicata_. every degree of union may be found amongst these associated organisms; from the one extreme in which the individuals can exist as well apart as together, to the other extreme in which the individuals are lost in the general mass. whilst each _bryozoon_ is tolerably independent of its neighbour, "in the compound _hydroida_, the lives of the polypes are subordinate to that of the polypdom."[ ] of the _salpidæ_ and _pyrosomidæ_, carpenter says:--"although closely attached to one another, these associated animals are capable of being separated by a smart shock applied to the sides of the vessel in which they are swimming.... in other species, however, the separate animals are imbedded in a gelatinous mass," and in one kind "there is an absolute union between the vascular systems of the different individuals."[ ] in the same manner that with a given aggregative tendency there is a limit to the size of groups, so is there a similarly-determined limit to the size of series of groups; and that spontaneous fission which we have seen in cells and groups of cells we here find repeated. in the lower _annelida_, for example, "after the number of segments in the body has been greatly multiplied by gemmation, a separation of those of the posterior portion begins to take place; a constriction forms itself about the beginning of the posterior third of the body, in front of which the alimentary canal undergoes a dilatation, whilst on the segment behind it a proboscis and eyes are developed, so as to form the head of the young animal which is to be budded off; and in due time, by the narrowing of the constriction, a complete separation is effected."[ ] not unfrequently in the _nais_ this process is repeated in the young one before it becomes independent of the parent. the higher _annelida_ are distinguished by the greater number of segments held in continuity; an obvious result of comparatively infrequent fission. in the class _myriapoda_, which stands next above, "there is no known instance of multiplication by fission."[ ] yet even here the law may be traced both in the number and structure of the segments. the length of the body is still increased after birth "by gemmation from (or partial fission of) the penultimate segment." the lower members of the class are distinguished from the higher by the greater extent to which this gemmation is carried. moreover, the growing aggregative tendency is seen in the fact, that each segment of the julus "is formed by the coalescence of two original segments,"[ ] whilst in the _scolopendridæ_, which are the highest of this class, "the head, according to mr. newport, is composed of eight segments, which are often consolidated into one piece;"[ ] both of which phenomena may be understood as arrests of that process of fission, which, if allowed to go a little further, would have produced distinct segments; and, if allowed to go further still, would have separated these segments into groups. § . remarking, first, how gradually this mode of multiplication disappears--how there are some creatures that spontaneously divide or not according to circumstances; others that divide when in danger (the several parts being capable of growing into complete individuals); others which, though not self-dividing, can live on in each half if artificially divided; and others in which only one of the divided halves can live--how, again, in the crustaceans the power is limited to the reproduction of lost limbs; how there are certain reptiles that can re-supply a lost tail, but only imperfectly; and how amongst the higher _vertebrata_ the ability to repair small injuries is all that remains--remarking thus much, let us now, by way of preparation for what is to follow, consider the significance of the foregoing facts taken in connection with the definition of life awhile since given. this spontaneous fission, which we have seen to be, in all cases, more or less destructive of individual life, is simply a cessation in the co-ordination of actions. from the single cell, the halves of whose nucleus, instead of continuing to act together, begin to repel each other, fly apart, establish distinct centres of assimilation, and finally cause the cell to divide; up to the annelidan, whose string of segments separates, after reaching a certain length; we everywhere see the phenomenon to be fundamentally this. the tendency to separate is the tendency not to act together, probably arising from inability to act together any longer; and the process of separation is the process of ceasing to act together. how truly non-co-ordination is the essence of the matter will be seen on observing that fission takes place more or less rapidly, according as the co-ordinating apparatus is less or more developed. thus, "the capability of spontaneous division is one of the most distinctive attributes of the acrite type of structure;"[ ] the acrite type of structure being that in which the neurine or nervous matter is supposed to be diffused through the tissues in a molecular state, and in which, therefore, there exists no distinct nervous or co-ordinating system. from this point upwards the gradual disappearance of spontaneous fission is clearly related to the gradual appearance of nerves and ganglia--a fact well exemplified by the several grades of _annelida_ and _myriapoda_. and when we remember that in the embryotic development of these classes, the nervous system does not make its appearance until after the rest of the organism has made great progress, we may even suspect that that coalescence of segments characteristic of the _myriapoda_, exhibits the co-ordinating power of the rapidly-growing nervous system overtaking and arresting the separative tendency; and doing this most where it (the nervous system) is most developed, namely, in the head. and here let us remark, in passing, how, from this point of view, we still more clearly discern the antagonism of individuation and reproduction. we before saw that the propagation of the race is at the expense of the individual: in the above facts we may contemplate the obverse of this--may see that the formation of the individual is at the expense of the race. this combination of parts that are tending to separate and become distinct beings--this union of many incipient minor individualities into one large individuality--is an arrest of reproduction--a diminution in the number produced. either these units may part and lead independent lives, or they may remain together and have their actions co-ordinated. either they may, by their diffusion, form a small, simple, and prolific race, or, by their aggregation, a large, complex, and infertile one. but manifestly the aggregation involves the infertility; and the fertility involves the smallness. § . the ability to multiply by spontaneous fission, and the ability to maintain individual life, are opposed in yet another mode. it is not in respect of size only, but still more in respect of structure, that the antagonism exists. higher organisms are distinguished from lower ones partly by bulk, and partly by complexity. this complexity essentially consists in the mutual dependence of numerous different organs, each subserving the lives of the rest, and each living by the help of the rest. instead of being made up of many like parts, performing like functions, as the crinoid, the star-fish, or the millipede, a vertebrate animal is made up of many unlike parts, performing unlike functions. from that initial form of a compound organism, in which a number of minor individuals are simply grouped together, we may, more or less distinctly, trace not only the increasing closeness of their union, and the gradual disappearance of their individualities in that of the mass, but the gradual assumption by them of special duties. and this "physiological division of labour," as it has been termed, has the same effect as the division of labour amongst men. as the preservation of a number of persons is better secured when, uniting into a society, they severally undertake different kinds of work, than when they are separate and each performs for himself every kind of work; so the preservation of a congeries of parts, which, combining into one organism, respectively assume nutrition, respiration, circulation, locomotion, as separate functions, is better secured than when those parts are independent, and each fulfils for itself all these functions. but the condition under which this increased ability to maintain life becomes possible is, that the parts shall cease to separate. while they are perpetually separating, it is clear that they cannot assume mutually subservient duties. and it is further clear that the more the tendency to separate diminishes, that is, the larger the groups that remain connected, _the more minutely and perfectly can that subdivision of functions which we call organization be carried out_. thus we see that in its most active form the ability to multiply is antagonistic to the ability to maintain individual life, not only as preventing increase of bulk, but also as preventing organization--not only as preventing homogeneous co-ordination, but as preventing heterogeneous co-ordination. § . to establish the unbroken continuity of this law of fertility, it will be needful, before tracing its results amongst the higher animals, to explain in what manner spontaneous fission is now understood, and what the cessation of it essentially means. originally, naturalists supposed that creatures which multiply by self-division, under any of its several forms, continue so to multiply perpetually. in many cases, however, it has latterly been shown that they do not do this; and it is now becoming a received opinion that they do not, and cannot, do this, in any case. a fertilised germ appears here, as amongst higher organisms, to be the point of departure; and that constant formation of new tissue implied in the production of a great number of individuals by fission, seems gradually to exhaust the germinal capacity in the same way that the constant formation of new tissue, during the development of a single mammal, exhausts it. the phenomena classified by steenstrup as "alternate generation," and since generalised by professor owen in his work "on parthenogenesis," illustrate this. the egg of a _medusa_ (jellyfish) develops into a polypoid animal called the _strobila_. this _strobila_ lives as the polype does, and, like it, multiplies rapidly by gemmation. after a great number of individuals has been thus produced, and when, as we must suppose, the germinal capacity is approaching exhaustion, each _strobila_ begins to exhibit a series of constrictions, giving it some resemblance to a rouleau of coin or a pile of saucers. these constrictions deepen; the segments gradually develop tentacula; the terminal segment finally separates itself, and swims away in the form of a young _medusa_; the other segments, in succession, do the same; and from the eggs which these _medusæ_ produce, other like series of polypoid animals, multiplying by gemmation, originate. in the compound polypes, in the _tunicata_, in the _trematoda_, and in the aphis, we find repeated, under various modifications, the same phenomenon. understanding then, this lowest and most rapid mode of multiplication to consist essentially in the production of a great number of individuals from a single germ--perceiving, further, that diminished activity of this mode of multiplication consists essentially in the aggregation of the germ-product into larger masses--and seeing, lastly, that the disappearance of this mode of multiplication consists essentially in the aggregation of the germ-product into _one_ mass--we shall be in a position to comprehend, amongst the higher animals, that new aspect of the law, under which increased individuation still involves diminished reproduction. progressing from those lowest forms of life in which a single ovum originates countless organisms, through the successive stages in which the number of organisms so originated becomes smaller and smaller; and finally arriving at a stage in which one ovum produces but one organism; we have now, in our further ascent, to observe the modified mode in which this same necessary antagonism between the ability to multiply, and the ability to preserve individual life, is exhibited. § . throughout both the animal and vegetable kingdoms, generation is effected "by the union of the contents of a 'sperm-cell' with those of a 'germ-cell;' the latter being that from within which the embryo is evolved, whilst the former supplies some material or influence necessary to its evolution."[ ] amongst the lowest vegetable organisms, as in the _desmideæ_, the _diatomaceæ_, and other families of the inferior _algæ_, those cells do not appreciably differ; and the application to them of the terms "sperm-cell" and "germ-cell" is hypothetical. from this point upwards, however, distinctions become visible. as we advance to higher and higher types of structure, marked differences arise in the character of these cells, in the organs evolving them, and in the position of these organs, which are finally located in separate sexes. doubtless a separation in the _functions_ of "sperm-cell" and "germ-cell" has simultaneously arisen. that change from homogeneity of function to heterogeneity of function which essentially constitutes progress in organization may be assumed to take place here also; and, indeed, it is probable that the distinction gradually established between these cells, in origin and appearance, is merely significant of, and consequent upon, the distinction that has arisen between them in constitution and office. let us now inquire in what this distinction consists. if the foundation of every new organism be laid by the combination of two elements, we may reasonably suspect that these two elements are typical of some two fundamental divisions of which the new organism is to consist. as nothing in nature is without meaning and purpose, we may be sure that the universality of this binary origin, signifies the universality of a binary structure. the simplest and broadest division of which an organism is capable must be that signified. what, then, must this division be? the proposed definition of organic life supplies an answer. if organic life be the co-ordination of actions, then an organism may be primarily divided into parts whose actions are co-ordinated, and parts which co-ordinate them--organs which are made to work in concert, and the apparatus which makes them so work--or, in other words, the assimilative, vascular, excretory, and muscular systems on the one hand, and the nervous system on the other. the justness of this classification will become further apparent, when it is remembered that by the nervous system alone is the individuality established. by it all parts are made one in purpose, instead of separate; by it the organism is rendered a conscious whole--is enabled to recognise its own extent and limits; and by it are all injuries notified, repairs directed, and the general conservation secured. the more the nervous system is developed, the more reciprocally subservient do the components of the body become--the less can they bear separating. and that which thus individuates many parts into one whole, must be considered as more broadly distinguished from the parts individuated, than any of these parts from each other. further evidence in support of this position may be drawn from the fact, that as we ascend in the scale of animal life, that is, as the co-ordination of actions becomes greater, we find the co-ordinating or nervous system becoming more and more definitely separated from the rest; and in the vertebrate or highest type of structure we find the division above insisted on distinctly marked. the co-ordinating parts and the parts co-ordinated are placed on opposite sides of the vertebral column. with the exception of a few ganglia, the whole of the nervous masses are contained within the neural arches of the vertebræ; whilst all the viscera and limbs are contained within, or appended to, the hæmal arches--the terms neural and hæmal having, indeed, been chosen to express this fundamental division. if, then, there be truth in the assumption that the two elements, which, by their union, give origin to a new organism, typify the two essential constituents of such new organism, we must infer that the sperm-cell and germ-cell respectively consist of co-ordinating matter and matter to be co-ordinated--neurine and nutriment. that apparent identity of sperm-cell and germ-cell seen in the lowest forms of life may thus be understood as significant to the fact that no extended co-ordination of actions exists in the generative product--each cell being a separate individual; and the dissimilarity seen in higher organic types may, conversely, be understood as expressive of, and consequent upon, the increasing degree of co-ordination exhibited.[ ] that the sperm-cell and germ-cell are thus contrasted in nature and function may further be suspected on considering the distinctive characteristics of the sexes. of the two elements they respectively contribute to the formation of a fertile germ, it may be reasonably supposed that each furnishes that which it possesses in greatest abundance and can best spare. well, in the greater size of the nervous centres in the male, as well as in the fact that during famines men succumb sooner than women, we see that in the male the co-ordinating system is relatively predominant. from the same evidence, as well as from the greater abundance of the cellular and adipose tissues in women, we may infer that the nutritive system predominates in the female.[ ] here, then, is additional support for the hypothesis that the sperm-cell, which is supplied by the male, contains co-ordinating matter, and the germ-cell, which is supplied by the female, contains matter to be co-ordinated. the same inference may, again, be drawn from a general view of the maternal function. for if, as we see, it is the office of the mother to afford milk to the infant, and during a previous period to afford blood to the foetus, it becomes probable that during a yet earlier stage it is still the function to supply nutriment, though in another form. indeed when, ascending gradually the scale of animal life, we perceive that this supplying of milk, and before that of blood, is simply a continuation of the previous process, we may be sure that, with nature's usual consistency, this process is essentially one from the beginning. quite in harmony with this hypothesis concerning the respective natures of the sperm-cell and germ-cell is a remark of carpenter's on the same point:-- "looking," he says, "to the very equal mode in which the characters of the two parents are mingled in _hybrid_ offspring, and to the certainty that the _material_ conditions which determine the development of the germ are almost exclusively female, it would seem probable that the _dynamical_ conditions are, in great part, furnished by the male."[ ] § . could nothing but the foregoing indirect evidence be adduced in proof of the proposition that the spermatozoon is essentially a neural element, and the ovum essentially a hæmal element, we should scarcely claim for it anything more than plausibility. on finding, however, that this indirect evidence is merely introductory to evidence of a quite direct nature, its significance will become apparent. adding to their weight taken separately the force of their mutual confirmation, these two series of proofs will be seen to give the hypothesis a high degree of probability. the direct evidence now to be considered is of several kinds. on referring to the description of the process of multiplication in monads, quoted some pages back (§ ), from professor owen, the reader will perceive that it is by the pellucid nucleus that the growth and reproduction of these single-celled creatures are regulated. the nucleus controls the circulation of the plasmatic fluid; the fission of the nucleus is the first step towards the formation of another cell; each half of the divided nucleus establishes round itself an independent current; and, apparently, it is by the repulsion of the nuclei that the separation into two individuals is finally effected. all which facts, when generalised, imply that the nucleus is the governing or _co-ordinating_ part. now, professor owen subsequently points out that the matter of the sperm-cell performs in the fertilised germ-cell just this same function which the nucleus performs in a single-celled animal. we find the absorption by a germ-cell of the contents of a sperm-cell "followed by the appearance of a pellucid nucleus in the centre of the opaque and altered germ-cell; we further see its successive fissions governed by the preliminary division of the pellucid centre;" and, led by these and other facts, professor owen thinks that "one cannot reasonably suppose that the nature and properties of the nucleus of the impregnated germ-cell and that of the monad can be different."[ ] and hence he further infers that "the nucleus of the monad is of a nature similar to, if not identical with," the matter of the spermatozoon. but we have seen that in the monad the nucleus is the co-ordinating part; and hence to say that the sperm-cell is, in nature, identical with it, is to say that the sperm-cell consists of co-ordinating matter. chemical analysis affords further evidence, though, from the imperfect data at present obtained, less conclusive evidence than could be wished. partly from the white and gray nervous substances having been analysed together instead of separately, and partly from the difficulty of isolating the efficient contents of the sperm-cells, a satisfactory comparison cannot be made. nevertheless, possessing in common, as they do, one element, by which they are specially characterised, the analysis, as far as it goes, supports our argument. the following table, which has been made up from data given in the _cyclopædia of anatomy and physiology, art._ nervous system, gives the proportion of this element in the brain in different conditions, and shows how important is its presence. +-----------------------------+--------+-------+-------+--------+-------+ | | in | in | in | in | in | | |infants.| youth.|adults.|old men.|idiots.| | +--------+-------+-------+--------+-------+ | solid constituents in a | | | | | | | hundred parts of the brain | . | . | . | . | . | | of these solid constituents | | | | | | | the phosphorus amounts to | . | . | . | . | . | | which gives a percentage of | | | | | | | phosphorus in the solid | | | | | | | constituents of | . | . | . | . | . | +-----------------------------+--------+-------+-------+--------+-------+ this connection between the quantity of phosphorus present and the degree of mental power exhibited, is sufficiently significant; and the fact that in the same individual the varying degrees of cerebral activity are indicated by the varying quantities of alkaline phosphates excreted by the kidneys,[ ] still more clearly shows the essentialness of phosphorus as a constituent of nervous matter. respecting the constitution of sperm-cells chemists do not altogether agree. one thing, however, is certain--that they contain unoxidized phosphorus; and also a fatty acid, that is not improbably similar to the fatty acid contained in neurine.[ ] in fact, there would seem to be present the constituents of that oleophosphoric acid which forms so distinctive an element of the brain. that a large quantity of binoxide of protein is also present, may be ascribed to the fact that a great part of the sperm-cell consists merely of the protective membrane and its locomotive appendage; the really efficient portion being but the central contents.[ ] evidence of a more conclusive nature--evidence, too, which will show in what direction our argument tends--is seen in the marked antagonism of the nervous and generative systems. thus, the fact that intense mental application, involving great waste of the nervous tissues, and a corresponding consumption of nervous matter for their repair, is accompanied by a cessation in the production of sperm-cells, gives strong support to the hypothesis that the sperm-cells consist essentially of neurine. and this becomes yet clearer on finding that the converse fact is true--that undue production of sperm-cells involves cerebral inactivity. the first result of a morbid excess in this direction is headache, which may be taken to indicate that the brain is out of repair; this is followed by stupidity; should the disorder continue, imbecility supervenes, ending occasionally in insanity. that the sperm-cell is co-ordinating matter, and the germ-cell matter to be co-ordinated, is, therefore, an hypothesis not only having much _à priori_ probability, but one supported by numerous facts. § . this hypothesis alike explains, and is confirmed by, the truth, that throughout the vertebrate tribes the degree of fertility varies inversely as the development of the nervous system. the necessary antagonism of individuation and reproduction does indeed show itself amongst the higher animals, in some degree in the manner hitherto traced; namely, as determining the total bulk. though the parts now thrown off, being no longer segments or gemmæ, are not obvious diminutions of the parent, yet they must be really such. under the form of internal fission, the separative tendency is as much opposed to the aggregative tendency as ever; and, _other things equal_, the greater or less development of the individual depends upon the less or greater production of new individuals or germs of new individuals. as in groups of cells, and series of groups of cells, we saw that there was in each species a limit, passing which, the germ product would not remain united; so in each species of higher animal there is a limit, passing which, the process of cell-multiplication results in the throwing off of cells, instead of resulting in the formation of more tissue. hence, taking an average view, we see why the smaller animals so soon arrive at a reproductive age, and produce large and frequent broods; and why, conversely, increased size is accompanied by retarded and diminished fertility. but, as above implied, it is not so much to the bulk of the body as a whole, as to the bulk of the nervous system, that fertility stands related amongst the higher animals. probably, indeed, it stands thus related in all cases; the difference simply arising from the fact, that whereas in the lower organisms, where the nervous system is not concentrated, its bulk varies as the bulk of the body, in the higher organisms it does not do so. be this as it may, however, we see clearly that, amongst the vertebrata, the bodily development is not the determining circumstance. in a fish, a reptile, a bird, and a mammal of the same weight, there is nothing like equality of fecundity. cattle and horses, arriving as they do so soon at a reproductive age, are much more prolific than the human race, at the same time that they are much larger. and whilst, again, the difference in size between the elephant and man is far greater, their respective powers of multiplication are less unlike. looking in these cases at the nervous systems, however, we find no such discrepancy. on learning that the average ratio of the brain to the body is--in fishes, to ; in reptiles, to ; in birds, to ; and in mammals, to ;[ ] their different degrees of fecundity are accounted for. though an ox will outweigh half-a-dozen men, yet its brain and spinal cord are far less than those of one man; and though in bodily development the elephant so immensely exceeds the human being, yet the elephant's cerebro-spinal system is only thrice the size attained by that of civilized men.[ ] unfortunately, it is impossible to trace throughout the animal kingdom this inverse relationship between the nervous and reproductive systems with any accuracy. partly from the fact that, in each case, the degree of fertility depends on three variable elements--the age at which reproduction begins, the number produced at a birth, and the frequency of the births; partly from the fact that, in respect to most animals, these data are not satisfactorily attainable, and that, when they are attainable, they are vitiated by the influence of domesticity; and partly from the fact that no precise measurement of the respective nervous systems has been made, we are unable to draw any but general and somewhat vague comparisons. these, however, as far as they go, are in our favour. ascending from beings of the acrite nerveless type, which are the most prolific of all, through the various invertebrate sub-kingdoms, amongst which spontaneous fission disappears as the nervous system becomes developed; passing again to the least nervous and most fertile of the vertebrate series--fishes, of which, too, the comparatively large-brained cartilaginous kinds multiply much less rapidly than the others; progressing through the more highly endowed and less prolific reptiles to the mammalia, amongst which the rodents, with their unconvoluted brains, are noted for their fecundity; and ending with man and the elephant, the least fertile and largest-brained of all--there seems to be throughout a constant relationship between these attributes. and indeed, on turning back to our _à priori_ principle, no other relationship appears possible. we found it to be the necessary law of maintenance of races, that the ability to maintain individual life and the ability to multiply vary inversely. but the ability to maintain individual life _is in all cases measured by the development of the nervous system_. if it be in good visceral organization that the power of self-preservation is shown, this implies some corresponding nervous apparatus to secure sufficient food. if it be in strength, there must be a provision of nerves and nervous centres answering to the number and size of the muscles. if it be in swiftness and agility, a proportionate development of the cerebellum is presupposed. if it be in intelligence, this varies with the size of the cerebrum. as in all cases co-ordination of actions constitutes the life, or, what is the same thing, the ability to maintain life; and as throughout the animal kingdom this co-ordination, under all its forms, is effected by nervous agents of some kind or other; and as each of these nervous agents performs but one function; it follows that in proportion to the number of the actions co-ordinated must be the number of nervous agents. hence the nervous system becomes the universal measure of the degree of co-ordination of actions; that is, of the life, or ability to maintain life. and if the nervous system varies directly as the ability to maintain life, it _must_ vary inversely as the ability to multiply.[ ] and here, assuming the constitution of the sperm-cell above inferred to be the true one, we see how the obverse _à priori_ principle is fulfilled. where, as amongst the lowest organisms, bulk is expressive of life, the antagonism of individuation and reproduction was broadly exhibited in the fact that the making of two or more new individuals was the _un_making of the original individual. and now, amongst the higher organisms, where bulk is no longer the measure of life, we see that this antagonism is between the neural elements thrown off, and that internal neural mass whose bulk _is_ the measure of life. the production of co-ordinating cells must be at the expense of the co-ordinating apparatus; and the aggregation of the co-ordinating apparatus must be at the expense of co-ordinating cells. how the antagonism affects the female economy is not so clear. possibly the provision required to be made for supplying nervous as well as other nutriment to the embryo, involves an arrest in the development of the nervous system; and if so, probably this arrest takes place early in proportion as the number of the coming offspring makes the required provision great: or rather, to put the facts in their right sequence, an early arrest renders the production of a numerous offspring possible. § . the law which we have thus traced throughout the animal kingdom, and which must alike determine the different fertilities of different species, and the variations of fertility in the same species, we have now to consider in its application to mankind. [_the remainder of the essay, which as implied, deals with the application of this general principle to the multiplication of the human race, need not be here reproduced. the subject is treated in full in part vi._] appendix b. the inadequacy of natural selection, etc., etc. [_in this appendix are included four essays originally published in the_ contemporary review _and subsequently republished as pamphlets. the first appeared under the above title in february and march, ; the second in may of that year under the title "prof. weismann's theories;" the third in december of that year under the title "a rejoinder to prof. weismann;" and the fourth in october, , under the title "weismannism once more." as these successive essays practically form parts of one whole, i have thought it needless to keep them separate by repeating their titles, and have simply marked them off from one another by the numbers i, ii, iii, iv. of course, as they are components of a controversy, some incompleteness arises from the absence of the essays to which portions of them were replies; but in each the course of the argument sufficiently indicates the counter-arguments which were met._] i. students of psychology are familiar with the experiments of weber on the sense of touch. he found that different parts of the surface differ widely in their ability to give information concerning the things touched. some parts, which yielded vivid sensations, yielded little or no knowledge of the sizes or forms of the things exciting them; whereas other parts, from which there came sensations much less acute, furnished clear impressions respecting the tangible characters, even of relatively small objects. these unlikenesses of tactual discriminativeness he ingeniously expressed by actual measurements. taking a pair of compasses, he found that if they were closed so nearly that the points were less than one-twelfth of an inch apart, the end of the forefinger could not perceive that there were two points: the two points seemed one. but when the compasses were opened so that the points were one-twelfth of an inch apart, then the end of the forefinger distinguished the two points. at the same time, he found that the compasses must be opened to the extent of two and a half inches, before the middle of the back could distinguish between two points and one. that is to say, as thus measured, the end of the forefinger has thirty times the tactual discriminativeness which the middle of the back has. between these extremes he found gradations. the inner surfaces of the second joints of the fingers can distinguish separateness of positions only half as well as the tip of the forefinger. the innermost joints are still less discriminating, but have powers of discrimination equal to that of the tip of the nose. the end of the great toe, the palm of the hand, and the cheek, have alike one-fifth of the perceptiveness which the tip of the forefinger has; and the lower part of the forehead has but one-half that possessed by the cheek. the back of the hand and the crown of the head are nearly alike in having but a fourteenth or a fifteenth of the ability to perceive positions as distinct, which is possessed by the finger-end. the thigh, near the knee, has rather less, and the breast less still; so that the compasses must be opened more than an inch and a half before the breast distinguishes the two points from one another. what is the meaning of these differences? how, in the course of evolution, have they been established? if "natural selection," or survival of the fittest, is the assigned cause, then it is required to show in what way each of these degrees of endowment has advantaged the possessor to such extent that not infrequently life has been directly or indirectly preserved by it. we might reasonably assume that in the absence of some differentiating process, all parts of the surface would have like powers of perceiving relative positions. they cannot have become widely unlike in perceptiveness without some cause. and if the cause alleged is natural selection, then it is necessary to show that the greater degree of the power possessed by this part than by that, has not only conduced to the maintenance of life, but has conduced so much that an individual in whom a variation has produced better adjustment to needs, thereby maintained life when some others lost it; and that among the descendants inheriting this variation, there was a derived advantage such as enabled them to multiply more than the descendants of individuals not possessing it. can this, or anything like this, be shown? that the superior perceptiveness of the forefinger-tip has thus arisen, might be contended with some apparent reason. such perceptiveness is an important aid to manipulation, and may have sometimes given a life-saving advantage. in making arrows or fish-hooks, a savage possessing some extra amount of it may have been thereby enabled to get food where another failed. in civilized life, too, a sempstress with well-endowed finger-ends might be expected to gain a better livelihood than one with finger-ends which were obtuse; though this advantage would not be so great as appears. i have found that two ladies whose finger-ends were covered with glove-tips, reducing their sensitiveness from one-twelfth of an inch between compass-points to one-seventh, lost nothing appreciable of their quickness and goodness in sewing. an experience of my own here comes in evidence. towards the close of my salmon-fishing days i used to observe what a bungler i had become in putting on and taking off artificial flies. as the tactual discriminativeness of my finger-ends, recently tested, comes up to the standard specified by weber, it is clear that this decrease of manipulative power, accompanying increase of age, was due to decrease in the delicacy of muscular co-ordination and sense of pressure--not to decrease of tactual discriminativeness. but not making much of these criticisms, let us admit the conclusion that this high perceptive power possessed by the forefinger-end may have arisen by survival of the fittest; and let us limit the argument to the other differences. how about the back of the trunk and its face? is any advantage derived from possession of greater tactual discriminativeness by the last than the first? the tip of the nose has more than three times the power of distinguishing relative positions which the lower part of the forehead has. can this greater power be shown to have any advantage? the back of the hand has scarcely more discriminative ability than the crown of the head, and has only one-fourteenth of that which the finger-tip has. why is this? advantage might occasionally be derived if the back of the hand could tell us more than it does about the shapes of the surfaces touched. why should the thigh near the knee be twice as perceptive as the middle of the thigh? and, last of all, why should the middle of the forearm, middle of the thigh, middle of the back of the neck, and middle of the back, all stand on the lowest level, as having but one-thirtieth of the perceptive power which the tip of the forefinger has? to prove that these differences have arisen by natural selection, it has to be shown that such small variation in one of the parts as might occur in a generation--say one-tenth extra amount--has yielded an appreciably greater power of self-preservation; and that those inheriting it have continued to be so far advantaged as to multiply more than those who, in other respects equal, were less endowed with this trait. does any one think he can show this? but if this distribution of tactual perceptiveness cannot be explained by survival of the fittest, how can it be explained? the reply is that, if there has been in operation a cause which it is now the fashion among biologists to ignore or deny, these various differences are at once accounted for. this cause is the inheritance of acquired characters. as a preliminary to setting forth the argument showing this, i have made some experiments. it is a current belief that the fingers of the blind, more practised in tactual exploration than the fingers of those who can see, acquire greater discriminativeness: especially the fingers of those blind who have been taught to read from raised letters. not wishing to trust to this current belief, i recently tested two youths, one of fifteen and the other younger, at the school for the blind in upper avenue road, and found the belief to be correct. i found that instead of being unable to distinguish between points of the compasses until they were opened to one-twelfth of an inch apart, both of them could distinguish between points when only one-fourteenth of an inch apart. they had thick and coarse skins; and doubtless, had the intervening obstacle, so produced, been less, the discriminative power would have been greater. it afterwards occurred to me that a better test would be furnished by those whose finger-ends are exercised in tactual perceptions, not occasionally, as by the blind in reading, but all day long in pursuit of their occupations. the facts answered expectation. two skilled compositors, on whom i experimented, were both able to distinguish between points when they were only one-seventeenth of an inch apart. thus we have clear proof that constant exercise of the tactual nervous structure leads to further development.[ ] now if acquired structural traits are inheritable, the various contrasts above set down are obvious consequences; for the gradations in tactual perceptiveness correspond with the gradations in the tactual exercises of the parts. save by contact with clothes, which present only broad surfaces having but slight and indefinite contrast, the trunk has scarcely any converse with external bodies, and it has but small discriminative power; but what discriminative power it has is greater on its face than on its back, corresponding to the fact that the chest and abdomen are much more frequently explored by the hands: this difference being probably in part inherited from inferior creatures; for, as we may see in dogs and cats, the belly is far more accessible to feet and tongue than the back. no less obtuse than the back are the middle of the back of the neck, the middle of the forearm, and the middle of the thigh; and these parts have but rare experiences of irregular foreign bodies. the crown of the head is occasionally felt by the fingers, as also the back of one hand by the fingers of the other; but neither of these surfaces, which are only twice as perceptive as the back, is used with any frequency for touching objects, much less for examining them. the lower part of the forehead, though more perceptive than the crown of the head, in correspondence with a somewhat greater converse with the hands, is less than one-third as perceptive as the tip of the nose; and manifestly, both in virtue of its relative prominence, in virtue of its contacts with things smelt at, and in virtue of its frequent acquaintance with the handkerchief, the tip of the nose has far greater tactual experience. passing to the inner surfaces of the hands, which, taken as wholes, are more constantly occupied in touching than are the back, breast, thigh, forearm, forehead, or back of the hand, weber's scale shows that they are much more perceptive, and that the degrees of perceptiveness of different parts correspond with their tactual activities. the palms have but one-fifth the perceptiveness possessed by the forefinger-ends; the inner surfaces of the finger-joints next the palms have but one-third; while the inner surfaces of the second joints have but one-half. these abilities correspond with the facts that whereas the inner parts of the hand are used only in grasping things, the tips of the fingers come into play not only when things are grasped, but when such things, as well as smaller things, are felt at or manipulated. it needs but to observe the relative actions of these parts in writing, in sewing, in judging textures, &c., to see that above all other parts the finger-ends, and especially the forefinger-ends, have the most multiplied experiences. if, then, it be that the extra perceptiveness acquired from actual tactual activities, as in a compositor, is inheritable, these gradations of tactual perceptiveness are explained. doubtless some of those who remember weber's results, have had on the tip of the tongue the argument derived from the tip of the tongue. this part exceeds all other parts in power of tactual discrimination: doubling, in that respect, the power of the forefinger-tip. it can distinguish points that are only one-twenty-fourth of an inch apart. why this unparalleled perceptiveness? if survival of the fittest be the ascribed cause, then it has to be shown what the advantages achieved have been; and, further, that those advantages have been sufficiently great to have had effects on the maintenance of life. besides tasting, there are two functions conducive to life, which the tongue performs. it enables us to move about food during mastication, and it enables us to make many of the articulations constituting speech. but how does the extreme discriminativeness of the tongue-tip aid these functions? the food is moved about, not by the tongue-tip, but by the body of the tongue; and even were the tip largely employed in this process, it would still have to be shown that its ability to distinguish between points one-twenty-fourth of an inch apart, is of service to that end, which cannot be shown. it may, indeed, be said that the tactual perceptiveness of the tongue-tip serves for detection of foreign bodies in the food, as plum-stones or as fish-bones. but such extreme perceptiveness is needless for the purpose. a perceptiveness equal to that of the finger-ends would suffice. and further, even were such extreme perceptiveness useful, it could not have caused survival of individuals who possessed it in slightly higher degrees than others. it needs but to observe a dog crunching small bones, and swallowing with impunity the sharp-angled pieces, to see that but a very small amount of mortality would be prevented. but what about speech? well, neither here can there be shown any advantage derived from this extreme perceptiveness. for making the _s_ and _z_, the tongue has to be partially applied to a portion of the palate next the teeth. not only, however, must the contact be incomplete, but its place is indefinite--may be half an inch further back. to make the _sh_ and _zh_, the contact has to be made, not with the tip, but with the upper surface of the tongue; and must be an incomplete contact. though, for making the liquids, the tip of the tongue and the sides of the tongue are used, yet the requisite is not any exact adjustment of the tip, but an imperfect contact with the palate. for the _th_, the tip is used along with the edges of the tongue; but no perfect adjustment is required, either to the edges of the teeth, or to the junction of the teeth with the palate, where the sound may equally well be made. though for the _t_ and _d_ complete contact of the tip and edges of the tongue with the palate is required, yet the place of contact is not definite, and the tip takes no more important share in the action than the sides. any one who observes the movements of his tongue in speaking, will find that there occur no cases in which the adjustments must have an exactness corresponding to the extreme power of discrimination which the tip possesses: for speech, this endowment is useless. even were it useful, it could not be shown that it has been developed by survival of the fittest; for though perfect articulation is an aid, yet imperfect articulation has rarely such an effect as to impede a man in the maintenance of his life. if he is a good workman, a german's interchanges of _b's_ and _p's_ do not disadvantage him. a frenchman who, in place of the sound of _th_, always makes the sound of _z_, succeeds as a teacher of music or dancing, no less than if he achieved the english pronunciation. nay, even such an imperfection of speech as that which arises from cleft palate, does not prevent a man from getting on if he is capable. true, it may go against him as a candidate for parliament, or as an "orator" of the unemployed (mostly not worth employing). but in the struggle for life he is not hindered by the effect to the extent of being less able than others to maintain himself and his offspring. clearly, then, even if this unparalleled perceptiveness of the tongue-tip is required for perfect speech, such use is not sufficiently important to have been developed by natural selection. how, then, is this remarkable trait of the tongue-tip to be accounted for? without difficulty, if there is inheritance of acquired characters. for the tongue-tip has, above all other parts of the body, unceasing experiences of small irregularities of surface. it is in contact with the teeth, and either consciously or unconsciously is continually exploring them. there is hardly a moment in which impressions of adjacent but different positions are not being yielded to it by either the surfaces of the teeth or their edges; and it is continually being moved about from some of them to others. no advantage is gained. it is simply that the tongue's position renders perpetual exploration almost inevitable; and by perpetual exploration is developed this unique power of discrimination. thus the law holds throughout, from this highest degree of perceptiveness of the tongue-tip to its lowest degree on the back of the trunk; and no other explanation of the facts seems possible. "yes, there is another explanation," i hear some one say: "they may be explained by _panmixia_." well, in the first place, as the explanation by _panmixia_ implies that these gradations of perceptiveness have been arrived at by the dwindling of nervous structures, there lies at the basis of the explanation an unproved and improbable assumption; and, in the second place, even were there no such difficulty, it may with certainty be denied that _panmixia_ can furnish an explanation. let us look at its pretensions. * * * * * it was not without good reason that bentham protested against metaphors. figures of speech in general, valuable as they are in poetry and rhetoric, cannot be used without danger in science and philosophy. the title of mr. darwin's great work furnishes us with an instance of the misleading effects produced by them. it runs:--_the origin of species by means of natural selection, or the preservation of favoured races in the struggle for life_. here are two figures of speech which conspire to produce an impression more or less erroneous. the expression "natural selection" was chosen as serving to indicate some parallelism with artificial selection--the selection exercised by breeders. now selection connotes volition, and thus gives to the thoughts of readers a wrong bias. some increase of this bias is produced by the words in the second title, "favoured races;" for anything which is favoured implies the existence of some agent conferring a favour. i do not mean that mr. darwin himself failed to recognize the misleading connotations of his words, or that he did not avoid being misled by them. in chapter iv of the _origin of species_, he says that, considered literally, "natural selection is a false term," and that the personification of nature is objectionable; but he thinks that readers, and those who adopt his views, will soon learn to guard themselves against the wrong implications. here i venture to think that he was mistaken. for thinking this, there is the reason that even his disciple, mr. wallace--no, not his disciple, but his co-discoverer, ever to be honoured--has apparently been influenced by them. when, for example, in combating a view of mine, he says that "the very thing said to be impossible by variation and natural selection has been again and again effected, by variation and artificial selection," he seems clearly to imply that the processes are analogous, and operate in the same way. now this is untrue. they are analogous only within certain narrow limits; and, in the great majority of cases, natural selection is utterly incapable of doing that which artificial selection does. to see this it needs only to de-personalise nature, and to remember that, as mr. darwin says, nature is "only the aggregate action and product of many natural laws [forces]." observe its relative shortcomings. artificial selection can pick out a particular trait, and, regardless of other traits of the individuals displaying it, can increase it by selective breeding in successive generations. for, to the breeder or fancier, it matters little whether such individuals are otherwise well constituted. they may be in this or that way so unfit for carrying on the struggle for life, that were they without human care, they would disappear forthwith. on the other hand, if we regard nature as that which it is, an assemblage of various forces, inorganic and organic, some favourable to the maintenance of life and many at variance with its maintenance--forces which operate blindly--we see that there is no such selection of this or that trait; but that there is a selection only of individuals which are, by the aggregate of their traits, best fitted for living. and here i may note an advantage possessed by the expression "survival of the fittest;" since this does not tend to raise the thought of any one character which, more than others, is to be maintained or increased; but tends rather to raise the thought of a general adaptation for all purposes. it implies the process which nature can alone carry on--the leaving alive of those which are best able to utilize surrounding aids to life, and best able to combat or avoid surrounding dangers. and while this phrase covers the great mass of cases in which there are preserved well-constituted individuals, it also covers those special cases which are suggested by the phrase "natural selection," in which individuals succeed beyond others in the struggle for life, by the help of particular characters which conduce in important ways to prosperity and multiplication. for now observe the fact which here chiefly concerns us, that survival of the fittest can increase any serviceable trait, only if that trait conduces to prosperity of the individual, or of posterity, or of both, _in an important degree_. there can be no increase of any structure by natural selection unless, amid all the slightly varying structures constituting the organism, increase of this particular one is so advantageous as to cause greater multiplication of the family in which it arises than of other families. variations which, though advantageous, fail to do this, must disappear again. let us take a case. keenness of scent in a deer, by giving early notice of approaching enemies, subserves life so greatly that, other things equal, an individual having it in an unusual degree is more likely than others to survive; and, among descendants, to leave some similarly endowed or more endowed, who again transmit the variation with, in some cases, increase. clearly this highly useful power may be developed by natural selection. so also, for like reasons, may quickness of vision and delicacy of hearing; though it may be remarked in passing that since this extra sense-endowment, serving to give early alarm, profits the herd as a whole, which takes the alarm from one individual, selection of it is not so easy, unless it occurs in a conquering stag. but now suppose that one member of the herd--perhaps because of more efficient teeth, perhaps by greater muscularity of stomach, perhaps by secretion of more appropriate gastric juices--is enabled to eat and digest a not uncommon plant which the others refuse. this peculiarity may, if food is scarce, conduce to better self-maintenance, and better fostering of young if the individual is a hind. but unless this plant is abundant, and the advantage consequently great, the advantages which other members of the herd gain from other slight variations may be equivalent. this one has unusual agility, and leaps a chasm which others balk at. that one develops longer hair in winter, and resists the cold better. another has a skin less irritated by flies, and can graze without so much interruption. here is one which has an unusual power of detecting food under the snow; and there is one which shows extra sagacity in the choice of a shelter from wind and rain. that the variation giving ability to eat a plant before unutilized, may become a trait of the herd, and eventually of a variety, it is needful that the individual in which it occurs shall have more descendants, or better descendants, or both, than have the various other individuals severally having their small superiorities. if these other individuals severally profit by their small superiorities, and transmit them to equally large numbers of offspring, no increase of the variation in question can take place: it must soon be cancelled. whether in the _origin of species_ mr. darwin has recognized this fact, i do not remember, but he has certainly done it by implication in his _animals and plants under domestication_. speaking of variations in domestic animals, he there says that "any particular variation would generally be lost by crossing, reversion, and the accidental destruction of the varying individuals, unless carefully preserved by man." (vol. ii, p. .) that which survival of the fittest does in cases like the one i have instanced, is to keep all faculties up to the mark, by destroying such individuals as have faculties in some respect below the mark; and it can produce development of some one faculty only if that faculty is predominantly important. it seems to me that many naturalists have practically lost sight of this, and assume that natural selection will increase _any_ advantageous trait. certainly a view now held by some assumes as much. the consideration of this view, to which the foregoing paragraph is introductory, may now be entered upon. this view concerns, not direct selection, but what has been called, in questionable logic, "reversed selection"--the selection which effects, not increase of an organ, but decrease of it. for as, under some conditions, it is of advantage to an individual and its descendants to have some structure of larger size, it may be, under other conditions--namely, when the organ becomes useless--of advantage to have it of smaller size; since, even if it is not in the way, its weight and the cost of its nutrition are injurious taxes on the organism. but now comes the truth to be emphasized. just as direct selection can increase an organ only in certain cases, so can reversed selection decrease it only in certain cases. like the increase produced by a variation, the decrease produced by one must be such as will sensibly conduce to preservation and multiplication. it is, for instance, conceivable that were the long and massive tail of the kangaroo to become useless (say by the forcing of the species into a mountainous and rocky habitat filled with brushwood), a variation which considerably reduced the tail might sensibly profit the individual in which it occurred; and, in seasons when food was scarce, might cause survival when individuals with large tails died. but the economy of nutrition must be considerable before any such result could occur. suppose that in this new habitat the kangaroo had no enemies; and suppose that, consequently, quickness of hearing not being called for, large ears gave no greater advantage than small ones. would an individual with smaller ears than usual, survive and propagate better than other individuals, in consequence of the economy of nutrition achieved? to suppose this is to suppose that the saving of a grain or two of protein per day would determine the kangaroo's fate. long ago i discussed this matter in the _principles of biology_ (§ ), taking as an instance the decrease of the jaw implied by the crowding of the teeth, and now proved by measurement to have taken place. here is the passage:-- "no functional superiority possessed by a small jaw over a large jaw, in civilized life, can be named as having caused the more frequent survival of small-jawed individuals. the only advantage which smallness of jaw might be supposed to give, is the advantage of economized nutrition; and this could not be great enough to further the preservation of men possessing it. the decrease of weight in the jaw and co-operative parts that has arisen in the course of many thousands of years, does not amount to more than a few ounces. this decrease has to be divided among the many generations that have lived and died in the interval. let us admit that the weight of these parts diminished to the extent of an ounce in a single generation (which is a large admission); it still cannot be contended that the having to carry an ounce less in weight, or having to keep in repair an ounce less of tissue, could sensibly affect any man's fate. and if it never did this--nay, if it did not cause a _frequent_ survival of small-jawed individuals where large-jawed individuals died, natural selection could neither cause nor aid diminution of the jaw and its appendages." when writing this passage in , i never dreamt that a quarter of a century later, the supposable cause of degeneration here examined and excluded as impossible, would be enunciated as an actual cause and named "reversed selection." one of the arguments used to show the adequacy of natural selection under its direct or indirect form consists of a counter-argument to the effect that inheritance of functionally-wrought changes, supposing it to be operative, does not explain certain of the facts. this is alleged by prof. weismann as a part justification for his doctrine of panmixia. concerning the "blind fish and amphibia" found in dark places, which have but rudimentary eyes "hidden under the skin," he argues that "it is difficult to reconcile the facts of the case with the ordinary theory that the eyes of these animals have simply degenerated through disuse." after giving instances of rapid degeneration of disused organs, he argues that if "the effects of disuse are so striking in a single life, we should certainly expect, if such effects can be transmitted, that all traces of an eye would soon disappear from a species which lives in the dark." doubtless this is a reasonable conclusion. to explain the facts on the hypothesis that acquired characters are inheritable, seems very difficult. one possible explanation may, indeed, be named. it appears to be a general law of organization that structures are stable in proportion to their antiquity--that while organs of relatively modern origin have but a comparatively superficial root in the constitution, and readily disappear if the conditions do not favour their maintenance, organs of ancient origin have deep-seated roots in the constitution, and do not readily disappear. having been early elements in the type, and having continued to be reproduced as parts of it during a period extending throughout many geological epochs, they are comparatively persistent. now the eye answers to this description as being a very early organ. but waiving possible explanations, let us take the particular instance cited by prof. weismann and see what is to be made of it. he writes:-- "the caverns in carniola and carinthia, in which the blind _proteus_ and so many other blind animals live, belong geologically to the jurassic formation; and although we do not exactly know when for example the _proteus_ first entered them, the low organization of this amphibian certainly indicates that it has been sheltered there for a very long period of time, and that thousands of generations of this species have succeeded one another in the caves. "hence there is no reason to wonder at the extent to which the degeneration of the eye has been already carried in the _proteus_; even if we assume that it is merely due to the cessation of the conserving influence of natural selection."[ ] let me first note a strange oversight on the part of prof. weismann. he points out that the caverns in question belong to the jurassic formation: apparently intending to imply that they have an antiquity related to that of the formation. but there is no such relation, except that the caverns cannot be older than the formation. they may have originated at any period since the containing strata were deposited; and they may be therefore relatively modern. but passing over this, and admitting that the _proteus_ has inhabited the caverns for an enormous period, what is to be said of the fact that their eyes have not disappeared entirely, as prof. weismann contends they should have done had the inheritance of the effects of disuse been all along operative? there is a very sufficient answer--the rudimentary eyes are not entirely useless. it seems that when the underground streams it inhabits are unusually swollen, some individuals of the species are carried out of the caverns into the open (being then sometimes captured). it is also said that the creatures shun the light; this trait being, i presume, observed when it is in captivity. now obviously, among individuals carried out into the open, those which remain visible are apt to be carried off by enemies; whereas, those which, appreciating the difference between light and darkness, shelter themselves in dark places, survive. hence the tendency of natural selection is to prevent the decrease of the eyes beyond that point at which they can distinguish between light and darkness. thus the apparent anomaly is explained. let me suggest, as another possible reason for persistence of rudimentary organs, that the principle of economy of growth will cause diminution of them only in proportion as their constituents are of value for other uses in the organism; and that in many cases their constituents are practically valueless. hence probably the reason why, in the case of stalk-eyed crustaceans, the eye is gone but the pedicle remains, or to use mr. darwin's simile, the telescope has disappeared but not its stand. * * * * * along with that inadequacy of natural selection to explain changes of structure which do not aid life in important ways, alleged in § of _the principles of biology_, a further inadequacy was alleged. it was contended that the relative powers of co-operative parts cannot be adjusted solely by survival of the fittest; and especially where the parts are numerous and the co-operation complex. in illustration it was pointed out that immensely developed horns, such as those of the extinct irish elk, weighing over a hundred-weight, could not, with the massive skull bearing them, be carried at the extremity of the outstretched neck without many and great modifications of adjacent bones and muscles of the neck and thorax; and that without strengthening of the fore-legs, too, there would be failure alike in fighting and in locomotion. and it was argued that while we cannot assume spontaneous increase of all these parts proportionate to the additional strains, we cannot suppose them to increase by variations, one at once, without supposing the creature to be disadvantaged by the weight and nutrition of parts that were for the time useless--parts, moreover, which would revert to their original sizes before the other needful variations occurred. when, in reply to me, it was contended that co-operative parts vary together, i named facts conflicting with this assertion--the fact that the blind cray-fish of the kentucky caves have lost their eyes but not the foot-stalks carrying them; the fact that the normal proportion between tongue and beak in certain selected varieties of pigeons is lost; the fact that lack of concomitance in decrease of jaws and teeth in sundry kinds of pet dogs, has caused great crowding of the teeth ("the factors of organic evolution," _essays_, i, - ). and i then argued that if co-operative parts, small in number and so closely associated as these are, do not vary together, it is unwarrantable to allege that co-operative parts which are very numerous and remote from one another vary together. after making this rejoinder i enforced my argument by a further example--that of the giraffe. tacitly recognizing the truth that the unusual structure of this creature must have been, in its most conspicuous traits, the result of survival of the fittest (since it is absurd to suppose that efforts to reach high branches could lengthen the legs), i illustrated afresh the obstacles to co-adaptation. not dwelling on the objection that increase of any components of the fore-quarters out of adjustment to the others, would cause evil rather than good, i went on to argue that the co-adaptation of parts required to make the giraffe's structure useful, is much greater than at first appears. this animal has a grotesque gallop, necessitated by the great difference in length between the fore and the hind limbs. i pointed out that the mode of action of the hind limbs shows that the bones and muscles have all been changed in their proportions and adjustments; and i contended that, difficult as it is to believe that all parts of the fore-quarters have been co-adapted by the appropriate variations, now of this part now of that, it becomes impossible to believe that all the parts in the hind-quarters have been simultaneously co-adapted to one another and to all the parts of the fore-quarters: adding that want of co-adaptation, even in a single muscle, would cause fatal results when high speed had to be maintained while escaping from an enemy. since this argument, repeated with this fresh illustration, was published in , i have met with nothing to be called a reply; and might, i think, if convictions usually followed proofs, leave the matter as it stands. it is true that, in his _darwinism_, mr. wallace has adverted to my renewed objection, and, as already said, contended that changes such as those instanced can be effected by natural selection, since such changes can be effected by artificial selection: a contention which, as i have pointed out, assumes a parallelism that does not exist. but now, instead of pursuing the argument further along the same line, let me take a somewhat different line. if there occurs some change in an organ, say by increase of its size, which adapts it better to the creature's needs, it is admitted that when, as commonly happens, the use of the organ demands the co-operation of other organs, the change in it will generally be of no service unless the co-operative organs are changed. if, for instance, there takes place such a modification of a rodent's tail as that which, by successive increases, produces the trowel-shaped tail of the beaver, no advantage will be derived unless there also take place certain modifications in the bulks and shapes of the adjacent vertebræ and their attached muscles, as well as, probably, in the hind limbs; enabling them to withstand the reactions of the blows given by the tail. and the question is, by what process these many parts, changed in different degrees, are co-adapted to the new requirements--whether variation and natural selection alone can effect the readjustment. there are three conceivable ways in which the parts may simultaneously change:--( ) they may all increase or decrease together in like degree; ( ) they may all simultaneously increase or decrease independently, so as not to maintain their previous proportions, or assume any other special proportions; ( ) they may vary in such ways and degrees as to make them jointly serviceable for the new end. let us consider closely these several conceivabilities. and first of all, what are we to understand by co-operative parts? in a general sense, all the organs of the body are co-operative parts, and are respectively liable to be more or less changed by change in any one. in a narrower sense, more directly relevant to the argument, we may, if we choose to multiply difficulties, take the entire framework of bones and muscles as formed of co-operative parts; for these are so related that any considerable change in the actions of some entails change in the actions of most others. it needs only to observe how, when putting out an effort, there goes, along with a deep breath, an expansion of the chest and a bracing up of the abdomen, to see that various muscles beyond those directly concerned are strained along with them. or, when suffering from lumbago, an effort to lift a chair will cause an acute consciousness that not the arms only are brought into action, but also the muscles of the back. these cases show how the motor organs are so tied together that altered actions of some implicate others quite remote from them. but without using the advantage which this interpretation of the words would give, let us take, as co-operative organs, those which are obviously such--the organs of locomotion. what, then, shall we say of the fore limbs and hind limbs of terrestrial mammals, which co-operate closely and perpetually? do they vary together? if so, how have there been produced such contrasted structures as that of the kangaroo, with its large hind limbs and small fore limbs, and that of the giraffe, in which the hind limbs are small and the fore limbs large--how does it happen that, descending from the same primitive mammal, these creatures have diverged in the proportions of their limbs in opposite directions? take, again, the articulate animals. compare one of the lower types, with its rows of almost equal-sized limbs, and one of the higher types, as a crab or a lobster, with limbs some very small and some very large. how came this contrast to arise in the course of evolution, if there was the equality of variation supposed? but now let us narrow the meaning of the phrase still further, giving it a more favourable interpretation. instead of considering separate limbs as co-operative, let us consider the component parts of the same limb as co-operative, and ask what would result, from varying together. it would in that case happen that, though the fore and hind limbs of a mammal might become different in their sizes, they would not become different in their structures. if so, how have there arisen the unlikenesses between the hind legs of the kangaroo and those of the elephant? or if this comparison is objected to, because the creatures belong to the widely different divisions of implacental and placental mammals, take the cases of the rabbit and the elephant, both belonging to the last division. on the hypothesis of evolution these are both derived from the same original form; but the proportions of the parts have become so widely unlike that the corresponding joints are scarcely recognized as such by the unobservant: at what seem corresponding places the legs bend in opposite ways. equally marked, or more marked, is the parallel fact among the _articulata_. take that limb of the lobster which bears the claw and compare it with the corresponding limb in an inferior articulate animal, or the corresponding limb of its near ally, the rock lobster, and it becomes obvious that the component segments of the limb have come to bear to one another in the one case, proportions immensely different from those they bear in the other case. undeniably, then, on contemplating the general facts of organic structure, we see that the concomitant variations in the parts of limbs, have not been of a kind to produce equal amounts of change in them, but quite the opposite--have been everywhere producing inequalities. moreover, we are reminded that this production of inequalities among co-operative parts, is an essential principle of development. had it not been so, there could not have been that progress from homogeneity of structure to heterogeneity of structure which constitutes evolution. we pass now to the second supposition:--that the variations in co-operative parts occur irregularly, or in such independent ways that they bear no definite relations to one another--miscellaneously, let us say. this is the supposition which best corresponds with the facts. glances at the faces around yield conspicuous proofs. many of the muscles of the face and some of the bones, are distinctly co-operative; and these respectively vary in such ways as to produce in each person a different combination. what we see in the face we have reason to believe holds in the limbs and in all other parts. indeed, it needs but to compare people whose arms are of the same lengths, and observe how stumpy are the fingers of one and how slender those of another; or it needs but to note the unlikenesses of gait of passers-by, implying small unlikenesses of structure; to be convinced that the relations among the variations of co-operative parts are anything but fixed. and now, confining our attention to limbs, let us consider what must happen if, by variations taking place miscellaneously, limbs have to be partially changed from fitness for one function to fitness for another function--have to be re-adapted. that the reader may fully comprehend the argument, he must here have patience while a good many anatomical details are set down. let us suppose a species of quadruped of which the members have, for immense past periods, been accustomed to locomotion over a relatively even surface, as, for instance, the "prairie-dogs" of north america; and let us suppose that increase of numbers has driven part of them into a region full of obstacles to easy locomotion--covered, say, by the decaying stems of fallen trees, such as one sees in portions of primeval forest. ability to leap must then become a useful trait; and, according to the hypothesis we are considering, this ability will be produced by the selection of favourable variations. what are the variations required? a leap is effected chiefly by the bending of the hind limbs so as to make sharp angles at the joints, and then suddenly straightening them; as any one may see on watching a cat leap on to the table. the first required change, then, is increase of the large extensor muscles, by which the hind limbs are straightened. their increases must be duly proportioned; for if those which straightened one joint become much stronger than those which straightened the other joint, the result must be collapse of the other joint when the muscles are contracted together. but let us make a large admission, and suppose these muscles to vary together; what further muscular change is next required? in a plantigrade mammal the metatarsal bones chiefly bear the reaction of the leap, though the toes may have a share. in a digitigrade mammal, however, the toes form almost exclusively the fulcrum, and if they are to bear the reaction of a higher leap, the flexor muscles which depress and bend them must be proportionately enlarged: if not, the leap will fail from want of a firm _point d'appui_. tendons as well as muscles must be modified; and, among others, the many tendons which go to the digits and their phalanges. stronger muscles and tendons imply greater strains on the joints; and unless these are strengthened, one or other, dislocation will be caused by a more vigorous spring. not only the articulations themselves must be so modified as to bear greater stress, but also the numerous ligaments which hold the parts of each in place. nor can the bodies of the bones remain unstrengthened; for if they have no more than the strengths needed for previous movements they will fail to bear more violent movements. thus, saying nothing of the required changes in the pelvis, as well as in the nerves and blood-vessels, there are, counting bones, muscles, tendons, ligaments, at least fifty different parts in each hind leg which have to be enlarged. moreover they have to be enlarged in unlike degrees. the muscles and tendons of the outer toes, for example, need not be added to so much as those of the median toes. now, throughout their successive stages of growth, all these parts have to be kept fairly well balanced; as any one may infer on remembering sundry of the accidents he has known. among my own friends i could name one who, when playing lawn-tennis, snapped the achilles tendon; another who, while swinging his children, tore some of the muscular fibres in the calf of his leg; another who, in getting over a fence, tore a ligament of one knee. such facts, joined with every one's experience of sprains, show that during the extreme exertions to which limbs are now and then subject, there is a giving way of parts not quite up to the required level of strength. how, then, is this balance to be maintained? suppose the extensor muscles have all varied appropriately; their variations are useless unless the other co-operative parts have also varied appropriately. worse than this. saying nothing of the disadvantage caused by extra weight and cost of nutrition, they will be causes of mischief--causes of derangement to the rest by contracting with undue force. and then, how long will it take for the rest to be brought into adjustment? as mr. darwin says concerning domestic animals:--"any particular variation would generally be lost by crossing, reversion, &c. ... unless carefully preserved by man." in a state of nature, then, favourable variations of these muscles would disappear again long before one or a few of the co-operative parts could be appropriately varied, much more before all of them could. with this insurmountable difficulty goes a difficulty still more insurmountable--if the expression may be allowed. it is not a question of increased sizes of parts only, but of altered shapes of parts, too. a glance at the skeletons of mammals shows how unlike are the forms of the corresponding bones of their limbs; and shows that they have been severally re-moulded in each species to the different requirements entailed by its different habits. the change from the structures of hind limbs fitted only for walking and trotting to hind limbs fitted also for leaping, implies, therefore, that, along with strengthenings of bones there must go alterations in their forms. now the fortuitous alterations of form which may take place in any bone are countless. how long, then, will it be before there takes place that particular alteration which will make the bone fitter for its new action? and what is the probability that the many required changes of shape, as well as of size, in bones will each of them be effected before all the others are lost again? if the probabilities against success are incalculable, when we take account only of changes in the sizes of parts, what shall we say of their incalculableness when differences of form also are taken into account? "surely this piling up of difficulties has gone far enough"; the reader will be inclined to say. by no means. there is a difficulty immeasurably transcending those named. we have thus far omitted the second half of the leap, and the provisions to be made for it. after ascent of the animal's body comes descent; and the greater the force with which it is projected up, the greater is the force with which it comes down. hence, if the supposed creature has undergone such changes in the hind limbs as will enable them to propel it to a greater height, without having undergone any changes in the fore limbs, the result will be that on its descent the fore limbs will give way, and it will come down on its nose. the fore limbs, then, have to be changed simultaneously with the hind. how changed? contrast the markedly bent hind limbs of a cat with its almost straight fore limbs, or contrast the silence of the spring on to the table with the thud which the fore paws make as it jumps off the table. see how unlike the actions of the hind and fore limbs are, and how unlike their structures. in what way, then, is the required co-adaptation to be effected? even were it a question of relative sizes only, there would be no answer; for facts already given show that we may not assume simultaneous increases of size to take place in the hind and fore limbs; and, indeed, a glance at the various human races, which differ considerably in the ratios of their legs to their arms, shows us this. but it is not simply a question of sizes. to bear the increased shock of descent the fore limbs must be changed throughout in their structures. like those in the hind limbs, the changes must be of many parts in many proportions; and they must be both in sizes and in shapes. more than this. the scapular arch and its attached muscles must also be strengthened and re-moulded. see, then, the total requirements. we must suppose that by natural selection of miscellaneous variations, the parts of the hind limbs will be co-adapted to one another, in sizes, shapes, and ratios; that those of the fore limbs will undergo co-adaptation similar in their complexity, but dissimilar in their kinds; and that the two sets of co-adaptations will be effected _pari passu_. if, as may be held, the probabilities are millions to one against the first set of changes being achieved, then it may be held that the probabilities are billions to one against the second being simultaneously achieved, in progressive adjustment to the first. there remains only to notice the third conceivable mode of adjustment. it may be imagined that though, by the natural selection of miscellaneous variations, these adjustments cannot be effected, they may nevertheless be made to take place appropriately. how made? to suppose them so made is to suppose that the prescribed end is somewhere recognized; and that the changes are step by step simultaneously proportioned for achieving it--is to suppose a designed production of these changes. in such case, then, we have to fall back in part upon the primitive hypothesis; and if we do this in part, we may as well do it wholly--may as well avowedly return to the doctrine of special creations. what, then, is the only defensible interpretation? if such modifications of structure produced by modifications of function as we see take place in each individual, are in any measure transmissible to descendants, then all these co-adaptations, from the simplest up to the most complex, are accounted for. in some cases this inheritance of acquired characters suffices by itself to explain the facts; and in other cases it suffices when taken in combination with the selection of favourable variations. an example of the first class is furnished by the change just considered; and an example of the second class is furnished by the case, before named, of development in a deer's horns. if, by some extra massiveness spontaneously arising, or by formation of an additional "point," an advantage is gained either for attack or defence, then, if the increased muscularity and strengthened structure of the neck and thorax, which wielding of these somewhat heavier horns produces, are in a greater or less degree inherited, and in several successive generations are by this process brought up to the required extra strength, it becomes possible and advantageous for a further increase of the horns to take place, and a further increase in the apparatus for wielding them, and so on continuously. by such processes only, in which each part gains strength in proportion to function, can co-operative parts be kept in adjustment, and be re-adjusted to meet new requirements. close contemplation of the facts impresses me more strongly than ever with the two alternatives--either there has been inheritance of acquired characters, or there has been no evolution. this very pronounced opinion will be met, on the part of some, by a no less pronounced demurrer, which involves a denial of possibility. it has been of late asserted, and by many believed, that inheritance of acquired characters cannot occur. weismann, they say, has shown that there is early established in the evolution of each organism such a distinctness between those component units which carry on the individual life and those which are devoted to maintenance of the species, that changes in the one cannot affect the other. we will look closely into his doctrine. basing his argument on the principle of the physiological division of labour, and assuming that the primary division of labour is that between such part of an organism as carries on individual life and such part as is reserved for the production of other lives, weismann, starting with "the first multicellular organism," says that--"hence the single group would come to be divided into two groups of cells, which may be called somatic and reproductive--the cells of the body as opposed to those which are concerned with reproduction." (_essays upon heredity_, i, p. .) though he admits that this differentiation "was not at first absolute, and indeed is not always so to-day," yet he holds that the differentiation eventually becomes absolute in the sense that the somatic cells, or those which compose the body at large, come to have only a limited power of cell-division, instead of an unlimited power which the reproductive cells have; and also in the sense that eventually there ceases to be any communication between the two further than that implied by the supplying of nutriment to the reproductive cells by the somatic cells. the outcome of this argument is that, in the absence of communication, changes induced in the somatic cells, constituting the individual, cannot influence the natures of the reproductive cells, and cannot therefore be transmitted to posterity. such is the theory. now let us look at a few facts--some familiar, some unfamiliar. his investigations led pasteur to the positive conclusion that the silkworm diseases are inherited. the transmission from parent to offspring resulted, not through any contamination of the surface of the egg by the body of the parent while being deposited, but resulted from infection of the egg itself--intrusion of the parasitic organism. generalized observations concerning the disease called _pébrine_, enabled him to decide, by inspection of the eggs, which were infected and which were not: certain modifications of form distinguishing the diseased ones. more than this; the infection was proved by microscopical examination of the contents of the egg; in proof of which he quotes as follows from dr. carlo vittadini:-- "il résulte de mes recherches sur les graines, à l'époque où commence le développement du germe, que les corpuscules, une fois apparus dans l'oeuf, augmentent graduellement en nombre, à mesure que l'embryon se développe; que, dans les derniers jours de l'incubation, l'oeuf en est plein, au point de faire croire que la majeure partie des granules du jaune se sont transformés en corpuscules. "une autre observation importante est que l'embryon aussi est souillé de corpuscules, et à un degré tel qu'on peut soupçonner que l'infection du jaune tire son origine du germe lui-même; en d'autres termes que le germe est primordialement infecté, et porte en lui-même ces corpuscules tout comme les vers adultes, frappés du même mal."[ ] thus, then the substance of the egg and even its innermost vital part, is permeable by a parasite sufficiently large to be microscopically visible. it is also of course permeable by the invisible molecules of protein, out of which its living tissues are formed, and by absorption of which they subsequently grow. but, according to weismann, it is _not_ permeable by those invisible units of protoplasm out of which the vitally active tissues of the parent are constituted: units composed, as we must assume, of variously arranged molecules of protein. so that the big thing may pass, and the little thing may pass, but the intermediate thing may not pass! a fact of kindred nature, unhappily more familiar, may be next brought in evidence. it concerns the transmission of a disease not infrequent among those of unregulated lives. the highest authority concerning this disease, in its inherited form, is mr. jonathan hutchinson; and the following are extracts from a letter i have received from him, and which i publish with his assent:-- "i do not think that there can be any reasonable doubt that a very large majority of those who suffer from inherited syphilis take the taint from the male parent.... it is the rule when a man marries who has no remaining local lesion, but in whom the taint is not eradicated, for his wife to remain apparently well, whilst her child may suffer. no doubt the child infects its mother's blood, but this does not usually evoke any obvious symptoms of syphilis.... i am sure i have seen hundreds of syphilitic infants whose mothers had not, so far as i could ascertain, ever displayed a single symptom." see, then, to what we are committed if we accept weismann's hypothesis. we must conclude, that whereas the reproductive cell may be effectually invaded by an abnormal living element in the parental organism, those normal living elements which constitute the vital protoplasm of the parental organism, cannot invade it. or if it be admitted that both intrude, then the implication is that, whereas the abnormal element can so modify the development as to cause changes of structure (as of the teeth), the normal element can cause no changes of structure![ ] we pass now to evidence not much known to the world at large, but widely known in the biological world, though known in so incomplete a manner as to be undervalued in it. indeed, when i name it, probably many will vent a mental pooh-pooh. the fact to which i refer is one of which record is preserved in the museum of the college of surgeons, in the shape of paintings of a foal borne by a mare not quite thoroughbred, to a sire which was thoroughbred--a foal which bears the markings of the quagga. the history of this remarkable foal is given by the earl of morton, f.r.s., in a letter to the president of the royal society (read november , ). in it he states that wishing to domesticate the quagga, and having obtained a male but not a female, he made an experiment. "i tried to breed from the male quagga and a young chestnut mare of seven-eighths arabian blood, and which had never been bred from; the result was the production of a female hybrid, now five years old, and bearing, both in her form and in her colour, very decided indications of her mixed origin. i subsequently parted with the seven-eighths arabian mare to sir gore ouseley, who has bred from her by a very fine black arabian horse. i yesterday morning examined the produce, namely, a two-year-old filly and a year-old colt. they have the character of the arabian breed as decidedly as can be expected, where fifteen-sixteenths of the blood are arabian; and they are fine specimens of that breed; but both in their colour and in the hair of their manes, they have a striking resemblance to the quagga. their colour is bay, marked more or less like the quagga in a darker tint. both are distinguished by the dark line along the ridge of the back, the dark stripes across the forehead, and the dark bars across the back part of the legs."[ ] lord morton then names sundry further correspondences. dr. wollaston, at that time president of the royal society, who had seen the animals, testified to the correctness of his description, and, as shown by his remarks, entertained no doubt about the alleged facts. but good reason for doubt may be assigned. there naturally arises the question--how does it happen that parallel results are not observed in other cases? if in any progeny certain traits not belonging to the sire, but belonging to a sire of preceding progeny, are reproduced, how is it that such anomalously inherited traits are not observed in domestic animals, and indeed in mankind? how is it that the children of a widow by a second husband do not bear traceable resemblances to the first husband? to these questions nothing like satisfactory replies seem forthcoming; and, in the absence of replies, scepticism, if not disbelief, may be held reasonable. there is an explanation, however. forty years ago i made acquaintance with a fact which impressed me by its significant implications, and has, for this reason i suppose, remained in my memory. it is set forth in the _journal of the royal agricultural society_, vol. xiv ( ), pp. _et seq._, and concerns certain results of crossing french and english breeds of sheep. the writer of the translated paper, m. malingie-nouel, director of the agricultural school of la charmoise, states that when the french breeds of sheep (in which were included "the _mongrel_ merinos") were crossed with an english breed, "the lambs present the following results. most of them resemble the mother more than the father; some show no trace of the father." joining the admission respecting the mongrels with the facts subsequently stated, it is tolerably clear that the cases in which the lambs bore no traces of the father were cases in which the mother was of pure breed. speaking of the results of these crossings in the second generation, "having per cent. of english blood," m. nouel says:--"the lambs thrive, wear a beautiful appearance, and complete the joy of the breeder.... no sooner are the lambs weaned than their strength, their vigour, and their beauty begin to decay.... at last the constitution gives way ... he remains stunted for life:" the constitution being thus proved unstable or unadapted to the requirements. how, then, did m. nouel succeed in obtaining a desirable combination of a fine english breed with the relatively poor french breeds? he took an animal from "flocks originally sprung from a mixture of the two distinct races that are established in those two provinces [berry and la sologne]," and these he "united with animals of another mixed breed ... which blended the tourangelle and native merino blood of" la beauce and touraine, and obtained a mixture of all four races "without decided character, without fixity ... but possessing the advantage of being used to our climate and management." putting one of these "mixed blood ewes to a pure new-kent ram ... one obtains a lamb containing fifty-hundredths of the purest and most ancient english blood, with twelve and a half hundredths of four different french races, which are individually lost in the preponderance of english blood, and disappear almost entirely, leaving the improving type in the ascendant.... all the lambs produced strikingly resembled each other, and even englishmen took them for animals of their own country." m. nouel goes on to remark that when this derived breed was bred with itself, the marks of the french breeds were lost. "some slight traces" could be detected by experts, but these "soon disappeared." thus we get proof that relatively pure constitutions predominate in progeny over much mixed constitutions. the reason is not difficult to see. every organism tends to become adapted to its conditions of life; and all the structures of a species, accustomed through multitudinous generations to the climate, food, and various influences of its locality, are moulded into harmonious co-operation favourable to life in that locality: the result being that in the development of each young individual, the tendencies conspire to produce the fit organization. it is otherwise when the species is removed to a habitat of different character, or when it is of mixed breed. in the one case its organs, partially out of harmony with the requirements of its new life, become partially out of harmony with one another; since, while one influence, say of climate, is but little changed, another influence, say of food, is much changed; and, consequently, the perturbed relations of the organs interfere with their original stable equilibrium. still more in the other case is there a disturbance in equilibrium. in a mongrel, the constitution derived from each source repeats itself as far as possible. hence a conflict of tendencies to evolve two structures more or less unlike. the tendencies do not harmoniously conspire, but produce partially incongruous sets of organs. and evidently where the breed is one in which there are united the traits of various lines of ancestry, there results an organization so full of small incongruities of structure and action, that it has a much-diminished power of maintaining its balance; and while it cannot withstand so well adverse influences, it cannot so well hold its own in the offspring. concerning parents of pure and mixed breeds respectively, severally tending to reproduce their own structures in progeny, we may therefore say, figuratively, that the house divided against itself cannot withstand the house of which the members are in concord. now if this is shown to be the case with breeds the purest of which have been adapted to their habitats and modes of life during some few hundred years only, what shall we say when the question is of a breed which has had a constant mode of life in the same locality for ten thousand years or more, like the quagga? in this the stability of constitution must be such as no domestic animal can approach. relatively stable as may have been the constitutions of lord morton's horses, as compared with the constitutions of ordinary horses, yet, since arab horses, even in their native country, have probably in the course of successive conquests and migrations of tribes become more or less mixed, and since they have been subject to the conditions of domestic life, differing much from the conditions of their original wild life, and since the english breed has undergone the perturbing effects of change from the climate and food of the east to the climate and food of the west, the organizations of the horse and mare in question could have had nothing like that perfect balance produced in the quagga by a hundred centuries of harmonious co-operation. hence the result. and hence at the same time the interpretation of the fact that analogous phenomena are not obvious among most domestic animals, or among ourselves; since both have relatively mixed, and generally extremely mixed, constitutions, which, as we see in ourselves, have been made generation after generation, not by the formation of a mean between two parents, but by the jumbling of traits of the one with traits of the other; until there exist no such conspiring tendencies among the parts as cause repetition of combined details of structure in posterity. expectation that scepticism might be felt respecting this alleged anomaly presented by the quagga-marked foal, had led me to think over the matter; and i had reached this interpretation before sending to the college of surgeons museum (being unable to go myself) to obtain the particulars and refer to the records. when there was brought to me a copy of the account as set forth in the _philosophical transactions_, it was joined with the information that there existed an appended account of pigs, in which a parallel fact had been observed. to my immediate inquiry--"was the male a wild pig?" there came the reply--"i did not observe." of course i forthwith obtained the volume, and there found what i expected. it was contained in a paper communicated by dr. wollaston from daniel giles, esq., concerning his "sow and her produce," which said that-- "she was one of a well-known black and white breed of mr. western, the member for essex. about ten years since i put her to a boar of the wild breed, and of a deep chestnut colour which i had just received from hatfield house, and which was soon afterwards drowned by accident. the pigs produced (which were her first litter) partook in appearance of both boar and sow, but in some the chestnut colour of the boar strongly prevailed. "the sow was afterwards put to a boar of mr. western's breed (the wild boar having been long dead). the produce was a litter of pigs, some of which, we observed with much surprise, to be stained and clearly marked with the chestnut colour which had prevailed in the former litter." mr. giles adds that in a second litter of pigs, the father of which was of mr. western's breed, he and his bailiff believe there was a recurrence, in some, of the chestnut colour, but admits that their "recollection is much less perfect than i wish it to be." he also adds that, in the course of many years' experience, he had never known the least appearance of the chestnut colour in mr. western's breed. what are the probabilities that these two anomalous results should have arisen, under these exceptional conditions, as a matter of chance? evidently the probabilities against such a coincidence are enormous. the testimony is in both cases so good that, even apart from the coincidence, it would be unreasonable to reject it; but the coincidence makes acceptance of it imperative. there is mutual verification, at the same time that there is a joint interpretation yielded of the strange phenomenon, and of its non-occurrence under ordinary circumstances. and now, in presence of these facts, what are we to say? simply that they are fatal to weismann's hypothesis. they show that there is none of the alleged independence of the reproductive cells; but that the two sets of cells are in close communion. they prove that while the reproductive cells multiply and arrange themselves during the evolution of the embryo, some of their germ-plasm passes into the mass of somatic cells constituting the parental body, and becomes a permanent component of it. further, they necessitate the inference that this introduced germ-plasm, everywhere diffused, is some of it included in the reproductive cells subsequently formed. and if we thus get a demonstration that the somewhat different units of a foreign germ-plasm permeating the organism, permeate also the subsequently formed reproductive cells, and affect the structures of the individuals arising from them, the implication is that the like happens with those native units which have been made somewhat different by modified functions: there must be a tendency to inheritance of acquired characters. one more step only has to be taken. it remains to ask what is the flaw in the assumption with which weismann's theory sets out. if, as we see, the conclusions drawn from it do not correspond to the facts, then, either the reasoning is invalid, or the original postulate is untrue. leaving aside all questions concerning the reasoning, it will suffice here to show the untruth of the postulate. had his work been written during the early years of the cell-doctrine, the supposition that the multiplying cells of which the _metazoa_ and _metaphyta_ are composed, become completely separate, could not have been met by a reasonable scepticism; but now, not only is scepticism justifiable, but denial is called for. some dozen years ago it was discovered that in many cases vegetal cells are connected with one another by threads of protoplasm--threads which unite the internal protoplasm of one cell with the internal protoplasms of cells around it is as though the pseudopodia of imprisoned rhizopods were fused with the pseudopodia of adjacent imprisoned rhizopods. we cannot reasonably suppose that the continuous network of protoplasm thus constituted has been produced after the cells have become adult. these protoplasmic connections must have survived the process of fission. the implication is that the cells forming the embryo-plant retained their protoplasmic connections while they multiplied, and that such connections continued throughout all subsequent multiplications--an implication which has, i believe, been established by researches upon germinating palm-seeds. but now we come to a verifying series of facts which the cell-structures of animals in their early stages present. in his _monograph of the development of peripatus capensis_, mr. adam sedgwick, f.r.s., reader in animal morphology at cambridge, writes as follows:-- "all the cells of the ovum, ectodermal as well as endodermal, are connected together by a fine protoplasmic reticulum." (p. ) "the continuity of the various cells of the segmenting ovum is primary, and not secondary; _i. e._, in the cleavage the segments do not completely separate from one another. but are we justified in speaking of cells at all in this case? _the fully segmented ovum is a syncytium, and there are not and have not been at any stage cell limits._" (p. ) "it is becoming more and more clear every day that the cells composing the tissues of animals are not isolated units, but that they are connected with one another. i need only refer to the connection known to exist between connective tissue cells, cartilage cells, epithelial cells, &c. and not only may the cells of one tissue be continuous with each other, but they may also be continuous with the cells of other tissues." (pp. - ) "finally, if the protoplasm of the body is primitively a syncytium, and the ovum until maturity a part of that syncytium, the separation of the generative products does not differ essentially from the internal gemmation of a protozoon, and the inheritance by the offspring of peculiarities first appearing in the parent, though not explained, is rendered less mysterious; for the protoplasm of the whole body being continuous, change in the molecular constitution of any part of it would naturally be expected to spread, in time, through the whole mass." (p. ) mr. sedgwick's subsequent investigations confirm these conclusions. in a letter of december , , passages which he allows me to publish run as follows:-- "all the embryological studies that i have made since that to which you refer confirm me more and more in the view that the connections between the cells of adults are not secondary connections, but primary, dating from the time when the embryo was a unicellular structure.... my own investigations on this subject have been confined to the arthropoda, elasmobranchii, and aves. i have thoroughly examined the development of at least one kind of each of these groups, and i have never been able to detect a stage in which the cells were not continuous with each other; and i have studied innumerable stages from the beginning of cleavage onwards." so that the alleged independence of the reproductive cells does not exist. the _soma_--to use weismann's name for the aggregate of cells forming the body--is, in the words of mr. sedgwick, "a continuous mass of vacuolated protoplasm;" and the reproductive cells are nothing more than portions of it separated some little time before they are required to perform their functions. thus the theory of weismann is doubly disproved. inductively we are shown that there _does_ take place that communication of characters from the somatic cells to the reproductive cells, which he says cannot take place; and deductively we are shown that this communication is a natural sequence of connections between the two which he ignores; his various conclusions are deduced from a postulate which is untrue. * * * * * from the title of this essay, and from much of its contents, nine readers out of ten will infer that it is directed against the views of mr. darwin. they will be astonished on being told that, contrariwise, it is directed against the views of those who, in a considerable measure, dissent from mr. darwin. for the inheritance of acquired characters, which it is now the fashion in the biological world to deny, was, by mr. darwin, fully recognized and often insisted on. such of the foregoing arguments as touch mr. darwin's views, simply imply that the cause of evolution which at first he thought unimportant, but the importance of which he increasingly perceived as he grew older, is more important than he admitted, even at the last. the neo-darwinists, however, do not admit this cause at all. let it not be supposed that this explanation implies any disapproval of the dissentients, considered as such. seeing how little regard for authority i have myself usually shown, it would be absurd in me to reflect in any degree upon those who have rejected certain of mr. darwin's teachings, for reasons which they have held sufficient. but while their independence of thought is to be applauded rather than blamed, it is, i think, to be regretted that they have not guarded themselves against a long-standing bias. it is a common trait of human nature to seek some excuse when found in the wrong. invaded self-esteem sets up a defence, and anything is made to serve. thus it happened that when geologists and biologists, previously holding that all kinds of organisms arose by special creations, surrendered to the battery opened upon them by _the origin of species_, they sought to minimise their irrationality by pointing to irrationality on the other side. "well, at any rate, lamarck was in the wrong." "it is clear that we were right in rejecting _his_ doctrine." and so, by duly emphasizing the fact that he overlooked "natural selection" as the chief cause, and by showing how erroneous were some of his interpretations, they succeeded in mitigating the sense of their own error. it is true their creed was that at successive periods in the earth's history, old floras and faunas had been abolished and others introduced; just as though, to use professor huxley's figure, the table had been now and again kicked over and a new pack of cards brought out. and it is true that lamarck, while he rejected this absurd creed, assigned for the facts reasons some of which are absurd. but in consequence of the feeling described, his defensible belief was forgotten and only his indefensible ones remembered. this one-sided estimate has become traditional; so that there is now often shown a subdued contempt for those who suppose that there can be any truth in the reasonings of a man whose general conception was partly sense, at a time when the general conceptions of his contemporaries were wholly nonsense. hence results unfair treatment--hence result the different dealings with the views of lamarck and of weismann. "where are the facts proving the inheritance of acquired characters?" ask those who deny it. well, in the first place, there might be asked the counter-question--where are the facts which disprove it? surely if not only the general structures of organisms, but also many of the modifications arising in them, are inheritable, the natural implication is that all modifications are inheritable; and if any say that the inheritableness is limited to those arising in a certain way, the _onus_ lies on them of proving that those otherwise arising are not inheritable.[ ] leaving this counter-question aside, however, it will suffice if we ask another counter-question. it is asserted that the dwindling of organs from disuse is due to the successive survivals in posterity of individuals in which the organs have varied in the direction of decrease. where now are the facts supporting this assertion? not one has been assigned or can be assigned. not a single case can be named in which _panmixia_ is a proved cause of diminution. even had the deductive argument for _panmixia_ been as valid as we have found it to be invalid, there would still have been required, in pursuance of scientific method, some verifying inductive evidence. yet, though not a shred of such evidence has been given, the doctrine is accepted with acclamation, and adopted as part of current biological theory. articles are written and letters published in which it is assumed that this mere speculation, justified by not a tittle of proof, displaces large conclusions previously drawn. and then, passing into the outer world, this unsupported belief affects opinions there too; so that we have recently had a right honourable lecturer who, taking for granted its truth, represents the inheritance of acquired characters as an exploded hypothesis, and proceeds to give revised views of human affairs. finally, there comes the reply that there _are_ facts proving the inheritance of acquired characters. all those assigned by mr. darwin, together with others such, remain outstanding when we find that the interpretation by _panmixia_ is untenable. indeed, even had that hypothesis been tenable, it would have been inapplicable to these cases; since in domestic animals, artificially fed and often overfed, the supposed advantage from economy cannot be shown to tell; and since, in these cases, individuals are not naturally selected during the struggle for life, in which certain traits are advantageous, but are artificially selected by man without regard to such traits. should it be urged that the assigned facts are not numerous, it may be replied that there are no persons whose occupations and amusements incidentally bring out such facts; and that they are probably as numerous as those which would have been available for mr. darwin's hypothesis, had there been no breeders and fanciers and gardeners who, in pursuit of their profits and hobbies, furnished him with evidence. it may be added that the required facts are not likely to be numerous, if biologists refuse to seek for them. see, then, how the case stands. natural selection, or survival of the fittest, is almost exclusively operative throughout the vegetal world and throughout the lower animal world, characterized by relative passivity. but with the ascent to higher types of animals, its effects are in increasing degrees involved with those produced by inheritance of acquired characters; until, in animals of complex structures, inheritance of acquired characters becomes an important, if not the chief, cause of evolution. we have seen that natural selection cannot work any changes in organisms save such as conduce in considerable degrees, directly or indirectly, to the multiplication of the stirp; whence failure to account for various changes ascribed to it. and we have seen that it yields no explanation of the co-adaptation of co-operative parts, even when the co-operation is relatively simple, and still less when it is complex. on the other hand, we see that if, along with the transmission of generic and specific structures, there tend to be transmitted modifications arising in a certain way, there is a strong _a priori_ probability that there tend to be transmitted modifications arising in all ways. we have a number of facts confirming this inference, and showing that acquired characters are inherited--as large a number as can be expected, considering the difficulty of observing them and the absence of search. and then to these facts may be added the facts with which this essay set out, concerning the distribution of tactual discriminativeness. while we saw that these are inexplicable by survival of the fittest, we saw that they are clearly explicable as resulting from the inheritance of acquired characters. and here let it be added that this conclusion is conspicuously warranted by one of the methods of inductive logic, known as the method of concomitant variations. for throughout the whole series of gradations in perceptive power, we saw that the amount of the effect is proportionate to the amount of the alleged cause. ii. apart from those more special theories of professor weismann i lately dealt with, the wide acceptance of which by the biological world greatly surprises me, there are certain more general theories of his--fundamental theories--the acceptance of which surprises me still more. of the two on which rests the vast superstructure of his speculations, the first concerns the distinction between the reproductive elements of each organism and the non-reproductive elements. he says:-- "let us now consider how it happened that the multicellular animals and plants, which arose from unicellular forms of life, came to lose this power of living for ever. "the answer to this question is closely bound up with the principle of division of labour which appeared among multicellular organisms at a very early stage.... "the first multicellular organism was probably a cluster of similar cells, but these units soon lost their original homogeneity. as the result of mere relative position, some of the cells were especially fitted to provide for the nutrition of the colony, while others undertook the work of reproduction." (_essays upon heredity_, i, p. ) here, then, we have the great principle of the division of labour, which is the principle of all organization, taken as primarily illustrated in the division between the reproductive cells and the non-reproductive or somatic cells--the cells devoted to the continuance of the species, and the cells which subserve the life of the individual. and the early separation of reproductive cells from somatic cells, is alleged on the ground that this primary division of labour is that which arises between elements devoted to species-life and elements devoted to individual life. let us not be content with words but look at the facts. when milne-edwards first used the phrase "physiological division of labour," he was obviously led to do so by perceiving the analogy between the division of labour in a society, as described by political economists, and the division of labour in an organism. every one who reads has been familiarized with the first as illustrated in the early stages, when men were warriors while the cultivation and drudgery were done by slaves and women; and as illustrated in the later stages, when not only are agriculture and manufactures carried on by separate classes, but agriculture is carried on by landlords, farmers, and labourers, while manufactures, multitudinous in their kinds, severally involve the actions of capitalists, overseers, workers, &c., and while the great function of distribution is carried on by wholesale and retail dealers in different commodities. meanwhile students of biology, led by milne-edwards's phrase, have come to recognize a parallel arrangement in a living creature; shown, primarily, in the devoting of the outer parts to the general business of obtaining food and escaping from enemies, while the inner parts are devoted to the utilization of food, and supporting themselves and the outer parts; and shown, secondarily, by the subdivision of these great functions into those of various limbs and senses in the one case, and in the other case into those of organs for digestion, respiration, circulation, excretion, &c. but now let us ask what is the essential nature of this division of labour. in both cases it is an _exchange of services_--an arrangement under which, while one part devotes itself to one kind of action and yields benefits to all the rest, all the rest, jointly and severally performing their special actions, yield benefits to it in exchange. otherwise described, it is a system of _mutual_ dependence: a depends for its welfare upon b, c, and d; b upon a, c, and d; and so with the rest: all depend upon each and each upon all. now let us apply this true conception of the division of labour, to that which professor weismann calls a division of labour. where is the _exchange of services_ between somatic cells and reproductive cells? there is none. the somatic cells render great services to the reproductive cells, by furnishing them with materials for growth and multiplication; but the reproductive cells render no services at all to the somatic cells. if we look for the _mutual_ dependence we look in vain. we find entire dependence on the one side and none on the other. between the parts devoted to individual life and the part devoted to species-life, there is no division of labour whatever. the individual works for the species; but the species works not for the individual. whether at the stage when the species is represented by reproductive cells, or at the stage when it is represented by eggs, or at the stage when it is represented by young, the parent does everything for it, and it does nothing for the parent. the essential part of the conception is gone: there is no giving and receiving, no exchange, no mutuality. but now suppose we pass over this fallacious interpretation, and grant professor weismann his fundamental assumption and his fundamental corollary. suppose we grant that because the primary division of labour is that between somatic cells and reproductive cells, these two groups are the first to be differentiated. having granted this corollary, let us compare it with the facts. as the alleged primary division of labour is universal, so the alleged primary differentiation should be universal too. let us see whether it is so. already, in the paragraph from which i have quoted above, a crack in the doctrine is admitted: it is said that "this differentiation was not at first absolute, and indeed it is not always so to-day." and then, on turning to page , we find that the crack has become a chasm. of the reproductive cells it is stated that--"in vertebrata they do not become distinct from the other cells of the body until the embryo is completely formed." that is to say, in this large and most important division of the animal kingdom, the implied universal law does not hold. much more than this is confessed. lower down the page we read--"there may be in fact cases in which such separation does not take place until after the animal is completely formed, and others, as i believe that i have shown, in which it first arises one or more generations later, viz., in the buds produced by the parent." so that in other great divisions of the animal kingdom the alleged law is broken; as among the _coelenterata_ by the _hydrozoa_, as among the _mollusca_ by the ascidians, and as among the _platyhelminthes_ by the trematode worms. following this admission concerning the _vertebrata_, come certain sentences which i partially italicize:-- "thus, as their development shows, a marked antithesis exists between the substance of the undying reproductive cells and that of the perishable body-cells. we cannot explain this fact except _by the supposition_ that each reproductive cell potentially contains two kinds of substance, which at a variable time after the commencement of embryonic development, separate from one another, and finally produce two sharply contrasted groups of cells." (p. ) and a little lower down the page we meet with the lines:-- "_it is therefore quite conceivable_ that the reproductive cells might separate from the somatic cells much later than in the examples mentioned above, without changing the hereditary tendencies of which they are the bearers." that is to say, it is "quite conceivable" that after sexless _cercariæ_ have gone on multiplying by internal gemmation for generations, the "two kinds of substance" have, notwithstanding innumerable cell-divisions, preserved their respective natures, and finally separate in such ways as to produce reproductive cells. here professor weismann does not, as in a case before noted, assume something which it is "easy to imagine," but he assumes something which it is difficult to imagine; and apparently thinks that a scientific conclusion may be thereon safely based. * * * * * associated with the assertion that the primary division of labour is between the somatic cells and the reproductive cells, and associated with the corollary that the primary differentiation is that which arises between them, there goes another corollary. it is alleged that there exists a fundamental distinction of nature between these two classes of cells. they are described as respectively mortal and immortal, in the sense that those of the one class are limited in their powers of multiplication, while those of the other class are unlimited. and it is contended that this is due to inherent unlikeness of nature. before inquiring into the truth of this proposition, i may fitly remark upon a preliminary proposition set down by professor weismann. referring to the hypothesis that death depends "upon causes which lie in the nature of life itself," he says:-- "i do not however believe in the validity of this explanation: i consider that death is not a primary necessity, but that it has been secondarily acquired as an adaptation. i believe that life is endowed with a fixed duration, not because it is contrary to its nature to be unlimited, but because the unlimited existence of individuals would be a luxury without any corresponding advantage." (p. ) this last sentence has a teleological sound which would be appropriate did it come from a theologian, but which seems strange as coming from a man of science. assuming, however, that the implication was not intended, i go on to remark that professor weismann has apparently overlooked a universal law of evolution--not organic only, but inorganic and super-organic--which implies the necessity of death. the changes of every aggregate, no matter of what kind, inevitably end in a state of equilibrium. suns and planets die, as well as organisms. the process of integration, which constitutes the fundamental trait of all evolution, continues until it has brought about a state which negatives further alterations, molar or molecular--a state of balance among the forces of the aggregate and the forces which oppose them.[ ] in so far, therefore, as professor weismann's conclusions imply the non-necessity of death, they cannot be sustained. but now let us consider the above-described antithesis between the immortal _protozoa_ and the mortal _metazoa_. an essential part of the theory is that the _protozoa_ can go on dividing and subdividing without limit, so long as the fit external conditions are maintained. but what is the evidence for this? even by professor weismann's own admission there is no proof. on p. he says:-- "i could only consent to adopt the hypothesis of rejuvenescence [achieved by conjugation], if it were rendered absolutely certain that reproduction by division could never under any circumstances persist indefinitely. but this cannot be proved with any greater certainty than the converse proposition, and hence, as far as direct proof is concerned, the facts are equally uncertain on both sides." but this is an admission which seems to be entirely ignored when there is alleged the contrast between the immortal _protozoa_ and the mortal _metazoa_. following professor weismann's method, it would be "easy to imagine" that occasional conjugation is in all cases essential; and this easily imagined conclusion might fitly be used to bar out his own. indeed, considering how commonly conjugation is observed, it may be held difficult to imagine that it can in any cases be dispensed with. apart from imaginations of either kind, however, here is an acknowledgment that the immortality of _protozoa_ is not proved; that the allegation has no better basis than the failure to observe cessation of fission; and that thus one term of the above antithesis is not a fact, but is only an assumption. and now what about the other term of the antithesis--the alleged inherent mortality of the somatic cells? this we shall, i think, find is no more defensible than the other. such plausibility as it possesses disappears when, instead of contemplating the vast assemblage of familiar cases which animals present, we contemplate certain less familiar and unfamiliar cases. by these we are shown that the usual ending of multiplication among somatic cells is due, not to an intrinsic cause, but to extrinsic causes. let us, however, first look at professor weismann's own statements:-- "i have endeavoured to explain death as the result of restriction in the powers of reproduction possessed by the somatic cells, and i have suggested that such restriction may conceivably follow from a limitation in the number of cell-generations possible for the cells of each organ and tissue." (p. ) "the above-mentioned considerations show us that the degree of reproductive activity present in the tissues is regulated by internal causes while the natural death of an organism is the termination--the hereditary limitation--of the process of cell-division, which began in the segmentation of the ovum." (p. ) now, though, in the above extracts there is mention of "internal causes" determining "the degree of reproductive activity" of tissue cells, and though, on page , the "causes of the loss" of the power of unlimited cell-production "must be sought outside the organism, that is to say, in the external conditions of life," yet the doctrine is that somatic cells have become constitutionally unfitted for continued cell-multiplication. "the somatic cells have lost this power to a gradually increasing extent, so that at length they became restricted to a fixed, though perhaps very large, number of cell-generations." (p. ) examination will soon disclose good reasons for denying this inherent restriction. we will look at the various causes which affect their multiplication, and usually put a stop to increase after a certain point is reached. there is first the amount of vital capital given by the parent; partly in the shape of a more or less developed structure, and partly in the shape of bequeathed nutriment. where this vital capital is small, and the young creature, forthwith obliged to carry on physiological business for itself, has to expend effort in obtaining materials for daily consumption as well as for growth, a rigid restraint is put on that cell-multiplication required for a large size. clearly, the young elephant, starting with a big and well-organized body, and supplied _gratis_ with milk during early stages of growth, can begin physiological business on his own account on a great scale; and by its large transactions his system is enabled to supply nutriment to its multiplying somatic cells until they have formed a vast aggregate--an aggregate such as it is impossible for a young mouse to reach, obliged as it is to begin physiological business in a small way. then there is the character of the food in respect of its digestibility and its nutritiveness. here, that which the creature takes in requires much grinding-up, or, when duly prepared, contains but a small amount of available matter in comparison with the matter that has to be thrown away; while there, the prey seized is almost pure nutriment, and requires but little trituration. hence, in some cases, an unprofitable physiological business, and in other cases a profitable one; resulting in small or large supplies to the multiplying somatic cells. further, there has to be noted the grade of visceral development, which, if low, yields only crude nutriment slowly distributed, but which, if high, serves by its good appliances for solution, depuration, absorption, and circulation, to yield to the multiplying somatic cells a rich and pure blood. then we come to an all-important factor, the cost of obtaining food. here large expenditure of energy in locomotion is necessitated, and there but little--here great efforts for small portions of food, and there small efforts for great portions: again resulting in physiological poverty or physiological wealth. next, beyond the cost of nervo-muscular activities in foraging, there is the cost of maintaining bodily heat. so much heat implies so much consumed nutriment, and the loss by radiation or conduction, which has perpetually to be made good, varies according to many circumstances--climate, medium (as air or water), covering, size of body (small cooling relatively faster than large); and in proportion to the cost of maintaining heat is the abstraction from the supplies for cell-formation. finally, there are three all-important co-operative factors, or rather laws of factors, the effects of which vary with the size of the animal. the first is that, while the mass of the body varies as the cubes of its dimensions (_proportions_ being supposed constant), the absorbing surface varies as the squares of its dimensions; whence it results that, other things equal, increase of size implies relative decrease of nutrition, and therefore increased obstacles to cell-multiplication.[ ] the second is a further sequence from these laws--namely, that while the weight of the body increases as the cubes of the dimensions, the sectional areas of its muscles and bones increase as their squares; whence follows a decreasing power of resisting strains, and a relative weakness of structure. this is implied in the ability of a small animal to leap many times its own length, while a great animal, like the elephant, cannot leap at all: its bones and muscles being unable to bear the stress which would be required to propel its body through the air. what increasing cost of keeping together the bodily fabric is thus entailed, we cannot say; but that there is an increasing cost, which diminishes the available, materials for increase of size, is beyond question.[ ] and then, in the third place, we have augmented expense of distribution of nutriment. the greater the size becomes, the more force must be exerted to send blood to the periphery; and this once more entails deduction from the cell-forming matters. here, then, we have nine factors, several of them involving subdivisions, which co-operate in aiding or restraining cell-multiplication. they occur in endlessly varied proportions and combinations; so that every species differs more or less from every other in respect of their effects. but in all of them the co-operation is such as eventually arrests that multiplication of cells which causes further growth; continues thereafter to entail slow decrease in cell-multiplication, accompanying decline of vital activities; and eventually brings cell-multiplication to an end. now a recognized principle of reasoning--the law of parsimony--forbids the assumption of more causes than are needful for explanation of phenomena; and since, in all such living aggregates as those above supposed, the causes named inevitably bring about arrest of cell-multiplication, it is illegitimate to ascribe this arrest to some inherent property in the cells. inadequacy of the other causes must be shown before an inherent property can be rightly assumed. for this conclusion we find ample justification when we contemplate types of animals which lead lives that do not put such decided restraints on cell-multiplication. first let us take an instance of the extent to which (irrespective of natures of cells as reproductive or somatic) cell-multiplication may go, where the conditions render nutrition easy and reduce expenditure to a minimum. i refer to the case of the _aphides_. though it is early in the season (march), the hothouses at kew have furnished a sufficient number of these to show that twelve of them weigh a grain--a larger number than would be required were they full-sized. citing professor owen, who adopts the calculations of tougard to the effect that by agamic multiplication "a single impregnated ovum of _aphis_ may give rise, without fecundation, to a quintillion of _aphides_," professor huxley says:-- "i will assume that an aphis weighs / of a grain, which is certainly vastly under the mark. a quintillion of _aphides_ will, on this estimate, weigh a quatrillion of grains. he is a very stout man who weighs two million grains; consequently the tenth brood alone, if all its members survive the perils to which they are exposed, contains more substance than , , stout men--to say the least, more than the whole population of china!"[ ] and had professor huxley taken the actual weight, one-twelfth of a grain, the quintillion of _aphides_ would evidently far outweigh the whole human population of the globe: five billions of tons being the weight, as brought out by my own calculation! of course i do not cite this in proof of the extent to which multiplication of somatic cells, descending from a single ovum, may go; because it will be contended, with some reason, that each of the sexless _aphides_, viviparously produced, arose by fission of a cell which had descended from the original reproductive cell. i cite it merely to show that when the cell-products of a fertilized ovum are perpetually divided and subdivided into small groups, distributed over an unlimited nutritive area, so that they can get materials for growth at no cost, and expend nothing appreciable in motion or maintenance of temperature, cell-production may go on without limit. for the agamic multiplication of _aphides_ has been shown to continue for four years, and to all appearance would be ceaseless were the temperature and supply of food continued without break. but now let us pass to analogous illustrations of cause and consequence, open to no criticism of the kind just indicated. they are furnished by various kinds of _entozoa_, of which take the _trematoda_, infesting molluscs and fishes. of one of them we read:--"_gyrodactylus_ multiplies agamically by the development of a young trematode within the body, as a sort of internal bud. a second generation appears within the first, and even a third within the second, before the young _gyrodactylus_ is born."[ ] and the drawings of steenstrup, in his _alternation of generations_, show us, among creatures of this group, a sexless individual the whole interior of which is transformed into smaller sexless individuals, which severally, before or after their emergence, undergo similar transformations--a multiplication of somatic cells without any sign of reproductive cells. under what circumstances do such modes of agamic multiplication, variously modified among parasites, occur? they occur where there is no expenditure whatever in motion or maintenance of temperature, and where nutriment surrounds the body on all sides. other instances are furnished by groups in which, though the nutriment is not abundant, the cost of living is almost unappreciable. among the _coelenterata_ there are the hydroid polyps, simple and compound; and among the _mollusca_ we have various types of ascidians, fixed and floating, _botryllidæ_ and _salpæ_. but now from these low animals in which sexless reproduction, and continued multiplication of somatic cells, is common, and one class of which is named "zoophytes," because its form of life simulates that of plants, let us pass to plants themselves. in these there is no expenditure in effort, there is no expenditure in maintaining temperature, and the food, some of it supplied by the earth, is the rest of it supplied by a medium which everywhere bathes the outer surface: the utilization of its contained material being effected _gratis_ by the sun's rays. just as was to be expected, we here find that agamogenesis may go on without end. numerous plants and trees are propagated to an unlimited extent by cuttings and buds; and we have sundry plants which cannot be otherwise propagated. the most familiar are the double roses of our gardens: these do not seed, and yet have been distributed everywhere by grafts and buds. hothouses furnish many cases, as i learn from an authority second to none. of "the whole host of tropical orchids, for instance, not one per cent. has ever seeded, and some have been a century under cultivation." again, we have the _acorus calamus_, "that has hardly been known to seed anywhere, though it is found wild all over the north temperate hemisphere." and then there is the conspicuous and conclusive case of _eloidea canadensis_ (alias _anacharis_,) introduced no one knows how (probably with timber), and first observed in , in several places; and which, having since spread over nearly all england, now everywhere infests ponds, canals, and slow rivers. the plant is dioecious, and only the female exists here. beyond all question, therefore, this vast progeny of the first slip or fragment introduced, sufficient to cover many square miles were it put together, is constituted entirely of somatic cells. hence, as far as we can judge, these somatic cells are immortal in the sense given to the word by professor weismann; and the evidence that they are so is immeasurably stronger than the evidence which leads him to assert immortality for the fissiparously-multiplying _protozoa_. this endless multiplication of somatic cells has been going on under the eyes of numerous observers for forty odd years. what observer has watched for forty years to see whether the fissiparous multiplication of _protozoa_ does not cease? what observer has watched for one year, or one month, or one week?[ ] even were not professor weismann's theory disposed of by this evidence, it might be disposed of by a critical examination of his own evidence, using his own tests. clearly, if we are to measure relative mortalities, we must assume the conditions to be the same and must use the same measure. let us do this with some appropriate animal--say man, as the most open to observation. the mortality of the somatic cells constituting the mass of the human body, is, according to professor weismann, shown by the decline and final cessation of cell-multiplication in its various organs. suppose we apply this test to all the organs: not to those only in which there continually arise bile-cells, epithelium-cells, &c., but to those also in which there arise reproductive cells. what do we find? that the multiplication of these last comes to an end long before the multiplication of the first. in a healthy woman, the cells which constitute the various active tissues of the body, continue to grow and multiply for many years after germ-cells have died out. if similarly measured, then, these cells of the last class prove to be more mortal than those of the first. but professor weismann uses a different measure for the two classes of cells. passing over the illegitimacy of this proceeding, let us accept his other mode of measurement, and see what comes of it. as described by him, absence of death among the _protozoa_ is implied by that unceasing division and subdivision of which they are said to be capable. fission continued without end, is the definition of the immortality he speaks of. apply this conception to the reproductive cells in a _metazoon_. that the immense majority of them do not multiply without end, we have already seen: with very rare exceptions they die and disappear without result, and they cease their multiplication while the body as a whole still lives. but what of those extremely exceptional ones which, as being actually instrumental to the maintenance of the species, are alone contemplated by professor weismann? do these continue their fissiparous multiplications without end? by no means. the condition under which alone they preserve a qualified form of existence, is that, instead of one becoming two, two become one. a member of series a and a member of series b, coalesce; and so lose their individualities. now, obviously, if the immortality of a series is shown if its members divide and subdivide perpetually, then the opposite of immortality is shown when, instead of division, there is union. each series ends, and there is initiated a new series, differing more or less from both. thus the assertion that the reproductive cells are immortal, can be defended only by changing the conception of immortality otherwise implied. even apart from these last criticisms, however, we have clear disproof of the alleged inherent difference between the two classes of cells. among animals, the multiplication of somatic cells is brought to an end by sundry restraining conditions; but in various plants, where these restraining conditions are absent, the multiplication is unlimited. it may, indeed, be said that the alleged distinction should be reversed; since the fissiparous multiplication of reproductive cells is necessarily interrupted from time to time by coalescence, while that of the somatic cells may go on for a century without being interrupted. * * * * * in the essay to which this is a postscript, conclusions were drawn from the remarkable case of the horse and the quagga, there narrated, along with an analogous case observed among pigs. these conclusions have since been confirmed. i am much indebted to a distinguished correspondent who has drawn my attention to verifying facts furnished by the offspring of whites and negroes in the united states. referring to information given him many years ago, he says:--"it was to the effect that the children of white women by a white father, had been _repeatedly_ observed to show traces of black blood, in cases when the woman had previous connection with [_i. e._ a child by] a negro." at the time i received this information, an american was visiting me; and, on being appealed to, answered that in the united states there was an established belief to this effect. not wishing, however, to depend upon hearsay, i at once wrote to america to make inquiries. professor cope of philadelphia has written to friends in the south, but has not yet sent me the results. professor marsh, the distinguished palæontologist, of yale, new haven, who is also collecting evidence, sends a preliminary letter in which he says:--"i do not myself know of such a case, but have heard many statements that make their existence probable. one instance, in connecticut, is vouched for so strongly by an acquaintance of mine, that i have good reason to believe it to be authentic." that cases of the kind should not be frequently seen in the north, especially nowadays, is of course to be expected. the first of the above quotations refers to facts observed in the south during slavery days; and even then, the implied conditions were naturally very infrequent. dr. w. j. youmans of new york has, on my behalf, interviewed several medical professors, who, though they have not themselves met with instances, say that the alleged result, described above, "is generally accepted as a fact." but he gives me what i think must be regarded as authoritative testimony. it is a quotation from the standard work of professor austin flint, and runs as follows:-- "a peculiar and, it seems to me, an inexplicable fact is, that previous pregnancies have an influence upon offspring. this is well known to breeders of animals. if pure-blooded mares or bitches have been once covered by an inferior male, in subsequent fecundations the young are likely to partake of the character of the first male, even if they be afterwards bred with males of unimpeachable pedigree. what the mechanism of the influence of the first conception is, it is impossible to say; but the fact is incontestable. the same influence is observed in the human subject. a woman may have, by a second husband, children who resemble a former husband, and this is particularly well marked in certain instances by the colour of the hair and eyes. a white woman who has had children by a negro may subsequently bear children to a white man, these children presenting some of the unmistakable peculiarities of the negro race."[ ] dr. youmans called on professor flint, who remembered "investigating the subject at the time his larger work was written [the above is from an abridgment], and said that he had never heard the statement questioned." some days before i received this letter and its contained quotation, the remembrance of a remark i heard many years ago concerning dogs, led to the inquiry whether they furnished analogous evidence. it occurred to me that a friend who is frequently appointed judge of animals at agricultural shows, mr. fookes, of fairfield, pewsey, wiltshire, might know something about the matter. a letter to him brought various confirmatory statements. from one "who had bred dogs for many years" he learnt that-- "it is a well known and admitted fact that if a bitch has two litters by two different dogs, the character of the first father is sure to be perpetuated in any litters she may afterwards have, no matter how pure-bred a dog may be the begetter." after citing this testimony, mr. fookes goes on to give illustrations known to himself. "a friend of mine near this had a very valuable dachshund bitch, which most unfortunately had a litter by a stray sheep-dog. the next year her owner sent her on a visit to a pure dachshund dog, but the produce took quite as much of the first father as the second, and the next year he sent her to another dachshund with the same result. another case:--a friend of mine in devizes had a litter of puppies, unsought for, by a setter from a favourite pointer bitch, and after this she never bred any true pointers, no matter of what the paternity was." [since the publication of this article additional evidences have come to hand. one is from the late prof. riley, state entomologist at washington, who says that telegony is an "established principle among well-educated farmers" in the united states, and who gives me a case in horse-breeding to which he was himself witness. mr. w. p. smith, writing from stoughton grange, guildford, but giving the results of his experiences in america, says that "the fact of a previous conception influencing subsequent offspring was so far recognised among american cattle-breeders" that it was proposed to raise the rank of any heifer that had borne a first calf by a thoroughbred bull, and though this resolution when brought before one of the chief societies was not carried, yet on all sides it was admitted that previous conceptions had effects of the kind alleged. mr. smith in another letter says:--"when i had a large mule and horse ranche in america i noticed that the foals of mares by horse stallions had a mulish appearance in those cases where the mare had previously given birth to a mule foal. common heifers who have had calves by a thoroughbred bull are apt thereafter to have well-bred calves even from the veriest scrubs." yet another very interesting piece of evidence is furnished by mr. w. sedgwick, m.r.c.s., in an article on "the influence of heredity in disease," published in the _british medical journal_ for feb. , , pp. - . it concerns the transmission of a malformation known among medical men as hypospadias. referring to a man belonging to a family in which this defect prevailed, he writes:--"the widow of the man from whom these three generations of hypospadians were descended married again, after an interval of eighteen months; and in this instance the second husband was not only free from the defect, but there was no history of it in his family. by this second marriage she had four hypospadiac sons and four hypospadiac grandsons; whilst there were seven grandsons and three great-grandsons who were not malformed."] coming from remote places, from those who have no theory to support, and who are some of them astonished by the unexpected phenomena, the agreement dissipates all doubt. in four kinds of mammals, widely divergent in their natures--man, horse, dog, and pig--we have this same seemingly-anomalous kind of heredity, made visible under analogous conditions. we must take it as a demonstrated fact that, during gestation, traits of constitution inherited from the father produce effects upon the constitution of the mother; and that these communicated effects are transmitted by her to subsequent offspring. we are supplied with an absolute disproof of professor weismann's doctrine that the reproductive cells are independent of, and uninfluenced by, the somatic cells; and there disappears absolutely the alleged obstacle to the transmission of acquired characters. * * * * * notwithstanding experiences showing the futility of controversy for the establishment of truth, i am tempted here to answer opponents at some length. but even could the editor allow me the needful space, i should be compelled, both by lack of time and by ill-health, to be brief. i must content myself with noticing a few points which most nearly concern me. referring to my argument respecting tactual discriminativeness, mr. wallace thinks that i-- "afford a glaring example of taking the unessential in place of the essential, and drawing conclusions from a partial and altogether insufficient survey of the phenomena. for this 'tactual discriminativeness,' which is alone dealt with by mr. spencer, forms the least important, and probably only an incidental portion of the great vital phenomenon of skin-sensitiveness, which is at once the watchman and the shield of the organism against imminent external dangers." (_fortnightly review_, april, , p. ) here mr. wallace assumes it to be self-evident that skin-sensitiveness is due to natural selection, and assumes that this must be admitted by me. he supposes it is only the unequal distribution of skin-discriminativeness which i contend is not thus accounted for. but i deny that either the general sensitiveness or the special sensitiveness results from natural selection; and i have years ago justified the first disbelief as i have recently the second. in "the factors of organic evolution" (_essays_, - ), i have given various reasons for inferring that the genesis of the nervous system cannot be due to survival of the fittest; but that it is due to the direct effects of converse between the surface and the environment; and that thus only is to be explained the strange fact that the nervous centres are originally superficial, and migrate inwards during development. these conclusions i have, in the essay mr. wallace criticizes, upheld by the evidence which blind boys and skilled compositors furnish; proving, as this does, that increased nervous development is peripherally initiated. mr. wallace's belief that skin-sensitiveness arose by natural selection, is unsupported by a single fact. he assumes that it _must_ have been so produced because it is all-important to self-preservation. my belief that it is directly initiated by converse with the environment, is supported by facts; and i have given proof that the assigned cause is now in operation. am i called upon to abandon my own supported belief and accept mr. wallace's unsupported belief? i think not. referring to my argument concerning blind cave-animals, professor lankester, in _nature_ of february , , writes:-- "mr. spencer shows that the saving of ponderable material in the suppression of an eye is but a small economy: he loses sight of the fact, however, that possibly, or even probably, the saving to the organism in the reduction of an eye to a rudimentary state is not to be measured by mere bulk, but by the non-expenditure of special materials and special activities which are concerned in the production of an organ so peculiar and elaborate as is the vertebrate eye." it seems to me that a supposition is here made to do duty as a fact; and that i might with equal propriety say that "possibly, or even probably," the vertebrate eye is physiologically cheap: its optical part, constituting nearly its whole bulk, consisting of a low order of tissue. there is, indeed, strong reason for considering it physiologically cheap. if any one remembers how relatively enormous are the eyes of a fish just out of the egg--a pair of eyes with a body and head attached; and if he then remembers that every egg contains material for such a pair of eyes; he will see that eye-material constitutes a very considerable part of the fish's roe; and that, since the female fish provides this quantity every year, it cannot be expensive. my argument against weismann is strengthened rather than weakened by contemplation of these facts. professor lankester asks my attention to a hypothesis of his own, published in the _encyclopædia britannica_, concerning the production of blind cave-animals. he thinks it can-- "be fully explained by natural selection acting on congenital fortuitous variations. many animals are thus born with distorted or defective eyes whose parents have not had their eyes submitted to any peculiar conditions. supposing a number of some species of arthropod or fish to be swept into a cavern or to be carried from less to greater depths in the sea, those individuals with perfect eyes would follow the glimmer of light and eventually escape to the outer air or the shallower depths, leaving behind those with imperfect eyes to breed in the dark place. a natural selection would thus be effected" in successive generations. first of all, i demur to the words "many animals." under the abnormal conditions of domestication, congenitally defective eyes may be not very uncommon; but their occurrence under natural conditions is, i fancy, extremely rare. supposing, however, that in a shoal of young fish, there occur some with eyes seriously defective. what will happen? vision is all-important to the young fish, both for obtaining food and for escaping from enemies. this is implied by the immense development of eyes just referred to; and the obvious conclusion to be drawn is that the partially blind would disappear. considering that out of the enormous number of young fish hatched with perfect eyes, not one in a hundred reaches maturity, what chance of surviving would there be for those with imperfect eyes? inevitably they would be starved or be snapped up. hence the chances that a matured or partially matured semi-blind fish, or rather two such, male and female, would be swept into a cave and left behind are extremely remote. still more remote must the chances be in the case of cray-fish. sheltering themselves as these do under stones, in crevices, and in burrows which they make in the banks, and able quickly to anchor themselves to weeds or sticks by their claws, it seems scarcely supposable that any of them could be carried into a cave by a flood. what, then, is the probability that there will be two nearly blind ones, and that these will be thus carried? then, after this first extreme improbability, there comes a second, which we may, i think, rather call an impossibility. how would it be possible for creatures subject to so violent a change of habitat to survive? surely death would quickly follow the subjection to such utterly unlike conditions and modes of life. the existence of these blind cave-animals can be accounted for only by supposing that their remote ancestors began making excursions into the cave, and, finding it profitable, extended them, generation after generation, further in: undergoing the required adaptations little by little.[ ] between dr. romanes and myself the first difference concerns the interpretation of "panmixia." clearer conceptions of these matters would be reached if, instead of thinking in abstract terms, the physiological processes concerned were brought into the foreground. beyond the production of changes in the sizes of parts by the selection of fortuitously-arising variations, i can see but one other cause for the production of them--the competition among the parts for nutriment. this has the effect that active parts are well-supplied and grow, while inactive parts are ill-supplied and dwindle.[ ] this competition is the cause of "economy of growth"; this is the cause of decrease from disuse; and this is the only conceivable cause of that decrease which dr. romanes contends follows the cessation of selection. the three things are aspects of the same thing. and now, before leaving this question, let me remark on the strange proposition which has to be defended by those who deny the dwindling of organs from disuse. their proposition amounts to this:--that for a hundred generations an inactive organ may be partially denuded of blood all through life, and yet in the hundredth generation will be produced of just the same size as in the first! there is one other passage in dr. romanes' criticism--that concerning the influence of a previous sire on progeny--which calls for comment. he sets down what he supposes weismann will say in response to my argument. "first, he may question the fact." well, after the additional evidence given above, i think he is not likely to do that; unless, indeed, it be that along with readiness to base conclusions on things "it is easy to imagine" there goes reluctance to accept testimony which it is difficult to doubt. second, he is supposed to reply that "the germ-plasm of the first sire has in some way or another become partly commingled with that of the immature ova"; and dr. romanes goes on to describe how there may be millions of spermatozoa and "thousands of millions" of their contained "ids" around the ovaries, to which these secondary effects are due. but, on the one hand, he does not explain why in such cases each subsequent ovum, as it becomes matured, is not fertilized by the sperm-cells present, or their contained germ-plasm, rendering all subsequent fecundations needless; and, on the other hand, he does not explain why, if this does not happen, the potency of this remaining germ-plasm is nevertheless such as to affect not only the next succeeding offspring, but all subsequent offspring. the irreconcilability of these two implications would, i think, sufficiently dispose of the supposition, even had we not daily multitudinous proofs that the surface of a mammalian ovarium is not a spermatheca. the third reply dr. romanes urges, is the inconceivability of the process by which the germ-plasm of a preceding male parent affects the constitution of the female and her subsequent offspring. in response, i have to ask why he piles up a mountain of difficulties based on the assumption that mr. darwin's explanation of heredity by "pangenesis" is the only available explanation preceding that of weismann? and why he presents these difficulties to me, more especially; deliberately ignoring my own hypothesis of physiological units? it cannot be that he is ignorant of this hypothesis, since the work in which it is variously set forth (_principles of biology_, §§ - ) is one with which he is well acquainted: witness his _scientific evidences of organic evolution_; and he has had recent reminders of it in weismann's _germ-plasm_, where it is repeatedly referred to. why, then, does he assume that i abandon my own hypothesis and adopt that of darwin; thereby entangling myself in difficulties which my own hypothesis avoids? if, as i have argued, the germ-plasm consists of substantially similar units (having only those minute differences expressive of individual and ancestral differences of structure), none of the complicated requirements which dr. romanes emphasizes exist; and the alleged inconceivability disappears. here i must end: not intending to say more, unless for some very urgent reason; and leaving others to carry on the discussion. i have, indeed, been led to suspend for a short time my proper work, only by consciousness of the transcendent importance of the question at issue. as i have before contended, a right answer to the question whether acquired characters are or are not inherited, underlies right beliefs, not only in biology and psychology, but also in education, ethics, and politics. iii. as a species of literature, controversy is characterised by a terrible fertility. each proposition becomes the parent of half a dozen; so that a few replies and rejoinders produce an unmanageable population of issues, old and new, which end in being a nuisance to everybody. remembering this, i shall refrain from dealing with all the points of professor weismann's answer. i must limit myself to a part; and that there may be no suspicion of a selection convenient to myself, i will take those contained in his first article. before dealing with his special arguments, let me say something about the general mode of argument which professor weismann adopts. the title of his article is "the all-sufficiency of natural selection."[ ] very soon, however, as on p. , we come to the admission, which he has himself italicised, "that _it is really very difficult to imagine this process of natural selection in its details_; and to this day it is impossible to demonstrate it in any one point." elsewhere, as on pp. and _à propos_ of other cases, there are like admissions. but now if the sufficiency of an assigned cause cannot in any case be demonstrated, and if it is "really very difficult to imagine" in what way it has produced its alleged effects, what becomes of the "all-sufficiency" of the cause? how can its all-sufficiency be alleged when its action can neither be demonstrated nor easily imagined? evidently to fit professor weismann's argument the title of the article should have been "the doubtful sufficiency of natural selection." observe, again, how entirely opposite are the ways in which he treats his own interpretation and the antagonist interpretation. he takes the problem presented by certain beautifully adapted structures on the anterior legs of "very many insects," which they use for cleansing their antennæ. these, he argues, cannot have resulted from the inheritance of acquired characters; since any supposed changes produced by function would be changes in the chitinous exo-skeleton, which, being a dead substance, cannot have had its changes transmitted. he then proceeds, very candidly, to point out the extreme difficulties which lie in the way of supposing these structures to have resulted from natural selection: admitting that an opponent might "say that it was absurd" to assume that the successive small variations implied were severally life-saving in their effects. nevertheless, he holds it unquestionable that natural selection has been the cause. see then the difference. the supposition that the apparatus has been produced by the inheritance of acquired characters is rejected _because_ it presents insuperable difficulties. but the supposition that the apparatus has been produced by natural selection is accepted, _though_ it presents insuperable difficulties. if this mode of reasoning is allowable, no fair comparison between diverse hypotheses can be made. with these remarks on professor weismann's method at large, let me now pass to the particular arguments he uses, taking them _seriatim_. * * * * * the first case he deals with is that of the progressive degradation of the human little toe. this he considers a good test case; and he proceeds to discuss an assigned cause--the inherited and accumulated effects of boot-pressure. without much difficulty he shows that this interpretation is inadequate; since fusion of the phalanges, which constitutes in part the progressive degradation, is found among peoples who go barefoot, and has been found also in egyptian mummies. having thus disposed of mr. buckman's interpretation, professor weismann forthwith concludes that the ascription of this anatomical change to the inheritance of acquired characters is disposed of, and assumes, as the only other possible interpretation, a dwindling "through panmixia": "the hereditary degeneration of the little toe is thus quite simply explained from my standpoint." it is surprising that professor weismann should not have seen that there is an explanation against which his criticism does not tell. if we go back to the genesis of the human type from some lower type of _primates_, we see that while the little toe has ceased to be of any use for climbing purposes, it has not come into any considerable use for walking and running. a glance at the feet of the sub-human _primates_ in general, shows that the inner digits are, as compared with those of men, quite small, have no such relative length and massiveness as the human great toes. leaving out the question of cause, it is manifest that the great toes have been immensely developed, since there took place the change from arboreal habits to terrestrial habits. a study of the mechanics of walking shows why this has happened. stability requires that the "line of direction" (the vertical line let fall from the centre of gravity) shall fall within the base, and, in walking, shall be brought at each step within the area of support, or so near it that any tendency to fall may be checked at the next step. a necessary result is that if, at each step, the chief stress of support is thrown on the outer side of the foot, the body must be swayed so that the "line of direction" may fall within the outer side of the foot, or close to it; and when the next step is taken it must be similarly swayed in an opposite way, so that the outer side of the other foot may bear the weight. that is to say, the body must oscillate from side to side, or waddle. the movements of a duck when walking or running show what happens when the points of support are wide apart. clearly this kind of movement conflicts with efficient locomotion. there is a waste of muscular energy in making these lateral movements, and they are at variance with the forward movement. we may infer, then, that the developing man profited by throwing the stress as much as possible on the inner sides of the feet; and was especially led to do this when going fast, which enabled him to abridge the oscillations: as indeed we now see in a drunken man. thus there was thrown a continually increasing stress upon the inner digits as they progressively developed from the effects of use; until now that the inner digits, so large compared with the others, bear the greater part of the weight, and being relatively near one another, render needless any marked swayings from side to side. but what has meanwhile happened to the outer digits? evidently as fast as the great toes have come more and more into play and developed, the little toes have gone more and more out of play and have been dwindling for--how long shall we say?--perhaps a hundred thousand years. so far, then, am i from feeling that professor weismann has here raised a difficulty in the way of the doctrine i hold, that i feel indebted to him for having drawn attention to a very strong evidence in its support. this modification in the form of the foot, which has occurred since arboreal habits have given place to terrestrial habits, shows the effects of use and disuse simultaneously. the inner digits have increased by use while the outer digits have decreased by disuse. * * * * * saying that he will not "pause to refute other apparent proofs of the transmission of acquired characters," professor weismann proceeds to deal with the argument which, with various illustrations, i have several times urged--the argument that the natural selection of fortuitously-arising variations cannot account for the adjustment of co-operative parts. very clearly and very fairly he summarises this argument as used in _the principles of biology_ in . admitting that in this case there are "enormous difficulties" in the way of any other interpretation than the inheritance of acquired characters, professor weismann before proceeding to assault this "last bulwark of the lamarckian principle," premises that the inheritance of acquired characters cannot be a cause of change because inactive as well as active parts degenerate when they cease to be of use: instancing the "skin and skin-armature of crabs and insects." on this i may remark in the first place that an argument derived from degeneracy of passive structures scarcely meets the case of development of active structures; and i may remark in the second place that i have never dreamt of denying the efficiency of natural selection as a cause of degeneracy in passive structures when the degeneracy is such as aids the prosperity of the stirp. making this parenthetical reply to his parenthetical criticism i pass to his discussion of this particular argument which he undertakes to dispose of. his _cheval de bataille_ is furnished him by the social insects--not a fresh one, however, as might be supposed from the way in which he mounts it. from time to time it has carried other riders, who have couched their lances with fatal effects as they supposed. but i hope to show that no one of them has unhorsed an antagonist, and that professor weismann fails to do this just as completely as his predecessors. i am, indeed, not sorry that he has afforded me the opportunity of criticising the general discussion concerning the peculiarities of these interesting creatures, which it has often seemed to me sets out with illegitimate assumptions. the supposition always is that the specialities of structures and instincts in the unlike classes of their communities, have arisen during the period in which the communities have existed in something like their present forms. this cannot be. it is doubtless true that association without differentiations of classes may pre-exist for co-operative purposes, as among wolves, and as among various insects which swarm under certain circumstances. hence we may suppose that there arise in some cases permanent swarms--that survival of the fittest will establish these constant swarms where they are advantageous. but admitting this, we have also to admit a gradual rise of the associated state out of the solitary state. wasps and bees present us with gradations. if, then, we are to understand how the organized societies have arisen, either out of the solitary state or out of undifferentiated swarms, we must assume that the differences of structure and instinct among the members of them arose little by little, as the social organization arose little by little. fortunately we are able to trace the greater part of the process in the annually-formed communities of the common wasp; and we shall recognize in it an all-important factor (ignored by professor weismann) to which the phenomena, or at any rate the greater part of them, are due. but before describing the wasp's annual history, let me set down certain observations made when, as a boy, i was given to angling, and, in july or august, sometimes used for bait "wasp-grubs," as they were called. after having had two or three days the combs or "cakes" of these, full of unfed larvæ in all stages of growth, i often saw some of them devouring the edges of their cells to satisfy their appetites; and saw others, probably the most advanced in growth, which were spinning the little covering caps to their cells, in preparation for assuming the pupa state. it is to be inferred that if, after a certain stage of growth has been reached, the food-supply becomes inadequate or is stopped altogether, the larva undergoes its transformation prematurely; and, as we shall presently see, this premature transformation has several natural sequences. let us return now to the wasp's family history. in the spring, a queen-wasp or mother-wasp which has survived the winter, begins to make a small nest containing four or more cells in which she lays eggs, and as fast as she builds additional cells, she lays an egg in each. presently, to these activities, is added the feeding of the larvæ: one result being that the multiplication of larvæ involves a restriction of the food that can be given to each. if we suppose that the mother-wasp rears no more larvæ than she can fully feed, there will result queens or mothers like herself, relatively few in number. but if we suppose that, laying more numerous eggs she produces more larvæ than she can fully feed, the result will be that when these have reached a certain stage of growth, inadequate supply of food will be followed by premature retirement and transformation into pupæ. what will be the characters of the developed insects? the first effect of arrested nutrition will be smaller size. this we find. a second effect will be defective development of parts that are latest formed and least important for the survival of the individual. hence we may look for arrested development of the reproductive organs--non-essential to individual life. and this expectation is in accord with what we see in animal development at large; for (passing over entirely sexless individuals) we see that though the reproductive organs may be marked out early in the course of development, they are not made fit for action until after the structures for carrying on individual life are nearly complete. the implication is, then, that an inadequately-fed and small larva will become a sterile imago. having noted this, let us pass to a remarkable concomitant. in the course of development, organs are formed not alone in the order of their original succession, but partly in the order of importance and the share they have to take in adult activities--a change of order called by haeckel "heterochrony." hence the fact that we often see the maternal instinct precede the sexual instinct. every little girl with her doll shows us that the one may become alive while the other remains dormant. in the case of wasps, then, premature arrest of development may result in incompleteness of the sexual traits, along with completeness of the maternal traits. what happens? leave out the laying of eggs, and the energies of the mother-wasp are spent wholly in building cells and feeding larvæ, and the worker-wasp forthwith begins to spend its life in building cells and feeding larvæ. thus interpreting the facts, we have no occasion to assume any constitutional difference between the eggs of worker-wasps and the eggs of queens; and that, their eggs are not different we see, first, in the fact that occasionally the worker-wasp is fertile and lays drone-producing eggs, and we see secondly that (if in this respect they are like the bees, of which, however, we have no proof) the larva of a worker-wasp can be changed into the larva of a queen-wasp by special feeding. but be this as it may, we have good evidence that the feeding determines everything. says dr. ormerod, in his _british social wasps_:-- "when the swarm is strong and food plentiful ... the well fed larvæ develop into females, full, large, and overflowing with fat. there are all gradations of size, from the large fat female to the smallest worker.... the larger the wasp, the larger and better developed, as the rule, are the female organs, in all their details. in the largest wasps, which are to be the queens of another year, the ovaries differ to all appearances in nothing but their size from those of the larger worker wasps.... small feeble swarms produce few or no perfect females; but in large strong swarms they are found by the score." (pp. - ) to this evidence add the further evidence that queens and workers pass through certain parallel stages in respect of their maternal activities. at first the queen, besides laying eggs, builds cells and feeds larvæ, but after a time ceases to build cells, and feeds larvæ only, and eventually doing neither one nor the other, only lays eggs, and is supplied with food by the workers. so it is in part with the workers. while the members of each successive brood, when in full vigour, build cells and feed larvæ, by-and-by they cease to build cells, and only feed larvæ: the maternal activities and instincts undergo analogous changes. in this case, then, we are not obliged to assume that only by a process of natural selection can the differences of structure and instinct between queens and workers be produced. the only way in which natural selection here comes into play is in the better survival of the families of those queens which made as many cells, and laid as many eggs, as resulted in the best number of half-fed larvæ, producing workers; since by a rapid multiplication of workers the family is advantaged, and the ultimate production of more queens surviving into the next year insured. the differentiation of classes does not go far among the wasps, because the cycle of processes is limited to a year, or rather to the few months of the summer. it goes further among the hive-bees, which, by storing food, survive from one year into the next. unlike the queen-wasp, the queen-bee neither builds cells nor gathers food, but is fed by the workers: egg laying has become her sole business. on the other hand the workers, occupied exclusively in building and nursing, have the reproductive organs more dwarfed than they are in wasps. still we see that the worker-bee occasionally lays drone-producing eggs, and that, by giving extra nutriment and the required extra space, a worker-larva can be developed into a queen-larva. in respect to the leading traits, therefore, the same interpretation holds. doubtless there are subsidiary instincts which are apparently not thus interpretable. but before it can be assumed that an interpretation of another kind is necessary, it must be shown that these instincts cannot be traced back to those pre-social types and semi-social types which must have preceded the social types we now see. for unquestionably existing bees must have brought with them from the pre-social state an extensive endowment of instincts, and, acquiring other instincts during the unorganized social state, must have brought these into the present organized social state. it is clear, for instance, that the cell-building instinct in all its elaboration was mainly developed in the pre-social stage; for the transition from species building solitary cells to those building combs is traceable. we are similarly enabled to account for swarming as being an inheritance from remote ancestral types. for just in the same way that, with under-feeding of larvæ, there result individuals with imperfectly developed reproductive systems, so there will result individuals with imperfect sexual instincts; and just as the imperfect reproductive system partially operates upon occasion, so will the imperfect sexual instinct. whence it will result that on the event which causes a queen to undertake a nuptial flight which is effectual, the workers may take abortive nuptial flights: so causing a swarm. and here, before going further, let us note an instructive class of facts related to the class of facts above set forth. summing up, in a chapter on "the determination of sex," an induction from many cases, professor geddes and mr. thompson remark that "such conditions as deficient or abnormal food," and others causing "preponderance of waste over repair ... tend to result in production of males;" while "abundant and rich nutrition" and other conditions which "favour constructive processes ... result in the production of females."[ ] among such evidences of this as immediately concern us, are these:--j. h. fabre found that in the nests of _osmia tricornis_, eggs at the bottom, first laid, and accompanied by much food, produced females, while those at the top, last laid, and accompanied by one-half or one-third the quantity of food, produced males,[ ] huber's observations on egg-laying by the honey-bee, show that in the normal course of things, the queen lays eggs of workers for eleven months, and only then lays eggs of drones: that is, when declining nutrition or exhaustion has set in. further, we have the above-named fact, shown by wasps and bees, that when workers lay eggs these produce drones only.[ ] special evidence, harmonizing with general evidence, thus proves that among the social insects the sex is determined by degree of nutrition while the egg is being formed. see then how congruous this evidence is with the conclusion above drawn; for it is proved that after an egg, predetermined as a female, has been laid, the character of the produced insect as a perfect female or imperfect female is determined by the nutrition of the larva. _that is, one set of differences in structures and instincts is determined by nutrition before the egg is laid, and a further set of differences in structures and instincts is determined by nutrition after the egg is laid._ we come now to the extreme case--that of the ants. is it not probable that the process of differentiation has been similar? there are sundry reasons for thinking so. with ants as with wasps and bees--the workers occasionally lay eggs; and an ant-community can, like a bee-community, when need be, produce queens out of worker-larvæ: presumably in the same manner by extra feeding. but here we have to add special evidence of great significance. for observe that the very facts concerning ants, which professor weismann names as exemplifying the formation of the worker type by selection, serve, as in the case of wasps, to exemplify its formation by arrested nutrition. he says that in several species the egg-tubes in the ovaries show progressive decrease in number; and this, like the different degrees of arrest in the ovaries of the worker-wasps, indicates arrest of larva-feeding at different stages. he gives cases showing that, in different degrees, the eyes of workers are less developed in the number of their facets than those of the perfect insects; and he also refers to the wings of workers as not being developed: remarking, however, that the rudiments of their wings show that the ancestral forms had wings. are not these traits also results of arrested nutrition? generally among insects the larvæ are either blind or have but rudimentary eyes; that is to say, visual organs are among the latest organs to arise in the genesis of the perfect organism. hence early arrest of nutrition will stop formation of these, while various more ancient structures have become tolerably complete. similarly with wings. wings are late organs in insect phylogeny, and therefore will be among those most likely to abort where development is prematurely arrested. and both these traits will, for the same reason, naturally go along with arrested development of the reproductive system. even more significant, however, is some evidence assigned by mr. darwin respecting the caste-gradations among the driver ants of west africa. he says:-- "but the most important fact for us is, that, though the workers can be grouped into castes of different sizes, yet they graduate insensibly into each other, as does the widely-different structure of their jaws."[ ] "graduate insensibly," he says; implying that there are very numerous intermediate forms. this is exactly what is to be expected if arrest of nutrition be the cause; for unless the ants have definite measures, enabling them to stop feeding at just the same stages, it must happen that the stoppage of feeding will be indefinite; and that, therefore, there will be all gradations between the extreme forms--"insensible gradations," both in size and in jaw-structure. in contrast with this interpretation, consider now that of professor weismann. from whichever of the two possible suppositions he sets out, the result is equally fatal. if he is consistent, he must say that each of these intermediate forms of workers must have its special set of "determinants," causing its special set of modifications of organs; for he cannot assume that while perfect females and the extreme types of workers have their different sets of determinants, the intermediate types of workers have not. hence we are introduced to the strange conclusion that besides the markedly-distinguished sets of determinants there must be, to produce these intermediate forms, many other sets slightly distinguished from one another--a score or more kinds of germ-plasm in addition to the four chief kinds. next comes an introduction to the still stranger conclusion, that these numerous kinds of germ-plasm, producing these numerous intermediate forms, are not simply needless but injurious--produce forms not well fitted for either of the functions discharged by the extreme forms: the implication being that natural selection has originated these disadvantageous forms! if to escape from this necessity for suicide, professor weismann accepts the inference that the differences among these numerous intermediate forms are caused by arrested feeding of the larvæ at different stages, then he is bound to admit that the differences between the extreme forms, and between these and perfect females, are similarly caused. but if he does this, what becomes of his hypothesis that the several castes are constitutionally distinct, and result from the operation of natural selection? observe, too, that his theory does not even allow him to make this choice; for we have clear proof that unlikenesses among the forms of the same species cannot be determined this way or that way by differences of nutrition. english greyhounds and scotch greyhounds do not differ from one another so much as do the amazon-workers from the inferior workers, or the workers from the queens. but no matter how a pregnant scotch greyhound is fed, or her pups after they are born, they cannot be changed into english greyhounds: the different germ-plasms assert themselves spite of all treatment. but in these social insects the different structures of queens and workers _are_ determinable by differences of feeling. therefore the production of their various castes does not result from the natural selection of varying germ-plasm. before dealing with professor weismann's crucial case--that co-adaptation of parts, which, in the soldier-ants, has, he thinks, arisen without inheritance of acquired characters--let me deal with an ancillary case which he puts forward as explicable by "panmixia alone." this is the "degeneration, in the warlike amazon-ants, of the instinct to search for food."[ ] let us first ask what have been the probable antecedents of these amazon-ants; for, as i have above said, it is absurd to speculate about the structures and instincts the species possesses in its existing organized social state without asking what structures and instincts it brought with it from its original solitary state and its unorganized social state. from the outset these ants were predatory. some variety of them led to swarm--probably at the sexual season--did not again disperse so soon as other varieties. those which thus kept together derived advantages from making simultaneous attacks on prey, and prospered accordingly. of descendants the varieties which carried on longest the associated state prospered most; until, at length, the associated state became permanent. all which social progress took place while there existed only perfect males and females. what was the next step? ants utilize other insects, and, among other ways of doing this, sometimes make their nests where there are useful insects ready to be utilized. giving an account of certain new zealand species of _tetramorium_, mr. w. w. smith says they seek out underground places where there are "root-feeding aphides and coccids," which they begin to treat as domestic animals; and further he says that when, after the pairing season, new nests are being formed, there are "a few ants of both sexes ... from two up to eight or ten."[ ] carrying with us this fact as a key, let us ask what habits will be fallen into by the conquering species of ants. they, too, will seek places where there are creatures to be utilized; and, finding it profitable, will invade the habitations not of defenceless creatures only, but of creatures whose powers of defence are inadequate--weaker species of their own order. a very small modification will affiliate their habits on habits of their prototypes. instead of being supplied with sweet substance excreted by the aphides they are supplied with sweet substance by the ants among which they parasitically settle themselves. how easily the subjugated ants may fall into the habit of feeding them, we shall see on remembering that already they feed not only larvæ but adults--individuals bigger than themselves. and that attentions kindred to these paid to parasitic ants may be established without difficulty, is shown us by the small birds which continue to feed a young cuckoo in their nest when it has outgrown them. this advanced form of parasitism grew up while there were yet only perfect males and females, as happens in the initial stage with these new zealand ants. what further modifications of habits were probably then acquired? from the practice of settling themselves where there already exist colonies of aphides, which they carry about to suitable places in the nest, like _tetramorium_, other ants pass to the practice of making excursions to get aphides, and putting them in better feeding places where they become more productive of saccharine matter. by a parallel step these soldier-ants pass from the stage of settling themselves among other ants which feed them, to the stage of fetching the pupæ of such ants to the nest: a transition like that which occurs among slave-making human beings. thus by processes analogous to those we see going on, these communities of slave-making ants may be formed. and since the transition from an unorganized social state to a social state characterized by castes, must have been gradual, there must have been a long interval during which the perfect males and females of these conquering ants could acquire habits and transmit them to progeny. a small modification accounts for that seemingly-strange habit which professor weismann signalizes. for if, as is observed, those ants which keep aphides solicit them to excrete a supply of ant-food by stroking them with the antennæ, they come very near to doing that which professor weismann says the soldier-ants do towards a worker--"they come to it and beg for food:" the food being put into their mouths in this last case as almost or quite in the first. and evidently this habit of passively receiving food, continued through many generations of perfect males and females, may result in such disuse of the power of self-feeding that this is eventually lost. the behaviour of young birds, during, and after, their nest-life, gives us the clue. for a week or more after they are full-grown and fly about with their parents, they may be seen begging for food and making no efforts to recognize and pick up food for themselves. if, generation after generation, feeding of them in full measure continued, they would not learn to feed themselves: the perceptions and instincts implied in self-feeding would be later and later developed, until, with entire disuse of them, they would disappear altogether by inheritance. thus self-feeding may readily have ceased among these soldier-ants before the caste-organization arose among them. with this interpretation compare the interpretation of professor weismann. i have before protested against arguing in abstracts without descending to concretes. here let us ask what are the particular changes which the alleged explanation by survival of the fittest involves. suppose we make the very liberal supposition that an ant's central ganglion bears to its body the same ratio as the human brain bears to the human body--say, one-fortieth of its weight. assuming this, what shall we assume to be the weight of those ganglion-cells and fibres in which are localized the perceptions of food and the suggestion to take it? shall we say that these amount to one-tenth of the central ganglion? this is a high estimate considering all the impressions which this ganglion has to receive, and all the operations which it has to direct. still we will say one-tenth. then it follows that this portion of nervous substance is one- th of the weight of its body. by what series of variations shall we say that it is reduced from full power to entire incapacity? shall we say five? this is a small number to assume. nevertheless we will assume it. what results? that the economy of nerve-substance achieved by each of these five variations will amount to one- th of the entire mass. making these highly favourable assumptions, what follows:--the queen-ant lays eggs that give origin to individuals in each of which there is achieved an economy in nerve-substance of one- th of its weight; and the implication of the hypothesis is that such an economy will so advantage this ant-community that in the competition with other ant-communities it will conquer. for here let me recall the truth before insisted upon, that natural selection can operate only on those variations which appreciably benefit the stirp. bearing in mind this requirement, is any one now prepared to say that survival of the fittest can cause this decline of the self-feeding faculty?[ ] not limiting himself to the darwinian interpretation, however, professor weismann says that this degradation may be accounted for by "panmixia alone." here i will not discuss the adequacy of this supposed cause, but will leave it to be dealt with by implication a few pages in advance, where the general hypothesis of panmixia will be reconsidered. and now, at length, we are prepared for dealing with professor weismann's crucial case--with his alleged disproof that co-adaptation of co-operative parts results from inheritance of acquired characters, because in the case of the amazon-ants, it has arisen where the inheritance of acquired characters is impossible. for after what has been said, it will be manifest that the whole question is begged when it is assumed that this co-adaptation has arisen since there existed among these ants an organized social state. unquestionably this organized social state pre-supposes a series of modifications through which it has been reached. it follows, then, that there can be no rational interpretation without a preceding inquiry concerning that earlier state in which there were no castes, but only males and females. what kinds of individuals were the ancestral ants--at first solitary, and then semi-social? they must have had marked powers of offence and defence. of predacious creatures, it is the more powerful which form societies, not the weaker. instance human races. nations originate from the relatively warlike tribes, not from the relatively peaceful tribes. among the several types of individuals forming the existing ant community, to which, then, did the ancestral ants bear the greatest resemblance? they could not have been like the queens, for these, now devoted to egg-laying, are unfitted for conquest. they could not have been like the inferior class of workers, for these, too, are inadequately armed and lack strength. hence they must have been most like these amazon-ants or soldier-ants, which now make predatory excursions--which now do, in fact, what their remote ancestors did. what follows? their co-adapted parts have not been produced by the selection of variations within the ant-community, such as we now see it. they have been inherited from the pre-social and early social types of ants, in which the co-adaptation of parts had been effected by inheritance of acquired characters. it is not that the soldier-ants have gained these traits; it is that the other castes have lost them. early arrest of development causes absence of them in the inferior workers; and from the queens they have slowly disappeared by inheritance of the effects of disuse. for, in conformity with ordinary facts of development, we may conclude that in a larva which is being so fed as that the development of the reproductive organs is becoming pronounced, there will simultaneously commence arrest in the development of those organs which are not to be used. there are abundant proofs that along with rapid growth of some organs others abort. and if these inferences are true, then professor weismann's argument falls to the ground. nay, it falls to the ground even if conclusions so definite as these be not insisted upon; for before he can get a basis for his argument he must give good reasons for concluding that these traits of the amazon-ants have _not_ been inherited from remote ancestors. one more step remains. let us grant him his basis, and let us pass from the above negative criticism to a positive criticism. as before, i decline to follow the practice of talking in abstracts instead of in concretes, and contend that, difficult as it may be to see how natural selection has in all cases operated, we ought, at any rate, to trace out its operation whenever we can, and see where the hypothesis lands us. according to professor weismann's admission, for production of the amazon-ant by natural selection, "_many parts must have varied simultaneously and in harmony with one another_;"[ ] and he names as such, larger jaws, muscles to move them, larger head, and thicker chitin for it, bigger nerves for the muscles, bigger motor centres in the brain, and, for the support of the big head, strengthening of the thorax, limbs, and skeleton generally. as he admits, all these parts must have varied simultaneously in due proportion to one another. what must have been the proximate causes of their variations? they must have been variations in what he calls the "determinants." he says:-- "we have, however, to deal with the transmission of parts which are _variable_ and this necessitates the assumption that just as many independent and variable parts exist in the germ-plasm as are present in the fully formed organism."[ ] consequently to produce simultaneously these many variations of parts, adjusted in their sizes and shapes, there must have simultaneously arisen a set of corresponding variations in the "determinants" composing the germ-plasm. what made them simultaneously vary in the requisite ways? professor weismann will not say that there was somewhere a foregone intention. this would imply supernatural agency. he makes no attempt to assign a physical cause for these simultaneous appropriate variations in the determinants: an adequate physical cause being inconceivable. what, then, remains as the only possible interpretation? nothing but _a fortuitous concourse of variations_; reminding us of the old "fortuitous concourse of atoms." nay, indeed, it is the very same thing. for each of the "determinants," made up of "biophors," and these again of protein-molecules, and these again of simpler chemical molecules, must have had its molecular constitution changed in the required way; and the molecular constitutions of all the "determinants," severally modified differently, but in adjustment to one another, must have been thus modified by "a fortuitous concourse of atoms." now if this is an allowable supposition in respect of the "determinants," and the varying organs arising from them, why is it not an allowable supposition in respect of the organism as a whole? why not assume "a fortuitous concourse of atoms" in its broad, simple form? nay, indeed, would not this be much the easier? for observe, this co-adaptation of numerous co-operative parts is not achieved by one set of variations, but is achieved gradually by a series of such sets. that is to say, the "fortuitous concourse of atoms" must have occurred time after time in appropriate ways. we have not one miracle, but a series of miracles! * * * * * of the two remaining points in professor weismann's first article which demand notice, one concerns his reply to my argument drawn from the distribution of tactual discriminativeness. in what way does he treat this argument? he meets it by an argument derived from hypothetical evidence--not actual evidence. taking the case of the tongue-tip, i have carefully inquired whether its extreme power of tactual discrimination can give any life-saving advantage in moving about the food during mastication, in detecting foreign bodies in it, or for purposes of speech; and have, i think, shown that the ability to distinguish between points one twenty-fourth of an inch apart is useless for such purposes. professor weismann thinks he disposes of this by observing that among the apes the tongue is used as an organ of touch. but surely a counter-argument equivalent in weight to mine should have given a case in which power to discriminate between points one twenty-fourth of an inch apart instead of one-twentieth of an inch apart (a variation of one-sixth) had a life-saving efficacy; or, at any rate, should have suggested such a case. nothing of the kind is done or even attempted. but now note that his reply, accepted even as it stands, is suicidal. for what has the trusted process of panmixia been doing ever since the human being began to evolve from the ape? why during thousands of generations has not the nervous structure giving this extreme discriminativeness dwindled away? even supposing it had been proved of life-saving efficacy to our simian ancestors, it ought, according to professor weismann's own hypothesis, to have disappeared in us. either there was none of the assumed special capacity in the ape's tongue, in which case his reply fails, or panmixia has not operated, in which case his theory of degeneracy fails. all this, however, is but preface to the chief answer. the argument drawn from the case of the tongue-tip, with which alone professor weismann deals, is but a small part of my argument, the remainder of which he does not attempt to touch--does not even mention. had i never referred to the tongue-tip at all, the various contrasts in discriminativeness which i have named, between the one extreme of the forefinger-tip and the other extreme of the middle of the back, would have abundantly sufficed to establish my case--would have sufficed to show the inadequacy of natural selection as a key and the adequacy of the inheritance of acquired characters. it seems to me, then, that judgment must go against him by default. practically he leaves the matter standing just where it did.[ ] the other remaining point concerns the vexed question of panmixia. confirming the statement of dr. romanes, professor weismann says that i have misunderstood him. already (_contemporary review_, may, , p. , and reprint, p. ) i have quoted passages which appeared to justify my interpretation, arrived at after much seeking.[ ] already, too, in this review (july, , p. ) i have said why i did not hit upon the interpretation now said to be the true one: i never supposed that any one would assume, without assigned cause, that (apart from the excluded influence of disuse) the _minus_ variations of a disused organ are greater than the _plus_ variations. this was a tacit challenge to produce reasons for the assumption. professor weismann does not accept the challenge, but simply says:--"in my opinion all organs are maintained at the height of their development only through uninterrupted selection" (p. ): in the absence of which they decline. now it is doubtless true that as a naturalist he may claim for his "opinion" a relatively great weight. still, in pursuance of the methods of science, it seems to me that something more than an opinion is required as the basis of a far-reaching theory.[ ] though the counter-opinion of one who is not a naturalist (as professor weismann points out) may be of relatively small value, yet i must here again give it, along with a final reason for it. and this reason shall be exhibited, not in a qualitative form, but in a quantitative form. let us quantify the terms of the hypothesis by weights; and let us take as our test case the rudimentary hind-limbs of the whale. zoologists are agreed that the whale has been evolved from a mammal which took to aquatic habits, and that its disused hind-limbs have gradually disappeared. when they ceased to be used in swimming, natural selection played a part--probably an important part--in decreasing them; since, being then impediments to movement through the water, they diminished the attainable speed. it may be, too, that for a period after disappearance of the limbs beneath the skin, survival of the fittest had still some effect. but during the latter stages of the process it had no effect; since the rudiments caused no inconvenience and entailed no appreciable cost. here, therefore, the cause, if professor weismann is right, must have been panmixia. dr. struthers, professor of anatomy at aberdeen, whose various publications show him to be a high, if not the highest, authority on the anatomy of these great cetaceans, has kindly taken much trouble in furnishing me with the needful data, based upon direct weighing and measuring and estimation of specific gravity. in the black whale (_balænoptera borealis_) there are no rudiments of hind-limbs whatever: rudiments of the pelvic bones only remain. a sample of the greenland right whale, estimated to weigh , lbs., had femurs weighing together ½ ozs.; while a sample of the razor-back whale (_balænoptera musculus_), feet long, and estimated to weigh , lbs., had rudimentary femurs weighing together one ounce; so that these vanishing remnants of hind-limbs weighed but one- , th part of the animal. now in considering the alleged degeneration by panmixia, we have first to ask why these femurs must be supposed to have varied in the direction of decrease rather than in the direction of increase. during its evolution from the original land-mammal, the whale has grown enormously, implying habitual excess of nutrition. alike in the embryo and in the growing animal, there must have been a chronic plethora. why, then, should we suppose these rudiments to have become smaller? why should they not have enlarged by deposit in them of superfluous materials? but let us grant the unwarranted assumption of predominant _minus_ variations. let us say that the last variation was a reduction of one-half--that in some individuals the joint weight of the femurs was suddenly reduced from two ounces to one ounce--a reduction of one- , th of the creature's weight. by inter-crossing with those inheriting the variation, the reduction, or a part of the reduction, was made a trait of the species. now, in the first place, a necessary implication is that this _minus_ variation was maintained in posterity. so far from having reason to suppose this, we have reason to suppose the contrary. as before quoted, mr. darwin says that "unless carefully preserved by man," "any particular variation would generally be lost by crossing, reversion, and the accidental destruction of the varying individuals."[ ] and mr. galton, in his essay on "regression towards mediocrity,"[ ] contends that not only do deviations of the whole organism from the mean size tend to thus disappear, but that deviations in its components do so. hence the chances are against such _minus_ variation being so preserved as to affect the species by panmixia. in the second place, supposing it to be preserved, may we reasonably assume that, by inter-crossing, this decrease, amounting to about a millionth part of the creature's weight, will gradually affect the constitutions of all razor-back whales distributed over the arctic seas and the north atlantic ocean, from greenland to the equator? is this a credible conclusion? for three reasons, then, the hypothesis must be rejected. thus, the only reasonable interpretation is the inheritance of acquired characters. if the effects of use and disuse, which are known causes of change in each individual, influence succeeding individuals--if functionally-produced modifications of structure are transmissible, as well as modifications of structure otherwise arising--then this reduction of the whale's hind limbs to minute rudiments is accounted for. the cause has been unceasingly operative on all individuals of the species ever since the transformation began. in one case see all. if this cause has thus operated on the limbs of the whale, it has thus operated in all creatures on all parts having active functions. * * * * * at the outset i intimated that i must limit my replies to those arguments of professor weismann which are contained in his first article. that those contained in his second might be dealt with no less effectually, did time and space permit, is manifest to me; but about the probability of this the reader must form his own judgment. my replies thus far may be summed up as follows:-- professor weismann says he has disproved the conclusion that degeneration of the little toe has resulted from inheritance of acquired characters. but his reasoning fails against an interpretation he overlooks. a profound modification of the hind limbs and their appendages must have taken place during the transition from arboreal habits to terrestrial habits; and dwindling of the little toe is an obvious consequence of disuse, at the same time that enlargement of the great toe is an obvious consequence of increased use. the entire argument based on the unlike forms and instincts presented by castes of social insects is invalidated by an omission. until probable conclusions are reached respecting the characters which such insects brought with them into the organized social state, no valid inferences can be drawn respecting characters developed during that state. a further large error of interpretation is involved in the assumption that the different caste-characters are transmitted to them in the eggs laid by the mother insect. while we have evidence that the unlike structures of the sexes are determined by nutrition of the germ before egg-laying, we have evidence that the unlike structures of classes are caused by unlikenesses of nutrition of the larvæ. that these varieties of forms do not result from varieties of germ-plasms, is demonstrated by the fact that where there are varieties of germ-plasms, as in varieties of the same species of mammal, no deviations in feeding prevent display of their structural results. for such caste-modifications as those of the amazon-ants, which are unable to feed themselves, there is a feasible explanation other than professor weismann's. the relation of common ants to their domestic animals--aphides and coccids--which yield them food on solicitation, does not differ widely from this relation between these amazon-ants and their domestic animals--the slave-ants. and the habit of being fed, contracted during the first stages of their parasitic life, when there were perfect males and females, may, during that stage, have become established by inheritance. meanwhile the opposed interpretation--that this incapacity has resulted from the selection of those ant-communities the queens of which laid eggs that had so varied as to entail this incapacity--implies that a scarcely appreciable economy of nerve-matter advantaged the stirp so greatly as to cause it to spread more than other stirps: an incredible supposition. as the outcome of these alternative interpretations we saw that the argument respecting the co-adaptation of co-operative parts, which professor weismann thinks is furnished to him by the amazon-ants, disappears. the ancestral ants were conquering ants. these founded the communities; and hence those members of the present communities which are most like them are the amazon-ants. if so, the co-adaptation of the co-operative parts was effected by inheritance during the solitary and semi-social stages. even were there no such solution, the opposed solution will be unacceptable. these simultaneous appropriate variations of the co-operative parts in sizes, shapes, and proportions, are supposed to be effected by simultaneous variations in the "determinants" of the germ-plasms; and in the absence of an assigned physical cause, this implies a fortuitous concourse of appropriate variations, which carries us back to a "fortuitous concourse of atoms." this may just as well be extended to the entire organism. the old hypothesis of special creations is more consistent and comprehensible. to rebut my inference drawn from the distribution of discriminativeness, professor weismann uses not an argument but the blank form of an argument. the ability to discriminate one twenty-fourth of an inch by the tongue-tip _may_ have been useful to the ape: no conceivable use being even suggested. and then the great body of my argument derived from the distribution of discriminativeness over the skin, which amply suffices, is wholly ignored. the tacit challenge i gave to name some facts in support of the hypothesis of panmixia--or even a solitary fact--is passed by. it remains a pure speculation having no basis but professor weismann's "opinion." when from the abstract statement of it we pass to a concrete test, in the case of the whale, we find that it necessitates an unproved and improbable assumption respecting _plus_ and _minus_ variations; that it ignores the unceasing tendency to reversion; and that it implies an effect out of all proportion to the cause. it is curious what entirely opposite conclusions men may draw from the same evidence. professor weismann thinks he has shown that the "last bulwark of the lamarckian principle is untenable." most readers will hold with me that he is, to use the mildest word, premature in so thinking. contrariwise my impression is that he has not shown either this bulwark or any other bulwark to be untenable; but rather that while his assault has failed it has furnished opportunity for strengthening sundry of the bulwarks. iv. among those who follow a controversy to its close, not one in a hundred turns back to its beginning to see whether its chief theses have been dealt with. very often the leading arguments of one disputant, seen by the other to be unanswerable, are quietly ignored, and attention is concentrated on subordinate arguments to which replies, actually or seemingly valid, can be made. the original issue is thus commonly lost sight of. more than once i have pointed out that, as influencing men's views about education, ethics, sociology, and politics, the question whether acquired characters are inherited is the most important question before the scientific world. hence i cannot allow the discussion with professor weismann to end in so futile a way as it will do if no summary of results is made. here, therefore, i propose to recapitulate the whole case in brief. primarily my purpose is to recall certain leading propositions which, having been passed by unnoticed, remain outstanding. i will turn, in the second place, to such propositions as have been dealt with; hoping to show that the replies given are invalid, and consequently that these propositions also remain outstanding. but something beyond a summing-up is intended. a few pages at the close will be devoted to setting forth new evidence which has come to light since the controversy commenced--evidence which many will think sufficient in itself to warrant a positive conclusion. * * * * * the fact that the tip of the fore finger has thirty times the power of discrimination possessed by the middle of the back, and that various intermediate degrees of discriminative power are possessed by various parts of the skin, was set down as a datum for my first argument. the causes which might be assigned for these remarkable contrasts were carefully examined under all their aspects. i showed in detail that the contrasts could not in any way be accounted for by natural selection. i further showed that no interpretation of them is afforded by the alleged process of panmixia: this has no _locus standi_ in the case. having proved experimentally, that ability of the fingers to discriminate is increased by practice, and having pointed out that gradations of discriminativeness in different parts correspond with gradations in the activities of the parts as used for tactual exploration, i argued that these contrasts have arisen from the organized and inherited effects of tactual converse with surrounding things, varying in its degrees according to the positions of the parts--in other words, that they are due to the inheritance of acquired characters. as a crowning proof i instanced the case of the tongue-tip, which has twice the discriminativeness of the forefinger-tip: pointing out that consciously, or semi-consciously, or unconsciously, the tongue-tip is perpetually exploring the inner surfaces of the teeth. singling out this last case, professor weismann made, or rather adopted from dr. romanes, what professed to be a reply but was nothing more than the blank form of a reply. it was said that though this extreme discriminativeness of the tongue-tip is of no use to mankind, it may have been of use to certain ancestral _primates_. no evidence of any such use was given; no imaginable use was assigned. it was simply suggested that there perhaps was a use. in my rejoinder, after indicating the illusory nature of this proceeding (which is much like offering a cheque on a bank where no assets have been deposited to meet it), i pointed out that had the evidence furnished by the tongue tip never been mentioned, the evidence otherwise furnished amply sufficed. i then drew attention to the fact that this evidence had been passed over, and tacitly inquired why. no reply.[ ] * * * * * in his essay on "the all-sufficiency of natural selection," professor weismann set out, not by answering one of the arguments i had used, but by importing into the discussion an argument used by another writer, which it was easy to meet. it had been contended that the smallness and deformity of the little toe are consequent upon the effects of boot-pressure, inherited from generation to generation. to this professor weismann made the sufficient reply that the fusion of the phalanges and otherwise degraded structure of the little toe, exist among peoples who go barefoot. in my "rejoinder" i said that though the inheritance of acquired characters does not explain this degradation in the way alleged, it explains it in a way which professor weismann overlooks. the cause is one which has been operating ever since the earliest anthropoid creatures began to decrease their life in trees and increase their life on the earth's surface. the mechanics of walking and running, in so far as they concern the question at issue, were analyzed; and it was shown that effort is economized and efficiency increased in proportion as the stress is thrown more and more on the inner digits of the foot and less and less on the outer digits. so that thus the foot furnishes us simultaneously with an instance of increase from use and of decrease from disuse; a further disproof being yielded of the allegation that co-operative parts vary together, since we have here co-operative parts of which one grows while the other dwindles. i ended by pointing out that, so far from strengthening his own case, professor weismann had, by bringing into the controversy this changed structure of the foot, given occasion for strengthening the opposite case. no reply. * * * * * we come now to professor weismann's endeavour to disprove my second thesis--that it is impossible to explain by natural selection alone the co-adaptation of co-operative parts. it is thirty years since this was set forth in _the principles of biology_. in § i instanced the enormous horns of the extinct irish elk, and contended that in this, and in kindred cases, where for the efficient use of some one enlarged part many other parts have to be simultaneously enlarged, it is out of the question to suppose that they can have all spontaneously varied in the required proportions. in "the factors of organic evolution," by way of enforcing this argument, which had, so far as i know, never been met, i dwelt upon the aberrant structure of the giraffe. and then, in the essay which initiated this controversy, i brought forward yet a third case--that of an animal which, previously accustomed only to walking, acquires the power of leaping. in the first of his articles in the _contemporary review_ (september, ), professor weismann made no direct reply, but he made an indirect reply. he did not attempt to show how there could have taken place in the stag the "harmonious variation of the different parts that co-operate to produce one physiological result" (p. ); but he contended that such harmonious variation _must_ have taken place, because the like has taken place in "the neuters of state-forming insects"--"animal forms which do not reproduce themselves, but are always propagated anew by parents which are unlike them" (p. ), and which therefore cannot have transmitted acquired characters. singling out those soldier-neuters which exist among certain kinds of ants, he described (p. ) the many co-ordinated parts required to make their fighting organs efficient. he then argued that the required simultaneous changes can "only have arisen by a selection of the parent-ants dependent on the fact that those parents which produced the best workers had always the best prospect of the persistence of their colony. no other explanation is conceivable; _and it is just because no other explanation is conceivable, that it is necessary for us to accept the principle of natural selection_" (pp. - ). [this passage initiated a collateral controversy, which, as continually happens, has greatly obscured the primary controversy. it became a question whether these forms of neuter insects have arisen as professor weismann assumes, or whether they have arisen from arrested development consequent upon innutrition. to avoid entanglements i must for the present pass over this collateral controversy, intending to resume it presently, when the original issues have been dealt with.] no one will suspect me of thinking that the inconceivability of the negation is not a valid criterion, since, in "the universal postulate," published in the _westminster review_ in and afterwards in _the principles of psychology_, i contended that it is the ultimate test of truth. but then in every case there has to be determined the question--is the negation inconceivable; and in assuming that it is so in the case named, lies the fallacy of the above-quoted passage. the three separate ways in which i dealt with this position of professor weismann are as follows:-- if we admit the assumption that the form of the soldier-ant has been developed since the establishment of the organized ant-community in which it exists, professor weismann's assertion that no other process than that which he alleges is conceivable, is true. but i pointed out that this assumption is inadmissible; and that no valid conclusion respecting the genesis of the soldier-ant can be drawn without postulating either the ascertained, or the probable, structure of those pre-social, or semi-social, ants from which the organized social ants have descended. i went on to contend that the pre-social type must have been a conquering type, and that therefore in all probability the soldier-ants represent most nearly the structures of those ancestral ants which existed when the society had perfect males and females and could transmit acquired characters, while the other members of the existing communities are degraded forms of the type. no reply. a further argument i used was that where there exist different castes among the neuter-ants, as those seen in the soldiers and workers of the driver ants of west africa, "they graduate insensibly into each other" alike in their sizes and in their structures; and that professor weismann's hypothesis implies a special set of "determinants" for each intermediate form. or if he should say that the intermediate forms result from mixtures of the determinants of the two extreme forms, there still remains the further difficulty that natural selection has maintained, for innumerable generations, these intermediate forms which are injurious deviations from the useful extreme forms. no reply. one further reason--fatal it seems to me--was urged in bar of his interpretation. no physical cause has been, or can be, assigned, why in the germ-plasm of any particular queen-ant, the "determinants" initiating these various co-operative organs, all simultaneously vary in fitting ways and degrees, and still less why there occur such co-ordinated variations generation after generation, until by their accumulated results these efficient co-operative structures have been evolved. i pointed out that in the absence of any assigned or assignable physical cause, it is necessary to assume a fortuitous concurrence of favourable variations, which means "a fortuitous concourse of atoms;" and that it would be just as rational, and much more consistent, to assume that the structure of the entire organism thus resulted. no reply. * * * * * it is reasonable to suspect that professor weismann recognized these difficulties as insuperable, for, in his romanes lecture on "the effect of external influences upon development," instead of his previous indirect reply, he makes a direct reply. reverting to the stag and its enlarging horns, he alleges a process by which, as he thinks, we may understand how, by variation and selection, all the bones and muscles of the neck, of the thorax, and of the fore-legs, are step by step adjusted in their sizes to the increasing sizes of the horns. he ascribes this harmonization to the internal struggle for nutriment, and that survival of the fittest which takes place among the parts of an organism: a process which he calls "_intra-individual_-selection, or more briefly--_intra-selection_" (p. ). "wilhelm roux has given an explanation of the cause of these wonderfully fine adaptations by applying the principle of selection to the parts of the organism. just as there is a struggle for survival among the individuals of a species, and the fittest are victorious, so also do even the smallest living particles contend with one another, and those that succeed best in securing food and place grow and multiply rapidly, and so displace those that are less suitably equipped" (p. ).[ ] that i do not explain as he does the co-adaptation of co-operative parts, professor weismann ascribes to my having overlooked this "principle of intra-selection"--an unlucky supposition, as we see. but i do not think that when recognizing it a generation ago, i should have seen its relevancy to the question at issue, had that issue then been raised, and i certainly do not see it now. full reproduction of professor weismann's explanation is impracticable, for it occupies several pages, but here are the essential sentences from it:-- "the great significance of intra-selection appears to me not to depend on its producing structures that are directly transmissible,--it cannot do that,--but rather consists in its causing a development of the germ-structure, acquired by the selection of individuals, which will be suitable to varying conditions.... we may therefore say that intra-selection effects the adaptation of the individual to its chance developmental conditions,--the suiting of the hereditary primary constituents to fresh circumstances" (p. ).... "but as the primary variations in the phyletic metamorphosis occurred little by little, the secondary adaptations would probably as a rule be able to keep pace with them. time would thus be gained till, in the course of generations, by constant selection of those germs the primary constituents of which are best suited to one another, the greatest possible degree of harmony may be reached, and consequently a definitive metamorphosis of the species involving all the parts of the individual may occur" (p. ). the connecting sentences, along with those which precede and succeed, would not, if quoted, give to the reader clearer conceptions than these by themselves give. but when disentangled from professor weismann's involved statements, the essential issues are, i think, clear enough. in the case of the stag, that daily working together of the numerous nerves, muscles, and bones concerned, by which they are adjusted to the carrying and using of somewhat heavier horns, produces on them effects which, as i hold, are inheritable, but which, as professor weismann holds, are not inheritable. if they are not inheritable, what must happen? a fawn of the next generation is born with no such adjustment of nerves, muscles and bones as had been produced by greater exercise in the parent, and with no tendency to such adjustment. consequently if, in successive generations, the horns go on enlarging, all these nerves, muscles, and bones, remaining of the original sizes, become utterly inadequate. the result is loss of life: the process of adaptation fails. "no," says professor weismann, "we must conclude that the germ-plasm has varied in the needful manner." how so? the process of "intra-individual selection," as he calls it, can have had no effect, since the cells of the soma cannot influence the reproductive cells. in what way, then, has the germ-plasm gained the characters required for producing simultaneously all these modified co-operative parts. well, professor weismann tells us merely that we must suppose that the germ-plasm acquires a certain sensitiveness such as gives it a proclivity to development in the requisite ways. how is such proclivity obtainable? only by having a multitude of its "determinants" simultaneously changed in fit modes. emphasizing the fact that even a small failure in any one of the co-operative parts may be fatal, as the sprain of an over-taxed muscle shows us, i alleged that the chances are infinity to one against the needful variations taking place at the same time. divested of its elaboration, its abstract words and technical phrases, the outcome of professor weismann's explanation is that he accepts this, and asserts that the infinitely improbable thing takes place! either his argument is a disguised admission of the inheritableness of acquired characters (the effects of "intra-selection") or else it is, as before, the assumption of a fortuitous concourse of favourable variations in the determinants--"a fortuitous concourse of atoms." * * * * * leaving here this main issue, i return now to that collateral issue named on a preceding page as being postponed--whether the neuters among social insects result from specially modified germ-plasms or whether they result from the treatment received during their larval stages. for the substantiation of his doctrine professor weismann is obliged to adopt the first of these alternatives; and in his romanes lecture he found it needful to deal with the evidence i brought in support of the second alternative. he says that "poor feeding is not the _causa efficiens_ of sterility among bees, but is merely the stimulus which _not only results in the formation of rudimentary ovaries, but at the same time calls forth all the other distinctive characters of the workers_" (pp. - ); and he says this although he has in preceding lines admitted that it is "true of all animals that they reproduce only feebly or not at all when badly and insufficiently nourished:" a known cause being thus displaced by a supposed cause. but professor weismann proceeds to justify his interpretation by experimentally-obtained evidence. he "reared large numbers of the eggs of a female blow-fly"; the larvæ of some he fed abundantly, but the larvæ of others sparingly; and eventually he obtained, from the one set flies of full size, and from the other small flies. nevertheless the small flies were fertile, as well as the others. here, then, was proof that innutrition had not produced infertility; and he contends that therefore among the neuter social insects, infertility has not resulted from innutrition. the argument seems strong, and to many will appear conclusive; but there are two differences which entirely vitiate the comparison professor weismann institutes. one of them has been pointed out by mr. cunningham. in the case of the blow-fly the food supplied to the larvæ though different in quantity was the same in quality; in the case of the social insects the food supplied, whether or not different in quantity, differs in quality. among bees, wasps, ants, &c., the larvæ of the reproductive forms are fed upon a more nitrogenous food than are the larvæ of the workers; whereas the two sets of larvæ of the blow-fly, as fed by professor weismann, were alike supplied with highly nitrogenous food. hence there did not exist the same cause for non-development of the reproductive organs. here, then, is one vitiation of the supposed parallel. there is a second. while the development of an embryo follows in a rude way the phyletic metamorphoses passed through by its ancestry, the order of development of organs is often gradually modified by the needs of particular species: the structures being developed in such order as conduces to self-sustentation and the welfare of offspring. among other results there arise differences in the relative dates of maturity of the reproductive system and of the other systems. it is clear, _à priori_, that it must be fatal to a species if offspring are habitually produced before the conditions requisite for their survival are fulfilled. and hence, if the life is a complex one, and the care taken of offspring is great, reproduction must be much longer delayed than where the life is simple and the care of offspring absent or easy. the contrast between men and oxen sufficiently illustrates this truth. now the subordination of the order of development of parts to the needs of the species, is conspicuously shown in the contrast between these two kinds of insects which professor weismann compares as though their requirements were similar. what happens with the blow fly? if it is able to suck up some nutriment, to fly tolerably, and to scent out dead flesh, various of its minor organs may be more or less imperfect without appreciable detriment to the species: the eggs can be laid in a fit place, and that is all that is wanted. hence it profits the species to have the reproductive system developed comparatively early--in advance, even, of various less essential parts. quite otherwise is it with social insects, which take such remarkable care of their young; or rather to make the case parallel--quite otherwise is it with those types from which the social insects have descended, bringing into the social state their inherited instincts and constitutions. consider the doings of the mason-wasp, or mason-bee, or those of the carpenter-bee. what, in these cases, must the female do that she may rear members of the next generation? there is a fit place for building or burrowing to be chosen; there is the collecting together of grains of sand and cementing them into a strong and water-proof cell, or there is the burrowing into wood and there building several cells; there is the collecting of food to place along with the eggs deposited in these cells, solitary or associated, including that intelligent choice of small caterpillars which, discovered and carried home, are carefully packed away and hypnotized by a sting, so that they may live until the growing larva has need of them. for all these proceedings there have to be provided the fit external organs--cutting instruments, &c., and the fit internal organs--complicated nerve-centres in which are located these various remarkable instincts, and ganglia by which these delicate operations have to be guided. and these special structures have, some if not all of them, to be made perfect and brought into efficient action before egg-laying takes place. ask what would happen if the reproductive system were active in advance of these ancillary appliances. the eggs would have to be laid without protection or food, and the species would forthwith disappear. and if that full development of the reproductive organs which is marked by their activity, is not needful until these ancillary organs have come into play, the implication, in conformity with the general law above indicated, is that the perfect development of the reproductive organs will take place later than that of these ancillary organs, and that if innutrition checks the general development, the reproductive organs will be those which chiefly suffer. hence, in the social types which have descended from these solitary types, this order of evolution of parts will be inherited, and will entail the results i have inferred. if only deductively reached, this conclusion would, i think, be fully justified. but now observe that it is more than deductively reached. it is established by observation. professor riley, ph.d., late government entomologist of the united states, in his annual address as president of the biological society of washington,[ ] on january , , said:-- "among the more curious facts connected with these termites, because of their exceptional nature, is the late development of the internal sexual organs in the reproductive forms." (p. .) though what has been shown of the termites has not been shown of the other social insects, which belong to a different order, yet, considering the analogies between their social states and between their constitutional requirements, it is a fair inference that what holds in the one case holds partially, if not fully, in the other. should it be said that the larval forms do not pass into the pupa state in the one case as they do in the other, the answer is that this does not affect the principle. the larva carries into the pupa state a fixed quantity of tissue-forming material for the production of the imago. if the material is sufficient, then a complete imago is formed. if it is not sufficient, then, while the earlier formed organs are not affected by the deficiency, the deficiency is felt when the latest formed organs come to be developed, and they are consequently imperfect. even if left without reply, professor weismann's interpretation commits him to some insuperable difficulties, which i must now point out. unquestionably he has "the courage of his opinions;" and it is shown throughout this collateral discussion as elsewhere. he is compelled by accumulated evidence to admit "that there is only _one_ kind of egg from which queens and workers as well as males arise."[ ] but if the production of one or other form from the same germ does not result from speciality of feeding, what does it result from? here is his reply:-- "we must rather suppose that the primary constituents of two distinct reproductive systems--_e. g._ those of the queen and worker--are contained in the germ-plasm of the egg."[ ] "the courage of his opinions," which professor weismann shows in this assumption, is, however, quite insufficient. for since he himself has just admitted that there is only one kind of egg for queens, workers, and males, he must at any rate assume three sets of "determinants." (i find that on a subsequent page he does so.) but this is not enough, for there are, in many cases, two if not more kinds of workers, which implies that four sets of determinants must co-exist in the same egg. even now we have not got to the extent of the assumption required. in the address above referred to on "social insects from psychical and evolutional points of view," professor riley gives us (p. ) the-- _forms in a termes colony under normal conditions._ . youngest larvæ. / \ / \ / \ . larvæ [of those] unfit . larvæ [that will be] fit for reproduction. for reproduction. / \ / \ / \ / \ . larvæ of . larvæ of . nymphs of . nymphs of nd workers. soldiers. st form. form. | | | . workers. . soldiers. . winged forms. | . true royal pairs. hence as, in this family tree, the royal pair includes male and female, it results that there are _five_ different adult forms (grassi says there are two others) arising from like eggs or larvæ; and professor weismann's hypothesis becomes proportionately complicated. let us observe what the complications are. it often happens in controversy--metaphysical controversy more than any other--that propositions are accepted without their terms having been mentally represented. in public proceedings documents are often "taken as read," sometimes with mischievous results; and in discussions propositions are often taken as thought when they have not been thought and cannot be thought. it sufficiently taxes imagination to assume, as professor weismann does, that two sets of "ids" or of "determinants" in the same egg are, throughout all the cell-divisions which end in the formation of the _morula_, kept separate, so that they may subsequently energize independently; or that if they are not thus kept separate, they have the power of segregating in the required ways. but what are we to say when three, four, and even five sets of "ids" or bundles of "determinants" are present? how is dichotomous division to keep these sets distinct; or if they are not kept distinct, what shall we say to the chaos which must arise after many fissions, when each set in conflict with the others strives to produce its particular structure? and how are the conquering determinants to find they ways out of the _mêlée_ to the places where they are to fulfil their organizing functions? even were they all intelligent beings and each had a map by which to guide his movements, the problem would be sufficiently puzzling. can we assume it to be solved by unconscious units? thus even had professor weismann shown that the special structures of the different individuals in an insect-community are not due to differences in the nurtures they receive, which he has failed to do, he would still be met by this difficulty in the way of his own view, in addition to the three other insuperable difficulties grouped together in a preceding section. * * * * * the collateral issue, which has occupied the largest space in the controversy, has, as commonly happens, begotten a second generation of collateral issues. some of these are embodied in the form of questions put to me, which i must here answer, lest it should be supposed that they are unanswerable and my view therefore untenable. in the notes he appends to his romanes lecture, professor weismann writes:-- "one of the questions put to spencer by ball is quite sufficient to show the utter weakness of the position of lamarckism:--if their characteristics did not arise among the workers themselves, but were transmitted from the pre-social time, how does it happen that the queens and drones of every generation can give anew to the workers the characteristics which they themselves have long ago lost?" (p. ). it is curious to see put forward in so triumphant a manner, by a professed naturalist, a question so easily disposed of. i answer it by putting another. how does it happen that among those moths of which the female has but rudimentary wings, she continues to endow the males of her species with wings? how does it happen, for example, that among the _geometridæ_, the peculiar structures and habits of which show that they have all descended from a common ancestor, some species have winged females and some wingless females; and that though they have lost the wings the ancestral females had, these wingless females convey to the males the normal developments of wings? or, still better, how is it that in the _psychidæ_ there are apterous worm-like females, which lay eggs that bring forth winged males of the ordinary imago form? if for males we read workers, the case is parallel to the cases of those social insects, the queens of which bequeath characteristics they have themselves lost. the ordinary facts of embryonic evolution yield us analogies. what is the most common trait in the development of the sexes? when the sexual organs of either become pronounced, the incipient ancillary organs belonging to the opposite sex cease to develop and remain rudiments, while the organs special to the sex, essential and nonessential, become fully developed. what, then, must happen with the queen-ant, which, through countless generations, has ceased to use certain structures and has lost them from disuse? if one of the eggs which she lays, capable, as professor weismann admits, of becoming queen, male, or worker of one or other kind, does not at a certain stage begin actively to develop its reproductive system, then those organs of the ancestral or pre-social type which the queen has lost begin to develop, and a worker results. another difficulty in the way of my view, supposed to be fatal, is that presented by the honey-ants--aberrant members of certain ant-colonies which develop so enormously the pouch into which the food is drawn, that the abdomen becomes little else than a great bladder out of which the head, thorax, and legs protrude. this, it is thought, cannot be accounted for otherwise than as a consequence of specially endowed eggs, which it has become profitable to the community for the queen to produce. but the explanation fits in quite easily with the view i have set forth. no one will deny that the taking in of food is the deepest of vital requirements, and the correlative instinct a dominant one; nor will any one deny that the instinct of feeding young is less deeply seated--comes later in order of time. so, too, every one will admit that the worker-bee or worker-ant before regurgitating food into the mouth of a larva must first of all take it in. hence, alike in order of time and necessity, it is to be assumed that development of the nervous structures which guide self-nutrition, precedes development of the nervous structures which guide the feeding of larvæ. what, then, will in some cases happen, supposing there is an arrested development consequent on innutrition? it will in some cases happen that while the nervous centres prompting and regulating deglutition are fully formed, the formation of those prompting and regulating the regurgitation of the food into the mouths of larvæ are arrested. what will be the consequence? the life of the worker is mainly passed in taking in food and putting it out again. if the putting out is stopped its life will be mainly passed in taking in food. the receptacle will go on enlarging and it will eventually assume the monstrous form that we see.[ ] here, however, to exclude misinterpretations, let me explain. i by no means deny that variation and selection have produced, in these insect-communities, certain effects such as mr. darwin suggested. doubtless ant-queens vary; doubtless there are variations in their eggs; doubtless differences of structure in the resulting progeny sometimes prove advantageous to the stirp, and originate slight modifications of the species. but such changes, legitimately to be assumed, are changes in single parts--in single organs or portions of organs. admission of this does not involve admission that there can take place numerous correlated variations in different and often remote parts, which must take place simultaneously or else be useless. assumption of this is what professor weismann's argument requires, and assumption of this we have seen to be absurd. before leaving the general problem presented by the social insects, let me remark that the various complexities of action not explained by inheritance from pre-social or semi-social types, are probably due to accumulated and transmitted knowledge. i recently read an account of the education of a butterfly, carried to the extent that it became quite friendly with its protector and would come to be fed. if a non-social and relatively unintelligent insect is capable of thus far consciously adjusting its actions, then it seems a reasonable supposition that in a community of social insects there has arisen a mass of experience and usage into which each new individual is initiated; just as happens among ourselves. we have only to consider the chaos which would result were we suddenly bereft of language, and if the young were left to grow up without precept and example, to see that very probably the polity of an insect community is made possible by the addition of intelligence to instinct, and the transmission of information through sign-language. * * * * * there remains now the question of _panmixia_, which stands exactly where it did when i published the "rejoinder to professor weismann." after showing that the interpretation i put upon his view was justified by certain passages quoted; and after pointing out that one of his adherents had set forth the view which i combated--if not as his view yet as supplementary to it; i went on to criticize the view as set forth afresh by professor weismann himself. i showed that as thus set forth the actuality of the supposed cause of decrease in disused organs, implies that _minus_ variations habitually exceed _plus_ variations--in degree or in number, or in both. unless it can be proved that such an excess ordinarily occurs, the hypothesis of _panmixia_ has no place; and i asked, where is the proof that it occurs. no reply. not content with this abstract form of the question i put it also in a concrete form, and granted for the nonce professor weismann's assumption: taking the case of the rudimentary hind limbs of the whale. i said that though, during those early stages of decrease in which the disused limbs were external, natural selection probably had a share in decreasing them, since they were then impediments to locomotion, yet when they became internal, and especially when they had dwindled to nothing but remnants of the femurs, it is impossible to suppose that natural selection played any part: no whale could have survived and initiated a more prosperous stirp in virtue of the economy achieved by such a decrease. the operation of natural selection being out of the question, i inquired whether such a decrease, say of one-half when the femurs weighed a few ounces, occurring in one individual, could be supposed in the ordinary course of reproduction to affect the whole of the whale species inhabiting the arctic seas and the north atlantic ocean; and so on with successive diminutions until the rudiments had reached their present minuteness. i asked whether such an interpretation could be rationally entertained. no reply. now in the absence of replies to these two questions it seems to me that the verdict must go against professor weismann by default. if he has to surrender the hypothesis of _panmixia_, what results? all that evidence collected by mr. darwin and others, regarded by them as proof of the inheritance of acquired characters, which was cavalierly set aside on the strength of this alleged process of panmixia, is reinstated. and this reinstated evidence, joined with much evidence since furnished, suffices to establish the repudiated interpretation. in the printed report of his romanes lecture, after fifty pages of complicated speculations which we are expected to accept as proofs, professor weismann ends by saying, in reference to the case of the neuter insects:-- "this case is of additional interest, as it may serve to convince those naturalists who are still inclined to maintain that acquired characters are inherited, and to support the lamarckian principle of development, that their view cannot be the right one. it has not proved tenable in a single instance" (p. ). most readers of the foregoing pages will think that since professor weismann has left one after another of my chief theses without reply, this is rather a strong assertion; and they will still further raise their eyebrows on remembering that, as i have shown, where he has given answers his answers are invalid. * * * * * and now we come to the additions which i indicated at the outset as having to be made--certain evidences which have come to light since this controversy commenced. when, by a remembered observation made in boyhood, joined with the familiar fact that worker-larvæ can be changed into the larvæ of queens by feeding, i was led to suggest that probably all the variations of form in the social insects are consequent on differences of nurture, i was unaware that observations and experiments were being made which have justified this suggestion. professor grassi has recently published accounts of the food-habits of two european species of termites, shewing that the various forms are due to feeding. he is known to be a most careful observer, and some of the most curious of his facts are confirmed by the collection of white ants exhibited by dr. david sharp, f.r.s., at the _soirée_ of the royal society in may last. he has favoured me with the following account of grassi's results, which i publish with his assent:-- "there is great variety as to the constituents of the community and economy of the species in white ants. one of the simplest conditions known is that studied by grassi in the case of the european species calotermes flavicollis. in this species there is no worker caste; the adult forms are only of two kinds, viz., soldiers, and the males and females; the sexes are externally almost indistinguishable, and there are males and females of soldiers as well as of the winged forms, though the sexual organs do not undergo their full development in any soldier whether male or female. "the soldier is not however a mere instance of simple arrested development. it is true that there is in it arrested development of the sexual organs, but this is accompanied by change of form of other parts--changes so extreme that one would hardly suppose the soldier to have any connection with either the young or the adult of the winged forms. "now according to grassi the whole of the individuals when born are undifferentiated forms (except as to sex), and each one is capable of going on the natural course of development and thus becoming a winged insect, or can be deviated from this course and made into a soldier; this is accomplished by the white ants by special courses of feeding. "the evidence given by grassi is not conclusive as to the young being all born alike; and it may be that there are some individuals born that could not be deviated from the natural course and made into soldiers. but there is one case which seems to show positively that the deviation grassi believes to occur is real, and not due to the selection by the ants of an individual that though appearing to our eyes undifferentiated is not really so. this is that an individual can be made into a soldier after it has visibly undergone one half or more of the development into a winged form. the termites can in fact operate on an individual that has already acquired the rudiments of wings and whose head is totally destitute of any appearance of the shape of the armature peculiar to the soldier, and can turn it into a soldier; the rudiments of the wings being in such a case nearly entirely re-absorbed." grassi has been for many years engaged in investigating these phenomena, and there is no reason for rejecting his statement. we can scarcely avoid accepting it, and if so, professor weismann's hypothesis is conclusively disposed of. were there different sets of "determinants" for the soldier-form and for the winged sexual form, those "determinants" which had gone a long way towards producing the winged sexual form, would inevitably go on to complete that form, and could not have their proclivity changed by feeding. [yet more evidence to the like effect has since become known. at the meeting of the entomological society, on march , (reported in _nature_, march ):-- "dr. d. sharp, f.r.s., exhibited a collection of white ants (_termites_), formed by mr. g. d. haviland in singapore, which comprised about twelve species, of most of which the various forms were obtained. he said that prof. grassi had recently made observations on the european species, and had brought to light some important particulars; and also that in the discussion that had recently been carried on between mr. herbert spencer and prof. weismann, the former had stated that in his opinion the different forms of social insects were produced by nutrition. prof. grassi's observations showed this view to be correct, and the specimens now exhibited confirmed one of the most important points in his observations. dr. sharp also stated that mr. haviland found in one nest eleven neoteinic queens--that is to say, individuals having the appearance of the queen in some respects, while in others they are still immature." another similarly conclusive verification i published in _nature_ for december , , under the title "the origin of classes among the 'parasol' ants." the letter ran as follows:-- "mr. j. h. hart is superintendent of the royal botanic gardens in trinidad. he has sent me a copy of his report presented to the legislative council in march, , and has drawn my attention to certain facts contained in it concerning the 'parasol' ants--the leaf-cutting ants which feed on the fungi developed in masses of the cut leaves carried to their nests. both mr. bates and mr. belt described these ants, but described, it seems, different, though nearly allied, species, the habits of which are partially unlike. as they are garden-pests, mr. hart was led to examine into the development and social arrangements of these ants; establishing, to that end, artificial nests, after the manner adopted by sir john lubbock. several of the facts set down have an important bearing on a question now under discussion. the following extracts, in which they are named, i abridge by omitting passages not relevant to the issue:-- "'the history of my nests is as follows: nos. and were both taken (august ) on the same day, while destroying nests in the gardens, and were portions of separate nests but of the same species. no. was procured on september , and is evidently a different although an allied species to nos. and . "'finding neither of my nests had a queen, i procured one from another nest about to be destroyed, and placed it with no. nest. it was received by the workers, and at once attended by a numerous retinue in royal style. on august i removed the queen from no. and placed it with no. , when it was again received in a most loyal manner.... "'ants taken from nos. and and placed with no. were immediately destroyed by the latter, and even the soldiers of no. , as well as workers or nurses, were destroyed when placed with nos. and . "'in nest no. , from which i removed the queen on august , there are now in the pupa stage several queens and several males. the forms of ant in nests nos. and are as follows: (_a_) queen, (_b_) male (both winged, but the queen loses its wings after marital flight), (_c_) large workers, (_d_) small workers, and (_e_) nurses. in nest no. i have not yet seen the queen or male, but it possesses--(_a_) soldier, (_b_) larger workers, (_c_) smaller workers, and (_d_) nurses; but these are different in form to those of nests no. and no. . probably we might add a third form of worker, as there are several sizes in the nest.... "'it is curious that in no. nest, from which the queen was removed on august , new queens and males are now being developed, while in no. nest, where the queen is at present, nothing but workers have been brought out, and if a queen larva or pupa is placed there it is at once destroyed, while worker larvæ or pupæ are amicably received. in no. all the eggs, larvæ, and pupæ collected with the nest have been hatched, and no eggs have since made their appearance to date. there is no queen with this nest.... on november i attempted to prove by experiment how small a number of "parasol" ants it required to form a new colony. i placed two dozen of ants (one dozen workers and one dozen nurses) in two separate nests, no. and no. . with no. i placed a few larvæ with a few rose petals for them to manipulate. with no. i gave a small piece of nest covered with mycelium. on the th these nests were destroyed by small foraging ants, known as the "sugar" or "meat" ant, and i had to remove them and replace with a new colony. my notes on these are not sufficiently lengthy to be of much importance. but i noted four eggs laid on the th, or two days after being placed in their new quarters; no queen being present. the experiment is being continued. i may mention that in no. nest, in which no fungus was present, the larvæ of all sizes appeared to change into the pupæ stage at once for want of food [a fact corresponding with the fact i have named as observed by myself sixty years ago in the case of wasp larvæ]. the circumstance tends to show that the development of the insect is influenced entirely by the feeding it gets in the larva stage. "'in nest no. before the introduction of a queen there were no eggs or larvæ. the first worker was hatched on october , or fifty-seven days afterwards, and a continual succession has since been maintained, but as yet (november ) no males or queens have made their appearance.' "in a letter accompanying the report, mr. hart says:-- "'since these were published, my notes go to prove that ants can practically manufacture at will, male, female, soldier, worker, or nurse. some of the workers are capable of laying eggs, and from these can be produced all the various forms as well as from a queen's egg. "'there does not, however, appear to be any difference in the character of the food; as i cannot find that the larger larvæ are fed with anything different to that given to the smaller.' "these results were obtained before the recent discussion of the question commenced, and joined with the other evidence entirely dispose of those arguments which prof. weismann bases on facts furnished by the social insects."] the other piece of additional evidence i have referred to, is furnished by two papers contributed to _the journal of anatomy and physiology_ for october and april , by r. havelock charles, m. d., &c. &c., professor of anatomy in the medical college, lahore. these papers set forth the differences between the leg-bones of europeans and those of the punjaub people--differences caused by their respective habits of sitting in chairs and squatting on the ground. he enumerates more than twenty such differences, chiefly in the structures of the knee-joint and ankle-joint. from the _résumé_ of his second paper i quote the following passages, which sufficiently show the data and the inferences:-- " . the habits as to sitting postures of europeans differ from those of their prehistoric ancestors, the cave-dwellers, &c., who probably squatted on the ground. " . the sitting postures of orientals are the same now as ever. they have retained the habits of their ancestors. the europeans have not done so. " . want of use would induce changes in form and size, and so, gradually, small differences would be integrated till there would be total disappearance of the markings on the european skeleton, as no advantage would accrue to him from the possession of facets on his bones fitting them for postures not practised by him. " . the facets seen on the bones of the panjabi infant or foetus have been transmitted to it by the accumulation of peculiarities gained by habit in the evolution of its racial type--in which an acquisition having become a permanent possession, 'profitable to the individual under its conditions of life,' is transmitted as a useful inheritance. " . these markings are due to the influence of certain positions, which are brought about by the use of groups of muscles, and they are the definite results produced by actions of these muscles. " . the effects of the use of the muscles mentioned in no. are transmitted to the offspring, for the markings are present in the _foetus-in-utero_, in the child at birth, and in the infant. " . the markings are instances of the transmission of acquired characters, which heritage in the individual, function subsequently develops." no other conclusion appears to me possible. _panmixia_, even were it not invalidated by its unwarranted assumption as above shown, would be out of court: the case is not a case of either increase or decrease of size but of numerous changes of form. simultaneous variation of co-operative parts cannot be alleged, since these co-operative parts have not changed in one way but in various ways and degrees. and even were it permissible to suppose that the required different variations had taken place simultaneously, natural selection cannot be supposed to have operated. the assumption would imply that in the struggle for existence, individuals of the european races who were less capable than others of crouching and squatting, gained by those minute changes of structure which incapacitated them, such advantages that their stirps prevailed over other stirps--an absurd supposition. and now i must once more point out that a grave responsibility rests on biologists in respect of the general question; since wrong answers lead, among other effects, to wrong beliefs about social affairs and to disastrous social actions. in me this conviction has unceasingly strengthened. though _the origin of species_ proved to me that the transmission of acquired characters cannot be the sole factor in organic evolution, as i had assumed in _social statics_ and in _the principles of biology_, published in pre-darwinian days, yet i have never wavered in the belief that it is a factor and an all-important factor. and i have felt more and more that since all the higher sciences are dependent on the science of life, and must have their conclusions vitiated if a fundamental datum given to them by the teachers of this science is erroneous, it behoves these teachers not to let an erroneous datum pass current: they are called on to settle this vexed question one way or other. the times give proof. the work of mr. benjamin kidd on _social evolution_, which has been so much lauded, takes weismannism as one of its data; and if weismannism be untrue, the conclusions mr. kidd draws must be in large measure erroneous and may prove mischievous. postscript.--since the foregoing pages have been put in type there has appeared in _natural science_ for september, an abstract of certain parts of a pamphlet by professor oscar hertwig, setting forth facts directly bearing on professor weismann's doctrine respecting the distinction between reproductive cells and somatic cells. in _the principles of biology_, § , i contended that reproductive cells differ from other cells composing the organism, only in being unspecialized. and in support of the hypothesis that tissue-cells in general have a reproductive potentiality, i instanced the cases of the _begonia phyllomaniaca_ and _malaxis paludosa_. in the thirty years which have since elapsed, many facts of like significance have been brought to light, and various of these are given by professor hertwig. here are some of them:-- "galls are produced under the stimulus of the insect almost anywhere on the surface of a plant. yet in most cases these galls, in a sense grown at random on the surface of a plant, when placed in damp earth will give rise to a young plant. in the hydroid _tubularia mesembryanthemum_, when the polyp heads are cut off, new heads arise. but if both head and root be cut off, and the upper end be inserted in the mud, then from the original upper end not head-polyps but root filaments will arise, while from the original lower end not root filaments but head-polyps will grow.... driesch, by separating the first two and the first four segmentation spheres of an _echinus_ ovum, obtained two or four normal plutei, respectively one half and a quarter of the normal size.... so, also, in the case of _amphioxus_, wilson obtained a normal, but proportionately diminished embryo with complete nervous system from a separated sphere of a two- or four- or eight celled stage.... chabry obtained normal embryos in cases where some of the segmentation-spheres had been artificially destroyed." these evidences, furnished by independent observers, unite in showing, firstly, that all the multiplying cells of the developing embryo are alike; and, secondly, that the soma-cells of the adult severally retain, in a latent form, all the powers of the original embryo-cell. if these facts do not disprove absolutely professor weismann's hypothesis, we may wonderingly ask what facts would disprove it? since hertwig holds that all the cells forming an organism of any species primarily consist of the same components, i at first thought that his hypothesis was identical with my own hypothesis of "physiological units," or, as i would now call them, constitutional units. it seems otherwise, however; for he thinks that each cell contains "only those material particles which are bearers of cell-properties," and that organs "are the functions of cell-complexes." to this it may be replied that the ability to form the appropriate cell-complexes, itself depends upon the constitutional units contained in the cells. appendix c. the inheritance of functionally-wrought modifications: a summary. the assertion that changes of structure caused by changes of function are transmitted to descendants is continually met by the question--where is the evidence? when some facts are assigned in proof, they are pooh-poohed as insufficient. if after a time the question is raised afresh and other facts are named, there is a like supercilious treatment of them. successively rejected in this way, the evidences do not accumulate in the minds of opponents; and hence produce little or no effect. when they are brought together, however, it turns out that they are numerous and weighty. we will group them into negative and positive. * * * * * negative evidence is furnished by those cases in which traits otherwise inexplicable are explained if the structural effects of use and disuse are transmitted. in the foregoing chapters and appendices three have been given. ( ) co-adaptation of co-operative parts comes first. this has been exemplified by the case of enlarged horns in a stag, by the case of an animal led into the habit of leaping, and in the case of the giraffe (cited in "the factors of organic evolution"); and it has been shown that the implied co-adaptations of parts cannot possibly have been effected by natural selection. ( ) the possession of unlike powers of discrimination by different parts of the human skin, was named as a problem to be solved on the hypothesis of natural selection or the hypothesis of panmixia; and it was shown that neither of these can by any twisting yield a solution. but the facts harmonize with the hypothesis that the effects of use are inherited. ( ) then come the cases of those rudimentary organs which, like the hind limbs of the whale, have nearly disappeared. dwindling by natural selection is here out of the question; and dwindling by panmixia, even were its assumptions valid, would be incredible. but as a sequence of disuse the change is clearly explained. failure to solve any _one_ of these three problems would, i think, alone prove the neo-darwinian doctrines untenable; and the fact that we have _three_ unsolved problems seems to me fatal. * * * * * from this negative evidence, turn now to the positive evidence. this falls into several groups. there are first the facts collected by mr. darwin, implying functionally-altered structures in domestic animals. the hypothesis of panmixia is, as we have seen, out of court; and therefore mr. darwin's groups of evidences are reinstated. there is the changed ratio of wing-bones and leg-bones in the duck; there are the drooping ears of cats in china, of horses in russia, of sheep in italy, of guinea-pigs in germany, of goats and cattle in india, of rabbits, pigs, and dogs in all long-civilized countries. though artificial selection has come into play where drooping has become a curious trait (as in rabbits), and has probably caused the greater size of ears which has in some cases gone along with diminished muscular power over them; yet it could not have been the initiator, and has not been operative on animals bred for profit. again there are the changes produced by climate; as instance, among plants, the several varieties of maize established in germany and transformed in the course of a few generations. facts of another class are yielded by the blind inhabitants of caverns. one who studies the memoir by mr. packard on _the cave fauna of north america_, &c., will be astonished at the variety of types in which degeneration or loss of the eyes has become a concomitant of life passed in darkness. a great increase in the force of this evidence will be recognized on learning that absence or extreme imperfection of visual organs is found also in creatures living in perpetual night at the bottoms of deep oceans. endeavours to account for these facts otherwise than by the effects of disuse we have seen to be futile. kindred evidence is yielded by decrease of the jaws in those races which have had diminished use of them--mankind and certain domestic animals. relative smallness in the jaws of civilized men, manifest enough on comparison, has been proved by direct measurement. in pet dogs--pugs, household spaniels--we find associated the same cause with the same effect. though there has been artificial selection, yet this did not operate until the diminution had become manifest. moreover there has been diminution of the other structures concerned in biting: there are smaller muscles, feeble zygomata, and diminished areas for insertion of muscles--traits which cannot have resulted from selection, since they are invisible in the living animal. in abnormal vision produced by abnormal use of the eyes we have evidence of another kind. that the germans, among whom congenital short sight is notoriously prevalent, have been made shortsighted by inheritance of modifications due to continual reading of print requiring close attention, is by some disputed. it is strange, however, that if there exists no causal connexion between them, neither trait occurs without the other elsewhere. but for the belief that there is a causal connexion we have the verifying testimony of oculists. from dr. lindsay johnson i have cited cases within his professional experience of functionally-produced myopia transmitted to children; and he asserts that other oculists have had like experiences. development of the musical faculty in the successive members of families from which the great composers have come, as well as in the civilized races at large, is not to be explained by natural selection. even when it is great, the musical faculty has not a life-saving efficiency as compared with the average of faculties; for the most highly gifted have commonly passed less prosperous lives and left fewer offspring than have those possessed of ordinary abilities. still less can it be said that the musical faculty in mankind at large has been developed by survival of the fittest. no one will assert that men in general have been enabled to survive and propagate in proportion as their musical appreciation was great. the transmission of nervous peculiarities functionally produced is alleged by the highest authorities--dr. savage, president of the neurological society, and dr. hughlings jackson. the evidence they assign confirms, and is confirmed by, that which the development of the musical faculty above named supplies. here, then, we have sundry groups of facts directly supporting the belief that functionally-wrought modifications descend from parents to offspring. * * * * * now let us consider the position of those darwinians who dissent from darwin, and who make light of all this evidence. we might naturally suppose that their own hypothesis is unassailable. yet, strange to say, they admit that there is no direct proof that any species has been established by natural selection. the proof is inferential only. the certainty of an axiom does not give certainty to the deductions drawn from it. that natural selection is, and always has been, operative is incontestable. obviously i should be the last person to deny that survival of the fittest is a necessity: its negation is inconceivable. the neo-darwinians, however, judging from their attitude, apparently assume that firmness of the basis implies firmness of the superstructure. but however high may be the probability of some of the conclusions drawn, none of them can have more than probability; while some of them remain, and are likely to remain, very questionable. observe the difficulties. ( ) the general argument proceeds upon the analogy between natural selection and artificial selection. yet all know that the first cannot do what the last does. natural selection can do nothing more than preserve those of which the _aggregate_ characters are most favourable to life. it cannot pick out those possessed of one particular favourable character, unless this is of extreme importance. ( ) in many cases a structure is of no service until it has reached a certain development; and it remains to account for that increase of it by natural selection which must be supposed to take place before it reaches the stage of usefulness. ( ) advantageous variations, not preserved in nature as they are by the breeder, are liable to be swamped by crossing or to disappear by atavism. now whatever replies are made, their component propositions cannot be necessary truths. so that the conclusion in each case, however reasonable, cannot claim certainty: the fabric can have no stability like that of its foundation. when to uncertainties in the arguments supporting the hypothesis we add its inability to explain facts of cardinal significance, as proved above, there is i think ground for asserting that natural selection is less clearly shown to be a factor in the origination of species than is the inheritance of functionally-wrought changes. * * * * * if, finally, it is said that the mode in which functionally-wrought changes, especially in small parts, so affect the reproductive elements as to repeat themselves in offspring, cannot be imagined--if it be held inconceivable that those minute changes in the organs of vision which cause myopia can be transmitted through the appropriately-modified sperm-cells or germ-cells; then the reply is that the opposed hypothesis presents a corresponding inconceivability. grant that the habit of a pointer was produced by selection of those in which an appropriate variation in the nervous system had occurred; it is impossible to imagine how a slightly-different arrangement of a few nerve-cells and fibres could be conveyed by a spermatozoon. so too it is impossible to imagine how in a spermatozoon there can be conveyed the , independent variables required for the construction of a single peacock's feather, each having a proclivity towards its proper place. clearly the ultimate process by which inheritance is effected in either case passes comprehension; and in this respect neither hypothesis has an advantage over the other. appendix d. on alleged "spontaneous generation," and on the hypothesis of physiological units. [_the following letter, originally written for publication in the_ north american review, _but declined by the editor in pursuance of a general rule, and eventually otherwise published in the united states, i have thought well to append to this first volume of the_ principles of biology. _i do this because the questions which it discusses are dealt with in this volume; and because the further explanations it furnishes seem needful to prevent misapprehensions._] _the editor of the north american review._ sir, it is in most cases unwise to notice adverse criticisms. either they do not admit of answers or the answers may be left to the penetration of readers. when, however, a critic's allegations touch the fundamental propositions of a book, and especially when they appear in a periodical having the position of the _north american review_, the case is altered. for these reasons the article on "philosophical biology," published in your last number, demands from me an attention which ordinary criticisms do not. it is the more needful for me to notice it, because its two leading objections have the one an actual fairness and the other an apparent fairness; and in the absence of explanations from me, they will be considered as substantiated even by many, or perhaps most, of those who have read the work itself--much more by those who have not read it. that to prevent the spread of misapprehensions i ought to say something, is further shown by the fact that the same two objections have already been made in england--the one by dr. child, of oxford, in his _essays on physiological subjects_, and the other by a writer in the _westminster review_ for july, . * * * * * in the note to which your reviewer refers, i have, as he says, tacitly repudiated the belief in "spontaneous generation;" and that i have done this in such a way as to leave open the door for the interpretation given by him is true. indeed the fact that dr. child, whose criticism is a sympathetic one, puts the same construction on this note, proves that your reviewer has but drawn what seems to be a necessary inference. nevertheless, the inference is one which i did not intend to be drawn. in explanation, let me at the outset remark that i am placed at a disadvantage in having had to omit that part of the system of philosophy which deals with inorganic evolution. in the original programme will be found a parenthetic reference to this omitted part, which should, as there stated, precede the _principles of biology_. two volumes are missing. the closing chapter of the second, were it written, would deal with the evolution of organic matter--the step preceding the evolution of living forms. habitually carrying with me in thought the contents of this unwritten chapter, i have, in some cases, expressed myself as though the reader had it before him; and have thus rendered some of my statements liable to misconstructions. apart from this, however, the explanation of the apparent inconsistency is very simple, if not very obvious. in the first place, i do not believe in the "spontaneous generation" commonly alleged, and referred to in the note; and so little have i associated in thought this alleged "spontaneous generation" which i disbelieve, with the generation by evolution which i do believe, that the repudiation of the one never occurred to me as liable to be taken for repudiation of the other. that creatures having _quite specific structures_ are evolved in the course of a few hours, without antecedents calculated to determine their specific forms, is to me incredible. not only the established truths of biology, but the established truths of science in general, negative the supposition that organisms having structures definite enough to identify them as belonging to known genera and species, can be produced in the absence of germs derived from antecedent organisms of the same genera and species. if there can suddenly be imposed on simple protoplasm the organization which constitutes it a _paramoecium_, i see no reason why animals of greater complexity, or indeed of any complexity, may not be constituted after the same manner. in brief, i do not accept these alleged facts as exemplifying evolution, because they imply something immensely beyond that which evolution, as i understand it, can achieve. in the second place, my disbelief extends not only to the alleged cases of "spontaneous generation," but to every case akin to them. the very conception of spontaneity is wholly incongruous with the conception of evolution. for this reason i regard as objectionable mr. darwin's phrase "spontaneous variation" (as indeed he does himself); and i have sought to show that there are always assignable causes of variation. no form of evolution, inorganic or organic, can be spontaneous; but in every instance the antecedent forces must be adequate in their quantities, kinds, and distributions, to work the observed effects. neither the alleged cases of "spontaneous generation," nor any imaginable cases in the least allied to them, fulfil this requirement. if, accepting these alleged cases of "spontaneous generation," i had assumed, as your reviewer seems to do, that the evolution of organic life commenced in an analogous way; then, indeed, i should have left myself open to a fatal criticism. this supposed "spontaneous generation" habitually occurs in menstrua that contain either organic matter, or matter originally derived from organisms; and such organic matter, proceeding in all known cases from organisms of a higher kind, implies the pre-existence of such higher organisms. by what kind of logic, then, is it inferrible that organic life was initiated after a manner like that in which _infusoria_ are said to be now spontaneously generated? where, before life commenced, were the superior organisms from which these lowest organisms obtained their organic matter? without doubting that there are those who, as the reviewer says, "can penetrate deeper than mr. spencer has done into the idea of universal evolution," and who, as he contends, prove this by accepting the doctrine of "spontaneous generation"; i nevertheless think that i can penetrate deep enough to see that a tenable hypothesis respecting the origin of organic life must be reached by some other clue than that furnished by experiments on decoction of hay and extract of beef. from what i do not believe, let me now pass to what i do believe. granting that the formation of organic matter, and the evolution of life in its lowest forms, may go on under existing cosmical conditions; but believing it more likely that the formation of such matter and such forms, took place at a time when the heat of the earth's surface was falling through those ranges of temperature at which the higher organic compounds are unstable; i conceive that the moulding of such organic matter into the simplest types, must have commenced with portions of protoplasm more minute, more indefinite, and more inconstant in their characters, than the lowest rhizopods--less distinguishable from a mere fragment of albumen than even the _protogenes_ of professor haeckel. the evolution of specific shapes must, like all other organic evolution, have resulted from the actions and reactions between such incipient types and their environments, and the continued survival of those which happened to have specialities best fitted to the specialities of their environments. to reach by this process the comparatively well-specialized forms of ordinary _infusoria_, must, i conceive, have taken an enormous period of time. to prevent, as far as may be, future misapprehension, let me elaborate this conception so as to meet the particular objections raised. the reviewer takes for granted that a "first organism" must be assumed by me, as it is by himself. but the conception of a "first organism," in anything like the current sense of the words, is wholly at variance with conception of evolution; and scarcely less at variance with the facts revealed by the microscope. the lowest living things are not properly speaking organisms at all; for they have no distinctions of parts--no traces of organization. it is almost a misuse of language to call them "forms" of life: not only are their outlines, when distinguishable, too unspecific for description, but they change from moment to moment and are never twice alike, either in two individuals or in the same individual. even the word "type" is applicable in but a loose way; for there is little constancy in their generic characters: according as the surrounding conditions determine, they undergo transformations now of one kind and now of another. and the vagueness, the inconstancy, the want of appreciable structure, displayed by the simplest of living things as we now see them, are characters (or absences of characters) which, on the hypothesis of evolution, must have been still more decided when, as at first, no "forms," no "types," no "specific shapes," had been moulded. that "absolute commencement of organic life on the globe," which the reviewer says i "cannot evade the admission of," i distinctly deny. the affirmation of universal evolution is in itself the negation of an "absolute commencement" of anything. construed in terms of evolution, every kind of being is conceived as a product of modifications wrought by insensible gradations on a pre-existing kind of being; and this holds as fully of the supposed "commencement of organic life" as of all subsequent developments of organic life. it is no more needful to suppose an "absolute commencement of organic life" or a "first organism," than it is needful to suppose an absolute commencement of social life and a first social organism. the assumption of such a necessity in this last case, made by early speculators with their theories of "social contracts" and the like, is disproved by the facts; and the facts, so far as they are ascertained, disprove the assumption of such a necessity in the first case. that organic matter was not produced all at once, but was reached through steps, we are well warranted in believing by the experiences of chemists. organic matters are produced in the laboratory by what we may literally call _artificial evolution_. chemists find themselves unable to form these complex combinations directly from their elements; but they succeed in forming them indirectly, by successive modifications of simpler combinations. in some binary compound, one element of which is present in several equivalents, a change is made by substituting for one of these equivalents an equivalent of some other element; so producing a ternary compound. then another of the equivalents is replaced, and so on. for instance, beginning with ammonia, n h_{ }, a higher form is obtained by replacing one of the atoms of hydrogen by an atom of methyl, so producing methyl-amine, n (c h_{ } h_{ }); and then, under the further action of methyl, ending in a further substitution, there is reached the still more compound substance dimethyl-amine, n (c h_{ }) (c h_{ }) h. and in this manner highly complex substances are eventually built up. another characteristic of their method is no less significant. two complex compounds are employed to generate, by their action upon one another, a compound of still greater complexity: different heterogeneous molecules of one stage, become parents of a molecule a stage higher in heterogeneity. thus, having built up acetic acid out of its elements, and having by the process of substitution described above, changed the acetic acid into propionic acid, and propionic into butyric, of which the formula is {c(ch_{ })(ch_{ })h} {co(ho) }; this complex compound, by operating on another complex compound, such as the dimethyl-amine named above, generates one of still greater complexity, butyrate of dimethyl-amine {c(ch)(ch_{ })h} n(ch_{ })(ch_{ })h. {co(ho) } see, then, the remarkable parallelism. the progress towards higher types of organic molecules is effected by modifications upon modifications; as throughout evolution in general. each of these modifications is a change of the molecule into equilibrium with its environment--an adaptation, as it were, to new surrounding conditions to which it is subjected; as throughout evolution in general. larger, or more integrated, aggregates (for compound molecules are such) are successively generated; as throughout evolution in general. more complex or heterogeneous aggregates are so made to arise, one out of another; as throughout evolution in general. a geometrically-increasing multitude of these larger and more complex aggregates so produced, at the same time results; as throughout evolution in general. and it is by the action of the successively higher forms on one another, joined with the action of environing conditions, that the highest forms are reached; as throughout evolution in general. when we thus see the identity of method at the two extremes--when we see that the general laws of evolution, as they are exemplified in known organisms, have been unconsciously conformed to by chemists in the artificial evolution of organic matter; we can scarcely doubt that these laws were conformed to in the natural evolution of organic matter, and afterwards in the evolution of the simplest organic forms. in the early world, as in the modern laboratory, inferior types of organic substances, by their mutual actions under fit conditions, evolved the superior types of organic substances, ending in organizable protoplasm. and it can hardly be doubted that the shaping of organizable protoplasm, which is a substance modifiable in multitudinous ways with extreme facility, went on after the same manner. as i learn from one of our first chemists, prof. frankland, _protein_ is capable of existing under probably at least a thousand isomeric forms; and, as we shall presently see, it is capable of forming, with itself and other elements, substances yet more intricate in composition, that are practically infinite in their varieties of kind. exposed to those innumerable modifications of conditions which the earth's surface afforded, here in amount of light, there in amount of heat, and elsewhere in the mineral quality of its aqueous medium, this extremely changeable substance must have undergone now one, now another, of its countless metamorphoses. and to the mutual influences of its metamorphic forms under favouring conditions, we may ascribe the production of the still more composite, still more sensitive, still more variously-changeable portions of organic matter, which, in masses more minute and simpler than existing _protozoa_, displayed actions verging little by little into those called vital--actions which protein itself exhibits in a certain degree, and which the lowest known living things exhibit only in a greater degree. thus, setting out with inductions from the experiences of organic chemists at the one extreme, and with inductions from the observations of biologists at the other extreme, we are enabled deductively to bridge the interval--are enabled to conceive how organic compounds were evolved, and how, by a continuance of the process, the nascent life displayed in these became gradually more pronounced. and this it is which has to be explained, and which the alleged cases of "spontaneous generation" would not, were they substantiated, help us in the least to explain. it is thus manifest, i think, that i have not fallen into the alleged inconsistency. nevertheless, i admit that your reviewer was justified in inferring this inconsistency; and i take blame to myself for not having seen that the statement, as i have left it, is open to misconstruction. * * * * * i pass now to the second allegation--that in ascribing to certain specific molecules, which i have called "physiological units," the aptitude to build themselves into the structure of the organism to which they are peculiar, i have abandoned my own principle, and have assumed something beyond the re-distribution of matter and motion. as put by the reviewer, his case appears to be well made out; and that he is not altogether unwarranted in so putting it, may be admitted. nevertheless, there does not in reality exist the supposed incongruity. before attempting to make clear the adequacy of the conception which i am said to have tacitly abandoned as insufficient, let me remove that excess of improbability the reviewer gives to it, by the extremely-restricted meaning with which he uses the word mechanical. in discussing a proposition of mine he says:-- "he then cites certain remarks of mr. paget on the permanent effects wrought in the blood by the poison of scarlatina and small-pox, as justifying the belief that such a 'power' exists, and attributes the repair of a wasted tissue to 'forces analogous to those by which a crystal reproduces its lost apex.' (neither of which phenomena, however, is explicable by mechanical causes.)" were it not for the deliberation with which this last statement is made, i should take it for a slip of the pen. as it is, however, i have no course left but to suppose the reviewer unaware of the fact that molecular actions of all kinds are now not only conceived as mechanical actions, but that calculations based on this conception of them, bring out the results that correspond with observation. there is no kind of re-arrangement among molecules (crystallization being one) which the modern physicist does not think of. and correctly reason upon, in terms of forces and motions like those of sensible masses. polarity is regarded as a resultant of such forces and motions; and when, as happens in many cases, light changes the molecular structure of a crystal, and alters its polarity, it does this by impressing, in conformity with mechanical laws, new motions on the constituent molecules. that the reviewer should present the mechanical conception under so extremely limited a form, is the more surprising to me because, at the outset of the very work he reviews, i have, in various passages, based inferences on those immense extensions of it which he ignores; indicating, for example, the interpretation it yields of the inorganic chemical changes effected by heat, and the organic chemical changes effected by light (_principles of biology_, § ). premising, then, that the ordinary idea of mechanical action must be greatly expanded, let us enter upon the question at issue--the sufficiency of the hypothesis that the structure of each organism is determined by the polarities of the special molecules, or physiological units, peculiar to it as a species, which necessitate tendencies towards special arrangements. my proposition and the reviewer's criticism upon it, will be most conveniently presented if i quote in full a passage of his from which i have already extracted some expressions. he says:-- "it will be noticed, however, that mr. spencer attributes the possession of these 'tendencies,' or 'proclivities,' to natural inheritance from ancestral organisms; and it may be argued that he thus saves the mechanist theory and his own consistency at the same time, inasmuch as he derives even the 'tendencies' themselves ultimately from the environment. to this we reply, that mr. spencer, who advocates the nebular hypothesis, cannot evade the admission of an absolute commencement of organic life on the globe, and that the 'formative tendencies,' without which he cannot explain the evolution of a single individual, could not have been inherited by the first organism. besides, by his virtual denial of spontaneous generation, he denies that the first organism was evolved out of the inorganic world, and thus shuts himself off from the argument (otherwise plausible) that its 'tendencies' were ultimately derived from the environment." this assertion is already in great measure disposed of by what has been said above. holding that, though not "spontaneously generated," those minute portions of protoplasm which first displayed in the feeblest degree that changeability taken to imply life, were evolved, i am _not_ debarred from the argument that the "tendencies" of the physiological units are derived from the inherited effects of environing actions. if the conception of a "first organism" were a necessary one, the reviewer's objection would be valid. if there were an "absolute commencement" of life, a definite line parting organic matter from the simplest living forms, i should be placed in the predicament he describes. but as the doctrine of evolution itself tacitly negatives any such distinct separation; and as the negation is the more confirmed by the facts the more we know of them; i do not feel that i am entangled in the alleged difficulty. my reply might end here; but as the hypothesis in question is one not easily conceived, and very apt to be misunderstood, i will attempt a further elucidation of it. much evidence now conspires to show that molecules of the substances we call elementary are in reality compound; and that, by the combination of these with one another, and re-combinations of the products, there are formed systems of systems of molecules, unimaginable in their complexity. step by step as the aggregate molecules so resulting, grow larger and increase in heterogeneity, they become more unstable, more readily transformable by small forces, more capable of assuming various characters. those composing organic matter transcend all others in size and intricacy of structure; and in them these resulting traits reach their extreme. as implied by its name _protein_, the essential substance of which organisms are built, is remarkable alike for the variety of its metamorphoses and the facility with which it undergoes them: it changes from one to another of its thousand isomeric forms on the slightest change of conditions. now there are facts warranting the belief that though these multitudinous isomeric forms of protein will not unite directly with one another, yet they admit of being linked together by other elements with which they combine. and it is very significant that there are habitually present two other elements, sulphur and phosphorus, which have quite special powers of holding together many equivalents--the one being pentatomic and the other hexatomic. so that it is a legitimate supposition (justified by analogies) that an atom of sulphur may be a bond of union among half-a-dozen different isomeric forms of protein; and similarly with phosphorus. a moment's thought will show that, setting out with the thousand isomeric forms of protein, this makes possible a number of these combinations almost passing the power of figures to express. molecules so produced, perhaps exceeding in size and complexity those of protein as those of protein exceed those of inorganic matter, may, i conceive, be the special units belonging to special kinds of organisms. by their constitution they must have a plasticity, or sensitiveness to modifying forces, far beyond that of protein; and bearing in mind not only that their varieties are practically infinite in number, but that closely allied forms of them, chemically indifferent to one another as they must be, may coexist in the same aggregate, we shall see that they are fitted for entering into unlimited varieties of organic structures. the existence of such physiological units, peculiar to each species of organism, is not unaccounted for. they are evolved simultaneously with the evolution of the organisms they compose--they differentiate as fast as these organisms differentiate; and are made multitudinous in kind by the same actions which make the organism they compose multitudinous, in kind. this conception is clearly representable in terms of the mechanical hypothesis. every physicist will endorse the proposition that in each aggregate there tends to establish itself an equilibrium between the forces exercised by all the units upon each and by each upon all. even in masses of substance so rigid as iron and glass, there goes on a molecular re-arrangement, slow or rapid according as circumstances facilitate, which ends only when there is a complete balance between the actions of the parts on the whole and the actions of the whole on the parts: the implications being that every change in the form or size of the whole, necessitates some redistribution of the parts. and though in cases like these, there occurs only a polar re-arrangement of the molecules, without changes in the molecules themselves; yet where, as often happens, there is a passage from the colloid to the crystalloid state, a change of constitution occurs in the molecules themselves. these truths are not limited to inorganic matter: they unquestionably hold of organic matter. as certainly as molecules of alum have a form of equilibrium, the octahedron, into which they fall when the temperature of their solvent allows them to aggregate, so certainly must organic molecules of each kind, no matter how complex, have a form of equilibrium in which, when they aggregate, their complex forces are balanced--a form far less rigid and definite, for the reason that they have far less definite polarities, are far more unstable, and have their tendencies more easily modified by environing conditions. equally certain is it that the special molecules having a special organic structure as their form of equilibrium, must be reacted upon by the total forces of this organic structure; and that, if environing actions lead to any change in this organic structure, these special molecules, or physiological units, subject to a changed distribution of the total forces acting upon them will undergo modification--modification which their extreme plasticity will render easy. by this action and reaction i conceive the physiological units peculiar to each kind of organism, to have been moulded along with the organism itself. setting out with the stage in which protein in minute aggregates, took on those simplest differentiations which fitted it for differently-conditioned parts of its medium, there must have unceasingly gone on perpetual re-adjustments of balance between aggregates and their units--actions and reactions of the two, in which the units tended ever to establish the typical form produced by actions and reactions in all antecedent generations, while the aggregate, if changed in form by change of surrounding conditions, tended ever to impress on the units a corresponding change of polarity, causing them in the next generation to reproduce the changed form--their new form of equilibrium. this is the conception which i have sought to convey, though it seems unsuccessfully, in the _principles of biology_; and which i have there used to interpret the many involved and mysterious phenomena of genesis, heredity, and variation. in one respect only am i conscious of having so inadequately explained myself, as to give occasion for a misinterpretation--the one made by the _westminster_ reviewer above referred to. by him, as by your own critic, it is alleged that in the idea of "inherent tendencies" i have introduced, under a disguise, the conception of "the archæus, vital principle, _nisus formativus_, and so on." this allegation is in part answered by the foregoing explanation. that which i have here to add, and did not adequately explain in the _principles of biology_, is that the proclivity of units of each order towards the specific arrangement seen in the organism they form, is not to be understood as resulting from their own structures and actions only; but as the product of these and the environing forces to which they are exposed. organic evolution takes place only on condition that the masses of protoplasm formed of the physiological units, and of the assimilable materials out of which others like themselves are to be multiplied, are subject to heat of a given degree--are subject, that is, to the unceasing impacts of undulations of a certain strength and period; and, within limits, the rapidity with which the physiological units pass from their indefinite arrangement to the definite arrangement they presently assume, is proportionate to the strengths of the ethereal undulations falling upon them. in its complete form, then, the conception is that these specific molecules, having the immense complexity above described, and having correspondently complex polarities which cannot be mutually balanced by any simple form of aggregation, have, for the form of aggregation in which all their forces are equilibrated, the structure of the adult organism to which they belong; and that they are compelled to fall into this structure by the co-operation of the environing forces acting on them, and the forces they exercise on one another--the environing forces being the source of the _power_ which effects the re-arrangement, and the polarities of the molecules determining the _direction_ in which that power is turned. into this conception there enters no trace of the hypothesis of an "archæus or vital principle;" and the principles of molecular physics fully justify it. it is, however, objected that "the living body in its development presents a long succession of _differing_ forms; a continued series of changes for the whole length of which, according to mr. spencer's hypothesis, the physiological units must have an 'inherent tendency.' could we more truly say of anything, 'it is unrepresentable in thought?'" i reply that if there is taken into account an element here overlooked, the process will not be found "unrepresentable in thought." this is the element of size or mass. to satisfy or balance the polarities of each order of physiological units, not only a certain structure of organism, but a certain size of organism is needed; for the complexities of that adult structure in which the physiological units are equilibrated, cannot be represented within the small bulk of the embryo. in many minute organisms, where the whole mass of physiological units required for the structure is present, the very thing _does_ take place which it is above implied _ought_ to take place. the mass builds itself directly into the complete form. this is so with _acari_, and among the nematoid _entozoa_. but among higher animals such direct transformations cannot happen. the mass of physiological units required to produce the size as well as the structure that approximately equilibrates them, is not all present, but has to be formed by successive additions--additions which in viviparous animals are made by absorbing, and transforming into these special molecules, the organizable materials directly supplied by the parent, and which in oviparous animals are made by doing the like with the organizable materials in the "food-yelk," deposited by the parent in the same envelope with the germ. hence it results that, under such conditions, the physiological units which first aggregate into the rudiment of the future organism, do not form a structure like that of the adult organism, which, when of such small dimensions, does not equilibrate them. they distribute themselves so as partly to satisfy the chief among their complex polarities. the vaguely-differentiated mass thus produced cannot, however, be in equilibrium. each increment of physiological units formed and integrated by it, changes the distribution of forces; and this has a double effect. it tends to modify the differentiations already made, bringing them a step nearer to the equilibrating structure; and the physiological units next integrated, being brought under the aggregate of polar forces exercised by the whole mass, which now approaches a step nearer to that ultimate distribution of polar forces which exists in the adult organism, are coerced more directly into the typical structure. thus there is necessitated a series of compromises. each successive form assumed is unstable and transitional: approach to the typical structure going on hand in hand with approach to the typical bulk. possibly i have not succeeded by this explanation, any more than by the original explanation, in making this process "representable in thought." it is manifestly untrue, however, that i have, as alleged, re-introduced under a disguise the conception of a "vital principle." that i interpret embryonic development in terms of matter and motion, cannot, i think, be questioned. whether the interpretation is adequate, must be a matter of opinion; but it is clearly a matter of fact, that i have not fallen into the inconsistency asserted by your reviewer. at the same time i willingly admit that, in the absence of certain statements which i have now supplied, he was not unwarranted in representing my conception in the way that he has done. ---- notes [ ] gross misrepresentations of this statement, which have been from time to time made, oblige me, much against my will, to add here an explanation of it. the last of these perversions, uttered in a lecture delivered at belfast by the rev. professor watts, d.d., is reported in the _belfast witness_ of december , ; just while a third impression of this work is being printed from the plates. the report commences as follows:--"dr. watts, after showing that on his own confession spencer was indebted for his facts to huxley and hooker, who," &c., &c. wishing in this, as in other cases, to acknowledge indebtedness when conscious of it, i introduced the words referred to, in recognition of the fact that i had repeatedly questioned the distinguished specialists named, on matters beyond my knowledge, which were not dealt with in the books at my command. forgetting the habits of antagonists, and especially theological antagonists, it never occurred to me that my expression of thanks to my friends for "information where my own was deficient," would be turned into the sweeping statement that i was indebted to them for my facts. had professor watts looked at the preface to the second volume (the two having been published separately, as the prefaces imply), he would have seen a second expression of my indebtedness "for their valuable criticisms, and for the trouble they have taken in _checking_ the numerous statements of fact on which the arguments proceed"--no further indebtedness being named. a moment's comparison of the two volumes in respect of their accumulations of facts, would have shown him what kind of warrant there was for his interpretation. doubtless the rev. professor was prompted to make this assertion by the desire to discredit the work he was attacking; and having so good an end in view, thought it needless to be particular about the means. in the art of dealing with the language of opponents, dr. watts might give lessons to monsignor capel and archbishop manning. _december th, ._ [ ] in this passage as originally written (in ) they were described as incondensible; since, though reduced to the density of liquids, they had not been liquefied. [ ] here and hereafter the word "atom" signifies a unit of something classed as an element, because thus far undecomposed by us. the word must not be supposed to mean that which its derivation implies. in all probability it is not a simple unit but a compound one. [ ] the name hydro-carbons was here used when these pages were written, thirty-four years ago. it was the name then current. in this case, as in multitudinous other cases, the substitution of newer words and phrases for older ones, is somewhat misleading. putting the thoughts of in the language of gives an illusive impression of recency. [ ] it will perhaps seem strange to class oxygen as a crystalloid. but inasmuch as the crystalloids are distinguished from the colloids by their atomic simplicity, and inasmuch as sundry gases are reducible to a crystalline state, we are justified in so classing it. [ ] the remark made by a critic to the effect that in a mammal higher temperature diminishes the rate of molecular change in the tissues, leads me to add that the exhalation i have alleged is prevented if the heat rises above the range of variation normal to the organism; since, then, unusually rapid pulsations with consequent inefficient propulsion of the blood, cause a diminished rate of circulation. to produce the effect referred to in the text, heat must be associated with dryness; for otherwise evaporation is not aided. general evidence supporting the statement i have made is furnished by the fact that the hot and dry air of the eastern deserts is extremely invigorating; by the fact that all the energetic and conquering races of men have come from the hot and dry regions marked on the maps as rainless; and by the fact that travellers in africa comment on the contrast between the inhabitants of the hot and dry regions (relatively elevated) and those of the hot and moist regions: active and inert respectively. [ ] the increase of respiration found to result from the presence of light, is probably an _indirect_ effect. it is most likely due to the reception of more vivid impressions through the eyes, and to the consequent nervous stimulation. bright light is associated in our experience with many of our greatest outdoor pleasures, and its presence partially arouses the consciousness of them, with the concomitant raised vital functions. [ ] to exclude confusion it may be well here to say that the word "atom" is, as before explained, used as the name for a unit of a substance at present undecomposed; while the word "molecule" is used as the name for a unit of a substance known to be compound. [ ] on now returning to the subject after many years, i meet with some evidence recently assigned, in a paper read before the royal society by mr. j. w. pickering, d.sc. (detailing results harmonizing with those obtained by prof. grimaux), showing clearly how important an agent in vital actions is this production of isomeric changes by slight changes of conditions. certain artificially produced substances, simulating proteids in other of their characters and reactions, were found to simulate them in coagulability by trifling disturbances. "in the presence of a _trace of neutral salt_ they coagulate on heating at temperatures very similar to proteid solutions." and it is shown that by one of these factitious organic colloids a like effect is produced in coagulating the blood, to that "produced by the intravenous injection of a nucleoproteid." [ ] after this long interval during which other subjects have occupied me, i now find that the current view is similar to the view above set forth, in so far that a small molecular disturbance is supposed suddenly to initiate a great one, producing a change compared to an explosion. but while, of two proposed interpretations, one is that the fuse is nitrogenous and the charge a carbo-hydrate, the other is that both are nitrogenous. the relative probabilities of these alternative views will be considered in a subsequent chapter. [ ] when writing this passage i omitted to observe the verification yielded of the conclusion contained in § concerning the part played in the vital processes by the nitrogenous compounds. for these vegeto-alkalies, minute quantities of which produce such great effects in exalting the functions (_e. g._, a sixteenth of a grain of strychnia is a dose), are all nitrogenous bodies, and, by implication, relatively unstable bodies. the small amounts of molecular change which take place in these small quantities of the vegeto-alkalies when diffused through the system, initiate larger amounts of molecular change in the nitrogenous elements of the tissues. but the evidence furnished a generation ago by these vegeto-alkalies has been greatly reinforced by far more striking evidence furnished by other nitrogenous compounds--the various explosives. these, at the same time that they produce by their sudden decompositions violent effects outside the organism, also produce violent effects inside it: a hundredth of a grain of nitro-glycerine being a sufficient dose. investigations made by dr. j. b. bradbury, and described by him in the bradshaw lecture on "some new vaso-dilators" (see _the lancet_, nov. , ), details the effects of kindred bodies--methyl-nitrate, glycol-dinitrate, erythrol-tetranitrate. the first two, in common with nitro-glycerine, are stable only when cool and in the dark--sunlight or warmth decomposes them, and they explode by rapid heating or percussion. the fact which concerns us here is that the least stable--glycol-dinitrate--has the most powerful and rapid physiological effect, which is proportionately transient. in one minute the blood-pressure is reduced by one-fourth and in four minutes by nearly two-thirds: an effect which is dissipated in a quarter of an hour. so that this excessively unstable compound, decomposing in the body in a very short time, produces within that short time a vast amount of molecular change: acting, as it seems, not through the nervous system, but directly on the blood-vessels. [ ] this interpretation is said to be disproved by the fact that the carbo-hydrate contained in muscle amounts to only about . of the total solids. i do not see how this statement is to be reconciled with the statement cited three pages back from professor michael foster, that the deposits of glycogen contained in the liver and in the muscles may be compared to the deposits in a central bank and branch banks. [ ] before leaving the topic let me remark that the doctrine of metabolism is at present in its inchoate stage, and that the prevailing conclusions should be held tentatively. as showing this need an anomalous fact may be named. it was long held that gelatine is of small value as food, and though it is now recognized as valuable because serving the same purposes as fats and carbo-hydrates, it is still held to be valueless for structural purposes (save for some inactive tissue); and this estimate agrees with the fact that it is a relatively stable nitrogenous compound, and therefore unfit for those functions performed by unstable nitrogenous compounds in the muscular and other tissues. but if this is true, it seems a necessary implication that such substances as hair, wool, feathers, and all dermal growths chemically akin to gelatine, and even more stable, ought to be equally innutritive or more innutritive. in that case, however, what are we to say of the larva of the clothes-moth, which subsists exclusively on one or other of these substances, and out of it forms all those unstable nitrogenous compounds needful for carrying on its life and developing its tissues? or again, how are we to understand the nutrition of the book-worm, which, in the time-stained leaves through which it burrows, finds no proteid save that contained in the dried-up size, which is a form of gelatine; or, once more, in what form is the requisite amount of nitrogenous substance obtained by the coleopterous larva which eats holes in wood a century old? [ ] this chapter and the following two chapters originally appeared in part iii of the original edition of the _principles of psychology_ ( ): forming a preliminary which, though indispensable to the argument there developed, was somewhat parenthetical. having now to deal with the general science of biology before the more special one of psychology, it becomes possible to transfer these chapters to their proper place. [ ] see _westminster review_ for april, .--art. iv. "a theory of population." see appendix a. [ ] this paragraph replaces a sentence that, in _the principles of psychology_, referred to a preceding chapter on "method;" in which the mode of procedure here indicated was set forth as a mode to be systematically pursued in the choice of hypotheses. this chapter on method is now included, along with other matter, in a volume entitled _various fragments_. [ ] speaking of "the general idea of _life_" m. comte says:--"cette idée suppose, en effet, non-seulement celle d'un être organisé de manière à comporter l'état vital, mais aussi celle, non moins indispensable, d'un certain ensemble d'influences extérieures propres à son accomplissement. une telle harmonie entre l'être vivant et le _milieu_ correspondant, caractérise evidemment la condition fondamentale de la vie." commenting on de blainville's definition of life, which he adopts, he says:--"cette lumineuse définition ne me paraît laisser rien d'important à désirer, si ce n'est une indication plus directe et plus explicite de ces deux conditions fondamentales co-relatives, nécessairement inséparables de l'état vivant, un _organisme_ déterminé et un _milieu_ convenable." it is strange that m. comte should have thus recognized the necessity of a harmony between an organism and its environment, as a _condition_ essential to life, and should not have seen that the continuous maintenance of such inner actions as will counterbalance outer actions, _constitutes_ life. [when the original edition was published dr. j. h. bridges wrote to me saying that in the _politique positive_, comte had developed his conception further. on p. , denying "le prétendu antagonisme des corps vivants envers leurs milieux inorganiques," he says "au lieu de ce conflit, on a reconnu bientôt que cette relation nécessaire constitue une condition fondamentale de la vie réelle, dont la notion systématique consiste dans une intime conciliation permanente entre la spontanéité intérieure et la fatalité extérieure." still, this "conciliation _permanente_" seems to be a "_condition_" to life; not that varying adjustment of changes which life consists in maintaining. in presence of an ambiguity, the interpretation which agrees with his previous statement must be chosen.] [ ] in further elucidation of this general doctrine, see _first principles_, § . [ ] in ordinary speech development is often used as synonymous with growth. it hence seems needful to say that development as here and hereafter used, means _increase of structure_ and not _increase of bulk_. it may be added that the word evolution, comprehending growth as well as development, is to be reserved for occasions when both are implied. [ ] this paragraph originally formed part of a review-article on "transcendental physiology," published in . [ ] when, in , the preceding chapter was written, it had not occurred to me that there needed an accompanying chapter treating of structure. the gap left by that oversight i now fill up. in doing this there have been included certain statements which are tacitly presupposed in the last chapter, and there may also be some which overlap statements in the next chapter. i have not thought it needful so to alter adjacent chapters as to remove these slight defects: the duplicated ideas will bear re-emphasizing. [ ] in connexion with this matter i add here a statement made by prof. foster which it is difficult to understand: "indeed it has been observed that a dormouse actually gained in weight during a hybernating period; it discharged during this period neither urine nor fæces, and the gain in weight was the excess of oxygen taken in over the carbonic acid given out." (_text-book of physiology_, th ed., part ii, page .) [ ] in the account of james mitchell, a boy born blind and deaf, given by james wardrop, f.r.s. (edin. ), it is said that he acquired a "preternatural acuteness of touch and smell." the deaf dr. kitto described himself as having an extremely strong visual memory: he retained "a clear impression or image of everything at which he ever looked." [ ] here, as in sundry places throughout this chapter, the necessities of the argument have obliged me to forestall myself, by assuming the conclusion reached in a subsequent chapter, that modifications of structure produced by modifications of function are transmitted to offspring. [ ] whether the _volvox_ is to be classed as animal or vegetal is a matter of dispute; but its similarity to the blastula stage of many animals warrants the claim of the zoologists. [ ] while the proof was in my hands there was published in _science progress_ an essay by dr. t. g. brodie on "the phosphorus-containing substances of the cell." in this essay it is pointed out that "nucleic acid is particularly characterized by its instability.... in the process of purification it is extremely liable to decompose, with the result that it loses a considerable part of its phosphorus. in the second place it is most easily split up in another manner in which it loses a considerable part of its nitrogen.... to avoid the latter source of error he [miescher] found that it was necessary to keep the temperature of all solutions down to °c., the whole time of the preparation." these facts tend strongly to verify the hypothesis that the nucleus is a source of perpetual molecular disturbance--not a regulating centre but a stimulating centre. [ ] the writing of the above section reminded me of certain allied views which i ventured to suggest nearly years ago. they are contained in the _westminster review_ for april, , in an article entitled "a theory of population deduced from the general law of animal fertility." it is there suggested that the "spermatozoon is essentially a neural element, and the ovum essentially a hæmal element," or, as otherwise stated, that the "sperm-cell is co-ordinating matter and the germ-cell matter to be co-ordinated" (pp. - ). and along with this proposition there is given some chemical evidence tending to support it. now if, in place of "neural" and "hæmal," we say--the element that is most highly phosphorized and the element that is phosphorized in a much smaller degree; or if, in place of co-ordinating matter and matter to be co-ordinated, we say--the matter which initiates action and the matter which is made to act; there is disclosed a kinship between this early view and the view just set forth. in the last part of this work, "laws of multiplication," which is developed from the essay referred to, i left out the portion containing the quoted sentences, and the evidence supporting the conclusion drawn. partly i omitted them because the speculation did not form an essential link in the general argument, and partly because i did not see how the suggested interpretation could hold of plants as well as of animals. if, however, the alleged greater staining capacity of the male generative nucleus in plants implies, as in other cases, that the male cell has a larger proportion of the phosphorized matter than the other elements concerned, then the difficulty disappears. as, along with the idea just named, the dropped portion of the original essay contains other ideas which seem to me worth preserving, i have thought it as well to reproduce it, in company with the chief part of the general argument as at first sketched out. it will be found in appendix a to this volume. [ ] unfortunately the word _heterogenesis_ has been already used as a synonym for "spontaneous generation." save by those few who believe in "spontaneous generation," however, little objection will be felt to using the word in a sense that seems much more appropriate. the meaning above given to it covers both metagenesis and parthenogenesis. [ ] prof. huxley avoids this difficulty by making every kind of genesis a mode of development. his classification, which suggested the one given above, is as follows:-- { growth { continuous { { { metamorphosis { development { { { metagenesis { { agamogenesis { { discontinuous { { parthenogenesis { gamogenesis [ ] the implication is that an essentially similar process occurs in those fragments of leaves used for artificial propagation. besides the begonias in general, i learn that various other plants are thus multiplied--citron and orange trees, _hoya carnosa_, _aucuba japonica_, _clianthus puniceus_, etc., etc. _bryophyllum calicinum_, _rochea falcata_, and _echeveria_. i also learn that the following plants, among others, produce buds from their foliage leaves:--_cardamine pratensis_, _nasturtium officinale_, _roripa palustris_, _brassica oleracea_, _arabis pumila_, _chelidonium majus_, _nymphæa guianensis_, _episcia bicolor_, _chirita sivensis_, _pinguicula backeri_, _allium_, _gagea_, _tolmia_, _fritillaria_, _ornithogalum_, etc. in _cardamine_ and several others, a complete miniature plant is at once produced; in other cases bulbils or similar detachable buds. [ ] among various examples i have observed, the most remarkable were among foxgloves, growing in great numbers and of large size, in a wood between whatstandwell bridge and crich, in derbyshire. in one case the lowest flower on the stem contained, in place of a pistil, a shoot or spike of flower-buds, similar in structure to the embryo-buds of the main spike. i counted seventeen buds on it; of which the first had three stamens, but was otherwise normal; the second had three; the third, four; the fourth, four; &c. another plant, having more varied monstrosities, evinced excess of nutrition with equal clearness. the following are the notes i took of its structure:-- st, or lowest flower on the stem, very large; calyx containing eight divisions, one partly transformed into a corolla, and another transformed into a small bud with bract (this bud consisted of a five-cleft calyx, four sessile anthers, a pistil, and a rudimentary corolla); the corolla of the main flower, which was complete, contained six stamens, three of them bearing anthers, two others being flattened and coloured, and one rudimentary; there was no pistil but, _in place of it_, a large bud, consisting of a three-cleft calyx of which two divisions were tinted at the ends, an imperfect corolla marked internally with the usual purple spots and hairs, three anthers sessile on this mal-formed corolla, a pistil, a seed vessel with ovules, and, growing to it, another bud of which the structure was indistinct. nd flower, large; calyx of seven divisions, one being transformed into a bud with bract, but much smaller than the other; corolla large but cleft along the top; six stamens with anthers, pistil, and seed-vessel. rd flower, large; six-cleft calyx, cleft corolla, with six stamens, pistil, and seed-vessel, with a second pistil half unfolded at its apex. th flower, large; divided along the top, six stamens. th flower, large; corolla divided into three parts, six stamens. th flower, large; corolla cleft, calyx six cleft, the rest of the flower normal. th, and all succeeding flowers, normal. while this chapter is under revision, another noteworthy illustration has been furnished to me by a wall-trained pear tree which was covered in the spring by luxuriant "foreright" shoots. as i learned from the gardener, it was pruned just as the fruit was setting. a large excess of sap was thus thrown into other branches, with the result that in a number of them the young pears were made monstrous by reversion. in some cases, instead of the dried up sepals at the top of the pear, there were produced good sized leaves; and in other cases the seed-bearing core of the pear was transformed into a growth which protruded through the top of the pear in the shape of a new shoot. [ ] in partial verification, mr. tansley writes:--"prof. klebs of basel has shown that in _hydrodictyon_, gametes can only be produced by the cells of a net when these are above a certain size and age; and then only under conditions unfavourable to growth, such as a feeble light or poverty of nutritive inorganic salts or absence of oxygen, or a low temperature in the water containing the plant. the presence of organic substances, especially sugar, also acts as a stimulus to the formation of gametes, and this is also the case in _vaucheria_. many other _algæ_ produce gametes mainly at the end of the vegetative season, when food is certainly difficult to obtain in their natural habitat, and we may well suppose that their assimilative power is waning. where, however, as is the case in _vaucheria_, the plant depends for propagation mainly on the production of fertilized eggs, we find the sexual organs often produced in conditions very favourable to vegetative growth, in opposition to those cases such as _hydrodictyon_, where the chief means of propagation is by zoospores. so that side by side with, and to some extent obscuring, the principle developed above we have a clear adaptation of the production of reproductive cells to the special circumstances of the case." [ ] this establishment by survival of the fittest of reproductive processes adapted to variable conditions, is indirectly elucidated by the habits of salmon. as salmon thrive in the sea and fall out of condition in fresh water (having during their sea-life not exercised the art of catching fresh-water prey), the implication is that the species would profit if all individuals ran up the rivers just before spawning time in november. why then do most of them run up during many preceding months? contemplation of the difficulties which lie in the way to the spawning grounds, will, i think, suggest an explanation. there are falls to be leaped and shallow rapids to be ascended. these obstacles cannot be surmounted when the river is low. a fish which starts early in the season has more chances of getting up the falls and the rapids than one which starts later; and, out of condition as it will be, may spawn, though not well. on the other hand, one which starts in october, if floods occur appropriately, may reach the upper waters and then spawn to great advantage; but in the absence of adequate rains it may fail altogether to reach the spawning grounds. hence the species profits by an irregularity of habits adapted to meet irregular contingencies. [ ] i owe to mr. (now sir john) lubbock an important confirmation of this view. after stating his belief that between crustaceans and insects there exists a physiological relation analogous to that which exists between water vertebrata and land-vertebrata, he pointed out to me that while among insects there is a definite limit of growth, and an accompanying definite commencement of reproduction, among crustaceans, where growth has no definite limit, there is no definite relation between the commencement of reproduction and the decrease or arrest of growth. [ ] while this chapter is passing through the press, i learn from mr. white cooper, that not only are near sight, long sight, dull sight, and squinting, hereditary; but that a peculiarity of vision confined to one eye is frequently transmitted: re-appearing in the same eye in offspring. [ ] an instance here occurs of the way in which those who are averse to a conclusion will assign the most flimsy reasons for rejecting it. rather than admit that the eyes of these creatures living in darkness have disappeared from lack of use, some contend that such creatures would be liable to have their eyes injured by collisions with objects, and that therefore natural selection would favour those individuals in which the eyes had somewhat diminished and were least liable to injury: the implication being that the immunity from the inflammations due to injuries would be so important a factor in life as to cause survival. and this is argued in presence of the fact that one of the most conspicuous among these blind cave-animals is a cray-fish, and that the cray-fish in its natural habitat is in the habit of burrowing in the banks of rivers holes a foot or more deep, and has its eyes exposed to all those possible blows and frictions which the burrowing involves! [ ] in addition to the numerous illustrations given by mr. sedgwick, here is one which colonel a. t. fraser published in _nature_ for nov. , , concerning two hindoo dwarfs:--"in speech and intelligence the dwarfs were indistinguishable from ordinary natives of india. from an interrogation of one of them, it appeared that he belonged to a family all the male members of which have been dwarfs for several generations. they marry ordinary native girls, and the female children grow up like those of other people. the males, however, though they develop at the normal rate until they reach the age of six, then cease to grow, and become dwarfs." [ ] this remarkable case appears to militate against the conclusion, drawn a few pages back, that the increase of a peculiarity by coincidence of "spontaneous variations" in successive generations, is very improbable; and that the special superiorities of musical composers cannot have thus arisen. the reply is that the extreme frequency of the occurrence among so narrow a class as that of musical composers, forbids the interpretation thus suggested. [ ] i omitted to name here a cause which may be still more potent in producing irregularity in the results of cousin-marriages. so far as i can learn, no attempt has been made to distinguish between such results as arise when the related parents from whom the cousins descend are of the same sex and those which arise when they are of different sexes. in the one case two sisters have children who intermarry; and in the other case a brother and a sister have children who intermarry. the marriages of cousins in these two cases may be quite dissimilar in their results. if there is a tendency to limitation of heredity by sex--if daughters usually inherit more from the mother than sons do, while sons inherit more from the father than from the mother, then two sisters will on the average of cases be more alike in constitution than a sister and a brother. consequently the descendants of two sisters will differ less in their constitutions than the descendants of a brother and a sister; and marriage in the first case will be more likely to prove injurious from absence of dissimilarity in the physiological units than marriage in the second. my own small circle of friends furnishes evidence tending to verify this conclusion. in one instance two cousins who intermarried are children of two sisters, and they have no offspring. in another the cousins who intermarried are children of two brothers, and they have no offspring. in the third case the cousins were descendants of two brothers and only one child resulted. [ ] _a propos_ of this sentence one of my critics writes:--"i cannot find in this book the statement as first made that the 'life of an individual is maintained by the unequal and ever-varying actions of incident forces on its different parts.' recent physiological work offers a startling example of the statement." to the question contained in the first sentence the answer is that i have not made the statement in the above words, but that it is implied in the chapter entitled "the degree of life varies as the degree of correspondence," and more especially in § , which, towards its close, definitely involves the statement. the verifying evidence my critic gives me is this:-- "prof. sherrington has shown that if the sensory roots of the spinal nerves are cut one by one there is at first no general effect produced. that is to say, the remainder of the nervous system continues to function as before. this condition (lack of general effect) persists until about six pairs have been cut. with the severance of the seventh pair, however, the whole central nervous system ceases to function, so that stimulation of intact sensory nerves produces no reflex action. after a variable period, but one of many hours duration, the power of functioning is recovered. that is to say, if the sensory impulses (from the skin, &c.) reaching the central nervous system are rapidly reduced in amount, there comes a point where those remaining do not suffice to keep the structure 'awake.' after a time, however, it adjusts itself to work with the diminished supply. similarly strumpell describes the case of a boy 'whose sensory inlets were all paralyzed except one eye and one ear.' when these were closed he instantly fell asleep." [ ] fifty years before the discovery of the röntgen rays and those habitually emanating from uranium, it had been observed by moser that under certain conditions the surfaces of metals receive permanent impressions from appropriate objects placed upon them. such facts show that the molecules of substances propagate in all directions special ethereal undulations determined by their special constitutions. [ ] this classification, and the three which follow it, i quote (abridging some of them) from prof. agassiz's "essay on classification." [ ] for explanations, see "illogical geology," _essays_, vol. i. how much we may be misled by assuming that because the remains of creatures of high types have not been found in early strata, such creatures did not exist when those strata were formed, has recently ( ) been shown by the discovery of a fossil sea-cow in the lower miocene of hesse-darmstadt. the skeleton of this creature proves that it differed from such sirenian mammals as the existing manatee only in very small particulars: further dwindling of disused parts being an evident cause. the same is true as regards, now, we consider that since the beginning of miocene days this aberrant type of mammal has not much increased its divergence from the ordinary mammalian type; if we then consider how long it must have taken for this large aquatic mammal (some eight or ten feet long) to be derived by modification from a land-mammal; and if then we contemplate the probable length of the period required for the evolution of that land-mammal out of a pre-mammalian type; we seem carried back in thought to a time preceding any of our geologic records. we are shown that the process of organic evolution has most likely been far slower than is commonly supposed. [ ] since this passage was written, in , there has come to light much more striking evidence of change from a more generalized to a less generalized type during geologic time. in a lecture delivered by him in , prof. huxley gave an account of the successive modifications of skeletal structure in animals allied to the horse. beginning with the _orohippus_ of the eocene formation, which had four complete toes on the front limb and three toes on the hind limb, he pointed out the successive steps by which in the _mesohippus_, _miohippus_, _protohippus_, and _pliohippus_, there was a gradual approach to the existing horse. [ ] several of the arguments used in this chapter and in that which follows it, formed parts of an essay on "the development hypothesis," originally published in . [ ] _studies from the morphological laboratory in the university of cambridge_, vol. vi, p. . [ ] _ibid._, p. . [ ] _studies from the morphological laboratory in the university of cambridge_, vol. vi, p. . [ ] early in our friendship (about ) prof. huxley expressed to me his conviction that all the higher articulate animals have twenty segments or somites. that he adhered to this view in , when his work on _the crayfish_ was published, is shown by his analysis there given of the twenty segments existing in this fluviatile crustacean; and adhesion to it had been previously shown in , when his work on _the anatomy of invertebrated animals_ was published. on p. of that work he writes:--"in the abdomen there are, at most, eleven somites, none of which, in the adult, bear ambulatory limbs. thus, assuming the existence of six somites in the head, the normal number of somites in the body of insects will be twenty, as in the higher _crustacea_ and _arachnida_." to this passage, however, he puts the note:--"it is open to question whether the podical plates represent a somite; and therefore it must be recollected that the total number of somites, the existence of which can be actually demonstrated in insects, is only seventeen, viz., four for the head, three for the thorax, and ten for the abdomen." i have changed the number twenty, which in the original edition occurred in the text, to the number seventeen in deference to suggestions made to me; though i find in dr. sharp's careful and elaborate work on the _insecta_, that viallanes and cholodkovsky agree with huxley in believing that there are six somites in the insect-head. the existence of a doubt on this point, however, does not essentially affect the argument, since there is agreement among morphologists respecting the _constancy_ of the total number of somites in insects. [ ] to avoid circumlocution i let these words stand, though they are not truly descriptive; for the prosperity of imported species is largely, if not mainly, caused by the absence of those natural enemies which kept them down at home. [ ] while these pages are passing through the press (in ), dr. hooker has obliged me by pointing out that "plants afford many excellent examples" of analogous transitions. he says that among true "water plants," there are found, in the same species, varieties which have some leaves submerged and some floating; other varieties in which they are all floating; and other varieties in which they are all submerged. further, that many plants characterized by floating leaves, and which have all their leaves floating when they grow in deeper water, are found with partly aerial leaves when they grow in shallower water; and that elsewhere they occur in almost dry soil with all their leaves aerial. [ ] it will be seen that the argument naturally leads up to this expression--survival of the fittest--which was here used for the first time. two years later (july, ) mr. a. r. wallace wrote to mr. darwin contending that it should be substituted for the expression "natural selection." mr. darwin demurred to this proposal. among reasons for retaining his own expression he said that i had myself, in many cases, preferred it--"continually using the words natural selection." (_life and letters_, &c., vol. iii, pp. - .) mr. darwin was quite right in his statement, but not right in the motive he ascribed to me. my reason for frequently using the phrase "natural selection," after the date at which the phrase "survival of the fittest" was first used above, was that disuse of mr. darwin's phrase would have seemed like an endeavour to keep out of sight my own indebtedness to him, and the indebtedness of the world at large. the implied feeling has led me ever since to use the expressions natural selection and survival of the fittest with something like equal frequency. [ ] i am indebted to mr. [now sir w.] flower for the opportunity of examining the many skulls in the museum of the college of surgeons for verification of this. unfortunately the absence, in most cases, of some or many teeth, prevented me from arriving at that specific result which would have been given by weighing a number of the under jaws in each race. simple inspection, however, disclosed a sufficiently-conspicuous difference. the under jaws of australians and negroes, when collated with those of englishmen, were visibly larger, not only relatively but absolutely. one australian jaw only seemed about of the same size as an average english jaw; and this (probably the jaw of a woman), belonging as it did to a smaller skull, bore a greater ratio to the whole body of which it formed part, than did an english jaw of the same actual size. in all the other cases, the under jaws of these inferior races (containing larger teeth than our own) were _absolutely_ more massive than our own--often exceeding them in all dimensions; and _relatively_ to their smaller skeletons were much more massive. let me add that the australian and negro jaws are thus strongly contrasted, not with all british jaws, but only with the jaws of the civilized british. an ancient british skull in the collection possesses a jaw almost or quite as massive as those of the australian skulls. all this is in harmony with the alleged relation between greater size of jaws and greater action of jaws, involved by the habits of savages. [in mr. f. howard collins carefully investigated this matter: measuring ten australian, ten ancient british, and ten recent english skulls in the college of surgeons museum. the result proved an absolute difference of the kind above indicated, and a far greater relative difference. to ascertain this last a common standard of comparison was established--an equal size of skull in all the cases; and then when the relative masses or cubic sizes of the jaws were calculated, the result which came out was this:--australian jaw, ; ancient british jaw, ; recent english jaw, . "hence," in the words of mr. collins, "the mass of the recent english jaw is, roughly speaking, half that of the australian relatively to that of the skull, and a ninth less than that of the ancient british." he adds verifying evidence from witnesses who have no hypothesis to support--members of the odontological society. the vice-president, mr. mummery, remarks of the australians that "the jaw-bones are powerfully developed, and large in proportion to the cranium."] [ ] as bearing on the question of the varieties of man, let me here refer to a paper on "the origin of the human races" read before the anthropological society, march st, , by mr. alfred wallace. in this paper, mr. wallace shows that along with the attainment of that intelligence implied by the use of implements, clothing, &c., there arises a tendency for modifications of brain to take the place of modifications of body: still, however, regarding the natural selection of spontaneous variations as the cause of the modifications. but if the foregoing arguments be valid, natural selection here plays but the secondary part of furthering the adaptations otherwise caused. it is true that, as mr. wallace argues, and as i have myself briefly indicated (see _westminster review_, for april, , pp. - ), the natural selection of races leads to the survival of the more cerebrally-developed, while the less cerebrally-developed disappear. but though natural selection acts freely in the struggle of one society with another; yet, among the units of each society, its action is so interfered with that there remains no adequate cause for the acquirement of mental superiority by one race over another, except the inheritance of functionally-produced modifications. [ ] _darwin and after darwin_, part ii, p. . [ ] _essays upon heredity_, vol. i, p. . [ ] in a letter published by dr. romanes in _nature_, for april , , he alleges three reasons why "as soon as selection is withdrawn from an organ the _minus_ variations of that organ outnumber the _plus_ variations." the first is that "the survival-mean must descend to the birth-mean." the interpretation of this is that if the members of a species are on the average born with an organ of the required size, and if they are exposed to natural selection, then those in which the organ is relatively small will some of them die, and consequently the mean size of the organ at adult age will be greater than at birth. contrariwise, if the organ becomes useless and natural selection does not operate on it, this difference between the birth-mean and the survival-mean disappears. now here, again, the _plus_ variations and their effects are ignored. supposing the organ to be useful, it is tacitly assumed that while _minus_ variations are injurious, _plus_ variations are not injurious. this is untrue. superfluous size of an organ implies several evils:--its original cost is greater than requisite, and other organs suffer; the continuous cost of its nutrition is unduly great, involving further injury; it adds needlessly to the weight carried and so again is detrimental; and there is in some cases yet a further mischief--it is in the way. clearly, then, those in which _plus_ variations of the organ have occurred are likely to be killed off as well as those in which _minus_ variations have occurred; and hence there is no proof that the survival-mean will exceed the birth-mean. moreover the assumption has a fatal implication. to say that the survival-mean of an organ is greater than the birth-mean is to say that the organ is greater _in proportion to other organs_ than it was at birth. what happens if instead of one organ we consider all the organs? if the survival-mean of a particular organ is greater than its birth-mean, the survival mean of each other organ must also be greater. thus the proposition is that every organ has become larger in relation to every other organ!--a marvellous proposition. i need only add that dr. romanes' inferences with respect to the two other causes--atavism and failing heredity--are similarly vitiated by ignoring the plus variations and their effects. [ ] _westminster review_, january, . see also _essays, &c._, vol. i, p. . [ ] "on orthogenesis and the impotence of natural selection in species-formation," pp. , , , . [ ] address to plymouth institution, at opening of session - . [ ] _westminster review_, april, . "progress: its law and cause." see also _essays_, vol. i. [ ] it may be needful to remark, that by the proposed expression it is intended to define--not life in its essence; but, life as manifested to us--not life as a _noumenon_: but, life as a _phenomenon_. the ultimate mystery is as great as ever: seeing that there remains unsolved the question--what _determines_ the co-ordination of actions? [ ] _prin. of phys._, nd edit., p. . [ ] _ibid._, rd edit., p . [ ] _ibid._, p. . [ ] agassiz and gould, p. . [ ] _prin. of phys._, rd edit., p. . [ ] "parthenogenesis," p. . [ ] _prin. of phys._, p. . [ ] _ibid._, p. . [ ] _ibid._, p. . [ ] "a general outline of the animal kingdom." by prof. t. r. jones, f. g. s., p. . [ ] carpenter. [ ] _prin. of phys._, p. . [ ] _ibid._, p. . [ ] _ibid._, p. . [ ] _ibid._, p. . [ ] _ibid._, p. . [ ] _ibid._, p. . [ ] _prin. of phys._, p. . [ ] _ibid._, p. . [ ] _ibid._, p. . [ ] _prin. of. phys._, p. . [ ] _ibid._, p. . [ ] _ibid._, nd edit., p. . [ ] _prin. of phys._, p. . [ ] _ibid._, p. . [ ] _ibid._, p. . [ ] _ibid._, p. . [ ] "a general outline of the animal kingdom." by professor t. r. jones, p. . [ ] _prin. of phys._, p. . [ ] should it be objected that in the higher plants the sperm-cell and germ-cell differ, though no distinct co-ordinating system exists, it is replied that there _is_ co-ordination of actions, though of a feeble kind, and that there must be some agency by which this is carried on. [ ] it is a significant fact that amongst the dioecious invertebrata, where the nutritive system greatly exceeds the other systems in development, the female is commonly the largest, and often greatly so. in some of the rotifera the male has no nutritive system at all. see _prin. of phys._, p. . [ ] _prin. of phys._, p. . [ ] "parthenogenesis," pp. , . [ ] "lectures on animal chemistry." by dr. bence jones. _medical times_, sept. th, . see also _prin. of phys._, p. . [ ] _cyclopædia of anatomy and physiology_, vol. iv, p. . [ ] from a remark of drs. wagner and leuckart this chemical evidence seems to have already suggested the idea that the sperm-cell becomes "metamorphosed into the central parts of the nervous system." but though they reject this assumption, and though the experiments of mr. newport clearly render it untenable, yet none of the facts latterly brought to light conflict with the hypothesis that the sperm-cell contains unorganized co-ordinating matter. [ ] quain's _elements of anatomy_, p. . [ ] the maximum weight of the horse's brain is lb. ozs.; the human brain weighs lbs., and occasionally as much as lbs.; the brain of a whale, feet long, weighed lbs. ozs.; and the elephant's brain reaches from lbs. to lbs. of the whale's fertility we know nothing; but the elephant's quite agrees with the hypothesis. the elephant does not attain its full size until it is thirty years old, from which we may infer that it arrives at a reproductive age later than man does; its period of gestation is two years, and it produces one at a birth. evidently, therefore, it is much less prolific than man. see müller's _physiology_ (baly's translation), p. , and quain's _elements of anatomy_, p. . [ ] that the size of the nervous system is the measure of the ability to maintain life, is a proposition that must, however, be taken with some qualifications. the ratio between the amounts of gray and white matter present in each case is probably a circumstance of moment. moreover, the temperature of the blood may have a modifying influence; seeing that small nervous centres exposed to rapid oxidation will be equivalent to larger ones more slowly oxidized. indeed, we see amongst mankind, that though, in the main, size of brain determines mental power, yet temperament exercises some control. there is reason to think, too, that certain kinds of nervous action involve greater consumption of nervous tissue than others; and this will somewhat complicate the comparisons. nevertheless, these admissions do not affect the generalization as a whole, but merely prepare us to meet with minor irregularities. [ ] let me here note in passing a highly significant implication. the development of nervous structures which in such cases take place, cannot be limited to the finger-ends. if we figure to ourselves the separate sensitive areas which severally yield independent feelings, as constituting a network (not, indeed, a network sharply marked out, but probably one such that the ultimate fibrils in each area intrude more or less into adjacent areas, so that the separations are indefinite), it is manifest that when, with exercise, the structure has become further elaborated, and the meshes of the network smaller, there must be a multiplication of fibres communicating with the central nervous system. if two adjacent areas were supplied by branches of one fibre, the touching of either would yield to consciousness the same sensation: there could be no discrimination between points touching the two. that there may be discrimination, there must be a distinct connection between each area and the tract of grey matter which receives the impressions. nay more, there must be, in this central recipient-tract, an added number of the separate elements which, by their excitements, yield separate feelings. so that this increased power of tactual discrimination implies a peripheral development, a multiplication of fibres in the trunk-nerve, and a complication of the nerve-centre. it can scarcely be doubted that analogous changes occur under analogous conditions throughout all parts of the nervous system--not in its sensory appliances only, but in all its higher co-ordinating appliances, up to the highest. [ ] _essays upon heredity_, p. . [ ] _les maladies des vers à soie_, par l. pasteur, vol. i, p. . [ ] curiously enough, weismann refers to, and recognizes, syphilitic infection of the reproductive cells. dealing with brown-séquard's cases of inherited epilepsy (concerning which, let me say, that i do not commit myself to any derived conclusions), he says:--"in the case of epilepsy, at any rate, it is easy to imagine [many of weismann's arguments are based on things 'it is easy to imagine'] that the passage of some specific organism through the reproductive cells may take place, as in the case of syphilis" (p. ). here is a sample of his reasoning. it is well known that epilepsy is frequently caused by some peripheral irritation (even by the lodging of a small foreign body under the skin), and that, among peripheral irritations causing it, imperfect healing is one. yet though, in brown-séquard's cases, a peripheral irritation caused in the parent by local injury was the apparent origin, weismann chooses gratuitously to assume that the progeny were infected by "some specific organism," which produced the epilepsy! and then though the epileptic virus, like the syphilitic virus, makes itself at home in the egg, the parental protoplasm is not admitted! [ ] _philosophical transactions of the royal society for the year _, part i, pp. - . [ ] it will, i suppose, be said that the non-inheritance of mutilations constitutes evidence of the kind here asked for. the first reply is that the evidence is conflicting, as it may well be. it is forgotten that to have valid evidence of non-inheritance of mutilations, it is requisite that both parents shall have undergone mutilation, and that this does not often happen. if they have not, then, assuming the inheritableness of mutilations, there would, leaving out other causes, be an equal tendency to appearance and non-appearance of the mutilation in offspring. but there is another cause--the tendency to reversion, which ever works in the direction of cancelling individual characters by the return to ancestral characters. so that even were the inheritance of mutilations to be expected (and for myself i may say that its occurrence surprises me), it could not be reasonably looked for as more than exceptional: there are two strong countervailing tendencies. but now, in the second place, let it be remarked that the inheritance or non-inheritance of mutilations is beside the question. the question is whether modifications of parts produced by modifications of functions are inheritable or not. and then, by way of disproof of their inheritableness, we are referred to cases in which the modifications of parts are not produced by modifications of functions, but are otherwise produced! [ ] see _first principles_, part ii, chap. xxii, "equilibration." [ ] _principles of biology_, § , (no. . april, ). [ ] _ibid._ this must not be understood as implying that while the mass increases as the cubes, the _quantity of motion_ which can be generated increases only as the squares; for this would not be true. the quantity of motion is obviously measured, not by the sectional areas of the muscles alone, but by these multiplied into their lengths, and therefore increases as the cubes. but this admission leaves untouched the conclusion that the ability to _bear stress_ increases only as the squares; and thus limits the ability to generate motion, by relative incoherence of materials. [ ] _the transactions of the linnæan society of london_, vol. xxii, p. . the estimate of reaumur, cited by kirby and spence, is still higher--"in five generations one aphis may be the progenitor of , , , descendants; and that it is supposed that in one year there may be twenty generations." (_introduction to entomology_, vol. i, p. ) [ ] _a manual of the anatomy of invertebrated animals_, by t. h. huxley, p. . [ ] respecting the _eloidea_ i learn that in --thirty years after it had become a pest--one solitary male plant was found in a pond near edinburgh; but "in an exhaustive inquiry on the plant made by dr. groenland, of copenhagen, he could find no trace of any male specimens having been found in europe other than the scotch." in waters from which the _eloidea_ has disappeared, it seems to have done so in consequence of the growth of an _alga_, which has produced turbid water unfavourable to it. that is to say, the decreased multiplication of somatic cells in some cases, is not due to any exhaustion, but is caused by the rise of enemies or adverse conditions; as happens generally with introduced species of plants and animals which multiply at first enormously, and then, without any loss of reproductive power, begin to decrease under the antagonizing influences which grow up. [ ] _a text book of human physiology._ by austin flint, m.d., ll.d. fourth edition. new york: d. appleton & co. . page . [ ] this supposition i find verified by mr. a. s. packard in his elaborate monograph on "the cave fauna of north america, &c.," as also in his article published in the _american naturalist_, september, ; for he there mentions "variations in _pseudotremia cavernarum_ and _tomocerus plumbeus_, found living near the entrance to caves in partial daylight." the facts, as accumulated by mr. packard, furnished a much more complete answer to prof. lankester than is above given, as, for example, the "blindness of _neotoma_, or the wood-rat of mammoth cave." it seems that there are also "cave beetles, with or without rudimentary eyes," and "eyeless spiders" and myriapods. and there are insects, as some "species of anophthalmus and adelops, whose larvæ are lacking in all traces of eyes and optic nerves and lobes." these instances cannot be explained as sequences of an inrush of water carrying with it the remote ancestors, some of which did not find their way out; nor can others of them be explained by supposing an inrush of air, which did the like. [ ] see "social organism" in _westminster review_ for january, ; also _principles of sociology_, § . [ ] _contemporary review_, september, . [ ] _evolution of sex_, p. . [ ] _souvenirs entomologiques_, ^{me} série, p. . [ ] _natural history of bees_, new ed., p. . [ ] _origin of species_, th ed., p. . [ ] _contemporary review_, september, , p. . [ ] _the entomologist's monthly magazine_, march, , p. . [ ] perhaps it will be alleged that nerve-matter is costly, and that this minute economy might be of importance. anyone who thinks this will no longer think it after contemplating a litter of half-a-dozen young rabbits (in the wild rabbit the number varies from four to eight); and on remembering that the nerve-matter contained in their brains and spinal cords, as well as the materials for building up the bones, muscles, and viscera of their bodies, has been supplied by the doe in the space of a month; at the same time that she has sustained herself and carried on her activities: all this being done on relatively poor food. nerve-matter cannot be so very costly then. [ ] _loc. cit._, p. . [ ] _the germ plasm_, p. . [ ] while professor weismann has not dealt with my argument derived from the distribution of discriminativeness on the skin, it has been criticized by mr. mckeen cattell, in the last number of _mind_ (october, ). his general argument, vitiated by extreme misconceptions, i need not deal with. he says:--"whether changes acquired by the individual are hereditary, and if so to what extent, is a question of great interest for ethics no less than for biology. but mr. spencer's application of this doctrine to account for the origin of species [!] simply begs the question. he assumes useful variations [!]--whether of structure or habit is immaterial--without attempting to explain their origin": two absolute misstatements in two sentences! the only part of mr. cattell's criticism requiring reply is that which concerns the "sensation-areas" on the skin. he implies that since weber, experimental psychologists have practically set aside the theory of sensation areas: showing, among other things, that relatively great accuracy of discrimination can be quickly acquired by "increased interest and attention.... practice for a few minutes will double the accuracy of discrimination, and practice on one side of the body is carried over to the other." to me it seems manifest that "increased interest and attention" will not enable a patient to discriminate two points where a few minutes before he could perceive only one. that which he can really do in this short time is to learn to discriminate between the _massiveness of a sensation_ produced by two points and the massiveness of that produced by one, and to _infer_ one point or two points accordingly. respecting the existence of sensation-areas marked off from one another, i may, in the first place, remark that since the eye originates as a dermal sac, and since its retina is a highly developed part of the sensitive surface at large, and since the discriminative power of the retina depends on the division of it into numerous rods and cones, each of which gives a separate sensation-area, it would be strange were the discriminative power of the skin at large achieved by mechanism fundamentally different. in the second place i may remark that if mr. cattell will refer to professor gustav retzius's _biologische untersuchungen_, new series, vol. iv (stockholm, ), he will see elaborate diagrams of superficial nerve-endings in various animals showing many degrees of separateness. i guarded myself against being supposed to think that the sensation-areas are sharply marked off from one another; and suggested, contrariwise, that probably the branching nerve-terminations intruded among the branches of adjacent nerve-terminations. here let me add that the intrusion may vary greatly in extent; and that where the intruding fibres run far among those of adjacent areas, the discriminativeness will be but small, while it will be great in proportion as each set of branching fibres is restricted more nearly to its own area. all the facts are explicable on this supposition. [ ] to save space and exclude needless complication i have omitted these passages from the preceding divisions of this appendix. [ ] though professor weismann does not take up the challenge, dr. romanes does. he says:--"when selection is withdrawn there will be no excessive _plus_ variations, because so long as selection was present the efficiency of the organ was maintained at its highest level: it was only the _minus_ variations which were then eliminated" (_contemporary review_, p. ). in the first place, it seems to me that the phrases used in this sentence beg the question. it says that "the efficiency of the organ was maintained at its _highest_ level"; which implies that the highest level (tacitly identified with the greatest size) is the best and that the tendency is to fall below it. this is the very thing i ask proof of. suppose i invert the idea and say that the organ is maintained at its right size by natural selection, because this prevents increase beyond the size which is best for the organism. every organ should be in due proportion, and the welfare of the creature as a whole is interfered with by excess as well as by defect. it may be directly interfered with--as for instance by too big an eyelid; and it may be indirectly interfered with, where the organ is large, by needless weight and cost of nutrition. in the second place the question which here concerns us is not what natural selection will do with variations. we are concerned with the previous question--what variations will arise? an organ varies in all ways; and, unless reason to the contrary is shown, the assumption must be that variations in the direction of increase are as frequent and as great as those in the direction of decrease. take the case of the tongue. certainly there are tongues inconveniently large, and probably tongues inconveniently small. what reason have we for assuming that the inconveniently small tongues occur more frequently than the inconveniently large ones? none that i can see. dr. romanes has not shown that when natural selection ceases to act on an organ the _minus_ variations in each new generation will exceed the _plus_ variations. but if they are equal the alleged process of panmixia has no place. [ ] _the variation of animals and plants under domestication_, vol. ii, p. . [ ] _journal of the anthropological institute_ for , p. . [ ] in "the all-sufficiency of natural selection" (_contemporary review_, sept., , p. ), professor weismann writes:--"i have ever contended that the acceptance of a principle of explanation is justified, if it can be shown that without it certain facts are inexplicable." unless, then, prof. weismann can show that the distribution of discriminativeness is otherwise explicable, he is bound to accept the explanation i have given, and admit the inheritance of acquired characters. [ ] prof. weismann is unaware that the view here ascribed to roux, writing in , is of far earlier date. in the _westminster review_ for january, , in an essay on "the social organism," i wrote:--"one more parallelism to be here noted, is that the different parts of a social organism, like the different parts of an individual organism, compete for nutriment; and severally obtain more or less of it according as they are discharging more or less duty." (see also _essays_, i, .) and then, in , in _the principles of sociology_, vol. i, § , i amplified the statement thus:--"all other organs, therefore, jointly and individually, compete for blood with each organ ... local tissue-formation (which under normal conditions measures the waste of tissue in discharging function) is itself a cause of increased supply of materials ... the resulting competition, not between units simply, but between organs, causes in a society, as in a living body, high nutrition and growth of parts called into greatest activity by the requirements of the rest." though i did not use the imposing phrase "intra-individual-selection," the process described is the same. [ ] _proceedings of the biological society of washington_, vol. ix. [ ] romanes lecture, p. . [ ] _ibid._, p. . [ ] this interpretation harmonizes with a fact which i learn from prof. riley, that there are gradations in this development, and that in some species the ordinary neuters swell their abdomens so greatly with food that they can hardly get home. transcriber's note in this etext the oe ligature is represented as [oe]. in the table of bat arm measurements, the graphic symbols for male and female are represented as [male] and [fem.]. proofreaders found a number of typographical errors which have been corrected, none of which affected the sense of the text. inconsistent hyphenation has been retained. [illustration: kentish plover with eggs and young. from the exhibit in the british natural history museum.] animal life and intelligence. by c. lloyd morgan, f.g.s., professor in and dean of university college, bristol; lecturer at the bristol medical school; president of the bristol naturalists' society, etc. author of "animal biology," "the springs of conduct," etc. boston, u. s. a. ginn & company, publishers. . to my father. preface. there are many books in our language which deal with animal intelligence in an anecdotal and conventionally popular manner. there are a few, notably those by mr. romanes and mr. mivart, which bring adequate knowledge and training to bear on a subject of unusual difficulty. in the following pages i have endeavoured to contribute something (imperfect, as i know full well, but the result of several years' study and thought) to our deeper knowledge of those mental processes which we may fairly infer from the activities of dumb animals. the consideration of animal intelligence, from the scientific and philosophical standpoint, has been my primary aim. but so inextricably intertwined is the subject of intelligence with the subject of life, the subject of organic evolution with the subject of mental evolution, so closely are questions of heredity and natural selection interwoven with questions of habit and instinct, that i have devoted the first part of this volume to a consideration of organic evolution. the great importance and value of professor weismann's recent contributions to biological science, and their direct bearing on questions of instinct, rendered such treatment of my subject, not only advisable, but necessary. moreover, it seemed to me, and to those whom i consulted in the matter, that a general work on animal life and intelligence, if adequately knit into a connected whole, and based on sound principles of science and of philosophy, would not be unwelcomed by biological students, and by that large and increasing class of readers who, though not professed students, follow with eager interest the development of the doctrine of evolution. incidentally, but only incidentally, matters concerning man, as compared with the dumb animals, have been introduced. it is contended that in man alone, and in no dumb animal, is the rational faculty, as defined in these pages, developed; and it is contended that among human-folk that process of natural selection, which is so potent a factor in the lower reaches of organic life, sinks into comparative insignificance. man is a creature of ideas and ideals. for him the moral factor becomes one of the very highest importance. he conceives an ideal self which he strives to realize; he conceives an ideal humanity towards which he would raise his fellow-man. he becomes a conscious participator in the evolution of man, in the progress of humanity. but while we must not be blind to the effects of new and higher factors of progress thus introduced as we rise in the scale of phenomena, we must at the same time remember that biological laws still hold true, though moral considerations and the law of duty may profoundly modify them. the eagle soars aloft apparently in defiance of gravitation; but the law of gravitation still holds good; and no treatment of the mechanism of flight which neglected it would be satisfactory. moral restraint, a higher standard of comfort, and a perception of the folly and misery of early and improvident marriage may tend to check the rate of growth of population: but the "law of increase" still holds good, as a law of the factors of phenomena; and malthus did good service to the cause of science when he insisted on its importance. we may guide or lighten the incidence of natural selection through competition; we may in our pity provide an asylum for the unfortunates who are suffering elimination; but we cannot alter a law which, as that of one of the factors of organic phenomena, still obtains, notwithstanding the introduction of other factors. however profoundly the laws of phenomena may be modified by such introduction of new and higher factors, the older and lower factors are still at work beneath the surface. and he who would adequately grasp the social problems of our time should bring to them a mind prepared by a study of the laws of organic life: for human beings, rational and moral though they may be, are still organisms; and man can in no wise alter or annul those deep-lying facts which nature has throughout the ages been weaving into the tissue of life. some parts of this work are necessarily more technical, and therefore more abstruse, than others. this is especially the case with chapters iii., v., and vi.; while, for those unacquainted with philosophical thought, perhaps the last chapter may present difficulties of a different order. with these exceptions, the book will not be beyond the ready comprehension of the general reader of average intelligence. i have to thank many kind friends for incidental help. thanks are also due to professor flower, who courteously gave permission that some of the exhibits in our great national collection in cromwell road might be photographed and reproduced; and to messrs. longmans for the use of two or three illustrations from my text-book of "animal biology." c. lloyd morgan. university college, bristol, october, . contents. chapter i. the nature of animal life. page the characteristics of animals the relation of animals to food-stuffs the relation of animals to the atmosphere the relation of animals to energy chapter ii. the process of life. illustration from respiration illustration from nutrition the utilization of the materials incorporated the analogy of a gas-engine. explosive metabolism chapter iii. reproduction and development. reproduction in the protozoa fission in the metazoa the regeneration of lost parts reproduction by budding sexual reproduction illustration of development parental sacrifice the law of increase chapter iv. variation and natural selection. the law of persistence the occurrence of variations application of the law of increase natural selection elimination and selection modes of natural elimination illustrated protective resemblance and mimicry selection proper illustrated the effects of natural selection isolation or segregation its modes, geographical, preferential and physiological its effects utility of specific characters variations in the intensity of the struggle for existence convergence of characters modes of adaptation: progress evolution and revolution chapter v. heredity and the origin of variations. heredity in the protozoa regeneration of lost parts sexual reproduction and heredity the problem of hen and egg reproductive continuity pangenesis modified pangenesis continuity of germ-plasm cellular continuity with differentiation the inheritance or non-inheritance of acquired characters origin of variations on the latter view hypothesis of organic combination the extrusion of the second polar cell the protozoan origin of variations how can the body influence the germ? is there sufficient evidence that it does? summary and conclusion chapter vi. organic evolution. the diversity of animal life the evolution theory natural selection: not to be used as a magic formula panmixia and disuse sexual selection or preferential mating use and disuse the nature of variations the inheritance of variations the origin of variations summary and conclusion chapter vii. the senses of animals. the primary object of sensation organic sensations and the muscular sense touch the temperature-sense taste smell hearing sense of rotation or acceleration sight restatement of theory of colour-vision variation in the limits of colour-vision the four types of "visual" organs problematical senses permanent possibilities of sensation chapter viii. mental processes in man. the physiological aspect the psychological aspect sensations: their localization, etc. perceptual construction conceptual analysis inferences perceptual and conceptual intelligence and reason chapter ix. mental processes in animals: their powers of perception and intelligence. the two factors in phenomena the basis in organic evolution perceptual construction in mammalia can animals analyze their constructs? the generic difference between the minds of man and brute perceptual construction in other vertebrates "understanding" of words perceptual construction in the invertebrates "the psychic life of micro-organisms" the inferences of animals intelligent not rational use of words defined language and analysis chapter x. the feelings of animals: their appetences and emotions. pleasure and pain: their organic limits their directive value an emotion exemplified sensitiveness and sensibility the expression of the emotions the postponement of action the three orders of emotion the capacities of animals for pleasure and pain sense-feelings some emotions of animals the necessity for caution in interpretation the sense of beauty can animals be moral? conclusion chapter xi. animal activities: habit and instinct. the nature of animal activities the outer and inner aspect the inherited organization habitual activities instinctive activities innate capacity blind prevision consciousness and instinct mr. romanes's treatment of instinct lapsed intelligence and modern views on heredity three factors in the origin of instinctive activities the emotional basis of instinct the influence of intelligence on instinct the characteristics of intelligent activities the place of volition perceptual and conceptual volition consciousness and consentience classification of activities chapter xii. mental evolution. is mind evolved from matter? kinesis and metakinesis monistic assumptions the nature of ejects the universe as eject metakinetic environment of mind conceptual ideas not subject to natural selection elimination through incongruity interneural evolution interpretations of nature can fetishism have had a natural genesis? the origin of interneural variations are acquired variations inherited? summary and conclusion list of illustrations. fig. page kentish plover with eggs and young: frontispiece . spiracles and air-tubes of cockroach . gills of mussel . a cell greatly magnified . am[oe]ba . egg-cell and sperm-cell . diagram of circulation . protozoa . hydra virides . aurelia: life-cycle . liver-fluke--embryonic stages . diagram of development . wing of bat (pipistrelle) . variations of the noctule . variations of the long-eared bat . variations of the pipistrelle . variations of the whiskered bat . variations adjusted to the standard of the noctule . caterpillar of a moth on an oak spray . locust resembling a leaf . mimicry of bees by flies . egg and hen . stag-beetles . tactile corpuscules . touch-hair of insect . taste-buds of rabbit . antennule of crayfish . diagram of ear . tail of mysis . leg of grasshopper . diagram of semicircular canals . the human eye . retina of the eye . variation in the limits of colour-vision . pineal eye . skull of melanerpeton . eyes and eyelets of bee . eye of fly . diagram of mosaic vision . direction-retina . antennary structures of hymenoptera animal life and intelligence. chapter i. the nature of animal life. i once asked a class of school-boys to write down for me in a few words what they considered the chief characteristics of animals. here are some of the answers-- . animals move about, eat, and grow. . animals eat, grow, breathe, feel (at least, most of them do), and sleep. . take a cat, for example. it begins as a kitten; it eats, drinks, plays about, and grows up into a cat, which does much the same, only it is more lazy, and stops growing. at last it grows old and dies. but it may have kittens first. . an animal has a head and tail, four legs, and a body. it is a living creature, and not a vegetable. . animals are living creatures, made of flesh and blood. combining these statements, we have the following characteristics of animals:-- . each has a proper and definite form, at present described as "a head and tail, four legs, and a body." . they breathe. . they eat and drink. . they grow. . they also "grow up." the kitten grows up into a cat, which is somewhat different from the kitten. . they move about and sleep. . they feel--"at least some of them do." . they are made of "flesh and blood." . they grow old and die. . they reproduce their kind. the cat may have kittens. . they are living organisms, but "not vegetables." now, let us look carefully at these characteristics, all of which were contained in the five answers, and were probably familiar in some such form as this to all the boys, and see if we cannot make them more general and more accurate. . _an animal has a definite form._ my school-boy friend described it as a head and tail, four legs, and a body. but it is clear that this description applies only to a very limited number of animals. it will not apply to the butterfly, with its great wings and six legs; nor to the lobster, with its eight legs and large pincer-claws; to the limbless snake and worm, the finned fish, the thousand-legs, the oyster or the snail, the star-fish or the sea-anemone. the animals to which my young friend's description applies form, indeed, but a numerically insignificant proportion of the multitudes which throng the waters and the air, and not by any means a large proportion of those that walk upon the surface of the earth. the description applies only to the backboned vertebrates, and not to nearly all of them. it is impossible to summarize in a sentence the form-characteristics of animals. the diversities of form are endless. perhaps the distinguishing feature is the prevalence of curved and rounded contours, which are in striking contrast to the definite crystalline forms of the inorganic kingdom, characterized as these are by plane surfaces and solid angles. we may say, however, that all but the very lowliest animals have each and all a proper and characteristic form of their own, which they have inherited from their immediate ancestors, and which they hand on to their descendants. but this form does not remain constant throughout life. sometimes the change is slight; in many cases, however, the form alters very markedly during the successive stages of the life of the individual, as is seen in the frog, which begins life as a tadpole, and perhaps even more conspicuously in the butterfly, which passes through a caterpillar and a chrysalis stage. still, these changes are always the same for the same kind of animal. so that we may say, each animal has a definite form and shape or series of shapes. . _animals breathe._ the essential thing here is that oxygen is taken in by the organism, and carbonic acid gas is produced by the organism. no animal can carry on its life-processes unless certain chemical changes take place in the substance of which it is composed. and for these chemical changes oxygen is essential. the products of these changes, the most familiar of which are carbonic acid gas and urea, must be got rid of by the process of excretion. respiration and excretion are therefore essential and characteristic life-processes of all animals. [illustration: fig. .--diagram of spiracles and air-tubes (tracheæ) of an insect (cockroach). the skin, etc., of the back has been removed, and the crop (cr.) and alimentary canal (al.c.) displayed. the air-tubes are represented by dotted lines. the ten spiracles are numbered to the right of the figure.] in us, and in all air-breathing vertebrates, there are special organs set apart for respiration and excretion of carbonic acid gas. these are the lungs. a great number of insects also breathe air, but in a different way. they have no lungs, but they respire by means of a number of apertures in their sides, and these open into a system of delicate branching tubes which ramify throughout the body. many organisms, however, such as fish and lobsters and molluscs, breathe the air dissolved in the water in which they live. the special organs developed for this purpose are the gills. they are freely exposed to the water from which they abstract the air dissolved therein. when the air dissolved in the water is used up, they sicken and die. there can be nothing more cruel than to keep aquatic animals in a tank or aquarium in which there is no means of supplying fresh oxygen, either by the action of green vegetation, or by a jet of water carrying down air-bubbles, or in some other way. and then there are a number of animals which have no special organs set apart for breathing. in them respiration is carried on by the general surface of the body. the common earthworm is one of these; and most microscopic organisms are in the same condition. still, even if there be no special organs for breathing, the process of respiration must be carried on by all animals. [illustration: fig. .--gills of mussel. o.g., outer gill; i.g., inner gill; mo., mouth; m., muscles for closing shell; ma., mantle; s., shell; f., foot; h., position of heart; e.s., exhalent siphon, whence the water passes out from the gill-chamber; i.s., inhalent siphon, where the water enters. the left valve of the shell has been removed, and the mantle cut away along the dark line.] . _they eat and drink._ the living substance of an animal's body is consumed during the progress of those chemical changes which are consequent upon respiration; and this substance must, therefore, be made good by taking in the materials out of which fresh life-stuff can be formed. this process is called, in popular language, feeding. but the food taken in is not identical with the life-stuff formed. it has to undergo a number of chemical changes before it can be built into the substance of the organism. in us, and in all the higher animals, there is a complex system of organs set aside for the preparation, digestion, and absorption of the food. but there are certain lowly organisms which can take in food at any portion of their surface, and digest it in any part of their substance. one of these is the am[oe]ba, a minute speck of jelly-like life-stuff, which lives in water, and tucks in a bit of food-material just as it comes. and there are certain degenerate organisms which have taken to a parasitic life, and live within the bodies of other animals. many of these can absorb the material prepared by their host through the general surface of their simple bodies. but here, again, though there may be no special organs set apart for the preparation, absorption, and digestion of food, the process of feeding is essential to the life of all animals. stop that process for a sufficient length of time, and they inevitably die. . _they grow._ food, as we have just seen, has to be taken in, digested, and absorbed, in order that the loss of substance due to the chemical changes consequent on respiration may be made good. but where the digestion and absorption are in excess of that requisite for this purpose, we have the phenomenon of growth. what are the characteristics of this growth? we cannot, perhaps, describe it better than by saying ( ) that it is organic, that is to say, a growth of the various organs of the animal in due proportion; ( ) that it takes place, not merely by the addition of new material (for a crystal grows by the addition of new material, layer upon layer), but by the incorporation of that new material into the very substance of the old; and ( ) that the material incorporated during growth differs from the material absorbed from without, which has undergone a preparatory chemical transformation within the animal during digestion. the growth of an animal is thus dependent upon the continued absorption of new material from without, and its transformation into the substance of the body. the animal is, in fact, a centre of continual waste and repair, of nicely balanced constructive and destructive processes. these are the invariable concomitants of life. only so long as the constructive processes outbalance the destructive processes does growth continue. during the greater part of a healthy man's life, for example, the two processes, waste and repair, are in equilibrium. in old age, waste slowly but surely gains the mastery; and at death the balanced process ceases, decomposition sets in, and the elements of the body are scattered to the winds or returned to mother earth. there are generally limits of growth which are not exceeded by any individuals of each particular kind of animal. but these limits are somewhat variable among the individuals of each kind. there are big men and little men, cart-horses and ponies, bloodhounds and lap-dogs. wild animals, however, when fully grown, do not vary so much in size. the period of growth is also variable. many of the lower backboned animals probably grow during the whole of life, but those which suckle their young generally cease growing after a fraction (in us from one-fourth to one-fifth) of the allotted span of life is past. . but animals not only grow--_they also "grow up."_ the kitten grows up into a cat, which is somewhat different from the kitten. we speak of this growing up of an animal as its _development_. the proportion of the various parts and organs progressively alter. the relative lengths of the arms and legs, and the relative size of the head, are not the same in the infant as in the man or woman. or, take a more marked case. in early spring there is plenty of frog-spawn in the ponds. a number of blackish specks of the size of mustard seeds are embedded in a jelly-like mass. they are frogs' eggs. they seem unorganized. but watch them, and the organization will gradually appear. the egg will be hatched, and give rise to a little fish-like organism. this will by degrees grow into a tadpole, with a powerful swimming tail and rounded head and body, but with no obvious neck between them. legs will appear. the tail will shrink in size and be gradually drawn into the body. the tadpole will have developed into a minute frog. there are many of the lower animals which go through a not less wonderful, if not more wonderful, metamorphosis. the butterfly or the silkworm moth, beginning life as a caterpillar and changing into a chrysalis, from which the perfect insect emerges, is a familiar instance. and hosts of the marine invertebrates have larval forms which have but little resemblance to their adult parents. such a series of changes as is undergone by the frog is called _metamorphosis_, which essentially consists in the temporary development of certain provisional embryonic organs (such as gills and a powerful swimming tail) and the appearance of adult organs (such as lungs and legs) to take their place. in metamorphosis these changes occur during the free life of the organism. but beneath the eggshell of birds and within the womb of mammals scarcely less wonderful changes are slowly but surely effected, though they are hidden from our view. there is no metamorphosis during the free life of the organism, but there is a prenatal _transformation_. the little embryo of a bird or mammal has no gills like the tadpole (though it has for a while gill-slits, pointing unmistakably to its fishy ancestry), but it has a temporary provisional breathing organ, called the allantois, pending the full development and functional use of its lungs. all the higher animals, in fact--the dog, the chick, the serpent, the frog, the fish, the lobster, the butterfly, the worm, the star-fish, the mollusc, it matters not which we select--take their origin from an apparently unorganized egg. they all, therefore, pass during their growth from a comparatively simple condition to a comparatively complex condition by a process of change which is called development. but there are certain lowly forms, consisting throughout life of little more than specks of jelly-like life-stuff, in which such development, if it occurs at all, is not conspicuous. . _they move about and sleep._ this is true of our familiar domestic pets. the dog and the cat, after periods of restless activity, curl themselves up and sleep. the canary that has all day been hopping about its cage, or perhaps been allowed the freedom of the dining-room, tucks its head under its wing and goes to sleep. the cattle in the meadows, the sheep in the pastures, the horses in the stables, the birds in the groves, all show alternating periods of activity and repose. but is this true of all animals? do all animals "move about and sleep"? the sedentary oyster does not move about from place to place; the barnacle and the coral polyp are fixed for the greater part of life; and whether these animals sleep or not it is very difficult to say. we must make our statement more comprehensive and more accurate. if we throw it into the following form, it will be more satisfactory: animals exhibit certain activities; and periods of activity alternate with periods of repose. i shall have more to say hereafter concerning the activities of animals. here i shall only say a few words concerning the alternating periods of repose. no organism can continue in ceaseless activity unbroken by any intervening periods of rest. nor can the organs within an organism, however continuous their activity may appear, work on indefinitely and unrestfully. the heart is apparently restless in its activity. but in every five minutes of the continued action of the great force-pump (ventricle) of the heart, two only are occupied in the efforts of contraction and work, while three are devoted to relaxation and repose. what we call sleep may be regarded as the repose of the higher brain-centres after the activity of the day's work--a repose in which the voluntary muscles share. the necessity for rest and repose will be readily understood. we have seen that the organism is a centre of waste and repair, of nicely balanced destructive and reconstructive processes. now, activity is accompanied by waste and destruction. but it is clear that these processes, by which the substance of the body and its organs is used up, cannot go on for an indefinite period. there must intervene periods of reconstruction and recuperation. hence the necessity of rest and repose alternating with the periods of more or less prolonged activity. . _they feel--"at least some of them do."_ the qualification was a wise one, for in truth, as we shall hereafter see, we know very little about the feelings of the lower organisms. the one animal of whose feelings i know anything definite and at first hand, is myself. of course, i believe in the feelings of others; but when we come to very lowly organisms, we really do not know whether they have feelings or not, or, if they do, to what extent they feel. shall we leave this altogether out of account? or can we throw it into some form which is more general and less hypothetical? this, at any rate, we know--that all animals, even the lowest, are sensitive to touches, sights, or sounds. it is a matter of common observation that their activities are generally set agoing under the influence of such suggestions from without. perhaps it will be objected that there is no difference between feeling and being sensitive. but i am using the word "sensitive" in a general sense--in that sense in which the photographer uses it when he speaks of a sensitive plate, or the chemist when he speaks of a sensitive test. when i say that animals are sensitive, i mean that they answer to touches, or sounds, or other impressions (what are called stimuli) coming from without. they may feel or not; many of them undoubtedly do. but that is another aspect of the sensitiveness. using the term, then, with this meaning, we may say, without qualification, that all animals are more or less sensitive to external influences. . _they are made of "flesh and blood."_ here we have allusion to the materials of which the animal body is composed. it is obviously a loose and unsatisfactory statement as it stands. an american is said to have described the difference between vertebrates and insects by saying that the former are composed of flesh and bone, and the latter of skin and squash. but even if we amend the statement that animals are made of "flesh and blood" by the addition of the words, "or of skin and squash," we shall hardly have a sufficiently satisfactory statement of the composition of the animal body. the essential constituent of animal (as indeed also of vegetable) tissues is protoplasm. this is a nearly colourless, jelly-like substance, composed of carbon, hydrogen, nitrogen, and oxygen, with some sulphur and phosphorus, and often, if not always, some iron; and it is permeated by water. protoplasm, together with certain substances, such as bony and horny matter, which it has the power of producing, constitutes the entire structure of simple organisms, and is built up into the organs of the bodies of higher animals. moreover, in these organs it is not arranged as a continuous mass of substance, but is distributed in minute separate fragments, or corpuscles, only visible under the microscope, called cells. these cells are of very various shapes--spherical, discoidal, polyhedral, columnar, cubical, flattened, spindle-shaped, elongated, and stellate. a great deal of attention has been devoted of late years to the minute structure of cells, and the great improvements in microscopical powers and appliances have enabled investigators to ascertain a number of exceedingly interesting and important facts. the external surface of a cell is sometimes, but not always in the case of animals, bounded by a film or membrane. within this membrane the substance of the cell is made up of a network of very delicate fibres (the _plasmogen_), enclosing a more fluid material (the _plasm_); and this network seems to be the essential living substance. in the midst of the cell is a small round or oval body, called the nucleus, which is surrounded by a very delicate membrane. in this nucleus there is also a network of delicate plasmogen fibres, enclosing a more fluid plasm material. at certain times the network takes the form of a coiled filament or set of filaments, and these arrange themselves in the form of rosettes and stars. in the meshwork of the net or in the coils of the filament there may be one or more small bodies (nucleoli), which probably have some special significance in the life of the cell. these cells multiply or give birth to new cells by dividing into two, and this process is often accompanied by special changes in the nucleus (which also divides) and by the arrangement of its network or filaments into the rosettes and stars before alluded to. instead, therefore, of the somewhat vague statement that animals are made of flesh and blood, we may now say that the living substance of which animals are composed is a complex material called protoplasm; that organisms are formed either of single cells or of a number of related cells, together with certain life-products of these cells; and that each cell, small as it is, has a definite and wonderful minute structure revealed by the microscope. [illustration: fig. .--a cell, greatly magnified. c.m., cell-membrane; c.p., cell-protoplasm; n.m., nuclear membrane; n.p., nuclear protoplasm; n.f., coiled nuclear filament.] . _animals grow old and die._ this is a familiar observation. apart from the fact that they are often killed by accident, by the teeth or claws of an enemy, or by disease, animals, like human beings, in course of time become less active and less vigorous; the vital forces gradually fail, and eventually the flame of life, which has for some time been burning dimmer and dimmer, flickers out and dies. but is this true of all animals? can we say that death--as distinct from being killed--is the natural heritage of every creature that lives? one of the simplest living creatures is the am[oe]ba. it consists of a speck of nucleated protoplasm, no larger than a small pin's head. simple as it is, all the essential life-processes are duly performed. it is a centre of waste and repair; it is sensitive and responsive to a stimulus; respiration and nutrition are effected in a simple and primitive fashion. it is, moreover, reproductive. first the nucleus and then the protoplasm of the cell divide, and in place of one am[oe]ba there are two. and these two are, so far as we can tell, exactly alike. there is no saying which is mother and which is daughter; and, so far as we can see at present, there is no reason why either should die. it is conceivable that am[oe]bæ never die, though they may be killed in immense numbers. hence it has been plausibly maintained that the primitive living cell is by nature deathless; that death is not the heritage of all living things; that death is indeed an acquisition, painful indeed to the individual, but, since it leaves the stage free for the younger and more vigorous individuals, conducive to the general good. [illustration: fig. .--am[oe]ba. . an am[oe]ba, showing the inner and outer substance (endosarc and ectosarc); a pseudopodium, p.s.; the nucleus, n.; and the contractile vesicle, c.v. . an am[oe]ba dividing into two. . the division just effected.] in face of this opinion, therefore, we cannot say that all animals grow old and die; but we may still say that all animals, with the possible exception of some of the lowest and simplest, exhibit, after a longer or a shorter time, a waning of the vital energies which sooner or later ends in death. . _animals reproduce their kind._ we have just seen the nature of reproduction in the simple unicellular am[oe]ba. the reproduction of the constituent cells in the complex multicellular organism, during its natural growth or to make good the inevitable loss consequent on the wear and tear of life, is of the same character. when we come to the higher organisms, reproduction is effected by the separation of special cells called egg-cells, or ova, from a special organ called the ovary; and these, in a great number of cases, will not develop into a new organism unless they be fertilized by the union with them in each case of another cell--the sperm-cell--produced by a different individual. the separate parents are called male and female, and reproduction of this kind is said to be sexual. [illustration: fig. .--egg-cell and sperm-cell. a, ovum or egg; b, spermatozoon or sperm.] the wonderful thing about this process is the power of the fertilized ovum, produced by the union of two minute cells from different parents, to develop into the likeness of these parents. this likeness, however, though it extends to minute particulars, is not absolute. the offspring is not exactly like either parent, nor does it present a precise mean between the characters of the two parents. there is always some amount of individual variability, the effects of which, as we shall hereafter see, are of wide importance. we are wont to say that these phenomena, the transmission of parental characteristics, together with a margin of difference, are due to heredity with variation. but this merely names the facts. how the special reproductive cells have acquired the secret of developing along special lines, and reproducing, with a margin of variability, the likeness of the organisms which produced them, is a matter concerning which we can at present only make more or less plausible guesses. scarcely less wonderful is the power which separated bits of certain organisms, such as the green freshwater hydra of our ponds, possess of growing up into the complete organism. cut a hydra into half a dozen fragments, and each fragment will become a perfect hydra. reproduction of this kind is said to be asexual. we shall have, in later chapters, to discuss more fully some of the phenomena of reproduction and heredity. for the present, it is sufficient to say that animals reproduce their kind by the detachment of a portion of the substance of their own bodies, which portion, in the case of the higher animals, undergoes a series of successive developmental changes constituting its life-history, the special nature of which is determined by inheritance, and the result of which is a new organism in all essential respects similar to the parent or parents. . _animals are living organisms, and "not vegetables."_ the first part of this final statement merely sums up the characteristics of living animals which have gone before. but the latter part introduces us to the fact that there are other living organisms than those we call animals, namely, those which belong to the vegetable kingdom. it might, at first sight, be thought a very easy matter to distinguish between animals and plants. there is no chance, for example, of mistaking to which kingdom an oak tree or a lion, a cabbage or a butterfly, belongs. but when we come down to the simpler organisms, those whose bodies are constituted by a single cell, the matter is by no means so easy. there are, indeed, lowly creatures which are hovering on the boundary-line between the two kingdoms. we need not discuss the nature of these boundary forms. it is sufficient to state that unicellular plants are spoken of as _protophyta_, and unicellular animals as _protozoa_, the whole group of unicellular organisms being classed together as _protista_. the animals whose bodies are formed of many cells in which there is a differentiation of structure and a specialization of function, are called _metazoa_, and the multicellular plants _metaphyta_. the relations of these groups may be thus expressed-- animals. plants. /\ /\ --------- ------------- -------------- ------------ metazoa. protozoa. protophyta. metaphyta. ----------- ------------- \/ protista. there are three matters with regard to the life-process of animals and plants concerning which a few words must be said. these are ( ) their relation to food-stuffs; ( ) their relation to the atmosphere; ( ) their relation to energy, or the power of doing work. with regard to the first matter, that of food-relation, the essential fact seems to be the dependence of animals on plants. plants can manufacture protoplasm out of its constituents if presented to them in suitable inorganic form scattered through earth and air and water. hence the peculiar features of their form, the branching and spreading nature of those parts which are exposed to the air, and the far-reaching ramifications of those parts which are implanted in the earth. hence, too, the flattened leaves, with their large available surface. animals are unable to manufacture protoplasm in this way. they are, sooner or later, dependent for food on plant-products. it is true that the carnivora eat animal food, but the animals they eat are directly or indirectly consumers of vegetable products. plants are nature's primary producers of organic material. animals utilize these products and carry them to higher developments. in relation to the atmosphere, animals require a very much larger quantity of oxygen than do plants. this, during the respiratory process, combines with carbon so as to form carbonic acid gas; and the atmosphere would be gradually drained of its oxygen and flooded with carbonic acid gas were it not that plants, through their green colouring matter (chlorophyll), under the influence of light, have the power of decomposing the carbonic acid gas, seizing on the carbon and building it into their tissues, and setting free the oxygen. thus are animals and green plants complementary elements in the scheme of nature.[a] the animal eats the carbon elaborated by the plant into organic products (starch and others), and breathes the oxygen which the plant sets free after it has abstracted the carbon. in the animal's body the carbon and oxygen recombine; its varied activities are thus kept going; and the resultant carbonic acid gas is breathed forth, to be again separated by green, growing plants into carbonaceous food-stuff and vitalizing oxygen. it must be remembered, however, that vegetable protoplasm, like animal protoplasm, respires by the absorption of oxygen and the formation of carbonic acid gas. but in green plants this process is outbalanced by the characteristic action of the chlorophyll, by which carbonic acid gas is decomposed. lastly, we have to consider the relations of animals and plants to energy. energy is defined as the power of doing work, and it is classified by physicists under two modes--potential energy, or energy of position; and kinetic energy, or energy of motion. the muscles of my arm contain a store of potential energy. suppose i pull up the weight of an old-fashioned eight-day clock. some of the potential energy of my arm is converted into the potential energy of the weight; that is, the raised weight is now in a position of advantage, and capable of doing work. it has energy of position, or potential energy. if the chain breaks, down falls the weight, and exhibits the energy of motion. but, under ordinary circumstances, this potential energy is utilized in giving a succession of little pushes to the pendulum to keep up its swing, and in overcoming the friction of the works. again, the energy of an electric current may be utilized in decomposing water, and tearing asunder the oxygen and hydrogen of which it is composed. the oxygen and hydrogen now have potential energy, and, if they be allowed to combine, this will manifest itself as the light and heat of the explosion. these examples will serve to illustrate the nature of the changes which energy undergoes. these are of the nature of transferences of energy from one body to another, and of transformations from one mode or manifestation to another. the most important point that has been established during this century with regard to energy is that, throughout all its transferences and transformations, it can be neither created nor destroyed. but there is another point of great importance. transformations of energy take place more readily in certain directions than in others. and there is always a tendency for energy to pass from the higher or more readily transformable to the lower or less readily transformable forms. when, for example, energy has passed to the low kinetic form of the uniformly distributed molecular motion of heat, it is exceedingly difficult, or practically impossible, to transform it into a higher and more available form. now, both animals and plants are centres of the transformation of energy; and in them energy, notwithstanding that it is being raised to a high position of potentiality, is constantly tending to be degraded to lower forms. hence the necessity of some source from which fresh stores of available energy may be constantly supplied. such a source is solar radiance. this it is which gives the succession of little pushes which keeps the pendulum of life a-swinging. and it is the green plants which, through their chlorophyll, are in the best position to utilize the solar energy. they utilize it in building up, from the necessary constituents diffused through the atmosphere and the soil, complex forms of organic material, of which the first visible product seems to be starch; and these not only contain large stores of potential energy, but are capable, when combined with oxygen, of containing yet larger stores. the animal, taking into its body these complex materials, and elaborating them together with oxygen into yet more complex and more unstable compounds, then, during its vital activity, makes organized use of the transformation of the potential energy thus stored into lower forms of energy. thus there go on side by side, in both animals and plants, a building up or synthesis of complex and unstable chemical compounds, accompanied by a storage of potential energy, and a breaking down or analysis of these compounds into lower and simpler forms, accompanied by a setting free of kinetic energy. but in the plant, synthetic changes and storage of energy are in excess, while in the animal, analytic changes and the setting free of kinetic energy are more marked. hence the variety and volume of animal activities. the building up of complex organic substances with abundance of stored energy may be roughly likened to the building up, by the child with his wooden bricks, of houses and towers and pyramids. the more complex they become the more unstable they are, until a touch will shatter the edifice and liberate the stored-up energy of position acquired by the bricks. thus, under the influence of solar energy, do plants build up their bricks of hydrogen, carbon, and oxygen into complex molecular edifices. animals take advantage of the structures so elaborated, modify them, add to them, and build yet more complex molecular edifices. these, at the touch of the appropriate stimulus, topple over and break down--not, indeed, into the elemental bricks, but into simpler molecular forms, and these again in later stages into yet simpler forms, which are then got rid of or excreted from the body. meanwhile the destructive fall of the molecular edifice is accompanied by the liberation of energy--as heat, maintaining the warmth of the body; as visible or hidden movements, in locomotion, for example, and the heart-beat; and sometimes as electrical energy (in electric fishes); as light (in phosphorescent animals and the glow-worm), or as sound. it is this abundant liberation of energy, giving rise to many and complex activities, which is one of the distinguishing features of animals as compared with plants. * * * * * we have now, i trust, extended somewhat and rendered somewhat more exact our common and familiar knowledge of the nature of animal life. in the next chapter we will endeavour to extend it still further by a consideration of the process of life. notes [a] an interesting problem concerning the atmosphere is suggested by certain geological facts. in our buried coal-seams and other carbonaceous deposits a great quantity of carbon, for the most part abstracted from the atmosphere, has been stored away. still greater quantities of carbon are imprisoned in the substance of our limestones, which contain, when pure, per cent. of this element. a large quantity of oxygen has also been taken from the atmosphere to combine with other elements during their oxidation. the question is--was the atmosphere, in the geological past, more richly laden with carbonic acid gas, of which some has entered into combination with lime to form limestone, while some has been decomposed by plants, the carbon being buried as coal, and the oxygen as products of oxidation? or, has the atmosphere been furnished with continuous fresh supplies of carbonic acid gas? chapter ii. the process of life. in the foregoing chapter, on "the nature of animal life," we have seen that animals breathe, feed, grow, are sensitive, exhibit various activities, and reproduce their kind. these may be regarded as primary life-processes, in virtue of which the animal characterized by them is a living creature. we have now to consider some of these life-processes--the sum of which we may term the process of life--a little more fully and closely. the substance that exhibits these life-processes is protoplasm, which exists in minute separate masses termed cells. it seems probable, however, that these cells, separate as they seem, are in some cases united to each other by minute protoplasmic filaments. in the higher animals the cells in different parts of the body take on different forms and perform different functions. like cells with like functions are also aggregated together into tissues. thus the surfaces of the body, external and internal, are bounded by or lined with epithelial tissue; the bones and framework of the body are composed of skeletal tissue; nervous tissue goes to form the brain and nerves; contractile tissue is found in the muscles; while the blood and lymph form a peculiar nutritive tissue. the organs of the body are distinct parts performing definite functions, such as the heart, stomach, or liver. an organ may be composed of several tissues. thus the heart has contractile tissue in its muscular walls, epithelial tissue lining its cavities, and skeletal tissue forming its framework. still, notwithstanding their aggregation into tissues and organs, it remains true that the body of one of the higher animals is composed of cells, together with certain cell-products, horny, calcareous, or other. the simplest animals, called protozoa, are, however, unicellular, each organism being constituted by a single cell. we must notice that, even during periods of apparent inactivity--for example, during sleep--many life-processes are still in activity, though the vigour of action may be somewhat reduced. when we are fast asleep, respiration, the heart-beat,[b] and the onward propulsion of food through the alimentary canal, are still going on. even at rest, the living animal is a _going_ machine. in some cases, however, as during the hibernating sleep of the dormouse or the bear, the vital activities fall to the lowest possible ebb. moreover, in some cases, the life-processes may be temporarily arrested, but again taken up when the special conditions giving rise to the temporary arrest are removed. frogs, for example, have been frozen, but have resumed their life-activities when subsequently thawed. let us take the function of respiration as a starting-point in further exemplification of the nature of the life-processes of animals. the organs of respiration, in ourselves and all the mammalia, are the lungs, which lie in the thoracic cavity of the chest, the walls of which are bounded by the ribs and breast-bone, its floor being formed of a muscular and movable partition, the diaphragm, which separates it from the stomach and other alimentary viscera in the abdominal region. the lungs fit closely, on either side of the heart, in this thoracic cavity; and when the size of this cavity is altered by movements of the ribs and diaphragm, air is either sucked into or expelled from the lungs through the windpipe, which communicates with the exterior through the mouth or nostrils. it is unnecessary to describe the minute structure of the lungs; suffice it to say that, in the mammal, they contain a vast number of tubes, all communicating eventually with the windpipe, and terminating in little expanded sacs or bags. around these little sacs courses the blood in a network of minute capillary vessels, the walls of which are so thin and delicate that the fluid they contain is only separated from the gas within the sacs by a film of organic tissue. the blood is a colourless fluid, containing a great number of round red blood-discs, which, from their minute size and vast numbers, seem to stain it red. they may be likened to a fleet of little boats, each capable of being laden with a freight of oxygen gas, while the stream in which they float is saturated with carbonic acid gas. this latter escapes into the air-sacs as the fluid courses through the delicate capillary tubes. whither goes the oxygen? whence comes the carbonic acid gas? the answer to these questions is found by following the course of the blood-circulation. the propulsion of the blood throughout the body is effected by the heart, an organ consisting, in mammals, of two receivers (auricles) into which blood is poured, and two powerful force-pumps (ventricles), supplied with blood from the receivers and driving it through great arteries to various parts of the body. there are valves between the receivers and the force-pumps and at the commencement of the great arterial vessels, which ensure the passage of the blood in the right direction. the two receivers lie side by side; the two force-pumps form a single muscular mass; and all four are bound up into one organ; but there is, during adult life, no direct communication between the right and left receivers or the right and left force-pumps. let us now follow the purified stream, with its oxygen-laden blood-discs, as it leaves the capillary tubes of the lungs. it generally collects, augmented by blood from other similar vessels, into large veins, which pour their contents into the left receiver. thence it passes on into the left force-pump, by which it is propelled, through a great arterial vessel and the numerous branches it gives off, to the head and brain, to the body and limbs, to the abdominal viscera; in short, to all parts of the body except the lungs. in all the parts thus supplied, the vessels at length break up into a delicate capillary network, so that the blood-fluid is separated from the tissue-cells only by the delicate organic film of the capillary walls. then the blood begins to re-collect into larger and larger veins. but a change has taken place; the blood-discs have delivered up to the tissues their freight of oxygen; the stream in which they float has been charged with carbonic acid gas. the veins leading from various parts of the body converge upon the heart and pour their contents into the right receiver; thence the blood passes into the right force-pump, by which it is propelled, by arteries, to the lungs. there the blood-discs are again laden with oxygen, the stream is again purified of its carbonic acid gas, and the blood proceeds on its course, to renew the cycle of its circulation. [illustration: fig. .--diagram of circulation. l.a., left auricle of the heart; l.v., left ventricle; h., capillary plexus of the head; b., capillary plexus of the body; a.c., alimentary canal; lr., liver; r.a., right auricle of the heart; r.v., right ventricle; lu., lungs.] now, if we study the process of respiration and that of circulation, with which it is so closely associated, in other forms of life, we shall find many differences in detail. in the bird, for example, the mechanism of respiration is different. there is no diaphragm, and the lungs are scarcely distensible. there are, however, large air-sacs in the abdomen, in the thoracic region, in the fork of the merry-thought, and elsewhere. these are distensible, and to reach them the air has to pass through the lungs, and as it thus passes through the delicate tubes of the lungs, it supplies the blood with oxygen and takes away carbonic acid gas. in the frog there is no diaphragm, and there are no ribs. the lungs are hollow sacs with honey-combed sides, and they are inflated from the mouth, which is used as a force-pump for this purpose. in the fish there are no lungs, respiration being effected by means of gills. in these organs the blood is separated from the water which passes over them (being gulped in by the mouth and forced out between the gill-covers) by only a thin organic film, so that it can take up the oxygen dissolved in the water, and give up to the water the carbonic acid it contains. in fishes, too, we have only one receiver and one force-pump, the blood passing through the gills on its way to the various parts of the body. in the lobster, again, there are gills, but the mechanism by which the water is drawn over them is quite different, and the blood passes through them on its way to the heart, after passing through the various organs of the body, not on its way from the heart, as in vertebrate fishes. the blood, too, has no red blood-discs. in the air-breathing insects the mechanism is, again, altogether different. the air, which obtains access to the body by spiracles in the sides (see fig. , p. ), is distributed by delicate and beautiful tubes to all parts of the organs; so that the oxygen is supplied to the tissues directly, and not through the intervention of a blood-stream. in the earthworm, on the other hand, there is a distributing blood-stream, but there is no mechanism for introducing the air within the body; while in some of the lowliest forms of life there is neither any introduction of air within the body nor any distribution by means of a circulating fluid. beginning, therefore, with the surface of the body simply absorbent of oxygen, we have the concentration of the absorbent parts in special regions, and an increase in the absorbent surface, either ( ) by the pushing out of processes into the surrounding medium, as in gills; or ( ) by the formation of internal cavities, tubes, or branching passages, as in lungs and the tracheal air-system of insects. what, then, is the essential nature of the respiratory process thus so differently manifested? clearly the supply of oxygen to the cellular tissue-elements, and, generally closely associated with this, the getting rid of carbonic acid gas. let us now glance at the life-processes which minister to nutrition, beginning, as before, with the mode in which these processes are effected in ourselves. the alimentary canal is a long tube running through the body from the mouth to the vent. in the abdominal region it is coiled upon itself, so that its great length may be conveniently packed away. opening into this tube are the ducts of certain glands, which secrete fluids which aid in the digestion of the food. into the mouth there open the ducts of the salivary glands, which secrete the saliva; in the stomach there are a vast number of minute gastric glands; in the intestine, besides some minute tubular glands, there are the ducts of the large liver (which secretes the bile) and the pancreas, or sweetbread. since, with the exception of the openings of these ducts, the alimentary canal is a closed tube, its contents, though lying within the body, are in a sense outside it, just as the fuel in a tubular boiler, though within the boiler, is really outside it. the organic problem, therefore, is how to get the nutritive materials through the walls of the tube and thus into the body. at an ordinary meal we are in the habit of consuming a certain amount of meat, with some fat, together with bread and potatoes, and perhaps some peas or beans and a little salt. this is followed by, say, milky rice-pudding, with which we take some sugar; and a cheese course may, perhaps, be added. the whole is washed down with water more or less medicated with other fluid materials. grouping these substances, there are ( ) water and salts, including calcium phosphate in the milk; ( ) meat, peas, milk, and cheese, all of which contain albuminous or allied materials; ( ) bread, potatoes, and rice, which contain starchy matters; and here we may place the sugar; ( ) fat, associated with the meat or contained in the cream of the milk. now, of all the materials thus consumed, only the water, salts, and sugar are capable, in their unaltered condition, of passing through the lining membrane of the alimentary canal, and thus of entering the body. the albuminous materials, the starchy matter, and the fat--that is to say, the main elements of the food--are, in their raw state, absolutely useless for nutritive purposes. the preparation of the food begins in the mouth. the saliva here acts upon some of the starchy matter, and converts it into a kind of sugar, which _can_ pass through the lining membrane of the alimentary canal, and thus enter the body. the fats and albuminous matters here remain unaltered, though they are torn to pieces by the mastication effected by the teeth. in the stomach the albuminous constituents of the meat are attacked by the gastric juice and converted into peptones; and in this new condition they, too, can soak through the lining membrane of the alimentary canal, and thus can enter the body. in the stomach all action on starch is arrested; but in the intestine, through the effect of a ferment contained in the pancreatic juice, this action is resumed, and the rest of the starch is converted into absorbable sugar. another principle contained in pancreatic juice takes effect on the albuminous matters, and converts them into absorbable peptones. the pancreatic juice also acts on the fats, converting them into an emulsion, that is to say, causing them to break up into exceedingly minute globules, like the butter globules in milk. it furthermore contains a ferment which splits up the fats into fatty acids and glycerine; and these fatty acids, with an alkaline carbonate contained in small quantities in pancreatic juice, form soluble soaps, which further aid in emulsifying fats. the bile also aids in emulsifying fats. the effect, then, of the various digestive fluids upon the food is to convert the starch, albuminous material, and fat into sugar, peptones, glycerine, and soap, and thus render them capable of passing through the lining membrane of the canal into the body. the materials thus absorbed are either taken up into the blood-stream or pass into a separate system of vessels called lacteals. all the blood which comes away from the alimentary canal passes into the liver, and there undergoes a good deal of elaboration in that great chemical laboratory of the body. the fluid in the lacteals passes through lymphatic glands, in which it too undergoes some elaboration before it passes into the blood-stream by a large vessel or duct. thus the blood, which we have seen to be enriched with oxygen in the lungs, is also enriched with prepared nutritive material through the processes of digestion and absorption in the alimentary organs and elaboration in the liver and lymphatic glands. here let us again notice that the details of the process of nutrition vary very much in different forms of life. in some mammals the organs of digestion are specially fitted to deal with a flesh diet; in others they are suited for a diet of herbs. in the graminivorous birds the grain is swallowed whole, and pounded up in the gizzard. the leech swallows nothing but blood. the earthworm pours out a secretion on the leaves, by which they are partially digested before they enter the body. many parasitic organisms have no digestive canal, the nutritive juices of their host being absorbed by the general external surface of the body. but the essential life-process is in all cases the same--the absorption of nutritive matter to be supplied to the cell or cells of which the organism is built up. thus in the mammal the blood, enriched with oxygen in the lungs, and enriched also with nutritive fluids, is brought, in the course of its circulation, into direct or indirect contact with all the myriads of living cells in the body. in the first place, the material thus supplied is utilized for and ministers to the growth of the organs and tissues. this growth is effected by the multiplication of the constituent cells. the cells themselves have a very limited power of growth. but, especially in the early stages of the life of the organism, when well supplied with nutriment, the cells multiply rapidly, by a process of fission, or the division of each cell into two daughter cells. the first part of the cell to divide is the nucleus, the protoplasmic network of which shows, during the process, curious and interesting arrangements and groupings of the fibres. when the nucleus has divided, the surrounding protoplasm is constricted, and separates into two portions, each of which contains a daughter nucleus. in addition to the multiplication of cells, there is the formation, especially during periods of growth, of certain products of cell-life and cell-activity. bone, for example, is a more or less permanent product of the activity of certain specialized cells. there is, perhaps, no more wonderful instance of rapid and vigorous growth than the formation of the antlers of deer. these splendid weapons and adornments are shed and renewed every year. in the spring, when they are growing, they are covered over with a dark skin provided with short, fine, close-set hair, and technically termed "the velvet." if you lay your hand on the growing antler, you will feel that it is hot with the nutrient blood that is coursing beneath it. it is, too, exceedingly sensitive and tender. an army of tens of thousands of busy living cells is at work beneath that velvet surface, building the bony antlers, preparing for the battles of autumn. each minute cell knows its work, and does it for the general good--so perfectly is the body knit into an organic whole. it takes up from the nutrient blood the special materials it requires; out of them it elaborates the crude bone-stuff, at first soft as wax, but ere long to become as hard as stone; and then, having done its work, having added its special morsel to the fabric of the antler, it remains embedded and immured, buried beneath the bone-products of its successors or descendants. no hive of bees is busier or more replete with active life than the antler of a stag as it grows beneath the soft, warm velvet. and thus are built up in the course of a few weeks those splendid "beams," with their "tynes" and "snags," which, in the case of the wapiti, even in the confinement of our zoological gardens, may reach a weight of thirty-two pounds, and which, in the freedom of the rocky mountains, may reach such a size that a man may walk, without stooping, beneath the archway made by setting up upon their points the shed antlers. when the antler has reached its full size, a circular ridge makes its appearance at a short distance from the base. this is the "burr," which divides the antler into a short "pedicel" next the skull, and the "beam" with its branches above. the circulation in the blood-vessels of the beam now begins to languish, and the velvet dies and peels off, leaving the hard, dead, bony substance exposed. then is the time for fighting, when the stags challenge each other to single combat, while the hinds stand timidly by. but when the period of battle is over, and the wars and loves of the year are past, the bone beneath the burr begins to be eaten away and absorbed, through the activity of certain large bone-eating cells, and, the base of attachment being thus weakened, the beautiful antlers are shed; the scarred surface skins over and heals, and only the hair-covered pedicel of the antler is left.[c] not only are there these more or less permanent products of cell-activity which are built up into the framework of the body; there are other products of a less enduring, but, in the case of some of them, not less useful character. the secretions, for example, which, as we have seen, minister in such an important manner to nutrition, are of this class. the salivary fluids, the gastric juice, the pancreatic products, and the bile,--all of these are products of cell-life and cell-activity. and then there are certain products of cell-life which must be cast out from the body as soon as possible. these are got rid of in the excretions, of which the carbonic acid gas expelled in the lungs and the waste-products eliminated through the kidneys are examples. they are the ultimate organic products of the combustion that takes place in the muscular, nervous, and other tissues. the animal organism has sometimes been likened to a steam-engine, in which the food is the fuel which enters into combustion with the oxygen taken in through the lungs. it may be worth while to modify and modernize this analogy--always remembering, however, that it is an analogy, and that it must not be pushed too far. in the ordinary steam-engine the fuel is placed in the fire-box, to which the oxygen of the air gains access; the heat produced by the combustion converts the water in the boiler into steam, which is made to act upon the piston, and thus set the machinery in motion. but there is another kind of engine, now extensively used, which works on a different principle. in the gas-engine the fuel is gaseous, and it can thus be introduced in a state of intimate mixture with the oxygen with which it is to unite in combustion. this is a great advantage. the two can unite rapidly and explosively. in gunpowder the same end is effected by mixing the carbon and sulphur with nitre, which contains the oxygen necessary for their explosive combustion. and this is carried still further in dynamite and gun-cotton, where the elements necessary for explosive combustion are not merely mechanically mixed, but are chemically combined in a highly unstable compound. but in the gas-engine, not only is the fuel and the oxygen thus intimately mixed, but the controlled explosions and the resulting condensation are caused to act directly on the piston, and not through the intervention of water in a boiler. whereas, therefore, in the steam-engine the combustion is to some extent external to the working of the machine, in the gas-engine it is to a large extent internal and direct. now, instead of likening the organism as a whole to a steam-engine, it is more satisfactory to liken each cell to a gas-engine. we have seen that the cell-substance around the nucleus is composed of a network of protoplasm, the plasmogen, enclosing within its meshes a more fluid material, the plasm. it is probable that this more fluid material is an explosive, elaborated through the vital activity of the protoplasmic network. during the period of repose which intervenes between periods of activity, the protoplasmic network is busy in construction, taking from the blood-discs oxygen, and from the blood-fluid carbonaceous and nitrogenous materials, and knitting these together into relatively unstable explosive compounds. these explosive compounds are like the mixed air and gas of the gas-engine. a rested muscle may be likened to a complex and well-organized battery of gas-engines. on the stimulus supplied through a nerve-channel a series of co-ordinated explosions takes place: the gas-engines are set to work; the muscular fibres contract; the products of the explosions (one of which is carbonic acid gas) are taken up and hurried away by the blood-stream; and the protoplasm sets to work to form a fresh supply of explosive material. long before the invention of the gas-engine, long before gun-cotton or dynamite were dreamt of, long before some chinese or other inventor first mixed the ingredients of gunpowder, organic nature had utilized the principle of controlled explosions in the protoplasmic cell. certain cells are, however, more delicately explosive than others. those, for example, on or near the external surface of the body--those, that is to say, which constitute the end organs of the special senses--contain explosive material which may be fired by a touch, a sound, an odour, the contact with a sapid fluid or a ray of light. the effects of the explosions in these delicate cells, reinforced in certain neighbouring nerve-knots (ganglionic cells), are transmitted down the nerves as along a fired train of gunpowder, and thus reach that wonderful aggregation of organized and co-ordinated explosive cells, the brain. here it is again reinforced and directed (who, at present, can say how?) along fresh nerve-channels to muscles, or glands, or other organized groups of explosives. and in the brain, somehow associated with the explosion of its cells, consciousness and the mind-element emerges; of which we need only notice here that it belongs to a _wholly different order of being_ from the physical activities and products with which we are at present concerned. no analogies between mechanical contrivances and organic processes can be pushed very far. to liken the organic cell to a gas-engine is better than to liken the organism to a steam-engine, because it serves to indicate the fact that the fuel does not simply combine with the oxygen in combustion, but that an unstable or explosive combination of "fuel" and oxygen is first formed; and again, because the effect of this is direct, and not through the intervention of any substance to which the combustion merely supplies the necessary heat. but beyond the fact that a kind of explosive is formed which, like a fulminating compound, can be fired by a touch, there is no very close analogy to be drawn. nor must we press the explosion analogy too far. the essential thing would seem to be this--which, perhaps, the analogy may have served to lead up to--that the vital protoplasmic network of the cell has the power of building up complex and unstable chemical compounds, which are probably stored in the plasm within the spaces between the threads of the network; and that these unstable compounds, under the influence of a stimulus (or, possibly, sometimes spontaneously) break down into simpler and more stable compounds.[d] in the case of muscle-cells, this latter change is accompanied by an alteration in length of the fibres and consequent movements in the organism, the products of the disruptive change being useless or harmful, and being, therefore, got rid of as soon as possible. but very frequently the products of explosive activity are made use of. in the case of bone-cells, one of the products of disruption is of permanent use to the organism, and constitutes the solid framework of the skeleton. in the case of the secreting cells of the salivary and other digestive glands, one of the disruptive products is of temporary value for the preparation of the food. it is exceedingly probable that these useful products of disruption, permanent or temporary, took their origin in waste products for which natural selection has found a use, and which have been, through natural selection, rendered more and more efficacious. this, however, is a question we are not at present in a position to discuss. in the busy hive of cells which constitutes what we call the animal body, there is thus ceaseless activity. during periods of apparent rest the protogen filaments of the cell-net are engaged in constructive work, building up fresh supplies of complex and unstable materials, which, during periods of apparent activity, break up into simpler and more stable substances, some of which are useful to the organism while others must be got rid of as soon as possible. from another point of view, the cells during apparent rest are storing up energy which is utilized by the organism during its periods of activity. the storing up of available energy may be likened to the winding up of a watch or clock; it is during apparent rest that the cell is winding itself up; and thus we have the apparent paradox that the cell is most active and doing most work when it is at rest. during the repose of an organ, in fact, the cells are busily working in preparation for the manifestation of energetic action that is to follow. just as the brilliant display of intellectual activity in a great orator is the result of the silent work of a lifetime, so is the physical manifestation of muscular power the result of the silent preparatory work of the muscle-cells.[e] one point to be specially noted is the varied activity of the cells. while they are all working for the general good of the organism, they are divided into companies, each with a distinct and definite kind of work. this is known as the physiological division of labour. it is accompanied by a morphological differentiation of structure. by the form of a cell, therefore, we can generally recognize the kind of work it has to perform. the unstable compounds produced by the various cells must also be different, though not much is known at present on this subject. the unstable compound which forms bone and that which forms the salivary ferment, the unstable matter elaborated by nerve-cells and that built up by muscle-cells, are in all probability different in their chemical nature. whether the formative plasmogen from which these different substances originate is in all cases the same or in different cases different, we do not know. it may, perhaps, seem strange that the products of cellular life should be reached by the roundabout process of first producing a very complex substance out of which is then formed a less complex substance, useful for permanent purposes, as in bone, or temporary purposes, as in the digestive fluids. it seems a waste of power to build up substances unnecessarily complex and stored with an unnecessarily abundant supply of energy. still, though we do not know that this course is adopted in all cases, there is no doubt that it is adopted in a great number of instances. and the reason probably is that by this method the organs are enabled to act under the influence of stimuli. they are thus like charged batteries ready to discharge under the influence of the slightest organic touch. in this way, too, is afforded a means by which the organ is not dependent only upon the products of the immediate activity of the protoplasm at the time of action, but can utilize the store laid up during a considerable preceding period. sufficient has now been said to illustrate the nature of the process of life. the fact that i wish to stand out clearly is that the animal body is stored with large quantities of available energy resident in highly complex and unstable chemical compounds, elaborated by the constructive energy of the formative protoplasm of its constituent cells. these unstable compounds, eminently explosive according to our analogy, are built up of materials derived from two different sources--from the nutritive matter (containing carbon, hydrogen, and nitrogen) absorbed in the digestive organs, and from oxygen taken up from the air in the lungs. the cells thus become charged with energy that can be set free on the application of the appropriate stimulus, which may be likened to the spark that fires the explosive. let us note, in conclusion, that it is through the blood-system, ramifying to all parts of the body, and the nerve-system, the ramifications of which are not less perfect, that the larger and higher organisms are knit together into an organic whole. the former carries to the cell the raw materials for the elaboration of its explosive products, and, after the explosions, carries off the waste products which result therefrom. the nerve-fibres carry the stimuli by which the explosive is fired, while the central nervous system organizes, co-ordinates, and controls the explosions, and directs the process of reconstruction of the explosive compounds. notes [b] it has before been noticed that the organs themselves have their periods of rest. the rhythm of rest and repose in the heart is not that of the activity and sleep of the organism, but that of the contraction and relaxation of the organ itself. [c] from a popular article of the author's on "horns and antlers," in _atalanta_. [d] it will be well here to introduce the technical terms for these changes. the general term for chemical actions occurring in the tissues of a living creature is _metabolism_; where the change is of such a nature that complex and unstable compounds are built up and stored for a while, it is called _anabolism_; where complex unstable compounds break up into less complex and relatively stable compounds, the term _katabolism_ is applied. we shall speak of anabolic changes as _constructive_; katabolic, as _disruptive_, or sometimes, _explosive_. [e] i do not mean, of course, to imply that there is no reconstruction during activity, but that it is then distinctly outbalanced by disruptive changes. chapter iii. reproduction and development. we have now to turn to a fresh aspect of animal life, that of reproduction; and it will be well to connect this process as closely as possible with the process of life in general, of which it is a direct outcome. it will be remembered that, in the last chapter, it was shown that the essential feature in the process of life is the absorption by living protoplasm of oxygen on the one hand and nutritive matter on the other hand, and the kneading of these together, in subtle metabolism, into unstable compounds, which we likened to explosives. this is the first, or constructive, stage of the life-process. thereupon follows the second, or disruptive, stage. the unstable compounds break down into more stable products,--they explode, according to our analogy; and accompanying the explosions are manifestations of motor activity--of heat, sometimes of light and electrical phenomena. but in the economy of nature the products of explosion are often utilized, and in the division of labour among cells the explosions of some of them are directed specially to the production of substances which shall be of permanent or temporary use--for digestion, as in the products of the salivary, gastric, and intestinal glands; for support, as in bone, cartilage, and skeletal tissue generally; or as a store of nutriment, in fat or yolk. the constructive products of protoplasmic activity seem for the most part to be lodged in the spaces between the network of formative protoplasm. the disruptive products--those of them, that is to say, which are of temporary or permanent value to the organism--accumulate either within the cell, sometimes at one pole, sometimes at the centre, as in the case of the yolk of eggs, or around the cell, as in the case of cartilage or bone. apart from and either preceding or accompanying these phenomena, is the growth or increase of the formative protoplasm itself; concerning which the point to be here observed is that it is not indefinite, but limited. this was first clearly enunciated by herbert spencer, and may be called spencer's law. in simplest expression it may thus be stated: _volume tends to outrun surface._ take a cube measuring one inch in the side; its volume is one cubic inch, its surface six square inches. eight such cubes will have a surface of ( Ã� ) forty-eight square inches. but let these eight be built into a larger cube, two inches in the side, and it will be found that the surface exposed is now only twenty-four square inches. while the volume has been increased eight times, the surface has been increased only four times. with increase of size, volume tends to outrun surface. but in the organic cell the nutritive material and oxygen are absorbed at the surface, while the explosive changes occur throughout its mass. increase of size, therefore, cannot be carried beyond certain limits, for the relatively diminished surface is unable to supply the relatively augmented mass with material for elaboration into unstable compounds. hence the cell divides to afford the same mass increased surface. this process of cell-division is called fission, and in some cases cleavage. we will now proceed to pass in review the phenomena of reproduction and development in animals. [illustration: fig. .--protozoa. a, vorticella extended. b, the same contracted. c, d, monads. e, am[oe]ba. f, _param[oe]cium_. g, _gregarina_. c.f., contractile fibre; c.v., contractile vesicle; d., disc; end., endoplast; f.v., food-vacuole; fl., flagellum; gu., gubernaculum; n., nucleus; p.a., potential anus; ps., (in a) peristome, (in e) pseudopodium; vs., vestibule.] attention has already been drawn to the difference between those lowly organisms, each of which is composed of a single cell--the protozoa, as they are termed--and those higher organisms, called metazoa, in which there are many cells with varied functions. confining our attention at first to the former group of unicellular animals, we find considerable diversities of form and habit, from the relatively large, sluggish, parasitic _gregarina_, to the active slipper-animalcule, or _param[oe]cium_, or the beautiful, stalked bell-animalcule, or _vorticella_; and from the small, slow-moving am[oe]ba to the minute, intensely active monad. in many cases reproduction is by simple fission, as in the am[oe]ba, where the nucleus first undergoes division; and then the whole organism splits into two parts, each with its own nucleus. in other cases, also numerous, the organism passes into a quiescent state, and becomes surrounded with a more or less toughened cyst. the nucleus then disappears, and the contents of the cyst break up into a number of small bodies or spores. eventually the cyst bursts, and the spores swarm forth. in the case of some active protozoa the minute creatures that swarm forth are more or less like the parent; but in the more sluggish kinds the minute forms are more active than the parent. thus in the case of the gregarina, the minute spore-products are like small am[oe]bæ; while in other instances the embryos, if so we may call them, have a whip-like cilium like the monads. very frequently, however, there is, in the protozoa, a further process, which would seem to be intimately associated with fission or the formation of spores, as the case may be. this is known as conjugation. among monads, for example, two individuals may meet together, conjugate, and completely fuse the one into the other. a triangular cyst results. after a while, the cyst bursts, and an apparently homogeneous fluid escapes. the highest powers of the microscope fail to disclose in it any germ of life; and there, at first sight, would seem to be an end of the matter. but wait and watch; and there will appear in the field of the microscope, suddenly and as if by magic, countless minute points, which prolonged watching shows to be growing. and when they have further grown, each distinct point is seen to be a monad. in the slipper-animalcule, conjugation is temporary. but during the temporary fusion of the two individuals important changes are said to occur. in these infusorians there is, beside the nucleus, a smaller body, the paranucleus. this, in the case of conjugating param[oe]cia, appears to divide into two portions, of which one is mutually exchanged. thus when two slipper-animalcules are in conjugation, the paranucleus of each breaks into two parts, _a_ and _b_, of which _a_ is retained and _b_ handed over in exchange. the old _a_ and the new _b_ then unite, and each param[oe]cium goes on its separate way. m. maupas, who has lately reinvestigated this matter, considers, as the result of his observations on another infusorian (_stylonichia_), that without conjugation these organisms become exhausted, and multiplication by fission comes to a standstill. if this be so, conjugation is, in these organisms, necessary for the continuance of the race. but richard hertwig has recently shown that this is, at any rate, not universally true. in the bell-animalcule, fission takes place in such a manner as to divide the bell into two equal portions. thus there are two bells to one stalk. but the fate of the two is not the same. one remains attached to the stalk, and expands into a complete vorticella. the other remains pear-shaped, and develops round the posterior region of the body a girdle of powerful vibratile cilia, by the lashing of which the animalcule tears itself away from the parent stem, and swims off through the water. after a short active existence, it settles down in a convenient spot, adhering by its posterior extremity. the hinder girdle of cilia is lost or absorbed, a stalk is rapidly developed, and the organism expands into a perfect vorticella. in some cases, however, the fission is of a different character, with different results. it may be very unequal, so that a minute, free-swimming animalcule is disengaged; or minute animalcules may result by repetition of division. in either case the minute form conjugates with an ordinary vorticella, its smaller mass being completely merged in the larger volume of its mate. there are, of course, many variations in detail in the modes of protozoan reproduction; but we may say that, omitting such details, reproduction is either by simple fission or by spore-formation; and that these processes are in some cases associated with, and perhaps dependent on, the temporary or permanent union of two individuals in conjugation. it is essential to notice that the results of fission or of spore-formation separate, each going on its own way. hence such development as we find in the protozoa results from differentiations within the limits of the single cell. thus the bell-animalcule has a well-defined and constant form; a definite arrangement of cilia round the rim and in the vestibule by which food finds entrance to the body. the outer layer of the body forms a transparent cuticle, beneath which is a so-called "myophan" layer, continuous with a contractile thread in the stalk. within the substance of the body is a pulsating cavity, or contractile vesicle, and a nucleus. such is the nature of the differentiation which may go on within the protozoan cell. when we pass to the metazoa, we find that the method of differentiation is different. these organisms are composed of many cells; and instead of the parts of the cell differentiating in several directions, the several cells differentiate each in its own special direction. this is known as the physiological division of labour. the cells merge their individuality in the general good of the organism. each, so to speak, cultivates some special protoplasmic activity, and neglects everything else in the attainment of this end. the adult metazoan, therefore, consists of a number of cells which have diverged in several, sometimes many, directions. in some of the lower metazoans, reproduction may be effected by fission. thus the fresh-water hydra is said to divide into two parts, each of which grows up into a perfect hydra. it is very doubtful, however, whether this takes place normally in natural life. but there is no doubt that if a hydra be artificially divided into a number of special pieces, each will grow up into a perfect organism, so long as each piece has fair samples of the different cells which constitute the body-wall. sponges and sea-anemones may also be divided and subdivided, each part having the power of reproducing the parts that are thus cut away. when a worm is cut in half by the gardener's spade, the head end grows a new tail; and it is even stated that a worm not only survived the removal of the first five rings, including the brain, mouth, and pharynx, but within fifty-eight days had completely regenerated these parts. higher up in the scale of metazoan life, animals have the power of regenerating lost limbs. the lobster that has lost a claw reproduces a new one in its stead. a snail will reproduce an amputated "horn," or tentacle, many times in succession, reproducing in each case the eye, with its lens and retina. even a lizard will regenerate a lost tail or a portion of a leg. in higher forms, regeneration is restricted to the healing of wounds and the mending of broken bones. closely connected with this process of regeneration of lost parts is the widely prevalent process of reproduction by budding. the cut stump of the amputated tentacle of the hydra or the snail buds forth a new organ. but in the hydra, during the summer months, under normal circumstances, a bud may make its appearance and give rise to a new individual, which will become detached from the parent, to lead a separate existence. in other organisms allied to the hydra the buds may remain in attachment, and a colony will result. this, too, is the result of budding in many of the sponges. in some worms, too, budding may occur. in the fresh-water worm (_chætogaster limnæi_) the animal, as we ordinarily see it, is a train of individuals, one budded off behind the other--the first fully developed, those behind it in various stages of development. the individuals finally separate by transverse division. another more lowly worm (_microstomum lineare_, a turbellarian) may bud off in similar fashion a chain of ten or fifteen individuals. in these cases budding is not far removed from fission. now, in the case of reproduction by budding, as in the hydra, a new individual is produced from some group of cells in the parent organism. from this it is but a step--a step, however, of the utmost importance--to the production of a new individual from a single cell from the tissues of the parental organism. such a reproductive cell is called an egg-cell, or ovum. in the great majority of cases, to enable the ovum to develop into a new individual, it is necessary that the egg-cell should conjugate or fuse with a minute, active sperm-cell, generally derived from a different parent. this process of fusion of germinal cells is called fertilization (see fig. , p. ). in sponges, the cells which become ova or sperms lie scattered in the mid-layer between the ciliated layers which line the cavities and spaces of the organism. sometimes the individual sponge produces only ova; sometimes only sperms; sometimes both, but at different periods. the cells which become ova increase in size, are passive, and rich in reserve material elaborated by their protoplasm. the cells which become sperms divide again and again, and thus produce minute active bodies, adance with restless motion. these opposite tendencies are repeated and emphasized throughout the animal kingdom--ova relatively large, passive, and accumulative of reserve material; sperms minute, active, and the result of repeated fission. the active sperm, when it unites with the ovum, imports into it a tendency to fission, or cleavage; but the resulting cells do not part and scatter--they remain associated together, and in mutual union give rise to a new sponge. [illustration: fig. .--hydra viridis. a, hydra half retracted, with a bud and an ovum attached to the shrunken ovary; b, a small hydra firmly retracted; c, a hydra fully extended. b., bud; f., foot; h.s., hypostome; ovm., ovum; ovy., ovary; t., tentacles; ts., testis.] in the hydra, generally near the foot or base of attachment, a rounded swelling often makes its appearance in autumn. within this swelling one central cell increases enormously at the expense of the others. it becomes an ovum. eventually it bursts through the swelling, but remains attached for a time. rarely in the same hydra, more frequently in another, one or two swellings may be seen higher up, beneath the circle of tentacles. within these, instead of the single ovum may be seen a swarm of sperms, minute and highly active. when these are discharged, one may fuse with and fertilize an ovum, occasionally in the same, but more frequently in another individual, with the result that it develops into a new hydra. here there are definite organs--an ovary and a testis--producing the ova or the sperms. but they are indefinite and not permanent in position. in higher forms of life the organs which are set apart for the production of ova or sperms become definite in position and definite in structure. occasionally, as in the snail, the same organ produces both sperms and ova, but then generally in separate parts of its structure. the two products also ripen at different times. not infrequently, as in the earthworm, each individual has both testes and ovaries, and thus produces both ova and sperms, but from different organs. the ova of one animal are, however, fertilized by sperms from another. but in the higher invertebrates and vertebrates there is a sex-differentiation among the individuals, the adult males being possessed of testes only and producing sperms, the adult females possessed of ovaries only and producing ova. there are also, in many cases, accessory structures for ensuring that the ova shall be fertilized by sperms, while sexual appetences are developed to further the same end. but however the matter may thus be complicated, the essential feature is the same--the union of a sluggish, passive cell, more or less laden with nutritive matter, with a minute active cell with an hereditary tendency to fission.[f] it is not, however, necessary in all cases that fertilization of the ovum should take place. the plant-lice, or _aphides_ of our rose trees, may produce generation after generation, and their offspring in turn reproduce in like manner, without any union or fusion of ovum or sperm. the same is true of the little water-fleas, or _daphnids_; while in some kinds of rotifers fertilization is said never to occur. it is a curious and interesting fact, which seems now to be established beyond question, that drone bees are developed from unfertilized ova, the fertilized ova producing either queens or workers, according to the nature of the food with which the grubs are supplied. where, as in the case of aphids and daphnids, fertilization occasionally takes place, it would seem that lowered temperature and diminished food-supply are the determining conditions. fertilization, therefore, generally takes place in the autumn; the fertilized ovum living on in a quiescent state during the winter, and developing with the warmth of the succeeding spring. in the artificial summer of a greenhouse, reproduction may continue for three or four years without the occurrence of any fertilization. [illustration: fig. .--aurelia: life-cycle. a, embryo; b, _hydra tuba_; c, _hydra tuba_, with medusoid segments; d, medusa separated to lead free existence.] mention may here be made of some peculiarly modified modes of reproduction among the metazoa. the aurelia is a well-known and tolerably common jelly-fish. these produce ova, which are duly fertilized by sperms from a different individual. a minute, free-swimming embryo develops from the ovum, which settles down and becomes a little polyp-like organism, the _hydra tuba_. as growth proceeds, this divides or segments into a number of separable, but at first connected, parts. as these attain their full development, first one and then another is detached from the free end, floats off, and becomes a medusoid aurelia. thus the fertilized ovum of aurelia develops, not into one, but into a number of medusæ,[g] passing through the _hydra tuba_ condition as an intermediate stage. many of the hydroid zoophytes, forming colonies of hydra-like organisms, give rise in the warm months to medusoid jelly-fish, capable of producing ova and sperms. fertilization takes place; and the fertilized ova develop into little hydras, which produce, by budding, new colonies. in these new colonies, again, the parts which are to become ovaries or testes float off, and ripen their products in free-swimming, medusoid organisms. such a rhythm between development from ova and development by budding is spoken of as an alternation of generations. the fresh-water sponge (_spongilla_) exhibits an analogous rhythm. the ova are fertilized by sperms from a different short-lived individual. they develop into sponges which have no power of producing ova or sperms. but on the approach of winter in europe, and of the dry season in india, a number of cells collect and group themselves into a so-called gemmule. round this is formed a sort of crust beset with spicules, which, in some cases, have the form of two toothed discs united by an axial shaft. when these gemmules have thus been formed, the sponge dies; but the gemmules live on in a quiescent state during the winter or the dry season, and with the advent of spring develop into sponges, male or female. these have the power of producing sperms or ova, but no power of producing gemmules. the power of producing ova, and that of producing gemmules, thus alternates in rhythmic fashion. [illustration: fig. .--liver-fluke: embryonic stages. (after a. p. thomas.) a. ovum: em., embryo; op., operculum. b. _limnæus truncatulus_ (natural size). c. free embryo: e.s., eye-spot; ex., excretory vessel; g.c., germinal cells; h.p., head-papilla. d. embryo preparing to become a sporocyst: g.c., germinal cells. e. sporocyst: g., gastrula; m., morula; re., redia. f. redia: b.o., birth-opening; ce., cercaria; col., collar; di., digestive sac; ph., pharynx; p.pr., posterior processes; re., daughter redia. g. cercaria: cys., cystogenous organ; di., digestive sac; o.s., oral sucker; p.s., posterior sucker; ph., pharynx.] but one more example of these modified forms of reproduction can here be cited (from the author's text-book on "animal biology"). the liver-fluke is a parasitic organism, found in the liver of sheep. here it reaches sexual maturity, each individual producing many thousands of eggs, which pass with the bile into the alimentary canal of the _host_, and are distributed over the fields with the excreta. here, in damp places, pools, and ditches, free and active embryos are hatched out of the eggs. each embryo (fig. , c., much enlarged) is covered with cilia, except at the anterior end, which is provided with a head-papilla (h.p.). when the embryo comes in contact with any object, it, as a rule, pauses for a moment, and then darts off again. but if that object be the minute water-snail, _limnæus truncatulus_ (fig. , b., natural size), instead of darting off, the embryo bores its way into the tissues until it reaches the pulmonary chamber, or more rarely the body-cavity. here its activity ceases. it passes into a quiescent state, and is now known as a _sporocyst_ (fig. , e.). the active embryo has degenerated into a mere brood-sac, in which the next generation is to be produced. for within the sporocyst special cells undergo division, and become converted into embryos of a new type, which are known as _rediæ_ (f.), and which, so soon as they are sufficiently developed, break through the wall of the sporocyst. they then increase rapidly in size, and browse on the digestive gland of the water-snail (known as the _intermediate host_), to which congenial spot they have in the mean time migrated. the series of developmental changes is even yet not complete. for within the rediæ (besides, at times, daughter rediæ) embryos of yet another type are produced by a process of cell-division. these are known as _cercariæ_ (fig. , g.). each has a long tail, by means of which it can swim freely in water. it leaves the intermediate host, and, after leading a short, active life, becomes encysted on blades of grass. the cyst is formed by a special larval organ, and is glistening snowy white. within the cyst lies the transparent embryonic liver-fluke, which has lost its tail in the process of encystment. the last chapter in this life-history is that in which the sheep crops the blade of grass on which the parasite lies encysted; whereupon the cyst is dissolved in the stomach of the host, the little liver-fluke becomes active, passes through the bile-duct into the liver of the sheep, and there, growing rapidly, reaches sexual maturity, and lays its thousands of eggs, from each of which a fresh cycle may take its origin. the sequence of phenomena is characterized by discontinuity of development. instead of the embryo growing up continuously into the adult, with only the atrophy of provisional organs (e.g. the gills and tail of the tadpole, or embryo frog), it produces germs from which the adult is developed. not merely provisional organs, but provisional organisms, undergo atrophy. in the case of the liver-fluke there are two such provisional organisms, the embryo sporocyst and the redia. we may summarize the life-cycle thus-- . _ovum_ laid in liver of sheep, passes with bile into intestine, and thence out with the excreta. . _free ciliated embryo_, in water or on damp earth, passes into pulmonary cavity of _limnæus truncatulus_, and develops into . _sporocyst_, in which secondary embryos are developed, known as . _rediæ_, which pass into the digestive glands of _limnæus_, and within which, besides daughter rediæ, there are developed tertiary embryos, or . _cercariæ_, which pass out of the intermediate host and become . _encysted_ on blades of grass, which are eaten by sheep. the cyst dissolves, and the young flukes pass into the liver of their host, each developing into . a _liver-fluke_, sexual, but hermaphrodite. here, again, we notice that one fertilized ovum gives rise to not one, but a number of liver-flukes. we must now pass on to consider the growth and development of organisms. simple growth results from the multiplication of similar cells. as the child, for example, grows, the framework of the body and the several organs increase in size by continuous cell-multiplication. development is differential growth; and this may be seen either in the organs or parts of an organism or in the cells themselves. as the child grows up into a man, there is a progressive change in his relative proportions. the head becomes relatively smaller, the hind limbs relatively longer, and there are changes in the proportional size of other organs. in the development of the embryo from the ovum, the differentiation is of a deeper and more fundamental character. cells at first similar become progressively dissimilar, and out of a primitively homogeneous mass of cells is developed a heterogeneous system of different but mutually related tissues. this view of development is, however, the outcome of comparatively modern investigation and perfected microscopical appliances. the older view was that development in all cases is nothing more than differential growth, that there is no differentiation of primitively similar into ultimately different parts. within the fertilized ovum of the horse or bird lay, it was supposed, in all perfection of structure, a miniature racer or chick, the parts all there, but too minute to be visible. all that was required was that each part should grow in due proportion. those who held this view, however, divided into two schools. the one believed that the miniature organism was contained within the ovum, the function of the sperm being merely to stimulate its subsequent developmental growth. the other held that the sperm was the miniature organism, the ovum merely affording the food-material necessary for its developmental growth. in either case, this unfolding of the invisible organic bud was the _evolution_ of the older writers on organic life. more than this. as messrs. geddes and thomson remind us,[h] "the germ was more than a marvellous bud-like miniature of the adult. it necessarily included, in its turn, the next generation, and this the next--in short, all future generations. germ within germ, in ever smaller miniature, after the fashion of an infinite juggler's box, was the corollary logically appended to this theory of preformation and unfolding." modern embryology has completely negatived any such view as that of preformation, and as completely established that the evolution is not the unfolding of a miniature germ, but the growth and differentiation of primitively similar cell-elements. in different animals, as might be expected, the manner and course of development are different. we may here illustrate it by a very generalized and so to speak diagrammatic description of the development of a primitive vertebrate. [illustration: fig. .--diagram of development. see text. the fine line across g. indicates the plane of section shown in h.] the ovum before fertilization is a simple spherical cell, without any large amount of nutritive material in the form of food-yolk (a.). it contains a nucleus. previous to fertilization, however, in many forms of life, portions of the nucleus, amounting to three parts of its mass, are got rid of in little "polar cells" budded off from the ovum. the import of this process we shall have to consider in connection with the subject of heredity. the sperm is also a nucleated cell; and on its entrance into the ovum there are for a short time two nuclei--the female nucleus proper to the ovum, and the male nucleus introduced by the sperm. these two unite and fuse to form a joint nucleus. thus the fertilized ovum starts with a perfect blending of the nuclear elements from two cells produced by different parents. then sets in what is known as the segmentation or cleavage of the ovum. first the nucleus and then the cell itself divides into two equal halves (b.), each of these shortly afterwards again dividing into two. we may call the points of intersection of these two planes of division the "poles," and the planes "vertical planes." we thus have four cells produced by two vertical planes (c.). the next plane of division is equatorial, midway between the poles. by this plane the four cells are subdivided into eight (d.). then follow two more vertical planes intermediate between the first two. by them the eight cells are divided into sixteen. these are succeeded by two more horizontal planes midway between the equator and the poles. thus we get thirty-two cells. so the process continues until, by fresh vertical and horizontal planes of division, the ovum is divided into a great number of cells. but meanwhile a cavity has formed in the midst of the ovum. this makes its appearance at about the eight-cell stage, the eight cells not quite meeting in the centre of the ovum. the central cavity so formed is thus surrounded by a single layer of cells, and it remains as a single layer throughout the process of segmentation, so that there results a hollow vesicle composed of a membrane constituted by a single layer of cells (e.). the cells on one side of the vesicle are rather larger than the others, and the next step in the process is the apparent pushing in of this part of the hollow sphere; just as one might take a hollow squash indiarubber ball, and push in one side so as to form a hollow, two-layered cup (f.). the vesicle, then, is converted into a cup, the mouth of which gradually closes in and becomes smaller, while the cup itself elongates (g.).[i] thus a hollow, two-layered, stumpy, worm-like embryo is produced, the outer layer of which may be ciliated, so that by the lashing of these cilia it is enabled to swim freely in the water. the inner cavity is the primitive digestive cavity. a cross-section through the middle of the embryo at this stage will show this central cavity surrounded by a two-layered body-wall (h.). a little later the following changes take place (j. k.): along a definite line on the surface of the embryo, marking the region of the back, the outer layer becomes thickened; the edges of the thickened band so produced rise up on either side, so as to give rise to a median groove between them; and then, overarching and closing over the groove, convert it into a tube. this tube is called the neural tube, because it gives rise to the central nervous system. in the region of the head it expands; and from its walls, by the growth and differentiation of the cells, there is formed--in the region of the head, the brain, and along the back, the spinal cord. immediately beneath it there is formed a rod of cells, derived from the inner layer. this rod, which is called the notochord, is the primitive axial support of the body. around it eventually is formed the vertebral column, the arches of the vertebræ embracing and protecting the spinal cord. meanwhile there has appeared between the two primitive body-layers a third or middle layer.[j] the cells of which it is composed arise from the inner layer, or from the lips of the primitive cup when the outer and inner layer pass the one into the other. this middle layer at first forms a more or less continuous sheet of cells between the inner and the outer layers. but ere long it splits into two sheets, of which one remains adherent to the inner layer and one to the outer layer. the former becomes the muscular part of the intestinal or digestive tube, the latter the lining of the body-wall. the space between the two is known as the body-cavity. beneath the throat the heart is fashioned out of this middle layer. very frequently--that is to say, in many animals--the opening by which the primitive digestive tube communicated with the exterior has during these changes closed up, so that the digestive cavity does not any longer communicate in any way with the exterior. this is remedied by the formation of a special depression or pit at the front end for the mouth, and a similar pit at the hinder end.[k] these pits then open into the canal, and communications with the exterior are thus established. the lungs and liver are formed as special outgrowths from the digestive tube. the ovaries or testes make their appearance _at a very early period_ as ridges of the middle layer projecting into the body-cavity. for some time it is impossible to say whether they will produce sperms or ova; and it is said that in many cases they pass through a stage in which one portion has the special sperm-producing, and another the special ovum-producing, structure. but eventually one or other prevails, and the organs become either ovaries or testes. thus from the outer layer of the primitive embryo is produced the outer skin, together with the hairs, scales, or feathers which it carries; from it also is produced the nervous system, and the end-organs of the special senses. from the inner layer is formed the digestive lining of the alimentary tube and the glands connected therewith; from it also the primitive axial support of the body. but this primitive support gives place to the vertebral column formed round the notochord; and this is of mid-layer origin. out of the middle layer are fashioned the muscles and framework of the body; out of it, too, the heart and reproductive organs. the tissues of many of the organs are cunningly woven out of cells from all three layers. the lens of the eye, for example, is a little piece of the outer layer pinched off and rendered transparent. the retina of that organ is an outgrowth from the brain, which, as we have seen, was itself developed from the outer layer. but round the retina and the lens there is woven from the middle layer the tough capsule of the eye and the circular curtain or iris. the lining cells of the digestive tube are cells of the inner layer, but the muscular and elastic coats are of middle-layer origin. the lining cells of the salivary glands arise from the outer layer where it is pushed in to form the mouth-pit; but the supporting framework of the glands is derived from the cells of the middle layer. enough has now been said to give some idea of the manner in which the different tissues and organs of the organism are elaborated by the gradual differentiation of the initially homogeneous ovum. the cells into which the fertilized egg segments are at first all alike; then comes the divergence between those which are pushed in to line the hollow of the cup, and those which form its outer layer. thereafter follows the differentiation of a special band of outer cells to form the nervous system, and a special rod, derived from the inner cells, to form the primitive axial support. and when the middle layer has come into existence, its cells group themselves and differentiate along special lines to form gristle or bone, blood or muscle. the description above given is a very generalized and diagrammatic description. there are various ways in which complexity is introduced into the developmental process. the store of nutritive material present in the egg, for example, profoundly modifies the segmentation so that where, as in the case of birds' eggs, there is a large amount of food-yolk, not all the ovum, but only a little patch on its surface, undergoes segmentation. in this little patch the embryo is formed. break open an egg upon which a hen has been sitting for five or six days, and you will see the little embryo chick lying on the surface of the yolk. the large mass of yolk to which it is attached is simply a store of food-material from which the growing chick may draw its supplies. for it is clear that the growing and developing embryo must obtain, in some way and from some source, the food-stuff for its nutrition. and this is effected, among different animals, in one of three ways. either the embryo becomes at a very early stage a little, active, voracious, free-swimming larva, obtaining for itself in these early days of life its own living; as is the case, for example, with the oyster or the star-fish. or the egg from which it is developed contains a large store of food-yolk, on which it can draw without stint; as is the case with birds. or else the embryo becomes attached to the maternal organism in such a way that it can draw on her for all the nutriment which it may require; as is the case with the higher mammals. in both these latter cases the food-material is drawn from the maternal organism, and is the result of parental sacrifice; but in different ways. in the case of the bird, the protoplasm of the ovum has acquired the power of storing up the by-products of its vital activity. the ovum of such an animal seems at first sight a standing contradiction to the statement, made some pages back, that the cell cannot grow to any great extent without undergoing division or fission; and this because volume tends to outrun surface. for the yolk of a bird's egg is a single cell, and is often of large size. but when we come to examine carefully these exceptional cases of very large cells--for what we call the yolk of an egg is, i repeat, composed of a single cell--we find that the formative protoplasm is arranged as a thin patch on one side of the yolk in the case of the bird's egg, or as a thin pellicle surrounding the yolk in the case of that of the lobster or the insect. all the rest is a product of protoplasmic life stowed away beneath the patch or within the pellicle. and this stored material is relatively stable and inert, not undergoing those vital disruptive changes which are characteristic of living formative protoplasm. the mass of formative protoplasm, even in the large eggs of birds, is not very great, and is so arranged as to offer a relatively extensive surface. all the rest, the main mass of the visible egg-yolk, is the stored product of a specialized activity of the formative protoplasm. but all this material is of parental origin--is elaborated from the nutriment absorbed and digested by the mother. thus we see, in the higher types of life, parental sacrifice, fosterage, and protection. for in the case of mammals and many birds, especially those which are born in a callow, half-fledged condition, even when the connection of mother and offspring is severed, or the supplies of food-yolk are exhausted, and the young are born or hatched, there is still a more or less prolonged period during which the weakly offspring are nourished by milk, by a secretion from the crop ("pigeon's milk"), or by food-stuff brought with assiduous care by the parents. there is a longer or shorter period of fosterage and protection--longer in the case of man than in that of any of the lower animals--ere the offspring are fitted to fend for themselves in life's struggle. and accompanying this parental sacrifice, first in supplying food for embryonic development, and then in affording fosterage and protection during the early stages of growth, there is, as might well be supposed, a reduction in the number of ova produced and of young brought forth or hatched. many of the lower organisms lay hundreds of thousands of eggs, each of which produces a living active embryo. the condor has but two downy fledglings in a year; the gannet lays annually but a single egg; while the elephant, in the hundred years of its life, brings forth but half a dozen young. we shall have to consider by what means these opposite tendencies (a tendency to produce enormous numbers of tender, ill-equipped embryos, and a tendency to produce few well-equipped offspring) have been emphasized. the point now to be noted is that every organism, even the slowest breeder that exists, produces more young than are sufficient to keep up the numbers of the species. if every pair of organisms gave birth to a similar pair, and if this pair survived to do likewise, the number of individuals in the species would have no tendency either to increase or to diminish. but, as a matter of fact, animals actually do produce from three or four times to hundreds or even thousands of times as many new individuals as are necessary in this way to keep the numbers constant. this is the _law of increase_. it may be thus stated: _the number of individuals in every race or species of animals is tending to increase._ practically this is only a tendency. by war, by struggle, by competition, by the preying of animals upon each other, by the stress of external circumstances, the numbers are thinned down, so that, though the births are many, the deaths are many also, and the survivals few. in the case of those species the numbers of which are remaining constant, out of the total number born only two survive to procreate their kind. we may judge, then, of the amount of extermination that goes on among those animals which produce embryos by the thousand or even the hundred thousand. the effects of this enormous death-rate on the progress of the race or species we shall have to consider in the next chapter, when the question of the differentiation of species is before us. there is one form of differentiation, however, which we may glance at before closing this chapter--the differentiation of sex. we are not in a position to discuss the ultimate causes of sex-differentiation, but we may here note the proximate causes as they seem to be indicated in certain cases. among honey-bees there are males (drones), fertile females (queens), and imperfect or infertile females (workers). it has now been shown, beyond question, that the eggs from which drones develop are not fertilized. the presence or absence of fertilization in this case determines the sex. during the nuptial flight, a special reservoir, possessed by the queen bee, is stored with sperms in sufficient number to last her egg-laying life. it is in her power either to fertilize the eggs as they are laid or to withhold fertilization. if the nuptial flight is prevented, and the reservoir is never stored with sperms, she is incapable of laying anything but drone eggs. the cells in which drones are developed are somewhat smaller than those for ordinary workers; but what may be the nature of the stimulus that prompts the queen to withhold fertilization we at present do not know. the difference between the fertile queen and the unfertile worker seems to be entirely a matter of nutrition. if all the queen-embryos should die, the workers will tear down the partitions so as to throw three ordinary worker-cells into one; they will destroy two of the embryos, and will feed the third on highly nutritious and stimulating diet; with the result that the ovaries and accessory parts are fully developed, and the grub that would have become an infertile worker becomes a fertile queen. and one of the most interesting points about this change, thus wrought by a stimulating diet, is that not only are the reproductive powers thus stimulated, but the whole organism is modified. size, general structure, sense-organs, habits, instincts, and character are all changed with the development of the power of laying eggs. the organism is a connected whole, and you cannot modify one part without deeply influencing all parts. this is the _law of correlated variation_. herr yung has made some interesting experiments on tadpoles. under normal circumstances, the relation of females to males is about to . but when the tadpoles were well fed on beef, the proportion of females to males rose so as to become to ; and on the highly nutritious flesh of frogs the proportion became to . a highly nutritious diet and plenty of it caused a very large preponderance of females. mrs. treat, in america, found that if caterpillars were half-starved before entering upon the chrysalis state, the proportion of males was much increased; while, if they were supplied with abundant nutritious food, the proportion of female insects was thereby largely increased. the same law is said to hold good for mammals. favourable vital conditions are associated with the birth of females; unfavourable, with that of males. herr ploss attempts to show that, among human folk, in hard times there are more boys born; in good times, more girls. on the whole, we may say that there is some evidence to show that in certain cases favourable conditions of temperature, and especially nutrition, tend to increase the number of females. we have seen that many animals pass through a stage where the reproductive organs are not yet differentiated into male and female, while in some there is a temporary stage where the outer parts of the organ produce ova and the inner parts sperms. we have also seen that the ova are cells where storage is in excess; the sperms are cells in which fission is in excess. favourable nutritive conditions may, therefore, not incomprehensibly lead to the formation of well-stored ova; unfavourable nutritive conditions, on the other hand, to the formation of highly subdivided sperms. by correlated variation,[l] the ova-bearing or sperm-bearing individuals then develop into the often widely different males and females. notes [f] professor geddes and mr. j. arthur thomson, in their interesting work on "the evolution of sex," regard the ovum in especial, and the female in general, as preponderatingly anabolic (see note, p. ); while the sperm in especial, and the male in general, are on their view preponderatingly katabolic. regarding, as i do, the food-yolk as a katabolic product, i cannot altogether follow them. the differentiation seems to me to have taken place along divergent lines of katabolism. in the ovum, katabolism has given rise to storage products; in the sperm, to motor activities associated with a tendency to fission. the contrast is not between anabolic and katabolic tendencies, but between storage katabolism and motor katabolism. nor do i think that "the essentially katabolic male-cell brings to the ovum a supply of characteristic waste products, or katastates, which stimulate the latter to division" (_l.c._, p. ). i believe that it brings an inherited tendency to fission, and thus reintroduces into the fertilized ovum the tendency which, as ovum, it had renounced in favour of storage katabolism. [g] on the other hand, three ova of the crustacean _apus_ are said to coalesce to form the single ovum from which one embryo develops. [h] "the evolution of sex," p. . [i] in some forms of life the opening of the cup marks the position of the future mouth: in others, of the future vent. in yet others it elongates into a slit, occupying the whole length of the embryo; the middle part of the slit closes up, and the opening at the far ends mark the position, the one of the future mouth, the other of the future vent. [j] in technical language, the outer layer of cells is called the _epiblast_, the inner layer the _hypoblast_, and the mid-layer between them the _mesoblast_. [k] in technical language, the opening by which the primitive digestive cavity (or _mesenteron_) communicates with the exterior is called the _blastopore_. when this closes, the new opening for the mouth is called the _stomod[oe]um_; that for the vent, the _proctod[oe]um_. [l] we have seen that when volume tends to outrun surface, fission may take place, whereby the same volume has increased surface. but in unfavourable nutritive conditions, the same surface which had before been sufficient for nutrition may become, under the less favourable circumstances, insufficient, and fission may again take place to give a larger absorbent surface. hence, possibly, the connection between insufficient nutriment and highly subdivided sperms. chapter iv. variation and natural selection. everything, so far as in it lies, said benedict spinoza, tends to persist in its own being. this is _the law of persistence_. it forms the basis of newton's first law of motion, which enunciates that, if a body be at rest, it will remain so unless acted on by some external force; or, if it be in motion, it will continue to move in the same straight line and at a uniform velocity unless it is acted on by some external force. practically every known body is thus affected by external forces; but the law of persistence is not thereby disproved. it only states what would happen under certain exceptional or perhaps impossible circumstances. to those ignorant of scientific procedure, it seems unsatisfactory, if not ridiculous, to formulate laws of things, not as they are, but as they might be. many well-meaning but not very well-informed people thus wholly misunderstand and mistake the value of certain laws of political economy, because in those laws (which are generalized statements of fact under narrowed and rigid conditions, and do not pretend to be inculcated as rules of conduct) benevolence, sentiment, even moral and religious duty, are intentionally excluded. these laws state that men, under motives arising out of the pursuit of wealth, will act in such and such a way, unless benevolence, sentiment, duty, or some other motive, lead them to act otherwise. such laws, which hold good, not for phenomena in their entirety, but for certain isolated groups of facts under narrowed conditions, are called _laws of the factors_ of phenomena. and since the complexity of phenomena is such that it is difficult for the human mind to grasp all the interlacing threads of causation at a single glance, men of science have endeavoured to isolate their several strands, and, applying the principle of analysis, without which reasoning is impossible, to separate out the factors and determine their laws. in this chapter we have to consider some of the factors of organic progress, and endeavour to determine their laws. the law of heredity may be regarded as that of persistence exemplified in a series of organic generations. when, as in the am[oe]ba and some other protozoa, reproduction is by simple fission, two quite similar organisms being thus produced, there would seem to be no reason why (modifications by surrounding circumstances being disregarded) hereditary persistence should not continue indefinitely. where, however, reproduction is effected by the detachment of a single cell from a many-celled organism, hereditary persistence[m] will be complete only on the condition that this reproductive cell is in some way in direct continuity with the cells of the parent organism or the cell from which that parent organism itself developed. and where, in the higher animals, two cells from two somewhat different parents coalesce to give origin to a new individual, the phenomena of hereditary persistence are still further complicated by the blending of characters handed on in the ovum and the sperm; still further complication being, perhaps, produced by the emergence in the offspring of characters latent in the parent, but derived from an earlier ancestor. and if characters acquired by the parents in the course of their individual life be handed on to the offspring, yet further complication will be thus introduced. it is no matter for surprise, therefore, that, notwithstanding the law of hereditary persistence, variations should occur in the offspring of animals. at the same time, it must be remembered that the occurrence of variations is not and cannot be the result of mere chance; but that all such variations are determined by some internal or external influences, and are thus legitimate and important subjects of biological investigation. in the next chapter we shall consider at some length the phenomena of heredity and the origin of variations. here we will accept them without further discussion, and consider some of their consequences. but even here, without discussing their origin, we must establish the fact that variations do actually occur. variations may be of many kinds and in different directions. in colour, in size, in the relative development of different parts, in complexity, in habits, and in mental endowments, organisms or their organs may vary. observers of mammals, of birds, and of insects are well aware that colour is a variable characteristic. but these colour-variations are not readily described and tabulated. in the matter of size the case is different. in mr. wallace's recent work on "darwinism" a number of observations on size-variations are collected and tabulated. as this is a point of great importance, i propose to illustrate it somewhat fully from some observations i have recently made of the wing-bones of bats. in carrying out these observations and making the necessary measurements, i have had the advantage of the kind co-operation of my friend mr. henry charbonnier, of clifton, an able and enthusiastic naturalist.[n] the nature of the bat's wing will be understood by the aid of the accompanying figure (fig. ). in the fore limb the arm-bone, or humerus, is followed by an elongated bone composed of the radius and ulna. at the outer end of the radius is a small, freely projecting digit, which carries a claw. this answers to the thumb. then follow four long, slender bones, which answer to the bones in the palm of our hand. they are the metacarpals, and are numbered ii., iii., iv., and v. in the tabulated figures in which the observations are recorded. the metacarpals of the second and third digits run tolerably close together, and form the firm support of the anterior margin of the wing. those of the third and fourth make a considerable angle with these and with each other, and form the stays of the mid part of the wing. beyond the metacarpals are the smaller joints or phalanges of the digits, two or three to each digit. the third digit forms the anterior point or apex of the wing. the fourth and fifth digits form secondary points behind this. between these points the wing is scalloped into bays. [illustration: fig. .--"wing" of bat (pipistrelle). hu., humerus, or arm-bone; ul., conjoined radius and ulna, a bone in the forearm; po., pollex, answering to our thumb; ii., iii., iv., v., second, third, fourth, and fifth digits of the manus, or hand. the figures are placed near the metacarpals, or palm-bones. these are followed by the phalanges. fe., femur or thigh-bone; ti., tibia, the chief bone of the shank. the digits of the pes, or foot, are short and bear claws. ca., calcar.] from the point of the fifth or last digit the leathery wing membrane sweeps back to the ankle. the bones of the hind limb are the femur, or thigh-bone, and the tibia (with a slender, imperfectly developed fibula). there are five toes, which bear long claws. from the ankle there runs backward a long, bony and gristly spur, which serves to support the membrane which stretches from the ankle to the tip (or near the tip) of the tail. thus the wing of the bat consists of a membrane stretched on the expanded or spread fingers of the hand, and sweeping from the point of the little finger to the ankle. behind the ankle there is a membrane reaching to the tip of the tail. this forms a sort of net in which some bats, at any rate, as i have myself observed, can catch insects. i have selected the wing of the bat to exemplify variation, ( ) because the bones are readily measured even in dried specimens; ( ) because they form the mutually related parts of a single organ; and ( ) because they offer facilities for the comparison of variations, not only among the individuals of a single species, but also among several distinct species. the method employed has been as follows: the several bones have been carefully measured in millimetres,[o] and all the bones tabulated for each species. such tables of figures are here given in a condensed form for three species of bats. bat-measurements (in millimetres). -------------------------------------------------------------------------- |r | p | nd | third | fourth | fifth |t | |a | o |digit.| digit. | digit. | digit. |i | |d u | l |------|--------------|--------------|------------|b | |i l | l | m | m | p | p | m | p | p | m | p | p |i | |u n | e | e | e | h | h | e | h | h | e | h | h |a | |s a | x | t | t | a | a | t | a | a | t | a | a |. | | . | . | a | a | l | l | a | l | l | a | l | l | | |a | | c | c | a | a | c | a | a | c | a | a | | |n | | a | a | n | n | a | n | n | a | n | n | | |d | | r | r | g | g | r | g | g | r | g | g | | | | | p | p | e | e | p | e | e | p | e | e | | | | | a | a | | s | a | | s | a | | s | | | | | l | l | | | l | | | l | | | | | | | . | . | . | ,| . | . | ,| . | . | ,| | | | | | | | | | | | | | | | | | | | | | . | | | . | | | . | | ------------------------------------------------------------------------- hairy-armed bat (_vesperugo leisleri_). | | . | | | | | | | | | | | |[male] | | | | | | | | . | | | | . | | " | | | | | | | | | . | | | | | " | . | | | . | | | | | | | | | | " | | | | | . | | | . | | | | . | |[fem.] | | . | . | | . | | | | . | | | . | | " | | | | | . | . | | . | | | | | | " horseshoe bat (_rhinolophus ferri-equinum_). | | | | | | | | | | | | | |[male] | | | | | | | | | | | | | |[fem.] | | | | | | | | | | | | | | " | | | | | | | | | | | | | | " | | | | | | | | | | | | | | ? lesser horseshoe bat (_rhinolophus hipposideros_). | | | | | | | | . | | | | | |[male] | | | | | | | | | | | | | | " | | | | . | | | | | | | | | | " ------------------------------------------------------------------------- it would be troublesome to the reader to pick out the meaning from these figures. i have, therefore, plotted in the measurements for four other species of bats in tabular form (figs. , , , ). fig. , for example, deals with the common large noctule bat, which may often be seen flying high up on summer evenings. now, the mean length of the radius and ulna in eleven individuals was . millimetres. suppose all the eleven bats had this bone (for the two bones form practically one piece) of exactly the same length. there would then be no variation. we may express this supposed uniformity by the straight horizontal line running across the part of the figure dealing with the radius and ulna. practically the eleven bats measured did not have this bone of the same length; in some of them it was longer, in others it was shorter than the mean. let us run through the eleven bats (which are represented by the numbers at the head of the table) with regard to this bone. the first fell below the average by a millimetre and a half, the length being fifty millimetres. this is expressed in the table by placing a dot or point three quarters of a division below the mean line. each division on the table represents two millimetres, or, in other words, the distance between any two horizontal lines stands for two millimetres measured. half a division, therefore, is equivalent to one measured millimetre; a quarter of a division to half a millimetre. the measurements are all made to the nearest half-millimetre. the second bat fell short of the mean by one millimetre. the bone measured . millimetres. the third exceeded the mean by a millimetre and a half; the fourth, by three millimetres and a half. the fifth was a millimetre and a half above the mean; and the sixth and seventh were both half a millimetre over the mean. the eighth fell short by half a millimetre; the ninth and tenth by a millimetre and a half; and the eleventh by two millimetres and a half. the points have been connected together by lines, so as to give a curve of variation for this bone. [illustration: fig .--the noctule (_vesperugo noctula_).] [illustration: fig. .--the long-eared bat (_plecotus auritus_).] [illustration: fig. .--the pipistrelle (_vesperugo pipistrellus_).] [illustration: fig. .--the whiskered bat (_vespertilio mystacinus_).] the other curves in these four tables are drawn in exactly the same way. the mean length is stated; and the amount by which a bone in any bat exceeds or falls short of the mean can be seen and readily estimated by means of the horizontal lines of the table. any one can reconvert the tables into figures representing our actual measurements. now, it may be said that, since some bats run larger than others, such variation is only to be expected. that is true. but if the bones of the wing all varied equally, _all the curves would be similar_. that is clearly not the case. the second metacarpal is the same length in and . but the third metacarpal is two millimetres shorter in than in . in the radius and ulna are _longer_ than in ; but the second metacarpal is _shorter_ in than in . a simple inspection of the table as a whole will show that there is a good deal of _independent_ variation among the bones. the amount of variation is itself variable, and in some cases is not inconsiderable. in the long-eared bats and in fig. , the phalanges of the third digit measured . millimetres in , and millimetres in --a difference of more than per cent. this is unusually large, and it is possible that there may have been some slight error in the measurements.[p] a difference of or per cent. is, however, not uncommon. in any case, the observations here tabulated show ( ) that variations of not inconsiderable amount occur among the related bones of the bat's wing; and ( ) that these variations are to a considerable extent independent of each other. so far we have compared a series of individuals of the same species of bat, each table in figs. - dealing with a distinct species. let us now compare the different species with each other. to effect such a comparison, we must take some one bone as our standard, and we must level up our bats for the purposes of tabulation. i have selected the radius and ulna as the standard. in both the noctule and the greater horseshoe bats the mean length of this bone is . millimetres. the bones of each of the other bats have been multiplied by such a number as will bring them up to the level of size in these two species. mr. galton, in his investigations on the variations of human stature, had to take into consideration the fact that men are normally taller than women. he found, however, that the relation of man to woman, so far as height is concerned, is represented by the proportion to . by multiplying female measurements by . , they were brought up to the male standard, and could be used for purposes of comparison. in the same way, by multiplying in each case by the appropriate number, i have brought all the species in the table (fig. ) up to the standard of the noctule. when so multiplied, the radius and ulna (selected as the standard of comparison) has the same length in all the species, and is hence represented by the horizontal line in the table. [illustration: fig. .--variations adjusted to the standard of the noctule.] compared with this as a standard, the mean length of the second metacarpal in the seven species is forty-three millimetres; that of the third metacarpal, forty-four millimetres; and so on. the amount by which each species exceeds or falls short of the mean is shown on the table, and the points are joined up as before. here, again, the table gives the actual measurements in each case. for example, if the mean length of the third metacarpal of the greater horseshoe bat be required, it is seen by the table to fall short of the mean by four horizontal divisions and a quarter, that is to say, by eight millimetres and a half. the length is therefore ( - - / ) . millimetres. now, it will be seen from the table that the variation in the mean length of the bones in different species is much greater than the individual variations in the members of the same species. the table also brings out in an interesting way the variation in the general character of the wing. the noctule, for example, is especially strong in the development of the second and third metacarpals, the phalanges of the third digit being also a little above the average. reference to the figure of the bat's wing on p. will show that these excellences give length to the wing. it fails, however, in the metacarpal and phalanges of the fifth digit, and in the length of the hind leg as represented by the tibia. on consulting the figure of the wing, it is seen that these are the bones which give breadth to the wing. here the noctule fails. its wing is, therefore, long and narrow. it is a swallow among bats. on the other hand, the horseshoe bats fail conspicuously in the second and third metacarpals, though they make up somewhat in the corresponding digits. on the whole, the wing is deficient in length. but the phalanges of the fourth and fifth digits, and the length of the hind limb represented by the tibia, give a corresponding increase of breadth. the wing is, therefore, relatively short and broad. the long-eared bat, again, has the third metacarpal and its digits somewhat above the mean, and therefore a somewhat more than average length. but it has the fifth metacarpal with its digit and also the tibia decidedly above the mean, and therefore more than average breadth. without possessing the great length of the noctule's wing, or the great breadth of that of the horseshoe, it still has a more than average length and breadth. the total wing-areas are very variable, the females having generally an advantage over the males. i do not feel that our measurements are sufficiently accurate to justify tabulation. taking, however, the radius and ulna as the standard for bringing the various species up to the same level, the greater horseshoe seems to have decidedly the largest wing-area; the noctule stands next; then come the lesser horseshoe and the long-eared bat; somewhat lower stands the hairy-armed bat; while the pipistrelle and the whiskered bat (both small species) stand lowest.[q] sufficient has now been said in illustration of the fact that variations in the lengths of the bones in the bat's wing do actually occur in the various individuals of one species; that the variations are independent; and that the different species and genera have the character of the wing determined by emphasizing, so to speak, variations in special directions. i make no apology for having treated the matter at some length. those who do not care for details will judiciously exercise their right of skipping. as before mentioned, mr. wallace has collected and tabulated other observations on size and length variations. and in addition to such variations, there are the numerous colour-variations that do not admit of being so readily tabulated. mr. cockerell tells us that among snail-shells, taking variations of banding alone, he knows of varieties of _helix nemoralis_ and of _h. hortensis_.[r] that variations do occur under nature is thus unquestionable. and it is clear that all variations necessarily fall under one of three categories. either they are of advantage to the organism in which they occur; or they are disadvantageous; or they are neutral, neither advantageous nor disadvantageous to the animal in its course through life. we must next revert to the fact to which attention was drawn in the last chapter, that every species is tending, through natural generation, to increase in numbers. even in the case of the slow-breeding elephant, the numbers tend to increase threefold in each generation; for a single pair of elephants give birth to three pairs of young. in many animals the tendency is to increase ten, twenty, or thirtyfold in every generation; while among fishes, amphibians, and great numbers of the lower organisms, the tendency is to multiply by a hundredfold, a thousandfold, or even in some cases ten thousandfold. but, as before noticed, this is only a tendency. the law of increase is a law of one factor in life's phenomena, the reproductive factor. in any area, the conditions of which are not undergoing change, the numbers of the species which constitute its fauna remain tolerably constant. they are not actually increasing in geometrical progression. there is literally no room for such increase. the large birth-rate of the constituent species is accompanied by a proportionate death-rate, or else the tendency is kept in check by the prevention of certain individuals from mating and bearing young.[s] now, the high death-rate is, to a large extent among the lower organisms and in a less degree among higher animals, the result of indiscriminate destruction. when the ant-bear swallows a tongue-load of ants, when the greenland whale engulfs some hundreds of thousands of fry at a gulp, when the bear or the badger destroys whole nests of bees,--in such cases there is wholesale and indiscriminate destruction. those which are thus destroyed are nowise either better or worse than those which escape. at the edge of a coral reef minute, active, free-swimming coral embryos are set free in immense numbers. presently they settle down for life. some settle on a muddy bottom, others in too great a depth of water. these are destroyed. the few which take up a favourable position survive. but they are no better than their less fortunate neighbours. the destruction is indiscriminate. so, too, among fishes and the many marine forms which produce a great number of fertilized eggs giving rise to embryos that are from an early period free-swimming and self-supporting. such embryos are decimated by a destruction which is quite indiscriminate. and again, to take but one more example, the liver-fluke, whose life-history was sketched in the last chapter, produces its tens or hundreds of thousands of ova. but the chances are enormously against their completing their life-cycle. if the conditions of temperature and moisture are not favourable, the embryo is not hatched or soon dies; even if it emerges, no further development takes place unless it chances to come in contact with a particular and not very common kind of water-snail. when it emerges from the intermediate host and settles on a blade of grass, it must still await the chance of that blade being eaten by a sheep or goat. it is said that the chances are eight millions to one against it, and for the most part its preservation is due to no special excellence of its own. the destruction is to a large extent, though not entirely, indiscriminate. even making all due allowance, however, for this indiscriminate destruction--which is to a large extent avoided by those higher creatures which foster their young--there remain more individuals than suffice to keep up the normal numbers of the species. among these there arises a struggle for existence, and hence what darwin named _natural selection_. "how will the struggle for existence"--i quote, with some omissions, the words of darwin--"act in regard to variation? can the principle of selection, which is so potent in the hands of man, apply under nature? i think that we shall see that it can act most efficiently. let the endless number of slight variations and individual differences be borne in mind; as well as the strength of the hereditary tendency. let it also be borne in mind how infinitely complex and close-fitting are the mutual relations of all organic beings to each other and to their physical conditions of life; and consequently what infinitely varied diversities of structure might be of use to each being under changing conditions of life. can it, then, be thought improbable, seeing that variations useful to man have undoubtedly occurred, that other variations, useful in some way to each being in the great and complex battle of life, should occur in the course of many successive generations? if such do occur, can we doubt (remembering that many more individuals are born than can possibly survive) that individuals having any advantage, however slight, over others, would have the best chance of surviving and of procreating their kind? on the other hand, we may feel sure that any variation in the least degree injurious would be rigidly destroyed. this preservation of favourable individual differences and variations, and the destruction of those which are injurious, i have called natural selection, or the survival of the fittest. variations neither useful nor injurious would not be affected by natural selection, and would be left either a fluctuating element, or would ultimately become fixed, owing to the nature of the organism and the nature of the conditions."[t] "the principle of selection," says darwin, elsewhere, "may conveniently be divided into three kinds. _methodical selection_ is that which guides a man who systematically endeavours to modify a breed according to some predetermined standard. _unconscious selection_ is that which follows from men naturally preserving the most valued and destroying the less valued individuals, without any thought of altering the breed. lastly, we have _natural selection_, which implies that the individuals which are best fitted for the complex and in the course of ages changing conditions to which they are exposed, generally survive and procreate their kind."[u] i venture to think that there is a more logical division than this. a man who is dealing with animals or plants under domestication may proceed by one of two well-contrasted methods. he may either select the most satisfactory individuals or he may reject the most unsatisfactory. we may term the former process _selection_, the latter _elimination_. suppose that a gardener is dealing with a bed of geraniums. he may either pick out first the best, then the second best, then the third, and so on, until he has selected as many as he wishes to preserve. or, on the other hand, he may weed out first the worst, then in succession other unsatisfactory stocks, until, by eliminating the failures, he has a residue of sufficiently satisfactory flowers. now, i think it is clear that, even if the ultimate result is the same (if, that is to say, he selects the twenty best, or eliminates all but the twenty best), the method of procedure is in the two cases different. selection is applied at one end of the scale, elimination at the other. there is a difference in method in picking out the wheat-grains (like a sparrow) and scattering the chaff by the wind. under nature both methods are operative, but in very different degrees. although the insect may select the brightest flowers, or the hen-bird the gaudiest or most tuneful mate, the survival of the fittest under nature is in the main the net result of the slow and gradual process of the elimination of the unfit.[v] the best-adapted are not, save in exceptional cases, selected; but the ill-adapted are weeded out and eliminated. and this distinction seems to me of sufficient importance to justify my suggestion that _natural selection_ be subdivided under two heads--_natural elimination_, of widespread occurrence throughout the animal world; and _selection proper_, involving the element of individual or special choice. the term "natural elimination" for the major factor serves definitely to connect the natural process with that struggle for existence out of which it arises. the struggle for existence is indeed the reaction of the organic world called forth by the action of natural elimination. organisms are tending to increase in geometrical ratio. there is not room or subsistence for the many born. the tendency is therefore held in check by elimination, involving the struggle for existence. and the factors of elimination are three: first, elimination through the action of surrounding physical or climatic conditions, under which head we may take such forms of disease as are not due to living agency; secondly, elimination by enemies, including parasites and zymotic diseases; and thirdly, elimination by competition. it will be convenient to give some illustrative examples of each of these. elimination through the action of surrounding physical conditions, taken generally, deals with the very groundwork or basis of animal life. there are certain elementary mechanical conditions which must be fulfilled by every organism however situated. any animal which fails to fulfil these conditions will be speedily eliminated. there are also local conditions which must be adequately met. certain tropical animals, if transferred to temperate or sub-arctic regions, are unable to meet the requirements of the new climatic conditions, and rapidly or gradually die. fishes which live under the great pressure of the deep sea are killed by the expansion of the gases in their tissues when they are brought to the surface. many fresh-water animals are killed if the lake in which they live be invaded by the waters of the sea. if the water in which corals live be too muddy, too cold, or too fresh--near the mouth of a great river on the australian coast, for example--they will die off. during the changes of climate which preceded and followed the oncoming of the glacial epoch, there must have been much elimination of this order. even under less abnormal conditions, the principle is operative. darwin tells us that in the winter of - four-fifths of the birds in his grounds perished from the severity of the weather, and we cannot but suppose that those who were thus eliminated were less able than others to cope with or stand the effects of the inclement climatic conditions. my colleague, mr. g. munro smith, informs me that, in cultivating microbes, certain forms, such as _bacillus violaceus_ and _micrococcus prodigiosus_, remain in the field during cold weather when other less hardy microbes have perished. the insects of madeira may fairly be regarded as affording another instance. the ground-loving forms allied to insects of normally slow and heavy flight have in madeira become wingless or lost all power of flight. those which attempted to fly have been swept out to sea by the winds, and have thus perished; those which varied in the direction of diminished powers of flight have survived this eliminating process. on the other hand, among flower-frequenting forms and those whose habits of life necessitate flight, the madeira insects have stronger wings than their mainland allies. here, since flight could not be abandoned without a complete change of life-habit, since all must fly, those with weaker powers on the wing have been eliminated, leaving those with stronger flight to survive and procreate their kind.[w] in kerguelen island mr. eaton has found that all the insects are incapable of flight, and most of them in a more or less wingless condition.[x] mr. wallace regards the reduction in the size of the wing in the isle of man variety of the small tortoiseshell butterfly as due to the gradual elimination of larger-winged individuals.[y] these are cases of elimination through the direct action of surrounding physical conditions. even among civilized human folk, this form of elimination is still occasionally operative--in military campaigns, for example (where the mortality from hardships is often as great as the mortality from shot or steel), in arctic expeditions, and in arduous travels. but in early times and among savages it must be a more important factor. elimination by enemies needs somewhat fuller exemplification. battle within battle must, throughout nature, as darwin says, be continually recurring with varying success. the stronger devour the weaker, and wage war with each other over the prey. in the battle among co-ordinates the weaker are eliminated, the stronger prevail. when the weaker are preyed upon by the stronger and a fair fight is out of the question, the slow and heavy succumb, the agile and swift escape; stupidity means elimination, cunning, survival; to be conspicuous, unless it be for some nasty or deleterious quality, is inevitably to court death: the sober-hued stand at an advantage. in these cases, if there be true selection at work, it is the selection of certain individuals, the plumpest and most toothsome to wit, for destruction, not for survival. this mode of elimination has been a factor in the development of protective resemblance and so-called mimicry, and we may conveniently illustrate it by reference to these qualities. if the hue of a creature varies in the direction of resemblance to the normal surroundings, it will render the animal less conspicuous, and therefore less liable to be eliminated by enemies. this is well seen in the larvæ or caterpillars of many of our butterflies and moths. it is not easy to distinguish the caterpillar of the clouded yellow, so closely does its colour assimilate to the clover leaves on which it feeds, nor that of the lulworth skipper on blades of grass. i would beg every visitor to the natural history museum at south kensington to look through the drawers containing our british butterflies and moths and their larvæ, in the further room on the basement, behind the inspiring statue of charles darwin. half an hour's inspection will serve to bring home the fact of protective resemblance better than many words. it may, however, be remarked that not all the caterpillars exhibit protective resemblance; and it may be asked--how have some of these conspicuous larvæ, that of the magpie moth, for example, escaped elimination? what is sauce for the lulworth goose should be sauce for the magpie gander. how is it that these gaudy and variable caterpillars, cream-coloured with orange and black markings, have escaped speedy destruction? because they are so nasty. no bird, or lizard, or frog, or spider would touch them. they can therefore afford to be bright-coloured. nay, their very gaudiness is an advantage, and saves them from being the subject of unpleasant experiments in the matter. other caterpillars, like the palmer-worms, are protected by barbed hairs that are intensely irritating. they, too, can afford to be conspicuous. but a sweet and edible caterpillar, if conspicuous, is eaten, and thus by the elimination of the conspicuous the numerous dull green or brown larvæ have survived. a walk through the bird gallery in the national collection will afford examples of protective resemblance among birds. look, for example, at the kentish plover with its eggs and young--faithfully reproduced in our frontispiece--and the way in which the creature is thus protected in early stages of its life will be evident. the stone-curlew, the ptarmigan, and other birds illustrate the same fact, which is also seen with equal clearness in many mammals, the hare being a familiar example. many oceanic organisms are protected through general resemblance. some, like certain medusæ, are transparent. the pellucid or transparent sole of the pacific (_achirus pellucidus_), a little fish about three inches long, is so transparent that sand and seaweed can be seen distinctly through its tissues. the salpa is transparent save for the intestine and digestive gland, which are brown, and look like shreds of seaweed. other forms, like the physalia, are cærulean blue. the exposed parts of flat-fish are brown and sandy coloured or speckled like the sea-bottom; and in some the sand-grains seem to adhere to the skin. so, too, with other fish. "looking _down_ on the dark back of a fish," says mr. a. r. wallace, "it is almost invisible, while to an enemy looking _up_ from below, the light under surface would be equally invisible against the light of clouds and sky." even some of the most brilliant and gaudiest fish, such as the coral-fish (_chætodon, platyglossus_, and others), are brightly coloured in accordance with the beautiful tints of the coral-reefs which form their habitat; the bright-green tints of some tropical forest birds being of like import. no conception of the range of protective resemblance can be formed when the creatures are seen or figured isolated from their surroundings. the zebra is a sufficiently conspicuous animal in a menagerie or a museum; and yet mr. galton assures us that, in the bright starlight of an african night, you may hear one breathing close by you, and be positively unable to see the animal. a black animal would be visible; a white animal would be visible; but the zebra's black and white so blend in the dusk as to render him inconspicuous. to cite but one more example, this time from the invertebrates. professor herdman found in a rock-pool on the west coast of scotland "a peculiarly coloured specimen of the common sea-slug (_doris tuberculata_). it was lying on a mass of volcanic rock of a dull-green colour, partially covered with rounded spreading patches of a purplish pink nullipore, and having numerous whitish yellow _spirorbis_ shells scattered over it--the general effect being a mottled surface of dull green and pink peppered over with little cream-coloured spots. the upper surface of the doris was of precisely the same colours arranged in the same way.... we picked up the doris, and remarked the brightness and the unusual character of its markings, and then replaced it upon the rock, when it once more became inconspicuous."[z] then, too, there are some animals with variable protective resemblance--the resemblance changing with a changing environment. this is especially seen in some northern forms, like the arctic hare and fox, which change their colour according to the season of the year, being brown in summer, white and snowy in winter. the chamæleon varies in colour according to the hue of its surroundings through the expansion and contraction of certain pigment-cells; while frogs and cuttle-fish have similar but less striking powers. mr. e. b. poulton's[aa] striking and beautiful experiments show that the colours of caterpillars and chrysalids reared from the same brood will vary according to the colour of their surroundings. [illustration: fig. .--caterpillar of a moth (_ennomos tiliaria_) on an oak-spray. (from an exhibit in the british natural history museum.)] if this process of protective resemblance be carried far, the general resemblance in hue may pass into special resemblance to particular objects. the stick-insect and the leaf-insect are familiar illustrations, though no one who has not seen them in nature can realize the extent of the resemblance. most of us have, at any rate, seen the stick-caterpillars, or loopers (fig. ), though, perhaps, few have noticed how wonderful is the protective resemblance to a twig when the larva is still and motionless, for the very reason that the resemblance is so marked that the organism at that time escapes, not only casual observation, but even careful search. fig. gives a representation of a locust with special protective resemblance to a leaf--not a perfect leaf, but a leaf with fungoid blotches. this insect and the stick-caterpillar may be seen in the insect exhibits on the basement at south kensington, having been figured from them by the kind permission of professor flower. [illustration: fig. .--a locust (_cycloptera speculata_) which closely resembles a leaf. (from an exhibit in the british natural history museum.)] perhaps one of the most striking instances of special protective resemblance is that of the malayan leaf-butterfly (_kallima paralecta_). so completely, when the wings are closed, does this insect resemble a leaf that it requires a sharp eye to distinguish it. these butterflies have, moreover, the habit of alighting very suddenly. as a recent observer (mr. s. b. t. skertchly) remarks, they "fly rapidly along, as if late for an appointment, suddenly pitch, close their wings, and become leaves. it is generally done so rapidly that the insect seems to vanish."[ab] instances might be multiplied indefinitely. mr. guppy thus describes a species of crab in the solomon islands: "the light purple colour of its carapace corresponds with the hue of the coral at the base of the branches, where it lives; whilst the light red colour of the big claws, as they are held up in their usual attitude, similarly imitates the colour of the branches. to make the guise more complete, both carapace and claws possess rude hexagonal markings which correspond exactly in size and appearance with the polyp-cells of the coral."[ac] when the special protective resemblance is not to an inanimate object, but to another organism, it is termed mimicry. it arises in the following way:-- many forms, especially among the invertebrates, escape elimination by enemies through the development of offensive weapons (stings of wasps and bees), a bitter taste (the heliconidæ among butterflies), or a hard external covering (the weevils among beetles). the animals which prey upon these forms learn to avoid these dangerous, nasty, or indigestible creatures; and the avoidance is often instinctive. it thus becomes an advantage to other forms, not thus protected, to resemble the animals that have these characteristics. such resemblance is termed mimicry, concerning which it must be remembered that the mimicry is unconscious, and is reached by the elimination of those forms which do not possess this resemblance. thus the _leptalis_, a perfectly sweet insect, closely resembles the _methona_, a butterfly producing an ill-smelling yellow fluid. the quite harmless _clytus arietis_, a beetle, resembles, not only in general appearance, but in its fussy walk, a wasp. the soft-skinned _doliops_, a longicorn, resembles the strongly encased _pachyrhyncus orbifex_, a weevil. the not uncommon fly _eristalis tenax_ (fig. ), is not unlike a bee, and buzzes in an unpleasantly suggestive manner.[ad] mimicry is not confined to the invertebrates. a harmless snake, the eiger-eter of dutch colonists at the cape, subsists mainly or entirely on eggs. the mouth is almost or quite toothless; but in the throat hard-tipped spines project into the gullet from the vertebræ of the column in this region. here the egg is broken, and there is no fear of losing the contents. now, there is one species of this snake that closely resembles the berg-adder. the head has naturally the elongated form characteristic of the harmless snakes. but when irritated, this egg-eater flattens it out till it has the usual viperine shape of the "club" on a playing-card. it coils as if for a spring, erects its head with every appearance of anger, hisses, and darts forward as if to strike its fangs into its foe, in every way imitating an enraged berg-adder. the snake is, however, quite harmless and inoffensive.[ae] here we have mimicry both in form and habit. another case of imperfect but no doubt effectual mimicry is given by mr. w. larden, in some notes from south america.[af] speaking of the rhea, or south american ostrich, he says, "one day i came across an old cock in a nest that it had made in the dry weeds and grass. its wings and feathers were loosely arranged, and looked not unlike a heap of dried grass; at any rate, the bird did not attract my attention until i was close on him. the long neck was stretched out close along the ground, the crest feathers were flattened, and an appalling hiss greeted my approach. it was a pardonable mistake if for a moment i thought i had come across a huge snake, and sprang back hastily under this impression." protective resemblance and mimicry have been considered at some length because, on the hypothesis of natural selection, they admirably illustrate the results which may be reached through long-continued elimination by enemies. sufficient has now been said to show that this form of elimination is an important factor. we are not at present considering the question how variations arise, or why they should take any particular direction. but granting the fact that variations may and do occur in all parts of the organism, it is clear that, in a group of organisms surrounded by enemies, those individuals which varied in the direction of swiftness, cunning, inconspicuousness,[ag] or resemblance to protected forms, would, other things being equal, stand a better chance of escaping elimination. elimination by competition is, as darwin well points out, keenest between members of the same group and among individuals of the same species, or between different groups or different species which have, so to speak, similar aims in life. while enemies of various kinds are preying upon weaker animals, and thus causing elimination among them, they are also competing one with another for the prey. while the slower and stupider organisms are succumbing to their captors, and thus leaving more active and cunning animals in possession of the field, the slower and stupider captors, failing to catch their cunning and active prey, are being eliminated by competition. while protective resemblance aids the prey to escape elimination by enemies, a correlative resemblance, called by mr. poulton aggressive resemblance, in the captors aids them in stealing upon their prey, and so gives advantage in competition. thus the hunting spider closely resembles the flies upon which he pounces, even rubbing his head with his fore legs after their innocent fashion. as in the case of protective resemblance, so, too, in its aggressive correlative, the resemblance may be general or special, or may reach the climax of mimicry. and since the same organism is not only a would-be captor, but sometimes an unwilling prey, the same resemblance may serve to protect it from its enemies and to enable it to steal upon its prey. the mantis, for example, gains doubly by its resemblance to the vegetation among which it lives. certain spiders, described by mr. h. o. forbes, in java, closely resemble birds'-droppings. this may serve to protect them from elimination by birds; but it also enables them to capture without difficulty unwary butterflies, which are often attracted by such excreta. a parasitic fly (_volucella bombylans_) closely resembles (fig. ) a bumble-bee (_bombus muscorum_), and is thus enabled to enter the nest of the bee without molestation. its larvæ feed upon the larvæ of the bee. the cuckoo bee _psithyrus rupestris_, an idle quean, who collects no pollen, and has no pollen-baskets, steals into the nest of the bumble-bee _bombus lapidarius_, and lays her eggs there. the resemblance between the two is very great, and it not only enables the mother bee to enter unmolested, but the young bees, when they are hatched, to escape. another bee (_nomada solidaginis_), which plays the cuckoo on _halictus cylindricus_, does not resemble this bee, but is wasp-like, and thus escapes molestation, not because it escapes notice, but because it looks more dangerous than it really is.[ah] many are the arts by which, in keen competition, organisms steal a march upon their congeners--not, be it remembered, through any conscious adaptation, but through natural selection by elimination. mr. poulton describes an asiatic lizard (_phrynocephalus mystaceus_) in which the "general surface resembles the sand on which it is found, while the fold of their skin at each angle of the mouth is of a red colour, and is produced into a flower-like shape exactly resembling a little red flower which grows in the sand. insects, attracted by what they believe to be flowers, approach the mouth of the lizard, and are, of course, captured."[ai] the fishing frog, or angler-fish, is possessed of filaments which allure small fry, who think them worms, into the neighbourhood of the great mouth in which they are speedily engulfed; and certain deep-sea forms discovered during the _challenger_ expedition have the lure illumined by phosphorescent light. [illustration: fig. .--mimicry of bees by flies. a, b, _bombus muscorum_; c, d, _volucella bombylans_; e, _eristalis tenax_; f, _apis mellifica_. the underwings of the hive bee (f) were invisible in the photograph from which the figure was drawn. (from an exhibit in the british natural history museum.)] we need say no more in illustration of the resemblances which have enabled certain organisms to escape elimination by competition. once more, be it understood that we are not at present considering _how_ any of these resemblances have been brought about; we are merely indicating that, given certain resemblances, advantageous either for captor or prey, those organisms which possess them not will have to suffer elimination--elimination by enemies, or else elimination by competition. the interaction between these two kinds of elimination is of great importance. hunters and hunted are both, so to speak, playing the game of life to the best of their ability. those who fail on either side are weeded out; and elimination is carried so far that those who are only as good as their ancestors are placed at a disadvantage as compared with their improving congeners. the standard of efficiency is thus improving on each side; and every improvement on the one side entails a corresponding advance on the other. nor is there only thus a competition for subsistence, and arising thereout a gradual sharpening of all the bodily and mental powers which could aid in seeking or obtaining food; there is also in some cases a competition for mates, reaching occasionally the climax of elimination by battle. there is, indeed, competition for everything which can be an object of appetence to the brute intelligence; and, owing to the geometrical tendency in multiplication--the law of increase--the competition is keen and unceasing. such, then, in brief, are the three main modes of elimination: elimination by physical and climatic conditions; elimination by enemies; elimination by competition. observe that it is a differentiating process. unlike the indiscriminate destruction before alluded to, the incidence of which is on all alike, good, bad, and indifferent, it separates the well-adapted from the ill-adapted, dooming the latter to death, and allowing the former to survive and procreate their kind. the destruction is not indiscriminate, but differential. let us now turn to cases of selection, properly so called, where nature is in some way working at the other end of the scale; where her method is not the elimination of the unfit, but the selection of the fit. such a case may be found on darwin's principles in brightly coloured flowers and fruits. "flowers," he says, "rank amongst the most beautiful productions of nature; but they have been rendered conspicuous in contrast with the green leaves, and, in consequence, at the same time beautiful, so that they may be easily observed by insects. i have come to this conclusion from finding it an invariable rule that, when a flower is fertilized by the wind, it never has a gaily coloured corolla. several plants habitually produce two kinds of flowers--one kind open and coloured, so as to attract insects; the other closed, not coloured, destitute of nectar, and never visited by insects. hence we may conclude that, if insects had not been developed on the face of the earth, our plants would not have been decked with beautiful flowers, but would have produced only such poor flowers as we see on our fir, oak, nut, and ash trees, on grasses, spinach, docks, and nettles, which are all fertilized through the agency of the wind. a similar line of argument holds good with fruits; that a ripe strawberry or cherry is as pleasing to the eye as to the palate; that the gaily coloured fruit of the spindle-wood tree, and the scarlet berries of the holly, are beautiful objects,--will be admitted by every one. but this beauty serves merely as a guide to birds and beasts, in order that the fruit may be devoured and manured seeds disseminated: i infer that this is the case from having as yet found no exception to the rule that seeds are always thus disseminated when embedded within a fruit of any kind (that is, within a fleshy or pulpy envelope), if it be coloured of any brilliant tint, or rendered conspicuous by being white or black."[aj] here we have a case of the converse of elimination--a case of genuine selection under nature. but even here the process of elimination also comes into play, for the visitations of flowers by insects involve cross-fertilization. the flowers of two distinct individuals of the same species of plants in this manner fertilize each other; and the act of crossing, as darwin firmly believed, though it is doubted by some observers nowadays, gives rise to vigorous seedlings, which consequently would have the best chance of flourishing and surviving--would best resist elimination by competition. so that we here have the double process at work; the fairest flowers being selected by insects, and those plants which failed to produce such flowers being eliminated as the relatively unfit. if we turn to the phenomena of what darwin termed sexual selection, we find both selection and elimination brought into play. by the law of battle, the weaker and less courageous males are eliminated so far as the continuation of their kind is concerned. by the individual choice of the females (on darwin's view, by no means universally accepted), the finer, bolder, handsomer, and more tuneful wooers are selected. let us again hear the voice of darwin himself. "most male birds," he says, "are highly pugnacious during the breeding season, and some possess weapons especially adapted for fighting with their rivals. but the most pugnacious and the best-armed males rarely or never depend for success solely on their power to drive away or kill their rivals, but have special means for charming the female. with some it is the power of song, or of emitting strange cries, or of producing instrumental music; and the males in consequence differ from the females in their vocal organs or in the structure of certain feathers. from the curiously diversified means for producing various sounds, we gain a high idea of the importance of this means of courtship. many birds endeavour to charm the females by love-dances or antics, performed on the ground or in the air, and sometimes at prepared places. but ornaments of many kinds, the most brilliant tints, combs and wattles, beautiful plumes, elongated feathers, top-knots, and so forth, are by far the commonest means. in some cases, mere novelty appears to have acted as a charm. the ornaments of the males must be highly important to them, for they have been acquired in not a few cases at the cost of increased danger from enemies, and even at some loss of power in fighting with their rivals[ak].... what, then, are we to conclude from these facts and considerations? does the male parade his charms with so much pomp and rivalry for no purpose? are we not justified in believing that the female exerts a choice, and that she receives the addresses of the male who pleases her most?"[al] here again, then, we have the combined action of elimination and selection. and now we may note that selection involves intelligence--involves the play of appetence and choice. hence it is that, when we come to consider the evolution of human-folk, the principle of elimination is so profoundly modified by the principle of selection. not only are the weaker eliminated by the inexorable pressure of competition, but we select the more fortunate individuals and heap upon them our favours. this enables us also to soften the rigour of the blinder law; to let the full stress of competitive elimination fall upon the worthless, the idle, the profligate, and the vicious; but to lighten its incidence on the deserving but unfortunate. both selection and elimination occurring under nature, but elimination having by far the wider scope, we may now inquire what will be their effect as regards the three modes of variation--advantageous, disadvantageous, and neutral. it must be remembered that these modes are relative and dependent upon circumstances, so that variations, neutral under certain conditions, may become relatively disadvantageous under other conditions. selection clearly leads to the preservation of advantageous variations alone, and these variations are advantageous in so far as they meet the taste of the selecting organism. for selection depends upon individual choice; and uniformity of selection is entirely dependent upon uniformity in the standard of taste. if, as darwin contends, the splendid plumage and tuneful notes of male birds are the result of a selection of mates by the hens, there must be a remarkable uniformity of taste among the hens of each particular species, since there is a uniformity of coloration among the cock-birds. it may be said that in all their mental endowments there is greater uniformity among animals than among men; and it is true that individuation has not been carried so far in them as in human-folk. still, careful observers of animals see in them many signs of individual character; and this uniformity in the standard of taste in each species of birds seems to many naturalists a real difficulty in the way of the acceptance of sexual selection. we shall, however, return to this point. for the present it is clear that selection chooses out advantageous variations, that the advantage is determined by the taste of the selector, and that uniform selection implies uniformity of taste. turning to elimination, it is clear that it begins by weeding out, first the more disadvantageous, then the less disadvantageous variations. it leaves both the advantageous and the neutral in possession of the field. i imagine that many, perhaps most, of the variations tabulated by mr. wallace and other observers belong to the neutral category. their fluctuating character seems to indicate that this is so. in any case, they are variations which have so far escaped elimination. and i think they are of great and insufficiently recognized importance. they permit, through interbreeding, of endless experiments in the combination of variations, some of which cannot fail to give favourable results. it is just possible that it may be asked--if in natural elimination there is nothing more than the weeding out of the unfit and the suppression of disadvantageous variations, where is the possibility of advance? the standard may thus be maintained, but where is the possibility of progress? such an objection would, however, imply forgetfulness of the fact that all the favourable variations remain to leaven the residual lump. given a mean, with plus and minus variations: if in any generations the minus variations are got rid of, the mixture of the mean with the plus variations will give a new mean nearer the plus or advantageous end of the scale than the old mean. by how much the favourable variations tend to raise the mean standard, by so much will the race tend to advance. but in this process i see no reason why the neutral variations should be eliminated, except in so far as, in the keen struggle for existence, they become relatively unfavourable. it is clear, however, that the intercrossing and interbreeding which occurs between average individuals on the one hand, and those possessing favourable variations on the other, while it tends gradually to raise the mean standard, tends also at the same time to reduce the advantageous variations towards the mean. it must tend to check advance by leaps and bounds, and to justify the adage, _natura nil facit per saltum_. at the same time, it will probably have a greater tendency to reduce to a mean level neutral variations indefinite in direction than advantageous variations definite in direction. still, it is a most important factor, and one not to be neglected. it tends to uniformity in the species, and checks individualism. it may act as a salutary brake on what we may figuratively term hasty and ill-advised attempts at progress. and at the same time, it favours repeated new experiments in the combination of variations, occasionally, we may suppose, with happy results. but it does more than this. it tends to check, and, if the offspring always possessed the blended character of both parents, would be absolutely fatal to, divergence of character within the interbreeding members of a species. and yet no fact is more striking than this divergence of character. it is seen in the diversified products of human selection; for example, among pigeons. it is seen in the freedom of nature. mr. wallace gives many examples. "among our native species," he says, "we see it well marked in the different species of titmice, pipits, and chats. the great titmouse, by its larger size and stronger bill, is adapted to feed on larger insects, and is even said sometimes to kill small and weak birds. the smaller and weaker coal-titmouse has adopted a more vegetarian diet, eating seeds as well as insects, and feeding on the ground as well as among trees. the delicate little blue titmouse, with its very small bill, feeds on the minutest insects and grubs, which it extracts from crevices of bark and from the buds of fruit trees. the marsh-titmouse, again, has received its name from the low and marshy localities it frequents; while the crested titmouse is a northern bird, frequenting especially pine forests, on the seeds of which trees it partially feeds. then, again, our three common pipits--the tree-pipit, the meadow-pipit, and the rock-pipit, or sea-lark--have each occupied a distinct place in nature, to which they have become specially adapted, as indicated by the different form and size of the hind toe and claw in each species. so the stone-chat, the whin-chat, and the wheat-ear are all slightly divergent forms of one type, with modifications in the shape of the wing, feet, and bill adapting them to slightly different modes of life."[am] there is scarcely a genus that does not afford examples of divergent species. the question then naturally occurs--how have these divergent forms escaped the swamping effects of intercrossing? that perfectly free intercrossing, between any or all of the individuals of a given group of animals, is, so long as the characters of the parents are blended in the offspring, fatal to divergence of character, is undeniable. through the elimination of less favourable variations, the swiftness, strength, and cunning of a race may be gradually improved. but no form of elimination can possibly differentiate the group into swift, strong, and cunning varieties, distinct from each other, so long as all three varieties freely interbreed, and the characters of the parents blend in the offspring. elimination may and does give rise to progress in any given group as a group; it does not and cannot give rise to differentiation and divergence, so long as interbreeding with consequent interblending of characters be freely permitted. whence it inevitably follows, as a matter of simple logic, that where divergence has occurred, intercrossing and interblending must in some way have been lessened or prevented. thus a new factor is introduced, that of _isolation_, or _segregation_. and there is no questioning the fact that it is of great importance.[an] its importance can, indeed, only be denied by denying the swamping effects of intercrossing, and such denial implies the tacit assumption that interbreeding and interblending are held in check by some form of segregation. the isolation explicitly denied is implicitly assumed. there are several ways in which isolation, or segregation, may be effected. isolation by geographical barriers is the most obvious. a stretch of water, a mountain ridge, a strip of desert land, may completely, or to a large extent, prevent any intercrossing between members of a species on either side of the barrier. the animals which inhabit the several islands of the galapagos archipelago are closely allied, but each island has its particular species or well-marked varieties. intercrossing between the several varieties on the different islands is prevented, and divergence is thus rendered possible and proceeds unchecked. it is said that in the zuyder zee a new variety of herrings, the fry of which are very small compared with open-sea herrings, is being developed. and the salmon introduced into tasmania seem to be developing a fresh variety with spots on the dorsal fin and a tinge of yellow on the adipose fin. in the wooded valleys of the sandwich islands there are allied but distinct species of land-shells. the valleys that are nearest each other furnish the most nearly related forms, and the degree of divergence is roughly measured by the number of miles by which they are separated. here there is little or no intercrossing between the slow-moving molluscs in adjoining valleys; none at all between those at any distance apart. but even if there are no well-marked physical barriers, the members of a species on a continent or large island tend to fall into local groups, between which, unless the animal be of a widely ranging habit, there will be little intercrossing. hence local varieties are apt to occur, and varieties show the first beginnings of that divergence which, if carried further and more deeply ingrained, results in the differentiation of species. geographically, therefore, we may have either complete isolation or local segregation, and in both cases the possibility of divergence. another mode of segregation arises also out of geographical conditions. if variations of habits occur (and structure is closely correlated with habit) such that certain individuals take to the mountains, others to the plains or valleys; or that certain individuals take to the forests, others to the open country; the probabilities are that the forest forms will interbreed frequently with each other, but seldom with those in the open, and so with the other varieties. the conditions of forest life or mountain life being thus similar throughout a large area, and life being through elimination slowly but surely adapted to its environment, there might thus arise two distinct varieties scattered throughout the length and breadth of the area, the one inhabiting the mountains, the other the forests. in illustration of this mode of segregation, we may take the case of two species of rats which have recently been found by mr. c. m. woodford on one of the solomon islands. these two quite distinct species are regarded by mr. oldfield thomas as slightly modified descendants of one parent species, the modifications resulting from the fact that of this original species some individuals have adopted a terrestrial, others an arboreal life, and their respective descendants have been modified accordingly. thus _mus rex_ lives in trees, has broad foot-pads, and a long rasp-like, probably semi-prehensile, tail; while _mus imperator_ lives on the ground, has smaller pads, and a short, smooth tail. the segregation of these two species has probably been effected by the difference of their mode of life, and each has been adapted to its special environment through the elimination of those individuals which were not in harmony with the condition of their life. it is probable that this mode of segregation has been an important one. and it is clear that in many cases competition would be a co-operating factor in this process, weaker organisms being forced into otherwise uncongenial habitats through the stress of competitive elimination, the weaker forms not perishing, but being eliminated from more favoured areas. protective coloration may also be a means of segregation. a species of insects having no protective resemblance might vary in two directions--in the direction of green tints, assimilating their hue to that of vegetation; and in the direction of sandy or dull earthy colours, assimilating them to the colour of the soil. in the one variety elimination would weed out all but the green forms, and these would be left to intercross. in the other variety, green forms would be eliminated, dull-brown forms being left to interbreed. stragglers from one group into the other would stand a chance of elimination before interbreeding was effected.[ao] in the case of birds whose freedom of flight gives them a wide range, sometimes almost a world-wide range, it would seem at first sight that their facilities for interbreeding and intercrossing are so great that divergence is well-nigh impossible. and yet the examples of divergence i cited from mr. wallace were taken from birds, and it is well known that divergence is particularly well shown in this class. but when the habits of birds are studied attentively, it is found that, wide as is their range, their breeding area is often markedly restricted. the sanderling and knot range freely during the winter throughout the northern hemisphere; but their breeding area is restricted to the north polar region. the interbreeding within this area keeps the species one and homogeneous, notwithstanding its wide range, and, at the same time, prevents intercrossing with allied species with different breeding-grounds. another most important mode of segregation among animals arises out of habitual or instinctive preferences. where varieties are formed there is a tendency for like to breed with like. in the falkland islands the differently coloured herds of cattle, all descended from the same stock, keep separate, and interbreed with each other, but not with individuals outside their own colour-caste. if two flocks of merino sheep and heath sheep be mixed together, they do not interbreed. in the forest of dean and in the new forest, the dark and pale coloured herds of fallow deer have never been known to intermingle.[ap] here we have a case of selective _segregation through preferential mating_, and may find therein the basis of sexual selection in its higher ranges as advocated by darwin. the question of sexual selection will, however, be briefly considered in the chapter on "organic evolution." at present what we have to notice is that, through preferential mating, segregation is effected. the forms that interbreed have a distinguishing colour. from this it is but a step to the possession, not merely of a distinguishing colour, but of distinguishing colour-markings. hence, through preferential mating, may arise those special markings which so frequently distinguish allied species. they not only enable _us_ to recognize species as distinct, but enable the species which possess them to recognize the members of their own kind. mr. wallace calls these diacritical marks _recognition-marks_, and gives many illustrative examples.[aq] they are especially noticeable in gregarious animals and in birds which congregate in flocks or which migrate together. mr. wallace considers that they "have in all probability been acquired in the process of differentiation for the purpose of checking the intercrossing of allied forms;" for "one of the first needs of a new species would be to keep separate from its nearest allies, and this could be more readily done by some easily seen external mark of difference." this language seems, however, to savour of teleology (that pitfall of the evolutionist). the cart is placed before the horse. the recognition-marks were, i believe, not produced to prevent intercrossing, but intercrossing has been prevented because of preferential mating between individuals possessing special recognition-marks. to miss this point is to miss an important segregation-factor. undoubtedly, other tendencies co-operate in maintaining the standard of the recognition-marks. stragglers who failed in the matter of recognition would get separated from their fellows, and stand a greater chance of elimination by enemies; young who failed in this respect would be in like condemnation. still, i cannot doubt that the foundations of recognition-marks were laid in preferential mating, and that in this we have an important factor in segregation. we may here note, in passing, as also arising out of preference, how the selection of flowers by insects may lead to segregation; for insects seem often to have habitual or instinctive colour-preferences. flowers of similar colour would be thus cross-fertilized, but would not intercross with those of different colour, whence colour-varieties might arise. it is important to note that in these cases there is a psychological factor in evolution. we have so far assumed that intercrossing of parents and interblending of their characters in the offspring always go together. this, we must now notice, is not always the fact. if a blue-eyed saxon marry a dark-eyed italian, the children will have blue eyes _or_ dark eyes, not eyes of an intermediate tint. the characters do not interblend. the _ancon_, or otter-sheep, a breed with a long body and short, bandy legs, appeared in massachusetts as a chance sport in a single lamb. the offspring of this ram were either ancons or ordinary sheep. the ancon characters did not blend. hence for a time a definite breed was maintained. we may call this mode of isolation _isolation by exclusive inheritance_. a further mode of isolation or segregation, for which mr. romanes[ar] claims a foremost, indeed, the foremost, place, is _physiological isolation_ as due to differential fertility. one among the many variations to which organisms are subject is a variation in fertility, which may reach the climax of absolute sterility. but it is clear that a sterile variation carries with it its own death-warrant, since the sterile individual leaves no descendants to inherit its peculiarity. relative infertility, too, unless it chances to be correlated with some unusual excellence, would be no advantage, would be transmitted to few descendants, and would tend to be extinguished. the same is not true, however, of differential fertility. "it is by no means rare," said darwin,[as] "to find certain males and females which will not breed together, though both are known to be perfectly fertile with other males and females." mr. romanes assumes, as a starting-point, the converse of this, namely, that certain males and females will breed together, though they are infertile with all other members of the species. suppose, then, a variety to arise which is perfectly fertile within the limits of the varietal form, but imperfectly fertile or infertile with the parent species. such a variety would have to run the risks of those ill effects which, as darwin showed,[at] are attendant upon close interbreeding. but mr. wallace points out[au] that these ill effects may not be so marked under nature as they are under domestication. suppose, then, that it escapes these ill effects. in this case, mr. romanes urges, it would neither be swamped by intercrossing nor die out on account of sterility. but although it could not be swamped by intercrossing, still, if it arose sporadically, here a case, there a case, and so on, the chances would be enormously against the perpetuation of the variety, unless some co-operating mode of segregation aided in bringing together the varying individuals. if, for example, there were a segregation of these variants in a particular habitat--all the variants meeting in some definite locality for breeding purposes; or if there were a further segregation through mutual preferences; or if, again, there were a further segregation in time the variety might obtain a firm footing. but without these co-operating factors it is clear that if one male and one female in a hundred individuals varied in this particular way, the chances would be at least forty-nine to one against their happening to mate together. it is interesting to note that almost the only particular example given by mr. romanes in illustration of his theory is one that involves the co-operation of one of these further segregation-factors. suppose, he says, the variation in the reproductive system is such that the season of flowering or of pairing becomes either advanced or retarded. this particular variation being inherited, the variety breeding, let us say, in may, the parent species in july, there would arise two races, each perfectly fertile within its own limits, but incapable of crossing with the other. thus is constituted "a barrier to intercrossing quite as effectual as a thousand miles of ocean." yes! a time-barrier instead of a space-barrier. the illustration is faulty, inasmuch as it introduces a mode of segregation other than that in question. i think it very improbable that differential fertility alone, without the co-operation of other segregation-factors, would give rise to separate varieties capable of maintaining themselves as distinct species. that distinct species are generally mutually infertile, or more frequently still, that their male offspring are sterile, is, however, an undoubted fact. but there are, exceptions. fertile hybrids between the sheep and the goat seem to be well authenticated. of rats darwin says that "in some parts of london, especially near the docks, where fresh rats are frequently imported, an endless variety of intermediate forms may be found between the brown, black, and snake rat, which are all three usually ranked as distinct species."[av] fertile hybrids have been produced between the green-tinted japanese and the long-tailed chinese pheasants. mr. thomas moore, of fareham, in hants, has been particularly successful in producing a hybrid breed between the golden pheasant (_thaumalia picta_), whose habitat is southern and south-eastern china, and the amherst pheasant (_thaumalia amherstiæ_), which is found in the mountains of yunnan and thibet. in answer to my inquiries, mr. moore kindly informs me that he "has bred the half-bred gold and amherst pheasant, crossed them again with gold, and recrossed them with half-bred amherst, and kept on crossing until only a strain of the gold pheasant remained. the result is that the birds so produced are far handsomer than either breed, since the feathers composing their tiplets as well as those under the chin are of so beautiful a colour that they beggar description. they all breed most freely, and are much more vigorous than the pure gold or amherst, and their tails reach a length of over three feet. they are also exceedingly prolific. out of a batch of forty-two eggs, forty chickens were hatched out, of which thirty-seven were reared to perfection." still, though there are exceptions, the general infertility of allied species when crossed is a fact in strong contrast with the marked fertility of varieties under domestication; concerning which, however, it should be noted that our domesticated animals have been selected to a very large extent on account of the freedom with which they breed in confinement, and that domestication has probably a tendency to increase fertility. the question, therefore, arises--is the infertility between species, and the general sterility of their male offspring, a secondary effect of their segregation? or is their segregation the direct effect of their differential fertility? the former is the general opinion; the latter is held by mr. romanes. he contends that sterility is the primary distinction of species, other specific characters being secondary, and regards it as a pure assumption to say that the secondary differences between species have been historically prior to the primary difference. i do not propose to discuss this question. while it seems to me in the highest degree improbable that differential fertility, apart from other co-operating factors, has been or could be a practical mode of segregation, it has probably been a not unimportant factor in association with other modes of segregation or isolation. suppose, for example, two divergent local varieties were to arise in adjacent areas, and were subsequently (by stress of competition or by geographical changes) driven together into a single area: we are justified in believing, from the analogy of the falkland island cattle, the forest of dean deer, and other similar observed habits, that preferential breeding, kind with kind, would tend to keep them apart. but, setting this on one side, let us say they interbreed. if, then, their unions are fertile, the isolation will be annulled by intercrossing--the two varieties will form one mean or average variety. but if the unions be infertile, the isolation will be preserved, and the two varieties will continue separate. suppose now, and the supposition is by no means an improbable one, that this has taken place again and again in the evolution of species: then it is clear that those varietal forms which had continued to be fertile together would be swamped by intercrossing; while those varietal forms which had become infertile would remain isolated. hence, in the long run, isolated forms occupying a common area would be infertile. or suppose, once more, that, instead of the unions between the two varietal forms being infertile, they are fertile, but give rise to sterile (mule) or degenerate offspring, as is said to be the case in the unions of japanese and ainos: then it is clear that the sterile or degenerate offspring of such unions would be eliminated, and intercrossing, even though it occurred, would be inoperative while breeding within the limits of the variety continued unchecked. sufficient has now been said concerning the modes of isolation and segregation, geographical, preferential, and physiological. we must now consider their effects. where the isolated varieties are under different conditions of life, there will be, through the elimination of the ill-adapted in each case, differential adoption to these different conditions. but suppose the conditions are similar: can there be divergence in this case? the supposition is a highly hypothetical one, because it postulates that all the conditions, climatal, environmental, and competitive, are alike, which would seldom, if ever, be likely to occur. let us, however, make the supposition. let us suppose that an island is divided into two equal halves by the submersion of a stretch of lowland running across it. then the only possible causes of divergence would lie in the organisms themselves[aw] thus divided into two equal groups. we have seen that variations may be advantageous, disadvantageous, or neutral. the neutral form a fluctuating, unfixed, indefinite body. but they afford the material with which nature may make, through intercrossing, endless experiments in new combinations, some of which may be profitable. such profitable variations would escape elimination, and, if not bred out by intercrossing, would be preserved. in any case, the variety would tend to advance through elimination as previously indicated. but in the two equal groups we are supposing to have become geographically isolated, the chances are many to one against the same successful experiments in combination occurring in each of the two groups. hence it follows that the progress or advance in the two groups, though analogous, would not be identical, and divergence would thus be possible under practically similar conditions of life. in his observations on the terrestrial molluscs of the sandwich islands, mr. gulick notes that different forms are found in districts which present essentially the same environment, and that there is no greater divergence when the climatic conditions are dissimilar than there is when those conditions are similar. as before noticed, the degree of divergence is, roughly speaking, directly as the distance the varietal forms are apart. again, darwin notes that the climate and environment in the several islands of the galapagos group are much the same, though each island has a somewhat divergent fauna and flora. these facts lend countenance to the view that divergence can and does occur under similar conditions of life, if there be isolation. they seem, also, so far as they go, to negative the view that the species is moulded directly by the external conditions. for, if this factor were powerful, it would override the effects of experimental combination of characters when the conditions were similar, and would give rise to well-marked varietal forms when the conditions were diverse. if we admit preferential breeding as a segregation-factor (and arising out of it sexual selection, in a modified form, as a determining one in the evolution of the plumage of male birds), it is evident that the standard of recognition-marks can only be maintained by a uniformity of preference or taste. still, the uniformity is not likely to be absolute. in this matter, as in others, variations will occur, and after the lapse of a thousand generations, in which elimination has been steadily at work, it is hardly probable that the recognition standard would remain absolutely unchanged. for, though there may not be any direct elimination in this particular respect, there might well be colour-eliminations in other (e.g. protective) respects, and the mental nature would not remain quite unchanged. moreover, we know that secondary sexual characters are remarkably subject to variation, as may be well seen in the case of ruffs (_machetes pugnax_) in the british natural history museum. in the case of our two islands with isolated faunas, therefore, if they formed separate breeding-areas for birds, the chances would be many to one against the change in the standard of recognition-marks being identical in each area. hence might arise those minute but definite specific distinctions which are so noteworthy in this class of the animal kingdom. instance the old and new world species of teal, the eastern and western species of curlew and whimbrel, and other cases numerous.[ax] this, in fact, is probably in many cases the true explanation of the occurrence of representative species, slight specific variations of the same form as it is traced across a continent or through an archipelago of islands. the question has been raised, and of late a good deal discussed, whether specific characters, those traits by which species are distinguishable, are always of use to the species which possess them. here it is essential to define what is meant by utility. characters may be of use in enabling the possessor to resist elimination; or, like the colours of flowers, they may be of use in attracting insects, and thus furthering selection; or, like recognition-marks, they may be of use in effecting segregation. this last form of utility is apt to be overlooked or lost sight of. in speaking of humming-birds, the duke of argyll says that "a crest of topaz is no better in the struggle for existence than a crest of sapphire. a frill ending in spangles of the emerald is no better in the battle of life than a frill ending in spangles of the ruby." but if these characters be recognition-marks, they may be of use in segregation. they are a factor in isolation. but it may be further asked--what is the use of the segregation? wherein lies the utility of the divergence into two forms? this question, however, involves a complete change of view-point. the question before us is whether specific characters are of use to the species which possesses them. to this question it is sufficient to answer that they are useful in effecting or preserving segregation, without which the species, _as a distinct species_, would cease to exist. we are not at present concerned with the question whether divergence in itself is useful or advantageous. if it be pressed, we must reply that, although divergence is undoubtedly of immense advantage to life in general, enabling, as darwin said, its varying and divergent forms to become adapted to many and highly diversified places in the economy of nature, still in many individual cases it is neither possible nor in any respect necessary to our conception of evolution to assign any grounds of utility or advantage for the divergence itself. in any case, we are dealing at present with the utility of specific characters to the species which possess them; and under the head of utility we are including usefulness in effecting or maintaining segregation. now, we have already seen that variations may be either advantageous (useful), or neutral (useless), or disadvantageous (worse than useless). the latter class we may here disregard; elimination will more or less speedily dispose of them. with regard to neutral (useless) variations, we must also note that they may be correlated with variations of the other two classes. if correlated with disadvantageous variations, they will be eliminated along with them; if correlated with advantageous variations, they will escape elimination (or will be selected) together with them. there remain neutral, or useless, variations, not correlated with either of the other two classes. are these in any cases distinctive of species? it is characteristic of specific distinctions that they are relatively constant. elimination, selection, or preferential breeding gives them relative fixity. on the other hand, it is characteristic of neutral variations that they are inconstant. there is nothing to give them fixity. it is, of course, conceivable that all the migrants to a new area were possessed of a useless neutral character, which those in the mother area did not possess; or that such a useless character was in them preponderant, and by intercrossing formed a less fluctuating, useless character than their progenitors exhibited. still, the extensive occurrence of such neutral, or useless, characteristics would be in the highest degree improbable. our ignorance often prevents us from saying in what particular way a character is useful. we must neither, on the one hand, demand proof that this, that, or the other specific character is useful, nor, on the other hand, demand negative evidence (obviously impossible to produce) that it is without utilitarian significance; but we may fairly request those who believe in the wide occurrence of useless specific characters to tell us by what means these useless characters have acquired their relative constancy and fixity. a suggestion on this head will be found in the next chapter. we must now pass on to consider briefly a most important factor in the struggle for existence. hitherto we have regarded this struggle as uniform in intensity; we must now regard it as variable, with alternations of good times and hard times, and indicate the causes to which such variations are due. with variations of climate, such as we know to occur from year to year, or from decade to decade, there are variations in the productiveness of the soil; and when we remember how closely interwoven are the web and woof of life, we shall see that the increased or diminished productiveness of any area will affect for good or ill all the life which that area supports. the introduction of new forms of life into an area, or their preponderance at certain periods owing to climatic or other conditions particularly favourable to them as opposed to other forms, may alter the whole balance of life in the district. we are often unable to assign any reason for the sudden increase or diminution of the numbers of a species; we can only presume that it is the result of some favourable or unfavourable change of conditions. thus mr. alexander becker[ay] has recently drawn attention to the fact that whereas for several years various species of grasshoppers appeared in great numbers in south-east russia, there came then one year of sudden death for most of them. they were sitting motionless on the grasses and dying. he gives similar cases of butterflies for a while numerous, and then rare, and states that a squirrel common near sarepta suddenly disappeared in the course of one summer, probably, he adds, succumbing to some contagious disease. such is the nice balance of life, that the partial disappearance of a single form may produce remarkable and little-expected effects. darwin amusingly showed how the clover crops might be beneficially affected by the introduction of a family of old maids into a parish. the clover is fertilized by humble-bees, the bees are preyed upon by mice; the relations between cats and mice, and between old maids and cats, are well known and familiar: more old maids, more cats; more cats, less mice; less mice, more humble-bees; more humble-bees, better fertilization. a little thing may modify the balance of life, and increase or diminish the struggle for existence, and the rigour of the process of elimination. but when we take a more extended view of the matter, and include secular changes of climate, the possible range of variation in the struggle for existence is seen to be enormously increased. it is well known to those who have followed the progress of geology, that in early kainozoic times a mild climate extended to within the arctic circle, while during the glacial epoch much of the north temperate zone was fast locked in ice, and the climate of the northern hemisphere was profoundly modified. the animals in the north temperate zone were driven southwards.[az] not only was there much elimination from the severe climatic conditions, but the migrants were driven southwards into areas already well stocked with life, and the competition for means of subsistence in these areas must have been rendered extremely severe. elimination was at a maximum. then followed the withdrawal of glacial conditions. the increasing geniality of the climate allowed an expansion of life within a given area, and the withdrawal of snow and ice further and further north set free new areas into which this expanding life could migrate and find subsistence. the hard times of the glacial period were succeeded by good times of returning warmth and an expanding area; and if, as some geologists believe, there was an inter-glacial period (or more than one such period) in the midst of the great ice age, then hard times and good times alternated during the glacial epoch. expansion and contraction of life-areas have also been effected again and again in the course of geological history by elevations and subsidences of the land. at the beginning of mesozoic times much of europe was dry land. in triassic and rhætic times there were lakes in england and in germany, and a warm mediterranean sea to the south. subsidence of the european area brought with it a lessened land-area and an increased sea-area: bad times and increased competition for land animals; good times and a widening life-area for marine forms of life. this continued, with minor variations, till its culmination in the cretaceous period. then came the converse process: the land-areas increased, the sea was driven back. a good time had come for terrestrial life; the marine inhabitants of estuaries and inland seas felt the pressure of increased competition in a lessening area. and so there emerged the continental europe of the beginning of the kainozoic era. and it is scarcely necessary to remind those who are in any degree conversant with geology that during tertiary times there have been alternate expansions and contractions of life-areas, marine and terrestrial, the former bringing good times, the latter hard times and a heightened struggle for existence. now, what would be the result of this alternation of good times and hard times? during good times varieties, which would be otherwise unable to hold their own, might arise and have time to establish themselves. in an expanding area migration would take place, local segregation in the colonial areas would be rendered possible, differential elimination in the different migration-areas would produce divergence. there would be diminished elimination of neutral variations, thus affording opportunities for experimental combinations. in general, good times would favour variation and divergence. intermediate between good times and hard times would come, in logical order, the times in which there is neither an expansion nor a contraction of the life-area. one may suppose that these are times of relatively little change. there is neither the divergence rendered possible by the expansion of life-area, nor the heightened elimination enforced by the contraction of life-area.[ba] elimination is steadily in progress, for the law of increase must still hold good. divergence is still taking place, for the law of variation still obtains. but neither is at its maximum. these are the good old-fashioned times of slow and steady conservative progress. they are, perhaps, well exemplified by the fauna of the carboniferous period, and it is not at all improbable that we are ourselves living in such a quiet, conservative period. on the other hand, hard times would mean increased elimination. during the exhibitions at south kensington there were good times for rats. but when the show was over, there followed times that were cruelly hard. the keenest competition for the scanty food arose, and the poor animals were forced to prey upon each other. "their cravings for food," we read in _nature_, "culminated in a fierce onslaught on one another, which was evidenced by the piteous cries of those being devoured. the method of seizing their victims was to suddenly make a raid upon one weaker or smaller than themselves, and, after overpowering it by numbers, to tear it in pieces." elimination by competition, passing in this way into elimination by battle, would, during hard times, be increased. none but the best organized and best adapted could hope to escape. there would be no room for neutral variations, which, in the keenness of the struggle, would be relatively disadvantageous. slightly divergent varieties, before kept apart through local segregation, would be brought into competition. the weakest would in some cases be eliminated. in other cases, the best-adapted individuals of each variety might survive. if their experiments in intercrossing, should such occur, gave rise to fertile offspring, more vigorous and better adapted than either parent-race, these would survive, and the parent-forms would be eliminated. but if such experiments in intercrossing gave rise to infertile, weakly offspring, these would be eliminated. thus sterility between species would become fixed. wherever, during the preceding good times, divergence had taken place in two different directions of adaptation, and some intermediate forms, fairly good in both directions, had been able to escape elimination, the chances are that these intermediates would be in hard times eliminated, and the divergent forms left in possession of the field. wherever, during good times, a species had acquired or retained a habit of flexibility, that habit would stand it in good stead in the midst of the changes wrought by hard times; but when it had, on the other hand, acquired rigidity (like the proverbially "inflexible goose"), it would be at a disadvantage in the stress of a heightened elimination. the alternation of good times and hard times may be illustrated by an example taken from human life. the introduction of ostrich-farming in south africa brought good times to farmers. whereupon there followed divergence in two directions. some devoted increased profits to improvements upon their farms, to irrigation works which could not before be afforded, and so forth. for others increased income meant increased expenditure and an easier, if not more luxurious, mode of life. then came hard times. others, in africa and elsewhere, learnt the secret of ostrich-farming. competition brought down profits, and elimination set in--of which variety need hardly be stated. i believe that the alternation of good times and hard times, during secular changes of climate and alternate expansions and contractions of life-areas through geological upheavals and depression of the land, has been a factor of the very greatest importance in the evolution of varied and divergent forms of life, and in the elimination of intermediate forms between adaptive variations. it now only remains in this chapter to say a few words concerning convergence, adaptation, and progress. convergence, which is the converse of divergence, is brought about through the adaptation of different forms of life to similar conditions of existence. the somewhat similar form of the body and fin-like limbs of fishes, of ancient reptiles (the ichthyosaurus and its allies), of whales, seals, and manatees, is a case in point. both birds, bats, and pterodactyls have keeled breastbones for the attachment of the large muscles for flight. a whole series of analogous adaptations, as the result of analogous modes of life, are found in the placental mammals of europe and asia, on the one hand, and the marsupial forms of australia on the other hand. the flying squirrel answers to the flying phalanger, the fox to the vulpine phalanger, the bear to the koala, the badger to the pouched badger, the rabbit to the bandicoot, the wolverine to the tasmanian devil, the weasel to the pouched weasel, the rats and mice to the kangaroo rats and mice, and so on. a familiar example of convergence is to be seen in our swallows and martins, on the one hand, and the swifts on the other. notwithstanding their superficial similarity in external form and habits, they are now generally regarded as belonging to distinct orders of birds. these are examples of convergence.[bb] animals of diverse descent and ancestry have, through similarity of surrounding conditions or of habits of life, become, in certain respects, assimilated. but some zoologists go further than this. they maintain that the same genus or species may, through adaptation to similar circumstances, be derived from dissimilar ancestors. some palæontologists, for example, believe that the horse has been independently evolved along parallel lines in europe and in america. professor cope considers that in the one continent _protohippus_, and in the other _hipparion_, was the immediate ancestor of _equus_. the probabilities are, however, so strongly against such a view, that it cannot be accepted until substantiated by stronger evidence than is yet forthcoming. a special and particular form of convergence, at any rate in certain obvious, if superficial, characters, has already been noticed in our brief consideration of mimicry. in the first place, among a number of closely related species of inedible butterflies, the tendency to divergence is checked, so far as external markings and coloration are concerned, that all may continue to profit by the resemblance, and that the numbers tasted by young birds in gaining their experience (for the avoidance seems to be at most incompletely instinctive) may be divided amongst all the species, thus lessening the loss to each. secondly, there may be a convergence of certain genera of distantly related inedible groups (e.g. among the heliconidæ and the danaidæ), which gain by being apparently one species, since the loss from young birds is shared between them. and lastly, there is the true mimicry of quite distinct families of butterflies, not themselves inedible, but sheltering themselves under the guise and sharing the bad reputation of the mimicked forms. such forms of convergence are in special adaptation to a very special environment. we must remember that in all cases adaptation is a matter of life and environment. and these, we may now note, may be related in one or more of three ways. in the first place, there is the adaptation of life to an unchanging environment; for example, the adaptation of all forms of life to the fixed and unchanging properties of inorganic matter. if we liken life to a statue and the environment to a mould in which it is cast, we have in this case a rigid mould and a plastic statue. secondly, the adaptation may be mutual, as, for example, when the structures of insects and flowers are fitted each to the other, or when the speed of hunters and hunted is steadily increased through the elimination of the slow in either group. here the mould and statue are both somewhat plastic, and yield to each other's influence. thirdly, the environment may be moulded to life. this, again, is only relative, since life never wholly loses its plasticity. the bird that builds a nest, the beaver that constructs a dam, the insect that gives rise to a gall,--these, so far, mould the environment to the needs of their existence. man in especial has the power, through his developed intelligence, of manufacturing his own environment. here the statue is relatively rigid, and the mould plastic. progress may be defined as continuous adaptation. in modern phrase, this is called evolution. the continuity makes the difference between evolution and revolution. both are natural. both occur in the organic, the social, and the intellectual sphere. evolution is the orderly progress of the organism or group of organisms, by which it becomes more and more in harmony with surrounding conditions. if the conditions become more and more complex, the organism will progress in complexity; but if the conditions be more and more simple, progress (if such it may still be called) will be towards simplicity of structure, unnecessary complexity being eliminated, or, in any case, disappearing. hence, in parasites and some forms of life which live under simple conditions, we have the phenomena of degeneration, or a passage from a more complex to a more simple condition. revolution in organic life is the destruction of one organism or group of organisms, and the replacement in its stead of a wholly different organism or group of organisms. during hard times there may be much revolution, or replacement of one set of organic forms by another set of organic forms. it was by revolution that the dominant reptiles of the mesozoic epoch were replaced by the dominant mammals of kainozoic times. it was by revolution that pterodactyls were supplanted by birds. revolution has exterminated many a group in geological ages. on the other hand, it was by evolution that the little-specialized eocene ungulates gave rise to the horse, the camel, and the deer; by divergent evolution that the bears and dogs were derived from common ancestors. palæontology testifies both to evolution and revolution.[bc] that history does the same, i need not stay to exemplify. the same laws also apply to systems of thought. darwinism has revolutionized our conceptions of nature. darwin placed upon a satisfactory basis a new order of interpretation of the organic world. by it other interpretations have been supplanted. and now this new conception is undergoing evolution, not without some divergence. in this chapter we have seen how evolution is possible under natural conditions. if the law of increase be true, if more are born than can survive to procreate their kind, natural selection is a logical necessity. we must not blame our forefathers for not seeing this. until geology had extended our conception of time, no such conclusions could be drawn. if organisms have existed but six or seven thousand years, and if in the last thousand years little or no change in organic life has occurred, the supposition that they could have originated by any such process as natural selection is manifestly absurd. lyell was the necessary precursor of darwin. given, then, increase and elimination throughout geological time, natural selection is a logical necessity. no one who adequately grasps the facts can now deny it. it is an unquestionable factor in organic evolution. whether it is the sole factor, is quite another matter, and one we will consider in the chapter on "organic evolution." notes [m] samuel butler in england, and ewald hering in prague, have ingeniously likened this hereditary persistence to "organic memory." what are ordinarily called memory, habit, instinct, and embryonic reconstruction, are all referable to the memory of organic matter. the analogy, if used with due caution, is a helpful one, what we call memory being the psychical aspect (under certain special organic and neural conditions) of what under the physical aspect we call persistence. [n] i have also to thank mr. edward wilson for kindly giving me the measurements of three or four bats in the bristol museum. [o] a millimetre is about / of an inch, or more exactly . inch. [p] in nearly all cases the measurements were checked by comparing the two wings. in one or two instances there were differences of as much as two or three millimetres between the bones of the two sides of the body, but in most cases they exactly corresponded. [q] we are anxious to extend our observations and to compare series of bats from different localities. if any of my readers should feel disposed to help us, by sending specimens (_with the locality duly indicated_) to mr. h. charbonnier, , the triangle south, clifton, bristol, we shall be grateful. [r] _nature_, vol. xli. p. . the variation in molluscs is often considerable. in one of the bays in the basement hall of the natural history museum is a series showing the variation in size, form, and sculpturing of _paludomus loricatus_, which is found in the streams of ceylon. these varieties have in former times been named as ten distinct species! [s] more observations and fuller knowledge on this latter point and on the relative numbers of the sexes in different species are much to be desired. it is clear that the number of offspring mainly depends upon the number of females. but if it be true that good times and favourable conditions lead to an increased production of females, while hard times and unfavourable conditions lead to a relative increase of males, then it is evident that good times will lead to a more rapid increase and hard times to a less rapid increase of the species. suppose, for example, in a particular district food and other conditions were especially favourable for frogs. among the well-nourished tadpoles there would be a preponderance of females. in the next generation the many females would produce abundant offspring (for one male may fertilize the ova laid by several females). there would be a greater number of tadpoles to compete for the same amount of nutriment. they would be less nourished. there would be less females; and in the succeeding generation a diminished number of tadpoles. thus to some extent a balance between the number of tadpoles and the amount of available nutrition would be maintained. these conclusions are, perhaps, too theoretical to be of much value, while the tendency here indicated would be but one factor among many. [t] "origin of species," pp. , . [u] "animals and plants under domestication," vol. ii. p. . [v] i may here draw attention to the fact that the bats whose wing-bone measurements were given above are those which have so far survived and escaped such elimination as is now in progress. [w] "origin of species," p. . [x] "darwinism," p. . [y] ibid. p. . [z] proceedings liverpool biological society, . [aa] since this chapter was written, mr. poulton has published his interesting and valuable work on "the colours of animals," from which i have contrived to insert one or two additional examples. [ab] _ann. and mag. nat. hist._, september, , p. , quoted by poulton, "colours of animals," p. . [ac] nature, vol. xxxv. p. . [ad] many other instances might be added. the hornet clear-wing moth (_sphecia apiformis_) mimics the hornet or wasp; the narrow-bordered bee-hawk moth (_sesia bombyliformis_) mimics a bumble-bee. these insects may be seen in the lepidoptera drawers in the natural history museum. but perhaps the most wonderful instance of insect-mimicry is that observed by mr. w. l. sclater, and given by mr. e. b. poulton, in his "colours of animals" (p. ), where a (probably) homopterous insect mimics a leaf-cutting ant, _together with its leafy burden_--a membranous expansion in the mimic closely resembling the piece of leaf carried by the particular kind of ant he resembles. [ae] the late mr. h. w. oakley first drew my attention to this snake. since then mr. hammond tooke has described the facts in _nature_, vol. xxxiv. p. . [af] _nature_, vol. xlii. p. . [ag] since the above was written and sent to press, there has been added, at the natural history museum, in the basement hall, a case illustrating the adaptation of external colouring to the conditions of life. all the animals, birds, etc., there grouped were collected in the egyptian desert, whence also the rocks, stones, and sand on which they are placed were brought. though somewhat crowded, they exemplify protective resemblance very well. [ah] i have to thank mr. h. a. francis for drawing my attention to this, and showing me the insects in his cabinet. [ai] "colours of animals," p. . [aj] "origin of species," p. . [ak] "descent of man," summary of chap. xvi. pt. ii. [al] ibid. chap. xiv. [am] "darwinism," p. . [an] its importance in artificial selection was emphasized by darwin: "the prevention of free crossing, and the intentional matching of individual animals, are the corner-stones of the breeder's art" ("animals and plants under domestication," ii. ). [ao] from the absence of interblending in some cases (to be considered shortly), both brown _and_ green forms may be produced; and under certain circumstances, even a power of becoming either brown _or_ green in the presence of appropriate stimuli. [ap] wallace, "darwinism," p. , where other examples are cited. [aq] ibid. pp. , _et seq._ [ar] _journal of the linnæan society_, vol. xix. no. : "zoology." [as] "animals and plants under domestication," p. . [at] ibid. chap. xvii. [au] "darwinism," p. . [av] "animals and plants under domestication," vol. ii. p. . for darwin's general conclusions on hybridism, see vol. ii. p. of the same work. [aw] "in every case there are two factors, namely, the nature of the organism and the nature of the conditions. the former seems to be much the more important; for nearly similar variations sometimes arise under, as far as we can judge dissimilar conditions; and, on the other hand, dissimilar variations arise under conditions which appear to be nearly uniform" ("origin of species," p. ). [ax] see "evolution without natural selection," by charles dixon. this author's facts are valuable; his theories are ill digested. [ay] _nature_, vol. xlii. p. . [az] we may here note, in passing, the fact that the changes of life-forms in a succession of beds points in nine cases out of ten rather to substitution through migration than to transmutation. still, there are notable cases of transmutation, as in the fresh-water _planorbes_ of steinhem, in wittenberg (described, after hilgendorf, by o. schmidt, "the doctrine of descent," p. ). [ba] i would ask historians whether there have not been, in english history, good times of free and beneficial divergence exemplified in diverse intellectual activity, hard times of rigorous elimination, and intermediate times of placid, somewhat humdrum conservatism. [bb] two more technical examples may be noticed in a note. ( ) professor haeckel has recently (_challenger_ reports, vol. xxviii.) shown that the siphonophora include two groups, closely resembling each other, but of different ancestry: (_a_) the disconanthæ, traceable to trachomedusoid ancestors; (_b_) the siphonanthæ, traceable to anthomedusoid ancestors like sarsia. ( ) m. paul pelseneer has been led to the conclusion that the pteropod molluscs also include two groups resembling each other, but of different ancestry: (_a_) the thecosomes, traceable to tornatellid ancestors; (_b_) the gymnosomes, traceable to aphysiid ancestors. in each case, the ancestral sea-slug has been modified for a free-swimming life. [bc] for evidence in copious abundance, see nicholson's "manual of palæontology," new edition, vol. i.: "vertebrata," by r. lydekker. chapter v. heredity and the origin of variations. the law of heredity, i have said above, may be regarded as that of persistence exemplified in a series of organic generations. variation results--it is clear that it must result--from some kind of differentiating influence. such statements as these, however, though they are true enough, do not help us much in understanding either heredity or variation. let us first notice that normal cases of reproduction exemplify both phenomena--heredity with variation; hereditary similarity to the parents in all essential respects, individual variations in minor points. this is seen in man. brothers and sisters may present family resemblances among each other and to their parents, but each has individual traits of feature and of character. only in particular cases of so-called "identical twins" are the variations so slight as not to be readily perceptible by even a casual observer. now, when we seek an explanation of these well-known facts, we may be tempted to find it in the supposition that the character of the parents does not remain constant, that the character influences the offspring, and that therefore the children born at successive periods will differ from each other, while twins born in the same hour will naturally resemble each other. as darwin himself says,[bd] "the greater dissimilarity of the successive children of the same family in comparison with twins, which often resemble each other in external appearance, mental disposition, and constitution, in so extraordinary a manner, apparently proves that the state of the parents at the exact period of conception, or the nature of the subsequent embryonic development, has a direct and powerful influence on the character of the offspring." but a little consideration will show that, though this might, in the absence of a better explanation, account for variation in character, it could not account for variation in form and feature, unless we regard these as in some way determined by the character. moreover, as we shall see presently, it is open to question whether acquired modifications of structure or character in the parent can in any way influence the offspring. again, in the litter of puppies born of the same bitch by the same dog there are individual variations, often as well marked as those in successive births. the facts, then, to be accounted for are--first, the close hereditary resemblance in all essential points of offspring to parent; and, secondly, the individual differences in minor points among the offspring produced simultaneously or successively by the same parents. these are the facts as they occur in the higher animals. it will be well to lead up to our consideration of them by a preliminary survey of the facts as they are exemplified by some of the lower organisms. in the simpler protozoa, where fission occurs, and where the organism is composed of a single cell, where also there is a single nucleus which apparently undergoes division into two equal and similar parts, it is easy to understand that the two organisms thus resulting from the halving of a single organism partake completely of its nature. if the fission of an am[oe]ba is such as to divide it into two similar parts, there is no reason why these two similar parts should not be in all respects alike, and should not, by the assimilation of new material, acquire the size and all the characteristics of the parent form. in the higher and more differentiated protozoa, the case is not quite so simple; for the two halves are not each like the whole parent, but have to be remodelled into a similar organism. but if we suppose, as we seem to have every right to suppose, that it is the nucleus that controls the formative processes in the cell, there is not much difficulty in understanding how, when the nucleus divides into two similar portions, each directs, so to speak, the similar refashioning of its own separated protoplasmic territory. from the protozoa we may pass to such a comparatively simple metazoon as the hydra. here the organism is composed, not of a single cell, but of a number of cells. these cells are, moreover, not all alike, but have undergone differentiation with physiological division of labour. there is an inner layer of large nutritive cells, and an outer layer of protective cells, some of which are conical with fine processes proceeding from the point of the cone; others are smaller, and fill in the interstices between the apices of the cones, while others have developed into thread-cells, each with a fine stinging filament. between the two layers there is a thin supporting lamella. the essential point we have here to notice is that there are two distinct layers with cells of different form and function. now, it has again and again been experimentally proved that if a hydra be divided into a number of fragments, each will grow up into a complete and perfect hydra. all that is essential is that, in the separated fragment, there shall be samples of the cells of both layers. under these conditions, the separated cells of the outer layer regenerate a complete external wall, and the separated cells of the inner layer similarly regenerate a complete internal lining. from these facts, it would appear that such a small adequately sampled fragment has the power, when isolated, of assimilating nutriment and growing by the multiplication of the constituent cells, and that the growth takes such lines that the original form of the hydra is reproduced. here we may note, by way of analogy, what takes place in the case of inorganic crystals. if a fragment of an alum crystal be suspended in a strong solution of alum, the crystal will be recompleted by the growth of new parts along the broken edges. we say that this is effected under the influence of molecular polarity. similarly, we may say that the fragment of the hydra rebuilds the complete form under the influence of an hereditary morphological tendency residing in the nuclei of the several cells. the case, though still comparatively simple, is more complex than that of the higher protozoa. there the divided nucleus in two separated cells directs each of these in hereditary lines of morphological growth. here not only do the cells and their nuclei divide, but they are animated by a common morphological principle, and in their multiplication _combine_ to form an organism possessing the ancestral symmetry. if, however, we call this an hereditary morphological tendency or a principle of symmetry; or, with the older physiologists, a _nisus formativus_; or, with darwin, "the co-ordinating power of the organization" (all of these expressions being somewhat unsatisfactory);--we must remember that these terms merely imply a play of molecular forces analogous to that which causes the broken crystal of alum to become recompleted in suitable solution. the inherent molecular processes in the nuclei[be] in the one case enable the cells to regenerate the hydra; the inherent molecular stresses in the crystalline fragment in the other case lead to the reproduction of the complete crystal. in either case there is no true explanation, but merely a restatement of the facts under a convenient name or phrase. the power of regeneration of lost parts, which is thus seen in the hydra, is also seen, in a less degree so far as amount is concerned, but in a higher degree so far as complexity goes, in animals far above the hydra in the scale of life. the lobster that has lost a claw, the snail whose tentacle has been removed, the newt which has been docked of a portion of its tail or a limb, are able more or less completely to regenerate these lost parts. and the regeneration may involve complex structures. with the tentacle of the snail the eye may be removed, and this, not once only, but a dozen times. after such mutilation, no part of the eye remains, though the stump of its nerve is, of course, left; still the perfect organ is reconstructed again and again, as often as the tentacle is removed. the cells at the cut end of the nerve-stump divide and multiply, as do also those of the surrounding tissues, and the growing nerve terminates in an optic cup, as it did previously under the influences of normal development before the mutilation. here we have phenomena analogous to, and in some respects more complex than, those which are seen in the regenerative process in hydra. it is well known, however, that, in the case of higher animals, in birds and mammals, this power of regenerating lost parts does not exist. when a bone is broken, osseous union of the broken pieces may indeed take place; and in flesh-wounds, the gash is filled in and heals over, not without permanent signs of its existence, as may often be seen in the faces of german students. but beyond this there is normally no regeneration. the soldier who has lost an arm in battle cannot return home and in quiet seclusion reproduce a new limb. that which seems to be among lower animals a well-established law of organic growth does not here obtain. this is probably due to the fact that the higher histological differentiation of the tissues in the more highly developed forms of life is a bar to regeneration. in their devotion to special and minute details of physiological work, the cells have, so to speak, forgotten their more generalized reproductive faculties. in any case, however the fact is to be explained, the higher organisms have in many cases almost completely lost the power of regenerating lost parts. but this loss of the regenerative power in the more highly differentiated animals does not alter or invalidate the law of organic growth we are considering. the law may be thus stated: _whenever, after mutilation, free growth of the mutilated surface occurs, that growth is directed in such lines as to reproduce the lost part and restore the symmetrical integrity of the organism._ this is a matter of heredity. and we may regard the hereditary reconstructive power as residing either ( ) in those cells at or adjoining the mutilated surface which are concerned in the regrowth of the lost part; or ( ) in the general mass of cells of the mutilated organism. there are difficulties in either view. professor sollas, supporting the former, says,[bf] "this power [in the snail] of growing afresh so complex and specialized an organ as an eye is certainly, at first sight, not a little astonishing, but it appears to be capable of a very simple explanation. the cells terminating the cut stump of the tentacle are the ancestors of those which are removed; a fresh series of descendants are derived from them, similarly related to the ancestral cells as their predecessors which they replace; the first generation of descendants become in turn ancestors to a second generation, similarly related to them as were the second tier of extirpated cells; and this process of descent being repeated, the completed organ will at length be rebuilt." this explanation is, however, misleading in its simplicity. the cells terminating the cut stump are not the direct ancestors of those which are removed, except in the same sense as gorillas are ancestors of men. they are rather collateral descendants of common ancestors. i think that professor sollas would probably agree that, though the lens and "retina" are of epiblastic (outer layer) origin, their relationship with the epiblastic cells at the cut stump is a somewhat distant one. in the reproduction of the lens the cell-heredity is not direct, but markedly indirect. and it is somewhat difficult to understand by what means the ordinary epiblastic cells of the cut stump, which have had no part in the special and peculiar work of lens-production, should be enabled to produce cell-offspring, some of which, and those in a special relation to other deeper-lying cells, possess this peculiar power. on the other hand, if we turn to the view that the reproduction is effected, not by the cells of the cut surface alone, but by the general mass of cells in the mutilated organism, we have to face the difficulty of understanding how the influence of cells other than those partaking in the regrowth can be brought to bear on these so as to direct their lines of development. if we say that the organism is pervaded by a principle of symmetry such that both growth and regrowth, whenever they take place, are constrained to follow the lines of ancestral symmetry, we are really doing little more than restating the facts without affording any real organic explanation. that which we want to know is in what organic way this symmetrical growth is effected--how the hereditary tendency is transmitted through the nuclear network which is concerned in cell-division. i do not think that we are at present in a position to give a satisfactory answer to this question. let us now return to the hydra, the artificial fission of which has suggested these considerations. multiplication in this way is probably abnormal. under suitable conditions, however, if well fed, the hydra normally multiplies by budding. at some spot, generally not far from the "foot," or base of attachment, a little swelling occurs, and the growth of the cells in this region takes such lines that a new hydra is formed. this is at first in direct connection with the parent stem, the two having a common internal cavity; but eventually it separates and lives a free existence as a distinct organism (see fig. , p. ). now, here we may notice, as an implication from these facts, that the size of the organism is limited. when the normal limits of size are reached, any further assimilation of nutriment ministers, not to the further growth of the organism, but to the formation of a new outgrowth, or bud. what determines that the outgrowth, or bud, should originate in this or that group of cells, we do not know. but, like the isolated fragment in the hydra subdivided by fission, the little group in which budding commences contains a fair sample of the various kinds of cells which constitute the hydra. and here, too, we see that their growth and development follow definite lines of hereditary symmetry. but there is a third method of multiplication in hydra: this is the sexual mode of reproduction, and occurs generally in the autumn. on the body-wall of certain individuals, near the tentacles, conical swellings appear. within these swellings are great numbers of minute sperms, with small oval heads and active, thread-like tails. they appear to originate from the interstitial cells of the outer layer (see p. ). nearer the foot, or base of attachment, and generally, but not quite always, in separate individuals, there are other larger swellings, different in appearance, of which there is generally only one in the same individual at the same time. each contains a single ovum, or egg-cell, surrounded by a capsule. it, too, and the cells which surround it would appear to be developed from the interstitial cells. it grows rapidly at the expense of the surrounding tissue, but when mature, it bursts through the enveloping capsule, and is freely exposed. a sperm-cell, which seems, in some cases at least, to be produced by the same individual, now unites with it; the egg-cell then begins to undergo division, becomes detached, falls to the bottom, and develops into a young hydra. here, then, we have that sexual mode of reproduction which occurs in all the higher animals. it is, however, in some respects peculiar in hydra. in the first place, the ovum is nearly always in other animals (but occasionally not in hydra) fertilized by the sperm from a separate and distinct individual. in the second place, the germinal cells are generally produced, not from the outer layer, but from the middle layer, which appears between the two primitive layers. in some allies of hydra, however, they take their origin in the inner layer; and it has been suggested that, even in hydra, the true germinal cells may migrate from the inner to the outer layer. but of this there does not seem to be at present sufficient evidence. in any case, however, the essential fact to bear in mind is that a new individual is produced by the union of a single cell produced by one organism and of another cell produced in most cases (but not always in the hydra) from a different individual. in the higher forms of animal life, the organisms are either female (egg-producing) or male (sperm-producing). but there are many hermaphrodite forms which produce both eggs and sperms, as in the common snail and earthworm. even in these cases, however, there are generally special arrangements by which it is ensured that the sperm from one individual should fertilize the ovum produced by another individual. * * * * * what, we must next inquire, is the relation in the higher forms of life--for we may now leave the special consideration of hydra--of the ovum or sperm to the organism which produces it? this is but one mode of putting a very old question--does the hen produce the egg, or does the egg produce the hen? of course, in a sense, both are true; for the hen produces an egg which, if duly fertilized, will develop into a new hen. but the question has of late been asked in a new sense; and many eminent naturalists reply, without hesitation--the egg produces the hen, but under no circumstances does the hen produce the egg. what, then, it may be asked, does produce the egg? to this it is replied--the egg was produced by a previous egg. at first sight, this may seem a mere quibble; for it may be said that, of course, if an egg produces a hen which contains other eggs, these eggs may be said to be produced by the first. but it is really more than a quibble, and we must do our best clearly to grasp what is meant. we have seen that, in development, the fertilized egg-cell undergoes division into two cells, each of which again divides into two, and so on, again and again, until from one there arises a multitude of cells. nor is this all. the multitude are organized into a whole. the constituent cells have different forms and structures, and perform diverse functions. some are skeletal, such as bone and connective tissue; some are protective, such as those which give rise to feathers or scales; some form nerves or nerve-centres; some, muscles; some give rise to glandular tissue; and lastly, some form the essential elements in reproduction. if, now, we express the development of tissues and the sequence of organisms in the following scheme, the continuity of the reproductive cells will be apparent:-- reproductive | skeletal and protective cells cell o< nerve and muscle cells | glandular and nutritive cells | skeletal, etc. | reproductive cells -------- o< nerve, etc. | glandular, etc. | s. | reproductive -- o< n. | gl. | r.-etc it is clear that there is a continuity of reproductive cells, which does not obtain with regard to nerve, gland, or skeleton. if, then, we class together as body-cells those tissue-elements which constitute what we ordinarily call the body, i.e. the head, trunk, limbs--all, in fact, except the reproductive cells, our scheme becomes-- reproductive | body cell o< | body | reproductive cells o< | b. | reproductive cells o< | r. from this, again, it is clear that the body does not produce the egg, or reproductive cell, but that the reproductive cell does produce the body. of course, it should be noted that we are here using the term "body" as distinguished from, and not as including, the reproductive cells. but this is convenient, in that it emphasizes the fact that the muscular, nervous, skeletal, and glandular cells take (on this view) no part whatever in producing those reproductive cells which are concerned in the continuance of the species. such, in brief, is the view that the egg produces the hen. we will return to it presently when we have glanced at the alternative view that the hen produces the egg. on this view, the reproductive elements are not merely cells, the result of normal cell-division, which have been set aside for the continuance of the species. they are, so to speak, the concentrated extract of the body, and contain minute or infinitesimal elements derived from all the different tissues of the organism which produces them. darwin[bg] suggested that all the cells of the various tissues produce minute particles called gemmules, which circulate freely throughout the body, but eventually find a home in the reproductive cells. just as the organism produces an ovum from which an organism like itself develops, so do the cells of the organism produce gemmules, which find their way to the ovum and become the germs of similar cells in the developing embryo. "the child, strictly speaking," says darwin, "does not grow into a man, but includes germs which slowly and successively become developed and form the man." "each animal may be compared with a bed of soil full of seeds, some of which soon germinate, some lie dormant for a period, whilst others perish." or, to vary the analogy, "an organic being is a microcosm--a little universe formed of a host of self-propagating organisms." this is darwin's provisional hypothesis of pangenesis, which has recently been accepted in a modified form by professor w. k. brooks in america, to some extent by de vries on the continent, by professor herdman of liverpool, and by other biologists. the ovum on this view is to be regarded as a composite germ containing the germs of the cellular constituents of the future organism. the scheme representing this view will stand thus-- reproductive | skeletal and protective cells | | sk. & pr. | | s. cell o< nerve and muscle cells >o< ne. & mu. >o< n. | glandular and nutritive cells | | gl. & nu. | | gl. it is clear that, on this hypothesis, we may frame an apparently simple and, on first sight, satisfactory theory of heredity. since all the body-cells produce gemmules, which collect in or give rise to the reproductive cells, and since each gemmule is the germ of a similar cell, what can be more natural than that the ovum, thus composed of representative cell-germs, should develop into an organism resembling the parent organism? modifications of structure acquired during the life of the organism would thus be transmitted from parent to offspring; for the modified cells of the parent would give rise to modified gemmules, which would thus hand on the modification. the inheritance of ancestral traits from grandparent or great-grandparent might be accounted for by supposing that some of the gemmules remained latent to develop in the second or third generation. the regeneration of lost parts receives also a ready explanation. if a part be removed by amputation, regrowth is possible because there are disseminated throughout the body gemmules derived from each part and from every organ. a stock of nascent cells or of partially developed gemmules may even be retained for this special purpose, either locally or throughout the body, ready to combine with the gemmules derived from the cells which come next in due succession. similarly, in budding, the buds may contain nascent cells or gemmules in a somewhat advanced stage of development, thus obviating the necessity of going through all the early stages in the genesis of tissues. the gemmules derived from each part being, moreover, thoroughly dispersed through the system, a little fragment of such an organism as hydra may contain sufficient to rebuild the complete organism; or, if it contains an insufficient number, we may assume that the gemmules, in their undeveloped state, are capable of multiplying indefinitely by self-division. finally, variations might arise from the superabundance of certain gemmules and the deficiency of others, and from the varying potency of the gemmules contained in the sperm and ovum. where the maternal and paternal gemmules are of equal potency, the cell resulting from their union will be a true mean between them; where one or other is prepotent, the resulting cell will tend in a corresponding direction. and since the parental cells are subject to modification, transmitted through the gemmules to the reproductive elements, it is clear that there is abundant room and opportunity for varietal combinations. it is claimed, as one of the chief advantages of some form of pangenetic hypothesis, that it, and it alone, enables us to explain the inheritance of characters or modifications of structure acquired by use (or lost by disuse) during the life of the organism, or imprinted on the tissues by environmental stresses. the evidence for the transmission of such acquired characters we shall have to consider hereafter. we may here notice, however, that at first sight the hypothesis seems to prove too little or too much. for while modifications of tissues, the effects of use and disuse, are said to be inherited, the total removal of tissues by amputation, even if repeated generation after generation, as in the docking of the tails of dogs and horses, formerly so common, does not have the effect of producing offspring similarly modified. professor weismann has recently amputated the tails of white mice so soon as they were born, for a number of generations, but there is no curtailment of this organ in the mice born of parents who had not only themselves suffered in this way, but whose parents, grandparents, and great-grandparents were all rendered tailless. the pangenetic answer to this objection is that gemmules multiply and are transmitted during long series of generations. we have only to suppose that the gemmules of distantly ancestral tails have been passing through the mutilated mice in a dormant condition, awaiting an opportunity to develop, and the constant reappearance of tails is seen to be no real anomaly. in this case the gemmules of the parental and grandparental tail are simply absent. but if the muscles of the parental tail were modified through unwonted use, the modified cells would give rise to modified gemmules, which would unite in generation with ancestral gemmules, and to a greater or less degree modify them. the difference is between the mere absence of gemmules and the presence of modified gemmules. and the fact that it takes some generations for the effects of use or disuse to become marked is (pangenetically) due to the fact that it takes some time for the modified gemmules to accumulate and be transmitted in sufficient numbers to affect seriously the numerous ancestral gemmules. the direction in which professor w. k. brooks has recently sought to modify darwin's pangenetic hypothesis may here be briefly indicated. he holds that it is under unwonted and abnormal conditions that the cells are stimulated to produce gemmules, and that the sperm is the special centre of their accumulation. hence it is the paternal influence which makes for variation, the maternal tendency being conservative. the reproductive cell is not merely or chiefly a microcosm of gemmules. it is a cell produced by ordinary cell-division from other reproductive cells. the ovum remains comparatively unaffected by changes in the body; but it receives from the sperm, with which it unites, gemmules from such tissues in the male as were undergoing special modification. the hen does not produce the egg; but the cock does produce the sperm; and the union of the two hits the happy mean between the conservatism of the one view and the progressive possibilities of the other. mr. francis galton, in ,[bh] suggested a modification of darwin's hypothesis, which included, as does that of professor brooks, the idea of germinal continuity which had been suggested by professor (now sir richard) owen, in . he calls the collection of gemmules in the fertilized ovum the "stirp." of the gemmules which constitute the stirp only a certain number, and they the most dominant, develop into the body-cells of the embryo. the rest are retained unaltered to form the germinal cells and keep up a continuous tradition. mr. galton's place in the history of theories of heredity can scarcely be placed too high. only one further modification of pangenesis can here be mentioned, namely, that proposed in by professor herdman, of liverpool. he suggested "that the body of the individual is formed, not by the development of gemmules alone and independently into cells, but by the gemmules in the cells causing, by their affinities and repulsions, these cells so to divide as to give rise to new cells, tissues, and organs." such are darwin's provisional hypothesis of pangenesis, and some more recent modifications thereof. bold and ingenious as was darwin's speculation, supported as it at first sight seems to be by organic analogies, it finds to-day but few adherents. with all our increased modern microscopical appliances, no one has ever seen the production of gemmules. although it appears sufficiently logical to say that, just as a large organism produces a small ovum, so does each small cell produce an exceedingly minute gemmule; when closely investigated, the analogy is not altogether satisfactory. it is denied, as we have seen, by many biologists that the organism does produce the ovum. multiplication is normally by definite, visible cell-division. nuclear fission can be followed in all its phases. but where is the nuclear fission in the formation of gemmules? it is true that the conjugation of monads is followed by the pouring forth of a fluid which must be crowded with germs from which new monads arise, and that these germs are so minute as to be invisible, even under high powers of the microscope. it might be suggested, then, that in every tissue some typical cell or cells might thus break up into a multitude of invisible gemmules. but there is at present no evidence that they do so. and even if this were the case, it would not bear out darwin's view, that every cell is constantly throwing off numerous gemmules. it is known, however, or at least generally believed, that there is a constant replacement of tissues during the life of the organism. it is said, for example, that in the course of seven years the whole cellular substance of the human body is entirely renewed. the fact is, i think, open to question. granting it, however, it might be suggested that the effete cells, ere they vanish, give rise to minute gemmules, which find their way to the ova. but it must be remembered that the new tissue-cells in the supposed successional renewal of the organs are the descendants of the old tissue-cells; that these are, therefore, already reproducing their kind directly; and that the formation of gemmules would thus be a special superadded provision for a future generation. still, there is no reason why cells should not have this double mode of reproduction, if any definite evidence of its existence could be brought forward. without such definite evidence, we may well hesitate before we accept it even provisionally. the existence of gemmules, then, is unproven, and their supposed mode of origin not in altogether satisfactory accordance with organic analogies. furthermore, the whole machinery of the scheme of heredity is complicated and hyper-hypothetical. it is difficult to read darwin's account of reversion, the inheritance of functionally acquired characters, and the non-inheritance of mutilation, or to follow his skilful manipulation of the invisible army of gemmules, without being tempted to exclaim--what cannot be explained, if this be explanation? and to ask whether an honest confession of ignorance, of which we are all so terribly afraid, be not, after all, a more satisfactory position. that the hen produces the egg, that "gemmules are collected from all parts of the system to constitute the sexual elements, and that their development in the next generation forms a new being," is further rendered improbable by direct observation upon the mode of origin of the germinal cells, ova, or sperms. it will be remembered that the view that the egg produces the hen, while the hen does not produce the egg, suggested the question--what, then, does produce the egg? to which the answer was--the egg is the product of a previous egg. on this view, then, the germinal cells, ova, or sperms are the direct and unmodified descendants of an ovum and sperm which have entered into fertile union. now, in certain cases, notably in the fly _chironomus_, studied by professor balbiani, but also in a less degree in some other invertebrate forms, it is possible to trace the continuity of the germinal cells with the fertilized ovum from which they are derived. in _chironomus_, for example, "at a very early stage in the embryo, the future reproductive cells are distinguishable and separable from the body-forming cells. the latter develop in manifold variety, into skin and nerve, muscle and blood, gut and gland; they differentiate, and lose almost all protoplasmic likeness to the mother ovum. but the reproductive cells are set apart; they take no share in the differentiation, but remain virtually unchanged, and continue unaltered the protoplasmic tradition of the original ovum."[bi] in such a case, then, observation flatly negatives the view that the germinal cells are "constituted" by gemmules collected from the body-cells, though, of course (on a modified pangenetic hypothesis), they might be the recipients of such gemmules. it is only in a minority of cases, however, that the direct continuity of germinal cells _as such_ is actually demonstrable. in the higher vertebrates, for instance, the future reproductive cells can first be recognized only after differentiation of some of the body-cells and the tissues they constitute is relatively advanced. while in cases of alternation of generations, "an entire asexual generation, or more than one, may intervene between one ovum and another." in all such cases the continuity of the chain of recognizably germinal cells cannot be actually demonstrated. the impracticability of actually demonstrating a continuity of germinal cells in the majority of cases has induced professor weismann to abandon the view that there is a continuity of germinal cells, and to substitute for it the view that there is a continuity of germ-plasm (_keimplasma_). "a continuity of germ-_cells_," he says,[bj] "does not now take place, except in very rare instances; but this fact does not prevent us from adopting a theory of the continuity of the germ-_plasm_, in favour of which much weighty evidence can be brought forward." it might, however, be suggested that, although a continuity of germ-cells cannot be _demonstrated_, such continuity may, nevertheless, obtain, the future germinal cells remaining undifferentiated, while the cells around them are undergoing differentiation. the comparatively slight differentiation of the body-cells in hydroids renders such a view by no means improbable. but professor weismann does not regard such an idea as admissible, at all events, in certain cases. "it is quite impossible," he says,[bk] "to maintain that the germ-cells of hydroids, or of the higher plants, exist from the time of embryonic development, as undifferentiated cells, which cannot be distinguished from others, and which are only differentiated at a later period." the number of daughter-cells in a colony of hydroid zoophytes is so great that "all the cells of the embryo must for a long time act as body-cells, and nothing else." moreover, actual observation (e.g. in _coryne_) convinces dr. weismann that ordinary body-cells are converted into reproductive cells. after describing the parts of the body-wall in which a sexual bud arises as in no way different from surrounding areas, he says, "rapid growth, then, takes place at a single spot, and some of the young cells thus produced _are transformed into germ-cells_ which did not previously exist as separate cells."[bl] this transformation of body-cells or their daughter-cells into germ-cells seems therefore, if it be admitted, to negative the continuity of germ-cells as such. but this fact, says weismann, does not prevent us from adopting a theory of the continuity of germ-plasm. "as a result of my investigations on hydroids," he says,[bm] "i concluded that the germ-plasm is present in a very finely divided and therefore invisible state in certain body-cells, from the very beginning of embryonic development, and that it is then transmitted, through innumerable cell-generations, to those remote individuals of the colony in which the sexual products are formed." this germ-plasm resides in the nucleus of the cell; and it would seem that by a little skilful manipulation it can be made to account for anything that has ever been observed or is ever likely to be observed. it is one of those convenient invisibles that will do anything you desire. the regrowth of a limb shows that the cells contained some of the original germ-plasm. a little sampled fragment of hydra has it in abundance. it lurks in the body-wall of the budding polyp. it is ever ready at call. it conveniently accounts for atavism, or reversion; for[bn] "the germ-plasm of very remote ancestors can occasionally make itself felt. even a very minute trace of a specific germ-plasm possesses the definite tendency to build up a certain organism, and will develop this tendency as soon as the nutrition is, for some reason, favoured above that of the other kinds of germ-plasm present in the nucleus." in place, then, of the direct continuity of germ-cells as distinct from body-cells, we have here the direct continuity of germ-plasm as opposed to body-plasm. the germ-plasm can give rise to body-plasm to any extent; the body-plasm can never give rise to germ-plasm. if it seems to do so, this is only because the nuclei of the body-cells contain some germ-plasm in an invisible form. the body-plasm dies; but the life of the germ-plasm is, under appropriate conditions, indefinitely continuous. so far as heredity is concerned, it matters not whether there be a continuity of germ-cells or of germ-plasma. in either case, the essential feature is that body-cells as such cannot give rise to the germ--that the hen cannot produce the egg. on either view, characters acquired by the body cannot be transmitted to the offspring through the ova or sperms. the annexed diagram illustrates how, on the view that the hen produces the egg, dints hammered into the body by the environment will be handed on; while, on the view that the hen does not produce the egg, the dints of the environment are not transmitted to the offspring. on the hypothesis of continuity, heredity is due to the fact that two similar things under similar conditions will give similar products. the ovum from which the mother is developed, and the ovum from which the daughter is developed, are simply two fragments separated at different times from the same continuous germ-plasm.[bo] both develop under similar circumstances, and their products cannot, therefore, fail to be similar. how variation is possible under these conditions we shall have to consider presently. [illustration: fig. .--egg and hen. i. "the egg produces the hen." ii. "the hen produces the egg." in i. the dints produced by the environment are not inherited; in ii. they are. the letters indicate successive individuals. the small round circles indicate the eggs.] now, although i value highly professor weismann's luminous researches, and read with interest his ingenious speculations, i cannot but regard his doctrine of the continuity of germ-plasm as a distinctly retrograde step. his germ-plasm is an unknowable, invisible, hypothetical entity. material though it be, it is of no more practical value than a mysterious and mythical germinal principle. by a little skilful manipulation, it may be made to account for anything and everything. the fundamental assumption that whereas germ-plasm can give rise to body-plasm to any extent, body-plasm can under no circumstances give rise to germ-plasm, introduces an unnecessary mystery. biological science should set its face against such mysteries. the fiction of two protoplasms, distinct and yet commingled, is, in my opinion, little calculated to advance our knowledge and comprehension of organic processes. for myself, i prefer to take my stand on protoplasmic unity and cellular continuity. the hypothesis of cellular continuity is one that the researches of embryologists tend more and more to justify. the fertilized ovum divides and subdivides, and, by a continuance of such processes of subdivision, gives rise to all the cells of which the adult organism is composed. it is true that in some cases, as in that of peripatus, as interpreted by mr. adam sedgwick, the cells of the embryo run together or remain continuous as a diffused protoplasmic mass with several or many nuclei. but this seemingly occurs only in early stages as a step towards the separation of distinct cells. and even if the process should be proved of far wider occurrence, it would not disprove the essential doctrine of cellular continuity. the nucleus is the essence of the cell. and the doctrine of cellular continuity emphasizes the fact that the nuclei of all the cells of the body are derived by a process of divisional growth from the first segmentation-nucleus which results from the union of the nuclei of the ovum and the sperm. in this sense, then, however late the germinal cells appear as such, they are in direct continuity with the germinal cell from which they, in common with all the cells of the organism, derive their origin. in this sense there is a true continuity of germ-cells. now, it has again and again been pointed out that the simple cell of which an am[oe]ba is composed is able to perform, in simple fashion, the various protoplasmic functions. it absorbs and assimilates food; it is contractile and responds to stimulation; it respires and exhibits metabolic processes; it undergoes fission and is reproductive. the metazoa are cell-aggregates; and in them the cells exemplify a physiological division of labour. they differentiate, and give rise to muscle and nerve, gut and gland, blood and connective or skeletal tissue, ova and sperms. are these germinal cells mysteriously different from all the other cells which have undergone differentiation? no. _they are the cells which have been differentiated and set apart for the special work of reproduction, as others have been differentiated and set apart for other protoplasmic functions._ cell-reproduction is, however, in the metazoa of two kinds. there is the direct reproduction of differentiated cells, by which muscle-cells, nerve-cells, or others reproduce their kind in the growth of tissues or organs; and there is the developmental reproduction, by which the germinal cells under appropriate conditions reproduce an organism similar to the parent. the former is in the direct line of descent from the simple reproduction of am[oe]ba. the latter is something peculiarly metazoan, and is, if one may be allowed the expression, specialized in its generality. that the metazoa are derived from the protozoa is generally believed. how they were developed is to a large extent a matter of speculation. but, however originating, their evolution involved the production, from cells of one kind, of cells of two or more kinds, co-operating in the same organism. whenever and however this occurred, the new phase of developmental reproduction must have had its origin. and if in cell-division there is any continuity of protoplasmic power, the faculty of producing diverse co-operating cells would be transmitted. on any view of the origin of the metazoa, this diverse or developmental reproduction is a new protoplasmic faculty; on any view, it must have been transmitted, for otherwise the metazoa would have ceased to exist. this new faculty of developmental reproduction, then, with the inception of the metazoa, takes its place among other protoplasmic faculties, and, with the progress of differentiation and the division of labour, will become the special business of certain cells. on this view, the specialization of the reproductive faculty and of germinal cells takes its place in line with other cell-differentiations with division of labour; and the difficulties of comprehending and following the process of differentiation in this matter are similar to those which attend physiological division of labour in general. it is probable that, in the lower metazoa, in which differentiation has not become excessively stereotyped, the power of developmental reproduction is retained by a great number of cells, even while it is being specialized in certain cells. hence the ability to produce lost parts and the reproduction of hydra by fission. but, on the other hand, the special differentiation of a tissue on particular lines has always a tendency to disqualify the cells from performing other protoplasmic faculties, and that of developmental reproduction among the number. i do not know of any definite, well-observed cases on record in the animal kingdom of ova or sperms being derived from cells which are highly differentiated in any other respect. in the vertebrata, the mesoblastic, or mid-layer, cells, from which the germinal epithelium arises, have certainly not been previously differentiated in any other line. and in the case of the hydroid zoophytes, quoted by professor weismann, the cells which give rise to the germinal products have never been so highly differentiated as to lose the protoplasmic faculty of developmental reproduction. some such view of developmental reproduction, based upon cellular continuity and the division of labour, seems to me more in accord with the general teachings of modern biology than a hypothetical and arbitrary distinction between a supposed germ-plasm and a supposed body-plasm. to which category, then, does this hypothesis belong? does it support the view that the hen produces the egg or that the egg produces the hen? undoubtedly the latter. it is based on cellular continuity, and is summarized by the scheme on p. . it adequately accounts for hereditary continuity, for there is a continuity of the germinal cells, the bearers of heredity. but how, it may be asked, on this view, or on any continuity hypothesis, are the origin of variations and their transmission to be accounted for? to this question we have next to turn. but before doing so, it will be well to recapitulate and summarize the positions we have so far considered. we saw at the outset that the facts we have to account for are those of heredity with variation. to lead up to the facts of sexual heredity, we considered fission, the regeneration of lost parts, and budding in the lower animals. we saw that, if a hydra be divided, each portion reproduces appropriately the absent parts. but we found it difficult to say whether this power resides, in such cases, in the cells along the plane of section or in the general mass of cells which constitute the regenerating portion. having led up to the sexual mode of reproduction, we inquired whether the egg produces the hen or the hen produces the egg. we saw that there is a marked difference between a _direct continuity_ of reproductive cells, giving rise to body-cells as by-products, and an _indirect continuity_ of reproductive cells, these cells giving rise to the hen, and then the hen to fresh reproductive cells, which, on this view, are to be regarded as concentrated essence of hen. darwin's hypothesis of pangenesis as exemplifying the latter view was considered at some length, and the modifications suggested by professor brooks, mr. galton, and professor herdman were indicated. the hypothesis, so far as it is regarded as a theory of the main facts of heredity, was rejected. it was then pointed out that only in a few cases has a direct continuity of germinal cells _as such_ been actually demonstrated. whence professor weismann has been led to elaborate his doctrine of the continuity of germ-plasm. this germ-plasm can give rise to, but cannot originate from, body-plasm. it may lurk in body-cells, which may, by its subsequent development, be transformed into germ-cells. but any external influences which may affect these body-cells produce no change on the germ-plasm which they may contain. we regarded this hypothesis as a retrograde step, much as we admire the genius of its propounder, and considered that the fiction of two protoplasms, distinct and yet commingled, is little calculated to advance our comprehension of organic processes. in the known and observed phenomena of cellular continuity and cell-differentiation, we found a sufficiently satisfactory hypothesis of heredity. the reproductive cells are the outcome of normal cell-division, and have been differentiated and set apart for the special work of developmental reproduction, as others have been differentiated and set apart for other protoplasmic functions. such a view adequately accounts for hereditary continuity, for there is a continuity of the germinal cells, the bearers of heredity. but how, we repeat, on this view or any other hypothesis of direct continuity, are the origin of variations and their transmission to be accounted for? * * * * * every individual organism reacts more or less markedly under the stress of environing conditions. the reaction may take the form of passive resistance, or it may be exemplified in the performance of specially directed motor-activities. the power to react in these ways is inborn; but the degree to which the power is exercised depends upon the conditions of existence, and during the life of the individual the power may be increased or diminished according to whether the conditions of life have led to its exercise or not. the effects of training and exercise on the performance of muscular feats and in the employment of mental faculties are too well known to need special exemplification. by manual labour the skin of the hand is thickened; and by long-continued handling of a rifle a bony growth caused by the weapon in drilling, the so-called _exercierknochen_ of the germans, is developed. now, it is clear that if these acquired structures or faculties are transmitted from parent to offspring, we have here a most important source and origin of variations--a source from which spring variations just in the particular direction in which they are wanted. the question is--are they transmitted? and if so, how? let us begin with the protozoa. dr. dallinger made some interesting experiments on monads. they extended over seven years, and were directed towards ascertaining whether these minute organisms could be gradually acclimatized to a temperature higher than that which is normal to them. commencing at ° fahr., the first four months were occupied in raising the temperature ° without altering the life-history. when the temperature of ° was reached, an adverse influence appeared to be exerted on the vitality and productiveness of the organism. the temperature being left constant for two months, they regained their full vigour, and by gradual stages of increase ° was reached in five months more. again, a long pause was necessary, and during the period of adaptation a marked development of vacuoles, or internal watery spaces, was noticed, on the disappearance of which it was possible to raise the temperature higher. thus by a series of advances, with periods of rest between, a temperature of ° fahr. was reached. it was estimated that the research extended over half a million generations. here, then, these monads became gradually acclimatized to a temperature more than double that to which their ancestors had been accustomed to--a temperature which brought rapid death to their unmodified relatives. now, in such observations it is impossible to exclude elimination. it is probable that there were numbers of monads which were unable to accommodate themselves to the changed conditions, and were therefore eliminated. but in any case, the fact remains that the survivors had, in half a million generations, acquired a power of existing at a temperature to which no individual in its single life could become acclimatized. here, then, we have the hereditary transmission of a faculty. but the organisms experimented on were protozoa. in them there is no distinction between germ-cell and body-cell. multiplication is by fission. and if the cell which undergoes fission has been modified, the two separate cell-organisms which result from that fission will retain the special modification. in such cases the transmission of acquired characters is readily comprehensible. we have an hereditary summation of effects. with the metazoa the case is different. in the higher forms the germinal cells are internal and sheltered from environing influences by the protecting body-wall. it is the body-cells that react to environmental stresses; it is muscle and nerve in which faculty is strengthened by use and exercise, or allowed to dwindle through neglect. the germ-cells are shielded from external influences. they lead a sheltered and protected life within the body-cavity. it is no part of their business to take part in either passive resistance or responsive activity. during the individual life, then, the body may be modified, may acquire new tissue, may by exercise develop enhanced faculties. but can the body so modified affect the germ-cells which it carries within it? biologists are divided on this question. some say that the body cannot affect the germ; others believe that it can and does do so. it might seem an easy matter to settle one way or another. but, in truth, it is by no means so easy. suppose that a man by strenuous exercise brings certain muscles to a high degree of strength or co-ordination. his son takes early to athletics, and perhaps excels his parent. is this a case of transmitted fibre and faculty? it may be. but how came it that the father took to athletics, and was enabled to develop so lithe and powerful a frame? it must have been "in him," as we say. in other words, it must have been a product of the germ-cells from which he was developed. and since his son was developed, in part at least, from a germ-cell continuous with these, what more natural than that he too should have an inherent athletic habit? every faculty that is developed in any individual is potential in the germ-stuff from which he springs; the tendency to develop any particular faculty is there too; and both faculty and tendency to exercise it are handed on by the continuity of germ-protoplasm or germ-cells. logically, there is no escape from the argument if put as follows: the body and all its faculties (i use the term "faculties" in the broadest possible sense) are the product of the germ; the acquisition of new characters or the strengthening of old faculties by the body is therefore a germinal product; there is continuity of the germs of parent and child; hence the acquisition by the child of characters acquired by the parent is the result of germinal or cellular continuity. it is not the acquired character which influences the germ, but the germ which develops what appears to be an acquired character. finally, if an acquired character, so called, is better developed in the child than in the parent, what is this but an example of variation? and if, in a series of generations, the acquired character continuously increases in strength, this must be due to the continued selection of favourable variations. it is clear that the organism that best uses its organs has, other things equal, the best chance of survival. it will therefore hand on to its offspring germinal matter with an inherent tendency to make vigorous use of its faculties. those who argue thus deny that the body-cells can in any way affect the germ-cells. to account for any continuous increase in faculty, they invoke variation and the selection of favourable varieties. what, then, we may now ask, is, on their view, the mode of origin of variations? in sexual reproduction, with the union of ovum and sperm, we seem to have a fertile source of variation. the parents are not precisely alike, and their individual differences are, _ex hypothesi_, germinal products. in the union of ovum and sperm, therefore, we see the union of somewhat dissimilar germs. and in sexual reproduction we have a constantly varying series of experiments in germinal combinations, some of which, we may fairly suppose, will be successful in giving rise to new or favourable variations. this view, however, would seem to involve an hypothesis which may be true, but which, in any case, should be indicated. for it is clear that if new or favourable variations arise in this way, the germinal union cannot be a mere mixture, but an organic combination. an analogy will serve to indicate the distinction implied in these phrases. it is well known that if oxygen and hydrogen be mixed together, at a temperature over °c., there will result a gaseous substance with characters intermediate between those of the two several gases which are thus commingled. but if they are made to combine, there will result a gas, water-vapour, with quite new properties and characters. in like manner, if, in sexual union, there is a mere mixture, a mere commingling of hereditary characters, it is quite impossible that new characters should result, or any intensification of existing characters be produced beyond the mean of those of ovum and sperm. if, for example, it be true, as breeders believe, that when an organ is strongly developed in both parents it is likely to be even more strongly developed in the offspring, and that weakly parts tend to become still weaker, this cannot be the result of germinal mixture. let us suppose, for the sake of illustration, that a pair of organisms have each an available store of forty units of growth-force, and that these are distributed among five sets of organs, _a_ to _e_, as in the first two columns. then the offspring will show the organs as arranged in the third column.[bp] parents. offspring. ----/\---- | | _a_ _b_ _c_ _d_ _e_ -- -- -- there is no increase in the set of organs _a_, which are strongly developed in both parents; and no decrease in the set of organs e, which are weakly developed in both parents. by sexual admixture alone there can be no increase or decrease beyond the mean of the two parental forms. if, then, the union of sperm and ovum be the source of new or more favourable variations other than or stronger than those of either parent, this must be due to the fact that the hereditary tendencies not merely commingle, but under favourable conditions combine, in some way different indeed from, but perhaps analogous to, that exemplified in chemical combination. such organic combination, as opposed to mere commixture, is altogether hypothetical, but it may be worth while to glance at some of its implications. if it be analogous to chemical combination, the products would be of a definite nature; in other words, the variations would be in definite directions. selection and elimination would not have to deal with variations in any and all directions, but would have presented to them variations specially directed along certain lines determined by the laws of organic combination. as professor huxley has said, "it is quite conceivable that every species tends to produce varieties of a limited number and kind, and that the effect of natural selection is to favour the development of some of these, while it opposes the development of others along their predetermined line of modification." mr. gulick[bq] and others have been led to believe in a tendency to divergent evolution residing in organic life-forms. such a tendency might be due to special modes of organic combination giving rise to particular lines of divergence. again, we have seen that some naturalists believe that specific characters are not always of utilitarian significance. but, as was before pointed out, on the hypothesis of all-round variation, there is nothing to give these non-useful specific characters fixity and stability, nothing to prevent their being swamped by intercrossing. if, however, on the hypothesis of combination, we have definite organic compounds, instead of, or as well as, mere hereditary mixtures; if, in other words, variations take definite lines determined by the laws of organic combination (as the nature and properties of chemical compounds are determined by the laws of chemical combination), then this difficulty disappears. there is no reason why a neutral divergence--one neither useful nor deleterious--should be selected or eliminated. and if its direction is predetermined, there is no reason why it should not persist, though, of course, it will not be kept at a high standard by elimination. it has again and again been pointed out as a difficulty in the path of natural selection that, in their first inception, certain characters or structures cannot yet be of sufficient utility to give the possessor much advantage in the struggle for existence. if, however, these be definite products of organic combination, this difficulty also disappears. so long as they are not harmful, they will not be eliminated, and by fortunate combinations will progress slowly until natural selection gets a hold on them and pushes them forward, developing to the full the inherent tendency. finally, we must notice that, on this hypothesis, our conception of panmixia, or intercrossing, would have to be modified. as generally held, this doctrine is based upon hereditary mixture, not organic combination. it is a doctrine of means and averages. there is a good deal of evidence that intercrossing does not, at least in all cases, produce mean or average results. and according to the hypothesis of organic combination, it need not always do so. according to this hypothesis, then, divergent modifications might arise and be perpetuated without the necessity of isolation. sterility might result from the fact that divergence had been carried so far that organic combination was no longer possible; reversion, due to intercrossing, from the fact that combinations long rendered impossible by the isolation of the necessary factors in distinct varieties, are again rendered possible when these varieties interbreed. on this hypothesis of organic combination, to which we shall recur in the chapter on "organic evolution," the varied forms of animal life are the outcome of definite organic products with definite organic structure, analogous to the definite chemical compounds with definite crystalline and molecular structure; and the analogy between the regeneration of hydra and the reconstruction of a crystal is carried on a step further. i do not say that i am myself at present prepared to adopt the hypothesis, at least in this crude form; but it is, perhaps, worth a passing consideration. its connection with mr. herbert spencer's doctrine of physiological units is obvious. the analogy there is with crystallization; here it is with chemical combination. we must now return to the point which gave rise to this digression, and repeat that mere hereditary commixture in the union of ovum and sperm cannot give rise to new characters or raise existing structures ( ) where there is free intercrossing beyond the mean of the species, and ( ) where there is rigorous elimination beyond the existing maximum of the species. variations beyond this existing maximum must be due to some other cause. professor weismann has suggested, as a cause of variation, the extrusion of the polar cells from the ovum. it has before been mentioned that, generally previous to fertilization, the ripe ovum buds off two minute polar bodies. the nucleus of the ovum divides, and one half is extruded in the first polar cell; the nucleus then (except in parthenogenetic[br] forms, where there is no union of ovum and sperm) again divides, and a second polar cell is extruded. in accordance with his special view of the absolute distinction between the body-plasm and the germ-plasm, the first polar cell is formed to carry off the body-plasm of the ovum-nucleus. for the ovum, besides being a germ-bearer, is a specialized cell, and its special form is determined by the body-plasm it contains. this is got rid of in the first polar cell, and nothing but germ-plasm remains. now, if nothing further took place, all the ova of this same individual containing similar germ-plasm would be identical, and similarly with all the sperms from the same parent. the union of these similar ova from one parent with similar sperms from another should therefore give rise to similar offspring. but the offspring are not all similar; they vary. professor weismann here makes use of the second polar cell.[bs] "a reduction of the germ-plasm," he says, "is brought about by its formation, a reduction not only in quantity, but above all, in the complexity of its constitution. by means of the second nuclear division, the excessive accumulation of different kinds of hereditary tendencies or germ-plasms is prevented. with the nucleus of the second polar body, as many different kinds of plasm are removed from the egg as will be afterwards introduced by the sperm-nucleus." "if, therefore, every egg expels half the number of its ancestral germ-plasms during maturation, the germ-cells of the same mother cannot contain the same hereditary tendencies, unless we make the supposition that corresponding ancestral germ-plasms chance to be retained by all eggs--a supposition that cannot be sustained." the two polar cells are therefore, on this view, of totally different character; and the nuclear division in each case of a special kind and _sui generis_. i do not think that the evidence afforded by observation lends much support to this view. but with that we are not here specially concerned. we have to consider how this reduction of the number of ancestral germ-plasms can further the kind of variation required. now, it is difficult to see, and professor weismann does not explain, how the getting rid of certain ancestral tendencies can give rise to new characters or the enhancement of old characters. one can understand how this "reducing division," as dr. weismann calls it, can reduce the level of now one and now another character. but how it can raise the level beyond that attained by either parent is not obvious. it is perhaps possible, though professor weismann does not, i think, suggest it, that, by a kind of compensation,[bt] the reduction of certain characters may lead to the enhancement of others. let us revert to the illustration on p. , where each individual has an available store of forty units of growth-force; and let us express by the minus sign the units lost in the parents by the extrusion of the polar cell and an analogous process which may occur in the genesis of the sperm. then the units of growth-force which may thus be lost by a "reducing division" in _b_, _c_, and _e_ may be, in the offspring, applied to the further growth of _a_; thus-- parents. offspring. -------/\------ | | _a_ _b_ - - _c_ - - _d_ _e_ - here the reduction of the characters _b_, _c_, and _e_ has led to the enhancement of _a_, which thus stands at a higher level than in either parent. on such an hypothesis we may, perhaps, explain the fact to which breeders of stock testify--that the organ strongly developed in both parents (_a_) is yet more strongly developed in some of their offspring, and that weakly parts (_e_) tend to become still weaker. i know not whether this way of putting the matter would commend itself to professor weismann or his followers; but some such additional hypothesis of transference of growth-force from one set of organs to another set of organs seems necessary to complete his hypothesis. professor weismann's view, then, assumes ( ) that the cell-division which gives rise to the ova in the ovary is so absolutely equal and similar that all ova have precisely the same characters; ( ) that the first polar cell leaves the germinal matter unaffected, merely getting rid of formative body-plasm; ( ) that the nuclear division giving rise to the second polar cell is unequal and dissimilar, effecting the differential reduction of ancestral germ-plasms. concerning all of which one can only say that it may be so, but that there is not much evidence that it is so. and, without strong confirmatory evidence, it is questionable whether we are justified in assuming these three quite different modes of nuclear division. there remains one more question for consideration, on the hypothesis that the germ-cells cannot in any special way be affected by the body-cells. in considering the union of ovum and sperm as a source of variation, we have taken for granted the existence of variations. we have been dealing with the mixture or combination of already existing variations. how were variations started in the first instance? we have already seen that in the protozoa parent and offspring are still, in a certain sense, one and the same thing; the child is a part, and usually half, of the parent. if, therefore, the individuals of a unicellular species are acted upon by any of the various external influences, it is inevitable that hereditary individual differences will arise in them; and, as a matter of fact, it is indisputable that changes are thus produced in these organisms, and that the resulting characters are transmitted. hereditary variability cannot, however, arise in the metazoa, in which the germ-plasm and the body-plasm are differentiated and kept distinct. it can only arise in the lowest unicellular organisms. but when once individual difference had been attained by these, it necessarily passed over into the higher organisms when they first appeared. sexual reproduction coming into existence at the same time, the hereditary differences were increased and multiplied, and arranged in ever-changing combinations. such is professor weismann's solution of the difficulty, told, for the most part, in his own words. i do not know that professor weismann has anywhere distinctly stated what he conceives to be the relation of body-plasm and germ-plasm in the protozoa. are the two as yet undifferentiated? this can hardly be so, seeing the fundamental distinction he draws between them. is it the germ-plasm or the body-plasm that is influenced by external stresses? if the former, does it transfer its influence to the body-plasm during the life of the individual? if the latter, then the body-plasm must either directly influence the germ-plasm in unicellular organisms (it would seem that, according to professor weismann, it cannot do so in the metazoa), or the changed body-plasm, which shares in the fission of the protozoon, must participate in that so-called immortality which is often said to be the special prerogative of germinal matter. these, however, are matters for professor weismann and his followers to settle. i regard the sharp distinction between body-plasm and germ-plasm as an interesting biological myth. for me, it is sufficient that the protoplasm of the protozoon is modified, and the modification handed on in fission. and it is clear that professor weismann is correct in saying that the commixture or combination of characters takes its origin among the protozoa. if the unicellular individuals are differently modified, however slightly, then, whenever conjugation occurs between two such individuals, there will be a commingling or combination of the different characters. the transmissible influence of the environment, however, ceases when the metazoon status is reached, and special cells are set apart for reproductive purposes--ceases, that is to say, in so far as the influence on the body is concerned. there may, of course, be still some direct[bu] influence on the germinal cells themselves. except for this further influence, the metazoon starts with the stock of variations acquired by that particular group of protozoa--whatever it may be--from which it originated. all future variations in even the highest metazoa arise from these. now, it is obvious that no mere commingling and rearrangement of protozoan characters could conceivably give rise to the indefinitely more complex metazoan characters. but if there be a combination and recombination of these elements in ever-varying groups, the possibilities are no longer limited. let us suppose that three simple protozoan characters were acquired. the mere commixture of these three could not give much scope for further variation. it would be like mixing carbon, oxygen, and hydrogen in varying proportions. but let them in some way combine, and you have, perhaps, such varied possibilities as are open to chemical combinations of oxygen, hydrogen, and carbon, whose name is legion, but whose character is determined by the laws of chemical combination. summing up now the origin of variations, apart from those which are merely individual, on the hypothesis that particular modifications of the body-cells cannot be transmitted to the germ-cells, we have-- . in protozoa, the direct influence of the environment and the induced development of faculty. . in metazoa-- (_a_) some direct and merely general influence of the environment on the germ, including under the term "environment" the nutrition, etc., furnished by the body. (_b_) the combination and recombination of elementary protoplasmic faculties (specific molecular groupings) acquired by the protozoa. (_c_) influences on the germ, the nature of which is at present unknown. * * * * * we may now pass on to consider the position of those who give an affirmative answer to the question--can the body affect the germ? two things are here required. first, definite evidence of the fact that the body does so affect the germ; i.e. that acquired characters are inherited. secondly, some answer to the question--how are the body-cells able to transmit their modifications to the germ-cells? we will take the latter first, assuming the former point to be admitted. let us clearly understand the question. an individual, in the course of its life, has some part of the epidermis, or skin, thickened by mechanical stresses, or some group of muscles strengthened by use, or the activity of certain brain-cells quickened by exercise: how are the special modifications of these cells, here, there, or elsewhere in the body, communicated to the germ, so that its products are similarly modified in the offspring? the following are some of the hypotheses which have been suggested:-- (_a_) darwin's pangenesis. (_b_) haeckel's perigenesis; spencer's physiological units. (_c_) the conversion of germ-plasm into body-plasm, and its return to the condition of germ-plasm (nägeli). (_d_) the unity of the organism. (_a_) concerning pangenesis, nothing need be added to what has already been said. although, as we have seen, it has been adopted with modifications by professor brooks; although mr. francis galton, a thinker of rare ability and a pioneer in these matters, while contending for continuity, admitted a little dose of pangenesis; although de vries has recently renewed the attempt to combine continuity and a modified pangenesis;--this hypothesis does not now meet with any wide acceptance. (_b_) with the pamphlet in which professor haeckel brought forward his hypothesis termed the perigenesis of the plastidule, i cannot claim first-hand acquaintance. according to professor ray lankester, who gave some account of it in _nature_,[bv] protoplasm is regarded by haeckel as consisting of certain organic molecules called plastidules. these plastidules are possessed of special undulatory movements, or vibrations. they are liable to have their undulations affected by every external force, and, once modified, the movement does not return to its pristine condition. by assimilation, they continually increase to a certain size and then divide, and thus perpetuate in the undulatory movement of successive generations the impressions or resultants due to the action of external agencies on the individual plastidules. on this view, then, the form and structure of the organism are due to the special mode of vibration of the constituent plastidules. this vibration is affected by external forces. the modified vibration is transmitted to the plastidules by the germ, which, therefore, produce a similarly modified organism. as mr. j. a. thomson says, "in metaphorical language, the molecules remember or persist in the rhythmic dance which they have learned." darwin's hypothesis was frankly and simply organic--the gemmules are little germs. this of professor haeckel tries to go deeper, and to explain organic phenomena in terms of molecular motion. mr. herbert spencer long ago suggested that, just as molecules are built up, through polarity, into crystals, so physiological units are built up, under the laws of organic growth, into definite and special organic forms. both views involve special units. with mr. herbert spencer, their "polarity" is the main feature; with professor haeckel, their "undulatory movements." according to mr. spencer, "if the structure of an organism is modified by modified function, it will impress some corresponding modification on the structures and polarities of its units."[bw] according to professor haeckel, the vibrations of the plastidules are permanently affected by external forces. in either case, an explanation is sought in terms of molecular science, or rather, perhaps, on molecular analogies. so far good. such "explanation," if hypothetical, may be suggestive. it may well be that the possibilities of fruitful advance will be found on these lines. but though, as general theories, these suggestions may be valuable, they do not help us much in the comprehension of our special point. to talk vaguely about "undulatory movements" or "polarities" does not enable us to comprehend with any definiteness how this particular modification of these particular nerve-cells is so conveyed to the germ that it shall produce an organism with analogous nerve-cells modified in this particular way. (_c_) the hypothesis that the germ-plasm may be converted into body-plasm, which, on its return again to the condition of germ-plasm, may retain some of the modifications it received as body-plasm, seems to be negatived, so far as most animals are concerned, by the facts of embryology and development. the distinction of germ-plasm and body-plasm i hold to be mythical. and there is no evidence that cells specially differentiated along certain lines can become undifferentiated again, and then contribute to the formation of ova or sperms. from the view-point of cell-differentiation, which seems to me the most tenable position, there does not seem any evidence for, or any probability of, the occurrence of any roundabout mode of development of the germinal cells which could enable them to pick up acquired characters _en route_. (_d_) we come now to the contention that the organism, being one and continuous, if any member suffers, the germ suffers with it. the organs of the body are not isolated or insulated; the blood is a common medium; the nerves ramify everywhere; the various parts are mutually dependent: may we not, therefore, legitimately suppose that long-continued modification of structure or faculty would soak through the organism so completely as eventually to modify the germ? the possibility may fairly be admitted. but how is the influence of the body brought to bear on the germ? the common medium of the blood, protoplasmic continuity, the influence of the products of chemical or organic change,--these are well enough as vague suggestions. but how do they produce their effects? once more, how is this increased power in that biceps muscle of the oarsman able to impress itself upon the sperms or the ova? no definite answer can be given. we are obliged to confess, then, that no definite and satisfactory answer can be given to the question--how can the body affect the germ so that this or that particular modification of body-cells may be transmitted to the offspring? we may make plausible guesses, or we may say--i know not how the transmission is effected; but there is the indubitable fact. this leads us to the evidence of the fact. it must be remembered that no one questions the modifiability of the individual. that the epidermis of the oarsman's hand is thickened and hardened; that muscles increase by exercise; that the capacity for thinking may be developed by steady application;--these facts nobody doubts. that well-fed fish grow to a larger size than their ill-fed brethren; that if the larger shin-bone (the tibia) of a dog be removed, the smaller shin-bone (the fibula) soon acquires a size equal to or greater than that of the normal tibia; that if the humerus, or arm-bone, be shifted through accident, a new or false joint will be formed, while the old cavity in which the head of the bone normally works, fills up and disappears; that canaries fed on cayenne pepper have the colour of the plumage deepened, and bullfinches fed on hemp-seed become black; that the common green amazonian parrot, if fed with the fat of siluroid fishes, becomes beautifully variegated with red and yellow; that climate affects the hairiness of mammals;--these and many other reactions of the individual organism in response to environing conditions, will be admitted by every one.[bx] that constitutional characters of germinal origin are inherited is also universally admitted. the difficulty is to produce convincing evidence that what is acquired is really inherited, and what is inherited has been really acquired. attempts have been made to furnish such evidence by showing that certain mutilations have been inherited. i question whether many of these cases will withstand rigid criticism. nor do i think that mutilations are likely to afford the right sort of evidence one way or the other. we must look to less abnormal influences. what we require is evidence in favour of or against the supposition that _modifications_ of the body-cells are transmitted to the germ-cells. now, these modifications must clearly be of such a nature as to be receivable by the cells without in any way destroying their integrity. the destruction or removal of cells is something very different from this. if it were proved that mutilations are inherited, this would not necessarily show that normal cell-modifications are transmissible. and if the evidence in favour of inherited mutilations breaks down, as i believe it does, this does not show that more normal modifications such as those with which we are familiar, as occurring in the course of individual life, are not capable of transmission. i repeat, we must not look to mutilations for evidence for or against the supposition that acquired characters are inherited. we must look to less abnormal influences. these readily divide themselves into two classes. the first includes the direct effects on the organism of the environment--effects, for example, wrought by changes of climate, alteration of the medium in which the organism lives, and so forth. the second comprises the effects of use and disuse--the changes in the organism wrought by the exercise of function. taking the former first, we have the remarkable case of _saturnia_, which was communicated to darwin by moritz wagner. mr. mivart thus summarizes it: "a number of pupæ were brought, in , to switzerland from texas of a species of _saturnia_, widely different from european species. in may, , the moths developed out of the cocoons (which had spent the winter in switzerland), and resembled entirely the texan species. their young were fed on leaves of _juglans regia_ (the texan form feeding on _juglans nigra_), and they changed into moths so different, not only in colour, but also in form, from their parents, that they were reckoned by entomologists as a distinct species."[by] professor mivart also reminds us that english oysters transported to the mediterranean are recorded by m. costa to have become rapidly like the true mediterranean oyster, altering their manner of growth, and forming prominent diverging rays; that setters bred at delhi from carefully paired parents had young with nostrils more contracted, noses more pointed, size inferior, and limbs more slender than well-bred setters ought to have; and that cats at mombas, on the coast of africa, have short, stiff hair instead of fur, while a cat from algoa bay, when left only eight weeks at mombas, underwent a complete metamorphosis--having parted with its sandy-coloured fur. very remarkable is the case of the brine-shrimp _artemia_, as observed and described by schmankewitsch. one species of this crustacean, _artemia salina_, lives in brackish water, while _a. milhausenii_ inhabits water which is much saltier. they have always been regarded as distinct species, differing in the form of the tail-lobes and the character of the spines they bear. and yet, by gradually altering the saltness of the water, either of them was transformed into the other in the course of a few generations. so long as the altered conditions remained the same, the change of form was maintained. many naturalists believe that climate has a direct and determining effect on colour, and contend or imply that it is hereditary. mr. j. a. allen correlates a decrease in the intensity of colour with a decrease in the humidity of the climate. mr. charles dixon, in his "evolution without natural selection," says, "the marsh-tit (_parus palustris_) and its various forms supply us with similar facts [illustrative of the effects of climate on the colours of birds]. in warm, pluvial regions we find the brown intensified; in dry, sandy districts it is lighter; whilst in arctic regions it is of variable degrees of paleness, until, in the rigorous climate of kamschatka, it is almost white." mr. dixon does not think that these changes are the result of natural selection. "depend upon it," he says, with some assurance,[bz] in considering a different case, "it is the white of the ptarmigan (modified by climatic influence) that has sent the bird to the snowy wastes and bare mountain-tops, and rigorously keeps it there; not the bird that has assumed, by a long process of natural selection, a white dress to conceal itself in such localities." professor eimer[ca] contends that in the nile valley the perfectly gradual transition in the colour of the inhabitants from brownish-yellow to black in passing from the delta to the soudan is particularly conclusive for the direct influence of climate, for the reason that various races of originally various colours dwell there. mr. a. r. wallace says[cb] of the island of celebes "that it gives to a large number of species and varieties (of papilionidæ) which inhabit it, ( ) an increase of size, and ( ) a peculiar modification in the form of the wings, which stamp upon the most dissimilar insects a mark distinctive of their common birthplace." but this similarity may largely, or at least in part, be due to mimicry. most interesting and valuable are the results of mr. e. b. poulton's experiments on caterpillars and chrysalids.[cc] they show that there is a definite colour-relation between the caterpillar (e.g. the eyed hawk-moth, _smerinthus ocellatus_) and its food-plant, adjustable within the limits of a single life; that the predominant colour of the food-plant is itself the stimulus which calls up a corresponding larval colour; that there is also a direct colour-relation between the chrysalids of the small tortoiseshell butterfly (_vanessa urticæ_) and the surrounding objects, the pupæ being dark grey, light grey, or golden, according to the nature and colour of the surroundings; and that the larvæ of the emperor moth (_saturnia carpini_) spin dark cocoons in dark surroundings, but white ones in lighter surroundings. these are but samples of the interesting results mr. poulton has obtained. what shall we say of such cases? some of them seem to indicate the very remarkable and interesting fact that changes of salinity of the medium, or changes of food, or the more general influence of a special climate, may modify organisms in _particular_ and little-related ways. the larvæ of a texan _saturnia_ fed on a new food-plant develop into imagos so modified as to appear new species. changes of salinity of the water modify one species of _artemia_ into another. if these be adaptations, the nature of the adaptation is not obvious. if the new character produced in this way be of utilitarian value, where the utility comes in is not clear. the facts need further confirmation and extension, which may lead to very valuable results. mr. poulton's observations, on the other hand, give us evidence of direct adaptation to colour-surroundings. but the effects are, in the main, restricted to the individual. what is hereditary is the power to assume one of two or three tints, that one being determined by the surrounding colour. his experiments neither justify a denial nor involve an assertion of the transmissibility of environmental influence. secondly, some of the cases above cited seem to show clearly that, under changed conditions of life, the changes which have been wrought in one generation may _reappear_ in the next. but are they inherited? is there sufficient evidence to show conclusively that the body-cells have been modified, and have handed on the modification to the germ? can we exclude the direct action of the more or less saline water, or the products of the unwonted food on the germinal cells? can we be sure that there is really a summation of results--that each generation is not affected _de novo_ in a similar manner? no one questions that the individual is modifiable, and that such modification is most readily effected in the early and plastic stages of life. if each plastic embryo is moulded in turn by similar influence, how can we conclusively prove hereditary summation? take a case that has been quoted in support of hereditary modification. greyhounds transported from england to the uplands of mexico are unable to course, owing to the rarity of the atmosphere. their pups are, however, able to run down the fleetest hares without difficulty. now, this may be due to the fact that the dogs acquire a certain amount of accommodation to a rare atmosphere, and hand on their acquired power to their offspring, which carry it on towards perfection. but it may also be due to the fact that the pups, subject from the moment of birth to the conditions of a rarified atmosphere, are developed in accordance with these conditions. or take another case that has been brought forward. english dogs are known in hot climates, like that of india, to degenerate in a few generations. let us suppose that these degenerate dogs are removed back to england, and that their pups, born in english air and in our temperate climate, are still degenerate: would not this, it may be asked, show that the influence of climate on the body is inherited? i do not think that such a case would be convincing. for the climate might well influence the germ through the body. the body being unhealthy and degenerate, the germ-cells must, one may suppose, suffer too. the degenerate pup born in england might well owe its degeneracy to effects wrought upon the germinal cells. in other words, such a case would indicate some _general_ influence of the environment (including the environing body) on the germ. it does not convince us that _particular_ modifications of body-cells as such are transmitted under normal and healthy conditions. on the whole, it seems to me that the evidence we at present possess on this head is not convincing or conclusive in favour of the effects on the body alone being transmitted to offspring. if cases can be brought forward in which there can be no direct influence on the germ, in which elimination is practically excluded, and in which there is a _gradual and increasing_ accommodation of successive generations of organisms to changed conditions _which remain constant_, then such transmission will be rendered probable. i do not know that there are observations of this kind of sufficient accuracy to warrant our accepting this conclusion as _definitely proved_. attention may here be drawn to a peculiar and remarkable mode of influence. if a pure-bred mare have foals by an ill-bred sire, they will be ill-bred. this we can readily understand. but if she subsequently have a foal by a perfectly well-bred sire, that foal, too, may in some cases be tainted by the blemish of the previous sire. so, too, with dogs. if a pure-bred bitch once produce a mongrel litter, no matter how carefully she be subsequently matched, she will have a tendency to give birth to pups with a mongrel taint. this subsequent influence of a previous sire is a puzzling fact. it may be that some of the male germ-nuclei are absorbed, and influence the germ-cells of the ovary. but this seems an improbable solution of the problem. it is more likely, perhaps, that in the close relation of mother and f[oe]tus during gestation, each influences the other (how it is difficult to say). on this view the bitch retains the influence of the mongrel puppies--is herself, in fact, partially mongrelized--and therefore mongrelizes subsequent litters. it would not be safe, however, to base any far-reaching conclusions on so peculiar a case, the explanation of which is so difficult. at all events, it is impossible to exclude the possibility of direct action on the germ, though the _particular_ nature of the results of such influence are noteworthy. we may pass now to the evidence that has been adduced in favour of a cumulative effect in the exercise of function, or of the inheritance of the results of use or disuse. here, again, it must be remembered that no one questions the effects of use and disuse in the individual. what we seek is convincing evidence that such effects are inherited. physiologically, the effects of use or disuse are, in the main, effects on the relative nutrition, and hence on the differential growth of organs. when an organ is well exercised, there is increased nutrition and increased growth of tissue, muscular, nervous, glandular, or other. when an organ is, so to speak, neglected, there is diminished blood-supply, diminished growth, and diminished functional power. the development of a complex activity would necessitate a complex adjustment of size and efficiency of parts, involving a nice balance of differential growth dependent on delicately regulated nutrition. what is the evidence that adjusted nutrition can be inherited? with regard to man, there is some evidence which bears upon this subject. mr. arbuthnot lane, in his valuable papers in the _journal of anatomy and physiology_, has shown that certain occupations, such as shoemaking, coal-heaving, etc., produce recognizable effects upon the skeleton, the muscular system, and other parts of the organization. and he believes[cd] that such effects are inherited, being very much more marked in the third generation than they were in the first. sir william turner informed professor herdman that, in his opinion, the peculiar habits of a tribe, such as tree-climbing among the australians, or those natives of the interior of new guinea whose houses are built in the upper branches of lofty trees, not only affect each generation individually, but have an intensified action through the influence of heredity.[ce] mr. francis galton's results mainly deal with human faculty; and though faculty has undoubtedly an organic basis, i do not propose to consider the evidence afforded by instinct, intelligence, or intellectual faculties in this chapter. mention should, however, be made of the interesting results of his study of twins. twins are either of the same sex, in which case they are remarkably alike, or of different sexes, in which case they are apt to differ even more widely than is usual with brothers and sisters. the former are believed to be developed from one ovum which has divided into two halves, each of which has given rise to a distinct individual; the latter from two different ova. mr. galton collected a large mass of statistics concerning twins of both classes. the result of this analysis seems to be that, in the case of "identical twins," the resemblances are not superficial, but extremely intimate; that they are not apt to be modified to any large extent by the circumstances of life; that where marked diversity sets in it is due to some form of illness; and, on the whole, that innate tendencies outmaster acquired modifications. "nature is far stronger than nurture within the limited range that i have been careful to assign to the latter." on the other hand, speaking of dissimilar twins, mr. galton says, "i have not a single case in which my correspondents speak of originally dissimilar characters having become assimilated through identity of nurture." "the impression that all this evidence leaves on the mind is one of some wonder whether nurture can do anything at all, beyond giving instruction and professional training." "there is no escape from the conclusion that nature prevails enormously over nurture where the differences of nurture do not exceed what is commonly to be found among persons of the same rank of society and in the same country."[cf] combining the results of messrs. lane and galton, we may say that it requires persistent and long-continued influence to modify the individual, and change, even by a little, the structure inherited or given by nature; but that if this structure is thus modified, there may be a tendency for such modification to increase by hereditary summation of effects. we require, however, further and fuller observations to render the evidence of such hereditary summation to any extent convincing. turning now from the evidence afforded by man[cg] to that afforded by animals, we may consider first that presented by domesticated breeds. they might be expected to afford exceptionally good examples. their modifiability and the readiness with which they interbreed are two of the determining causes of their selection for domestication. they have, moreover, been placed under new conditions of life, and they undoubtedly exhibit changes of structure, many of which darwin[ch] regarded as attributable to the effects of use and disuse. in domestic ducks, the relative weight and strength of the wing-bones have been diminished, while conversely the weight and strength of the leg-bones have been increased. the bones of the shoulder-girdle have been decreased in weight and "the prominence of the crest of the sternum, relatively to its length, is also much reduced in all the domestic breeds. these changes," says darwin, "have evidently been caused by the lessened use of the wings." the shoulder-girdle and breast-bone of domestic fowls have been similarly reduced. after a careful consideration of numerous facts concerning the brains of rabbits, darwin concluded that this "most important and complicated organ in the whole organization is subject to the law of decrease in size from disuse." and sir j. crichton browne has recently shown that, in the wild duck, the brain is nearly twice as heavy in proportion to the body as it is in the comparatively imbecile domestic duck. in pigs, the nature of the food supplied during many generations has apparently affected the length of the intestines; for, according to cuvier, their length to that of the body in the wild boar is as to , in the common domestic boar as . to , and in the siam breed as to . with regard to horses, darwin tells us that "veterinarians are unanimous that horses are affected with spavins, splints, ring-bones, etc., from being shod and from travelling on hard roads, and they are almost unanimous that a tendency to these malformations is transmitted." these are samples of the effects of domestication. it has been suggested, however, that, quite apart from any diminution from disuse, the reduction of size in parts or organs may be the result of the absence or cessation of selection. if an organ be subject to selection, the mean size in adult creatures will be that of the selected individuals; but if selection ceases, it will be the mean of those born. let us suppose that nine individuals are born, and that the size of some organ varies in these from , the most efficient, to , the least efficient. the birth-mean will therefore be, as shown on the left-hand side of the following table, at the level of number , four being more efficient, and four less efficient. but if, of these nine, six be eliminated, then the mean of the survivals will be as shown on the right-hand side of the table:-- --survival-mean. } birth-mean-- } } eliminated individuals. } } } the result, then, of the cessation of selection will be to reduce the survival-mean to the birth-mean, and that without any necessary effect of disuse. but unless this be accompanied by a tendency to diminution due to economy of growth or some other cause, this cannot produce any well-marked or considerable amount of reduction. i very much question, for example, whether the cessation of selection, even with the co-operation of the principle of economy of growth, will adequately account for the reduction to nearly one-half its original proportion of the brain of the duck. the subject will be more fully discussed, however, in the next chapter. there is probably but little tendency for disused parts to be reduced in size through artificial selection. an imbecile duck does not probably taste nicer than one with bigger brains. on the other hand, the increase of size in organs may presumably, in certain cases, be increased by selection. pigs, for example, have been selected according to their fattening capacity. those with longer intestines, and therefore increased absorbent surface, may well have an advantage in this respect. hence, in selecting pigs for fattening, breeders may have been unconsciously selecting those with the longest intestines. of course, on this view, the longer intestine must be there to be selected, and the increased length must be due to variation. but this may be all-round variation (cause unknown), not variation in one direction, the result of increased function. another point that has to be taken into consideration is the amount of _individual_ increment or decrement, owing to individual use or disuse, apart from any possible summation of results. seeing, then, that it is difficult to estimate the amount of purely individual increment or decrement, and that it is difficult, if not impossible, to exclude the disturbing effects of cessation of selection with economy of growth on the one hand, reducing the size of organs, and artificial selection on the other hand, increasing the size or efficiency of parts, it is clear that such cases cannot afford convincing evidence that the observed variations are the directly inherited results of use and disuse. indeed, i am not aware of any experiments or direct observations on animals which are individually conclusive in favour of the hereditary summation of functionally produced modifications. it may, however, be said--although no absolutely convincing experiments or observations are forthcoming (for, from the nature of the case, it is almost impossible logically to prove that this interpretation of the facts is alone possible), still there are cases which are much more readily explained on the hypothesis that the effects of use and disuse are inherited, than on any other hypothesis. but, so far as professor weismann and his followers are concerned, such an argument is wholly beside the question. they are ready to admit that inherited modifications of the body, if they could be proved, would render the explanation of many results of evolution much easier. it would, no doubt, they say, be easier to account for the shifting of the eye of a flat-fish from one side of the head to the other on the supposition that individual efforts were inherited, until, by an hereditary summation of effort, the eye at last came round. the question is--are we justified in accepting the easier explanation if it be based on a mere assumption, at present unproved, the _modus operandi_ of which is inexplicable? let us consider very briefly these two points--first, the "mere assumption;" secondly, "the inexplicable _modus operandi_." is there any reason why we should not assume the inheritance of effects of use or disuse as a working hypothesis, if it is not in opposition to any known biological law, and if it does enable us to explain certain observed phenomena? i see no such reason. we do not know enough about the causes of variation to be rigidly bound by the law of parcimony. i am not aware of any biological law that would render the acceptance of this view as a provisional hypothesis unjustifiable. but how, it is asked, can we accept it if its _modus operandi_ is inexplicable? i question the validity of this argument. i fear our knowledge of organic nature is not at present so full and exact as to justify us in excluding an hypothesis because we are not able to give an adequate answer to the question--how are these effects produced? of course, if it can be shown that no _modus operandi_ is possible, there is an end of the matter. but who shall dare thus to limit the possibilities of organic nature? and, if possible, then that natural selection in which the neo-darwinians place their sole trust would certainly develop so advantageous a mode of influence. it is clear that a species sensitive to every shock of the environment on the organism would be unstable, and hence at a disadvantage. but, on the other hand, the ability to answer by adaptation to long-continued and persistent environmental influence or to oft-repeated and consistent performance of function would be so distinct an advantage to the species which possessed it, that, if it lay within the possibilities of organic nature, natural selection, always, as we are told, on the look out for every possible advantage, would assuredly seize upon it and develop it. those who believe in the absolute sway of natural selection have not at present given any adequate answer to the question--how are particular variations (e.g. the twisted skull of flat-fish) produced? they say that constitutional variations, which are alone inheritable, are due to variations in the germs. when asked how these variations are produced, they are forced to reply--we cannot say. but when it is suggested that they may be in some unknown way transmitted to the germ from the body, they are up in arms, and exclaim--you have no right to believe that, or ask us to believe it, unless you can tell us plainly how the effect is produced. unable themselves to give the _modus operandi_ of the origin of particular variations, they demand the exact _modus operandi_ from those who suggest that variations may arise through this mode of influence of the body on the germ. we shall have to consider this question from a more general standpoint in the next chapter on "organic evolution." we may now very briefly summarize some of the results we have reached in this chapter. the ova and sperms are specially differentiated cells which have, in the division of labour, retained and emphasized the function of developmental reproduction. there is a continuity of such cells. the cells which become ova or sperms have never become differentiated into anything else. hereditary similarity is due to the fact that parents and offspring are derived eventually from the same germinal cells. variation in the existing world is partly due to sexual union. but if there be mere admixture, new characters cannot arise in this way, nor can old characters be strengthened beyond the existing maximum. some mode of organic combination (analogous to chemical combination) might afford an explanation of the occurrence of new variations and the increase of existing characters. in the protozoa there may be a summation of the effects of the environment in succeeding generations. there is no convincing evidence that in the metazoa special modifications of the body so influence the germ as to become hereditary. but there is no reason why such influence should not be assumed as a provisional hypothesis. notes [bd] "animals and plants under domestication," vol ii. p. . [be] or in certain "physiological units" (herbert spencer), or "plastidules" (haeckel), which may be regarded as organic molecules exhibiting their special properties under vital conditions. [bf] _nature_, vol. xxxix. p. . [bg] darwin, "animals and plants under domestication," nd edit., vol. ii. chap. xxvii., from which the following description and quotations are taken. [bh] for an excellent account of the genesis and growth of the modern views of heredity, see mr. j. arthur thomson's paper on "the history and theory of heredity:" proceedings of the royal society of edinburgh, . [bi] geddes and thomson, "the evolution of sex," p. . [bj] weismann, "essays on heredity," english translation, p. . [bk] weismann, "essays on heredity," p. . [bl] a few pages earlier (p. ) in the same essay, professor weismann says, "a sudden transformation of the nucleo-plasm of a somatic cell into that of a germ-cell would be almost as incredible as the transformation of a mammal into an am[oe]ba." this at first sight does not seem quite consistent with the subsequent sentence which i have quoted in the text; for here, at any rate, the daughters of "mammals" are said to be converted into "am[oe]bæ." but this is no doubt because the am[oe]bæ (germ-plasms) are _contained in_ the mammals (body-cells). (see the quotations that follow in the text.) [bm] weismann, "essays on heredity," p. . [bn] weismann, "essays on heredity," p. . [bo] it will, of course, be understood that a minute fragment of germ-plasm is capable of almost unlimited growth by assimilation of nutritive material, its properties remaining unchanged during such growth. [bp] latency is here neglected. mr. francis galton has shown, statistically, that the offspring, among human folk, inherit / from each parent, / from each grandparent, and the remaining / from more remote ancestors. in domesticated animals, reversion to characters of distant ancestors sometimes occurs. this, however, does not invalidate the argument in the text, which is that sexual admixture tends towards the mean of the race (ancestors included), and cannot be credited with new and unusually favourable variations. the prepotency of one parent is also here neglected. [bq] see his valuable paper on "divergent evolution," lin. soc. zool., no. cxx. [br] one parthenogenetic form--the drone--has been shown by blochmann to extrude a second polar cell. this observation is in serious opposition to dr. weismann's theory. [bs] weismann, "essays on heredity," pp. , . [bt] the law of compensation of growth or balancement was suggested at nearly the same time by goethe and geoffrey saint-hilaire. the application in the text has not, so far as i know, been before suggested. [bu] darwin spoke of changed conditions acting "directly on the organization or indirectly through the reproductive system." now, since professor weismann has taught us to reconsider these questions, we speak of such conditions as acting directly on the germ or indirectly through the body. the germ is no longer subordinate to the body, but the body to the germ. [bv] july , . since reprinted in "the advancement of science," p. . [bw] herbert spencer, "principles of biology," vol. i. p. . [bx] mr. j. a. thomson has published a most valuable "synthetic summary of the influence of the environment upon the organism" (proceedings royal physiological society, edinburgh: vol. ix. pt. , ). the case of the amazonian parrots was communicated to darwin by mr. wallace ("animals and plants under domestication," vol. ii. p. ). [by] st. george mivart, "on truth," p. . [bz] _op. cit._, p. . i venture to say, "with some assurance," because charles darwin, who had also considered this matter, writes, "who will pretend to decide how far the thick fur of arctic animals, or their white colour, is due to the direct action of a severe climate, and how far to the preservation of the best-protected individuals during a long succession of generations?" ("animals and plants under domestication," p. ). [ca] "organic evolution," english translation, p. . [cb] "contributions to natural selection," p. . [cc] since this was written, mr. poulton has described his results in an interesting volume on "the colours of animals" (_q.v._). [cd] see _journal of anatomy and physiology_, vol. xxii. p. . [ce] see professor herdman's inaugural address, liverpool biological society, . [cf] francis galton, "inquiries into human faculty," p. . [cg] that the epidermis is thicker on the palms of the hands and the soles of the feet in the infant long before birth, may be attributable to the inherited effects of use or pressure. it can hardly be held that the thickening of the skin in these parts is of elimination value. [ch] the instances cited are from "animals and plants under domestication." chapter vi. organic evolution. it is difficult to realize the wealth, the variety, the diversity, of "animal life." even if we endeavour to pass in review all that we have seen in woodland and meadow, in pond or pool, in the air, on the earth, in the waters, in temperate or tropical regions; even when we try to remember the results of all anatomical and microscopic investigation displaying new wonders and new diversities hidden from ordinary and unaided vision; even when we call to mind the multifarious contents, recent and fossil, of all the natural history museums we have ever visited, and throw in such mental pictures as we have formed of all the diverse adaptations we have read about or heard described;--even so we cannot but be conscious that not one-tenth, not one-hundredth, part of the diversity and variety of animal life has passed before our mental vision even in sample. it is said that our greatest living poet once, when a young man, left his companions to gaze into the waters of a clear, still pool. "what an imagination god has!" he said, as he rejoined his friends. fit observation for the poet, whose sensitive nature must be keenly alive to the varied endowments which nature has lavishly showered upon her animate children. certain it is that words, mere words, can never present, though they may aid in recalling, an adequate picture of either the wealth or the beauty of animal life. fortunately for those who visit london (and who nowadays does not?), we have, in our national collection in south kensington, the means of getting some insight into the wealth of life. and much is being done there to aid the imagination and to facilitate study for those who are not professed students. many of the birds are now to be seen set in their natural surroundings, with their life-history illustrated. our frontispiece is taken from one of these cases. and this admirable system will, no doubt, so far as space permits, be extended; and, perhaps, dramatic incidents may be introduced, like those (notably in the life of heron and hawk) which form so marked a feature in the little museum at exeter. anything which leads us to understand the life of animals, and to go forth and study it for ourselves, has an educational value. in our national museum, again, much is being wisely done to illustrate the diversity and variety of structure and the principles that underlie them. observe, as you enter the central hall, the case containing stuffed specimens of ruffs (_machetes pugnax_). among the young autumn birds there is not much difference between males and females, the male being distinguished chiefly by its somewhat larger size. nor do the old birds, male and female, differ much during the winter months. but in pairing-time, may and june, the females are somewhat richer in colour; while the males not only don the ruff to which the bird owes its popular name, but develop striking colour-tints. among different individuals it will be seen that the colour-variation is tolerably wide; but the same individual keeps strictly, we are told, in successive seasons, to the same summer dress. note, next, in a bay to the right, the great variety of form, ornamentation, and colouring among the molluscan shells there exhibited. observe that the rich colours are often hidden during life by the dull epidermis. half an hour's attentive study of these varied molluscan forms will give a better idea of the beauty and diversity of these life-products than pages of mere description. pass on, too, to note, in a further bay to the right, the extraordinary modifications of the antenna, or feeler, in insects. there is the long, whip-like form in the locust; the clubbed whip in the ant-lion and the butterfly; the feathered form in certain moths and flies; the hooked form characteristic of the sphinx-moths; the many-leaf form in the lamellicorn beetles, like the cockchafer; and the feathered plate of other beetles. equally wonderful are the diverse developments of the mouth-organs of insects, the spiral tube of the butterfly or moth, the strong jaws of the great beetles, the lancets of the gnat, the sucking-disc of the fly,--all of them special modifications of the same set of structures. then, in the same bay, note some of the striking differences between the males and females of certain insects. in some there is an extraordinary difference in size (e.g. the locust _xiphocera_, and the moth _attacus_); in others, like the stag-beetle, it is the size of the jaws that distinguishes the males; in others, again, the most notable differences are in the length, development, or complexity of the antennæ, or feelers; in some beetles the males have great horns on the head or thorax; while in many butterflies it is in richness of colour that the difference chiefly lies--the brilliant green of the _ornithoptera_ there exhibited contrasting strongly with the sober brown of his larger mate. the fact that the special characteristics of the male, which we have seen to be variable in the ruff, are also variable among insects, is well exemplified in the case of the stag-beetle, in some males of which the mandibles are far larger than in others. this is shown in fig. , which is copied from the series displayed in the british museum, by the kind permission of professor flower. [illustration: fig. .--variations in the size of, and especially in the head and mandibles of, the male stag-beetle (_lucanus cervus_). (from an exhibit in the british natural history museum.)] crossing the hall to where the vertebrate structures are displayed, the development of hair, of feathers, of teeth, the modifications of the skull and of legs, wings, and fins are being exemplified. note here and elsewhere the special adaptations of structure, of which we may select two examples. the first is that seen in the _balistes_, or trigger-fish. the anterior dorsal fin is reduced to three spines, of which that which lies in front is a specially modified weapon of defence, while that which follows it is the so-called trigger. these two are so hinged to the underlying interspinous bones and so related to each other that, when once the defensive spine in front is erected, it cannot be forced down until the trigger is lowered. the second example of special adaptation is well displayed in specimens of the mud-tortoise _trionyx_. between the last vertebra of the neck and the first fixed vertebra of the dorsal series is a beautiful hinge-joint, enabling the neck to be bent back, s-fashion, when the creature withdraws its head within the carapace. these are only one or two particular instances of what any one who will visit the national museum may see for himself admirably displayed and illustrated. no one can, one would suppose, pass through the galleries in cromwell road and remain quite insensible to the beauties of animal life. beauty of form and beauty of colour are conspicuously combined in many species of birds and insects. and much of this colour-beauty and splendid iridescence is known to be due to minute scales, to thin films of air or fluid, and to microscopically fine lines developed upon scales or feathers. but there is one phase of beauty which cannot be exhibited in the museum--the beauty that comes of life as opposed to death. for this we must go out into the free air of nature, where the animals not only have lived, but are still instinct with the glow of life, and where the silence of the museum galleries is replaced by the song of birds and the hum of insect-wings. how have this wealth, this diversity, this beauty, this manifold activity, which we summarize under the term "animal life," been produced? if we answer this question in a word--the word "evolution"[ci]--we must remember that this word merely expresses our belief in a general fact; and we must not forget that many questions remain behind, all centering round that little question, to which an adequate answer is so difficult to give, the question--how? reduced to its simplest expression, the doctrine of evolution merely states that the animal world as it exists to-day is naturally developed out of the animal world as it existed yesterday, and will in turn develop into the animal world as it shall exist to-morrow. this is the central belief of the evolutionist. no matter what moment in the past history of life you select, the life at that moment was in the act of insensibly passing from the previous towards a future condition. then at once arises the question--does life remain the same yesterday, to-day, and to-morrow? a thousand indubitable facts at once make answer--no! underlying the law of continuity there is a law of change. life to-day is not what it was yesterday, nor will it be to-morrow the same as to-day. what, then, is the nature of this change? if it be replied that the change must be either for the better or the worse, we shall have to answer the further question--better or worse in what respects? let us narrow our view from the contemplation of life as a whole to the more particular consideration of an organism as one of its constituent units. the individual life of that organism depends on (some would say consists in) its ceaseless adaptation to surrounding circumstances. the circumstances remaining the same, or only varying within constant limits, the adaptation may be _more_ or _less_ perfect. a change in the direction of more perfect adaptation will be a change for the better, a tendency to less perfect adaptation will be a change for the worse. but the relation of an organism to its circumstances or environment is itself subject to change. the environment itself may alter, or the organism may be brought into relation with a new environment. we have to consider not only the changes in an organism in the direction of more or less perfect adaptation to its environment, but also changes in the environment. these changes are in the direction of increased simplicity or of increased complexity. so that we may say that the modification of life is in the direction of more or of less complete adaptation to simpler or to more complex conditions. where the adaptation advances to more complex conditions, we speak of elaboration; where it retrogrades to less complex conditions, we speak of degeneration; but both fall under the head of evolution in its more general sense. viewed as a whole, there can be little doubt that the general tendency of evolution is towards more complete adaptation to more diverse and complex environment. and this tendency is accompanied by a general increase of differentiation and of integration; of differentiation whereby the constituent elements of life, whether cells, tissues, organs, organisms, or groups of organisms, become progressively more specialized and more different from one another; of integration whereby these elements become progressively more interdependent one on the other. we may conveniently sum up the tendency towards more perfect adaptation to more complex circumstances in the word _progress_; the tendency to differentiation in the word _individuality_; and the tendency to integration in the word _association_. nobody now doubts the propositions thus briefly summarized, and it is therefore unnecessary to bring forward evidence in their favour. we may pass, then, to the question--how? evolution being continuity, associated with change, tending in certain directions, and accompanied by certain processes, how has it been effected? what are its methods? _natural selection._ natural selection claims a foremost place. we have already devoted a chapter to its consideration. animals vary; more are born than can survive to procreate their kind; hence a struggle for existence, in which the weaker and less adapted are eliminated, the stronger and better adapted surviving to continue the race. it is scarcely possible to over-estimate what darwin's labour and genius have done for the study of animal life. through darwin's informing spirit, biology has become a science. but now we must be on our guard. so long as natural selection was winning its way to acceptance, every application of the theory had to be made with caution, and was subjected to keen, if sometimes ignorant, criticism. now there is, perhaps, some danger lest it should suffer the nemesis of triumphant creeds, and be used blindly as a magic formula. first, we should be careful not to use the phrase, "of advantage to the species," vaguely and indefinitely, but should in all cases endeavour clearly to indicate wherein lies the particular advantage, and how its possession enables the organism to escape elimination; next, we must remember that the advantage must be immediate and present, prospective advantage being, of course, inoperative; then we must endeavour to show that the advantage is really sufficient to decide the question of elimination or non-elimination; lastly, we must distinguish between indiscriminate and differential destruction, between mere numerical reduction by death or otherwise and selective elimination. ( ) in illustration of the first point, we may select a passage from the writings of even so great a biologist as professor weismann. as is well known, professor weismann believes that senility and death are no part of the natural heritage of animal life, but have been introduced among the metazoa on utilitarian grounds. in his earlier papers, he attributed the introduction of death, and the tissue-degeneration that precedes it, to the direct action of natural selection.[cj] more lately, he attributes it to the cessation of selection.[ck] concerning this later view, we shall have somewhat to say presently; we may now consider the former as an example of too indefinite a use of such phrases as "of advantage to the species." "worn-out individuals," says professor weismann, "are not only valueless to the species, but they are even harmful, for they take the places of those which are sound. hence, by the operation of natural selection, the life of our hypothetically immortal individual would be shortened by the amount which was useless to the species. it would be reduced to a length which would afford the most favourable conditions of existence of as large a number as possible of vigorous individuals at the same time." this may be so, but, as it stands, the _modus operandi_ is not given, and is not obvious. we start with a hypothetically immortal metazoon. barring accidents, it will go on existing indefinitely. but you cannot bar accidents for an indefinite time; hence, the longer the individual lives, the more defective and crippled it becomes. there is neither natural decay nor natural death here. the organism is gradually crippled through accident and injury. but the crippled individuals are harmful to the species, because they take the places of those which are sound. therefore, says professor weismann, natural decay and death step in to take them off before they have time to become cripples. now, the point i wish to notice is that there is no definite statement how or why natural decrepitude should thus be introduced. we must remember that it is not until a late stage in evolution that, through the association of its members, groups of organisms compete with other groups. in the earlier stages, when we must suppose decrepitude and death to arise on professor weismann's hypothesis, the law of the struggle for existence is--each for himself against all. the question, therefore, is--what advantage _to the individual_ is there in natural decay and death to enable it, through the possession of these attributes, to escape elimination? surely none as such. at the same time, it is quite conceivable that natural decay and death may be the penalty the individual has to pay for increased strength and vitality in the early stages of life. this, probably, was professor weismann's meaning. but, if so, it would surely have been better to state the matter in such a way as to lay the chief stress on the really important feature, and to say that, through natural selection, those individuals have survived which exhibited predominant strength and vitality for a shortened period, even at the expense of natural decay and death. the increased life-power, not the seeds of decay and death, was that which natural selection picked out for survival, or rather that which elimination allowed to survive. in such ways--a short life with heightened activity being of advantage to some forms, a more prolonged existence at a lower level of vitality being essential to others--natural selection may have determined in some degree the relative longevity of different organisms. that it caused the introduction of senility as a preparation for death is a less tenable hypothesis. and here we may note, in passing, that in using the phrase, "of advantage to the race or species," we must steadily bear in mind the fact that it is with _individuals_ that the process of elimination deals. in the individual it is that every modification must make good its claim to existence and transmission. where the principle of association for mutual benefit obtains, as in the case of social insects, it is still the individual that must resist elimination. self-sacrifice, whether conscious or unconscious, must not be carried so far as to lead to the elimination of the self-sacrificing individual, for in this event it cannot but defeat its own ends. within these limits, self-sacrifice is of advantage, as in the case of parental self-sacrifice, in that it enables certain other individuals to escape elimination. we should endeavour, then, not to use the phrase, "of advantage to the species," vaguely and indefinitely, but to indicate in what particular ways certain individuals are to be so advantaged as to escape the nemesis of elimination. ( ) the second point that i mentioned above scarcely needs exemplification. that the advantage which enables an organism to escape elimination must be present and existent, not merely prospective, is obvious. still, the mistake is sometimes made. i have heard it stated that feathers were evolved for the sake of flight. but clearly, unless the wing sprang into existence already sufficiently developed for flight, this would be impossible. the same is true of the first stages of many structures which could not be of service for the purpose and use to which they were subsequently turned. not impossibly, the earliest "wings" were for diving, and flight was, so to speak, an after-thought. undoubtedly, structures which have been fostered under the wing of one form of advantage have been subsequently applied to new purposes, and fostered through new modes of adaptation. teeth, for example, are probably modified scales, such as are found in the thorn-back skate. but the early development of these scales could have had no reference to their future application to purposes subservient to alimentation. again, such and such a structure is sometimes spoken of as a "prevision against emergencies." in his interesting and valuable work on "the colours of animals," for example, mr. e. b. poulton says, "dimorphism [in the larvæ of butterflies and moths] is also valuable in another way: the widening range of a species may carry it into countries in which one of its forms may be especially well concealed, while in other countries the other form may be more protected. thus a dimorphic form is more fully provided against emergencies than one with only a single form." and after giving, as an example, the fact that the convolvulus hawk-moth has a browner and a greener form of caterpillar, of which the browner is more prevalent under european conditions, and the greener under those which obtain in the canary islands, mr. poulton adds, "this result appears to have been brought about by the ordinary operation of natural selection, leading to the extermination of the less-protected variety." now, i do not mean for one moment to imply that so careful and able a naturalist as mr. poulton believes that any character has been evolved through natural selection in prevision for future emergencies. but i do think that his statement is open to this criticism. ( ) it is sometimes said, in bold metaphor, that natural selection is constantly on the watch to select any modification, however slight, which is of advantage to the species. and it is true that elimination is ceaselessly operative. but it is equally certain that the advantage must be of sufficient value to decide the question whether its possessor should be eliminated or should escape elimination. if it does not reach this value, natural selection, watch she never so carefully, can make no use of it. elimination need not, however, be to the death; exclusion from any share in continuing the species is sufficient. to breed or not to breed, that is the question. any advantage affecting this essential life-function will at once catch the eye of a vigilant natural selection. but it must be of sufficient magnitude for the machinery of natural selection to deal with. that machinery is the elimination of a certain proportion of the individuals which are born. which shall be eliminated, and which shall survive, depends entirely on the way in which the individuals themselves come out in life's competitive examination. the manner in which that examination is conducted is often rude and coarse, too rough-and-ready to weigh minute and infinitesimal advantages. what must be the value of a favourable or advantageous modification to decide the question of elimination, to make it an _available advantage_, must remain a matter of conjecture. it will vary with the nature and the pressure of the eliminative process. and perhaps it is scarcely too much to say that, at present, we have not observational grounds on which to base a reliable estimate in a single instance. we must not let our conviction of its truth and justice blind us to the fact that natural selection is a logical inference rather than a matter of direct observation. a hundred are born, and two survive; the ninety-eight are eliminated in the struggle for existence; we may therefore infer that the two escaped elimination in virtue of their possession of certain advantageous characters. there is no flaw in the logic that has thus convinced the world that natural selection is a factor in evolution. but by what percentage of elimination-marks the second of the two successful candidates beats the senior on the list of failures we do not know. we can only see that, on the hypothesis of natural selection, it must have been sufficiently appreciable to determine success or failure. ( ) and then, to come to our fourth point, we must remember that, apart from the differentiating process of elimination, there is much fortuitous destruction. a hundred are born, and but two survive. but of the ninety-eight which die, and fail to procreate, how many are eliminated, how many are fortuitously destroyed, we do not find it easy to say. and indiscriminate destruction gets rid of good, bad, and indifferent alike. it is a mistake to say that of the hundred born the two survivors are necessarily the very best of the lot. it is quite possible that indiscriminate destruction got rid of ninety of all sorts, and left only ten subject to the action of a true elimination. "in the majority of birds," says professor weismann, "the egg, as soon as it is laid, becomes exposed to the attacks of enemies; martens and weasels, cats and owls, buzzards and crows, are all on the look out for it. at a later period, the same enemies destroy numbers of the helpless young, and in winter many succumb in the struggle against cold and hunger, or to the numerous dangers which attend migration over land and sea--dangers which decimate the young birds." there is here, first, a certain amount of fortuitous destruction; secondly, some selection applied to the eggs; thirdly, a selection among the very young nestlings; and, fourthly, a selection among the young migratory birds. what may be the proportion of elimination to destruction at each stage it is difficult to say. among the eggs and fry of fishes fortuitous destruction probably very far outbalances the truly differentiating process. _panmixia and disuse._ we may now pass on to consider shortly some of the phenomena of degeneration, and the dwindling or disappearance of structures which are no longer of use. many zoologists believe, or until lately have believed, that disuse is itself a factor in the process. just as the well-exercised muscle is strengthened, so is the neglected muscle rendered weak and flabby. until recently it was generally held that the effects of such use or disuse are inherited. but now professor weismann has taught us, if not to doubt ourselves, at least to admit that doubt is permissible. on the older view, the gradual dwindling of unused parts was readily comprehensible. but now, if professor weismann is right, we must seek another explanation of the facts; and, in any case, we may be led to recognize other factors (than that of disuse alone) in the process. professor weismann regards panmixia, or free intercrossing, when the preserving influence of natural selection is suspended, as the efficient cause of a reduction or deterioration in the organ concerned. and mr. romanes had, in england, drawn attention to the fact that the "cessation of natural selection" would lead to some dwindling of the organ concerned, since it was no longer kept up to standard. in illustration of his panmixia, professor weismann says, "a goose or duck must possess strong powers of flight in the natural state, but such powers are no longer necessary for obtaining food when it is brought into the poultry-yard, so that a rigid selection of individuals with well-developed wings at once ceases among its descendants. hence, in the course of generations, a deterioration of the organs of flight must necessarily ensue, and the other members and organs of the bird will be sensibly affected."[cl] and, again, "as at each stage of retrogressive transformation individual fluctuations always occur, a continued decline from the original degree of development will inevitably, although very slowly, take place, until the last remnant finally disappears."[cm] now, i think it can be shown that panmixia, or the cessation of selection, alone cannot affect much reduction. it can only affect a reduction from the "survival-mean" to the "birth-mean." this was referred to in the chapter on "heredity and the origin of variations," but may be again indicated. suppose the number of births among wild ducks be represented by the number nine, of which six are eliminated through imperfections in the organs of flight. let us place the nine in order of merit in this respect, as is done in the table on p. . the average wing-power of the nine will be found in no. , there being four ducks with superior wing-power ( - ), and four with inferior wing-power ( - ). the birth-mean will therefore be at the level of no. , as indicated to the left of the table. but if six ducks with the poorest wings be eliminated, only three survive. the average wing-power will now be found in no. , one duck being superior and one inferior to it in this respect. it is clear that this survival-mean is at a level of higher excellence than the birth-mean. now, when the ducks are placed in a poultry-yard, selection in the matter of flight ceases, and, since all nine ducks survive, the survival-mean drops to the birth-mean. we may variously estimate this retrogression; but it cannot be a large percentage--i should suppose, in the case under consideration, one or two per cent. at most. but professor weismann says, "a _continued_ decline from the original degree of development must inevitably take place." it is not evident why such decline should continue. if variations continue in the same proportion as before, the birth-mean will be preserved, since there are as many positive or favourable variations above the mean as there are negative or unfavourable variations below the mean. a continuous decline must result from a preponderance of negative over positive variations, and for this some other principle, such as atavism, or reversion to ancestral characters, must be called in. but in the case of so long-established and stable an organ as that of flight, fixed and rendered constant through so many generations, it is hardly probable that reversion would be an important factor. mr. galton has calculated that among human-folk the offspring inherits one-fourth from each parent, one-sixteenth from each grandparent, leaving one-fourth to be contributed by more remote ancestors. there is no doubt, however, that among domesticated animals reversion occurs to characters which have been lost for many generations. but we should probably have to go a very long way back in the ancestry of wild ducks for any marked diminution in wing-power. it must be remembered that, in the case of the artificial selection of domesticated animals, man has been working against and not with the stream of ancestral tendency. reversion in their case is towards a standard which was long maintained and had become normal before man's interference. reversion in domesticated ducks should therefore be towards the greater wing-power of their normal ancestry before domestication, not in the direction of lessened wing-power and diminished wing-structure. the whole question of reversion is full of interest, and needs further investigation. in the dwindling of disused structures, mr. romanes has suggested "failure of heredity" as an efficient cause. i find it difficult, however, to distinguish this failure of heredity from the effects of disuse. to what other cause is the failure of heredity due? if natural selection has intervened to hasten this failure, this can only be because the failure is advantageous, since it permits the growth-force to be applied more advantageously elsewhere. and this involves a different principle. even so it is difficult to exclude the possibility (to put it no stronger) that the diversion of growth-force from a less useful to a more useful organ is in part due to the use of the one and the disuse of the other. but of disuse mr. romanes says, "there is the gravest possible doubt lying against the supposition that any really inherited decrease is due to the inherited effects of disuse." we may fairly ask mr. romanes, therefore, to explain to what cause the failure of heredity is due. in any case, professor weismann and his school are not likely to accept this failure of heredity as an efficient factor in the process. nor is professor weismann likely to fall back upon any innate tendency to degeneration. unless, therefore, some cause be shown why the negative variations should be prepotent over the positive variations, we must, i think, allow that unaided panmixia cannot affect any great amount of reduction. in this connection we may notice professor weismann's newer view of the introduction of bodily mortality. he says, "the problem is very easily solved if we seek assistance from the principle of panmixia. as soon as natural selection ceases to operate upon any character, structural or functional, it begins to disappear. as soon, therefore, as the immortality of somatic [body-] cells became useless, they would begin to lose this attribute."[cn] even granting that panmixia could continuously reduce the size of ducks' wings, it is not easy to see how it could get rid of immortality. the essence of the idea of panmixia is that, when the natural selection which has raised an organ to a high functional level, and sustains it there, ceases or is suspended, the organ drops back from its high level. but on professor weismann's hypothesis, immortality has neither been produced nor is it sustained by natural selection. how, therefore, the cessation of selection can cause the disappearance of immortality--a character with which natural selection has had nothing whatever to do--professor weismann does not explain. he seems to be using "panmixia" in the same vague way that, in his previous explanation, he used "natural selection." if panmixia alone cannot, to any very large extent, reduce an organ no longer sustained by natural selection, to what efficient cause are we to look? mr. romanes has drawn attention to the reversal of selection as distinguished from its mere cessation. when an organ is being improved or sustained by selection, elimination weeds out all those which have the organ in an ill-developed form. under a reversal of selection, elimination will weed out all those which possess the organ well developed. in burrowing animals, the eyes may have been reduced in size, or even buried beneath the skin, through a reversal of selection. the tuco-tuco (_ctenomys_), a burrowing rodent of south america, is frequently blind. one which darwin kept alive was in this condition, the immediate cause being inflammation of the nictitating membrane. "as frequent inflammation of the eyes," says darwin, "must be injurious to any animal, and as eyes are certainly not necessary to animals having subterranean habits, a reduction in their size, with the adhesion of the eyelids and growth of fur over them, might in such cases be an advantage; and, if so, natural selection would aid the effect of disuse."[co] granting that the inflammation of the eyes is a sufficient disadvantage to lead to elimination, such cases may be assigned to the effects of a reversal of selection. perhaps the best instances of the reversal of selection are to be found in the insects of wind-swept islands, in which, as we have already seen (p. ), the power of flight has been gradually reduced or even done away with. such instances are, however, exceptional. and one can hardly suppose that such reversal of selection can be very far-reaching in its effects, at least, through any direct disadvantage from the presence of the organ. one can hardly suppose that the presence of an eye in a cave-dwelling fish[cp] could be of such direct disadvantage as to lead to the elimination of those members which still possess this structure. but may it not be of indirect disadvantage? may not this structure be absorbing nutriment which would be more advantageously utilized elsewhere? this is darwin's principle of economy. granting its occurrence, is it effective? we may put the matter in this way: the crustacea which have been swept into a dark cave may be divided into three classes so far as fortuitous variations of eyes and antennæ are concerned. first, those which preserve eyes and antennæ in the original absolute and relative proportion and value; secondly, those in which, while the eyes remain the same, the antennæ are longer and more sensitive; thirdly, those in which, while the antennæ are longer and more sensitive, the eyes are reduced in size and elaboration. according to the principle of economy, the third class have sufficient advantage over the first and second to enable them to survive and escape the elimination which removes those with fully developed eyes. it may be so. we cannot estimate the available advantage with sufficient accuracy to deny it. but we may fairly suppose that, in general, it is only where the useless organ in question is of relatively large size, and where nutriment is deficient, that economy of growth is an important factor. we may here note the case of the hermit crab as one which exemplifies degeneration through the reversal of natural selection. this animal, as is well known, adopts an empty whelk-shell or other gasteropod shell as its own. the hinder part of the body which is thus thrust into the shell loses its protective armour, and is quite soft. professor weismann seems to regard this loss of the hardened cuticle as due entirely to panmixia. if what has been urged above has weight, this explanation cannot be correct. no amount of promiscuous interbreeding of crabs could reduce the cuticle to a level indefinitely below that of any of the interbreeding individuals. but it is clear that an armour-sheathed "tail" would be exceedingly ill adapted to thrusting into a whelk-shell. hence there would, by natural selection, be an adaptation to new needs, involving not the higher development of cuticle, but the reverse. so far as the cuticle is concerned, it is a case of reversed selection. whether this reversal alone will adequately account for the facts is another matter. mr. herbert spencer has made a number of observations and measurements of the jaws of pet dogs, which lead him to conclude that there has been a reduction in size and muscular power due to disuse. the creatures being fed on sops, have no need to use to any large extent the jaw-muscles. in this case, he argues, the principle of economy is not likely to be operative, since the pampered pet habitually overeats, and has therefore abundant nutriment and to spare to keep up the jaws. it is possible, however, that artificial selection has here been a factor. there may have been a competition among the old ladies who keep such pets to secure the dear little dog that never bites, while the nasty little wretch that does occasionally use his jaws for illegitimate purposes may have been speedily eliminated. pet dogs are, moreover, a pampered, degenerate, and for the most part unhealthy race, often deteriorated by continued in-breeding, so that we must not build too much on mr. spencer's observations, interesting as they undoubtedly are. there is one feature about the reduction of organs which must not be lost sight of. they are very apt to persist for a long time as remnants or vestiges. the pineal gland is the vestigial remnant of a structure connected with the primitive, median, or pineal eye. the whalebone whales and the duck-bill platypus have teeth which never cut the gum and are of no functional value. with regard to these, it may be asked--if disuse leads to the reduction of unused structures, how comes it that it has not altogether swept away these quite valueless structures? in considering this point, we must notice the unfortunate and misleading way in which disuse is spoken of as if it were a positive determinant, instead of the mere absence of free and full and healthy exercise. few will question the fact that in the individual, if an organ is to be kept up to its full standard of perfection, it must be healthily and moderately exercised; and that, if not so exercised, it will not only cease to increase in size, but will tend to degenerate. the healthy, functionally valuable tissue passes into the condition of degenerate, comparatively useless tissue. now, those who hold that the inheritance of functional modifications is still a tenable hypothesis, carry on into the history of the race that which they find to hold good in the history of the individual. they believe that, in the race, the continued functional activity of an organ is necessary for the maintenance of the integrity and perfection of its structure, and that, if not so exercised, the organ will inevitably tend to dwindle to embryonic proportions and to degenerate. the healthy, functionally valuable tissue passes at last into the condition of degenerate, comparatively useless tissue. the force of heredity will long lead to the production in the embryo of the structure which, in the ancestral days of healthy exercise, was to be of service to the organism. at this stage of life the conditions have not changed. the degeneration sets in at that period when the ancestral use is persistently denied. there is no reason why "disuse" should in all cases remove all remnants of a structure; but if the presence of the degenerate tissue is a source of danger to the organism which possesses it, that organism will be eliminated, and those ( ) which possess it in an inert, harmless form, or ( ) in which it is absent, will survive. thus natural selection (which will fall under mr. romanes's reversed selection) will step in--will in some cases reduce the organ to a harmless and degenerate rudiment, and in others remove the last vestiges of the organ. on the whole, even taking into consideration the effects of panmixia, of reversed selection, and of the principle of economy, the reduction of organs is difficult to explain, unless we call into play "disuse" as a co-operating factor. _sexual selection, or preferential mating._ it is well known that, in addition to and apart from the primary sexual differences in animals, there are certain secondary characters by which the males, or occasionally the females, are conspicuous. the antlers of stags, the tail of the peacock, the splendid plumes of the male bird of paradise, the horns or pouches of lizards, the brilliant frilled crest of the newt, the gay colours of male sticklebacks, the metallic hues of male butterflies, and the large horns or antennæ of other insects,--these and many other examples which will at once occur to the reader are illustrations of the fact. as a contribution towards the explanation of this order of phenomena, darwin brought forward his hypothesis of sexual selection, of which there are two modes. in the first place, the males struggle together for their mates; in this struggle the weakest are eliminated; those possessed of the most efficient weapons of offence and defence escape elimination. in the second place, the females are represented as exercising individual choice, and selecting (in the true sense of the word) those mates whose bright colours, clear voices, or general strength and vigour render them most pleasing and attractive. for this mode i shall employ the term "preferential mating." combining these two in his summary, darwin says, "it has been shown that the largest number of vigorous offspring will be reared from the pairing of the strongest and best-formed males, victorious in contests over other males, with the most vigorous and best-nourished females, which are the first to breed in the spring. if such females select the more attractive and, at the same time, vigorous males, they will rear a larger number of offspring than the retarded females, which must pair with the less vigorous and less attractive males. so it will be if the more vigorous males select the more attractive and, at the same time, healthy and vigorous females; and this will especially hold good if the male defends the female, and aids in providing food for the young. the advantage thus gained by the more vigorous pairs in rearing a larger number of offspring has apparently sufficed to render sexual selection efficient."[cq] with regard to the first of the two modes, little need be said. there can be no question that there are both elimination by battle and elimination by competition in the struggle for mates. it is well known that the emperor moth discovers his mate by his keen sense of smell residing probably in the large, branching antennæ. there can be little doubt that, if an individual is deficient in this sense, or misinterprets the direction in which the virgin female lies, he will be unsuccessful in the competition for mates; he will be eliminated from procreation. and it is a familiar observation of the poultry-yard that the law of battle soon determines which among the cock birds shall procreate their kind. the law of battle for mates is, indeed, an established fact among many animals, especially those which are polygamous, and the elimination of the unfit in this respect is a logical necessity. it is when we come to the second of the two modes, that which involves selection proper, that we find differences of opinion among naturalists. darwin, as we have seen, suggested that those secondary sexual characters which can be of no value in aiding their possessor to escape elimination by combat result from the preferential choice of the female, the female herself remaining comparatively unaffected. but mr. wallace made an exceedingly valuable suggestion with regard to these comparatively dull colours of the female. he pointed out that conspicuousness (unless, as we have seen, accompanied by some protective character, such as a sting or a bitter taste) increased the risk of elimination by enemies. now, the males, since they are generally the stronger, more active, and more pugnacious, could better afford to run this risk than their mates. they could to some extent take care of themselves. moreover, when impregnation was once effected, the male's business in procreation was over. not so the female; she had to bear the young or to lay the eggs, often to foster or nourish her offspring. not only were her risks greater, but they extended over a far longer period of time. hence, according to mr. wallace, the dull tints of the females, as compared with those of the males, are due to natural selection eliminating the conspicuous females in far greater proportion than the gaudy males. there is clearly no reason why this view should not be combined with darwin's; preferential mating being one factor, natural elimination being another factor; both being operative at the same time, and each contributing to that marked differentiation of male and female which we find to prevail in certain classes of the animal kingdom. but mr. wallace will not accept this compromise. he rejects preferential mating altogether, or, in any case, denies that through its agency secondary sexual characters have been developed. he admits, of course, the striking and beautiful nature of some of these characters; he admits that the male in courtship takes elaborate pains to display all his finery before his would-be mate; he admits that the "female birds may be charmed or excited by the fine display of plumage by the males;" but he concludes that "there is no proof whatever that slight differences in that display have any effect in determining their choice of a partner."[cr] how, then, does mr. wallace himself suppose that these secondary sexual characters have arisen? his answer is that "ornament is the natural outcome and direct product of superabundant health and vigour," and is "due to the general laws of growth and development."[cs] at which one rubs one's eyes and looks to the title-page to see that mr. wallace's name is really there, and not that of professor mivart or the duke of argyll. for, if the plumage of the argus pheasant and the bird of paradise is due to the general laws of growth and development, why not the whole animal? if darwin's sexual selection is to be thus superseded, why not messrs. darwin and wallace's natural selection? must we not confess that mr. wallace, for whose genius i have the profoundest admiration, has here allowed himself to confound together the question of origin and the question of guidance or direction? natural selection by elimination and sexual selection through preferential mating are, supposing them to be _veræ causæ_, guiding or selecting agencies. given the variations, however caused, these agencies will deal with them, eliminating some, selecting others, with the ultimate result that those specially fitted for their place in nature will survive. neither the one nor the other deals with the origin of variations. that is a wholly different matter, and constitutes the leading biological problem of our day. mr. wallace's suggestion is one which concerns the origin of variations, and as such is worthy of careful consideration. it does not touch the question of their guidance into certain channels or the maintenance of specific standards. concerning this mr. wallace is silent or confesses ignorance. "why, in allied species," he says, "the development of accessory plumes has taken different forms, we are unable to say, except that it may be due to that individual variability which has served as the starting-point for so much of what seems to us strange in form or fantastic in colour, both in the animal and vegetable world."[ct] it is clear, however, that "individual variability" cannot be regarded as a _vera causa_ of the maintenance of a specific standard--a standard maintained _in spite of_ variability. the only directive agency (apart from that of natural selection) to which mr. wallace can point is that suggested by mr. alfred tylor, in an interesting, if somewhat fanciful, posthumous work on "coloration in animals and plants," "namely, that diversified coloration follows the chief lines of structure, and changes at points, such as the joints, where function changes." but even if we admit that coloration-bands or spots originate at such points or along such lines--and the physiological rationale is not altogether obvious--even if we admit that in butterflies the spots and bands usually have reference to the form of the wing and the arrangement of the nervures, and that in highly coloured birds the crown of the head, the throat, the ear-coverts, and the eyes have usually distinct tints, still it can hardly be maintained that this affords us any adequate explanation of the _specific_ colour-tints of the humming-birds, or the pheasants, or the papilionidæ among butterflies. if, as mr. wallace argues, the immense tufts of golden plumage in the bird of paradise owe their origin to the fact that they are attached just above the point where the arteries and nerves for the supply of the pectoral muscles leave the interior of the body, are there no other birds in which similar arteries and nerves are found in a similar position? why have these no similar tufts? and why, in the birds of paradise themselves, does it require four years (for it takes so long for the feathers of the male to come to maturity) ere these nervous and arterial influences take effect upon the plumage? finally, one would inquire how the colour is determined and held constant in each species. the difficulty of the tylor-wallace view, even as a matter of origin, is especially great in those numerous cases in which the colour is determined by delicate lines, thin plates, or thin films of air or fluid.[cu] under natural selection, as we have seen, the development of colour is fostered under certain conditions. the colour is either protective, rendering the organism inconspicuous amid its normal surroundings, or it is of warning value, advertising the organism as inedible or dangerous, or, in the form of recognition-marks, it is of service in enabling the members of a species to recognize each other. now, in the case of both warning colour and recognition-marks, their efficacy depends upon the perceptual powers of animals. unless there be a rapidly acquired and close association of the quality we call nastiness with the quality we call gaudiness (though, for the animal, there is no such _isolation_ of these qualities as is implied in our words [cv]), such that the sight of the gaudy insect suggests that it will be unpleasant to eat, the gaudiness will be of no avail. and if there is any truth in the doctrine of mimicry, the association is particular. it is not merely that bright colours are suggestive of a nasty taste. the insect-eating birds associate nastiness especially with certain markings and coloration--"the tawny _danais_, the barred _heliconias_, the blue-black _euplæas_, and the fibrous _acræas_;" and this is proved by the fact that sweet insects mimicking these particular forms are thereby protected. so, too, with recognition-marks. if the bird or the mammal have not sufficient perceptive powers to distinguish between the often not very different recognition-marks, of what service can they be? recognition-marks and mimicry seem, therefore, to show that in the former case many animals, and in the latter the insect-eating birds, mammals, lizards, and other animals concerned, have considerable powers of perception and association. among other associations are those which are at the base of what i have termed preferential mating. we must remember how deeply ingrained in the animal nature is the mating instinct. _we_ may find it difficult to distinguish closely allied species. but the individuals of that species are led to mate together by an impelling instinct that is so well known as to elicit no surprise. instinct though it be, however, the mating individuals must recognize each other in some way. the impulse that draws them together must act through perceptual agency. it is not surprising, therefore, to find, when we come to the higher animals, that, built upon this basis, there are well-marked mating preferences. and this, as we have before pointed out, following wallace, is an efficient factor in segregation. let us, however, hear mr. wallace himself in the matter. there is, he says,[cw] "a very powerful cause of isolation in the mental nature--the likes and dislikes--of animals; and to this is probably due the fact of the rarity of hybrids in a state of nature. the differently coloured herds of cattle in the falkland islands, each of which keeps separate, have been already mentioned. similar facts occur, however, among our domestic animals, and are well known to breeders. professor low, one of the greatest authorities on our domesticated animals, says, 'the female of the dog, when not under restraint, makes selection of her mate, the mastiff selecting the mastiff, the terrier the terrier, and so on.' and again, 'the merino sheep and the heath sheep of scotland, if two flocks are mixed together, each will breed with its own variety.' mr. darwin has collected many facts illustrating this point.[cx] one of the chief pigeon-fanciers in england informed him that, if free to choose, each breed would prefer pairing with its own kind. among the wild horses in paraguay those of the same colour and size associate together; while in circassia there are three races of horses which have received special names, and which, when living a free life, almost always refuse to mingle and cross, and will even attack one another. in one of the faröe islands, not more than half a mile in diameter, the half-wild native black sheep do not readily mix with imported white sheep. in the forest of dean and in the new forest the dark and pale coloured herds of fallow deer have never been known to mingle; and even the curious ancon sheep, of quite modern origin, have been observed to keep together, separating themselves from the rest of the flock when put into enclosures with other sheep. the same rule applies to birds, for darwin was informed by the rev. w. d. fox that his flocks of white and chinese geese kept distinct. this constant preference of animals for their like, even in the case of slightly different varieties of the same species, is evidently a fact of great importance in considering the origin of species by natural selection, since it shows us that, so soon as a slight differentiation of form or colour has been effected, isolation will at once arise by the selective association of the animals themselves." mr. wallace thus allows, nay, he lays no little stress on, preferential mating, and his name is associated with the hypothesis of recognition-marks. but he denies that preferential mating, acting on recognition-marks, has had any effect in furthering a differentiation of form or colour. he admits that so soon as a slight differentiation of form or colour has been effected, segregation will arise by the selective association of the animals themselves; but he does not admit that such selective association can carry the differentiation further. now, it is clear that mating preferences must be either fixed or variable. if fixed, how can differentiation occur in the same flock or herd? and how can selective association be a means of isolation? or, granting that differentiation has occurred, if the mating preferences are then stereotyped, all further differentiation, so far as colour and form are concerned, will be rendered impossible; for divergent modifications, not meeting the stereotyped standard of taste, will for that reason fail to be perpetuated. we must admit, then, that these mating preferences are subject to variation. and now we come to the central question with regard to sexual selection by means of preferential mating. what guides the variation along special lines leading to heightened beauty? this, i take it, is the heart and centre of mr. wallace's criticism of darwin's hypothesis. sexual selection of preferential mating involves a standard of taste; that standard has advanced from what we consider a lower to what we consider a higher æsthetic level, not along one line, but along many lines. what has guided it along these lines? not as in any sense affording a direct answer to this question, but for illustrative purposes, we may here draw attention to what seems to be a somewhat parallel case, namely, the development of flowers through insect agency. in his "origin of species," darwin contended that flowers had been rendered conspicuous and beautiful in order to attract insects, adding, "hence we may conclude that, if insects had not been developed on the earth, our plants would not have been decked with beautiful flowers, but would have produced only such poor flowers as we see on our fir, oak, nut, and ash trees, on grasses, docks, and nettles, which are all fertilized through the agency of the wind." "the argument in favour of this view," says mr. wallace,[cy] who quotes this passage, "is now much stronger than when mr. darwin wrote;" and he cites with approval the following passage from mr. grant allen's "colour-sense:" "while man has only tilled a few level plains, a few great river-valleys, a few peninsular mountain slopes, leaving the vast mass of earth untouched by his hand, the insect has spread himself over every land in a thousand shapes, and has made the whole flowering creation subservient to his daily wants. his buttercup, his dandelion, and his meadowsweet grow thick in every english field. his thyme clothes the hillside; his heather purples the bleak grey moorland. high up among the alpine heights his gentian spreads its lakes of blue; amid the snows of the himalayas his rhododendrons gleam with crimson light. even the wayside pond yields him the white crowfoot and the arrowhead, while the broad expanses of brazilian streams are beautified by his gorgeous water-lilies. the insect has thus turned the whole surface of the earth into a boundless flower-garden, which supplies him from year to year with pollen or honey, and itself in turn gains perpetuation by the baits that it offers to his allurement."[cz] mr. grant allen is perfectly correct in stating that the insect has produced all this beauty. it is the result of insect choice, a genuine case of selection as contrasted with elimination. and when we ask in this case, as we asked in the case of the beautiful colours and forms of animals, what has guided their evolution along lines which lead to such rare beauty, we are given by mr. wallace himself the answer, "the preferential choice of insects." if these insects have been able to produce through preferential selection all this wealth of floral beauty (not, indeed, for the sake of the beauty, but incidentally in the practical business of their life), there would seem to be no _a priori_ reason why the same class and birds and mammals should not have been able to produce, through preferential selection, all the wealth of animal beauty. it should be noted that the answer to the question is in each case a manifestly incomplete one. for if we say that these forms of beauty, floral and animal, have been selected through animal preferences, there still remains behind the question--how and why have the preferences taken these _æsthetic_ lines? to which i do not see my way to a satisfactory answer, though some suggestions in the matter will be made in a future chapter.[da] at present all we can say is this--to be conspicuous was advantageous, since it furthered the mating of flowers and animals. to be diversely conspicuous was also advantageous. as mr. wallace says, "it is probably to assist the insects in keeping to one flower at a time, which is of vital importance to the perpetuation of the species, that the flowers which bloom intermingled at the same season are usually very distinct, both in form and colour."[db] but conspicuousness is not beauty. and the question still remains--from what source comes this tendency to beauty? leaving this question on one side, we may state the argument in favour of sexual selection in the following form: the generally admitted doctrine of mimicry involves the belief that birds and other insect-eating animals have delicate and particular perceptual powers. the generally received doctrine of the origin of flowers involves the belief that their diverse forms and markings result from the selective choice of insects. there are a number of colour and form peculiarities in animals that cannot be explained by natural selection through elimination. there is some evidence in favour of preferential mating or selective association. it is, therefore, permissible to hold, as a provisional hypothesis, that just as the diverse forms of flowers result from the preferential choice of insects, so do the diverse secondary sexual characters of animals result, in part at least, from the preferential choice of animals through selective mating. if this be admitted, then the elaborate display of their finery by male birds, which mr. wallace does admit, may fairly be held to have a value which he does not admit. for if preferential mating is _à priori_ probable, such display may be regarded as the outcome of this mode of selection. at the same time, it may be freely admitted that more observations are required. in a recent paper, "on sexual selection in spiders of the family _attidæ_,"[dc] by george w. and elizabeth g. peckham, a full, not to say elaborate, description is given of the courtship, as they regard it, of spiders. the "love-dances" and the display of special adornments are described in detail. and the observers, as the result, be it remembered, of long and patient investigation and systematic study, come to the conclusion that female spiders exercise selective choice in their mates. and courtship must be a serious matter for spiders, for if they fail to please, they run a very serious risk of being eaten by the object of their attentions. some years ago i watched, on the cape flats, near capetown, the courtship of a large spider (i do not know the species). in this case the antics were strange, and, to me, amusing; but they seemed to have no effect on the female spider, who merely watched him. once or twice she darted forward towards him, but he, not liking, perhaps, the gleam in her eyes, retreated hastily. eventually she seemed to chase him off the field. we must remember how difficult it is to obtain really satisfactory evidence of mating preferences in animals. in most cases we must watch the animals undisturbed, and very rarely can we have an opportunity of determining whether one particular female selects her mate out of her various suitors. we watch the courtship in this, that, or the other case. in some we see that it is successful; in others that it is unsuccessful. how can we be sure that in the one case it was through fully attaining, in the other through failing to reach, the standard of taste? and yet it is evidence of this sort that mr. wallace demands. after noting the rejection by the hen of male birds which had lost their ornamental plumage, he says, "such cases do not support the idea that males with the tail-feathers a trifle longer, or the colours a trifle brighter, are generally preferred, and that those which are only a little inferior are as generally rejected,--and this is what is absolutely needed to establish the theory of the development of these plumes by means of the choice of the female."[dd] if mr. wallace requires direct observational evidence of this kind, i do not suppose he is likely to get any large body of it. but one might fairly ask him what body of direct observational evidence he has of natural selection. the fact is that direct observational evidence is, from the nature of the processes involved, almost impossible to produce in either case. natural selection is an explanation of organic phenomena reached by a process of logical inference and justified by its results. it is not claimed for the hypothesis of selective mating that it has a higher order of validity. _use and disuse._ as we have already seen, biologists are divided into two schools, one of which maintains that the effects of use and disuse[de] have been a potent factor in organic evolution; the other, that the effects of use and disuse are restricted to the individual. my own opinion is that we have not a sufficient body of carefully sifted evidence to enable us to dogmatize on the subject, one way or the other. but, the position of strict equilibrium being an exceedingly difficult and some would have us believe an undesirable attitude of mind, i may add that i lean to the view that use and disuse, if persistent and long-continued, take effect, not only on the individual, but also on the species. it is scarcely necessary to give examples of the kind of change which, according to the lamarckian school, are wrought by use and disuse. any organ persistently used will have a tendency, on this view, to become in successive generations more and more adapted to its functional work. to give but one example. it is well known that certain hoofed creatures are divisible into two groups--first, those which, like the horse, have in each limb one large and strong digit armed with a solid hoof; and, secondly, those which, like the ox, have in each limb two large digits, so that the hoof is cloven or split. it is also well known that the ancestral forms from which both horse-group and ox-group are derived were possessed of five digits to each limb. professor cope regards the differentiation of these two groups as the result of the different modes of use necessitated by different modes of life. "the mechanical effect," he says, "of walking in the mud is to spread the toes equally on opposite sides of the middle line. this would encourage the equal development of the digits on each side of the middle line, as in the cloven-footed types. in progression on hard ground the longest toe (the third) will receive the greatest amount of shock from contact with the earth."[df] hence the solid-hoofed types. here, then, the middle digit in the horse-group, or two digits in the ox-group, having the main burden to bear, increase through persistent use, while the other digits dwindle through disuse.[dg] on the other hand, one who holds the opposite view will say--i do not believe that use and disuse have had anything whatever to do with the matter. fortuitous variations in these digits have taken place. the conditions have determined which variations should be preserved. in the horse, variations in the direction of increase of functional value of the mid digit, and variations in the simultaneous decrease of the functional value of the lateral digits, have been of advantage, and have therefore survived the eliminating process of natural selection. now, since it is quite clear, in this and numberless similar cases, that we can explain the facts either way, it is obviously not worth while to spend much time or ingenuity in devising such explanations. they are not likely to convince any one worth convincing. what we need is ( ) crucial cases which can only be explained one way or the other; or ( ) direct observation or experiment leading to the establishment of one hypothesis or the other (or both). . crucial cases are very difficult to find. we cannot exclude the element of use or disuse, for on both hypotheses it is essential. the difference is that one school says the organ is developed in the species _by_ use; the other school says it is developed _for_ use. what we must seek is, therefore, the necessary exclusion of natural selection; and that is not easy to prove, in any case, to a darwinian. if it can be shown that there exist structures which are of use, but not of vital importance (that is to say, which have not what i called above the _available advantage_ necessary to determine the question of elimination or not-elimination), then we are perhaps able to exclude the influence of natural selection. i think, if anywhere, such cases are to be found in faculties and instincts;[dh] and as such they must be considered in a later chapter. i will, however, here cite one case in illustration of my meaning. we have seen that certain insects are possessed of warning colours, which advertise their nastiness to the taste. birds avoid these bright but unpleasant insects, and though there is some individual learning, there seems to be an instinctive avoidance of these unsavoury morsels. there is hesitation before tasting; and one or two trials are sufficient to establish the association of gaudiness and nastiness. moreover, mr. poulton and others have shown that, under the stress of keen hunger, these gaudy insects may be eaten, and apparently leave no ill effects. birds certainly instinctively avoid bees and wasps; and yet the sting of these insects can seldom be fatal. it is, therefore, improbable that nastiness or even the power of stinging can have been an eliminating agency. in the development of the instinctive avoidance, natural selection through elimination seems to be excluded, and the inheritance of individual experience is thus rendered probable. as before pointed out, it is not enough to say that a nasty taste or a sting in the gullet is disadvantageous; it must be shown that the disadvantage has an eliminating value. from my experiments (feeding frogs on nasty caterpillars, and causing bees to sting chickens), i doubt the eliminating value in this case. hence elimination by natural selection seems, i repeat, to be excluded, and the inheritance of individual experience rendered probable. mr. herbert spencer has contended that, in certain modifications, natural selection is excluded on the grounds of the extreme complexity of the changes, and adduces the case of the irish "elk" with its huge antlers, and the giraffe with its specially modified structure. he points out that in either case the conspicuous modification--the gigantic antlers or the long neck--involves a multitude of changes affecting many and sometimes distant parts of the body. not only have the enormous antlers involved changes in the skull, the bones of the neck, the muscles, blood-vessels, and nerves of this region, but changes also in the fore limbs; while the long neck of the giraffe has brought with it a complete change of gait, the co-ordinated movements of the hind limbs sharing in the general modification. mr. spencer, therefore, argues that it is difficult to believe that these multitudinous co-ordinated modifications are the result of fortuitous variations seized upon by natural selection. for natural selection would have to wait for the fortunate coincidence of a great number of distinct parts, all happening to vary just in the particular way required. that natural selection should seize upon the favourable modification of a particular part is comprehensible enough; that two organs should coincidently vary in favourable directions we can understand; that half a dozen parts should, in a few individuals among the thousands born, by a happy coincidence, vary each independently in the right way is conceivable; but that the whole organization should be remodelled by fortunately coincident and fortuitously favourable variations is not readily comprehensible. it may be answered--notwithstanding all this, we know that such happy coincidences have occurred, for there is the resulting giraffe. the question, however, is not whether these modifications have occurred or not, but whether they are due to fortuitous variation alone, or have been guided by functional use. the argument seems to me to have weight.[di] still, we should remember that among neuter ants--for example, in the sauba ant of south america (_oecodoma cephalotes_)--there are certain so-called soldiers with relatively enormous heads and mandibles. the possession of these parts so inordinately developed must necessitate many correlated changes. but these cannot be due to inherited use, since such soldiers are sterile. furthermore, according to professor weismann, natural selection is really working, not on the organism at large, but on the germ-plasm which produces it; and it is conceivable that the variation of one or more of the few cells in early embryonic life may introduce a great number of variations in the numerous derivative cells. in explanation of my meaning, i will quote a paragraph from a paper of mr. e. b. poulton's on "theories of heredity."[dj] "it appears," he says, "that, in some animals, the great groups of cells are determined by the first division [of the ovum in the process of cleavage[dk]]; in others, the right and left sides, or front and hind ends of the body; while the cells giving rise to the chief groups on each side would then be separated at some later division. this is not theory, but fact; for roux has recently shown that, if one of the products of the first division of the egg of a frog be destroyed with a hot needle, development is not necessarily arrested, but, when it proceeds, leads to the formation of an embryo from which either the right or the left side is absent. when the first division takes place in another direction, either the hind or the front half was absent from the embryo which was afterwards produced. after the next division, when four cells were present, destruction of one produced an embryo in which one-fourth was absent." now, it is conceivable that a single modification or variation of the primitive germ might give rise to many correlated modifications or variations of the numerous cells into which it develops; just as an apparently trivial incident in childhood or youth may modify the whole course of a man's subsequent life. it is difficult, indeed, to see how this could be effected; to understand what could be the nature of a modification of the germ which could lead simultaneously to many favourable variations of bones, muscles, blood-vessels, and nerves in different parts of the body. this, however, is a question of the origin of variations; and it is, at any rate, conceivable that, just as by the extirpation with a hot needle of one cell of the cleaved frog's ovum all the anterior part of the body should be absent in development, so by the appropriate modification of this one cell, or the germinal matter which produced it, all the anterior part of the body should be appropriately modified. these considerations, perhaps, somewhat weaken the force of mr. spencer's argument, which is not quite so strong now as it was when the "principles of biology" was published. ( ) we may pass now to the evidence afforded by direct observation and experiment. there is little enough of it. the best results are, perhaps, those which have been incidentally reached in the poultry-yard and on the farm in the breeding of domesticated animals. we have seen that, under these circumstances, certain parts or organs have very markedly diminished in size and efficiency; others have as markedly increased. of the former, or decrease in size and efficiency, the imbecile ducks with greatly diminished brains have been already mentioned. mr. herbert spencer draws attention[dl] to the diminished efficiency in ear-muscles, giving rise to the drooping ears of many domesticated animals. "cats in china, horses in parts of russia, sheep in italy and elsewhere, the guinea-pig formerly in germany, goats and cattle in india, rabbits, pigs, and dogs in all long-civilized countries, have dependent ears."[dm] since many of these animals are habitually well fed, the principle of economy of growth seems excluded. indeed, the ears are often unusually large; it is only their motor muscles that have dwindled either relatively or absolutely. if what has been urged above be valid, panmixia cannot have been operative; since panmixia _per se_ only brings about regression to mediocrity. if the effects in these two cases, ducks' brains and dogs' ears, be not due to disuse, we know not at present to what they are due. in the correlative case of increase by use, we find it exceedingly difficult to exclude the disturbing effects of artificial selection. the large and distended udders of cows, the enhanced egg-laying powers of hens, the fleetness or strength of different breeds of horses,--all of these have been subjects of long-continued, assiduous, and careful selection. one cannot be sure whether use has co-operated or not. sufficient has now, i think, been said to show the difficulty of deciding this question, the need of further observation and discussion, and the necessity for a receptive rather than a dogmatic attitude; and sufficient, also, to indicate my reasons for leaning to the view that use and disuse, long-continued and persistent, may be a factor in organic evolution. _the nature of variations._ the diversity of the variations which are possible, and which actually occur in animal life, is so great that it is not easy to sum up in a short space the nature of variations. without attempting anything like an exhaustive classification, we may divide variations into three classes. . _superficial variations_ in colour, form, etc., not necessarily in any way correlated with . _organic variations_ in the size, complexity, and efficiency of the organs of the body; . _reproductive and developmental variations._ any of these variations, if sufficient in amount and value to determine the question of elimination or not-elimination, selection or not-selection, may be seized upon by natural selection. our domesticated animals exemplify very fully the superficial variations which, through man's selection, have in many cases been segregated and to some extent stereotyped. it is unnecessary to do more than allude to the variations in form and coloration of dogs, cattle, fowls, and pigeons. these variations are not _necessarily_ in any way correlated with any deeper organic variations. they are, however, in many cases so correlated. for example, the form of the pouter pigeon is correlated with the increased size of the crop, the length of the beak carries with it a modification of the tongue, the widely expanded tail of the fantail carries with it an increase in the size and number of the caudal vertebræ. and here we might take the whole series of secondary sexual characters. these and their like may be said to be direct correlations. but there are also correlations which are seemingly indirect, their connection being apparently remote. that in pigeons the size of the feet should vary with the size of the beak; that the length of the wing and tail feathers should be correlated; that the nakedness of the young should vary with the future colour of the plumage; that white dogs should be subject to distemper, and white fowls to the "gapes;" that white cats with blue eyes should be nearly always deaf;--in these cases the correlation is indirect. but from the existence of correlation, whether direct or indirect, it follows that variations seldom come singly. the organism is so completely a unity that the variation of one part, even in superficial matters, affects directly or indirectly other parts. in the freedom of nature such superficial variations are not so obvious. but among the invertebrates they are not inconsiderable. the case of land-snails, already quoted, may again be cited. taking variations in banding alone, mr. cockerell knows of varieties of _helix nemoralis_ and of _h. hortensis_. still, among the wild relatives of our domestic breeds of animals and birds the superficial variations are decidedly less marked. and this is partly due to the fact that they are in a state of far more stable equilibrium than our domestic products, and partly to the constant elimination of all variants which are thereby placed at a serious or vital disadvantage. white rats, mice, or small birds, in temperate regions, would soon be seized upon by hawks and other enemies. if the eggs and young of the kentish plover, shown in our frontispiece, were white or yellowish, like the eggs and young of our fowls, they would soon be snapped up. the varied protective resemblances, general and special, have been brought about by the superficial variations of organisms, and the elimination of those which, from non-variation or wrong variation, remained conspicuous. we need only further notice one thing here, namely, that, in the case of special resemblance to an inorganic object or to another organism, the variations of the several parts must be very closely, and sometimes completely, correlated. the correlations, however, need not, perhaps, have been simultaneous--the resemblance having been gradually perfected by the filling in of additional touches, first one here, then another there, and so on. concerning "organic variations," little need be said. it is clear that an organ or limb may vary in size, such variation carrying with it a correlative variation in power; or it may vary in complexity--the teeth of the horse tribe, for example, having increased in complexity, while their limbs have been rendered less complex; or it may vary in efficiency through the more perfect correlation and co-ordination of its parts. the evidence of such variations from actual observation is far less in amount than that of superficial variations. and this is not to be wondered at, since in many cases it can only be obtained by careful anatomical investigation. nevertheless, anatomists, both human and comparative, are agreed that such variations do occur. and no one can examine such a collection as that of the royal college of surgeons without acknowledging the fact. thirdly, "reproductive and developmental variations" are of very great importance. the following are among the more important modifications which may occur in the animal kingdom. . variations in the mode of reproduction, sexual or asexual. . variations in the mode of fertilization. . variations in the number of fertilized ova produced. . variations in the amount of food-yolk and in the way in which it is supplied. . variations in the time occupied in development. . variations in the time at which reproduction commences. . variations in the duration and amount of parental protection and fosterage. . variations in the period at which secondary sexual characters and the maximum efficiency of the several organs is reached. it is impossible here to discuss these modes of variation _seriatim_. i shall therefore content myself with but a few remarks on the importance of protection and fosterage. it is not too much to say that, without fosterage and protection, the higher forms of evolution would be impossible. if you are to have a highly evolved form, you must allow time for its evolution from the egg; and that development may go on without let or hindrance, you must supply the organism with food and lighten the labour of self-defence. most of the higher organisms are slow in coming to maturity, passing through stages when they are helpless and, if left to themselves, would inevitably fall a prey to enemies. in those animals in which the system of fosterage and protection has not been developed a great number of fertilized ova are produced, only a few of which come to maturity. it might be suggested that this is surely an advantage, since the greater the number produced the greater the chances of favourable variations taking place. but it has before been pointed out that these great numbers are decimated, and more than decimated, not by elimination, but by indiscriminate destruction; embryos, good, bad, and indifferent, being alike gobbled up by those who had learnt the secret of fostering their young. the alternative has been between producing great numbers[dn] of embryos which soon fend for themselves, and a few young who are adequately provided for during development. and the latter have proved the winners in life's race. if we compare two flat-fishes belonging to very different groups, the contrast here indicated will be readily seen. the skate is a member of the shark tribe, flattened symmetrically from above downwards. it lays, perhaps, eighty to a hundred eggs. each of these is large, and has a rich supply of nutritive food-yolk. each is also protected by a horny case with pointed corners--the so-called sea-purse of seaside visitors. these are committed by the skate to the deep, and are not further cared for. but the abundant supply of food-yolk gives the little skate which emerges a good start in life. on the other hand, the turbot, one of the bony fishes, flattened from side to side with an asymmetrical head, lays several millions of eggs, which float freely in the open sea. these are minute and glassy, and not more than one-thirtieth of an inch in diameter. when the fishes are hatched, they are not more than about one-fifth of an inch in length. the slender stock of food-yolk is soon used up, and henceforth the little turbot (at present more like a stump-nosed eel than a turbot) has to get its own living. hundreds of thousands of them are eaten by other fishes. or, if we compare such different vertebrates as a frog, a sparrow, and a mouse, we find that the frog produces a considerable number of fertilized ova, though few in comparison with the turbot, each provided with a small store of food-yolk. the tiny tadpoles very soon have to obtain their own food and run all the risks of destruction. few survive. the sparrow lays a few eggs; but each is supplied with a large store of food-yolk, sufficient to meet its developmental needs until, under the fostering influence of maternal warmth, it is hatched. even on emerging from the eggs, the callow fledglings enjoy for a while parental protection and fosterage, and, when sent forth into the world, are very fairly equipped for life's struggle. the mouse produces minute eggs with little or no food-yolk; but they undergo development within the womb of the mother, and are supplied with nutrient fluids elaborated within the maternal organism. even when born, they are cherished for a while and supplied with food-milk by the mother. the higher stages of this process involve a mental element, and are developed under the auspices of intelligence or instinct. but the lower stages, the supply of food-yolk and intra-uterine protection, are purely organic. a hen cannot by instinctive or intelligent forethought increase the amount of food-yolk stored up in the ovum, any more than the lily, which, by an analogous process, stores up in its bulb during one year material for the best part of next year's growth, can increase this store by a mental process. it cannot therefore be questioned that variations in the amount of capital with which an embryo is provided in generation would very materially affect its chances of escaping elimination by physical circumstances, by enemies, and by competition. nor can it be questioned that variations in the time occupied in reaching maturity would, other things equal, not a little affect the chances of success of an organism in the competition of life. hence we have the phenomena of what may be termed acceleration and retardation in development. these terms have, however, been used by american zoologists, notably professors hyatt and cope, in a somewhat different and wider sense; for they include not merely time-changes, but also the loss of old characters or the acquisition of new characters. "it is evident," says professor cope, "that the animal which adds something to its structure which its parents did not possess has grown more than they; while that which does not attain to all the characteristics of its ancestors has grown less than they." "if the embryonic form be the parent, the advanced descendant is produced by an increased rate of growth, which phenomenon is called 'acceleration'; but if the embryonic type be the offspring, then its failure to attain the condition of the parent is due to the supervention of a slower rate of growth; to this phenomenon the term 'retardation' is applied." "i believe that this is the simplest mode of stating and explaining the law of variation: that some forms acquire something which their parents did not possess; and that those which acquire something additional have to pass through more numerous stages than their ancestors; and those which lose something pass through fewer stages than their ancestors; and these processes are expressed by the terms 'acceleration' and 'retardation.'"[do] it is clear, however, that we have here something more than acceleration and retardation of development in the ordinary sense of these words. it would be, therefore, more convenient to use the term "acceleration" for the condensation of _the same series_ of developmental changes into a shorter period of time; "retardation" for the lengthening of the period in which _the same series_ of changes are effected; and "arrested development" for those cases in which the young are born in an immature or embryonic condition. whether there is any distinct tendency, worthy of formulation as a law, for organisms to acquire, as a result of protracted embryonic development, definite characteristics which their ancestors did not possess, i think very questionable. if so, this will fall under the head of the origin of variations. that acceleration, in the sense in which i have used the term, does occur as a variation is well known. "with our highly improved breeds of all kinds," says darwin,[dp] "the periods of maturity and reproduction have advanced with respect to the age of the animal; and in correspondence with this, the teeth are now developed earlier than formerly, so that, to the surprise of agriculturalists, the ancient rules for judging of the age of an animal by the state of its teeth are no longer trustworthy." "disease is apt to come on earlier in the child than in the parent; the exceptions in the other direction being very much rarer."[dq] professor weismann contends that the time of reproduction has been accelerated through natural selection, since the shorter the time before reproduction, the less the number of possible accidents. we may, perhaps, see in the curious cases of reproduction during an otherwise immature condition, extreme instances of acceleration. the axolotl habitually reproduces in the gilled, or immature condition. some species of insects reproduce before they complete their metamorphoses. and the females of certain beetles (_phengodini_) are described by professor riley as larviform.[dr] precocity is variation in the direction of acceleration, and that condensed development which is familiar in the embryos of so many of the higher animals may be regarded as the result of variations constantly tending in the same direction. that there are fewer examples of retardation is probably due to the fact that nature has constantly favoured those that can do the same work equally well in a shorter time than their neighbours. but there can be no doubt that, accompanying that fosterage and protection which is of such marked import in the higher animals, there is also much retardation. and as bearing upon the supposed law of variation as formulated by messrs. hyatt and cope, it should be noted that this retardation or _decreased_ rate of growth leads to the production of the more advanced descendant. _the inheritance of variations._ given the occurrence of variations in certain individuals of a species, we have the alternative logical possibilities of their being inherited or their not being inherited. the latter alternative seems at first sight to be in contradiction to the law of persistence. sir henry holland, seeing this, remarked that the real subject of surprise is, not that a character should be inherited, but that any should ever fail to be inherited.[ds] intercrossing may diminish a character, and sooner or later practically obliterate it: annihilate it at once and in the first generation it cannot. this logical view, however, ceases to be binding if we admit, with professor weismann, that variations may be produced in the body without in any way affecting the germ. it is also vitally affected if we believe that the hen does not produce the egg, though she may, perhaps, modify the eggs inside her; for the modification of the hen (i.e. the variety in question) may not be of the right nature or of sufficient strength to impress itself upon the germinal matter of the egg. we may at once admit, then, that acquired variations need not be inherited. passing to innate variations--variations, that is to say, which are the outcome of normal development from the fertilized ovum--must they be inherited, at any rate, in some degree? it seems to me that they must, on the hypothesis that sexual generation involves simply the blending or commingling of the characters handed on in the ovum or the sperm. the only cases where this would _apparently_ fail to hold good would be where the ovum and the sperm handed on exactly opposite tendencies--a variation in excess contributed by the male precisely counterbalancing a variation in the opposite direction contributed by the female parent. even here the tendency is inherited, though it is counterbalanced. on the hypothesis of "organic combination" before alluded to (p. ), variations might, however, in the union of ovum and sperm, be not only neutralized, but augmented. if the variation be, so to speak, a definite organic compound resulting from a fortunate combination of characters in ovum and sperm, it might either fail altogether, or be repeated in an enfeebled form, or augmented in the offspring, according as the new conditions of combination were unfavourable or favourable. whether innate variations ever actually fail to be inherited, even in an enfeebled form, it is very difficult to say; for if this, that, or the other variation fail to be thus inherited, it is difficult to exclude the possibility of its being an acquired variation not truly innate. certainly variations seem sometimes to appear in one generation, and not to be inherited at all. and, as we have seen, mr. romanes appeals to a gradual failure of heredity, apart from intercrossing, to explain the diminution of disused organs. that a variation strongly developed in both parents is apt to be augmented in the offspring is commonly believed by breeders. darwin was assured that to get a good jonquil-coloured canary it does not answer to pair two jonquils, as the colour then comes out too strong, or is even brown. moreover,[dt] "if two crested canaries are paired, the young birds rarely inherit this character; for in crested birds a narrow space of bare skin is left on the back of the head, where the feathers are upturned to form the crest, and, when both parents are thus characterized, the bareness becomes excessive, and the crest itself fails to be developed." on the whole, it would seem that variations may either be neutralized or augmented in inheritance; but the determining causes are not well understood. another fact to be noticed with regard to the inheritance of variations is that some characters blend in the offspring, while others apparently fail to do so. mr. francis galton,[du] speaking of human characters, gives the colour of the skin as an instance of the former, that of the eyes as an example of the latter. if a negro marries a white woman, the offspring are mulattoes. but the children of a light-eyed father and a dark-eyed mother are either light-eyed or dark-eyed. their eyes do not present a blended tint. among animals the colour of the hair or feathers is often a mean or blended tint; but not always. darwin gives the case of the pairing of grey and white mice, the offspring of which are not whitish-grey, but piebald. if you cross a white and a black game bird, the offspring are either black or white, neither grey nor piebald. sir r. heron crossed white, black, brown, and fawn-coloured angora rabbits, and never once got these colours mingled in the same animal, but often all four colours in the same litter. he also crossed "solid-hoofed" and ordinary pigs. the offspring did not possess all four hoofs in an intermediate condition; but two feet were furnished with properly divided and two with united hoofs.[dv] professor eimer[dw] has noticed that, in the crossing of striped and unstriped varieties of the garden snail, _helix hortensis_, the offspring are either striped or unstriped, not in an intermediate or faintly striped condition. these facts are of no little importance. they tend to minimize, for some characters at least, the effects of intercrossing. the variations which present this trait may be likened to stable organic compounds, which may be inherited or not inherited, but which cannot be watered down by admixture and intercrossing. it is well known[dx] that, in , a ram-lamb was born in massachusetts, with short, crooked legs and a long back, like a turn-spit dog. from this one lamb[dy] the _otter_, or _ancon_, breed was raised. when sheep of this breed were crossed with other breeds, the lambs, with rare exceptions, perfectly resembled one parent or the other. of twin lambs, even, one has been found to resemble one parent, and the second the other. all that the breeder has to do is to eliminate those which do not possess the required character. and very rarely do the lambs of ancon parents fail to be true-bred. now, it can scarcely fail that such sports occur in nature. and if they are stable compounds, they will not be readily swamped by intercrossing. it only requires some further isolation to convert the sporting individuals into a distinct and separate variety. now, darwin tells us that the ancons have been observed to keep together, separating themselves from the rest of the flock when put into enclosures with other sheep. here, then, we have preferential mating as the further isolating factor. i feel disposed, therefore, to agree with mr. galton when he says,[dz] "the theory of natural selection might dispense with a restriction for which it is difficult to see either the need or the justification, namely, that the course of evolution always proceeds by steps that are severally minute, and that become effective only through accumulation. that the steps _may_ be small, and that they _must_ be small, are very different views; it is only to the latter that i object, and only when the indefinite word 'small' is used in the sense of 'barely discernible,' or as small as compared with such large sports as are known to have been the origins of new races." connected, perhaps, with the phenomena we have just been considering is that of _prepotency_.[ea] it is found that, when two individuals of the same race or of different races are crossed, one has a preponderant influence in determining the character of the offspring. thus the famous bull favourite is believed to have had a prepotent influence on the short-horn race; and the improved short-horns possess great power in impressing their likeness on other breeds. the phenomena are in some respects curiously variable. in fowls, silkiness of feathers seems to be at once bred out by intercrossing between silk-fowl and any other breed. but in the silky variety of the fan-tail pigeon this character seems prepotent; for, when the variety is crossed with any other small-sized race, the silkiness is invariably transmitted. one may fairly suppose that prepotent characters have unusual stability; but to what causes this stability is due we are at present ignorant. lastly, we have to consider the phenomenon of _latency_. this is the lying hid of characters and their subsequent emergence. we may distinguish three forms of latency. . where characters lie hid till a certain period of life, and then normally emerge. . where the characters normally lie hid throughout life, but are, under certain circumstances, abnormally developed. . where the characters lie hid throughout life, but appear in the offspring or (sometimes distant) descendants. latency is often closely connected with correlated variations. secondary sexual characters, for example, are correlated with the functional maturity or activity of the reproductive organs. they therefore lie hid until these organs are mature and ready for activity. when they are restricted to the male, they normally remain latent throughout the life of the female, but reappear in her male offspring. under abnormal conditions, such as the removal of the essentially male organs, the secondary sexual characters correlated with them do not appear, or appear in a lessened and modified form. the males may even, under such circumstances, acquire female characters. thus capons take to sitting, and will bring up young chickens. conversely, females which have lost their ovaries through disease or from other causes sometimes acquire secondary sexual characters proper to the male. characters thus normally latent abnormally emerge. mr. bland sutton[eb] gives a case of a hen golden pheasant which "presented the resplendent dress of the cock, but her plumage was not quite so brilliant; she had no spurs, and the iris was not encircled by the ring of white so conspicuous in the male." her ovary was no larger than a split pea. a curious instance of latent characters correlated with sex is seen in hive bees. the worker bee differs from the female in the rudimentary condition of the sexual organs, in size and form, and in the higher development of the sense-organs. but it is well known that, if a very young worker grub be fed on "royal jelly," she will develop into a perfect queen. not only are the sexual organs stimulated to increased growth and functional activity, but the correlated size and condition of the sense-organs are likewise acquired. the characters of queen and worker are latent in the grub. according to the nature of the food it receives, the one set of characters or the other emerges. professor yung's tadpoles and mrs. treat's butterflies (_ante_, p. ) afford similar instances. we come now to those cases of latency in which this obvious correlation does not occur. they afford examples of reversion to more or less remote ancestral characters. in some cases the cause of such reversion--such unexpected emergence of characters, which have remained latent through several, perhaps many, generations--is quite unknown. in others, at any rate among domesticated animals, the determining condition of such reversion is the crossing of distinct breeds. darwin gives[ec] an instance of reversion, on the authority of mr. r. walker. he bought a black bull, the son of a black cow with white legs, white belly, and part of the tail white; and in a calf, the gr-gr-gr-gr-grandchild of this cow, was born, coloured in the same very peculiar manner, all the intermediate offspring having been black. in man partial reversions are not infrequent. an additional pair of lumbar ribs is sometimes developed, and in such cases the fan-shaped tendons which are normally connected with the transverse processes of the vertebræ are replaced by functional levator muscles. since it is probable that the ancestor of man had more than the twelve pairs of ribs that are normally present in the human species, we may, perhaps, fairly regard the supernumerary rib as a reversion. but it may be a new sport on old lines. the occasional occurrence in scotland of red grouse with a large amount of white in the winter plumage, especially on the under parts, is justly regarded by mr. wallace[ed] as a good example of reversion or latency in wild birds. there can be little doubt that, as he suggests, the scotch red grouse is derived from a form which, like the wide-ranging willow grouse, has white winter plumage. during the glacial epoch this would be an advantage. "but when the cold passed away, and our islands became permanently separated from the mainland, with a mild and equable climate, and very little snow in winter, the change to white at that season became hurtful, rendering the birds more conspicuous, instead of serving as a means of concealment." the red grouse has lost its white winter dress; but occasional reversions point to the ancestral habit. that crossing tends to produce reversion is a fact familiar to breeders and fanciers, and one which is emphasized by darwin. when pigeons are crossed, there is a strong tendency to revert to the slatey-blue tint and black bars of the ancestral rock-pigeon. there is always a tendency in sheep to revert to a black colour, and this tendency is emphasized when different breeds are crossed. the crossing of the several equine species (horse, ass, etc.) "tends in a marked manner to cause stripes to appear on various parts of the body, especially on the legs," and this _may be_ a reversion to the condition of a striped and zebra-like ancestor. professor jaeger described a good case with pigs. "he crossed the japanese, or masked breed, with the common german breed, and the offspring were intermediate in character. he then recrossed one of these mongrels with a pure japanese, and in the litter thus produced one of the young resembled in all its characters a wild pig; it had a long snout and upright ears, and was striped on the back. it should be borne in mind that the young of the japanese breed are not striped, and that they have a short muzzle and ears remarkably dependent."[ee] darwin crossed a black spanish cock with a white silk hen. one of the offspring almost exactly resembled the _gallus bankiva_, the remote ancestor of the parents. such cases would seem to show that in our domestic breeds ancestral traits lie latent. the crossing of distinct varieties may either neutralize the variations artificially selected, and thus allow the ancestral characters which have been masked by them to reappear; or they may allow the elements of the ancestral traits, long held apart in separate breeds by domestication, to recombine with the consequent emergence of the normal characters of the wild species. but, in truth, any attempted explanations of the facts are little better than guess-work. there are the facts. and the importance of crossing as a determining condition in domesticated animals should make us cautious in applying reversion, as it occurs in such cases, to wild species which live under more stable conditions where crossing is of rare occurrence. _the origin of variations._ the subject of the origin of variations is a difficult one, one concerning which comparatively little is known, and one on which i am not able to throw much light. taking a simple animal cell as our starting-point, we have already seen that it performs, in primitive fashion, certain elementary and essential protoplasmic activities, and gives rise to certain products of cell-life. in the metazoa, which are co-ordinated aggregates of animal cells, together with some of their products, there is seen a division of labour and a differentiation of structure among the cells. we see, then, that variation among these related cells has led to differences in size, in form, in transparency, and in function; while the cell-products have been differentiated into those which are of lifelong value, such as bone, cartilage, connective tissue, horn, chitin, etc., together with a variety of colouring matters; those which are of temporary value, such as the digestive secretions, fat, etc.; and those which are valueless or noxious, such as carbonic acid gas and urea, which are excreted as soon as possible. here are already a number of important and fundamental variations to be accounted for. let us notice that, wide as the variations are, they are to a large extent hedged in by physical, chemical, and organic limitations. we have already seen that the size of cells is to a large extent limited, because during growth mass tends to outrun surface; and because, while disruptive changes occur throughout the mass, nutriment and oxygen must be absorbed by the surface. this is a physical limitation. since the products of cell-life and cell-activity are chemical products, it is clear that they can only be produced under the fixed limitations of chemical combination; and though in organic products these limitations are not so rigid as among inorganic substances, still that there are limitations no chemist is likely to question. the organic limitations are to the varied, but not very numerous, modes of protoplasmic activity. probably, even at the threshold of metazoan life, such variations did not affect only individual cells, but rather groups of cells. in other words, the differentiation was at once and primarily a tissue-differentiation. what do we know, however, about the primitive tissue-differentiation of the earliest metazoa? hardly anything. we may fairly suppose that the first marked difference to appear was that between the outside and the inside. in the formation of an embryo this is the first differentiation we notice. from the beginning of segmentation or, in any case, very early, the outer-layer cells become marked off from the inner-layer cells. the next step was, perhaps, the formation of the mid-layer between the outer and inner. but how further differentiations were effected we really do not know, though we may guess a little. this, perhaps, we may fairly surmise--that fresh differentiations presupposed previous differentiations, and formed the basis of yet further differentiations. thus calcified cartilage presupposes cartilage, and leads up to the formation of true bone. in all this, however, we are very much in the dark. we can watch, always with fresh wonder, the genesis of tissues in the development of the embryo; but we do not at present know much of the mode of their primitive genesis in the early days of organic evolution: how can we, then, pretend to understand their origins? if we speculate at all on the matter, we are led to the view that the variations must be primarily due to the differential incidence of mechanical stresses and physical or chemical influences. it may be admitted that this is little more than saying that they are due to some physical cause. still, this at least may be taken as certain for what it is worth--that the primitive tissue-differentiations are due to physical or chemical influences, direct or indirect, on the protoplasm of the cell. here is one mode of the origin of variations. i do not wish to reopen the question whether these variations originate in the germ or in the body. i content myself with indicating the difference, from this standpoint, between the two views. take, for example, the end-organs of the special senses, which respond explosively to physical influences in ways we shall have to consider more fully in the next chapter. if we hold that variations originating in the body may be transmitted through the germ to the offspring, then we may say that these variations are the direct result of the incidence of the physical or molecular vibrations on the protoplasm. but if we believe, with professor weismann, that all variations originate in the germ, then the variations in the end-organs of the special senses, fitting them to be the recipients of special modes of influence, result from physical effects upon the germ of purely fortuitous origin, that is to say, wholly unrelated to the end in view. the rods and cones of the retina are due to purely chance variations, impressed by some chemical or physical causes completely unknown on the germinal protoplasmic substance. those individuals which did not have these chance variations have been eliminated. it matters not that the rods and cones are believed to have reached their present excellence through many intermediate steps from much simpler beginnings. the fact remains that the origin of all these step-like variations was fortuitous, and not in any way the direct outcome of the physical influences which their products, the rods and cones, have become fitted to receive. i am not at present prepared to accept this theory of the germinal origin of all tissue-variations. whether use and disuse are to be regarded as sources of origin of variations is, again, a matter in which there is wide difference of opinion. but if we admit that any variations can take their origin in the body (as distinguished from the germ), then there is no _à priori_ reason for rejecting use and disuse as factors. as such, we are, i think, justified, in the present state of our knowledge, in reckoning them, at all events, provisionally. it is clear, however, that they are a proximate, not an ultimate, source of origin. i mean that the structures must be there before they can be either strengthened or weakened by use or disuse. they are at most a source of positive or negative variations of existing structures. they cannot be a direct source of origin of superficial variations. gain or loss of colour; form-variations not correlated with organic variations;--these cannot be directly due to use or disuse. it is in the nervous and muscular systems and the glandular organs that use and disuse are mainly operative. when, however, organs are brought into relation, or fail to be brought into relation, to their appropriate stimuli, we speak of this, too, as use and disuse. we say, for example, that persistent disuse may impair the essential tissues of the recipient end-organs of the special senses, implying that these tissues require to be brought into continued relation to the appropriate stimuli in order that their efficiency be maintained. so, too, we say that the epidermis is thickened by use, meaning that it is brought into relation with certain mechanical stresses. through correlation, too, the effects of use and disuse may be widespread. thus increase in the size of a group of muscles may be correlated with increase in the size of the bones to which they are in relation. in fact, so knit together and co-ordinated is the organism into a unity, it is probable that hardly any variation could take place through use or disuse without modifying to some extent the whole organic being. once more, let it be clearly remembered that a large and important school of zoologists reject altogether use or disuse as a factor in variation. they believe that those germs are selected through natural selection in which there is an increased tendency to use or disuse of certain organs. in this, however, we are all agreed. the real question is what is the source of origin of this tendency. on the view of germinal origin, we are forced back on unknown physical or chemical influences in no wise related in origin (though, of course, related in result) with the use or disuse to which they give rise. so far the main distinction between the two biological schools seems to be that the one, placing the origin of variation in the body-tissues, regards the variations as evoked in direct reaction to physical or chemical influences; while the other, placing the origin of variation in the germ, regards the variations as of fortuitous origin. i do not use the phrase, "of fortuitous origin," as in any sense discrediting the theory. i am not attempting the cheap artifice of damning a view that does not happen to be my own with a phrase or a nickname. and i therefore hasten to point out what variations i do believe to have had a fortuitous origin. the phrase is often misunderstood, and they will serve to explain its meaning. if the reader will kindly refer to the tables of variations in the bats' wings (figs. - ), he will see that there are a great number of bones which vary in length and vary independently. and if he will also refer to fig. , in which seven species of bats are compared, he will see that the differences arise from the increased length of one set of bones in one species and another set of bones in another species. now, let us suppose that the long, swallow-like wing of the noctule, a high flyer with rapid wing-strokes, that catches insects in full flight, and the broad wings of the horse-shoe, a low flyer, flapping slowly, and, at any rate, sometimes catching insects on the ground, and covering them with its wings as with a net; let us suppose, i say, that to each species its special form of wing is an advantage. among thousands of independent variations in the lengths of the bones there would be occasional combinations of variations, giving either increased length or increased breadth to the wing. in the noctule, the former would tend to be selected; in the horse-shoe, the latter. thus the wing of the noctule would be lengthened, and that of the horse-shoe broadened, through the selection of fortuitous combinations of variations which chanced to be favourable. now, each individual bone-variation is, we believe, due to some special cause; but the fortunate combination is fortuitous, due to what we term "mere chance." darwin believed that chance, in this sense, played a very important part in the origin of those favourable variations for which, as he said, natural selection is constantly and unceasingly on the watch. and there can be little question that darwin was right. we must now consider very briefly some of the proximate causes of variations. in most of these cases we cannot hope to unravel the nexus of causation. when a plexus of environing circumstances acts upon a highly organized living animal, the most we can do in the present state of knowledge is to note--we cannot hope to explain--the effects produced. all readers of darwin's works know well how insistent he was that the nature of the organism is more important than the nature of the environing conditions. "the organization or constitution of the being which is acted on," he says,[ef] "is generally a much more important element than the nature of the changed conditions in determining the nature of the variation." and, again,[eg] "we are thus driven to conclude that in most cases the conditions of life play a subordinate part in causing any particular modification; like that which a spark plays when a mass of combustible matter bursts into flame--the nature of the flame depending on the combustible matter, and not on the spark." recent investigations have certainly not lessened the force of darwin's contention. from which there follows the corollary that the vital condition of the organism is a fact of importance. darwin was led to believe that among domesticated animals and plants good nutritive conditions were favourable to variation. "of all the causes which induce variability," he says,[eh] "excess of food, whether or not changed in nature, is probably the most powerful." darwin also held that the male is more variable than the female--a view that has been especially emphasized by professor w. k. brooks. mr. wallace, as we have already seen, regards the secondary sexual characters of male birds as the direct outcome of superabundant health and vigour. "there is," he says,[ei] "in the adult male a surplus of strength, vitality, and growth-power which is able to expend itself in this way without injury." and messrs. geddes and thomson contend[ej] that "brilliancy of colour, exuberance of hair and feathers, activity of scent-glands, and even the development of weapons, are in origin and development outcrops of a male as opposed to a female constitution." there is, i think, much truth in these several views thus brought into apposition. vigour and vitality, predominant activity and the consequent disruptive changes, with their abundant by-products utilized in luxuriant outgrowths and brilliant colours, are probably important sources of variation. they afford the material for natural selection and sexual selection to deal with. these guide the variations in specific directions. for i am not prepared to press the theory of organic combination so far as to believe that this alone has served to give definiteness to the specific distinctions between secondary sexual characters, though it may have been to some extent a co-operating factor. this, however, is a question apart from that of origin. superabundant vigour may well, i think, have been a source of _origin_, not only of secondary sexual characters, but of many other forms of variation. and while these forms of variation may be the special prerogative of the male, we may perhaps see, in superabundant female vigour, a not less important source of developmental and embryonic variations in the offspring. the characteristic selfishness of the male applies his surplus vitality to the adornment of his own person; the characteristic self-sacrifice of the mother applies her surplus vitality to the good of her child. here we may have the source and origin of those variations in the direction of fosterage and protection which we have seen to have such important and far-reaching consequences in the development of organic life. the storage of yolk in the ovum, the incubation of heavily yolked eggs, the self-sacrificing development in the womb, the elaboration of a supply of food-milk,--all these and other forms of fosterage may well have been the outcome of superabundant female vigour, the advantages of which are thus conferred upon the offspring. we may now proceed to note, always remembering the paramount importance of the organism, some of the effects produced by changes in the environment. the most striking and noteworthy feature about the effects of changes of climate and moisture, changes of salinity of the water in aquatic organisms, and changes of food-stuff, is that, when they produce any effect at all, they give rise to _definite_ variations. only one or two examples of each can here be cited. mr. merrifield,[ek] experimenting with moths (_selenia illunaria_ and _s. illustraria_), finds that the variations of temperature to which the pupa, and apparently also the larva, are subjected tend to produce "very striking differences in the moths." on the whole, cold "has a tendency, operating possibly by retardation, to produce or develop a darker hue in the perfect insect; if so, it may, perhaps, throw some light on the mechanism so often remarked in north-country examples of widely distributed moths." mr. cockerell[el] regards moisture as the determining condition of a certain phase of melanism, especially among lepidoptera. the same author states that the snail "_helix nemoralis_ was introduced from europe into lexington, virginia, a few years ago. under the new conditions it varied more than i have ever known it to do elsewhere, and up to the present date ( ) varieties have been discovered there. of these, no less than are new, and unknown in europe, the native country of the species." the effects of the salinity of the water on the brine-shrimp _artemia_ have already been mentioned. one species with certain characteristics was transformed into another species with other characteristics by gradually altering the saltness of the water. so, too, in the matter of food, the effects of feeding the caterpillars of a texan species of _saturnia_ on a new food-plant were so marked that the moths which emerged were reckoned by entomologists as a new species. the point, i repeat, to be especially noted about these cases and others which might be cited,[em] is that the variation produced is a definite variation. very probably it is generally, or perhaps always, produced in the embryonic or larval period of life. in some cases the variation seems to be transmissible, though definite and satisfactory proofs of this are certainly wanting. still, we may say that if the changed conditions be maintained, the resulting variation will also be maintained. under these conditions, at least, the variation is a stable one. it is probable that, apart from preferential mating, the varieties thus produced will tend to breed together rather than to be crossed with the parent form or varieties living under different conditions. in this way varieties may sometimes arise by definite and perhaps considerable leaps under the influence of changed conditions. we must not run the adage, _natura nil facit per saltum_, too hard, nor interpret _saltum_ in too narrow a sense. it is true, and we may repeat the statement of the fact for the sake of emphasis, that we do not know how or why this or that particular variation should result from this or that change of climate, environment, or food-stuff; nor do we know why certain variations (such as that which produced the ancon breed of sheep) should be stable, while other variations are peculiarly unstable. but in this we are not worse off than we are in the study of inorganic nature. we do not know why calcite should crystallize in any particular one of its numerous varieties of crystalline form; we do not know why some of these are more stable than others. we may be able to point to some of the conditions, but we cannot be said to understand why arragonite should be produced under some circumstances, calcite under others; or why the same constituents should assume the form of augite in some rocks, and hornblende in other rocks. we are hedged in by ignorance; and perhaps one of our chief dangers, becoming with some people a besetting sin, is that of pretending to know more than we are at present in a position to know. our very analogies by which we endeavour to make clear our meaning may often seem to imply an unwarrantable assumption of knowledge. in the last chapter i used the term "organic combination," and drew a chemical analogy. i wished to indicate the particularity and the stability of certain variations, and the possibility of new departures through new combinations of variations, the new departure not being necessarily anything like a mean between the combining variations.[en] i trust that this will not be misunderstood as a new chemico-physical theory of organic forms. i have some fear lest i should be represented as maintaining that a giraffe or a peacock is a definite organic compound, with its proper organic form, in exactly the same way as a rhombohedron of calcite or a rhombic dodecahedron of garnet is a definite chemical compound, with its proper crystalline form. all that the analogy is intended to convey is that variations seem, under certain circumstances, to be definite and stable, and may possibly combine rather than commingle. _summary and conclusion._ it only remains to bring this chapter to a close with a few words of summary and conclusion. the diversity of animal life must first be grasped. we believe that this diversity is the result of a process or processes of evolution. evolution is the term applied to continuity of development. it involves adaptation; and adaptation to an unchanging environment may become more and more perfect. but the environment to which organisms are adapted also changes. where the change is in the direction of complexity, we have elaboration; where it is in the direction of simplicity, we have degeneration. of these elaboration is the more important. it involves both a tendency to differentiation giving rise to individuality, and a tendency to integration giving rise to association. continued elaboration is progress; and this is opposed to degeneration. the factors of evolution fall under two heads--origin and guidance. the origin of variations lies in mechanical stresses, and chemical or physical influences. whether these act on the body (and are transmitted by inheritance) or only on the germ, is a question which divides biologists into two schools. in the latter case all variations are fortuitous; in the former the development of tissue-variations has been in direct response to the physical or chemical influences. there are, however, in any case fortuitous combinations of variations. whether use and disuse are factors of origin is also a debatable point. those who believe that physical influences on the body are transmissible believe also that the effects of use and disuse are transmissible. the vital vigour of the organism is a determining condition of importance. the vital vigour of males has favoured the origin of secondary sexual characters; that of females, the fostering and protection of young, and therefore the development in them of vital vigour. the almost universally admitted factor in guidance is natural selection. but we must be careful not to use it as a mere formula. whether sexual selection is also a factor is still a matter of opinion. without it the specific character and constancy of secondary sexual features are at present unexplained. if inherited use and disuse are admitted as factors in origin, they must also be admitted as important factors in guidance. questions of origin and guidance should, so far as is possible, be kept distinct. these terms, however, apply to the origin and guidance of variations. in the origin of species guidance is a factor, no doubt a most important factor. the title of darwin's great work was, therefore, perfectly legitimate. and those who say that natural selection plays no part in the origin of species are, therefore, undoubtedly in error. notes [ci] it is beyond the scope of this book to give the _evidences_ of evolution. such evidence from embryology, from distribution, and from palæontology, is now abundant. for palæontological evidence, see nicholson's "manual of palæontology," rd edit., especially the second volume on "vertebrates," by r. lydekker. [cj] weismann, "essays on heredity," p. . [ck] ibid. p. . [cl] weismann, "essays on heredity," p. . [cm] ibid. p. . see also a discussion in _nature_, in which mr. romanes and professor ray lankester took part, beginning vol. xli. p. . [cn] weismann, "essay on heredity," p. . [co] "origin of species," p. . [cp] with regard to blind cave-fish, professor ray lankester has suggested that some selection has been effected. those animals whose sight-sensitiveness enabled them to detect a glimmer of light would escape to the exterior, leaving those with congenitally weak sight to remain and procreate in the darkness of the cave. [cq] darwin, "descent of man," pt. ii. chap. viii. [cr] "darwinism," chap. x. [cs] "darwinism," p. . messrs. geddes and thomson, "the evolution of sex," p. , also contend that "combative energy and sexual beauty rise _pari passu_ with male katabolism." [ct] "darwinism," p. . [cu] mr. poulton, who takes a similar line of argument in his "colours of animals," lays special stress upon the production of _white_ (see p. ). [cv] see chapter viii. [cw] "darwinism," p. . [cx] see "animals and plants under domestication," vol. ii. p. . [cy] "darwinism," p. . [cz] "the colour-sense," by grant allen, p. . [da] that on "the emotions of animals" (x.). [db] "darwinism," p. . [dc] natural history society of wisconsin, vol. i. ( ). [dd] "darwinism," p. . [de] on the negative character of disuse, see p. . [df] cope, "origin of the fittest," p. . [dg] it would appear, from certain passages of his "darwinism," that mr. a. r. wallace (e.g. p. , note) holds or held similar views. "the genera _ateles_ and _colobus_," he says, "are two of the most purely arboreal types of monkeys, and it is not difficult to conceive that the constant use of the elongated fingers for climbing from tree to tree, and catching on to branches while making great leaps, might require all the nervous energy and muscular growth to be directed to the fingers, the small thumb remaining useless." i should also have quoted mr. wallace's account of the twisting round of the eyes of flat-fishes--where he says that the constant repetition of the effort of twisting the eye towards the upper side of the head, when the bony structure is still soft and flexible, causes the eye gradually to move round the head till it comes to the upper side--had he not subsequently disclaimed this explanation (see _nature_, vol. xl. p. ). it is possible that mr. wallace, notwithstanding the words "constant use" in the passage i have quoted, merely intends to imply that the elongated fingers are of advantage in climbing, and are thus subject to natural selection, the thumb diminishing through economy of growth. [dh] i find, on rereading one of his articles, that i have here unwittingly adopted one of mr. romance's arguments (see _nature_, vol. xxxvi. p. ). the instance mr. romanes cites is the curious habit of dogs turning round before they lie down. [di] mr. darwin, while contending that the modifications need not all have been simultaneous, says, "although natural selection would thus tend to give the male elk its present structure, yet it is probable that the inherited effects of use, and of the mutual action of part on part, have been equally or more important" ("animals and plants under domestication," vol. ii. p. ). [dj] _midland naturalist_, november, . [dk] see _ante_, p. . [dl] _nature_, vol. xli. p. . [dm] "animals and plants under domestication," vol. ii. p. . [dn] in the third chapter we saw that in such cases not only are there an enormous number of ova produced, but that (e.g. in aurelia and the liver-fluke) each ovum produces, through the intervention of asexual multiplication, many individuals. [do] cope, "origin of the fittest," pp. , , and . [dp] "animals and plants under domestication," vol. ii. p. . [dq] ibid. p. . [dr] _nature_, vol. xxxvi. p. . [ds] quoted from "medical notes and reflections," , p. , by darwin, "animals and plants under domestication," vol. i. p. . [dt] darwin, "animals and plants under domestication," vol. i. p. . [du] "natural inheritance," p. . [dv] darwin, "animals and plants under domestication," vol. ii. p. . [dw] "organic evolution," mr. cunningham's translation, p. . [dx] darwin, "animals and plants under domestication," vol. i. p. . [dy] similarly, from a chance sport of a one-eared rabbit, anderson formed a breed which steadily produced one-eared rabbits ("animals and plants under domestication," vol. i. p. ). this is an example of asymmetrical variation. variations are generally, but not always, symmetrical. superficial colour-variations are sometimes asymmetrical. gasteropod molluscs are nearly always asymmetrically developed. among insects, _anisognathus_ affords an example of the asymmetrical development of the mandible. our right-handedness is a mark of asymmetry. [dz] "natural inheritance," p. . [ea] see "animals and plants under domestication," vol. ii. p. , from which illustrations are taken. [eb] "evolution and disease," p. . [ec] "animals and plants under domestication," vol. ii. p. . [ed] "darwinism," p. . [ee] darwin, "animals and plants under domestication," vol. ii. pp. , . [ef] "animals and plants under domestication," vol. ii. p. . [eg] ibid. p. . the phenomena of the seasonal dimorphism of butterflies and moths show that changes of temperature (and perhaps moisture, etc.) determine very striking differences in these insects. [eh] "animals and plants under domestication," vol. ii. p. . [ei] "darwinism," p. . [ej] "evolution of sex," p. . [ek] "incidental observations in pedigree moth-breeding," f. merrifield. transactions entomological society, , pt. i. p. , _et seq._ [el] _nature_, vol. xli. p. . [em] see professor meldola's edition of professor weismann's "studies in the theory of descent," and mr. cunningham's translation of professor eimer's "organic evolution." [en] see darwin, "animals and plants under domestication," vol. ii. p. . chapter vii. the senses of animals. it is part of the essential nature of an animal to be receptive and responsive. the forces of nature rain their influence upon it; and it reacts to their influence in certain special ways. other organisms surround it, compete with it, contend with it, strive to prey upon it, and occasionally lend it their aid. it has to adjust itself to this complex environment. there are two kinds of organic response--one more or less permanent, the other temporary and transient. we have already seen something of the former, by which the tissues (the epidermis of the oarsman's hand, and the muscles of his arm) respond to the call made upon them. the response is here gradual, and the effects on the organism more or less enduring. this, however, is not the kind of response with which we have now to deal. what we have now to consider is that rapid response, transient, but of the utmost importance, by means of which the organism directly answers to certain changes in the environment by the performance of certain activities. the parts specially set aside and adapted to receive special modes of influence of the environment are the sense-organs. we human folk get so much pleasure from and through the employment of our sense-organs, that it is important to remember that the primary object of the process of reception of the influences from without was not the æsthetic one of ministering to the enjoyment of life by the recipient organism, but the essentially practical one of enabling that organism to respond to these influences. in other words, the _raison d'être_ of the sense-organs is to set agoing suitable activities--activities in due response to the special stimuli. in this chapter we shall consider the modes in which the special sense-organs are fitted to receive the influences of the environment, deferring to a future chapter the consideration of the resulting activities. for the present we take these activities for granted, observing them only in so far as they give us a clue to the sense-reaction by which they are originated. in this chapter, too, we shall deal, for the most part, with the physiological aspects of sensation. in all other organisms than ourselves, that is to say, than each one of us individually for himself, the psychological accompaniments of the physiological reactions of the sense-organs are matters of inference. still, so closely and intimately associated are the physiological and the psychological aspects, that the exclusion of all reference to the latter would be impracticable, or, if practicable, unadvisable. what is practicable and advisable is to remember that, even if the two are mentioned in a breath, the physiological and the psychological belong to distinct orders of being. in addition to the time-honoured "five senses," there are certain _organic sensations_, so called, which take their origin within the body. these are, for the most part, somewhat vague and indefinite. they do not arise immediately and in direct response to changes in the environment, but indicate conditions of the internal organs. such are hunger, thirst, nausea, fatigue, and various forms of discomfort. although they are of vital importance to the organism, prompting it to perform certain actions or to desist from others, they need not detain us here. more definite than these, but still of internal origin, is the _muscular sense_. this, too, is of continual service to every active animal. by it information is given as to the energy of contraction of the muscles, and of the amount of movement effected--not to mention the rapidity and duration of the muscular effort. by it the position, or changes of position, of the motor-organs are indicated. it is obvious, therefore, that the sensations obtained in this way, some of which are exceedingly delicate, are an important guide to the organism in the putting forth of its activities. it is through the muscular sense that we maintain an upright position. it is through an educated and refined muscular sense that the juggler and the acrobat can perform their often surprising feats. concerning the physiology of the muscular sense, we have at present no very definite knowledge. some have held that we judge of muscular movements by the amount of effort required to initiate them; but it is much more probable that there are special sensory nerves, whose terminations are either in the muscles themselves or in the membranes which surround them. * * * * * we come now to the special senses. of these we will take first the _sense of touch_. through this sense we are made aware of bodies solid or liquid (or perhaps gaseous) which are actually in contact with the skin or its infoldings at the mouth, nostrils, etc. there are considerable differences in the sensitiveness of the skin in different parts of its surface; some parts, like the filmy membrane which covers the eye, being very sensitive, while others, like the horny skin that covers the heel of a man who is accustomed to much walking, are relatively callous. different from this is the delicacy of the sense of touch. this delicacy is really the power of discrimination, and therefore involves some mental activity. but it is also dependent upon the distribution of the recipient end-organs of the nerve. the highest pitch of delicacy is reached in the tip of the tongue, which is about sixty times as delicate as the skin of the back. the power of discrimination is tested in the following way: the points of a pair of compasses are blunted, and with them the skin is lightly touched. when the points are close together, the sensation is of one object; when they are more divergent, each point is felt as distinct from the other. on the thigh and in the middle of the back, two distinct points of contact are not felt unless the compass-tips are about - / inches ( . millimetres) apart. when the divergence is inches, they are felt as one. with the tip of the tongue, however, we can distinguish the two separate points when they are only / of an inch ( . millimetre) apart. for the finger-tip the distance is about / of an inch ( millimetres); for the tip of the nose, about / of an inch ( . millimetres); for the forehead, a little less than an inch ( . millimetres); and so on. shut your eyes, and allow a friend to draw the compass with the points about / an inch apart, from the forehead to the tip of your nose, or (setting the points about / of an inch apart) from the ball of your thumb to the finger-tip. the increasing delicacy and power of discrimination is readily felt, and it is difficult to believe that the compasses are not being slowly opened. it is beyond the purpose of this chapter to describe minutely the nature and structure of the nerve-ends in the sense-organs. this is a matter of minute anatomy, or histology. a full description of them as they occur in man will be found in any standard text-book of physiology; while sir john lubbock's "senses of animals" gives much information concerning, and many illustrations of, the minute structure of the sense-organs in the invertebrates. here i can only touch very briefly on some of the more important points. one of the larger nerves of the body (e.g. the sciatic nerve), consists of a bundle of nerve-threads collected from a considerable area; some of these (motor threads) end in muscles, others (sensory threads) in the skin or its neighbourhood. each nerve-thread has a central axis-fibre, which is surrounded by a fatty, insulating medullary sheath, and this by a delicate primitive sheath. in some parts of the skin the sensory nerve-threads lose their medullary sheath, and end in very fine branches between the cells of the tissue. in other cases the cells near their termination are specially modified to form tactile cells, or tactile corpuscles, in contact with or surrounding the axis-fibre or its expansion (fig. ). [illustration: fig. .--tactile corpuscles. . in the beak of a goose. . in the finger of a man. . in the mesentery of a cat.] hairs are delicate organs of touch, though, of course, this is not their only function. they act as little levers embedded in the skin. turning now to the vertebrate animals other than man, we find in them a sense of touch closely analogous to our own. as in us, so in them, the specially mobile parts are eminently sensitive and delicate; for instance, the lips in many animals, such as the horse, and the finger-like organ at the end of the elephant's trunk. in some of them special hairs are largely developed as organs of touch, as in the whiskers of the cat and the long hairs on the rabbit's lip. with the aid of these the rabbit finds its way in the darkness of its burrow; and it is said that, deprived of these organs, the poor animal blunders about, and is unable to steer its course in the dark. the wing of the bat is very sensitive to touch; and it is supposed that it is through this sense that the bat is able to direct its course in the darkness of caves. miss caroline bolton thus describes an experimental trial of this power of the bat at which she was herself present. a room, about twenty feet by sixteen, was arranged with strings crossing each other in all directions so as to form a network with about sixteen inches space between the strands. to each string was attached a bell in such a way that the slightest touch would make it ring. one corner of the room was left free for those who were present at the experiment. a bat, measuring about one foot from the tip of one wing to that of the other, was let loose in the room when it was quite dark, "and it was distinctly heard flying about all over the room, but never once did it touch a string or stop flying. it several times came quite near to the spectators, so that they could feel the vibration of the air in their faces. the experiment was continued for half an hour. then, when the door was opened and light let in, the bat stopped flying, and settled down in the darkest corner." now, here it may be said that, although the room was dark to human spectators, there may have been light enough for a bat to see his way. the cruel experiments of spalanzani, however, who put out the eyes of bats and obtained a similar result, seem to show that the animal is guided by some sense other than that of sight. [illustration: fig. .--touch-hair of insect. t.h., touch-hair; cu., cuticle; h.y., hypodermis; g., ganglion-cell connected with nerve passing into the cavity of the touch-hair (after miall). the ganglion is often surrounded by several--eight or less--accessory cells, which are not figured here.] the crustaceans and many insects are covered with a dense armour, and it might be supposed that in them there could be no sense of touch. but this sense is by no means absent. seated on the tough integument are delicate little hairs, to the base of which a nerve-fibril passes through a perforation in the integument. these are specially numerous in the antennæ of insects. in yet lower organisms we know in some cases the manner in which they are sensitive to touch; but in a great number of cases, although observation shows that they are thus sensitive, we know nothing definite as to how the surface is specially fitted to receive the stimuli. even the primitive am[oe]ba, however, is sensitive in the sense spoken of on p. ; that is to say, it reacts under the influence of a stimulus. * * * * * closely associated with the sense of touch is the _temperature-sense_. goldschneider and others have shown that on the skin of the human hand, for example, there are special points that are sensitive to heat and cold. some of these little specialized areas are sensitive to cold; others are sensitive to heat; and neither of these seem to be sensitive to pressure. it therefore seems probable that special nerve-fibrils are set apart for the temperature-sense; but of the manner in which these fibrils terminate little or nothing is known. let us note that this temperature-sense, unlike the sense of touch, may make us aware of distant bodies. it is, then, what we may term a _telæsthetic_ sense in contradistinction to a contact-sense. it is stimulated by a molecular throb; the throbbing body may be in contact, but it may be as distant as the sun, in which case the molecular pulsations are brought to us on waves of æther. whether these waves act directly on the nerve end-organs, or indirectly on them through the warming of the skin-surface in which they terminate, we cannot say for certain. but if the hand be held before a heated stove and be sheltered from the heat by a screen, the removal of the screen, even for the fraction of a second, gives rise to a strong stimulation of the temperature-sense, though the skin-surface be not appreciably raised in temperature. hence it is probable that the end-organs are stimulated directly, and not indirectly. concerning the temperature-sense in the lower animals, nothing definite is known. but it is impossible to see our familiar pets basking in the sunshine, or a butterfly sunning itself on a bright summer's day, without feeling confident that the temperature-sense is a channel of keen enjoyment. as before mentioned, however, this is not to be regarded as the primary end in sensation. the primary end is not life-enjoyment, but life-preservation. and we must regard the temperature-sense as developed in the first instance to enable the organism to escape from the ill effects of deleterious heat or cold, and to seek those temperature-conditions which are most helpful to the continued and healthful fulfilment of the process of life. * * * * * the _sense of taste_ is called into play by certain soluble substances, or liquids, which must come in contact with the specialized nerve-endings. under normal circumstances, the sense of taste is closely associated with that of smell, the result of the combination of the two special senses being a _flavour_. the _bouquet_ of a choice wine, the flavour of a peach, involve both senses; quinine involves taste alone; and garlic and vanilla are nearly, if not quite, tasteless,--what we call their taste is in reality their action on the organ of smell. it is difficult to classify tastes. sweet, bitter, salt, alkaline, sour, acid, astringent, acrid,--these are the prominent and characteristic varieties. [illustration: fig. .--taste-buds of rabbit. i., section across part of the pleated patch (enlarged); ii., taste-buds further enlarged.] this sense is generally localized in or near the mouth; in us mainly in the tongue. one manner, but not the only manner, in which the nerves in this region terminate is in the minute flask-shaped taste-buds, which have near one end, where they reach the surface, a funnel-shaped opening, the taste-pore. they are made up of elongated cells, some of which near the centre are spindle-shaped, and are called taste-cells. they are found chiefly round the large circumvallate papillæ; but in the rabbit and some other animals they are collected in the folds of a little ridged or pleated patch--the _papilla foliata_--on each side of the tongue near the cheek-teeth. it is probable that the stimulation of the end-organs of taste is effected by the special mode of molecular vibration due to the chemical nature of the sapid substance. mr. j. b. haycroft, in a paper read before the royal society of edinburgh,[eo] suggests that "a group of salts of similar chemical properties have their molecules in a similar vibrating condition, giving rise to similar colours and similar tastes." "thus the chlorides and sulphates of a series of similar elements--called a group of elements by mendeljeff--have similar tastes." the delicacy of the sense of taste in man has been the subject of investigation by messrs. e. h. s. bailey and e. l. nichols.[ep] they give the following table:-- i. quinine-- male observers detected part in , parts of water. female " " " , " " ii. cane-sugar-- male observers " " " " female " " " " " iii. sulphuric acid-- male observers " " , " " female " " " , " " iv. bicarbonate of sodium-- male observers " " " " female " " " " " v. common salt-- male observers " " , " " female " " " , " " the above figures represent means or averages of a great number of individuals. there was very considerable variation for some tastes. in the case of the bitter of quinine, the maximum delicacy was the detection of part in , , parts of water; the minimum part in , parts of water. except in the case of salt, the sense was more delicate in women than in men. it is not stated whether the men tested were smokers. it does not seem necessary to say anything concerning the sense of taste in the lower mammalia. in birds and reptiles the sense of taste does not appear to be highly developed. parrots are, perhaps, better off in this respect than the majority of their class; and the ducks have special organs on the edges of the beak, which seem to minister to this sense. a python at the zoological gardens, partially blind owing to a change of skin, is said to have struck at an animal, but to have only succeeded in capturing its blanket. this, however, it constricted, and proceeded to swallow with abundant satisfaction. it may here be mentioned that the scales and skin of many fishes are provided with sense-organs which very closely resemble the taste-buds of higher animals. they occur in the head and along the "lateral line" which runs down the side of the fish, and may be readily seen, for example, in the cod. mr. bateson's[eq] careful observations at plymouth gave, however, no indication of the possession of an olfactory or gustatory function, and their place in the sensory economy of the fish remains problematical. in or near the mouth similar end-organs are found to be somewhat variously developed in different fishes--on the palate and lips, on the gill-bars, more rarely on the tongue, and on the barbels of the rockling and the pout. how far any or all of these have a gustatory function remains to be proved. anglers and fishermen, however, from their everyday experience, and naturalists from special observations, do not doubt that fishes have a sense of taste. professor herdman's recent experiments on feeding fishes with nudibranchs[er] (naked molluscs) seem to show, for example, that the fishes concerned, including shannies, flat-fish, cod, rockling, and others, have a sense of taste leading them to reject these molluscs as nasty. they show, too, that some of the nudibranchs (_doris_, _ancula_, _eolis_) are protected by warning coloration. our knowledge of the sense of taste among the lower (invertebrate) animals is imperfect, and is largely based rather on observation of their habits than on the evidence of anatomical structure. here, again, comes in the difficulty of distinguishing between taste and smell. but even if the caterpillars which refuse to eat all but one or two special herbs, or the races of bloodsuckers which seem to have individual and special tastes, are guided in part by an olfactory sense, there is much evidence which seems to admit of no alternative explanation. moisten, for example, the antennæ of a cockroach with a solution of epsom salts or quinine, and watch him suck it off; or repeat f. will's experiments on bees, tempting them with sugar, and then perfidiously substituting pounded alum. the way in which these little insects splutter and spit suggests that, whatever may be the psychological effect, the physiological effect is analogous to that produced in us by an exceedingly nasty taste. here smell would seem to be excluded. forel, moreover, mixed strychnine with honey, and offered it to his ants. the smell of the honey attracted them, but when they began to feed, the effect of the taste was at once evident. the organs of taste in insects are probably certain minute pits, in each of which is a delicate taste-hair, which, in some cases, is perforated at the free end. they occur in the maxillæ and tongue in ants and bees, and on the proboscis of the fly. in many of the invertebrates, the crayfish and the earthworm, for example--to take two instances from very different groups--observation seems to show that a sense of taste is developed, for they have marked and decided food-preferences. nevertheless, the existence of special organs for this purpose has not been definitely proved. the sense of taste no doubt ministers to the enjoyment of life. but, presumably, it has been developed in subservience to the process of nutrition. primarily, taste was not an end in itself, but was to guide the organism in its selection of food that could be assimilated. nice and nasty were at first, and still are to a large extent, synonymous with good-for-eating and not-good-for-eating. with unwonted substances, however, its testimony may be false. sugar of lead is sweet, but fatal. brought to a new country, cattle often eat, apparently with relish, poisonous plants. still, under normal circumstances, the testimony of taste is reliable. * * * * * the _sense of smell_ is, to a large extent, telæsthetic. it is true that the stimulation of the end-organs is effected by actual contact with the odoriferous vapour. but since this vapour may be given off from an odoriferous body at some distance from the organism, such as a flower or a decomposing carcase, it is clear that the sense gives information of the existence of such bodies before they themselves come in contact with us. primitively, we may suppose that it was developed in connection with that sense of taste with which, as we have seen, it is so closely associated. in this respect smell is a kind of anticipatory taste. but it has now other ends, apart from those which are purely æsthetic. in us it may serve as a warning of a pestilential atmosphere; in many organisms, such as the deer, it gives warning of the presence of enemies; in many again, and some insects among the number, it is the guiding sense in the search for mates. the organ of smell in ourselves and in all the mammalia is the delicate membrane that covers the turbinal bones in the nose. it contains cells with a largish nucleus, around which the protoplasm is mainly collected. a filament passes from this to the surface, and ends in a fine hair or cilium (or a group of hairs or cilia in birds and amphibia); a second filament runs downwards into the deeper parts of the tissue, and may pass into a nerve-fibril. in us and air-breathing creatures, the substance which excites the sensation of smell must be either gaseous or in a very fine state of division; but in water-breathers the substance exciting this sensation--or, in any case, one of anticipatory taste--may be in solution. the sensitiveness of the olfactory membrane is very remarkable. a grain of musk will scent a room for years, and yet have not sensibly lost in weight. drs. emil fischer and penzoldt found that our olfactory nerves are capable of detecting the / , , part of a milligramme of chlorophenol, and the / , , part of a milligramme, or about one thirty-thousand-millionth of a grain, of mercaptan. it may be that to such substances our olfactory sensibility is especially delicate. not much is known concerning the manner in which the end-organs of smell are stimulated. as in the case of taste, it is probably a matter of molecular vibration; and professor william ramsay has suggested that the end-organs are stimulated by vibrations of a lower order than those which give rise to sensations of light and heat. he has also drawn attention to the fact that to produce a sensation of smell, the substance must have a molecular weight at least fifteen times that of hydrogen. it is well known that the sense of smell is in some of the mammalia exceedingly acute. the dog can track his master through a crowded thoroughfare. the interesting experiments of mr. romanes[es] show that, under ordinary conditions of civilized life, the smell of boot-leather is a factor, and the dog tracks his master's boots. in one case, the boots were soaked in oil of aniseed, but this to us powerful scent did not overcome the normal odour of the master's boots. mr. w. j. russell, in a subsequent number of the same periodical, describes how his pug could find a small piece of biscuit by scent, and this odour of biscuit was not overmastered by a strong smell of eau-de-cologne. deer-stalkers know well how keen is the sense of smell in the antlered ruminants. we must not, however, be too ready to conclude, from these observations, that the olfactory membrane is absolutely more sensitive in such animals than it is in man. it may well be that, though they are so keen to detect certain scents, they are dull to those which affect us powerfully. it is quite possible that the odour of aniseed or eau-de-cologne is--possibly from the fact that their end-organs are not attuned to these special molecular vibrations--out of their range of smell. their special interests in life have led to the cultivation of extreme sensibility to special tones of olfactory sensation. under unusual circumstances, man may cultivate unwonted modes of utilizing the sense of smell. a boy, james mitchell, who was born blind, deaf, and dumb, and who was mainly dependent on the sense of smell for keeping up some connection with the external world, observed the presence of a stranger in the room, and formed his opinion of people from their characteristic smell. on the whole, therefore, we may, perhaps, conclude that the variations in sensitiveness are mainly relative to the needs of life. in birds the sense of smell is but little developed, notwithstanding all that most interesting naturalist, charles waterton, wrote on the subject. vultures seem unable to discover the presence of food which is hidden from their sight. probably reptiles share with them this dulness of the sense of smell. it has already been remarked that, in the case of aquatic animals, there is probably little distinction between taste and smell. it would be well, perhaps, to restrict the word "smell" to the stimuli produced by vapours or air-borne particles, and to use the phrase "telæsthetic taste," or simply "taste," for those cases where the effects are produced through the medium of solution. in this case, however, the point to be specially noticed is that taste in aquatic animals becomes a telæsthetic sense, informing the organism of the presence of more or less distant food. thus, if you stir with your finger the water in which leeches are living, they will soon flock to the spot, showing that the telæsthetic sense is associated with an appreciation of direction. if a stick be used to stir the water, they do not take any notice of it. mr. w. bateson[et] has shown that there are many fishes, among which are the dog-fish, skate, conger eel, rockling, loach, sole, and sterlet, which habitually seek their food by scent (telæsthetic taste), aided to some extent by touch, and but little, if at all, by sight. "none of these fishes ever starts in quest of food when it is first put into the tank, but waits for an interval, doubtless until the scent has been diffused through the water. having perceived the scent of food, they swim vaguely about, and appear to seek it by examining the whole area pervaded by the scent, having seemingly no sense of the direction whence it proceeds." i venture to think that further observation and experiment may show that such a sense of direction does in some cases exist. some years ago i was fishing in simon's bay, at the cape, with a long casting-line. the sea was unusually calm, and the water clear as crystal. beneath me was a clear patch of granite, two or three yards across, surrounded by tangled seaweed. evening was coming on, and i was just going to put up my tackle when i saw a long dark fish slowly sail into the open space and take up his position at one side. my line was out, baited, i think, with a piece of cuttle-fish, and i tried to draw it into the clear space, but only succeeded in bringing it to within a foot or so of the side furthest from the fish. there it got hitched in the weed; but the fish being still undisturbed, i awaited further developments. after two or three minutes the fish slowly turned, crossed the pool, and remained motionless for a few moments; then he proceeded straight to the bait; and in a few minutes i had landed a dog-fish between four and five feet long. i did not then know that the dog-fish sought its food mainly or solely by scent (taste); but in any case i do not think in this instance he could have seen the bait, hidden as it was amid the seaweed. although i am aware, and have already mentioned, that mr. bateson's observations do not support the view that the sense-organs of the lateral line minister to this telæsthetic sense, still i think that further observations and experiments may show that these sense-organs are "olfactory," and that the lateral development may be in relation to the appreciation of the direction in which the food lies. it is, however, a difficult matter to determine, and the few experiments i have made are so far inconclusive. much has been written concerning the sense of smell in insects. that they possess such a sense few will be disposed to doubt. the classical observations of huber show that bees are affected by the smell of honey, and that the penetrating odour of fresh bee-poison will throw a whole hive into a state of commotion. he was of opinion that the impunity with which his assistant, francis burnens, performed his various operations on bees was due to the gentleness of his motions, and the habit of repressing his respiration, it being the odour transmitted by the breath to which the bees objected. sir john lubbock formed a little bridge of paper, and suspended over it a camel's-hair brush containing scent, and then put an ant at one end. she ran forward, but stopped dead short when she came to the scented brush. dr. mccook introduced a pellet of blotting-paper saturated with eau-de-cologne into the neighbourhood of some pavement-ants, who were engaged in a free fight. the effect was instantaneous; in a very few seconds the warriors had unclasped mandibles, relaxed their hold of their enemies' legs, antennæ, or bodies. the correct localization of the sense of smell has been a matter of difficulty. kirby and spence localized it at the extremity of the "nose," between it and the upper lip. that the nose, they naïvely remark, corresponds with the so-named part in mammalia, both from its situation and often from its form, must be evident to every one who looks at an insect. lehman, cuvier, and others, misled by the fact that the organ of smell is in us localized at the entrance of the air-track, supposed that at or near the spiracles of insects were the organs of smell. modern research tends more and more clearly to localize the sense of smell, as first suggested by réaumur, in the feelers or antennæ, and in some cases also in the palps. if the antennæ of a cockroach be extirpated or coated with paraffin, he no longer rushes to food, and takes little notice of, and will sometimes even walk over, blotting-paper moistened with turpentine or benzoline, which a normal insect cannot approach without agitation. there can be little doubt that it is by means of its large branching antennæ that the male emperor moth (_saturnia carpini_) is able to find its mate.[eu] if a collector take a virgin female into a locality frequented by these moths, he will soon be surrounded by twenty or thirty males; but if the moth be not a virgin, he will at most see one or two males. the sense of smell is thus delicate enough to distinguish the fertilized from the unfertilized female, and has associated with it a sense of direction by which the insect is guided to the right spot. carrion flies whose antennæ have been removed fail to discover putrid flesh; and e. hasse has observed that male humble-bees whose antennæ have been removed cannot discover the females. the sensory elements are lodged in pits or cones, which may be filled with liquid, peculiar sensory rods or hairs being associated with the nerve-endings. of these pits the queen-bee has, according to mr. cheshire, , the worker , and the drone nearly , , on each antennæ. on the antennæ of the male cockchafer, hauser estimates the number to be , . in the aquatic crayfish there are, besides the long antennæ, smaller antennules, each of which has two filaments, an inner and an outer. on the under surface of most of the joints of the outer filament there are two bunches of minute, curiously flattened organs, which were regarded by leydig, their discoverer, as olfactory. observation, too, seems to confirm the view that the sense of smell (or telæsthetic taste) is located in the antennule. i tried on a crayfish the following experiment: when it was at rest at the bottom of its tank, i allowed a current of pure water (the water in which it lived) to flow from a pipette over its antennæ and antennules. the antennæ moved slowly, but the antennules remained motionless. i then took some water in which a cod's head had been boiled, and allowed some of this to stream over the antennæ and antennules. the former moved slightly as before, but the antennules were thrown into a rapid up-and-down jerky vibration, and shortly afterwards the crayfish began moving about the bottom of its tank. if only one antennule be thus stimulated, or stimulated to a higher degree than the other, the crayfish seems generally (but not always) to turn to that side in search of food. mr. bateson[ev] has shown to how large an extent shrimps and prawns seek their food by smell, and states that a prawn, though blind, will often find his way back to his proper place, and stay in it. in the snail the anterior pair of "horns," or tentacles, are said to be olfactory. near the end of each is a large ganglion, or nerve-knot, from which fibres pass to the surface, in which there are said to be developed sensory knobs. snails, however, from which these tentacles have been removed are apparently still possessed of a sense of smell. certain lobed processes round the mouth have been regarded as the seat of olfactory sensation, but this is doubtful. in the foot of the snail, the part on which it glides, there is a hollow gland, and in this there are special cells, each of which gives off a delicate rod, enlarging at the free end into a ciliated knob. these are regarded as sensory and, it may be, olfactory. in shell-fish like the mussel, in which the water is sucked in by an inhalent tube or siphon, and ejected through an exhalent siphon above it (see fig. , p. ), there is at the entrance of the incoming current a thin layer of elongated cells which are described as olfactory, and are in association with a special ganglion. olfactory depressions have been described in some worms. but in a great number of the lower invertebrates very little or nothing is known concerning a sense of smell. * * * * * _hearing_ is a telæsthetic sense. through it we become aware of certain vibratory states of more or less distant objects. the vibrations of these bodies are transferred to the air or other medium surrounding the body, and are transmitted through the air or other medium to the ear. the sound-waves traverse the air at a rate of metres ( feet) in a second; but they travel about four times as fast in water. if the vibration is periodic or regular, the sound is called a tone; non-periodic or irregular sounds are noises. the pitch of a tone is determined by the number of vibrations in a second. the lowest or gravest tone most of us can hear is that where there are about vibrations in a second; twice this number give us a tone of an octave higher; twice this again, another octave; and so on. in musical composition, tones from about to about vibrations per second are employed. this is a range of somewhat over six octaves. but many of us are capable of hearing sounds over a range of about ten octaves, that is to say, from to , vibrations per second. the upper limit of hearing is, however, very variable. some people are deaf to tones of more than , or , vibrations per second.[ew] others may hear shrill tones of , , or even in rare cases , . i could as a boy hear the shrill squeak of a bat; now i am quite deaf to it. a friend of mine in south africa was unable to hear the piping of the frogs in the pond, which was to me so loud as almost to drown the tones of his voice. apart from the pitch of a note is its quality. the same note struck on different instruments or sung by different persons has a different ring. this is determined by the number and intensity of overtones, or partials, which are associated with the fundamental tone. suppose the deep fundamental tone of vibrations be sounded; with it there may be associated overtones, eight or nine in number, all of which are simple multiples (twice, thrice, four times, and so on) of the fundamental . the effects of these on the organ of hearing fuse or combine with the predominant effect of the fundamental tone. in harmonious chords, also, two or more fundamental tones, with their accompaniment of partials, blend in sensation so completely that it requires a keen musical ear and some training to analyze them into their component elements. the delicacy of discrimination of tones is greatest in the mid-region of hearing; and there is much individual variation in accuracy of ear. i have made experiments on many individuals to test their powers in this respect. i found some who were unable, in the mid-region of hearing, to state which was the higher of two notes sounded on a violin, the tones of which were separated by a major third, and in one case by a fifth. with notes on the piano the discrimination was more delicate, and yet more delicate when the notes were sung. in such cases tone-discrimination is deficient; and between these and the musician, who is stated to be able to distinguish tones separated by only / of a tone, there are many intermediate stages. it is beyond my purpose to describe, in more than a very general way, the nature of the auditory apparatus of man. the vibrations of the air are received by the drum-membrane, which lies in the auditory passage. from this it is transmitted, by a chain of small bones, to the inner auditory apparatus. this consists of two small membranous sacs, with one of which three membranous looped tubes, the semicircular canals, are connected; with the other is connected a spiral tube, the cochlear canal. these membranous sacs and canals are filled with fluid, and are surrounded by the fluid which fills the bony cavity in which they lie. this bony cavity has two little windows, one oval and the other round, across each of which a membrane is stretched. the oval membrane is in connection with the chain of auditory bones; and when this is made to vibrate in and out, the membrane of the round window vibrates out and in. thus the fluid around and within the membranous sacs and canals is set in vibration. and the parts are so arranged that the vibrations, in passing from the oval to the round membrane, must run up one side and down the other side of the cochlear canal. as they run down they set in vibration a delicate membrane which is supported on beautiful arched rods (the organs of corti). and this membrane contains a number of special hair-cells, so called because they bear minute hair-like structures. these are the special end-organs of hearing. it has been suggested that the fibres of the membrane on the arched rods, which are of different lengths and may be stretched with differing degrees of tension, respond to vibrations of different pitch. thus the hair-cells on that particular part of the membrane would be stimulated, and the note might be appreciated in its true position in the scale. we must now pass on to consider the sense of hearing in animals. that the mammalia have this sense well developed is a matter of familiar observation, and in some of them, such as the horse and the deer, it is exceedingly acute. the form and movements of the external ear also enable many of the mammalia to collect and attend to sounds from special directions. the mammalia possess also the power of tone-discrimination, as is shown by the fact that our domesticated animals recognize different modulations of the human voice, and that wild creatures distinguish tones or noises of different quality. a newfoundland dog, possessed by a friend of mine, always howled when the tenor d was struck on the piano, or sung. and théophile gautier reports that one of his cats could not endure the note g, and always put a reproving and silencing paw on the mouth of any one who sang it. in birds the sense of hearing is not only very sensitive, but the power of discrimination is exceedingly delicate. no one who has watched a thrush listening for worms can doubt that her ear is highly sensitive. the astonishing accuracy with which many birds imitate, not only the song of other birds, but such unwonted sounds as the clink of glasses or the ring of quoits, shows that the delicacy in discrimination has reached a high level of development. in birds, however, the cochlear canal has not the same development that it has in mammals, and there are no arched rods--no organs of corti. nothing special is to be noted concerning the sense of hearing in the reptiles, amphibia, and fishes. in all (with the exception of the lowly lancelet) the auditory organ is developed. we shall, however, presently see reason to question whether the possession of an "auditory organ," with well-developed semicircular canals, necessarily indicates the power of hearing. and mr. bateson's recent experiments at plymouth[ex] seem to indicate that fishes are not so sensitive in this respect as anglers[ey] are wont to believe. "the sound made by pebbles rattling inside an opaque glass tube does not attract or alarm pollack; neither are they affected by the sharp sound made by letting a hanging stone tap against an opaque glass plate standing vertically in the water." carp at potsdam are, indeed, said to come to be fed at the sound of a bell. but mr. bateson well remarks that this "can scarcely be taken to prove that the sound of the bell was heard by them, unless it be clearly proven that the person about to feed them was hidden from their sight." there is clearly room for further observation and experiment in this matter. turning to the invertebrata, we find, even in creatures as low down in the scale of life as jelly-fish, around the margin of the umbrella in certain medusa, simple auditory organs. in some cases they are pits containing otoliths (minute calcareous or other bodies, which are supposed to be set a-dance by the sound-vibrations); in others there is a closed sac with one or more otoliths; in others, again, they are modified tentacles, partially or completely enclosed in a hood. all these are generally regarded as auditory, there being specially modified cells of the nature of hair-cells. we shall see, however, that another interpretation of organs containing otoliths is at any rate possible. for the present, we will follow the usual interpretation, and regard them as auditory. vesicular organs containing otoliths are found near the cerebral ganglia in some of the worms and their relations. but the common earthworm, though it appears to be sensitive to sound, does not appear to have any such organs. molluscan shell-fish are generally provided with auditory organs. in the fresh-water mussel it is found in the muscular foot. it can be more readily seen in the _cyclas_, if the transparent foot of this small mollusc be examined under the microscope. it is a small sac containing an otolith. mr. bateson found that the mollusc _anomia_ "can be made to shut its shell by smearing the finger on the glass of the tank so as to make a creaking sound. the animals shut themselves thus when the object on which they were fixed was hung in the water by a thread." in the snail and its allies the auditory sac is found in close connection with the nerve-collar that surrounds the gullet. in the cuttle-fishes it is found embedded in the cartilage of the head. [illustration: fig. .--antennule of crayfish. i.j., inner joint; o.j., outer joint; ol., olfactory setæ; ol'., the same, enlarged; au.op., auditory opening in the basal division, which has been cut open to show au.s., the auditory sac; au.n., auditory nerve branching to the two ridges beset with auditory hairs; au.h., auditory hair, enlarged. (after howes.)] [illustration: fig. .--diagram of ear. t.m., tympanic membrane, to which is attached a chain of small bones stretching across the cavity of the drum, the innermost of which, st., fits into the "oval window." the vibrations are transmitted up one side and down the other side of the cochlear canal, c.c., and thus reach the "round window," f.r.; s.c. is one of the semicircular canals, the other two are omitted; e.t. is the eustachian tube connecting the cavity of the drum with the mouth-cavity.] in the lobster or crayfish the auditory organs are found at the base of the smaller feelers or antennules. they are little sacs formed by an infolding of the external integument (see fig. , p. ). beautifully feathered auditory hairs project into the sac along specialized ridges, and the sac in many cases contains grains of sand which play the part of otoliths. hensen seems to have proved that shrimps collect the grains of sand and place them in the auditory sac for this purpose. the curious shrimp-like _mysis_ has two beautiful auditory sacs in its tail. these are provided with auditory hairs. hensen watched these under the microscope while a musical scale was sounded, and found that the special hairs responded each to a certain note. when this particular note was sounded the hair was thrown into such violent vibration as to become invisible, but by other notes it was unaffected. [illustration: fig. .--tail of _mysis_. au., auditory organ.] [illustration: fig. .--leg of grasshopper. ty., tympanic membrane.] passing now to insects, we may first note that grasshoppers and crickets have an auditory organ on the front leg. these are provided with tympanic membranes, and the breathing-tubes, or tracheæ, are so arranged that the pressure of the air is equalized on the two sides of the membrane--just as in us and other vertebrates the same end is effected by a tube which runs from the interior of the drum of the ear to the mouth-cavity (see fig. ). in the organ within the leg there is a group of cells, followed by a row of similar cells which diminish regularly in size from above downwards. each is in connection with a nerve-fibril, and contains a delicate auditory rod. it has been suggested that the diminution in size of the cells may have reference to the appreciation of different notes, but nothing definite is known on the matter. ants, too, have an auditory organ, as shown by sir john lubbock, in the tibia of the front leg. but in locusts it is situated on the first segment of the abdomen. in flies there are a number of vesicles, generally regarded as auditory (but by some as olfactory), at the base of the rudimentary hind wings--the so-called halteres, or balancers. observation seems to point to the fact that in most insects the sense of hearing is lodged in the feelers, or antennæ. kirby made the following observation on a little moth: "i made," he says, "a quiet, not loud, but distinct noise; the antenna nearest to me immediately moved towards me. i repeated the noise at least a dozen times, and it was followed every time by the same motion of that organ, till at length the insect, being alarmed, became more agitated and violent in its motions." hicks wrote, in , "whoever has observed a tranquilly proceeding capricorn beetle which is suddenly surprised by a loud sound, will have seen how immovably outward it spreads its antennæ, and holds them porrect, as it were, with great attention, as long as it listens." the same observer described certain highly specialized organs in the antennæ of the hymenoptera (ants, bees, and wasps), which he thus describes: "they consist," he says, "of a small pit leading into a delicate tube, which, bending towards the base, dilates into an elongated sac having its end inverted." of these remarkable organs, sir john lubbock says there are about twelve in the terminal segment, and he has suggested that they may serve as microscopic stethoscopes. mayer, experimenting with the feathered antenna of the male mosquito, found that some of the hairs were thrown into vigorous vibration when a note with vibrations per second was sounded. and sir john lubbock, who quotes this observation, adds,[ez] "it is interesting that the hum of the female gnat corresponds nearly to this note, and would consequently set the hairs in vibration." the same writer continues, "moreover, those auditory hairs are most affected which are at right angles to the direction from which the sound comes. hence, from the position of the antennæ and the hairs, a sound would act most intensely if it is directly in front of the head. suppose, then, a male gnat hears the hum of a female at some distance. perhaps the sound affects one antenna more than the other. he turns his head until the two antennæ are equally affected, and is thus able to direct his flight straight towards the female." it is difficult to determine the range of hearing in the lower organisms. but it is quite possible, nay, very probable, that the superior limit of auditory sensation is much more extended in insects than it is in man. we know that many insects, such as the cicadas, the crickets and grasshoppers, many beetles, the death's-head moth, the death-watch, and others, make, in one way or another, sounds audible to us. but there may be many insect-sounds--we may not call them voices--which, though beyond our limits of hearing, are nevertheless audible to insects. at the other end of the scale, on the other hand, slow pulsations may be appreciated--for example, by aquatic creatures--by means of what we term the auditory organs, in a way that is not analogous to the sensation of sound in us. it may be noted that auditory organs are dotted about the body somewhat promiscuously in the various invertebrates. we have seen that auditory organs, or what are generally believed to be such, are found in the foot of bivalves, in the antennules of lobsters, in the fore legs of crickets and ants, in the abdomen of locusts, in the balancers of flies, and in the tail of _mysis_. but when we come to consider the matter, there is no reason why the organ of hearing should be in any special part of the body. the waves of sound rain in upon the organism from all sides. there is no great advantage in having the organs of hearing in the line of progression, as with sight, where the rays come in right lines; nor in having them in close association with the mouth, as in the case of the organ of smell. closely connected with the organ of hearing in vertebrates is the organ of another and but recently recognized sense. in briefly describing the auditory apparatus in man, mention was made of three curved membranous loops, the so-called semicircular canals. a few more words must now be said about them and the membranous sac with which they are connected. the sac lies in a somewhat irregular cavity in a bone at the side of the head, in the walls of which are five openings leading into curved tunnels in the bone in which lie the membranous loops. the planes in which the three semicircular canals lie are nearly at right angles to each other, and they are called respectively the horizontal, the superior, and the posterior. the two latter unite at one end before they reach the sac; hence there are five, and not six, openings into the cavity. at one end of each semicircular canal is a swelling, or ampulla, in each of which is a ridge, or crest, abundantly supplied with hair-cells. and in a little recess in the sac there is, occupying its floor, its front wall, and part of its outer wall, a patch of hair-cells covered by a gelatinous material with numerous small crystalline otoliths. the only other point that calls for notice is that the membranous sac does not fit closely in the bony cavity in which it lies, while the diameter of the membranous semicircular canals is considerably less than that of their bony tunnels, except at the ampullæ, or swellings, where they fit pretty closely. both the bony cavity and the membranous labyrinth (as it is called) are filled with fluid. from its close connection with the organ of hearing, this apparatus was for long regarded as in some way auditory in its function, and it was surmised that it enabled us to perceive the direction from which the sound came. but how it could do so was not clear. in m. flourens made the observation that the injury or division of a membranous canal gave rise in the patient to rotatory movements of the animal round an axis at right angles to the plane of the divided canal; and he, therefore, suggested that the canals might be concerned in the co-ordination of movement. they are now regarded as the organs of a sense of rotation or acceleration. that we have such a sense of rotation has been proved experimentally.[fa] let a man, blindfolded, sit on a smooth-running turn-table. when it begins to rotate he feels that he is being moved round, but if the rotation be continued at the same rate, this feeling quickly dies away. if the rotation be increased, he again feels as if he were being moved round, but this again soon dies away. further increase gives a fresh sensation, which in turn subsides, and the man may then be spinning round rapidly, and be perfectly unconscious of the fact. he is only aware that he has been gently turned round a little two or three times. now let the speed of rotation be slackened. he has a sensation of being gently turned round a little in the opposite direction. each time the speed is lessened he has this sense of being turned the reverse way. from these experiments we see that what we are conscious of is change of rate of rotation, or, in technical language, acceleration, positive or negative. [illustration: fig. .--diagram of semicircular canals. a. bony labyrinth of human ear (after sömmering). c, c., the cochlea; s.c., superior semicircular canal; p.c., posterior semicircular canal; h.c., horizontal semicircular canal; a, a, a, their swellings, or ampullæ; f.o., f.r., fenestra ovalis and rotunda (oval and round windows) in the vestibule. b. diagram of semicircular canal to illustrate effect of rotation. the large arrows indicate the direction of the rotation. the small arrow to the left indicates the resulting flow of the inner fluid into the ampulla; that to the right, the flow of the outer fluid into the vestibule.] from professor crum brown's paper in_ nature_ i transcribe, with some verbal modifications, his account of how the semicircular canals enable us to feel these changes of motion. let us consider the action of one canal. if the head be rotated about a line at right angles to the plane of the canal, with the ampulla leading, there will be a tendency for the fluid within the sac to flow into the ampulla, and for the fluid around the semicircular canal to flow into the cavity in which the sac lies. these movements will conspire to stretch the membranous ampulla, and thus to stimulate the hair-cells. this stretching will not take place in that canal if the rotation be in the reverse direction. but on the opposite side of the head is another canal in the same plane, but turned the other way. in the reversed rotation the ampulla in this canal will lead, and its hair-cells will be stimulated. thus by means of the two canals on either side of the head in the same plane, rotation in either direction can be appreciated. and since there are two other pairs of semicircular canals in two other planes, rotation in any direction will be recognized by means of one or more of the six canals. it is thus by means of the semicircular canals that we can appreciate acceleration of rotatory motion.[fb] but we can also appreciate acceleration of movements of translation--forwards or backwards, up or down. and professor mach has suggested that it is through the stimulation of the hair-cells in the patch in the sac itself (the so-called _macula acustica_) that we are able to appreciate these changes. the otoliths, held loosely and lightly in position by the gelatinous substance in which they are embedded, may, through their inertia, aid in the stimulation of the sense-hairs. and this naturally suggests the question whether those sense-organs in the invertebrates which contain otoliths may not be regarded with more probability as organs for the appreciation of changes of motion than as auditory organs. this for some years has been my own belief. i have always felt a difficulty in understanding how the otoliths are set a-dance by auditory vibrations. but their inertia would materially aid in the appreciation of changes of motion. in some forms the otoliths are held in suspension in a gelatinous material. in others--the molluscs, for example--the otolith (which is generally single) is retained in a free position by ciliary action. in aquatic creatures an organ for the appreciation of changes of motion might be of more service than an auditory organ. and if one be permitted to speculate, one may surmise that the sense of hearing may be a refinement of the sense through which changes of motion are appreciated. first would come a sense of movements of the organism in the medium through the stimulation of the sense-hairs by the relative motion of the otolith; then these sense-hairs, with increased delicacy, might appreciate shocks in the medium; and, eventually, those more delicate shocks which we know as auditory waves. in this way we might account for the fact that in the vertebrates the same organ, through different parts of its structure, appreciates both change of motion and auditory vibrations. and thus the organs in the invertebrata which are generally regarded as auditory, and for which has been suggested the function of reacting to changes of motion, may, in truth, subserve both purposes--may be organs in which the differentiation i have hinted at is taking place. * * * * * _sight_, like hearing, is a telæsthetic sense. through it we become aware of certain vibratory states of more or less distant objects. the medium by means of which these vibrations are transmitted is not, as in the case of hearing, the air, but the æther which pervades all space. the rate of transmission is about , miles in a second. that which answers in vision to pitch in hearing is colour. the lowest, or gravest, light-tone to which we are sensitive is deep red, where the number of vibrations per second is about billions ( , , , , ). the highest, or most acute, light-tone is violet, with about billion vibrations in a second. if white light be passed through a prism, the rays are classified according to their vibration-periods, and are spread out in a spectrum, or band of rainbow colours. but different individuals vary, as we shall presently see, in their sensibility to the lowest and the highest vibrations. some people are, moreover, relatively or absolutely insensible to certain colours, generally either red or green. such persons are said to be colour-blind. when the rainbow colours are combined in due proportion, or when pairs or sets of them are combined in certain ways, white light is produced. we saw that in the case of sound-waves, when the number of vibrations in a second is doubled, the sound is raised in pitch by an octave. using this term in an analogous way for colour-tones, we may say the range in average vision is about one octave--that is, from about billion to about billion vibrations in a second. but, though these are the limits in human vision, we know of the existence of many octaves of radiant energy physically in continuity with the light-vibrations. photography has made us acquainted with ultra-violet vibrations up to about billions per second--an octave above the violet. and professor langley's observations with the bolometer indicate the existence of waves with as low a vibration-period as one billion per second, and even here, in all probability, the limit has not been reached. to the vibrations more rapid than those that are concerned in the sensation of violet, the human organism is apparently in no manner sensitive. but to infra-red vibrations down to about thirty billions per second the nerves of the skin respond through the temperature-sense. we shall have to return to these limits of sensation at the close of this chapter. [illustration: fig. .--the human eye. horizontal section, to show general structure.] [illustration: fig. .--retina of the eye. enlarged section of minute fragment. b., back of retina next the outer coat; l.r.c., layer of rods and cones; i.l., intermediate layers; l.g.c., layer of ganglion-cells; l.n.f., layer of nerve-fibres; f., front of retina, the surface turned towards the pupil.] the human eye is a nearly spherical organ, capable of tolerably free movements of rotation in its socket. what we may call the outer case, which is white and opaque elsewhere, is quite transparent in front. through this transparent window may be seen the coloured iris, in the centre of which is a circular aperture, the pupil. the size of the pupil changes with the amount of light--it dilates or contracts, according as the light is less or more intense. just behind it, and still in the front part of the eye, is the transparent lens, the convexity of the anterior surface of which can be altered in the accommodation of the organ for near or far vision. the space between the lens and iris and the corneal window of the eye is filled with a watery fluid. behind the lens there is a transparent, semi-fluid, jelly-like material, filling the rest of the chamber of the eye. at the back of the eye is spread out the sensitive membrane--the retina. the structure of this membrane is very complicated, and cannot be described here. it is, however, indicated in fig. . for our present purpose it is sufficient to note that here are the end-organs of the optic nerve; that these consist of a number of delicate rods and cones; and that these rods and cones do not face in the direction from which the light comes, but face towards the back of the eyeball, where a pigmented substance is developed. the rays of light are thus focussed through the retina on to this pigmented substance; the ends of the rods and cones are stimulated; and the stimulation is handed on, augmented in certain intermediate ganglia, to the delicate transparent nerve-fibres in the front of the retina. these collect to a certain spot, where they pass through the retina to form the optic nerve. where they pass through the retina there can, of course, be no rods and cones. and in this spot there is no power of vision. it is the blind spot. the reality of its existence can easily be proved. make a dot on a piece of writing-paper, and about three inches to the left of it place a threepenny or sixpenny bit. close the right eye, and look with the left eye at the dot. the sixpenny bit will also be seen, but not distinctly. keep the eye fixed on the dot, and move the head slowly away from the paper. at a distance of about ten inches the coin will completely disappear from view. its image then falls on the blind spot. the organ of vision, then, in us consists of an essential sensory membrane, the retina, with its delicate rods and cones; and an accessory apparatus for focussing an inverted image on to the sensitive surface of the retina. the surface is not, however, equally sensitive, or, in any case, does not give an equal power of discrimination, throughout its whole extent. this is seen in the experiment above described. when we look at the dot we see the coin, but not distinctly. the area of clear and distinct vision is, in fact, very small, constituting the yellow spot about / of an inch ( millimetres) long, and / of an inch (. millimetre) broad. and even within this small area there is a still more restricted area of most acute sensibility only / of an inch (. millimetre) in diameter. nevertheless, within this minute area there are some two thousand cones, the rods being here absent. in carefully examining an object we allow this area of acute vision to range over it. hence the extreme value of that delicate mobility which the eye possesses--a mobility that is accompanied by muscular sensations of great nicety. we saw that the sense of touch in the tongue is sufficiently delicate to enable us to recognize, as two, points of contact separated by / of an inch ( . millimetre). what, in similar terms, is the delicacy of sight? at what distance apart, on the most delicate part of the retina, can two points of stimulation be recognized as distinct from each other? if the points of stimulation be not less than / of an inch (. millimetre) apart, they can be distinguished as two. below this they fuse into one. the diameter of the end of a single cone in the yellow spot is also about / of an inch (. millimetre). with regard to the mode in which the stimulation of the retinal elements is effected, we have no complete knowledge. certain observations of boll and kühne, however, show that when an animal is killed in the dark the retina has a peculiar purple colour which is at once destroyed if the retina be exposed to light. if a rabbit be killed at the moment when the image, say, of a window, is formed on the retina, and the membrane at once plunged in a solution of alum, the image may be fixed, and an "optogram" of the window may be seen on the retina. the discharge of the colour of the retinal purple may be regarded as the sign of a chemical change effected by the impact of the light-vibrations. but in the yellow spot there seems to be no visual purple. it is, indeed, developed only in the rods, not in the cones. here, probably, chemical or metabolic changes occur without the obvious sign of the bleaching of retinal purple. in the dusk-loving owl the retinal purple is well developed, but in the bat it is said to be absent. we saw that in the case of hearing the auditory organ is fitted to respond to air-borne vibrations varying from about thirty to thirty thousand per second. and though the details of the process are at present not well understood, it is believed that certain parts of the recipient surface are fitted to respond to low tones, other parts to intermediate tones, and yet others to high tones. thus the reception is serial. if there be two pianos near each other, accurately in tune, any note struck on one will set the corresponding note vibrating in the other.[fc] the auditory organ may be likened to this second piano. special parts respond to special tones. now, in the case of vision, the conditions are different. the reception cannot be serial. as i range my eye over a flower-bed, i bring the area of distinct vision on to a number of different colours, and these are seen to be distinct, though they are received on the same part of the retinal surface. it might, perhaps, be suggested that special cones were set apart for each shade of colour. but there are only some two thousand cones in the central area of most acute vision, and lyons silk-manufacturers prepare pattern cards containing as many shades of coloured silks. so that there would be only one cone to each colour. and herschel thought that the workers on the mosaics of the vatican could distinguish at least thirty thousand different shades of colour! there are also many phenomena of colour-blending which show that colour-reception cannot in any sense be serial. how, then, are we to account for our wide range of colour-sensation? just as the blending by the artist on his palette of a limited number of pigments gives him the wide range of colour seen on his canvas, so the blending of a few colour-tones may give us the many shades we are able to distinguish. the smallest number of fundamental colour-tones which will fairly well account for the phenomena of colour-vision, is three. and these three are red, green, and blue or violet. these are the three so-called primary colours. all others are produced from these elements by blending. to explain our ability to appreciate differences of colour, then, it is supposed, on the hypothesis of young and von helmholtz, that three kinds of nerve-fibres exist in the retina, the stimulation of which gives respectively, red, green, and violet in consciousness. professor mckendrick, interpreting von helmholtz, gives[fd] the following scheme:-- " . red excites strongly the fibres sensitive to red, and feebly the other two. " . yellow excites moderately the fibres sensitive to red and green, feebly the violet. " . green excites strongly the fibres sensitive to green, feebly the other two. " . blue excites moderately the fibres sensitive to green and violet, feebly the red. " . violet excites strongly the fibres sensitive to violet, feebly the other two. " . when the excitation is nearly equal for the three kinds of fibres, the sensation is white." this theory cannot be regarded as more than a provisional hypothesis. still, by its means we can explain many colour-phenomena. it is well known, for example, that if we gaze steadily at a red object, and then look aside at a grey surface, an after-image of the object will be seen of a blue colour. according to the theory, the red fibres have been tired and cannot so readily answer to stimulation. over this part of the retina, therefore, the effect of grey light is to stimulate normally the fibres sensitive to green and violet, but only slightly those sensitive to red, owing to their tired condition. the result will be, as we see from the above scheme ( ), the sensation of blue. colour-blind people, on this view, are those in whom one set of the fibres, generally the red or the green, are lacking or ill developed. we may, perhaps, with advantage restate this theory in terms of chemical change, or metabolism. on this view three kinds of "explosives" are developed in the retinal cones; for it is seemingly the cones, rather than the rods, which are concerned in colour-vision. all three explosive substances are unstable; but one, which we may call r., is especially unstable for the longer waves of the spectrum; another, g., for the waves of mid-period; a third, v., for those of smallest wave-length. suppose that r. only were developed. if, then, we were to look at a band of light spread out in spectrum wave-lengths, we should see a band[fe] of monochromatic _r_. light. its centre would be bright, and here would be the maximum instability of r. on either side it would fade away. the lateral edges of the spectrum would be the limits of the instability of r. if g. only were developed, we should see only a band of monochromatic _g._ light. its centre would not coincide with that for r., but would lie in a region of smaller wave-length. here would be the maximum instability for g. on either side the green would fade away. its lateral edges would mark the limits of the instability of g. but though their centres would not coincide, the r. band and the g. band would to a large extent overlap. similarly with the band for v. it, too, would have its centre of maximum instability and its lateral edges of lessening instability. its centre would lie in a region of yet smaller wave-length than that for g. and the _v._ band would overlap the green and the red. normally, all three bands are developed, and their blended overlapping gives the colours of the rainbow. for this reason the monochromatic bands _r._, _g._, and _v._ are unknown to us in experience. all the colour-tints we know are blended tints. what we call full-red light causes strong disruptive change in r., but decomposes slightly g., and probably also, but in much less degree, v. whether r., g., and v. are all three present in each cone, or whether they are each developed in separate cones, we do not know for certain. nor are we certain that there are separate nerve-fibres for the transmission of stimuli due to r., g., and v. when we look steadily at a red object we cause the disruption of r.; and since it takes some time for the reformation and reconstitution of this explosive substance, on turning the eye to a grey surface, g. and v. are alone, or in preponderating proportions, caused to undergo disruption. hence the phenomena of complementary after-images. it is not merely a matter of the tiring of certain nerve-fibres, but a using-up of the explosive material in certain of the cones. what is called _colour-blindness_ is probably due to one of several abnormal conditions. it is _possible_ that in some cases r., g., or v. may be entirely absent. more frequently they are in abnormal proportions. they probably vary in their sensitiveness, and not improbably in the wave-period to which they show the maximum response. to test the variation, if any, in the limits of instability for r. and v., or in any case in the limits of colour-vision at the red end and at the violet end of the spectrum, in apparently normal individuals, my friend and colleague, mr. a. p. chattock, made, at my suggestion, a number of observations on some of the students of the university college, bristol, to whom my best thanks are due for their kind willingness to be submitted to experiment. the instrument used[ff] was a single-prism spectro-goniometer. in the accompanying diagram (fig. ) the results of some of these observations are graphically shown. the middle part of the spectrum, between the wave-lengths and millionths of a millimetre, is omitted, only the red end and the violet end being shown. the observations on thirty-four individuals, seventeen men and seventeen women, all under thirty years of age, are given for both eyes. the left-hand vertical line of each pair stands for the right eye in each case. to the left of the table are placed the wave-lengths in millionths of a millimetre. [illustration: fig. .--variation in the limits of colour-vision.] take, for example, the first pair of vertical lines. the individual whose colour-range they represent could detect red light in the spectrum up to millionths of a millimetre wave-length for the right eye, and up to for the left; and could detect violet light down to and . beyond these limits all was dark. but the last individual in the series, while his range in the violet was about the same, could only detect red light up to and millionths of a millimetre. his spectrum was so much shorter. it is seen that there is more variation at the red end than at the violet end of the spectrum, and this notwithstanding that the violet rays are more spread out by the prism than the red rays. it is seen that the two eyes are often markedly different. this is not due to inaccuracy of observation, for certain individuals in which this occurred were tested several times with similar results. it is seen that the variations at the red end and the violet end are often independent, and that the absolute length of the visible spectrum differs in different individuals. the following table presents these observations and a few others in another light:-- table of maxima and minima in wave-lengths, expressed in millionths of a millimetre. ------------------------------------------------------------------------- | violet | red | |----------------------+----------------------| no. of |highest| mean | lowest|highest| mean |lowest|individuals |-------+------+-------+-------+-------+------+----------- women under | . | . | . | | . | | men " " | . | . | . | | . | | women over | . | . | . | | . | | men " " | . | . | . | | . | | ---------------------------------------+--------------------------------- n { right eye | | { left eye | | ------------------------------------------------------------------------- the individual n showed signs of colour-blindness, and is therefore not included in the table, but entered separately. he was unable to recognize the c line of the hydrogen spectrum (wave-length ), which was brilliantly obvious to the normal eye. these observations[fg] need further confirmation and extension. we intend to continue the investigation each session. they are, however, sufficient to show that in some individuals r. undergoes disruptive change on the impact of light-waves which have no noticeable effect on the retina of other individuals. it is impossible here to do more than just touch the fringe of the difficult subject of colour-vision. and the only further fact that can here be noticed is that trichromatic colour-vision is apparently in us limited to the yellow-spot and its immediate neighbourhood. around this is an area which is said to be bichromatic--all of us being, for this area, more or less green-blind. in the peripheral area around this, colour is indistinguishable, and we are only sensitive to light and shade. so far as the structure of the retina is concerned, we may notice in this connection that in the central region of most complete trichromatic vision there are cones only; around the yellow spot each cone is surrounded by a circle of rods; and further out into the peripheral region by two, three, or more circles of rods. concerning the sense of sight in the lower mammals little need be said. in many cases the acuteness of vision is remarkable. mr. romanes's experiments on sally, the bald-headed chimpanzee at regent's park, led him to conclude that she was colour-blind, but i question whether the experiments described quite justify this conclusion. sir john lubbock was unable to teach his intelligent dog van to distinguish between coloured cards; but the failure was as complete when the cards were marked respectively with one, two, or three dark bands. we are not justified, therefore, in ascribing the failure to colour-blindness. the real failure, probably, was in each case to make the animal understand what was wanted. bulls are, at any rate, credited with strong colour-antipathies, and insect-eating mammals are probably not defective in the colour-sense. it is said that nocturnal animals, such as mice, bats, and hedgehogs, have no retinal cones; and if the cones are associated with colour-vision, they may not improbably be unable to distinguish colours. some moles are blind (e.g. the cape golden mole). but the common european mole, though the eyes are exceedingly minute ( / of an inch in diameter), has the organ fairly developed, and is even said not to be very short-sighted. it is protected by long hairs when the animal is burrowing, and is only used when it comes to the surface of the ground. it is probably in birds that vision reaches its maximum of acuteness. a tame jackdaw will show signs of uneasiness when seemingly nothing is visible in the sky. presently, far up, a mere speck in the blue, a hawk will come within the range of far-sighted human vision. steadily watch the speck as the hawk soars past, until it ceases to be visible; the jackdaw will still keep casting his eye anxiously upward for some little time. he may be only watching for the possible reappearance of the hawk. but just as he saw it before man could see it, so probably he still watches it after, to man's sight, it has become invisible. so, too, for nearer minute objects, the swift, as it wheels through the summer air, presumably sees the minute insects which constitute its food. and every one must have noticed how domestic fowls will pick out from among the sand-grains almost infinitesimal crumbs. it is probable that the area of acute vision is much more widely diffused over the retina of birds than it is with us. in any case, the cones are more uniformly and more abundantly distributed over the general retinal surface. an exceedingly interesting and important peculiarity in the retina of birds, which they share with some reptiles and fishes, is the development, in the cones, of coloured globules. "the retinæ of many birds, especially of the finch, the pigeon, and the domestic fowl, have been carefully examined by dr. waelchli, who finds that near the centre green is the predominant colour of the cones, while among the green cones red and orange ones are somewhat sparingly interspersed, and are nearly always arranged alternately--a red cone between two orange ones, and _vice versâ_. in a surrounding portion, called by dr. waelchli the red zone, the red and orange cones are arranged in chains, and are larger and more numerous than near the yellow spot; the green ones are of smaller size, and fill up the interspaces. near the periphery the cones are scattered, the three colours about equally numerous and of equal size, while a few colourless cones are also seen. dr. waelchli examined the optical properties of the coloured cones by means of the micro-spectroscope, and found, as the colours would lead us to suppose, that they transmitted only the corresponding portions of the spectrum; and it would almost seem, excepting for the few colourless cones at the peripheral part of the retina, that the birds examined must have been unable to see blue, the whole of which would be absorbed by their colour-globules."[fh] these facts are of exceeding interest. they seem to show that for these birds the retinal explosives are not the same as for us. they are r., o., and g. moreover, the colour-globules will have the effect of excluding the phenomena of overlapping. for each kind of cone the spectrum must be limited to the narrow spectral band transmissible through the associated colour-globule. if these facts be so, it is not too much to say that the colour-vision of birds must be so utterly different from that of human beings, that, being human beings, we are and must remain unable to conceive its nature. the factors being different, and the blending of the factors by overlap being, by specially developed structures, lessened or excluded, the whole set of resulting phenomena must be different from ours. and this is a fact of the utmost importance when we consider the phenomena of sexual selection among birds, and those theories of coloration in insects which involve a colour-sense in birds. concerning the sense of sight in reptiles and in amphibians, little need here be said. at near distances some of them undoubtedly have great accuracy of vision. this is, perhaps, best seen in the chamæleon. in this curious animal the eyes are conical, and each moves freely, independently of the other. the eyelids encase the organ, except for a minute opening, looking like a small ink-spot at the blunted apex of the cone. the animal catches the insects on which it feeds by darting on to them its long elastic tongue and slinging them back into the mouth, glued to its sticky tip. its aim is unerring, but it never strikes until both eyes come to rest on the prey, and great accuracy of vision must accompany the great accuracy of aim. frogs and toads capture their prey in a somewhat similar way; and a great number of reptiles and amphibians are absolutely dependent for their subsistence on the acuteness and accuracy of their vision, which is, however, on the whole, markedly inferior to that of birds. in fishes, from their aquatic habit, the lens and dioptric apparatus are specially modified, in accordance with the denser medium in which they live; and one curious fish, the surinam sprat, is stated to have the upper part of the lens suited for aerial, and the lower part for aquatic vision. mr. bateson[fi] has made some interesting observations on the sense of sight in fishes. he finds that in the great majority of fishes the shape and size of the pupil do not alter materially in accordance with the intensity of the light. the chief exceptions are among the elasmobranchs (dog-fishes and skates). in the torpedo the lower limb of the iris rises so as almost to close the pupil, leaving a horizontal slit at the upper part of the eye. in the rough dog-fish, the angel-fish, and the nurse-hound, the pupil closes by day, forming merely an oblique slit. in the skate a fern-like process descends from the upper limb of the iris. the contraction in these cases does not seem to take place rapidly as in land vertebrates, but slowly and gradually. among diurnal fishes belonging to the group of the bony fishes (teleosteans), the turbot, the brill, and the weever have a semicircular flap from the upper edge of the iris, which partially covers the pupil by day, but is almost wholly retracted at night. none of the fishes observed by mr. bateson appears to distinguish food (worms) at a greater horizontal distance than about four feet, and for most of them the vertical limit seemed to be about three feet; but the plaice at the bottom of the tank perceived worms when at the surface of the water, being about five feet above them. most of them exhibited little power of seeing an object below them. but though the distance of clear vision seems to be so short for small objects in the water, many of these fish (plaice, mullet, bream) notice a man on the other side of the room, distant about fifteen feet from the window of the tank. the sight of some fishes, such as the wrasses (_labridæ_), is admirably adapted for vision at very close quarters. "i have often seen," says mr. bateson, "a large wrasse search the sand for shrimps, turning sideways, and looking with either eye independently, like a chamæleon. its vision is so good that it can see a shrimp with certainty when the whole body is buried in grey sand excepting the antennæ and antenna-plates. it should be borne in mind that, if the sand be fine, a shrimp will bury itself absolutely, digging with its swimmerets, kicking the sand forwards with its chelæ, finally raking the sand over its back, and gently levelling it with its antennæ; but if the least bit be exposed, the wrasses will find it in spite of its protective coloration." * * * * * [illustration: fig. .--pineal eye. modified eye-scale of a small lizard, _varanus benekalensis_. (after baldwin spencer.)] although it is probably not functional in any existing form, mention must here be made of the median or pineal eye. on the head of the common slow-worm, or blind-worm, there is a dark patch surrounding a brighter spot. this is the remnant of a median eye. it has been found in varying states of degeneration in many reptiles (fig. ), and in a yet more vestigial form in some fishes and amphibia. it is connected with a curious structure, associated with the brain of all vertebrates, and called the pineal gland. descartes thought that this was the seat of the soul; but modern investigation shows it to be a structure which has resulted from the degeneration of that part of the brain which was connected with the median eye. there is some reason to suppose that, in ancient life-forms, like the ichthyosaurus, and plesiosaurus, and the labyrinthodont amphibians, it was large and functional. in any case, there is a large hole in the skull (fig. ) through which the nervous connection with the brain may have been established. the structure of the eye is not similar to that of the lateral eye, but more like that of some of the invertebrates. to these invertebrates we must now turn. [illustration: fig. .--skull of _melanerpeton_. a labyrinthodont amphibian from the permian of bohemia (after fritsch). Ã� . pa., the parietal foramen.] * * * * * insects have eyes of two kinds. if we examine with a lens the head of a bee, we shall see, on either side, the large compound or facetted eye; but in addition to these there is on the forehead or vertex a triangle of three small, bright, simple eyes, or ocelli. these ocelli, or eyelets, differ, in different insects, as to the details of their structure; but in general they consist of a lens produced by the thickening of the integumentary layer which is at the same time rendered transparent. behind this lies the so-called vitreous body, composed of transparent cells, and then follows the retina, in which there are a number of rods, the recipient ends of which are turned towards the rays of light, and not away from them as in the vertebrate. spiders have from six to eight ocelli, arranged in a pattern on the top of the head. facetted eyes are not found in them. [illustration: fig. .--eyes and eyelets of bee. a. drone. b. worker.] these facetted eyes, which are found in both insects and crustacea, have apparently a more complex structure than the ocelli. externally--in the bee, for example--the surface is seen to be divided up into a great number of hexagonal areas, each of which is called a facet, and forms (in some insects, but not in all) a little lens. of these the queen bee has on each side nearly five thousand; the worker some six thousand; and the drone upwards of twelve thousand; while a dragon-fly (_Ã�schna_) is stated to have twenty thousand. beneath each facet (in transverse section, fig. ) is a crystalline cone, its base applied to the lens, its apex embraced by a group of elongated cells, in the midst of which is a nerve-rod which is stated to be in direct connection with the fibres of the optic nerve. dark pigment is developed around the crystalline cones. and retinal purple is said to be present in the cells which underlie it. with regard to these facetted eyes there has been much discussion. the question is--is each facetted organ an eye, or is it an aggregate of eyes? to this question the older naturalists answered confidently--an aggregate. a simple experiment seems to warrant this conclusion. if the facetted surface be cleared of its internal structures (the crystalline cones, etc.) and placed under the microscope, each lens may, at a suitable distance of the object-glass, be made to give a separate image of such an object as a candle reflected in the mirror of the microscope. if each lens thus gives an image, is not each the focussing apparatus of a single eye? but a somewhat more difficult experiment points in another direction. if the facetted cornea be removed _with the crystalline cones still attached_ (grenacher was able to do it with a moth's eye), and placed under the microscope, when the instrument is focussed at the point of the cone (where the nerve-rod comes), a spot of light, and not an image, is seen. no image can be seen unless the microscope be focussed for the centre of the cone; and here there are no structures capable of receiving it and transmitting corresponding waves of change to the "brain." [illustration: fig. .--eye of fly. transverse section through head. (after hickson.)] but what, it may be asked, can be the purpose of an eye-structure which gives, not an image, but merely a spot of light? the answer to this question can only be found when it is remembered that there are thousands of these facets and cones giving thousands of spots of light. the somewhat divergent cones and facets of the insect's eye (fig. ) embrace, as a whole, an extended field of vision; each has its special point in that field; and each conveys to the nerve-rod which lies beneath it a stimulation in accordance with the brightness, or intensity, or quality of that special point of the field to which it is directed. the external field of vision is thus reproduced in miniature mosaic at the points of the crystalline cones--thus there is produced by the juxtaposition of contiguous points a stippled image. and it must be remembered that, even in human vision, the stimulation is not that of a continuum, but is stippled with the fine stippling of the ends of the rods and cones. in insect-vision the stippling is far coarser, and the image is produced on different principles. [illustration: fig. .--diagram of mosaic vision.] in the vertebrate the image is produced by a lens; in the insect's eye, by the elongated cones. how this is effected will be readily seen with the aid of the diagram. at _a b_ are a number of transparent rods, separated by pigmented material absorbent of light. they represent the crystalline cones. at _c d_ is an arrow placed in front of them; at _e f_ is a screen placed behind them. rays of light start in all directions from any point, _c_, of the arrow; but of these only that which passes straight down one of the transparent rods reaches the screen. those which pass obliquely into other rods are absorbed by the pigmented material. similarly with rays starting from any other point of the arrow. only those which, in each case, pass straight down one of the rods reach the screen. thus there is produced a reduced stippled image, _c' d'_, of the arrow. there has been a good deal of discussion as to the relative functions of the ocelli and the facetted eyes of insects. the view generally held is that the ocelli are specially useful in dark places and for near vision; while the facetted eyes are for more distant sight and for the ascertainment of space-relations. how the two sets of impressions are correlated and co-ordinated in insect-consciousness, who can say?[fj] the interesting observations of sir john lubbock seem to show that insects can distinguish between different colours. "amongst other experiments," he says,[fk] "i brought a bee to some honey which i placed on a slip of glass laid on blue paper, and about three feet off i placed a similar drop of honey on orange paper. with a drop of honey before her a bee takes two or three minutes to fill herself, then flies away, stores up the honey, and returns for more. my hives were about two hundred yards from the window, and the bees were absent about three minutes or even less. after the bee had returned twice, i transposed the papers; but she returned to the honey on the blue paper. i allowed her to continue this for some time, and then again transposed the papers. she returned to the old spot, and was just going to alight, when she observed the change of colour, pulled herself up, and without a moment's hesitation darted off to the blue. no one who saw her at that moment could have the slightest doubt about her perceiving the difference between the two colours." passing now to the crustacea, we find in them eyes of the same type as in insects; but in the higher crustacea ocelli are absent. in the crabs and lobsters the eyes are seated on little movable pedestals; in the former the crystalline cones are very long, in the latter they are short. there can be little doubt that vision is by no means wanting in acuteness in an animal which, like the lobster, can dart into a small hole in the rocks with unerring aim from a considerable distance. the experiments of sir john lubbock have shown that the little water-flea (_daphnia_) can distinguish differences of colour, yellows and greens being preferred to blues or reds. among the molluscs there are great differences in the power of sight. most bivalves, like the mussel, are blind. interesting stages in the development of the eye may be seen in such forms as the limpet, _trochus_ and _murex_. the limpet has simply an optic pit, the _trochus_ a pit nearly closed at the orifice and filled with a vitreous mass, and the _murex_ a spherical organ completely closed in with a definite lens. the snail has a well-developed eye on the hinder and longer horn or tentacle. but it does not seem to be aware of the presence of an object until it is brought within a quarter of an inch or less of the tentacle. in all probability the eye does little more than enable the snail to distinguish between light and dark. and the same may be said of the eye of many of the molluscs. in some, however, the cuttle-fishes and their allies, the eye is so highly developed that it has been compared with that of the vertebrate. there is an iris with a contractile pupil. and the ganglion with which it is connected forms a large part of the so-called brain. the powers of accurate vision in these higher forms are probably considerable. it is interesting to note that whereas in the cuttle-fishes and most molluscs, the rods of the retina are turned towards the light, in _pecten_, _onchidium_ (a kind of slug), and some others, they are, as in vertebrates, turned from the light. in _pecten_ the nerve to supply the retina bends round its edge at one side. but in _onchidium_ it pierces the retina as in vertebrates. in worms, eyes are sometimes present, sometimes absent. in star-fishes and their allies they often occur. in medusæ (jelly-fish) they are sometimes found on the margin of the umbrella. even in lowly organisms, like the infusoria, eye-spots not unfrequently occur. we must remember, however, that, in these lower forms of life, the organs spoken of as eyes or eye-spots merely enable the possessor to distinguish light from darkness. even when eyes or eye-spots are not developed, the organism seems to be in some cases sensitive to light--using the word "sensitive," once more, in its merely physical acceptation. the earthworm, for example, though it has no eyes, is distinctly sensitive to light; and the same has been shown to be the case with other eyeless organisms. graber holds that his experiments demonstrate that the eyeless earthworm can distinguish between different colours--in other words, is differentially sensitive to light-waves of different vibration-period--preferring red to blue or green, and green to blue. and the same observer has shown that animals provided with eyes--the newt, for example--can distinguish between light and darkness by the general surface of the skin. m. dubois, by a number of experiments on the blind _proteus_ of the grottoes of carniola, has shown that the sensitiveness of its skin to light is about half that of its rudimentary eyes; and, further, that this sensibility varies with the colour of the light employed, being greatest for yellow light.[fl] we have not been able to do more than make a rapid survey of the sense of sight as it seems to be developed in the invertebrates and lower animals. the visual organs differ, not only in structure, but in principle. we may, i think, distinguish four types. . organs for the mere appreciation of light or darkness (shadow), exemplified by pigment-spots, with or without concentrating apparatus. . organs for the appreciation of the direction of light or shadow, with or without a lens. the simple retinal eyes of gasteropods, and perhaps in some cases the ocelli of insects, probably belong to this class. . true eyes, or organs in which a retinal image is formed, through the instrumentality of a lens, as in vertebrates and cephalopods. . the facetted eyes of insects, in which a stippled image is formed, on the principle of mosaic vision. unfortunately, all these are called indiscriminately eyes, or organs of vision. an infusorian or a snail is said to see. but the terms "eye," "vision," "sight," imply that final excellence to which only the higher animals, each on its own line, have attained. this final excellence probably has its basis and earliest inception in the fact that the functional activity of protoplasm is heightened in the presence of ætherial vibrations. if, then, we imagine, as a starting-point, a primitive transparent organism with a general susceptibility to the influence of light-vibrations, the formation within its tissues of pigment-granules absorbent of light will render the spots where they occur specially sensitive to the ætherial vibrations. special refraction-globules would also act as minute lenses, focussing the light, and thus concentrating it upon certain spots. [illustration: fig. .--direction-retina. simple retina for distinguishing the direction of the source of light or of shadow.] in many of the lower animals we find such organs, belonging to our first category, and constituting either eye-spots of pigmented material or simple lenses covering a pigmented area. if we call these eyes, we must remember that in all probability they have no power of what we call vision--only a power of distinguishing light from dark. where, however, there exists beneath the lens a so-called retina, that is, a layer of rod-like endings of a nerve, it might, at first sight, be thought that there, at any rate, we have true vision. but in all probability, in a great number of cases the retinal rods are simply for the purpose of rendering the organism sensitive, not only to the presence of light, but to its direction. light straight ahead (_a_) stimulates the middle rods; from one side (_b_, _c_) it is focussed on the rods of the opposite side of the retina; and similarly for intermediate positions. the presence of a retinal layer is thus no infallible sign of a power of vision as apart from mere sensibility to light. indeed, in a great number of cases, from the convexity and position of the lens, the formation of an image is impossible. only when it can be shown that a more or less definite image can be focussed on the retina, or can be formed on the principle of mosaic vision, can we justly surmise that a power of true vision is present. i doubt whether this can be shown to be unquestionably the case in any forms but the higher arthropods, the cuttle-fishes and their allies, and the vertebrates. there is one more point for consideration before we leave the sense of sight--are the limits of vision the same in the lower forms of life as they are in man? or, to put the question in a more satisfactory form--are the limits of sensibility to light-vibrations the same in them as in us? m. paul bert concluded that they are. but sir john lubbock has, i think, conclusively shown that they are not. for the full evidence the reader is referred to his "senses of animals."[fm] his experiments on ants, with which those of m. forel are in complete accordance, satisfied him that these little animals are sensitive to the ultra-violet rays which lie beyond the range of our vision. other experiments with fresh-water fleas (_daphnia_) showed that they have colour-preferences, green and yellow being the favourite colours. the daphnias were placed in a shallow wooden trough, divided by movable partitions of glass into divisions. over this was thrown a spectrum of rainbow colours. the partitions were removed, and the daphnias allowed to collect in the differently illuminated parts of the trough. the partitions were then inserted, and the number of crustaceans in each division counted. the following numbers resulted from five such experiments:-- dark. violet. blue. green. yellow. red. special experiments seem to show that their limits of vision at the red end of the spectrum coincide approximately with ours; but at the violet end their spectrum is longer than ours. sir john covered up the visible spectrum, so as to render it dark, and gave the daphnias the option of collecting in this dark space or in the ultra-violet. to human eyes both were alike dark. but not so to the daphnian eye; for while only collected in the covered part, were found in the ultra-violet. the width of the violet visible to man was two inches. sir john divided the ultra-violet into three spaces of two inches each. of the daphnias, were in the space nearest the violet, in the next space, and none in the furthest of the three spaces. from which it would seem that, though these little creatures are sensitive to light of higher vibration-period than that which affects the human eye, their limits do not very far exceed ours. we have seen that human beings differ not a little in their limits of violet-susceptibility. we may presume that sir john lubbock and those who assisted him in these experiments were normal in this respect. but it is possible that some individuals could have perceived a faint purple where there was darkness to them, and that the majority of the daphnias were collected in the region just beyond the partition between ultra-violet and darkened violet. still, there is no cause for doubting the general conclusion that daphnias are sensible to ultra-violet rays beyond the limits of human vision. * * * * * [illustration: fig. .--antennary structures of hymenoptera. (after lubbock.) a., cuticle; b., hypodermis; c., ordinary hair; d., tactile hair; e., cone; f., depressed hair lying over g. cup with rudimentary hair at the base; h., simple cup; i., champagne-cork-like organ of forel; k., flask-like organ; l., papilla, with a rudimentary hair at the apex.] sir john lubbock has an interesting chapter on problematical organs of sense. in the antennæ of ants and bees there are modified hairs and pits in the integument (at least eight different types, according to sir john lubbock), the sensory nature of which is undoubted. but what the sensory nature in each case may be is more or less problematical. many worms have sense-hairs or bristles of the use of which we are ignorant. some organs described as tactile or olfactory in the lower invertebrates are so described on a somewhat slender basis of evidence. the sense-value of the bright marginal beads of sea-anemones is unknown. even in animals as high in the scale of life as fishes, there is a complete set of sense-organs--the muciparous canals, in the head and along the lateral line down the side, the function of which we can only guess. by some they are regarded as olfactory; by others, as fitted to respond to vibrations or shocks of greater wave-length than the auditory organ can appreciate; by others, as of importance for the equilibration or balancing of the fish. it will thus be seen that, apart from the possibility of unknown receptive organs as completely hidden from anatomical and microscopic scrutiny as the end-organs of our temperature-sense, there are in the lower animals organs which may be fitted to receive modes of influence to which we human folk are not attuned. and what are the physical possibilities? we have seen that, through the telæsthetic senses--hearing, vision, and the temperature-sense--we are made aware of the vibrations of distant bodies, the effects of which are borne to us on waves of air or of æther. the limits of hearing with us are between thirty and about forty thousand (or perhaps, in very rare cases, fifty thousand) vibrations per second. but these are by no means the limits of vibrations of the same class. by experiments with sensitive flames,[fn] lord rayleigh has detected vibrations of fifty-six thousand per second; and mr. w. f. barrett has shown that a sensitive flame two feet long is sensitive to vibrations beyond the limit of his own hearing and that of several of his friends who were present at the experiment. we have some reason to suppose that vibrations too rapid to be audible by man are audible by insects, but not much is known with regard to the exact limits. the following table shows what is known concerning the æther-vibrations. the figures are those given by professor langley:-- wave-lengths number of in thousandths vibrations quality of radiations. of a per second effects on man. millimetre. in billions. limit of photography, artificial source . none known limit of photography, solar source . none known limit of violet to normal eyes . } limit of red to normal eyes . } vision. probable inferior limit of temperature-sensations . [fo] temperature-sense longest waves hitherto recognised with bolometer . none known from this table it will be seen that, apart from the possible extension of sight beyond human limits, there are possibilities of another sense for the ultra-violet actinic vibrations as different from sight as is the infra-red temperature-sense. moreover, the temperature-sense for us has no scale; there is nothing corresponding to pitch in sound or colour in sight. it may not be so with lower organisms. insects, for example, may be sensitive to tones of heat. the bee may enjoy a symphony of solar radiance. i am not saying that it is so; i am merely suggesting possibilities which we have not sufficient knowledge to authoritatively deny. we have no right to impose the limits of human sensation on the entire organic world. insects may have "permanent possibilities of sensation" denied to us. even within our limits there may be, as we have already seen, great and inconceivable differences. we saw that our own colour-sensations are probably due to the blending and overlapping in different proportions of three primitive monochromatic bands, but that in all probability in birds the bands are different, and overlapping is largely prevented. their colour-phenomena must be inconceivably different from ours. and what shall we say of the colour-vision of invertebrates? are we justified in supposing that for them, as for us, r., g., and v. are the unstable explosives, and that they are present in the same proportions as with us? if not, their colour-world cannot be the same as ours. of the same order it probably is. and all that we can hope to do is to show, as has been shown, that colours which differently affect us affect them also differently. * * * * * in conclusion, we may return to the point from which we set out. the organism is fitted to respond to certain influences of the external world. the organs for the reception of these influences are the sense-organs. when they are stimulated waves of change are transmitted inwards to the great nerve-centres; they are there co-ordinated, and issue thence to muscles or glands. thus the organism is fitted to respond to the influences from without. the activities of organisms are in response to stimulation. we have seen that the cells of the organic tissues are like little packets of explosives, and that the changes which occur in the organism may be likened to their explosion and the setting free of the energy stored up in them. the end-organs of the special senses may be regarded as charged with explosives of extreme sensitiveness. some are fired by a touch; the molecular vibrations of sapid or odorous particles explode others; yet others are fired by the coarser vibrations of sound; others, once more, by the energy of the ætherial waves. the visual purple is a highly unstable chemical compound of this kind; expose it for a moment to light, and it topples over to a new molecular arrangement, the colour being at the same time discharged. if the retina has been removed from the body, this is all that happens. but if (in the frog) it be replaced on the choroid layer from which it has been stripped, the visual purple is reformed. the explosive is thus reconstructed and the sensibility is restored. thus, as fast as the explosives are fired off by sense-stimuli, so fast in normal life are they reconstituted and the sensibility restored. meanwhile the explosion at the end-organs has fired the train of explosives in the nerve, and created molecular explosive disturbances in the brain. thence the explosive waves pass down other nerves to muscles or glands, and, giving rise therein to further explosions, take effect in the activities of the organism. we shall have to consider these activities hereafter. we must now turn to the psychical or mental accompaniments of the explosive disturbances in the brain or other aggregated mass of nerve-cells. notes [eo] see abstract in _nature_, vol. xxxiv. p. . [ep] see _nature_, vol. xxxvii. p. . [eq] "sense-organs and perception of fishes:" journal of marine biological association, new series, vol. i. no. , p. . [er] _nature_, vol. xlii. p. . [es] _nature_, vol. xxxvi. p. . [et] journal of marine biological association, new series, vol. i. no. , p. . [eu] mr. s. klein mentions a similar fact in connection with _bombyx quercus_ (_nature_, vol. xxxv. p. ). [ev] journal of marine biological association, new series, vol. i. no. , p. . [ew] a friend of mine informs me that his limit is about , per second, , being quite inaudible. [ex] journal of marine biological association, new series, vol. i. no. , p. . [ey] of course, anglers will say that what may be true for pollack and other coarse and vulgar sea-fish does not apply to king salmon or prince trout. [ez] "senses of animals," p. . [fa] see a very interesting and lucid paper by professor crum brown, whose name is intimately connected with this subject, in _nature_, vol. xl. p. . [fb] it is interesting to note that in the blind-fish (_amblyopsis spelæus_) the semicircular canals are, according to wyman, unusually large. [fc] the dampers must, of course, be lifted by depressing the loud pedal. [fd] "special physiology," p. . [fe] a band and not a line, because r. is unstable to the impact of a considerable range of light-vibrations. [ff] mr. chattock has kindly supplied me with the following note:-- "readings at the violet end were taken at the extremity of the lavender rays, at the point where the faint band of lavender light seemed to end off about half-way across the field of view (the cross-wires being invisible). "at the red end the cross-wires were always visible, and were in each case set to the point where the top horizontal edge of the spectrum lost its definition. "other things equal, the 'red' readings should be more reliable than the violet, therefore, from the greater definiteness of the point observed, and the means of observing it. but against this has to be set off the fact that the extreme violet rays were spread out by the prism used more than eight times as much as the red rays. "in any case, the wide differences observed in the 'red' readings are much greater than could have been due to misunderstanding or careless observation--as shown by setting the instrument to maximum and minimum readings, and noting the very obvious difference between them apparent to a normal eye. the same conclusion is rather borne out by the closer (average) agreement between the two eyes of the same individual than between those of different persons. "the source of light was the central portion of an ordinary argand burner." [fg] the variations above indicated throw light on a fact to which lord rayleigh has directed attention. the yellow of the spectrum may be matched by a blending of spectral red and spectral green; but the proportions in which these spectral colours must be mixed differ for different individuals. the complementary colours for different individuals are also not precisely the same. [fh] "colour-vision and colour-blindness," r. brudenell carter (_nature_, vol. xlii. p. ). [fi] journal of marine biological association, new series, vol. i. nos. and . his experiments with regard to the colour-sense in fishes gave, for the most part, negative results. [fj] we must remember how largely the antennæ are used when an insect is finding its way about. watch, for example, a wasp as it climbs over your plate. if the antennæ be removed, it seems to stumble about blindly. the antennæ seem almost to take the place of eyes at close quarters. [fk] "senses of animals," p. . [fl] see _nature_, vol. xli. p. . [fm] chap. x. p. . [fn] the observations are not yet published, and i have to thank lord rayleigh for his courtesy in allowing me to make use of this fact. [fo] professor langley finds that the maximum effect with a radiating source at ° c. is at about . thousandths of a millimetre wave-length. " ° c. " " . " " " " ° c. " " . " " " we are sensitive to radiations from a body at ° c. but when the temperature falls below the normal temperature of the body we are not sensitive to heat-vibrations, but to loss of heat from the surface exposed. the limit of sensibility to heat-vibrations, therefore, probably lies between . and thousandths of a millimetre. i have taken about . as the limit. chapter viii. mental processes in man. i have already drawn attention to the fact that the primary end and object of the reception of the influences (_stimuli_) of the external world, or environment, is to enable the organism to answer or respond to these special modes of influence, or stimuli. in other words, their purpose is to set agoing certain activities. now, in the unicellular organism, where both the reception and the response are effected by one and the same cell, the activities are for the most part simple, though even among these protozoa there are some which show no little complexity of response. where, however, the organism is composed of a number of cells, in which a differentiation of structure and a specialization of function have been effected, certain cells are set apart as _recipients_, while other cells are set apart to respond (_respondents_). there is thus the necessity of a channel of communication between the two. hence yet other cells (_transmitters_), arranged end to end, form a line of connection and communication between the group of receiving cells and the group of responding cells, and constitute what we term a _nerve_. that which is transmitted may still be called a stimulus, each cell being stimulated in turn by its neighbour. thus a stimulus must be first received and then transmitted. but little observation is required to convince us of the fact that, in the higher creatures, a very simple stimulus may give rise to a very complex response. a light pin-prick will cause a vigorous leap in a healthy frog--a leap that involves a most intricate, accurate, and complex co-ordination of muscular activities. and anatomical investigation shows us that in such creatures there is always, in the course of the channel of communication or transmission, a group of closely connected cells, which play the part of co-ordinants. in the vertebrate animals these co-ordinants are collected in the brain and spinal cord. in the insects, crustaceans, and worms they are arranged in a knotted chain running close to the under surface of the body. to this central nervous system, as it is called, nerves (afferent nerves) run inwards from the recipient organs. from it nerves (efferent nerves) run outwards to the organs of response. and in it the transmitted stimuli, brought in by the afferent nerves, are modified, through intervention of the co-ordinants, into stimuli carried out by the efferent nerves. a simple stimulus may create a great commotion among the co-ordinants of the central nervous system, and give rise to many and complex stimuli going out to the muscles and other organs of response. how this is effected is one of the many wonders of the animal mechanism. we believe that the connection and co-ordinations have gradually been established during a long process of development and evolution, reaching back far into the past. how, we can at present scarcely guess. we must picture to ourselves, then, in the animal organism, a multitude of nerve-fibres running inwards from all the end-organs of the special senses, from the muscles, and from the internal organs, and all converging on the central nervous system. and we must picture to ourselves a multitude of nerve-fibres passing outwards from the central system, and diverging to supply the muscles, glands, and other organs which are to respond to the stimulation from without. we must picture the fibres coming from or going to related parts or organs collecting together to form nerves and nerve-trunks, which are, however, only bundles of isolated nerve-fibres. and, lastly, we must picture the central nerve-system itself co-ordinating and organizing the stimuli brought into it by afferent nerves, from the organs of special sense, and handing over the resultants by efferent nerves to the organs of special activities. so far we have purely physiological effects, many of which occur with surprising accuracy and precision when an organism is in a state of unconsciousness. place your finger in the palm of a sleeping child, and the fingers will close over it without the child awaking to consciousness. if, in a frog, the brain of which has been extirpated, the side be touched with a drop of acid, the leg of that side will be drawn up, and the foot will be used to wipe away the acid. and if that leg be held and prevented from reaching the side, the other leg will be brought round so as to try and bring the foot within reach of the irritated spot. the actions are, however, in all probability, purely physiological, and are performed in complete absence of consciousness. * * * * * when we turn from the physiological to the psychological aspect of the question, we enter a new world, the world of consciousness, wherein the impressions received by the recipient organs (no longer regarded as mere stimuli, but as the elements of consciousness) are co-ordinated and organized, and are built up into those sensations and perceptions through which the objects of the external world take origin and shape. it is with this process that we have now to deal; and we will deal with it first in man. the first fact to notice is that, apart from sense-stimuli received and exciting consciousness, we have also the revival of past impressions. this revival is the germ of memory. what exactly is the physical basis of memory, how the effects of stimuli in consciousness come to be registered, we do not know. it is clearly a matter that falls under the general law of persistence; but in what organic manner we are largely ignorant. still, there can be no question of the fact that, quite apart from impressions due to immediate influences of the environment _now_ acting on our recipient organs, we have also revivals of bygone influences of the environment--shadows or after-images of previous modes of influence. without this process of registration and revival, stimuli could never give rise to sensations and perceptions such as we know them. without it experience would be impossible. we may say, then, that impressions (resulting from stimuli) and their revival in memory are the bricks of the house of knowledge; and these are built up through experience into what we call the world of things around us. there may be and is a certain amount of mortar, supplied by the builder, in addition to the elementary bricks. but without the bricks no house of knowledge could be built. let us now examine the bricks and the building. from what we have already learnt in the chapter on "the senses of animals," it is clear that the impressions and their revivals in memory have differences in quality. here, on the very threshold of the subject, we must pause. they have differences of quality. but in consciousness these differences must be distinguished. and this involves their recognition and discrimination, presupposing, therefore, a corresponding faculty, however simple, on the part of the recipient. without cognition and recognition (twin sisters, born in the same hour) we can never get beyond mere impressions; which may, indeed, be differentiated physically, as different stimuli due to diverse action of the environment, but are psychically undifferentiated. this recognition and discrimination is thus the primary activity of the recipient mind. here is already some of the mortar supplied by the builder. memory is absolutely essential to the process. the sense-impression of external origin gives rise to an impression of similarity or dissimilarity, which is part of the internal reaction to the external stimulus. thus impressions are raised to the level of _sensations_. a sensation is an impression that has been discriminated from others, and recognized as being of such and such a nature. the impressions of the sense-organs as we know them are thus not mere impressions, but impressions raised to the level of sensations, in so far as they are recognized and discriminated. let us now glance at some of the differences in quality recognized in sensation. first, we have the broadly distinguished groups of touches and pressures, temperature-sensations, tastes, smells, sounds, sights, muscular sensations, and organic sensations from internal parts of the body. and then, within each of these groups, there are the more or less delicate and distinct shades of quality, well exemplified in vision by the different colour-sensations, in hearing by notes of different pitch, and in smell by the varieties of scents and odours. many of those sensations, moreover, which are apparently simple, are in reality compound. there are differences of quality in the note a as sounded on a violin, a piano, and a flute; and these differences are due to different admixtures of overtones, which fuse with the fundamental tone and alter its timbre. so, too, with vision. the sensation given by a white disc is a compound sensation, due to waves of different period, which separately would give sensations of colour. sensations, then, differ in quality. they also differ in quantity or intensity. this needs little illustration. as evening falls, the sight-sensations derived from the surrounding objects grow more and more feeble. they may remain the same in quality, but the quantity or intensity gradually diminishes. so, too, in music, the pianos and fortes give us differences in intensity of sound-sensations. sensations also differ in duration. the stimulation may be either prolonged or instantaneous. two or more sensations may, moreover, be simultaneous or successive. just as they may be either similar or different in quality and in intensity, so they may be either simultaneous or successive in time. simultaneous sensations are best exemplified in vision and through touch; successive sensations are given most clearly by the sense of hearing, through which we recognize a sequence of sounds. and then, again, sensations not only differ in time, but they seem also to differ in place. a sensation of touch may be referred to different parts of the body--the hand, the foot, or the forehead. but here we open up an important question--where do we feel a sensation, such as, for example, that of pressure on the skin? common sense answers, without hesitation, that we feel it at the particular part of the body which is affected by the external stimulus. i feel the pen with which i write with my finger-tips. and common sense is perfectly right from its own point of view. but it is a well-known fact that a person whose leg has been amputated experiences at times tickling and uneasiness in the absent member. this is due to irritation of the nerve-ends in the stump of the limb. but the sensations are referred outwards to the normal source of origin of impressions, the effects of which were carried inwards by the nerve affected. we shall have to consider hereafter the nature of the relation between physiological and psychological processes--the connection of mind and body. assuming for the present that psychical processes have a physical basis in physiological processes, the fact given above and others of like implication seem to show that the sensation has for its physiological basis some nerve-change in the central nervous system--in us, no doubt, in the brain. of course, it must be remembered that the sensation, as felt, is a mental fact (using the word "mental" in its broadest sense, as belonging to the psychical as opposed to the physiological series). but it would seem that the physiological accompaniment of this mental fact is some nerve-change in the brain. this nerve-change is caused by a stimulus having its origin in the end-organ of the afferent nerve, and we naturally refer the impression outwards to the place of its source of origin under ordinary and normal conditions. in other words, we _localize_ it. that is what common sense means when it says that we feel pressure at the finger-tips. to account for this process of localization, it is supposed that every sensation, apart from its special quality as a touch, a taste, or a smell, has a more or less defined spatial quality, or local sign, dependent upon the part of the body to which the stimulus is applied. these local signs have, doubtless, in the long run, been established by experience--if under this term we may include a more or less unconscious process, the outcome of evolution. but they are so rapidly established in the individual, that we are forced to conclude that we inherit very highly developed aptitudes for localization. the refinement of localization is very different in the different senses. in smell and taste there seems no more than a general localization in the organ affected--the nose or the mouth. in hearing there is not much more, unless we regard the discrimination of pitch as a mode of localization. in touch (and temperature) the refinement is much higher, but it varies with the part of the body affected. if the back be touched by two points less than two inches and a third apart, the sensation will be that of a single point; the finger-tips, however, can distinguish two points separated by less than one-tenth of an inch; and the tip of the tongue is still more refined in its power of discrimination, distinguishing as two, points separated by less than the twenty-fifth part of an inch. so that the tongue is about sixty times as refined in its discrimination as the skin of the back. moreover, the delicacy of localization may be cultivated, so that in some cases the refinement may, by practice, be doubled. when we come to sight, the refinement of localization reaches its maximum, the local signs in the retina showing the highest stage of differentiation, the distance on the retina between two points distinguishable by local signs being, according to helmholtz, not much more than / of an inch (. millimetre), which nearly corresponds with the space between two cones in the yellow spot. we must remember that the presentations of sense are in all cases given in a stippled form, that is, by the stimulation of a number of separate and distinct points. in vision the stippling is very fine, owing to the minute size and close setting of the retinal cones. in the case of hearing, the stippling, if we may so extend the use of this term, is also very fine, as is shown by the fact that musicians can, according to weber, distinguish notes separated in the scale of sounds by only one-sixtieth part of a musical tone. in touch the stippling is comparatively coarse. but in all cases there is a stippling; and yet from these stippled sensations the mind in all cases elaborates a continuum. the visual image is continuous, notwithstanding the retinal stippling and the existence of the blind spot. when we lay our hands on a smooth table we fill in the interstices between the sensational points, and feel the surface as continuous. in all cases out of the stippled sense-stimuli we form a continuum. the next thing that we have to note is that it is not so much the sensation itself, as that which gives origin to it, that we habitually refer outwards to the recipient end of the afferent fibre. in referring a sensation of touch to a certain part of the skin, it is of something touching us that we seem to be immediately conscious. we refer the stimulus to an object in the external world, which we localize, and which we believe to have given rise to the sensation. this, however, is more clearly seen in the case of vision. when we look through the window and see an object such as a house before us, we do not habitually localize the sensation in a certain part of the retina, but we refer the object to a particular position more or less distant in the world around us. this projection of the object outwards in a right line from the eyes is really a marvellous process, though the wonder of it is lost in its familiarity. it is the outcome of the experience of hundreds of generations. and the experience is not gained through vision alone, but through this in combination with other senses and activities. we see an object, but we have to go to it before we can touch it. it is not in contact with us, but distant from us. its outness and distance is a matter of what is termed the geometry of the senses; and this geometry has been elaborated through many generations of organized beings, from data given by sight, touch, and the muscular sense. it is true that i can now estimate the distance of the house without going to it; but my eyes go to it, and i can feel them go. the panes of my window are separated by iron bars. as i look from them to the distant house and back to them again, i can feel my eyes going from one to the other. the lens of the eye is adjusted for near or far distance by the action of a ciliary muscle, through which its anterior surface can be flattened, returning again by its own elasticity to the more convex form when the muscle ceases to act. each eye, moreover, is moved in its orbit by six eye-muscles, and in normal vision the two eyes act as one organ. for near distances they converge; for far distances there is less convergence. through the muscular sense, which is here extraordinarily delicate, we can feel the amount of accommodation and convergence; and thus we can feel the eyes going to or coming from a near and a distant object. of course, we are aided in judging or estimating distances by the apparent size of the object when the real size is known, by the clearness of its outlines in a slightly hazy atmosphere, and so forth. but apart from such judgments, it would probably be impossible to perceive that an object is near or distant in the absence of muscles of accommodation and convergence affording the data of the muscular sense. not only the distance of two objects from the eye, but their distance apart, can be measured by the aid of the muscular sense as we move the eyes from one to the other. and in us this is so delicate that, according to weber, a distinct muscular sensation is attached to a displacement of a sensitive point of the yellow spot through less than / of an inch. now, if it be true that the consciousness aroused by objects around us, through sensation, is an accompaniment of certain physiological changes in the brain, it is clear that the localization of their points of origin in special parts of the skin, and the outward projection of the objects exciting vision, is an act of the mind quite distinct from the mere passive response in consciousness which we call an impression, and more complex than that mental activity which, through discrimination and recognition, converts the bare impression into a sensation. it is, in fact, part of that mental process which is called _perception_.[fp] sensation has nothing to do with the objects around us as such; it is by perception that we are aware of their existence. let us now follow the process of perception a little further, always remembering that it involves certain activities of the mind. these activities are too often ignored. we often speak of the senses as the avenues of knowledge, and john bunyan, likening the soul to a citadel, spoke of the five gateways of knowledge, eye-gate, ear-gate, mouth-gate, smell-gate, and feel-gate. hence arises a vague notion that through the eye-gate, for example, a sort of picture of the external object somehow enters the mind. and this idea is no doubt fostered by the fact that an inverted image of the object is formed on the retina, though how the inverted image is turned right way up again in passing into the mind bothers some people not a little.[fq] a much closer analogy is this: something stands without and knocks at the doorway of sense, and from the nature of the knocks we learn somewhat concerning that which knocks. in other words, at the bidding of certain stimuli from without we construct that mental product which we call the object of sense. it is of these mental constructions--"_constructs_"[fr] i will call them for convenience--that i have now to speak. in a fruiterer's shop on the opposite side of a street i see an orange. that is to say, certain cones of the retina of my eye are stimulated by light-waves of a yellow quality, and at the bidding of these stimuli i construct the object which i call an orange. that object is distant, roundish, yellow, resisting and yet somewhat soft, with a peculiar smell, and possessed of a taste of its own. now, it is obvious that i cannot see all these qualities of the orange, as we call them. i construct the object on reception of certain light-waves which are focussed on the retina of my eye. if i go to the orange, however, i can test the correctness of my construct by the senses of touch, smell, and taste. but what led me to construct an object with these qualities? experience has taught me that these qualities are grouped together in special ways in an orange. i constructed that particular object through what is termed the principle of association. i have learnt that these qualities are grouped together in certain relations to each other, and when i actually receive sight-stimuli of a certain quality, grouped in certain ways, they immediately call up the memories of the associated qualities. that which is actually received is a mere suggestion, the rest is suggested in memory through association. the object might be suggested through other senses. i come into a dining-room after dessert, and the object is suggested through smell. or my little son says, "open your mouth and shut your eyes, and see what the fairies will send you;" and an orange is suggested by taste. in all these cases the object is constructed at the bidding of certain sensations, which suggest to my mind the associated qualities. the object is a _construct_. and here let us notice that we ascribe the form, the resistance, the taste, the smell, to the object. we do not say or think, "sight-sensations inform me that there is something which i call an orange, and which is capable of exciting in me sensations of touch, taste, and smell;" but we say, "there is an orange, which _has_ such and such a taste, smell, and feel." in other words, we refer these sensations, related in certain ways, outwards to the object, and name them qualities of the object that we see. but remember, that we do not necessarily or normally say or think anything about it. we just inevitably construct the object, what we build in to the construct depending upon association through experience. at this stage, perhaps, common sense steps in, and, shaking his head, says, with characteristic bluntness, "nonsense; you'll never persuade me that the things i see and feel around me are nothing but fictions of my own mind. i don't construct them, as you call it; there they are for me to see and feel and taste if i will." now, common sense is a sturdy, hard-headed individual, with whom i desire to keep on friendly terms. and i therefore hasten to explain that i most fully agree with every word that he says. the orange that i see before me is not a mere fiction of my mind. i can, if i will, take it up, feel it, smell it, and taste it. if it will satisfy common sense, i will say that it is the idea of the orange that i construct. only i think that common sense, who has a horror of roundabout and indirect statements, will not like my saying, "i am receiving certain visual sensations related in certain ways, which lead me to construct an idea of an orange." he will prefer my saying simply, "i see an orange." since what he wants me to call our ideas of things answer point for point to the things as they actually exist for us human-folk, it is not only more satisfactory but more correct to merge the two in one, and speak directly and simply of the object. the object is a thing i construct. that it is real may be proved by submitting it to the test of all the senses that i have. and what do i mean by "real"? i mean that what it is for me it is also for you and any other normally constituted human being. this is, in truth, the only common-sense criterion of objective reality. some people are colour-blind, and tell us that a rose is not red, but green. we reply that it is really red, but that, through a defect of sight, they cannot distinguish its redness. here we take the normal human being as a standard for objective reality. for him the rose is red. and this is the only practical criterion that we have. this, however, does not satisfy some people, who think that the objects around them have the same reality, independent of man, that they have for us human-folk. annihilate, they say, every human being--nay, all life--and the objects will remain as they are, and retain the same reality. yes, the same reality; which means that if just one fortunate fellow escaped annihilation, he would find them all just as they were. and this nobody doubts. nevertheless, it is (to me, at least) inconceivable that things independently of us are what they appear to us. think of what we learnt about the sensations. they all arose in stimulations of the end-organs of special sense. thence the explosive waves of change passed inwards to the brain, and somewhere therein gave rise to mental products. these mental products, the accompaniments of nerve-changes, can in no sense be like the outside something which gave rise to them. they are symbols of that outside something. and it is these symbols that we build up into objects. hence i said that it is not only more satisfactory and convenient, but more correct, to speak directly of the object as constructed, and not our idea of the object. the mental product _is_ the object for us, not only for me, but for you and all normal human beings, since the object is the same for all of us. and hence, also, i said that the analogy of gateways, through which pictures of objects gain access to the mind, was false and misleading, and that a truer analogy is that something stands without and knocks at the doorway of sense, and that from the nature of the knocks we learn somewhat concerning that which knocks. the person inside can never open the door to see what manner of thing it is which knocks. but he can build up a most cunning symbolism of knocks which shall suffice for all practical purposes. in other words, the object-world, symbolic though it is, which you and i and the rest of us construct at the bidding of something without us (the existence of which i assume), is amply sufficient for all our practical needs, and constitutes the only practical reality for human-folk. i am well aware that there are many people who cannot bring themselves to believe in, or even to listen without impatience to, the view that the world we see around us is a world of phenomena. it is absurd, they say, to tell us that yonder tulip, as an object, is in any sense dependent on our perception of it. there it is, and there it would have been had man never been created. can one conceive that the new species of fossil, which was only yesterday disentombed from the strata in which it has lain buried for long ages, is dependent on man's observation for its qualities as an object? to say that it was "constructed" by the lucky geologist who was fortunate enough first to set eyes on it is sheer nonsense. its shelly substance protected a bivalve mollusc millions of years before man appeared upon the earth. when we see the orange in the fruiterer's shop, the sight of it merely reminds us of its other qualities--its taste, its smell, its weight, and the rest, which are essentially its own, and no endowments of ours--nowise bestowed upon it by us. i have no hope of convincing, and not much desire to convince, one who thus objects. i would merely ask him how and when he stepped outside his own consciousness to ascertain that these things are so. does he believe that consciousness is an accompaniment of certain nervous processes in the grey cortex of the brain? if so, let him tell us how these conscious accompaniments resemble (not merely symbolize, but _resemble_) tulips and oranges and fossil molluscs. if not, let him propound his new theory of consciousness. let it not be supposed that i am denying the existence, and the richly diversified existence, of the external world. we are fully justified, i think, in believing that, corresponding to the diversity of mental symbolism, there is a rich diversity of external existence. but its nature i hold that we can never know. the objects that we see are the joint products of two factors--the external existence and the percipient mind. we cannot eliminate the latter factor so as to see what the external factor is like without it. those who, like professor mivart,[fs] say that we can eliminate the percipient factor, and that the external world without it is just the same as it is with it, are content to reduce the human mind, in the matter of perception, to the level of a piece of looking-glass. there are some people who seek to get behind phenomena by an appeal to evolution. it will not do nowadays, they say, to make the human mind a starting-point in these considerations; for the human mind is the product of evolution, and throughout that evolution has been step by step moulded to the external world. the external world has, therefore, the prior existence, and to it our perceptions have to conform. all this is quite true; but it is beside the point. mind has, throughout the process of evolution, been moulded to the external world; our perceptions do conform to outside existences. but they conform, not in exact resemblance, but in mental symbolism. they do not copy, but they correspond to, external existences. it is just because, throughout the long ages of evolution, mind has lived and worked in this symbolic world that common sense is unable to shake off the conviction that this is the only possible world, and exists as such independently of mental processes. the world of phenomena _is_ the world in which we, as conscious beings, live and move. no one denies it. but it is none the less a symbolic world; none the less a world which mind has constructed in the sense that it is an inalienable factor in its being. each of us, when we perceive an object, repeats and summarizes the constructive process which it has been the end of mental evolution to compass. hence it is that, at the bidding of a simple impression, percepts or constructs take origin and shape in the mind. in taking possession of this faculty in the early years of life, we are entering upon a rich ancestral heritage. but if what i have been urging has truth, what we call objects are human constructs, and cannot by any manipulation be converted into anything else. i will now take another and more complex case of construction, which will bring out some other facts about what i have termed "constructs." i hear in the street a piercing howl, which suggests a dog in pain. rising from my seat and going to the window, i see a white terrier with a black patch over the left eye limping down the road on three legs. now, what was the nature of the construct framed at the bidding of the piercing howl? a dog in pain. but what dog? the nature of the howl suggested a small dog; but there was nothing further to particularize him. the construct was, therefore, exceedingly vague and ill defined, and was not rendered definite and particular till i went to the window, and saw that it was a white terrier with a black patch over the eye. the howl, moreover, suggested certain activities of the dog. the construct was not merely a passive, inanimate object, like the orange, but an object capable of performing, and actually performing, certain actions. here, again, we can only say that it is through experience that special activities are associated with certain objects. just as the construct orange is capable of exciting sensations of taste, so the construct dog is capable of doing certain things and performing certain actions, that is, of affecting us in certain further ways. but, further, the howl suggested a dog in pain. no amount of sensations entering into any manner of relations could give me that element of the construct. i can neither see, touch, taste, smell, nor hear pain in another being. pain is entirely subjective and known only to the sufferer. but i have been a sufferer. i have experienced pain and pleasure. and just as my experiences, individual and ancestral, lead me to project into inanimate objects certain qualities, the products of my sensations, so do my experiences, individual and ancestral, lead me to project into certain animals feelings analogous to those i have myself experienced. this is sometimes described as an inference. but if we call this an inference, then we must, i think, call the taste, smell, and feel of the orange i see before me inferences. in both cases the inference, if we so call it, enters at once into the immediate construct. and when i went to the window and saw the dog limping down the street, i saw also a small boy, with arm drawn back, in the act of throwing a stone. in other words, i saw the objects in the scene before me standing in certain relations to each other. i concluded that the boy had thrown a stone at the dog and was about to throw another. in other words, i saw the scene before me as part of a sequence of events. one more example i will give to bring out another and important feature in the mental process. strolling before breakfast in early spring in my friend's garden, there is borne to me on the morning air a whiff of violet fragrance. not only does this lead me to construct violets, but it reminds me of a scene in my childhood with which the scent of these flowers was closely associated. not only is the object constructed, but a scene with which their fragrant odour has been associated is _reconstructed_ in memory. the violets are immediate constructs or presentations of sense; the remembered scene is a _reconstruct_ or representation in memory. so, too, when i heard a piercing howl in the street, the dog i constructed was a vague presentation of sense; but the street in which i instinctively placed him was a reconstruct or representation in memory. the difference between a construct or presentation of sense, and a reconstruct or representation in memory, is that the former is directly suggested through the immediate action of some quality or activity of the object, while the latter is indirectly suggested through some intermediate agency. before proceeding further, let us review the conclusions we have thus far reached. through the action of certain surroundings on our sensitive organization, we receive certain impressions, and among these impressions and others revived in memory we recognize certain similarities or differences in quality, in intensity, in order of sequence, and in source of origin. the sensations which thus originate are mental facts in no sense resembling their causes, but representing them in mental symbolism. the consciousness of similarity or difference is no part of the impression, but a further mental fact arising out of the impression, and with it giving origin to sensation. it deals with the relation of impressions among each other and to the recipient. it involves recognition and discrimination. its basis is laid in memory. the sensations are instantly localized, referred to objects, and projected outwards, mainly through the instrumentality of the muscular sense. the mental symbolism is thus built into the objects around us, and constructs are formed. but into the tissue of these constructs are woven, not only the sensations immediately received, but much that is only suggested through association as the outcome of past experience, individual and ancestral. the constructs and their associated reconstructs are thus endowed with qualities which have practical reality, since they are not for me only, but for you and for mankind. they are, therefore, in a sense independent of _me_, but nowise independent of _man_.[ft] some of the constructs are endowed with activities, and some with feelings akin to our own. finally, in the field of vision which we construct or reconstruct, the objects are seen to stand in relationship to each other, and the scene as a whole is perceived to be part of an orderly sequence of events. we have already got a long way beyond the impressions with which we started; and yet, if i may trust my own experience, such construction as i have described is direct and immediate. a child of four or five would not only construct as much, but might not improbably go a long way further, and say, "naughty boy to throw a stone at poor doggie!" it is, i say, direct and immediate, and it implies a wonderful amount of mental activity. some people seem to imagine that in the simpler forms of perception, as when i see an orange on the table, the mind is as passive as the sensitive plate in a photographer's camera. this surely is not so. it is a false and shallow psychology which teaches it. just as a light pin-prick may set agoing complex physical activities in the frog, so may comparatively simple visual sensations give rise to complex mental activities in construction and reconstruction. it is to emphasize this mental activity that i have persistently used the terms "construct" and "construction." and i wish to emphasize it still further by saying that without the active and constructive mind no such process of construction or reconstruction is possible or (i speak for myself) conceivable. we might just as well suppose that the frog could leap away on stimulation of a pin-prick in the absence of its complex bodily organization, as that sensation could give rise to construction and reconstruction in the absence of a highly organized mind. we have seen that when a howl suggested the construct dog, that construct was vague and undefined; but when i went to the window and saw the terrier, the construct became particularized and defined. this seems to me the normal order of development: first the vague, general, and indefinite; then the particular, special, and defined. that which is immediately suggested at the bidding of sensations received is always more or less general; it only becomes specialized on further examination physical or mental--first a dog or an orange; then this dog or this orange. the more unfamiliar the object, the more vague and indefinite the construct. the more familiar the object, and the further our examination of it is carried, the more particular and defined the construct. i would, therefore, mark two stages in the process of construction: first, the formation of constructs by immediate association, more or less vague, indefinite, and ill defined; and, secondly, the definition of constructs by examination, by which they are rendered more definite, particular, and special, and supplemented by intelligent inferences. i need not stay here to point out the immense importance of this process of defining and particularizing constructs, or the length to which it may be carried; nor need i pause to indicate how, through memory and association, representative or reconstructive elements crowd in to link or weave the constructs into more or less vivid and brilliant scenes. but i have next to notice that out of this intelligent examination arises a new, distinct mental process, _the analysis of constructs_. this process involves the paying of special attention to certain qualities of objects, to the intentional exclusion of other qualities. when i cease to examine an orange as a construct, and pay attention to its colour or its taste to the exclusion of other properties, with the purpose of comparing this colour or taste with other colours and tastes, i am making a step in analysis. so, too, when i consider the form of an orange for the purpose of comparing it with the form of the earth, i am making a step in analysis. and, again, when i consider the howl of the dog with the object of comparing it with other sounds, i am making a step in analysis. we may call the process by which we select a certain quality, and consider it by itself to the neglect of other qualities, _isolation_, and the products of the process we may term _isolates_.[fu] this process could not be initiated till a large body of constructive and reconstructive experience had been gained. but once initiated, there is no end to the process. we pick to pieces all the phenomena of nature, all the qualities and relationships of objects, the activities and functions of animals, the mental phenomena of which we are conscious in ourselves. we isolate the qualities, relationships, feelings; and we name the isolates we obtain. hence arises all our science, all our higher thought. in the terms which we apply to our isolates consists the richness of our language. we _name_ the isolates; that is, we apply to each an arbitrary symbol to stand for the isolated quality or relation. all words (except the obviously onomatopoetic, such as "bow-wow," "cuckoo," etc.) are arbitrary symbols associated with objects, or qualities, or relations, or other phenomena. and abstract names of isolates are, so to speak, the pegs on which we hang the qualities we have separated by analysis and isolation, while class-names are pegs upon which we can hang a group of similars reached by the process of isolation; for all classing and grouping of objects, or qualities, or relations involves, so far as the process is a conscious one, the principle of analysis. in classing objects, we group them in reference to certain characters which they have in common, disregarding certain other characters in which they differ. we group together, for example, sights, or sounds, or smells, and distinguish them from each other and from tastes and touches. and then we go further, and class all these together as sensations having certain characteristics in common whereby they are distinguished from perceptions of relation and so forth. perhaps it may be objected that classification comes much earlier in the mental process than i am now putting it. it may be said that the recognition of a sensation as a touch, or a smell, or a sound involves a classification of sensations in these categories, and that the simple perception of an orange involves the placing of the object in this class of bodies. and, undoubtedly, we have here the germs of the process. sensation and perception give us the materials for classification; the perception of similarity and difference gives us the _sine quâ non_ of the process. nevertheless, although there may be an earlier unconscious grouping of phenomena, it is only when the mind is specially directed to these materials, with the object of grouping them according to their similarities, that we can speak of classification proper--conscious and intentional classification, as opposed to unconscious grouping. and this involves the intentional selection of the points of similarity, and discarding or neglecting the points of difference. it involves the process of analysis or isolation. there is a vast difference between the perceptual recognition of objects as similar, and conceptual classification on grounds of similarity. just as the recognition of a sensation as now and not then, or here and not there, or as due to something outside us, gives us the germs from which, on ultimate analysis, our ideas of time, space, and causation are reached; so does the recognition of these sensations as of this kind and not that give us the germ from which, on analysis, the process of classification may arise. true, conscious, scientific classification is late in development. and here let us notice that the conclusions we have reached in this chapter are the outcome of analysis and classification. the sensations with which we started are isolates. in considering their quality, intensity, sequence, we were isolating and classifying these special modes of their existence. localization and outward projection involved isolation. we simply see the orange before us. to understand and explain how we come to see it as we do see it involves a somewhat subtle analysis. we perceive it to be yellow, round, resistant; and then, isolating these qualities, we reach conceptions of yellowness, roundness, and resistance, quite apart from oranges. throughout our description the terms we used were very largely terms denoting classified isolates. lastly, having enormously increased our knowledge by this process of isolation, we proceed to build in the knowledge thus gained to the structure of our constructs. this is the third and last stage in construction. the first stage is the formation of indefinite constructs by immediate association; the second is the definition of constructs by examination; and the third is the completion of constructs by synthesis. and the further this process of analysis and isolation is carried, the more we are, so to speak, floated off from the immediate objects of sense into the higher regions of abstract thought. furthermore, by recombining our isolates in new modes and under new relations, we reach the splendid results of constructive imagination. in the brief description which i have now given of our mental processes, i have for the most part avoided certain terms which are current in the science of psychology. it will be well here to say a few words concerning these words and their use. the process of _sensation_ is sometimes defined as the mere reception of a sense-stimulus. but it is more convenient, and more in accordance with common usage, to call the simple result of a stimulus an impression, and to apply the term "sensation" to the discrimination and recognition of the impressions as of such and such a quality. sensation, then, is the reception and discrimination of impressions which result from certain modes of influence (stimuli) brought to bear on our organization. viewed in this way, therefore, even sensation involves a distinct reaction of the mind; it implies the first stage of mental activity. but when the sensations are given objective significance, when they suggest the existence of an object-world without us, they enter the field of _perception_. here the discriminated sense-impression is, to use the words of mr. sully, "supplemented by an accompaniment or escort of revived sensations, the whole aggregate of actual and revived sensations being solidified or integrated into the form of a _percept_; that is, an apparently immediate apprehension or cognition of an object now present in a particular locality or region of space."[fv] throughout the whole process of the formation of constructs by immediate association, and their definition by examination, we were dealing with perception and percepts. but when we reach the stage when particular qualities were isolated, then we enter the field of _conception_. the isolates are _concepts_. class-names, reached through processes involving isolation, stand for concepts. and completed constructions, involving synthesis of the results of analysis, contain conceptual elements. the word "concept," however, is used in different senses by different authors. mr. sully says,[fw] for example, "a concept, otherwise called a general notion, or a general idea, is the representation in our minds answering to a general name, such as 'soldier,' 'man,' 'animal.'... thus the concept 'soldier' is connected in my mind with the representations of various individual soldiers known to me. when i use the word 'soldier,' ... what is in my mind is a kind of composite image formed by the fusion or coalescence of many images of single objects, in which individual differences are blurred, and only the common features stand out distinctly.... this may be called a typical or generic image." but noiré, quoted by professor max müller,[fx] taking another illustration, says, "all trees hitherto seen by me leave in my imagination a mixed image, a kind of ideal presentation of a tree. quite different from this is my concept, which is never an image." i follow noiré; and i hold that the image, in so far as it is an image, whether simple or composite,[fy] is a percept; but that, in so far as there enter into the idea of the soldier or the tree elements which have been isolated by analysis, just in so far does the word "soldier" or "tree" stand for a concept. how far a word stands for a percept, and how far there enter conceptual elements, depends to a large extent on the level of intelligence of the hearer. the moment educated and intellectual folk begin to think about their words, or the objects for which they stand, conceptual elements are sure to crowd in. there is one more feature of these mental processes in man, and that by no means the least important, that remains for brief consideration. i began by saying that the primary end and object of the reception of the influences of the external world, or environment, is to enable the organism to answer to them in activity. we saw that the sight of an orange suggests, through association, its taste; and that the validity of the association could be verified by going to the orange and tasting it. we saw, too, that when i heard a dog howl in the street, and, going to the window, saw a small boy with a stone in his hand, i concluded that he was going to throw it at the dog. what i wish now to elicit is that out of perceptions through association there arise certain expectations, and that the activities of organisms are moulded in accordance with these expectations. it is clear that these expectations or anticipations belong partly to the presentative or constructive order, and partly to the reconstructive or representative order. they are in some cases directly suggested by the presentations of sense; they are also built up out of representations which have become associated with the constructs in memory and through experience. but what we have here especially to notice about them is that, in the latter case, they involve more or less distinctly the element which we, in the language of our developed thought, call causation. there is a sequence of events, and the perception of certain of these gives rise, through association and experience, to an expectation of certain succeeding phenomena. expectations are, therefore, the outcome of the linked nature of phenomena. and when we come eventually to think about the phenomena, and how they are linked together into a chain (successional) or web (coexistent), we reach the conception of causation as the connecting thread. in early stages of the mental process, such a conception does not emerge. nevertheless, the phenomena are perceived as linked or woven. and the mental process by which we pass from any perceived event or existence to other preceding, concomitant, or subsequent events or existences linked or woven with it in the chain or web of phenomena, we call _inference_.[fz] when, for example, i find a footprint in the sand, i infer that a man has passed that way; and when the clouds are heaped up heavy and black, i infer that a storm is about to burst upon us. concerning inference, of which i shall have more to say in the next chapter, i have now to note that it is of two kinds: first, perceptual inference, or inference from direct experience; secondly, conceptual inference, or inference based on experience, but reached through the exercise of the reasoning faculties. the latter involves the process of analysis or isolation; the former does not. there is a marked difference between the two. perceptual inferences are the outcome of practical experience, but do not go beyond such practical experience. conceptual inferences are also based on experience, but they predict occurrences never before experienced. perceptual inferences, again, deal with matters practically; but conceptual thought explains them. the expectation of a storm when the thunder-clouds are heavy is a case of perceptual inference. it is the outcome of a long-established association, and is not reached by a process of reasoning involving an analysis of the phenomena. but if, though the sky is clear, a west wind and a rapidly falling barometer lead me to predict rain, the inference is conceptual, and gained by me or for me by a process of reasoning; for the barometer was the outcome of the analysis of phenomena. in the mind of the rough sailor-lad, however, the fall of the mercury and the succeeding storm may be connected by mere perceptual inference, the phenomena being simply associated together. if, however, there is any attempt at explanation, correct or incorrect, there is so far a conceptual element. in a little fishing-village on our south coast, a benevolent lady presented the fishermen with a fitzroy barometer. i happened shortly after to remark to one of the men that the summer had been unusually stormy. "yes, sir," he said, "it has. but then, you see, the weather hasn't no chance against that new glass." here there was an attempted explanation of the phenomena. the falling glass was conceived as somehow causing bad weather. it is hard to draw the line between perceptual and conceptual inferences, or rather to say, in this or that case, to which class the inference belongs, because man, through language, lives in a conceptual atmosphere. moreover, the same result may, in different cases, be reached by perceptual or by conceptual inference. a child who had seen a great number of ascending balloons might, on seeing a balloon, expect it to ascend by a perceptual inference; but a man, knowing that the balloon was full of a gas lighter than air, might expect it to ascend through the exercise of conceptual inference. and just as in adult civilized life our constructs have more and more conceptual elements built into them, so do our inferences become more and more reasoned. it is probable that in an adult englishman every inference has a larger or smaller dose of the conceptual element. with the development of language we state our inferences in the form of propositions, and call them judgments. "every proposition," says mr. sully,[ga] "is made up of two principal parts: ( ) the subject, or the name of that about which something is asserted; ( ) the predicate, or the name of that which is asserted. thus, when we affirm, 'this knife is blunt,' we affirm or predicate the fact of being blunt of a certain subject, namely, 'this knife.' similarly, when we say, 'air corrodes,' we assert or predicate the power of corroding of the subject 'air.'" the proposition always involves conceptual elements; for the predicate of a proposition is always an abstract idea or general notion. propositions so formed may then become links in a chain of reasoning. "to reason is," says mr. sully,[gb] "to pass from a certain judgment or certain judgments to a new one." and so passing on from judgment to judgment, we may ascend to the higher levels of abstract thought. according to mr. sully's definition, therefore, we start from a judgment or judgments in the process of reasoning. the formation of a judgment (conceptual inference) is, however, the first step in a continuous process; and i propose, under this term, "reason,"[gc] to include this first step also. the formation of a conceptual inference i regard as the first stage of reason. any mental process involving conceptual inference i shall call _rational_. in contradistinction to this, i shall use the term "intelligence" for the processes by which perceptual inferences are reached. an intelligent act is an act performed as the outcome of merely perceptual inference. a rational act is the outcome of an inference which contains a conceptual element. notes [fp] i use this term in a broad sense, as the process involved in the formation of what i shall term _constructs_. [fq] and i may add it is not an easy matter to explain to those who have not considered such questions. it is a matter of the correlation of the testimony of the sense-organs. a boy stands before me. i go to him and touch him, and pass my hands downwards from head to foot. then i stand a little way off and look at him. his image on my retina is inverted. but as i run my eye over him i direct my eye downwards to his feet and upwards to his head. i am not conscious that the stimuli are running _upwards_ along the retinal image. thus my eye-muscles and my other muscular and tactile sensations seem to tell me that he is one way upwards. the image on my retina tells me, though i am not conscious of the fact, that he is the other way upwards. but he cannot be both! the testimony of one sense has to give way. one standard or the other has to be adopted. practically that of touch and the muscular sensations is unconsciously selected, and sight-sensations are habitually interpreted in terms of this standard. so long as the two are sufficiently accurately correlated, the practical requirements of the case are met. and it is well known that it is not difficult, with a little practice, to establish a new correlation. this is indeed done every day by the microscopist, for whom the images are all reversed by his instrument. he very soon learns, however, that to move the object, as seen, to the left, he must push it to the right. a new correlation is rapidly and correctly established. [fr] i use this term because the word "percept" is used in different senses by different writers, e.g. by mr. mivart and mr. romanes. [fs] "let the perception be considered to be made up of x + y; x being the ego, or self, and y the object. the mind has the power of supplying its own - x, and so we get (through the imagination of the mind and the object) x + y - x, or y pure and simple" (mivart, "on truth," p. ). mr. mivart devotes a whole section of this work to the defence of ordinary common-sense realism. the above assertion seems to contain the essence of his teaching in the matter. [ft] if it be said that the object does exist independently of man, though not in the phenomenal guise under which we know it, i would reply--not so; for it is to the existence _under this phenomenal guise_ that we apply the word "object." in philosophical language, the existence, stripped of its phenomenal aspect, is called the _ding an sich_. its essential character is its independence of man; and hence its unknowability. [fu] i avoid, for the present, the use of the terms "abstraction" and "abstract idea" because they are employed in different senses by different authors. [fv] "outlines of psychology," p. . [fw] ibid. p. . [fx] "science of thought," p. . [fy] for compound or generic ideas "not consciously fixed and signed by means of an abstract name," mr. romanes ("mental evolution in man," p. ) has suggested the term "recept." in the photographic psychology which he adopts, the percept is an individual and particular photograph, the recept a generalized or composite photograph. "the word 'recept,'" he says, "is seen to be appropriate to the class of ideas in question, because, in receiving such ideas, the mind is passive." this, it will be observed, is in opposition to the teaching of this chapter, in which the activity of the mind in perception has been insisted on. mr. romanes's recepts answer in part to what i have termed _constructs_, which, as we have seen, are, as a rule, from the first general rather than particular, and in part to concepts reached through analysis. mr. romanes, for example, speaks of ideas of principles (e.g. the principle of the screw) and ideas of qualities (e.g. good-for-eating and not-good-for-eating) as recepts (p. ). on the other hand, mr. mivart ("the origin of human reason," p. ; see also his work "on truth") terms such generic affections "sensuous universals." it may be well to append mr. romanes's and mr. mivart's tabular statements. _mr. romanes._ { general, abstract, or notional = concepts. ideas { complex, compound, or mixed = recepts, or generic ideas. { simple, particular, or concrete = memories of percepts. _mr. mivart._ { general or true universals = concepts. ideas { particular or individual = percepts. { groups of actual experiences } sensitive { combined with sensuous } = sensuous universals, cognitive { reminiscences } or recepts. affections { groups of simply juxtaposed } = sense-perceptions, { actual experiences } or sencepts. in mr. mivart's terminology, the representations of the lower group are "mental images" or "phantasmata." the term "consciousness" is by him restricted to the higher region of ideas, the term "consentience" being applied to the faculty by which cognitive affections are felt, unified, and grouped without consciousness. _there is a difference in kind_, according to mr. mivart, between "consentience" and "consciousness;" and the former could therefore never develop into the latter, nor the latter be evolved from the former. for this reason (because of the philosophy it is intended to carry with it) i shall not employ the word "consentience," which would otherwise be a useful term. [fz] we do not speak of the filling in the complement of a percept (the construction of the object at the bidding of a simple impression) as a matter of conscious inference. i do not consciously _infer_ that yonder moss-rose is scented. scent is an integral part of the construct. from the appearance of the rose, i may, however, infer that a rose-chafer has disturbed its petals. the complement of the percept, if inferred at all, is unconsciously inferred. [ga] "outlines of psychology," p. . [gb] "outlines of psychology," p. . [gc] mr. romanes adopts a different use of the terms "reason" and "rational," to which allusion will be made in the next chapter. chapter ix. mental processes in animals: their powers of perception and intelligence. two things i have been especially anxious to bring out prominently in the foregoing chapter: first, that the world we see around us is a joint product of two factors--the outward existence, on the one hand, and our active mind on the other; and secondly, that our mental processes and products fall under two categories--on the one hand, perception, giving rise to percepts, perceptual inferences, and intelligence, and on the other, conception (involving the analysis of phenomena), giving rise to concepts, conceptual inferences, and reason. now, i am anxious that the former--to take that first--should be laid hold of and really grasped as an indubitable fact. it is implied in the word "phenomena," that is to say, appearances. we can only know the world as it appears to us; and the world is for us what it appears. there is nothing here in conflict with common sense; the practical reality of phenomena is altered no whit. suppose philosophy tries to get behind phenomena, so as to get a peep at the world beyond. suppose carlyle tells us that "all visible things are emblems; what thou seest is not there on its own account; strictly taken, is not there [as such] at all; matter exists only spiritually, and to represent some idea and _body_ it forth." has he altered the reality of the phenomena themselves? not in the smallest degree. suppose the materialist gives us his analysis of phenomena. are not the phenomena he analyzes still the same, still equally real? no matter how far he analyzes phenomena, behind phenomena he cannot get. the materialist resolves all phenomena into matter in motion or into energy, and says that these are the only real existences. but they are no more real (they are a good deal less real to most of us) than the phenomena with which he started. how can the results of analysis be more real than that which is analyzed? moreover, the matter and energy are still phenomena, and involve, as such, the percipient mind. do what you will, you cannot get rid of the mental factor in phenomena. it is possible that my use of the word "construct," my saying that the object is a thing which each of us constructs at the suggestion of certain sense-stimuli, may lead some to suppose that the process is in some sense an arbitrary one. this, however, would be a misconception. the process under normal conditions is just as inevitable as is, under normal conditions, the fall of a stone to the ground. the law of construction for human-folk is as much a law of nature as the law of gravitation. both laws are condensed statements of the facts of the case. there is nothing arbitrary, lawless, or unnatural in the one or the other; the phrase merely emphasizes the essential presence of the mental factor. if this principle be once thoroughly grasped, it will be seen how shallow and misleading is the view that the world is just reflected in consciousness unchanged as in a mirror, or faithfully photographed as on a sensitive plate. this is to reduce the human mind, which is surely no whit _less_ complex than the human body, to the condition of a mere passive recipient instead of a vital and active agent in the construction of man's world. the next point we have to consider is why we believe, as you and i practically do believe, that the world of phenomena exists as such, not merely for you and for me, but for man. is it not because we believe in the practical unity of mankind? is it not because we believe that, greatly as the conceptual and intellectual superstructure may differ in different individuals, the perceptual basis and foundation are practically identical? the senses and sense-organs give, in all normal individuals, sense-data, which differ only within comparatively narrow limits; and though the intellectual and moral world of the bushman and the north australian may differ profoundly from those of shakespeare and pascal, the perceptual world is, we have every reason to suppose, within these narrow limits, the same. this we may fairly believe; but even so there must be, nay, we know that there are, very great differences in the interpretation of the perceptual world. the individual cannot divest himself of the intellectual and conceptual part of his nature. we, for whom phenomena are more or less conditioned by science, find it difficult to think ourselves into the position of the savage, whose perceptual world is conditioned by crude superstition. the elements of his perceptual world are the same as ours, but the light of knowledge in which we view them is, for him, very dim. when we try to realize his world we find it exceedingly difficult. and when we come to the lower animals--even those nearest us in the scale of life--the difficulties are enormously increased. the sense-data are probably much the same, but they are combined in different proportions. olfactory sensation must, one would suppose, be built into the constructs of the dog and the deer to an extent which we cannot at all realize. and then, as mr. p. g. hamerton has well said, we have to take into account the immensity of the ignorance of animals. that ignorance, in combination with perfect perceptual clearness (ignorance and mental clearness are quite compatible) and with inconceivably strong instincts, produces a creature whose mental states we can never accurately understand. i am tempted here to give the instance mr. hamerton quotes[gd] in illustration of the ignorance of animals. "the following account of the behaviour of a cow," he says, "gives a glimpse of the real nature of the animal. these long-tailed cows, say messrs. huc and gabet, are so restive and difficult to milk, that to keep them at all quiet the herdsman has to give them a calf to lick meanwhile. but for this device, not a single drop of milk can be obtained from them. one day a llama herdsman, who lived in the same house as ourselves, came with a long dismal face to announce that his cow had calved during the night, and that, unfortunately, the calf was dying. it died in the course of the day. the llama forthwith skinned the poor beast and stuffed it with hay. this proceeding surprised us at first, for the llama had by no means the air of a man likely to give himself the luxury of a cabinet of natural history. when the operation was completed, we found that the hay-calf had neither feet nor head; whereupon it occurred to us that, after all, it was perhaps a pillow that the llama contemplated. we were in error, but the error was not dissipated till the next morning, when our herdsman went to milk his cow. seeing him issue forth, the pail in one hand and the hay-calf under the other arm, the fancy occurred to us to follow him. his first proceeding was to put the hay-calf down before the cow. he then turned to milk the cow herself. the mamma at first opened enormous eyes at her beloved infant; by degrees she stooped her head towards it, then smelt at it, sneezed three or four times, and at last proceeded to lick it with the most delightful tenderness. this spectacle grated against our sensibilities; it seemed to us that he who first invented this parody upon one of the most touching incidents in nature must have been a man without a heart. a somewhat burlesque circumstance occurred one day to modify the indignation with which this treachery inspired us. by dint of caressing and licking her little calf, the tender parent one fine morning unripped it. the hay issued from within, and the cow, manifesting not the slightest surprise nor agitation, proceeded tranquilly to devour the unexpected provender." are we surprised at the want of surprise on the part of the cow? why should we be? what knows she of anatomy or of physiology? if she could think at all about the matter, she would, no doubt, have expected her calf to be composed of condensed milk. but failing that, why not hay? she had presumably some little experience of _putting_ hay inside. why not _find_ hay inside; and, finding hay, why not enjoy the good provender thus provided? but clearly we must not expect the brutes to possess knowledge to which they cannot attain about matters which in no wise concern their daily life. "in our estimates of the characters of animals," continues mr. hamerton, in his comments on this anecdote, "we always commit one of two mistakes--either we conclude that the beasts have great knowledge because they are so clever, or else we fancy that they must be stupid because they are so ignorant." "the main difficulty in conceiving the mental states of animals," says the same observer, "is that the moment we think of them as _human_, we are lost." yes, but the pity of it is that we cannot think of them in any other terms than those of human consciousness. the only world of constructs that we know is the world constructed by man. "to newton and to newton's dog, diamond," said carlyle, "what a different pair of universes! while the painting in the optical retina of both was most likely the same." different, indeed; if we can be permitted, without extravagance, to speak of the universe as existing at all for diamond, or allowed, except in hyperbole, to set side by side a conception of ultimate generality, like the universe, the summation of all conceptions, and "the painting in the optical retina." carlyle's meaning is, however, clear enough. given two different minds and the same facts, how different are the products! in the construct formed on sight of the simplest object, we give far more than we receive; and what we give is a special resultant of inheritance and individual acquisition. no two of us give quite the same in amount or in quality. it is not too much to say that for no two human beings is the world we live in quite the same. and if this be so of human-folk, how different must be the world of man from the world of the dog--the world of newton from the world of diamond! and we must remember that it is not merely that the same world is differently mirrored in different minds, but that they are two different worlds. if there is any truth in what i have urged in the last chapter, we _construct_ the world that we see. the sensations are, as we have seen, mental facts, in no sense resembling their causes, but representing them in mental symbolism. percepts are the elaborated products of this mental symbolism. the question, then, is not--how does the world mirror itself in the mind of the dog? but rather--how far does the symbolic world of the dog resemble the symbolic world of man? how far is his symbolism the same as ours? only by fully grasping the fact that the external world of objects does not exist independently of us (though something exists which we thus symbolize), shall we realize the greatness of the difficulty which stands in the path of the student of animal psychology. so long as we are content to accept john bunyan's crude analogy of the gateways of sense, the difficulty is comparatively small. there is the outside world self-existent and independent; a knowledge of it comes into the mind through the five gateways of sense--a picture of it through the eye-gate, and so on. the dog has also five similar gateways. the world for him is, therefore, much the same as for us. but this is not a true analogy. the world we see around us is a joint product of an external existence, the independent nature of which we can never know, and the human mind. it is something we construct in mental symbolism. how far does the dog construct a similar world? the answer to this question must, as it seems to me, be largely speculative. and what help have we towards answering it? that afforded by the theory of organic evolution. if we accept that theory, and accept also the view that mental or psychical products are the inseparable concomitants of certain organic or physiological processes, then we have a basis from which to start. that basis i adopt. unfortunately, we have at present but little particular knowledge of the correlation of psychical and physiological processes. we cannot, by the dissection of a brain, draw much in the way of valid and detailed inference as to the nature of the psychical processes which accompany its physiological action. fortunately, however, on the other hand, there are certain physical manifestations which do aid us, and that not a little, in drawing inferences from the physical to the mental. for organisms exhibit certain activities, and from these activities we can infer to some extent the character of the mental processes by which they are prompted. we are wont, in observing the actions of our fellow-men, to draw conclusions (often, alas! erroneous) as to the mental processes which accompany them. we are ourselves active, and we are immediately conscious of the modes of consciousness which accompany our actions. thus the activities of organisms give us some clue to their mental processes, and it is through observation of their physical activities that we gain nearly all that is of particular value concerning the mental activities of animals. these activities we shall have to consider more fully in a future chapter. in the present chapter we shall consider them only so far as they give us information concerning the perceptual world (or worlds) of animals, and the nature of the inferences which we may suppose animals to draw from the phenomena which fall within their observation. i think that, from the fundamental identity of life-stuff, or protoplasm, in all forms of animal life, and from the observed similarity of nerves and nerve-cells when nervous tissue has been developed, and again from the essential resemblance of life-processes in all animal organisms, we are justified in believing that mental or conscious processes, when they emerge, are essentially similar in kind. exactly when they do emerge in the ascending branches of the great tree of animal life it is exceedingly difficult, if not quite impossible, to determine. and it is, i fancy, quite impossible for us so to divest ourselves of the complexity of human consciousness as to imagine what the simplicity of the emergent consciousness in very lowly organisms is like. but i think that we may fairly believe that some dim form of discrimination is the germ from which the spreading tree of mind shall develop.[ge] i assume, then, that, granting the theory of evolution, the early stages of the process of construction--discrimination, localization, and outward projection--are the same in kind throughout the whole range of animal life, wherever we are justified in surmising that psychical processes occur, and the power of registration and revival in memory has been established. as will be gathered, however, from what i have already said, i hold that the nature of the constructs produced is and must be for us human-folk, since we are human-folk, to a large extent a matter of speculation. remembering this, then, endeavouring never to lose sight of it for a moment, let us consider what we may fairly surmise concerning the constructs and the process of construction in animals. * * * * * there can be no question that the animals nearest us in the scale of life--the higher mammalia--form constructs analogous to, if not closely resembling, ours. i do not think the resemblance can be in any sense close, seeing to how large an extent our constructs are literally our _handi_-work. for though in many animals the tongue and lips are delicate organs of touch--not to mention the trunk of the elephant--and though in the monkeys and many rodents the hands are used for grasping, still we have no reason to suppose that in any other mammal the geometrical sense of touch plays so determining a part in the formation of constructs as in man. on the other hand, in the dog and the deer, for example, not only must the marvellously acute sense of smell have a far higher suggestive value, but smells and odours must, one would suppose, be built into the constructs in a far larger proportion. but although their constructs may not closely resemble ours, the constructs of animals may, i believe, be fairly regarded as closely analogous to our own. and as with us, so with them, a comparatively simple and meagre suggestion may give rise, through association in experience, to the construction of a complex object. and again, as with us, so with them, the suggested construct may be very vague and indefinite. a dog, for example, is lying asleep upon the mat, and hears an unfamiliar step in the porch without. there can be no question that this suggests the construct man. but from the very nature of the case, this must be vague and indefinite. so, too, when a chamois, bounding across the snow-fields, stops suddenly when he scents the distant footprints of the mountaineer, the construct that he forms cannot be in any way particularized--no more particularized than is to me the sheep that i hear bleating in the meadow behind yonder wall. and no one is likely to question the fact that animals habitually proceed from this first stage--the formation of constructs by immediate association--to the second stage of construction--the defining of constructs by examination. in many of the deer tribe, notably the prong-horn of america, this tendency is so strongly developed that they may be lured to their destruction by setting up a strange and unfamiliar object which, as we put it, may excite their curiosity. a strange noise or appearance will make a dog uneasy until he has by examination satisfied himself of the nature of that which produces it. of this an instance fell under my observation a few days ago. my cat was asleep on a chair, and my little son was blowing a toy horn. the cat, without moving, mewed uneasily. i told my boy to continue blowing. the cat grew more uneasy, and at last got up, stretched herself, and turned towards the source of discomfort. she stood looking at my boy for a minute as he blew. then curling herself up, she went to sleep again, and no amount of blowing disturbed her further. similarly, mr. romanes's dog was cowed at the sound of apples being shot on to the floor of a loft above the stable; but when he was taken to the place, and saw what gave rise to the sound, he ceased to be disquieted by it. every one must have seen animals defining their constructs by examination. a monkey will spend hours in the examination of an old bottle or a bit of looking-glass. at the zoological gardens connected with the national museum at washington, a monkey was observed with a female opossum on his knee. he had discovered the slit-like opening of the marsupial pouch, and took out first one and then another of the young, looked them over carefully, and replaced them without injury.[gf] there may possibly be some difference of opinion as to whether animals are able to infuse into their constructs of other animals the element of feeling. one would, perhaps, fain believe that the beasts of prey were wholly unaware of the pain they inflict on other organisms. but i question whether any close observer of animals could hold this view. even if it were supposed that when two dogs fight they are blind to the pain they are inflicting on each other, their mock-fighting seems to imply a consciousness of the pain they might inflict, but avoid inflicting. and many of us have presumably had experiences analogous to the following: a favourite terrier of mine was once brought home to me so severely gashed in the abdominal region that i felt it necessary to sew up the wound. in his pain the poor dog turned round and seized my hand, but he checked himself before the teeth had closed upon me tightly, and piteously licked my hand. for myself, i cannot doubt that animals project into each other the shadows of the feelings of which they are themselves conscious. the fact that dogs may be deceived by pictures[gg] shows that they may be led through the sense of sight to form false constructs, that is to say, constructs which examination shows to be false. through my friend and colleague, mr. a. p. chattock, i am able to give a case in point. i quote from a letter received by mr. chattock: "your father asks me to tell you about our old spaniel dash and the picture. i remember it well, though it must be somewhere, about half a century ago. we had just unpacked and placed on the old square pianoforte, which then stood at the end of the dining-room, the well-known print of landseer's 'a distinguished member of the humane society.' when dash came into the room and caught sight of it, he rushed forward, and jumped on the chair which stood near, and then on the pianoforte in a moment, and then turned away with an expression, as it seemed to us, of supreme disgust." i think we may say, then, that the higher animals are able to proceed a long way in the formation and definition of highly complex constructs analogous to, but probably differing somewhat from, those which we form ourselves. these constructs, moreover, through association with reconstructs or representations, link themselves in trains, so that a sensation or group of sensations may suggest a series of reconstructs or a series of remembered phenomena. we here approach the question of inferences, of which more anon. but in this connection passing reference may be made to the phenomena of dreaming. dogs and some other animals undoubtedly seem to dream. the nature of dreaming may, perhaps, be best illustrated by a rough analogy. professor clifford likened the human consciousness to a rope made up of a great number of occasionally interlacing strands. let us picture such a rope floating in water. much of it is submerged; only the upper part is visible at the surface. this upper part is like the series of mental phenomena of which we are distinctly conscious. below this lie other series in the half-submerged state of subconsciousness. deeper still lie unconscious physiological processes capable of emerging into the shadow of subconsciousness or the light of distinct consciousness. now picture this rope gradually slipping round as it floats, so that now one part, now another, sees the light. this is analogous to the musing state, when we allow our thoughts to wander unchecked by any effort of attention. attention is the faculty by which we steady the rope, so that one particular strand is kept continuously uppermost. the inattentive mind is one in which the rope keeps slipping round and refuses to be steadied in this manner; and in unquiet sleep, when the faculty of attention is dormant, the strands come quite irregularly and haphazard to the surface, and we have the phantasmagoria of dreams. in the dog or the ape the rope is presumably incomparably simpler. but that it is of the nature of a rope we may, perhaps, not improbably surmise. interest and the attention it commands steady the rope. animals differ widely in their power of attention, as every one knows who has endeavoured to educate his pets. darwin tells us that those who buy monkeys from the zoological gardens, to teach them to perform, will give a higher price if they are allowed a short time in which to select those in which the power of attention is most developed. and when animals dream, their consciousness-rope is slipping round unsteadily. that they do apparently dream is, so far, evidence of their possessing linked chains of memories. in speaking of the faculty of attention in animals, it may be well to note that attention is of two kinds--perceptual or direct, and conceptual or indirect. in perceptual attention its motive is directly suggested by the object which stimulates this concentration of the faculties; a menacing dog, for example, stimulates my perceptual attention. in conceptual attention the motive is ulterior and indirect. the concentrated attention which a man devotes to the acquisition of sanscrit does not arise directly out of the symbols over which he pores; it is of intellectual origin. in the normal life of animals the attention is of the perceptual order; it is a direct stimulation of the faculties through a perceptual presentation of sense or representation in memory which gives rise to an appetence or aversion. the importance of such a faculty is obvious. as m. ribot well says, it is no less than a condition of life. the carnivorous animal that had not its attention roused on sight of prey would stand but a poor chance of survival; the prey that had not its attention roused by the approach of its natural enemy would stand but a poor chance of escape. the emperor moth that had not its attention roused by the scent of the virgin female would stand but a poor chance of propagating its species. we are not, however, at present in a position further to discuss this matter. for there is a factor in the process which we shall have to consider more fully hereafter--the emotional factor. the hungry lion is in a very different position, so far as attention is concerned, from the satiated animal. the force and volume of the attention depends not merely, or even mainly, upon the intensity of the stimulus, but on the emotional state of the recipient organism. endeavour to divert the attention of any animal which is intent upon some action connected with the main business of its life--nutrition, self-defence, or the propagation of the species--the force of attention will at once be obvious. in the training of animals (and young children) artificial associations, pleasurable or painful, have to be established in connection with certain actions. abnormal appetences and aversions have to be introduced into the mental constitution. in this process much depends on the plasticity of the constitution. in the absence of such plasticity it is impossible to establish new associations. we have seen that words are arbitrary[gh] symbols, which we associate with objects, or qualities, or actions. can animals, we may ask, form such arbitrary associations? there can be little question that they can. many of the higher animals understand perfectly some of _our_ words. the word "cat" or "rats" will suggest a construct to the dog on which he may take very vigorous action. how far they are able to communicate with each other is a somewhat doubtful matter. but the signs by which such communication is effected are probably far less arbitrary. and, in any case, the communication would seem to refer only to the here and the now. a dog may be able to suggest to his companion the fact that he has descried a worriable cat; but can a dog tell his neighbour of the delightful worry he enjoyed the day before yesterday? i imagine that what a dog can suggest to his neighbour is what we symbolize by the simple expression "come." but i am fully aware that other observers will interpret the facts in a different way. here is an anecdote that is communicated to me by mr. robert hall warren, of bristol. "my grandfather," he says, "a merchant of this city, or, as thomas poole, of stowey, would have preferred calling him, 'a tradesman,' had two dogs, one a small one and another larger, who, being fierce, rejoiced in the appropriate name of boxer. on one of his business journeys into cornwall he took the smaller dog with him, and for some reason left it at an inn in devonshire, promising to call for him on his return from cornwall. when he did so, the landlord apologized for the absence of the dog, and said that, some time after my grandfather left, the little dog fought with the landlord's dog, and came off much the worse for the fight. he then disappeared, and some time afterwards returned with another and larger dog, who set upon his enemy, and, i think, killed him. then the two dogs walked off, and were no more seen. from the description given, my grandfather had no doubt that the larger dog was boxer, and, on returning home, found that the little dog had come back, and that both dogs had gone away, and, after a time, had returned home, where he found them." now, some will say that the little dog told boxer all about it; but i am inclined to believe that the facts may be explained by the communication "come." dogs can also communicate their wishes to us. the action of begging in dogs is a mode of communication with us. mr. romanes tells of a dog that was found opposite a rabbit-hutch begging for rabbits. when i was at the diocesan college near capetown, a retriever, scamp, used to come in and sit with the lecturers at supper. he despised bread, but used to get an occasional bone, which he was not, however, allowed to eat in the hall. he took it to the door, and stood there till it was opened for him. on one occasion he heard without the excited barking of the other dogs. he trotted round the hall, picked up a piece of bread which one of the boys had dropped, and stood with it in his mouth at the door. when it was opened, he dropped the bread, and raced off into the darkness to join in the fun. in a similar way, but with less marked intelligence, i have seen a dog begging before a door which he wished opened. my cat has been taught to touch the handle of the door with his paw when he wishes to leave the room. mr. arthur lee, of bristol, tells me that a favourite cat has a habit of knocking for admittance by raising the door-mat and letting it fall. this is an action similar to those communicated by several observers to _nature_, where cats have learnt either to knock for admittance or to ring the bell--an action which, as my friend, mr. j. clifton ward, informed me, was also performed by a dog of his. i think, therefore, that it is unquestionable that the higher animals are able to associate arbitrary signs with certain objects and actions, and to build these signs into the constructs that they form. sir john lubbock has tried some experiments with his intelligent black poodle van, with the object of ascertaining how far the dog could be taught to communicate his wishes by means of printed cards. "i took," he says,[gi] "two pieces of cardboard, about ten inches by three, and on one of them printed in large letters the word 'food,' leaving the other blank. i then placed the two cards over two saucers, and in the one under the 'food' card put a little bread-and-milk, which van, after having his attention called to the card, was allowed to eat. this was repeated over and over again till he had had enough. in about ten days he began to distinguish between the two cards. i then put them on the floor, and made him bring them to me, which he did readily enough. when he brought the plain card, i simply threw it back; while, when he brought the 'food' card, i gave him a piece of bread, and in about a month he had pretty well learned to realize the difference. i then had some other cards printed with the words 'out,' 'tea,' 'bone,' 'water,' and a certain number also with words to which i did not intend him to attach any significance, such as 'nought,' 'plain,' 'ball,' etc. van soon learned that bringing a card was a request, and soon learned to distinguish between the plain and printed cards; it took him longer to realize the difference between words, but he gradually got to recognize several, such as 'food,' 'out,' 'bone,' 'tea,' etc. if he was asked whether he would like to go out for a walk, he would joyfully fish up the 'out' card, choosing it from several others, and bring it to me or run with it in evident triumph to the door. "a definite numerical statement always seems to me clearer and more satisfactory than a mere general assertion. i will, therefore, give the actual particulars of certain days. twelve cards were put on the floor, one marked 'food' and one 'tea.' the others had more or less similar words. i may again add that every time a card was brought, another similarly marked was put in its place. van was not pressed to bring cards, but simply left to do as he pleased.[gj] "day . van brought 'food' times, 'tea' times. " . " " " " . " " " " " " . " " " " " " . " " " " " " . " " " " " 'nought' once. " . " " " " " " . " " " " " " . " " " " " " . " " " " " 'door' once. " . " " " " " " . " " " " " -- -- "thus, out of times, he brought 'food' times, 'tea' times, and [one out of] the other cards only twice. moreover, the last time he was wrong he brought a card--namely, 'door'--in which three letters out of four were the same as in 'food.'" these experiments and observations are of great interest. but, of course, no stress whatever must be laid on the fact that _words_ chanced to be printed on the cards instead of any other arrangements of lines. i draw attention to this because i have heard sir john lubbock's interesting experiments quoted, in conversation, as evidence that the dog understands the meaning of words, not only spoken, but written! what they show is that van is able, under human guidance, to associate certain arbitrary symbols with certain objects of appetence; and, desiring the object, will bring its symbol. it would have been better, i think, because less misleading to the general public, had sir john lubbock selected other arbitrary symbols than the printed words we employ. then no one could have run away with the foolish notion that the dog _understands_ the meaning of these words. no doubt if they had been written in greek or hebrew, some people would have been interested, but not surprised, to learn that a dog can be taught to understand with perfect ease these languages! the next question is--have the higher animals the power of analyzing their constructs and forming isolates or abstract ideas of qualities apart from the constructs of which these qualities are elements? can we say, with mr. romanes,[gk] "all the higher animals have general ideas of 'good-for-eating' and 'not-good-for-eating,' _quite apart from any particular objects of which either of these qualities happens to be characteristic_"? or with leroy,[gl] that a fox "will see snares when there are none; his imagination, distorted by fear, will produce deceptive shapes, to which he will attach _an abstract notion of danger_"? now, this is a most difficult question to answer. but it seems to me that, if we take the term "abstract idea" in the sense in which i have used the word "isolate," we must answer it firmly, but not dogmatically (this is the last subject in the world on which to dogmatize), in the negative. fully admitting, nay, contending, that this is a matter in which it is exceedingly difficult to obtain anything like satisfactory evidence, i fail to see that we have any grounds for the assertion that the higher animals have abstract ideas of "good-for-eating" or "not-good-for-eating," quite apart from any particular objects of which either of these qualities happens to be characteristic.[gm] the particular example is well chosen, since the idea of food is a dominant one in the mind of the brute. there can be no question that the quality of eatability is built in by the dog into a great number of his constructs. but i question whether this quality can be isolated by the dog, and can exist in his mind divorced from the eatables which suggest it. if it can, then the dog is capable of forming a concept as i have defined the term. i can quite understand that a hungry dog, prowling around for food, has, suggested by his hunger, vague representations in memory of things good to eat, in which the element of eatability is predominant and comparatively distinct, while the rest is vague and indistinct. and that this is a concept in mr. sully's use of the term, i admit. but it appears to me that there is a very great difference between a perceptual construct with eatability predominant and the rest vague, and a conceptual isolate or abstract idea of eatability quite apart from any object or objects of which this quality is characteristic. and to mark the difference, i venture to call the prominent quality a _predominant_ as opposed to the _isolate_ when the quality is floated off from the object. _no doubt it is out of this perceptual prominence of one characteristic and vagueness of its accompaniments that conceptual isolation of this one characteristic has grown, as i believe, through the naming of predominants._ but i should draw the line between the one and the other somewhere distinctly above the level of intelligence that is attained by any dumb animal. i am not prepared either to affirm or deny that this line should be drawn exactly between brute intelligence and human intelligence and reason, though i strongly incline to the view that it should. i am not sure that every savage and yokel is capable of isolation, that he raises the predominant to the level of the isolate, or abstract idea. i am not sure that these simple folk submit the phenomena of nature around them, and of their own mental states to analysis. but they have in language the instrument which can enable them to do so, even if individually some of them have not the faculty for using language for this purpose. that is, however, a different question. but i do not at present see satisfactory evidence of the fact that animals form isolates, and i think that the probability is that they are unable to do so. i am, therefore, prepared to say, with john locke, that this abstraction "is an excellency which the faculties of brutes do by no means attain to." i am anxious, however, not to exaggerate my divergence, more apparent, i believe, than real, from so able a student of animal psychology as mr. romanes. let me, therefore, repeat that it is the power of analysis--the power of isolating qualities of objects, the power of forming "abstract ideas quite apart from the particular objects of which the particular qualities happen to be characteristic," as i understand these words--that i am unable to attribute to the brute. animals can and do, i think, form predominants; they have not the power of isolation. furthermore, it seems to me that this capacity of analysis, isolation, and abstraction constitutes in the possessor a new mental departure, which we may describe as constituting, not merely a specific, but a generic difference from lower mental activities. i am not prepared, however, to say that there is a difference in kind between the mind of man and the mind of the dog. this would imply a difference in origin or a difference in the essential nature of its being. there is a great and marked difference in kind between the material processes which we call physiological and the mental processes we call psychical. they belong to wholly different orders of being. i see no reason for believing that mental processes in man differ thus in kind from mental processes in animals. but i do think that we have, in the introduction of the analytic faculty, so definite and marked a new departure that we should emphasize it by saying that the faculty of perception, in its various specific grades, differs generically from the faculty of conception. and believing, as i do, that conception is beyond the power of my favourite and clever dog, i am forced to believe that his mind differs generically from my own. * * * * * passing now to the other vertebrates, the probabilities are that their perceptual processes are essentially similar to those of the higher animals; but, in so far as these creatures differ more and more widely from ourselves, we may, perhaps, fairly infer that their constructs are more and more different from ours. still, the thrush that listens attentively on the lawn and hops around a particular spot must have a vague construct of the worm he hopes to have a more particular acquaintance with ere long. the cobra that i watched on the basal slopes of table mountain, and that raised his head and expanded his hood when i pitched a pebble on to the granite slope over which he was gliding, must have had a vague percept suggested thereby. the trout that leaps at your fly so soon as it touches the water must have a vague percept of an eatable insect which suggests his action. the carp[gn] that come to the sound of a bell must have, suggested by that sound, vague percepts of edible crumbs. and no one who has watched as a lad the fish swimming curiously round his bait can doubt that they are by examination defining their percepts, and drawing unsatisfactory inferences of a perceptual nature. and here let us notice that the whole set of phenomena which have been described in previous chapters under the heads of recognition-marks, of warning coloration, and of mimicry, involve close and accurate powers of perception. recognition-marks are developed for the special purpose of enabling the organisms concerned rapidly and accurately to form particular perceptual constructs. of what use would warning coloration be if it did not serve to suggest to the percipient the disagreeable qualities with which it is associated? the very essence of the principle of mimicry is that misleading associations are suggested. here a false construct, untrue to fact, that is to say, one that verification would prove to be false, is formed; just as a well-executed imitation orange, in china or in soap, may lead a child to form a false construct, one that is proved to be incorrect so soon as the suggestions of sight are submitted to verification by touch, smell, and taste. no one who has carefully watched the habits of birds can have failed to notice how they submit a doubtful object to examination. probably the avoidance of insects protected by warning colours is not perfectly instinctive. i have seen young birds, after some apparent hesitation, peck once or twice doubtfully at such insects. a young baboon with whom i experimented at the cape seemed to have an undefined aversion to certain caterpillars, which he could not be induced to taste, though he smelt at them. scorpions he darted at, twisted off the sting, and ate with greedy relish. if nudibranchs and other marine invertebrates be protectively coloured, there must be corresponding perceptual powers in the fishes that are thus led to avoid them; for there seems to be definite avoidance, and not merely indifference. this, however, might be made the subject of further experiment, not only with fishes, but with other animals. i tried some chickens with currant-moth caterpillars, to each of which i tied with thread a large looper. some of them would have nothing to do with the unwonted combination. but one persistently pecked at the looper, and tried to detach it from its fellow-prisoner. though, on the whole, there was some tendency for aversion to the currant-moth caterpillar to overmaster the appetence for the looper, i was not altogether satisfied with the result of the experiment. but i think that if the protectively coloured larva had been regarded with mere indifference (i.e. neither aversion nor appetence), the appetence for the loopers should have made the chickens seize them at once. to return to fishes. it is probably difficult or impossible for us to imagine what their constructs are like; but that they, too, proceed to define them by examination seems to be a legitimate inference from some of their actions. mr. bateson says, "the rockling searches [for food] by setting its filamentous pelvic fins at right angles to the body, and then swimming about, feeling with them. if the fins touch a piece of fish or other soft body, the rockling turns its head round and snaps it up with great quickness. it will even turn round and examine uneatable substances, as glass, etc., which come in contact with its fins, and which presumably seem to it to require explanation."[go] and, speaking of the sole, the same observer says,[gp] "in searching for food the sole creeps about on the bottom by means of the fringe of fin-rays with which its body is edged, and, thus slowly moving, it raises its head upwards and sideways, and gently pats the ground at intervals, feeling the objects in its path with the peculiar viliform papillæ which cover the lower (left) side of its head and face. in this way it will examine the whole surface of the floor of the tank, stopping and going back to investigate pieces of stick, string, or other objects which it feels below its cheek." if we admit the fact that carp come to be fed at the sound of a bell, we have evidence that some fishes can associate an arbitrary sound with the advent of things good to eat. but it is, perhaps, better at present to regard the fact as one requiring verification. that some birds can associate arbitrary signs with their percepts will be admitted by all who have watched their habits. and from its peculiar and almost unique power of articulation, the parrot shows us that not only may the words suggest a construct, but that the sight of the construct may suggest the word that it has heard associated with the object by man. mr. romanes gives evidence which satisfies him that a parrot which had associated the word "bow-wow" with a particular dog, uttered this sound when another dog entered the room. the word was here suggested at sight, not of the same object, but of an object which the bird recognized as similar. a somewhat similar case is furnished by one of my own correspondents (miss mabel westlake). "we left london," she says, "in december, , and brought our grey parrot with us; but left behind with a friend our favourite cat, a dark tortoiseshell with a white breast, the forehead clearly marked with a division down the middle to the tip of the nose. this led to our calling her 'demi.' for a week or two after our arrival in bristol, a black-and-white cat belonging to the people formerly living here frequented the house. the parrot seemed delighted to see this cat, which was larger than our old cat, and called it dem, as she had been accustomed to do in london. from that time until the commencement of january ( ), which was over a year, the parrot had not seen a cat that we are aware of, nor had we heard her call it for a long time. about six weeks ago, as i was coming along kingsdown parade, a large black kitten followed me home. we took it in and fed it. the next day it came into the room where the parrot was, and she immediately said 'puss! puss! puss! hullo, dear!' and during the day called it by the same name, 'dem! dem! dem!' that she had called our cat in london." we may here notice that, in most of the tricks which animals are taught to perform, the action is suggested by a form of words (or the tone and manner in which they are uttered). mr. john g. naish, j.p., of ilfracombe,[gq] has taught his cockatoo the following trick (i quote mr. naish's own words): "i give him a shilling, which he puts into the slit of a money-box. this is 'enlisting.' after that, i say to him, 'will you die for the queen, like a loyal soldier?' then he lies on his back, with his paws together, for as long as i hold up my finger. 'now live for your master!' he takes hold of my finger and resumes his erect posture. last year i took him into the street near my house, and collected on our 'hospital saturday.' he worked for more than an hour before he became impatient. and then he would do no more, but flung the coins over his head or at the giver in the funniest way. he went to sleep for a long time after that performance; and when he awoke and i took him, he covered my face with kisses, as if he was glad to find his bad dream was over." the weariness and failure to perform the trick when tired, and the long sleep which succeeded, are interesting points. what i wish especially to notice is, however, that the actions are suggested by certain forms of words; but that there is no evidence that the form of words is in any sense understood. when the onlooker sees a bird lie on its back when asked if it will die for the queen, and get up again when told to live for its master, he is apt to think that, since _he_ understands the form of words, the bird must understand them too. but i am convinced that mr. naish's intelligent cockatoo could have been taught with equal ease to lie down at the command "abracadabra," and to stand up again at "hocus pocus." tricks taught to animals involve the performing animal and the human onlooker. the form of words introduced is _for the sake of the latter_, not for the sake of the former. so much has been written concerning the intelligence of the parrot, and so much has been said concerning its imitative power of speech, that i must say somewhat on this head. i have received from miss mildred sturge, of clifton, an interesting account of an african west coast parrot which was possessed by miss tregelles, of falmouth. this parrot used the phrases it had learnt appropriately in time and place. "at dinner, when he saw the vegetable-dishes, he generally said, 'polly wants potato;' at tea he would say, 'polly wants cake,' or 'polly's sop,' or 'polly's toast.' our grandmother's house was not far from the station, and almost before people could hear it, polly would announce, 'grandmamma, the train is coming,' and presently the train would quietly go by. besides repeating much poetry, polly made new editions by putting lines together from different authors; but the remarkable thing was that he always got the right rhyme. one of his favourite mixtures was, 'sing a song of sixpence' and 'i love little pussy.' one day my mother overheard-- "'four and twenty blackbirds, when they die, go to that world above, baked in a pie.'" now, we must not underrate nor overrate the evidence afforded by parrot-talk. the rhyme-association is interesting; but since we cannot suppose that the poetry is more to the parrot than a linked series of sounds, there does not seem much evidence of intelligence here, though the evidence of memory is important. the correct association of words and phrases with appropriate objects and actions is of great interest. but the fact that they are words and phrases does not give them a higher value than that of imitative actions in the dog or other animal. what parrot-talk does give us evidence of is ( ) remarkable powers of memory; ( ) an almost unique power of articulation; ( ) a great faculty of imitation; ( ) and some intelligence in the association of certain linked sounds which we call phrases with certain objects or actions. the teaching of phrases to the parrot is certainly not more remarkable than the teaching of clever tricks to many birds. but the fact that word-sounds are articulated throws a glamour over these special tricks, and leads some people to speak of the parrot's using language, instead of saying that the parrot can imitate some of the sounds made by man, and can associate these sounds with certain objects. * * * * * coming now to the invertebrates, much has been written concerning the psychology and intelligence of ants and bees. what shall we say concerning their constructs? for reasons already given, i think we may suppose that they are analogous to ours; but it can scarcely be that they in any way closely resemble ours. their sense-organs are constructed on a different plan from ours; they have probably senses of which we are wholly ignorant. is it conceivable, by any one who has grasped the principle of construction, that with these differently organized senses and these other senses than ours, the world they construct can much resemble the world we construct? remember how largely our perceptual world is the product of our geometrical senses--of our delicate and accurate sense of touch, and of our binocular vision, with its delicate and accurate muscular adjustments. remember how largely these muscular adjustments enter into our perceptual world as constructed in vision. and then remember, on the other hand, that the bee is encased in a hard skin (the chitinous exoskeleton), and that its tactile sensations are mainly excited by means of touch-hairs seated thereon. remember its compound eye with mosaic vision, coarser by far than our retinal vision, and its ocelli of problematical value, and the complete absence of muscular adjustment in either the one or the other. can we conceive that, with organs so different, anything like a similar perceptual world can be elaborated in the insect mind? i for one cannot. admitting, therefore, that their perceptions may be fairly surmised to be analogous, that their world is the result of construction, i do not see how we can for one moment suppose that the perceptual world they construct can in any accurate sense be said to resemble ours. for all that, the processes of discrimination, localization, outward projection; the formation of vague constructs, their definition through experience, and the association of reconstructs or representations;--all these processes are presumably similar in kind to those of which we have evidence in ourselves. in considering such organisms as ants and bees, however, we must be careful to avoid the error of supposing that, because they happen to have no backbones, they are necessarily low in the scale of life and intelligence. the tree of life has many branches, and, according to the theory of evolution, these divergent branches have been growing up side by side. there is no reason whatever why the bee and the ant, in their branch of life, should not have attained as high a development of structure and intelligence as the elephant or the dog in their branch of life. i do not say that they have. as it is difficult to compare their structure, in complexity and efficiency, with that of vertebrates, so is it difficult to compare their intelligence. the mere matter of size may have necessitated the condensation of intelligence into instinct in a far higher degree than was required in the big-brained mammals. still, their intelligence, though of a different order and on a different plane, may well be as high. and darwin has said that the so-called brain of the ant may perhaps be regarded as the most wonderful piece of matter in the world. that ants have some power of communication seems to be proved by the interesting experiments of sir john lubbock. he found that they could carry information to the nest of the presence of larvæ, and that the greater the number of larvæ to be fetched, the greater the number of ants brought out to fetch them in a given time. on one occasion sir john lubbock put an ant to some larvæ. "she examined them carefully, and went home without taking one. at this time no other ants were out of the nest. in less than a minute she came out again with eight friends, and the little group made straight for the heap of larvæ. when they had gone two-thirds of the way, i imprisoned the marked ant; the others hesitated a few minutes, and then, with curious quickness, returned home." this is only one observation out of many; and it shows ( ) that since the marked ant took no larva home, she must have given information which led the others to come out--unless we can suppose that the smell of the larvæ she had examined still hung about her; and ( ) that the communication was not detailed, and probably was no more than "come," for, when the leader of the party was removed, the rest knew not[gr] where to go--very possibly knew not why they had been summoned. passing now to creatures of lower organization, it is exceedingly difficult so to divest ourselves of our own special mental garments as to imagine what their simple and rudimentary constructs are like. perhaps we may fairly surmise that, as visual, olfactory and auditory organs develop, and differentiate from a common basis of more simple sensation, the process of outward projection has its rudimentary inception. the earthworm, which finds its way to favourite food-stuffs buried in the earth in which it lives, would seem to possess the power of outward projection in a dim and possibly not very definite form. through their marginal bodies--simple auditory or visual organs--the medusæ may have a rudimentary form of this capacity. in any case, they seem to have the power of localization. mr. romanes says,[gs] "a medusa being an umbrella-shaped animal, in which the whole of the surface of the handle and the whole of the concave surface of the umbrella is sensitive to all kinds of stimulation, if any point in the last-named surface is gently touched with a camel-hair brush or other soft (or hard) object, the handle or manubrium is (in the case of many species) immediately moved over to that point, in order to examine or brush away the foreign body." and the same author thus describes[gt] the process of discrimination in the sea-anemone: "i have observed that if a sea-anemone is placed in an aquarium tank, and allowed to fasten upon one side of the tank near the surface of the water, and if a jet of sea-water is made to play continuously and forcibly upon the anemone from above, the result, of course, is that the animal becomes surrounded by a turmoil of water and air-bubbles. yet, after a short time, it becomes so accustomed to this turmoil that it will expand its tentacles in search of food, just as it does when placed in calm water. if now one of the expanded tentacles is gently touched with a solid body, all the others close around that body in just the same way as they would were they expanded in calm water. that is to say, the tentacles are able to discriminate between the stimulus which is supplied by the turmoil of the water, and that which is supplied by their contact with the solid body, and they respond to the latter stimulus notwithstanding that it is of incomparably less intensity than the former." here, in discrimination, we reach the lowest stage of mental activity. it is exceedingly difficult, however, to determine how far such simple responses to stimuli are merely organic, and how far there enters a psychological element. i ought not, perhaps, to pass over in perfect silence the subject of protozoan psychology. m. binet has published a little book on "the psychic life of micro-organisms," in the preface of which he says, "we could, if it were necessary, take every single one of the psychical faculties which m. romanes reserves for animals more or less advanced on the zoological scale, and show that the _greater part_ of these faculties belonged equally to micro-organisms." he says that "there is not a single infusory that cannot be frightened, and that does not manifest its fear by a rapid flight through the liquid of the preparation," and he speaks of infusoria fleeing "in all directions like a flock of frightened sheep." he attributes memory to _folliculina_, and instinct "of great precision" to _difflugia_. he regards some of these animalculæ as "endowed with memory and volition," and he describes the following stages:-- " . the perception of the external object. " . the choice made between a number of objects. " . the perception of their position in space. " . movements calculated either to approach the body and seize it or to flee from it." but when we have got thus far, we are brought up by the following sentence: "we are not in a position to determine whether these various acts are accompanied by consciousness, or whether they follow as simple physiological processes." since, therefore, the fear, memory, instinct, perception, and choice, spoken of by m. binet, may be merely physiological processes (though, of course, they _may be_ accompanied by some dim unimaginable form of consciousness), it seems scarcely necessary to say more about them here. * * * * * i have now said all that is necessary, and all that i think justified by the modest scope of this work, concerning the process of construction in animals, and the nature of the constructs we may presume that they form. the process i hold to be similar in kind throughout the animal kingdom wherever we may presume that it occurs at all. but the products of the process seem to me to be presumably widely different. if we steadily bear in mind the fact that the world of man is a joint product of an external existence and the human mind, and then ask whether it is conceivable that the joint products of this external existence and the dog-mind, the bird-mind, the fish-mind, the bee-mind, or the worm-mind are exactly or even closely similar, we must, it seems to me, answer the question with an emphatic negative. * * * * * we will now consider the nature of the inferences of animals. it will be remembered that a distinction was drawn between perceptual inferences and inferences involving a conceptual element. as i use the words, perceptual inferences are a matter, at most, of intelligence; but conceptual inferences involve the higher faculty of reason. it will be necessary here to say somewhat more than i have already said concerning inference. when i see an orange, that object is mentally constructed at the bidding of certain sight-sensations. all that is actually received is the stimulus of the retinal elements; the rest is suggested and supplied by the activity of the mind. it is sometimes said that this complementary part of the perception is inferred. so, too, when i hear a howl in the street which suggests the construct dog, it may be said that i infer the presence of the dog. and again, when the dog is perceived to be in pain, it may be said that this is an inference. now, although the use of the word "inference" to denote the complementary part of a percept seems a little contrary to ordinary usage, still there are some advantages in so--with due qualification--employing it. but since, as it seems to me, the characteristic of the inference, if so we style it, in the formation of constructs by immediate association is its unconscious nature (i.e. unconscious as a process) we may perhaps best meet the case by speaking of these as unconscious inferences. when the inference is not immediate and unconscious, but involves a more individual conscious act of the mind in the perceptual sphere, we may speak of it as intelligent; and when the inference can only be reached by analysis and the use of concepts, we may call it rational. defining, therefore, "inference" as the passing of the mind from something immediately given to something not given but suggested through association and experience, we have thus three stages of inference: ( ) unconscious inference on immediate construction (perceptual); ( ) intelligent inference, dealing with constructs and reconstructs (perceptual); and ( ) rational inference, implying analysis and isolation (conceptual). concerning unconscious inferences in animals, i need add nothing to that which i have already said concerning the process of construction. it is concerning the intelligent inferences[gu] of animals that i have now to speak. i do not propose here to bring forward a number of new observations on the highly intelligent actions which animals are capable of performing. mr. romanes has given us a most valuable collection of anecdotes on the subject in his volume on "animal intelligence." it is more to my purpose to discuss some of the more remarkable of these, and endeavour to get at the back of them, so as to estimate what are the mental processes involved. in doing so, the principle i adopt is to assume that the inferences are perceptual, unless there seem to be well-observed facts which necessitate the analysis of the phenomena, the formation of isolates, and therefore the employment of reason (_as i have above defined it_). in doing this, i shall _seem_ to differ very widely from mr. romanes and other interpreters of animal habits and intelligence. but i believe that the divergence is less wide than it seems. i believe that it is largely, but i fear not entirely, a question of the terms we employ. why, then, rediscuss the question under these new terms? because i believe that such rediscussion may place the matter in a fresh and, perhaps, clearer light. the question of the relation of animal intelligence to human reason is one upon which there is a good deal of disagreement, and one that has been discussed and rediscussed. i seek to put it in a somewhat new light. i have endeavoured to define carefully and accurately the terms i use, and the sense in which i use them. i have coined for my own purposes unfamiliar terms such as "construct," "isolate," and "predominant," that i might thereby be enabled to avoid the use of terms which, from the different senses in which they are employed by different writers, have become invested with a certain ambiguity. i trust, therefore, that even those with whom i seem most to disagree will allow that my aim has not been mere disputation, but scientific accuracy and precision in a difficult subject where these qualities are of essential importance. i take first some observations communicated by mr. h. l. jenkins to mr. romanes, since, though they raise a point which we have already shortly considered, they form a transition from unconscious to perceptual inferences. speaking of the intelligence of the elephant, mr. jenkins says,[gv] "what i particularly wish to observe is that there are good grounds for supposing that elephants possess abstract ideas; for instance, i think it is impossible to doubt that they acquire, through their own experience, notions of hardness and weight." he then details observations which show that elephants at first hand up things of all kinds to their mahouts with considerable force, but that after a time the soft articles are handed up rapidly and forcibly as before, but that hard and heavy things are handed up gently. "i have purposely," he says, "given elephants things to lift which they could never have seen before, and they were all handled in such a manner as to convince me that they recognized such qualities as hardness, sharpness, and weight." now, the question i wish here to ask is--do the observations of mr. jenkins, the nature of which i have indicated, afford good or sufficient reasons for supposing that these animals possess abstract ideas? and i reply--that depends upon what is meant by abstract ideas. if it is implied that the abstract ideas are _isolates_; that is, qualities considered quite apart from the objects of which they are characteristic, i think not. but if mr. jenkins means that elephants, in a practical way, "recognize such qualities as hardness, sharpness, and weight" as _predominant_ elements in the constructs they form, i am quite ready to agree with him. i much question, however, whether there is any conscious inference in the matter. the elephant sees a new object, and unconsciously and instinctively builds the element hardness or weight into the construct that he forms. and he shows his great intelligence by dealing in an appropriate manner with the object thus recognized. but i do not think any reasoning is required; that is to say, any process involving an analysis of the phenomena with subsequent synthesis, any introduction of the conceptual element. let us consider next an observation which shows a very high degree of perceptual intelligence on the part of the dog. several observers have described dogs, which had occasion to swim across a stream, entering the water at such a point as to allow for the force of the current. and both dr. rae and mr. fothergill communicated to mr. romanes instances[gw] of the dog's observing whether the tide was ebbing or flowing, and acting accordingly. now, i believe that the dog performs this action through intelligence, and that man explains it by reason. the dog has presumably had frequent experience of the effect of the stream in carrying him with it. he has been carried beyond the landing-place, and had bother with the mud; but when he has entered the stream higher up, he has nearly, if not quite, reached the landing-stage. his keen perceptions come to his aid, and he adjusts his action nicely to effect his purpose. on the bank sits a young student watching him. he sees in the dog's action a problem, which he runs over rapidly in his mind. velocity of stream, two miles an hour. width, one-eighth of a mile. dog takes ten minutes to swim one-eighth of a mile. distance flowed by the stream in ten minutes, one-third of a mile. clever dog that! he allows just about the right distance. a little short, though! has rather a struggle at the end. the dog intelligently performs the feat; the lad reasons it out. i do not know whether i am making my point sufficiently clear. a wanton boy is constantly throwing stones at birds and all sorts of objects. he does not know much about the force of gravitation or the nature of the curve his stone marks out; but he allows pretty accurately for the fall of the stone during its passage through the air. he acquires a catapult; and, being an intelligent lad, he perceives that he must aim a little above the object he wishes to hit. this is a perceptual inference. reason may subsequently step in and explain the matter, or very possibly, being human, sparks of reason fly around his intelligent action. am i using the word "reason" in an unnatural and forced sense? i think not. my use is in accord with the normal use of the word by educated people. two men are working in the employ of a mechanical engineer. listen to their employer as he describes them. "a most intelligent fellow is a; he does everything by rule of thumb; but he's wonderfully quick at perceiving the bearing of a new bit of work; he sees the right thing to do, though he cannot tell you why it should be done. now, b is a very different man; he is slow, but he reasons everything out. a knows the right thing to do; and b can tell you why it must be done. a has the keenest intelligence, but b the clearest reasoning faculty. if i have occasion to question them about any mechanical contrivance, a says, 'let me see it work;' but b says, 'let me think it out.'" in other words, a, the intelligent man, deals with phenomena as wholes, and his perceptual inferences are rapid and exact; while b, the reasoner, analyzes the phenomena, and draws conceptual inferences about them. let us take next dr. rae's[gx] most interesting description of the cunning of arctic foxes. these clever animals, he tells us, soon learn to avoid the ordinary steel and wooden traps. the hudson bay trappers, therefore, set gun-traps. the bait is laid on the snow, and connected with the trigger of the gun by a string fifteen or twenty feet long, five or six inches of slack being left to allow for contraction from moisture. the fox, on taking up the bait, discharges the gun and is shot. but, after one or more foxes have been shot, the cunning beasts often adopt one of two devices. either they gnaw through the string, and then take the bait; or they tunnel in the snow at right angles to the line of fire, and pull the bait _downwards_, thus discharging the gun, but remaining uninjured. this is regarded by dr. rae as a wonderful instance of "abstract reasoning." here, again, it is the "abstract reasoning" that i question. do the clever foxes resemble the intelligent workman a, or the abstract reasoner b? i believe that their actions are the result of perceptual inferences. they adopt their cunning devices _after one or more foxes have been shot_. their keen perceptions (let me repeat that the perceptions of wild animals are extraordinarily keen) lead them to see that this food, quiet as it seems, has to be taken with caution. with regard to the devices adopted, i think we need further information. do arctic foxes tunnel in the snow for any other purposes? what is the proportion of those who adopt this device to those who gnaw through the string? have careful and reliable observers watched the foxes? or are their actions, as described by dr. rae, inferences, on the part of the trappers, from the state of matters they found when they came round to examine their traps? without fuller information on these points, it is undesirable to discuss the case further. even if we had full details, however, we should be as little able to get at the process of perceptual inference in the case of the fox as we are in the case of the intelligent workman, who sees the right thing to do, but cannot tell you how he reached the conclusion. no one can watch the actions of a clever dog without seeing how practical he is. he is carrying your stick in his mouth, and comes to a stile. a young puppy will go blundering with the stick against the stile, and, perhaps, go back home, or get through the bars and leave the stick behind. but practical experience has taught the clever dog better. he lays down the stick, takes it by one end, and draws it backwards through the opening at one side of the stile. a friend tells me of a dog which was carrying a basket of eggs. he came to a stile which he was accustomed to leap, poked his head through the stile, deposited the basket, ran back a few yards, took the stile at a bound, picked up the basket, and continued on his course. "intelligent fellow!" i exclaim. "yes," says my friend, "he _knew the eggs would break_ if he attempted to leap with the basket!" this is just the little gratuitous, unwarrantable, human touch which is so often filled in, no doubt in perfect good faith, by the narrators of anecdotes. against such interpolations we must be always on our guard. it is so difficult not to introduce a little dose of reason. mr. romanes obtained from the zoological gardens at regent's park a very intelligent capuchin monkey, on which his sister made a series of most interesting and valuable observations. this monkey on one occasion got hold of a hearth-brush, and soon found the way to unscrew the handle. after long trial, he succeeded in screwing it in again, and throughout his efforts always turned the handle the right way for screwing. having once succeeded, he unscrewed it and screwed it in again several times in succession, each time with greater ease. a month afterwards he unscrewed the knob of the fender and the bell-handle beside the mantelpiece. commenting on these actions, mr. romanes speaks[gy] of "the keen satisfaction which this monkey displayed when he had succeeded in making any little discovery, such as that of the mechanical principle of the screw." i once watched, near the little village of ceres, in south africa, a dung-beetle trundling his dung-ball over an uneven surface of sand. the ball chanced to roll into a sand hollow, from which the beetle in vain attempted to push it out. the sides were, however, too steep. leaving the ball, he butted down the sand at one side of the hollow, so as to produce an inclined plane of much less angle, up which he then without difficulty pushed his unsavoury sphere. now, it seems to me that, if we say, with mr. romanes, that the brown capuchin discovered the _principle of the screw_, we must also say that the dung-beetle that i observed in south africa was acquainted with the _principle of the inclined plane_. such an expression, i contend, involves an unsatisfactory misuse of terms. a mechanical principle is a concept,[gz] and as such, in my opinion, beyond the reach of the brute--monkey or beetle. that of which the monkey is capable is the perceptual recognition of the fact that certain actions performed in certain ways produce certain results. why they do so he neither knows nor cares to know. what the brown capuchin discovered was not the principle of the screw, but that the action of screwing produced the results he desired--a very different matter. my friend, mr. s. h. swayne, tells me that the elephant at the clifton zoo, having taking a tennis-racket from a boy who had been plaguing him, broke it by leaning it against a step and deliberately stepping on it in the middle, where it was unsupported. a most intelligent action. and it would have been a capital piece of exercise for the lad's reasoning power, had he been required to analyze the matter, to show why the elephant's action had the desired effect, and set forth the principle involved. i do not think the elephant himself possesses the faculty requisite for such a piece of reasoning. he is content with the practical success of his actions; principles are beyond him. i will now give two instances of intelligence in vertebrates which exemplify phases of inference somewhat different from those which we have so far considered. mr. watson, in his "reasoning power of animals,"[ha] tells of an elephant which was suffering from eye-trouble, and nearly blind. a dr. webb operated on one eye, the animal being made to lie down for the purpose. the pain was intense, and the great beast uttered a terrific roar. but the effect was satisfactory, for the sight was partially restored. on the following day the elephant lay down of himself, and submitted quietly to a similar operation on the other eye. no doubt the elephant's action here was, in part, the result of its wonderful docility and training. but there was also probably the inference that, since dr. webb had already given him relief, he would do so again. the anticipation of relief outmastered the anticipation of immediate discomfort or pain. i do not think, however, that any one is likely to contend that any rational analysis of the phenomena is necessarily involved in the elephant's behaviour. the other instance i will quote was communicated by mr. george bidie to _nature_.[hb] he there gives an account of a favourite cat which, during his absence, was much plagued by two boys. about a week before his return the cat had kittens, which she hid from her tormentors behind the book-shelves in the library. but when he returned she took them one by one from this retreat, and carried them to the corner of his dressing-room where previous litters had been deposited and nursed. here abnormal circumstances and the reign of anarchy and persecution forced her to adopt a hiding-place where she might bring forth her young; but the return of normal conditions, sovereignty, and order led her to take up her old quarters under the protection of her master. now, look at the description i have given in explanation of her conduct. see how it bristles with conceptual terms: "abnormal," with its correlative "normal;" "anarchy and persecution," "protection" and "order." all this, i believe, is mine, and not the cat's. for her there was a practical perception, in the one case of plaguing boys, in the other case of protecting master; and her action was the direct outcome of these perceptions through the employment of her intelligence. some stress has been laid on the occasional use of tools by animals. mr. peal[hc] observed a young elephant select a bamboo stake, and utilize it for detaching a huge elephant-leech which had fixed itself beneath the animal's fore leg near the body. "leech-scrapers are," he says, "used by every elephant daily." he also saw an elephant select and trim a shoot from the jungle, and use it as a switch for flapping off flies. how far, we may ask, do such actions imply "a conscious knowledge of the relation between the means employed and the ends attained"?[hd] that, again, depends upon how much or how little is implied in this phrase. a boy picks up a stone and throws it at a bird; he comes home and unlocks the garden-gate with a key; he enters his room, and removes the large "liddell and scott" which he uses as a convenient object to keep the lid of his play-box shut; he opens the box, and cuts himself a slice of cake with his pocket-knife. then he goes to his tutor, who is teaching him about means and ends, and their relation to each other. he is told that the throwing of the stone was the means by which the death of the bird, or the end, was to be accomplished; that the use of the knife was the means by which the end in view, the severance of a piece of cake, was to be effected, and so on. he is led to see that the employment of a great many different things, differing in all sorts of ways--stones, keys, lexicons, and knives--may be classified together as means; and that a great many various effects, the death of a bird or the cutting a bit of cake, may be regarded as ends. he is told that when he thinks of the means and the ends together, as means and end, he will be thinking of their relationship. and it is explained to him that means and ends and their relationships are concepts, and involve the exercise of his reasoning powers. weary and sick to death of concepts and relationships and reason, at length he escapes to the garden. picking up a light stick, he sweeps off the heads of some peculiarly aggravating poppies, and determines to think no more of means and ends, continuing to use the stick meanwhile as a most appropriate means to the end of decapitating the poppies. by all which i mean to imply that there is a great difference between selecting and using a tool for an appropriate purpose, and possessing a conscious knowledge of the relation between the means employed and the ends attained. i do not think that any conception of means, or end, or relationship is possible to the brute. but i believe that the elephant can perceive that this stick will serve to remove that leech. and if this is what mr. romanes means by its possessing a conscious knowledge of the relation between the means employed and the ends attained, then i am, so far, at one with him in the interpretation of the facts, though i disagree with his mode of expressing them. i do not propose to consider particular instances of intelligent inferences as displayed by the invertebrates. bees in the manipulation of their comb, ants in the economy of their nest, spiders in the construction of their web and the use they make of their silken ropes, show powers of intelligent adaptation which cannot fail to excite our wonder and admiration. but apart from the fact that insect psychology is more largely conjectural than that of the more intelligent mammals, a consideration of these actions would only lead me to reiterate the opinion above frequently expressed. in a word, i regard the bees in their cells, the ants in their nests, the spiders in their webs, as workers of keen perceptions and a high order of practical intelligence. but i do not, as at present advised, believe that they reason upon the phenomena they deal with so cleverly. intelligent they are; but not rational. once more, let me repeat that the sense in which i use the words "rational" and "reason" must be clearly understood and steadily borne in mind. mr. romanes uses them in a different sense. "reason," he says,[he] "is the faculty which is concerned in the intentional adaptation of means to ends. it therefore implies the conscious knowledge of the relation between means employed and ends attained, and may be exercised in adaptation to circumstances novel alike to the experience of the individual and to that of the species. in other words, it implies the power of perceiving analogies or ratios, and is in this sense equivalent to the term 'ratiocination,' or the faculty of deducing inferences from a perceived equivalency of relations. this latter is the only sense of the word that is strictly legitimate." it is not my intention to criticize this use of the term "reason." whether animals are capable of a conscious knowledge of the relation between means employed and ends attained, depends, as we have already seen, upon how much is implied by the word "knowledge"--whether the knowledge is perceptual or conceptual. my only care is to indicate what seem to me the advantages of the usage (legitimate or illegitimate) i adopt. i repeat, then, that the introduction of the process of analysis appears to me to constitute a new departure in psychological evolution; that the process differs generically from the process of perceptual construction on which it is grafted. and i hold that, this being so, we should mark the departure in every way that we can. i mark it by a restriction of the word "intelligence" to the inferences formed in the field of perception; and the use of the word "reason" when conceptual analysis supervenes. whether i am justified in so doing, whether my usage is legitimate or not, i must leave others to decide. but, adopting this usage, i see no grounds for believing that the conduct of animals, wonderfully intelligent as it is, is, in any instances known to me, rational. i say that the introduction of the process of analysis appears to me to constitute a new departure. this, however, must not be construed to involve any breach of continuity. i do not believe that there is or has been any such breach of continuity. take a somewhat analogous case. i regard the introduction of aerial respiration in animal life as a new departure. organisms which had hitherto been water-breathers became air-breathers. but i do not imagine that there was any breach of continuity in respiration. the tadpole begins life as a water-breather only; the frog into which he develops is an air-breather; but there is no breach of continuity between the one state and the other. so, too, the little child dwells in the perceptual sphere; the man into whom he develops is capable of conceptual thought; but there is no breach of continuity in the mental life of the child. it is true that, with all our talk on the subject, we cannot say exactly when in this continuous mental life the new departure is made. but this is no proof whatever that there is no new departure. in a sigmoidal curve there is a new departure where the convex passes into the concave. we may find it difficult to mark the exact point of change. but that does not invalidate the fact that the change does actually take place. if i be asked how, in the course of mental evolution, the new departure was rendered possible, i reply--through language. the first step was, i imagine, _the naming of predominants_. if noiré and professor max müller be correct in their views, language took its origin in the association of an uttered sound with certain human activities. the action thus named was, so to speak, floated off by its sign. by diacritical marks attached to the word, the agent, the action, and the object of the action were distinguished, and thus came to be differentiated the one from the other. inseparable in fact, they came henceforth to be separable in thought. here was analysis in the germ. the action or activity was isolated, and henceforth stood forth as an element in abstract thought. all the busy world around was interpreted in terms of activities. the host of heaven and all the powers of earth were named according to their predominant activities. the moon became the measurer, the sun the shining one, the wind the one who bloweth, the fire the purifier, and so forth. our verbs and nouns, then, being named predominants (agents, actions, or objects), adjectives and adverbs were subsequently introduced to qualify these by naming a quality less predominant, or to indicate the how, the when, and the where. when once the different activities and different qualities came to be named or symbolized, they were, as i say, floated off from the agents or objects, and through isolation entered the conceptual sphere. _the named predominant became an isolate._ body and mind became separable in thought; the self was differentiated from the not-self; the mind was turned inwards upon itself through the isolation of its varying phases; and the consciousness of the brute became the self-consciousness of man. language, and the analytical faculty it renders possible, differentiates man from the brute. "if a brute," says mr. mivart,[hf] "could think 'is,' brute and man would be brothers. 'is' as the copula of a judgment implies the mental separation and recombination of two terms that only exist united in nature, and can, therefore, never have impressed the sense except as one thing. and 'is,' considered as a substantive verb, as in the example, 'this man is,' contains in itself the application of the copula of judgment to the most elementary of all abstractions--'thing' or 'something.' yet if a being has the power of thinking 'thing' or 'something,' it has the power of transcending space and time by dividing or decomposing the phenomenally one. here is the point where instinct [intelligence] ends and reason begins." i regard this as one of the truest and most pregnant sentences that mr. mivart has written. and when once the logos had entered into the mind of man, and made him man, it slowly but surely permeated his whole mental being. hence language is not only involved in our concepts, but also in our percepts, in so far as they are ours. professor max müller goes so far as to question whether an unnamed percept is possible. and adult intellectual man is so permeated by the logos that i am not prepared to disagree with him when he says that he has no unnamed perceptions. nevertheless, the actions of the speechless child and our dumb companions show that they (children and animals) are capable of forming mental products of the perceptual order. but here, once more, we must not forget that it is in terms of these adult human percepts that we interpret the percepts of children and animals; that in doing so we cannot divest ourselves of the garment of our conceptual thought, that we cannot banish the logos, and that, therefore, these percepts other than ours cannot be identical with ours, though they are of the same order, saving their conceptual element. we may put the matter thus-- ( ) _x_ Ã� dog-mind } { percepts to be interpreted in terms ( ) _x_ Ã� cat-mind } = { of ( ), being analogous thereto but ( ) _x_ Ã� infant-mind } { not identical therewith. ( ) _x_ Ã� adult human mind = the percepts of psychologists, named or namable. if the views that i have thus very briefly sketched (for i have no right to offer an opinion on a question of linguistic science) be correct, language has made analysis, isolation, and conceptual thought possible. but there may have been a transitory stage when the word-signs stood for predominants, not yet for isolates. granting the possibility or probability of this, i am prepared to follow professor max müller in his contention that language and thought, from the close of that stage onward, are practically inseparable, and have advanced hand-in-hand. it is true that i can now think out a chemical or physical problem without the use of words--the stages of the experimental work being visualized, just as a chess-player may think out a game in pictures of the successive moves. but, historically, i believe the power to do this has been acquired through language; and if i am able temporarily to isolate and analyze without language, thought being at times a little ahead of naming, yet the fact remains that language is absolutely necessary to make such advances good, if not for me, at any rate for man. and here i would make one more suggestion. professor max müller, as the result of analysis of the aryan language, finds a comparatively small number of roots which he says are in all cases symbolic of concepts. yes, for us now they symbolize concepts. but in their inception may they not have been symbolic of predominants? have we not in them the signs for predominants not yet converted for the primitive utterers into isolates? may not these have been the stepping-stones from the perceptual predominants of animal man, to the conceptual isolates of rational man? or, to modify the analogy, may they not have been the embryonic wings by which the human race were floated off from the things of sense into the free but tenuous air of abstract thought? lastly, before taking leave of the subject of this chapter, i am most anxious that it should not be thought that, in contending that intelligence is not reason, i wish in any way to disparage intelligence. nine-tenths at least of the actions of average men are intelligent and not rational. do we not all of us know hundreds of practical men who are in the highest degree intelligent, but in whom the rational, analytic faculty is but little developed? is it any injustice to the brutes to contend that their inferences are of the same order as those of these excellent practical folk? in any case, no such injustice is intended; and if i deny them self-consciousness and reason, i grant to the higher animals perceptions of marvellous acuteness and intelligent inferences of wonderful accuracy and precision--intelligent inferences in some cases, no doubt, more perfect even than those of man, who is often distracted by many thoughts. notes [gd] "chapters on animals," p. . [ge] or perhaps we may say, in the language of analogy, that when the germinal psychoplasm of some dim form of organic memory is fertilized by the union therewith of the more active male element of discrimination, a process of segmentation of the psychoplasm sets in by which, in process of differentiation, the tissues and organs of the mind are eventually developed. [gf] _nature_, vol. xxxviii. p. . [gg] for examples, see romanes's "animal intelligence," p. . [gh] i use the word "arbitrary" in the sense that they form no part of the normal construct such as would be formed by the animal. [gi] "the senses of animals," p. . [gj] as i understand the observations here tabulated, the twelve cards lay always within van's reach and sight. an ordinary untrained dog would have taken no notice of them. but van, when he wanted food or tea, went and fetched the appropriate card, and got what he wanted in exchange. in twelve days he only made two mistakes, bringing "nought" once and "door" once. [gk] "mental evolution in man," p. . [gl] "intelligence of animals," p. . [gm] mr. romanes also says ("mental evolution in animals," p. ), "this abstract idea of ownership is well developed in many if not in most dogs." by an abstract idea of ownership i understand a conception of ownership which, to modify mr. romanes's phrase, is quite apart from any objects or persons of which such ownership happens to be characteristic. even if we believe that a dog can regard this or that man as his owner, or this or that object as his master's property, still even this seems to me a very different thing from his possessing an abstract idea of ownership. [gn] doubt has recently been thrown on this fact. mr. bateson has shown that some fishes do not hear well, and has suggested that the carp may be attracted by seeing people come to the edge of the pond. [go] journal of marine biological association, new series, vol. i. no. , p. . i should not myself have used the word "explanation." [gp] ibid. vol. i. no. , p. . [gq] i have to thank this gentleman for a most interesting account of the intelligence of his favourite bird. [gr] professor max müller suggests to me that perhaps the ants were frightened. [gs] "mental evolution in animals," p. . [gt] ibid. p. . [gu] these fall under the "practical intelligence" of mr. mivart. all their intelligent activities, in his view, are performed by the exercise of merely sensitive faculties, through their "consentience." i agree to so large an extent with mr. mivart in his estimate of animal intelligence, and in his psychological treatment, that i the more regret our wide divergence when we come to the philosophy of the subject. i am with him in believing that conception and perception, in the sense he uses the words, are beyond the reach of the brute. but i see no reason to suppose that these higher faculties differ _in kind_ from the lower faculties possessed by animals. they differ generically, but not in kind. i believe that, through the aid of language, the higher faculties have been developed and evolved from the lower faculties. here, therefore, i have to part company from mr. mivart. [gv] romanes, "animal intelligence," p. . [gw] "animal intelligence," p. . [gx] "animal intelligence," p. ; and _nature_, vol. xix. p. . [gy] "animal intelligence," p. . [gz] mr. romanes regards it as, in the case of the capuchin, a _recept_. but when he speaks of a generic idea of causation, and generic ideas of principles, and of qualities as recepts, i find it exceedingly difficult to follow him. they seem to me to be concepts supposed to be formed in the absence of language. [ha] page . [hb] vol. xx. p. . [hc] _nature_, vol. xxi. p. . [hd] romanes, "animal intelligence," p. : definition of _reason_. [he] "mental evolution in animals," p. . [hf] "lessons from nature," pp. , . chapter x. the feelings of animals: their appetences and emotions. there is one aspect of the mental processes of men and animals that we have so far left unnoticed--the aspect of feeling, the aspect of pleasure and pain. quite distinct from, and yet intimately associated with, our perception of a beautiful scene, is the pleasure we derive therefrom; and quite distinct from, and yet inseparably bound up with, our perception of a discordant clang, is the painful effect that it produces. we have, however, no separate organs for the appreciation of pleasure and pain. these feelings arise out of, and are bound up with, our sensations, our perceptions, and especially with the conscious exercise of our bodily activities. there may be, at any rate in some cases, separate nerves for the appreciation of the pleasurable and the painful; but even if this be so, these shades of feeling are so closely associated with our other activities, mental and bodily, that we may for the present regard them simply as the accompaniments of these activities. the question has been raised and much discussed whether all our activities are accompanied by some shade or colouring of feeling, pleasurable on the one hand, or painful on the other; or whether some of these activities may not be indifferent in this respect, affording us neither pleasure nor pain. put in this way, i think we may say that there may be activities which are thus indifferent. but if it be asked whether, in addition to the pleasurable and painful feelings, there is a third class of _feelings_, which we may call indifferent or neutral, i am inclined to answer it in the negative. i hold that every feeling, as such, must belong either to the painful or pleasurable class, and that if the pleasurable and painful, so to speak, exactly balance each other, then feeling, as such, does not emerge into consciousness at all. for, as lotze says, "we apply the name 'feelings' exclusively to states of pleasure and pain, in contrast with sensations as [the elements of] indifferent perceptions of a certain content." the broadest division of the feelings is, therefore, into pleasurable on the one hand, and painful on the other. another general question with regard to the feelings is--with what condition or state of the bodily organization are they associated? in answer to this question we may say ( ) that any very violent and abnormal stimulus produces pain; ( ) that the conditions of pleasure are to be sought within the limits of the healthy and normal exercise of the bodily functions and mental activities; ( ) that within these limits the changes of activity consequent upon the rhythmic flow of normal organic processes bring with them, in the aggregate, pleasure, the delight of healthy life; ( ) that within these limits, again, we experience pleasure or pain, enjoyment or weariness, ease or discomfort, happiness or unhappiness, with the continued rise and fall of our life-tide. for, as spinoza says, "we live in perpetual mutation, and are called happy or unhappy according as we change for the better or the worse." so long as our activities remain at a dead level, there is indifference--neither pleasure nor pain. a rise of the tide of activity brings pleasure, a fall the reverse. lastly, we may say ( ) that beyond the limits of healthy and normal exercise there is, on the one hand, excessive exercise which, carried far enough, may give rise, first to fatigue, and then to acute pain; and, on the other hand, deficient exercise, which may produce that dull and numb form of pain which we call discomfort, or a sense of craving or want. pleasures and pains may thus be either massive or acute, diffused or locally concentrated. on the whole, we may say, with mr. grant allen,[hg] that "the acute pains, as a class, arise from the action of surrounding destructive agencies; the massive pains, as a class, from excessive function or insufficient nutriment." but since massive pains, when pushed to an extreme, merge into the acute class, "the two classes are rather indefinite in their limits, being simply a convenient working distinction, not a natural division." "massive pleasure can seldom or never attain the intensity of massive pain, because the organism can be brought down to almost any point of innutrition or exhaustion; but its efficient working cannot be raised very high above the average. similarly, any special organ or plexus of nerves can undergo any amount of violent disruption or wasting away, giving rise to very acute pains; but organs are very seldom so highly nurtured and so long deprived of their appropriate stimulant as to give rise to very acute pleasure." the amount of pleasure varies, according to mr. grant allen, whose discussion of the subject is, perhaps, the best and clearest we have, directly as the number of nerve-fibres involved, and inversely as the natural frequency of their excitation. no doubt the principles above sketched out are somewhat vague and general; but we are scarcely justified in formulating any that are more precise and exact. accepting now the theory of evolution, we may say, furthermore, that during the long process of the moulding of life to its environment, there has been a constant tendency to associate pleasure with such actions as contribute towards the preservation and conservation of the individual and the race, and to associate pain with such actions as tend to the destruction or detriment of the individual or the race. for there can be little doubt that pleasure and pain are the primary incentives to action. without the association of pleasure with conservative action, and pain with detrimental action, it is difficult to conceive how the evolution of conscious creatures would be possible. conservative action, if it is to be persisted in by a conscious creature, must be associated directly or indirectly with pleasurable feelings; nay, more, if it is to be persistently persevered in, its non-performance must be associated with that dull form of pain which we call a craving or want. only under such conditions could activities which tend to the survival of the individual and the race be fostered and furthered. it must be remembered, however, that such association is founded on experience, and has no necessary validity beyond experience. that quinine, though unpleasant to the taste, is, under certain circumstances, beneficial to the individual, and that acetate of lead, though sweet-tasted, is harmful, cannot be fairly urged in opposition to this principle, since the effects of these drugs form no part of the normal experience of the individual and the race. nor can it be fairly objected that animals transported to new countries often eat harmful and poisonous plants presumably because they are nice; for these plants form part of an unwonted environment. nor, again, is the fact that the association of pleasure with conservative action and pain with harmful action is not always perfect, in any sense fatal to the general principle. for the establishment of the association is still in progress; and with the increase in the complexity of life its accurate establishment is more and more difficult. no one is likely to contend that what appears to be a general principle must also be an invariable rule. the general principle is that under the joint influence of pleasure (attractive) and pain (repellent) the needle of animal life sets towards the pole of beneficial action. that the needle does not always point true only illustrates the fact that life-activities are still imperfect. let us notice that it is under the joint action of pleasure and pain that the needle sets. we must not think only of the positive aspect, and neglect the negative. what we know as wants, cravings, appetites, desires, and dissatisfactions, are dull and continuous pains,[hh] which tend to drive us to actions by which they shall be annulled, and the performance of which shall give us the pleasures of gratification. dr. martineau regards a felt want as a mainspring of our energy. "life," he says,[hi] "is a cluster of wants, physical, intellectual, affectional, moral, each of which may have, and all of which may miss, the fitting object. is the object withheld or lost? there is pain: is it restored or gained? there is pleasure: does it abide or remain constant? there is content. the two first are cases of disturbed equilibrium, and are so far dynamic that they will not rest till they reach the third, which is their posture of stability and their true end." to this i would only add that the content which follows on the keen pleasure of satisfaction is evanescent, and ere long lapses into indifference, on which in due time follows the dull pain resulting from the recurrent pressure of the want or desire. it is clear that, in introducing these wants and desires, we are entering the sphere of the emotions, and it is sometimes said that the emotions have their basis in pleasure and pain. if by this it is meant that the emotions often exhibit more or less prominently one or other of these two aspects of feeling, we may agree with the statement. it will be well, however, to lead up to our consideration of the emotions by taking a general review of the manner in which the organism responds to external stimuli. a dog is lying dreamily on the lawn in the sunshine. suddenly he raises his head, pricks his ears, scents the air, looks fixedly at the hedge, and utters a low growl. place your hand upon his shoulder, and you will find that his muscles are all a-tremble. he can restrain himself no longer, and darts through the hedge. you follow him, look over the hedge, and see that it is his old enemy, the butcher's cur. they are moving slowly past each other, head down, teeth bared, back roughened. you whistle softly. such a whistle would generally bring him bounding to your feet. but now it is apparently unheard. the two dogs have a short scuffle, and the cur slinks off. your dog races after him; but after a few minutes returns, jumps up at you playfully, and then lies down again on the grass. but every now and then, for ten minutes or so, he raises his head and growls softly. let us briefly analyze the dog's actions, reading into them, conjecturally, the accompaniments in consciousness. as he lies on the lawn, he receives a sense-stimulus, auditory or olfactory, which gives rise to the construction of the percept dog (perhaps particularized through olfactory discrimination). about the formation of constructs or percepts, however, we have already said enough; we have now to consider their effects. the head is raised, the ears pricked, and so on. the dog is on the alert. his attention is roused. what are the physiological effects? certain motor-activities or tendencies to activity. these are of two kinds--first, in connection with the sense-organs, the muscles of which are brought into play in such a way as to bring the organs to bear upon the exciting object; secondly, in connection with many other muscles, which are innervated, so as to be ready to act rapidly and forcibly. the first motor-effect, that on the muscles of the sense-organs, is a very characteristic physical concomitant of the psychological state which we term "attention;" the second effect, the incipient innervation of muscles likely to be called into play, is equally characteristic of the psychological state we call alertness. meanwhile an emotional state is rising in the mind of the dog. we may call it, conjecturally, anger and combativeness. but what we name it does not much signify for our present purpose. it has a growing tendency to work itself out in a series of definitely directed actions. and this reaches its point of culmination when the dog rushes through the hedge and stands with bared teeth before his antagonist. a whole set of appropriate muscles are now strongly innervated. there is probably a double innervation--an innervation prompting to activity and an innervation inhibiting or restraining from activity. the attention is so concentrated that he heeds not, probably hears not, his master's whistle. he is keenly on the alert. then he sees his chance; the inhibition or restraint is withdrawn, and he flies at his opponent. the emotional tendency works itself out in action. even after he has resumed his place on the lawn, memories of the emotional state return, and lead him to lift his head, slightly bare his teeth, and growl. now, with regard to the emotional state here indicated, we may notice, first, that it is initiated by a percept; secondly, that associations of pleasure or pain are by no means the most important or predominant characteristics; thirdly, that the motor-tendencies seem to be essential, the emotional state being the psychological aspect of these motor-tendencies; and, fourthly, that we should perhaps be justified in speaking of a presentative emotion when the percept which gives rise to the emotion is presentative; and a representative emotion where the originating percept is represented in memory. and with regard to the attention which was incidentally introduced, we may notice that it, too, has motor-concomitants, and that it is directly associated with the emotional state. if no emotional state is aroused by a percept, attention is not specially directed to the object. the concentration of the attention is directly proportional to the intensity of the emotion evoked. emotions, then, would seem from this illustration to be certain psychological states which accompany activities or tendencies to activity. they are evoked by appropriate objects perceived or remembered. where the tendency is towards the object, as in the sexual emotions, we may speak of it as an _appetence_; where it is away from the object, as in the emotion of fear, we may speak of it as an _aversion_. appetences are normally pleasurable; aversions, painful. it is clear that the organism must be in a condition fitting it to carry out its various activities. and this condition is more or less variable. in the terms of our previous analogy (chapter ii.) the tissues are "explosive." after a series of explosions have taken place in a tissue, its store of explosive material becomes exhausted, and a powerful stimulus is required to liberate further energy in the exhausted tissue. a period of rest is required to enable the plasmogen to generate a fresh store of explosive material. as this store increases to its maximum pitch, the tissue becomes more and more ready to respond at the slightest touch. responsiveness to external stimuli is spoken of as _sensitiveness_; emotional responsiveness is called _sensibility_. what we have before spoken of as a want or craving is a state of heightened sensibility, which often gives rise to a painful state of general uneasiness. it may also give rise to perceptual representations in memory, as may be seen in the dreams experienced during a state of extreme sexual sensibility. if we seek a basis for the emotional states, therefore, we shall find it in sensibility rather than in pleasure and pain. the motor-accompaniments of the emotional states have long been known under the title of the "expression" of the emotions. the term is too deeply rooted to be altered; but we may notice that what is called the expression of an emotion is really _its partial fulfilment in action_. some psychologists, dissatisfied with the term "expression of the emotions," as seeming to imply that the emotion is one thing and its expression another, go so far as to say that the motor-accompaniments are the objective aspect of what, under its subjective aspect, is the emotion. it is quite possible, however, to experience an emotion without any motor-accompaniments at all. nevertheless, there is, i believe, in such cases an unfulfilled tendency to action. a most important feature in general physiology and psychology is _the postponement or suppression of action_. the physiological faculty on which it is based is inhibition. i do not propose to discuss the somewhat conflicting views on the physiological mechanism of inhibition. it is, however, a fact of far-reaching importance which no one is likely to deny. in its higher ranges it is the objective basis and aspect of self-restraint. a stimulus gives rise to sensation and perception; the perception gives origin to an emotional state; and the emotional state is fulfilled in appropriate motor-activities. the process is a continuous one, and, in the absence of inhibition, would in all cases inevitably fulfil itself. but through the faculty of inhibition, the final state of activity may be postponed or suppressed. we may place side by side the physiological series and the accompanying psychological series thus-- stimulus of } --> nervous processes in brain --> { stimulus of sense-organ } { motor-organs. consciousness of } <-- perception, emotion --> { consciousness of sense-stimulus } { activity. the arrows pointing away from perception and emotion are intended to indicate the fact that the consciousness of sense-stimulus on the one hand, and of activity on the other hand, are accompaniments of the nervous processes in the brain, and are referred outwards to the sense-organ or the motor-organ, as the case may be. it must be remembered that the two series, physiological and psychological, belong to distinct phenomenal orders. if one speaks of emotion being fulfilled in activity, and thus seems to jump from the psychological to the physiological series, one does so merely to avoid the appearance of pedantry. now, by the postponement or suppression of action, the process is either arrested in its middle phase, the motor-organs not being innervated at all, or, as i believe to be more probable, the motor-organs are doubly innervated, a stimulus to activity being counteracted by an inhibitory stimulus, the two neutralizing each other either in the motor-organ or the efferent nerves which convey the stimuli. in any case, there is no consciousness[hj] of activity. and the mind occupies itself more and more completely with the central processes, perception, and emotion, and also, in human beings, conceptual thoughts and emotions. nevertheless, at any rate _so long as we confine ourselves to the perceptual sphere_, these processes have their normal fulfilments in action, and, if they become sufficiently intense, actually do so fulfil themselves. now, since the emotions with which we are now dealing (we may call them emotions in the perceptual sphere) are stages in the fulfilment of activities (though the activities themselves may be suppressed), it is clear that there may be as many emotional states as there are modes of activity. hence, no doubt, the extreme difficulty of anything like a satisfactory classification of these emotions, especially when the activities are regarded as a merely extraneous expression. moreover, when certain emotions reach a high pitch of intensity, they may defeat their own object, and give rise, not to definite well-executed motor-activities, but to helpless contradictory actions, affections of glandular and other organs, and a general condition of collapse. the emotion of fear, for example, will lead to motor-activities tending to remove a man from the source of danger; but when it reaches the degree of dread, or its culmination terror, the effects are markedly different. the countenance pales, the lips tremble, the pupils of the eyes become dilated, and there is an uncomfortable sensation about the roots of the hair. the bowels are often strongly affected, the heart palpitates, respiration labours, the secretions of the glands are deranged, the mouth becomes dry, and a cold sweat bursts from the skin. the muscles cease to obey the will, and the limbs will scarcely support the weight of the body. here we have all the effects of a prolonged struggle to escape. just as such a prolonged struggle will at length produce these motor and other effects accompanied by the emotion of terror; so, if the emotion of terror be produced directly, these motor and other effects are seen to accompany it. mr. charles richardson, the well-known engineer of the severn tunnel, has recorded several instances of railway servants and others being so affected by the approach of a train or engine that they have been unable to save themselves by getting out of the way, though there was ample time to do so. this may have been through the effect of terror. but one man, who was nearly killed in this way, only just saving himself in time, informed me that he experienced no feeling of terror; he was unable to explain why, but he couldn't help watching the train as it darted towards him. in this case it seems to have been a sort of hypertrophy of attention. his attention was so rivetted that he was unable to make, or rather he felt no desire to make, the appropriate movements. he said, "i had to shake myself, and only did so just in time. for in another moment the express would have been on me. when it had passed, i came over all a cold sweat, and felt as helpless as a baby. i was frightened enough _then_." cases of so-called fascination in animals may be due in some cases to terror, but more often, perhaps, to a hypertrophy of attention, such as is seen in the hypnotic state. speaking of the effects of artificial light on fish, mr. bateson says,[hk] "bass, pollack, mullet, and bream generally get quickly away at first, but if they can be induced to look steadily at the light with both eyes, they generally sink to the bottom of the tank, and on touching the bottom commonly swim away.... in the case of mullet, effects apparently of a mesmeric character sometimes occur, for a mullet which has sunk to the bottom as described will sometimes lie there quite still for a considerable time. at other times it will slowly rise in the water until it floats with its dorsal fin out of the water, as though paralyzed.... when the light is first shown, turbot generally take no notice of it, but after about a quarter of an hour i have three times seen a turbot swim up, and lie looking into the lamp steadily. it seemed to be seized with an irresistible impulse like that of a moth to a candle, and throws itself open-mouthed at the lamp." as a boy i used frequently to "mesmerize" chickens by making them look at a chalk mark. they would then lie for some time perfectly motionless. some such effect has, perhaps, led to the instinct displayed by some animals of "shamming dead." returning now to the emotions as displayed in man, we may take one more example in anger. this is an emotion that arises from the idea of evil having been inflicted or threatened. "under moderate anger," says darwin, "the action of the heart is a little increased, the colour heightened, and the eyes become bright. the respiration is likewise a little hurried; and as all the muscles serving for this purpose act in association, the wings of the nostrils are sometimes raised to allow of a free draught of air; and this is a highly characteristic sign of indignation. the mouth is commonly compressed, and there is almost always a frown on the brow. instead of the frantic gestures of extreme rage, an indignant man unconsciously throws himself into an attitude ready for attacking or striking his enemy, whom he will, perhaps, scan from head to foot in defiance. he carries his head erect, with his chest well expanded, and the feet planted firmly on the ground. with europeans the fists are generally clenched." "under rage the action of the heart is much accelerated, or, it may be, much disturbed. the face reddens, or it becomes purple from the impeded return of the blood, or may turn deadly pale. the respiration is laboured, the chest heaves, and the dilated nostrils quiver. the whole body often trembles. the voice is affected. the teeth are clenched or ground together, and the muscular system is commonly stimulated to violent, almost frantic, action. but the gestures of a man in this state usually differ from the purposeless writhings and struggles of one suffering from an agony of pain; for they represent more or less plainly the act of striking or fighting with an enemy." these examples will serve to remind the reader of the nature of those complex aggregates of organized feelings which we call emotions, and will also show the close connection of these emotions with the associated bodily movements and activities which constitute their normal fulfilment. so close is this connection, that the assumption of the appropriate attitude will conjure up a faint revival of the associated emotion. let any one stand with squared shoulders, clenched fists, and set muscles, and he will find the respiration affected, and perhaps also the heart-beat, and will experience a faint revival of the emotion of anger. very different will be his feelings as he reseats himself, abandons his limbs to a posture of leisurely repose, and allows a pleasant smile to steal over his features. the next point to notice about these emotions is that they are to a large extent instinctive, and are evidenced in the infant at so early a period that individual acquisition is out of the question. in any case, the basis of sensibility is innate. as mr. sully says,[hl] "there are instinctive capacities of emotion of different kinds, answering to such well-marked classes of feeling as fear, anger, and love. these emotions arise uniformly when the appropriate circumstances occur, and for the most part very early in life. thus there is an instinctive disposition in the child to feel in the particular way known as anger or resentment when he is annoyed or injured." in this, as in other cases of instinctive action, of which we shall have more to say in the next chapter, it is, of course, impossible to say for certain how far the activities observed are associated with psychological states. the activities are undoubtedly instinctive. and their performance by an adult would be accompanied by an emotional state. it is, therefore, probable that in the very young child they have their emotional concomitants. still, we must remember that oft-repeated actions tend to become automatic, that the accompanying consciousness sinks into evanescence, and that it is, therefore, _possible_ that the emotional state may not have that vividness which the activities seem to bespeak. there only remains, before passing on to consider the feelings and emotions of animals, to indicate what mr. sully terms[hm] "the three orders of emotion." the first order comprises the individual and personal emotions--those which are self-interested and have sole reference to the individual who feels, enjoys, or suffers. they take origin in percepts, either in presentations of sense or representations in memory. the second order introduces the sympathetic emotions. they are evoked on sight of the sufferings or emotional states of others. if we see a woman insulted, we are filled with indignation; and this emotion has a sympathetic origin. the third order comprises the complex feelings known as _sentiments_. they have reference to certain qualities of objects or activities of individuals which inspire admiration or disapprobation. they are abstract in their nature, and belong to the conceptual sphere. such are love of truth, beauty, virtue, liberty, justice. to become operative on conduct, however, they need, at any rate in the case of most people, to be particularized and individualized, or brought within the perceptual sphere, ere they arouse anything that is emotional in much more than in name. as dr. mccosh has well said, "no man ever had his heart kindled by the abstract idea of loveliness, or sublimity, or moral excellence, or any other abstraction. that which calls forth our admiration is a lovely scene; that which raises wonder or awe is a grand scene; that which calls forth love is not loveliness in the abstract, but a lovely and loving person; that which evokes moral approbation is not virtue in the abstract, but a virtuous agent performing a virtuous act. the contemplation of the beautiful and the good cannot evoke deep or lively emotion. he who would create admiration for goodness must exhibit a good being performing a good action." * * * * * turning now to the lower animals, the first question that suggests itself is--what are their capacities for pleasure and pain? a very difficult question to answer. we cannot, i think, hope to know how much or how little the invertebrates feel--to what degree they are psychologically sensitive. even among the higher vertebrates we are very apt, i imagine, to over-estimate the intensity of their feelings. among human-folk it is not he who halloas loudest that is necessarily most hurt. and it is only through the expression of their feelings in cries and gestures that we can conjecture the feelings of animals. there are grounds for supposing that savages are far less keenly sensitive than civilized people. and we have some reason for believing and hoping that our dumb companions are less sensitive to pain than we are. mr. g. a. rowell, for example, in his "essay on the beneficent distribution of the sense of pain," tells us that "a post-horse came down on the road with such violence that the skin and sinews of both the fore fetlock joints were so cut that, on his getting up again, the bones came through the skin, and the two feet turned up at the back of the legs, the horse walking upon the ends of its leg-bones. the horse was put into a field close by, and the next morning it was found quietly feeding about the field, with the feet and skin forced some distance up the leg-bones, and, where it had been walking about, the holes made in the ground by the leg-bones were three or four inches deep." mr. lamont gives a somewhat similar observation in the case of the reindeer. "on one occasion," he says, "we broke one of the fore feet of an old fat stag from an unseen ambush; his companions ran away, and the wounded deer, after making some attempts to follow them, which the softness of the ground and his own corpulence prevented him doing, looked about him a little, and then, seeing nothing, actually began to graze on his three remaining legs, as if nothing had happened of sufficient consequence to keep him from his dinner." colonel sir charles w. wilson, in his work "from korti to khartoum," gives similar instances with regard to camels. "the most curious thing," he says,[hn] "was that they showed no alarm, and did not seem to mind being hit. one heard a heavy thud, and, looking round, saw a stream of blood oozing out of the wound, but the camel went on chewing his cud as if nothing at all had happened, not even giving a slight wince to show he was in pain." and, again,[ho] "i heard the rush of the shot through the air, and then a heavy thud behind me. i thought at first it had gone into the field-hospital; but, on looking round, found it had carried away the lower jaw of one of the artillery camels, and then buried itself in the ground. the poor brute walked on as if nothing had happened, and carried its load to the end of the day." with regard to this question, then, of the susceptibility of animals to pleasure and pain, no definite answer can be given. that they feel more or less acutely we may be sure; how keenly they feel we cannot tell; but it is better to over-estimate than to under-estimate their sensitiveness. in any case, whether their pain be acute or dull, whether their pleasures be intense or the reverse, we should do all in our power to increase the pleasures and diminish the pains of the dumb creatures who so meekly and willingly minister to our wants. that the bodily feelings and wants occupy a large relative space in the conscious life of brutes can scarcely be questioned. on the one hand are the dull pains resulting from the organic wants and appetences, and driving the animal to their gratification; the keen pleasure that accompanies this gratification, when intelligence is so far developed that it can be foreseen, being a pull in the same direction. and on the other hand are the pleasures of the normal and healthy exercise of the sense-organs and bodily activities giving rise to the pleasures of existence, the joys of active and vigorous life. in the main, these bodily feelings, or sense-feelings, as they are sometimes called, seem to cluster round three chief centres--food, sex, and the free exercise of the bodily activities, including in some cases what seems to be play. give a wild creature liberty and the opportunity of gratifying its appetites; allow its bodily functions the alternating rhythm of healthy and vigorous exercise and restorative repose; and its life is happy and joyous. it is not troubled by the pressure of unfulfilled ideals. the very struggle for existence, keen as it often is, by calling into play the full exercise of the activities, ministers to the health and happiness of brutes as well as men. sir w. r. grove has preached [hp] the advantages of antagonism. speaking of the rabbit, he says, "to keep itself healthy, it must exert itself for its food; this, and perhaps avoiding its enemies, gives it exercise and care, brings all its organs into use, and thus it acquires its most perfect form of life. an estate in somersetshire, which i once took temporarily, was on the slope of the mendip hills. the rabbits on one part of it, that on the hillside, were in perfect condition, not too fat nor too thin, sleek, active, and vigorous, and yielding to their antagonists, myself and family, excellent food. those in the valley, where the pasturage was rich and luxuriant, were all diseased, most of them unfit for human food, and many lying dead on the fields. they had not to struggle for life; their short life was miserable and their death early; they wanted the sweet uses of adversity--that is, of antagonism." without endorsing the view that these rabbits were unhealthy _only_ because they had too much food and comfort--for the food, though abundant, may have been in some way noxious, and the damp situation may have been prejudicial--we may still believe that a struggle for life is better for animals (and men) than unlimited ease and plenty. under the influence, then, of these bodily pleasures and wants, the activities of animals are drawn out and guided. as darwin says, in his autobiography,[hq] "an animal may be led to pursue that course of action which is most beneficial to the species by suffering, such as pain, hunger, thirst, and fear; or by pleasure, as in eating and drinking, and in the propagation of the species; or by both means combined, as in the search for food. but pain or suffering of any kind, if long continued, causes depression, and lessens the power of action, yet it is adapted to make a creature guard itself against any great or sudden evil. pleasurable sensations, on the other hand, may be long continued without any depressing effect; on the contrary, they stimulate the whole system to increased action. hence it has come to pass that most or all sentient beings have been developed in such a manner, through natural selection, that pleasurable sensations serve as their habitual guides. we see this in the pleasure from exertion, even occasionally of great exertion, of the body or mind--in the pleasure of our daily meals, and especially in the pleasure derived from sociability, and from loving our families. the sum of such pleasures as these, which are habitual or frequently recurrent, give, as i can hardly doubt, to most sentient beings an excess of happiness over misery, although they occasionally suffer much. such suffering is quite compatible with belief in natural selection; which is not perfect in its action, but tends only to render each species as successful as possible in the battle for life with other species, in wonderfully complex and changing circumstances." passing now from the bodily feelings and wants to the emotions, there can be no question that the simpler emotions, of which i have taken fear and anger as typical, are shared with us by the dumb brutes. and the interesting observations of mr. douglas spalding showed beyond doubt that they are instinctive--their manifestation being prior to, and not the outcome of, individual experience. writing in _macmillan's magazine_, he says, "a young turkey, which i had adopted when chirping within the uncracked shell, was, on the morning of the tenth day of its life, eating a comfortable breakfast from my hand, when the young hawk in a cupboard just beside us gave a shrill 'chip! chip! chip!' like an arrow, the poor turkey shot to the other side of the room, and stood there, motionless and dumb with fear, until the hawk gave a second cry, when it darted out at the open door right to the extreme end of the passage, and there, silent and crouched in a corner, remained for ten minutes. several times during the course of that day it again heard these alarming sounds, and in every instance with similar manifestations of fear." and as an example of combined fear and anger, mr. spalding says, "one day last month, after fondling my dog, i put my hand into a basket containing four blind kittens three days old. the smell my hand had carried with it sent them puffing and spitting in a most comical fashion." a remarkable instance of inherited antipathy in the dog was communicated by dr. huggins to mr. darwin. he possessed an english mastiff, kepler, which was brought when six weeks old from the stable in which he was born. the first time dr. huggins took him out he started back in alarm at the first butcher's shop he had ever seen, and throughout his life he manifested the strongest and strangest antipathy to butchers and all that pertained to them. on inquiry, dr. huggins ascertained that in the father, in the grandfather, and in two half-brothers of kepler the same curious antipathy was innate. of these, paris, a half-brother, on one occasion, at hastings, sprang at a gentleman who came into the hotel at which his master was staying. the owner caught the dog, and apologized, saying he had never known him to behave thus before except when a butcher came into the house. the gentleman at once said that was his business. that many animals display affection towards their offspring and their mates, towards man and towards other companions, is a matter of familiar observation. often the attachments are strange, as of cats and horses, or contrary to instinctive tendencies, as between cats and dogs. sometimes they are capricious, as when mr. romanes's wounded widgeon conceived a strong, persistent, and unremitting attachment to a peacock;[hr] or even insane, as where a pigeon became the victim of an infatuation for a ginger-beer bottle. strong attachment to man is often exhibited. every one knows the story which mr. darwin tells[hs] of the little monkey who bravely rushed at the dreaded baboon which had attacked his keeper. a friend of my own (the rev. george h. r. fisk, of capetown) tells me the following story (which may be added to the many similar cases reported of dogs) concerning a favourite cat he had as a boy. it happened that the children of the house, my friend among the number, were confined to their room by measles. their mother remained with the children by day and night until they were convalescent. she then came down and resumed her usual daily life, but was shocked at the appearance of the cat, which was little more than skin and bones, and would not touch food or milk. the cat seemed to know that mrs. fisk could help her, and gave her no peace till she had taken her upstairs to the convalescent patients. to mrs. fisk's surprise, the cat snarled and beat the young master with her paws. why the cat chose this peculiar method of venting her feelings it is difficult to say. but immediately afterwards she went down into the kitchen, ate the meat and drank the milk which she had before refused to touch. early next morning she mewed outside the young master's room; and, having gained admittance, sat at the foot of the bed until he woke, and then licked his face and hair. this leads us on to the class of sympathetic emotions. for the sympathetic emotions are those which centre, not round the self, but round some other self in whose welfare an interest is, in some way and for some reason, aroused. not long ago, at the hamburg zoological gardens, i saw two baboons fighting savagely. one at last retreated vanquished, with his arm somewhat deeply gashed. he climbed to a corner of the cage and sat down, moodily licking his wound. thither followed him a little capuchin, and, though his bigger friend took mighty little notice of his overtures, seemed anxious to comfort him, nestling against him, and laying his head against his side. so far as one could judge, it was not curiosity, but sympathy, that prompted his action. the following example of sympathetic action on the part of a dog towards a stranger-dog is communicated to me by mrs. mann, a friend of mine at the cape. carlo was a favourite black retriever, and a highly intelligent animal. "one day," says mrs. mann, "a miserable-looking white dog came into our yard. carlo went up to him, looking displeased, dog-fashion, and ready to fly at the intruder. it was clear, however, that some communication passed between them, for carlo's wrath seemed disarmed, and he trotted into the kitchen, coming out again with a chop-bone (one with a good deal of meat on it) which the cook had given him. on looking into the yard, the miserable cur was seen enjoying the bone, carlo sitting straight up watching him with a look of satisfaction."[ht] that dogs feel sympathy with man will scarcely be questioned by any one who has known the companionship of these four-footed friends. at times they seem instinctively to grasp our moods, to be silent with us when we are busy, to lay their shaggy heads on our knees when we are worried or sad, and to be quickened to fresh life when we are gay and glad--so keen are their perceptions. their life with man has implanted in them some of the needs of social beings; and as they are ever ready to sympathize with us, so do they rejoice in our sympathy. to be deprived of that sympathy, to be neglected, to have no attention bestowed on them, is to some dogs a punishment more bitter than direct reproof. mr. romanes quotes[hu] an account given him by mrs. e. picton of a skye terrier who had the greatest aversion to being washed, snarling and biting during the operation. threats, beating, and starvation were all of no avail; but the animal was reduced to submission by persistent neglect on the part of his mistress. at the end of a week or ten days he looked wretched and forlorn, and yielded himself quite quietly and patiently to one of the roughest ablutions it had ever been his lot to experience. so far i have been content to credit animals with very general and simple forms of emotion--anger, fear, antipathy, affection, and some form of sympathy. if, on the perusal of familiar anecdotes, we also credit them with jealousy, envy, emulation, pride, resentment, cruelty, deceitfulness, and other more complex emotional states, we must remember that every one of these, as we know them, is essentially human. it is necessary to insist on the need of caution and the danger of anthropomorphism. this is, perhaps, even more necessary in the case of the emotions than in that of the perceptions, which we have before considered. even among men, different individuals and different races probably vary far more in their emotions than in their perceptions. the emotions of civilized man have assumed their present form in the midst of complex social surroundings. they one and all bear ineffaceably stamped upon them the human image and superscription. in terms of these complex human emotions we have to decipher the simpler emotional states of the lower animals. we call them by the same names; we think of them as like unto those that we experience. and we can do no otherwise, if we are to consider them at all. but let us not lose sight of the fact that all we can ever hope to see in the mirror of the animal mind is a distorted image of our own mental and emotional features. and since the mirrors are of varying and unknown curvature, we can never hope to be in a position accurately to estimate the amount of distortion. remembering this, it is always well to look narrowly at every anecdote of animal intelligence and emotion, and endeavour _to distinguish observed fact from observer's inference_. if we take the great number of stories illustrative of revenge, consciousness of guilt, an idea of caste, deceitfulness, cruelty, and so forth, in the higher mammalia, we shall find but few that do not admit of a different interpretation from that given by the narrator. a cat's treatment of a mouse is adduced by a number of witnesses as illustrative of cruelty; but others see in this conduct, not cruelty, but practice and training in an important branch of the business of cat-life. that is to say, the act, though objectively cruel from the human standpoint, is not on this view performed from a motive of cruelty. some time ago i ventured to stroke the nose of a little lion-cub which had tottered, kitten-like, to the bars of its cage. "i wish," i said shortly afterwards to a distinguished animal painter, "you could have caught the look of conscious dignity (i speak anthropomorphically) with which the lioness turned and seemed to say, 'how dare you meddle with my child!'" "i have seen such a look and attitude," said mr. nettleship; "but i attributed it, not to pride, but to fear." mr. romanes quotes,[hv] as typically illustrative of an "idea of caste," the case of mr. st. john's retriever, which struck up an acquaintance with a rat-catcher and his cur, but at once cut his humble friends, and denied all acquaintanceship with them, on sight of his master. i, on the other hand, should regard this case as parallel with that which i have noted a hundred times. my dogs would go out with the nurse and children when i was busy or absent; but if i appeared within sight, they raced to me. the stronger affection prevailed. a dog is described[hw] as "showing a deliberate design of deceiving," because he hobbled about the room as if lame and suffering from pain in his foot. i would suggest that there was no pretence, no "deliberate design of deceit," in this case, but a direct association of ideas between a hobbling gait and more sympathy and attention than usual. i am not denying objective deceitfulness to the dog any more than i deny objective cruelty to the cat. my only question is whether the _motive_ is deceit. we must not forget that the deceitful intent is a piece, not of the observed fact, but of the observer's inference. mr. romanes, for example, tells[hx] of a black retriever who was asleep, or apparently asleep, in the kitchen of a certain dignitary of the church. the cook, who had just trussed a turkey for roasting, was suddenly called away. during her temporary absence, "the dog carried off the turkey to the garden, deposited it in a hollow tree, and at once returned to resume his place by the fire, where he pretended to be asleep as before." unfortunately, a perfidious gardener had watched him, and brought back the turkey, so that the retriever did not enjoy the feast he had reserved for a quiet and undisturbed moment. assuming that the gardener and cook were accurate in their statement of fact, the deceitful intent is an inference on their part, or that of the dignitary of the church, or mr. romanes. i do not deny its correctness from the objective standpoint. deceitfulness is apparently exhibited by children at a very tender age. but for us civilized adults deceit and its converse, truthfulness in action, mean something a good deal more definite than for dogs and infants. animals are often described as harbouring feelings of revenge and vindictiveness. to test this in the elephant, captain shipp gave an elephant a sandwich of cayenne pepper. "he then waited," says mr. romanes,[hy] "for six weeks before again visiting the animal, when he went into the stable, and began to fondle the elephant as he had previously been accustomed to do. for a time no resentment was shown, so that the captain began to think that the experiment had failed; but at last, watching an opportunity, the elephant filled his trunk with dirty water, and drenched the captain from head to foot." here the facts are that an injury was received, and that the retaliation followed after an interval of six weeks. the inference seems to be that the elephant harboured feelings of revenge or vindictiveness during this period. it may have been so. it may be, however, that the elephant never once pictured the captain during the six weeks; but, on seeing him again, remembered the injury, and, as we say, paid him out. but what we understand by revenge and vindictiveness is the keeping of an injury before the mind for the express purpose of ultimately avenging it. and this the elephant, to say the least of it, may not have done. in miss romanes's interesting observations on the cebus monkey, she says,[hz] "he bit me in several places to-day when i was taking him away from my mother's bed after his morning's game there. i took no notice; but he seemed ashamed of himself afterwards, hiding his face in his arms, and sitting quiet for a time." but, in a footnote, we read, "on subsequent observation, i find this quietness was not due to shame at having bitten me; for whether he succeeds in biting any person or not, he always sits quiet and dull-looking after a fit of passion, being, i think, fatigued." i quote this to illustrate the difference which i am endeavouring to insist upon between observed fact and observer's inference. mr. romanes comments[ia] on the remarkable change which has been produced in the domestic dog as compared with wild dogs, with reference to the enduring of pain. "a wolf or a fox will sustain the severest kinds of physical suffering without giving utterance to a sound, while a dog will scream when any one accidentally treads upon its toes. this contrast," says mr. romanes, "is strikingly analogous to that which obtains between savage and civilized man: the north american indian, and even the hindoo, will endure without a moan an amount of physical pain--or, at least, bodily injury--which would produce vehement expressions of suffering from a european. and, doubtless, the explanation is in both cases the same; namely, that refinement of life engenders refinement of nervous organization, which renders nervous lesions more intolerable." i cannot accept this as the most probable explanation. in the first place, the human beings referred to have different ideals in the matter of conduct under pain and suffering. the american indian and the hindoo have a stoic ideal, which does not influence the average european. on the other hand, the dog, from his association with man, has learnt more and more to give expression to his feelings in barks, whines, and yelpings. to howl at every little pain would do a wolf no good, but rather advertise him to his enemies; to howl when his toes are trodden on makes most men look where they are stepping, and probably pet the sufferer for his pains. in the one case, to howl is disadvantageous; in the other, it is advantageous. i do not, however, put forward my own explanation as necessarily more correct than that given by mr. romanes (though i regard it myself as more probable). my object is to show that it is possible for two observers to regard the same activities of animals, and read into them different psychological accompaniments. throughout the sections of mr. romanes's work which deal with the emotions, i feel myself forced at almost every turn to question the validity of his inferences. from all that i have said in the last chapter, it will be gathered that i am not prepared to credit our dumb companions with a single _sentiment_. a sense of beauty, a sense of the ludicrous, a sense of justice, and a sense of right and wrong,--these abstract emotions or sentiments, _as such_, are certainly impossible to the brute, if, as i have contended, he is incapable of isolation and analysis. but, as we have already seen, even with us these emotions have to be particularized and brought within the perceptual sphere ere they are strongly operative on conduct. we are not roused to indignation by an abstract sense of injustice, but by the particular performance of an unjust deed. even so, however, the emotional state aroused carries with it in us some of the spirit of the conceptual sphere from which it has descended. the analogous emotions in animals cannot possess, if i am right, any tincture of this conceptual spirit. and since we cannot divest ourselves of our conceptual spirituality, we cannot justly estimate what these emotional states, in dog or ape, are like. remembering this, let us see what can be said in favour of a perceptual sense of injustice, guilt, the ludicrous, and the beautiful. in evidence of a sense of justice, we have the oft-quoted case of the turnspit-dog reported by arago the astronomer.[ib] this dog refused, with bared teeth, to enter out of his turn the drum by the revolution of which the spit was rotated. m. arago, for whom the pullet on the spit was being dressed, requested that the dog's companion, after turning the spit for a short time, should be released. whereupon the dog who had before been so refractory seemed satisfied that his turn for drudgery had come, and, entering the wheel of his own accord, began without hesitation to turn it as usual. many will be prepared to maintain that dogs resent unjust chastisement. a gentleman i met near rio de janeiro possessed a dog whose sensitiveness was such that, after a reproof, he would leave the house, and sometimes not return for several days. his owner assured me of his belief that in such cases the reproof had always been undeserved; and he told me of one definite instance in which the reproof--never more than verbal--had been for a theft which was afterwards found to have been committed by his garden-boy. on this occasion the dog was away for three days, and returned in a wretched and miserable condition. what shall we say of such cases? seeing how complex is what we call a sense of justice, i am not prepared to credit the dog therewith; and i am disposed to regard such actions as i have just described as the result of a breach of normal association. dogs, like men, are creatures of habit; and breaches of normal association--occurrences contrary to expectation--give rise to uneasiness, dissatisfaction, and consequent resentment. conversely, many of the cases where dogs and other animals are said to know when they have done wrong, and to suffer the pricks of conscience, may probably be satisfactorily explained by association. when my friend, coming down into his drawing-room, sees tim's "guilty" look, he suspects that the dog has, contrary to rule, been taking a nap on one of the chairs; and his suspicions are not a little strengthened by the unnatural warmth of the easiest armchair. "ah! tim always knows when he has done wrong," says my friend. but not improbably the association in tim's mind is a direct one between a nap on that chair and his master's displeasure. what tim knows is, perhaps, not that he _has_ done wrong, but that he will "catch it." it is the expectation of a reproof, or something more, that gives rise to his look of conscious guilt. in the same way, the look of "conscious rectitude" we often see in some dogs may be due to the anticipation of a word of commendation. and, in general, i fancy that the association in an animal's mind is between the performance of a given act and the occurrence of certain consequences. when this association becomes definite it must, i imagine, draw after it a dislike of such actions as have been accompanied by evil consequences, and a delight in such actions as have been accompanied by pleasant consequences. and eventually this dislike or delight is transferred from his own actions to the similar actions of others. thus dogs punish their puppies for acts of uncleanliness, while cats are even more particular in this respect. a correspondent in _nature_[ic] gives a case of a cat chastising by a violent blow with her paw her kitten, who was about to enjoy a herring which had been set down before the fire to keep hot. so, too, according to mr. darwin,[id] "when the baboons in abyssinia plunder a garden, they silently follow their leader, and, if an imprudent young animal makes a noise, he receives a slap from the others to teach him silence and obedience." and mr. schaub communicated to professor nipher[ie] a case of a black-and-tan terrier bitch, whose pup had stolen a stocking from his bedroom, and who followed the young offender, took the stocking from him, and returned it to the owner. her action gave evidence, he says, of displeasure at the action of the pup. and mr. schaub contrived to have the offence committed on many successive mornings, the same performance being repeated each time. in this connection i will give two anecdotes of carlo, communicated to me by mrs. mann. "once i came upon carlo sitting in the dining-room doorway, dulceline, the cat, angrily watching him from the stairs, and also evidently having an eye on a leg of mutton half dragged off the dish on the dining-table. carlo had clearly caught the thief in the act. he was on guard; and he seemed much relieved when higher powers came on the scene. honesty seemed part of carlo's nature. in this matter we never had to give him any lessons. nor could he bear to see dishonesty in others. one sunday, one of the little girls saw carlo coming along looking so anxiously at her that she knew he wanted her to come. she therefore followed him, and carlo took her to the store-room, the door of which her sister had left open. in the doorway carlo stopped, and looked first up at his mistress and then into the store-room, as much as to say, 'what can we think of this?' and truly there was a certain little black-and-tan terrier, whose principles were by no means of a high order, regaling himself with some cold meat that he had dragged on to the floor. toby knew he was in the wrong, and tried to flee. but carlo stopped him as he endeavoured to fly past. and when toby was thereupon duly slapped, carlo sat straight up, with a face of conscious rectitude." these anecdotes, communicated to me by a lady of culture and intelligence, illustrate how, in describing the actions of animals, phraseology only, in strictness, applicable to the psychology of man, is unwittingly and almost unavoidably employed. toby's "_principles_ were not of a high order," yet he "_knew he was in the wrong_," while carlo watched him receive his punishment, and "sat straight up, with a face of _conscious rectitude_." coming now to a sense of humour or a sense of the ludicrous, darwin himself said,[if] "dogs show what may fairly be called a sense of humour, as distinguished from mere play; if a bit of stick or other such object be thrown to one, he will often carry it away for a short distance; and then, squatting down with it on the ground close before him, will wait until his master comes close to take it away. the dog will seize it and rush away in triumph, repeating the same man[oe]uvre, and evidently enjoying the practical joke." mr. romanes had a dog who used to perform certain self-taught tricks, "which clearly had the object of exciting laughter. for instance, while lying on his side and violently grinning, he would hold one leg in his mouth. under such circumstances, nothing pleased him so much as having his joke duly appreciated, while, if no notice was taken of him, he would become sulky." to these i may add an observation of my own. i used sometimes, when staying at lancaster with a friend, to take his dog sambo, a highly intelligent retriever, to the seashore. his chief delight there was to bury small crabs in the sand, and then stand watching till a leg or a claw appeared above the surface, upon which he would race backwards and forwards, giving short barks of keen enjoyment. this i saw him do on many occasions. he always waited till a helpless leg appeared, and then bounded away as if he could not contain the canine laughter that was in him. who shall say, however, what was passing through the mind of the dog in any of these three cases? the motive of mr. darwin's dog may have been to prolong the game, though i expect there was something more than this. mr. romanes's dog exemplified, perhaps, the sense of satisfaction at being noticed. sambo's performance is now, as it was years ago, beyond me. but a sense of humour, involving a delicate appreciation of the minor incongruities of life, is, i imagine, too subtle an emotion for even sambo. i pass now to the sense of beauty, and i shall consider this at greater length, because of its bearing on sexual selection and the origin of floral beauty. the interesting experiments of sir john lubbock already alluded to seem to establish the fact that bees have certain colour-preferences. blue and pink are the most attractive colours; yellow and red are in less favour. no doubt these preferences have arisen in association with the flowers from which the bees obtain their nectar. they have a practical basis of biological value. but there seems no doubt that certain colours are now for them more attractive than others. bees and other insects are, undoubtedly, attracted by flowers; these flowers excite in us an æsthetic pleasure; the bees are, therefore, supposed to be attracted to the flowers through their possession of an æsthetic sense. now, this does not necessarily follow. it is the nectar, not the beauty of the flower, that attracts the bee. so long as the flower is sufficiently _conspicuous_ to be rapidly distinguished by the insect, the conditions of the case are met so far as insect psychology is concerned. the fact remains, however, that the flowers thus conspicuous to the insect are fraught with beauty _for us_. in the case of sexual selection among birds, again, i believe that the gorgeous plumage has its basis of origin in that pre-eminent vitality which mr. tylor and mr. wallace have insisted on. but, as before indicated, this will not serve to explain its special character for each several species of birds. here, again, conspicuousness and recognition are unquestionably factors. but that the bright plumage of male birds awakens emotional states in the hens, that it probably also arouses sexual appetence, seems to be shown by the manner in which the finery is displayed by the male before the female. i think it is probable, also, that pleasure, becoming thus associated with bright colours in the mate, is also aroused by bright colours in other associations. thus the gardener bower-bird, described by dr. beccari,[ig] collects in front of its bower flowers and fruits of bright and varied colours. it removes everything unsightly, and strews the ground with moss, among which it places the bright objects from among which the cock bird is said to select daily gifts for his mate's acceptance! dr. gould states that certain humming-birds decorate their nests "with the utmost taste," weaving into their structure beautiful pieces of flat lichen. if by crediting birds with a sense of beauty we mean that in them pleasurable emotions may be aroused on sight of objects which we regard as beautiful, i am not prepared to deny them such a sense of beauty, nay, i fully believe that such pleasurable feelings are aroused in them. when, however, it is said that the gorgeous plumage of male birds has been produced by the æsthetic choice of their mates, i am not so ready to agree. a consciously æsthetic motive has not, i believe, been a determining cause. the mate selected has been that which has excited the strongest sexual appetence; his beauty has probably not, as such, been distinctly present to consciousness. here, then, we have again the question which arose in connection with floral beauty--how is it that the sight of the mates selected by hen birds excites in us, in so many cases, an æsthetic pleasure? it is clear that this is a matter rather of human than of animal or comparative psychology. as such, except for purposes of illustration, it does not fall within the scope of this work. i can, therefore, say but a few words on the subject. the view that i think erroneous is that either floral beauty or the beauty of secondary sexual characters has been produced on æsthetic grounds, that is to say, for the sake of the beauty they are seen by man to possess. it is, therefore, to the point to draw attention to the fact that many of the objects and scenes which excite in us this æsthetic sense have certainly not been produced for the sake of their beauty. their beauty is an adjunct, a by-product of rarest excellence, but none the less a by-product. nothing can be more beautiful in its way than a well-grown beech or lime tree; and yet it cannot be held to have been produced for its beauty's sake. the leaves of many trees, shrubs, and plants are scarcely less beautiful than the flowers. but _they_ cannot have been produced by the æsthetic choice of insects. from the depth of a mine there may be brought up a specimen of ruby copper ore, or malachite, or a nest of quartz crystals, or an agate, or a piece of veined serpentine, which shall be at once pronounced a delight to the eye. but for the eye it was not evolved. the grandeur of alpine scenery, the charm of a winding river, the pleasing undulations of a flowing landscape,--no one can say that these were evolved for the sake of their beauty. the fact of their being beautiful is, therefore, no proof that the blue gentian, or the red admiral, or the robin redbreast were evolved for the sake of, or by means of, the beauty that they possess. again, one leading feature in the beauty of flowers is their symmetry. the beauty is, so to speak, kaleidoscopic beauty. it is not so much the single veined or marbled petal that is so lovely, as the group of similar petals symmetrically arranged. but this symmetry can hardly be said to have been selected for its æsthetic value; it is rather part of the natural symmetry of the plant. even with butterflies and birds and beasts the symmetrical element is an important one in their beauty.[ih] i must not attempt to analyze our sense of beauty or endeavour to trace its origin. it appears to involve a pleasurable stimulation of the sense-organs concerned, together with perceptions of symmetry, of diversity and contrast, and of proportion, with a basis of unity. it is rich in suggestions and associations. it is heightened by sympathy. a beautiful scene is doubly enjoyable if a congenial companion is by our side. "the whole effect of a beautiful object, so far as we can explain it," says mr. sully,[ii] "is an harmonious confluence of these delights of sense, intellect, and emotion, in a new combination. thus a beautiful natural object, as a noble tree, delights us by its gradations of light and colour, the combination of variety with symmetry in its contour or form, the adaptation of part to part, or the whole to its surroundings; and, finally, by its effect on the imagination, its suggestions of heroic persistence, of triumph over the adverse forces of wind and storm. similarly, a beautiful painting delights the eye by supplying a rich variety of light and shade, of colour, and of outline; gratifies the intellect by exhibiting a certain plan of composition, the setting forth of a scene or incident with just the fulness of detail for agreeable apprehension; and, lastly, touches the many-stringed instrument of emotion by an harmonious impression, the several parts or objects being fitted to strengthen and deepen the dominant emotional effect, whether this be grave or pathetic on the one hand, or light and gay on the other. the effect of beauty, then, appears to depend on a simultaneous presentment in a single object of a well-harmonized mass of pleasurable material or pleasurable stimulus for sense, intellect, and emotion." this, too, is what i understand by an æsthetic sense of beauty; and if a hen bird has her sexual appetence evoked by the bright display of her mate, the emotional state she experiences is something very different from what we know as a sense of beauty. the adjective "æsthetic" should in any case, i think, be resolutely excluded in any discussion of sexual selection. Ã�sthetics, like conceptual thought, accompany the suppression or postponement of action. as we have already seen, the normal and primitive series is ( ) sense-stimulus; ( ) certain nerve-processes in the brain which are associated with perception and emotion; and ( ) certain resulting activities. by the suppression of action the mind comes to occupy itself more and more completely with the central processes. perception blossoms forth into conceptual thought; emotion blossoms forth into æsthetics. "'throughout the whole range of sensations, perceptions, and emotions which we do not class as _æsthetic_,'[ij] says mr. herbert spencer, 'the states of consciousness serve simply as aids and stimuli to guidance and action. they are transitory, or, if they persist in consciousness some time, they do not monopolize the attention; that which monopolizes the attention is something ulterior, to the effecting of which they are instrumental. but in the states of mind we class as æsthetic the opposite attitude is maintained towards the sensations, perceptions, and emotions. these are no longer links in the chain of states which prompt and guide conduct. instead of being allowed to disappear with merely passing recognition, they are kept in consciousness and dwelt upon, their natures being such that their continued presence in consciousness is agreeable.' the action which is the normal consequent on sensation is here postponed or suppressed; and thus we are enabled to make knowledge or beauty an end to be sought for its own sake; and thus, too, we are able to make progress, otherwise impossible, in science and in art. sensations and perceptions are the roots from which spring the sturdy trunk of action, the expanded leaves of knowledge, and the fair blossoms of art. the leaves and the flowers are the terminal products along certain lines of development; but the function of the leaves is to minister to the growth of the wood, and the function of the flowers is to minister to the continuance and well-being of the race. so, too, in human affairs. knowledge and art are justified by their influence on conduct; truth and beauty must ever guide us towards right living; and æsthetics are true or false according as they lead towards a higher or a lower standard of moral life."[ik] * * * * * to sum up, then, concerning this difficult subject, the following are the propositions on which i would lay stress: ( ) what we term an æsthetic sense of beauty involves a number of complex perceptual, conceptual, and emotional elements. ( ) the fact that a natural object excites in us this pleasurable emotion does not carry with it the implication that the object was evolved for the sake of its beauty. ( ) even if we grant, as we fairly may, that brightly coloured flowers, in association with nectar, have been objects of appetence to insects; and that brilliant plumage, in association with sexual vigour, has been a factor in the preferential mating of birds;--this is a very different thing from saying that, either in the selection of flowers by insects, or in the selection of their mates by birds, a consciously æsthetic motive has been a determining cause. ( ) in fine, though animals may be incidentally attracted by beautiful objects, they have no æsthetic sense of beauty. a sense of beauty is an abstract emotion. Ã�sthetics involve ideals; and to ideals, if what has been urged in these pages be valid, no brute can aspire. what applies thus to æsthetics applies also to ethics. few, however, will be found to contend that animals can be moral or immoral, or have any moral ideas properly so called. mr. romanes does indeed state, in the table he prefixes to his works on mental evolution, that the anthropoid apes and dogs are capable of "indefinite morality." he leaves this to be explained, however, in a future work. in the published instalment of "mental evolution in man" he seems to contend,[il] or, at least, admit, "that the fundamental concepts of morality are of later origin than the names by which they have been baptized." but he says nothing of indefinite morality, which still remains for consideration in another work. in the mean while we may, i think, confidently assume that ethics, like conceptual thought and æsthetics, are beyond the reach of the brute. morality is essentially a matter of ideals, and these belong to the conceptual sphere. * * * * * i have now said enough[im] to indicate what i mean by advocating the exercise of extreme caution in our inferences concerning the emotional states of animals. we must remember, first, how liable to error are our inferences in these matters; we must remember, next, how complex and essentially human are our own emotions. i do not for one moment deny that in animals are to be found the perceptual germs of even the higher emotional states. nevertheless, if we employ, in our interpretation of the actions of animals, such terms as "consciousness of guilt," "sense of right and wrong," "idea of justice," "deceitfulness," "revenge," "vindictiveness," "shame," and the rest, we must not forget that these terms stand for human products, that they are saturated with conceptual thought, and that they must be to a large extent emptied of their meaning before they can become applicable to the emotional consciousness of brutes. notes [hg] "physiological Ã�sthetics:" chapter on "pleasure and pain." [hh] all of these, at any rate, satisfy mr. herbert spencer's definition. pleasure he describes as a feeling which we seek to bring into consciousness and retain there; pain, as a feeling which we seek to get out of consciousness and keep out. [hi] "types of ethical theory," vol. ii. p. . [hj] such consciousness of activity is probably associated with the innervation of afferent, not efferent, nerves. [hk] journal of marine biological association, new series, vol. i. no. , pp. , . [hl] "outlines of psychology," p. . [hm] ibid. p. . [hn] page . [ho] page . [hp] _nature_, vol. xxxvii. p. . [hq] vol. i. p. , under date . [hr] "mental evolution in animals," p. . [hs] "descent of man," pt. i. chap. iii. [ht] miss nellie maclagan describes how her newfoundland similarly took a roll to a hungry pauper-friend (_nature_, vol. xxviii. p. ). mr. duncan stewart gives (_nature_, vol. xxviii. p. ) the case of a cat who used frequently to provide her blind mother with food. sir harry lumsden states that during the cold autumn of some tame partridges in aberdeenshire brought two wild coveys to be fed near the doorstep of the house. and a case has been communicated to me by miss agnes tanner, of clifton, of a thrush that pulled up worms on the lawn for a lame companion. [hu] "animal intelligence," p. . [hv] "animal intelligence," p. . [hw] ibid. p. . [hx] ibid. p. . [hy] "animal intelligence," p. . [hz] "animal intelligence," p. . [ia] ibid. p. . [ib] "animal intelligence," p. . [ic] mr. alexander mackennal, vol. xxi. p. . [id] "descent of man," pt. i. chap. iii., quoted from brehm's "thierleben." [ie] _nature_, vol. xxviii. p. . [if] "descent of man," quoted by romanes, p. . [ig] _nature_, vol. xl. p. . [ih] another example of beauty which can hardly be said to have been evolved for beauty's sake is to be seen in birds' eggs. mr. henry seebohm regards the bright colours of some birds' eggs as a difficulty in the way of the current interpretation of organic nature. "few eggs," he says (_nature_, vol. xxxv. p. ), "are more gorgeously coloured [than those of the guillemot], and no eggs exhibit such a variety of colour. [they are sometimes of a bluish green, marbled or blotched with full brown or black; sometimes white streaked with brown; sometimes pale green or almost white with only the ghosts of blotches and streaks; and sometimes the reddish brown extends so as to form the ground-tint which is blotched with deeper brown.] it is impossible to suppose that protective selection can have produced colours so conspicuous on the white ledges of chalk cliffs; and sexual selection must have been equally powerless. it would be too ludicrous a suggestion to suppose that a cock guillemot fell in love with a plain-coloured hen because he remembered that last season she laid a gay-coloured egg." if we connect colour with metabolic changes, its occurrence in association with the products of the highly vascular oviduct will not be surprising. some _guidance_ is, however, on the principles advocated in chapter vi., required to maintain a standard of coloration. in many cases such guidance is found in protective selection, as in the plover's eggs in our frontispiece. in the guillemot's egg such protective selection seems to be absent, and, as mr. seebohm himself says, "no eggs exhibit such a variety of colour." in our present connection, however, the point to be noticed is that many eggs are undoubtedly beautiful. but they cannot have been in any way selected for the sake of their beauty. [ii] "outlines of psychology," p. . [ij] i should add, "or as _conceptual thought_." [ik] this paragraph is quoted from the author's "springs of conduct," p. . [il] page . [im] i have said nothing about the emotions of invertebrates, because i have nothing special to say. they have, no doubt, emotions analogous to fear, anger, and so on. but it is difficult to interpret their actions. the "angry" wasp is, perhaps, a good deal more frightened than furious. sir john lubbock's interesting experiments seem to show that ants have what is termed the instinct of play. but this admirable observer has rendered it probable that sympathy and affection in ants and bees have been somewhat exaggerated. chapter xi. animal activities: habit and instinct. so soon as one of the higher animals comes into the world a number of simple vital activities are already in progress or are at once initiated. some of these are what are termed "automatic actions," or actions which take their origin within the organ which manifests the activity; such are the heart-beat and the rhythmical contractions of the intestines by which the food is pushed onwards through the alimentary canal. some are reflex, or responsive, actions, taking origin from a stimulus coming from without; such are the contraction of the pupil of the eye under bright light, the pouring forth of the secretions on the presence of food in the alimentary canal, taking the breast, sneezing, and so forth. some are partly automatic and partly reflex; such is the rhythm of respiration. in addition to these vital activities, there is a vast body of more complex activities, for the performance of which the animal brings with it innate capacities. some of these, which we term "instinctive," are performed at once and without any individual training, as when a chicken steps out into the world, runs about, and picks up food without learning or practice. others, which we term "habitual," are more or less rapidly learnt, and are then performed without forethought or attention. the store of innate capacity is often very large; and a multitude of activities are ere long performed with ease and certainty so soon as the animal has learnt to use the organization it thus inherits. and lastly, built upon this as a basis, by recombining of old activities in new modes, and by special application of the activities to special circumstances, we have the activities which we term "intelligent;" and here again the activities are sometimes divided into two classes, answering respectively to the reflex and the automatic, but on a higher plane, according as they are responsive to stimuli coming more or less directly from without, or spontaneous and taking their origin from within. but it is probably rather the remoteness and indirectness of the responsive element than its absence that characterizes these spontaneous activities. another classification of activities is into voluntary and involuntary. voluntary actions are consciously performed for the attainment of some more or less definite end or object. involuntary actions, though they may be accompanied by consciousness, and though they may be apparently purposive, are performed without intention. notwithstanding the conscious element, they may, perhaps, be regarded as rather physiological than psychological. the simple vital activities belong to this class. but some are much more complex. if, when i am watching the cobra at the zoo, it suddenly strikes at the glass near my face, i involuntarily start back. the action is apparently purposive, that is to say, an observer of the action would perceive that it was performed for a definite end, the removal from danger; it is also accompanied by consciousness; but it is unintentional, no representation of the end to be gained or the action to be performed being at the moment of action framed by the mind. on the other hand, if i perform a voluntary act, such as selecting and lighting a cigar, there is first a desire or motive directed to a certain end in view, involving an ill-defined representation of the means by which that end may be achieved; and this is followed by the fulfilment of the desire through the application of the means to the performance of the act. in the carrying out of voluntary activities, then, both perception and emotional appetence are involved. there are construction and reconstruction, memory and anticipation, and interwoven therewith the motive elements of appetence or aversion. it is emotion that gives force and power to the motive. and this must be regarded as the dynamic element in voluntary activity, while intelligence is the directive element. feeling is the horse in the carriage of life, and intelligence the coachman. let us here note that, in speaking of the activities of animals and the motives by which they are prompted, we are forced, if we would avoid pedantry, to leap backwards and forwards across the chasm which separates the mental from the physical. motives, as we know them, are mental phenomena; the activities, as we see them, are physical phenomena. the two sets of phenomena belong to distinct phenomenal categories. in ordinary speech, when we pass and repass from motives to actions, and from actions to the feelings they may give rise to, we are apt to be forgetful of the depth of the chasm we so lightly leap. and this is no doubt because the chasm, though so infinitely deep, is so infinitely narrow. there are, however, no physical analogies by which we can explain the connection between the physical and the mental, between body and mind. the so-called connection is, in reality, as i believe, identity. viewed from without, we have a series of physical and physiological phenomena; felt from within, we have a series of mental and psychological phenomena. it is the same series viewed from different aspects. this is no explanation; it is merely a way, and, as i believe, the correct way, of stating the facts. why certain physiological phenomena should have a totally different aspect to the organism in which they occur from that which they offer to one who watches them from without, is a question which i hold to be insoluble. all we have to remember, however, is that, in passing from the mental to the physical, we are changing our point of view. the series may be set down thus-- _external aspect_: physical stimulus--> interneural processes--> activities. \ / \ / _inner aspect_: \ / \ / accompanying consciousness <--mental states--> accompanying consciousness the physical stimulus and the resulting activities are occurrences in the external world, and more or less lie open to our view. but the intervening physical and physiological neural processes are hidden from us. as occurring in ourselves, however, the mental states which are the inner aspects of these neural processes stand out clearly in the light of consciousness. when, therefore, we are watching the life-activities of others, we naturally fill in between the physical stimulus and the activities, not the neural processes of which we are so ignorant, but mental states analogous to those of which we are conscious under similar conditions. thus we leap from the physical to the mental, and back again to the physical, as represented by the diagonal lines in the above scheme. and there can be no objection to our doing so if we bear in mind that we are thus changing our point of view. the human organism, then--for at present we may regard the matter from man's own position--is a wonderfully delicate piece of organization, with mental (inner) and physical (outer) aspects. it is in a condition of the most delicate equipoise. under the influence of a perception associated with an appetence, or of a conception accompanied by a desire, it is thrown into a state of unstable equilibrium; the performance of the action which leads to the fulfilment or satisfaction of the appetence or the desire restores the stability of the system. the instability is caused by the conjoint action of an attraction towards some state represented as desirable, and a repulsion from the existing state which is relatively undesirable. in some cases the attraction, and in others the repulsion, is predominant. when we are in an uncomfortable position, the discomfort is predominant, and we seek relief by changing our attitude. when the bright sunshine tempts us to go out for a walk, the attraction is predominant. but if the uncomfortable attitude is enforced and prolonged, we have a mental representation of the relief we long for; and this is attractive. and if we have work which keeps us indoors, the irksome restraint brings with it an aversion to our present lot. inseparably associated with the appetence or aversion there is a representation of the activity which constitutes the fulfilment of the emotion. on the physiological side this is probably an incipient excitation of the muscles or other organs concerned in the requisite actions. the miser's fingers itch to clutch the gold, the possession of which he desires. our muscles twitch as we long to join in the race or the active contention of a game of football. our horse grows restive as the hunt goes by. our dog can scarce restrain himself from racing after the rabbits in the park. under the influence of emotion, then, the body is prepared for activity, the organs and muscles are beginning to be innervated, and, if the appetence or desire be sufficiently strong, the appropriate actions are initiated, and the organism tends to pass from the state of unstable equilibrium arising out of a pressing need to the stable condition of satisfied appetence. the function of the will in this process we shall have briefly to consider presently. let us here notice, with regard to the activities, what we have before seen with regard to the process of perceptual construction. we there noticed that, at the bidding of a relatively simple suggestion, a complex object may be constructed by the mind. this presupposes a highly complex mental organization ready to be set in motion by the appropriate stimulus. the organization has been established by association and through evolution in the individual and his ancestors. it is the same with the activities. they, too, are the outcomes of associations and experiences established and registered during generations of ancestral predecessors. at the bidding of the appropriate stimulus arousing impulse or appetence, a train of activities of great intricacy may be set agoing with remarkable accuracy and precision. it is true that a certain amount of individual education is required to draw out and establish the latent powers of the body, as also of the mind; but _the ability is inborn_, and only requires to be cultivated. every one of us inherits an organization rendering him capable of performing a vast amount of mental construction and a great number of bodily activities. all he has to do is to learn how to use it and to make himself master of the powers that are given him. at first, the acquisition of this mastery over the innate powers, even in the performance of comparatively simple muscular adjustments, may require a good deal of attention and practice. but, as time goes on, the frequent repetition of the ordinary activities of everyday life leads to their easier and easier performance. in simple responsive actions the appropriate activity follows readily on the appropriate stimulus. and, ere long, many acts which at first required intelligent attention are performed easily and without consciousness of effort or definite intention. a close association between certain oft-recurring stimuli and the appropriate response in activity is thus established, and the action follows on the stimulus without hesitation or trouble. with fuller experience and further practice in the ordinary avocations of life, the responsive activities link themselves more and more closely in association, become more and more complex, are combined in series and classes of activity of greater length and accuracy, and thus become organized into _habits_. under this head fall those activities which we learn with difficulty in childhood, and perform with ease in after-life. at first voluntary and intentional, they have become, or are becoming, through frequency and uniformity of performance, more or less involuntary and unintentional. "the work of the world is," we are told, "for the most part done by people of whom nobody ever hears. the political machine and the social machine are under the ostensible control of personages who are well to the front; but these brilliant beings would be sorely perplexed, and the machinery would soon come to a standstill, but for certain experienced, unambitious, and unobtrusive members of society." so is it also in the economy of animal life. the work of life is--to paraphrase mr. norris's words--for the most part done by habits of which nobody ever thinks. the bodily organization is ostensibly under the control of intellect and reason; but these brilliant qualities would be sorely perplexed, and the machinery would soon come to a standstill, but for certain unobtrusive, habitual activities which are already as well trained in the routine work of life as are the permanent clerks in the routine work of a government office. the importance of the establishment of these habitual activities is immense. as the muscular and other responses of ordinary everyday life become habitual, the mind is, so to speak, set free from any special care with regard to their regulation and co-ordination, and can be concentrated on the end to be attained by such activities. the cat that is creeping stealthily upon the bird has all her attention rivetted on the object of her appetence, and has not to trouble herself about the movements of her body and limbs. when the swallows are wheeling over our heads in the summer air, their sweeping curves and graceful evolutions are not the outcome of careful planning, but are just the normal exercise of activities which from long practice have become habitual. to swim, to skate, to cycle, to row, to play the piano or the violin,--all these require our full attention at first. but with practice they become habitual, and during their performance the attention may be devoted to quite other matters. this is a great gain. without it complex trains of activities could not be performed with ease by man or beast. when once habits have been firmly established, their normal performance is accompanied by a sense of satisfaction. but if their performance is prevented or thwarted, there arises a sense of want or dissatisfaction. the pining of a caged wild animal for liberty is a craving for the free performance of its habitual activities. in an animal born into captivity the craving is probably less intense, though, for reasons which will presently become evident, it is presumably by no means absent. animals are, to a very large extent, creatures of habit. much of the pleasure of their existence lies in the performance of habitual activities. our zoological gardens, interesting as they are to us, are probably centres of an amount of misery and discomfort, from unfulfilled promptings of habit and instinct, which we can hardly realize. from habitual activities we may pass by easy steps to those which are instinctive. both habits and instincts, or, to use a more convenient and satisfactory mode of expression for our present purpose, both habitual and instinctive activities, are based upon innate capacity. but whereas habitual activities always require some learning and practice, and very often some intelligence, on the part of the individual, instinctive activities are performed without instruction or training, through the exercise of no intelligent adaptation on the part of the performer, and either at once and without practice (perfect instincts) or by self-suggested trial and practice (incomplete instincts).[in] there is some little difficulty in distinguishing between instinctive activities and reflex actions. mr. herbert spencer defines or describes instinct as compound reflex action. mr. romanes defines instinct as reflex action into which there is imported the element of consciousness. but, on the one hand, many instincts involve something more than compound reflex action, since there is an organized sequence of activities; and, on the other hand, the difficulty (which mr. romanes admits) or impossibility (as i contend) of applying the criterion of consciousness renders unsatisfactory the introduction of the mental element as distinctive. i would say, therefore, that ( ) reflex actions are those comparatively isolated activities which are of the nature of organic or physiological responses to more or less definite stimuli, and which involve rather the several organs of the organism than the activities of the organism as a whole; and that ( ) instinctive activities are those organized trains or sequences of co-ordinated activities which are performed by the individual in common with all the members of the same more or less restricted group, in adaptation to certain circumstances, oft-recurring or essential to the continuance of the species. these instinctive activities may, as i have said, be performed at once and without practice (perfect instincts) or by self-suggested trial and practice (incomplete instincts). most young mammals require some little practice in the use of their limbs before they are able to walk or run. but young pigs run about instinctively so soon as they are born. thunberg, the south african traveller, relates, on the testimony of an experienced hunter, the case of a female hippopotamus which was shot the moment she had given birth to a calf. "the hottentots," he said, "who imagined that after this they could catch the calf alive, immediately rushed out of their hiding-place to lay hold of it; but, though there were several of them, the new-born calf got away from them, and at once made the best of its way to the river." even in cases where some practice is apparently necessary, the activities may be, and often are, perfectly instinctive. they cannot, however, be performed immediately on birth, because the nervous and muscular mechanism is not at that time sufficiently developed. they might, perhaps, with advantage be termed "deferred instincts." if time be given for this development, the activities are carried out at once and without practice. throw a new-born puppy into the river, and, after some helpless floundering, he will be drowned. throw his brother when fully grown into the river, and, though he may never have been in the water in his life, he will swim to shore. he has not to learn to swim; this is with him an instinctive activity. the dog inherits the power which the boy must with some little difficulty acquire. he probably has to pay no special attention to the muscular adjustments involved. the act is accompanied by consciousness, but not that directed consciousness we call "attention." when the boy has acquired the habit, he is scarcely conscious of the special muscular co-ordinations as he swims across the river; he is only conscious of a desire to pick the water-lilies near the further bank. birds, especially those which are called pr[oe]coces, in contradistinction from the altrices, which are hatched in a helpless, callow condition, come into the world prepared at once to perform complex activities. mr. spalding writes,[io] "a chicken that had been made the subject of experiments on hearing [having been blindfolded at birth] was unhooded when nearly three days old. for six minutes it sat chirping and looking about it; at the end of that time it followed with its head and eyes the movements of a fly twelve inches distant; at ten minutes it made a peck at its own toes, and the next instant it made a vigorous dart at the fly, which had come within reach of its neck, and seized and swallowed it at the first stroke; for seven minutes more it sat calling and looking about it, when a hive-bee, coming sufficiently near, was seized at a dart, and thrown some distance much disabled. for twenty minutes it sat on the spot where its eyes had been unveiled without attempting to walk a step. it was then placed on rough ground, within sight and call of a hen with a brood of its own age. after standing chirping for about a minute, it started off towards the hen, displaying as keen a perception of the qualities of the outer world as it was ever likely to possess in after-life. it never required to knock its head against a stone to discover that there was 'no road that way.' it leaped over the smaller obstacles that lay in its path, and ran round the larger, reaching the mother in as nearly straight a line as the nature of the ground would permit. this, let it be remembered, was the first time it had ever walked by sight."[ip] mr. spalding's experiments also proved that, even among the altrices, young birds do not require to be taught to fly, but fly instinctively so soon as the bodily organization is sufficiently developed to render this activity possible. he kept young swallows caged until they were fully fledged, and then allowed them to escape. they flew straight off at the first attempt. they exhibited the instinctive power of flight in a perfect but deferred form. it is, however, among the higher invertebrates--especially among the insects, and of them pre-eminently in the social hymenoptera, ants and bees, that the most remarkable and complete instincts are seen. there is, however, a tendency to ascribe all the habits of ants and bees to instinct, often, as it seems to me, without sufficient evidence that they are performed without instruction, and through no imitation or intelligent adjustment. this is, perhaps, a survival of the old-fashioned view that all the mental activities of the lower animals are performed from instinct, whereas all the activities of human beings are to be regarded as rational or intelligent. in popular writings and lectures, for example, we frequently find some or all of the following activities of ant-life ascribed to instinct: recognition of members of the same nest; powers of communication; keeping aphides for the sake of their sweet secretion; collection of aphid eggs in october, hatching them out in the nest, and taking them in the spring to the daisies, on which they feed, for pasture; slave-making and slave-keeping, which, in some cases, is so ancient a habit that the enslavers are unable even to feed themselves; keeping insects as beasts of burden, e.g. a kind of plant-bug to carry leaves; keeping beetles, etc., as domestic pets; habits of personal cleanliness, one ant giving another a brush-up, and being brushed-up in return; habits of play and recreation; habits of burying the dead; the storage of grain and nipping the budding rootlet to prevent further germination; the habits described by dr. lincecum, and to a large extent confirmed by dr. mccook,[iq] that texan ants go forth into the prairie to seek for the seeds of a kind of grass of which they are particularly fond, and that they take these seeds to a clearing which they have prepared, and then sow them for the purpose, six months afterwards, of reaping the grain which is the produce of their agriculture; the collection by other ants of grass to form a kind of soil on which there subsequently grows a species of fungus upon which they feed; the military organization of the ecitons of central america; and so forth. now, the description of the habits of ants forms one of the most interesting chapters in natural history. but to lump them together in this way, as illustrations of instinct, is a survival of an old-fashioned method of treatment. that they have to a very large extent _an innate basis_ may be readily admitted. but at present we are hardly in a position to say how far they are instinctive, that is, performed by each individual straight off, and without imitation, instruction, or intelligence; how far habitual, that is, performed after some little training and practice; how far there is the intelligent element of special adaptation to special circumstances; how far they are the result of imitation; to what extent, if any, individual training and instruction are factors in the process. to put the matter in another way. suppose that an intelligent ant were to make observations on human activities as displayed in one of our great cities or in an agricultural district. seeing so great an amount of routine work going on around him, might he not be in danger of regarding all this as evidence of blind instinct? might he not find it difficult to obtain satisfactory evidence of the establishment of our habits, of the fact that this routine work has to some extent to be learnt? might he not say (perhaps not wholly without truth), "i can see nothing whatever in the training of the children of these men to fit them for their life-activities. the training of their children has no more apparent bearing upon the activities of their after-life than the feeding of our grubs has on the duties of ant-life. and although we must remember," he might continue, "that these large animals do not have the advantage which we possess of awaking suddenly, as by a new birth, to their full faculties, still, as they grow older, now one and now another of their instinctive activities are unfolded and manifested. they fall into the routine of life with little or no training as the period proper to the various instincts arrives. if learning thereof there be, it has at present escaped our observation. and such intelligence as their activities evince (and many of them do show remarkable adaptation to uniform conditions of life) would seem to be rather ancestral than of the present time; as is shown by the fact that many of the adaptations are directed rather to past conditions of life than to those which now hold good. in the presence of new emergencies to which their instincts have not fitted them, these poor men are often completely at a loss. we cannot but conclude, therefore, that, although shown under somewhat different and less favourable conditions, instinct occupies fully as large a space in the psychology of man as it does in that of the ant, while their intelligence is far less unerring and, therefore, markedly inferior to our own." of course, the views here attributed to the ant are very absurd. but are they much more absurd than the views of those who, on the evidence which we at present possess, attribute all the varied activities of ant-life to instinct? take the case of the ecitons, or military ants, or the harvesting ants, or the ants that keep draught-bugs as beasts of burden: have we sufficient evidence to enable us to affirm that these activities are purely instinctive and not habitual? that they are to a large extent innate, few are likely to deny; but then our own habitual acts have a basis that is, to a very large extent, innate. the question is not whether they have an innate basis, but whether all the varied man[oe]uvres of the military ants, for example, are displayed to the full without any learning or imitation, without teaching and without intelligence on the part of every individual in the army.[ir] that in some cases there is something very like a training or education of the ant when it emerges from the pupa condition is rendered probable by the observations of m. forel. as mr. romanes says,[is] "the young ant does not appear to come into the world with a full instinctive knowledge of all its duties as a member of a social community. it is led about the nest and 'trained to a knowledge of domestic duties, especially in the case of larvæ.' later on, the young ants are taught to distinguish between friends and foes. when an ants' nest is attacked by foreign ants, the young ones never join in the fight, but confine themselves to removing the pupæ; and that the knowledge of hereditary enemies is not wholly instinctive in ants is proved by the following experiment, which we owe to forel. he put young ants belonging to three different species into a glass case with pupæ of six other species--all the species being naturally hostile to one another. the young ants did not quarrel, but worked together to tend the pupæ. when the latter hatched out, an artificial colony was formed of a number of naturally hostile species, all living together after the manner of the 'happy families' of the showmen." i have said that the varied activities of ants, though they may not in all cases be truly instinctive, are nevertheless the outcome of certain innate capacities. it seems to me necessary to distinguish carefully between innate capacity and instinct. every animal comes into the world with an innate capacity to perform the activities which have been necessary for the maintenance of the normal existence of its ancestors. this is part of its inherited organization. only when these activities are performed at the bidding of impulse, through no instruction and from no tendency to imitation, can they, strictly speaking, be termed instinctive. the more uniform the conditions of ancestral life, and the more highly developed the organism when it enters upon the scene of active existence, the more likely are the innate capacities to manifest themselves at once and without training as perfect instincts. among birds, the pr[oe]coces, which reach a high state of development within the egg, and among insects, those which undergo complete metamorphosis, and emerge from the pupa or chrysalis condition fully formed and fully equipped for life, display the greatest tendency to exhibit activities which are truly and perfectly instinctive. but man, whose ancestors have lived and worked under such complex conditions, and who comes into the world in so helpless and immature a state, though his innate capacities are enormous, exhibits but few and rudimentary instincts. one marked characteristic of many of the habits and instincts of the lower animals is the large amount of blind prevision (if one may be allowed the expression) which they display. by blind prevision i mean that preparation for the future which, if performed through intelligence or reason, we should term "foresight," but which, since it is performed prior to any individual experience of the results, is done, we must suppose, in blind obedience to the internal impulse. the sphex, a kind of wasp-like insect, forms a little mud chamber in which she lays her eggs. she goes forth, finds a spider, stings it in such a way that it is paralyzed but not killed, and places it in the chamber for her unborn young, which she will never see. the hen incubates her eggs, though she may never have seen a chicken in her life. the caterpillars of an african moth weave a collective cocoon as large as a melon. all unite to weave the enveloping husk; each forms its separate cocoon within the shell, and all these separate cocoons are arranged round branch-passages or corridors, by which the moths, when they emerge from the chrysalis condition, may escape. another caterpillar, that of a butterfly (_thekla_) feeds within the pomegranate, but with silken threads attaches the fruit to the branch of the tree, lest, when withered, it should fall before the metamorphosis is complete. an ichneumon fly, mentioned by kirby and spence, "deposits its eggs in the body of a larva hidden between the scales of a fir-cone, which it can never have seen, and yet knows where to seek;" and thus provision is made for young which it will never know. instances of such blind prevision might be quoted by the score. it is idle to speculate as to the accompaniments of consciousness of such acts. if it be asked--may there not be associated with the performance of the instinctive activity of incubation an inherited memory of a generalized chick? we can only answer that we do not know, but that we guess not.[it] there is, however, one association, in the case of these and other instincts, which we may fairly surmise to be frequent, though, for reasons to be specified hereafter, it is probably not invariable. just as we saw to be the case with habits, so too with instinctive activities, their performance is not infrequently associated with pleasurable feeling, their non-performance with pain and discomfort and a sense of craving or want. the animal prevented from performing its instinctive activities is often apparently unquiet, uneasy, and distressed. hence i said that the animals in our zoological gardens, even if born and reared in captivity, may exhibit a craving for freedom and a yearning to perform their instinctive activities. this craving may be regarded as a blind and vague impulse, prompting the animal to perform those activities which are for its own good and for the good of the race to which it belongs. the satisfaction of the craving, the gratification of the blind impulse, is accompanied by a feeling of relief and ease. thus where a motive emerges at all into consciousness, that from which we may presume that instinctive activities are performed is not any foreknowledge of their end and purpose, but the gratification of an immediate and pressing need, the satisfaction of a felt want. * * * * * we have, so far, been concerned merely with the various kinds of activity presented by men and animals, and with some of their characteristics. the organism, in virtue of its organization, has an inherited groundwork of innate capacity. surrounding circumstances and commerce with the world draw out and develop the activities which the innate capacity renders possible. first, there are automatic and reflex actions, which are comparatively isolated activities in response to definite stimuli, external or internal. secondly, there are those organized trains or sequences of co-ordinated activities which are performed by the individual in common with all the members of the same more or less restricted group, in adaptation to certain circumstances, oft-recurring or essential to the continuance of the species. these are the instinctive activities. but no hard-and-fast line can be drawn between them and reflex actions. the instinctive activities may be either perfect or relatively imperfect, according to the accuracy of their adaptation to the purpose for which the activity is performed; but in either case they are carried out without learning or practice. in some cases, however, they cannot be performed until the organization is more perfectly developed than it is at birth; but when the proper time arrives they are perfect, and require no practice; these may be termed "deferred instincts." where some practice, but only a little, is required, the instinctive activities may be regarded as incomplete; and these pass into those activities which require at first a good deal of practice, learning, and attention, but eventually run off smoothly and without special attention, at times almost or quite unconsciously. these are habitual activities. finally, we have those activities which are performed in special adaptation to special circumstances. these are intelligent activities. all of these may be, and the last, the intelligent actions, invariably are, accompanied by consciousness. the habitual activities, and those which are incompletely instinctive, are also, we may presume, accompanied by consciousness during the process of their organization and establishment. it is possible, however, that some of the perfectly instinctive activities may be performed unconsciously. when we consider how perfectly organized such activities are, and when we also remember that perfectly organized habitual activities are frequently in us unconscious, we shall see cause for suspecting that instinctive activities may, at any rate in some cases, be unconscious. no doubt the conditions of consciousness are not well understood. but let us accept mr. romanes's suggestion, that a physiological concomitant is ganglionic delay. "now what," he asks,[iu] "does this greater consumption of time imply? it clearly implies," he answers, "that the nervous mechanism concerned has not been fully habituated to the performance of the response required, and therefore that, instead of the stimulus merely needing to touch the trigger of a ready-formed apparatus of response (however complex this may be), it has to give rise in the nerve-centre to a play of stimuli before the appropriate response is yielded. in the higher planes of conscious life this play of stimuli in the presence of difficult circumstances is known as indecision; but even in a simple act of consciousness--such as signalling a perception--more time is required by the cerebral hemispheres in supplying an appropriate response to a non-habitual experience, than is required by the lower nerve-centres for performing the most complicated of reflex actions by way of response to their habitual experience. in the latter case the routes of nervous discharge have been well worn by use; in the former case these routes have to be determined by a complex play of forces amid the cells and fibres of the cerebral hemispheres. and this complex play of forces, which finds its physiological expression in a lengthening of the time of latency, finds also a psychological expression in the rise of consciousness." now, since in many instinctive activities the stimulus "merely needs to touch the trigger of a ready-formed apparatus of response," i think that they _may_ be unconscious. and mr. romanes thus himself supplies the reason for rejecting his own definition of instinct as "reflex action into which there is imported the element of consciousness." of course, logically, mr. romanes can reply, "it is merely a question of where we draw the line; if the activity is unconscious, it is a reflex action; if conscious, it is an instinct." i think this unsatisfactory, ( ) because the criterion of consciousness, from its purely inferential nature, is practically impossible of application with accuracy; ( ) because the same series of activities may probably at one time be unconscious and at another time conscious; and ( ) because many actions which are almost universally regarded as reflex actions may at times be accompanied by consciousness, and would then have, on mr. romanes's view, to be regarded as instincts. having made this initial criticism, i may now state that i regard mr. romanes's treatment of instinct as most admirable and masterly. building upon the foundation laid by charles darwin, he has worked out the theory of instinct in a manner at once broad and yet minute, lucid and yet close, definite in doctrine and yet not blind to difficulties. if i say that it is a piece of work worthy of the great master whose devoted disciple mr. romanes has proved himself, i am according it the highest praise in my power. i have ventured in this volume to criticize some of mr. romanes's conclusions in the field of animal intelligence. and lest i should seem to undervalue his work, lest our few divergences should seem to hide our many parallelisms, i take this opportunity of testifying to my great and sincere admiration of the results of his careful and exact observations, his patient and thoughtful inferences, and his lucid and often luminous exposition. i do not propose to go over the ground so exhaustively covered by mr. romanes in his discussion of instinct. i shall first endeavour shortly to set forth his conclusions, and then review the subject in the light of modern views of heredity. admitting that some instincts may have arisen from the growth, extension, and co-ordination of reflex actions, mr. romanes regards the majority of instincts as of two-fold origin--first, from the natural selection of fortuitous unintelligent activities which chanced to be profitable to the agent (primary instincts); and, secondly, from the inheritance of habitual activities intelligently acquired. these are the secondary instincts, comprising activities which have become instinctive through lapsed intelligence. in illustration of primary instincts, mr. romanes cites the instinct of incubation. "it is quite impossible," he says,[iv] "that any animal can ever have kept its eggs warm with the intelligent purpose of hatching out their contents, so that we can only suppose that the incubating instinct began by warm-blooded animals showing that kind of attention to their eggs which we find to be frequently shown by cold-blooded animals.... those individuals which most constantly cuddled or brooded over their eggs would, other things equal, have been most successful in rearing progeny; and so the incubating instinct would be developed without there ever having been any intelligence in the matter." many of the instincts which exhibit what i have termed above "blind prevision" must, it would seem, belong completely or in the main to this class. the instincts of female insects, which lead them to anticipate by blind prevision the wants of offspring they will never see; the instincts of the caterpillars, which lead them to make provision for the chrysalis or imago condition of which they can have no experience; the instinct of a copepod crustacean, which lays its eggs in a brittle-star, that they may therein develop, probably in the brood-sac, and may even destroy the reproductive powers of the host for the future good of her own offspring--these and many others would seem to have no basis in individual experience. in illustration of the second class of instincts, those due to lapsed intelligence, mr. romanes cites the case of birds living on oceanic islands, which at first show no fear of man, but which acquire in a few generations an instinctive dread of him--for the wildness or tameness may become truly instinctive. "if," says dr. rae,[iw] "the eggs of a wild duck are placed with those of a tame one under a hen to be hatched, the ducklings from the former, on the very day they leave the egg, will immediately endeavour to hide themselves, or take to the water if there is any water, should any person approach, whilst the young from the tame duck's eggs will show little or no alarm, indicating in both cases a clear instance of instinct or 'inherited memory.'" it must not be supposed that these two modes of origin are mutually exclusive, and that any particular instinct must belong either to the one class or the other. on the contrary, many instincts have, as it were, a double root--the principle of selection combining with that of lapsing intelligence in the formation of a joint result. intelligence may thus give a new direction to a primary instinct, and, the intelligent modification being inherited, what is practically a new instinct may arise. conversely, selection may tend to preserve those individuals which perform some intelligent action, and may, therefore, aid the lapsing of intelligence in establishing and stereotyping an instinct. referring the reader to mr. romanes's work for the examples and illustrations by which he enforces his views, we may now proceed to consider the subject in the light of recently developed theories of heredity. we have seen that a school of biologists has arisen who deny the inheritance of acquired characters. but mr. romanes's secondary instincts depend upon the inheritance of habits intelligently acquired. by the school of professor weismann, therefore (if we may so call it without injustice to mr. francis galton), secondary instincts, in so far as any individual acquisition is concerned, are denied. opposed to this school are those who lay great stress on the inheritance of acquired characters. some of them seem driven to the opposite extreme in the matter of instinct, and appear to hold that instincts are entirely (or let us say almost entirely) due to lapsed intelligence. professor eimer, of tübingen, for example, says,[ix] "i describe as automatic actions those which, originally performed consciously and voluntarily, in consequence of frequent practice, come to be performed unconsciously and involuntarily.... such acquired automatic actions can be inherited. instinct is inherited faculty, especially is inherited habit." in his discussion of the subject, professor eimer seems to make no express allusion to primary instincts. and he regards at any rate some of those which are classed by mr. romanes as primary, as due to lapsed intelligence. "every bird," he says,[iy] "must, from the first time it hatches its eggs, draw the conclusion that young will also be produced from the eggs which it lays afterwards, and this experience must have been inherited as instinct." he says[iz] that the infant takes the breast and sucks "in accordance with its acquired and inherited faculties." he believes[ja] that "the original progenitors of our cuckoo, when they began to lay their eggs in other nests, acted by reflection and with design." regarding the mason-wasps and their allies, which sting larvæ in the ganglia which govern muscular action, and thus provide their young with paralyzed but living prey, he exclaims,[jb] "what a wonderful contrivance! what calculation on the part of the animal must have been necessary to discover it!" of the storing instincts of bees he remarks,[jc] "selection cannot here have had much influence, since the workers do not reproduce. in order to make these favourable conditions constant, insight and reflection on the part of the animals, and inheritance of these faculties, were necessary." and he concludes,[jd] "thus, according to the preceding considerations, automatic action may be described as habitual voluntary action; instinct, as inherited habitual voluntary action, or the capacity for such action." professor eimer would not probably deny the co-operation of natural selection in the establishment of these instincts, but he throws it altogether into the background. now, such a view seems to me wholly untenable. many of the instincts of insects are performed only once in the course of each individual life. can it be supposed that the weaving of a cocoon by the caterpillar is mainly a matter of lapsed intelligence? even if we credit the hen bird with the amount of reflection supposed by professor eimer, can we grant to the ancestors of the ichneumon fly such far-reaching observation and intelligence as really to foresee (not by blind prevision, but through intelligent foresight) the future development of the eggs which she lays in a caterpillar? are we to suppose that the instinctive action of the young cuckoo, which, _the day after it is hatched_, will eject all the other occupants of a hedge-accentor's nest,[je] can have had its origin in lapsed intelligence? if, because of their purposive character, we are to regard such instincts as of intelligent origin, may we not be told that through intelligent design the pike has beset its jaws, palate, and gill-arches with innumerable teeth, all backwardly directed for the purpose of holding its slippery prey; and the eagle has protected its eye with a bony ring of sclerotic plates, like the holder of an optician's watch-glass? if mimicry in form and colour is due to natural selection, why not mimicry in habits and activities? if _structures_ of a wonderfully purposive character have been evolved without the intelligent co-operation of the organisms which possess them, why not some of the highly purposive _activities_? and here the disciple of the school of professor weismann will echo and extend the question, and will say, "yes! why not _all_ instinctive activities? you are ready to admit," he will continue, "that many instincts, wonderfully purposive in their nature, are of primary origin, that is due to natural selection; why, then, invoke any other mode of origin? if lapsed intelligence be excluded in these cases, why introduce it at all? why not admit, what our theory of heredity demands, that[jf] 'all instinct is entirely due to the operation of natural selection, and has its foundation, not upon inherited experiences, but upon the variations of the germ'?" professor weismann's contention needs much more serious consideration than that of professor eimer. i think there is force in the _à priori_ argument (as an _à priori_ argument) that since very complex instincts are probably of primary origin, there is no _à priori_ necessity for the introduction of the hypothesis of lapsed intelligence. let me first illustrate this further. a certain beetle (_sitaris_) lays its eggs at the entrance of the galleries excavated by a kind of bee (_anthophora_), each gallery leading to a cell. the young larvæ are hatched as active little insects, with six legs, two long antennæ, and four eyes, very different from the larvæ of other beetles. they emerge from the egg in the autumn, and remain in a sluggish condition till the spring. at that time (in april) the drones of the bee emerge from the pupæ, and as they pass out through the gallery the sitaris larvæ fasten upon them. there they remain till the nuptial flight of the anthophora, when the larva passes from the male to the female bee. then again they await their chance. the moment the bee lays an egg, the sitaris larva springs upon it. "even while the poor mother is carefully fastening up her cell, her mortal enemy is beginning to devour her offspring; for the egg of the anthophora serves not only as a raft, but as a repast. the honey, which is enough for either, would be too little for both; and the sitaris, therefore, at its first meal, relieves itself from its only rival. after eight days the egg is consumed, and on the empty shell the sitaris undergoes its first transformation, and makes its appearance in a very different form.... it changes into a white, fleshy grub, so organized as to float on the surface of the honey, with the mouth beneath and the spiracles above the surface.... in this state it remains until the honey is consumed;"[jg] and, after some further metamorphoses, develops into a perfect beetle in august. now, it seems to me difficult to understand how, at any stage of this long series of highly adaptive, instinctive activities, lapsed intelligence can have been a factor. and therefore i say, if such a complex series[jh] can have resulted from natural selection and non-intelligent adaptation, i see no _à priori_ reason why any instinct, no matter how complex, should not have had a like origin. let us, however, next consider whether professor weismann's theory of the origin of instincts necessarily altogether excludes intelligence as a co-operating factor. the essential point on which that theory is absolutely insistent is that what is handed on through inheritance is _an innate, and not an individually acquired, character_. now, since intelligent actions are characteristically individual, and performed in special adaptation to special circumstances, it would seem, at first sight, that the intelligent modification of an instinct could not, on professor weismann's view, be handed on. let us consider whether this must be so. speaking of ants and bees, darwin pointed out that their instincts could not possibly have been acquired by inherited habit, since they are performed by neuter insects, that is, by undeveloped females incapable of laying eggs and continuing their race. for a habit to pass into an instinct by inheritance, it is obviously necessary that the organism which performs the habitual actions should be capable of producing offspring by which these actions might be inherited. but in this case the parental forms do not possess these instincts, while the neuter insects which do possess them are sterile. and how does mr. darwin meet this difficulty? "it is lessened, or, as i believe, disappears," he says,[ji] "when it is remembered that selection may be applied to the family as well as to the individual. breeders of cattle wish the flesh and fat to be well marbled together; an animal thus characterized has been slaughtered, but the breeder has gone with confidence to the same stock, and has succeeded. such faith may be placed in the power of selection, that a breed of cattle always yielding oxen with extraordinarily long horns could, it is probable, be formed by carefully watching which individual bulls and cows, when matched, produced oxen with the longest horns; and yet no one ox would ever have propagated his kind.... hence we may conclude that slight modifications of structure or of instinct, correlated with the sterile condition of certain members of the community, have proved advantageous; consequently, the fertile males and females have flourished, and transmitted to their fertile offspring a tendency to produce sterile members with the same modifications. this process must have been repeated many times, until that prodigious amount of difference between the fertile and sterile females of the same species has been produced which we see in many social insects." now let us apply this illustration to the case of habits intelligently acquired. instead of the possession of long horns, suppose the performance of some habitual action be observed in the oxen. then, by carefully watching which individual bulls and cows, when matched, produced oxen which performed this intelligent habitual action, a breed of cattle always yielding oxen which possessed this habit might, on darwin's principles, be produced. the intelligence of oxen might in this way be enhanced. such faith may be placed in the power of selection that a breed of cattle always yielding oxen of marked intelligence could, it is possible, be formed by carefully watching which individual bulls and cows, when matched, produced the most intelligent oxen; and yet no ox would ever have propagated its kind. regarding, then, a nest of ants or bees as a social community, mutually dependent on each other, and subject to natural selection, that community would best escape elimination in which the queen produced two sets of offspring--one set in which the procreative faculty was predominant to the partial exclusion of intelligence, and another in which intelligent activities were predominant to the exclusion of propagation. it is possible that i have weakened my case by introducing such a difficult problem as the instincts of neuter insects. and i would beg the reader to remember that this is only incidental. what i wish to indicate is that among the many variations to which organisms are subject, there are variations in their intelligent activities; that these are of elimination value, those animals which conspicuously possess them escaping elimination in its several modes; that those survivors which thus escape elimination are likely to hand on, through inheritance, that intelligence which enabled them to survive; that if, throughout a series of generations, such intelligence be applied to some definite end, nervous channels will tend to be definitely established, and the intelligent activity will more and more readily become habitual; that eventually, through the lapsing of intelligence, these habitual activities may become so fixed and stereotyped as to become instinctive; that intelligence has thus been a factor in the establishment of these instinctive activities; that throughout the sequence there is no inheritance of anything individually acquired, the intelligent variations being throughout of germinal origin; and that, therefore, in the origin of instincts, the co-operation of intelligence and the lapsing of intelligence are not excluded on the principles advocated by professor weismann. what, then, is excluded? any _individually acquired increment_, either in the intelligence displayed or the stereotyping process. the subject of instinct and of animal intelligence has not at present been considered at any great length by professor weismann, but, judging by the general tenor of his writings, i take it that what he demands is definite proof that such individually acquired increment _is actually inherited_. as before indicated in the chapter on "heredity," such proof it is, from the nature of the case, almost impossible to produce. suppose that we find evidence of a gradually increasing application of intelligence to some important life-activity, or a more and more defined stereotyping of some incompletely habitual or instinctive action; how are we to prove that the increment in either case is due to the inheritance of individual acquisitions, not to the selection of favourable innate (that is to say, germinal) variations? such a hopeless task may at once be abandoned. are we, then, to leave the question as insoluble? i think not. it is still open to us to consider whether there are any cases in which the inheritance of acquired modifications is a more probable hypothesis than the selection of favourable germinal variations. now, the acquisition of an instinctive dread of man, and the loss of this instinctive timidity under domestication, seem to be of this kind. and yet i doubt whether the evidence on this head is _convincing_. for the loss of instinctive timidity, professor weismann may invoke the aid of panmixia. but if there is truth in what i have already urged on this head, panmixia will not adequately account for the facts. on the other hand, he may contend that the instinctive dread is not due to the inheritance of individually acquired experience, but to the selection of the wilder birds and animals through the persistent elimination of those which are tame. and in support of this view, he may quote darwin himself, who says,[jj] "it is surprising, considering the degree of persecution which they have occasionally suffered during the last one or two centuries, that the birds of the falklands and galapagos have not become wilder; it shows that the fear of man is not soon acquired." it is questionable, however, whether this persecution, admittedly occasional, can have much elimination value. there is, however, the element of imitation and instruction to be taken into account, and the difficulty of proving that the timidity is really instinctive. it has frequently been observed that birds become, after a while, quite fearless of trains. here elimination is practically excluded; but it has to be proved that this fearlessness is truly instinctive. professor eimer says,[jk] "in my garden every sparrow and every crow know me from afar because i persecute these birds. once, in the presence of a friend, i shot a crow from the roof of my house, while the pigeons and starlings on the same roof, to the great astonishment of my friend, to whom i had predicted it, remained perfectly quiet. they had learned by frequent experience at what my gun was aimed, and knew that it did not threaten them." there is nothing in this interesting observation, however, to show that what the pigeons had learnt had, by _inherited_ experience, become instinctive. and professor weismann will not, in all probability, be prepared to accept as a logical inference "that this instinct of fear, because it can be dispelled by experience, must be founded on inherited, acquired experience."[jl] fully admitting, then, that this is a matter of relative probability, and that the observations and inferences in this matter are not by themselves convincing, i still think that the balance of probability is here on the side of some inheritance of experience. take next such an instinctive habit as that which dogs display of turning round in a narrow circle ere they lie down. in its origin the instinct probably arose with the object of preparing a couch in the long grass. now, is this habit of elimination value? can we suppose that it arose through the elimination of those ancestral animals which failed to perform this habit? i find it difficult to accept this view, though it is just possible that the animals which did this thereby escaped the observation of their enemies. it is also possible that this originally was a merely purposeless habit, a strange trick of manner, which has been inherited, and rendered constant and fixed. here again, however, i think the balance of probability is that the habit was intelligently acquired and inherited. i have before drawn attention to the more or less incompletely instinctive avoidance, by birds and lizards, of insects with warning coloration. that the avoidance is not perfectly instinctive is shown by the fact that young birds sometimes taste these caterpillars or insects. but a very small basis of experience, often a single case, is sufficient to establish the association. and in young chicks the avoidance of bees and wasps seems to be perfectly instinctive. the effects on the young birds, however, can hardly be of elimination value. mr. poulton offered unpalatable insects "to animals from which all other food was withheld. under these circumstances, the insects were eaten, although often after many attempts, and evidently with the most intense disgust."[jm] i have caused bees to sting young chickens; the result was extreme discomfort, but in no cases permanent injury or death. if, then, the instinct is not of elimination value, that is to say, not such as to save the possessors from elimination, how can it have been established by natural selection? and if not due to natural selection, to what can it be due, save inherited antipathy? natural selection is such a far-reaching and ubiquitous factor in organic evolution, that it is not likely that many cases can be found in which the play of elimination can be rigidly excluded. but there are not a few in which elimination does not appear to be the most important factor. mr. g. l. grant has recently observed that the sparrows near auckland, new zealand, have taken to burrowing holes in sand-cliffs, like the sand-martin. the cliff-swallow of the eastern united states has almost ceased to build nests in the cliffs, like its progenitors, and now avails itself of the protection afforded by the eaves of houses. the surviving beavers in europe are said to have abandoned the instinct of building huts and dams. the race being no longer sufficiently numerous to live in communities, the survivors live in deep burrows. in russian lapland, under the persecution of hunters, the reindeer are reported to be abandoning the tundras, or open lichen-covered tracts, for the forests. the kea (_nestor notabilis_), a brush-tongued parrot of new zealand, which normally feeds on honey, fruits, and berries, has, since the introduction of sheep, taken to a carnivorous diet. it is said to have begun by pecking at the sheep-skins hung out to dry; subsequently it began to attack living sheep; and now it has learnt to tear its way down to the fat which surrounds the kidneys. this habit, far from being the result of elimination, is rapidly leading to the elimination of the bird that has so strangely adopted it. now, although in these cases elimination has, i think, been a quite subordinate factor, i do not adduce them as convincing evidence that acquired habits are hereditary. instruction and imitation in each successive generation may well have come into play. there is no proof that they are even incompletely instinctive. but i think that these are the kinds of activities, renewed and careful observations and, if possible, experiments on which, may lead to more decisive results. it would probably not be difficult to ascertain how far the carnivorous habit of the kea has become hereditary, and how far it is performed in the absence of instruction and without the possibility of imitation. i confess that when i look round upon the varied habits of birds and mammals, when i see the frigate bird robbing the fish-hawk of the prey that it has captured from the sea, the bald-headed chimpanzee adopting a diet of small birds, a _semnopithecus_ in the mergui archipelago eating crustacea and mollusca, and the koypu, a rodent, living on shell-fish; when i consider the divergence of habits in almost every group of organisms, the ground-pigeons, rock-pigeons, and wood-pigeons, seed-eating pigeons and fruit-eating pigeons; the carrion-eating, insect-eating, and fruit-eating crows; the aquatic and terrestrial kingfishers, some living on fish, some on insects, some on reptiles;[jn] the divergent habits of the ring-ousel and the water-ousel; and the peculiar habits of blood-sucking bats;--when i see these and a thousand other modifications and divergences of habit, i question whether the theory that they have all arisen through the elimination of those forms which failed to possess them may not be pushed too far; i am inclined to believe that the inheritance of acquired modifications has been a co-operating factor. it is not enough to say that these habits are all useful to their several possessors. _it has to be shown that they are of elimination value_--that their possession or non-possession has made all the difference between survival and elimination. on the whole, then, as the result of a careful consideration of the subject of instinctive and habitual activities, and in accordance with my general view of organic evolution as set forth in previous chapters, i am disposed to accept the inheritance of individually acquired modifications of habit as a working hypothesis. i do not think that absolutely convincing evidence thereof can at present be produced. but to the best of my judgment, the probabilities are in favour of the inheritance of modifications of existing activities, due to intelligence, instruction, and imitation; always provided that the exercise of these modified activities is sufficiently frequent and definite to give rise to habits in the individual. i recognize three factors in the origin of instinctive activities-- . elimination through natural selection. . selection through preferential mating. . the inheritance of individually acquired modifications. of these i consider the first quite incontrovertible; the second as highly probable; and the third as probable in a less degree. in all three, intelligence may or may not have been a factor. some of the habits which have survived elimination under the first factor may have been originally intelligent, some of them from the first unintelligent. some of the love-antics (so called), which, through their tendency to excite sexual appetence in the female, have been selected under the second factor, may have had a basis in intelligence; many of them probably have not. and though the great majority of individually acquired modifications of habits have owed their origin to intelligent direction, still it is conceivable that some of them have not. an animal may have been forced by circumstances to modify its habits, without any exercise of intelligence; and this modification, forced, through changed conditions, upon all the members of a species, may, through inheritance, have passed into the stereotyped condition of an instinct. under each factor, then, we have two several categories. . elimination { _a._ of unintelligent activities. { _b._ of intelligent activities. . selection { _a._ of unintelligent activities. { _b._ of intelligent activities. . inheritance { _a._ of unintelligent activities. { _b._ of intelligent activities. in all cases, however, where intelligence has been a co-operating factor, this intelligence has lapsed so soon as the activity became truly instinctive. from the co-operation of the factors it is almost impossible to give examples which shall illustrate the exclusive action of any one. the following table must therefore be regarded as indicating the probable predominance of the factor indicated:-- . { _a._ caterpillars spinning cocoons. { _b._ instincts of social hymenoptera. . { _a._ drumming of snipe. { _b._ procedure of queensland bower-bird. . { _a._ ants forming nests in trees in flooded parts of siam. { _b._ instinctive fear of man. in speaking of the instinct of caterpillars spinning cocoons as unintelligent, i am regarding the final purpose of the activity. intelligence may very possibly have come into play in modifying the details of procedure. in giving the drumming of snipe as an example of unintelligent activities furthered by selection, i am assuming that it has a sexual import, and that the activity correlated with a narrowing of the tail-feathers was not, in its inception, intelligently performed with the object of exciting sexual appetence in the hen. the case of the ants of siam is given by mr. romanes,[jo] on the authority of lonbière, who says "that in one part of that kingdom, which lies open to great inundations, all the ants made their settlements upon trees; no ants' nests are to be seen anywhere else." now, this modification of habits may have been the result of intelligence; or it may have been forced upon the ants by circumstances. the floods drove them on to the trees; the instinctive impulse to build a settlement was imperative; hence the settlement had to be formed on the trees, because the ground was flooded. the difficulty of ascertaining whether intelligence has or has not been a factor is simply part of the inherent difficulty of comparative psychology--a difficulty on which sufficient stress has already been laid in an earlier chapter. the great majority of the instinctive activities of animals have arisen through a co-operation of the factors, and it is exceedingly difficult in any individual case to assign to the factors their several values. and here we must once more notice that the separation off of the instinctive activities from the other activities of animals is merely a matter of convenience in classification. in the living organism the activities--automatic actions, reflex actions, incompletely and perfectly established instincts, habits, and intelligent activities--are unclassified and commingled. they are going on at the same time, shading the one into the other, untrammelled by the limits imposed by a scientific method of treatment. once more, too, we must notice that the activities of animals are essentially the outcome and fulfilment of emotional states. when the emotional sensibility is high, the resulting activities are varied and vigorous. as we have before seen, this high state of emotional sensibility is correlated with a highly charged and sensitive condition of the organic explosives elaborated by the plasmogen of the cells. after repose, and at certain periodic times, this state of exalted sensibility is apt to occur. it is exemplified in the so-called instinct of play, which manifests itself in varied activities in the early morning, in early life, and in the returning warmth of spring--at such times, in fact, as the life-tide is in full flood. but perhaps the activities which result from a highly wrought state of sensibility are best seen at the periodic return of sexual appetence or impulse in animals of various grades of life and intelligence. many organisms, at certain periods of the year, and in presence of their mates, are thrown into a perfect frenzy of sexual appetence. the love-antics of birds have been so frequently described that i will merely quote from darwin[jp] mr. strange's account of the satin bower-bird: "at times the male will chase the female all over the aviary, then go to the bower, pick up a gay feather or a large leaf, utter a curious kind of note, set all his feathers erect, run round the bower, and become so excited that his eyes appear ready to start from his head; he continues opening first one wing, and then the other, uttering a low, whistling note, and, like the domestic cock, seems to be picking up something from the ground, until at last the female goes gently towards him." instances might be quoted from almost all classes of the animal kingdom. many fish display "love-antics," for example, the gay-suited, three-spine stickleback, whose excitement is apparently intense. newts display similar activities. even the lowly snail makes play with its love-darts (_spiculæ amoris_), practical tangible darts of glistening carbonate of lime. mr. george w. peckham has recently described[jq] the extraordinary "love-dance" of a spider (_saitis pulex_). "on may we found a mature female, and placed her in one of the larger boxes; and the next day we put a male in with her. he saw her as she stood perfectly still, twelve inches away; the glance seemed to excite him, and he at once moved towards her; when some four inches from her he stood still, and then began the most remarkable performances that an amorous male could offer to an admiring female. she eyed him eagerly, changing her position from time to time, so that he might be always in view. he, raising his whole body on one side by straightening out the legs, and lowering it on the other by folding the first two pairs of legs up and under, leaned so far over as to be in danger of losing his balance, which he only maintained by sidling rapidly towards the lowered side. the palpus, too, on this side was turned back to correspond to the direction of the legs nearest it. he moved in a semicircle for about two inches, and then instantly reversed the position of the legs, and circled in the opposite direction, gradually approaching nearer and nearer to the female. now she dashes towards him, while he, raising his first pair of legs, extends them upward and forward as if to hold her off, but withal slowly retreats. again and again he circles from side to side, she gazing towards him in a softer mood, evidently admiring the grace of his antics. this is repeated until we have counted a hundred and eleven circles made by the ardent little male. now he approaches nearer and nearer, and when almost within reach whirls madly around and around her, she joining and whirling with him in a giddy maze. again he falls back and resumes his semicircular motions, with his body tilted over; she, all excitement, lowers her head and raises her body, so that it is almost vertical; both draw nearer; she moves slowly under him, he crawling over her head, and the mating is accomplished." it can scarcely be doubted that such antics, performed in presence of the female and suggested at sight of her, serve to excite in the mate sexual appetence. if so, it can, further, scarcely be doubted that there are degrees of such excitement, that certain antics excite sexual appetence in the female less fully or less rapidly than others; yet others, perhaps, not at all. if so, again, it can hardly be questioned that those antics which excite most fully or most rapidly sexual appetence in the female will be perpetuated through the selection of the male which performs them. this is sexual selection through preferential mating. and, i think, the importance of these activities, their wide range, and their perfectly, or at any rate incompletely instinctive nature, justifies me in emphasizing this factor in the origin of instinctive activities. it has hitherto, i think, not received the attention it deserves in discussions of instinct. a few more words may here be added to what has already been said on the influence of intelligence on instinct. the influence may be twofold--it may aid in making or in unmaking instincts. we have seen that instincts may be modified through intelligent adaptation. a little dose of judgment, as huber phrased it, often comes into play. the cell-building instinct of bees is one which is remarkably stereotyped; and yet it may be modified in intelligent ways to meet special circumstances. when, for example, honey-bees were forced to build their comb on the curve, the cells on the convex side were made of a larger size than usual, while those on the concave side were smaller than usual. huber constrained his bees to construct their combs from below upwards, and also horizontally, and thus to deviate from their normal mode of building. the nest-construction of birds, again, may be modified in accordance with special circumstances. and, perhaps, it is scarcely too much to say that, whenever intelligence comes on the scene, it may be employed in modifying instinctive activities and giving them special direction. now, suppose the modifications are of various kinds and in various directions, and that, associated with the instinctive activity, a tendency to modify it _indefinitely_ be inherited. under such circumstances intelligence would have a tendency to break up and render plastic a previously stereotyped instinct. for the instinctive character of the activities is maintained through the constancy and uniformity of their performance. but if the normal activities were thus caused to vary in different directions in different individuals, the offspring arising from the union of these differing individuals would not inherit the instinct in the same purity. the instincts would be imperfect, and there would be an inherited tendency to vary. and this, if continued, would tend to convert what had been a stereotyped instinct into innate capacity; that is, a general tendency to certain activities (mental or bodily), the exact form and direction of which is not fixed, until by training, from imitation or through the guidance of individual intelligence, it became habitual. thus it may be that it has come about that man, with his enormous store of innate capacity, has so small a number of stereotyped instincts. but while intelligence, displayed under its higher form of originality, may, in certain cases, lead to all-round variation, tending to undermine instinct and render it less stereotyped, intelligence, under its lower form of imitation, has the opposite tendency. for young animals are more likely to imitate the habits of their own species than the foreign habits of other species, and such imitation would therefore tend towards uniformity. imitation is probably a by no means unimportant factor in the development of habits and instincts. mr. a. r. wallace, in his "contributions to the theory of natural selection," contends that the nest-building habit in birds is, to a large extent, kept constant by imitation. the instinctive motive is there, but the stereotyped form is maintained through imitation of the structure of the nest in which the builders were themselves reared. mr. weir, however, writing to mr. darwin, in , says in a letter, which mr. romanes quotes,[jr] "the more i reflect on mr. wallace's theory, that birds learn to make their nests because they have themselves been reared in one, the less inclined do i feel to agree with him.... it is usual with canary-fanciers to take out the nest constructed by the parent birds, and to place a felt nest in its place, and, when the young are hatched and old enough to be handled, to place a second clean nest, also of felt, in the box, removing the other. this is done to prevent acari. but i never knew that canaries so reared failed to make a nest when the breeding-time arrived. i have, on the other hand, marvelled to see how like a wild bird's the nests are constructed. it is customary to supply them with a small set of materials, such as moss and hair. they use the moss for the foundation, and line with the finer materials, just as a wild goldfinch would do, although, making it in a box, the hair alone would be sufficient for the purpose. i feel convinced nest-building is a true instinct." on the other hand, mr. charles dixon, quoted[js] in mr. wallace's "darwinism," speaking of chaffinches which were taken to new zealand and turned out there, says, "the cup of the nest is small, loosely put together, apparently lined with feathers, and the walls of the structure are prolonged for about eighteen inches, and hang loosely down the side of the supporting branch. the whole structure bears some resemblance to the nests of the hang-birds (_icteridæ_), with the exception that the cavity is at the top. clearly these new zealand chaffinches were at a loss for a design when fabricating their nest. they had no standard to work by, no nests of their own kind to copy, no older birds to give them any instruction, and the result is the abnormal structure i have just described." there is more evidence in favour of the view that the song of birds is, in part at least, imitative. that it has an innate basis is certain; and that it may be truly instinctive is shown by mr. couch's observation of a goldfinch which had never heard the song of its own species, but which sang the goldfinch-song, though tentatively and imperfectly. on the other hand, imitation is undoubtedly a factor. the hon. daines barrington says ( ), "i have educated nestling linnets under the three best singing larks--the skylark, woodlark, and titlark--every one of which, instead of the linnet's song, adhered entirely to that of their respective instructors. when the note of the titlark linnet was thoroughly fixed, i hung the bird in a room with two common linnets for a quarter of a year. they were in full song, but the titlark linnet adhered steadfastly to that of the titlark." mr. wallace, who quotes this, adds,[jt] "for young birds to acquire a new song correctly, they must be taken out of hearing of their parents very soon, for in the first three or four days they have already acquired some knowledge of the parent's notes, which they afterwards imitate." dureau de la malle, as quoted by mr. romanes,[ju] describes how he taught a starling the "marseillaise," and from this bird all the other starlings in a canton to which he took it are stated to have learned the air! that dogs, monkeys, and other mammalia have powers of imitation needs no illustration. and when we remember that it is only the imitation of strange and unusual actions that arrests our attention, while the imitation of normal activities is likely to pass unnoticed, we may, i think, fairly surmise that imitation is by no means an unimportant factor in the acquisition and development of habits. and where the young animal is surrounded during the early plastic and imitative period of life by its own kith and kin, imitation will undoubtedly have a conservative tendency. the education of young animals by their parents has also a conservative tendency. mr. spalding's observations show that the flight of birds is instinctive; but the parent birds normally aid the development of the instincts by instruction. ants, as we have seen, are instructed in the business of ant-life. dogs and cats train their young. and darwin tells us, on the authority of youatt,[jv] that lambs turned out without their mothers are very liable to eat poisonous herbs. we may say, then, with regard to the influence of intelligence on instinctive activities, that it may lead them to vary along certain definite lines of increased adaptation; that it may, in some cases, lead them to vary along divergent lines, and hence tend to render stereotyped instincts more plastic; and that, through imitation and instruction, it may tend to render instinctive habits more uniform in a community, and hence, if the habits are tending to vary under changed circumstances in a given direction, may tend to draw the habits of all the members of the community in that given direction. and with regard to the more general question of the variation of habits and instincts, we may say that, in addition to those variations in the origin and direction of which intelligence is a factor, there are other variations which take their origin without the influence of intelligence under the stress of changing circumstances, and yet others which may arise as we say "fortuitively" or "by chance," that is, from some cause or causes whereof we are at present ignorant, and which do not appear to be evoked directly by the stress of environing circumstances. granting, however, the existence of these variations in whatsoever way arising, and granting the influence of natural selection, of sexual selection, and perhaps of the inheritance of individually acquired modifications, those variations which are for the good of the race or species in which they occur will have a tendency to be perpetuated, while those which are detrimental will be weeded out and will tend to disappear. passing on now to consider the characteristics of those activities which we term "intelligent," we may first notice what mr. charles mercier, in "the nervous system and the mind," calls the four criteria of intelligence. intelligence is manifested, he says, first, in the novelty of the adjustments to external circumstances; secondly, in the complexity; thirdly, in the precision; and fourthly, in dealing with the circumstances in such a way as to extract from them the maximum of benefit. now, i think it is clear that, when it is our object to distinguish intelligent from instinctive activities, the precision of the adjustment cannot be regarded as a criterion of intelligence. many instinctive acts are wonderfully precise. the sphex is said to stab the spider it desires to paralyze with unerring aim in the central nerve-ganglion. other species, which paralyze crickets and caterpillars, pierce them in three and nine places respectively, according to the number of the ganglia. and yet this seems to be a purely instinctive action. so, too, to take but one more example, there is surely no lack of precision in the cell-making instinct of bees. we may say, then, that, granting that an action is intelligent, the precision of the adjustment is a criterion of the level of intelligence; but that, since there may be instinctive actions of wonderful precision, this criterion is not distinctive of intelligence. nay, more, there are many reflex actions of marvellous precision and accuracy of adjustment; and there can be no question of intelligence, individual or ancestral, in many of these. nor can we regard prevision (which is sometimes advanced as a criterion of intelligence) as specially distinctive of intelligent acts regarded objectively in the study of the activities of animals. for, as we have already seen, there are many instincts which display an astonishing amount of what i ventured to term "blind prevision"--instance the instinctive regard for the welfare of unborn offspring, and the instinctive preparation for an unknown future state in the case of insect larvæ. nor, again, is the complexity of the adjustment distinctive of intelligence as opposed to instinct. the case of the sitaris, before given, the larva of which attaches itself to a male bee, passes on to the female, springs upon the eggs she lays, eats first the egg and then the store of honey,--this case, i say, affords us a series of sufficiently marked complexity. this instinct, the paralyzing, but not killing outright, of prey by the sphex; the marvellous economy of wax in the cell-building of the honey-bee; the affixing to their body, by crabs, of seaweed (_stenorhynchus_), of ascidians (an australian _dromia_), of sponge (_dromia vulgaris_), of the cloaklet anemone (_pagurus prideauxii_); and other cases too numerous for citation;--these show, too, that the circumstances may be dealt with in such a way as to extract from them the maximum of benefit, probably without intelligence. it would be quite impossible intelligently to improve upon the manner of dealing with the circumstances displayed in many instinctive activities, even those which we have reason to believe were evolved without the co-operation of intelligence. there remain, therefore, the novelty of the adjustment and the individuality displayed in these adjustments. and here we seem to have the essential features of intelligent activities. the ability to perform acts in special adaptation to special circumstances, the power of exercising individual choice between contradictory promptings, and the individuality or originality manifested in dealing with the complex conditions of an ever-changing environment,--these seem to be the distinctive features of intelligence. on the other hand, in instinctive actions there seems to be no choice; the organism is impelled to their performance through impulse, as by a stern necessity; they are so far from novel that they are performed by every individual of the species, and have been so performed by their ancestors for generations; and, in performing the instinctive action, the animal seems to have no more individuality or originality than a piece of adequately wound clockwork. it may be said that, in granting to animals a power of individual choice, we are attributing to them free-will; and surely (it may be added), after denying to them reason, we cannot, in justice and in logic, credit them with this, man's choicest gift. i shall not here enter into the free-will controversy. i shall be content with defining what i mean by saying that animals have a power of individual choice. two weather-cocks are placed on adjoining church pinnacles, two clouds are floating across the sky, two empty bottles are drifting down a stream. none of these has any power of individual choice. they are completely at the mercy of external circumstances. on the other hand, two dogs are trotting down the road, and come to a point of divergence; one goes to the right hand, the other to the left hand. here each exercises a power of individual choice as to which way he shall go. or, again, my brother and i are out for a walk, and our father's dog is with us. after a while we part, each to proceed on his own way. pincher stands irresolute. for a while the impulse to follow me and the impulse to follow my brother are equal. then the former impulse prevails, and he bounds to my side. he has exercised a power of individual choice. if any one likes to call this yielding to the stronger motive an exercise of free-will, i, for one, shall not say him nay. what i wish specially to notice about it is that we have here a sign of individuality. there is no such individuality in inorganic clouds or empty bottles. choice is a symbol of individuality; and individuality is a sign of intelligence. but though i decline here to enter into the free-will controversy, i may fairly be asked where i place volition in the series between external stimulus and resulting activity; and what i regard as the concomitant physiological manifestation. i doubt whether i shall be able to say anything very satisfactory in answer to these questions. i shall have to content myself with little more than stating how the problem presents itself to my mind. i believe that volition is intimately bound up and associated with inhibition. i go so far as to say that, without inhibition, volition properly so called has no existence. when the series follows the inevitable sequence-- stimulus: perception: emotion: fulfilment in action --the act is involuntary. and such it must ever have remained, had not inhibition been evolved, had not an alternative been introduced, thus-- /fulfilment in action. stimulus: perception: emotion \inhibition of action. at the point of divergence i would place volition. volition is the faculty of the forked way. there are two possibilities--fulfilment in action or inhibition. i can write or i can cease writing; i can strike or i can forbear. and my poor little wounded terrier, whose gashed side i was sewing up, clumsily, perhaps, but with all the gentleness and tenderness i could command, could close his teeth on my hand or could restrain the action. i have here, so to speak, reduced the matter to its simplest expression. it is really more complex. for volition involves an antagonism of motives, one or more prompting to action, one or more prompting to restraint. the organism yields to the strongest prompting, acts or refrains from acting according as one motive or set of motives or the other motive or set of motives prevails; in other words, according as the stimuli to action or the inhibitory stimuli are the more powerful. and then we must remember that the perceptual volition of animals becomes in us the conceptual volition of man. an animal can choose, and is probably conscious of choosing. this is its perceptual volition. man not only chooses, and is conscious of choosing, but can _reflect upon his choice_; can see that, under different circumstances, his choice would have been different; can even fancy that, under the same circumstances (external and internal), his choice might have been different. this is conceptual volition. just as spinoza said that desire is appetence with consciousness of self; so may we say that the volition of contemplative man is the volition of the brute with consciousness of self. no animal has consciousness of self; that is to say, no animal can reflect on its own conscious states, and submit them to analysis with the formation of isolates. self-consciousness involves a conception of self, persistent amid change, and isolable in thought from its states. it involves the isolation in thought of phenomena not isolable in experience. we can think about the self as distinct from its conscious states and the bodily organization; but they are no more separable in experience than the rose is separable from its colour or its scent. such isolation is impossible to the brute. an animal is conscious of itself as suffering, but the consciousness is perceptual. there is no separation of the self as an entity distinct from the suffering which is a mere accident thereof; no conception of a self which may suffer or not suffer, may act or may not act, may be connected with the body or may sever that connection. just as there is a vast difference between the perception of an object as here and not there, of an occurrence as now and not then, of a touch as due to a solid body; and the conception of space, time, and causation; so is there a vast difference between a perception of an injury as happening to one's self, and a conception of self as the actual or possible subject of painful consciousness. this difference is clearly seen by mr. mivart, who therefore speaks of the _consentience_ of brutes as opposed to the _consciousness_ of man. consciousness he regards as conceptual; consentience as perceptual.[jw] and, as before stated, i should be disposed to accept his nomenclature, were it not for its philosophical implications. for mr. mivart regards the difference between consciousness and consentience as a difference _in kind_, whereas i regard it as a generic difference. i believe that consentience (perceptual consciousness) can pass and has passed into consciousness (conceptual consciousness); but mr. mivart believes that between the two there is a great gulf fixed, which no evolutionary process could possibly bridge or span. the perceptual volition of animals, then, is a state of consciousness arising when, as the outcome of perception and emotion, motor-stimuli prompting to activity conflict with inhibitory stimuli restraining from activity. the animal chooses or yields to the stronger motive, and is conscious of choosing. but it cannot reflect upon its choice, and bother its head about free-will. this involves conceptual thought. when physiologists have solved the problem of inhibition, they will be in a position to consider that of volition. at present we cannot be said to know much about it from the physiological standpoint. still, as before indicated, the fact of inhibition is unquestionable and of the utmost importance. it has before been pointed out that through inhibition, through the suppression or postponement of action, there has been rendered possible that reverberation among the nervous processes in the brain which is the physiological concomitant of æsthetic and conceptual thought. we have just seen that, in association with inhibition, the faculty of volition has been developed. and we may now notice that the postponement or suppression of action is one of the criteria of intelligent as opposed to instinctive or impulsive activities. this is, however, subordinate to the criterion of novelty and individuality. granting, then, that an action is shown to be intelligent from the novelty of the adjustments involved, and from the individuality displayed in dealing with complex circumstances (instinctive adjustments being long-established and lacking in originality), we may say that the level of intelligence is indicated by the complexity of the adjustments; their precision; the rapidity with which they are made; the amount of prevision they display; and in their being such as to extract from the surrounding conditions the maximum of benefit. * * * * * before closing this chapter, i will give a classification of involuntary and voluntary activities:-- --------------------------------------------------------------------- | |initiation. |motive. |result. | --------------------------------------------------------------------- |a. involuntary |sense-stimulus |unconscious |automatic or| | (automatic and | | reaction |reflex act | | reflex) | | of nerve-centres |reflex act | --------------------------------------------------------------------- |b. involuntary |percept |impulse (perhaps |involuntary | | (habitual and |(perhaps |lapsed) |activity | | instinctive) |lapsed) | | | --------------------------------------------------------------------- |c. voluntary |percept |appetence |voluntary | |(perceptual) | | |activity | --------------------------------------------------------------------- |d. voluntary |concept |desire |conduct | |(conceptual) | | | | --------------------------------------------------------------------- in the involuntary acts classed as automatic and reflex, the initiation and the result may be accompanied by consciousness, but the intermediate mental link which answers to the motive in higher activities is, i think, unconscious. in habitual and instinctive activities the consciousness of the percept and the impulse may in some cases have become evanescent, or, to use g. h. lewes's phrase, have lapsed. in the case of some instincts, originating by the natural selection of unintelligent activities, the perceptual element may never have emerged, and the initiation may have been a mere sense-stimulus. the division of voluntary activities into perceptual and conceptual follows on the principles adopted and developed in this work. as to the terminology employed, i agree with mr. s. alexander[jx] that it is convenient to reserve the terms "desire" and "conduct" for use in the higher conceptual plane. animals, i believe, are incapable of this higher desire and this higher conduct. it only remains to note that it is within the limits of the fourth class (of voluntary activities initiated by concepts) that morality takes its origin. morality is a matter of ideals. moral progress takes its origin in a state of dissatisfaction with one's present moral condition, and of desire to reach a higher standard. the man quite satisfied with himself has not within him this mainspring of progress. the chief determinant of the moral character of any individual is the _ideal self_ he keeps steadily in view as the object of moral desire--the standard to be striven for, but never actually attained. notes [in] i use the term "incomplete," and not "imperfect," because mr. romanes, in his admirable discussion of the subject, applies the term "imperfect instinct" to cases where the instinct is not perfectly adapted to the end in view (see "mental evolution in animals," p. ). [io] _macmillan's magazine_, february, . professor eimer, in his "organic evolution" (english translation, p. ), narrates similar experiences. [ip] mr. w. larden states, in _nature_ (vol. xlii.), that his brother extracted, from the oviduct of a vivora de la cruz snake in the west indies, two young snakelets six inches long. both, though thus from their mother's oviduct untimely ripped, threatened to strike, and made the burring noise with the tail, characteristic of the snake. [iq] dr. mccook confirms the observation that the clearings are kept clean, that the ant-rice alone is permitted to grow on them, and that the produce of this crop is carefully harvested; but he thinks that the ant-rice sows itself, and is not actually planted by the ants (see sir john lubbock's "scientific lectures," nd edit., p. ). [ir] the experiments, both of sir john lubbock and mr. romanes, show that the homing instinct of bees is largely the result of individual observation. taken to the seashore at no great distance from the hive, where the objects around them, however, were unfamiliar (since the seashore is not the place where flowers and nectar are to be found), the bees were nonplussed and lost their way. similarly, the migration of birds "is now," according to mr. wallace, "well ascertained to be effected by means of vision, long flights being made on bright moonlight nights, when the birds fly very high, while on cloudy nights they fly low, and then often lose their way" ("darwinism," p. ). this, of course, does not explain the migratory instinct--the internal prompting to migrate--but it indicates that the carrying out of the migratory impulse is, in part at least, intelligent. [is] "animal intelligence," p. . [it] the american expression, "i guess," is often far truer to fact than its english equivalent, "i think." [iu] "mental evolution in animals," pp. , . [iv] "mental evolution in animals," p. . [iw] _nature_, vol. xxviii. p. , quoted in "mental evolution in animals," footnote, p. . [ix] "organic evolution," pp. , . [iy] ibid. p. . [iz] ibid. p. . [ja] ibid. p. . [jb] ibid. p. . [jc] ibid. p. . [jd] "organic evolution," p. . the late g. h. lewes held somewhat similar views. [je] see mr. john hancock, natural history transactions, northumberland, durham, and newcastle-on-tyne, vol. viii. ( ); and _nature_, vol. xxxiii. p. . [jf] weismann, "on heredity," p. . [jg] m. fabre, as interpreted by sir john lubbock, "scientific lectures," nd edit., p. . [jh] in further illustration of the fact that purposiveness and complex adaptation of activities is no criterion of present or past direction by intelligence, we may draw attention to the action of the leucocytes, or white blood-corpuscles. metchnikoff found that in the water-flea (_daphnia_), affected by spores of _monospora bicuspidata_, a kind of yeast which passes from the intestinal canal into the body-cavity, the leucocytes attacked and devoured the conidia. if a conidium were too much for one cell, a plasmodium, or compound giant-cell, was formed to repel the invader. the same thing occurs in anthrax, the bacilli being attacked and devoured by the leucocytes. "if we summarize," says mr. bland sutton ("general pathology," pp. , ), "the story of inflammation as we read it zoologically, it should be likened to a battle. the leucocytes are the defending army, their roads and lines of communication the blood-vessels. every composite organism maintains a certain proportion of leucocytes as representing its standing army. when the body is invaded by bacilli, bacteria, micrococci, chemical or other irritants, information of the aggression is telegraphed by means of the vaso-motor nerves, and leucocytes rush to the attack; reinforcements and recruits are quickly formed to increase the standing army, sometimes twenty, thirty, or forty times the normal standard. in the conflict, cells die and often are eaten by their companions; frequently the slaughter is so great that the tissue becomes burdened by the dead bodies of the soldiers in the form of pus, the activity of the cell being testified by the fact that its protoplasm often contains bacilli, etc., in various stages of destruction. these dead cells, like the corpses of soldiers who fall in battle, later become hurtful to the organism they were in their lifetime anxious to protect from harm, for they are fertile sources of septicæmia and pyæmia--the pestilence and scourge so much dreaded by operative surgeons." now, if the leucocytes were separate organisms, whose habits were being described, some might suppose that they were actuated by intelligence, individual or inherited. but in this case the activities are purely physiological. the marshalling of the cells during the growth of tissue (e.g. the antler of a stag before described) is of like import. and dr. verworn has shown that when a (presumably weak) electric current is passed through a drop of water containing protozoa, they will, when the current is closed, flock towards the negative pole, and when the current is opened will travel towards the positive pole. the implication of all this is that vital phenomena may be intensely purposive, and yet afford no evidence or indication of the present or ancestral play of intelligence. [ji] "origin of species," p. . [jj] see appendix to mr. romanes's "mental evolution in animals," p. . [jk] "organic evolution," p. . [jl] ibid. p. . [jm] "colours of animals," p. . [jn] wallace's "darwinism," p. . [jo] "mental evolution in animals," p. . [jp] "descent of man," pt. ii. chap. xiii. [jq] george w. and elizabeth g. peckham, "occasional papers of the natural history of wisconsin," vol. i. ( ), p. . [jr] "mental evolution in animals," p. . [js] "darwinism," p. , from _nature_, vol. xxxi. p. . [jt] "contributions," etc., p. . [ju] "mental evolution in animals," p. . [jv] "on sheep," p. . [jw] in the sense in which i have used the word; not as he uses it himself. [jx] "moral order and progress." chapter xii. mental evolution. the phrase "mental evolution" clearly implies the existence of somewhat concerning which evolution can be predicated; and the adjective "mental" further implies that this somewhat is that which we term "mind." what is this mind which is said to be evolved? and out of what has it been evolved? can we say that matter, when it reaches the complexity of the grey cortex of the brain, becomes at last self-conscious? may we say that mind is evolved from matter, and that when the dance of molecules reaches a certain intensity and intricacy consciousness is developed? i conceive not. "if a material element," says mr. a. r. wallace,[jy] "or a combination of a thousand material elements in a molecule, are alike unconscious, it is impossible for us to believe that the mere addition of one, two, or a thousand other material elements to form a more complex molecule could in any way tend to produce a self-conscious existence. the things are radically distinct. to say that mind is a product or function of protoplasm, or of its molecular changes, is to use words to which we can attach no clear conception. you cannot have in the whole what does not exist in any of the parts; and those who argue thus should put forth a definite conception of matter, with clearly enunciated properties, and show that the necessary result of a certain complex arrangement of the elements or atoms of that matter will be the production of self-consciousness. there is no escape from this dilemma--either all matter is conscious, or consciousness is something distinct from matter; and in the latter case, its presence in material forms is a proof of the existence of conscious beings, outside of and independent of what we term 'matter.'" there is a central core of truth in mr. wallace's argument which i hold to be beyond question, though i completely dissent from the conclusion which he draws from it. i do not believe that the existence of conscious beings, outside of and independent of what we term "matter," is a tenable scientific hypothesis. in which case, mr. wallace will reply, "you are driven on to the other horn of the dilemma, and must hold the preposterous view that all matter is conscious." now, i venture to think that the use here of the word "conscious" is prejudicial to the fair consideration of the view which i hold in common with many others of far greater insight than i can lay claim to. and it seems to me that we cannot fairly discuss this question without the introduction of terms which, from their novelty, are devoid of the inevitable implications associated with "mind" and "consciousness" and their correlative adjectives. such terms, therefore, i venture to suggest, not with a view to their general acceptance, but to enable me to set forth, without arousing at the outset antagonistic prejudice, that hypothesis which alone, as it seems to me, meets the conditions of the case. according to the hypothesis that is known as _the monistic hypothesis_, the so-called connection between the molecular changes in the brain and the concomitant states of consciousness is assumed to be identity. professor huxley suggested the term "neuroses" for the molecular changes in the brain, and "psychoses" for the concomitant states of consciousness. according to materialism, psychosis is a product of neurosis; but according to monism, neither is psychosis a product of neurosis, nor is neurosis a product of psychosis, but _neurosis is psychosis_. they are identical. what an external observer might perceive as a neurosis of my brain, i should at the same moment be feeling as a psychosis. the neurosis is the outer or objective aspect; the psychosis is the inner or subjective aspect. it is almost impossible to illustrate this assumption by any physical analogies. perhaps the best is that of a curved surface. the convex side is quite different from the concave side. but we cannot say that the concavity is produced by the convexity, or that the convexity is caused by the concavity. the convex and the concave are simply different aspects of the same curved surface. so, too, are molecular brain-changes (neuroses) and the concomitant states of consciousness (psychoses) simply different aspects of the same waves on the troubled sea of being. again, we may liken the brain-changes to spoken or written words, and the states of consciousness to the meaning which underlies them. the spoken word is, from the physical point of view, a mere shudder of sound in the air; but it is also, from the conceptual point of view, a fragment of analytic thought. now, we believe that the particular kind of molecular motion which we call neurosis, or brain-action, has been evolved. evolved from what? from other and simpler modes of molecular motion. complex neuroses have been evolved from less complex neuroses; these from simple neuroses; these, again, from organic modes of motion which can no longer be called neuroses at all; and these, once more, from modes of motion which can no longer be called organic. and from what have psychoses, or states of consciousness, been evolved? complex psychoses have been evolved from less complex psychoses; these from simple psychoses; these, again, from--what? we are stopped for want of words to express our meaning. we believe that psychoses have been evolved. evolved from what? from other and simpler modes of--something which answers on the subjective side to motion. we can hardly say "of consciousness;" for consciousness answers to a _particular_ mode of motion called neurosis. so that unless we are prepared to say that all modes of motion are neuroses, we can hardly say that all modes of that which answers on the subjective side to motion are conscious. i shall venture, therefore, to coin a word[jz] to meet my present need. it is generally admitted that physical phenomena, including those which we call physiological, can be explained (or are explicable) in terms of energy. it is also generally admitted that consciousness is something distinct from, nay, belonging to a wholly different phenomenal order from, energy. and it is further generally admitted that consciousness is nevertheless in some way closely, if not indissolubly, associated with special manifestations of energy in the nerve-centres of the brain. now, we call manifestations of energy "kinetic" manifestations, and we use the term "kinesis" for physical manifestations of this order. similarly, we may call concomitant manifestations of the mental or conscious order "metakinetic," and may use the term "metakinesis" for all manifestations belonging to this phenomenal order. according to the monistic hypothesis, _every mode of kinesis has its concomitant mode of metakinesis, and when the kinetic manifestations assume the form of the molecular processes in the human brain, the metakinetic manifestations assume the form of human consciousness_. i am, therefore, not prepared to accept the horn of mr. wallace's dilemma in the form in which he states it. all matter is not conscious, because consciousness is the metakinetic concomitant of a highly specialized order of kinesis. but every kinesis has an associated metakinesis; and _parallel to the evolution of organic and neural kinesis there has been an evolution of metakinetic manifestations culminating in conscious thought_. paraphrasing the words of professor max müller,[ka] i say, "like descartes, like spinoza, like leibnitz, like noiré, i require two orders of phenomena only, but i define them differently, namely, as kinesis and metakinesis. according to these two attributes of the noumenal, philosophy has to do with two streams of evolution--the subjective and the objective. neither of them can be said to be prior.... the two streams of evolution run parallel, or, more correctly, the two are one stream, looked at from two opposite shores." and again,[kb] "like noiré, i would go hand-in-hand with spinoza, and carry away with me this permanent truth, that metakinesis can never be the product of kinesis (materialism), nor kinesis the product of metakinesis (spiritualism), but that the two are inseparable, like two sides of one and the same substance." according to this view, the two distinct phenomenal orders, the kinetic and the metakinetic, are distinct only as being different phenomenal manifestations of the same noumenal series. matter, the unknown substance[kc] of kinetic manifestations, disappears as unnecessary; spirit, the unknown substance of metakinetic manifestations, also disappears; both are merged in the unknown substance of being--unknown, that is to say, in itself and apart from its objective and subjective manifestations. it will, no doubt, be objected that the final identity of neuroses and psychoses is an assumption. it is pure assumption, it will be said, that these molecular nervous processes, and those percepts and emotions which are their concomitants, are simply different aspects, outer and inner, objective and subjective, physiological and psychological, of the same noumenal series. this must fully and freely be admitted. any and every explanation of the connection of mind and body is based on an assumption. the common-place view of two distinct entities, a mind which can act on the body and a body which influences the mind, is a pure assumption. the philosophic view, that there are two entities, body and mind, that neither can act on the other, but that there is a pre-established harmony between the activities of the one and the activities of the other, is, again, a pure assumption. the materialistic view, that matter becomes at last self-conscious, is a pure assumption. the idealistic view, that the world of phenomena has no existence save as a fiction of my own mind, is, once more, a pure assumption. it is not a question of making or of not making an initial assumption; _that we must do in any case_. the question is--which assumption yields the most consistent and harmonious results? again, an answer will, no doubt, be demanded by some people to the question--_how_ does that which, objectively considered, is neurosis become subjectively felt as psychosis? is not the identification of neurosis and psychosis a begging of the question, unless the _how_, the _modus operandi_, is explained? if, in the latter query, by "begging the question" the adoption of an initial assumption is meant, i have already answered it in the affirmative. to the direct question--how does the objective neurosis become conscious as a subjective psychosis?--while freely admitting that i do not know, i enter the protest that it is philosophically an illegitimate question; for an answer is impossible without transcending consciousness. an illustration will, perhaps, make my meaning clear. suppose that a sentient being be enclosed within a sphere of opaque but translucent ground glass, into the substance of which there are wrought certain characters. suppose that external to this there is another similar but larger sphere, similarly inscribed, and that a second sentient being is enclosed in the space between the two spheres. by an attentive study of the two spheres, this second sentient being arrives at the conclusion that the markings on the convex surface of the inner sphere answer to the markings on the concave surface of the outer sphere; and he is led to the conviction that what _he_ sees as markings on the convex, the being within the sphere sees as markings on the concave. he is, however, perplexed by the question--how can this be? he is acquainted with a certain inner surface and a certain outer surface. he is led to correlate the markings of the one with the markings of the other. but the question how the two can have such different aspects is beyond his solution. puzzle as he may, he can never solve it. it can only be solved (and how simple then the solution!) by a being _outside both spheres_, who can see what the enclosed being, "cabin'd, cribb'd, confined," could never see, namely, that the characters were wrought in the translucent glass of the spheres. by which parable, imperfect as it is, i would teach that we can never learn how kinetic manifestations have a metakinetic aspect without getting outside ourselves to view kinesis and metakinesis from an independent standpoint. or, in the words of sir w. r. hamilton,[kd] "how consciousness in general is possible; and how, in particular, the consciousness of self and the consciousness of something different from self are possible ... these questions are equally unphilosophical, as they suppose the possibility of a faculty exterior to consciousness and conversant about its operations." the only course open to us, then, in this difficult but important problem is to make certain assumptions, and see how far a consistent hypothesis may be based upon them. i make, therefore, the following assumptions: first, that there is a noumenal system of "things in themselves" of which all phenomena, whether kinetic or metakinetic, are manifestations. secondly, that whenever in the curve of noumenal sequences kinetic manifestations (convexities) appear, there appear also concomitant metakinetic manifestations (concavities). thirdly, that when kinetic manifestations assume the integrated and co-ordinated complexity of the nerve-processes in certain ganglia of the human brain, the metakinetic manifestations assume the integrated and co-ordinated complexity of human consciousness. fourthly, that what is called "mental evolution" is the metakinetic aspect of what is called brain or interneural evolution. it would require far more space than i can here command to deal adequately with these assumptions, and meet the objections which have been and are likely to be raised against them. i must content myself with drawing attention to one or two which seem at once obvious and yet easily met. it may be asked--what advantage has such a view over realistic materialism? why not assume that neural processes, when they reach a certain complexity, give rise to or produce consciousness? first of all, i think, the objection raised by mr. wallace, in the passage before quoted, to materialism is unanswerable. secondly, realistic materialism ignores the fact that kinetic manifestations for us human-folk are phenomena of consciousness. to this we will return presently. thirdly, realistic materialism, and any view which regards the physical series as one which is independent of the psychical accompaniments, and which regards consciousness as in any sense a by-product of neural processes, are open to an objection which was forcibly stated by the late professor herbert.[ke] "it is clearly impossible," he says, "for those ... who teach that consciousness is [a by-product and] never the cause of physical change, to dispute that the actions, words and gestures of every individual of the human race would have been exactly what they have been in the absence of mind; had mind been wanting [had the by-product never emerged], the same empires would have risen and fallen, the same battles would have been fought and won, the same literature, the same masterpieces of painting and music would have been produced, the same religious rites would have been performed, and the same indications of friendship and affection given. to this absurdity physical science [realistic materialism] stands committed." i believe that professor herbert's argument, of which this passage is a summary, is, as against realistic materialism, sound and unanswerable. finally, as professor max müller has well observed,[kf] "materialism may in one sense be said to be a grammatical blunder; it is a misapplication of a word which can be used in an oblique sense only, but which materialists use in the nominative. in another sense it is a logical blunder, because it rests on a confusion between the objective and the subjective. matter can never be a subject, it can never know, because the name was framed to signify what is the object of our knowledge or what can be known." materialism, then, for more than one sufficient reason, stands condemned. it should be stated, however, that professor herbert seems to regard the monistic view i am advocating as committed to the absurdity indicated in the passage i have quoted. i am convinced that he was here in error. indeed, he seems to have failed to see the full bearing of the monistic hypothesis; for while he combats it, he comes very near adopting it himself. with this, however, i have no concern. i have only to show that, on the assumptions above set down, we are not committed to the "absurdity" of supposing that intelligence and consciousness have had no influence on the course of events in organic evolution--that they have only felt the inevitable sequence of physical phenomena without in any way influencing it. according to the monistic hypothesis, kinesis and metakinesis are co-ordinate. the physiologist may explain all the activities of men and animals in terms of kinesis. the psychologist may explain all the thoughts and emotions of man in terms of metakinesis. they are studying the different phenomenal aspects of the same noumenal sequences. it is just as absurd to say that kinetic manifestations would have been the same in the absence of metakinesis, as to say that the metakinetic manifestations, the thoughts and emotions, would have been the same in the absence of kinesis. it is just as absurd to say that the physical series would have been the same in the absence of mind, as to say that the mental series would have been the same in the absence of bodily organization. for on this view consciousness is no mere by-product of neural processes, but is simply one aspect of them. you cannot abstract (except in thought and by analysis) metakinesis from kinesis; for when you have taken away the one, you have taken the other also. to speak of the organic activities being conceivably the same in the absence of consciousness, is like saying that the outer curve of a soap-bubble would be the same in the absence of the inner curve. whatever hypothetical existences this statement may be true of, it assuredly is not true of soap-bubbles. to pass on from this point to another, it is possible--i trust not probable, but still not impossible--that some one may say, "but how, on this view, can perception be accounted for? granted that in the neural processes of the individual organism kinesis is accompanied by those metakinetic manifestations which we term 'consciousness,' how will this account for our perception of a distant object? yonder scarlet geranium is a centre of kinetic manifestations; it is fifty yards and more away. how can i here, by any metakinetic process, perceive the kinesis that is going on out there?" for one who can ask this question, i have written the chapter on "mental processes in man," and have used the term "construct," in vain. in vain have i endeavoured to explain that the seat of all mental processes is somewhere within the brain; in vain have i indicated the nature of localization and outward projection; in vain have i reiterated that the object is a thing we construct through a (metakinetic) activity of the mind; in vain have i insisted that our knowledge is merely _symbolic_ of the noumenal existence; and perhaps in vain shall i again endeavour to make my meaning clear. when we say that we perceive an object, the mental process (perception) is the metakinetic equivalent of certain kinetic changes among the brain-molecules. the object, _as an object_ (as a phenomenon or appearance), is there generated. as before stated, i assume the existence of a noumenal system of which the noumenal existence, symbolized as object, is a part. but what we term the object is a certain phase of metakinesis accompanying certain kinetic nerve-processes in the brain. in other words, phenomena are states of consciousness, and cannot, for the percipient, be anything else. "it comes to this, then," an idealist will interpose: "states of consciousness are metakinetic; phenomena are states of consciousness; therefore phenomena are metakinetic. your kinesis vanishes, and you are one with us, a pure idealist." before showing _wherein_ i am not a pure idealist, let me state _why_ i am not. for the pure idealist, phenomena being states of consciousness, _and nothing more_, the world around resolves itself into an individual dream. were i to hold this view, this pen which i hold, this table at which i write, the spreading trees outside my window, my little sons whose merry voices i can hear in the garden, my very body and limbs, all are merely states of my own consciousness. this i am not prepared to accept. do what i will, i cannot believe that such an interpretation of the facts is true. for this reason i make my first assumption that there is a noumenal system of things in themselves, of which all phenomena, whether kinetic or metakinetic, are manifestations. i differ from the pure idealist in that i believe that phenomena, besides being states of consciousness, _have another, namely, a kinetic, aspect_. what are for me states of consciousness are for you neural processes in my brain. these are, again, for you states of consciousness; but still for some one else they are kinetic processes. and an ordinary extraneous object, like this table, is the phenomenal aspect to me of a noumenal existence; and since that noumenal existence appears to you also in like phenomenal guise, the table is an object for you as well as for me, and not only for us, but for all sentient beings similarly constituted. the world we live in is a world of phenomena; and it has a phenomenal reality every whit as valid as the noumenal reality which underlies it. and that phenomenal reality has two aspects--an inner aspect as metakinesis, and an outer aspect as kinesis. i must not here further develop the manner in which the hypothesis of monism presents itself to my mind. i will only, before passing on to consider mental or metakinetic evolution, draw passing attention to two matters. we have seen that professor hering and mr. samuel butler have suggested "organic memory" as a conception useful for the comprehension of embryonic reconstruction in development and other such matters (see p. ). on the hypothesis of monism, this may be regarded as a kinetic manifestation of that which in memory rises to the metakinetic level of consciousness. the other matter is of far wider import. monism affords a consistent and comprehensible theory of the ego, or conscious self--that which endures amid the flux and reflux of our conscious states. the ego, or self, is that metakinetic unity which answers to, or is the inner aspect of, the kinetic unity of the organism.[kg] only here and there, in fleeting and changing series, does the metakinesis rise to the level of consciousness. but the metakinetic unity is as completely one, indivisible, and enduring, as is the physical organism which is its kinetic counterpart. no one questions that there is an enduring organism of which certain visible activities are occasional manifestations; no one who has adequately grasped the teachings of monism can question that the enduring ego, of which certain states of consciousness are occasional manifestations, is the metakinetic equivalent of the organic kinesis. this solution of a problem which baffles alike materialists and idealists is, as it seems to me, as satisfactory as it is simple. and now let us pass on to consider the question of mental or metakinetic evolution. what, on the principles above laid down, can we be said to know or have learnt about it? the inevitable isolation of the individual mind has long been recognized. "such is the nature of spirit, or that which acts," says bishop berkeley, "that it cannot be itself perceived, but only by the effects that it produceth." "thinking things, as such," writes kant, "can never occur in the outward phenomena; we can have no outward perception of their thoughts, consciousness, desires; for all this is the domain of the inward sense." how comes it, then, that there is nothing of which, practically speaking, we are more firmly convinced than that our neighbours have each a consciousness more or less similar to our own? certain it is that no one can come into sensible contact with his brother's personality and essential spirit. my brother's soul can never stand to me in the relation of object. subject he never can be to any but himself. what, then, is he--his metakinetic self, not his kinetic material body--to me? in clifford's convenient phrase, he is an eject. and what is an eject? an eject is a more or less modified image of myself, that i see mirrored, as in a glass darkly, in the human-folk around me. into every human brother i breathe the spirit of this eject, and he becomes henceforth to me a living soul. or, if this mode of presentation does not meet with approval, i will say that an eject is that metakinetic unity i infer as identically associated with the organic and kinetic unity of my brother's living body. and i base the close metakinetic correspondence that i infer on the close kinetic correspondence that i observe. but since the only form or kind of metakinesis that i know is that of human self-conscious personality, it is certain that the metakinetic eject is an image of myself; it is and must be, in a word, anthropomorphic. too much stress can scarcely, i think, be laid on the human, nay, even the individual, nature of the eject. all other-mind i am bound to think of in terms of my own mind. the men and women i see around me are like curved mirrors, in which i see an altered reflection of my own mental features. by certain signs i may be able to infer in this or that human mirror graces or imperfections that i lack. but throughout my survey of human nature, every estimate of intellectual or moral elevation or degradation that i form must ever be measured in terms of my own subjective base-line. my conception of humanity must always be, not only anthropomorphic, but idiomorphic. once more, let it be remembered that the metakinesis that rises to the level of consciousness is that which forms the inner aspect of the neural kinesis of my brain or yours. for each of us, then, that metakinesis is the only possible metakinesis which we can know as such and at first-hand. and for the pure idealist it is the only metakinesis which he can know at all. not so with us. we have assumed a noumenal system of "things in themselves," of which all phenomena, whether kinetic or metakinetic, are manifestations. we have assumed that kinesis cannot emerge into the light of being without casting its inseparable metakinetic shadow. we have assumed that when the kinetic manifestations assume the integrated and co-ordinated complexity of nerve-processes in certain ganglia of the human brain, the metakinetic manifestations assume the integrated and co-ordinated complexity of human consciousness. human physiology is teaching us more clearly every day that all human activities are, physically speaking, the outcome of neural processes. such neural processes are in us conscious. therefore, granting our assumptions, the conclusion that my neighbour is a conscious self, just as i am, is not only legitimate, but (as we see from the daily conduct of men) inevitable. in other words, certain kinetic phenomena have for us inevitable metakinetic implications. now, when we pass from man to the lower animals, the metakinetic implications become progressively less inevitable and less forcible as the kinesis becomes more dissimilar from that which obtains in the human organism. the only metakinesis that we know directly is our own human consciousness. in terms of this we have to interpret all other forms of metakinesis. it is unnecessary to go over again the ground that has already been covered in previous chapters, in which we have endeavoured to give some account of what seem to us the legitimate inferences concerning the mental processes in animals. the point on which i wish here to insist is that, outside ourselves, we can only know metakinesis in and through its correlative kinesis. underlying kinetic evolution, we see that, on the hypothesis of monism, there must have been metakinetic evolution. but of this mental or metakinetic evolution we neither have nor can have independent evidence. such evolution is the inevitable monistic corollary from kinetic evolution. more than this it is not and cannot be. and only on the monistic hypothesis, as it seems to me, is it admissible to believe in mental evolution,[kh] properly so called. but does not, it may be asked, the hypothesis of monism, if carried to its logical conclusion, involve the belief in a world-consciousness on the one hand, and a crystal-consciousness on the other? if, according to the hypothesis, every form of kinesis has also its metakinetic aspect, "must we not maintain," in the words of mr. j. a. symonds, "that the universe being in one rhythm, things less highly organized than man possess consciousness in the degree of their descent, less acute than man's? must we not also surmise that ascending scales of existence, more highly organized, of whom we are at present ignorant, are endowed with consciousness superior to man's? is it incredible that the globe on which we live is vastly more conscious of itself than we are of ourselves; and that the cells which compose our corporeal frame are gifted with a separate consciousness of a simpler kind than ours?" to such questions w. k. clifford replied with an emphatic negative. "unless we can show," he said, as interpreted by mr. romanes,[ki] "in the disposition of the heavenly bodies some morphological resemblance to the structure of a human brain, we are precluded from rationally entertaining any probability that self-conscious volition belongs to the universe." i conceive that both parties, opposed as they seem, are logically right; and i venture to think that the terms i have suggested will help us here. mr. symonds used the word "consciousness" to signify metakinesis in general; clifford used it to signify that particular kind of metakinesis which in the human brain rises to the level of consciousness. not only is it not inconceivable, but it is a logical necessity on the hypothesis of monism, that answering to the kinetic rhythm of the universe there is a metakinetic rhythm; but unless the gyrations of the spheres have some kinetic resemblance to the dance of molecules in the human brain, the metakinesis cannot be inferred to be similar to the consciousness of man. similarly, with regard to the supposed self-consciousness of the so-called social organism. mr. romanes, in his article on "the world as an eject,"[kj] leads up to his conception of a world-eject through the conception of a society-eject--an eject, he tells us, that, for aught that any one of its constituent personalities can prove to the contrary, may possess self-conscious personality of the most vivid character. its constituent human minds may be born into it, and die out of it, as do the constituent cells of the human body; it may feel the throes of war and famine, rejoice in the comforts of peace and plenty; it may appreciate the growth of civilization in its passage from childhood to maturity. this, of course, may be so; or it may not. who can tell? but clifford was on firm monistic ground when he maintained that, unless the kinesis be similar, we have no grounds for inferring similarity of metakinesis. the study of kinesis leads us to recognize different kinds or modes of its manifestation. there is one mode of kinesis in the circling of the planets around the sun, another mode of kinesis in the orderly evolutions of a great army, another mode in the throb of a great printing-press; there is one mode of kinesis in the quivering molecules of the intensely heated sun, another in the wire that flashes our thought to america, and yet another in the molecular vibrations of the human brain. all are of the same order, all are kinetic. but they differ so widely in mode that each requires separate, patient, and long-continued study. so is it, we may conclude, with metakinesis. there may be, nay, there must be, many modes. but our knowledge is confined to one mode--that in which the metakinesis assumes the form of human consciousness. i have been led to discuss this matter in order further to indicate the inevitable limits of our knowledge of metakinetic evolution. our conclusions may be thus summarized: first, we can know directly only one product of metakinetic evolution--that revealed in our own consciousness. secondly, the process of metakinetic evolution must be reached, if reached at all, indirectly through a study of kinetic evolution. thirdly, we have no right to infer a mode of metakinesis analogous to human consciousness, unless the mode of kinesis is analogous to that which is observed in neural processes. and, fourthly, the closer the kinetic resemblance we observe, the closer the metakinetic resemblance we may infer. * * * * * the last point we have to notice, and it is by no means an unimportant one, is that, just as the kinetic evolution of the organism must be studied in reference to its kinetic environment, so, too, must the metakinetic evolution of mind be studied in reference to its metakinetic or mental environment. of course, in ordinary speech, and even in careful scientific description, we are forced, if we would avoid pedantry, to skip backwards and forwards from the kinetic to the metakinetic. we speak of a kinetic cow giving rise to metakinetic fear, and this determining certain kinetic activities. why we thus interpose a mental link in a physical series has already been explained. the physical cow we know, the physical activities we know, the physical neuroses we scarcely know at all. on the other hand, fear we have ourselves experienced, and know well. hence we introduce the mental link that we know in place of the physical link of which we are ignorant. and there can be no harm in our doing so when we are working on the practical, and not the philosophical plane. but when we are striving to go deeper, and are employing that gift of analysis which is man's prerogative, in order to proceed to a higher and more complete synthesis,--then we must be careful to keep separate those processes which analysis discloses to be distinct. and i repeat that, on the philosophical plane of thought, we must remember that _metakineses are determined by other metakineses, and by them alone_. the reader who has kept his head among these slippery places will at once see that this is and must be so; for, as we have already seen (p. ), all phenomena are states of consciousness, whatever else they may also be. the cow, as a phenomenon, is a _construct_, a product of mental activity, and woven out of states of consciousness. for the pure idealist she is this and nothing more. but for us she is a real external entity, manifested through phenomenal kineses. hence in ordinary speech we separate the kinetic cow from its metakinetic symbols in consciousness (the convex from the concave aspect), and call the former the cow itself, and the latter our idea of the cow. but, as before maintained, my idea of an object is for me the object. and this is now justified by our deeper analysis. the physiologist, dealing with organic phenomena in terms of motion (kinesis), proclaims that the physical series is complete, that there is no necessity for the introduction of feeling which is at best but a by-product. the idealist, dealing with the processes of thought and emotion in terms of consciousness, proclaims that his series is complete--an external material universe is an unnecessary encumbrance. each proclaims a half-truth; each sees that half of the truth which alone is visible from his special standpoint. monism combines the two (and is, of course, scouted by both). it sees not only that the one series does not in any case interfere with the other, but that the conception of such an interference involves an impossibility and incongruity. as soon could one speak of the convexities of one side of a curved surface interfering with the corresponding concavities of the other side, as of the metakinetic series interfering with the kinetic series, which is its other aspect. but if the one cannot interfere with the other, neither can the one exist without the other. to apply the same analogy, as well might one speak of the convexities of a curved surface existing without the concavities of its other side, as of the kinetic phenomena of organic life as being conceivably the same in the absence of conscious intelligence. remembering, then, that just as the environment of kinetic phenomena is itself kinetic, with which consciousness can in no wise interfere, so is the environment of metakinetic phenomena, perception, thought, and emotion, itself metakinetic. let us now proceed to consider some of the implications. we have already seen that, in what we may regard as the earlier phases of organic and mental life, the series between stimulus and activity is a simple one, which may be kinetically represented thus-- stimulus-->neural processes-->motor-activities; but that when inhibition is developed, there arises an alternative, thus-- / motor-activities. stimulus-->neural processes \ inhibition thereof. and we further saw that, as a result of this inhibition, the entering stimuli, instead of, as it were, rapidly running out of the organism in motor-activities, set up a more and more complex series of diffused and reverberating neural processes in the brain or other central ganglia. from the metakinetic view-point these diffused and reverberating neural processes in the brain culminate in consciousness as thought, æsthetic emotion, and the higher conceptual mental activities. deeply as these influence conduct, they are, to a large extent, independent of conduct. a man's thoughts and æsthetic yearnings may be of the truest and purest; but in the moment of temptation and action, when stimuli crowding in run through rapidly to action, he falls away. his conduct belies his ideals. nevertheless, the ideals were there, but too far away in the region of thought and abstract æsthetics to be operative in action. now, we may divide the metakinetic concomitants of neural processes into two categories: first, those which are intimately associated with neural processes directly leading to motor-activities; secondly, those which are, so to speak, floated off from these into the region of thought and æsthetic emotion, and which are therefore associated with neural processes only indirectly or remotely leading to motor-activities. both have, of course, kinetic equivalents in neural processes, but the former are directly associated with activities and conduct, and the latter are not. let me exemplify. interpretations of nature, theories, hypotheses, belong to the latter class. their association with activities is in the main indirect. whether we believe in materialism, idealism, or monism, our conduct is much the same. people got out of the way of falling stones, and guarded against being caught by the incoming tide, before science comprised both phenomena under the theory of gravitation. the conduct of human-folk was not much altered by the replacement of the geocentric by a heliocentric explanation of the solar system. it matters not much how a man explains the lightning's flash so long as he avoids being struck. the bird continues to soar quite irrespective of man's prolonged discussion of how it can be explained on mechanical principles. and in general the practical activities of mankind remain much the same (i do not say quite the same, for there are remote and indirect results of the greatest importance in the long run) whatever their particular theory of the universe may be. now, let us note the implication. we have said a good deal in earlier chapters about natural elimination and selection. to which category of neural kineses do they apply--to those associated with practical results; or to those associated with theoretical results (supposing these to obtain below the level of man); or to both? clearly to those associated with practical results. it matters not what theories a lion, or an adder, or a spider hold (supposing, again, that they are capable of theorizing, which i doubt). its practical activities determine whether it survives or not. so, too, with men, _so far as they are subject to natural elimination_. it matters not what may be the nature of their thoughts, their æsthetic yearnings, their ideals. according to their practical conduct, they are eliminated or escape elimination. in other words, elimination or natural selection applies only remotely or indirectly to the human race regarded as theorists, æsthetes, or interpreters of nature. before proceeding to indicate to what laws our theories and interpretations of nature and moral ideals are subject, we may note that there are sundry activities of man, the outcome of his conceptual thought and emotion, which are also, under the conditions of social life, to a large extent beyond the pale of elimination. i refer to the æsthetic activities--music, painting, sculpture, and the like; in a word, the activities associated with art, literature, and pure science. these, in the main, take rank alongside the ideas of which they are the outward expression. natural selection, which deals with practical, life-preserving, and life-continuing activities, has little to say to them. they are neutral variations which, so far as elimination is concerned, are neither advantageous nor disadvantageous, and, therefore, remain unmolested. we may, therefore, fully agree with mr. wallace, when he says,[kk] "we conclude, then, that the present gigantic development of the mathematical faculty [as also of the musical and artistic faculties] is wholly unexplained by the theory of natural selection, and must be due to some altogether distinct cause." nay, we may go further, and say that it is only by misunderstanding the range of natural selection as an eliminator that any one could suppose that these faculties could be explained by that theory. we must admit, then, that there are certain neural kineses which, from the fact that they are unassociated with life-preserving and life-continuing activities, are not subject to the law of elimination; and in the development of which natural selection cannot have been an essential factor. these, in their metakinetic aspect, are conceptual thoughts, emotions, and ideas. remembering the distinction drawn in the chapter on "organic evolution" between _origin_ and _guidance_, let us proceed to inquire, first, how these ideas have been guided to their present development; and, secondly, how we may suppose these special variations to have originated. to understand their development, we must understand their environment. the environment of metakineses is, as we have already seen, constituted by other metakineses. what we have now to note is that the environment of conceptual ideas, as such, is constituted by other ideas. the immediate environment of an hypothesis is other hypotheses; of a moral ideal, other moral ideals; of an æsthetic thought, other æsthetic thoughts; of a religious conception, other religious conceptions. but not only are ideas environed by ideas of their own order; they are environed by ideas of other orders. thus a scientific hypothesis or a moral ideal may be in harmony or conflict with religious conceptions, and its fate may be thereby determined; or a religious conception may be in harmony or conflict with psychological principles, and its acceptance or rejection thereby determined. so that we may say, in general, that _the environment of an idea is the system of ideas among which it is introduced_. of course, it must be clearly understood that it is with the individual mind that we are dealing. the scientific ideas, moral ideals, æsthetic standards, religious conceptions, of a tribe, nation, or other community, are simply representative, either of the general views of the majority of the individuals, or more frequently of a majority among a cultivated minority. in any case, we have seen that metakineses are and must be an individual matter. for each individual there is a separate ideal world. through certain activities, notably language spoken or written, men can symbolize to each other the ideas that are taking metakinetic shape in their own minds. all-important, however, as is this power of intercommunication by means of language, it does not a whit alter the fact that the idea and its environment have to work out their relations to each other separately in each individual mind. my neighbour may symbolize, through language, his ideas in such a form that similar ideas may be called up in my mind; but it is there that they have to make good their claim for acceptance in the environment of the system of ideas among which they are introduced. now, what is the guiding principle of the evolution and development of ideas in the world of their metakinetic environment? is there any principle analogous to that of elimination which we have seen to be of such high importance in organic evolution? i believe that there is. _an idea is accepted or rejected according to its congruity or incongruity with the system of ideas among which it is introduced._ the process has, perhaps, closer analogy with elimination than with selection, inasmuch as it would seem to proceed by the rejection of the incongruous, leaving both the congruous and the neutral. an idea or hypothesis may be accepted, at any rate provisionally, so long as it is not in contradiction to the theories and beliefs already existing in the mind. it may, however, be objected that this view is at variance with the familiar observation that there are many excellent people who hold and maintain theories which are exceedingly incongruous, which seem, indeed, to us mutually antagonistic. yes, _to us_. brought into the environment of _our_ system of ideas, one or other of these antagonistic views would be eliminated through incongruity. not so, however, with those who hold both. amid the environment of a less logical and less coherent system of ideas, both can find admission, if not as congruous, still as neutral. a sense of their incongruity is not aroused. but there are some people, it may be said, who consciously hold views which they admit to be incongruous; who base all their scientific reasonings on a continuity of causation, but who, nevertheless, believe in miraculous interruptions of that continuity. in this case, however, the incongruity is made congruous in a higher synthesis. they belie themselves when they suppose that they are holding incongruous views. stated at length, what they admit is that miraculous interventions are incongruous, not for them, but for those whose whole system of thought is cast in another mould than theirs--for the materialist and the infidel. i cannot discuss the matter further here. this is not the place to show, or attempt to show, how the evolution of systems of thought has caused, or is causing, certain ideas, such as that of slavery, religious persecution, the moral and physical degradation of our poor, to reach that degree of incongruity which we signify as abhorrent; or how that evolution has caused yet more primitive ideas to seem positively repulsive. nor is it the place to show, or attempt to show, how the advance of scientific knowledge has been constantly accompanied by the elimination of incongruous conceptions. i must content myself with the brief indication i have given of the principle of elimination through incongruity as applied to ideas. it may be said that such a principle does not account for the origin of the new congruous ideas, but only for the getting rid of old incongruous ideas. quite true. but i have grievously failed in my exposition of natural selection through elimination if i have not made it evident that this objection (if that can be called an objection which, in truth, is none) lies also at the door of darwin's generalization.[kl] now, from all that has been said in this chapter, it will be seen that, on the hypothesis of monism, we cannot regard organic and mental evolution as continuous the one into the other, but rather as parallel the one with the other--as the kinetic and metakinetic manifestations of the same process. organic evolution is a matter of structure and activity. if the structure or the activity be not attuned to the environing conditions, it will be eliminated, those sufficiently well attuned surviving. turning to the metakinetic aspect, we have seen that there are certain mental processes which are directly and closely associated with activities. their evolution will be intimately associated with organic evolution. for if these processes lead to ill-attuned activities, the organism will be eliminated; and thus the evolution of well-attuned activities and their corresponding mental states will proceed side by side. we may, therefore, say, not incorrectly, that these lower phases of mental evolution are subject to the law of natural selection. but when the neural processes which intervene between stimulus and activity become more complex and more roundabout; when, instead of being directly and closely associated with life-preserving activities, they are associated indirectly and remotely;--then they become, step by step, removed from their subjection to natural selection. and when, in man, the metakineses associated with these neural kineses assume the form of hypotheses, theories, interpretations of nature, moral ideals, and religious conceptions, these are, except in so far as they lead to activities which may conduce to elimination, no longer subject to the law of natural selection, unless we use this term in a somewhat metaphorical, or at least extended, sense. they are subject, as we have seen, to a new process of elimination through incongruity. similarly with that wide range of conduct in man which is the outcome of his conceptual life, and is removed from those merely life-preserving activities which are still, to some extent, under the influence of natural elimination. conduct is here modified in accordance with the conceptual system of which it is the outcome and outward expression. and this higher conduct is subject, not to elimination through natural selection, but to elimination through incongruity. slavery would never have been abolished through natural selection; by this means the modest behaviour of a chaste woman could not have been developed. to natural selection neither the factory acts nor the artistic products in this year's academy were due; by this process were determined neither the conduct of john howard nor that of florence nightingale. some evolutionists have done no little injury to the cause they have at heart by vainly attempting to defend the untenable position that natural selection has been a prime factor in the higher phases of human conduct. i believe that natural selection has had little or nothing to do with them as such. they are the outcome of conceptual ideas, and are subject to the same process of elimination through incongruity. so soon as, in the course of mental evolution, the idea of slavery became incongruous, and in certain minds abhorrent and repulsive, steps were taken to check the conduct which was the outward expression of this idea. so, too, in other cases. the reformer must not, however, be too far in advance of his generation, if his reform is to be practically carried out. when his ideas are so "advanced" as to be incongruous with those of all but a very small minority of his contemporaries, even they are forced to confess that the nation is not yet ripe for the changes they contemplate. no one will question that artistic products are the outcome of artistic ideas. in the slow and difficult progress of a new school of painting or of music, we see exemplified the rejection of the new ideas through their incongruity with the old-fashioned artistic systems. only gradually do there grow up new generations for whom these new ideas are not incongruous. for them the old-fashioned systems become incongruous; and if the school becomes dominant, artistic products embodying the old ideas are eliminated through incongruity. we are not all alike. our mental systems are different. one artist will introduce into his canvas effects which, to the eye of another, will at once strike a jarring note of incongruity. to some minds the institution of slavery presents no incongruity. there are not wanting men for whom the degrading moral and physical conditions under which many of our poor are forced to live and work present little or no incongruity. to the russian, english fidelity to the marriage vow is said to be as incongruous as, to an english woman, is the harem of an eastern potentate. in the higher phases of human conduct, then, the activities are subject to the law of the ideas of which they are the outcome--the law of elimination through incongruity. i have said that natural selection has little or nothing to do with these higher phases of conduct. but has not human selection through preferential mating? i believe that it has; and i trust that it will have a still greater influence in the future. it is one of the noblest privileges of woman, for with her mainly lies the choice, that she may aid in raising humanity to a higher level. if once the idea of marrying for anything but pure affection could become utterly incongruous to woman's mental nature; and if once the idea of perpetuating any form of moral, intellectual, or physical deformity could become equally incongruous; the bettering of humanity, through the exclusion of the deformed in body and mind from any share in its continuance must inevitably follow. here, again, ideas would determine conduct. and what, we may now proceed to ask, is the physiological or kinetic aspect of this metakinetic process? the answer to this question involves the conception of what i would term "interneural evolution." just as the environment of a conceptual idea is constituted by other conceptual ideas, so is the environment of its neural concomitant constituted by the other neural processes in the brain. just as no idea can get itself accepted if it be in incongruity with the system of ideas among which it is introduced, so, too, can no neural process become established if it be not in harmony with the other neural processes of the cerebral hemispheres. the brain is a microcosm; its neural processes are interrelated; and _the environment of any neural process is constituted by other neural processes_. a little consideration will show that this must be so; that it is only the physical or kinetic aspect of what is freely admitted when the mental or metakinetic aspect is under consideration. if it be admitted that states of consciousness are determined by other states of consciousness, and that states of consciousness are the concomitants of certain neural processes in the brain, it follows as a logical necessity that brain-neuroses, however originating, are determined in their evolution by other brain-neuroses; and that there has been a brain or interneural evolution, distinct from and yet intimately associated with the evolution of other bodily structures and activities. the more closely and directly brain-neuroses are associated with immediate activities, the more closely implicated is interneural evolution in the process of organic elimination through natural selection. but when long trains of neuroses take place in only remote and distant connection with other bodily activities, they are removed from the process of elimination through natural selection, and interneural evolution is allowed to proceed comparatively untrammelled. i have already indicated my belief that abstraction (isolation), analysis, and conceptual ideas have been rendered possible through language, and are excellences unto which the lower animals do not attain. hence i regard this comparatively untrammelled phase of interneural evolution as something essentially human, something which differentiates man from brute. and i would correlate man's greatly developed brain--inexplicable, i think, by natural selection alone--with this later and special phase of interneural evolution. even in the lowest savage this brain-evolution has proceeded a long way. i am not fitted in this matter to offer an opinion which would carry much weight. but from all that i have read i gather that savages have in all cases elaborated a complex--often a highly complex--interpretation of nature and theory of things. the interpretation may seem _bizarre_ and incongruous enough _to us_, full of fetishism and strange superstitions, but it is an interpretation; to the savage it presents no incongruity; to him the incongruity is in the oddly assorted beliefs of the missionary. his system of ideas is, in fact, one of the many possible systems to which mental evolution may give rise. for what we call systems of thoughts, interpretations of nature, theories of things, are so many genera and species which have resulted from this later phase of metakinetic evolution. our methods are at present too coarse, our powers too limited, to enable us to determine these species from their kinetic aspect. the brains of kaffir and boer, of ploughboy and merchant, of materialist and idealist, are too subtly wrought to enable us to trace the systems of kineses which were the concomitants of their scheme of beliefs. but we can learn something of the genera and species from their metakinetic aspect as symbolized through language and other bodily activities. they fall into certain groups, fetishistic, spiritualistic, materialistic, idealistic, monistic, and so on, and within these groups there are sub-divisions. this is not the place to consider them or discuss their characteristics. what i wish to note about them is that, diverse as they seem and are, each is a coherent product of mental evolution. in each, all that is incongruous to itself has been or is being eliminated. there are some people, however, who are surprised at the incongruity of interpretations of nature among each other. fetishism, they say, has been proved to be utterly false. it constitutes a hideous and grotesque delirium. how can that which is utterly and completely false to nature have had a natural evolution? now, for the _élite_ of the aryan race, whose systems of ideas have been moulded in accordance with the conceptions of modern science, no doubt the fetishism of the poor savage seems sufficiently incongruous and grotesque. so, too, does the system of ideas of the right rev. bishop of ---- appear no doubt, to the learned and eminent professor ----, and _vice versâ_. and so, too, no doubt, does the system of ideas of the white man (who introduces firearms and firewater, and preaches the gospel of forgiveness and temperance) appear to the poor savage. each in his degree wonders how this falsity, this incongruity, can have had a natural genesis. but in each case the falsity and the incongruity is not within the system itself, but between different systems. once more, i repeat that if the individual nature of the systems of ideas be not adequately grasped, the nature of mental evolution will not be apprehended. states of consciousness can only be determined by other states of consciousness; and states of consciousness are for the individual subject, and for him alone. conceptual ideas are states of consciousness; and "falsity to nature" means, and can only mean, incongruity with the environing states of consciousness in the individual mind. for the savage there is no falsity to nature in his fetishism. the idea presents no incongruity with his system of ideas; no more incongruity than filed teeth, flattened head, or pierced nose do to his standard of beauty. it is with _our_ system of ideas (i.e. mine or yours) that his fetishism is false and incongruous. the falsity or incongruity, i repeat, is not within the system itself, but between different systems. it may still, however, be said--only one interpretation of nature can be true; all others must be false. and the falsity is not merely incongruity with other ideas in other systems of thought or belief; it is falsity to the plain and obvious facts of nature. we may freely admit that only one interpretation of nature can be true. but who is to determine which? who can decide the question between monist and materialist? who dare arbitrate between the bishop and the professor? the criterion of fitness in this case, as in others, is survival; and who can say what existing interpretation of nature (if any) shall outlive all its competitors? who can say what will be the nature of the further evolution of any existing philosophical creed? the elimination of the false is a slow and gradual process; and many degenerate systems of ideas may linger on in the darker corners of the world of men. false or out of harmony as they seem to be with the higher phases of development; false or out of harmony as they would be with a different and more exalted environment; they are not false or out of harmony with the environment in the midst of which we find them; they are not false or out of harmony with "the plain and obvious facts of nature," as these exist for the ill-developed or savage mind. the plain and obvious facts of nature, as interpreted by men of science in , have simply no existence for the untutored or the savage intellect. for him they have not emerged into the light of consciousness. but while we cannot blame the savage for entertaining ideas which are false to facts which for him have no existence, we may none the less believe that his system of ideas is not among those which are destined to become predominant species. so far as we can judge, the winning species among systems of ideas and interpretations of nature are those in which the greatest number of ideas are fused into harmonious synthesis; in which all the ideas are congruous, few or none neutral; and in which the abstract or conceptual ideas, when brought into contact with concrete or perceptual states of consciousness, are found to be in harmony and congruity therewith. there is one more question in this connection on which i must say a few words. how, it may be asked, has the world become peopled, for many primitive and savage folk, with a crowd of immaterial spiritual essences, so that it is scarcely too much to say that, for some of these peoples, everything has its double; and there is no material existence that has not its spiritual counterpart? i would connect this almost universal tendency with the origin of abstract ideas (isolates) through language. when the named predominant gave rise to the isolate (see p. ), it could scarcely fail that the primitive speakers and thinkers should tend to regard those qualities or properties which they could isolate in thought (conceptually) as also isolable in fact (perceptually). and we may well suppose, though this is, of course, hypothetical, that one of the earliest severances to be thus effected through isolation was the severance of mind and body. the first phenomena that the nascent reason would endeavour to explain would probably be those of daily life and almost hourly experience. many familiar facts would seem to point to the temporary or permanent divorce of the part which is conscious and feels, from the part which is tangible and visible. during wakeful life the two are closely associated. the visible part, or body, is conscious. but during sleep, or under the influence of a heavy blow, the visible part, which before was conscious, is conscious no longer. the conscious part is, therefore, absent, but returns again after a while. on death the conscious part returns no more. the divorce of the two has become permanent. and then comes in the confirmatory testimony of dreams. in dreams the savage has seen his enemy, though that enemy's body was far away. here, then, is the spirit which has left the body during sleep. in dreams also the slain enemy or the dead chief appears. the spirit, permanently divorced from the body, still walks the earth in spirit-guise. many occurrences would seem like the fulfilled threats of dead enemies or the fulfilled promises of dead ancestors. how can these be explained? are they not produced by the ghost of the departed enemy, by the spirit of the deceased ancestor? and if these spirits are still powerful to act, why not petition them to act in certain ways? probably primitive man would explain all activities anthropomorphically. what knows he of gravitation or the laws of the winds? he knows himself as agent, and attributes his activities to the immaterial spirit within him; for when this is absent during sleep or in death these activities cease. all acting things might, therefore, come to be regarded as dual in their nature--possessed of a sensible material bodily part, and an insensible active spiritual part. and thus the whole world might be peopled with living existences of the spiritual order. now, whether the fetishistic faith arose in some such way as this or not--and we can never know how it arose, but can only guess--there would be nothing in such primitive explanations which would violate the law of congruity. they would have, therefore, a perfectly natural genesis. the attempted interpolation at such a stage of primitive reason of any modern scientific conception would be futile. it would at once be rejected through incongruity. the history of scientific conceptions seems to show that they were first adopted with regard to phenomena on the very horizon of thought--in regions, that is to say, most remote from the central citadel of the soul. only gradually have they, little by little, encroached upon this centre; and the application of them to physiology and psychology is a matter of quite modern times. even to-day only a minority, but an increasing minority, of thinkers are prepared indissolubly to unite the mind and body, so long divorced in thought, so completely united, as many of us believe, in their essential being. i have now, i trust, illustrated at sufficient length the principle of elimination through incongruity in interneural and its associated metakinetic or mental evolution. this, however, like natural selection, is a matter of guidance; we have still to consider the question of origin. in truth, we know too little on the subject to enable us to discuss it with much profit. from the kinetic or organic point of view, neural variations take their place among the other variations, the origin of which, as we have already found, is so hard to account for. there may be a tendency for neural vibrations to mutually influence each other (like two clocks placed side by side), and thus gradually to drag each other into one harmonious and congruous rhythm. but this, though not improbable, is purely hypothetical. there is the hypothesis of the inheritance of acquired variations, the increased congruity acquired by the parent being in some degree transmitted to the offspring. there is the view which mr. wallace adopts[km] with regard to the origin of accessory plumes, that such variations may be due to "a surplus of strength, vitality, and growth-power, which is able to expend itself in this way without injury," and not without profit. the development of the social habit, the mutual aid and protection thus afforded, may well have left a balance of the life-energy, previously employed in individual self-preservation, available for this purpose. and then there is always the hypothesis of favourable fortuitous variations to fall back upon. on only one of these points do i propose to say a few words--that of the possible inheritance of acquired variations. let us restate the problem here for the sake of clearness. there is, according to the suggestion put forward in this chapter, an interneural evolution, leading to an harmonious development of the neuroses in the individual brain. but this special evolution of the brain is nowise independent of the more general evolution of the body. the human being, as an organism, is still subject to natural elimination and human selection. elimination through the action of surrounding physical conditions, although it has played some part in the evolution of man, is not a factor of the first importance. elimination through enemies is more important, but has not much bearing on the question at present before us--the evolution of the conceptual. elimination by competition, again, though a factor of yet greater importance in human evolution, has, nevertheless, so far as individuals are concerned, but little bearing on our present question. few are eliminated through the absence of the conceptual faculty. natural elimination, then, is, as mr. wallace well pointed out, practically excluded in this matter. no doubt, in the struggle between tribes and nations, that community is most likely to be successful in which there is rational guidance. no doubt, during the earlier phases of the development of man on our islands, the elimination of the irrational was a factor in progress. but if we take the last three centuries of english history, i doubt whether it can be shown that there has been much elimination determined by the relative absence of conceptual ideas and emotions. human selection has been a much more important factor. those individuals which showed the higher types of intellectual thought have been constantly selected. riches, rank, and social position have been bestowed upon them. of course, there have been exceptions; great intellects have been allowed to languish in their lifetime, and have only obtained recognition through their works after death. but every day there is less chance of a genius dying in a garret. and the best intellects, being thus selected and chosen out from among their fellow-men, form to some extent a distinct social class. segregation is thus effected; and intermarriage takes place within this intellectual caste, with the result that the conditions are eminently favourable for the inheritance of intellectual qualities. now, is this process of selection of the intellectual, this segregation into a caste, and the inheritance of innate intellectual qualities sufficient to account for the facts of intellectual progress; or must we call in to our aid the inheritance of individual increments? i confess i cannot say. direct and satisfactory evidence, one way or the other, is almost impossible to obtain. must we, then, leave the question undecided? i think we must so far as direct evidence is concerned. i may have a general belief that there has been some transmission of acquired increment of intellectual faculty. but unless i can substantiate it by definite facts, i cannot expect to convince any one who holds the opposite view. and definite facts of sufficient cogency i am unable to adduce. it is practically impossible to exclude the influence of human selection; and unless we can do this the followers of dr. weismann will not be satisfied. still, general belief--which means the net result of one's consideration of the subject--counts for something. we must remember the question is one of origin, and not of guidance. the guidance of human selection is unquestioned and unquestionable. but when we consider the intellectual progress of the last three centuries, and ask whether all this has originated in fortuitous brain-variations, which human selection has simply picked out from the total mass of available material, an affirmative answer seems to me a little difficult of acceptance. there seems to have been a definite tendency to vary in this particular direction, a general raising of the intellectual level, which is difficult to account for unless it be due to the persistent employment of the intellectual faculties. to put the matter in another way. i do not think that, during the last three centuries, there has been a large amount of elimination of the unintellectual. such elimination as there has been of this nature has probably been more than compensated by the slower rate of multiplication of the intellectual classes. elimination, then, in this matter may be practically disregarded. but it is obvious that selection, without the removal or exclusion of the non-selected, does nothing to alter the _general level_[kn] with regard to the particular quality or faculty concerned. it is merely a classification of the individuals in order of merit in this particular respect. it is, in a word, a segregation-factor. it arranges the individuals in classes, but it does not alter the position of the mean around which they vary. let me explain by means of an analogous case. fifty boys, who have been admitted to a public school, await examination in a class-room. they are at present unclassified, but there is a mean of ability among the whole fifty. a week afterwards they are distributed in different forms. some are selected for a higher form, others have to take a lower place. but though selection has classified the material, it has not altered the position of the mean of ability among the fifty boys. this can only be done by expelling a certain number or excluding them from the school. granted, therefore, that elimination is practically excluded, human selection can at most classify the individuals according to their intellectual faculties. it cannot raise the mean standard of intellectuality. if, therefore, this mean standard has been raised during the last three centuries, there has been a tendency to vary in this particular direction, which _may_,[ko] to say the least of it, be due to the inheritance of individual increment. i am, of course, aware that the matter is complicated by the increased and increasing diffusion of knowledge through the printing-press and by the extension and improvement of education. but education, to take that first, though it may raise the level of each generation, can have no cumulative effect. for the effects of education cannot, on professor weismann's hypothesis, be inherited. you may educate brain and muscle in the individual, but his heir will inherit no good or ill effects therefrom. each generation goes back and starts from the old level. there is no summation of effect; or, if there is, it tells so far against professor weismann. and with regard to the diffusion of knowledge, this, though it brings more grist to the intellectual mill, can have no effect in raising the mean standard of excellence in the mill itself. there is more to grind; but this does not improve the grinding apparatus; or, if it does, it tells so far against professor weismann's hypothesis. to vary the analogy, the diffusion of knowledge increases the store of available food; but it does not bring with it any additional power of digesting the food; or, if it does, it may be through inherited increments of mean digestive power. it may, however, be maintained that there is no conclusive proof that the mean intellectual level of englishmen to-day is any higher than it was in the days of the tudors. if so, of course, my argument falls to the ground. i have no desire to dogmatize on the subject. i merely set down the reasons, such as they are, and for what they are worth, which lead me to entertain a general belief that the intellectual progress of englishmen during the past three hundred years has been in part due to the inheritance of individually acquired faculty. * * * * * mental evolution, then, is the metakinetic equivalent of interneural, or, in us vertebrates, brain-evolution. the brain forms a kinetic system in some sense independent of, and yet in constant touch with, the kinetic system of the world around. its kineses, though they do not resemble, yet more or less accurately represent or symbolize, the kineses of the surrounding universe. as the kineses of the world around are interdependent and harmonious, so are the neural kineses of the brain interdependent and harmonious. and no modification of this kinesis which is out of harmony with the kinetic system already established in the brain can be incorporated with that existing system. such attempted modification is eliminated through incongruity. associated with this brain-kinesis, and forming its inner aspect, is a metakinetic system in which the higher manifestations rise to the level of full consciousness; others form sub-conscious states; others are unconscious. but the whole form a coherent system answering to the coherent kinetic system. consciousness is thus associated only with the phenomena of that kinetic microcosm which we call the brain (or other interneural system). obviously, therefore, it does not and cannot deal directly with anything outside the brain. its knowledge is solely and entirely a knowledge of the representative occurrences of the interneural system. but out of these occurrences a surrounding world of phenomena is constructed in mental symbolism. the brain itself, however, is part of the world of phenomena thus constructed in mental symbolism; and the world, therefore, dissolves in pure idealism, leaving only a fleeting series of states of consciousness, if we do not assume the existence of a system of "things in themselves" (noumena), of which kineses and metakineses are the phenomenal manifestations. whether the "things in themselves" in any sense resemble their phenomenal manifestations, we cannot say. it is as difficult philosophically to conceive that they can as it is practically to conceive that they do not. and since, whether they do or do not, the world we live in is phenomenal; since it is to phenomena that we have to adapt our conduct; since it is with phenomena that all our thoughts and emotions have reference; since the world we construct in mental symbolism is the world in which we live and move and have our being; it is not only convenient, but logically justifiable, to call this world of phenomena the really existing world for us human-folk and other sentient organisms. as in the kinetic interneural system, or brain, so, too, in the metakinetic system, no modification of the metakinesis which is out of harmony with the existing metakinesis can be incorporated therewith. such attempted modification is eliminated through incongruity. in the lower stages of mental evolution, those which belong to the perceptual sphere, where the neuroses are closely connected with the life-preserving activities of the organism, the survival or non-survival of the system of neuroses is largely dependent on the fitness of the associated activities to the conditions of life. but in the higher stages of mental evolution, those which belong to the conceptual sphere, the connection of certain brain-neuroses with life-preserving motor-activities becomes less close and direct. the corresponding ideas, thoughts, and emotions become floated off into a more abstract region. here the system of ideas, as such, that is to say, so far as they are removed from life-preserving activities, is determined mainly by the law of congruity. but there are several such systems. there are, indeed, as many systems as there are minds; but these may be classified in several distinct groups, which we may liken to genera and species. these are the various interpretations of nature, theories of things, and the like; the systems of ideas, thoughts, conceptions, emotions, beliefs, which, as we say, belong to us, each and all, and which determine to which metakinetic species we belong. these are the highest products of mental evolution; and among them there is, so to speak, a struggle, if not for existence, at any rate for prevalence. which shall eventually prevail--a spiritual interpretation of nature, a material interpretation, a monistic interpretation, or other, who shall say? but, so far as we can judge, the winning species among systems of ideas and interpretations of nature are likely to be those in which the greatest number of ideas are fused into harmonious synthesis; in which all the ideas are congruous; and in which the abstract or conceptual ideas, when brought into contact with concrete or perceptual states of consciousness, are found to be in harmony and congruity therewith. notes [jy] "contributions to the theory of natural selection," p. . [jz] i consider that an apology is needed for the coinage of this and of two or three other words, such as "construct," "isolate," and "predominant." i can only say that in each case i endeavoured to avoid them, but found that i could not make my meaning clear, or bring out the point i wished to emphasize without them. [ka] "science of thought," pp. , . [kb] "science of thought," p. . [kc] i use "substance" here in its philosophical sense. [kd] quoted in professor veitch's "hamilton," p. . [ke] t. m. herbert, "the realistic assumptions of modern science examined," nd edit., p. . [kf] "science of thought," p. . [kg] strictly speaking, of the brain; but since the brain has no organic independence of the body, it is best here to focus attention on the unity of the organism. [kh] i ought not to pass over without notice the "psychological scale" which mr. romanes introduces in a table prefixed to "mental evolution in animals." it would be unjust to criticize this too closely, for it is admittedly provisional and tentative. if such a scheme is to be framed, i would suggest that the various phyla of the animal kingdom be kept distinct. i question, however, whether any one can produce a scheme which any other independent observer will thoroughly endorse. and i am inclined to think that the wisest plan is to tabulate the kinetic manifestations which we can actually observe rather than the metakineses of which we can have no independent knowledge. [ki] _contemporary review_, july, . see clifford's "lectures and essays," vol. i. pp. and ; vol. ii. p. . [kj] _contemporary review_, july, . [kk] "darwinism," p. . [kl] in both cases, the question to which an answer is suggested is not--what variations will arise? but--what variations will survive? [km] "darwinism," p. . it is strange that mr. wallace did not apply this view to the mathematical and artistic faculties discussed in his last chapter. it is true that such application tends to undermine the argument there developed. but mr. wallace is far too great and conscientious a thinker to be influenced by such a consideration. [kn] if elimination of the unintellectual (not necessarily of the unintelligent) may be excluded, and if the unintellectual increase by natural generation more rapidly than the intellectual, the general level of intellectuality must, on professor weismann's principles, be steadily _falling_. [ko] it _may_ also, in part, be due to "organic combination." index. a abstract ideas, , acceleration, sense of, acceleration and retardation, _achirus pellucidus_, acquired characters, are they transmitted? ; habits, are they inherited? ; variations in the intellectual sphere, _acræa_, activities, organic basis of comparative psychology, ; of animals, ; voluntary and involuntary, classification of, adaptation, analogous, ; modes of, ; special, examples of, ; to varying environment, advantage must be particular, ; must be immediate and not prospective, ; must be "available," , _Ã�schna_, Ã�sthetic preferences in insects and birds, ; aspect of sensation, not primary, ; motive not present to animal consciousness, alexander, mr. s., "moral order and progress," allen, mr. grant, on evolution of flowers, ; on pleasure and pain, allen, mr. j. a., on colour and humidity, alternation of generations, _amblyopsis spelæus_, american school of evolutionists, am[oe]ba, how it feeds, ; reproduction of, , ; diagram of, ; protoplasmic functions of, amphibia, labyrinthodont, anabolism, constructive process, analysis (mental), ancon sheep, anderson, mr., on one-eared rabbits, anemone, sea, reproduction of, ; marginal beads of, ; discrimination by, anger and rage, animal life, nature of, ; diversity of, animal intelligence, differs generically from man's reason, animals, characteristics of, ; divided into protozoa and metazoa, ; and plants, their relation to food-stuffs, the atmosphere, and energy, ; intelligent not rational, ; capacities for pleasure and pain, animistic ideas of savages, how developed, _anisognathus_, _anomia_, ant, sauba, of south america, ; sense of taste in, ; sense of smell in, ; auditory organ of, ; intelligence of, ; activities often described as instinctive, ; neuter insects, ; siamese, antagonism, advantages of, antennæ of insects, modifications of, ; of emperor moth, ; organ of hearing in, ; modified hairs of, antennule of crayfish, _anthophora_, anticipation, antlers of deer in illustration of growth, aphides, absence of fertilization in reproduction, appetence and aversion, , _apus_, aquatic organisms, respiration in, ; sense of smell in, arago, m., observation on turnspit dog, arctic hare and fox, ; animals, colours of, ; fox, cunning of, argyle, duke of, on humming-birds, _artemia salina_ and _milhausenii_, artistic faculties and natural selection, ; products, evolution of, association a tendency to integration, ; perceptual and mimicry, ; and recognition marks, _ateles_, atmosphere, relations of animals and plants to, attacus, attention, _attidæ_, auditory apparatus in man, aurelia, life-cycle of, australian mammals and others convergent, automatic action, available advantage, , aversion and appetence, , b baboon, experiments with, bacilli attacked by leucocytes, _bacillus violaceus_, bailey, mr. e. h. s., on taste, balbiani on _chironomus_, _balistes_, barrett, mr. w. f., on sensitive-flame experiment, barrier, geographical, ; time, in physiological isolation, barrington, the hon. daines, on song of linnet, bateson, mr. w., on lateral line, ; on fishes hunting by scent, ; on smell in shrimps, etc., ; on hearing in fishes, ; on hearing in _anomia_, ; on sight in fishes, ; on rockling and sole, ; on fascination in fishes, bats, tabulated measurements of wing-bones of, - ; wings, fortuitous variations in, ; experiment with, beauty, standard of, ; sense of, beaver, change of habit in, beccari on gardener bower bird, becker, alexander, on variations in the balance of life, bees, divergent development of, ; cuckoo, ; latency in, ; sense of taste, ; sense of smell, ; smell-hollows, ; eyes and eyelets of, ; intelligence of, ; colour preferences in, ; homing faculty in, ; neuter insects, beetles of madeira, ; stag-, variability of male, ; observations on dung-, begging in dogs, berkeley, bishop, quoted, bert, m. paul, limits of sensibility to light, bidie, mr. george, anecdote of cat, binet, m., "psychic life of micro-organisms," birds, influence of food-yolk on development of, ; divergence among, ; breeding area of comparatively restricted, ; humming, duke of argyle on, ; destruction of eggs of, ; game-, white and black crossed, ; taste in, ; smell in, ; hearing in, ; sight in, ; colour-vision in, ; gardener bower, ; humming, nests of, ; perfect instincts of pr[oe]coces, ; love antics of satin bower, ; nests of, ; song of, blochmann on the development of the drone, blood, circulation of, body as distinguished from reproductive cells, boll and kÃ�hne, messrs., on retinal purple, bolton, miss caroline, on the bat, _bombus muscorum_, ; _lapidarius_, _bombyx quercus_, bower bird, , brain, ; decreased, of rabbits and ducks, ; a microcosm, brehm's, thierleben, quotation from, brine shrimp, modified by salinity of water, brooks, prof. w. k., his modification of pangenesis, ; on the greater variability of the male, brown, prof. crum, on sense of acceleration, browne, sir j. crichton, on ducks, budding, reproduction by, ; in relation to heredity, bull, "favourite," prepotent, ; reversion in, bunyan, john, on gateways of knowledge, butler, mr. samuel, on organic memory, , butterfly, protective resemblance in, ; mimicry in, c camel, wounded, canary, crested, ; nest building of, capon, taking to sitting, capuchin monkey, miss romanes's observation on, ; sympathy in, carlyle, quoted, , carp at potsdam, carter, dr. brudenell, quoted, caste, idea of, in dog, cat, effect of african climate on, ; defining its percept, ; communication, ; intelligence of, ; and mouse, ; punishing kitten, caterpillars, protective resemblance in, cattle of falkland islands, causation, cell, diagram of animal, ; controlled explosions in, cessation of selection, effects of, _chætodon_, _chætogaster limnæi_, reproduction of, chaffinch, nest of new zealand, chamæleon, chance, change of conditions, characters, specific, charbonnier, mr, henry, measurements of bats, chattock, mr. a. p., his experiments on colour-vision, ; letter to, on dog and picture, cheshire, mr., on smell-hollows in bees, chickens' aversion to protected caterpillars, ; perfectly instinctive activities, _chironomus_, reproductive cells of, choice, circulation of the blood, classification, clifford, w. k., on human consciousness, ; on the eject, ; on "world-consciousness," clover and bees, _clytus arietis_, cockchafer, smell-hollows of, cockerell, mr., on variations in snails, ; on effects of moisture, cockroach, diagram of trachea or air-tubes of, ; sense of taste in, ; sense of smell in, cocoon, collective, _colobus_, colour, protective resemblance in, ; warning of inedibility, ; dependent on humidity, ; direct action of climate on, ; development of, ; blindness, , ; phenomena of, combination, organic, hypothesis of, , communication in dogs, ; in bees, compensation of growth, competition, elimination through, concept, , conception, conceptual conduct and evolution, condor, rate of increase of, conduct, ; influence of thought and æsthetics on, ; conceptual, and natural selection, congruity, principle of, conjugation in protozoa, ; of ovum and sperm-cell, consciousness, ; and consentience, , ; as a criterion of instinct, consentience, , construct and construction (mental), ; three stages of, ; inevitable nature of, ; in mammals, continuity of reproductive cells, ; germ-plasm, ; cellular, ; in mental development, convergence, phenomena of, co-ordinants, cope, prof., on the effects of use, ; and hyatt, prof., on retardation and acceleration, correlated variation, , corti, organ of, _coryne_, prof. weismann on, couch, mr., on goldfinch song, crab, protective resemblance in, ; hermit, ; habit of decking itself, crayfish, smell in, ; auditory organ of, crossing, effect on reversion, cruelty in cat, objective, crustacea, eyes of, _ctenomys_, cuckoo, the name onomatopoetic, ; habits intelligent, ; ejecting young birds, curiosity in prong-horn, cuttlefish, eyes of, _cyclas_, _cycloptera speculata_, locust resembling leaf, d dallinger, dr., his temperature-experiments on monads, _danais_, daphnids, absence of fertilization in reproduction of, ; colour-vision in, , ; leucocytes of, darwin, charles. natural selection and the struggle for existence, ; divides the principle of selection into three kinds, ; on selection of flowers and fruits by insects, ; on sexual selection, ; on prevention of free crossing in breeding, ; on differential fertility, ; on london rats, ; on galapagos archipelago, ; on diverse adaptation, ; on the influence of old maids on clover crops, ; on the influence of parent on offspring, ; on the co-ordinating power of her organization, ; hypothesis of pangenesis, ; on fur of arctic animals, ; changes of structure attributed to use and disease, ; on blindness of tuco-tuco, ; on the principle of economy, ; on sexual selection, ; on preferential mating, ; on evolution of flowers, ; on co-ordinated variations in the elk, ; on acceleration, ; on ancon sheep, ; on prepotency, ; on reversion, ; on the effects of crossing, ; on fortuitous variation, ; on the subordination of the conditions to the organism, ; on the greater variability of male, ; on attention in monkeys, ; on brain of ant, ; on gestures of anger and rage, ; on pleasures and pains of animals, ; on bravery of a monkey, ; on abyssinian baboons, ; on sense of humour in the dog, ; on neuter insects, ; on selection of oxen, ; on acquisition of fear of man by birds, ; on satin bower bird, death, natural introduction of, , deceit in dogs, degeneration, desert animals, inconspicuousness of, descartes on pineal gland, desire, , destruction, indiscriminate, as opposed to elimination, development of organisms distinct from growth, ; reproduction and, ; is differential growth, ; of a vertebrate, diagrammatic account of, ; comparative, of some vertebrates, de vries, , differentiation in protozoa, ; in metazoa, ; during development, ; of reproductive cells, ; and integration, ; of tissues, _difflugia_, dimorphism in larvæ, discrimination in the sense of touch, ; hearing, ; sight, ; its fundamental nature, ; in sea-anemone, disease, elimination by, display, disuse, panmixia and, ; negative and not positive, ; use and, divergence among birds, illustrated from wallace, ; through diverse adaptation, dixon, mr. charles, effects of climate on the colours of birds, ; on chaffinch nests, dog, effect of indian climate on, , ; greyhounds in mexico, ; sense of smell in, , ; vague percept of, ; and the feelings of other animals, ; and pictures, ; powers of communication, ; swimming rivers, ; cleverness of, ; sympathy in, ; idea of caste, deceit, ; endurance of pain, ; sense of justice in, ; punishing pup, ; sense of humour in, ; swimming a deferred instinct in, ; turning round to make a couch, dog-fish, sense of smell in, domestication, variations effected by, , ; crossing and reversion, _doris tuberculata_, dreaming, ; and the animistic hypothesis, _dromia vulgaris_, drones developed from unfertilized ova, ; second polar cell extruded, dubois, m., on _proteus_, ducks, sir j. crichton browne on, ; dr. rae on instinctive wildness of, duration of life, e eagle, sclerotic plates of, ear, earthworm, respiration in, , ; regeneration of lost parts, ; sensitive to light, ; outward projection in, eaton, rev. a. e., on insects of kerguelen island, ecitons, economy, principle of, education of ants, ; of young animals, egg and hen, problem of, egg-cell and sperm-cell, diagram of, ; conditions which determine production of, eggs, influence of food-yolk on mode of development of, ; destruction of birds, ego, or self, eimer, prof., on inhabitants of nile valley, ; on _helix hortensis,_ ; on instinct, ; on differential dread in birds, eject, meaning of, elaboration, elephant, rate of increase of, ; intelligence of, , ; use of tools by, ; vindictiveness in, elimination, as opposed to selection, ; its three modes, ; as a factor in the origin of instinct, ; of ideas through incongruity, ; as applied to the intellectual faculties, embryology negatives preformation, emotions exemplified, ; the expression of, ; three orders of, ; in vertebrata, encystment, , ends and means, energy, relations of animals and plants to, _ennomos tiliaria_, caterpillar, protective resemblance of, environment, direct effects of on the organism, ; changes of, in relation to the organism, ; are effects of direct or indirect? ; instances of effects of, _equus_, _eristalis tenax_, ethics in animals, _euplæa_, evolution of older writers, ; and revolution, ; organic, ; meaning of term, ; mental, ; organic and mental not continuous, ; interneural, excrement of birds, resemblance of spider to, excretion, an essential life-process, , expectation, experience dependent on memory, expression of the emotions, eye, structure of in man, ; in mole, ; pineal, ; in insects, ; facetted, ; in crustacea, ; in molluscs, ; four types of, f fabre, m., on _sitaris_, facetted eye, factors of phenomena, laws of, falkland islands, cattle of, ; birds of, fear, dread and terror, ; instinct of, feelings of animals, , female. _see_ sex-differentiation. female and male insects, differences between, ; vigour expended on offspring, fertilization, nature of, ; absent in parthenogenetic forms, fertility, differential, darwin and romanes on, ; of hybrids, fetishism, its natural genesis, fischer, dr. emil, on smell, fish, respiration in, ; protective resemblance in, ; amount of food-yolk in eggs of, ; skate and turbot compared, ; sense of taste in, ; sense of smell in, ; sense of hearing in, ; sense of sight in, ; fascination in, ; love-antics of, fisk, rev. g. h. r., on sympathy in cat, fission, a process of cell-division, ; in protozoa, ; in metazoa, flight, instinctive nature of, flourens, m., on function of semicircular canals, flowers and fruits, selection of, ; evolved through insect agency, _folliculina_, food-stuffs, relations of animals and plants to, ; nature of and digestion of, food-yolk, influence of, on development, ; the result of parental sacrifice, forbes, h. o., on javan spiders, forel, m., on taste of ants, ; on vision of daphnids, ; on happy family of ants, form-characteristics of animals, fortuitous variation, fosterage and protection, ; result of female self-sacrifice, fothergill, mr., on dogs swimming rivers, fowl, variations in, attributed by darwin to use, ; crossing of, , fox, cunning of, francis, mr. h. a., fritsch, dr., fig. of skull of _melanerpeton_, frog, development of, ; arrest of life in, ; respiration in, ; fishing, or angler-fish, ; modified development of, ; effects of simple stimulus on, fruits and flowers, selection of, g gabet, messrs. huc and, on llama cow, galapagos archipelago, species and varieties in, ; climate of, _gallus bankiva_, galton, mr. francis, on the coloration of the zebra, ; his modification of pangenesis, ; numerical estimate of inheritance, , ; his investigations on twins, ; on blended characters, ; on the steps of evolution, ganglia, gannet, rate of increase of, gas-engine, analogy of, gautier, théophile, his cat, geddes, prof. patrick, and thomson, j. a., on anabolism and katabolism, ; quoted, , , gemmules, pangenetic, generations, alternation of, generic idea, geographical barriers a means of segregation, geological changes, influence on natural selection, germ-plasm, continuity of, ; convenience of, gills of mussel, ; as respiratory organs, giraffe, co-ordinated variations in, glacial epoch, effects of, gland, pineal, goldfinch, song of, goldschneider, on temperature-sense, gould, dr., on humming-birds' nests, graber, dr., on colour-sensitiveness of earthworm, grant, mr. g. l., on new zealand sparrows, grasshopper, auditory organ of, _gregarina_, reproduction in, grenacher, dr., experiment on moth's eye, grouse, white plumage in, due to reversion, grove, sir w. r., on antagonism, growth of organisms, ; illustration of a deer's antler, ; law of, after mutilation, guidance distinguished from origin, guillemot, eggs of, gulick, rev. j. t., on landshells of sandwich islands, ; on tendency to divergence, guppy, mr., on crab of solomon islands, h habits of animals, habitual activities, ; sense of satisfaction in performance of, haeckel, prof., plastidules of, ; theory of perigenesis, _halictus cylindricus_, hamerton, mr. p. g., on the ignorance of animals, hamilton, sir wm., quoted, hancock, mr. john, on instinct of cuckoo, hasse, e., on bumble-bees, hauser, on cockchafer, haycroft, mr. j. b., on taste, hearing, sense of, _heliconia_, _helix_, _nemoralis_ and _hortensis_, variation of, , , , helmholtz, von, on colour, ; on local signs of retina, hen and egg, problem of, hensen, on shrimps, herbert, prof. t. m., quoted, herdman, prof., on sea-slug (_doris_), ; his modification of pangenesis, ; on warning coloration in nudibranchs, heredity, an organic application of the law of persistence, ; and the origin of variations, ; in protozoa, ; and regeneration of lost parts, ; failure of, ; and instinct, hering, edward, on organic memory, , heron, sir r., on crossing rabbits, herschell, sir john, on colour, hertwig, richard, observations on infusoria, hicks, on capricorn beetle, hicks' organ, hickson, dr., fig. of eye of fly, _hipparion_, hippopotamus, instinctive activities in, holland, sir henry, on inheritance, homing faculty of bees, horse, two different evolutions of, ; effects of use on digits of, ; sense of pain in, howse, prof., antennule of crayfish, huber, pierre, on smell in bees, ; judgment and instinct, huc and gabet, messrs., on llama cow, huggins, dr., his dog kepler, humming-birds, humour, sense of, in dog, huxley, t. h., on limitation of variations, ; on neurosis and psychosis, hyatt, prof., on acceleration and retardation, hybrids, fertility of, hydra, reproduction of, , ; diagram of, ; artificial division of, ; budding in, ; sexual reproduction of, hydra tuba, and medusa of aurelia, hydroids, development of, ; weismann on, hymenoptera, antennary structures of, ; instincts of social, , i ichneumon fly, instinct of, _ichthyosaurus_, pineal eye of, _icteridæ_, idea of an object, ideas, conceptual, their environment, ; the law of their evolution, idealism, ignorance of animals, image, inverted in retina, imagination, constructive, imitation as a factor in habit or instinct, , immortality of protozoa, incongruity, elimination by, increase, law of, incubation, instinct of, individuality, a tendency to differentiation, inference, conscious and unconscious, ; in animals, infertility of isolated forms, infusoria, reproduction in, inheritance, exclusive, a means of isolation, ; of variations, ; of acquired habits, ; of acquired increments of intellectual faculty, inhibition, ; as a condition of volition, innate capacity, ; its importance, insects, tracheal respiration of, , ; wingless, of madeira, ; of kerguelen island, ; mimicry and protective resemblance in, , ; segregation by colour, ; antennæ of, ; mouth-organs of, ; and the evolution of flowers, ; sense of touch in, ; taste in, ; smell in, ; hearing in, ; sight in, ; perceptual powers of, ; neuter, instinct and available advantage, ; consideration of, ; perfect, imperfect, and incomplete, ; deferred, ; blind prevision in, ; gratification in performance of, ; consciousness and, ; primary and secondary, ; three factors in the origin of, ; as influenced by intelligence, ; by imitation, ; by education, ; as distinguished from intelligence, instinctive emotion, , integration and differentiation, intellectual development, intelligence involved in selection, ; distinguished from reason, , ; lapsed, involved in instinct, ; as influencing instinct, ; criteria of, interbreeding and intercrossing, interneural evolution, interpretations of nature, genera and species of, isle of man, tortoiseshell butterfly of, isolates, , isolation, organic, or segregation, ; mental, or abstraction, j jaeger, dr., on crossing of pigs, jenkins, mr. h. l., on the elephant, judgment, k _kallima paralecta_, leaf-butterfly, kant, quoted, katabolism, a disruption or explosive process, kea, of new zealand, keimplasma. _see_ germ-plasm kentish plover, , kepler, dr. huggins's dog, kerguelen island, wingless insects of, kinesis, kingfishers, kirby and spence, localization of smell in insects, ; on hearing in a moth, ; on instinct of ichneumon fly, kittens, instinctive antipathy to dog, klein, mr. s., on _bombyx quercus_, kÃ�hne, messrs. boll and, on retinal purple, l labyrinthodont amphibia, pineal eye in, lamont, on reindeer, lane, dr. arbuthnot, on influence of certain trades on structure, langley, prof., on ætherial vibrations, language, ; the instrument of analysis, ; its origin and effects, lankester, prof. e. ray, his description of perigenesis, ; on blind cave-fish, lapsing of intelligence, larden, w., on the rhea, ; on instinct in a snakelet, larmarckian school, larvæ, dimorphism in, latency, phenomena of, lateral line of fishes, leaf-butterfly, lee, mr. arthur, on communication in cat, _leptalis_, leroy, on abstract notion of danger in fox, leucocytes, _role_ of, lewes, g. h., , leydig, on antennule of crayfish, life, duration of, due to natural selection, life-area, expansion and contraction of, limits of vision, ; of sensation, _limnæus truncatulus_, lincecum, dr., on habits of texan ants, linnet, song of, lion, observation on, liver-fluke, life-history of, local signs, localization, ; in animals, ; in medusa, locke, on difference between man and brute, logos makes man human, lonbiÃ�re, on instincts of siamese ants, lotze, quoted, lubbock, sir j., "senses of animals," ; sense of smell in ants, ; auditory organ of ant, ; on hicks's organ, ; on colour-sense in dog, ; in insects, ; in daphnia, ; on limits of colour-vision, ; on antennary structures in hymenoptera, ; on power of communication in dog, ; in ants, ; on colour preferences in bees, ; on instinct of play and sympathy in ants, ; on homing faculty in bees, ; on sitaris, _lucanus cervus_, ludicrous, sense of, in dog, lumsden, sir harry, on partridges, lyell, the necessary precursor of darwin, m mach, prof., on macula acustica, _machetes pugnax_, , mackennal, mr. alexander, observation on a cat, maclagan, miss nellie, on sympathetic action in dog, madeira, wingless insects of, male, _see_ sex-differentiation male and female insects, differences between, ; greater variability in, ; vigour and vitality of, in secondary sexual characters, malle, dureau de la, on starling, mammals, respiration in, ; early nutrition of, the result of parental sacrifice, ; convergence in, ; sense of smell in, ; hearing in, ; sight in, ; perceptions of, man, elimination by physical circumstances, ; alternation of good and bad times, ; reversion in, mann, mrs., on sympathetic action of dog, ; anecdotes of dogs, mantis, protective and aggressive resemblance in, marsupials of australia, martineau, dr., on wants, materialism, , mathematical faculty and natural selection, maupas, m., observations on infusoria, mayer, on mosquito, mccook, dr., sense of smell in ants, ; habits of texan ants, mccosh, dr., quoted, means and ends, medusa, ; sense of hearing in, ; eyes of, ; localization by, _melanerpeton_, meldola, prof. r., memory, the revival of past impressions, ; organic, butler and hering on, mental evolution, mercier, dr. charles, on the criteria of intelligence, merrifield, mr., experiments on moths, metabolism, metakinesis, metamorphosis and transformation, metaphyta, metazoa, _methona_, miall, prof., fig. of touch-hair of an insect, mice, white and grey, crossed, microbes, elimination among, _micrococcus prodigiosus_, _microstomum lineare_, reproduction in, mimicry, ; as evidence of perceptual association, , mind, out of what evolved? mineral crystals, analogy of, mitchell, james, his delicate sense of smell, mivart, prof. st. george, on saturnia, ; on common-sense realism, ; on ideas, etc., ; on "practical intelligence," ; on man and brute, ; on consciousness and consentience, modifiability of individual organism, modifications of antennæ and mouth-organs of insects, mole, eye of, mollusks, variety of, ; sense of smell in, ; hearing in, ; sight in, monads, reproduction of, ; temperature experiments with, mongrelization, monistic hypothesis, monkey, _ateles_ and _colobus_ digits of, ; examining marsupial pouch, ; attention in, ; capuchin, intelligence of, _monospora bicuspidata_, moore, mr. thomas, on hybrids between amherst and golden pheasants, mosaic vision, mouth-organs of insects, muciparous canals of fishes, mÃ�ller, prof. max, "science of thought," ; on percepts, ; on language and thought, ; paraphrased, ; on materialism, _murex_, _mus rex_ and _imperator_, musical and artistic faculty, mussel, freshwater, gills of, ; olfactory organ of, mutilation, law of growth after, ; not the best kind of evidence of transmitted modifications, n nÃ�geli, naish, mr. john g., on the cockatoo, natural selection, variation and, ; two modes, elimination and selection proper, ; and the effects of use and disuse, ; not to be used as a magic formula, ; and instinct, ; and human thought, nerves, briefly described, ; afferent and efferent, _nestor notabilis_, nests of bower-bird and humming-bird, ; instinctive building of, nettleship, mr., on a lion, neural processes, environment of, neurosis and psychosis, neuter insects, new zealand sparrow, ; parrot, ; chaffinch, nichols, on taste, noctule, noirÃ�, on concepts, _nomada solidaginis_, norris, mr. w. e., quoted, noumena, or "things in themselves," nucleus of animal cell, ; as controlling formative process in, nutrition in illustration of the process of life, o object, nature of, , ocelli in insects, _oecodoma cephalotes_, _onchidium_, optogram, organic combination, hypothesis of, , organic evolution, ; as basis of comparative psychology, organic growth, organism, unity of, as regards body and germ, ; relation of, to environment, organization, co-ordinating power of, ; of bodily and mental activities, origin, distinguished from guidance, origin of species, origin of organic variations, ; of metakinetic or mental variations, _ornithoptera_, otoliths, , owen, sir richard, suggested germinal continuity, oyster-embryo set free early, ; variation of mediterranean, p _pachyrhyncus orbifex_, _pagurus prideauxii_, pain, massive and acute, ; capacities of animals for, pangenesis, panmixia and disuse, _papilionidæ_, paradise, birds of, paranucleus in protozoa, _param[oe]cium_, reproduction in, parasites, how they feed, parental sacrifice in birds and mammals, ; its limits, parrot, intelligence of, parthenogenetic forms, no second polar cell in, ; the drone an exception, _parus palustris_, peal, mr. s., on use of tools by elephant, peckham, mr. g. w., on love-antics of a spider, , _pecten_, pelagic animals, colours of, penzoldt, dr., on smell, percept, , perception, , ; in animals, perceptual association, perigenesis of the plastidule, _peripatus_, persistence, law of, pheasant, hybrids between amherst and golden, ; golden, hen with cock's plumage, _phengodini_, phenomenal nature of object, , , photographic psychology, , _phrynocephalus mystaceus_, physiological isolation, physiological and psychological activities, ; series, , picton, mrs. e., on skye terrier, pigeons, correlated variations in, ; silky fantail prepotent, pigs, intestines of, ; crossing of, , pike, teeth of, pineal gland, , pipistrelle, wing of, pipits as illustrating divergence, pitch, musical, plasm, plasmogen, _platyglossus_, play, instinct of, pleasure and the special senses, ; massive and acute, ; capacities of animals for, _plecotus auritus_, _plesiosaurus_, pineal eye of, ploss, herr, on sex-differentiation in man, plover, kentish, , polar cells, extrusion of, ; and variation, postponement of action, poulton, mr. e. b., on colours of animals, ; on _phrynocephalus mystaceus_, ; on caterpillars and chrysalids, ; dimorphism in larvæ, ; observations on edibility of caterpillars, ; "theories of heredity," quotation from, ; on the eating of unpalatable insects, predominant defined, ; and language, preferential mating, a means of segregation, ; and sexual selection, preformation and evolution of older writers, prepotency, presentations of sense, previous sire, effect of, prevision as a criterion of intelligence, principles, mechanical, process of life, progress, or continuous adaptation, ; adaptation to more complex circumstances, pronghorn, curiosity in, proposition, protective resemblance and mimicry, ; general resemblance, ; variable resemblance, ; special resemblance, ; to another organism, ; coloration, a means of segregation, protection, fosterage and, _proteus_, sensitive to light, protista, _protohippus_, protophyta, protoplasm, protozoa, nature of, ; transmission of acquired faculty in, ; origin of metazoan variations in, ; psychology of, _psithyrus rupestris_, psychological and physiological activities, ; series, , psychoses and neuroses, ptarmigan, on colour of, r rabbit, brain of, ; angora crossed, ; one-eared, ; deprived of long lip-hairs, ; _papilla foliata_ of, ; effects of superabundant food on, rae, dr., on dogs swimming rivers, ; on "abstract reasoning" in the fox, ; on wild and tame ducklings, rage and anger, ramsay, dr. wm., on smell, rats of solomon islands, ; of the london docks, ; at south kensington, rayleigh, lord, on colour-blending, ; on sensitive-flame experiments, ; reality, meaning of term, reason distinguished from intelligence, , ; as defined by mr. romanes, recepts, , recognition-marks, ; involve perception, reconstructs and reconstruction (mental), reflex action, ; and instinct, regeneration of lost parts, ; in relation to heredity, ; law of growth concerned in, reindeer wounded, ; change of habit in, remnants or vestiges, reproduction, nature of, ; and development, ; in the protozoa, ; in the metazoa, ; by budding, ; sexual, ; peculiar modifications of, ; developmental, reproductive cells, continuity of, resemblance, protective, ; aggressive, respiration an essential life-process, ; in illustration of the process of life, retardation and acceleration, retina of man, ; of birds, retinal purple, revenge, reversion, revolution and evolution, rhea, neck resembling snake, _rhinolophus ferri-equinum_, _hipposideros_, rhyme-association in parrot, ribot, m., on attention, richardson, mr. charles, on railway servants killed by train, riley, prof., on _phengodini_, romanes, prof. g. j., on physiological isolation, ; on the cessation of selection, ; on the failure of heredity, ; on the reversal of selection, ; on sense of smell in dog, on colour-sense in chimpanzee, ; on ideas, ; on dog cowed by noise, ; on abstract ideas in animals, ; on parrot, ; on localization and discrimination, ; examples of animal intelligence considered, ; on abstract ideas in the capuchin, ; definition of reason, ; on strange attachments in birds, ; on some emotions in animals, ; on endurance of pain in dogs and wolves, ; on sense of humour in dog, ; on indefinite morality in animals, ; definition of instinct, ; on education of ant, ; on homing faculty of bees, ; on consciousness and instinct, ; summary of his conclusions on instinct, ; on instincts of siamese ants, ; his psychological scale, ; on the world as an eject, rotation, sense of, rotifers, absence of fertilization in reproduction, roux, on extirpation of cleavage-cell of frog's egg, rowell, g. a., on "beneficent distribution of pain," ruffs, variability of males, , russell, mr. w. j., on smell in the dog, s _saitis pulex_, salinity of water, effects of, on brine-shrimp, salmon, new variety of, in tasmania, _saturnia_, modification of, by changed food, ; _carpini_ (emperor moth), savages, fetishistic belief in, schaub, mr., observations on a terrier, schmankewitsch on artemia, sclater, mr. w. l., on mimicry in an insect, sedgwick, mr. adam, on development of peripatus, seebohm, mr. h., on birds' eggs, segregation, selection, as compared with elimination, ; illustrated, ; artificial, ; cessation of, ; reversal of, ; sexual, or preferential mating, , ; as a factor in the origin of instinct, ; as applied to the intellectual faculties, _selenia_, _illunaria_, and _illustraria_, self, the, or ego, self-consciousness, semicircular canals, , senility, introduction of, sensation defined, , sense-feelings of animals, senses of animals, ; organic and muscular, ; touch, ; temperature-sense, ; taste, ; smell, ; hearing, ; sight, ; contact and telæsthetic, ; problematical, sensibility, ; variations of, sensitive, special use of the term, sensitiveness and sensibility, sentiments, ; in animals, sex-differentiation, sexual union of ovum and sperm a source of variations, ; characters, secondary, ; selection, shame in monkey, sheep, youatt on, quoted, shells, land, of sandwich islands, shipp, captain, experiment on an elephant, sight, sense of, _sitaris_, instinct of, skertchly, mr. s. b. j., on leaf-butterfly, slave-making ants, smell, sense of, _smerinthus ocellatus_, smith, mr. g. munro, on elimination among microbes, snail, variations in banding of shells, ; sense of smell in, ; auditory sac, ; eye of, ; _spiculæ amoris_ of, snakes, mimicry in, snipe, drumming of, sollas, dr. w. j., on regeneration of tentacle in snail, somatic, or body-cells, sommering, fig. of semicircular canals, spalanzani, his experiments on bats, spalding, douglas, on instinctive emotions, ; on perfect instincts of chicken, ; on deferred instinct in swallow, sparrows in new zealand, specific characters, utility of, ; constancy of, spencer, mr. baldwin, fig. of pineal eye, spencer, mr. herbert, law associated with his name, ; physiological units, , ; on lap-dogs, ; on the irish elk and giraffe, ; on diminution in ear-muscles, ; definition of pleasure and pain, ; on æsthetics, ; on instinct and reflex action, sperm-cell and egg-cell, ; conditions which determine production of, sphex, instinct of, , spiders, hunting, mimicry in, ; javan, mr. h. o. forbes on, ; love-antics of, , ; ocelli of, spinoza, quoted, , , sponges, reproduction of, , _spongilla_, reproduction of, spore-formation, reproduction by, squirrel of sarepta, stag-beetles, variation in males of, star-fish, embryo set free early, starling, modified song of, st. john, observations on a retriever, _stenorhynchus_, sterility, how developed, stewart, mr. duncan, on sympathy in cat, stimuli, strange, mr., on love-antics of satin bower-bird, striped ancestor of equidæ, struggle for existence, ; variations in the intensity of, sturge, miss mildred, on the parrot, _stylonichia_, observations of m. maupas on, sully, mr. james, on concepts, ; on propositions, ; on judgment and reason, ; on emotion, ; on æsthetic sense of beauty, sutton, mr. bland, on hen pheasant like the male, ; on the action of leucocytes, swallow and swift, convergence in, ; cliff, of united states, swayne, mr. s. h., on the elephant, symbolic nature of mental products, symonds, mr. j. a., on "world-consciousness," sympathy in animals, t tameness, instinctive, tanner, miss agnes, on a thrush, tasmanian salmon, taste, standard of, , ; sense of, teeth of pike, temperature-sense, terror, _thaumalia picta_ and _amherstiæ_, _thekla_, instinct of, "things in themselves," or noumena, thomas, mr. oldfield, on rats of solomon islands, thomson, mr. j. a., prof. patrick geddes, and, on anabolism and katabolism, ; quoted, , , ; his "history and theory of heredity," thought, thrush, hearing in, ; sympathy in, thunberg on young hippopotamus, tissues of the body, tooke, mr. hammond, on egg-eating snake, tools, use of, by animals, touch, sense of, transformation and metamorphosis, transparency of some marine organisms, treat, mrs., her experiments on caterpillars, tricks, _trionyx_, _trochus_, tuco-tuco, turner, sir wm., on new guinea natives, turkey, instinctive emotion in the, twins, mr. galton's investigations on, tylor, alfred, on coloration in animals and plants, u udders, enlarged, of cows, ultra-violet rays, unicellular organism. _see_ protozoa unity of organism, , use and disuse, , utility of specific characters, v _vanessa urticæ_, _varanus benegalensis_, variation, correlated, ; and natural selection, ; tabulated by a. r. wallace, ; in wing-bones of bats, ; advantageous, neutral, and disadvantageous, ; in climatal and geographical conditions, ; secular, in climate and life area, ; effect of good times and hard times on, ; heredity and the origin of, ; a source of, in use and disuse, ; sexual union, a mode of origin of, ; in definite directions, ; produced by extrusion of second polar cell, ; protozoan origin of, ; due to the action of environment, ; to the effects of use and disuse, ; to domestication, ; in male stag-beetles, ; in mating preferences, ; co-ordinated in irish "elk" and giraffe, ; nature of, ; in amount of developmental capital, ; inheritance of, ; origin of, ; limitations of, ; fortuitous, in bat's wing, ; definite direction of, ; in limits of colour-vision, ; in habits and instincts, , ; in mental evolution, vertebrata, diagrammatic account of development of, verworn, dr., on protozoa, _vespertilio mystacinus_, _vesperugo leisleri_, _vesperugo noctula_, _vesperugo pipistrellus_, vigour and vitality, application of, in male, ; in female, vindictiveness, vision, ; mosaic, volition, _volucella bombylans_, voluntary and involuntary activities, _vorticella_, w waelchli, dr., on colour-globules in birds, wallace, mr. a. r., tabulations of variations, ; on tortoiseshell butterfly of isle of man, ; on protective colours in fishes, ; on divergence among birds, ; on recognition-marks, ; on papilionidæ of celebes, ; on the dull colours of hen birds, ; on origin of secondary sexual characters, ; and a. tylor on physiological guidance, ; on preferential mating, ; on reversion in grouse, ; on migration in birds, ; on nest-building in birds, ; on the song of birds, ; on materialism, ; on mathematical and artistic faculties, , walker, r., on reversion in bull, ward, mr. j. clifton, on dog, warning-coloration, ; involves perception, warren, mr. robert hall, a dog anecdote, wasp, use of antennæ, waste and repair essential life-processes, water, changes of salinity in, water-ousel, waterton, charles, watson, "reasoning power of animals," webb, dr., his operation on an elephant, weber, on musical discrimination, ; on muscular sensation in eye, weir, mr. jenner, on nest-building in birds, weismann, dr., on continuity of germ-plasm, ; on distinctness of germ-plasm from body-plasm, ; on meaning of second polar cell, ; on protozoan origin of variations ; on the introduction of senility and death, ; on the distinction of birds' eggs, ; on the effects of panmixia, ; on acceleration, ; his views applied to instinct, ; the intellectual faculties, westlake, miss mabel, on the parrot, whiskered bat, white, in arctic forms, ; mr. poulton on production of, ; in grouse, instance of reversion, wildness of birds, instinctive, will, f., on taste in bees, wilson, sir charles w., on wounded camels, wilson, edward, measurements of bats, wing-bones of bats, measurement of, in illustration of variation, words, "understanding" of, by animals, wrasse, keenness of vision of, x xiphocera, y youatt "on sheep," young, thomas, his colour-vision theory, yung, his experiments on tadpoles, z zebra, inconspicuousness of, in dusk, zuyder zee, new variety of herrings in, london: printed by william clowes and sons, limited, stamford street and charing cross. [illustration: compare the unfavorable artificial environment of a crowded city with the more favorable environment of the country.] a civic biology presented in problems by george william hunter, a.m. head of the department of biology, de witt clinton high school, city of new york. author of "elements of biology," "essentials of biology," etc. [illustration: printer's logo] american book company new york cincinnati chicago copyright, , by george william hunter. copyright, , in great britain. * * * * * hunter, civic biology. w. p. dedicated to my fellow teachers of the department of biology in the de witt clinton high school whose capable, earnest, unselfish and inspiring aid has made this book possible foreword to teachers a course in biology given to beginners in the secondary school should have certain aims. these aims must be determined to a degree, first, by the capabilities of the pupils, second, by their native interests, and, third, by the environment of the pupils. the boy or girl of average ability upon admission to the secondary school is not a thinking individual. the training given up to this time, with but rare exceptions, has been in the forming of simple concepts. these concepts have been reached didactically and empirically. drill and memory work have been the pedagogic vehicles. even the elementary science work given has resulted at the best in an interpretation of some of the common factors in the pupil's environment, and a widening of the meaning of some of his concepts. therefore, the first science of the secondary school, elementary biology, should be primarily the vehicle by which the child is taught to solve problems and to think straight in so doing. no other subject is more capable of logical development. no subject is more vital because of its relation to the vital things in the life of the child. a series of experiments and demonstrations, discussed and applied as definite concrete problems which have arisen within the child's horizon, will develop power in thinking more surely than any other subject in the first year of the secondary school. but in our eagerness to develop the power of logical thinking we must not lose sight of the previous training of our pupil. up to this time the method of induction, that handmaiden of logical thought, has been almost unknown. concepts have been formed deductively by a series of comparisons. all concepts have been handed down by the authority of the teacher or the text; the inductive search for the unknown is as yet a closed book. it is unwise, then, to directly introduce the pupil to the method of induction with a series of printed directions which, though definite in the mind of the teacher because of his wider horizon, mean little or nothing as a definite problem to the pupil. the child must be brought to the appreciation of the problem through the deductive method, by a comparison of the future problem with some definite concrete experience within his own field of vision. then by the inductive experiment, still led by a series of oral questions, he comes to the real end of the experiment, the conclusion, with the true spirit of the investigator. the result is tested in the light of past experiment and a generalization is formed which means something to the pupil. for the above reason the laboratory problems, which naturally precede the textbook work, should be separated from the subject matter of the text. a textbook in biology should serve to verify the student's observations made in the laboratory, it should round out his concept or generalization by adding such material as he cannot readily observe and it should give the student directly such information as he cannot be expected to gain directly or indirectly through his laboratory experience. for these reasons the laboratory manual has been separated from the text. "the laboratory method was such an emancipation from the old-time bookish slavery of pre-laboratory days that we may have been inclined to overdo it and to subject ourselves to a new slavery. it should never be forgotten that the laboratory is simply a means to the end; that the dominant thing should be a consistent chain of ideas which the laboratory may serve to elucidate. when, however, the laboratory assumes the first place and other phases of the course are made explanatory to it, we have taken, in my mind, an attitude fundamentally wrong. the question is, not what _types_ may be taken up in the laboratory to be fitted into the general scheme afterwards, but what _ideas_ are most worth while to be worked out and developed in the laboratory, if that happens to be the best way of doing it, or if not, some other way to be adopted with perfect freedom. too often our course of study of an animal or plant takes the easiest rather than the most illuminating path. what is easier, for instance, particularly with large classes of restless pupils who apparently need to be kept in a condition of uniform occupation, than to kill a supply of animals, preferably as near alike as possible, and set the pupils to work drawing the dead remains? this method is usually supplemented by a series of questions concerning the remains which are sure to keep the pupils busy a while longer, perhaps until the bell strikes, and which usually are so planned as to anticipate any ideas that might naturally crop up in the pupil's mind during the drawing exercise. "such an abuse of the laboratory idea is all wrong and should be avoided. the ideal laboratory ought to be a retreat for rainy days; a substitute for out of doors; a clearing house of ideas brought in from the outside. any course in biology which can be confined within four walls, even if these walls be of a modern, well-equipped laboratory, is in some measure a failure. living things, to be appreciated and correctly interpreted, must be seen and studied in the open where they will be encountered throughout life. _the place where an animal or plant is found is just as important a characteristic as its shape or function._ impossible field excursions with large classes within school hours, which only bring confusion to _inflexible_ school programs, are not necessary to accomplish this result. properly administered, it is without doubt one of our most efficient devices for developing biological ideas, but the laboratory should be kept in its proper relation to the other means at our disposal and never be allowed to degenerate either into a place for vacuous drawing exercises or a biological morgue where dead remains are viewed."--_dr. h. e. walter._ for the sake of the pupil the number of technical and scientific terms has been reduced to a minimum. the language has been made as simple as possible and the problems made to hinge upon material already known, by hearsay at least, to the pupil. so far as consistent with a well-rounded course in the essentials of biological science, the interests of the children have been kept in the foreground. in a recent questionnaire sent out by the author and answered by over three thousand children studying biology in the secondary schools of connecticut, massachusetts, new jersey, and new york by far the greatest number gave as the most interesting topics those relating to the care and functions of the human body and the control and betterment of the environment. as would be expected, boys have different biological interests from girls, and children in rural schools wish to study different topics from those in congested districts in large communities. the time has come when we must frankly recognize these interests and adapt the content of our courses in biology to interpret the _immediate_ world of the pupil. with this end in view the following pages have been written. this book shows boys and girls living in an urban community how they may best live within their own environment and how they may coöperate with the civic authorities for the betterment of their environment. a logical course is built up around the topics which appeal to the average normal boy or girl, topics given in a logical sequence so as to work out the solution of problems bearing on the ultimate problem of the entire course, that of preparation for citizenship in the largest sense. seasonal use of materials has been kept in mind in outlining this course. field trips, when properly organized and later used as a basis for discussion in the classroom, make a firm foundation on which to build the superstructure of a course in biology. the normal environment, its relation to the artificial environment of the city, the relations of mutual give and take existing between plants and animals, are better shown by means of field trips than in any other way. field and museum trips are enjoyed by the pupils as well. these result in interest and in better work. the course is worked up around certain great biological principles; hence insects may be studied when abundant in the fall in connection with their relations to green plants and especially in their relation to flowers. in the winter months material available for the laboratory is used. saprophytic and parasitic organisms, wild plants in the household, are studied in their relations to mankind, both as destroyers of food, property and life and as man's invaluable friends. the economic phase of biology may well be taken up during the winter months, thus gaining variety in subject matter and in method of treatment. the apparent emphasis placed upon economic material in the following pages is not real. it has been found that material so given makes for variety, as it may be assigned as a topical reading lesson or simply used as reference when needed. cyclic work in the study of life phenomena and of the needs of organisms for oxygen, food, and reproduction culminates, as it rightly should, in the study of life-processes of man and man's relation to his environment. in a course in biology the difficulty comes not so much in knowing what to teach as in knowing what _not_ to teach. the author believes that he has made a selection of the topics most vital in a well-rounded course in elementary biology directed toward civic betterment. the physiological functions of plants and animals, the hygiene of the individual within the community, conservation and the betterment of existing plant and animal products, the big underlying biological concepts on which society is built, have all been used to the end that the pupil will become a better, stronger and more unselfish citizen. the "spiral" or cyclic method of treatment has been used throughout, the purpose being to ultimately build up a number of well-rounded concepts by constant repetition but with constantly varied viewpoint. the sincere thanks of the author is extended to all who have helped make this book possible, and especially to the members of the department of biology in the de witt clinton high school. most of the men there have directly or indirectly contributed their time and ideas to help make this book worth more to teachers and pupils. the following have read the manuscript in its entirety and have offered much valuable constructive criticism: dr. herbert e. walter, professor of zoölogy in brown university; miss elsie kupfer, head of the department of biology in wadleigh high school; george c. wood, of the department of biology in the boys' high school, brooklyn; edgar a. bedford, head of department of biology in the stuyvesant high school; george e. hewitt, george t. hastings, john d. mccarthy, and frank m. wheat, all of the department of biology in the de witt clinton high school. thanks are due, also, to professor e. b. wilson, professor g. n. calkins, mr. william c. barbour, dr. john a. sampson, w. c. stevens, and c. w. beebe, dr. alvin davison, and dr. frank overton; to the united states department of agriculture; the new york aquarium; the charity organization society; and the american museum of natural history, for permission to copy and use certain photographs and cuts which have been found useful in teaching. dr. charles h. morse and dr. lucius j. mason, of the de witt clinton high school, prepared the hygiene outline in the appendix. frank m. wheat and my former pupil, john w. teitz, now a teacher in the school, made many of the line drawings and took several of the photographs of experiments prepared for this book. to them especially i wish to express my thanks. at the end of each of the following chapters is a list of books which have proved their use either as reference reading for students or as aids to the teacher. most of the books mentioned are within the means of the small school. two sets are expensive: one, _the natural history of plants_, by kerner, translated by oliver, published by henry holt and company, in two volumes, at $ ; the other, _plant geography upon a physiological basis_, by schimper, published by the clarendon press, $ ; but both works are invaluable for reference. for a general introduction to physiological biology, parker, _elementary biology_, the macmillan company; sedgwick and wilson, _general biology_, henry holt and company; verworn, _general physiology_, the macmillan company; and needham, _general biology_, comstock publishing company, are most useful and inspiring books. two books stand out from the pedagogical standpoint as by far the most helpful of their kind on the market. no teacher of botany or zoölogy can afford to be without them. they are: lloyd and bigelow, _the teaching of biology_, longmans, green, and company, and c. f. hodge, _nature study and life_, ginn and company. other books of value from the teacher's standpoint are: ganong, _the teaching botanist_, the macmillan company; l. h. bailey, _the nature study idea_, doubleday, page, and company; and mcmurry's _how to study_, houghton mifflin company. contents chapter page foreword to teachers i. some reasons for the study of biology ii. the environment of plants and animals iii. the interrelations of plants and animals iv. the functions and composition of living things v. plant growth and nutrition--the causes of growth vi. the organs of nutrition in plants--the soil and its relation to roots vii. plant growth and nutrition--plants make food viii. plant growth and nutrition--the circulation and final uses of food by plants ix. our forests, their uses and the necessity of their protection x. the economic relation of green plants to man xi. plants without chlorophyll in their relation to man xii. the relations of plants to animals xiii. single-celled animals considered as organisms xiv. division of labor, the various forms of plants and animals xv. the economic importance of animals xvi. an introductory study of vertebrates xvii. heredity, variation, plant and animal breeding xviii. the human machine and its needs xix. foods and dietaries xx. digestion and absorption xxi. the blood and its circulation xxii. respiration and excretion xxiii. body control and habit formation xxiv. man's improvement of his environment xxv. some great names in biology appendix suggested course with time allotment and sequence of topics for course beginning in fall suggested syllabus for course in biology beginning in february and ending the next january hygiene outline weights, measures, and temperatures suggestions for laboratory equipment index a civic biology i. the general problem--some reasons for the study of biology what is biology?--_biology is the study of living beings, both plant and animal._ inasmuch as man is an animal, the study of biology includes the study of man in his relations to the plants and the animals which surround him. most important of all is that branch of biology which treats of the mechanism we call the human body,--of its parts and their uses, and its repair. this subject we call _human physiology_. why study biology?--although biology is a very modern science, it has found its way into most high schools; and an increasingly large number of girls and boys are yearly engaged in its study. these questions might well be asked by any of the students: why do i take up the study of biology? of what practical value is it to me? besides the discipline it gives me, is there anything that i can take away which will help me in my future life? human physiology.--the answer to this question is plain. if the study of biology will give us a better understanding of our own bodies and their care, then it certainly is of use to us. that phase of biology known as _physiology_ deals with the uses of the parts of a plant or animal; human physiology and hygiene deal with the uses and care of the parts of the human animal. the prevention of sickness is due in a large part to the study of hygiene. it is estimated that over twenty-five per cent of the deaths that occur yearly in this country could be averted if _all_ people lived in a hygienic manner. in its application to the lives of each of us, as a member of our family, as a member of the school we attend, and as a future citizen, a knowledge of hygiene is of the greatest importance. relations of plants to animals.--but there are other reasons why an educated person should know something about biology. we do not always realize that if it were not for the green plants, there would be no animals on the earth. green plants furnish food to animals. even the meat-eating animals feed upon those that feed upon plants. how the plants manufacture this food and the relation they bear to animals will be discussed in later chapters. plants furnish man with the greater part of his food in the form of grains and cereals, fruits and nuts, edible roots and leaves; they provide his domesticated animals with food; they give him timber for his houses and wood and coal for his fires; they provide him with pulp wood, from which he makes his paper, and oak galls, from which he may make ink. much of man's clothing and the thread with which it is sewed together come from fiber-producing plants. most medicines, beverages, flavoring extracts, and spices are plant products, while plants are made use of in hundreds of ways in the useful arts and trades, producing varnishes, dyestuffs, rubber, and other products. bacteria in their relation to man.--in still another way, certain plants vitally affect mankind. tiny plants, called _bacteria_, so small that millions can exist in a single drop of fluid, exist almost everywhere about us,--in water, soil, food, and the air. they play a tremendous part in shaping the destiny of man on the earth. they help him in that they act as scavengers, causing things to decay; thus they remove the dead bodies of plants and animals from the surface of the earth, and turn this material back to the ground; they assist the tanner; they help make cheese and butter; they improve the soil for crop growing; so the farmer cannot do without them. but they likewise sometimes spoil our meat and fish, and our vegetables and fruits; they sour our milk, and may make our canned goods spoil. worst of all, they cause diseases, among others tuberculosis, a disease so harmful as to be called the "white plague." fully one half of all yearly deaths are caused by these plants. so important are the bacteria that a sub-division of biology, called _bacteriology_, has been named after them, and hundreds of scientists are devoting their lives to the study of bacteria and their control. the greatest of all bacteriologists, louis pasteur, once said, "it is within the power of man to cause all parasitic diseases (diseases mostly caused by bacteria) to disappear from the world." his prophecy is gradually being fulfilled, and it may be the lot of some boys or girls who read this book to do their share in helping to bring this condition of affairs about. the relation of animals to man.--animals also play an important part in the world in causing and carrying disease. animals that cause disease are usually tiny, and live in other animals as _parasites_; that is, they get their living from their hosts on which they feed. among the diseases caused by parasitic animals are malaria, yellow fever, the sleeping sickness, and the hookworm disease. animals also _carry_ disease, especially the flies and mosquitoes; rats and other animals are also well known as spreaders of disease. from a money standpoint, animals called insects do much harm. it is estimated that in this country alone they are annually responsible for $ , , worth of damage by eating crops, forest trees, stored food, and other material wealth. the uses of animals to man.--we all know the uses man has made of the domesticated animals for food and as beasts of burden. but many other uses are found for animal products, and materials made from animals. wool, furs, leather, hides, feathers, and silk are examples. the arts make use of ivory, tortoise shell, corals, and mother-of-pearl; from animals come perfumes and oils, glue, lard, and butter; animals produce honey, wax, milk, eggs, and various other commodities. the conservation of our natural resources.--still another reason why we should study biology is that we may work understandingly for the conservation of our natural resources, especially of our forests. the forest, aside from its beauty and its health-giving properties, holds water in the earth. it keeps the water from drying out of the earth on hot days and from running off on rainy days. thus a more even supply of water is given to our rivers, and thus freshets are prevented. countries that have been deforested, such as china, italy, and parts of france, are now subject to floods, and are in many places barren. on the forests depend our supply of timber, our future water power, and the future commercial importance of cities which, like new york, are located at the mouths of our navigable rivers. plants and animals mutually helpful.--most plants and animals stand in an attitude of mutual helpfulness to one another, plants providing food and shelter for animals; animals giving off waste materials useful to plants in the making of food. we also learn that plants and animals need the same conditions in their surroundings in order to live: water, air, food, a favorable temperature, and usually light. the life processes of both plants and animals are essentially the same, and the living matter of a tree is as much alive as is the living matter in a fish, a dog, or a man. biology in its relation to society.--again, the study of biology should be part of the education of every boy and girl, because society itself is founded upon the principles which biology teaches. plants and animals are living things, taking what they can from their surroundings; they enter into competition with one another, and those which are the best fitted for life outstrip the others. animals and plants tend to vary each from its nearest relative in all details of structure. the strong may thus hand down to their offspring the characteristics which make them the winners. health and strength of body and mind are factors which tell in winning. man has made use of this message of nature, and has developed improved breeds of horses, cattle, and other domestic animals. plant breeders have likewise selected the plants or seeds that have varied toward better plants, and thus have stocked the earth with hardier and more fruitful domesticated plants. man's dominion over the living things of the earth is tremendous. this is due to his understanding the principles which underlie the science of biology. finally the study of biology ought to make us better men and women by teaching us that unselfishness exists in the natural world as well as among the highest members of society. animals, lowly and complex, sacrifice their comfort and their very lives for their young. in the insect communities the welfare of the individual is given up for the best interests of the community. the law of mutual give and take, of sacrifice for the common good, is seen everywhere. this should teach us, as we come to take our places in society, to be willing to give up our individual pleasure or selfish gain for the good of the community in which we live. thus the application of biological principles will benefit society. ii. the environment of plants and animals _problem.--to discover some of the factors of the environment of plants and animals._ _(a) environment of a plant._ _(b) environment of an animal._ _(c) home environment of a girl or boy._ laboratory suggestions _laboratory demonstrations._--factors of the environment of a living plant or animal in the vivarium. _home exercise._--the study of the factors making up my own environment and how i can aid in their control. environment.--each one of us, no matter where he lives, comes in contact with certain surroundings. air is everywhere around us; light is necessary to us, so much so that we use artificial light at night. the city street, with its dirty and hard paving stones, has come to take the place of the soil of the village or farm. water and food are a necessary part of our surroundings. our clothing, useful to maintain a certain temperature, must also be included. all these things--air, light, heat, water, food--together make up our _environment_. [illustration: an unfavorable city environment.] all other animals, and all plants as well, are surrounded by and use practically the same things from their environment as we do. the potted plant in the window, the goldfish in the aquarium, your pet dog at home, all use, as we will later prove, the factors of their environment in the same manner. air, water, light, a certain amount of heat, soil to live in or on, and food form parts of the surroundings of _every_ living thing. [illustration: an experiment that shows the air contains about four fifths nitrogen.] [illustration: apparatus for separating water by means of an electric current into the two elements, hydrogen and oxygen.] the same elements found in plants and animals as in their environment.--it has been found by chemists that the plants and animals as well as their environment may be reduced to about eighty very simple substances known as _chemical elements_. for example, the air is made up largely of two elements, _oxygen_ and _nitrogen_. water, by means of an electric current, may be broken up into two elements, _oxygen_ and _hydrogen_. the elements in water are combined to make a _chemical compound_. the oxygen and nitrogen of the air are not so united, but exist as separate gases. if we were to study the chemistry of the bodies of plants and animals and of their foods, we would find them to be made up of certain chemical elements combined in various complex compounds. these elements are principally _carbon_, _hydrogen_, _oxygen_, _nitrogen_, and perhaps a dozen others in very minute proportions. but the same elements present in the living things might also be found in the environment, for example, water, food, the air, and the soil. it is logical to believe that living things use the chemical elements in their surroundings and in some wonderful manner build up their own bodies from the materials found in their environment. how this is done we will learn in later chapters. [illustration: chart to show the percentage of chemical elements in the human body.] what plants and animals take from their environment. air.--it is a self-evident fact that animals need air. even those living in the water use the air dissolved in the water. a fish placed in an air-tight jar will soon die. it will be proven later that plants also need air in order to live. [illustration: the effect of water upon the growth of trees. these trees were all planted at the same time in soil that is sandy and uniform. they are watered by a small stream which runs from left to right in the picture. most of the water soaks into the ground before reaching the last trees.] water.--we all know that water must form part of the environment of plants and animals. it is a matter of common knowledge that pets need water to drink; so do other animals. every one knows we must water a potted plant if we expect it to grow. water is of so much importance to man that from the time of the caesars until now he has spent enormous sums of money to bring pure water to his cities. the united states government is spending millions of dollars at the present time to bring by irrigation the water needed to support life in the western desert lands. [illustration: the effect of light upon a growing plant.] light as condition of the environment.--light is another important factor of the environment. a study of the leaves on any green plant growing near a window will convince one that such plants grow toward the light. all green plants are thus influenced by the sun. other plants which are not green seem either indifferent or are negatively influenced (move away from) the source of light. animals may or may not be attracted by light. a moth, for example, will fly toward a flame, an earthworm will move away from light. some animals prefer a moderate or weak intensity of light and live in shady forests or jungles, prowling about at night. others seem to need much and strong light. and man himself enjoys only moderate intensity of light and heat. look at the shady side of a city street on any hot day to prove this statement. heat.--animals and plants are both affected by heat or the absence of it. in cold weather green plants either die or their life activities are temporarily suspended,--the plant becomes _dormant_. likewise small animals, such as insects, may be killed by cold or they may _hibernate_ under stones or boards. their life activities are stilled until the coming of warm weather. bears and other large animals go to sleep during the winter and awake thin and active at the approach of warm weather. animals or plants used to certain temperatures are killed if removed from those temperatures. even man, the most adaptable of all animals, cannot stand great changes without discomfort and sometimes death. he heats his houses in winter and cools them in summer so as to have the amount of heat most acceptable to him, _i.e._ about ° fahrenheit. [illustration: vegetation in northern russia. the trees in this picture are nearly one hundred years old. they live under conditions of extreme cold most of the year.] the environment determines the kind of animals and plants within it.--in our study of geography we learned that certain luxuriant growths of trees and climbing plants were characteristic of the tropics with its moist, warm climate. no one would expect to find living there the hardy stunted plants of the arctic region. nor would we expect to find the same kinds of animal life in warm regions as in cold. the surroundings determine the kind of living things there. plants or animals _fitted to live_ in a given locality will probably be found there if they have had an opportunity to reach that locality. if, for example, temperate forms of life were introduced by man into the tropics, they would either die or they would gradually change so as to become fitted to live in their new environment. sheep with long wool fitted to live in england, when removed to cuba, where conditions of greater heat exist, soon died because they were not fitted or _adapted_ to live in their changed environment. [illustration: plant life in a moist tropical forest. notice the air plants to the left and the resurrection ferns on the tree trunk.] adaptations.--plants and animals are not only fitted to live under certain conditions, but each part of the body may be fitted to do certain work. i notice that as i write these words the fingers of my right hand grasp the pen firmly and the hand and arm execute some very complicated movements. this they are able to do because of the free movement given through the arrangement of the delicate bones of the wrist and fingers, their attachment to the bones of the arm, a wonderful complex of muscles which move the bones, and a directing nervous system which plans the work. because of the peculiar fitness in the structure of the hand for this work we say it is adapted to its function of grasping objects. each part of a plant or animal is usually fitted for some particular work. the root of a green plant, for example, is fitted to take in water by having tiny absorbing organs growing from it, the stems have pipes or tubes to convey liquids up and down and are strong enough to support the leafy part of the plant. each part of a plant does work, and is fitted, by means of certain structures, to do that work. it is because of these adaptations that living things are able to do their work within their particular environment. plants and animals and their natural environment.--those of us who have tried to keep potted plants in the schoolroom know how difficult it is to keep them healthy. dust, foreign gases in the air, lack of moisture, and other causes make the artificial environment in which they are placed unsuitable for them. a goldfish placed in a small glass jar with no food or no green water plants soon seeks the surface of the water, and if the water is not changed frequently so as to supply air the fish will die. again the artificial environment lacks something that the fish needs. each plant and animal is limited to a certain environment because of certain individual needs which make the surroundings fit for it to live in. [illustration: a natural barrier on a stream. no trout would be found above this fall. why not?] changes in environment.--most plants and animals do not change their environment. trees, green plants of all kinds, and some animals remain fixed in one spot practically all their lives. certain tiny plants and most animals move from place to place, either in air, water, on the earth or in the earth, but they maintain relatively the same conditions in environment. birds are perhaps the most striking exception, for some may fly thousands of miles from their summer homes to winter in the south. other animals, too, migrate from place to place, but not usually where there are great changes in the surroundings. a high mountain chain with intense cold at the upper altitudes would be a barrier over which, for example, a bear, a deer, or a snail could not travel. fish like trout will migrate up a stream until they come to a fall too high for them to jump. there they must stop because their environment limits them. [illustration: a new apartment house, with out-of-door sleeping porch.] man in his environment.--man, while he is like other animals in requiring heat, light, water, and food, differs from them in that he has come to live in a more or less artificial environment. men who lived on the earth thousands of year ago did not wear clothes or have elaborate homes of wood or brick or stone. they did not use fire, nor did they eat cooked foods. in short, by slow degrees, civilized man has come to live in a changed environment from that of other animals. the living together of men in communities has caused certain needs to develop. many things can be supplied in common, as water, milk, foods. wastes of all kinds have to be disposed of in a town or city. houses have come to be placed close together, or piled on top of each other, as in the modern apartment. fields and trees, all outdoor life, has practically disappeared. man has come to live in an artificial environment. care and improvement of one's environment.--man can modify or change his surroundings by making this artificial environment favorable to live in. he may heat his dwellings in winter and cool them in summer so as to maintain a moderate and nearly constant temperature. he may see that his dwellings have windows so as to let light and air pass in and out. he may have light at night and shade by day from intense light. he may have a system of pure water supply and may see that drains or sewers carry away his wastes. he may see to it that people ill with "catching" or _infectious_ diseases are isolated or _quarantined_ from others. this care of the artificial environment is known as _sanitation_, while the care of the _individual_ for himself within the environment is known as _hygiene_. it will be the chief end of this book to show girls and boys how they may become good citizens through the proper control of personal hygiene and sanitation. reference books elementary hunter, _laboratory problems in civic biology_. american book company. hough and sedgwick, _elements of hygiene and sanitation_. ginn and company. jordan and kellogg, _animal life_. appleton. sharpe, _a laboratory manual for the solution of problems in biology_, _p. _. american book company. tolman, _hygiene for the worker_. american book company. advanced allen, _civics and health_. ginn and company. iii. the interrelations of plants and animals _problem.--to discover the general interrelations of green plants and animals._ _(a) plants as homes for insects._ _(b) plants as food for insects._ _(c) insects as pollinating agents._ laboratory suggestions _a field trip_:--object: to collect common insects and study their general characteristics; to study the food and shelter relation of plant and insects. the pollination of flowers should also be carefully studied so as to give the pupil a general viewpoint as an introduction to the study of biology. _laboratory exercise._--examination of simple insect, identification of parts--drawing. examination and identification of some orders of insects. _laboratory demonstration._--life history of monarch and some other butterflies or moths. _laboratory exercise._--study of simple flower--emphasis on work of essential organs, drawing. _laboratory exercise._--study of mutual adaptations in a given insect and a given flower, _e.g._ butter and eggs and bumble bee. _demonstration of examples of insect pollination._ the object of a field trip.--many of us live in the city, where the crowded streets, the closely packed apartments, and the city playgrounds form our environment. it is very artificial at best. to understand better the _normal environment_ of plants or animals we should go into the country. failing in this, an overgrown city lot or a park will give us much more closely the environment as it touches some animals lower than man. we must then remember that in learning something of the natural environment of other living creatures we may better understand our own environment and our relation to it. on any bright warm day in the fall we will find insects swarming everywhere in any vacant lot or the less cultivated parts of a city park. grasshoppers, butterflies alighting now and then on the flowers, brightly marked hornets, bees busily working over the purple asters or golden rod, and many other forms hidden away on the leaves or stems of plants may be seen. if we were to select for observation some partially decayed tree, we would find it also inhabited. beetles would be found boring through its bark and wood, while caterpillars (the young stages of butterflies or moths) are feeding on its leaves or building homes in its branches. everywhere above, on, and under ground may be noticed small forms of life, many of them insects. let us first see how we would go to work to identify some of the common forms we would be likely to find on plants. then a little later we will find out what they are doing on these plants. [illustration: an insect viewed from the side. notice the head, thorax, and abdomen. what other characters do you find?] how to tell an insect.--a bee is a good example of the group of animals we call _insects_. if we examine its body carefully, we notice that it has three regions, a front part or _head_, a middle part called the _thorax_, and a hind portion, jointed and hairy, the _abdomen_. we cannot escape noting the fact that this insect has wings with which it flies and that it also has legs. the three pairs of legs, which are jointed and provided with tiny hooks at the end, are attached to the thorax. two pairs of delicate wings are attached to the upper or _dorsal_ side of the thorax. the thorax and indeed the entire body, is covered with a hard shell of material similar to a cow's horn, there being no skeleton inside for the attachment of muscles. if we carefully watch the abdomen of a living bee, we notice it move up and down quite regularly. the animal is breathing through tiny breathing holes called _spiracles_, placed along the side of the thorax and abdomen. bees also have compound eyes. wings are not found on all insects, but all the other characters just given are marks of the great group of animals we call _insects_. [illustration: part of the compound eye of an insect (highly magnified).] forms to be looked for on a field trip.--inasmuch as there are over , different species or kinds of insects, it is evident that it would be a hopeless task for us even to think of recognizing all of them. but we can learn to recognize a few examples of the common forms that might be met on a field trip. in the fields, on grass, or on flowering plants we may count on finding members from six groups or _orders_ of insects. these may be known by the following characters. the order _hymenoptera_ (membrane wing) to which the bees, wasps, and ants belong is the only insect group the members of which are provided with true stings. this sting is placed in a sheath at the extreme hind end of the abdomen. other characteristics, which show them to be insects, have been given above. butterflies or moths will be found hovering over flowers. they belong to the order _lepidoptera_ (scale wings). this name is given to them because their wings are covered with tiny scales, which fit into little sockets on the wing much as shingles are placed on a roof. the dust which comes off on the fingers when one catches a butterfly is composed of these scales. the wings are always large and usually brightly colored, the legs small, and one pair is often inconspicuous. these insects may be seen to take liquid food through a long tubelike organ, called the _proboscis_, which they keep rolled up under the head when not in use. the young of the butterfly or moth are known as _caterpillars_ and feed on plants by means of a pair of hard jaws. grasshoppers, found almost everywhere, and crickets, black grasshopper-like insects often found under stones, belong to the order _orthoptera_ (straight wings). members of this group may usually be distinguished by their strong, jumping hind legs, by their chewing or biting mouth parts, and by the fact that the hind wings are folded up under the somewhat stiffer front wings. [illustration: forms of life to be met on a field trip. _a_, the red-legged locust, one of the _orthoptera_; _o_, the egg-layer, about natural size. _b_, the honey bee, one of the _hymenoptera_, about natural size. _c_, a bug, one of the _hemiptera_, about natural size. _d_, a butterfly, an example of the _lepidoptera_, slightly reduced. _e_, a house fly, an example of the _diptera_, about twice natural size. _f_, an orb-weaving spider, about half natural size. (this is not an insect, note the number of legs.) _g_, a beetle, slightly reduced, one of the _coleoptera_.] another group of insects sometimes found on flowers in the fall are flies. they belong to the order _diptera_ (two wings). these insects are usually rather small and have a single pair of gauzy wings. flies are of much importance to man because certain of their number are disease carriers. bugs, members of the order _hemiptera_ (half wings), have a jointed proboscis which points backward between the front legs. they are usually small and may or may not have wings. the beetles or _coleoptera_ (sheath wings), often mistaken for bugs by the uneducated, have the first pair of hardened wings meeting in a straight line in the middle of the back, the second pair of wings being covered by them. beetles are frequently found on goldenrod blossoms in the fall. other forms of life, especially _spiders_, which have four pairs of walking legs, _centipedes_ and _millepedes_, both of which are wormlike and have many pairs of legs, may be found. try to discover members of the six different orders named above. collect specimens and bring them to the laboratory for identification. why do insects live on plants?--we have found insect life abundant on living green plants, some visiting flowers, others hidden away on the stalks or leaves of the plants. let us next try to find out _why_ insects live among and upon flowering green plants. the life history of the milkweed butterfly.--if it is possible to find on our trip some growing milkweed, we are quite likely to find hovering near, a golden brown and black butterfly, the monarch or milkweed butterfly (_anosia plexippus_). its body, as in all insects, is composed of three regions. the monarch frequents the milkweed in order to lay eggs there. this she may be found doing at almost any time from june until september. egg and larva.--the eggs, tiny hat-shaped dots a twentieth of an inch in length, are fastened singly to the underside of milkweed leaves. some wonderful instinct leads the animal to deposit the eggs on the milkweed, for the young feed upon no other plant. the eggs hatch out in four or five days into rapid-growing wormlike caterpillars, each of which will shed its skin several times before it becomes full size. these caterpillars possess, in addition to the three pairs of true legs, additional pairs of _prolegs_ or caterpillar legs. the animal at this stage is known as a _larva_. formation of pupa.--after a life of a few weeks at most, the caterpillar stops eating and begins to spin a tiny mat of silk upon a leaf or stem. it attaches itself to this web by the last pair of prolegs, and there hangs in the dormant stage known as the _chrysalis_ or _pupa_. this is a resting stage during which the body changes from a caterpillar to a butterfly. [illustration: monarch butterfly: adults, larvæ, and pupa on their food plant, the milkweed. (from a photograph loaned by the american museum of natural history.)] the adult.--after a week or more of inactivity in the pupa state, the outer skin is split along the back, and the adult butterfly emerges. at first the wings are soft and much smaller than in the adult. within fifteen minutes to half an hour after the butterfly emerges, however, the wings are full-sized, having been pumped full of blood and air, and the little insect is ready after her wedding flight to follow her instinct to deposit her eggs on a milkweed plant. plants furnish insects with food.--food is the most important factor of any animal's environment. the insects which we have seen on our field trip feed on the green plants among which they live. each insect has its own particular favorite food plant or plants, and in many cases the eggs of the insect are laid on the food plant so that the young may have food close at hand. some insects prefer the rotted wood of trees. an american zoölogist, packard, has estimated that over kinds of insects live upon oak trees alone. everywhere animals are engaged in taking their nourishment from plants, and millions of dollars of damage is done every year to gardens, fruits, and cereal crops by insects. [illustration: damage done by insects. these trees have been killed by boring insects.] all animals depend on green plants.--but insects in their turn are the food of birds; cats and dogs may kill birds; lions or tigers live on still larger defenseless animals as deer or cattle. and finally comes man, who eats the bodies of both plants and animals. but if we reduce this search after food to its final limit, we see that green plants provide _all_ the food for animals. for the lion or tiger eats the deer which feeds upon grass or green shoots of young trees, or the cat eats the bird that lives on weed seeds. green plants supply the food of the world. later by experiment we will prove this. homes and shelter.--after a field trip no one can escape the knowledge that plants often give animals a home. the grass shelters millions of grasshoppers and countless hordes of other small insects which can be obtained by sweeping through the grass with an insect net. some insects build their homes in the trees or bushes on which they feed, while others tunnel through the wood, making homes there. spiders build webs on plants, often using the leaves for shelter. birds nest in trees, and many other wild animals use the forest as their home. man has come to use all kinds of plant products to aid him in making his home, wood and various fibers being the most important of these. what do animals do for plants?--so far it has seemed that green plants benefit animals and receive nothing in return. we will later see that plants and animals _together_ form a balance of life on the earth and that one is necessary for the other. certain substances found in the body wastes from animals are necessary to the life of a green plant. insects and flowers.--certain other problems can be worked out in the fall of the year. one of these is the biological interrelations between insects and flowers. it is easy on a field trip to find insects lighting upon flowers. they evidently have a reason for doing this. to find out why they go there and what they do when there, it will be first necessary for us to study flowers with the idea of finding out what the insects get from them, and what the flowers get from the insects. [illustration: a section of a flower, cut lengthwise. in the center find the pistil with the ovary containing a number of ovules. around this organ notice a circle of stalked structures, the stamens; the knobs at the end contain pollen. the outer circles of parts are called the petals and sepals, as we go from the inside outward.] the use and structure of a flower.--it is a matter of common knowledge that flowers form fruits and that fruits contain seeds. they are, then, very important parts of certain plants. our field trip shows us that flowers are of various shapes, colors, and sizes. it will now be our problem first to learn to know the parts of a flower, and then find out how they are fitted to attract and receive insect visitors. the floral envelope.--in a flower the expanded portion of the flower stalk, which holds the parts of the flower, is called the _receptacle_. _the green leaflike parts covering the unopened flower are called the sepals._ together they form the _calyx_. _the more brightly colored structures are the petals._ together they form the _corolla_. the corolla is of importance, as we shall see later, in making the flower conspicuous. frequently the petals or corolla have bright marks or dots which lead down to the base of the cup of the flower, where a sweet fluid called _nectar_ is made and secreted. it is principally this food substance, later made into honey by bees, that makes flowers attractive to insects. the essential organs.--a flower, however, could live without sepals or petals and still do the work for which it exists. certain _essential organs_ of the flower are within the so-called floral envelope. they consist of the _stamens_ and _pistil_, the latter being in the center of the flower. the structures with the knobbed ends are called _stamens_. in a single stamen the boxlike part at the end is the _anther_; the stalk which holds the anther is called the _filament_. the anther is in reality a hollow box which produces a large number of little grains called _pollen_. each pistil is composed of a rather stout base called the _ovary_, and a more or less lengthened portion rising from the ovary called the _style_. the upper end of the style, which in some cases is somewhat broadened, is called the _stigma_. the free end of the stigma usually secretes a sweet fluid in which grains of pollen from flowers of the same kind can grow. insects as pollinating agents.--insects often visit flowers to obtain pollen as well as nectar. in so doing they may transfer some of the pollen from one flower to another of the same kind. this transfer of pollen, called _pollination_, is of the greatest use to the plant, as we will later prove. no one who sees a hive of bees with their wonderful communal life can fail to see that these insects play a great part in the life of the flowers near the hive. a famous observer named sir john lubbock tested bees and wasps to see how many trips they made daily from their homes to the flowers, and found that the wasp went out on visits during a working day of hours, while the bee made but a few less visits, and worked only a little less time than the wasp worked. it is evident that in the course of so many trips to the fields a bee must light on hundreds of flowers. [illustration: bumblebees. _a_, queen; _b_, worker; _c_, drone.] adaptations in a bee.--if we look closely at the bee, we find the body and legs more or less covered with tiny hairs; especially are these hairs found on the legs. _when a plant or animal structure is fitted to do a certain kind of work, we say it is adapted to do that work._ the joints in the leg of the bee adapt it for complicated movements; the arrangement of stiff hairs along the edge of a concavity in one of the joints of the leg forms a structure well adapted to hold pollen. in this way pollen is collected by the bee and taken to the hive to be used as food. but while gathering pollen for itself, the dust is caught on the hairs and other projections on the body or legs and is thus carried from flower to flower. the value of this to a flower we will see later. field work.--is color or odor in a flower an attraction to an insect?--sir john lubbock tried an experiment which it would pay a number of careful pupils to repeat. he placed a few drops of honey on glass slips and placed them over papers of various colors. in this way he found that the honeybee, for example, could evidently distinguish different colors. bees seemed to prefer blue to any other color. flowers of a yellow or flesh color were preferred by flies. it would be of considerable interest for some student to work out this problem with our native bees and with other insects by using paper flowers and honey or sirup. test the keenness of sight in insects by placing a white object (a white golf ball will do) in the grass and see how many insects will alight on it. try to work out some method by which you can decide whether a given insect is attracted to a flower by odor alone. the sight of the bumblebee.--the large eyes located on the sides of the head are made up of a large number of little units, each of which is considered to be a very simple eye. the large eyes are therefore called the _compound eyes_. all insects are provided with compound eyes, with simple eyes, or in most cases with both. the simple eyes of the bee may be found by a careful observer between and above the compound eyes. insects can, as we have already learned, distinguish differences in color at some distance; they can see _moving_ objects, but they do not seem to be able to make out form well. to make up for this, they appear to have an extremely well-developed sense of smell. insects can distinguish at a great distance odors which to the human nose are indistinguishable. night-flying insects, especially, find the flowers by the odor rather than by color. [illustration: the head of a bee. _a_, antennæ or "feelers"; _e_, compound eye; _s_, simple eye; _m_, mouth parts; _t_, tongue.] mouth parts of the bee.--the mouth of the bee is adapted to take in the foods we have mentioned, and is used for the purposes for which man would use the hands and fingers. the honeybee laps or sucks nectar from flowers, it chews the pollen, and it uses part of the mouth as a trowel in making the honeycomb. the uses of the mouth parts may be made out by watching a bee on a well-opened flower. suggestions for field work.--in any locality where flowers are abundant, try to answer the following questions: how many bees visit the locality in ten minutes? how many other insects alight on the flowers? do bees visit flowers of the same kinds in succession, or fly from one flower on a given plant to another on a plant of a different kind? if the bee lights on a flower cluster, does it visit more than one flower in the same cluster? how does a bee alight? exactly what does the bee do when it alights? [illustration: flower cluster of "butter and eggs."] butter and eggs (_linaria vulgaris_).--from july to october this very abundant weed may be found especially along roadsides and in sunny fields. the flower cluster forms a tall and conspicuous cluster of orange and yellow flowers. the corolla projects into a spur on the lower side; an upper two-parted lip shuts down upon a lower three-parted lip. the four stamens are in pairs, two long and two short. [illustration: diagram to show how the bee pollinates "butter and eggs." the bumblebee, upon entering the flower, rubs its head against the long pair of anthers (_a_), then continuing to press into the flower so as to reach the nectar at (_n_) it brushes against the stigma (_s_), thus pollinating the flower. inasmuch as bees visit other flowers in the same cluster, cross-pollination would also be likely. why?] certain parts of the corolla are more brightly colored than the rest of the flower. this color is a guide to insects. butter and eggs is visited most by bumblebees, which are guided by the orange lip to alight just where they can push their way into the flower. the bee, seeking the nectar secreted in the spur, brushes his head and shoulders against the stamens. he may then, as he pushes down after nectar, leave some pollen upon the pistil, thus assisting in _self-pollination_. visiting another flower of the cluster, it would be an easy matter accidentally to transfer this pollen to the stigma of another flower. in this way pollen is carried by the insect to another flower of the same kind. this is known as _cross-pollination_. _by pollination we mean the transfer of pollen from an anther to the stigma of a flower. self-pollination is the transfer of pollen from the anther to the stigma of the same flower; cross-pollination is the transfer of pollen from the anthers of one flower to the stigma of another flower on the same or another plant of the same kind._ [illustration: a wild orchid, a flower of the type from which charles darwin worked out his theory of cross-pollination by insects.] history of the discoveries regarding pollination of flowers.--although the ancient greek and roman naturalists had some vague ideas on the subject of pollination, it was not until the first part of the nineteenth century that a book appeared in which a german named conrad sprengel worked out the facts that the structure of certain flowers seemed to be adapted to the visits of insects. certain facilities were offered to an insect in the way of easy foothold, sweet odor, and especially food in the shape of pollen and nectar, the latter a sweet-tasting substance manufactured by certain parts of the flower known as the nectar glands. sprengel further discovered the fact that pollen could be and was carried by the insect visitors from the anthers of the flower to its stigma. it was not until the middle of the nineteenth century, however, that an englishman, charles darwin, applied sprengel's discoveries on the relation of insects to flowers by his investigations upon cross-pollination. the growth of the pollen on the stigma of the flower results eventually in the production of seeds, and thus new plants. many species of flowers are self-pollinated and do not do so well in seed production if cross-pollinated, but charles darwin found that some flowers which were self-pollinated did not produce so many seeds, and that the plants which grew from their seeds were smaller and weaker than plants from seeds produced by cross-pollinated flowers of the same kind. he also found that plants grown from cross-pollinated seeds tended to _vary_ more than those grown from self-pollinated seed. this has an important bearing, as we shall see later, in the production of new varieties of plants. microscopic examination of the stigma at the time of pollination also shows that the pollen from another flower usually germinates before the pollen which has fallen from the anthers of the same flower. this latter fact alone in most cases renders it unlikely for a flower to produce seeds by its own pollen. darwin worked for years on the pollination of many insect-visited flowers, and discovered in almost every case that showy, sweet-scented, or otherwise attractive flowers were adapted or fitted to be cross-pollinated by insects. he also found that, in the case of flowers that were inconspicuous in appearance, often a compensation appeared in the odor which rendered them attractive to certain insects. the so-called carrion flowers, pollinated by flies, are examples, the odor in this case being like decayed flesh. other flowers open at night, are white, and provided with a powerful scent. thus they attract night-flying moths and other insects. other examples of mutual aid between flowers and insects.--many other examples of adaptations to secure cross-pollination by means of the visits of insects might be given. the mountain laurel, which makes our hillsides so beautiful in late spring, shows a remarkable adaptation in having the anthers of the stamens caught in little pockets of the corolla. the weight of the visiting insect on the corolla releases the anther from the pocket in which it rests so that it springs up, dusting the body of the visitor with pollen. [illustration: the condition of stamens and pistils on the spiked loosestrife (_lythrum salicaria_).] in some flowers, as shown by the primroses or primula of our hothouses, the stamens and pistils are of different lengths in different flowers. short styles and long or high-placed filaments are found in one flower, and long styles with short or low-placed filaments in the other. pollination will be effected only when some of the pollen from a low-placed anther reaches the stigma of a short-styled flower, or when the pollen from a high anther is placed upon a long-styled pistil. there are, as in the case of the loosestrife, flowers having pistils and stamens of three lengths. pollen only grows on pistils of the same length as the stamens from which it came. the milkweed or butterfly weed already mentioned is another example of a flower adapted to insect pollination.[ ] footnote : for an excellent account of cross-pollination of this flower, the reader is referred to w. c. stevens, _introduction to botany_. orchids are well known to botanists as showing some very wonderful adaptations. a classic easily read is darwin, _on the fertilization of orchids_. [illustration: the pronuba moth within the yucca flower.] a very remarkable instance of insect help is found in the pollination of the yucca, a semitropical lily which lives in deserts (to be seen in most botanic gardens). in this flower the stigmatic surface is above the anther, and the pollen is sticky and cannot be transferred except by insect aid. this is accomplished in a remarkable manner. a little moth, called the _pronuba_, after gathering pollen from an anther, deposits an egg in the ovary of the pistil, and then rubs its load of pollen over the stigma of the flower. the young hatch out and feed on the young seeds which have grown because of the pollen placed on the stigma by the mother. the baby caterpillars eat some of the developing seeds and later bore out of the seed pod and escape to the ground, leaving the plant to develop the remaining seeds without further molestation. [illustration: the pronuba pollinating the pistil of the yucca.] the fig insect (_blastophaga grossorum_) is another member of the insect tribe that is of considerable economic importance. it is only in recent years that the fruit growers of california have discovered that the fertilization of the female flowers is brought about by a gallfly which bores into the young fruit. by importing the gallflies it has been possible to grow figs where for many years it was believed that the climate prevented figs from ripening. [illustration: pod of yucca showing where the young pronubas escaped.] other flower visitors.--other insects besides those already mentioned are pollen carriers for flowers. among the most useful are moths and butterflies. projecting from each side of the head of a butterfly is a fluffy structure, the palp. this collects and carries a large amount of pollen, which is deposited upon the stigmas of other flowers when the butterfly pushes its head down into the flower tube after nectar. the scales and hairs on the wings, legs, and body also carry pollen. [illustration: a humming bird about to cross-pollinate a lily.] flies and some other insects are agents in cross-pollination. humming birds are also active agents in some flowers. snails are said in rare instances to carry pollen. man and the domesticated animals undoubtedly frequently pollinate flowers by brushing past them through the fields. [illustration: a cornfield showing staminate and pistillate flowers, the latter having become grains of corn. an ear of corn is a bunch of ripened fruits.] pollination by the wind.--not all flowers are dependent upon insects or other animals for cross-pollination. many of the earliest of spring flowers appear almost before the insects do. such flowers are dependent upon the wind for carrying pollen from the stamens of one flower to the pistil of another. most of our common trees, oak, poplar, maple, and others, are cross-pollinated almost entirely by the wind. flowers pollinated by the wind are generally inconspicuous and often lack a corolla. the anthers are exposed to the wind and provided with much pollen, while the surface of the stigma may be long and feathery. such flowers may also lack odor, nectar, and bright color. can you tell why? imperfect flowers.--some flowers, the wind-pollinated ones in particular, are imperfect; that is, they lack either stamens or pistils. again, in some cases, imperfect flowers having stamens only are alone found on one plant, while those flowers having pistils only are found on another plant of the same kind. in such flowers, cross-pollination must of necessity follow. many of our common trees are examples. [illustration: the flower of "lady washington" geranium, in which stamens and pistil ripen at different times, thus insuring cross-pollination. _a_, flower with ripe stamens; _b_, flower with stamens withered and ripe pistil.] other cases.--the stamens and pistil ripen at different times in some flowers. the "lady washington" geranium, a common house plant, shows this condition. here also cross-pollination must take place if seeds are to be formed. summary.--if we now collect our observations upon flowers with a view to making a summary of the different devices flowers have assumed to prevent self-pollination and to secure cross-pollination, we find that they are as follows:-- _( ) the stamens and pistils may be found in separate flowers, either on the same or on different plants._ _( ) the stamens may produce pollen before the pistil is ready to receive it, or vice versa._ _( ) the stamens and pistils may be so placed with reference to each other that pollination can be brought about only by outside assistance._ artificial cross-pollination and its practical benefits to man.--artificial cross-pollination is practiced by plant breeders and can easily be tried in the laboratory or at home. first the anthers must be carefully removed from the bud of the flower so as to eliminate all possibility of self-pollination. the flower must then be covered so as to prevent access of pollen from without; when the ovary is sufficiently developed, pollen from another flower, having the characters desired, is placed on the stigma and the flower again covered to prevent any other pollen reaching the flower. the seeds from this flower when planted _may_ give rise to plants with the best characters of each of the plants which contributed to the making of the seeds. reference books elementary hunter, _laboratory problems in civic biology_. american book company. andrews, _a practical course in botany_, pages - . american book company. atkinson, _first studies of plant life_, chaps. xxv-xxvi. ginn and company. coulter, _plant life and plant uses_, pages - . american book company. dana, _plants and their children_, pages - . american book company. lubbock, _flowers, fruits, and leaves_, part i. the macmillan company. needham, _general biology_, pages - . the comstalk publishing company. newell, _a reader in botany_, part ii, pages - . ginn and company. sharpe, _a laboratory manual in biology_, pages - . american book company. advanced bailey, _plant breeding_. the macmillan company. campbell, _lectures on the evolution of plants_. the macmillan company. coulter, barnes, and cowles, _a textbook of botany_, part ii. american book company. darwin, _different forms of flowers on plants of the same species_, d. appleton and company. darwin, _fertilization in the vegetable kingdom_, chaps. i and ii. d. appleton and company. darwin, _orchids fertilized by insects_, d. appleton and company. lubbock, _british wild flowers_. the macmillan company. müller, _the fertilization of flowers_. the macmillan company. iv. the functions and composition of living things _problems.--to discover the functions of living matter._ _(a) in a living plant._ _(b) in a living animal._ laboratory suggestions _laboratory study of a living plant._--any whole plant may be used; a weed is preferable. _laboratory demonstration or home study._--the functions of a living animal. _demonstration._--the growth of pollen tubes. _laboratory exercise._--the growth of the mature ovary into the fruit, _e.g._ bean or pea pod. a living plant and a living animal compared.--a walk into the fields or any vacant lot on a day in the early fall will give us first-hand acquaintance with many common plants which, because of their ability to grow under somewhat unfavorable conditions, are called _weeds_. such plants--the dandelion, butter and eggs, the shepherd's purse--are particularly well fitted by nature to produce many of their kind, and by this means drive out other plants which cannot do this so well. on these or other plants we find feeding several kinds of animals, usually insects. if we attempt to compare, for example, a grasshopper with the plant on which it feeds, we see several points of likeness and difference at once. both plant and insect are made up of parts, each of which, as the stem of the plant or the leg of the insect, appears to be distinct, but which is a part of the whole living plant or animal. each part of the living plant or animal which has a separate work to do is called an _organ_. thus plants and animals are spoken of as living _organisms_. [illustration: a weed--notice the unfavorable environment.] functions of the parts of a plant.--we are all familiar with the parts of a plant,--the root, stem, leaves, flowers, and fruit. but we may not know so much about their uses to the plant. each of these structures differs from every other part, and each has a separate work or function to perform for the plant. _the root holds the plant firmly in the ground and takes in water and mineral matter from the soil; the stem holds the leaves up to the light and acts as a pathway for fluids between the root and leaves; the leaves, under certain conditions, manufacture food for the plant and breathe; the flowers form the fruits; the fruits hold the seeds, which in turn hold young plants which are capable of reproducing adult plants of the same kind._ the functions of an animal.--as we have already seen, the grasshopper has a head, a jointed body composed of a middle and a hind part, three pairs of jointed legs, and two pairs of wings. obviously, the wings and legs are used for movement; a careful watching of the hind part of the animal shows us that breathing movements are taking place; a bit of grass placed before it may be eaten, the tiny black jaws biting little pieces out of the grass. if disturbed, the insect hops away, and if we try to get it, it jumps or flies away, evidently seeing us before we can grasp it. hundreds of little grasshoppers on the grass indicate that the grasshopper can reproduce its own kind, but in other respects the animal seems quite unlike the plant. the animal moves, breathes, feeds, and has sensation, while _apparently_ the plant does none of these. it will be the purpose of later chapters to prove that the functions of plants and animals are in many respects similar and that _both plants_ and _animals breathe_, _feed_, and _reproduce_. [illustration: section through the blade of a leaf. _e_, cells of the upper surface; _d_, cells of the lower surface; _i_, air spaces in the leaf; _v_, vein in cross sections; _p_, green cells.] organs.--if we look carefully at the organ of a plant called a leaf, we find that the materials of which it is composed do not appear to be everywhere the same. the leaf is much thinner and more delicate in some parts than in others. holding the flat, expanded blade away from the branch is a little stalk, which extends into the blade of the leaf. here it splits up into a network of tiny "veins" which evidently form a framework for the flat blade somewhat as the sticks of a kite hold the paper in place. if we examine under the compound microscope a thin section cut across the leaf, we shall find that the veins as well as the other parts are made up of many tiny boxlike units of various sizes and shapes. these smallest units of building material of the plant or animal disclosed by the compound microscope are called _cells_. the organs of a plant or animal are built of these tiny structures. [illustration: several cells of _elodea_, a water plant. _chl._, chlorophyll bodies; _c.s._, cell sap; _c.w._, cell wall; _n._, nucleus; _p._, protoplasm. the arrows show the direction of the protoplasmic movement.] tissues.[ ]--the cells which form certain parts of the veins, the flat blade, or other portions of the plant, are often found in groups or collections, the cells of which are more or less alike in size and shape. such a collection of cells is called a _tissue_. examples of tissues are the cells covering the outside of the human body, the muscle cells, which collectively allow of movement, bony tissues which form the framework to which the muscles are attached, and many others. footnote : _to the teacher._--any simple plant or animal tissue can be used to demonstrate the cell. epidermal cells may be stripped from the body of the frog or obtained by scraping the inside of one's mouth. the thin skin from an onion stained with tincture of iodine shows well, as do thin sections of a young stem, as the bean or pea. one of the best places to study a tissue and the cells of which it is composed is in the leaf of a green water plant, _elodea_. in this plant the cells are large, and not only their outline, but the movement of the living matter within the cells, may easily be seen, and the parts described in the next paragraph can be demonstrated. [illustration: a cell. _ch._, chromosomes; _c.w._, cell wall; _n._, nucleus; _p._, protoplasm.] cells.--_a cell may be defined as a tiny mass of living matter containing a nucleus, either living alone or forming a unit of the building material of a living thing._ the living matter of which all cells are formed is known as _protoplasm_ (formed from two greek words meaning _first form_). if we examine under a compound microscope a small bit of the water plant _elodea_, we see a number of structures resembling bricks in a wall. each "brick," however, is really a plant cell bounded by a thin wall. if we look carefully, we can see that the material inside of this wall is slowly moving and is carrying around in its substance a number of little green bodies. this moving substance is living matter, the protoplasm of the cell. the green bodies (the _chlorophyll_ bodies) we shall learn more about later; they are found only in plant cells. all plant and animal cells appear to be alike in the fact that every living cell possesses a structure known as the _nucleus_ (pl. _nuclei_), which is found within the body of the cell. this nucleus is not easy to find in the cells of _elodea_. within the nucleus of all cells are found certain bodies called _chromosomes_. these chromosomes in a given plant or animal are always constant in number. these chromosomes are supposed to be the bearers of the qualities which we believe can be handed down from plant to plant and from animal to animal, in other words, the inheritable qualities which make the offspring like its parents. how cells form others.--cells grow to a certain size and then split into two new cells. in this process, which is of very great importance in the growth of both plants and animals, the nucleus divides first. the chromosomes also divide, each splitting lengthwise and the parts going in equal numbers to each of the two cells formed from the old cell. in this way the matter in the chromosomes is divided equally between the two new cells. then the rest of the protoplasm separates, and two new cells are formed. this process is known as _fission_. it is the usual method of growth found in the tissues of plants and animals. [illustration: stages in the division of one cell to form two. which part of the cell divides first? what seems to become of the chromosomes?] cells of various sizes and shapes.--plant cells and animal cells are of very diverse shapes and sizes. there are cells so large that they can easily be seen with the unaided eye; for example, the root hairs of plants and eggs of some animals. on the other hand, cells may be so minute, as in the case of the plant cells named bacteria, that several million might be present in a few drops of milk. the forms of cells may be extremely varied in different tissues; they may assume the form of cubes, columns, spheres, flat plates, or may be extremely irregular in shape. one kind of tissue cell, found in man, has a body so small as to be quite invisible to the naked eye, although it has a prolongation several feet in length. such are some of the cells of the nervous system of man and other large animals, as the ox, elephant, and whale. varying sizes of living things.--plant cells and animal cells may live alone, or they may form collections of cells. some plants are so simple in structure as to be formed of only one kind of cells. usually living organisms are composed of several groups of different kinds of cells. it is only necessary to call attention to the fact that such collections of cells may form organisms so tiny as to be barely visible to the eye; as, for instance, some of the small flowerless plants or many of the tiny animals living in fresh water or salt water. on the other hand, among animals, the bulk of the elephant and whale, and among plants the big trees of california, stand out as notable examples. the large plants and animals are made up of _more_, not necessarily larger, cells. what protoplasm can do.--it responds to influences or stimulation from without its own substance. both plants and animals are sensitive to touch or stimulation by light, heat or cold, certain chemical substances, gravity, and electricity. green plants turn toward the source of light. some animals are attracted to light and others repelled by it; the earthworm is an example of the latter. _protoplasm is thus said to be irritable._ _protoplasm has the power to contract and to move._ muscular movement is a familiar instance of this power. movement may also take place in plants. some plants fold up their leaves at night; others, like the sensitive plant, fold their leaflets when touched. _protoplasm can form new living matter out of food._ to do this, food materials must be absorbed into the cells of the living organism. to make protoplasm, it is evident that the same chemical elements must enter into the composition of the food substances as are found in living matter. the simplest plants and animals have this wonderful power as certainly developed as the most complex forms of life. _protoplasm, be it in plant or animal, breathes and throws off waste materials._ when a living thing does work oxygen unites with food in the body; the food is burned or _oxidized_ and work is done by means of the energy released from the food. the waste materials are _excreted_ or passed out. plants and animals alike pass off the carbon dioxide which results from the oxidation of food and of parts of their own bodies. animals eliminate wastes containing nitrogen through the skin and the kidneys. _protoplasm can reproduce, that is, form other matter like itself._ new plants are constantly appearing to take the places of those that die. the supply of living things upon the earth is not decreasing; reproduction is constantly taking place. in a general way it is possible to say that plants and animals reproduce in a very similar manner. the importance of reproduction.--reproduction is the final process that plants and animals are called upon to perform. without the formation of _new_ living things no progress would be possible on the earth. we have found that insects help flowering plants in this process. let us now see exactly what happens when pollen is placed by the bee on the stigma of another flower of the same kind. to understand this process of reproduction in flowers, we must first study carefully pollen grains from the anther of some growing flower. [illustration: pollen grains of different shapes and sizes.] pollen.--pollen grains of various flowers, when seen under the microscope, differ greatly in form and appearance. some are relatively large, some small, some rough, others smooth, some spherical, and others angular. they all agree, however, in having a thick wall, with a thin membrane under it, the whole inclosing a mass of protoplasm. at an early stage the pollen grain contains but a single cell. a little later, however, two nuclei may be found in the protoplasm. hence we know that at least two cells exist there, one of which is called the sperm cell; its nucleus is the sperm nucleus. [illustration: a pollen grain greatly magnified. two nuclei are found (_n_, _n'_) at this stage of its growth.] [illustration: three stages in the germination of the pollen grain. the nuclei in the tube in ( ) are the sperm nuclei. drawn under the compound microscope.] growth of pollen grains.--under certain conditions a pollen grain will grow or germinate. this growth can be artificially produced in the laboratory by sprinkling pollen from well-opened flowers of sweet pea or nasturtium on a solution of parts of sugar to of water. left for a few hours in a warm and moist place and then examined under the microscope, the grains of pollen will be found to have germinated, a long, threadlike mass of protoplasm growing from it into the sugar solution. the presence of this sugar solution was sufficient to induce growth. when the pollen grain germinates, the nuclei enter the threadlike growth (this growth is called the pollen tube; see figure). one of the nuclei which grows into the pollen tube is known as the _sperm nucleus_. [illustration: fertilization of the ovule. a flower cut down lengthwise (only one side shown). the pollen tube is seen entering the ovule. _a_, anther; _f_, filament; _pg_, pollen grain; _s_, stigmatic surface; _pt_, pollen tube; _st_, style; _o_, ovary; _m_, micropyle; _sp_, space within ovary; _e_, egg cell; _p_, petal; _s_, sepal.] fertilization of the flower.--if we cut the pistil of a large flower (as a lily) lengthwise, we notice that the style appears to be composed of rather spongy material in the interior; the ovary is hollow and is seen to contain a number of rounded structures which appear to grow out from the wall of the ovary. these are the _ovules_. the ovules, under certain conditions, will become _seeds_. an explanation of these conditions may be had if we examine, under the microscope, a very thin section of a pistil, on which pollen has begun to germinate. the central part of the style is found to be either hollow or composed of a soft tissue through which the pollen tube can easily grow. upon germination, the pollen tube grows downward through the spongy center of the style, follows the path of least resistance to the space within the ovary, and there enters the ovule. it is believed that some chemical influence thus attracts the pollen tube. when it reaches the ovary, the sperm cell penetrates an ovule by making its way through a little hole called the _micropyle_. it then grows toward a clear bit of protoplasm known as the _embryo sac_. the embryo sac is an ovoid space, microscopic in size, filled with semifluid protoplasm containing several nuclei. (see figure.) _one of the nuclei, with the protoplasm immediately surrounding it, is called the egg cell._ it is this cell that the sperm nucleus of the pollen tube grows toward; ultimately the sperm nucleus reaches the egg nucleus and unites with it. _the two nuclei, after coming together, unite to form a single cell. this process is known as fertilization._ this single cell formed by the union of the pollen tube cell or sperm and the egg cell is now called a _fertilized egg_. development of ovule into seed.--_the primary reason for the existence of a flower is that it may produce seeds from which future plants will grow. after fertilization the ovule grows into a seed._ the first beginning of the growth of the seed takes place at the moment of fertilization. from that time on there is a growth of the fertilized egg within the ovule which makes a baby plant called the _embryo_. _the embryo will give rise to the adult plant._ [illustration: the fruit of the locust, a bean-like fruit. _p_, the attachment to the placenta; _s_, the stigma.] a typical fruit,--the pea or bean pod.--if a withered flower of any one of the pea or bean family is examined carefully, it will be found that the pistil of the flower continues to grow after the rest of the flower withers. if we remove the pistil from such a flower and examine it carefully, we find that it is the ovary that has enlarged. the space within the ovary has become nearly filled with a number of nearly ovoid bodies, attached along one edge of the inner wall. these we recognize as the young seeds. the pod of a bean, pea, or locust illustrates well the growth from the flower. the pod, which is in reality a ripened ovary with other parts of the pistil attached to it, is considered as a _fruit_. by definition, _a fruit is a ripened ovary and its contents together with any parts of the flower that may be attached to it_. the chief use of the fruit to the flower is to hold and to protect the seeds; it may ultimately distribute them where they can reproduce young plants. [illustration: the development of an apple. notice that in this fruit additional parts besides the ovary (_o_) become part of the fruit. certain outer parts of the flower, the sepals (_s_) and receptacle, become the fleshy part of the fruit, while the ovary becomes the core. stages numbered to are in the order of development.] the necessity of fruit and seed dispersal to a plant.--we have seen that the chief reason for flowers, from the plant's standpoint, is to produce fruits which contain seeds. reproduction and the ultimate scattering of fruits and seeds are absolutely necessary in order that colonies of plants may reach new localities. it is evident that plants best fitted to scatter their seeds, or place fruits containing the seeds some little distance from the parent plants, are the ones which will spread most rapidly. a plant, if it is to advance into new territory, must get its seeds there first. plants which are best fitted to do this are the most widely distributed on the earth. how seeds and fruits are scattered.--seed dispersal is accomplished in many different ways. some plants produce enormous numbers of seeds which may or may not have special devices to aid in their scattering. most weeds are thus started "in pastures new." some prolific plants, like the milkweed, have _seeds_ with a little tuft of hairlike down which allows them to be carried by the wind. others, as the omnipresent dandelion, have their _fruits_ provided with a similar structure, the pappus. some plants, as the burdock and clotbur, have fruits provided with tiny hooks which stick to the hair of animals, thus proving a means of transportation. most fleshy fruits contain indigestible seeds, so that when the fruits are eaten by animals the seeds are passed off from the body unharmed and may, if favorably placed, grow. nuts of various kinds are often carried off by animals, buried, and forgotten, to grow later. such are a few of the ways in which seeds are scattered. all other things being equal, the plants best equipped to scatter seeds or fruits are those which will drive out other plants in a given locality. because of their adaptations they are likely to be very numerous, and when unfavorable conditions come, for that reason, if for no other, are likely to survive. such plants are best exemplified in the weeds of the grassplots and gardens. reference books elementary hunter, _laboratory problems in civic biology_. american book company. andrews, _a practical course in botany_, pages - . american book company. atkinson, _first studies of plant life_, chaps. xxv-xxvi. ginn and company. bailey, _lessons with plants_, part iii, pages - . the macmillan company. coulter, _plant life and plant uses_. american book company. dana, _plants and their children_, pages - . american book company. lubbock, _flowers, fruit, and leaves_, part i. the macmillan company. newell, _a reader in botany_, part ii, pages - . ginn and company. advanced bailey, _plant breeding_. the macmillan company. campbell, _lectures on the evolution of plants_. the macmillan company. coulter, barnes, and cowles, _a textbook of botany_, part ii. american book company. darwin, _different forms of flowers on plants of the same species_. appleton. darwin, _fertilization in the vegetable kingdom_, chaps. i and ii. appleton. darwin, _orchids fertilized by insects_. d. appleton and company. müller, _the fertilization of flowers_. the macmillan company. v. plant growth and nutrition. causes of growth _problem.--what causes a young plant to grow?_ _(a) the relation of the young plant to its food supply._ _(b) the outside conditions necessary for germination._ _(c) what the young plant does with its food supply._ _(d) how a plant or animal is able to use its food supply._ _(e) how a plant or animal prepares food to use in various parts of the body._ laboratory suggestions _laboratory exercise._--examination of bean in pod. examination and identification of parts of bean seed. _laboratory demonstration._--tests for the nutrients: starch, fats or oils, protein. _laboratory demonstration._--proof that such foods exist in bean. _home work._--test of various common foods for nutrients. tabulate results. _extra home work by selected pupils._--factors necessary for germination of bean. demonstration of experiments to class. _demonstration._--oxidation of candle in closed jar. test with lime water for products of oxidation. _demonstration._--proof that materials are oxidized within the human body. _demonstration._--oxidation takes place in growing seeds. test for oxidation products. oxygen necessary for germination. _laboratory exercise._--examination of corn on cob, the corn grain, longitudinal sections of corn grain stained with iodine to show that embryo is distinct from food supply. _demonstration._--test for grape sugar. _demonstration._--grape sugar present in growing corn grain. _demonstration._--the action of diastase on starch. conditions necessary for action of diastase. what makes a seed grow.--the general problem of the pages that follow will be to explain how the baby plant, or _embryo_, formed in the seed as the result of the fertilization of the egg cell, is able to grow into an adult plant. two sets of factors are necessary for its growth: first, the presence of food to give the young plant a start; second, certain stimulating factors outside the young plant, such as water and heat. [illustration: three views of a kidney bean, the lower one having one cotyledon removed to show the hypocotyl and plumule.] if we open a bean pod, we find the seeds lying along one edge of the pod, each attached by a little stalk to the inner wall of the ovary. if we pull a single bean from its attachment, we find that the stalk leaves a scar on the coat of the bean; this scar is called the _hilum_. the tiny hole near the hilum is called the _micropyle_. turn back to the figure (page ) showing the ovule in the ovary. find there the little hole through which the pollen tube reached the embryo sac. this hole is identical with the micropyle in the seed. the thick outer coat (the _testa_) is easily removed from a soaked bean, the delicate coat under it easily escaping notice. the seed separates into two parts; these are called the _cotyledons_. if you pull apart the cotyledons very carefully, you find certain other structures between them. the rodlike part is called the _hypocotyl_ (meaning _under the cotyledons_). this will later form the root (and part of the stem) of the young bean plant. the first true leaves, very tiny structures, are folded together between the cotyledons. that part of the plant above the cotyledons is known as the _plumule_ or _epicotyl_ (meaning _above the cotyledons_). all the parts of the seed within the seed coats together form the _embryo_ or young plant. a bean seed contains, then, a tiny _plant_ protected by a tough coat. food in the cotyledons.--the problem now before us is to find out how the embryo of the bean is adapted to grow into an adult plant. up to this stage of its existence it has had the advantage of food and protection from the parent plant. now it must begin the battle of life alone. we shall find in all our work with plants and animals that the problem of food supply is always the most important problem to be solved by the growing organism. let us see if the embryo is able to get a start in life (which many animals get in the egg) from food provided for it within its own body. organic nutrients.--organic foods (those which come from living sources) are made up of two kinds of substances, the _nutrients_ or food substances and _wastes_ or _refuse_. an egg, for example, contains the white and the yolk, composed of nutrients, and the shell, which is waste. the organic nutrients are classed in three groups. _carbohydrates_, foods which contain carbon, hydrogen, and oxygen in a certain fixed proportion (c{ }h{ }o{ } is an example). they are the simplest of these very complex chemical compounds we call organic nutrients. starch and sugar are common examples of carbohydrates. _fats and oils._--these foods are also composed of carbon, hydrogen, and oxygen in a proportion which enables them to unite readily with oxygen. _proteins._--a third group of organic foods, proteins, are the most complex of all in their composition, and have, besides carbon, oxygen, and hydrogen, the element nitrogen and minute quantities of other elements. [illustration: starch grains in the cells of a potato tuber.] test for starch.--if we boil water with a piece of laundry starch in a test tube, then cool it and add to the mixture two or three drops of iodine solution,[ ] we find that the mixture in the test tube turns purple or deep blue. it has been discovered by experiment that starch, and no _other known substance_, will be turned purple or dark blue by iodine. therefore, iodine solution has come to be used as a test for the presence of starch. footnote : iodine solution is made by simply adding a few crystals of the element iodine to per cent alcohol; or, better, take by weight gram of iodine crystals, / gram of iodide of potassium, and dilute to a dark brown color in weak alcohol ( per cent) or distilled water. [illustration: test for starch.] starch in the bean.--if we mash up a little piece of a bean cotyledon which has been previously soaked in water, and test for starch with iodine solution, the characteristic blue-black color appears, showing the presence of the starch. if a little of the stained material is mounted in water on a glass slide under the compound microscope, you will find that the starch is in the form of little ovoid bodies called _starch grains_. the starch grains and other food products are made use of by the growing plant. test for oils.--if the substance believed to contain oil is rubbed on brown paper or is placed on paper and then heated in an oven, the presence of oil will be known by a translucent spot on the paper. [illustration: test for protein.] protein in the bean.--another nutrient present in the bean cotyledon is _protein_. several tests are used to detect the presence of this nutrient. the following is one of the best known:-- place in a test tube the substance to be tested; for example, a bit of hard-boiled egg. pour over it a little strong ( per cent) nitric acid and heat gently. note the color that appears--a lemon yellow. if the egg is washed in water and a little ammonium hydrate added, the color changes to a deep orange, showing that a protein is present. if the protein is in a liquid state, its presence may be proved by heating, for when it coagulates or thickens, as does the white of an egg when boiled, protein in the form of an _albumin_ is present. another characteristic protein test easily made at home is burning the substance. if it burns with the odor of burning feathers or leather, then protein forms part of its composition.[ ] footnote : other tests somewhat more reliable, but much more delicate, are the biuret test and test with millon's reagent. a test of the cotyledon of a bean for protein food with nitric acid and ammonium hydrate shows us the presence of this food. beans are found by actual test to contain about per cent of protein, per cent of carbohydrates, and about per cent oils. the young plant within a pea or bean is thus shown to be well supplied with nourishment until it is able to take care of itself. in this respect it is somewhat like a young animal within the egg, a bird or fish, for example. beans and peas as food for man.--so much food is stored in legumes (as beans and peas) that man has come to consider them a very valuable and cheap source of food. study carefully the following table:-- nutrients furnished for ten cents in beans and peas at certain prices per pound ========================================================================= | | ten cents will pay for | prices |------------------------------------------ food materials | per | total | | | as purchased | pound | food | protein | fat |carbohydrates | |material | | | ---------------------|--------|---------|---------|--------|------------- | _cents_| _pounds_| _pounds_|_pounds_| _pounds_ kidney beans, dried | | . | . | . | . lima beans, fresh, | | | | | shelled | | . | . | -- | . lima beans, dried | | . | . | . | . string beans, fresh, | | | | | cents per peck | | . | . | . | . beans, baked, canned | | . | . | . | . lentils, dried | | . | . | . | . peas, green, in pod, | | | | | cents per peck | | . | . | . | . peas, dried | | . | . | . | . ========================================================================= [illustration: a series of early stages in the germination of the kidney bean.] germination of the bean.--if dry seeds are planted in sawdust or earth, they will not grow. a moderate supply of water must be given to them. if seeds were to be kept in a freezing temperature or at a very high temperature, no growth would take place. a moderate temperature and a moderate water supply are most favorable for their development. [illustration: bean seedlings. the older seedlings at the left have used up all of the food supply in the cotyledons.] if some beans were planted so that we might make a record of their growth, we would find the first signs of germination to be the breaking of the testa and the pushing outward of the hypocotyl to form the first root. a little later the hypocotyl begins to curve downward. a later stage shows the hypocotyl lifting the cotyledon upward. in consequence the hypocotyl forms an arch, dragging after it the bulky cotyledons. the stem, as soon as it is released from the ground, straightens out. from between the cotyledons the budlike plumule or epicotyl grows upward, forming the first true leaves and all of the stem above the cotyledons. as growth continues, we notice that the cotyledons become smaller and smaller, until their food contents are completely absorbed into the young plant. the young plant is now able to care for itself and may be said to have passed through the stages of germination. what makes an engine go.--if we examine the sawdust or soil in which the seeds are growing, we find it forced up by the growing seed. evidently work was done; in other words, _energy_ was released by the seeds. a familiar example of release of energy is seen in an engine. coal is placed in the firebox and lighted, the lower door of the furnace is then opened so as to make a draft of air which will reach the coal. you know the result. the coal burns, heat is given off, causing the water in the boiler to make steam, the engine wheels to turn, and work to be done. let us see what happens from the chemical standpoint. coal, organic matter.--coal is made largely from dead plants, long since pressed into its present hard form. it contains a large amount of a chemical element called carbon, the presence of which is characteristic of all organic material. [illustration: the limewater test. the tube at the right shows the effect of the carbon dioxide.] oxidation, its results.--when things containing carbon are lighted, they burn. if we place a lighted candle which contains carbon in a closed glass jar, the candle soon goes out. if we then carefully test the air in the jar with a substance known as _limewater_,[ ] the latter, when shaken up with the air in the jar, turns milky. this test proves the presence in the jar of a gas, known as _carbon dioxide_. this gas is formed by the carbon of the candle uniting with the oxygen in the air. when the oxygen of the air in the jar was used up, the flame went out, showing that oxygen is necessary to make a thing burn. this uniting of oxygen with some other substance is called _oxidation_. footnote : limewater can be made by shaking up a piece of quicklime the size of your fist in about two quarts of water. filter or strain the limewater into bottles and it is ready for use. [illustration: diagram to show that when a piece of wood is burned it forms water and carbon dioxide.] oxidation possible without a flame.--but a flame is not necessary for oxidation. iron, if left in a damp place, becomes rusty. a union between the oxygen in the water or air and the iron makes what is known as iron oxide or rust. this is an example of _slow oxidation_. oxidation in our bodies.--if we expel the air from our lungs through a tube into a bottle of limewater, we notice the limewater becomes milky. evidently carbon dioxide is formed in our own bodies and oxidation takes place there. is it fair to believe that the heat of our body (for example, . ° fahrenheit under the tongue) is due to oxidation within the body, and that the work we do results from this chemical process. if so, what is oxidized? energy comes from foods.--from the foregoing experiment it is evident that food is oxidized within the human body to release energy for our daily work. is it not logical to suppose that all living things, both plant and animal, release energy as the result of oxidation of foods within their cells? let us see if this is true in the case of the pea. food oxidized in germinating seeds.--if we take equal numbers of soaked peas, placed in two bottles, one tightly stoppered, the other having no stopper, both bottles being exposed to identical conditions of light, temperature, and moisture, we find that the seeds in both bottles start to germinate, but that those in the closed bottle soon stop, while those in the open jar continue to grow almost as well as similar seeds placed in an open dish would. [illustration: experiment that shows the necessity for air in germination.] why did not the seeds in the covered jar germinate? to answer this question, let us carefully remove the stopper from the stoppered jar and insert a lighted candle. the candle goes out at once. the surer test of limewater shows the presence of carbon dioxide in the jar. the carbon of the foodstuffs of the pea united with the oxygen of the air, forming carbon dioxide. growth stopped as soon as the oxygen was exhausted. the presence of carbon dioxide in the jar is an indication that a very important process which we associate with animals rather than plants, that of _respiration_, is taking place. the seed, in order to release the energy locked up in its food supply, must have oxygen, so that the oxidation of the food may take place. _hence a constant supply of fresh air is an important factor in germination._ it is important that air should penetrate between the grains of soil around a seed. the frequent stirring of the soil enables the air to reach the seed. air also acts upon some materials in the soil and puts them in a form that the germinating seed can use. this necessity for oxygen shows us at least one reason why the farmer plows and harrows a field and one important use of the earthworm. explain. [illustration: a grain of corn cut lengthwise. _c_, cotyledon; _e_, endosperm; _h_, hypocotyl; _p_, plumule.] structure of a grain of corn.--examination of a well-soaked grain of corn discloses a difference in the two flat sides of the grain. a light-colored area found on one surface marks the position of the embryo; the rest of the grain contains the food supply. the interesting thing to remember here is that the food supply is _outside_ of the embryo. a grain cut lengthwise perpendicular to the flat side and then dipped in weak iodine shows two distinct parts, an area containing considerable starch, the _endosperm_, and the embryo or young plant. careful inspection shows the hypocotyl and plumule (the latter pointing toward the free end of the grain) and a part surrounding them, the _single_ cotyledon (see figure). here again we have an example of a fitting for future needs, for in this fruit the one seed has at hand all the food material necessary for rapid growth, although the food is here outside the embryo. [illustration: longitudinal section of young ear of corn. _o_, the fruits; _s_, the stigmas; _sh_, the sheath-like leaves; _st_, the flower stalk. (after sargent.)] endosperm the food supply of corn.--we find that the one cotyledon of the corn grain does not serve the same purpose to the young plant as do the two cotyledons of the bean. although we find a little starch in the corn cotyledon, still it is evident from our tests that the endosperm is the chief source of food supply. the study of a thin section of the corn grain under the compound microscope shows us that the starch grains in the endosperm are large and regular in size. when the grain has begun to grow, examination shows that the starch grains near the edge of the cotyledon are much smaller and quite irregular, having large holes in them. we know that the germinating grain has a much sweeter taste than that which is not growing. this is noticed in sprouting barley or malt. we shall later find that, in order to make use of starchy food, a plant or animal must in some manner change it over to sugar. this change is necessary, because starch will not dissolve in water, while sugar will; in this form substances can pass from cell to cell in the plant and thus distribute the food where it is needed. [illustration: test for grape sugar.] a test for grape sugar.--place in a test tube the substance to be tested and heat it in a little water so as to dissolve the sugar. add to the fluid twice its bulk of fehling's solution,[ ] which has been previously prepared. heat the mixture, which should now have a blue color, in the test tube. if grape sugar is present in considerable quantity, the contents of the tube will turn first a greenish, then yellow, and finally a brick-red color. smaller amounts will show less decided red. no other substance than sugar will give this reaction. if benedict's test[ ] is used, a colored precipitate will appear in the test tube after boiling. footnotes and : directions for making these solutions will be found in hunter's _laboratory problems in civic biology_. starch changed to grape sugar in the corn.--that starch is being changed to grape sugar in the germinating corn grain can easily be shown if we cut lengthwise through the embryos of half a dozen grains of corn that have just begun to germinate, place them in a test tube with some fehling's solution, and heat almost to the boiling point. they will be found to give a reaction showing the presence of sugar along the edge of the cotyledon and between it and the endosperm. digestion.--this change of starch to grape sugar in the corn is a process of _digestion_. if you chew a bit of unsweetened cracker in the mouth for a little time, it will begin to taste sweet, and if the chewed cracker, which we know contains starch, is tested with fehling's solution, some of the starch will be found to have changed to grape sugar. here, again, a process of digestion has taken place. in both the corn and in the mouth, the change is brought about by the action of peculiar substances known as digestive ferments, or _enzymes_. such substances have the power under certain conditions to change insoluble foods--solids--into soluble substances--liquids. the result is that substances which before digestion would not dissolve in water now will dissolve. [illustration: a germinating corn grain. _c_, cotyledon; _h_, growing root (_hypocotyl_); _p_, growing stem (_plumule_); _s_, endosperm; _d.s._, digested starch; _p.r._, primary root; _s.r._, secondary root; _r.h._, root hairs.] the action of diastase on starch.--the enzyme found in the cotyledon of the corn, which changes starch to grape sugar, is called _diastase_. it may be separated from the cotyledon and used in the form of a powder. to a little starch in half a cup of water we add a very little ( gram) of diastase and put the vessel containing the mixture in a warm place, where the temperature will remain nearly constant at about ° fahrenheit. on testing part of the contents at the end of half an hour, and the remainder the next morning, for starch and for grape sugar, we find from the morning test that the starch has been almost completely changed to grape sugar. starch and warm water alone under similar conditions will not react to the test for grape sugar. digestion has the same purpose in plants and animals.--in our own bodies we know that solid foods taken into the mouth are broken up by the teeth and moistened by saliva. if we could follow that food, we would find that eventually it became part of the blood. it was made soluble by digestion, and in a liquid form was able to reach the blood. once a part of the body, the food is used either to release energy or to build up the body. summary.--we have seen: . that seeds, in order to grow, must possess a food supply either in or around their bodies. . that this food supply must be oxidized before energy is released. . that in cases where the food is not stored at the point where it is to be oxidized the food must be digested so that it may be transported from one part to another in the same plant. the life processes of plants and animals, so far, may be considered as alike; they both feed, breathe (oxidize their food), do work, and grow. reference books elementary hunter, _laboratory problems in civic biology_. american book company. andrews, _a practical course in botany_, pages - . american book company. atkinson, _first studies of plant life_, chap. xxx. ginn and company. bailey, _botany_, chaps. xx, xxx. the macmillan company. beal, _seed dispersal_. ginn and company. bergen and davis, _principles of botany_, chaps. xx, xxx. ginn and company. coulter, _plant life and plant uses_. american book company. dana, _plants and their children_. american book company. mayne and hatch, _high school agriculture_. american book company. lubbock, _flowers, fruits, and leaves_. the macmillan company. newell, _reader in botany_, pages - . ginn and company. sharpe, _a laboratory manual in biology_, pages - . american book company. advanced bailey, _the evolution of our native fruits_. the macmillan company. bailey, _plant breeding_. the macmillan company. coulter, barnes, and cowles, _a textbook of botany_, vol. i. american book company. de candolle, _origin of cultivated plants_. d. appleton and company. duggar, _plant physiology_. the macmillan company. farmers' bulletins, nos. , , , . u. s. department of agriculture. hodge, _nature study and life_, chaps. x, xx. ginn and company. kerner (translated by oliver), _natural history of plants_. henry holt and company, vols. vol. ii, part . sargent, _corn plants_. houghton, mifflin, and company. vi. the organs of nutrition in plants--the soil and its relation to the roots _problem.--what a plant takes from the soil and how it gets it._ _(a) what determines the direction of growth of roots?_ _(b) how is the root built?_ _(d) what is in the soil that a root might take out?_ _(e) why is nitrogen necessary, and how is it obtained?_ laboratory suggestions _demonstration_.--roots of bean or pea. _demonstration or home experiment_.--response of root to gravity and to water. what part of root is most responsive? _laboratory work_.--root hairs, radish or corn, position on root, gross structure only. drawing. _demonstration._--root hair under compound microscope. _demonstration._--apparatus illustrating osmosis. _demonstration or a home experiment._--organic matter present in soil. _demonstration._--root tubercles of legume. _demonstration._--nutrients present in some roots. uses of the root.--if one of the seedlings of the bean spoken of in the last chapter is allowed to grow in sawdust and is given light, air, and water, sooner or later it will die. soil is part of its natural environment, and the roots which come in contact with the soil are very important. it is the purpose of this chapter to find out just how the young plant is fitted to get what it needs from this part of its environment; namely, the soil. the development of a bean seedling has shown us that the root grows first. _one of the most important functions of the root to a young seed plant is that of a holdfast, an anchor to fasten it in the place where it is to develop._ it has many other uses, as the taking in of water with the mineral and organic matter dissolved therein, the storage of food, climbing, etc. all functions other than the first one stated arise after the young plant has begun to develop. [illustration: a root system, showing primary and secondary roots.] root system.--if you dig up a young bean seedling and carefully wash the dirt from the roots, you will see that a long root is developed as a continuation of the hypocotyl. this root is called the _primary_ root. other smaller roots which grow from the primary root are called _secondary_, or _tertiary_, depending on their relation to the first root developed. downward growth of root. influence of gravity.--most of the roots examined take a more or less downward direction. we are all familiar with the fact that the force we call gravity influences life upon this earth to a great degree. does gravity act on the growing root? this question may be answered by a simple experiment. [illustration: revolve this figure in the direction of the arrows to see if the roots of the radish respond to gravity.] plant mustard or radish seeds in a pocket garden, place it on one edge and allow the seeds to germinate until the root has grown to a length of about half an inch. then turn it at right angles to the first position and allow it to remain for one day undisturbed. the roots now will be found to have turned in response to the change in position, that part of the root near the growing point being the most sensitive to the change. this experiment seems to indicate that the roots are influenced to grow downward by the force of gravity. experiments to determine the influence of moisture on a growing root.--the objection might well be interposed that possibly the roots in the pocket garden[ ] grew downward after water. that moisture has an influence on the growing root is easily proved. footnote : _the pocket garden._--a very convenient form of pocket germinator may be made as follows. obtain two cleaned four by five negatives (window glass will do); place one flat on the table and place on this half a dozen pieces of colored blotting paper cut to a size a little less than the glass. now cut four thin strips of wood to fit on the glass just outside of the paper. next moisten the blotter, place on it some well-soaked radish, mustard seeds or barley grains, and cover with the other glass. the whole box thus made should be bound together with bicycle tape. seeds will germinate in this box and with care may live for two weeks or more. plant bird seed, mustard or radish seed in the underside of a sponge, which should be kept wet, and may be suspended by a string under a bell jar in the schoolroom window. note whether the roots leave the sponge to grow downward, or if the moisture in the sponge is sufficient to counterbalance the force of gravity. water a factor which determines the course taken by roots.--_water, as well as the force of gravity, has much to do with the direction taken by roots._ water is always found below the surface of the ground, but sometimes at a great depth. most trees, and all grasses, have a greater area of surface exposed by the roots than by the branches. the roots of alfalfa, a cloverlike plant used for hay in the western states, often penetrate the soil after water for a distance of ten to twenty feet below the surface of the ground. fine structure of a root.[ ]--when we examine a delicate root in thin longitudinal section under the compound microscope, we find the entire root to be made up of cells, the walls of which are uniformly rather thin. over the lower end of the root is found a collection of cells, most of which are dead, loosely arranged so as to form a cap over the growing tip. this is evidently an adaptation which protects the young and actively growing cells just under the root cap. in the body of the root a central cylinder can easily be distinguished from the surrounding cells. in a longitudinal section a series of tubelike structures may be found within the central cylinder. these structures are cells which have grown together at the small end, the long axis of the cells running the length of the main root. in their development the cells mentioned have grown together in such a manner as to lose their small ends, and now form continuous hollow tubes with rather strong walls. other cells have come to develop greatly thickened walls; these cells give mechanical support to the tubelike cells. collections of such tubes and supporting woody cells together make up what are known as _fibrovascular bundles_. footnote : sections of tradescantia roots are excellent for demonstration of these structures. [illustration: cross section of a young taproot; _a_, _a_, root hairs; _b_, outer layer of bark; _c_, inner layer of bark; _d_, wood or central cylinder.] root hairs.--careful examination of the root of one of the seedlings of mustard, radish, or barley grown in the pocket germinator shows a covering of tiny fuzzy structures. these structures are very minute, at most to millimeters in length. they vary in length according to their position on the root, the most and the longest root hairs being found near the point marked _r. h._ in the figure. these structures are outgrowths of the outer layer of the root (the _epidermis_), and are of very great importance to the living plant. [illustration: young embryo of corn, showing root hairs (_r. h._) and growing stem (_p._).] structure of a root hair.--a single root hair examined under a compound microscope will be found to be a long, round structure, almost colorless in appearance. the wall, which is very flexible and thin, is made up of cellulose, a substance somewhat like wood in chemical composition, through which fluids may easily pass. clinging close to the cell wall is the protoplasm of the cell. the interior of the root hair is more or less filled with a fluid called _cell sap_. forming a part of the living protoplasm of the root hair, sometimes in the hairlike prolongation and sometimes in that part of the cell which forms the epidermis, is found a _nucleus_. the protoplasm and nucleus are alive; the cell wall formed by the living matter in the cell is dead. _the root hair is a living plant cell_ with a wall so delicate that water and mineral substances from the soil can pass through it into the interior of the root. [illustration: diagram of a root hair; _cs_, cell sap; _cw_, cell wall; _p_, protoplasm; _n_, nucleus; _s_, particles of soil.] how the root absorbs water.--the process by which the root hair takes up soil water can better be understood if we make an artificial root hair large enough to be easily seen. an egg with part of the outer shell removed so as to expose the soft skinlike membrane underneath is an example. better, an artificial root hair may be _made_ in the following way. pour some soft celloidin into a test tube; carefully revolve the test tube so that an even film of celloidin dries on the inside. this membrane is removed, filled with white of egg, and tied over the end of a rubber cork in which a glass tube has previously been inserted. when placed in water, it gives a very accurate picture of the root hair at work. after a short time water begins to rise in the tube, having passed through the film of celloidin. if grape sugar, salt, or some other substance which will dissolve in water were placed in the water outside the artificial root hair, it could soon be proved by test to pass through the wall and into the liquid inside. osmosis.--to explain this process we must remember that gases and liquids of different densities, when separated by a membrane, tend to flow toward each other and mingle, the greater flow always being in the direction of the denser medium. _the process by which two gases or fluids, separated by a membrane, tend to pass through the membrane and mingle with each other, is called osmosis._ the method by which the root hairs take up soil water is exactly the same process. it is by osmosis. the white of the egg is the best possible substitute for living matter; the celloidin membrane separating the egg from the water is much like the delicate membrane-like wall which separates the protoplasm of the root hair from the water in the soil surrounding it. the fluid in the root hair is denser than the soil water; hence the greater flow is toward the interior of the root hair.[ ] footnote : for an excellent elementary discussion of osmosis see moore, _physiology of man and other animals_. henry holt and company. [illustration: the soil particles are each surrounded with a delicate film of water. how might the root hairs take up this water?] passage of soil water within the root.--we have already seen that in an exchange of fluids by osmosis the greater flow is always toward the denser fluid. thus it is that the root hairs take in more fluid than they give up. the cell sap, which partly fills the interior of the root hair, is a fluid of greater density than the water outside in the soil. when the root hairs become filled with water, the density of the cell sap is lessened, and the cells of the epidermis are thus in a position to pass along their supply of water to the cells next to them and nearer to the center of the root. these cells, in turn, become less dense than their inside neighbors, and so the transfer of water goes on until the water at last reaches the central cylinder. here it is passed over to the tubes of the woody bundles and started up the stem. the pressure created by this process of osmosis is sufficient to send water up the stem to a distance, in some plants, of to feet. cases are on record of water having been raised in the birch a distance of feet. physiological importance of osmosis.--it is not an exaggeration to say that osmosis is a process not only of great importance to a plant, but to an animal as well. foods are digested in the food tube of an animal; that is, they are changed into a soluble form so that they may pass through the walls of the food tube and become part of the blood. the inner lining of part of the food tube is thrown into millions of little fingerlike projections which look somewhat, in size at least, like root hairs. these fingerlike processes are (unlike a root hair) made up of many cells. but they serve the same purpose as the root hairs, for they absorb liquid food into the blood. this process of absorption is largely by osmosis. without the process of osmosis we should be unable to use much of the food we eat. composition of soil.--if we examine a mass of ordinary loam carefully, we find that it is composed of numerous particles of varying size and weight. between these particles, if the soil is not caked and hard packed, we can find tiny spaces. in well-tilled soil these spaces are constantly being formed and enlarged. they allow air and water to penetrate the soil. if we examine soil under the microscope, we find considerable water clinging to the soil particles and forming a delicate film around each particle. in this manner most of the water is held in the soil. [illustration: inorganic soil is being formed by weathering.] how water is held in soil.--to understand what comes in with the soil water, it will be necessary to find out a little more about soil. scientists who have made the subject of the composition of the earth a study, tell us that once upon a time at least a part of the earth was molten. later, it cooled into solid rock. soil making began when the ice and frost, working alternately with the heat, chipped off pieces of rock. these pieces in time became ground into fragments by action of ice, glaciers, running water, or the atmosphere. this process is called weathering. weathering is aided by oxidation. a glance at almost any crumbling stones will convince you of this, because of the yellow oxide of iron (rust) disclosed. so by slow degrees this earth became covered with a coating of what we call inorganic soil. later, generation after generation of tiny plants and animals which lived in the soil died, and their remains formed the first organic materials of the soil. [illustration: this picture shows how the forests help to cover the inorganic soil with an organic coating. explain how.] you are all familiar with the difference between the so-called rich soil and poor soil. the dark soil contains more dead plant and animal matter, which forms the portion called _humus_. [illustration: apparatus for testing the capacity of soils to take in and retain moisture.] humus contains organic matter.--it is an easy matter to prove that black soil contains organic matter, for if an equal weight of carefully dried humus and soil from a sandy road is heated red-hot for some time and then reweighed, the humus will be found to have lost considerably in weight, and the sandy soil to have lost very little. the material left after heating is inorganic material, the organic matter having been burned out. soil containing organic materials holds water much more readily than inorganic soil, as a glance at the accompanying figure shows. if we fill each of the vessels with a given weight (say grams each) of gravel, sand, barren soil, rich loam, leaf mold, and grams of dry, pulverized leaves, then pour equal amounts of water ( c.c.) on each and measure all that runs through, the water that has been retained will represent the water supply that plants could draw on from such soil. [illustration: soil particles cling to root hairs. why?] the root hairs take more than water out of the soil.--if a root containing a fringe of root hairs is washed carefully, it will be found to have little particles of soil still clinging to it. examined under the microscope, these particles of soil seem to be cemented to the sticky surface of the root hair. the soil contains, besides a number of chemical compounds of various mineral substances,--lime, potash, iron, silica, and many others,--a considerable amount of organic material. acids of various kinds are present in the soil. these acids so act upon certain of the mineral substances that they become dissolved in the water which is absorbed by the root hairs. root hairs also give off small amounts of acid. an interesting experiment may be shown (see figure on page ) to prove this. a solution of _phenolphthalein_ loses its color when an acid is added to it. if a growing pea be placed in a tube containing some of this solution the latter will quickly change from a rose pink to a colorless solution. a plant needs mineral matter to make living matter.--living matter (protoplasm), besides containing the chemical elements carbon, hydrogen, oxygen, and nitrogen, contains a very minute proportion of various elements which make up the basis of certain minerals. these are calcium (lime), sulphur, iron, potassium, magnesium, phosphorus, sodium, and chlorine. that plants will not grow well without certain of these mineral substances can be proved by the growth of seedlings in a so-called nutrient solution.[ ] such a solution contains all the mineral matter that a plant uses for food. if certain ingredients are left out of this solution, the plants placed in it will not live. footnote : see hunter's _laboratory problems in civic biology_ for list of ingredients. [illustration: effect of root hairs on phenolphthalein solution. the change of color indicates the presence of acid.] nitrogen in a usable form necessary for growth of plants.--a chemical element needed by the plant to make protoplasm is nitrogen. the air can be proven by experiment to be made up of about four fifths nitrogen, but this element cannot be taken from either soil, water, or air in a pure state, but is usually obtained from the organic matter in the soil, where it exists with other substances in the form of _nitrates_. ammonia and other organic compounds which contain nitrogen are changed by two groups of little plants called _bacteria_, first into nitrites and then nitrates.[ ] footnote : it has recently been discovered that under some conditions these bacteria are preyed upon by tiny one-celled animals (_protozoa_) living in the soil and are so reduced in numbers that they cannot do their work effectively. if, then, the soil is heated artificially or treated with antiseptics so as to kill the protozoa, the bacteria which escape multiply so rapidly as to make the land much richer than before. [illustration: diagram to show how the nitrogen-fixing bacteria prepare nitrogen for use by plants; _t_, tubercles.] relation of bacteria to free nitrogen.--it has been known since the time of the romans that the growth of clover, peas, beans, and other legumes in soil causes it to become more favorable for growth of other plants. the reason for this has been discovered in late years. on the roots of the plants mentioned are found little swellings or nodules; in the nodules exist millions of bacteria, which take nitrogen from the atmosphere and fix it so that it can be used by the plant; that is, they assist in forming nitrates for the plants to use. only these bacteria, of all the living plants, have the power to take the free nitrogen from the air and make it over into a form that can be used by the roots. as all the compounds of nitrogen are used over and over again, first by plants, then as food for animals, eventually returning to the soil again, or in part being turned into free nitrogen, it is evident that any _new_ supply of usable nitrogen must come by means of these nitrogen-fixing bacteria. rotation of crops.--the facts mentioned above are made use of by careful farmers who wish to make as much as possible from a given area of ground in a given time. such plants as are hosts for the nitrogen-fixing bacteria are planted early in the season. later these plants are plowed in and a second crop is planted. the latter grows quickly and luxuriantly because of the nitrates left in the soil by the bacteria which lived with the first crop. for this reason, clover is often grown on land in which it is proposed to plant corn, the nitrogen left in the soil thus giving nourishment to the young corn plants. in scientifically managed farms, different crops are planted in a given field on different years so that one crop may replace some of the elements taken from the soil by the previous crop. this is known as rotation of crops.[ ] the annual yield of the average farm may thus be greatly increased. footnote : that crop rotation is not primarily a process to conserve the fertility of the soil, but is a sanitary measure to prevent infection of the soil, is the latest belief of the scientist. [illustration: nitrogen in the soil is necessary for plants. explain from this diagram how nitrogen is put into the soil by some plants and taken out by others.] five of the elements necessary to the life of the plant which may be taken out of the soil by constant use are calcium, nitrogen, phosphorus, potassium, and sulphur. several methods are used by the farmer to prevent the exhaustion of these and other raw food materials from the soil. one method known as _fallowing_ is to allow the soil to remain idle until bacteria and oxidation have renewed the chemical materials used by the plants. this is an expensive method, if land is dear. the most common method of enriching soil is by means of fertilizing material rich in plant food. manure is most frequently used, but many artificial fertilizers, most of which contain nitrogen in the form of some nitrate, are used, because they can be more easily transported and sold. such are ground bone, guano (bird manure), nitrate of soda, and many others. these also contain other important raw food materials for plants, especially potash and phosphoric acid. both of these substances are made soluble so as to be taken into the roots by the action of the carbon dioxide in the soil. the indirect relation of this to the city dweller.--all of us living in the city are aware of the importance of fresh vegetables, brought in from the neighboring market gardens. but we sometimes forget that our great staple crops, wheat and other cereals, potatoes, fruits of all kinds, our cotton crop, and all plants we make use of grow directly in proportion to the amount of raw food materials they take in through the roots. when we also remember that many industries within the cities, as mills, bakeries, and the like, as well as the earnings of our railways and steamship lines, are largely dependent on the abundance of the crops, we may recognize the importance of what we have read in this chapter. food storage in roots of commercial importance.--some plants, as the parsnip, carrot, and radish, produce no seed until the second year, storing food in the roots the first year and using it to get an early start the following spring, so as to be better able to produce seeds when the time comes. this food storage in roots is of much practical value to mankind. many of our commonest garden vegetables, as those mentioned above, and the beet, turnip, oyster plant, sweet potato and many others, are of value because of the food stored. the sugar beet has, in europe especially, become the basis of a great industry. reference books elementary hunter, _laboratory problems in civic biology_. american book company. bigelow, _applied biology_. the macmillan company. coulter, _plant life and plant uses_, chaps. iii, iv. american book company. mayne and hatch, _high school agriculture_. american book company. moore, _the physiology of man and other animals_. henry holt and company. sharpe, _laboratory manual in biology_, pp. - . american book company. advanced coulter, barnes, and cowles, _a textbook of botany_, part ii. amer. book co. duggar, _plant physiology_. the macmillan company. goodale, _physiological botany_. american book company. green, _vegetable physiology_, chaps. v, vi. j. and a. churchill. kerner-oliver, _natural history of plants_. henry holt and company. macdougal, _plant physiology_. longmans, green, and company. vii. plant growth and nutrition--plants make food _problem.--where, when, and how do green plants make food?_ _(a) how and why is moisture given off from leaves?_ _(b) what is the reaction of leaves to light?_ _(c) what is made in green leaves in the sunlight?_ _(d) what by-products are given off in the above process?_ _(e) other functions of leaves._ laboratory suggestions _demonstration._--water given off by plant in sunlight. loss of weight due to transpiration measured. _laboratory exercise._-- (_a_) gross structure of a leaf. (_b_) study of stoma and lower epidermis under microscope. (_c_) study of cross section to show cells and air spaces. _demonstration._--reaction of leaves to light. _demonstration._--light necessary to starch making. _demonstration._--air necessary to starch making. _demonstration._--oxygen a by-product of starch making. what becomes of the water taken in by the roots?--we have seen that more than pure water has been absorbed through the root hairs into the roots. what becomes of this water and the other substances that have been absorbed? this question may be partly answered by the following experiments. [illustration: apple twigs split to show the course of colored water up the stem.] passage of fluids up the stem.--if any young growing shoots (young seedlings of corn or pea, or the older stems of garden balsam, touch-me-not, or sunflower) are placed in red ink (eosin), and left in the sun for a few hours, the red ink will be found to have passed up the stem. if such stems were examined carefully, it would be seen that the colored fluid is confined to collections of woody tubes immediately under the inner bark. water evidently rises in that part of the stem we call the wood. [illustration: experiment to prove that water is given off through the leaves of a green plant.] water given off by evaporation from leaves.--take some well-watered potted green plant, as a geranium or hydrangea, cover the pot with sheet rubber, fastening the rubber close to the stem of the plant. next weigh the plant with the pot. then cover it with a tall bell jar and place the apparatus in the sun. in a few minutes drops of moisture are seen to gather on the _inside_ of the jar. if we now weigh the potted plant, we find it weighs less than before. obviously the loss comes from the water lost, and evidently this water escapes as vapor from either the stem or leaves. [illustration: the skeleton of a leaf. _m.r._, the midrib; _p._, the leafstalk; _v._, the veins.] the structure of a leaf.--in the experiment with the red ink mentioned above we will find that the fluid has gone out into the skeleton or framework of the leaf. let us now examine a leaf more carefully. it shows usually ( ) a flat, broad _blade_, which may take almost any conceivable shape; ( ) a _stem_ which spreads out in the blade ( ) in a number of _veins_. [illustration: section through the blade of a leaf as seen under the compound microscope. _s_, air spaces, which communicate with the outside air; _v_, vein in cross section; _s.t._, breathing hole (stoma); _e_, outer layer of cells; _p_, green cells.] the cell structure of a leaf.--the under surface of a leaf seen under the microscope usually shows numbers of tiny oval openings. these are called _stomata_ (singular _stoma_). two cells, usually kidney-shaped, are found, one on each side of the opening. these are the _guard cells_. by change in shape of these cells the opening of the stoma is made larger or smaller. larger irregular cells form the _epidermis_, or outer covering of the leaf. study of the leaf in cross section shows that these stomata open directly into air chambers which penetrate between and around the loosely arranged cells composing the underpart of the leaf. the upper surface of leaves sometimes contains stomata, but more often they are lacking. the under surface of an oak leaf of ordinary size contains about , , stomata. under the upper epidermis is a layer of green cells closely packed together (called collectively the _palisade layer_). these cells are more or less columnar in shape. under these are several rows of rather loosely placed cells just mentioned. these are called collectively the _spongy tissue_. if we happen to have a section cut through a vein, we find this composed of a number of tubes made up of, and strengthened by, thick-walled cells. the veins are evidently a continuation of the tubes of the stem out into the blade of the leaf. evaporation of water.--during the day an enormous amount of water is taken up by the roots and passed out through the leaves. so great is this excess at times that a small grass plant on a summer's day evaporates more than its own weight in water. this would make nearly half a ton of water delivered to the air during twenty-four hours by a grass plot twenty-five by one hundred feet, the size of the average city lot. according to ward, an oak tree may pass off two hundred and twenty-six times its own weight in water during the season from june to october. from which surface of the leaf is water lost?--in order to find out whether water is passed out from any particular part of the leaf, we may remove two leaves of the same size and weight from some large-leaved plant[ ]--a mullein was used for the illustrations given below--and cover the upper surface of one leaf and the lower surface of the other with vaseline. the leaf stalks of each should be covered with wax or vaseline, and the two leaves exactly balanced on the pans of a balance which has previously been placed in a warm and sunny place. within an hour the leaf which has the upper surface covered with vaseline will show a loss of weight. examination of the surface of a mullein leaf shows us that the _lower surface of the leaf is provided with stomata_. it is through these organs, then, that water is passed out from the tissues of the leaf. footnote : the "rubber plant" leaf is an easily obtainable and excellent demonstration. [illustration: experiment to show through which surface of a leaf water passes off.] factors in transpiration.--the amount of water lost from a plant varies greatly under different conditions. the humidity of the air, its temperature, and the temperature of the plant all affect the rate of transpiration. the stomata also tend to close under some conditions, thus helping to prevent evaporation. but there seems to be no certain regulation of this water loss. consequently plants droop or wilt on hot dry days because they cannot obtain water rapidly enough from the soil to make up for the loss through the leaves. [illustration: diagrams of a stoma. _a_, surface view of a closed stoma; _b_, the same stoma opened. (after hanson.) _c_, diagrams of a transverse section through a stoma, dotted lines indicate the closed position of the guard cells, the heavy lines the open condition. (after schwendener.)] green plants food makers.--we have previously stated that green plants are the great food makers for themselves and for animals. we are now ready to attack the problem of how green plants _make_ food. the sun a source of energy.--we all know the sun is a source of most of the energy that is released on this earth in the form of heat or light. every boy knows the power of a "burning glass." solar engines have not come into any great use as yet, because fuel is cheaper, but some day we undoubtedly will directly harness the energy of the sun in everyday work. actual experiments have shown that vast amounts of energy are given to the earth. when the sun is highest in the sky, energy equivalent to one hundred horse power is received by a plot of land twenty-five by one hundred feet, the size of a city lot. plants receive and use much of this energy by means of their leaves. effect of light on plants.--in young plants which have been grown in total darkness, no green color is found in either stems or leaves, the latter often being reduced to mere scales. the stems are long and more or less reclining. we can explain the changed condition of the seedling grown in the dark only by assuming that light has some effect on the protoplasm of the seedling and induces the growth of the green part of the plant. if seedlings have been growing on a window sill, or where the light comes in from one side, you have doubtless noticed that the stem and leaves of the seedlings incline in the direction from which the light comes. the experiment pictured shows this effect of light very plainly. a hole was cut in one end of a cigar box and barriers were erected in the interior of the box so that the seeds planted in the sawdust received their light by an indirect course. the young seedling in this case responded to the influence of the stimulus of light so as to grow out finally through the hole in the box into the open air. this growth of the stem to the light is of very great importance to a growing plant, because, as we shall see later, food making depends largely on the amount of sunlight the leaves receive. [illustration: two stages in an experiment to show that green plants grow toward the light.] effect of light on leaf arrangement.--it is a matter of common knowledge that green leaves turn toward the light. place growing pea seedlings, oxalis, or any other plants of rapid growth near a window which receives full sunlight. within a short time the leaves are found to be in positions to receive the most sunlight possible. careful observation of any plant growing outdoors shows us that in almost every case the leaves are so disposed as to get much sunlight. the ivy climbing up the wall, the morning-glory, the dandelion, and the burdock all show different arrangements of leaves, each presenting a large surface to the light. leaves are often definitely arranged, fitting in between one another so as to present their upper surface to the sun. such an arrangement is known as a _leaf mosaic_. in the case of the dandelion, a _rosette_ or whorled cluster of leaves is found. in the horse-chestnut, where the leaves come out opposite each other, the older leaves have longer petioles than the young ones. in the mullein the entire plant forms a cone. the old leaves near the bottom have long stalks, and the little ones near the apex come out close to the main stalk. in every case each leaf receives a large amount of light. other modifications of these forms may easily be found on any field trip. [illustration: a lily, showing long narrow leaves.] [illustration: the dandelion, showing a whorled arrangement of long irregular leaves.] starch made by a green leaf.--if we examine the palisade layer of the leaf, we find cells which are almost cylindrical in form. in the protoplasm of such cells are found a number of little green-colored bodies, which are known as _chloroplasts_ or _chlorophyll bodies_. if we place the leaf in wood alcohol, we find that the bodies still remain, but that the color is extracted, going into the alcohol and giving to it a beautiful green color. the chloroplasts are, indeed, simply part of the protoplasm of the cell colored green. these bodies are of the greatest importance directly to plants and indirectly to animals. _the chloroplasts, by means of the energy received from the sun, manufacture starch out of certain raw materials._ these raw materials are soil water, which is passed up through the bundles of tubes into the veins of the leaf from the roots, and carbon dioxide, which is taken in through the stomata or pores, which dot the under surface of the leaf. a plant with variegated leaves, as the coleus, makes starch only in the green part of the leaf, even though these raw materials reach all parts of the leaf. [illustration: an experiment to show the effect of excluding light (but not air) from the leaves of a green plant. the result of this experiment is seen in the next picture. (experiment performed by c. dobbins and a. schwartz.)] [illustration: starchless area in a leaf caused by excluding sunlight by means of a strip of black cloth.] light and air necessary for starch making.--if we pin strips of black cloth, such as alpaca, over some of the leaves of a growing hydrangea which has previously been placed in a dark room for a few hours, and then put the plant in direct sunlight for an hour or two, we are ready to test for starch. we then remove some of the covered leaves and extract the chlorophyll with wood alcohol (because the green color of the chlorophyll interferes with the blue color of the starch test). a test then shows that starch is present only in the portions of the leaves exposed to sunlight. from this experiment we infer that the sun has something to do with starch making in a leaf. the necessity of a part of the air (carbon dioxide) for starch making may also easily be proved, for the parts of leaves covered with vaseline will be found to contain no starch, while parts of the leaf without vaseline, but exposed to the sun and air, do contain starch. [illustration: diagram to show starch making. read the text carefully and then explain this diagram.] air is necessary for the process of starch making in a leaf, not only because carbon dioxide gas is absorbed (there are from three to four parts in ten thousand present in the atmosphere), but also because the leaf is alive and must have oxygen in order to do work. this oxygen it takes from the air around it. [illustration: diagram to illustrate the formation of starch in a leaf.] comparison of starch making and milling.--the manufacture of starch by the green leaf is not easily understood. the process has been compared to the milling of grain. in this case the mill is the green part of the leaf. the sun furnishes the motive power, the chloroplasts constitute the machinery, and soil water and carbon dioxide are the raw products taken into the mill. the manufactured product is starch, and a certain by-product (corresponding to the waste in a mill) is also given out. this by-product is oxygen. to understand the process fully, we must refer to a small portion of the leaf shown below. here we find that the cells of the green layer of the leaf, under the upper epidermis, perform most of the work. the carbon dioxide is taken in through the stomata and reaches the green cells by way of the intercellular spaces and by osmosis from cell to cell. water reaches the green cells through the veins. it then passes into the cells by osmosis, and there becomes part of the cell sap. the light of the sun easily penetrates to the cells of the palisade layer, giving the energy needed to make the starch. this whole process is a very delicate one, and will take place only when external conditions are favorable. for example, too much heat or too little heat stops starch making in the leaf. this building up of food and the release of oxygen by the plant in the presence of sunlight is called _photosynthesis_. [illustration: diagram (after stevens) to illustrate the processes of breathing and food making in the cells of a green leaf in the sunlight.] manufacture of fats.--inasmuch as tiny droplets of oil are found _inside_ the chlorophyll bodies in the leaf, we believe that fats, too, are made there, probably by a transformation of the starch already manufactured. protein making and its relation to the making of living matter.--protein material is a food which is necessary to form protoplasm. protein food is present in the leaf, and is found in the stem or root as well. proteins can apparently be manufactured in any of the cells of green plants, the presence of light not seeming to be a necessary factor. how it is manufactured is a matter of conjecture. the minerals brought up in the soil water form part of its composition, and starch or grape sugar give three elements (c, h, and o). the element nitrogen is taken up by the roots as a nitrate (nitrogen in combination with lime or potash). proteins are probably not made directly into protoplasm in the leaf, but are stored by the cells of the plant and used when needed, either to form new cells in growth or to repair waste. while plants and animals obtain their food in different ways, they probably make it into living substance (_assimilate_ it) in exactly the same manner. [illustration: an example of how a tree may exert energy. this rock has been split by the growing tree.] foods serve exactly the same purposes in plants and in animals; they either build living matter or they are burned (oxidized) to furnish energy (power to do work). if you doubt that a plant exerts energy, note how the roots of a tree bore their way through the hardest soil, and how stems or roots of trees often split open the hardest rocks, as illustrated in the figure above. starch-making and its relation to human welfare.--leaves which have been in darkness show starch to be present soon after exposure to light. a corn plant sends to grams of reserve material into the ears in a single day. the formation of fruit, and especially the growth of the grain fields, show the economic importance of this fact. not only do plants make their own food and store it away, but they make food for animals as well. and the food is stored in such a stable form that it may be sent to all parts of the world in the form of grain or other fruits. animals, herbivorous and flesh-eating, man himself, all are dependent upon the starch-making processes of the green plant for the ultimate source of their food. when we remember that in in the united states the total value of all farm crops was over $ , , , , and when we realize that these products came from the air and soil through the energy of the sun, we may begin to realize why as city boys and girls the study of plant biology is of importance to us. [illustration: experiment to show that oxygen is given off by green plants in the sunlight.] green plants give off oxygen in sunlight.--in still another way green plants are of direct use to us in the city. during this process of starch-making oxygen is given off as a by-product. this may easily be proven by the following experiment.[ ] place any green water plant in a battery jar partly filled with water, cover the plants with a glass funnel and mount a test tube full of water over the mouth of the funnel. then place the apparatus in a warm sunny window. bubbles of gas are seen to rise from the plant. after two or three hours of hot sun, enough of the gas can be obtained by displacement of the water to make the oxygen test. footnote : immediate success with this experiment will be obtained if the water has been previously charged with carbon dioxide. that oxygen is given off as a by-product by green plants is a fact of far-reaching importance. city parks are true "breathing spaces." the green covering of the earth is giving to animals an element that they must have, while the animals in their turn are supplying to the plants carbon dioxide, a compound used in food-making. thus a widespread relation of mutual helpfulness exists between plants and animals. respiration by leaves.--all living things require oxygen. it is by means of the oxidation of food materials within the plant's body that the energy used in growth and movement is released. a plant takes in oxygen largely through the stomata of the leaves, to a less extent through the _lenticels_ or breathing holes in the stem, and through the roots. thus rapidly growing tissues receive the oxygen necessary for them to perform their work. the products of oxidation in the form of carbon dioxide are also passed off through these same organs. it can be shown by experiment that a plant uses up oxygen in the darkness; in the light the amount of oxygen given off as a by-product in the process of starch-making is, of course, much greater than the amount used by the plant. summary.--from the above paragraphs it is seen that a leaf performs the following functions: ( ) breathing, or the taking in of oxygen and passing off of carbon dioxide; ( ) starch-making, with the incidental passing out of oxygen; ( ) formation of proteins, with their digestion and assimilation to form new tissues; and ( ) the transpiration of water. reference books elementary hunter, _laboratory problems in civic biology_. american book company. andrews, _a practical course in botany_, pages - . american book company. coulter, _a textbook of botany_, pages - . d. appleton and company. coulter, _plant life and plant uses_. american book company. dana, _plants and their children_, pages - . american book company. sharpe, _a laboratory manual in biology_, pages - . american book company. stevens, _introduction to botany_, pages - . d. c. heath and company. advanced clement, _plant physiology and ecology_. henry holt and company. coulter, barnes, and cowles, _a textbook of botany_, part ii, and vol. ii. american book company. darwin, _insectivorous plants_. d. appleton and company. duggar, _plant physiology_. the macmillan company. goodale, _physiological botany_, pages - and - . american book company. green, _vegetable physiology_. j. and a. churchill. lubbock, _flowers, fruits, and leaves_, last part. the macmillan company. macdougal, _practical textbook of plant physiology_. longmans, green, and company. report of the division of forestry, u.s. department of agriculture, . ward, _the oak_. d. appleton and company. viii. plant growth and nutrition--the circulation and final uses of food by plants _problem.--how green plants store and use the food they make._ _(a) what are the organs of circulation?_ _(b) how and where does food circulate?_ _(c) how does the plant assimilate its food?_ laboratory suggestions _laboratory exercise._--the structure (cross section) of a woody stem. _demonstration._--to show that food passes downward in the bark. _demonstration._--to show the condition of food passing through the stem. _demonstration._--plants with special digestive organs. the circulation and final uses of foods in green plants.--we have seen that cells of green plants make food and that such cells are mostly in the leaves. but _all_ parts of the bodies of plants grow. roots, stems, leaves, flowers, and fruits grow. seeds are storehouses of food. we must now examine the stem of some plant in order to see how food is distributed, stored, and finally used in the various parts of the plant. the structure of a woody stem.--if we cut a cross section through a young willow or apple stem, we find it shows three distinct regions. the center is occupied by the spongy, soft _pith_; surrounding this is found the rather tough _wood_, while the outermost area is _bark_. more careful study of the bark reveals the presence of three layers--an outer layer, a middle green layer, and an inner fibrous layer, the latter usually brown in color. this layer is made up largely of tough fiberlike cells known as _bast_ fibers. the most important parts of this inner bark, so far as the plant is concerned, are many tubelike structures known as _sieve tubes_. these are long rows of living cells, having perforated sievelike ends. through these cells food materials pass downward from the upper part of the plant, where they are manufactured. [illustration: section of a twig of box elder three years old, showing three annual growth rings. the radiating lines (_m_) which cross the wood (_w_) represent the pith rays, the principal ones extending from the pith in the center to the cortex or bark. (from coulter's _plant relations_.)] in the wood will be noticed (see figure) a number of lines radiating outward from the pith toward the bark. these are thin plates of pith which separate the wood into a number of wedge-shaped masses. these masses of wood are composed of many elongated cells, which, placed end to end, form thousands of little tubes connecting the leaves with the roots. in addition to these are many thick-walled cells, which give strength to the mass of wood. the bundles of tubes with their surrounding hard walled cells are the continuation of the bundles of tubes which are found in the root. in sections of wood which have taken several years to grow, we find so-called _annual rings_. the distance between one ring and the next (see figure) usually represents the amount of growth in one year. growth takes place from an actively dividing layer of cells, known as the _cambium layer_. this layer forms wood cells from its inner surface and bark from its outer surface. thus new wood is formed as a distinct ring around the old wood. use of the outer bark.--the outer bark of a tree is protective. the cells are dead, the heavy woody skeletons serving to keep out cold and dryness, as well as prevent the evaporation of fluids from within. the bark also protects the tree from attack of other plants or animals which might harm it. most trees are provided with a layer of corky cells. this layer in the cork oak is thick enough to be of commercial importance. the function of the corky layer in preventing evaporation is well seen in the case of the potato, which is a true stem, though found underground. if two potatoes of equal weight are balanced on the scales, the skin having been peeled from one, the peeled potato will be found to lose weight rapidly. this is due to loss of water, which is held in by the skin of the unpeeled potato (see right hand figure below). there are also small breathing holes known as _lenticels_ scattered through the surface of the bark. these can easily be seen in a young woody stem of apple, beech, or horse-chestnut. [illustration: experiment to show that the skin of the potato (a stem) retards evaporation.] proof that food passes down the stem.--if freshly cut willow twigs are placed in water, roots soon begin to develop from that part of the stem which is under water. if now the stem is girdled by removing the bark in a ring just above where the roots are growing, the latter will eventually die, and new roots will appear above the girdled area. the food material necessary for the outgrowth of roots evidently comes from above, and the passage of food materials takes place in a downward direction just outside the wood in the layer of bark which contains the bast fibers and sieve tubes. this experiment with the willow explains why it is that trees die when girdled so as to cut the sieve tubes of the inner bark. the food supply is cut off from the protoplasm of the cells in the part of the tree below the cut area. many of the canoe birches of our adirondack forest are thus killed, girdled by thoughtless visitors. in the same manner mice and other gnawing animals kill fruit trees. food substances are also conducted to a much less extent in the wood itself, and food passes from the inner bark to the center of the tree by way of the pith plates. this can be proved by testing for starch in the pith plates of young stems. it is found that much starch is stored in this part of the tree trunk. [illustration: experiment to show that food material passes down in the inner bark.] in what form does food pass through the stem?--we have already seen that materials in solution (those substances which will dissolve in the water) will pass from cell to cell by the process of osmosis. this is shown in the experiment illustrated in the figure. two thistle tubes are partly filled, one with starch and water, the other with sugar and water, and a piece of parchment paper is tied over the end of each. the lower ends of both tubes are placed in a glass dish under water. after twenty-four hours, the water in the dish is tested for starch, and then for sugar. we find that only the sugar, which has been dissolved by the water, can pass through the membrane. [illustration: experiment to show osmosis of sugar (right hand tube) and non-osmosis of starch (left hand tube).] digestion.--much of the food made in the leaves is stored in the form of starch. but starch, being insoluble, cannot be passed from cell to cell in a plant. it must be changed to a soluble form, for otherwise it could not pass through the delicate cell membranes. this is accomplished by the process of _digestion_. we have already seen that starch is changed to grape sugar in the corn by the action of a substance (an enzyme) called _diastase_. this process of digestion seemingly may take place in all living parts of the plant, although most of it is done in the leaves. in the bodies of all animals, including man, starchy foods are changed in a similar manner, but by other enzymes, into soluble grape sugar. the food material may be passed in a soluble form until it comes to a place where food storage is to take place, then it can be transformed to an insoluble form (starch, for example); later, when needed by the plant in growth, it may again be transformed and sent in a soluble form through the stem to the place where it will be used. in a similar manner, protein seems to be changed and transferred to various parts of the plant. some forms of protein substance are _soluble_ and others _insoluble_ in water. white of egg, for example, is slightly soluble, but can be rendered insoluble by heating it so that it coagulates. insoluble proteins are digested within the plant; how and where is but slightly understood. in a plant, soluble proteins pass down the sieve tubes in the bast and then may be stored in the bast or medullary rays of the wood in an insoluble form, or they may pass into the fruit or seeds of a plant, and be stored there. [illustration: diagram to show the areas in a plant through which the raw food materials pass up the stem and food materials pass down.] what forces water up the stem.--we have seen that the process of osmosis is responsible for taking in soil water, and that the enormous absorbing surface exposed by the root hairs makes possible the absorption of a large amount of water. frequently this is more than the weight of the plant in every twenty-four hours. experiments have been made which show that at certain times in the year this water is in some way forced up the tiny tubes of the stem. during the spring season, in young and rapidly growing trees, water has been proved to rise to a height of nearly ninety feet. the force that causes this rise of water in stems is known as _root pressure_. the greatest factor, however, is transpiration of water from leaves. this evaporation of water in the form of vapor seems to result in a kind of suction on the column of water in the stem. in the fall, after the leaves have gone, much less water is taken in by roots, showing that an intimate relation exists between the leaves and the root. summary of the functions of green plants.--the processes which we have just described (with the exception of food making) are those which occur in the lives of any plant or animal. all plants and animals breathe, they oxidize their foods to release energy, carbon dioxide being given off as the result of the union of the carbon in the foods with the oxygen of the air. both plants and animals digest their food; plants may do this in the cells of the root, stem, and leaf. digestion must always occur so that food can be moved in a soluble condition from cell to cell in the plant's body. [illustration: leaf of sundew closing over a captured insect.] [illustration: the venus fly trap, showing open and closed leaves.] plants with special digestive organs.--some plants have special organs of digestion. one of these, the sundew, has leaves which are covered on one side with tiny glandular hairs. these attract insects and later serve to catch and digest the nitrogenous matter of these insects by means of enzymes poured out by the same hairs. another plant, the venus fly trap, catches insects in a sensitive leaf which folds up and holds the insect fast until enzymes poured out by the leaf slowly digest it. still others, called pitcher plants, use as food the decayed bodies of insects which fall into their cuplike leaves and die there. in this respect plants are like those animals which have certain organs in the body set apart for the digestion of food. assimilation.--the assimilation of foods, or making of foods into living matter, is a process we know very little about. we know it takes place in the living cells of plants and animals. but how foods are changed into living matter is one of the mysteries of life which we have not yet solved. excretion.--the waste and repair of living matter seems to take place in both plants and animals. when living plants breathe, they give off carbon dioxide. in the process of starch-making, oxygen might be considered the waste product. water is evaporated from leaves and stems. the leaves fall and carry away waste mineral substances which they contain. [illustration: the embryos of (_a_) the morning glory, (_b_) the barberry, (_c_) the potato, (_d_) the four o'clock, showing the position of their food supply. (after gray.)] reproduction.--finally, both plants and animals have organs of reproduction. we have seen that the flower gives rise, after pollination, to a fruit which holds the seeds. these seeds hold the _embryo_. thus the young plant is doubly protected for a time and is finally thrown off in the seed with enough food to give it a start in life. in much the same way we will find that animals reproduce, either by laying eggs which contain an _embryo_ and food to start it in life or, as in the higher animals, by holding and protecting the embryo within the body of the mother until it is born, a helpless little creature, to be tenderly nourished by the mother until able to care for itself. the life cycle.--ultimately both plants and animals grow old and die. some plants, for example the pea or bean, live but a season; others, such as the big trees of california, live for hundreds of years. some insects exist as adults but a day, while the elephant is said to live almost two hundred years. the span of life from the time the plant or animal begins to grow until it dies is known as the _life cycle_. reference books elementary hunter, _laboratory problems in civic biology_. american book company. andrews, _a practical course in botany_, pages - . american book company. atkinson, _first studies of plant life_, chaps. iv, v, vi, viii, xxi. ginn. coulter, _plant life and plant uses_, chap. v. american book company. dana, _plants and their children_, pages - . american book company. mayne and hatch, _high school agriculture_. american book company. hodge, _nature study and life_, chaps. ix, x, xi. ginn and company. macdougal, _the nature and work of plants_. the macmillan company. advanced apgar, _trees of the united states_, chaps. ii, v, vi. american book company. coulter, barnes, and cowles, _a textbook of botany_, vol. i. american book company. duggar, _plant physiology_. the macmillan company. ganong, _the teaching botanist_. the macmillan company. goebel, _organography of plants_, part v. clarendon press. goodale, _physiological botany_. american book company. gray, _structural botany_, chap. v. american book company. kerner-oliver, _natural history of plants_. henry holt and company. strasburger, noll, schenck, and karston, _a textbook of botany_. the macmillan company. ward, _the oak_. d. appleton and company. yearbook, u. s. department of agriculture, , , - . ix. our forests, their uses and the necessity for their protection _problem.--man's relations to forests._ _(a) what is the value of forests to man?_ _(b) what can man do to prevent forest destruction?_ laboratory suggestions demonstration of some uses of wood. optional exercise on structure of wood. method of cutting determined by examination. home work on study of furniture trim, etc. visit to museum to study some economic uses of wood. visit to museum or field trip to learn some common trees. [illustration: a forest in north carolina. (u. s. g. s.)] the economic value of trees. protection and regulation of water supply.--trees form a protective covering for parts of the earth's surface. they prevent soil from being washed away, and they hold moisture in the ground. the devastation of immense areas in china and considerable damage by floods in parts of switzerland, france, and in pennsylvania has resulted where the forest covering has been removed. no one who has tramped through our adirondack forest can escape noticing the differences in the condition of streams surrounded by forest and those which flow through areas from which trees have been cut. the latter streams often dry up entirely in hot weather, while the forest-shaded stream has a never failing supply of crystal water. [illustration: working to prevent erosion after the removal of the forest in the french alps.] [illustration: erosion at sayre, pennsylvania, by the chemung river. (photograph by w. c. barbour.)] the city of new york owes much of its importance to its position at the mouth of a great river with a harbor large enough to float the navies of the world. this river is supplied with water largely from the adirondack and catskill forests. should these forests be destroyed, it is not impossible that the frequent freshets which would follow would so fill the hudson river with silt and débris that the ship channels in the bay, already costing the government hundreds of thousands of dollars a year to keep dredged, would become too shallow for ships. if this _should_ occur, the greatest city in this country would soon lose its place and become of second-rate importance. the story of how this very thing happened to the old greek city of poseidonia is graphically told in the following lines:-- "it was such a strange, tremendous story, that of the greek poseidonia, later the roman pæstum. long ago those adventuring mariners from greece had seized the fertile plain, which at that time was covered with forests of great oak and watered by two clear and shining rivers. they drove the italian natives back into the distant hills, for the white man's burden even then included the taking of all the desirable things that were being wasted by incompetent natives, and they brought over colonists--whom the philosophers and moralists at home maligned, no doubt, in the same pleasant fashion of our own day. and the colonists cut down the oaks, and plowed the land, and built cities, and made harbors, and finally dusted their busy hands and busy souls of the grime of labor and wrought splendid temples in honor of the benign gods who had given them the possessions of the italians and filled them with power and fatness. "every once in so often the natives looked lustfully down from the hills upon this fatness, made an armed snatch at it, were driven back with bloody contumely, and the heaping of riches upon riches went on. and more and more the oaks were cut down--mark that! for the stories of nations are so inextricably bound up with the stories of trees--until all the plain was cleared and tilled; and then the foothills were denuded, and the wave of destruction crept up the mountain sides, and they, too, were left naked to the sun and the rains. "at first these rains, sweeping down torrentially, unhindered by the lost forests, only enriched the plain with the long-hoarded sweetness of the trees; but by and by the living rivers grew heavy and thick, vomiting mud into the ever shallowing harbors, and the land soured with the undrained stagnant water. commerce turned more and more to deeper ports, and mosquitoes began to breed in the brackish soil that was making fast between the city and the sea. "who of all those powerful landowners and rich merchants could ever have dreamed that little buzzing insects could sting a great city to death? but they did. fevers grew more and more prevalent. the malaria haunted population went more and more languidly about their business. the natives, hardy and vigorous in the hills, were but feebly repulsed. carthage demanded tribute, and rome took it, and changed the city's name from poseidonia to pæstum. after rome grew weak, saracen corsairs came in by sea and grasped the slackly defended riches, and the little winged poisoners of the night struck again and again, until grass grew in the streets, and the wharves crumbled where they stood. finally, the wretched remnant of a great people wandered away into the more wholesome hills, the marshes rotted in the heat and grew up in coarse reeds where corn and vine had flourished, and the city melted back into the wasted earth."[ ] footnote : elizabeth bisland and anne hoyt, _seekers in sicily_. john lane company. [illustration: result of deforestation in china. this land has been ruined by erosion. (carnegie institution research in china.)] prevention of erosion by covering of organic soil.--we have shown how ungoverned streams might dig out soil and carry it far from its original source. examples of what streams have done may be seen in the deltas formed at the mouths of great rivers. the forest prevents this by holding the water supply and letting it out gradually. this it does by covering the inorganic soil with humus or decayed organic material. in this way the forest floor becomes like a sponge, holding water through long periods of drought. the roots of the trees, too, help hold the soil in place. the gradual evaporation of water through the stomata of the leaves cools the atmosphere, and this tends to precipitate the moisture in the air. eventually the dead bodies of the trees themselves are added to the organic covering, and new trees take their place. [illustration: the forest regions of the united states.] other uses of the forest.--in some localities forests are used as windbreaks and to protect mountain towns against avalanches. in winter they moderate the cold, and in summer reduce the heat and lessen the danger from storms. birds nesting in the woods protect many valuable plants which otherwise might be destroyed by insects. forests have great commercial importance. pyrogallic and other acids are obtained from trees, as are tar, creosote, resin, turpentine, and many useful oils. the making of maple sirup and sugar forms a profitable industry in several states. the forest regions of the united states.--the combined area of all the forests in the united states, exclusive of alaska, is about , , acres. this seemingly immense area is rapidly decreasing in acreage and in quality, thanks to the demands of an increasing population, a woeful ignorance on the part of the owners of the land, and wastefulness on the part of cutters and users alike. a glance at the map on page shows the distribution of our principal forests. washington ranks first in the production of lumber. here the great douglas fir, one of the "evergreens," forms the chief source of supply. in the southern states, especially louisiana and mississippi, yellow pine and cypress are the trees most lumbered. which states produce the most hardwoods? from which states do we get most of our yellow pine, spruce, red fir, redwood? where are the heaviest forests of the united states? [illustration: transportation of lumber in the west. a logging train.] [illustration: transportation of lumber in the east. logs are mostly floated down rivers to the mills.] uses of wood.--even in this day of coal, wood is still by far the most used fuel. it is useful in building. it outlasts iron under water, in addition to being durable and light. it is cheap and, with care of the forests, inexhaustible, while our mineral wealth may some day be used up. distilled wood gives wood alcohol. partially burned wood is charcoal. in our forests much of the soft wood (the cone-bearing trees, spruce, balsam, hemlock, and pine), and poplars, aspens, basswood, with some other species, make paper pulp. the daily newspaper and cheap books are responsible for inroads on our forests which cannot well be repaired. it is not necessary to take the largest trees to make pulp wood. hence many young trees of not more than six inches in diameter are sacrificed. of the hundreds of species of trees in our forests, the conifers are probably most sought after for lumber. pine, especially, is probably used more extensively than any other wood. it is used in all heavy construction work, frames of houses, bridges, masts, spars and timber of ships, floors, railway ties, and many other purposes. cedar is used for shingles, cabinetwork, lead pencils, etc.; hemlock and spruce for heavy timbers and, as we have seen, for paper pulp. another use for our lumber, especially odds and ends of all kinds, is in the packing-box industry. it is estimated that nearly per cent of all lumber cut ultimately finds its way into the construction of boxes. hemlock bark is used for tanning. the hard woods--ash, basswood, beech, birch, cherry, chestnut, elm, maple, oak, and walnut--are used largely for the "trim" of our houses, for manufacture of furniture, wagon or car work, and endless other purposes. [illustration: diagrams of sections of timber. _a_, cross section; _b_, radial; _c_, tangential. (from pinchot, u. s. dept. of agriculture.)] methods of cutting timber.--a glance at the diagram of the sections of timber shows us that a tree may be cut radially through the middle of the trunk or tangentially to the middle portion. most lumber is cut tangentially. in wood cut in this manner the yearly rings take a more or less irregular course. the grain in wood is caused by the fibers not taking straight lines in their course in the tree trunk. in many cases the fibers of the wood take a spiral course up the trunk, or they may wave outward to form little projections. boards cut out of such a piece of wood will show the effect seen in many of the school desks, where the annual rings appear to form elliptical markings. quite a difference in color and structure is often seen between the heartwood, composed of the dead walls of cells occupying the central part of the tree trunk, and the sapwood, the living part of the stem. [illustration: section of a tree trunk showing knot.] knots.--knots, as can be seen from the diagram, are branches which at one time started in their outward growth and were for some reason killed. later, the tree, continuing in its outward growth, surrounded them and covered them up. a dead limb should be pruned before such growth occurs. the markings in bird's-eye maple are caused by buds which have not developed, and have been overgrown with the wood of the tree. destruction of the forest.--_by waste in cutting._--man is responsible for the destruction of one of this nation's most valuable assets. this is primarily due to wrong and wasteful lumbering. hundreds of thousands of dollars' worth of lumber is left to rot annually because the lumbermen do not cut the trees close enough to the ground, or because through careless felling of trees many other smaller trees are injured. there is great waste in the mills. in fact, man wastes in every step from the forest to the finished product. [illustration: a forest in the far west totally destroyed by fire and wasteful lumbering.] _by fire._--indirectly, man is responsible for fire, one of the greatest enemies of the forest. most of the great forest fires of recent years, the losses from which total in the hundreds of millions, have been due either to railroads or to carelessness in making fires in the woods. it is estimated that in forest lands traversed by railroads from per cent to per cent of the fires are caused by coal-burning locomotives. for this reason laws have been made in new york state requiring locomotives passing through the adirondack forest preserve to burn oil instead of coal. this has resulted in a considerable reduction in the number of fires. in addition to the loss in timber, the fires often burn out the organic matter in the soil (the "duff") forming the forest floor, thus preventing the growth of forest there for many years to come. in new york and other states fires are fought by an organized corps of fire wardens, whose duty it is to watch the forest and to fight forest fires. other enemies.--other enemies of the forest are numerous fungus plants, insect parasites which bore into the wood or destroy the leaves, and grazing animals, particularly sheep. wind and snow also annually kill many trees. forestry.--in some parts of central europe, the value of the forests was seen as early as the year a.d., and many towns consequently bought up the surrounding forests. the city of zurich has owned forests in its vicinity for at least years and has found them a profitable investment. in this country only recently has the importance of preserving and caring for our forests been noted by our government. now, however, we have a forest survey of the department of agriculture and numerous state and university schools of forestry which are rapidly teaching the people of this country the best methods for the preservation of our forests. the federal government has set aside a number of tracts of mountain forest in some of the western states, making a total area of over , , acres. new york has established for the same purpose the adirondack park, with nearly , , acres of timberland. pennsylvania has one of , acres, and many other states have followed their example. [illustration: the forest primeval. trees are killing each other in the struggle for light and air.] [illustration: a german beech forest. the trees are kept thinned out so as to allow the young trees to get a start. contrast this with the picture above.] methods for keeping and protecting the forests.--forests should be kept thinned. too many trees are as bad as too few. they struggle with one another for foothold and light, which only a few can enjoy. in cutting the forest, it should be considered as a harvest. the oldest trees are the "ripe grain," the younger trees being left to grow to maturity. several methods of renewing the forest are in use in this country. ( ) trees may be cut down and young ones allowed to sprout from cut stumps. this is called coppice growth. this growth is well seen in parts of new jersey. ( ) areas or strips may be cut out so that seeds from neighboring trees are carried there to start new growth. ( ) forests may be artificially planted. two seedlings planted for every tree cut is a rule followed in europe. ( ) the most economical method is that shown in the lower picture on page , where the largest trees are thinned out over a large area so as to make room for the younger ones to grow up. the greatest dangers to the forests are from fire and from careless cutting, and these dangers may be kept in check by the efficient work of our national and state foresters. [illustration: we must protect our city trees. this tree was badly wounded by being gnawed by a horse.] a city's need for trees.--the city of paris, well known as one of the most beautiful of european capitals, spends over $ , annually in caring for and replacing some of the , trees owned by the city. all over the united states the city governments are beginning to realize what european cities have long known, that trees are of great value to a city. they are now following the example of european cities by planting trees and by protecting the trees after they are planted. thousands of city trees are annually killed by horses which gnaw the bark. this may be prevented by proper protection of the trunk by means of screens or wire guards. chicago has appointed a city forester, who has given the following excellent reasons why trees should be planted in the city:-- ( ) trees are beautiful in form and color, inspiring a constant appreciation of nature. ( ) trees enhance the beauty of architecture. ( ) trees create sentiment, love of country, state, city, and home. ( ) trees have an educational influence upon citizens of all ages, especially children. ( ) trees encourage outdoor life. ( ) trees purify the air. ( ) trees cool the air in summer and radiate warmth in winter. ( ) trees improve climate and conserve soil and moisture. ( ) trees furnish resting places and shelter for birds. ( ) trees increase the value of real estate. ( ) trees protect the pavement from the heat of the sun. ( ) trees counteract adverse conditions of city life. let us all try to make arbor day what it should be, a day for caring for and planting trees, for thus we may preserve this most important heritage of our nation. reference books elementary hunter, _laboratory problems in civic biology_. american book company. mayne and hatch, _high school agriculture_. american book company. murrill, _shade trees_, bul. , cornell university agricultural experiment station. pinchot, _a primer of forestry_, division of forestry, u. s. department of agriculture. advanced apgar, _trees of the united states_, chaps. ii, v, vi. american book company. coulter, barnes, and cowles, _a textbook of botany_, part i and vol. ii. american book company. goebel, _organography of plants_, part v. clarendon press. strasburger, noll, schenck, and karston, _a textbook of botany_. the macmillan company. ward, _timber and some of its diseases_. the macmillan company. yearbook, u. s. department of agriculture, division of forestry, buls. , , , , , , , , . x. the economic relation of green plants to man _problems.--how green plants are useful to man._ _(a) as food._ _(b) for clothing._ _(c) other uses._ _how green plants are harmful to man._ suggested laboratory work if a commercial museum is available, a trip should be planned to work over the topics in this chapter. the school collection may well include most of the examples mentioned, both of useful and harmful plants. a study of weeds and poisonous plants should be taken up in actual laboratory work, either by collection and identification or by demonstration. green plants have a "dollar and cents" value.--to the girl or boy living in the city green plants seem to have little direct value. although we see vegetables for sale in stores and we know that fruits have a money value, we are apt to forget that the wealth of our nation depends more upon its crops than it does on its manufactories and business houses. the economic or "dollars and cents" value of plants is enormous and far too great for us to comprehend in terms of figures. we have already seen some of the uses to mankind of the products of the forest; let us now consider some other plant products. [illustration: cabbage onions lettuce leaves used as food.] leaves as food.--grazing animals feed almost entirely on tender shoots or leaves, blades of grass, and other herbage. certain leaves and buds are used by man as food. lettuce, beet tops, kale, spinach, broccoli, are examples. a cabbage head is nothing but a big bud which has been cultivated by man. an onion is a compact budlike mass of thickened leaves which contain stored food. [illustration: celery kohl-rabi potato sugar cane stems used as food.] stems as food.--a city child would, if asked to name some stem used as food, probably mention asparagus. we sometimes forget that one of our greatest necessities, cane sugar, comes from the stem of sugar cane. over seventy pounds of sugar is used each year by every person in the united states. to supply the growing demand beets are now being raised for their sugar in many parts of the world, so that nearly half the total supply of sugar comes from this source. maple sugar is a well-known commodity which is obtained by boiling the sap of sugar maple until it crystallizes. over , tons of maple sugar is obtained every spring, vermont producing about per cent of the total output. the sago palm is another stem which supports the life of many natives in africa. another stem, living underground, forms one of man's staple articles of diet. this is the potato. roots as food.--roots which store food for plants form important parts of man's vegetable diet. beets, radishes, carrots, parsnips, sweet potatoes, and many others might be mentioned. the following table shows the proportion of foods in some of the commoner roots and stems:-- -------------------------------------------------------------------- | water| proteins| carbohydrates| fat | mineral matter -------------+------+---------+--------------+-----+---------------- potato | | . | | . | . carrot | | . | | . | . parsnip | | . | . | . | . turnip | . | . | . | . | . onion | | . | . | . | . sweet potato | | . | . | . | . beet | . | . | . | . | . -------------------------------------------------------------------- [illustration: wheat nuts pear melon seeds and fruits used for food.] fruits and seeds as foods.--our cereal crops, corn, wheat, etc., have played a very great part in the civilization of man and are now of so much importance to him as food products that bread made from flour from the wheat has been called the "staff of life." our grains are the cultivated progeny of wild grasses. domestication of plants and animals marks epochs in the advance of civilization. the man of the stone age hunted wild beasts for food, and lived like one of them in a cave or wherever he happened to be; he was a nomad, a wanderer, with no fixed home. he may have discovered that wild roots or grains were good to eat; perhaps he stored some away for future use. then came the idea of growing things at home instead of digging or gathering the wild fruits from the forest and plain. the tribes which first cultivated the soil made a great step in advance, for they had as a result a fixed place for habitation. the cultivation of grains and cereals gave them a store of food which could be used at times when other food was scarce. the word "cereal" (derived from ceres, the roman goddess of agriculture) shows the importance of this crop to roman civilization. from earliest times the growing of grain and the progress of civilization have gone hand in hand. as nations have advanced in power, their dependence upon the cereal crops has been greater and greater. "indian corn," says john fiske, in _the discovery of america_, "has played a most important part in the history of the new world. it could be planted without clearing or plowing the soil. there was no need of threshing or winnowing. sown in tilled land, it yields more than twice as much food per acre as any other kind of grain. this was of incalculable advantage to the english settlers in new england, who would have found it much harder to gain a secure foothold upon the soil if they had had to begin by preparing it for wheat or rye." to-day, in spite of the great wealth which comes from our mineral resources, live stock, and manufactured products, the surest index of our country's prosperity is the size of the corn and wheat crop. according to the last census, the amount of capital invested in agriculture was over $ , , , , while that invested in manufacture was less than one half that amount. corn.--about three billion bushels of corn were raised in the united states during the year . this figure is so enormous that it has but little meaning to us. in the past half century our corn crop has increased over per cent. illinois and iowa are the greatest corn-producing states, each having a yearly record of over four hundred million bushels. the figure on this page shows the principal corn-producing areas in the united states. [illustration: indian corn production--percentage] indian corn is put to many uses. it is a valuable food. it contains a large proportion of starch, from which glucose (grape sugar) and alcohol are made. machine oil and soap are made from it. the leaves and stalk are an excellent fodder; they can be made into paper and packing material. mattresses can be stuffed with the husks. the pith is used as a protective belt placed below the water line of our huge battleships. corn cobs are used for fuel, one hundred bushels having the fuel value of a ton of coal. [illustration: wheat crop in united states--percentage source] wheat.--wheat is the crop of next greatest importance in size. nearly seven hundred millions of bushels were raised in this country in , representing a total money value of over $ , , . seventy-two per cent of all the wheat raised comes from the north central states and california. about three fourths of the wheat crop is exported, nearly one half of it to great britain, thus indirectly giving employment to thousands of people on railways and steamships. wheat has its chief use in its manufacture into flour. the germ, or young wheat plant, is sifted out during this process and made into breakfast foods. flour making forms the chief industry of minneapolis, minnesota, and of several other large and wealthy cities in this country. [illustration: a field of rice, showing the conditions of culture.] other grains.--of the other grain and cereals raised in this country, oats are the most important crop, over one billion bushels having been produced in . barley is another grain, a staple of some of the northern countries of europe and asia. in this country, it is largely used in making malt for the manufacture of beer. rye is the most important cereal crop of northern europe, russia, germany, and austro-hungary producing over per cent of the world's supply. one of the most important grain crops for the world (although relatively unimportant in the united states) is rice. the fruit of this grasslike plant, after thrashing, screening, and milling, forms the principal food of one third of the human race. moreover, its stems furnish straw, its husks make a bran used as food for cattle, and the grain, when fermented and distilled, yields alcohol. garden fruits.--green plants and especially vegetables have come to play an important part in the dietary of man. the diseases known as scurvy and beri-beri, the latter the curse of the far eastern navies, have been largely prevented by adding vegetables and fruit juices to the dietary of the sailors. people in this country are beginning to find that more vegetables and less meat are better than the meat diet so often used. market gardening forms the lucrative business of many thousands of people near our great cities. some of the more important fruits are squash, cucumbers, pumpkins, melons, tomatoes, peppers, strawberries, raspberries, and blackberries. the latter fruits bring in an annual income of $ , , to our market gardeners. beans and peas are important as foods because of their relatively large amount of protein. canning green corn, peas, beans, and tomatoes has become an important industry. [illustration: picking apples, an important crop in some parts of the united states.] orchard and other fruits.--in the united states over one hundred and seventy-five million bushels of apples are grown every year. pears, plums, apricots, peaches, and nectarines also form large orchards, especially in california. nuts form one of our important articles of food, largely because of the large amount of protein contained in them. the grape crop of the world is commercially valuable, because of the raisins and wine produced. the culture of lemons, oranges, and grapefruit has come in recent years to give a living to many people in this country as well as in other parts of the world. figs, olives, and dates are staple foods in the mediterranean countries and are sources of wealth to the people there, as are coconuts, bananas, and many other fruits in tropical countries. beverages and condiments.--the coffee and cacao beans, and leaves of the tea plant, products of tropical regions, form the basis of very important beverages of civilized man. pepper, black and red, mustard, allspice, nutmegs, cloves, and vanilla are all products manufactured from various fruits or seeds of tropical plants. alcoholic liquors are produced from various plants in different parts of the world, the dried fruit of the hop vine being an important product of new york state used in the making of beer. raw materials.--besides use as food, green plants have many other uses. many of our city industries would not be in existence, were it not for certain plant products which furnish the raw materials for many manufacturing industries. many cities of the east and south, for example, depend upon cotton to give employment to thousands of factory hands. [illustration: cotton crop in united states--percentage source cotton crop in united states--percentage consumption] cotton.--of our native plant products cotton is probably of the most importance to the outside world. over eleven million bales of five hundred pounds each are raised annually. the cotton plant thrives in warm regions. its commercial importance is gained because the seeds of the fruit have long filaments attached to them. bunches of these filaments, after treatment, are easily twisted into threads from which are manufactured cotton cloth, muslin, calico, and cambric. in addition to the fiber, cottonseed oil, a substitute for olive oil, is made from the seeds, and the refuse remaining makes an excellent cattle fodder. [illustration: map showing the spread of the cotton boll weevil. it was introduced from mexico about . what proportion of the cotton raising belt was infected in ?] cotton boll weevil.--the cotton crop of the united states has rather recently been threatened with destruction by a beetle called the cotton boll weevil. this insect, which bores into the young pod of the cotton, develops there, stunting the growth of the fruit to such an extent that seeds are not produced. the loss in texas alone is estimated at over $ , , a year. the boll weevil, because of the protection offered by the cotton boll, is very difficult to exterminate. the weevils are destroyed by birds, the infected bolls and stalks are burnt, millions are killed each winter by cold, other insects prey on them, but at the present time they are one of the greatest pests the south knows. [illustration: mexican cotton boll weevil. much enlarged, above; natural size, below. (herrick.)] the control of this pest seems to depend upon early planting so that the crop has an opportunity to ripen before the insects in the boll grow large enough to do harm. ultimately the boll weevil may do more good than harm by bringing into the market a type of cotton plant that ripens very early. vegetable fibers.--among the most important are manila hemp, which comes from the leaf-stalks of a plant of the banana family and true hemp, which is the bast or woody fiber of a plant cultivated in most warm parts of the earth. flax is also an important fiber plant, grown largely in russia and other parts of europe (see picture on next page). from the bast fibers of the stem of this herb linen cloth is made. [illustration: flax grown for fiber.] vegetable oils.--some of the same plants which give fiber also produce oil. cotton seed oil pressed from the seeds, linseed oil from the seeds of the flax plant, and coconut oil (the covering of the nut here producing the fiber) are examples. [illustration: poison ivy, a climbing plant which is poisonous to touch. notice the leaves in threes.] some harmful green plants.--we have seen that on the whole green plants are useful to man. there are, however, some that are harmful. for example, the poison ivy is extremely poisonous to touch. the poison ivy is a climbing plant which attaches itself to the trees or walls by means of tiny air roots which grow out from the stem. it is distinguished from its harmless climbing neighbor, the virginia creeper, by the fact that its leaves are notched in _threes_ instead of _fives_. every boy and girl should know poison ivy. numerous other poisonous common plants are found, but one other deserves special notice because of its presence in vacant city lots. the jimson weed (_datura_) is a bushy plant, from two to five feet high, bearing large leaves. it has white or purplish flowers, and later bears a four-valved seed pod containing several hundred seeds. these plants contain a powerful poison, and people are often made seriously ill by eating the roots or other parts by mistake. weeds.--from the economic standpoint the green plants which do the greatest damage are weeds. those plants which provide best for their young are usually the most successful in life's race. plants which combine with the ability to scatter many seeds over a wide territory the additional characteristics of rapid growth, resistance to dangers of extreme cold or heat, attacks of enemies, inedibility, and peculiar adaptations to cross-pollination or self-pollination, are usually spoken of as weeds. they flourish in the sterile soil of the roadside and in the fertile soil of the garden. by means of rapid growth they kill other plants of slower growth by usurping their territory. slow-growing plants are thus actually exterminated. many of our common weeds have been introduced from other countries and have, through their numerous adaptations, driven out other plants which stood in their way. such is the russian thistle. a single plant of this kind will give rise to over , seeds. first introduced from russia in , it spread so rapidly that in twenty years it had appeared as a common weed over an area of some twenty-five thousand square miles. it is now one of the greatest pests in our northwest. reference books elementary hunter, _laboratory problems in civic biology_. american book company. gannet, _commercial geography_. american book company. sargent, _plants and their uses_. henry holt and company. toothaker, _commercial raw materials_. ginn and company. u. s. dept. of agriculture, farmers' bulletin , _thirty poisonous plants of the united states_, v. k. chestnut. bulletin . _two hundred weeds, how to know them and how to kill them_, l. h. dewey. advanced bailey, _cyclopedia of american agriculture_. the macmillan company. xi. plants without chlorophyll in their relation to man _problems.--(a) how molds and other saprophytic fungi do harm to man._ _(b) what yeasts do for mankind._ _(c) a study of bacteria with reference to_ _( ) conditions favorable and unfavorable to growth._ _( ) their relations to mankind._ _( ) some methods of fighting harmful bacteria and diseases caused by them._ laboratory suggestions _field work._--presence of bracket fungi and chestnut canker. _home experiment._--conditions favorable to growth of mold. _laboratory demonstration._--growth of mold, structure, drawing. _home experiment or laboratory demonstration._--conditions unfavorable for growth of molds. _demonstration._--process of fermentation. _microscopic demonstration._--growing yeast cells. drawing. _home experiment._--conditions favorable for growth of yeast. _home experiment._--conditions favorable for growth of yeast in bread. _demonstration and experiment._--where bacteria may be found. _demonstration._--methods of growth of bacteria, pure cultures and colonies shown. _demonstration._--foods preferred by bacteria. _demonstration._--conditions favorable for growth of bacteria. _demonstration._--conditions unfavorable for growth of bacteria. _demonstration by charts, diagrams, etc._--the relation of bacteria to disease in a large city. colorless plants are useful and harmful to man the fungi.--we have found that green plants on the whole are useful to mankind. but not all plants are green. most of us are familiar with the edible mushroom sold in the markets or the so-called "toadstools" found in parks or lawns. these plants contain no chlorophyll and hence do not make their own food. they are members of the plant group called _fungi_. such plants are almost as much dependent upon the green plants for food as are animals. but the fungi require for the most part dead organic matter for their food. this may be obtained from decayed vegetable or animal material in soil, from the bodies of dead plants and animals, or even from foods prepared for man. fungi which feed upon _dead_ organic material are known as _saprophytes_. examples are the mushrooms, the yeasts, molds, and some bacteria, of which more will be learned later. [illustration: chestnut trees in a new york city park; killed by a parasite, the chestnut canker.] some parasitic fungi.--other fungi (and we will find this applies to some animals as well) prefer _living_ plants or animals for their food. thus a tiny plant, recently introduced into this country, known as the chestnut canker, is killing our chestnut trees by the thousands in the eastern part of the united states. it produces millions of tiny reproductive cells known as _spores_; these spores, blown about by the wind, light on the trees, sprout, and send in under the bark a threadlike structure which sucks in the food circulating in the living cells, eventually causing the death of the tree. _a plant or animal which lives at the expense of another living plant or animal is called a parasite._ the chestnut canker is a dangerous parasite. later we shall see that animal and plant parasites destroy yearly crops and trees valued at hundreds of millions of dollars and cause untold misery and suffering to humanity. [illustration: shelf fungi. (photographed by w. c. barbour.)] another fungus which does much harm to the few trees found in large towns and cities is the shelf or bracket fungus. the part of the body visible on the tree looks like a shelf or bracket, hence the name. this bracket is in reality the reproductive part of the plant; on its lower surface are formed millions of little bodies called _spores_. these spores are capable, under favorable conditions, of reproducing new plants. the true body of the plant, a network of threads, is found under the bark. this fungus begins its life as a spore in some part of the tree which has become _diseased_ or _broken_. once established, it spreads rapidly. there is no remedy except to kill the tree and burn it, so as to destroy the spores. many fine trees, sound except for a slight bruise or other injury, are annually infected and eventually killed. in cities thousands of trees become infected through careless hitching of horses so that the horse may gnaw the tree, thus exposing a fresh surface on which spores may obtain lodgment and grow (see page ). suggestions for field work.--a field trip to a park or grove near home may show the great destruction of timber by this means. count the number of perfect trees in a given area. compare it with the number of trees attacked by the fungus. does the fungus appear to be transmitted from one tree to another near at hand? in how many instances can you discover the point where the fungus first attacked the tree? fungi of our homes.--but not all fungi are wild. some have become introduced into our homes and these live on food or other materials. _these plants are very important because of their relation to life in a town or crowded city._[ ] footnote : experiments on conditions favorable to growth of mold should be introduced here. [illustration: bread mold; _r_, rhizoids; _s_, fruiting bodies containing spores.] the growth of bread mold.--if a piece of moist bread is exposed to the air of the schoolroom, or in your own kitchen for a few minutes and then covered with a glass tumbler and kept in a warm place, in a day or two a fuzzy whitish growth will appear on the surface of the bread. this growth shortly turns black. if we now examine a little piece of the bread with a lens or low-powered microscope, we find a tangled mass of threads (the _mycelium_) covering the surface of the bread. from this mass of threads project tiny upright stalks bearing round black bodies, the fruit. little rootlike structures known as _rhizoids_ dip down into the bread, and absorb food for its threadlike body. the upright threads with the balls at the end contain many tiny bodies called _spores_. these spores have been formed by the division of the protoplasm making up the fruiting bodies into many separate cells. when grown under favorable conditions, the spores will produce more mycelia, which in turn bear fruiting bodies. physiology of the growth of mold.--molds, in order to grow rapidly, need oxygen, moisture, and moderate heat. they seem to prefer dark, damp places where there is not a free circulation of air, for if the bell jar is removed from growing mold for even a short time, the mold wilts. too great or very little heat will prevent growth and kill everything except the spores. they obtain their food from the material on which they live. this they are able to do by means of digestive enzymes given out by the rootlike parts, by means of which the molds cling to the bread. these digestive enzymes change the starch of the bread to sugar and the protein to a soluble form which will pass by osmosis into cells of the mold. thus the mold is able to absorb food material. these foods are then used to supply energy and make protoplasm. this seems to be the usual method by which saprophytes make use of the materials on which they live. what can molds live on?--we have seen that black mold lives upon bread. we would find that it or some other mold (_e.g._ green or blue mold) live upon decaying or overripe fruit,--apples, peaches, and plums being especially susceptible to their growth. molds feed upon all cakes or breads, upon meat, cheese, and many raw vegetables. they are almost sure to grow upon flour if it is allowed to get damp. moisture seems necessary for their growth. jelly is a substance particularly favorable to molds for this reason. shoes, leather, cloth, paper, or even moist wood will give food enough to support their growth. at least one troublesome disease, _ringworm_, is due to the growth of molds in the skin. what mold does to foods.--mold usually changes the taste of the material it grows upon, rendering it "musty" and sometimes unfit to eat. eventually it will spoil food completely because decay sets in. decay, as we will see later, is not entirely due to mold growth, but is usually caused by another group of organisms, the _bacteria_. molds, however, in feeding _do_ cause chemical changes which result in decay or putrefaction. some molds are useful. they give the flavor to roquefort, gorgonzola, camembert, and brie cheeses. but on the whole molds are pests which the housekeeper wishes to get rid of. how to prevent molds.[ ]--as we have seen, moisture is favorable for mold growth; conversely, dryness is unfavorable. inasmuch as the spores of mold abound in the air, materials which cannot be kept dry should be covered. jelly after it is made should at once be tightly covered with a thin layer of paraffin, which excludes the air and possible mold spores. or waxed paper may be fastened over the surface of the jelly so as to exclude the spores. to prevent molds from attacking fresh fruit, the surface of the fruit should be kept dry and, if possible, each piece of fruit should be wrapped in paper. why? heating with dry heat to ° for a few moments will kill any mold spores that happen to be in food. moldy food, if heated after removing surface on which the mold grew, is perfectly good to eat. footnote : an experiment to show conditions unfavorable for growth of molds should be shown at this point. dry dusting or sweeping will raise dust, which usually contains mold spores. use a dampened broom or dust cloth frequently in the kitchen if you wish to preserve foods from molds. other moldlike fungi.--mildews are near relatives of the molds found in our homes. they may attack leather, cloth, etc., in a damp house. other allied forms may do damage to living plants. some of these live upon the lilac, rose, or willow. these fungi do not penetrate the host plant to any depth, for they obtain their food from the outer layer of cells in the leaf of their host and cover the leaves with the whitish threads of the mycelium. hence they may be killed by means of applications of some fungus-killing fluid, as bordeaux mixture.[ ] among the useful plants preyed upon by mildews are the plum, cherry, and peach trees. (the diseases known as black knot and peach curl are thus caused.) another important member of this group is the tiny parasite found on rye and other grains, which gives us the drug ergot. footnote : see goff and mayne, _first principles of agriculture_, page , for formula of bordeaux mixture. among other parasitic fungi are rusts and smuts. wheat rust is probably the most destructive parasitic fungus. indirectly this parasite is of considerable importance to the citizen of a great city because of its effect upon the price of wheat. yeasts in their relation to man fermentation.--it is of common knowledge to country boys or girls that the juice of fresh apples, grapes, and some other fruits, if allowed to stand exposed to the air for a short time will _ferment_. that is, the sweet juice will begin to taste sour and to have a peculiar odor, which we recognize as that of alcohol. the fermenting juice appears to be full of bubbles which rise to the surface. if we collect enough of these bubbles of gas to make a test, we find it to be carbon dioxide. evidently something changed some part of the apple or grape, the sugar, (c{ }h{ }o{ }), into alcohol, (c{ }h{ }o), and carbon dioxide, (co{ }). this chemical process is known as _fermentation_. [illustration: apparatus to show effect of fermentation. _n_, molasses, water and yeast plants; _c_, bubbles of carbon dioxide.] yeast causes fermentation.--let us now take a compressed yeast cake, shake up a small portion of it in a solution of molasses and water, and fill a fermentation tube with the mixture. leave the tube in a warm place overnight. in the morning a gas will be found to have been collected in the closed end of the tube (see figure on page ). the taste and odor of the liquid shows alcohol to be present, and the gas, if tested, is proven carbon dioxide. evidently yeast causes fermentation. what are yeasts?--if now part of the liquid from the fermentation tube which contains the settlings be drawn off, a drop placed on a slide and a little weak iodine added and the mixture examined under the compound microscope, two kinds of structures will be found (see figure below), starch grains which are stained deep blue, and other smaller ovoid structures of a brownish yellow color. the latter are yeast plants. [illustration: yeast and starch grains. notice that the starch grains around which are clustered yeast cells have been rounded off by the yeast plants. how do you account for this?] size and shape, manner of growth, etc.--the common compressed yeast cake contains millions of these tiny plants. in its simplest form a yeast plant is a single cell. the shape of such a plant is ovoid, each cell showing under the microscope the granular appearance of the protoplasm of which it is formed. look for tiny clear areas in the cells; these are vacuoles, or spaces filled with fluid. the nucleus is hard to find in a yeast cell. many of the cells seem to have others attached to them, sometimes there being several in a row. yeast cells reproduce very rapidly by a process of budding, a part of the parent cell forming one or more smaller daughter cells which eventually become free from the parent. conditions favorable to growth of yeast.--_experiment._--label three pint fruit jars a, b, and c. add one fourth of a compressed yeast cake to two cups of water containing two tablespoonfuls of molasses or sugar. stir the mixture well and divide it into three equal parts and pour them into the jars. place covers on the jars. put jar a in the ice box on the ice, and jar b over the kitchen stove or near a radiator; pour the contents of jar c into a small pan and boil for a few minutes. pour back into c, cover and place it next to b. after forty-eight hours, look to see if any bubbles have made their appearance in any of the jars. if the experiment has been successful, only jar b will show bubbles. after bubbles have begun to appear at the surface, the fluid in jar b will be found to have a sour taste and will smell unpleasantly. the gas which rises to the surface, if collected and tested, will be found to be carbon dioxide. the contents of jar b have fermented. evidently, the growth of yeast will take place only under conditions of moderate warmth and moisture. carbohydrates necessary to fermentation.--sugar must be present in order for fermentation to take place. the wild yeasts cause fermentation of the apple or grape juice because they live on the skin of the apple or grape. various peoples recognize this when they collect the juice of certain fruits and, exposing it to the air, allow it to ferment. such is the _saki_ or rice wine of the japanese, the _tuba_ or sap of the coconut palm of the filipinos and the _pulqué_ of the mexicans. beer and wine making.--brewers' yeasts are cultivated with the greatest care; for the different flavors of beer seem to depend largely upon the condition of the yeast plants. beer is made in the following manner. sprouted barley, called malt, in which the starch of the grain has been changed to grape sugar by digestion, is killed by drying in a hot kiln. the malt is dissolved in water, and hops are added to give the mixture a bitter taste. now comes the addition of the yeast plants, which multiply rapidly under the favorable conditions of food and heat. fermentation results on a large scale from the breaking down of the grape sugar, the alcohol remaining in the fluid, and the carbon dioxide passing off into the air. at the right time the beer is stored either in bottles or casks, but fermentation slowly continues, forming carbon dioxide in the bottles. this gives the sparkle to beer when it is poured from the bottle. in wine making the wild yeasts growing on the skin of the grapes set up a slow fermentation. it takes several weeks before the wine is ready to bottle. in sparkling wines a second fermentation in the bottles gives rise to carbon dioxide in such quantity as to cause a decided frothing when the bottle is opened. commercial yeast.--cultivated yeasts are now supplied in the home as compressed or dried yeast cakes. in both cases the yeast plants are mixed with starch and other substances and pressed into a cake. but the compressed yeast cake must be used fresh, as the yeast plants begin to die rapidly after two or three days. the dried yeast cake, while it contains a much smaller number of yeast plants, is nevertheless probably more reliable if the yeast cannot be obtained fresh. [illustration: _a_ _b_ _c_] the cut illustrates an experiment that shows how yeast plants depend upon food in order to grow. in each of three fermentation tubes were placed an equal amount of a compressed yeast cake. then tube _a_ was filled with distilled water, tube _b_ with a solution of glucose and water, and tube _c_ with a nutrient solution containing nitrogenous matter as well as glucose. the quantity of gas (co{ }) in each tube is an index of the amount of growth of the yeast cells. in which tube did the greatest growth take place? bread making.--most of us are familiar with the process of bread making. the materials used are flour, milk or water or both, salt, a little sugar to hasten the process of fermentation, or "_rising_," as it is called, some butter or lard, and yeast. after mixing the materials thoroughly by a process called "kneading," the bread is put aside in a warm place (about ° fahrenheit) to "rise." if we examine the dough at this time, we find it filled with holes, which give the mass a spongy appearance. the yeast plants, owing to favorable conditions, have grown rapidly and filled the cavities with carbon dioxide. alcohol is present, too, but this is evaporated when the dough is baked. the baking cooks the starch of the bread, drives off the carbon dioxide and alcohol, and kills the yeast plants, besides forming a protective crust on the loaf. sour bread.--if yeast cakes are not fresh, sour bread may result from their use. in such yeast cakes there are apt to be present other tiny one-celled plants, known as _bacteria_. certain of these plants form acids after fermentation takes place. the sour taste of the bread is usually due to this cause. the remedy would be to have fresh yeast, to have good and fresh flour, and to have clean vessels with which to work. importance of yeasts.--yeasts in their relation to man are thus seen to be for the most part useful. they may get into canned substances put up in sugar and cause them to "work," giving them a peculiar flavor. but they can be easily killed by heating to the temperature of boiling. on the other hand, yeast plants are necessary for the existence of all the great industries which depend upon fermentation. and best of all they give us leavened bread, which has become a necessity to most of mankind. bacteria in their relation to man what bacteria do and where they may be found.--a walk through a crowded city street on any warm day makes one fully alive to odors which pervade the atmosphere. some of these unpleasant odors, if traced, are found to come from garbage pails, from piles of decaying fruit or vegetables, or from some butcher shop in which decayed meat is allowed to stand. this characteristic phenomena of decay is one of the numerous ways in which we can detect the presence of bacteria. these tiny plants, "man's invisible friends and foes," are to be found "anywhere, but not everywhere," in nature. they swarm in stale milk, in impure water, in soil, in the living bodies of plants and animals and in their dead bodies as well. most "catching" diseases we know to be caused directly by them; the processes of decay, souring of milk, acid fermentation, the manufacture of nitrogen for plants are directly or indirectly due to their presence. it will be the purpose of the next paragraphs to find some of the places where bacteria may be found and how we may know of their presence. [illustration: a steam sterilizer.] how we catch bacteria to study them.--to study bacteria it is first necessary to find some material in which they will grow, then kill all living matter in this food material by heating to boiling point ( °) for half an hour or more (this is called _sterilization_), and finally protect the _culture medium_, as this food is called, from other living things that might grow upon it. one material in which bacteria seem to thrive is a mixture of beef extract, digested protein and gelatine or agar-agar, the latter a preparation derived from seaweed. this mixture, after sterilization, is poured into flat dishes with loose-fitting covers. these _petri_ dishes, so called after their inventor, are the traps in which we collect and study bacteria. where bacteria might grow.--expose a number of these sterilized dishes, each for the same length of time, to some of the following conditions: (_a_) exposed to the air of the schoolroom. (_b_) exposed in the halls of the school while pupils are passing. (_c_) exposed in the halls of the school when pupils are not moving. (_d_) exposed at the level of a dirty and much-used city street. (_e_) exposed at the level of a well-swept and little-used city street. (_f_) exposed in a city park. (_g_) exposed in a factory building. (_h_) dirt from hands placed in dish. (_i_) rub interior of mouth with finger and touch surface of dish. (_j_) touch surface of dish with decayed vegetable or meat. (_k_) touch surface of dish with dirty coin or bill. (_l_) place in dish two or three hairs from boy's head. this list might be prolonged indefinitely. [illustration: colonies of bacteria growing in a petri dish.] now let us place all of the dishes together in a moderately warm place (a closet in the schoolroom will do) and watch for results. after a day or two little spots, brown, yellow, white, or red, will begin to appear. these spots, which grow larger day by day, are _colonies_ made up of millions of bacteria. but probably each colony arose from a single bacterium which got into the dish when it was exposed to the air. how we may isolate bacteria of certain kinds from others.--in order to get a number of bacteria of a given kind to study, it becomes necessary to grow them in what is known as a pure culture. this is done by first growing the bacteria in some medium such as beef broth, gelatin, or on potato.[ ] then as growth follows the colonies of bacteria appear in the culture media or the beef broth becomes cloudy. if now we wish to study one given form, it becomes necessary to isolate them from the others. this is done by the following process: a platinum needle is first passed through a flame to _sterilize_ it; that is, to kill all living things that may be on the needle point. then the needle, which cools very quickly, is dipped in a colony containing the bacteria we wish to study. this mass of bacteria is quickly transferred to another sterilized plate, and this plate is immediately covered to prevent any other forms of bacteria from entering. when we have succeeded in isolating a certain kind of bacterium in a given dish, we are said to have a _pure culture_. having obtained a pure culture of bacteria, they may easily be studied under the compound microscope. footnote : for directions for making a culture medium, see hunter, _laboratory problems in civic biology_. culture tubes may be obtained, already prepared, from parke, davis, and company or other good chemists. [illustration: a pure culture of bacteria. notice that the bacteria are all the same size and shape.] size and form.--in size, bacteria are the most minute plants known. a bacterium of average size is about / of an inch in length, and perhaps / of an inch in diameter. some species are much larger, others smaller. a common spherical form is / of an inch in diameter. they are so small that several million are often found in a single drop of impure water or sour milk. three well-defined forms of bacteria are recognized: a spherical form called a _coccus_, a rod-shaped bacterium, the _bacillus_, and a spiral form, the _spirillum_. some bacteria are capable of movement when living in a fluid. such movement is caused by tiny lashlike threads of protoplasm called _flagella_. the flagella project from the body, and by a rapid movement cause locomotion to take place. bacteria reproduce with almost incredible rapidity. it is estimated that a single bacterium, by a process of division called _fission_, will give rise to over , , others in twenty-four hours. under unfavorable conditions they stop dividing and form rounded bodies called spores. this spore is usually protected by a wall and may withstand very unfavorable conditions of dryness or heat; even boiling for several minutes will not kill some forms. [illustration: a figure to show the relative size and shape of ( ) a green mold, ( ) yeast cells, and ( ) different forms of bacteria; _b_, bacillus; _c_, coccus; _s_, spirillum forms. the yeast and bacteria are drawn to scale, they are much enlarged in proportion to the green mold, being actually much smaller than the mold spores seen at the top of the picture.] where bacteria are most numerous.--as the result of our experiments, we can make some generalizations concerning the presence of bacteria in our own environment. they are evidently present in the air, and in greater quantity in air that is moving than quiet air. why? that they stick to particles of dust can be proven by placing a little dust from the schoolroom in a culture dish. bacteria are present in greater numbers where crowds of people live and move, the air from dusty streets of a populous city contains many more bacteria than does the air of a village street. the air of a city park contains relatively few bacteria as compared with the near-by street. the air of the woods or high mountains fewer still. why? our previous experiment has shown that dirt on our hands, the mouth and teeth, decayed meat and vegetables, dirty money, the very hairs of our head are all carriers of bacteria. fluids the favorite home of bacteria.--tap water, standing water, milk, vinegar, wine, cider all can be proven to contain bacteria by experiments similar to those quoted above. spring or artesian well water would have very few, if any, bacteria, while the same quantity of river water, if it held any sewage, might contain untold millions of these little organisms. foods preferred by bacteria.--if bacteria are living and contain no chlorophyll, we should expect them to obtain protein food in order to grow. such is not always the case, for some bacteria seem to be able to build up protein out of simple inorganic nitrogenous substances. if, however, we take several food substances, some containing much protein and others not so much, we will find that the bacteria cause decay in the proteins almost at once, while other food substances are not always attacked by them. [illustration: growth of bacteria in a drop of impure water allowed to run down a sterilized culture in a dish.] what bacteria do to foods.--when bacteria feed upon a protein they use part of the materials in the food so that it falls to pieces and eventually rots. the material left behind after the bacteria have finished their meal is quite different from its original form. it is broken down by the action of the bacteria into gases, fluids, and some solids. it has a characteristic "rotten" odor and it has in it poisons which come as a result of the work of the bacteria. these poisonous wastes, called _ptomaines_, we shall learn more about later. conditions favorable and unfavorable to the growth of bacteria.--moisture and dryness.--_experiment_.--take two beans, remove the skins, crush one, soak the second bean overnight and then crush it. place in test tubes, one dry, the second with water. leave in a warm place two or three days, then smell each tube. in which is decay taking place? in which tube are bacteria at work? how do you know? moisture.--moisture is an absolute need for bacterial growth, consequently keeping material dry will prevent the growth of germs upon its surface. foods, in order to decay, must contain enough water to make them moist. bacteria grow most freely in fluids. light.--if we cover one half of a petri dish in which bacteria are growing with black paper and then place the dish in a light warm place for a few days, the growth of bacteria in the light part of the dish will be found to be checked, while growth continues in the covered part. it is a matter of common knowledge that disease germs thrive where dirt and darkness exist and are killed by any long exposure to sunlight. this shows us the need of light in our homes, especially in our bedrooms. air.--we have seen that plants need oxygen in order to perform the work that they do. this is equally true of all animals. but not all bacteria need _air_ to live; in fact, some are killed by the presence of air. just how these organisms get the oxygen necessary to oxidize their food is not well understood. the fact that some bacteria grow without air makes it necessary for us to use the one sure weapon we have for their extermination, and that is heat. heat.--_experiment._--take four cultures containing bouillon, inoculate each tube with bacteria and plug each tube with absorbent cotton. place one tube in the ice box, a second tube in a dark closet at a moderate temperature, a third in a warm place (about ° fahrenheit), and boil the contents of the fourth tube for ten minutes, then place it with tube number two. in which tubes does growth take place most rapidly? why? bacteria grow very slowly if at all in the temperature of an ice box, very rapidly at the room temperature of from ° to ° and much less rapidly at a higher temperature. all bacteria except those which have formed spores can be instantly killed as soon as boiling point is reached, and most spores are killed by a few minutes boiling. sterilization.--the practical lessons drawn from _sterilization_ are many. we know enough now to boil our drinking water if we are uncertain of its purity; we sterilize any foods that we believe might harbor bacteria, and thus keep them from spoiling. the industry of canning is built upon the principle of sterilization. canning.--canning is simply a method by which first the bacteria in a substance are killed by heating and then the substance is put into vessels into which no more bacteria may gain entrance. this is usually done at home by boiling the fruit or vegetable to be canned either in salt and water or with sugar and water, either of which substances aids in preventing the growth of bacteria. the time of boiling will be long or short, depending upon the materials to be canned. some vegetables, as peas, beans, and corn, are very difficult to can, probably because of spores of bacteria which may be attached to them. fruits, on the other hand, are usually much easier to preserve. after boiling for the proper time, the food, now free from all bacteria, must be put into jars or cans that are themselves absolutely _sterile_ or free from germs. this is done by first boiling the jars, then pouring the boiling hot material into the hot jars and sealing them so as to prevent the entrance of bacteria later. uses of canning.--canning as an industry is of immense importance to mankind. not only does it provide him with fruits and vegetables at times when he could not otherwise get them, but it also cheapens the cost of such things. it prevents the waste of nature's products at a time when she is most lavish with them, enabling man to store them and utilize them later. canning has completely changed the life of the sailor and the soldier, who in former times used to suffer from various diseases caused by lack of a proper balance of food. [illustration: pasteurizing milk. why should this be done?] pasteurization.--milk is one of the most important food supplies of a great city. it is also one of the most difficult supplies to get in good condition. this is in part due to the fact that milk is produced at long distances from the city and must be brought first from farms to the railroads, then shipped by train, again taken to the milk supply depot by wagon, there bottled, and again shipped by delivery wagons to the consumers. when we remember that much of the milk used in new york city is forty-eight hours old and when we realize that bacteria grow _very_ rapidly in milk, we see the need of finding some way to protect the supply so as to make it safe, particularly for babies and young children. this is done by _pasteurization_, a method named after the french bacteriologist louis pasteur. to pasteurize milk we heat it to a temperature of not over ° fahrenheit for from ten minutes to half an hour. by such a process all harmful germs will be killed and the keeping qualities of the milk greatly lengthened. most large milk companies pasteurize their city supply by a rapid pasteurization at a much higher temperature, but this method slightly changes the flavor of the milk. cold storage.--man has also come to use cold to keep bacteria from growing in foods. the ice box at home and cold storage on a larger scale enables one to keep foods for a more or less lengthy period. if food is frozen, as in cold storage, it might keep without growth of bacteria for years. but fruits and vegetables cannot be frozen without spoiling their flavor. and all foods after freezing seem particularly susceptible to the bacteria of decay. for that reason products taken from cold storage must be used at once. ptomaines.--many foods get their flavor from the growth of molds or bacteria in them. cheese, butter, the gamey taste of certain meats, the flavor of sauerkraut, are all due to the work of bacteria. but if bacteria are allowed to grow so as to become very numerous, the ptomaines which result from their growth in foods may poison the person eating such foods. frequently ptomaine poisoning occurs in the summer time because of the rapid growth of bacteria. much of the indigestion and diarrhoea which attack people during the summer is doubtless due to this kind of poisoning. preservatives.[ ]--this leads us to ask if we may not preserve food in ways other than those mentioned so as to protect ourselves from danger of ptomaine poisoning. many substances check the development of bacteria and in this way they _preserve_ the food. preservatives are of two kinds, those harmless to man and those that are poisonous. of the former, salt and sugar are examples; of the latter, formaldehyde and possibly benzoic acid. footnote : perform experiment here to determine the value of different preservatives. use sugar, salt, vinegar, boracic acid, benzoic acid, formaldehyde, and alcohol. sugar.--we have noted the use of sugar in canning. small amounts of sugar will be readily attacked by yeasts, molds, and bacteria, but a to per cent solution will effectually keep out bacteria. preserves are fruits boiled in about their own weight of sugar. condensed milk is preserved by the sugar added to it; so are candied and, in part, dried fruits. salt.--salt has been used for centuries to keep foods. meats are smoked, dried, and salted; some are put down in strong salt solutions. fish, especially cod and herring, are dried and salted. the keeping of butter is also due to the salt mixed with it. vinegar is another preservative. it, like salt, changes the flavor of materials kept in it and so cannot come into wide use. spices are also used as preservatives. harmful preservatives.--certain chemicals and drugs, used as preservatives, seem to be on the border line of harmfulness. such are benzoic acid, borax, or boracic acid. such drugs _may_ be harmless in small quantities, but unfortunately in canned goods we do not always know the amount used. the national government in passed what is known as the pure food law, which makes it illegal to use any of these preservatives (excepting benzoic acid in very small amounts). food which contains this preservative will be so labeled and should not be given to children or people with weak digestion. unfortunately people do not always read the labels and thus the pure food law is ineffective in its working. infrequently formaldehyde or other preservatives are used in milk. such treatment renders milk unfit for ordinary use and is an illegal process. disinfectants.[ ]--frequently it becomes necessary to destroy bacteria which cause diseases of various kinds. this process is called _disinfecting_. the substances commonly used are carbolic acid, formalin or formaldehyde, lysol, and bichloride of mercury. of these, the last named is the most powerful as well as the most dangerous to use. as it attacks metal, it should not be used in a metal pail or dish. it is commonly put up in tablets which are mixed to form a to solution. such tablets should be carefully safeguarded because of possible accidental poisoning. footnote : experiment to determine the most effective disinfectants. use tubes of bouillon containing different strength solutions of formaldehyde, lysol, iodine, carbolic acid, and bichloride of mercury. results. conclusions. formaldehyde used in liquid form is an excellent disinfectant. when burned in a formalin candle, it sets free an intensely pungent gas which is often used for disinfecting sick rooms after the patient has been removed. carbolic acid is perhaps the best disinfectant of all. if used in a solution of about part to of water, it will not burn the skin. it is of particular value to disinfect skin wounds, as it heals as well as cleanses when used in a weak solution. its rather pleasant odor makes it useful to cover up unpleasant smells of the sick room. the fumes of burning sulphur, which are so often used for disinfecting, are of little real value. [illustration: this shows how organic matter is broken down by bacteria so it may be used again by green plants.] bacteria cause decay.--let us next see in what ways the bacteria directly influence man upon the earth. have you ever stopped to consider what life would be like on the earth if things did not decay? the sea would soon be filled and the land covered with dead bodies of plants and animals. conditions of life would become impossible and living things on the earth would cease to exist. fortunately, bacteria cause decay. all organic matter, in whatever form, is sooner or later decomposed by the action of untold millions of bacteria which live in the air, water, and soil. these soil bacteria are most numerous in rich damp soils containing large amounts of organic material. they are very numerous around and in the dead bodies of plants and animals. to a considerable degree, then, these bacteria are useful in feeding upon these dead bodies, which otherwise would soon cover the surface of the earth to the exclusion of everything else. bacteria may thus be scavengers. they oxidize organic materials, changing them to compounds that can be absorbed by plants and used in building protoplasm. without bacteria and fungi it would be impossible for life to exist on the earth, for green plants would be unable to get the raw food materials in forms that could be used in making food and living matter. in this respect bacteria are of the greatest service to mankind. [illustration: microscopic appearance of ordinary milk, showing fat globules and bacteria which cause the souring of milk.] relation to fermentation.--they may incidentally, as a result of this process of decay, continue the process of fermentation begun by the yeasts. in making vinegar the yeasts first make alcohol (see page ) which the bacteria change to acetic acid. the lactic acid bacteria, which sour milk, changing the milk sugar to an acid, grow very rapidly in a warm temperature; hence milk which is cooled immediately and kept cool or which is pasteurized and kept in a cool place will not sour readily. why? these same lactic acid bacteria may be useful when they sour the milk for the cheese maker. other useful bacteria.--certain bacteria give flavor to cheese and butter, while still other bacteria aid in the "curing" of tobacco, in the production of the dye indigo, in the preparation of certain fibers of plants for the market, as hemp, flax, etc., in the rotting of animal matter from the skeletons of sponges, and in the process of tanning hides to make leather. [illustration: a field of alfalfa, a plant which harbors the nitrogen-fixing bacteria.] nitrogen-fixing bacteria.--still other bacteria, as we have seen before, "change over" nitrogen in organic material in the soil and even the free nitrogen of the air so that it can be used by plants in the form of a compound of nitrogen. the bacteria living in tubercles on the roots of clover, beans, peas, etc., have the power of thus "fixing" the free nitrogen in the air found between particles of soil. this fact is made use of by farmers who rotate their crops, growing first a crop of clover or other plants having root tubercles, which produce the bacteria, then plowing these in and planting another crop, as wheat or corn, on the same area. the latter plants, making use of the nitrogen compounds there, produce a larger crop than when grown in ground containing less nitrogenous material. bacteria cause disease.--the most harmful bacteria are those which cause diseases of plants and animals. certain diseases of plants--blights, rots, and wilts--are of bacterial nature. these do much annual damage to fruits and other parts of growing plants useful to man as food. but by far the most important are the bacteria which cause disease in man. they accomplish this by becoming parasites in the human body. millions upon millions of bacteria exist in the human body at all times--in the mouth, on the teeth, in the blood, and especially in the lower part of the food tube. some in the food tube are believed to be useful, some harmless, and some harmful; others in the mouth cause decay of the teeth, while a few kinds, if present in the body, may cause disease. [illustration: tubercles on the roots of the soy bean. they contain the nitrogen-fixing bacteria. (fletcher's soils.) copyright by doubleday, page and company.] it is known that bacteria, like other living things, feed and give off organic waste from _their own_ bodies. this waste, called a _toxin_, is poison to the host on which the bacteria live, and it is usually the production of this toxin that causes the symptoms of disease. some forms, however, break down tissues and plug up the small blood vessels, thus causing disease. diseases caused by bacteria.--it is estimated that bacteria cause annually over per cent of the deaths of the human race. as we will later see, a very large proportion of these diseases might be prevented if people were educated sufficiently to take the proper precautions to prevent their spread. these precautions might save the lives of some , , of people yearly in europe and america. tuberculosis, typhoid fever, diphtheria, pneumonia, blood poisoning, syphilis, and a score of other germ diseases ought not to exist. a good deal more than half of the present misery of this world might be prevented and this earth made cleaner and better by the coöperation of the young people now growing up to be our future home makers. [illustration: a single cell scraped from the roof of the mouth and highly magnified. the little dots are bacteria, most of which are harmless. notice the comparative size of bacteria and cell.] how we take germ diseases.--germ or contagious diseases either enter the body by way of the mouth, nose, or other body openings, or through a break in the skin. they may be carried by means of air, food, or water, but are usually _transmitted directly_ from the person who has the disease to a well person. this may be done through personal contact or by handling articles used by the sick person or by drinking or eating foods which have received some of the germs. from this it follows that if we know the methods by which a given disease is communicated, we may protect ourselves from it and aid the civic authorities in preventing its spread. [illustration: deaths from tuberculosis compared with other contagious diseases in the city of new york in .] tuberculosis.--the one disease responsible for the greatest number of deaths--perhaps one seventh of the total on the globe--is tuberculosis. it is estimated that of all people alive in the united states to-day, , , will die of this disease. but this disease is slowly but surely being overcome. it is believed that within perhaps one hundred years, with the aid of good laws and sanitary living, it will be almost extinct. [illustration: this curve shows a decreasing death rate from tuberculosis. explain.] tuberculosis is caused by the growth of bacteria, called the _tubercle bacilli_, within the lungs or other tissues of the human body. here they form little tubers full of germs, which close up the delicate air passages in the lungs, while in other tissues they give rise to hip-joint disease, scrofula, lupus, and other diseases, depending on the part of the body they attack. tuberculosis may be contracted by taking the bacteria into the throat or lungs or possibly by eating meat or drinking milk from tubercular cattle. especially is it communicated from a consumptive to a well person by kissing, by drinking or eating from the same cup or plate, using the same towels, or in coming in direct contact with the person having the germs in his body. although there are always some of the germs in the air of an ordinary city street, and though we may take some of these germs into our bodies at any time, yet the bacteria seem able to gain a foothold only under certain conditions. it is only when the tissues are in a worn-out condition, when we are "run down," as we say, that the parasite may obtain a foothold in the lungs. even if the disease gets a foothold, it is quite possible to cure it if it is taken in time. the germ of tuberculosis is killed by exposure to bright sunlight and fresh air. thus the course of the disease may be arrested, and a permanent cure brought about, by a life in the open air, the patient sleeping out of doors, taking plenty of nourishing food and very little exercise. see also chapter xxiv. [illustration: this figure shows how sewage from a cesspool (_c_) might get into the water supply: _lm_, layer of rock; _w_, wash water.] typhoid fever.--one of the most common germ diseases in this country and europe is typhoid fever. this is a disease which is conveyed by means of water and food, especially milk, oysters, and uncooked vegetables. typhoid fever germs live in the intestine and from there get into the blood and are carried to all parts of the body. a poison which they give off causes the fever so characteristic of the disease. the germs multiply very rapidly in the intestine and are passed off from the body with the excreta from the food tube. if these germs get into the water supply of a town, an epidemic of typhoid will result. among the recent epidemics caused by the use of water containing typhoid germs have been those in butler, pa., where persons were made ill; ithaca, n. y., with cases; and watertown, n. y., where over cases occurred. another source of infection is milk. frequently epidemics have occurred which were confined to users of milk from a certain dairy. upon investigation it was found that a case of typhoid had occurred on the farm where the milk came from, that the germs had washed into the well, and that this water was used to wash the milk cans. once in the milk, the bacteria multiplied rapidly, so that the milkman gave out cultures of typhoid in his milk bottles. proper safeguarding of our water and milk supply is necessary if we are to keep typhoid away. blood poisoning.--the bacterium causing blood poisoning is another toxin-forming germ. it lives in dust and dirt and is often found on the skin. it enters the body through cuts or bruises. it seems to thrive best in less oxygen than is found in the air. it is therefore important not to close up with court-plaster wounds which such germs may have entered. it, with typhoid, is responsible for four times as many deaths as bullets and shells in time of battle. the wonderfully small death rate of the japanese army in their war with russia was due to the fact that the japanese soldiers always boiled their drinking water before using it, and their surgeons always dressed all wounds on the battlefield, using powerful antiseptics in order to kill any bacteria that might have lodged in the exposed wounds. [illustration: this figure shows how a milk route might be instrumental in spreading diphtheria. _x_ is a farm on which a case of diphtheria occurred that was responsible for all the cases along milk routes _a_ and _f_ in hyde park, dorchester, and milton. how would you explain this?] other diseases.--many other diseases have been traced to bacteria. diphtheria is one of the best known. as it is a throat disease, it may easily be conveyed from one person to another by kissing, putting into the mouth objects which have come in contact with the mouth of the patient, or by food into which the germs have been carried. another disease which probably causes more misery in the world than any other germ disease is syphilis. hundreds of thousands of new-born babies die annually or grow up handicapped by deformities from this dread scourge. syphilis and gonorrhea, both diseases of the same sort and contracted in the same manner, hand down to innocent wives and still more innocent children a heritage of disease "even unto the third and fourth generation." grippe, pneumonia, whooping cough, and colds are believed to be caused by bacteria. other diseases, as malaria, yellow fever, sleeping sickness, and probably smallpox, scarlet fever, and measles, are due to the attack of one-celled animal parasites. of these we shall learn later in chapter xv. immunity.--it has been found that after an attack of a germ disease the body will not soon be again attacked by the same disease. this immunity, of which we will learn more later, seems to be due to a manufacture in the blood of substances which fight the bacteria or their poisons. if a person keeps his body in good physical condition and lives carefully, he will do much toward acquiring this natural immunity. acquired immunity.--modern medicine has discovered means of protecting the body from some contagious diseases. vaccination as protection against smallpox, the use of antitoxins (of which more later) against diphtheria, and inoculation against typhoid are all ways in which we may be protected against diseases. methods of fighting germ diseases.--as we have seen, diseases produced by bacteria may be caused by the bacteria being _directly_ transferred from one person to another, or the disease may obtain a foothold in the body from food, water, or by taking them into the blood through a cut or a wound or a body opening. it is evident that as individuals we may each do something to prevent the spread of germ diseases, especially in our homes. we may keep our bodies, especially our hands and faces, clean. sweeping and dusting may be done with damp cloths so as not to raise a dust; our milk and water, when from a suspicious supply, may be _sterilized_ or pasteurized. wounds through which bacteria might obtain foothold in the body should be washed with some _antiseptic_ such as carbolic acid ( part to water), which kills the germs. in a later chapter we shall learn more of how we may coöperate with the authorities to combat disease and make our city or town a better place in which to live.[ ] footnote : teachers may take up parts or all of chapter xxiv at this point. i have found it advisable to repeat much of the work on bacteria _after_ the students have taken up the study of the human organism. reference books elementary hunter, _laboratory problems in civic biology_. american book company. bigelow, _introduction to biology_. the macmillan company. conn, _bacteria, yeasts, and molds in the home_. ginn and company. conn, _story of germ life_. d. appleton and company. davison, _the human body and health_. american book company. frankland, _bacteria in daily life_. longmans, green, and company. overton, _general hygiene_. american book company. prudden, _dust and its dangers_. g. p. putnam's sons. prudden, _the story of the bacteria_. g. p. putnam's sons. ritchie, _primer of sanitation_. world book company. sharpe, _laboratory manual in biology_, pages - . american book company. advanced conn, _agricultural bacteriology_. p. blakiston's sons and company. coulter, barnes, and cowles, _a textbook of botany_, vol. i. american book company. de bary, _comparative morphology and biology of the fungi, mycetozoa, and bacteria_. clarendon press. duggar, _fungous diseases of plants_. ginn and company. hough and sedgwick, _the human mechanism_. ginn and company. hutchinson, _preventable diseases_. houghton, mifflin and company. lee, _scientific features of modern medicine_. columbia university press. muir and ritchie, _manual of bacteriology_. the macmillan company. newman, _the bacteria_. g. p. putnam's sons. sedgwick, _principles of sanitary science and public health_. the macmillan company. xii. the relations of plants to animals _problems.--to determine the general biological relations existing between plants and animals._ _(a) as shown in a balanced aquarium._ _(b) as shown in hay infusion._ suggestions for laboratory work _demonstration of life in a "balanced" and "unbalanced" aquarium._--determination of factors causing balance. _demonstration of hay infusion._--examination to show forms of animal and plant life. tabular comparison between balanced aquarium and hay infusion. some ways in which plants affect animals.--we have been studying the life of plants in order better to understand the life of animals and men. we have seen first that green plants play indirectly a tremendous part in man's welfare by supplying him with food. we have found that the colorless plants directly affected his welfare by causing disease, and by causing decay, thus making usable the nitrogen locked up in dead bodies of plants and animals, and by some even supplying nitrogen from the atmosphere. the dependence of animals upon plants has been shown and the interdependence of plants on animals has also been seen in cross-pollination and in the supply of raw food materials to plants by animals. study of a balanced aquarium.--perhaps the best way for us to understand the interrelation between plants and animals is to study an aquarium in which plants and animals live and in which a balance has been established between the plant life on one side and animal life on the other. aquaria containing green pond weeds, either floating or rooted, a few snails, some tiny animals known as water fleas, and a fish or two will, if kept near a light window, show this relation. [illustration: a balanced aquarium. explain the term "balanced."] we have seen that green plants under favorable conditions of sunlight, heat, moisture, and with a supply of raw food materials, give off oxygen as a by-product while manufacturing food in their green cells. we know the necessary raw materials for starch manufacture are carbon dioxide and water, while nitrogenous material is necessary for the making of proteins within the plant. in previous experiments we have proved that carbon dioxide is given off by any living thing when oxidation occurs in the body. the crawling snails and the swimming fish give off carbon dioxide, which is dissolved in the water; the plants themselves, at all times, oxidize food within their bodies, and so must _pass off_ some carbon dioxide. the green plants in the daytime _use up_ the carbon dioxide obtained from the various sources and, with the water taken in, manufacture starch. while this process is going on, oxygen is given off to the water of the aquarium, and this free oxygen is used by the animals there. [illustration: this diagram shows that plants and animals on the earth hold the same relation to each other as plants and animals in a balanced aquarium. explain the diagram in your notebook.] [illustration: the carbon and oxygen cycle in the balanced aquarium. trace by means of the arrows the carbon from the time plants take it in as co{ } until animals give it off. show what happens to the oxygen.] but the plants are continually growing larger. the snails and fish, too, eat parts of the plants. thus the plant life gives food to the animals within the aquarium. the animals give off certain nitrogenous wastes of which we shall learn more later. these materials, with other nitrogenous matter from the dead parts of the plants or animals, form part of the raw material used for protein manufacture in the plant. this nitrogenous matter is prepared for use by several different kinds of bacteria which first break the dead bodies down and then give it to the plants in the form of soluble nitrates. the green plants manufacture food, the animals eat the plants and give off organic waste, from which the plants in turn make their food and living matter. the plants give off oxygen to the animals, and the animals give carbon dioxide to the plants. thus a balance exists between the plants and animals in the aquarium. make a table to show this balance. [illustration: the relations between green plants and animals.] relations between green plants and animals.--what goes on in the aquarium is an example of the relation existing between all green plants and all animals. everywhere in the world green plants are making food which becomes, sooner or later, the food of animals. man does not feed to a great extent upon leaves, but he eats roots, stems, fruits, and seeds. when he does not feed directly upon plants, he eats the flesh of plant eating animals, which in turn feed directly upon plants. and so it is the world over; the plants are the food makers and supply the animals. green plants also give a very considerable amount of oxygen to the atmosphere every day, which the animals may use. [illustration: the nitrogen cycle. trace the nitrogen from its source in the air until it gets back again into the air.] the nitrogen cycle.--the animals in their turn supply much of the carbon dioxide that the plant uses in starch making. they also supply some of the nitrogenous matter used by the plants, part being given the plants from the dead bodies of their own relatives and part being prepared from the nitrogen of the air through the agency of bacteria, which live upon the roots of certain plants. these bacteria are the only organisms that can take nitrogen from the air. thus, in spite of all the nitrogen of the atmosphere, plants and animals are limited in the amount available. and the available supply is used over and over again, perhaps in nitrogenous food by an animal, then it may be given off as organic waste, get into the soil, and be taken up by a plant through the roots. eventually the nitrogen forms part of the food supply in the body of the plant, and then may become part of its living matter. when the plant dies, the nitrogen is returned to the soil. thus the usable nitrogen is kept in circulation.[ ] footnote : a small amount of nitrogen gas is returned to the atmosphere by the action of the decomposing bacteria on the ammonia compounds in the soil. (see figure of nitrogen cycle.) symbiosis.--we have seen that in the balanced aquarium the animals and plants, in a wide sense, form a sort of unconscious partnership. _this process of living together for mutual advantage is called symbiosis._ some animals thus combine with plants; for example, the tiny animal known as the hydra with certain of the one-celled algæ, and, if we accept the term in a wide sense, all green plants and animals live in this relation of mutual give and take. animals also frequently live in this relation to each other, as the crab, which lives within the shell of the oyster; the sea anemones, which are carried around on the backs of some hermit crabs, aiding the crab in protecting it from its enemies, and being carried about by the crab to places where food is plentiful. [illustration: life in the late stage of a hay infusion. _b_, bacteria, swimming or forming masses of food upon which the one-celled animals, the paramoecia, are feeding; _g_, gullet; _f.v._, food vacuole; _c.v._, contractile vacuole; _p_, pleurococcus; _p.d._, pleurococcus dividing. (drawn from nature by j. w. teitz.)] a hay infusion.--still another example of the close relation between plants and animals may be seen in the study of a hay infusion. if we place a wisp of hay or straw in a small glass jar nearly full of water, and leave it for a few days in a warm room, certain changes are seen to take place in the contents of the jar; after a little while the water gets cloudy and darker in color, and a scum appears on the surface. if some of this scum is examined under the compound microscope, it will be found to consist almost entirely of bacteria. these bacteria evidently aid in the decay which (as the unpleasant odor from the jar testifies) is beginning to take place. as we have learned, bacteria flourish wherever the food supply is abundant. the water within the jar has come to contain much of the food material which was once within the leaves of the grass,--organic nutrients, starch, sugar, and proteins, formed in the leaf by the action of the sun on the chlorophyll of the leaf, and now released into the water by the breaking down of the walls of the cells of the leaves. the bacteria themselves release this food from the hay by causing it to decay. after a few days small one-celled animals appear; these multiply with wonderful rapidity, so that in some cases the surface of the water seems to be almost white with active one-celled forms of life. if we ask ourselves where these animals come from, we are forced to the conclusion that they must have been in the water, in the air, or on the hay. hay is dried grass and may have been cut in a field near a pool containing these creatures. when the pool dried up, the wind may have scattered some of these little organisms in the dried mud or dust. some may have existed in a dormant state on the hay and the water awakened them to active life. in the water, too, there may have been some living cells, plants and animals. at first the multiplication of the tiny animals within the hay infusion is extremely rapid; there is food in abundance and near at hand. after a few days more, however, several kinds of one-celled animals may appear, some of which prey upon others. consequently a struggle for life takes place, which becomes more and more intense as the food from the hay is used up. eventually the end comes for all the animals unless some green plants obtain a foothold within the jar. if such a thing happens, food will be manufactured within their bodies, a new food supply arises for the animals within the jar, and a balance of life may result. reference books elementary hunter, _laboratory problems in civic biology_. american book company. sharpe, _a laboratory manual for the solution of problems in biology_, pp. - . american book company. advanced eggerlin and ehrenberg, _the fresh water aquarium and its inhabitants_. henry holt and company. furneaux, _life in ponds and streams_. longmans, green, and company. parker, _biology_. the macmillan company. sedgwick and wilson, _biology_. henry holt and company. xiii. single-celled animals considered as organisms _problems.--to determine:_ _(a) how a one-celled animal is influenced by its environment._ _(b) how a single cell performs its functions._ _(c) the structure of a single-celled animal._ laboratory suggestions _laboratory study._--study of paramoecium under compound microscope in its relation to food, oxygen, etc. determination of method of movement, turning, avoiding obstructions, sensitiveness to stimuli. drawings to illustrate above points. _laboratory demonstration._--living paramoecium to show structure of cell. demonstration with carmine to show food vacuoles, and action of cilia. use of charts and stained specimens to show other points of cell structure. laboratory demonstration of fission. [illustration: pleurococcus. a very simple plant cell.] the simplest plants.--we have seen that perhaps the simplest plant would be exemplified by one of the tiny bacteria we have just read about. a typical one-celled plant, however, would contain green coloring matter or chlorophyll, and would have the power to manufacture its own food under conditions giving it a moderate temperature, a supply of water, oxygen, carbon dioxide, and sunlight. such a simple plant is the _pleurococcus_, the "green slime" seen on the shady sides of trees, stones, or city houses. this plant would meet one definition of a cell, as it is a minute mass of protoplasm containing a nucleus. it is surrounded by a wall of a woody material formed by the activity of the living matter within the cell. it also contains a little mass of protoplasm colored green. of the work of the chlorophyll in the manufacture of organic food we have already learned. such is a simple plant cell. let us now examine a simple animal cell in order to compare it with that of a plant. where to find paramoecium.--if we examine very carefully the surface of a hay infusion, we are likely to notice in addition to the scum formed of bacteria, a mass of whitish tiny dots collected along the edge of the jar close to the surface of the water. more attentive observation shows us that these objects move, and that they are never found far from the surface. the life habits of paramoecium.--if we place on a slide a drop of water containing some of these moving objects and examine it under the compound microscope, we find each minute whitish dot is a cell, elongated, oval, or elliptical in outline and somewhat flattened. this is a one-celled animal known as the _paramoecium_ or the slipper animalcule (because of its shape). seen under the low power of the microscope, it appears to be extremely active, rushing about now rapidly, now more slowly, but seemingly always taking a definite course. the narrower end of the body (the _anterior_) usually goes first. if it pushes its way past any dense substance in the water, the cell body is seen to change its shape temporarily as it squeezes through. response to stimuli.--many of these little creatures may be found collected around masses of food, showing that they are attracted by it. in another part of the slide we may find a number of the paramoecia lying close to the edge of an air bubble with the greatest possible amount of their surface exposed to its surface. these animals are evidently taking in oxygen by osmosis. they are breathing. a careful inspection of the jar containing paramoecia shows thousands of tiny whitish bodies collected near the surface of the jar. in the paramoecium, as in the one-celled plants, the protoplasm composing the cell responds to certain agencies acting upon it, coming from without; these agencies we call _stimuli_. such stimuli may be light, differences of temperature, presence of food, electricity, or other factors of its surroundings. plant and animal cells may react differently to the same stimulus. in general, however, we know that protoplasm is _irritable_ to some of these factors. to severe stimuli, protoplasm usually responds by _contracting_, another power which it possesses. we know, too, that plant and animal cells take in food and change the food to protoplasm, that is, that they _assimilate_ food; and that they may waste away and repair themselves. finally, we know that new plant and animal cells are _reproduced_ from the original bit of protoplasm, a single cell. [illustration: a paramoecium. _c.v._, contractile vacuole; _f.v._, food vacuole; _m_, mouth; _ma.n._, macronucleus; _mi.n._, micronucleus; _w.v._, water vacuole.] the structure of paramoecium.--the cell body is almost transparent, and consists of semifluid protoplasm which has a granular grayish appearance under the microscope. this protoplasm appears to be bounded by a very delicate membrane through which project numerous delicate threads of protoplasm called _cilia_. (these are usually invisible under the microscope). the locomotion of the paramoecium is caused by the movement of these cilia, which lash the water like a multitude of tiny oars. the cilia also send particles of food into a funnel-like opening, the _gullet_, on one side of the cell. once inside the cell body, the particles of food materials are gathered into little balls within the almost transparent protoplasm. these masses of food seem to be inclosed within a little area containing fluid, called a _vacuole_. other vacuoles appear to be clear; these are spaces in which food has been digested. one or two larger vacuoles may be found; these are the _contractile vacuoles_; their purpose seems to be to pass off waste material from the cell body. this is done by pulsation of the vacuole, which ultimately bursts, passing fluid waste to the outside. solid wastes are passed out of the cell in somewhat the same manner. no breathing organs are seen, because osmosis of oxygen and carbon dioxide may take place anywhere through the cell membrane. the nucleus of the cell is not easily visible in living specimens. in a cell that has been stained it has been found to be a double structure, consisting of one large and one small portion, called, respectively, the _macronucleus_ and the _micronucleus_. [illustration: paramoecium dividing by fission. _m_, mouth; _mac._, macronucleus; _mic._, micronucleus. (after sedgwick and wilson.)] reproduction of paramoecium.--sometimes a paramoecium may be found in the act of dividing by the process known as _fission_, to form two new cells, each of which contains half of the original cell. this is a method of _asexual_ reproduction. the original cell may thus form in succession many hundreds of cells in every respect like the original parent cell. [illustration: amoeba, with pseudopodia (_p._) extended; _ec_, ectoplasm; _end_, endoplasm; the dark area (_n._) is the nucleus. (from a photograph loaned by professor g. n. calkins.)] amoeba.[ ]--in order to understand more fully the life of a simple bit of protoplasm, let us take up the study of the _amoeba_, a type of the simplest form of animal life. unlike the plant and animal cells we have examined, the amoeba has no fixed form. viewed under the compound microscope, it has the appearance of an irregular mass of granular protoplasm. its form is constantly changing as it moves about. this is due to the pushing out of tiny projections of the protoplasm of the cell, called _pseudopodia_ (false feet). the locomotion is accomplished by a streaming or flowing of the semifluid protoplasm. the pseudopodia are pushed forward in the direction which the animal is to go, the rest of the body following. in the central part of the cell is the nucleus. this important organ is difficult to see except in cells that have been stained. footnote : amoebæ _may_ be obtained from the hay infusion, from the dead leaves in the bottom of small pools, from the same source in fresh-water aquaria, from the roots of duckweed or other small water plants, or from green algæ growing in quiet localities. no _sure_ method of obtaining them can be given. although but a single cell, still the amoeba appears to be aware of the existence of food when it is near at hand. food may be taken into the body at any point, the semifluid protoplasm simply rolling over and engulfing the food material. within the body, as in the paramoecium, the food becomes inclosed within a fluid space or vacuole. the protoplasm has the power to take out such material as it can use to form new protoplasm or give energy. circulation of food material is accomplished by the constant streaming of the protoplasm within the cell. [illustration: amoeba, showing the changes which take place during division of the cell. the dark body in each figure is the nucleus; the transparent circle, the contractile vacuole; the large granular masses, the food vacuoles. much magnified.] the cell absorbs oxygen from the water by osmosis through its delicate membrane, giving up carbon dioxide in return. thus the cell "breathes" through any part of its body covering. waste nitrogenous products formed within the cell when work is done are passed out by means of the contractile vacuole. the amoeba, like other one-celled organisms, reproduces by the process of fission. a single cell divides by splitting into two others, each of which resembles the parent cell, except that they are of less bulk. when these become the size of the parent amoeba, they each in turn divide. this is a kind of asexual reproduction. when conditions unfavorable for life come, the amoeba, like some one-celled plants, encysts itself within a membranous wall. in this condition it may become dried and be blown through the air. upon return to a favorable environment, it begins life again, as before. in this respect it resembles the spore of a plant. [illustration: vorticella. _e_, gullet; _n_, nucleus; _cv_, contractile vacuole; _a_, axis; _s_, sheath; _fv_, food vacuole. (from herrick'_s general zoölogy_.)] the cell as a unit.--in the daily life of a one-celled animal we find the single cell performing all the general activities which we shall later find the many-celled animal is able to perform. in the amoeba no definite parts of the cell appear to be set off to perform certain functions; but any part of the cell can take in food, can absorb oxygen, can change the food into protoplasm, and excrete the waste material. the single cell is, in fact, an organism able to carry on the business of living almost as effectually as a very complex animal. complex one-celled animals.--in the paramoecium we find a single cell, but we find certain parts of the cell having certain definite functions: the cilia are used for locomotion; a definite part of the cell takes in food, while the waste passes out at another definite spot. in another one-celled animal called _vorticella_, part of the cell has become elongated and is contractile. by this stalk the little animal is fastened to a water plant or other object. the stalk may be said to act like a muscle fiber, as its sole function seems to be movement; the cilia are located at one end of the cell and serve to create a current of water which will bring food particles to the mouth. here we have several parts of the cell, each doing a different kind of work. this is known as _physiological division of labor_. habitat of protozoa.--protozoa are found almost everywhere in shallow water, especially close to the surface. they appear to be attracted near to the surface by the supply of oxygen. every fresh-water lake swarms with them; the ocean contains countless myriads of many different forms. use as food.--they are so numerous in lakes, rivers, and the ocean as to form the food for many animals higher in the scale of life. almost all fish that do not take the hook and that travel in schools, or companies, migrating from one place to another, live partly on such food. many feed on slightly larger animals, which in turn eat the protozoa. such fish have on each side of the mouth attached to the gills a series of small structures looking like tiny rakes. these are called the _gill rakers_, and aid in collecting tiny organisms from the water as it passes over the gills. the whale, the largest of all mammals, strains protozoans and other small animals and plants out of the water by means of hanging plates of whalebone or baleen, the slender filaments of which form a sieve from the top to the bottom of the mouth. protozoa cause disease.--protozoa of certain kinds play an important part in causing malaria, yellow fever, and other diseases, as we shall see later.[ ] (see page .) footnote : teachers may find it expedient to take up the study of protozoan diseases at this point. reference books elementary hunter, _laboratory problems in civic biology_. american book company. davison, _human body and health_. american book company. jordan, kellogg and heath, _animal studies_. d. appleton and company. sharpe, _laboratory manual_, pp. - . american book company. advanced calkins, _the protozoa_. macmillan company. jennings, _study of the lower organisms_. carnegie institution report. parker, _lessons in elementary biology_. the macmillan company. wilson, _the cell in development and inheritance_. the macmillan company. xiv. division of labor. the various forms of plants and animals _problems.--the development and forms of plants._ _the development of a simple animal._ _what is division of labor? in what does it result?_ _how to know the chief characters of some great animal groups._ laboratory suggestions _a visit to a botanical garden or laboratory demonstration._--some of the forms of plant life. review of essential facts in development of bean or corn embryo. _demonstration._--charts or models showing the development of a many-celled animal from egg through gastrula stage. _demonstration._--types which illustrate increasing complexity of body form and division of labor. _museum trip._--to afford pupil a means of identification of examples of principal phyla. this should be preceded by objective demonstration work in school laboratory. reproduction in plants.--although there are very many plants and animals so small and so simple as to be composed of but a single cell, by far the greater part of the animal and plant world is made up of individuals which are collections of cells living together. [illustration: a cell of pond scum. how might it divide to form a long thread made up of cells?] in a simple plant like the pond scum, a string or filament of cells is formed by a single cell dividing crosswise, the two cells formed each dividing into two more. eventually a long thread of cells is thus formed. at times, however, a cell is formed by the union of two cells, one from each of two adjoining filaments of the plant. at length a hard coat forms around this cell, which has now become a _spore_. the tough covering protects it from unfavorable changes in the surroundings. later, when conditions become favorable for its germination, the spore may form a new filament of pond scum. in molds, in yeasts, and in the bacteria we also found spores could be formed by the protoplasm of the plant cutting up into a number of tiny spores. these spores are called _asexual_ (without sex) because they are not formed by the union of two cells, and may give rise to other tiny plants like themselves. still other plants, mosses and ferns, give rise to two kinds of spores, sexual and asexual. all of these collectively are called _spore plants_. [illustration: the formation of spores in pond scum. _zs_, zygospore; _f_, fusion in progress.] reproduction in seed plants.--another great group of plants we have studied, plants of varied shapes and sizes, produce seeds. they bear flowers and fruits. [illustration: the formation and growth of a plant embryo. , the sperm and egg cell uniting; , a fertilized egg; , two cells formed by division; , four cells formed from two; , a many-celled embryo; , young plant; _h_, hypocotyl; _p_, plumule; _c_, cotyledons.] the embryo develops from a single fertilized "egg," growing by cell division into two, four, eight, and a constantly increasing number of cells until after a time a baby plant is formed, which as in the bean, either contains some stored food to give it a start in life, or, as in the corn, is surrounded with food which it can digest and absorb into its own tiny body. we have seen that these young plants in the seed are able to develop when conditions are favorable. furthermore, the young of each kind of plant will eventually develop into the kind of plant its parent was and into no other kind. thus the plant world is divided into many tribes or groups. plants are placed in groups.--if we plant a number of peas so that they will all germinate under the same conditions of soil, temperature, and sunlight, the seedlings that develop will each differ one from another in a slight degree.[ ] but in a general way they will have many characters in common, as the shape of the leaves, the possession of tendrils, form of the flower and fruit. a _species_ of plants or animals is a group of individuals so much alike in their characters that they might have had the same parents. individuals of such species differ slightly; for no two individuals are exactly alike. footnote : note to teachers.--a trip to the botanical garden or to a museum should be taken at this time. [illustration: a colony of trilliums, a flowering plant. (photograph by w. c. barbour.)] [illustration: rock fern, _polypody_. notice the underground stem giving off roots from its lower surface, and leaves (_c_), (_s_), from its upper surface.] species are grouped together in a larger group called a genus. for example, many kinds of peas--the wild beach peas, the sweet peas, and many others--are all grouped in one genus (called _lathyrus_, or vetchling) because they have certain structural characteristics in common. plant and animal genera are brought together in still larger groups, the classification based on general likenesses in structure. such groups are called, as they become successively larger, _family_, _order_, and _class_. thus both the plant and animal kingdoms are grouped into divisions, the smallest of which contains individuals very much alike; and the largest of which contains very many groups of individuals, the groups having some characters in common. this is called a system of classification. classification of the plant kingdom.--the entire plant kingdom has been divided into four sub-kingdoms by botanists:-- . _spermatophytes._ { _angiosperms_, true flowering plants. { _gymnosperms_, the pines and their allies. . _pteridophytes._ the fern plants and their allies. . _bryophytes._ the moss plants and their allies . _thallophytes._ the thallophytes form two groups: the algæ and the fungi; the algæ being green, while the fungi have no chlorophyll. [illustration: rockweed, a brown algæ, showing its distribution on rocks below highwater mark.] the extent of the plant kingdom can only be hinted at; each year new species are added to the lists. there are about , species of flowering plants and nearly as many flowerless plants. the latter consist of over species of fernlike plants, some , species of mosses, over lichens (plants consisting of a partnership between algæ and fungi), approximately , species of fungi, and about , species of algæ. [illustration: a moss plant. _g_, the moss body; _s_, the spore-bearing stalk (fruiting body).] development of a simple animal.--many-celled animals are formed in much the same way as are many-celled seed plants. a common bath sponge, an earthworm, a fish, or a dog,--each and all of them begin life in the same manner. in a many-celled animal the life history begins with a single cell, the fertilized egg. as in the flowering plant, this cell has been formed by the union of two other cells, a tiny (usually motile) cell; the _sperm_, and a large cell, the _egg_. after the egg is fertilized by a sperm cell, it splits into two, four, eight, and sixteen cells; as the number of cells increases, a hollow ball of cells called the _blastula_ is formed; later this ball sinks in on one side, and a double-walled cup of cells, now called a _gastrula_, results. practically all animals pass through the above stages in their development from the egg, although these stages are often not plain to see because of the presence of food material (yolk) in the egg. in animals the body consists of three layers of cells: those of the outside, developed from the outer layer of the gastrula, are called _ectoderm_, which later gives rise to the skin, nervous system, etc.; an inner layer, developed from the inner layer of the gastrula, the _endoderm_, which forms the lining of the digestive organs, etc.; a middle layer, called the _mesoderm_, lying between the ectoderm and the endoderm, is also found. in higher animals this layer gives rise to muscles, the skeleton, and parts of other internal structures. [illustration: stages in the development of a fertilized egg into the gastrula stage. read your text, then draw these stages and name each stage.] physiological division of labor.--if we compare the amoeba and the paramoecium, we find the latter a more complex organism than the former. an amoeba may take in food through any part of the body; the paramoecium has a definite gullet; the amoeba may use any part of the body for locomotion; the paramoecium has definite parts of the cell, the cilia, fitted for this work. since the structure of the paramoecium is more complex, we say that it is a "higher" animal. in the vorticella, a still more complex cell, part of the cell has grown out like a stalk, has become contractile, and acts like muscle. [illustration: photograph of a living _vorticella_, showing the contractile stalk and the cilia around the mouth. compare this figure with that of the paramoecium. which cell shows greater division of labor?] as we look higher in the scale of life, we invariably find that certain parts of a plant or animal are set apart to do certain work, and only that work. just as in a community of people, there are some men who do rough manual work, others who are skilled workmen, some who are shopkeepers, and still others who are professional men, so among plants and animals, wherever _collections_ of cells live together to form an organism, there is division of labor, some cells being fitted to do one kind of work, while others are fitted to do work of another sort. this is called physiological division of labor. [illustration: enlarged lengthwise section of the hydra, a very simple animal which shows slight division of labor. _ba_, base; _b_, bud; _m_, mouth; _ov_, ovary; _sp_, spermary.] [illustration: different forms of tissue cells. _c_, bone making cells; _e_, epithelial cells; _f_, fat cells; _l_, liver cells; _m_, muscle cell; _i_, involuntary; _v_, voluntary; _n_, nerve cell; _c b_, cell body; _n.f._, nerve fiber; _t.b._, nerve endings; _w_, colorless blood cells.] as we have seen, the higher plants are made up of a vast number of cells of many kinds. collections of cells alike in structure and performing the same function we have called a _tissue_. examples of animal tissues are the highly contractile cells set apart for movement, _muscles_; those which cover the body or line the inner parts of organs, the skin, or _epithelium_; the cells which form secretions or _glands_ and the sensitive cells forming the _nervous_ tissues. frequently several tissues have certain functions to perform in conjunction with one another. the arm of the human body performs movement. to do this, several tissues, as muscles, nerves, and bones, must act together. a collection of tissues performing certain work we call an _organ_. [illustration: part of a sponge, showing how cells perform division of labor. _ect_, ectoderm; _mes_, mesoderm; _end_, endoderm; _c.c._, ciliated cells, which take in food by means of their flagellæ or large cilia (_fla_).] in a simple animal like a sponge, division of labor occurs between the cells; some cells which line the pores leading inward create a current of water, and feed upon the minute organisms which come within reach, other cells build the skeleton of the sponge, and still others become eggs or sperms. in higher animals more complicated in structure and in which the tissues are found working together to form organs, division of labor is much more highly specialized. in the human arm, an organ fitted for certain movements, think of the number of tissues and the complicated actions which are possible. the most extreme division of labor is seen in the organism which has the most complex actions to perform and whose organs are fitted for such work, for there the cells or tissues which do the particular work do it quickly and very well. in our daily life in a town or city we see division of labor between individuals. such division of labor may occur among other animals, as, for example, bees or ants. but it is seen at its highest in a great city or in a large business or industry. in the stockyards of chicago, division of labor has resulted in certain men performing but a single movement during their entire day's work, but this movement repeated so many times in a day has resulted in wonderful accuracy and speed. thus division of labor obtains its end. organs and functions common to all animals.--the same general functions performed by a single cell are performed by a many-celled animal. but in the many-celled animals the various functions of the single cell are taken up by the organs. in a complex organism, like man, the organs and the functions they perform may be briefly given as follows:-- ( ) the organs of _food taking_: food may be taken in by individual cells, as those lining the pores of the sponge, or definite parts of a food tube may be set apart for this purpose, as the mouth and parts which place food in the mouth. ( ) the organs of _digestion_: the food tube and collections of cells which form the glands connected with it. the enzymes in the fluids secreted by the latter change the foods from a solid form (usually insoluble) to that of a _fluid_. such fluid may then pass by osmosis, through the walls of the food tube into the blood. ( ) the organs of _circulation_: the tubes through which the blood, bearing its organic foods and oxygen, reaches the tissues of the body. in simple animals, as the sponge and hydra, no such organs are needed, the fluid food passing from cell to cell by osmosis. ( ) the organs of _respiration_: the organs in which the blood receives oxygen and gives up carbon dioxide. the outer layer of the body serves this purpose in very simple animals; gills or lungs are developed in more complex animals. ( ) the organs of _excretion_: such as the kidneys and skin, which pass off nitrogenous and other waste matters from the body. ( ) the organs of _locomotion_: muscles and their attachments and connectives; namely, tendons, ligaments, and bones. ( ) the organs of _nervous control_: the central nervous system, which has control of coördinated movement. this consists of scattered cells in low forms of life; such cells are collected into groups and connected with each other in higher animals. ( ) the organs of _sense_: collections of cells having to do with the reception and transmission of sight, hearing, smell, taste, touch, pressure, and temperature sensations. ( ) the organs of _reproduction_: the sperm and egg-forming organs. almost all animals have the functions mentioned above. in most, the various organs mentioned are more or less developed, although in the simpler forms of animal life some of the organs mentioned above are either very poorly developed or entirely lacking. but in the so-called "higher" animals each of the above-named functions is assigned to a certain organ or group of organs. the work is done better and more quickly than in the "lower" animals. division of labor is thus a guide in helping us to determine the place of animals in the groups that exist on the earth. the animal series.--we have found that a one-celled animal can perform certain functions in a rather crude manner. man can perform these same functions in an extremely efficient manner. division of labor is well worked out, extreme complexity of structure is seen. between these two extremes are a great many groups of animals which can be arranged more or less as a series, showing the gradual evolution or development of life on the earth. it will be the purpose of the following pages to show the chief characteristics of the great groups of the animal kingdom. [illustration: the glasslike skeleton of a _radiolarian_, a protozoan. (from model at american museum of natural history.)] i. protozoa.--animals composed of a single cell, reproducing by cell division. the following are the principal classes of protozoa, examples of which we may have seen or read about:-- class i. _rhizopoda_ (greek for _root-footed_). having no fixed form, with pseudopodia. either naked as _amoeba_ or building limy (_foraminifera_) or glasslike skeletons (_radiolaria_). class ii. _infusoria (in infusions)._ usually active ciliated protozoa. examples, _paramoecium_, _vorticella_. class iii. _sporozoa (spore animals)._ parasitic and usually nonactive. example, _plasmodium malariæ_. [illustration: a horny fiber sponge. notice that it is a colony. one fourth natural size.] ii. sponges.--because the body contains many pores through which water bearing food particles enters, these animals are called _porifera_. they are classed according to the skeleton they possess into limy, glasslike, and horny fiber sponges. the latter are the sponges of commerce. with but few exceptions sponges live in salt water and are never free swimming. [illustration: sea anemones. one half natural size. the right hand specimen is expanded and shows the mouth surrounded by the tentacles. the left hand specimen is contracted. (from model at the american museum of natural history.)] iii. coelenterates.--the hydra and its salt-water allies, the jellyfish, hydroids, and corals, belong to a group of animals known as the _coelenterata_. the word "coelenterate" (_coelom_ = body cavity, _enteron_ = food tube) explains the structure of the group. they are animals in which the real body cavity is lacking, the animal in its simplest form being little more than a bag. some examples are the hydra, shown on page , salt-water forms known as hydroids, colonial forms which have part of their life free swimming as jellyfish; sea anemones and coral polyps, tiny colonial hydra like forms which build a living or secreted covering. iv. worms.--the wormlike animals are grouped into _flatworms_, _roundworms_, and segmented or _jointed_ worms. (_a_) flatworms are sometimes parasitic, examples being the tapeworm and liver fluke. they are usually small, ribbon- or leaf-like and flat and live in water. (_b_) roundworms, minute threadlike creatures, are not often seen by the city girl or boy. vinegar eels, the horsehair worm, the pork worm or trichina and the dread hookworm are examples. (_c_) segmented worms are long, jointed creatures composed of body rings or segments. examples are the earthworm, the sandworm (known to new york boys as the fishworm), and the leeches or bloodsuckers. [illustration: a jointed worm. the sandworm. slightly reduced.] [illustration: the common starfish seen from below to show the tube feet. about one half natural size.] [illustration: the crayfish, a crustacean. _a_, antenna; _m_, mouth; _e_, compound stalked eye; _ch_, pincher claw; _c.p._, cephalothorax; _ab_, abdomen; _c.f._, caudal fin. a little reduced.] v. echinoderms.--these are spiny-skinned animals, which live in salt water. they are still more complicated in structure than the worms and may be known by the spines in their skin. they show radial symmetry. starfish or sea urchins are examples. vi. arthropods.--these animals are distinguished by having jointed body and legs. they form two great groups. the higher forms of the _crustacea_ have only two regions in the body, a fused head and thorax, called the _cephalothorax_, and an abdominal region. a second group is the _insecta_, of which we know something already. crustacea breathe by means of _gills_, which are structures for taking oxygen out of the water, while adult insects breathe through air tubes called _trachea_. two smaller groups of arthropods also exist, the _arachnida_, consisting of spiders, scorpions, ticks, and mites, and the _myriapoda_, examples being the "thousand leggers" found in some city houses. [illustration: a common snail, a mollusk. (from a photograph by davison.)] vii. mollusca.--another large group is the mollusca. this phylum gets its name from the soft, unsegmented body (_mollis_ = soft). mollusks usually have a shell, which may be of one piece, as a snail, or two pieces or _valves_, as the clam or oyster. [illustration: the skeleton of a dog; a typical vertebrate.] viii. the vertebrates.--all of the animals we have studied thus far agree in having whatever skeleton or hard parts they possess on the outside of the body. collectively, they are called _invertebrates_. this exoskeleton differs from the main or axial skeleton of the higher animals, the latter being inside of the body. the exoskeleton is dead, being secreted by the cells lining the body, while the endoskeleton is, in part at least, alive and is capable of growth, _e.g._ a broken arm or leg bone will grow together. but a man has certain parts of the skeleton, as nails or hair, formed by the skin and in addition possesses inside bones to which the muscles are attached. some of the bones are arranged in a flexible column in the _dorsal_ (the back) side of the body. this _vertebral column_, as it is called, is distinctive of all _vertebrates_. within its bony protection lies the delicate central nervous system, and to this column are attached the big bones of the legs and arms. the vertebrate animals deserve more of our attention than other forms of life because man himself is a vertebrate. [illustration: the sand shark, an elasmobranch. note the slits leading from the gills. (from a photograph loaned by the american museum of natural history.)] five groups or classes of vertebrates exist. _fishes_, _amphibians_, _reptiles_, _birds_, and _mammals_. let us see how to distinguish one class from another. [illustration: the sturgeon, a ganoid fish.] fishes.--fishes are familiar animals to most of us. we know that they live in the water, have a backbone, and that they have fins. they breathe by means of gills, delicate organs fitted for taking oxygen out of the water. the heart has two chambers, an auricle and a ventricle. they have a skin in which are glands secreting mucus, a slimy substance which helps them go through the water easily. they usually lay very many eggs. classification of fishes order i. _the elasmobranchs._ fishes which have a soft skeleton made of cartilage and exposed gill slits. examples: sharks, skates, and rays. order ii. _the ganoids._ fishes which once were very numerous on the earth, but which are now almost extinct. they are protected by platelike scales. examples: gars, sturgeon, and bowfin. [illustration: a bony fish.] order iii. _the teleosts, or bony fishes._ they compose per cent of all living fishes. in this group the skeleton is bony, the gills are protected by an operculum, and the eggs are numerous. most of our common food fishes belong to this class. order iv. _the dipnoi, or lung fishes._ this is a very small group. in many respects they are more like amphibians than fishes, the swim bladder being used as a lung. they live in tropical africa, south america, and australia, inhabiting the rivers and lakes there. characteristics of amphibia.--the frog belongs to the class of vertebrates known as amphibia. as the name indicates (_amphi_, both, and _bia_, life), members of this group live both in water and on land. in the earlier stages of their development they take oxygen into the blood by means of gills. when adult, however, they breathe by means of lungs. at all times, but especially during the winter, the skin serves as a breathing organ. the skin is soft and unprotected by bony plates or scales. the heart has three chambers, two auricles and one ventricle. most amphibians undergo a complete metamorphosis, or change of form, the young being unlike the adults. [illustration: newt. (from a photograph loaned by the american museum of natural history.) about natural size.] [illustration: the leopard frog, an amphibian.] classification of amphibia order i. _urodela._ amphibia having usually poorly developed appendages. tail persistent through life. examples: mud puppy, newt, salamander. order ii. _anura._ tailless amphibia, which undergo a metamorphosis, breathing by gills in larval state, by lungs in adult state. examples: toad and frog. characteristics of reptilia.--these animals are characterized by having scales developed from the skin. in the turtle they have become bony and are connected with the internal skeleton. reptiles always breathe by means of lungs, differing in this respect from the amphibians. they show their distant relationship to birds in that their large eggs are incased in a leathery, limy shell. [illustration: box tortoise, a land reptile. (from photograph loaned by the american museum of natural history.) about one fourth natural size.] [illustration: the gila monster, a poisonous lizard. about one twelfth natural size.] classification of reptiles order i. _chelonia_ (turtles and tortoises). flattened reptiles with body inclosed in bony case. no teeth or sternum (breastbone). examples: snapping turtle, box tortoise. order ii. _lacertilia_ (lizards). body covered with scales, usually having two-paired appendages. breathe by lungs. examples: fence lizard, horned toad. [illustration: the common garter snake. reduced to about one tenth natural size.] order iii. _ophidia_ (snakes). body elongated, covered with scales. no limbs present. examples: garter snake, rattlesnake. order iv. _crocodilia._ fresh-water reptiles with elongated body and bony scales on skin. two-paired limbs. examples: alligator, crocodile. birds.--birds among all other animals are known by their covering of feathers and the presence of wings. the feathers are developed from the skin. these aid in flight, and protect the body from the cold. [illustration: adaptations in the bills of birds. could we tell anything about the food of a bird from its bill? do these birds all get their food in the same manner? do they all eat the same kind of food?] the form of the bill in particular shows adaptation to a wonderful degree. a duck has a flat bill for pushing through the mud and straining out the food; a bird of prey has a curved or hooked beak for tearing; the woodpecker has a sharp, straight bill for piercing the bark of trees in search of the insect larvæ which are hidden underneath. birds do not have teeth. [illustration: common tern and young, showing nesting and feeding habits. (from group at american museum of natural history.)] the rate of respiration, of heartbeat, and the body temperature are all higher in the bird than in man. man breathes from twelve to fourteen times per minute. birds breathe from twenty to sixty times a minute. because of the increased activity of a bird, there comes a necessity for a greater and more rapid supply of oxygen, an increased blood supply to carry the material to be used up in the release of energy, and a means of rapid excretion of the wastes resulting from the process of oxidation. birds are large eaters, and the digestive tract is fitted to digest the food quickly, by having a large crop in which food may be stored in a much softened condition. as soon as the food is part of the blood, it may be sent rapidly to the places where it is needed, by means of the large four-chambered heart and large blood vessels. the high temperature of the bird is a direct result of this rapid oxidation; furthermore, the feathers and the oily skin form an insulation which does not readily permit of the escape of heat. this insulating cover is of much use to the bird in its flights at high altitudes, where the temperature is often very low. birds lay eggs and usually care for their young. [illustration: african ostrich, one of the largest living birds.] classification of birds order i. _cursores._ running birds with no keeled breastbone. examples: ostrich, cassowary. order ii. _passeres._ perching birds; three toes in front, one behind. over one half of all species of birds are included in this order. examples: sparrow, thrush, swallow. order iii. _gallinæ._ strong legs; feet adapted to scratching. beak stout. examples: jungle fowl, grouse, quail, domestic fowl. order iv. _raptores._ birds of prey. hooked beak. strong claws. examples: eagle, hawk, owl. order v. _grallatores._ waders. long neck, beak, and legs. examples: snipe, crane, heron. order vi. _natatores._ divers and swimmers. legs short, toes webbed. examples: gull, duck, albatross. order vii. _columbinæ._ like gallinæ, but with weaker legs. examples: dove, pigeon. order viii. _pici._ woodpeckers. two toes point forward, two backward, and adaptation for climbing. long, strong bill. order ix. _psittaci._ parrots, hooked beak and fleshy tongue. order x. _coccyges._ climbing birds, with powerful beak. examples: kingfisher, toucan, and cuckoo. order xi. _macrochires._ birds having long-pointed wings, without scales on metatarsus. examples: swift, humming bird, and goatsucker. mammals.--dogs and cats, sheep and pigs, horses and cows, all of our domestic animals (and man himself) have characters of structure which cause them to be classed as mammals. they, like some other vertebrates, have lungs and warm blood. they also have a hairy covering and bear young developed to a form similar to their own,[ ] and nurse them with milk secreted by glands known as the _mammary glands_; hence the term "mammal." footnote : with the exception of the monotremes. [illustration: the bison, an almost extinct mammal.] adaptations in mammalia.--of the thirty-five hundred species, most inhabit continents; a few species are found on different islands, and some, as the whale, inhabit the ocean. they vary in size from the whale and the elephant to tiny shrew mice and moles. adaptations to different habitat and methods of life abound; the seal and whale have the limbs modified into flippers, the sloth and squirrel have limbs peculiarly adapted to climbing, while the bats have the fore limbs modeled for flight. lowest mammals.--the lowest are the monotremes, animals which lay eggs like the birds, although they are provided with hairy covering like other mammals. such are the australian spiny anteater and the duck mole. all other mammals bring forth their young developed to a form similar to their own. the kangaroo and opossum, however, are provided with a pouch on the under side of the body in which the very immature, blind, and helpless young are nourished until they are able to care for themselves. these pouched animals are called _marsupials_. the other mammals may be briefly classified as follows:-- classification of higher mammals order i. _edentata._ toothless or with very simple teeth. examples: anteater, sloth, armadillo. order ii. _rodentia._ incisor teeth chisel-shaped, usually two above and two below. examples: beaver, rat, porcupine, rabbit, squirrel. order iii. _cetacea._ adapted to marine life. examples: whale, porpoise. order iv. _ungulata._ hoofs, teeth adapted for grinding. examples: (_a_) odd-toed, horse, rhinoceros, tapir; (_b_) even-toed, ox, pig, sheep, deer. order v. _carnivora._ long canine teeth, sharp and long claws. examples: dog, cat, lion, bear, seal, and sea lion. order vi. _insectivora._ example: mole. order vii. _cheiroptera._ fore limbs adapted to flight, teeth pointed. example: bat. order viii. _primates._ erect or nearly so, fore appendage provided with hand. examples: monkey, ape, man. [illustration: the geological history of the horse. (after mathews, in the american museum of natural history.) ask your teacher to explain this diagram.] increasing complexity of structure and of habits in plants and animals.--in our study of biology so far we have attempted to get some notion of the various factors which act upon living things. we have seen how plants and animals interact upon each other. we have learned something about the various physiological processes of plants and animals, and have found them to be in many respects identical. we have found grades of complexity in plants from the one-celled plant, bacterium or pleurococcus, to the complicated flowering plants of considerable size and with many organs. so in animal life, from the protozoa upward, there is constant change, and the change is toward greater complexity of structure and functions. an insect is a higher type of life than a protozoan, because its structure is more complex and it can perform its work with more ease and accuracy. a fish is a higher type of animal than the insect for these same reasons, and also for another. the fish has an internal skeleton which forms a pointed column of bones on the _dorsal_ side (the back) of the animal. it is a vertebrate animal. [illustration: the evolutionary tree. modified from galloway. copy this diagram in your notebook. explain it as well as you can.] the doctrine of evolution.--we have now learned that animal forms may be arranged so as to begin with very simple one-celled forms and culminate with a group which contains man himself. this arrangement is called the _evolutionary series_. evolution means change, and these groups are believed by scientists to represent stages in complexity of development of life on the earth. geology teaches that millions of years ago, life upon the earth was very simple, and that gradually more and more complex forms of life appeared, as the rocks formed latest in time show the most highly developed forms of animal life. the great english scientist, charles darwin, from this and other evidence, explained the theory of evolution. this is the belief that simple forms of life on the earth slowly and gradually gave rise to those more complex and that thus ultimately the most complex forms came into existence. the number of animal species.--over , species of animals are known to exist to-day, as the following table shows. protozoa , arachnids , sponges , crustaceans , coelenterates , mollusks , echinoderms , fishes , flat-worms , amphibians , roundworms , reptiles , annelids , birds , insects , mammals , myriapods , ------- total , man's place in nature.--although we know that man is separated mentally by a wide gap from all other animals, in our study of physiology we must ask where we are to place man. if we attempt to classify man, we see at once he must be placed with the vertebrate animals because of his possession of a vertebral column. evidently, too, he is a mammal, because the young are nourished by milk secreted by the mother and because his body has at least a partial covering of hair. anatomically we find that we must place man with the apelike mammals, because of these numerous points of structural likeness. the group of mammals which includes the monkeys, apes, and man we call the _primates_. although anatomically there is a greater difference between the lowest type of monkey and the highest type of ape than there is between the highest type of ape and the lowest savage, yet there is an immense mental gap between monkey and man. instincts.--mammals are considered the highest of vertebrate animals, not only because of their complicated structure, but because their instincts are so well developed. monkeys certainly seem to have many of the mental attributes of man. professor thorndike of columbia university sums up their habits of learning as follows:-- "in their method of learning, although monkeys do not reach the human stage of a rich life of ideas, yet they carry the animal method of learning, by the selection of impulses and association of them with different sense-impressions, to a point beyond that reached by any other of the lower animals. in this, too, they resemble man; for he differs from the lower animals not only in the possession of a new sort of intelligence, but also in the tremendous extension of that sort which he has in common with them. a fish learns slowly a few simple habits. man learns quickly an infinitude of habits that may be highly complex. dogs and cats learn more than the fish, while monkeys learn more than they. in the number of things he learns, the complex habits he can form, the variety of lines along which he can learn them, and in their permanence when once formed, the monkey justifies his inclusion with man in a separate mental genus." evolution of man.--undoubtedly there once lived upon the earth races of men who were much lower in their mental organization than the present inhabitants. if we follow the early history of man upon the earth, we find that at first he must have been little better than one of the lower animals. he was a nomad, wandering from place to place, feeding upon whatever living things he could kill with his hands. gradually he must have learned to use weapons, and thus kill his prey, first using rough stone implements for this purpose. as man became more civilized, implements of bronze and of iron were used. about this time the subjugation and domestication of animals began to take place. man then began to cultivate the fields, and to have a fixed place of abode other than a cave. the beginnings of civilization were long ago, but even to-day the earth is not entirely civilized. the races of man.--at the present time there exist upon the earth five races or varieties of man, each very different from the other in instincts, social customs, and, to an extent, in structure. these are the ethiopian or negro type, originating in africa; the malay or brown race, from the islands of the pacific; the american indian; the mongolian or yellow race, including the natives of china, japan, and the eskimos; and finally, the highest type of all, the caucasians, represented by the civilized white inhabitants of europe and america. reference books elementary hunter, _laboratory problems in civic biology_, american book company. bulletin of u. s. department of agriculture, _division of biological survey_, nos. , , , . davison, _practical zoölogy_. american book company. ditmars, _the reptiles of new york_. guide leaflet . amer. mus. of nat. history. sharpe, _a laboratory manual in biology_, pp. - , american book company. walker, _our birds and their nestlings_. american book company. walter, h. e. and h. a., _wild birds in city parks_. published by authors. advanced apgar, _birds of the united states_. american book company. beebe, _the bird_. henry holt and company. ditmars, _the reptile book_. doubleday, page and company. hegner, _zoölogy_. the macmillan company. hornaday, _american natural history_. jordan and evermann, _food and game fishes_. doubleday, page and company. parker and haswell, _textbook of zoölogy_. the macmillan company. _riverside natural history._ houghton, mifflin and company. weed and dearborn, _relation of birds to man_. lippincott. xv. the economic importance of animals _problems.--i. to determine the uses of animals._ _(a) indirectly as food._ _(b) directly as food._ _(c) as domesticated animals._ _(d) for clothing._ _(e) other direct economic uses._ _(f) destruction of harmful plants and animals._ _--ii. to determine the harm done by animals._ _(a) animals destructive to those used for food._ _(b) animals harmful to crops and gardens._ _(c) animals harmful to fruit and forest trees._ _(d) animals destructive to stored food or clothing._ _(e) animals indirectly or directly responsible for disease._ laboratory suggestions inasmuch as this work is planned for the winter months the laboratory side must be largely museum and reference work. it is to be expected that the teacher will wish to refer to much of this work at the time work is done on a given group. but it is pedagogically desirable that the work as planned should be _varied_. interest is thus held. outlines prepared by the teacher to be filled in by the student are desirable because they lead the pupil to individual selection of what seems to _him_ as important material. opportunity should be given for laboratory exercises based on original sources. the pupils should be made to use reports of the u. s. department of agriculture, the biological survey, various states reports, and others. special home laboratory reports may be well made at this time, for example: determination at a local fish market of the fish that are cheap and fresh at a given time. have the students give reasons for this. study conditions in the meat market in a similar manner. other local food conditions may also be studied first hand. uses of animals indirect use as food.--just as plants form the food of animals, so some animals are food for others. man may make use of such food directly or indirectly. many mollusks, as the barnacle and mussel, are eaten by fishes. other fish live upon tiny organisms, water fleas and other small crustaceans. these in turn feed upon still smaller animals, and we may go back and back until finally we come to the protozoa and one-celled water plants as an ultimate source of food. direct use as food. lower forms.--the forms of life lower than the crustacea are of little use directly as food, although the chinese are very fond of one of the echinoderms, a holothurian. [illustration: north american lobster. this specimen, preserved at the u. s. fish commission at woods hole, was of unusual size and weighed over twenty pounds.] crustacea as food.--crustaceans, however, are of considerable value for food, the lobster fisheries in particular being of importance. the lobster is highly esteemed as food, and is rapidly disappearing from our coasts as the result of overfishing. between twenty and thirty million are yearly taken on the north atlantic coast. this means a value at present prices of about $ , , . laws have been enacted in new york and other states against overfishing. egg-carrying lobsters must be returned to the water; all smaller than six to nine inches in length (the law varies in different states) must be put back; other restrictions are placed upon the taking of the animals, in hope of saving the race from extinction. some states now hatch and care for the young for a period of time; the united states bureau of fisheries is also doing much good work, in the hope of restocking to some extent the now almost depleted waters. several other common crustaceans are near relatives of the crayfish. among them are the shrimp and prawn, thin-shelled, active crustaceans common along our eastern coast. in spite of the fact that they form a large part of the food supply of many marine animals, especially fish, they do not appear to be decreasing in numbers. they are also used as food by man, the shrimp fisheries in this country aggregating over $ , , yearly. [illustration: the edible blue crab. (from a photograph loaned by the american museum of natural history.)] another edible crustacean of considerable economic importance is the blue crab. crabs are found inhabiting muddy bottoms; in such localities they are caught in great numbers in nets or traps baited with decaying meat. they are, indeed, among our most valuable sea scavengers, although they are carnivorous hunters as well. the young crabs differ considerably in form from the adult. they undergo a complete _metamorphosis_ (change of form). immediately after molting or shedding of the outer shell in order to grow larger, crabs are greatly desired by man as an article of food. they are then known as "shedders," or soft-shelled crabs. [illustration: the oyster.] mollusks as food.--oysters are never found in muddy localities, for in such places they would be quickly smothered by the sediment in the water. they are found in nature clinging to stones or on shells or other objects which project a little above the bottom. here food is abundant and oxygen is obtained from the water surrounding them. hence oyster raisers throw oyster shells into the water and the young oysters attach themselves. in some parts of europe and this country where oysters are raised artificially, stakes or brush are sunk in shallow water so that the young oyster, which is at first free-swimming, may escape the danger of smothering on the bottom. after the oysters are a year or two old, they are taken up and put down in deeper water as seed oysters. at the age of three and four years they are ready for the market. the oyster industry is one of the most profitable of our fisheries. nearly $ , , a year has been derived during the last decade from such sources. hundreds of boats and thousands of men are engaged in dredging for oysters. three of the most important of our oyster grounds are long island sound, narragansett bay, and chesapeake bay. [illustration: this diagram shows how cases of intestinal disease (typhoid and diarrhoea) have been traced to oysters from a locality where they were "fattened" in water contaminated with sewage. (loaned by american museum of natural history.)] sometimes oysters are artificially "fattened" by placing them on beds near the mouths of fresh-water streams. too often these streams are the bearers of much sewage, and the oyster, which lives on microscopic organisms, takes in a number of bacteria with other food. thus a person might become infected with the typhoid bacillus by eating raw oysters. state and city supervision of the oyster industry makes this possibility very much less than it was a few years ago, as careful bacteriological analysis of the surrounding water is constantly made by competent experts. clams.--other bivalve mollusks used for food are clams and scallops. two species of the former are known to new yorkers, one as the "round," another as the "long" or "soft-shelled" clams. the former (_venus mercenaria_) was called by the indians "quahog," and is still so called in the eastern states. the blue area of its shell was used by the indians to make wampum, or money. the quahog is now extensively used as food. the "long" clam (_mya arenaria_) is considered better eating by the inhabitants of massachusetts and rhode island. this clam was highly prized as food by the indians. the clam industries of the eastern coast aggregate nearly $ , , a year. the dredging for scallops, another molluscan delicacy, forms an important industry along certain parts of the eastern coast. [illustration: salmon leaping a fall on their way to their spawning beds. (photographed by dr. john a. sampson.)] fish as food.--fish are used as food the world over. from very early times the herring were pursued by the norsemen. fresh-water fish, such as whitefish, perch, pickerel, pike, and the various members of the trout family, are esteemed food and, especially in the great lake region, form important fisheries. but by far the most important food fishes are those which are taken in salt water. here we have two types of fisheries, those where the fish comes up a river to spawn, as the salmon, sturgeon, or shad, and those in which fishes are taken on their feeding grounds in the open ocean. herring are the world's most important catch, though not in this country. here the salmon of the western coast is taken to the value of over $ , , a year. cod fishing also forms an important industry; over men being employed and over $ , , of codfish being taken each year in this country. [illustration: globe fisheries.] hundreds of other species of fish are used as food, the fish that is nearest at hand being often the cheapest and best. why, for example, is the flounder so cheap in the new york markets? in what waters are the cod and herring fisheries, sardine, oyster, sponge, pearl oyster? (see chart on page .) amphibia and reptiles as food.--frogs' legs are esteemed a delicacy. certain reptiles are used as food by people of other nationalities, the iguana, a mexican lizard, being an example. many of the sea-water turtles are of large size, the leatherback and the green turtle often weighing six hundred to seven hundred pounds each. the flesh of the green turtle and especially of the diamond-back terrapin, an animal found in the salt marshes along our southeastern coast, is highly esteemed as food. unfortunately for the preservation of the species, these animals are usually taken during the breeding season when they go to sandy beaches to lay their eggs. birds as food.--birds, both wild and domesticated, form part of our food supply. unfortunately our wild game birds are disappearing so fast that we should not consider them as a source of food. our domestic fowls, turkey, ducks, etc., form an important food supply and poultry farms give lucrative employment to many people. eggs of domesticated birds are of great importance as food, and egg albumin is used for other purposes,--clarifying sugars, coating photographic papers, etc. mammals as food.--when we consider the amount of wealth invested in cattle and other domesticated animals bred and used for food in the united states, we see the great economic importance of mammals. the united states, argentina, and australia are the greatest producers of cattle. in this country hogs are largely raised for food. they are used fresh, salted, smoked as ham and bacon, and pickled. sheep, which are raised in great quantities in australia, argentina, russia, uruguay, and this country, are one of the world's greatest meat supplies. goats, deer, many larger game animals, seals, walruses, etc., give food to people who live in parts of the earth that are less densely populated. domesticated animals.-- when man emerged from his savage state on the earth, one of the first signs of the beginning of civilization was the domestication of animals. the dog, the cow, sheep, and especially the horse, mark epochs in the advance of civilization. beasts of burden are used the world over, horses almost all over the world, certain cattle, as the water buffalo, in tropical malaysia; camels, goats, and the llama are also used as draft animals in some other countries. [illustration: feeding silkworms. the caterpillars are the white objects in the trays.] man's wealth in many parts of the world is estimated in terms of his cattle or herds of sheep. so many products come from these sources that a long list might be given, such as meats, milk, butter, cheese, wool, or other body coverings, leather, skins, and hides used for other purposes. great industries are directly dependent upon our domesticated animals, as the making of shoes, the manufacture of woolen cloth, the tanning industry, and many others. uses for clothing.--the manufacture of silk is due to the production of raw silk by the silkworm, the caterpillar of a moth. it lives upon the mulberry and makes a cocoon from which the silk is wound. the chinese silkworm is now raised to a slight extent in southern california. china, japan, italy, and france, because of cheaper labor, are the most successful silk-raising countries. the use of wool gives rise to many great industries. after the wool is cut from the sheep, it has to be washed and scoured to get out the dirt and grease. this wool fat or lanoline is used in making soap and ointments. the wool is next "carded," the fibers being interwoven by the fine teeth of the carding machine or "combed," the fibers here being pulled out parallel to each other. carded wool becomes woolen goods; combed wool, worsted goods. the wastes are also utilized, being mixed with "shoddy" (wool from cloth cuttings or rags) to make woolen goods of a cheap grade. goat hair, especially that of the angora and the cashmere goat, has much use in the clothing industries. camel's hair and alpaca are also used. [illustration: polar bear, a fur-bearing mammal which is rapidly being exterminated. why?] fur.--the furs of many domesticated and wild animals are of importance. the carnivora as a group are of much economic importance as the source of most of our fur. the fur seal fisheries alone amount to many millions of dollars annually. otters, skunks, sables, weasels, foxes, and minks are of considerable importance as fur producers. even cats are now used for fur, usually masquerading under some other name. the fur of the beaver, one of the largest of the rodents or gnawing mammals, is of considerable value, as are the coats of the chinchilla, muskrats, squirrels, and other rodents. the fur of the rabbit and nutria are used in the manufacture of felt hats. the quills of the porcupines (greatly developed and stiffened hairs) have a slight commercial value. conservation of fur-bearing animals needed.--as time goes on and the furs of wild animals become scarcer and scarcer through overkilling, we find the need for protection and conservation of many of these fast-vanishing wild forms more and more imperative. already breeding of some fur-bearing animals has been tried with success, and cheap substitutes for wild animal skins are coming more and more into the markets. black-fox breeding has been tried successfully in prince edward island, canada, $ to $ being given for a single skin. skunk, marten, and mink are also being bred for the market. game preserves in this country and canada are also helping to preserve our wild fur-bearing animals. animal oils.--whale oil, obtained from the fat or "blubber" of whales, is used extensively for lubricating. neat's-foot oil comes from the feet of cattle and is also used in lubrication. tallow and lard, two fats from cattle, sheep, and pigs, have so many well-known uses that comment is unnecessary. cod-liver oil is used medically and is well known. but it is not so widely known that a fish called the menhaden or "moss bunkers" of the atlantic coast produces over , , gallons of oil every year and is being rapidly exterminated in consequence. hides, horns, hoofs, etc.--leathers, from cattle, horses, sheep, and goats, are used everywhere. leather manufacture is one of the great industries of the eastern states, hundreds of millions of dollars being invested in its manufacturing plants. horns and bones are utilized for making combs, buttons, handles for brushes, etc. glue is made from the animal matter in bones. ivory, obtained from elephant, walrus, and other tusks, forms a valuable commercial product. it is largely used for knife handles, piano keys, combs, etc. perfumes.--the musk deer, musk ox, and muskrat furnish a valuable perfume called musk. civet cats also give us a somewhat similar perfume. ambergris, a basis for delicate perfumes, comes from the intestines of the sperm whale. protozoa.--the protozoa have played an important part in rock building. the chalk beds of kansas and other chalk formations are made up to a large extent of the tiny skeletons of _protozoa_, called _foraminifera_. some limestone rocks are also composed in large part of such skeletons. the skeletons of some species are used to make a polishing powder. sponges.--the sponges of commerce have the skeleton composed of tough fibers of material somewhat like that of cow's horn. this fiber is elastic and has the power of absorbing water. in a living state, the horny fiber sponge is a dark-colored fleshy mass, usually found attached to rocks. the warm waters of the mediterranean sea and the west indies furnish most of our sponges. the sponges are pulled up from their resting place on the bottom, by means of long-handled rakes operated by men in boats or are secured by divers. they are then spread out on the shore in the sun, and the living tissues allowed to decay; then after treatment consisting of beating, bleaching, and trimming, the bath sponge is ready for the market. some forms of coral are of commercial value. the red coral of the mediterranean sea is the best example. [illustration: in some countries little metal images of buddha are placed within the shells of living pearl oysters or clams. over these the mantle of the animal secretes a layer of mother of pearl as is shown in the picture.] pearls and mother of pearl.--pearls are prized the world over. it is a well-known fact that even in this country pearls of some value are sometimes found within the shells of the fresh-water mussel and the oyster. most of the finest, however, come from the waters around ceylon. if a pearl is cut open and examined carefully, it is found to be a deposit of the mother-of-pearl layer of the shell around some central structure. it has been believed that any foreign substance, as a grain of sand, might irritate the mantle at a given point, thus stimulating it to secrete around the substance. it now seems likely that most perfect pearls are due to the growth within the mantle of the clam or oyster of certain parasites, stages in the development of a flukeworm. the irritation thus set up in the tissue causes mother of pearl to be deposited around the source of irritation, with the subsequent formation of a pearl. the pearl-button industry in this country is largely dependent upon the fresh-water mussel, the shells of which are used. this mussel is being so rapidly depleted that the national government is working out a means of artificial propagation of these animals. honey and wax.--honeybees[ ] are kept in hives. a colony consists of a queen, a female who lays the eggs for the colony, the drones, whose duty it is to fertilize the eggs, and the workers. footnote : their daily life may be easily watched in the schoolroom, by means of one of the many good and cheap observation hives now made to be placed in a window frame. directions for making a small observation hive for school work can be found in hodge, _nature study and life_, chap. xiv. bulletin no. , u. s. department of agriculture, entitled _the honey bee_, by frank benton, is valuable for the amateur beekeeper. it may be obtained for twenty-five cents from the superintendent of documents, union building, washington, d.c. [illustration: cells of honeycomb, queen cell on right at bottom.] the cells of the comb are built by the workers out of wax secreted from the under surface of their bodies. the wax is cut off in thin plates by means of the wax shears between the two last joints of the hind legs. these cells are used to place the eggs of the queen in, one egg to each cell, and the young are hatched after three days, to begin life as footless white grubs. the young are fed for several days, then shut up in the cells and allowed to form pupæ. eventually they break their cells and take their place as workers in the hive, first as nurses for the young and later as pollen gatherers and honey makers. we have already seen (pages to ) that the honeybee gathers nectar, which she swallows, keeping the fluid in her crop until her return to the hive. here it is forced out into cells of the comb. it is now thinner than what we call honey. to thicken it, the bees swarm over the open cells, moving their wings very rapidly, thus evaporating some of the water. a hive of bees have been known to make over thirty-one pounds of honey in a single day, although the average is very much less than this. it is estimated from twenty to thirty millions of dollars' worth of honey and wax are produced each year in this country. cochineal and lac.--among other products of insect origin is cochineal, a red coloring matter, which consists of the dried bodies of a tiny insect, one of the plant lice which lives on the cactus plants in mexico and central america. the lac insect, another one of the plant lice, feeds on the juices of certain trees in india and pours out a substance from its body which after treatment forms shellac. shellac is of much use as a basis for varnish. gall insects.--oak galls, growths caused by the sting of wasp-like insects, give us products used in ink making, in tanning, and in making pyrogallic acid which is much used in developing photographs. insects destroy harmful plants or animals.--some forms of animal life are of great importance because of their destruction of harmful plants or animals. [illustration: an insect friend of man. an ichneumon fly boring in a tree to lay its eggs in the burrow of a boring insect harmful to that tree.] a near relative of the bee, called the ichneumon fly, does man indirectly considerable good because of its habit of laying its eggs and rearing the young in the bodies of caterpillars which are harmful to vegetation. some of the ichneumons even bore into trees in order to deposit their eggs in the larvæ of wood-boring insects. it is safe to say that the ichneumons save millions of dollars yearly to this country. several beetles are of value to man. most important of these is the natural enemy of the orange-tree scale, the ladybug, or ladybird beetle. in new york state it may often be found feeding upon the plant lice, or aphids, which live on rosebushes. the carrion beetles and many water beetles act as scavengers. the sexton beetles bury dead carcasses of animals. ants in tropical countries are particularly useful as scavengers. insects, besides pollinating flowers, often do a service by eating harmful weeds. thus many harmful plants are kept in check. we have noted that they spin silk, thus forming clothing; that in many cases they are preyed upon, and that they supply an enormous multitude of birds, fishes, and other animals with food. [illustration: the common toad, an insect eater.] use of the toad.--the toad is of great economic importance to man because of its diet. no less than eighty-three species of insects, mostly injurious, have been proved to enter into the dietary. a toad has been observed to snap up one hundred and twenty-eight flies in half an hour. thus at a low estimate it could easily destroy one hundred insects during a day and do an immense service to the garden during the summer. it has been estimated by kirkland that a single toad may, on account of the cutworms which it kills, be worth $ . each season it lives, if the damage done by each cutworm be estimated at only one cent. toads also feed upon slugs and other garden pests. birds eat insects.--the food of birds makes them of the greatest economic importance to our country. this is because of the relation of insects to agriculture. a large part of the diet of most of our native birds includes insects harmful to vegetation. investigations undertaken by the united states department of agriculture (division of biological survey) show that a surprisingly large number of birds once believed to harm crops really perform a service by killing injurious insects. even the much maligned crow lives to some extent upon insects. swallows in the southern states kill the cotton-boll weevil, one of our worst insect pests. our earliest visitor, the bluebird, subsists largely on injurious insects, as do woodpeckers, cuckoos, kingbirds, and many others. the robin, whose presence in the cherry tree we resent, during the rest of the summer does much good by feeding upon noxious insects. birds use the food substances which are most abundant around them at the time.[ ] footnote : the following quotation from i. p. trimble, _a treatise on the insect enemies of fruit and shade trees_, bears out this statement: "on the fifth of may, , ... seven different birds ... had been feeding freely upon small beetles.... there was a great flight of beetles that day; the atmosphere was teeming with them. a few days after, the air was filled with ephemera flies, and the same species of birds were then feeding upon them." during the outbreak of rocky mountain locusts in nebraska in - , professor samuel aughey saw a long-billed marsh wren carry thirty locusts to her young in an hour. at this rate, for seven hours a day, a brood would consume locusts per day, and the passerine birds of the eastern half of nebraska, allowing only twenty broods to the square mile, would destroy daily , , of the pests. the average locust weighs about fifteen grains, and is capable each day of consuming its own weight of standing forage crops, which at $ per ton would be worth $ . . this case may serve as an illustration of the vast good that is done every year by the destruction of insect pests fed to nestling birds. and it should be remembered that the nesting season is also that when the destruction of injurious insects is most needed; that is, at the period of greatest agricultural activity and before the parasitic insects can be depended on to reduce the pests. the encouragement of birds to nest on the farm and the discouragement of nest robbing are therefore more than mere matters of sentiment; they return an actual cash equivalent, and have a definite bearing on the success or failure of the crops.--_year book of the department of agriculture._ [illustration: food of some common birds. which of the above birds should be protected by man and why?] birds eat weed seeds.--not only do birds aid man in his battles with destructive insects, but seed-eating birds eat the seeds of weeds. our native sparrows (not the english sparrow), the mourning dove, bobwhite, and other birds feed largely upon the seeds of many of our common weeds. this fact alone is sufficient to make birds of vast economic importance. not all birds are seed or insect feeders. some, as the cormorants, ospreys, gulls, and terns, are active fishers. near large cities gulls especially act as scavengers, destroying much floating garbage that otherwise might be washed ashore to become a menace to health. the vultures of india and semitropical countries are of immense value as scavengers. birds of prey (owls) eat living mammals, including many rodents; for example, field mice, rats, and other pests. extermination of our native birds.--within our own times we have witnessed the almost total extermination of some species of our native birds. the american passenger pigeon, once very abundant in the middle west, is now extinct. audubon, the greatest of all american bird lovers, gives a graphic account of the migration of a flock of these birds. so numerous were they that when the flock rose in the air the sun was darkened, and at night the weight of the roosting birds broke down large branches of the trees in which they rested. to-day not a single wild specimen of this pigeon can be found, because they were slaughtered by the hundreds of thousands during the breeding season. the wholesale killing of the snowy egret to furnish ornaments for ladies' headwear is another example of the improvidence of our fellow-countrymen. charles dudley warner said, "feathers do not improve the appearance of an ugly woman, and a pretty woman needs no such aid." wholesale killing for plumage, eggs, and food, and, alas, often for mere sport, has reduced the number of our birds more than one half in thirty states and territories within the past fifteen years. every crusade against indiscriminate killing of our native birds should be welcomed by all thinking americans. the recent mclane bill which aims at the protection of migrating birds and the bird-protecting clause of the recently passed tariff bill shows that this country is awaking to the value of her bird life. without the birds the farmer would have a hopeless fight against insect pests. the effect of killing native birds is now well seen in italy and japan, where insects are increasing and do greater damage each year to crops and trees. of the eight hundred or more species of birds in the united states, only six species of hawks (cooper's and the sharp-shinned hawk in particular), and the great horned owl, which prey upon useful birds; the sapsucker, which kills or injures many trees because of its fondness for the growing layer of the tree; the bobolink, which destroys yearly $ , , worth of rice in the south; the crow, which feeds on crops as well as insects; and the english sparrow, may be considered as enemies of man. the english sparrow.--the english sparrow is an example of a bird introduced for the purpose of insect destruction, that has done great harm because of its relation to our native birds. introduced at brooklyn in for the purpose of exterminating the cankerworm, it soon abandoned an insect diet and has driven out most of our native insect feeders. investigations by the united states department of agriculture have shown that in the country these birds and their young feed to a large extent upon grain, thus showing them to be injurious to agriculture. dirty and very prolific, it already has worked its way from the east as far as the pacific coast. in this area the bluebird, song sparrow, and yellowbird have all been forced to give way, as well as many larger birds of great economic value and beauty. the english sparrow has become a pest especially in our cities, and should be exterminated in order to save our native birds. it is feared in some quarters that the english starling which has recently been introduced into this country may in time prove a pest as formidable as the english sparrow. [illustration: this shows how some snakes (constrictors) kill and eat their prey. (series photographed by c. w. beebe and clarence halter.)] food of snakes.--probably the most disliked and feared of all animals are the snakes. this feeling, however, is rarely deserved, for, on the whole, our common snakes are beneficial to man. the black snake and the milk snake feed largely on injurious rodents (rats, mice, etc.), the pretty green snake eats injurious insects, and the little dekay snake feeds partially on slugs. if it were not that the rattlesnake and the copperhead are venomous, they also could be said to be useful, for they live on english sparrows, rats, mice, moles, and rabbits. food of herbivorous animals.--we must not forget that other animals besides insects and birds help to keep down the rapidly growing weeds. herbivorous animals the world over destroy, besides the grass which they eat, untold multitudes of weeds, which, if unchecked, would drive out the useful occupants of the pasture, the grasses and grains. harm done by animals economic loss from insects.--the money value of crops, forest trees, stored foods, and other material destroyed annually by insects is beyond belief. it is estimated that they get one tenth of the country's crops, at the lowest estimate a matter of some $ , , yearly. "the common schools of the country cost in the sum of $ , , , and all higher institutions of learning cost less than $ , , , making the total cost of education in the united states considerably less than the farmers lost from insect ravages. "furthermore, the yearly losses from insect ravages aggregate nearly twice as much as it costs to maintain our army and navy; more than twice the loss by fire; twice the capital invested in manufacturing agricultural implements; and nearly three times the estimated value of the products of all the fruit orchards, vineyards, and small fruit farms in the country."--slingerland. the total yearly value of all farm and forest products in new york is perhaps $ , , , and the one tenth that the insects get is worth $ , , . insects which damage garden and other crops.--the grasshoppers and the larvæ of various moths do considerable harm here, especially the "cabbage worm," the cutworm, a feeder on all kinds of garden truck, and the corn worm, a pest on corn, cotton, tomatoes, peas, and beans. among the beetles which are found in gardens is the potato beetle, which destroys the potato plant. this beetle formerly lived in mexico upon a wild plant of the same family as the potato, and came north upon the introduction of the potato into colorado, evidently preferring cultivated forms to wild forms of this family. [illustration: cotton-boll weevil. _a_, larva; _b_, pupa; _c_, adult. enlarged about four times. (photographed by davison.)] the one beetle doing by far the greatest harm in this country is the cotton-boll weevil. imported from mexico, since it has spread over eastern texas and into louisiana. the beetle lays its eggs in the young cotton fruit or boll, and the larvæ feed upon the substance within the boll. it is estimated that if unchecked this pest would destroy yearly one half of the cotton crop, causing a loss of $ , , . fortunately, the united states department of agriculture is at work on the problem, and, while it has not found any way of exterminating the beetle as yet, it has been found that, by planting more hardy varieties of cotton, the crop matures earlier and ripens before the weevils have increased in sufficient numbers to destroy the crop (see page ). the bugs are among our most destructive insects. the most familiar examples of our garden pests are the squash bug; the chinch bug, which yearly does damage estimated at $ , , , by sucking the juice from the leaves of grain; and the plant lice, or aphids. one, living on the grape, yearly destroys immense numbers of vines in the vineyards of france, germany, and california. [illustration: female tussock moth which has just emerged from the cocoon at the left, upon which it has deposited over two hundred eggs. (photograph by davison.)] [illustration: caterpillar of tussock moth. (photograph by davison.)] insects which harm fruit and forest trees.--great damage is annually done trees by the larvæ of moths. massachusetts has already spent over $ , , in trying to exterminate the imported gypsy moth. the codling moth, which bores into apples and pears, is estimated to ruin yearly $ , , worth of fruit in new york alone, which is by no means the most important apple region of the united states. among these pests, the most important to the dweller in a large city is the tussock moth, which destroys our shade trees. the caterpillar may easily be recognized by its hairy, tufted red head. the eggs are laid on the bark of shade trees in what look like masses of foam. (see figure on page .) by collecting and burning the egg masses in the fall, we may save many shade trees the following year. the larvæ of some moths damage the trees by boring into the wood of the tree on which they live. such are the peach, apple, and other fruit-tree borers common in our orchards. many beetle larvæ also live in trees and kill annually thousands of forest and shade trees. the hickory borer threatens to kill all the hickory trees in the eastern states. among the bugs most destructive to trees are the scale insect and the plant lice. the san josé scale, a native of china, was introduced into the fruit groves of california about and has spread all over the country. a ladybird beetle, which has also been imported, is the most effective agent in keeping this pest in check. insects of the house or storehouse.--weevils are the greatest pests, frequently ruining tons of stored corn, wheat, and other cereals. roaches will eat almost anything, even clothing; they are especially fond of all kinds of breadstuffs. the carpet beetle is a recognized foe of the housekeeper, the larvæ feeding upon all sorts of woolen material. the larvæ of the clothes moth do an immense amount of damage, especially to stored clothing. fleas, lice, and particularly bedbugs are among man's personal foes. besides being unpleasant they are believed to be disease carriers and as such should be exterminated.[ ] footnote : directions for the treatment of these pests may be found in pamphlets issued by the u. s. department of agriculture. food of starfish.--starfish are enormously destructive to young clams and oysters, as the following evidence, collected by professor a. d. mead, of brown university, shows. a single starfish was confined in an aquarium with fifty-six young clams. the largest clam was about the length of one arm of the starfish, the smallest about ten millimeters in length. in six days every clam in the aquarium was devoured. hundreds of thousands of dollars' damage is done annually to the oysters in connecticut alone by the ravages of starfish. during the breeding season of the clam and oyster the boats dredge up tons of starfish which are thrown on shore to die or to be used as fertilizer. the relations of animals to disease [illustration: the life history of the malarial parasite. this cut of the malarial parasite shows parts of the body of the mosquito and of man. to understand the life history begin at the point where the mosquito injects the crescent-shaped bodies into the blood of man. notice that after the spores are released from the corpuscles of man two kinds of cells _may be formed_. these are probably a sexual stage. development within the body of the mosquito will only take place when the parasite is taken into its body at this sexual stage.] the cause of malaria.--the study of the life history and habits of the protozoa has resulted in the finding of many parasitic forms, and the consequent explanation of some kinds of disease. one parasitic protozoan like an amoeba is called _plasmodium malariæ_. it causes the disease known as malaria. when a mosquito (the _anopheles_) sucks the blood from a person having malaria this parasite passes into the stomach of the mosquito. after completing a part of its life history within the mosquito's body the parasite establishes itself within the glands which secrete the saliva of the mosquito. after about eight days, if the infected mosquito bites a person, some of the parasites are introduced into the blood along with the saliva. these parasites enter the corpuscles of the blood, increase in size, and then form spores. the rapid process of spore formation results in the breaking down of the blood corpuscles and the release of the spores, and the poisons they manufacture, into the blood. this causes the chill followed by the fever so characteristic of malaria. the spores may again enter the blood corpuscles and in forty-eight or seventy-two hours repeat the process thus described, depending on the kind of malaria they cause. the only cure for the disease is _quinine_ in rather large doses. this kills the parasites in the blood. but quinine should not be taken except under a physician's directions. [illustration: how to distinguish the harmless mosquito (_culex_), _a_, from the malarial mosquito (_anopheles_), _b_, when at rest. notice the position of legs and body.] the malarial mosquito.--fortunately for mankind, not all mosquitoes harbor the parasite which causes malaria. the harmless mosquito (_culex_) may be usually distinguished from the mosquito which carries malaria (_anopheles_) by the position taken when at rest. culex lays eggs in tiny rafts of one hundred or more eggs in any standing water; thus the eggs are distinguished from those of anopheles, which are not in rafts. rain barrels, gutters, or old cans may breed in a short time enough mosquitoes to stock a neighborhood. the larvæ are known as wigglers. they breathe through a tube in the posterior end of the body, and may be recognized by their peculiar movement when on their way to the surface to breathe. the pupa, distinguished by a large thoracic region, breathes through a pair of tubes on the thorax. the fact that both larvæ and pupæ take air from the surface of the water makes it possible to kill the mosquito during these stages by pouring oil on the surface of the water where they breed. the introduction of minnows, gold fish, or other small fish which feed upon the larvæ in the water where the mosquitoes breed will do much to free a neighborhood from this pest. draining swamps or low land which holds water after a rain is another method of extermination. some of the mosquito-infested districts around new york city have been almost freed from mosquitoes by draining the salt marshes where they breed. long shallow trenches are so built as to tap and drain off any standing water in which the eggs might be laid. in this way the mosquito has been almost exterminated along some parts of our new england coast. [illustration: swamps are drained and all standing water covered with a film of oil in order to exterminate mosquitoes. why is the oil placed on the surface of the water?] since the beginning of historical times, malaria has been prevalent in regions infested by mosquitoes. the ancient city of rome was so greatly troubled by periodic outbreaks of malarial fever that a goddess of fever came to be worshiped in order to lessen the severity of what the inhabitants believed to be a divine visitation. at the present time the malaria of italy is being successfully fought and conquered by the draining of the mosquito-breeding marshes. by a little carefully directed oiling of water a few boys may make an almost uninhabitable region absolutely safe to live in. why not try it if there are mosquitoes in your neighborhood? yellow fever and mosquitoes.--another disease carried by mosquitoes is yellow fever. in the year there were , cases and , deaths in the united states, mostly in alabama, louisiana, and mississippi. during the french occupation of the panama canal zone the work was at a standstill part of the time because of the ravages of yellow fever. before the war with spain thousands of people were ill in cuba. but to-day this is changed, and yellow fever is under almost complete control, both here and in the canal zone, where the mosquito (_stegomyia_) which carries yellow fever exists. [illustration: notice the difference in the number of yearly deaths from yellow fever before and _after_ the american occupation of havana.] this is due to the experiments during the summer of of a commission of united states army officers, headed by dr. walter reed. of these men one, dr. jesse lazear, gave up his life to prove experimentally that yellow fever was caused by mosquitoes. he allowed himself to be bitten by a mosquito that was known to have bitten a yellow fever patient, contracted the disease, and died a martyr to science. others, soldiers, volunteered to further test by experiment how the disease was spread, so that in the end dr. reed was able to prove to the world that if mosquitoes could be prevented from biting people who had yellow fever the disease could not be spread. the accompanying illustration shows the result of this knowledge for the city of havana. for years havana was considered one of the pest spots of the west indies. visitors shunned this port and commerce was much affected by the constant menace of yellow fever. at the time of the american occupation after the war with spain, the experiments referred to above were undertaken. the city was cleaned up, proper sanitation introduced, screens placed in most buildings, and the breeding places of the mosquitoes were so nearly destroyed that the city was practically free from mosquitoes. the result, so far as yellow fever was concerned, was startling, as you can see by reference to the chart. notice also the rise in the death rate when the young cuban republic took control. how do you account for that? we all know what american scientific medicine and sanitation is doing in panama and in the philippines. [illustration: stegomyia, the carrier of yellow fever. (after howard.)] other protozoan diseases.--many other diseases of man are probably caused by parasitic protozoans. dysentery of one kind appears to be caused by the presence of an amoeba-like animal in the digestive tract which comes usually through an impure water supply. smallpox, rabies, and possibly other diseases are caused by protozoans. smallpox, which was once the most dreaded disease known to man, because of its spread in epidemics, has been conquered by _vaccination_, of which we shall learn more later. the death rate from rabies or hydrophobia has in a like manner been greatly reduced by a treatment founded on the same principles as vaccination and invented by louis pasteur. another group of protozoan parasites are called _trypanosomes_. these are parasitic in insects, fish, reptiles, birds, and mammals in various parts of the world. they cause various diseases of cattle and other domestic animals, being carried to the animal in most cases by flies. one of this family is believed to live in the blood of native african zebras and antelopes; seemingly it does them no harm. but if one of these parasites is transferred by the dreaded tsetse fly to one of the domesticated horses or cattle of the colonist of that region, death of the animal results. another fly carries a species of trypanosome to the natives of central africa, which causes "the dreaded and incurable sleeping sickness." this disease carries off more than fifty thousand natives yearly, and many europeans have succumbed to it. its ravages are now largely confined to an area near the large central african lakes and the upper nile, for the fly which carries the disease lives near water, seldom going more than feet from the banks of streams or lakes. the british government is now trying to control the disease in uganda by moving all the villages at least two miles from the lakes and rivers. among other diseases that may be due to protozoans is kala-agar, a fever in hot asiatic countries which is probably carried by the bedbug, and african tick fever, probably carried by a small insect called the tick. bubonic plague, one of the most dreaded of all infectious diseases, is carried to man by fleas from rats. in this country many fatal diseases of cattle, as "tick," or texas cattle fever, are probably caused by protozoans. [illustration: life history of house flies, showing from left to right the eggs, larvæ, pupæ, and adult flies. (photograph, about natural size, by overton.)] the fly a disease carrier.--we have already seen that mosquitoes of different species carry malaria and yellow fever. another rather recent addition to the black list is the house fly or typhoid fly. we shall see later with what reason this name is given. the development of the typhoid fly is extremely rapid. a female may lay from one hundred to two hundred eggs. these are usually deposited in filth or manure. dung heaps about stables, privy vaults, ash heaps, uncared-for garbage cans, and fermenting vegetable refuse form the best breeding places for flies. in warm weather, the eggs hatch a day or so after they are laid and become larvæ, called maggots. after about one week of active feeding, these wormlike maggots become quiet and go into the pupal stage, whence under favorable conditions they emerge within less than another week as adult flies. the adults breed at once, and in a short summer there may be over ten generations of flies. this accounts for the great number. fortunately relatively few flies survive the winter. the membranous wings of the adult fly appear to be two in number, a second pair being reduced to tiny knobbed hairs called balancers. the head is freely movable, with large compound eyes. the mouth parts form a proboscis, which is tonguelike, the animal obtaining its food by lapping and sucking. the foot shows a wonderful adaptation for clinging to smooth surfaces. two or three pads, each of which bears tubelike hairs that secrete a sticky fluid, are found on its under surface. it is by this means that the fly is able to walk upside down, and carry bacteria on its feet. [illustration: the foot of a fly, showing the hooks, hairs, and pads which collect and carry bacteria. the fly doesn't wipe his feet.] [illustration: colonies of bacteria which have developed in a culture medium upon which a fly was allowed to walk.] the typhoid fly a pest.--the common fly is recognized as a pest the world over. flies have long been known to spoil food through their filthy habits, but it is more recently that the very serious charge of spread of diseases, caused by bacteria, has been laid at their door. in a recent experiment two young men from the connecticut agricultural station found that a single fly might carry on its feet anywhere from to , , bacteria, the average number being over , , . not all of these germs are harmful, but they might easily include those of typhoid fever, tuberculosis, summer complaint, and possibly other diseases. a recent pamphlet published by the merchants' association in new york city shows that the rapid increase of flies during the summer months has a definite correlation with the increase in the number of cases of summer complaint. observations in other cities seem to show the increase in number of typhoid cases in the early fall is due, in part at least, to the same cause. a terrible toll of disease and death may be laid at the door of the typhoid fly. [illustration: showing how flies may spread disease by means of contaminating food.] recently the stable fly has been found to carry the dread disease known as infantile paralysis. [illustration: there were typhoid cases in jacksonville, florida, in , in , in first months of . to per cent of outdoor toilets were made fly proof during winter of . account for the decrease in typhoid after the flies were kept out of the toilets.] remedies.--cleanliness which destroys the breeding place of flies, the frequent removal and destruction of garbage, rubbish, and manure, covering of all food when not in use and especially the _careful_ screening of windows and doors during the breeding season, will all play a part in the reduction of flies. to the motto "swat the fly" should be added, "remove their breeding places!" [illustration: flea which transmits bubonic plague from rat to man.] other insect disease carriers.--fleas and bedbugs have been recently added to those insects proven to carry disease to man. bubonic plague, which is primarily a disease of rats, is undoubtedly transmitted from the infected rats to man by the fleas. fleas are also believed to transmit leprosy although this is not proven. to rid a house of fleas we must first find their breeding places. old carpets, the sleeping places of cats or dogs or any dirty unswept corner may hold the eggs of the flea. the young breed in cracks and crevices, feeding upon organic matter there. eventually they come to live as adults on their warm-blooded hosts, cats, dogs, or man. evidently destruction of the breeding places, careful washing of all infected areas, the use of benzine or gasoline in crevices where the larvæ may be hid are the most effective methods of extermination. pets which might harbor fleas should be washed frequently with a weak (two to three per cent) solution of creolin. bedbugs are difficult to prove as an agent in the transmission of disease but their disgusting habits are sufficient reason for their extermination. it has been proven by experiment that they may spread typhoid and relapsing fevers. they prefer human blood to other food and have come to live in bedrooms and beds because this food can be obtained there. they are extremely difficult to exterminate because their flat body allows them to hide in cracks out of sight. wooden beds are thus better protection for them than iron or brass beds. boiling water poured over the cracks when they breed or a mixture of strong corrosive sublimate four parts, alcohol four parts and spirits of turpentine one part, are effective remedies. how the harm done by insects is controlled.--the combating of insects is directed by several bodies of men, all of which have the same end in view. these are the bureau of entomology of the united states department of agriculture, the various state experiment stations, and medical and civic organizations. the bureau of entomology works in harmony with the other divisions of the department of agriculture, giving the time of its experts to the problems of controlling insects which, for good or ill, influence man's welfare in this country. the destruction of the malarial mosquito and control of the typhoid fly; the destruction of harmful insects by the introduction of their natural enemies, plant or animal; the perfecting of the honeybee (see hodge, _nature study and life_, page ), and the introduction of new species of insects to pollinate flowers not native to this country (see _blastophaga_, page ), are some of the problems to which these men are now devoting their time. all the states and territories have, since , established state experiment stations, which work in coöperation with the government in the war upon injurious insects. these stations are often connected with colleges, so that young men who are interested in this kind of natural science may have opportunity to learn and to help. the good done by these means directly and indirectly is very great. bulletins are published by the various state stations and by the department of agriculture, most of which may be obtained free. the most interesting of these from the high school standpoint are the farmers' bulletins, issued by the department of agriculture, and the nature study pamphlets issued by the cornell university in new york state. [illustration: this diagram shows how bubonic plague is carried to man. explain the diagram.] animals other than insects may be disease carriers.--the common brown rat is an example of a mammal, harmful to civilized man, which has followed in his footsteps all over the world. starting from china, it spread to eastern europe, thence to western europe, and in it had obtained a lodgment in this country. in seventy-five years it reached the pacific coast, and is now fairly common all over the united states, being one of the most prolific of all mammals. rats are believed to carry bubonic plague, the "black death" of the middle ages, a disease estimated to have killed , , people during the fourteenth century. the rat, like man, is susceptible to plague; fleas bite the rat and then biting man transmit the disease to him. a determined effort is now being made to exterminate the rat because of its connection with bubonic plague. other parasitic animals cause disease.--besides parasitic protozoans other forms of animals have been found that _cause_ disease. chief among these are certain round and flat worms, which have come to live as parasites on man and other animals. a one-sided relationship has thus come into existence where the worm receives its living from the host, as the animal is called on which the parasite lives. consequently the parasite frequently becomes fastened to its host during adult life and often is reduced to a mere bag through which the fluid food prepared by its host is absorbed. sometimes a complicated life history has arisen from their parasitic habits. such is seen in the life history of the liver fluke, a flatworm which kills sheep, and in the tapeworm. [illustration: the life cycle of a tapeworm. ( ) the eggs are taken in with filthy food by the pig; ( ) man eats undercooked pork by means of which the bladder worm ( ) is transferred to his own intestine ( ).] cestodes or tapeworms.--these parasites infest man and many other vertebrate animals. the tapeworm (_tænia solium_) passes through two stages in its life history, the first within a pig, the second within the intestine of man. the developing eggs are passed off with wastes from the intestine of man. the pig, an animal with dirty habits, may take in the worm embryos with its food. the worm develops within the intestine of the pig, but soon makes its way into the muscle or other tissues. it is here known as a bladderworm. if man eats raw or undercooked pork containing these worms, _he_ may become a host for the tapeworm. thus during its complete life history it has two hosts. another common tapeworm parasitic on man lives part of its life as an embryo within the muscles of cattle. the adult worm consists of a round headlike part provided with hooks, by means of which it fastens itself to the wall of the intestine. this head now buds off a series of segmentlike structures, which are practically bags full of sperms and eggs. these structures, called _proglottids_, break off from time to time, thus allowing the developing eggs to escape. the proglottids have no separate digestive systems, but the whole body surface, bathed in digested food, absorbs it and is thus enabled to grow rapidly. [illustration: _trichinella spiralis_ imbedded in human muscle. (after leuckart.)] roundworms.--still other wormlike creatures called roundworms are of importance to man. some, as the vinegar eel found in vinegar, or the pinworms parasitic in the lower intestine, particularly of children, do little or no harm. the pork worm or _trichina_, however, is a parasite which may cause serious injury. it passes through the first part of its existence as a parasite in a pig or other vertebrate (cat, rat, or rabbit), where it lies, covered within a tiny sac or _cyst_, in the muscles of its hosts. if raw pork containing these worms is eaten by man, the cyst is dissolved off by the action of the digestive fluids, and the living trichina becomes free in the intestine of man. here it reproduces and the young bore their way through the intestine walls and enter the muscles, causing inflammation there. this causes a painful and often fatal disease known as _trichinosis_. the hookworm.--the discovery by dr. c. w. stiles of the bureau of animal industry, that the laziness and shiftlessness of the "poor whites" of the south is partly due to a parasite called the _hookworm_, reads like a fairy tale. the people, largely farmers, become infected with a larval stage of the hookworm, which develops in moist earth. it enters the body usually through the skin of the feet, for children and adults alike, in certain localities where the disease is common, go barefoot to a considerable extent. a complicated journey from the skin to the intestine now follows, the larvæ passing through the veins to the heart, from there to the lungs; here they bore into the air passages and eventually work their way by way of the windpipe into the intestine. one result of the injury of the lungs is that many thus infected are subject to tuberculosis. the adult worms, once in the food tube, fasten themselves and feed upon the blood of their host by puncturing the intestine wall. the loss of blood from this cause is not sufficient to account for the bloodlessness of the person infected, but it has been discovered that the hookworm pours out a poison into the wound which prevents the blood from clotting rapidly (see page ); hence a considerable loss of blood occurs from the wound after the worm has finished its meal and gone to another part of the intestine. [illustration: a family suffering from hookworm.] the cure of the disease is very easy; thymol is given, which weakens the hold of the worm, this being followed by epsom salts. for years a large area in the south undoubtedly has been retarded in its development by this parasite; hundreds of millions of dollars and thousands of lives have been needlessly sacrificed. "the hookworm is not a bit spectacular: it doesn't get itself discussed in legislative halls or furiously debated in political campaigns. modest and unassuming, it does not aspire to such dignity. it is satisfied simply with ( ) lowering the working efficiency and the pleasure of living in something like two hundred thousand persons in georgia and all other southern states in proportion; with ( ) amassing a death rate higher than tuberculosis, pneumonia, or typhoid fever; with ( ) stubbornly and quite effectually retarding the agricultural and industrial development of the section; with ( ) nullifying the benefit of thousands of dollars spent upon education; with ( ) costing the south, in the course of a few decades, several hundred millions of dollars. more serious and closer at hand than the tariff; more costly, threatening, and tangible than the negro problem; making the menace of the boll weevil laughable in comparison--it is preëminently the problem of the south."--_atlanta constitution._ animals that prey upon man.--the toll of death from animals which prey upon or harm man directly is relatively small. snakes in tropical countries kill many cattle and not a few people. the bite of the rattlesnake of our own country, although dangerous, seldom kills. the dreaded cobra of india has a record of over two hundred and fifty thousand persons killed in the last thirty-five years. the indian government yearly pays out large sums for the extermination of venomous snakes, over two hundred thousand of which have been killed during a single year. [illustration: a flesh-eating reptile, the alligator.] alligators and crocodiles.--these feed on fishes, but often attack large animals, as horses, cows, and even man. they seek their prey chiefly at night, and spend the day basking in the sun. the crocodiles of the ganges river in india levy a yearly tribute of many hundred lives from the natives. carnivorous animals such as lions and tigers still inflict damage in certain parts of the world, but as the tide of civilization advances, their numbers are slowly but surely decreasing so that as important factors in man's welfare they may be considered almost negligible. reference books elementary hunter, _laboratory problems in civic biology_. american book company. beebe, _the bird_. henry holt and company. bigelow, _applied biology_. macmillan and company. davison, _practical zoölogy_. american book company. herrick, _household insects and methods of control_. cornell reading courses. hornaday, _our vanishing wild life_. new york zoölogical society. hodge, _nature study and life_. ginn and company. kipling, _captains courageous_. charles scribner's sons. sharpe, _laboratory manual_, pp. - , - , - . american book company. stone and cram, _american animals_. doubleday, page and company. toothaker, _commercial raw materials_. ginn and company. advanced flower, _the horse_. d. appleton and company. hornaday, _the american natural history_. macmillan and company. jordan, _fishes_. henry holt and company. jordan and evermann, _american food and game fishes_. doubleday, page and company. schaler, _domesticated animals, their relations to man and to his advancement in civilization_. charles scribner's sons. xvi. the fish and frog, an introductory study of vertebrates _problems._--_to determine how a fish and a frog are fitted for the life they lead._ _to determine some methods of development in vertebrate animals._ _(a) fishes._ _(b) frogs._ _(c) other animals._ laboratory suggestions _laboratory exercise._--study of a living fish--adaptations for protection, locomotion, food getting, etc. _laboratory demonstration._--the development of the fish or frog egg. _visit to the aquarium._--study of adaptations, economic uses of fishes, artificial propagation of fishes. two methods of breathing in vertebrates.--vertebrate animals have at least two methods of getting their oxygen. in other respects their life processes are nearly similar. of all vertebrates fishes are the only ones fitted to breathe all their lives under water. other vertebrates are provided with lungs and take their oxygen directly from the air.[ ] we will next take up the study of a fish to see how it is fitted for its life in the water. footnote : with the exception of a few lungless salamanders. most salamanders get much of their supply of oxygen through their moist skins. study of a fish the body.--one of our common fresh-water fish is the bream, or golden shiner. the body of the bream runs insensibly into the head, the neck being absent. the long, narrow body with its smooth surface fits the fish admirably for its life in the water. certain cells in the skin secrete mucus or slime, another adaptation. the position of the scales, overlapping in a backward direction, is yet another adaptation which aids in passing through the water. its color, olive above and bright silver and gold below, is protective. can you see how? [illustration: the bream. _a_, dorsal fin; _b_, caudal fin; _c_, anal fin; _d_, pelvic fin; _e_, pectoral fin.] the appendages and their uses.--the appendages of the fish consist of paired and unpaired fins. the paired fins are four in number, and are believed to correspond in position and structure with the paired limbs of a man. note the illustration above and locate the paired _pectoral_ and _pelvic_ fins. (these are so called because they are attached to the bones forming the pectoral and pelvic girdles. see page .) find, by comparison with the figure, the _dorsal_, _anal_, and _caudal_ fins. how many unpaired fins are there? the flattened, muscular body of the fish, tapering toward the caudal fin, is moved from side to side with an undulating motion which results in the forward movement of the fish. this movement is almost identical with that of an oar in sculling a boat. turning movements are brought about by use of the lateral fins in much the same way as a boat is turned. we notice the dorsal and other single fins are evidently useful in balancing and steering. the senses.--the position of the eyes at the side of the head is an evident advantage to the fish. why? the eye is globular in shape. such an eye has been found to be very nearsighted. thus it is unlikely that a fish is able to perceive objects at any great distance from it. the eyes are unprotected by eyelids, but the tough outer covering and their position afford some protection. feeding experiments with fishes show that a fish becomes aware of the presence of food by smelling it as well as by seeing it. the nostrils of a fish can be proved to end in little pits, one under each nostril hole. thus they differ from our own, which are connected with the mouth cavity. in the catfish, for example, the _barbels_, or horns, receive sensations of smell and taste. they do not perceive odors as we do for a fish perceives only substances that are dissolved in the water in which it lives. the senses of taste and touch appear to be less developed than the other senses. along each side of most fishes is a line of tiny pits, provided with sense organs and connected with the central nervous system of the fish. this area, called the _lateral line_, is believed to be sensitive to mechanical stimuli of certain sorts. the "ear" of the fish is under the skin and serves partly as a balancing organ. food getting.--a fish must go after its food and seize it, but has no structures for grasping except the teeth. consequently we find the teeth small, sharp, and numerous, well adapted for holding living prey. the tongue in most fishes is wanting or very slightly developed. breathing.--a fish, when swimming quietly or when at rest, seems to be biting when no food is present. a reason for this act is to be seen when we introduce a little finely powdered carmine into the water near the head of the fish. it will be found that a current of water enters the mouth at each of these biting movements and passes out through two slits found on each side of the head of the fish. investigation shows us that under the broad, flat plate, or _operculum_, forming each side of the head, lie several long, feathery, red structures, the _gills_. [illustration: diagram of the gills of a fish. (_h_), the heart which forces the blood into the tubes (_v_), which run out into the gill filaments. a gill bar (_g_) supports each gill. the blood after exchanging its carbon dioxide for oxygen is sent out to the cells of the body through the artery (_a_).] gills.--if we examine the gills of any large fish, we find that a single gill is held in place by a bony arch, made of several pieces of bone which are hinged in such a way as to give great flexibility to the gill arch, as the support is called. covering the bony framework, and extending from it, are numerous delicate filaments covered with a very thin membrane or skin. into each of these filaments pass two blood vessels; in one blood flows downward and in the other upward. blood reaches the gills and is carried away from these organs by means of two large vessels which pass along the bony arch previously mentioned. in the gill filament the blood comes into contact with the free oxygen of the water bathing the gills. an exchange of gases through the walls of the gill filaments results in the loss of carbon dioxide and a gain of oxygen by the blood. the blood carries oxygen to the cells of the body and (as work is done by the cells as a result of the oxidation of food) brings carbon dioxide back to the gills. gill rakers.--if we open wide the mouth of any large fish and look inward, we find that the mouth cavity leads to a funnel-like opening, the gullet. on each side of the gullet we can see the gill arches, guarded on the inner side by a series of sharp-pointed structures, the _gill rakers_. in some fishes in which the teeth are not well developed, there seems to be a greater development of the gill rakers, which in this case are used to strain out small organisms from the water which passes over the gills. many fishes make such use of the gill rakers. such are the shad and menhaden, which feed almost entirely on _plankton_, a name given to the small plants and animals found by millions in the water. digestive system.--the gullet leads directly into a baglike stomach. there are no salivary glands in the fishes. there is, however, a large liver, which appears to be used as a digestive gland. this organ, because of the oil it contains, is in some fishes, as the cod, of considerable economic importance. many fishes have outgrowths like a series of pockets from the intestine. these structures, called the _pyloric cæca_, are believed to secrete a digestive fluid. the intestine ends at the vent, which is usually located on the under side of the fish, immediately in front of the anal fin. [illustration: a fish opened to show _h_, the heart; _g_, the gills; _l_, the liver; _s_, the stomach; _i_, the intestine; _o_, the ovary; _k_, the kidney, and _b_, the air bladder.] swim bladder.--an organ of unusual significance, called the _swim bladder_, occupies the region just dorsal to the food tube. in young fishes of many species this is connected by a tube with the anterior end of the digestive tract. in some forms this tube persists throughout life, but in other fishes it becomes closed, a thin, fibrous cord taking its place. the swim bladder aids in giving the fish nearly the same weight as the water it displaces, thus buoying it up. the walls of the organ are richly supplied with blood vessels, and it thus undoubtedly serves as an organ for supplying oxygen to the blood when all other sources fail. in some fishes (the _dipnoi_, page ) it has come to be used as a lung. circulation of the blood.--in the vertebrate animals the blood is said to circulate in the body, because it passes through a more or less closed system of tubes in its course around the body. in the fishes the heart is a two-chambered muscular organ, a thin-walled _auricle_, the receiving chamber, leading into a thick-walled muscular _ventricle_ from which the blood is forced out. the blood is pumped from the heart to the gills; there it loses some of its carbon dioxide; it then passes on to other parts of the body, eventually breaking up into very tiny tubes called _capillaries_. from the capillaries the blood returns, in tubes of gradually increasing diameter, toward the heart again. the body cells lie between the smallest branches of the capillaries. thus they get from the blood food and oxygen and return to the blood the wastes resulting from oxidation within the cell body. during its course some of the blood passes through the kidneys and is there relieved of part of its nitrogenous waste. circulation of blood in the body of the fish is rather slow. the temperature of the blood being nearly that of the surrounding media in which the fish lives, the animal has incorrectly been given the term "cold-blooded." nervous system.--as in all other vertebrate animals, the brain and spinal cord of the fish are partially inclosed in bone. the central nervous system consists of a _brain_, with nerves connecting the organs of sight, taste, smell, and hearing, and such parts of the body as possess the sense of touch; a _spinal cord_; and _spinal nerves_. nerve cells located near the outside of the body send in messages to the central system, which are there received as sensations. cells of the central nervous system, in turn, send out messages which result in the movement of muscles. skeleton.--in the vertebrates, of which the bony fish is an example, the skeleton is under the skin, and is hence called an _endoskeleton_. it consists of a bony framework, the vertebral column which protects the spinal cord and certain attached bones, the ribs, with other spiny bones to which the unpaired fins are attached. the paired fins are attached to the spinal column by two collections of bones, known respectively as the _pectoral_ and _pelvic girdles_. the bones in the main skeleton serve in the fish for the attachment of powerful muscles, by means of which locomotion is accomplished. in most fishes, the _exoskeleton_, too, is well developed, consisting usually of scales, but sometimes of bony plates. food of fishes.--we have already seen that in a balanced aquarium the balance of food was preserved by the plants, which furnished food for the tiny animals or were eaten by larger ones,--for example, snails or fish. the smaller animals in turn became food of larger ones. the nitrogen balance was maintained through the excretions of the animals and their death and decay. the marine world is a great balanced aquarium. the upper layer of water is crowded with all kinds of little organisms, both plant and animal. some of these are microscopic in size; others, as the tiny crustaceans, are visible to the eye. on these little organisms some fish feed entirely, others in part. such are the menhaden[ ] (bony, bunker, mossbunker of our coast), the shad, and others. other fishes are bottom feeders, as the blackfish and the sea bass, living almost entirely upon mollusks and crustaceans. still others are hunters, feeding upon smaller species of fish, or even upon their weaker brothers. such are the bluefish, squeteague or weakfish, and others. footnote : it has been discovered by professor mead of brown university that the increase in starfish along certain parts of the new england coast was in part due to overfishing of menhaden, which at certain times in the year feed almost entirely on the young starfish. what is true of salt-water fish is equally true of those inhabiting our fresh-water streams and lakes. it is one of the greatest problems of our bureau of fisheries to discover this relation of various fishes to their food supplies so as to aid in the conservation and balance of life in our lakes, rivers, and seas. migration of fishes.--some fishes change their habitat at different times during the year, moving in vast schools northward in summer and southward in the winter. in a general way such migrations follow the coast lines. examples of such migratory fish are the cod, menhaden, herring, and bluefish. the migrations are due to temperature changes, to the seeking after food, and to the spawning instinct. some fish migrate to shallower water in the summer and to deeper water in the winter; here the reason for the migration is doubtless the change in temperature. [illustration: development of a trout. , the embryo within the egg; , the young fish just hatched with the yoke sac still attached; , the young fish.] the egg-laying habits of the bony fishes.--the eggs of most bony fishes are laid in great numbers, varying from a few thousand in the trout to many hundreds of thousands in the shad and several millions in the cod. the time of egg-laying is usually spring or early summer. at the time of spawning the male usually deposits milt, consisting of millions of sperm cells, in the water just over the eggs, thus accomplishing fertilization. some fishes, as sticklebacks, sunfish, toadfish, etc., make nests, but usually the eggs are left to develop by themselves, sometimes attached to some submerged object, but more frequently free in the water. in some eggs a tiny oil drop buoys up the egg to the surface, where the heat of the sun aids development. they are exposed to many dangers, and both eggs and developing fish are eaten, not only by birds, fish of other species, and other water inhabitants, but also by their own relatives, and even parents. consequently a very small percentage of eggs ever produce mature fish. the relation of the spawning habits to economic importance of fish.--the spawning habits of fish are of great importance to us because of the economic value of fish to mankind, not only directly as a food, but indirectly as food for other animals in turn valuable to man. many of our most desirable food fishes, notably the salmon, shad, sturgeon, and smelt, pass up rivers from the ocean to deposit their eggs, swimming against strong currents much of the way, some species leaping rapids and falls, in order to deposit their eggs in localities where the conditions of water and food are suitable, and the water shallow enough to allow the sun's rays to warm it sufficiently to cause the eggs to develop. the chinook salmon of the pacific coast, the salmon used in the western canning industry, travels over a thousand miles up the columbia and other rivers, where it spawns. the salmon begin to pass up the rivers in early spring, and reach the spawning beds, shallow deposits of gravel in cool mountain streams, before late summer. here the fish, both males and females, remain until the temperature of the water falls to about ° fahrenheit. the eggs and milt are then deposited, and the old fish die, leaving the eggs to be hatched out later by the heat of the sun's rays. need of conservation.--the instinct of this and other species of fish to go into shallow rivers to deposit their eggs has been made use of by man. at the time of the spawning migration the salmon are taken in vast numbers, for the salmon fisheries net over $ , , annually. but the need for conservation of this important national asset is great. the shad have within recent time abandoned their breeding places in the connecticut river, and the salmon have been exterminated along our eastern coast within the past few decades. it is only a matter of a few years when the western salmon will be extinct if fishing is continued at the present rate. more fish must be allowed to reach their breeding places. to do this a closed season on the rivers of two or three days out of each seven while the shad or the salmon run would do much good. the sturgeon, the eggs of which are used in the manufacture of the delicacy known as _caviar_, is an example of a fish that is almost extinct in this part of the world. other food fish taken at the breeding season are also in danger. [illustration: artificial fertilization of fish eggs.] artificial propagation of fishes.--fortunately, the government through the bureau of fisheries, and various states by wise protective laws and by artificial propagation of fishes, are beginning to turn the tide. certain days of the week the salmon are allowed to pass up the columbia unmolested. closed breeding seasons protect our trout, bass, and other game fish, also the catching of fish under a certain size is prohibited. [illustration: early development of salmon. natural size.] many fish hatcheries, both government and state, are engaged in artificially fertilizing millions of fish eggs of various species and protecting the young fry until they are of such size that they can take care of themselves, when they are placed in ponds or streams. this artificial fertilization is usually accomplished by first squeezing out the ripe eggs from a female into a pan of water; in a similar manner the milt or sperm cells are obtained, and poured over the eggs. the eggs are thus fertilized. they are then placed in receptacles supplied with running water and left to develop under favorable conditions. shortly after the egg has segmented (divided into many cells) the embryo may be seen developing on one side of the egg. the rest of the egg is made up of food or yolk, and when the baby fish hatches it has for some time the yolk attached to its ventral surface. eventually the food is absorbed into the body of the fish. the development of the fish is direct, the young fish becoming an adult without any great change in form. the young fry are kept under ideal conditions until later, when they are shipped, sometimes thousands of miles, to their new homes. note to teacher.--it is suggested that in the spring term the frog be studied, but if animal biology be taken up during the fall term the fish only might be used. the frog adaptations for life.--the most common frog in the eastern part of the united states is the leopard frog. it is recognized by its greenish brown body with dark spots, each spot being outlined in a lighter-colored background. in spite of the apparent lack of harmony with their surroundings, their color appears to give almost perfect protection. in some species of frogs the color of the skin changes with the surroundings of the frog, another means of protection. adaptations for life in the water are numerous. the ovoid body, the head merging into the trunk, the slimy covering (for the frog is provided, like the fish, with mucus cells in the skin), and the powerful legs with webbed feet, are all evidences of the life which the frog leads. locomotion.--you will notice that the appendages have the same general position on the body and same number of parts as do your own (upper arm, forearm, and hand; thigh, shank, and foot, the latter much longer relatively than your own). note that while the hand has four fingers, the foot has five toes, the latter connected by a web. in swimming the frog uses the stroke we all aim to make when we are learning to swim. most of the energy is liberated from the powerful backward push of the hind legs, which in a resting position are held doubled up close to the body. on land, locomotion may be by hopping or crawling. [illustration: this diagram shows how the frog uses its tongue to catch insects.] sense organs.--the frog is well provided with sense organs. the eyes are large, globular, and placed at the side of the head. when they are closed, a delicate fold, or third eyelid, called the _nictitating membrane_, is drawn over each eye. frogs probably see best moving objects at a few feet from them. their vision is much keener than that of the fish. the external ear (_tympanum_) is located just behind the eye on the side of the body. frogs hear sounds and distinguish various calls of their own kind, as is proved by the fact that frogs recognize the warning notes of their mates when any one is approaching. the inner ear also has to do with balancing the body as it has in fishes and other vertebrates. taste and smell are probably not strong sensations in a frog or toad. they bite at moving objects of almost any kind when hungry. the long flexible tongue, which is fastened at the front, is used to catch insects. experience has taught these animals that moving things, insects, worms, and the like, make good food. these they swallow whole, the tiny teeth being used to hold the food. touch is a well-developed sense. they also respond to changes in temperature under water, remaining there in a dormant state for the winter when the temperature of the air becomes colder than that of the water. breathing.--the frog breathes by raising and lowering the floor of the mouth, pulling in air through the two nostril holes. then the little flaps over the holes are closed, and the frog swallows this air, forcing it down into the baglike lungs. the skin is provided with many tiny blood vessels, and in winter, while the frogs are dormant at the bottom of the ponds, it serves as the only organ of respiration. [illustration: internal organs of a frog: m, mouth; t, tongue; lu, lungs; h, heart; st, stomach; i, small intestine; l, liver; g, gall bladder; p, pancreas; c, cloaca; b, urinary bladder; s, spleen; k, kidney; od, oviduct; o, ovary; br, brain; sc, spinal cord; ba, back bone.] the food tube and its glands.--the mouth leads like a funnel into a short tube, the _gullet_. on the lower floor of the mouth can be seen the slitlike _glottis_ leading to the lungs. the gullet widens almost at once into a long _stomach_, which in turn leads into a much coiled intestine. this widens abruptly at the lower end to form the _large intestine_. the latter leads into the _cloaca_ (latin, _sewer_), into which open the _kidneys_, _urinary bladder_, and reproductive organs (_ovaries_ or _spermaries_). several _glands_, the function of which is to produce digestive fluids, open into the food tube. these digestive fluids, by means of the ferments or enzymes contained in them, change insoluble food materials into a soluble form. this allows of the absorption of food material through the walls of the food tube into the blood. the glands (having the same names and uses as those in man) are the _salivary glands_, which pour their juices into the mouth, the _gastric glands_ in the walls of the stomach, and the _liver_ and _pancreas_, which open into the intestine. circulation.--the frog has a well-developed heart, composed of a thick-walled muscular ventricle and two thin-walled auricles. the heart pumps the blood through a system of closed tubes to all parts of the body. blood enters the right auricle from all parts of the body; it then contains considerable carbon dioxide; the blood entering the left auricle comes from the lungs, hence it contains a considerable amount of oxygen. blood leaves the heart through the ventricle, which thus pumps some blood containing much and some containing little oxygen. before the blood from the tissues and lungs has time to mix, however, it leaves the ventricle and by a delicate adjustment in the vessels leaving the heart most of the blood containing much oxygen is passed to all the various organs of the body, while the blood deficient in oxygen, but containing a large amount of carbon dioxide, is pumped to the lungs, where an exchange of oxygen and carbon dioxide takes place by osmosis. in the tissues of the body wherever work is done the process of burning or oxidation must take place, for by such means only is the energy necessary to do the work released. food in the blood is taken to the muscle cells or other cells of the body and there oxidized. the products of the burning--carbon dioxide--and any other organic wastes given off from the tissues must be eliminated from the body. as we know, the carbon dioxide passes off through the lungs and to some extent through the skin of the frog, while the nitrogenous wastes, poisons which must be taken from the blood, are eliminated from it in the kidneys. change of form in development of the frog.--not all vertebrates develop directly into an adult. the frog, for example, changes its form completely before it becomes an adult. this change in form is known as a _metamorphosis_. let us examine the development of the common leopard frog. [illustration: development of a frog. , two cell stage; , four cell stage; , cells are formed, notice the upper cells are smaller; in ( ) the lower cells are seen to be much larger because of the yolk; , the egg has continued to divide and has formed a gastrula; , , the body is lengthening, head is seen at the right hand end; , the young tadpole with external gills; , , the gills are internal, hind legs beginning to form; , the hind legs show plainly; , , , later stages in development; , the adult frog. figures , , , , , , and are very much enlarged. (drawn after leukart and kny by frank m. wheat.)] the eggs of this frog are laid in shallow water in the early spring. masses of several hundred, which may be found attached to twigs or other supports under water, are deposited at a single laying. immediately before leaving the body of the female they receive a coating of jellylike material, which swells up after the eggs are laid. thus they are protected from the attack of fish or other animals which might use them as food. the upper side of the egg is dark, the light-colored side being weighted down with a supply of yolk (food). the fertilized egg soon segments (divides into many cells), and in a few days, if the weather is warm, these eggs have each grown into an oblong body which shows the form of a tadpole. shortly after the tadpole wriggles out of the jellylike case and begins life outside the egg. at first it remains attached to some water weed by means of a pair of suckerlike projections; later a mouth is formed, and the tadpole begins to feed upon algæ or other tiny water plants. at this time, about two weeks after the eggs were laid, gills are present on the outside of the body. soon after, the external gills are replaced by gills which grow out under a fold of the skin which forms an operculum somewhat as in the fish. water reaches the gills through the mouth and passes out through a hole on the left side of the body. as the tadpole grows larger, legs appear, the hind legs first, although for a time locomotion is performed by means of the tail. in the leopard frog the change from the egg to adult is completed in one summer. in late july or early august, the tadpole begins to eat less, the tail becomes smaller (being absorbed into other parts of the body), and before long the transformation from the tadpole to the young frog is complete. in the green frog and bullfrog the metamorphosis is not completed until the beginning of the second summer. the large tadpoles of such forms bury themselves in the soft mud of the pond bottom during the winter. shortly after the legs appear, the gills begin to be absorbed, and lungs take their place. at this time the young animal may be seen coming to the surface of the water for air. changes in the diet of the animal also occur; the long, coiled intestine is transformed into a much shorter one. the animal, now insectivorous in its diet, becomes provided with tiny teeth and a mobile tongue, instead of keeping the horny jaws used in scraping off algæ. after the tail has been completely absorbed and the legs have become full grown, there is no further structural change, and the metamorphosis is complete. [illustration: at the left is a hen's egg, opened to show the embryo at the center (the spot surrounded by a lighter area). at the right is an english sparrow one day after hatching.] development of birds.--the white of the hen's egg is put on during the passage of the real egg (which is in the yolk or yellow portion) to the outside of the body. before the egg is laid a shell is secreted over its surface. if the fertilized egg of a hen be broken and carefully examined, on the surface of the yolk will be found a little circular disk. this is the beginning of the growth of an _embryo_ chick. if a series of eggs taken from an incubator at periods of twenty-four hours or less apart were examined, this spot would be found at first to increase in size; later the little embryo would be found lying on the surface. still later small blood vessels could be made out reaching into the yolk for food, the tiny heart beating as early as the second day of incubation. after about three weeks of incubation the little chick hatches; that is, breaks the shell, and emerges in almost the same form as the adult. [illustration: the embryo (_e_) of a mammal, showing the absorbing organ in black, the branch-like processes which absorb blood from the mother being shown at (_v_); _ct_, the tube connecting the embryo with the absorbing organ or placenta.] development of a mammal.--in mammals after fertilization the egg undergoes development within the body of the mother. instead of blood vessels connecting the embryo with the yolk as in the chick, here the blood vessels are attached to an absorbing organ, known as the _placenta_. this structure sends branch-like processes into the wall of the _uterus_ (the organ which holds the embryo) and absorbs nourishment and oxygen by osmosis from the blood of the mother. after a length of time which varies in different species of mammals (from about three weeks in a guinea pig to twenty-two months in an elephant), the young animal is expelled by muscular contraction of the uterus, or is born. the young, usually, are born in a helpless condition, then nourished by milk furnished by the mother until they are able to take other food. thus we see as we go higher in the scale of life fewer eggs formed, but those few eggs are more carefully protected and cared for by the parents. the chances of their growth into adults are much greater than in the cases when many eggs are produced. reference books elementary hunter, _laboratory problems in civic biology_. american book company. bigelow, _introduction to biology_. the macmillan company. cornell _nature study leaflets_. bulletins xvi, xvii. davison, _practical zoölogy_, pages - . american book company. hodge, _nature study and life_, chaps. xvi, xvii. ginn and company. sharpe, _laboratory manual_, pp. , - . american book company. advanced dickerson, _the frog book_. doubleday, page and company. holmes, _the biology of the frog_. the macmillan company. jordan, _fishes_. henry holt and company. morgan, _the development of the frog's egg_. the macmillan company. needham, _general biology_. comstock publishing company. xvii. heredity, variation, plant and animal breeding _problems.--to determine what makes the offspring of animals or plants tend to be like their parents._ _to determine what makes the offspring of animals and plants differ from their parents._ _to learn about some methods of plant and animal breeding._ _(a) by selection._ _(b) by hybridizing._ _(c) by other methods._ _to learn about some methods of improving the human race._ _(a) by eugenics._ _(b) by euthenics._ suggestions for laboratory work _laboratory exercise._--on variation and heredity among members of a class in the schoolroom. _laboratory exercise._--on construction of curve of variation in measurements from given plants or animals. _laboratory demonstration._--stained egg cells (_ascaris_) to show chromosomes. _laboratory demonstrations._--to illustrate the part played in plant or animal breeding by (_a_) selection. (_b_) hybridizing. (_c_) budding and grafting. _laboratory demonstration._--from charts to illustrate how human characteristics may be inherited. heredity and eugenics heredity and what it means.--as i look over the faces of the boys in my class i notice that each boy seems to be more or less like each other boy in the class; he has a head, body, arms, and legs, and even in minor ways he resembles each of the other boys in the room. moreover, if i should ask him i have no doubt but that he would tell me that he resembled in many respects his mother or father. likewise if i should ask his _parents_ whom he resembled, they would say, "i can see his grandmother or his grandfather in him." this wonderful force which causes the likeness of the child to its parents and to _their_ parents we call _heredity_. heredity causes the plants as well as animals to be like their parents. if we trace the workings of heredity in our own individual case, we will probably find that we are molded like our ancestors not only in physical characteristics but in mental qualities as well. the ability to play the piano or to paint is probably as much a case of inheritance as the color of our eyes or the shape of our nose. we are a complex of physical and mental characters, received in part from all our ancestors. [illustration: variations in the catalpa caterpillar. (photographed, natural size, by davison.)] variation.--but i notice another thing; no boy in the class before me is _exactly_ like any other boy, even twins having minute differences. in this wonderful mold of nature each one of us tends to be slightly different from his or her parents. each plant, each animal, varies to a greater or lesser degree from its immediate ancestors and may vary to a very great degree. this factor in the lives of plants and animals is called _variation_. heredity and variation are the cornerstones on which all the work in the improvement of plants and animals, including man himself, are built. the bearers of heredity.--we have seen that somewhere in every living cell is a structure known as a nucleus. in this nucleus, which is a part of the living matter of the cell, are certain very minute structures always present, known as _chromosomes_. these chromosomes (so called because they take up color when stained) are believed to be the structures which contain the _determiners_ of the qualities which may be passed from parent plant to offspring or from animal to animal; in other words, the qualities that are inheritable (see page ). the germ cells.--but it has been found that certain cells of the body, the egg and the sperm cells, before uniting contain only half as many chromosomes as do the body cells. in preparing for the process of fertilization, half of these elements have been eliminated, so that when the egg and sperm cell are united they will have the full number of chromosomes that the other cells have. if the chromosomes carry the determiners of the characters which are inheritable, then it is easy to see that a fertilized egg must contain an equal number of chromosomes from the bodies of each parent. consequently characteristics from each parent are handed down to the new individual. this seems to be the way in which nature succeeds in obtaining variation, by providing cell material from two different individuals. offspring are part of their ancestors.--we can see that if you or i receive characteristics from our parents and they received characteristics from their parents, then we too must have some of the characteristics of the grandparents, and it is a matter of common knowledge that each of us does have some trait or lineament which can be traced back to our grandfather or grandmother. indeed, as far back as we are able to go, ancestors have added something. [illustration: comparison of sexual and asexual cell reproduction] charles darwin and natural selection.--the great englishman charles darwin was one of the first scientists to realize how this great force of heredity applied to the development or evolution of plants and animals. he knew that although animals and plants were like their ancestors, they also tended to vary. in nature, the variations which best fitted a plant or animal for life in its own environment were the ones which were handed down because those having variations which were not fitted for life in that particular environment would die. thus nature seized upon favorable variations and after a time, as the descendants of each of these individuals also tended to vary, a new species of plant or animal, fitted for the place it had to live in, would be gradually evolved. mutations.--recently a new method of variation has been discovered by a dutch naturalist, named hugo de vries. he found that new species of plants and animals arise suddenly by "mutations" or steps. this means that new species instead of arising from very slight variations, continuing during long periods of years (as darwin believed), might arise very suddenly as a very great variation which would at once breed true. it is easily seen that such a condition would be of immense value to breeders, as new plants or animals quite unlike their parents might thus be formed and perpetuated. it will be one of the future problems of plant and animal breeders to isolate and breed "mutants," as such organisms are called. [illustration: improvement in corn by selection. to the left, the corn improved by selection from the original type at the right.] artificial selection.--darwin reasoned that if nature seized upon favorable variants, then man, by selecting the variations he wanted, could form new varieties of plants or animals much more quickly than nature. and so to-day plant or animal breeders _select_ the forms having the characters they wish to perpetuate and breed them together. this method used by plant and animal breeders is known as _selection_. selective planting.--_by selective planting we mean choosing the best plants and planting the seed from these plants with a view of improving the yield._ in doing this we must not necessarily select the most perfect fruits or grains, but must select seeds from the _best plants_. a wheat plant should be selected not from its yield alone, but from its ability to stand disease and other unfavorable conditions. in a mr. fultz, of pennsylvania, found three heads of beardless or bald wheat while passing through a large field of bearded wheat. these were probably _mutants_ which had lost the chaff surrounding the kernel. mr. fultz picked them out, sowed them by themselves, and produced a quantity of wheat now known favorably all over the world as the fultz wheat. in selecting wheat, for example, we might breed for a number of different characters, such as more starch, or more protein in the grain, a larger yield per acre, ability to stand cold or drought or to resist plant disease. each of these characters would have to be sought for separately and could only be obtained after long and careful breeding. the work of mendel (see page ) when applied to plant breeding will greatly shorten the time required to produce better plants of a given kind. by careful seed selection, some western farmers have increased their wheat production by per cent. this, if kept up all over the united states, would mean over $ , , a year in the pockets of the farmers. hybridizing.--we have already seen that pollen from one flower may be carried to another of the same species, thus producing seeds. if pollen from one plant be placed on the pistil of another of an _allied_ species or variety, fertilization _may_ take place and new plants be eventually produced from the seeds. this process is known as _hybridizing_, and the plants produced by this process known as _hybrids_. [illustration: in hybridizing, all of the flower is removed at the line (_w_) except the pistil (_p_). then pollen from another flower of a nearly related kind is placed on the pistil and the pollinated flower covered up with a paper bag. can you explain why?] hybrids are extremely variable, rarely breed from seeds, and often are apparently quite unlike either parent plant. they must be grown for several years, and all plants that do not resemble the desired variety must be killed off, if we expect to produce a hybrid that will breed more plants like itself. luther burbank, the great hybridizer of california, destroys tens of thousands of plants in order to get one or two with the characters which he wishes to preserve. thus he is yearly adding to the wealth of this country by producing new plants or fruits of commercial value. a number of years ago he succeeded in growing a new variety of potato, which has already enriched the farmers of this country about $ , , . one of his varieties of black walnut trees, a very valuable hard wood, grows ten to twelve times as rapidly as ordinary black walnuts. with lumber yearly increasing in price, a quick growing tree becomes a very valuable commercial product. among his famous hybrids are the plumcot, a cross between an apricot and a plum, his numerous varieties of berries and his splendid "climax" plum, the result of a cross between a bitter chinese plum and an edible japanese plum. but none of burbank's products grow from seeds; they are all produced _asexually_, from hybrids by some of the processes described in the next paragraph. the department of agriculture and its methods.--the department of agriculture is also doing splendid work in producing new varieties of oranges and lemons, of grain and various garden vegetables. the greatest possibilities have been shown by department workers to be open to the farmer or fruit grower through hybridizing, and by budding, grafting, or slipping. [illustration: steps in budding. _a_, twig having suitable buds to use; _b_, method of cutting out bud; _c_, how the bark is cut; _d_, how the bark is opened; _e_, inserting the bud; _f_, the bud in place; _g_, the bud properly bound in place.] _budding._--if a given tree, for example, produces a kind of fruit which is of excellent quality, it is possible sometimes to attach parts of the tree to another strong tree of the same species that may not bear good fruit. this is done by _budding_. a t-shaped incision is cut in the bark; a bud from the tree bearing the desired fruit is placed in the cut and bound in place. when a shoot from the embedded bud grows out the following spring, it is found to have all the characters of the tree from which it was taken. [illustration: steps in tongue grafting. _a_, the two branches to be formed; _b_, a tongue cut in each; _c_, fitted together; _d_, method of wrapping.] grafting.--of much the same nature is grafting. here, however, a small portion of the stem of the closely allied tree is fastened into the trunk of the growing tree in such a manner that the two cut layers just under the bark will coincide. this will allow of the passage of food into the grafted part and insure the ultimate growth of the twig. grafting and budding are of considerable economic value to the fruit grower, as it enables him to produce at will, trees bearing choice varieties of fruit.[ ] footnote : for full directions for budding and grafting, see goff and mayne, _first principles of agriculture_, chap. xix, mayne and hatch, _high school agriculture_, pp. - , or hodge, _nature study and life_, pages - . other methods.--other methods of plant propagation are by means of runners, as when strawberry plants strike root from long stems that run along the ground; layering, where roots may develop on covered up branches of blackberry or raspberry plants; slips, roots developing from stems which are cut off and placed in moist sand; from tubers, as in planting potatoes; and by means of bulbs, as the tulip or hyacinth. all of the above means of propagation are asexual and are of importance in our problem of plant breeding. [illustration: plant breeding plots. (minnesota experiment station.)] the work of gregor mendel.--fifty years ago, an austrian monk, gregor mendel, found in breeding garden peas that these plants passed on certain _fixed characters_, as the shape of the seed, the color of the pod when ripe, and others, and that when two pea plants of different characters were crossed, one of these characters would be likely to appear in the offspring of the second generation in the ratio of three to one. such characters as would appear to the exclusion of others in the first crossing of the plants were called _dominant_, the ones not appearing, _recessive_ characteristics. when these seeds were again sown the ones bearing a recessive characteristic would produce only peas with this recessive characteristic, but the ones with a dominant characteristic might give rise to a pure dominant or to offspring having partly a dominant and partly a recessive character; pure dominants being to the mixed offspring in the ratio of to . the pure dominants if bred with others like themselves would produce only pure dominants, but the cross breeds would again produce mixed offspring of three kinds in the ratio of one dominant to two cross breeds and one recessive. the feature of this work that interests us is that _unit_ characters are passed along by heredity in the germ cells _pure_, that is, unchanged, from one generation to another, and independently of each other. [illustration: illustration of mendel's law.] determiners of character.--a child then resembles his parents in some definite particulars because certain _determiners_ of characters have been present in the germ cells of one of the parents. if the determiner of a certain character is _absent_ from the germ cells of both parents, it will be _absent_ in _all_ of their offspring. these discoveries of mendel are of the greatest importance in plant and animal breeding because they enable the breeder to isolate certain characters and by proper selection to breed varieties which have these desired characters, instead of waiting for a _chance_ union of the desired characters by nature. animal breeding.--it has been pointed out that the domestication of wild animals, the horse, cattle, sheep, goats, and the dog, marked a great advance in civilization in the history of the earth's peoples. as the young of these animals came to be bred in captivity the peoples owning them would undoubtedly pick out the strongest and best of the offspring, killing off the others for food. thus they came unconsciously to select and aid nature in producing a stronger and better stock. later man began to recognize certain characters that he wished to have in horses, dogs, or cattle, and so by slow processes of breeding and "crossing" or hybridizing one nearly allied form with another the numerous groups of domesticated animals began to appear. [illustration: what has resulted from artificial selection among dogs. (after romanes.)] in darwin's time animal breeding was so far advanced that he got his ideas of selection by nature in evolution from the artificial selection practiced by animal breeders. a glance at the pictures will give some idea of the changes that have taken place in the form of some animals since man began to breed them a few thousand years ago. [illustration: the four-toed ancestor of the present horse, restored from a study of its fossil skeleton. (after knight in american museum of natural history.)] some domesticated animals.--our domesticated dogs are descended from a number of wolflike forms in various parts of the world. all the present races of cats, on the other hand, seem to be traced back to egypt. modern horses are first noted in europe and asia, but far older forms flourished on the earth in former geologic periods. it is interesting to note that america was the original home of the horse, although at the time of the earliest explorers the horse was unknown here, the wild horse of the western plains having arisen from horses introduced by the spaniards. long ages ago, the first ancestors of the horse were probably little animals about the size of a fox. the earliest horse we have knowledge of had four toes on the fore and three toes on the hind foot. thousands of years later we find a larger horse, the size of a sheep, with a three-toed foot. by gradual changes, caused by the tendency of the animals to vary and by the action of the surroundings upon the animal in preserving these variations, there was eventually produced our present horse, an animal with legs adapted for rapid locomotion, with feet particularly fitted for the life in open fields, and with teeth which serve well to seize and grind herbage. knowledge of this sort was also used by darwin to show that constant changes in the form of animals have been taking place since life began on the earth. the horse, which for some reason disappeared in this country, continued to exist in europe, and man, emerging from his early savage condition, began to make use of the animal. we know the horse was domesticated in early biblical times, and that he soon became one of man's most valued servants. in more recent times, man has begun to change the horse by breeding for certain desired characteristics. in this manner have been established and improved the various types of horses familiar to us as draft horses, coach horses, hackneys, and the trotters. it is needless to say that all the various domesticated animals have been tremendously changed in a similar manner since civilized man has come to live on the earth. when we realize the very great amount of money invested in domesticated animals; that there are over , , each of sheep, cattle, and swine and over , , horses owned in this country, then we may see how very important a part the domestic animals play in our lives. improvement of man.--if the stock of domesticated animals can be improved, it is not unfair to ask if the health and vigor of the future generations of men and women on the earth might not be improved by applying to them the laws of selection. this improvement of the future race has a number of factors in which we as individuals may play a part. these are personal hygiene, selection of healthy mates, and the betterment of the environment. personal hygiene.--in the first place, good health is the one greatest asset in life. we may be born with a poor bodily machine, but if we learn to recognize its defects and care for it properly, we may make it do its required work effectively. if certain muscles are poorly developed, then by proper exercise we may make them stronger. if our eyes have some defect, we can have it remedied by wearing glasses. if certain drugs or alcohol lower the efficiency of the machine, we can avoid their use. with proper _care_ a poorly developed body may be improved and do effective work. eugenics.--when people marry there are certain things that the individual as well as the race should demand. the most important of these is freedom from germ diseases which might be handed down to the offspring. tuberculosis, syphilis, that dread disease which cripples and kills hundreds of thousands of innocent children, epilepsy, and feeble-mindedness are handicaps which it is not only unfair but criminal to hand down to posterity. the science of being well born is called _eugenics_. [illustration: in this and the following diagrams the circle represents a female, the square a male. n means normal; f means feeble-minded; a, alcoholic; t, tubercular; _sx_, sexually immoral; _sy_, having syphilis. this chart shows the record of a certain family for three generations. a normal woman married an alcoholic and tubercular man. he must have been feeble-minded also as two of his children were born feeble-minded. one of these children married another feeble-minded woman, and of their five children two died in infancy and three were feeble-minded. (after davenport.)] [illustration: this chart shows that feeble-mindedness is a characteristic sure to be handed down in a family where it exists. the feeble-minded woman at the top left of the chart married twice. the first children from a normal father are all normal, but the other children from an alcoholic father are all feeble-minded. the right-hand side of the chart shows a terrible record of feeble-mindedness. should feeble-minded people be allowed to marry? (after davenport.)] the jukes.--studies have been made on a number of different families in this country, in which mental and moral defects were present in one or both of the original parents. the "jukes" family is a notorious example. the first mother is known as "margaret, the mother of criminals." in seventy-five years the progeny of the original generation has cost the state of new york over a million and a quarter of dollars, besides giving over to the care of prisons and asylums considerably over a hundred feeble-minded, alcoholic, immoral, or criminal persons. another case recently studied is the "kallikak" family.[ ] this family has been traced back to the war of the revolution, when a young soldier named martin kallikak seduced a feeble-minded girl. she had a feeble-minded son from whom there have been to the present time descendants. of these were sexually immoral, confirmed drunkards, epileptics, and _feeble-minded_. the man who started this terrible line of immorality and feeble-mindedness later married a normal quaker girl. from this couple a line of descendants have come, with _no_ cases of feeble-mindedness. the evidence and the moral speak for themselves! footnote : the name kallikak is fictitious. parasitism and its cost to society.--hundreds of families such as those described above exist to-day, spreading disease, immorality, and crime to all parts of this country. the cost to society of such families is very severe. just as certain animals or plants become parasitic on other plants or animals, these families have become parasitic on society. they not only do harm to others by corrupting, stealing, or spreading disease, but they are actually protected and cared for by the state out of public money. largely for them the poorhouse and the asylum exist. they take from society, but they give nothing in return. they are true parasites. the remedy.--if such people were lower animals, we would probably kill them off to prevent them from spreading. humanity will not allow this, but we do have the remedy of separating the sexes in asylums or other places and in various ways preventing intermarriage and the possibilities of perpetuating such a low and degenerate race. remedies of this sort have been tried successfully in europe and are now meeting with success in this country. blood tells.--eugenics show us, on the other hand, in a study of the families in which are brilliant men and women, the fact that the descendants have received the _good_ inheritance from their ancestors. the following, taken from davenport's _heredity in relation to eugenics_, illustrates how one family has been famous in american history. in elizabeth tuttle, "of strong will, and of extreme intellectual vigor, married richard edwards of hartford, conn., a man of high repute and great erudition. from their one son descended another son, jonathan edwards, a noted divine, and president of princeton college. of the descendants of jonathan edwards much has been written; a brief catalogue must suffice: jonathan edwards, jr., president of union college; timothy dwight, president of yale; sereno edwards dwight, president of hamilton college; theodore dwight woolsey, for twenty-five years president of yale college; sarah, wife of tapping reeve, founder of litchfield law school, herself no mean lawyer; daniel tyler, a general in the civil war and founder of the iron industries of north alabama; timothy dwight, second, president of yale university from to ; theodore william dwight, founder and for thirty-three years warden of columbia law school; henrietta frances, wife of eli whitney, inventor of the cotton gin, who, burning the midnight oil by the side of her ingenious husband, helped him to his enduring fame; merrill edwards gates, president of amherst college; catherine maria sedgwick of graceful pen; charles sedgwick minot, authority on biology and embryology in the harvard medical school; edith kermit carow, wife of theodore roosevelt; and winston churchill, the author of _coniston_ and other well-known novels." [illustration: this record shows the inheritance of artistic ability (black circles and squares). (after davenport.)] of the daughters of elizabeth tuttle distinguished descendants also came. robert treat paine, signer of the declaration of independence; chief justice of the united states morrison r. waite; ulysses s. grant and grover cleveland, presidents of the united states. these and many other prominent men and women can trace the characters which enabled them to occupy the positions of culture and learning they held back to elizabeth tuttle. euthenics.--euthenics, the betterment of the environment, is another important factor in the production of a stronger race. the strongest physical characteristics may be ruined if the surroundings are unwholesome and unsanitary. the slums of a city are "at once symptom, effect, and cause of evil." a city which allows foul tenements, narrow streets, and crowded slums to exist will spend too much for police protection, for charity, and for hospitals. every improvement in surroundings means improvement of the chances of survival of the race. in the spring of the health department and street-cleaning department of the city of new york coöperated to bring about a "clean up" of all filth, dirt, and rubbish from the houses, streets, and vacant lots in that city. during the summer of the health department reported a smaller percentage of deaths of babies than ever before. we must draw our own conclusions. clean streets and houses, clean milk and pure water, sanitary housing, and careful medical inspection all do their part in maintaining a low rate of illness and death, thus reacting upon the health of the citizens of the future. it will be the purpose of the following pages to show how we may best care for our own bodies and how we may better the environment in which we are placed. reference books elementary hunter, _laboratory problems in civic biology_. american book company. bailey, _plant breeding_. macmillan and company. harwood, _new creations in plant life_. the macmillan company. jordan, _the heredity of richard roe_. american unitarian association. sharpe, _laboratory manual_, pp. - , - . american book company. advanced allen, _civics and health_. ginn and company. coulter, castle, east, tower, and davenport, _heredity and eugenics_. university of chicago press. davenport, _heredity in relation to eugenics_. henry holt and company. de vries, _plant breeding_. open court publishing company. goddard, _the kallikak family_. the macmillan company. kellicott, _the social direction of human evolution_. appleton company. punnet, _mendelism_. the macmillan company. richards, helen m., _euthenics, the science of controllable environment_. walter, _genetics_. the macmillan company. xviii. the human machine and its needs _problem.--to obtain a general understanding of the parts and uses of the bodily machine._ laboratory suggestions _demonstration._--review to show that the human body is a complex of cells. _laboratory demonstration_ by means of (_a_) human skeleton and (_b_) manikin to show the position and gross structure of the chief organs of man. man and his environment.--in the last chapter we saw that one factor in the improvement of man lies in giving him better surroundings. it will be the purpose of the following chapters to show how man is fitted to live in the environment in which he is placed. he comes in contact with air, light, water, soil, food, and shelter which make his somewhat artificial environment; he must adapt himself to get the best he can out of this environment. the needs of living things.--we have already found that the primary needs of plants and animals are the same. they both need food, they both need to digest their food and to have it circulate in a fluid form to the cells where it will be used. they both need oxygen so as to release the energy locked up in their food. and they both need to reproduce so that their kind may be continued on the earth. what is true of plants and other animals is true of man. the needs of simple and complex animals the same.--the simplest animal, a single cell, has the same needs as the most complex. the _cell_ paramoecium feeds, digests, oxidizes its food, and releases energy. the _cells_ of the human body built up into tissues have the same needs and perform the same functions as the paramoecium. it is the _cells_ of the body working together in groups as tissues and organs that make the complicated actions of man possible. division of labor has arisen because of the complex needs and work of the organism. [illustration: the human body seen from the side in longitudinal section.] the human body a machine.--in all animals, and the human animal is no exception, the body has been likened to a machine in that it turns over the _latent_ or potential energy stored up in food into _kinetic_ energy (mechanical work and heat), which is manifested when we perform work. one great difference exists between an engine and the human body. the engine uses fuel unlike the substance out of which it is made. the human body, on the other hand, uses for fuel the same substances out of which it is formed; it may, indeed, use part of its own substance for food. it must as well do more than purely mechanical work. the human organism must be so delicately adjusted to its surroundings that it will react in a ready manner to stimuli from without; it must be able to utilize its fuel (food) in the most economical manner; it must be fitted with machinery for transforming the energy received from food into various kinds of work; it must properly provide the machine with oxygen so that the fuel will be oxidized, and the products of oxidation must be carried away, as well as other waste materials which might harm the effectiveness of the machine. most important of all, the human machine must be able to repair itself. in order to understand better this complicated machine, the human body, let us briefly examine the structure of its parts and thus get a better idea of the interrelation of these parts and of their functions. the skin.--covering the body is a protective structure called the skin. covered on the outside with dead cells, yet it is provided with delicate sense organs, which give us perception of touch, taste, smell, pressure, and temperature. it also aids in getting wastes out of the body by means of its sweat glands and plays an important part in equalizing the temperature of the body. [illustration: skeleton of a man. _cr._, cranium; _cl._, clavicle; _st._, sternum; _h._, humerus; _v.c._, vertebral column; _r._, radius; _u._, ulna; _p._, pelvic girdle; _c._, carpals; _m._, metacarpals; _ph._, phalanges; _f._, femur; _fi._, fibula; _t._, tibia; _tar._, tarsals; _mt._, metatarsals.] bones and muscles.--the body is built around a framework of bones. these bones, which are bound together by tough _ligaments_, fall naturally into two great groups, the bones of the body proper, vertebral column, ribs, breast bone, and skull, which form the _axial_ skeleton, and the appendages, two sets of bones which form the framework of the arms and legs, which with the bones which attach them to the axial skeleton form the _appendicular_ skeleton. to the bones are attached the muscles of the body. movement is accomplished by contraction of muscles, which are attached so as to cause the bones to act as levers. bones also protect the nervous system and other delicate organs. they also help to give form and rigidity to the body. [illustration: diagram showing action of biceps muscle. _a_, contracted; _b_, extended; _h_, humerus; _s_, scapula.] hygiene of muscles and bones.--young people especially need to know how to prevent certain defects which are largely the result of bad habits of posture. standing erect is an example of a good habit, round shoulders a bad habit of this sort. the habit of a wrong position of bones and muscles once formed is very hard to correct. this can best be done by certain corrective exercises at home or in the gymnasium. round shoulders is most common among people whose occupation causes them to stoop. drawing, writing, and a wrong position when at one's desk are among the causes. exercises which strengthen the back muscles and cause the head to be kept erect are helpful in forming the habit of erect carriage. slight curvature of the spine either backward or forward is helped most by exercises which tend to straighten the body, such as stretching up with the hands above the head. lateral curvature of the spine, too often caused by a "hunched-up" position at the school desk, may also be corrected by exercises which tend to lengthen the spinal column. [illustration: three classes of levers in the human body; bones and muscles act together. _a_, a lever of the first class; _b_, a lever of the second class; _c_, a lever of the third class.] it is the duty of every girl and boy to have good posture and erect carriage, not only because of the better state of health which comes with it, but also because one's self-respect demands that each one of us makes the best of the gifts that nature has given us. an erect head, straight shoulders, and elastic carriage go far toward making their owner both liked and respected. [illustration: bad posture in the schoolroom may cause permanent injury to the spine.] other body structures.--in spaces between the muscles are found various other structures,--blood vessels, which carry blood to and from the great pumping station, the heart, and thence to all parts of the body; connective tissue, which holds groups of muscle or other cells together; fat cells, scattered in various parts of the body; various gland cells, which manufacture enzymes; and the cells of the nervous system, which aid in directing the body parts. body cavity.--within the body is a cavity, which in life is almost completely filled with various organs. a thin wall of muscle called the _diaphragm_ divides the body cavity into two unequal spaces. in the upper space are found the _heart_ and _lungs_, in the lower, the digestive tract with its glands, the _liver_, _kidneys_, and other structures (see page ). digestion, absorption, and excretion.--running through the body is a food tube in which undigested food is placed and from which digested or liquid food is absorbed into the blood so that the cells of the various organs which do the work may receive food. emptying into this food tube are various groups of gland cells, which pour digestive fluids over the solid foods, thus aiding in changing them to liquids. solid wastes are passed out through the posterior end of the food tube, while liquid wastes are excreted by means of glands called _kidneys_. work done by cells.--food, prepared in the digestive tract, and oxygen from the lungs are taken by the blood to the cells. bathed in liquid food, the cells do their work; they promote the oxidization of food and the exchange of carbon dioxide for oxygen in the blood, while other wastes of the cells are given off, to pass eventually through the kidneys and out of the body. the nervous system.--the smooth working of the bodily machine is due to another set of structures which direct the working of the parts so that they will act in unison. this director is the nervous system. we have seen that, in the simplest of animals, one cell performs the functions necessary to its existence. in the more complex animals, where groups of cells form tissues, each having a different function, a nervous system is developed. _the functions of the human nervous system are:_ ( ) _the providing of man with sensation, by means of which he gets in touch with the world about him;_ ( ) _the connecting of organs in different parts of the body so that they act as a united and harmonious whole;_ ( ) _the giving to the human being a will, a provision for thought._ coöperation in word and deed is the end attained. we are all familiar with examples of the coöperation of organs. you see food; the thought comes that it is good to eat; you reach out, take it, raise it to the mouth; the jaws move in response to your will; the food is chewed and swallowed. while digestion and absorption of the food are taking place, the nervous system is still in control. the nervous system also regulates pumping of blood over the body, respiration, secretion of glands, and, indeed, every bodily function. man is the highest of all animals because of the extreme development of the nervous system. man is the thinking animal, and as such is master of the earth. reference reading for this and succeeding chapters on human biology elementary hunter, _laboratory problems in civic biology_. american book company. davison, _the human body and health_. american book company. gulick, _the gulick hygiene series_. ginn and company. overton, _general hygiene_. american book company. ritchie, _human physiology_. world book company. sharpe, _laboratory manual in botany_, pages - . american book company. advanced halliburton, _kirk's handbook of physiology_. p. blakiston's son and company. hough and sedgwick, _the human mechanism_. ginn and company. howell, _physiology_, d edition. w. b. saunders company. schafer, _textbook of physiology_. the macmillan company. stiles, _nutritional physiology_. w. b. saunders company. verworn, _general physiology_. the macmillan company. xix. foods and dietaries _problems.--a study of foods to determine:--_ _(a) their nutritive value._ _(b) the relation of work, environment, age, sex, and digestibility of foods to diet._ _(c) their relative cheapness._ _(d) the daily calorie requirement._ _(e) food adulteration._ _(f) the relation of alcohol to the human system._ laboratory suggestions _laboratory exercise._--composition of common foods. the series of food charts supplied by the united states department of agriculture makes an excellent basis for a laboratory exercise to determine common foods rich in (_a_) water, (_b_) starch, (_c_) sugar, (_d_) fats or oils, (_e_) protein, (_f_) salts, (_g_) refuse. _demonstration._--method of using bomb calorimeter. _laboratory and home exercise._--to determine the best individual balanced dietary (using standard of atwater, chittenden, or voit) as determined by the use of the -calorie portion. _demonstration._--tests for some common adulterants. _demonstration._--effect of alcohol on protein, _e.g._ white of egg. _demonstration._--alcohol in some patent medicines. _demonstration._--patent medicines containing acetanilid. determination of acetanilid. why we need food.--a locomotive engine takes coal, water, oxygen, from its environment. a living plant or animal takes organic food, water, and oxygen from its environment. both the living and nonliving machine do the same thing with this fuel or food. they oxidize it and release the energy in it. but the living organism in addition may use the food to repair parts that have broken down or even build new parts. _thus food may be defined as something that releases energy or that forms material for the growth or repair of the body of a plant or animal._ the millions of cells of which the body is composed must be given material which will form more living matter or material which can be oxidized to release energy when muscle cells move, or gland cells secrete, or brain cells think. [illustration: the composition of milk. why is it considered a good food?] nutrients.--certain nutrient materials form the basis of food of both plants and animals. these have been stated to be _proteins_ (such as lean meat, eggs, the gluten of bread), _carbohydrates_ (starches, sugars, gums, etc.), _fats_ and _oils_ (both animal and vegetable), _mineral matter_ and _water_. proteins.--protein substances contain the element nitrogen. hence such foods are called nitrogenous foods. man must form the protoplasm of his body (that is, the muscles, tendons, nervous system, blood corpuscles, the living parts of the bone and the skin, etc.) in part at least from nitrogenous food. some of this he obtains by eating the flesh of animals, and some he obtains directly from plants (for example, peas and beans). proteins are the only foods available for tissue building. they may be oxidized to release energy if occasion requires it. fats and oils.--fats and oils, both animal and vegetable, are the materials from which the body derives part of its energy. the chemical formula of a fat shows that, compared with other food substances, there is very little oxygen present; hence the greater capacity of this substance for uniting with oxygen. the rapid burning of fat compared with the slower combustion of a piece of meat or a piece of bread illustrates this. a pound of butter releases over twice as much energy to the body as does a pound of sugar or a pound of steak. human fatty tissue is formed in part from fat eaten, but carbohydrate or even protein food may be changed and stored in the body as fat. carbohydrates.--we see that the carbohydrates, like the fats, contain carbon, hydrogen, and oxygen. _carbohydrates are essentially energy-producing foods._ they are, however, of use in building up or repairing tissue. it is certainly true that in both plants and animals such foods pass directly, together with foods containing nitrogen, to repair waste in tissues, thus giving the needed proportion of carbon, oxygen, and hydrogen to unite with the nitrogen in forming the protoplasm of the body. [illustration: three portions of foods, each of which furnishes about the same amount of nourishment.] inorganic foods.--water forms a large part of almost every food substance. it forms about five sixths of a normal daily diet. the human body, by weight, is about two thirds water. about per cent of the blood is water. water is absolutely essential in passing off waste of the body. when we drink water, we take with it some of the inorganic salts used by the body in the making of bone and in the formation of protoplasm. sodium chloride (table salt), an important part of the blood, is taken in as a flavoring upon our meats and vegetables. phosphate of lime and potash are important factors in the formation of bone. phosphorus is a necessary substance for the making of living matter, milk, eggs, meat, whole wheat, and dried peas and beans containing small amounts of it. iron also is an extremely important mineral, for it is used in the building of red blood cells. meats, eggs, peas and beans, spinach, and prunes, are foods containing some iron. some other salts, compounds of calcium, magnesium, potassium, and phosphorus, have been recently found to aid the body in many of its most important functions. the beating of the heart, the contraction of muscles, and the ability of the nerves to do their work appear to be due to the presence of minute quantities of these salts in the body. uses of nutrients.--the following table sums up the uses of nutrients to man:[ ]-- protein forms tissue \ white of eggs (albumen), (muscles, tendon, } curd of milk (casein), lean and probably } all serve as meat, gluten of wheat, etc. fat) } _fuel_ and yield } _energy_ in form fats form fatty tissue. } of heat and muscular fat of meat, butter, olive oil, } strength. oils of corn and wheat, etc. } } carbohydrates transformed into } sugar, starch, etc. fat. / mineral matters (ash) aid in forming bone, phosphates of lime, potash, assist in digestion, soda, etc. aid in absorption and in other ways help the body parts do their work. water used as a vehicle to carry nutrients, and enters into the composition of living matter. footnote : adapted from atwater, _principles of nutrition and nutritive value of food_, u. s. department of agriculture, . common foods contain the nutrients.--we have already found in our plant study that various plant foods are rich in different nutrients, carbohydrates forming the chief nutrient in the foods we call cereals, breads, cake, fleshy fruits, sugars, jellies, and the like. fats and oils are most largely found in nuts and some grains. animal foods are our chief supply of protein. white of egg and lean meat are almost pure protein and water. proteins are most abundant, as we should expect, in those plants which are richly supplied with nitrogen; peas and beans, and in grains and nuts. fats, which are melted into oils at the temperature of the body, are represented by the fat in meats, bacon, pork, lard, butter, and vegetable oils. water.--water is, as we have seen, a valuable part of food. it makes up a very high percentage of fresh fruits and vegetables; it is also present in milk and eggs, less abundant in meats and fish, and is lowest in dried foods and nuts. the amount of water in a given food is often a decided factor in the cost of the given food, as can easily be seen by reference to the chart on page . refuse.--some foods bought in the market may contain a certain unusable portion. this we call refuse. examples of refuse are bones in meat, shells of eggs or of shellfish, the covering of plant cells which form the skins of potatoes or other vegetables. the amount of refuse present also plays an important part in the values of foods for the table. the table[ ] on page gives the percentages of organic nutrients, water, and refuse present in some common foods. [illustration: table of food values. determine the percentage of water in codfish, loin of beef, milk, potatoes. percentage of refuse in leg of mutton, codfish, eggs, and potatoes. what _is_ the refuse in each case? find three foods containing a high percentage of protein; of fat; of carbohydrate. find some food in which the proportions of protein, fat, and carbohydrate are combined in a good proportion.] fuel values of nutrients.--in experiments performed by professor atwater and others, and in the appended tables, the value of food as a source of energy is stated in heat units called _calories_. _a calorie is the amount of heat required to raise the temperature of one kilogram of water from zero to one degree centigrade._ this is about equivalent to raising one pound four degrees fahrenheit. the fuel value of different foods may be computed in a definite manner. this is done by burning a given portion of a food (say one gram) in the apparatus known as a _calorimeter_. by this means may be determined the number of degrees the temperature of a given amount of water is raised during the process of burning. it has thus been found that a gram of fat will liberate . calories of heat, while a gram of starch or sugar only about calories. the burning value of fat is, therefore, over twice that of carbohydrates. in a similar manner protein has been shown to have about the same fuel value as carbohydrates, _i.e._ calories to a gram.[ ] footnote and : w. o. atwater, _principles of nutrition and nutritive value of food_, u. s. department of agriculture, . the relation of work to diet.--it has been shown experimentally that a man doing hard, muscular work needs more food than a person doing light work. the mere exercise gives the individual a hearty appetite; he eats more and needs more of all kinds of food than a man or boy doing light work. especially is it true that the person of sedentary habits, who does brain work, should be careful to eat less food and food that will digest easily. his protein food should also be reduced. rich or hearty foods may be left for the man who is doing hard manual labor out of doors, for any extra work put on the digestive organs takes away just so much from the ability of the brain to do its work. [illustration: foods of plant origin. select foods containing a high percentage of protein, with a high percentage of carbohydrates, with a high percentage of water. do vegetable foods contain much fat? which of the above-mentioned foods have the highest burning value?] [illustration: foods largely of animal origin. compare with the previous chart with reference to amount of protein, carbohydrate, and fat in foods. compare the burning value of plant and animal foods. compare the relative percentage of water in both kinds of foods.] [illustration: the composition of milk.] the relation of environment to diet.--we are all aware of the fact that the body seems to crave more food in winter than in summer. the temperature of the body is maintained at . ° in winter as in summer, but much more heat is lost from the body in cold weather. hence feeding in winter should be for the purpose of maintaining our fuel supply. we need heat-producing food, and we need _more_ food in winter than in summer. we may use carbohydrates for this purpose, as they are economical and digestible. the inhabitants of cold countries get their heat-releasing foods largely from fats. in tropical countries and in hot weather little protein should be eaten and a considerable amount of fresh fruit used. the relation of age to diet.--as we will see a little later, age is a factor not only in determining the kind but the amount of food to be used. young children require far less food than do those of older growth or adults. the body constantly increases in weight until young manhood, or womanhood, then its weight remains nearly stationary, varying with health or illness. it is evident that food in adults simply repairs the waste of cells and is used to supply energy. elderly people need much less protein than do younger persons. but inasmuch as the amount of food to be taken into the body should be in proportion to the body weight, it is also evident that growing children do not, as is popularly supposed, need as much food as grown-ups. the relation of sex to diet.--as a rule boys need more food than girls, and men than women. this seems to be due to, first, the more active muscular life of the man and, secondly, to the greater amount of fat in the tissues of the woman, making loss of heat less. larger bodies, because of greater surface, give off more heat than smaller ones. men are usually larger in bulk than are women,--another reason for more food in their case. the relation of digestibility to diet.--animal foods in general may be said to be more completely digested within the body than plant foods. this is largely due to the fact that plant cells have woody walls that the digestive juices cannot act upon. cereals and legumes are less digestible foods than are dairy products, meat, or fish. this does not mean necessarily that these foods would not agree with you or me but that in general the body would get less nourishment out of the total amount available. the agreement or disagreement of food with an individual is largely a personal matter. i, for example, cannot eat raw tomatoes without suffering from indigestion, while some one else can digest tomatoes but not strawberries. each individual should learn early in life the foods that disagree with him personally and leave such foods out of his dietary. for "what is one man's meat may be another man's poison." the relation of cost of food to diet.--it is a mistaken notion that the best foods are always the most expensive. a glance at the table (page ) will show us that both fuel value and tissue-building value is present in some foods from vegetable sources, as well as in those from animal sources, and that the vegetable foods are much cheaper. the american people are far less economical in their purchase of food than most other nations. nearly one half of the total income of the average workingman is spent on food. not only does he spend a large amount on food, but he wastes money in purchasing the wrong kinds of food. a comparison of the daily diets of persons in various occupations in this and other countries shows that as a rule we eat more than is necessary to supply the necessary fuel and repair, and that our workingmen eat more than those of other countries. another waste of money by the american is in the false notion that a large proportion of the daily dietary should be meat. many people think that the most expensive cuts of meat are the most nutritious. the falsity of this idea may be seen by a careful study of the tables on pages and . the best dietary.--inasmuch as all living substance contains nitrogen, it is evident that protein food must form a part of the dietary; but protein alone is not usable. if more protein is eaten than the body requires, then immediately the liver and kidneys have to work overtime to get rid of the excess of protein which forms a poisonous waste harmful to the body. we must take foods that will give us, as nearly as possible, the proportion of the different chemical elements as they are contained in protoplasm. it has been found, as a result of studies of atwater and others, that a man who does muscular work requires a little less than one quarter of a pound of protein, the same amount of fat, and about one pound of carbohydrate to provide for the growth, waste, and repair of the body and the energy used up in one day. the daily calorie requirement.--put in another way, atwater's standard for a man at light exercise is food enough to yield calories; of these, calories are from protein, calories from fat, and calories from carbohydrate. that is, for every calories furnished by the food, are from protein, from fat, and from carbohydrate. in exact numbers, the day's ration as advocated by atwater would contain about grams or . ounces protein, grams or . ounces fat, and grams or ounces carbohydrate. professor chittenden of yale university, another food expert, thinks we need proteins, fats, and carbohydrates in about the proportion of to to , thus differing from atwater in giving less protein in proportion. chittenden's standard for the same man is food to yield a total of calories, of which protein furnishes calories, fat calories, and carbohydrates calories. for every calories furnished by the food, are from protein, from fat, from carbohydrate. in actual amount the chittenden diet would contain . ounces protein, . ounces fat, and ounces carbohydrate. a german named voit gives as ideal calories from proteins, from fat, and from carbohydrate, out of every calories; this is nearer our actual daily ration. in addition, an ounce of salt and nearly one hundred ounces of water are used in a day. [illustration: table showing the cost of various foods. using this table, make up an economical dietary for one day, three meals, for a man doing moderate work. give reasons for the amount of food used and for your choice of foods. make up another dietary in the same manner, using expensive foods. what is the difference in your bill for the day?] a mixed diet best.--knowing the proportion of the different food substances required by man, it will be an easy matter to determine from the tables and charts shown you the best foods for use in a mixed diet. meats contain too much nitrogen in proportion to the other substances. in milk, the proportion of proteins, carbohydrates, and fats is nearly right to make protoplasm; a considerable amount of mineral matter being also present. for these reasons, milk is extensively used as a food for children, as it combines food material for the forming of protoplasm with mineral matter for the building of bone. some vegetables (for example, peas and beans) contain a large amount of nitrogenous material but in a less digestible form than is found in some other foods. vegetarians, then, are correct in theory when they state that a diet of vegetables may contain everything necessary to sustain life. but a mixed diet containing meat is healthier. a purely vegetable diet contains much waste material, such as the cellulose forming the walls of plant cells, which is indigestible. it has been recently discovered that the outer coats of some grains, as rice, contain certain substances (enzymes) which aid in digestion. in the case of polished rice, when this outer coat is removed the grain has much less food value. daily fuel needs of the body.--it has been pointed out that the daily diet should differ widely according to age, occupation, time of year, etc. the following table shows the daily fuel needs for several ages and occupations:-- daily calorie needs (approximately) . for child under years calories . for child from - years calories . for child from - years calories . for child from - years calories . for child from - (woman, light work, also) calories . for boy ( - ), girl ( - ), man, sedentary calories . for boy ( - ) (man, light muscular work) calories . for man, moderately active muscular work calories . for farmer (busy season) to calories . for ditchers, excavators, etc. to calories . for lumbermen, etc. and more calories normal heat output.--the following table gives the result of some experiments made to determine the hourly and daily expenditure of energy of the average normal grown person when asleep and awake, at work or at rest:-- average normal output of heat from the body ========================================================== | average conditions of muscular activity | calories | per hour -------------------------------------------+-------------- man at rest, sleeping | calories man at rest, awake, sitting up | calories man at light muscular exercise | calories man at moderately active muscular exercise | calories man at severe muscular exercise | calories man at very severe muscular exercise | calories ========================================================== it is very simple to use such a table in calculating the number of calories which are spent in twenty-four hours under different bodily conditions. for example, suppose the case of a clerk or school teacher leading a relatively inactive life, who sleeps for hours × calories = works at desk hours × calories = reads, writes, or studies hours × calories = walks or does light exercise hours × calories = ---- this comes out, as we see, very close to example of the table[ ] on page . footnote : the above tables have been taken from the excellent pamphlet of the cornell reading course, no. , _human nutrition_. how we may find whether we are eating a properly balanced diet.--we already know approximately our daily calorie needs and about the proportion of protein, fat, and carbohydrate needed. dr. irving fisher of yale university has worked out a very easy method of determining whether one is living on a proper diet. he has made up a number of tables, in which he has designated portions of food, each of which furnishes calories of energy. the tables show the proportion of protein, fat, and carbohydrate in each food, so that it is a simple matter by using such a table to estimate the proportions of the various nutrients in our dietary. we may depend upon taking somewhere near the proper amount of food if we take a diet based upon either atwater's, chittenden's, or voit's standard. one of the most interesting and useful pieces of home work that you can do is to estimate your own personal dietary, using the tables giving the -calorie portion to see if you have a properly balanced diet. from the table on page make out a simple dietary for yourself for one day, estimating your own needs in calories and then picking out -calorie portions of food which will give you the proper proportions of protein, fat, and carbohydrate. table of calorie portions--modified from fisher ============================================================================ | |wt. in| cal. furnished| price | | oz. |----+----+-----+--------+------ |port. containing | | | |car- | | food | calories | cal.|pro-| fat|bohy-| lb. | cal. | | port.|tein| |drate| | por. ------------------+------------------+------+----+----+-----+--------+------ oysters | doz. | . | | | | . | . bean soup | / small serving | . | | | | | . cream of corn | / ordin. serv. | . | | | | | . vegetable soup | / ordin. serv. | . | | | | | . cod fish (fresh) |ordin. serv. | | | | | . | . salmon (canned) |small serv. | . | | | | . | . chicken | / large serv. | . | | | | . | . veal cutlet | / large serv. | . | | | | . | . beef, corned | / large serv. | . | | | | . | . beef, sirloin | small serv. | . | | | | . | . beef, round | small serv. | . | | | | . | . ham, lean | ordin. serv. | . | | | | . | . lamb chops | / ordin. serv. | . | | | | . | . mutton, leg | ordin. serv. | . | | | | . | . eggs, boiled | large egg | . | | | |. doz.| . eggs, scrambled | - / ordin. serv.| . | | | |. doz.| . beans, baked | side dish | . | | | | . | . potatoes, mashed | ordin. serv. | . | | | | . | . macaroni | / large serv. | . | | | | . | . potato salad | ordin. serv. | . | | | | . | . tomatoes, sliced | large serv. | . | | | | . | . rolls, plain | large roll | . | | | |. doz.| . butter | ordin. pat | . | | . | | . | . wheat bread | small slice | . | | | | . | . chocolate cake | / ord. sq. piece| . | | | | . | . gingerbread | / ord. sq. piece| . | | | | . | . custard pudding | ordin. serv. | . | | | | . | . rice pudding | very small serv.| . | | | | . | . apple pie | / piece | . | | | | | . cheese, american | - / cu. in. | . | | | | . | . crackers (soda) | crackers | . | | | | . | . currant jelly | heap. spoons | . | | | | . | . sugar | teaspoons | . | | | | . | . milk as bought | small glass | . | | | | . | . milk, cond., sweet| teaspoons | . | | | | | . oranges | large one | . | | | | | . peanuts | double ones | . | | | | | . almonds, shelled | - | . | | | | | . ============================================================================ from the preceding table plan a well-balanced and cheap dietary for one day for a family of five, two adults and three children. make a second dietary for the same time and same number of people which shall give approximately the same amount of tissue and energy producing food from more expensive materials. food waste in the kitchen.--much loss occurs in the improper cooking of foods. meats especially, when overdone, lose much of their flavor and are far less easily digested than when they are cooked rare. the chief reasons for cooking meats are that the muscle fibers may be loosened and softened, and that the bacteria or other parasites in the meat may be killed by the heat. the common method of frying makes foods less digestible. stewing is an economical as well as healthful method. a good way to prepare meat, either for stew or soup, is to place the meat, cut in small pieces, in cold water, and allow it to simmer for several hours. rapid boiling toughens the muscle fibers by the too rapid coagulation of the albuminous matter in them, just as the white of egg becomes tough when boiled too long. boiling and roasting are excellent methods of cooking meat. in order to prevent the loss of the nutrients in roasting, it is well to baste the meat frequently; thus a crust is formed on the outer surface of the meat, which prevents the escape of the juices from the inside. vegetables are cooked in order that the cells containing starch grains may be burst open, thus allowing the starch to be more easily attacked by the digestive fluids. inasmuch as water may dissolve out nutrients from vegetable tissues, it is best to boil them rapidly in a small amount of water. this gives less time for the solvent action to take place. vegetables should be cooked with the outer skin left on when it is possible. adulterations in foods.--the addition of some cheaper substance to a food, or the subtraction of some valuable substance from a food, with the view to cheating the purchaser, is known as _adulteration_. many foods which are artificially manufactured have been adulterated to such an extent as to be almost unfit for food, or even harmful. one of the commonest adulterations is the substitution of grape sugar (glucose) for cane sugar. glucose, however, is not a harmful adulterant. it is used largely in candy making. flour and other cereal foods are sometimes adulterated with some cheap substitutes, as bran or sawdust. alum is sometimes added to make flour whiter. probably the food which suffers most from adulteration is milk, as water can be added without the average person being the wiser. by means of an inexpensive instrument known as a _lactometer_, this cheat may easily be detected. in most cities, the milk supply is carefully safeguarded, because of the danger of spreading typhoid fever from impure milk (see chapter xx). before the pure food law was passed in , milk was frequently adulterated with substances like formalin to make it keep sweet longer. such preservatives are harmful, and it is now against the law to add anything whatever to milk. coffee, cocoa, and spices are subject to great adulteration; cottonseed oil is often substituted for olive oil; butter is too frequently artificial; while honey, sirups of various kinds, cider and vinegar, have all been found to be either artificially made from cheaper substitutes or to contain such substitutes. pure food laws.--thanks to the national pure food and drug law passed by congress in , and to the activity of various city and state boards of health, the opportunity to pass adulterated foods on the public is greatly lessened. this law compels manufacturers of foods or medicines to state the composition of their products on the labels placed on the jars or bottles. so if a person reads the label he can determine exactly what he is getting for his money. impure water.--great danger comes from drinking impure water. this subject has already been discussed under bacteria, where it was seen that the spread of typhoid fever in particular is due to a contaminated water supply. as citizens, we must aid all legislation that will safeguard the water used by our towns and cities. boiling water for ten minutes or longer will render it safe from all organic impurities. stimulants.--we have learned that food is anything that supplies building material or releases energy in the body; but some materials used by man, presumably as food, do not come under this head. such are tea and coffee. when taken in moderate quantities, _they produce a temporary increase in the vital activities_ of the person taking them. this is said to be a stimulation; and material taken into the digestive tract, producing this, is called a _stimulant_. in moderation, tea and coffee appear to be harmless. some people, however, cannot use either without ill effects, even in small quantity. it is the _habit_ formed of relying upon the stimulus given by tea or coffee that makes them a danger to man. cocoa and chocolate, although both contain a stimulant, are in addition good foods, having from per cent to per cent of protein, from per cent to per cent fat, and over per cent carbohydrate in their composition. is alcohol a food?--the question of the use of alcohol has been of late years a matter of absorbing interest and importance among physiologists. a few years ago dr. atwater performed a series of very careful experiments by means of the respiration calorimeter, to ascertain whether alcohol is of use to the body as food.[ ] in these experiments the subjects were given, instead of their daily allotment of carbohydrates and fats, enough alcohol to supply the same amount of energy that these foods would have given. the amount was calculated to be about two and one half ounces per day, about as much as would be contained in a bottle of light wine.[ ] this alcohol was administered in small doses six times during the day. professor atwater's results may be summed up briefly as follows:-- . the alcohol administered was almost all oxidized in the body. . the potential energy in the alcohol was transformed into heat or muscular work. . the body did about as well with the rations including alcohol as it did without it. footnote : alcohol is made up of carbon, oxygen, and hydrogen. it is very easily oxidized, but it cannot, as is shown by the chemical formula, be of use to the body in tissue building, because of its lack of nitrogen. footnote : alcoholic beverages contain the following proportions of alcohol: beer, from to per cent; wine, from to per cent; liquors, from to per cent. patent medicines frequently contain as high as per cent alcohol. (see page .) the committee of fifty eminent men appointed to report on the physiological aspects of the drink problem reported that a large number of scientific men state that they are in the habit of taking alcoholic liquor in small quantities, and many report that they do not _feel_ harm thereby. a number of scientists seem to agree that within limits alcohol may be a kind of food, although a very _poor_ food. on the other hand, we know that although alcohol may technically be considered as a food, it is a very unsatisfactory food and, as the following statements show, it has an effect on the body tissues which foods do not have. professor chittenden of yale college, in discussing the food problem of alcohol, writes as follows: "it is true that alcohol in moderate quantities may serve as a food, _i.e._ it can be oxidized with the liberation of heat. it may to some extent take the place of fat and carbohydrates, but it is not a perfect substitute for them, and for this reason alcohol has an action that cannot be ignored. it reduces liver oxidation. it therefore presents a dangerous side wholly wanting in carbohydrates and fat. the latter are simply burned up to carbonic acid and water or are transformed to glycogen and fat, but alcohol, although more easily oxidized, is at all times liable to obstruct, in a measure at least, the oxidative processes of the liver and probably of other tissues also, thereby throwing into the circulation bodies, such as uric acid, which are harmful to health, a fact which at once tends to draw a distinct line of demarcation between alcohol and the two non-nitrogenous foods, fat and carbohydrates. another matter must be emphasized, and it is that the form in which alcohol is taken is of importance. port wine, for instance, has more influence on the amount of uric acid secreted than an equivalent amount of alcohol has in some other form. to conclude: as an adjunct to the ordinary daily diet of the healthy man alcohol cannot be considered as playing the part of a true non-nitrogenous food."--quoted in _american journal of inebriety_, winter, . effect of alcohol on living matter.--if we examine raw white of egg, we find a protein which closely resembles protoplasm in its chemical composition; it is called albumen. add to a little albumen in a test tube some per cent alcohol and notice what happens. as soon as the alcohol touches the albumen the latter coagulates and becomes hard like boiled white of egg. shake the alcohol with the albumen and the entire mass soon becomes a solid. this is because the alcohol draws the water out of the albumen. it has been shown that albumen is somewhat like protoplasm in structure and chemical composition. strong alcohol acts in a similar manner on living matter when it is absorbed by the living body cells. it draws water from them and hardens them. it has a chemical and physical action upon living matter. alcohol a poison.--but alcohol is also in certain quantities a poison. _a commonly accepted definition of a poison is that it is any substance which, when taken into the body, tends to cause serious detriment to health, or the death of the organism._ that alcohol may do this is well known by scientists. it is a matter of common knowledge that alcohol taken in small quantities does not do any _apparent_ harm. but if we examine the vital records of life insurance companies, we find a large number of deaths directly due to alcohol and a still greater number due in part to its use. in the united states every year there are a third more deaths from alcoholism and cirrhosis of the liver (a disease _directly_ caused by alcohol) than there are from typhoid fever. the poisonous effect is not found in small doses, but it ultimately shows its harmful effect. hardening of the arteries, an old-age disease, is rapidly becoming in this country a disease of the middle aged. from it there is no escape. it is chiefly caused by the cumulative effect of alcohol. the diagram following, compiled by two english life insurance companies that insure moderate drinkers and abstainers, shows the death rate to be considerably higher among those who use alcohol. [illustration: abstainers live longer than moderate drinkers.] dr. kellogg, the founder of the famous battle creek sanitarium, points out that strychnine, quinine, and many other drugs are oxidized in the body but surely cannot be called foods. the following reasons for not considering alcohol a food are taken from his writings:-- " . a habitual user of alcohol has an intense craving for his accustomed dram. without it he is entirely unfitted for business. one never experiences such an insane craving for bread, potatoes, or any other particular article of food. " . by continuous use the body acquires a tolerance for alcohol. that is, the amount which may be imbibed and the amount required to produce the characteristic effects first experienced gradually increase until very great quantities are sometimes required to satisfy the craving which its habitual use often produces. this is never the case with true foods.... alcohol behaves in this regard just as does opium or any other drug. it has no resemblance to a food. " . when alcohol is withdrawn from a person who has been accustomed to its daily use, most distressing effects are experienced.... who ever saw a man's hand trembling or his nervous system unstrung because he could not get a potato or a piece of cornbread for breakfast? in this respect, also, alcohol behaves like opium, cocaine, or any other enslaving drug. " . alcohol lessens the appreciation and the value of brain and nerve activity, while food reënforces nervous and mental energy. " . alcohol as a protoplasmic poison lessens muscular power, whereas food increases energy and endurance. " . alcohol lessens the power to endure cold. this is true to such a marked degree that its use by persons accompanying arctic expeditions is absolutely prohibited. food, on the other hand, increases ability to endure cold. the temperature after taking food is raised. after taking alcohol, the temperature, as shown by the thermometer, is lowered. " . alcohol cannot be stored in the body for future use, whereas all food substances can be so stored. " . food burns slowly in the body, as it is required to satisfy the body's needs. alcohol is readily oxidized and eliminated, the same as any other oxidizable drug." [illustration: experiment (by davison) to show how the nicotine in six cigarettes was sufficient to kill this fish. the smoke from the cigarettes was passed through the water in which the fish is swimming.] the use of tobacco.--a well-known authority defines a narcotic as a substance "_which directly induces sleep, blunts the senses, and, in large amounts, produces complete insensibility_." tobacco, opium, chloral, and cocaine are examples of narcotics. tobacco owes its narcotic influence to a strong poison known as nicotine. its use in killing insect parasites on plants is well known. in experiments with jellyfish and other lowly organized animals, the author has found as small a per cent as one part of nicotine to one hundred thousand parts of sea water to be sufficient to profoundly affect an animal placed within it. the illustration here given shows the effect of nicotine upon a fish, one of the vertebrate animals. nicotine in a pure form is so powerful a poison that two or three drops would be sufficient to cause the death of a man by its action upon the nervous system, especially the nerves controlling the beating of the heart. this action is well known among boys training for athletic contests. the heart is affected; boys become "short-winded" as a result of the action on the heart. it has been demonstrated that tobacco has, too, an important effect on muscular development. the stunted appearance of the young smoker is well known. [illustration: the amounts of alcohol in some liquors and in some patent medicines. _a_, beer, %; _b_, claret, %; _c_, champagne, %; _d_, whisky, %; _e_, well-known sarsaparilla, %; _f_, _g_, _h_, much-advertised nerve tonics, %, %, %; _i_, another much-advertised sarsaparilla, %; _j_, a well-known tonic, %; _k_, _l_, bitters, %, % alcohol.] use and abuse of drugs.--the american people are addicted to the use of drugs, and especially patent medicines. a glance at the street-car advertisements shows this. most of the medicines advertised contain alcohol in greater quantity than beer or wine, and many of them have opium, morphine, or cocaine in their composition. paregoric and laudanum, medicines sometimes given to young children, are examples of dangerous drugs that contain opium. dr. george d. haggard of minneapolis has shown by many analyses that a large number of the so-called "malts," "malt extracts," and "tonics," including several of the best known and most advertised on the market, are simply disguised beers and, frequently, very poor beers at that. these drugs, in addition to being harmful, affect the person using them in such a manner that he soon feels the need for the drug. thus the drug habit is formed,--a condition which has wrecked thousands of lives. a number of articles on patent medicines recently appeared in a leading magazine and have been collected and published under the title of _the great american fraud_. in this booklet the author points out a number of different kinds of "cures" and patent medicines. the most dangerous are those headache or neuralgia cures containing _acetanilid_. this drug is a heart depresser and should not be used without medical advice. another drug which is responsible for habit formation is _cocaine_. this is often found in catarrh or other cures. alcohol is the basis of all tonics or "bracers." every boy and girl should read this booklet so as to be forearmed against evils of the sort just described. reference reading on foods hunter, _laboratory problems in civic biology._ american book company. allen, _civics and health._ ginn and company. bulletin , american school of home economics, chicago. cornell university reading course, buls. and , _human nutrition._ davison, _the human body and health._ american book company. jordan, _the principles of human nutrition._ the macmillan company. kehler, l. f., _habit-forming agents._ farmers' bulletin , u. s. dept. of agri. lusk, _science and nutrition._ w. b. saunders company. norton, _foods and dietetics._ american school of home economics. olsen, _pure foods._ ginn and company. sharpe, _a laboratory manual for the solution of problems in biology,_ pp. - . american book company. stiles, _nutritional physiology._ w. b. saunders company. _the great american fraud._ american medical association, chicago. _the propaganda for reform in proprietary medicines._ am. medical association. farmers' bulletin: numbers , , , , , , , , , , , , . reprint from yearbook, , atwater, _dietaries in public institutions._ reprint from yearbook, , milner, _cost of food related to its nutritive value._ experiment station, circular , langworthy, _functions and uses of food._ xx. digestion and absorption _problems.--to determine where digestion takes place by examining_:-- _(a) the functions of glands._ -(b) the work done in the mouth._ -(c) the work done in the stomach._ -(d) the work done in the small intestine._ -(e) the function of the liver._ _to discover the absorbing apparatus and how it is used._ laboratory suggestions _demonstration of food tube of man_ (manikin).--comparison with food tube of frog. drawing (comparative) of food tube and digestive glands of frog and man. _demonstration of simple gland._--(microscopic preparation.) _home experiment and laboratory demonstration._--the digestion of starch by saliva. conditions favorable and unfavorable. _demonstration experiment._--the digestion of proteins with artificial gastric juice. conditions favorable and unfavorable. _demonstration._--an emulsion as seen under the compound microscope. _demonstration._--emulsification of fats with artificial pancreatic fluid. digestion of starch and protein with artificial pancreatic fluid. _demonstration_ of "tripe" to show increase of surface of digestive tube. _laboratory or home exercise._--make a table showing the changes produced upon food substances by each digestive fluid, the reaction (acid or alkaline) of the fluid, when the fluid acts, and what results from its action. purpose of digestion.--we have learned that starch and protein food of plants are formed in the leaves. a plant, however, is unable to make use of the food in this condition. before it can be transported from one part of the plant body to another, it is changed into a soluble form. in this state it can be passed from cell to cell by the process of osmosis. much the same condition exists in animals. in order that food may be of use to man, it must be changed into a state that will allow of its passage in a soluble form through the walls of the alimentary canal, or food tube. this is done by the enzymes which cause digestion. it will be the purpose of this chapter to discover where and how digestion takes place in our own body. alimentary canal.--in all vertebrate animals, including man, food is taken in the mouth and passed through a _food tube_ in which it is digested. this tube is composed of different portions, named, respectively, as we pass from the _mouth_ downward, the _gullet_, _stomach_, _small_ and _large intestine_, and _rectum_. [illustration: the digestive tract of the frog and man. _gul_, gullet; _s_, stomach; _l_, liver; _g_, gall bladder; _p_, pancreas; _sp_, spleen; _si_, small intestine; _li_, large intestine; _v_, appendix; _a_, anus.] comparison of food tube of a frog and man.--if we compare the food tube of a dissected frog with the food tube of man (as shown by a manikin or chart), we find part for part they are much the same. but we notice that the intestines of man, both small and large, are relatively longer than in the frog. we also notice in man the body cavity or space in which the internal organs rest is divided in two parts by a wall of muscle, the _diaphragm_, which separates the heart and lungs from the other internal organs. in the frog no muscular diaphragm exists. in the frog we can see plainly the silvery transparent _mesentery_ or double fold of the lining of the body cavity in which the organs of digestion are suspended. numerous blood vessels can be found especially in the walls of the food tube. glands.--in addition to the alimentary canal proper, we find a number of _digestive glands_, varying in size and position, connected with the canal. [illustration: diagram of a gland. _i_, the common tube which carries off the secretions formed in the cells lining the cavity _c_; _a_, arteries carrying blood to the glands; _v_, veins taking blood away from the glands.] what a gland does. enzymes.--in man there are the saliva gland of the mouth, the gastric glands of the stomach, the pancreas and liver, the two latter connected with the small intestine, and the intestinal glands in the walls of the intestine. besides glands which aid in digestion there are several others of which we will speak later. as we have already learned, a gland is a collection of cells which takes up material from within the body and manufactures from it something which is later poured out as a secretion. an example of a gland in plants is found in the nectar-secreting cells of a flower. certain substances, called _enzymes_, formed by glands cause the digestion of food. the enzymes secreted by the cells of the glands and poured out into the food tube act upon insoluble foods so as to change them to a soluble form. they are the product of the activity of the cell, although they are not themselves alive. we do not know much about enzymes themselves, but we can observe what they do. some enzymes render soluble different foods, others work in the blood, still others probably act within any cell of the body as an aid to oxidation, when work is done. enzymes are very sensitive to changes in temperature and to the degree of _acidity_ or _alkalinity_[ ] of the material in which they act. we will find that the enzymes found in glands in the mouth will not act long in the stomach because of the change from an alkaline surrounding in the mouth to that of an acid in the stomach. enzymes seem to be able to work indefinitely, providing the surroundings are favorable. a small amount of digestive fluid, if it had long enough to work, could therefore digest an indefinite amount of food. footnote : the teacher should explain the meaning of these terms. gland structure.--the entire inner surface of the food tube is covered with a soft lining of _mucous membrane_. this is always moist because certain cells, called _mucus cells_, empty out their contents into the food tube, thus lubricating its inner surface. when a large number of cells which have the power to secrete fluids are collected together, the surface of the food tube may become indented at this point to form a pitlike _gland_. often such depressions are branched, thus giving a greater secreting surface, as is seen in the figure on page . the cells of the gland are always supplied with blood vessels and nerves, for the secretions of the glands are under the control of the nervous system. how a gland secretes.--we must therefore imagine that as the blood goes to the cells of a gland it there loses some substances which the gland cells take out and make over into the particular enzyme that they are called upon to manufacture. under certain conditions, such as the sight or smell of food, or even the desire for it, the activity of the gland is stimulated. it then pours out its secretion containing the digestive enzyme. thus a gland does its work. salivary glands.--we are all familiar with the substance called _saliva_ which acts as a lubricant in the mouth. saliva is manufactured in the cells of three pairs of glands which empty into the mouth, and which are called, according to their position, the _parotid_ (beside the ear), the _submaxillary_ (under the jawbone), and the _sublingual_ (under the tongue). [illustration: experiment showing non-osmosis of starch in tube _a_, and osmosis of sugar in tube _b_.] digestion of starch.--if we collect some saliva in a test tube, add to it a little starch paste, place the tube containing the mixture for a few minutes in tepid water, and then test with fehling's solution, we shall find grape sugar present. careful tests of the starch paste and of the saliva made separately will usually show no grape sugar in either. if another test be made for grape sugar, in a test tube containing starch paste, saliva, and a few drops of any weak acid, the starch will be found not to have changed. the digestion or change of starch to grape sugar is caused by the presence in the saliva of an _enzyme_, or _digestive ferment_. you will remember that starch in the growing corn grain was changed to grape sugar by an enzyme called _diastase_. here a similar action is caused by an enzyme called _ptyalin_. this ferment acts _only_ in an alkaline medium at about the temperature of the body. [illustration: the mouth cavity of man. _e_, eustachian tube; _hp_, hard palate; _sp_, soft palate; _ut_, upper teeth; _bc_, buccal cavity; _lt_, lower teeth; _t_, tongue; _ph_, pharynx; _ep_, epiglottis; _lx_, voice box; _oe_, gullet; _tr_, trachea.] mouth cavity in man.--in our study of a frog we find that the mouth cavity has two unpaired and four paired tubes leading from it. these are (_a_) the _gullet_ or food tube, (_b_) the _windpipe_ (in the frog opening through the _glottis_), (_c_) the paired nostril holes (_posterior nares_), (_d_) the paired _eustachian tubes_, leading to the ear. all of these openings are found in man. in man the mouth cavity, and all internal surfaces of the food tube, are lined with a _mucous membrane_. the _mucus_ secreted from gland cells in this lining makes a slippery surface so that the food may slip down easily. the roof of the mouth is formed in front by a plate of bone called the _hard palate_, and a softer continuation to the back of the mouth, the _soft palate_. these separate the nose cavity from that of the mouth proper. the part of the space back of the soft palate is called the _pharynx_, or throat cavity. from the pharynx lead off the _gullet_ and _windpipe_, the former back of the latter. the lower part of the mouth cavity is occupied by a muscular tongue. examination of its surface with a looking-glass shows it to be almost covered in places by tiny projections called _papillæ_. these papillæ contain organs known as _taste buds_, the sensory endings of which determine the taste of substances. the tongue is used in moving food about in the mouth, and in starting it on its way to the gullet; it also plays an important part in speaking. [illustration: i. teeth of the upper jaw, from below. _ , _, incisors; _ _, canine; _ , _, premolars; _ , , _, molars. ii. longitudinal section of a tooth. _e_, enamel; _d_, dentine; _c_, cement; _p_, pulp cavity.] the teeth.--in man the teeth, unlike those of the frog, are used in the mechanical preparation of the food for digestion. instead of holding prey, they crush, grind, or tear food so that more surface may be given for the action of the digestive fluids. the teeth of man are divided, according to their functions, into four groups. in the center of both the upper and lower jaw in front are found eight teeth with chisel-like edges, four in each jaw; these are the _incisors_, or cutting teeth. next is found a single tooth on each side (four in all); these have rather sharp points and are called the _canines_. then come two teeth on each side, eight in all, called _premolars_. lastly, the _flat-top molars_, or grinding teeth, of which there are six in each jaw. food is caught between irregular projections on the surface of the molars and crushed to a pulpy mass. hygiene of the mouth.--food should simply be chewed and relished, with no thought of swallowing. there should be no more effort to prevent than to force swallowing. it will be found that if you attend only to the agreeable task of extracting the flavors of your food, nature will take care of the swallowing, and this will become, like breathing, involuntary. the instinct by which most people eat is perverted through the "hurry habit" and the use of abnormal foods. thorough mastication takes time, and therefore one must not feel hurried at meals if the best results are to be secured. the stopping point for eating should be at the _earliest_ moment after one is really satisfied. care of the teeth.--it has been recently found that fruit acids are very beneficial to the teeth. vinegar diluted to about half strength with water makes an excellent dental wash. clean your teeth carefully each morning and before going to bed. use dental silk after meals. we must remember that the bacteria which cause decay of the teeth are washed down into the stomach and may do even more harm there than in the mouth. how food is swallowed.--after food has been chewed and mixed with saliva, it is rolled into little balls and pushed by the tongue into such position that the muscles of the throat cavity may seize it and force it downward. food, in order to reach the gullet from the mouth cavity, must pass over the opening into the windpipe. when food is in the course of being swallowed, the upper part of this tube forms a trapdoor over the opening. when this trapdoor is not closed, and food "goes down the wrong way," we choke, and the food is expelled by coughing. [illustration: peristaltic waves on the gullet of man. (a bolus means little ball.)] the gullet, or esophagus.--like the rest of the food tube the gullet is lined by soft and moist mucous membrane. the wall is made up of two sets of muscles,--the inside ones running around the tube; the outer layer of muscle taking a longitudinal course. after food leaves the mouth cavity, it gets beyond our direct control, and the muscles of the gullet, stimulated to activity by the presence of food in the tube, push the food down to the stomach by a series of contractions until it reaches the stomach. these wavelike movements (called _peristaltic_ movements) are characteristic of other parts of the food tube, food being pushed along in the stomach and the small intestine by a series of slow-moving muscular waves. peristaltic movement is caused by muscles which are not under voluntary nervous control, although anger, fear, or other unpleasant emotions have the effect of slowing them up or even stopping them entirely. stomach of man.--the stomach is a pear-shaped organ capable of holding about three pints. the end opposite to the gullet, which empties into the small intestine, is provided with a ring of muscle forming a valve called the _pylorus_. there is also another ring of muscle guarding the entrance to the stomach. gastric glands.--if we open the stomach of the frog, and remove its contents by carefully washing, its wall is seen to be thrown into folds internally. between the folds in the stomach of man, as well as in the frog, are located a number of tiny pits. these form the mouths of the _gastric glands_, which pour into the stomach a secretion known as the _gastric juice_. the gastric glands are little tubes, the lining of which secretes the fluid. when we think of or see appetizing food, this secretion is given out in considerable quantity. the stomach, like the mouth, "waters" at the sight of food. gastric juice is slightly acid in its chemical reaction, containing about . per cent free _hydrochloric acid_. it also contains two very important enzymes, one called _pepsin_, and another less important one called _rennin_. action of gastric juice.--if protein is treated with artificial gastric juice at the temperature of the body, it will be found to become swollen and then gradually to change to a substance which is soluble in water. this is like the action of the gastric juice upon proteins in the stomach. the other enzyme of gastric juice, called _rennin_, curdles or coagulates a protein found in milk; after the milk is curdled, the pepsin is able to act upon it. "junket" tablets, which contain rennin, are used in the kitchen to cause this change. [illustration: a peptic gland, from the stomach, very much magnified. _a_, central or chief cell, which makes pepsin; _b_, border cells, which make acid. (from miller's _histology_.)] the hydrochloric acid found in the gastric juice acts upon lime and some other salts taken into the stomach with food, changing them so that they may pass into the blood and eventually form the mineral part of bone or other tissue. the acid also has a decided antiseptic influence in preventing growth of bacteria which cause decay, and some of which might cause disease. movement of walls of stomach.--the stomach walls, provided with three layers of muscle which run in an oblique, circular, and longitudinal direction (taken from the inside outward), are well fitted for the constant churning of the food in that organ. here, as elsewhere in the digestive tract, the muscles are involuntary, muscular action being under the control of the so-called _sympathetic nervous system_. food material in the stomach makes several complete circuits during the process of digestion in that organ. contrary to common belief, the greatest amount of food is digested _after_ it leaves the stomach. but this organ keeps the food in it in almost constant motion for a considerable time, a meal of meat and vegetables remaining in the stomach for three or four hours. while movement is taking place, the gastric juice acts upon proteins, softening them, while the constant churning movement tends to separate the bits of food into finer particles. ultimately the semifluid food, much of it still undigested, is allowed to pass in small amounts through the pyloric valve, into the small intestine. this is allowed by the relaxation of the ringlike muscles of the pylorus. experiments on digestion in the stomach.--some very interesting experiments have recently been made by professor cannon of harvard with reference to movements of the stomach contents. cats were fed with material having in it bismuth, a harmless chemical that would be visible under the x-ray. it was found that shortly after food reached the stomach a series of waves began which sent the food toward the pyloric end of the stomach. if the cat was feeling happy and well, these contractions continued regularly, but if the cat was cross or bad tempered, the movements would stop. this shows the importance of _cheerfulness_ at meals. other experiments showed that food which was churned into a soft mass was only permitted to leave the stomach when it became thoroughly permeated by the gastric juice. it is the _acid_ in the partly digested food that causes the stomach valve to open and allow its contents to escape little by little into the small intestine. the partly digested food in the small intestine almost immediately comes in contact with fluids from two glands, the liver and pancreas. we shall first consider the function of the pancreas. position and structure of the pancreas.--the most important digestive gland in the human body is the pancreas. the gland is a rather diffuse structure; its duct empties by a common opening with the bile duct into the small intestine, a short distance below the pylorus. in internal structure, the pancreas resembles the salivary glands. [illustration: appearance of milk under the microscope, showing the natural grouping of the fat globules. in the circle a single group is highly magnified. milk is one form of an emulsion. (s. m. babcock, wis. bul. no. .)] work done by the pancreas.--starch paste added to artificial pancreatic fluid and kept at blood heat is soon changed to sugar. protein, under the same conditions, is changed to a peptone. fats, which so far have been unchanged except to be melted by the heat of the body, are changed by the action of the pancreas into a form which can pass through the walls of the food tube. if we test pancreatic fluid, we find it strongly _alkaline_ in its reaction. if two test tubes, one containing olive oil and water, the other olive oil and a weak solution of caustic soda, an _alkali_, be shaken violently and then allowed to stand, the oil and water will quickly separate, while the oil, caustic soda, and water will remain for some time in a milky _emulsion_. if this emulsion be examined under the microscope, it will be found to be made of millions of little droplets of fat, floating in the liquid. the presence of the caustic soda helped the forming of the emulsion. pancreatic fluid similarly emulsifies fats and changes them into soft soaps and fatty acids. fat in this form may be absorbed. the process of this transformation is not well understood. conditions under which the pancreas does its work.--the secretion from this gland seems to be influenced by the overflow of acid material from the stomach. this acid, on striking the lining of the small intestine, causes the formation in its walls of a substance known as _secretin_. this secretin reaches the blood and seems to stimulate all the glands pouring fluid into the intestine to do more work. a pint or more of pancreatic fluid is secreted every day. the intestinal fluid.--three different pancreatic enzymes do the work of digestion, one acting on starch, another on protein, and a third on fats. it has been found that some of these enzymes will not do their work unless aided by the _intestinal_ fluid, a secretion formed in glands in the walls of the small intestine. this fluid, though not much is known about it, is believed to play an important part in the digestion of all kinds of foods left undigested in the small intestine. liver.--the liver is the largest gland in the body. in man, it hangs just below the diaphragm, a little to the right side of the body. during life, its color is deep red. it is divided into three lobes, between two of which is found the _gall bladder_, a thin-walled sac which holds the _bile_, a secretion of the liver. bile is a strongly alkaline fluid of greenish color. it reaches the intestine through the same opening as the pancreatic fluid. almost one quart of bile is passed daily into the digestive canal. the color of bile is due to certain waste substances which come from the destruction of worn-out red corpuscles of the blood. this destruction takes place in the liver. [illustration: diagram of a bit of the wall of the small intestine, greatly magnified, _a_, mouths of intestinal glands; _b_, villus cut lengthwise to show blood vessels and lacteal (in center); _e_, lacteal sending branches to other villi; _i_, intestinal glands; _m_, artery; _v_, vein; _l_, _t_, muscular coats of intestine wall.] functions of bile.--the action of bile is not very well known. it has the very important faculty of aiding the pancreatic fluid in digestion, though alone it has slight if any digestive power. certain substances in the bile aid especially in the absorption of fats. bile seems to be mostly a waste product from the blood and as such incidentally serves to keep the contents of the intestine in a more or less soft condition, thus preventing extreme constipation. the liver a storehouse.--perhaps the most important function of the liver is the formation within it of a material called _glycogen_, or animal starch. the liver is supplied by blood from two sources. the greater amount of blood received by the liver comes directly from the walls of the stomach and intestine to this organ. it normally contains about one fifth of all the blood in the body. this blood is very rich in food materials, and from it the cells of the liver take out sugars to form glycogen.[ ] glycogen is stored in the liver until such a time as a food is needed that can be quickly oxidized; then it is changed to sugar and carried off by the blood to the tissue which requires it, and there used for this purpose. glycogen is also stored in the muscles, where it is oxidized to release energy when the muscles are exercised. footnote : it is known that glycogen _may_ be formed in the body from protein, and possibly from fatty foods. the absorption of digested food into the blood.--the object of digestion is to change foods from an insoluble to a soluble form. this has been seen in the study of the action of the various digestive fluids in the body, each of which is seen to aid in dissolving solid foods, changing them to a fluid, and, in case of the bile, actually assisting them to pass through the wall of the intestine. a small amount of digested food may be absorbed by the blood in the blood vessels of the walls of the stomach. most of the absorption, however, takes place through the walls of the small intestine. structure of the small intestine.--the small intestine in man is a slender tube nearly twenty feet in length and about one inch in diameter. if the chief function of the small intestine is that of absorption, we must look for adaptations which increase the absorbing surface of the tube. this is gained in part by the inner surface of the tube being thrown into transverse folds which not only retard the rapidity with which food passes down the intestine, but also give more absorbing surface. but far more important for absorption are millions of little projections which cover the inner surface of the small intestine. the villi.--so numerous are these projections that the whole surface presents a velvety appearance. collectively, these structures are called the _villi_ (singular _villus_). they form the chief organs of absorption in the intestine, several thousand being distributed over every square inch of surface. by means of the folds and villi the small intestine is estimated to have an absorbing surface equal to twice that of the surface of the body. between the villi are found the openings of the _intestinal glands_. internal structure of a villus.--the internal structure of a villus is best seen in a longitudinal section. we find the outer wall made up of a thin layer of cells, the _epithelial_ layer. it is the duty of these cells to absorb the semifluid food from within the intestine. underneath these cells lies a network of very tiny blood vessels, while inside of these, occupying the core of the villus, are found spaces which, because of their white appearance after absorption of fats, have been called _lacteals_. (see figure, page .) [illustration: diagram to show how the nutrients reach the blood.] absorption of foods.--let us now attempt to find out exactly how foods are passed from the intestines into the blood. food substances in solution may be soaked up as a sponge would take up water, or they may pass by osmosis into the cells lining the villus. these cells break down the peptones into a substance that will pass into and become part of the blood. once within the villus, the sugars and digested proteins pass through tiny blood vessels into the larger vessels comprising the _portal circulation_. these pass through the liver, where, as we have seen, sugar is taken from the blood and stored as glycogen. from the liver, the food within the blood is sent to the heart, from there is pumped to the lungs, from there returns to the heart, and is pumped to the tissues of the body. a large amount of water and some salts are also absorbed through the walls of the stomach and intestine as the food passes on its course. the fats in the form of soaps and fatty acids pass into the space in the center of the villus. later they are changed into fats again, probably in certain groups of gland cells known as _mesenteric_ glands, and eventually reach the blood by way of the thoracic duct without passing through the liver. large intestine.--the large intestine has somewhat the same structure as the small intestine, except that it lacks the villi and has a greater diameter. considerable absorption, however, takes place through its walls as the mass of food and refuse material is slowly pushed along by the muscles within its walls. vermiform appendix.--at the point where the small intestine widens to form the large intestine, a baglike pouch is formed. from one side of this pouch is given off a small tube about four inches long, closed at the lower end. this tube, the rudiment of what is an important part of the food tube in the lower vertebrates, is called the _vermiform appendix_. it has come to have unpleasant notoriety in late years, as the site of serious inflammation. constipation.--in the large intestine live millions of bacteria, some of which make and give off poisonous substances known as toxins. these substances are easily absorbed through the walls of the large intestine, and, when they pass into the blood, cause headaches or sometimes serious trouble. hence it follows that the lower bowel should be emptied of this matter as frequently as possible, at least once a day. constipation is one of the most serious evils the american people have to deal with, and it is largely brought about by the artificial life which we lead, with its lack of exercise, fresh air, and sleep. fruit with meals, especially at breakfast, plenty of water between meals and before breakfast, exercise, particularly of the abdominal muscles, and regular habits will all help to correct this evil. hygienic habits of eating; the causes and prevention of dyspepsia.--from the contents of the foregoing chapter it is evident that the object of the process of digestion is to break up solid food so that it may be absorbed to form part of the blood. any habits we may form of thoroughly chewing our food will evidently aid in this process. undoubtedly much of the distress known as dyspepsia is due to too hasty meals with consequent lack of proper mastication of food. the message of mr. horace fletcher in bringing before us the need of proper mastication of food and the attendant evils of overeating is one which we cannot afford to ignore. it is a good rule to go away from the table feeling a little hungry. eating too much overtaxes the digestive organs and prevents their working to the best advantage. still another cause of dyspepsia is eating when in a _fatigued_ condition. it is always a good plan to rest a short time before eating, especially after any hard manual work. we have seen how great a part unpleasant emotions play in preventing peristaltic movements of the food tube. conversely, pleasant conversation, laughter, and fun will help you to digest your meal. eating between meals is condemned by physicians because it calls the blood to the digestive organs at a time when it should be more active in other parts of the body. effect of alcohol on digestion.--it is a well-known fact that alcohol extracts water from tissues with which it is in contact. this fact works much harm to the interior surface of the food tube, especially the walls of the stomach, which in the case of a hard drinker are likely to become irritated and much toughened. in very small amounts alcohol stimulates the secretion of the salivary and gastric glands, and thus appears to aid in digestion. the following results of experiments on dogs, published in the _american journal of physiology_, vol. i, professor chittenden of yale university gives as "strictly comparable," because "they were carried out in succession on the same day." they show that alcohol retards rather than aids in digestion:-- ========================================================================== number of experiment | / lb. meat with water| / lb. meat with | | dilute alcohol ------------------------+------------------------+------------------------ xvii [alpha] : a.m. | digested in hours | xvii [beta] : p.m. | | digested in : hours xviii [alpha] : a.m. | digested in : hours | xviii [beta] : p.m. | | digested in : hours xix [alpha] : a.m. | digested in : hours | xix [beta] : p.m. | | digested in : hours xx [alpha] : a.m. | | digested in : hours xx [beta] : p.m. | digested in : hours | vi [alpha] : a.m. | | digested in : hours vi [beta] : p.m. | digested in : hours | ------------------------+------------------------+------------------------ average | : hours | : hours ------------------------+------------------------+------------------------ as a result of his experiments, professor chittenden remarks: "we believe that the results obtained justify the conclusion that gastric digestion as a whole is not materially modified by the introduction of alcoholic fluids with the food. in other words, the unquestionable acceleration of gastric secretion which follows the ingestion of alcoholic beverages is, as a rule, counterbalanced by the inhibitory effect of the alcoholic fluids upon the chemical process of gastric digestion, with perhaps at times a tendency towards preponderance of inhibitory action." others have come to the same or stronger conclusions as to the undesirable action of alcohol on digestion, as a result of their own experiments. effect of alcohol on the liver.--the effect of heavy drinking upon the liver is graphically shown in the following table prepared by the scientific temperance federation of boston, mass.:-- [illustration: proportion of deaths from disease in a certain area due to alcohol. the black area shows deaths due to alcohol.[ ]] footnote : does not include deaths from general alcoholic paralysis or other organic diseases due to alcohol. liver cirrhosis due to alcohol conservatively estimated at per cent of total cases. "alcoholic indulgence stands almost if not altogether in the front rank of the enemies to be combated in the battle for health."--professor william t. sedgwick. xxi. the blood and its circulation _problems.--to discover the composition and uses of the different parts of the blood._ _to find out the means by which the blood is circulated about the body._ laboratory suggestions _demonstration._--structure of blood, fresh frog's blood and human blood. drawings. _demonstration._--clotting of blood. _demonstration._--use of models to demonstrate that the heart is a force pump. _demonstration._--capillary circulation in web of frog's foot or tadpole's tail. drawing. _home or laboratory exercise._--on relation of exercise on rate of heart beat. function of the blood.--the chief function of the digestive tract is to change foods to such form that they can be absorbed through the walls of the food tube and become part of the blood.[ ] footnote : this change is due to the action of certain enzymes upon the nutrients in various foods. but we also find that peptones are changed back again to proteins when once in the blood. this appears to be due to the _reversible_ action of the enzymes acting upon them. (see page .) if we examine under the microscope a drop of blood taken from the frog or man, we find it made up of a fluid called _plasma_ and two kinds of bodies, the so-called _red corpuscles_ and _colorless corpuscles_, floating in this plasma. composition of plasma.--the plasma of blood is found to be largely (about per cent) water. it also contains a considerable amount of protein, some sugar, fat, and mineral material. it is, then, the medium which holds the fluid food that has been absorbed from within the intestine. this food is pumped to the body cells where, as work is performed, oxidation takes place and heat is given off as a form of energy. the almost constant temperature of the body is also due to the blood, which brings to the surface of the body much of the heat given off by oxidation of food in the muscles and other tissues. when the blood returns from the tissues where the food is oxidized, the plasma brings back with it to the lungs part of the carbon dioxide liberated where oxidation has taken place. some waste products, to be spoken of later, are also found in the plasma. [illustration: human blood as seen under the high power of the compound microscope; at the extreme right is a colorless corpuscle.] the red blood corpuscle; its structure and functions.--the red corpuscle in the blood of the frog is a true cell of disklike form, containing a nucleus. the red corpuscle of man is made in the red marrow of bones and in its young stages has a nucleus. in its adult form, however, it lacks a nucleus. its form is that of a biconcave disk. so small and so numerous are these corpuscles that over five million are found in a cubic centimeter of normal blood. they make up almost one half the total volume of the blood. the color, which is found to be a dirty yellow when separate corpuscles are viewed under the microscope, is due to a protein material called _hæmoglobin_. hæmoglobin contains a large amount of iron. it has the power of uniting very readily with oxygen whenever that gas is abundant, and, after having absorbed it, of giving it up to the surrounding media, when oxygen is there present in smaller amounts than in the corpuscle. this function of carrying oxygen is the most important function of the red corpuscle, although the red corpuscle also removes part of the carbon dioxide from the tissues on their return to the lungs. the taking up of oxygen is accompanied by a change in color of the mass of corpuscles from a dull red to a bright scarlet. clotting of blood.--if fresh beef blood is allowed to stand overnight, it will be found to have separated into two parts, a dark red, almost solid _clot_ and a thin, straw-colored liquid called _serum_. serum is found to be made up of about per cent water, per cent protein, per cent other organic foods, and per cent mineral substances. in these respects it very closely resembles the fluid food that is absorbed from the intestines. if another jar of fresh beef blood is poured into a pan and briskly whipped with a bundle of little rods (or with an egg beater), a stringy substance will be found to stick to the rods. this, if washed carefully, is seen to be almost colorless. tested with nitric acid and ammonia, it is found to contain a protein substance which is called _fibrin_. blood plasma, then, is made up of a fluid portion of serum, and fibrin, which, although in a fluid state in the blood vessels within the body, coagulates when blood is removed from the blood vessels. this coagulation aids in making a blood clot. a clot is simply a mass of fibrin threads with a large number of corpuscles tangled within. the clotting of blood is of great physiological importance, for otherwise we might bleed to death even from a small wound. blood plates.--in blood within the circulatory system of the body, the fibrin is held in a fluid state called _fibrinogen_. an enzyme, acting upon this fibrinogen, the soluble protein in the blood, causes it to change to an insoluble form, the fibrin of the clot. this change seems to be due to the action of minute bodies in the blood known as _blood plates_. under abnormal conditions these blood plates break down, releasing some substances which eventually cause this enzyme to do its work. [illustration: a small artery (_a_) breaking up into capillaries (_c_) which unite to form a vein (_v_). note at (_p_) several colorless corpuscles, which are fighting bacteria at that point.] the colorless corpuscle; structure and functions.--a colorless corpuscle is a cell irregular in outline, the shape of which is constantly changing. these corpuscles are somewhat larger than the red corpuscles, but less numerous, there being about one colorless corpuscle to every three hundred red ones. they have the power of movement, for they are found not only inside but outside the blood vessels, showing that they have worked their way between the cells that form the walls of the blood tubes. [illustration: a colorless corpuscle catching and eating germs.] a russian zoölogist, metchnikoff, after studying a number of simple animals, such as medusæ and sponges, found that in such animals some of the cells lining the inside of the food cavity take up or engulf minute bits of food. later, this food is changed into the protoplasm of the cell. metchnikoff believed that the colorless corpuscles of the blood have somewhat the same function. this he later proved to be true. like the amoeba, they feed by engulfing their prey. this fact has a very important bearing on the relation of colorless corpuscles to certain diseases caused by bacteria within the body. if, for example, a cut becomes infected by bacteria, inflammation may set in. colorless corpuscles at once surround the spot and attack the bacteria which cause the inflammation. if the bacteria are few in number, they are quickly eaten by certain of the colorless corpuscles, which are known as _phagocytes_. if bacteria are present in great quantities, they may prevail and kill the phagocytes by poisoning them. the dead bodies of the phagocytes thus killed are found in the pus, or matter, which accumulates in infected wounds. in such an event, we must come to the aid of nature by washing the wound with some antiseptic, as weak carbolic acid or hydrogen peroxide. antibodies and their uses.--in case of disease where, for example, fever is caused by poison given off from bacteria we find the cells of the body manufacture and pour into the blood a substance known as an _antibody_. this substance does not of necessity kill the harmful germs or even stop their growth. it does, however, unite with the toxin or poison given off by the germs and renders it entirely harmless. function of lymph.--the tissues and organs of the body are traversed by a network of tubes which carry the blood. inside these tubes is the blood proper, consisting of a fluid plasma, the colorless corpuscles, and the red corpuscles. outside the blood tubes, in spaces between the cells which form tissues, is found another fluid, which is in chemical composition very much like plasma of the blood. this is the _lymph_. it is, in fact, fluid food in which some colorless amoeboid corpuscles are found. blood gives up its food material to the lymph. this it does by passing it through the walls of the capillaries. the food is in turn given up to the tissue cells, which are bathed by the lymph. [illustration: the exchange between blood and the cells of the body.] some of the amoeboid corpuscles from the blood make their way between the cells forming the walls of the capillaries. _lymph, then, is practically blood plasma plus some colorless corpuscles. it acts as the medium of exchange between the blood proper and the cells in the tissues of the body._ by means of the food supply thus brought, the cells of the body are able to grow, the fluid food being changed to the protoplasm of the cells. by means of the oxygen passed over by the lymph, oxidation may take place within the cells. lymph not only gives food to the cells of the body, but also takes away carbon dioxide and other waste materials, which are ultimately passed out of the body by means of the lungs, skin, and kidneys. internal secretions.--in addition to all the functions given above, the blood has recently been shown to carry the secretions of a number of glands through which it passes, although these glands have no ducts to carry off their secretions. these internal secretions seem _absolutely necessary_ for the health of the body. several glands, the thyroid, adrenal bodies, the testes, and ovaries, as well as the pancreas, give off these remarkable substances. the amount of blood and its distribution.--blood forms, by weight, about one sixteenth of the body. this would be about four quarts to a body weight of pounds. normally, about one half of the blood of the body is found in or near the organs lying in the body cavity below the diaphragm, about one fourth in the muscles, and the rest in the head, heart, lungs, large arteries, and veins. blood temperature.--the temperature of blood in the human body is normally about . ° fahrenheit when tested under the tongue by a thermometer, although the temperature drops almost two degrees after we have gone to sleep at night. it is highest about p.m. and lowest about a.m. in fevers, the temperature of the body sometimes rises to °; but unless this temperature is soon reduced, death follows. any considerable drop in temperature below the normal also means death. body heat results from the oxidation of food, and the circulation of blood keeps the temperature nearly uniform in all parts of the body. cold-blooded animals.--in animals which are called cold-blooded, the blood has no fixed temperature, but varies with the temperature of the medium in which the animal lives. frogs, in the summer, may sit for hours in water with a temperature of almost °. in winter, they often endure freezing so that the blood and lymph within the spaces under the loose skin are frozen into ice crystals. this change in body temperature is evidently an adaptation to the mode of life. circulation of the blood in man.--the blood is the carrying agent of the body. like a railroad or express company, it takes materials from one part of the human organism to another. this it does by means of the organs of circulation,--the heart and blood vessels. these blood vessels are called _arteries_ where they carry blood away from the heart, _veins_ where they bring blood back to the heart, and _capillaries_ where they connect the larger blood vessels. the organs of circulation thus form a system of connected tubes through which the blood flows. the heart; position, size, protection.--the heart is a cone-shaped muscular organ about the size of a man's fist. it is located immediately above the diaphragm, and lies so that the muscular apex, which points downward, moves while beating against the fifth and sixth ribs, just a little to the left of the midline of the body. this fact gives rise to the notion that the heart is on the left side of the body. the heart is surrounded by a loose membranous bag called the _pericardium_, the inner lining of which secretes a fluid in which the heart lies. when, for any reason, the pericardial fluid is not secreted, inflammation arises in that region. [illustration: diagram showing the front half of the heart cut away: _a_, aorta; _l_, arteries to the lungs; _la_, left auricle; _lv_, left ventricle; _m_, tricuspid valve open; _n_, bicuspid or mitral valve closed; _p_ and _r_, veins from the lungs; _ra_, right auricle; _rv_, right ventricle; _v_, vena cava. arrows show direction of circulation.] internal structure of heart.--if we should cut open the heart of a mammal down the midline, we could divide it into a right and a left side, _each of which would have no internal connection with the other_. each side is made up of an upper thin-walled portion with a rather large internal cavity, the _auricle_, which opens into a lower smaller portion with heavy muscular walls, the _ventricle_. communication between auricles and ventricles is guarded by little flaps or _valves_. the auricles receive blood from the veins. the ventricles pump the blood into the arteries. the heart in action.--the heart is constructed on the same plan as a force pump, the valves preventing the reflux of blood into the auricle when it is forced out of the ventricle. blood enters the auricles from the veins because the muscles of that part of the heart relax; this allows the space within the auricles to fill. almost immediately the muscles of the ventricles relax, thus allowing blood to pass into the chambers within the ventricles. then, after a short pause, during which time the muscles of the heart are resting, a wave of muscular contraction begins in the auricles and ends in the ventricles, with a sudden strong contraction which forces the blood out into the arteries. blood is kept on its course by the valves, which act in the same manner as do the valves in a pump. the blood is thus made to pass into the arteries upon the contraction of the ventricle walls. [illustration: the heart is a force pump; prove it from these diagrams.] [illustration: i. circulation in a fish. _g_, gills; _c_, capillaries of the body. notice the two-chambered heart. ii. the circulation in a frog. _l_, the lungs; _c_, the capillaries. notice the heart has three chambers. what is the condition of blood leaving the ventricle to go to the cells of the body? iii. the circulation in man. _h_, head; _a_, arms; _l_, lungs; _s_, stomach; _li_, liver; _k_, kidney; _s.i._, small intestine; _l.i._, large intestine; _le_, legs; _ _, right auricle; _ _, right ventricle; _ _, left ventricle; _ _, left auricle; _ _, dorsal aorta; _ _, vein to lungs. ] the course of the blood in the body.--although the two sides of the heart are separate and distinct from each other, yet every drop of blood that passes through the right heart likewise passes later through the left heart. there are two distinct systems of circulation in the body. the _pulmonary circulation_ takes the blood through the right auricle and ventricle, to the lungs, and passes it back to the left auricle. this is a relatively short circulation, the blood receiving in the lungs its supply of oxygen, and there giving up some of its carbon dioxide. the greater circulation is known as the _systemic circulation_; in this system, the blood leaves the left ventricle through the great dorsal _aorta_. a large part of the blood passes directly to the muscles; some of it goes to the nervous system, kidneys, skin, and other organs of the body. it gives up its supply of food and oxygen in these tissues, receives the waste products of oxidation while passing through the capillaries, and returns to the right auricle through two large vessels known as the _venæ cavæ_. it requires only from twenty to thirty seconds for the blood to make the complete circulation from the ventricle back again to the starting point. this means that the entire volume of blood in the human body passes three or four thousand times a day through the various organs of the body.[ ] footnote : see hough and sedgwick, _the human mechanism_, page . portal circulation.--some of the blood, on its way back to the heart, passes to the walls of the food tube and to its glands. from there it is sent with its load of absorbed food to the liver. here the vein which carries the blood (called the portal vein) breaks up into capillaries around the cells of the liver, when it gives up sugar to be stored as glycogen. from the liver, blood passes directly to the right auricle. the _portal circulation_, as it is called, is the only part of the circulation where the blood passes through two sets of capillaries on its way from auricle to auricle. [illustration: capillary circulation in the web of a frog's foot, as seen under the compound microscope. _a_, _b_, small veins; _c_, pigment cells in the skin; _d_, capillaries in which the oval corpuscles are seen to follow one another in single series.] circulation in the web of a frog's foot.--if the web of the foot of a live frog or the tail of a tadpole is examined under the compound microscope, a network of blood vessels will be seen. in some of the larger vessels the corpuscles are moving rapidly and in spurts; these are _arteries_. the arteries lead into smaller vessels hardly greater in diameter than the width of a single corpuscle. this network of _capillaries_ may be followed into larger _veins_ in which the blood moves regularly. this illustrates the condition in any tissue of man where the arteries break up into capillaries, and these in turn unite to form veins. structure of the arteries.--a distinct difference in structure exists between the arteries and the veins in the human body. the arteries, because of the greater strain received from the blood which is pumped from the heart, have thicker muscular walls, and in addition are very elastic. cause of the pulse.--the _pulse_, which can easily be detected by pressing the large artery in the wrist or the small one in front of and above the external ear, is caused by the gushing of blood through the arteries after each pulsation of the heart. as the large arteries pass away from the heart, the diameter of each individual artery becomes smaller. at the very end of their course, these arteries are so small as to be almost microscopic in size and are very numerous. there are so many that if they were placed together, side by side, their united diameter would be much greater than the diameter of the large artery (_aorta_) which passes blood from the left side of the heart. this fact is of very great importance, for the force of the blood as it gushes through the arteries becomes very much less when it reaches the smaller vessels. this gushing movement is quite lost when the capillaries are reached, first, because there is so much more space for the blood to fill, and second, because there is considerable friction caused by the very tiny diameter of the capillaries. capillaries.--the capillaries form a network of minute tubes everywhere in the body, but especially near the surface and in the lungs. it is through their walls that the food and oxygen pass to the tissues, and carbon dioxide is given up to the plasma. they form the connection that completes the system of circulation of blood in the body. function and structure of the veins.--if the arteries are supply pipes which convey fluid food to the tissues, then the veins may be likened to drain pipes which carry away waste material from the tissues. extremely numerous in the extremities and in the muscles and among other tissues of the body, they, like the branches of a tree, become larger and unite with each other as they approach the heart. [illustration: valves in a vein. notice the thin walls of the vein.] if the wall of a vein is carefully examined, it will be found to be neither so thick nor so tough as an artery wall. when empty, a vein collapses; the wall of an artery holds its shape. if you hold your hand downward for a little time and then examine it, you will find that the veins, which are relatively much nearer the surface than are the arteries, appear to be very much knotted. this appearance is due to the presence of tiny valves within. these valves open in the direction of the blood current, but would close if the direction of the blood flow should be reversed (as in case a deep cut severed a vein). as the pressure of blood in the veins is much less than in the arteries, the valves thus aid in keeping the flow of blood in the veins toward the heart. the higher pressure in arteries and the suction in the veins (caused by the enlargement of the chest cavity in breathing) are the chief factors which cause a steady flow of blood through the veins in the body. lymph vessels.--the lymph is collected from the various tissues of the body by means of a number of very thin-walled tubes, which are at first very tiny, but after repeated connection with other tubes ultimately unite to form large ducts. these lymph ducts are provided, like the veins, with valves. the pressure of the blood within the blood vessels forces continually more plasma into the lymph; thus a slow current is maintained. on its course the lymph passes through many collections of gland cells, the _lymph glands_. in these glands some impurities appear to be removed and colorless corpuscles made. the lymph ultimately passes into a large tube, the _thoracic duct_, which flows upward near the ventral side of the spinal column, and empties into the large subclavian vein in the left side of the neck. another smaller lymph duct enters the right subclavian vein. [illustration: the lymph vessels; the dark spots are lymph glands: _lac_, lacteals; _rc_, thoracic duct.] the lacteals.--we have already found that part of the digested food (chiefly carbohydrates, proteins, salts, and water) is absorbed directly into the blood through the walls of the villi and carried to the liver. fat, however, is passed into the spaces in the central part of the villi, and from there into other spaces between the tissues, known as the _lacteals_. the lacteals carry the fats into the blood by way of the thoracic duct. the lacteals and lymph vessels have in part the same course. it will be thus seen that lymph at different parts of its course would have a very different composition. the nervous control of the heart and blood vessels.--although the muscles of the heart contract and relax without our being able to stop them or force them to go faster, yet in cases of sudden fright, or after a sudden blow, the heart may stop beating for a short interval. this shows that the heart is under the control of the nervous system. two sets of nerve fibers, both of which are connected with the central nervous system, pass to the heart. one set of fibers accelerates, the other slows or inhibits, the heart beat. the arteries and veins are also under the control of the sympathetic nervous system. this allows of a change in the diameter of the blood vessels. thus, blushing is due to a sudden rush of blood to the surface of the body caused by an expansion of the blood vessels at the surface. the blood vessels of the body are always full of blood. this results from an automatic regulation of the diameter of the blood tubes by a part of the nervous system called the _vasomotor nerves_. these nerves act upon the muscles in the walls of the blood vessels. in this way, each vessel adapts itself to the amount of blood in it at a given time. after a hearty meal, a large supply of blood is needed in the walls of the stomach and intestines. at this time, the arteries going to this region are dilated so as to receive an extra supply. when the brain performs hard work, blood is supplied in the same manner to that region. hence, one should not study or do mental work immediately after a hearty meal, for blood will be drawn away to the brain, leaving the digestive tract with an insufficient supply. indigestion may follow as a result. the effect of exercise on the circulation.--it is a fact familiar to all that the heart beats more violently and quickly when we are doing hard work than when we are resting. count your own pulse when sitting quietly, and then again after some brisk exercise in the gymnasium. exercise in moderation is of undoubted value, because it sends the increased amount of blood to such parts of the body where increased oxidation has been taking place as the result of the exercise. the best forms of exercise are those which give as many muscles as possible work--walking, out-of-door sports, any exercise that is not violent. exercise should not be attempted immediately after eating, as this causes a withdrawal of blood from the digestive tract to the muscles of the body. neither should exercise be continued after becoming tired, as poisons are then formed in the muscles, which cause the feeling we call _fatigue_. remember that extra work given to the heart by extreme exercise may injure it, causing possible trouble with the valves. [illustration: stopping flow of blood from an artery by applying a tight bandage (ligature) between the cut and the heart.] treatment of cuts and bruises.--blood which oozes slowly from a cut will usually stop flowing by the natural means of the formation of a clot. a cut or bruise should, however, be washed in a weak solution of carbolic acid or some other antiseptic in order to prevent bacteria from obtaining a foothold on the exposed flesh. if blood, issuing from a wound, gushes in distinct pulsations, then we know that an artery has been severed. to prevent the flow of blood, a tight bandage known as a _tourniquet_ must be tied between the cut and the heart. a handkerchief with a knot placed over the artery may stop bleeding if the cut is on one of the limbs. if this does not serve, then insert a stick in the handkerchief and twist it so as to make the pressure around the limb still greater. thus we may close the artery until the doctor is called, who may sew up the injured blood vessel. the effect of alcohol upon the blood.--it has recently been discovered that alcohol has an extremely injurious effect upon the colorless corpuscles of the blood, lowering their ability to fight disease germs to a marked degree. this is well seen in a comparison of deaths from certain infectious diseases in drinkers and abstainers, the percentage of mortality being much greater in the former. dr. t. alexander macnichol, in a recent address, said:-- "massart and bordet, metchnikoff and sims woodhead, have proved that alcohol, even in very dilute solution, prevents the white blood corpuscles from attacking invading germs, thus depriving the system of the coöperation of these important defenders, and reducing the powers of resisting disease. the experiments of richardson, harley, kales, and others have demonstrated the fact that one to five per cent of alcohol in the blood of the living human body in a notable degree alters the appearance of the corpuscular elements, reduces the oxygen bearing elements, and prevents their reoxygenation." alcohol weakens resistance to disease.--in acute illnesses, grippe, fevers, blood poisoning, etc., substances formed in the blood termed "antibodies" antagonize the action of bacteria, facilitating their destruction by the white blood cells and neutralizing their poisonous influence. in a person with good "resistance" this protective machinery, which we do not yet thoroughly understand, works with beautiful precision, and the patient "gets well." experiments by scientific experts have demonstrated that alcohol restrains the formation of these marvelous antibodies. alcohol puts to sleep the sentinels that guard your body from disease. the effect of alcohol on the circulation.--alcoholic drinks affect the very delicate adjustment of the nervous center's controlling the blood vessels and heart. even very dilute alcohol acts upon the muscles of the tiny blood vessels; consequently, more blood is allowed to enter them, and, as the small vessels are usually near the surface of the body, the habitual redness seen in the face of hard drinkers is the ultimate result. "the first effect of diluted alcohol is to make the heart beat faster. this fills the small vessels near the surface. a feeling of warmth is produced which causes the drinker to feel that he was warmed by the drink. this feeling, however, soon passes away, and is succeeded by one of chilliness. the body temperature, at first raised by the rather rapid oxidation of the alcohol, is soon lowered by the increased radiation from the surface. "the immediate stimulation to the heart's action soon passes away and, like other muscles, the muscles of the heart lose power and contract with less force after having been excited by alcohol."--macy, _physiology_. alcohol, when brought to act directly on heart muscle, lessens the force of the beat. it may even cause changes in the tissues, which eventually result in the breaking of the walls of a blood vessel or the plugging of a vessel with a blood clot. this condition may cause the disease known as _apoplexy_. effects of tobacco upon the circulation.--"the frequent use of cigars or cigarettes by the young seriously affects the quality of the blood. the red blood corpuscles are not fully developed and charged with their normal supply of life-giving oxygen. this causes paleness of the skin, often noticed in the face of the young smoker. palpitation of the heart is also a common result, followed by permanent weakness, so that the whole system is enfeebled, and mental vigor is impaired as well as physical strength."--macy, _physiology_. xxii. respiration and excretion _problems.--a study of respiration to find out:-- (a) what changes in blood and air take place within the lungs. (b) the mechanics of respiration. a study of ventilation to discover:-- (a) the reason for ventilation. (b) the best method of ventilation. a study of the organs of excretion._ laboratory suggestions _demonstration._--comparison of lungs of frog with those of bird or mammal. _experiment._--the changes of blood within the lungs. _experiment._--changes taking place in air in the lungs. _experiment._--the use of the ribs in respiration. _demonstration experiment._--what causes the filling of air sacs of the lungs? _demonstration experiment._--what are the best methods of ventilating a room? _demonstration._--best methods of dusting and cleaning. _demonstration._--beef or sheep's kidney to show areas. necessity for respiration.--we have seen that plants and animals need oxygen in order that the life processes may go on. food is oxidized to release energy, just as coal is burned to give heat to run an engine. as a draft of air is required to make fire under the boiler, so, in the human body, oxygen must be given so that food in tissues may be oxidized to release energy used in work. this oxidation takes place in the cells of the body, be they part of a muscle, a gland, or the brain. _blood, in its circulation to all parts of the body, is the medium which conveys the oxygen to that place in the body where it will be used._ [illustration: air passages in the human lungs. _a_, larynx; _b_, trachea (or windpipe); _c_, _d_, bronchi; _e_, bronchial tubes; _f_, cluster of air cells.] the organs of respiration in man.--we have alluded to the fact that the lungs are the organs which give oxygen to the blood and take from it carbon dioxide. the course of the air passing to the lungs in man is much the same as in the frog. air passes through the nose, and into the windpipe. this cartilaginous tube, the top of which may easily be felt as the adam's apple of the throat, divides into two _bronchi_. the bronchi within the lungs break up into a great number of smaller tubes, the _bronchial tubes_, which divide somewhat like the small branches of a tree. the bronchial tubes, indeed all the air passages, are lined with ciliated cells. the cilia of these cells are constantly in motion, beating with a quick stroke toward the outer end of the tube, that is, toward the mouth. hence any foreign material will be raised from the throat first by the action of the cilia and then by coughing or "clearing the throat." the bronchi end in very minute air sacs, little pouches having elastic walls, into which air is taken when we inspire, or take a deep breath. in the walls of these pouches are numerous capillaries, the ends of arteries which pass from the heart into the lung. _it is through the very thin walls of the air sacs that an interchange of gases takes place which results in the blood giving up part of its load of carbon dioxide, and taking up oxygen in its place._ this exchange appears to be aided by the presence of an enzyme in the lung tissues. this is another example of the various kinds of work done by the enzymes of the body. [illustration: diagram to show what the blood loses and gains in one of the air sacs of the lungs.] changes in the blood within the lungs.--blood, after leaving the lungs, is much brighter red than just before entering them. the change in color is due to a taking up of oxygen by the _hæmoglobin_ of the red corpuscles. changes taking place in blood are obviously the reverse of those which take place in air in the lungs. every hundred cubic centimeters of blood going into the lungs contains to c.c. of oxygen, to c.c. of carbon dioxide, and to c.c. of nitrogen. the same amount of blood passing out of the lungs contains c.c. of oxygen, c.c. of carbon dioxide, and to c.c. of nitrogen. the water, of which about half a pint is given off daily, is mostly lost from the blood. changes in air in the lungs.--air is much warmer after leaving the lungs than before it enters them. breathe on the bulb of a thermometer to prove this. expired air contains a considerable amount of moisture, as may be proved by breathing on a cold polished surface. this it has taken up in the air sacs of the lungs. the presence of carbon dioxide in expired air may easily be detected by the limewater test. air such as we breathe out of doors contains, by volume:-- nitrogen . oxygen . carbon dioxide . argon . water vapor (average) . air expired from the lungs contains:-- nitrogen . oxygen . carbon dioxide . water vapor argon in other words, there is a loss between and per cent oxygen, and nearly a corresponding gain in carbon dioxide, in expired air. there are also some other organic substances present. [illustration: the respiration of cells.] cell respiration.--it has been shown, in the case of very simple animals, such as the _amoeba_, that when oxidation takes place in a cell, work results from this oxidation. the oxygen taken into the lungs is not used there, but is carried by the blood to such parts of the body as need oxygen to oxidize food materials in the cells. since work is done in the cells of the body, food and oxygen are therefore required. the quantity of oxygen used by the body is nearly dependent on the amount of work performed. oxygen is constantly taken from the blood by tissues in a state of rest and is used up when the body is at work. this is suggested by the fact that in a given time a man, when working, gives off more oxygen (in carbon dioxide) than he takes in during that time. while work is being done certain wastes are formed in the cell. carbon dioxide is given off when carbon is burned. but when proteins are burned, another waste product containing nitrogen is formed. this must be passed off from the cells, as it is a poison. here again the lymph and blood, the common carriers, take the waste material to points where it may be _excreted_ or passed out of the body. the mechanics of respiration. the pleura.--the lungs are covered with a thin elastic membrane, the _pleura_. this forms a bag in which the lungs are hung. between the walls of the bag and the lungs is a space filled with lymph. by this means the lungs are prevented from rubbing against the walls of the chest. [illustration: the chest cavity (_a_) at the time of a full breath; (_b_), after an expiration. explain how the cavity for lungs is made larger.] breathing.--in every full breath there are two distinct movements, inspiration (taking air in) and expiration (forcing air out). in man an inspiration is produced by the contraction of the muscles between the ribs, together with the contraction of the diaphragm, the muscular wall just below the heart and lungs; this results in pulling down the diaphragm and pulling upward and outward of the ribs, thus making the space within the chest cavity larger. the lungs, which lie within this cavity, are filled by the air rushing into the larger space thus made. that this cavity is larger than it was at first may be demonstrated by a glance at the accompanying figure. an expiration is simpler than an inspiration, for it requires no muscular effort; the muscles relax, the breastbone and ribs sink into place, while the diaphragm returns to its original position. [illustration: apparatus to show the mechanics of breathing.] a piece of apparatus which illustrates to a degree the mechanics of breathing may be made as follows: attach a string to the middle of a piece of sheet rubber. tie the rubber over the large end of a bell jar. pass a glass y-tube through a rubber stopper. fasten two small toy balloons to the branches of the tube. close the small end of the jar with the stopper. adjust the tube so that the balloons shall hang free in the jar. if now the rubber sheet is pulled down by means of the string, the air pressure in the jar is reduced and the toy balloons within expand, owing to the air pressure down the tube. when the rubber is allowed to go back to its former position, the balloons collapse. [illustration: diagram showing the relative amounts of tidal, complemental, reserve, and residual air. the brace shows the average lung capacity for the adult man.] rate of breathing and amount of air breathed.--during quiet breathing, the rate of inspiration is from fifteen to eighteen times per minute; this rate largely depends on the amount of physical work performed. about cubic inches of air are taken in and expelled during the ordinary quiet respiration. the air so breathed is called _tidal air_. in a "long" breath, we take in about cubic inches in addition to the tidal air. this is called _complemental air_. by means of a forced expiration, it is possible to expel from to cubic inches more than tidal air; this air is called _reserve air_. what remains in the lungs, amounting to about cubic inches, is called the _residual air_. the value of deep breathing is seen by a glance at the diagram. it is only by this means that we clear the lungs of the reserve air with its accompanying load of carbon dioxide. respiration under nervous control.--the muscular movements which cause an inspiration are partly under the control of the will, but in part the movement is beyond our control. the nerve centers which govern inspiration are part of the sympathetic nervous system. anything of an irritating nature in the trachea or larynx will cause a sudden expiration or cough. when a boy runs, the quickened respiration is due to the fact that oxygen is used up rapidly and a larger quantity of carbon dioxide is formed. the carbon dioxide in the blood stimulates the nervous center which has control of respiration to greater activity, and quickened inspiration follows. need of ventilation.--during the course of a day the lungs lose to the surrounding air nearly two pounds of carbon dioxide. this means that about three fifths of a cubic foot is given off by each person during an hour. when we are confined for some time in a room, it becomes necessary to get rid of this carbon dioxide. this can be done only by means of proper ventilation. a considerable amount of moisture is given off from the body, and this moisture in a crowded room is responsible for much of the discomfort. the air becomes humid and uncomfortable. it has been found that by keeping the air in motion in such a room (as through the use of electric fans) much of this discomfort is obviated. the presence of impurities in the air of a room may easily be determined by its odor. the odor of a poorly ventilated room is due to organic impurities given off with the carbon dioxide. this, fortunately, gives us an index of the amount of waste material in the air. among the factors which take oxygen from the air in a closed room and produce carbon dioxide are burning gas or oil lamps and stoves, and the presence of a number of people. [illustration: three ways of ventilating a room. _i_, inlet for air; _o_, outlet for air. which is the best method of ventilation? explain.] proper ventilation.--ventilation consists in the removal of air that has been used, and the introduction of a fresh supply to take its place. heated air rises, carrying with it much of the carbon dioxide and other impurities. a good method of ventilation for the home is to place a board two or three inches high between the lower sash and the frame of a window or to have the window open an inch or so at the top and the bottom. an open fireplace in a room aids in ventilation because of the constant draft up the flue. sweeping and dusting.--it is very easy to demonstrate the amount of dust in the air by following the course of a beam of light in a darkened room. we have already proved that spores of mold and yeast exist in the air. that bacteria are also present can be proved by exposing a sterilized gelatin plate to the air in a schoolroom for a few moments.[ ] footnote : expose two sterilized dishes containing culture media; one in a room being swept with a damp broom, and the other in a room which is being swept in the usual manner. note the formation of colonies of bacteria in each dish. in which dish does the more abundant growth take place? many of the bacteria present in the air are active in causing diseases of the respiratory tract, such as diphtheria, membranous croup, and tuberculosis. other diseases, as colds, bronchitis (inflammation of the bronchial tubes), and pneumonia (inflammation of the tiny air sacs of the lungs), are also caused by bacteria. [illustration: plate culture exposed for five minutes in a school hall where pupils were passing to recitations. each spot is a colony of bacteria or mold.] dust, with its load of bacteria, will settle on any horizontal surface in a room not used for three or four hours. dusting and sweeping should always be done with a damp cloth or broom, otherwise the bacteria are simply stirred up and sent into the air again. the proper watering of streets before they are swept is also an important factor in health. much dust is composed largely of dried excreta of animals. soft-coal smoke does its share to add to the impurities of the air, while sewer gas and illuminating gas are frequently found in sufficient quantities to poison people. pure air is, as can be seen, almost an impossibility in a great city. [illustration: a sleeping porch, an ideal way to get fresh air at night.] how to get fresh air.--as we know, green plants give off in the sunlight considerable more oxygen than they use, and they use up carbon dioxide. the air in the country is naturally purer than in the city, as smoke and bacteria are not so prevalent there, and the plants in abundance give off oxygen. in the city the night air is purer than day air, because the factories have stopped work, the dust has settled, and fewer people are on the streets. the old myth of "night air" being injurious has long since been exploded, and thousands of people of delicate health, especially those who have weak throat or lungs, are regaining health by sleeping out of doors or with the windows wide open. the only essential in sleeping out of doors or in a room with a low temperature is that the body be kept warm and the head be protected from strong drafts by a nightcap or hood. proper ventilation at _all_ times is one of the greatest factors in good health. change of air.--persons in poor health, especially those having tuberculosis, are often cured by a change of air. this is not always so much due to the composition of the air as to change of occupation, rest, and good food. mountain air is dry, and relatively free from dust and bacteria, and often helps a person having tuberculosis. air at the seaside is beneficial for some forms of disease, especially hay fever and bone tuberculosis. many sanitariums have been established for this latter disease near the ocean, and thousands of lives are being annually saved in this way. [illustration: unfavorable sleeping conditions. explain why unfavorable.] ventilation of sleeping rooms.--sleeping in close rooms is the cause of much illness. beds ought to be placed so that a constant supply of fresh air is given without a direct draft. this may often be managed with the use of screens. bedroom windows should be thrown open in the morning to allow free entrance of the sun and air, bedclothes should be washed frequently, and sheets and pillow covers often changed. bedroom furniture should be simple, and but little drapery allowed in the room. hygienic habits of breathing.--every one ought to accustom himself upon going into the open air to inspire slowly and deeply to the full capacity of the lungs. a slow expiration should follow. take care to force the air out. breathe through the nose, thus warming the air you inspire before it enters the lungs and chills the blood. repeat this exercise several times every day. you will thus prevent certain of the air sacs which are not often used from becoming hardened and permanently closed. relation of proper exercise to health.--we are all aware that exercise in moderation has a beneficial effect upon the human organism. the pale face, drooping shoulders, and narrow chest of the boy or girl who takes no regular exercise is too well known. exercise, besides giving direct use of the muscles, increases the work of the heart and lungs, causing deeper breathing and giving the heart muscles increased work; it liberates heat and carbon dioxide from the tissues where the work is taking place, thus increasing the respiration of the tissues themselves, and aids mechanically in the removal of wastes from tissues. it is well known that exercise, when taken some little time after eating, has a very beneficial effect upon digestion. exercise and especially games are of immense importance to the nervous system as a means of rest. the increasing number of playgrounds in this country is due to this acknowledged need of exercise, especially for growing children. proper exercise should be moderate and varied. walking in itself is a valuable means of exercising certain muscles, so is bicycling, but neither is ideal as the _only_ form to be used. _vary_ your exercise so as to bring different muscles into play, take exercise that will allow free breathing out of doors if possible, and the natural fatigue which follows will lead you to take the rest and sleep that every normal body requires. exercise should always be limited by fatigue, which brings with it fatigue poisons. this is nature's signal when to rest. if one's use of diet and air is proper, the fatigue point will be much further off than otherwise. one should learn to _relax_ when not in activity. the habit produces rest, even between exertions very close together, and enables one to continue to repeat those exertions for a much longer time than otherwise. the habit of lying down when tired is a good one. the relation of tight clothing to correct breathing.--it is impossible to breathe correctly unless the clothing is worn loosely over the chest and abdomen. tight corsets and tight belts prevent the walls of the chest and the abdomen from pushing outward and interfere with the drawing of air into the lungs. they may also result in permanent distortion of parts of the skeleton directly under the pressure. other organs of the body cavity, as the stomach and intestines, may be forced downward, out of place, and in consequence cannot perform their work properly. suffocation and artificial respiration.--suffocation results from the shutting off of the supply of oxygen from the lungs. it may be brought about by an obstruction in the windpipe, by a lack of oxygen in the air, by inhaling some other gas in quantity, or by drowning. a severe electric shock may paralyze the nervous centers which control respiration, thus causing a kind of suffocation. in the above cases, death often may be prevented by prompt recourse to artificial respiration. to accomplish this, place the patient on his back with the head lower than the body; grasp the arms near the elbows and draw them _upward_ and _outward_ until they are stretched above the head, on a line with the body. by this means the chest cavity is enlarged and an inspiration produced. to produce an expiration, carry the arms downward, and press them against the chest, thus forcing the air out of the lungs. this exercise, regularly repeated every few seconds, if necessary for hours, has been the source of saving many lives. common diseases of the nose and throat.--catarrh is a disease to which people with sensitive mucous membrane of the nose and throat are subject. it is indicated by the constant secretion of mucus from these membranes. frequent spraying of the nose and throat with some mild antiseptic solutions is found helpful. chronic catarrh should be attended to by a physician. often we find children breathing entirely through the mouth, the nose being seemingly stopped up. when this goes on for some time the nose and throat should be examined by a physician for _adenoids_, or growths of soft masses of tissue which fill up the nose cavity, thus causing a shortage of the air supply for the body. many a child, backward at school, thin and irritable, has been changed to a healthy, normal, bright scholar by the removal of adenoids. sometimes the tonsils at the back of the mouth cavity may become enlarged, thus shutting off the air supply and causing the same trouble as we see in a case of adenoids. the simple removal of the obstacle by a doctor soon cures this condition. (see page .) organs of excretion.--all the life processes which take place in a living thing result ultimately, in addition to giving off of carbon dioxide, in the formation of organic wastes within the body. the retention of these wastes which contain nitrogen, is harmful to animals. in man, the skin and kidneys remove this waste from the body, hence they are called the organs of excretion. [illustration: longitudinal section through a kidney.] the human kidney.--the human kidney is about four inches long, two and one half inches wide, and one inch in thickness. its color is dark red. if the structure of the medulla and cortex (see figure above) is examined under the compound microscope, you will find these regions to be composed of a vast number of tiny branched and twisted tubules. the outer end of each of these tubules opens into the _pelvis_, the space within the kidney; the inner end, in the cortex, forms a tiny closed sac. in each sac, the outer wall of the tube has grown inward and carried with it a very tiny artery. this artery breaks up into a mass of capillaries. these capillaries, in turn, unite to form a small vein as they leave the little sac. each of these sacs with its contained blood vessels is called a _glomerulus_. [illustration: diagram of kidney circulation, showing a glomerulus and tubule: _a_, artery bringing blood to part; _b_, capillary bringing blood to glomerulus; _b'_, vessel continuing with blood to vein; _c_, vein; _t_, tubule; _g_, glomerulus.] wastes given off by the blood in the kidney.--in the glomerulus the blood loses by osmosis, through the very thin walls of the capillaries, first, a considerable amount of water (amounting to nearly three pints daily); second, a nitrogenous waste material known as urea; third, salts and other waste organic substances, uric acid among them. these waste products, together with the water containing them, are known as _urine_. the total amount of nitrogenous waste leaving the body each day is about twenty grams. it is passed through the _ureter_ to the _urinary bladder_; from this reservoir it is passed out of the body, through a tube called the _urethra_. after the blood has passed through the glomeruli of the kidneys it is purer than in any other place in the body, because, before coming there, it lost a large part of its burden of carbon dioxide in the lungs. after leaving the kidney it has lost much of its nitrogenous waste. so dependent is the body upon the excretion of its poisonous material that, in cases where the kidneys do not do their work properly, death may ensue within a few hours. [illustration: diagram of a section of the skin. (highly magnified.)] structure and use of sweat glands.--if you examine the palm of your hand with a lens, you will notice the surface is thrown into little ridges. in these ridges may be found a large number of very tiny pits; these are the pores or openings of the sweat-secreting glands. from each opening a little tube penetrates deep within the epidermis; there, coiling around upon itself several times, it forms the sweat gland. close around this coiled tube are found many capillaries. from the blood in these capillaries, cells lining the wall of the gland take water, and with it a little carbon dioxide, urea, and some salts (common salt among others). this forms the excretion known as _sweat_. the combined secretions from these glands amount normally to a little over a pint during twenty-four hours. at all times, a small amount of sweat is given off, but this is evaporated or is absorbed by the underwear; as this passes off unnoticed, it is called _insensible perspiration_. in hot weather or after hard manual labor the amount of perspiration is greatly increased. regulation of heat of the body.--the bodily temperature of a person engaged in manual labor will be found to be but little higher than the temperature of the same person at rest. we know from our previous experiments that heat is released. muscles, nearly one half the weight of the body, release about five sixths of their energy as heat. at all times they are giving up some heat. how is it that the bodily temperature does not differ greatly at such times? the temperature of the body is largely regulated by means of the activity of the sweat glands. the blood carries much of the heat, liberated in the various parts of the body by the oxidation of food, to the surface of the body, where it is lost in the evaporation of sweat. in hot weather the blood vessels of the skin are dilated; in cold weather they are made smaller by the action of the nervous system. the blood thus loses water in the skin, the water evaporates, and we are cooled off. _the object of increased perspiration, then, is to remove heat from the body._ with a large amount of blood present in the skin, perspiration is increased; with a small amount, it is diminished. hence, we have in the skin an automatic regulator of bodily temperature. sweat glands under nervous control.--the sweat glands, like the other glands in the body, are under the control of the sympathetic nervous system. frequently the nerves dilate the blood vessels of the skin, thus helping the sweat glands to secrete, by giving them more blood. "thus regulation is carried out by the nervous system determining, on the one hand, the _loss_ by governing the supply of blood to the skin and the action of the sweat glands; and on the other, the _production_ by diminishing or increasing the oxidation of the tissues."--foster and shore, _physiology_. colds and fevers.--the regulation of blood passing through the blood vessels is under control of the nervous system. if this mechanism is interfered with in any way, the sweat glands may not do their work, perspiration may be stopped, and the heat from oxidation held within the body. the body temperature goes up, and a fever results. [illustration: _a_, blood vessels in skin normal; _b_, when congested.] if the blood vessels in the skin are suddenly cooled when full of blood, they contract and send the blood elsewhere. as a result a congestion or cold may follow. colds are, in reality, a congestion of membranes lining certain parts of the body, as the nose, throat, windpipe, or lungs. when suffering from a cold, it is therefore important not to chill the skin, as a full blood supply should be kept in it and so kept from the seat of the congestion. for this reason hot baths (which call the blood to the skin), the avoiding of drafts (which chill the skin), and warm clothing are useful factors in the care of colds. hygiene of the skin.--the skin is of importance both as an organ of excretion and as a regulator of bodily temperature. the skin of the entire body should be bathed frequently so that this function of excretion may be properly performed. pride in one's own appearance forbids a dirty skin. for those who can stand it, a cold sponge bath is best. soap should be used daily on parts exposed to dirt. exercise in the open air is important to all who desire a good complexion. the body should be kept at an even temperature by the use of proper underclothing. wool, a poor conductor of heat, should be used in winter, and cotton, which allows of a free escape of heat, in summer. cuts, bruises, and burns.--in case the skin is badly broken, it is necessary to prevent the entrance and growth of bacteria. this may be done by washing the wound with weak antiseptic solutions such as per cent _carbolic acid_, per cent _lysol_, or _peroxide of hydrogen_ (full strength). these solutions should be applied immediately. a burn or scald should be covered at once with a paste of baking soda, with olive oil, or with a mixture of limewater and linseed oil. these tend to lessen the pain by keeping out the air and reducing the inflammation. summary of changes in blood within the body.--we have already seen that red corpuscles in the lungs lose part of their load of carbon dioxide that they have taken from the tissues, replacing it with oxygen. this is accompanied by a change of color from purple (in blood which is poor in oxygen) to that of bright red (in richly oxygenated blood). other changes take place in other parts of the body. in the walls of the food tube, especially in the small intestine, the blood receives its load of fluid food. in the muscles and other working tissues the blood gives up food and oxygen, receiving carbon dioxide and organic waste in return. in the liver, the blood gives up its sugar, and the worn-out red corpuscles which break down are removed (as they are in the spleen) from the circulation. in glands, it gives up materials used by the gland cells in their manufacture of secretions. in the kidneys, it loses water and nitrogenous wastes (_urea_). in the skin, it also loses some waste materials, salts, and water. "the effect of alcohol on body heat.--it is usually believed that 'taking a drink' when cold makes one warmer. but such is not the case. in reality alcohol lowers the temperature of the body by dilating the blood vessels of the skin. it does this by means of its influence on the nervous system. it is, therefore, a mistake to drink alcoholic beverages when one is extremely cold, because by means of this more bodily heat is allowed to escape. "because alcohol is quickly oxidized, and because heat is produced in the process, it was long believed to be of value in maintaining the heat of the body. a different view now prevails as the result of much observation and experiment. physiologists show by careful experiments that though the temperature of the body rises during digestion of food, it is lowered for some hours when alcohol is taken. the flush which is felt upon the skin after a drink of wine or spirits is due in part to an increase of heat in the body, but also to the paralyzing effect of the alcohol upon the capillary walls, allowing them to dilate, and so permitting more of the warm blood of the interior of the body to reach the surface. there it is cooled by radiation, and the general temperature is lowered."--macy, _physiology_. effect of alcohol on respiration.--alcohol tends to congest the membrane of the throat and lungs. it does this by paralyzing the nerves which take care of the tiny blood vessels in the walls of the air tubes and air sacs. the capillaries become full of blood, the air spaces are lessened, and breathing is interfered with. the use of alcohol is believed by many physicians to predispose a person to tuberculosis. certainly this disease attacks drinkers more readily than those who do not drink. alcohol interferes with the respiration of the cells because it is oxidized very quickly within the body as it is quickly absorbed and sent to the cells. so rapid is this oxidation that it interferes with the oxidation of other substances. using alcohol has been likened to burning kerosene in a stove; the operation is a dangerous one. effects of tobacco on respiration.--tobacco smoke contains the same kind of poisons as the tobacco, with other irritating substances added. it is extremely irritating to the throat; it often causes a cough, and renders it more liable to inflammation. if the smoke is inhaled more deeply, the vaporized nicotine is still more readily absorbed and may thus produce greater irritation in the bronchi and lungs. cigarettes are worse than other forms of tobacco, for they contain the same poisons with others in addition. effect of alcohol on the kidneys.--it is said that alcohol is one of the greatest causes of disease in the kidneys. the forms of disease known as "fatty degeneration of the kidney" and "bright's disease" are both frequently due to this cause. the kidneys are the most important organs for the removal of nitrogenous waste. alcohol unites more easily with oxygen than most other food materials, hence it takes away oxygen that would otherwise be used in oxidizing these foods. imperfect oxidation of foods causes the development and retention of poisons in the blood which it becomes the work of the kidneys to remove. if the kidneys become overworked, disease will occur. such disease is likely to make itself felt as rheumatism or gout, both of which are believed to be due to waste products (poisons) in the blood. poisons produced by alcohol.--when too little oxygen enters the draft of the stove, the wood is burned imperfectly, and there are clouds of smoke and irritating gases. so, if oxygen unites with the alcohol and too little reaches the cells, instead of carbon dioxide, water, and urea being formed, there are other products, some of which are exceedingly poisonous and which the kidneys handle with difficulty. the poisons retained in the circulation never fail to produce their poisonous effects, as shown by headaches, clouded brain, pain, and weakness of the body. the word "intoxication" means "in a state of poisoning." these poisons gradually accumulate as the alcohol takes oxygen from the cells. the worst effects come last, when the brain is too benumbed to judge fairly of their harm. reference books elementary hunter, _laboratory problems in civic biology_. american book company. davison, _human body and health_. american book company. gulick, _hygiene series, emergencies, good health_. ginn and company. hough and sedgwick, _the human mechanism_. ginn and company. macy, _general physiology_. american book company. ritchie, _human physiology_. world book company. xxiii. body control and habit formation _problems.--how is body control maintained? (a) what is the mechanism of direction and control? (b) what is the method of direction and control? (c) what are habits? how are they formed and how broken? (d) what are the organs of sense? what are their uses? (e) how does alcohol affect the nervous system?_ laboratory suggestions _demonstration._--sensory motor reactions. _demonstration._--nervous system. models and frog dissections. _demonstration._--neurones under compound microscope (optional). _demonstration._--reflex acts are unconscious acts: show how conscious acts may become habitual. _home exercise_ in habit forming. _the senses.--home exercises._--( ) to determine areas most sensitive to touch. ( ) to determine or map out hot and cold spots on an area on the wrist. ( ) to determine functions of different areas on tongue. _demonstration._--show how eye defects are tested. _laboratory summary._--the effects of alcohol on the nervous system. the body a self-directed machine.--throughout the preceding chapters the body has been likened to an engine, which, while burning its fuel, food, has done work. if we were to carry our comparison further, however, the simile ceases. for the engineer runs the engine, while the bodily machine is self-directive. moreover, most of the acts we perform during a day's work are results of the automatic working of this bodily machine. the heart pumps; the blood circulates its load of food, oxygen, and wastes; the movements of breathing are performed; the thousand and one complicated acts that go on every day within the body are _seemingly_ undirected. [illustration: the central nervous system.] automatic activity.--in addition to this, numbers of other of our daily acts are not thought about. if we are well-regulated body machines, we get up in the morning, automatically wash, clean our teeth, dress, go to the toilet, get our breakfast, walk to school, even perform such complicated processes as that of writing, without _thinking_ about or _directing_ the machine. in these respects we have become creatures of habit. certain acts which once we might have learned consciously, have become automatic. but once at school, if we are really making good in our work in the classroom, we begin a higher control of our bodily functions. automatic control acts no longer, and sensation is not the only guide--for we now begin to make _conscious choice_; we weigh this matter against another,--in short, we _think_. parts of the nervous system.--this wonderful self-directive apparatus placed within us, which is in part under control of our will, is known as the nervous system. in the vertebrate animals, including man, it consists of two divisions. one includes the brain, spinal cord, the cranial and spinal nerves, which together make up the _cerebro-spinal nervous system_. the other division is called the _sympathetic nervous system_ and has to do with those bodily functions which are beyond our control. every group of cells in the body that has work to do (excepting the floating cells of the blood) is directly influenced by these nerves. our bodily comfort is dependent upon their directive work. the organs which put us in touch with our surroundings are naturally at the _surface_ of the body. small collections of nerve cells, called _ganglia_, are found in all parts of the body. these nerve centers are connected, to a greater or less degree, with the surface of the body by the nerves, which serve as pathways between the end organs of touch, sight, taste, etc., and the centers in the brain or spinal cord. thus sensation is obtained. sensations and reactions.--we have already seen that simpler forms of life perform certain acts because certain outside forces acting upon them cause them to _react_ to the stimulus from without. the one-celled animal responds to the presence of food, to heat, to oxygen, to other conditions in its surroundings. an earthworm is repelled by light, is attracted by food. all animals, including man, are put in touch with their surroundings by what we call the organs of sensation. the senses of man, besides those we commonly know as those of sight, hearing, taste, smell, and touch, are those of temperature, pressure, and pain. it is obvious that such organs, if they are to be of use to an animal, must be at the outside of the body. thus we find eyes and ears in the head, and taste cells in the mouth, while other cells in the nose perceive odors, and still others in the skin are sensitive to heat or cold, pressure or pain. but this is not all. strangely enough, we do not see with our eyes or taste with our taste cells. these organs receive the sensations, and by means of a complicated system of greatly elongated cell structures, the message is sent inward, relayed by other elongated cells until the sensory message reaches an inner station, in the central nervous system. we see and hear and smell in our brain. let us next examine the structure of the nerve cells or _neurons_ part of which serve as pathways for these messages. [illustration: diagram of a neuron or nerve unit.] neurones.--a nerve cell, like other cells in the body, is a mass of protoplasm containing a nucleus. but the body of the nerve cell is usually rather irregular in shape, and distinguished from most other cells by possessing several delicate, branched protoplasmic projections called _dendrites_. one of these processes, the axon, is much longer than the others and ends in a muscle or organ of sensation. the axon forms the pathway over which nervous impulses travel to and from the nerve centers. a nerve consists of a bundle of such tiny axons, bound together by connective tissue. as a nerve ganglia is a center of activity in the nervous system, so a cell body is a center of activity which may send an impulse over this thin strand of protoplasm (the axon) prolonged many hundreds of thousands of times the length of the cell. some neurones in the human body, although visible only under the compound microscope, give rise to axons several feet in length. because some bundles of axons originate in organs that receive sensations and send those sensations to the central nervous system, they are called _sensory nerves_. other axons originate in the central nervous system and pass outward as nerves producing movement of muscles. these are called _motor nerves_. the brain of man.--in man, the central nervous system consists of a brain and spinal cord inclosed in a bony case. from the brain, twelve pairs of nerves are given off; thirty-one pairs more leave the spinal cord. the brain has three divisions. the _cerebrum_ makes up the largest part. in this respect it differs from the cerebrum of the frog and other vertebrates. it is divided into two lobes, the _hemispheres_, which are connected with each other by a broad band of nerve fibers. the outer surface of the cerebrum is thrown into folds or _convolutions_ which give a large surface, the cell bodies of the neurons being found in this part of the cerebrum. holding the cell bodies and fibers in place is a kind of connective tissue. the inner part (white in color) is composed largely of fibers which pass to other parts of the brain and down into the spinal cord. under the cerebrum, and dorsal to it, lies the little brain, or _cerebellum_. the two sides of the cerebellum are connected by a band of nerve fibers which run around into the lower hindbrain or _medulla_. this band of fibers is called the _pons_. the medulla is, in structure, part of the spinal cord, and is made up largely of fibers running longitudinally. the sympathetic nervous system.--connected with the central nervous system is that part of the nervous apparatus that controls the muscles of the digestive tract and blood vessels, the secretions of gland cells, and all functions which have to do with life processes in the body. this is called the sympathetic nervous system. functions of the parts of the central nervous system of the frog.--from careful study of living frogs, birds, and some mammals we have learned much of what we know of the functions of the parts of the central nervous system in man. it has been found that if the entire brain of a frog is destroyed and separated from the spinal cord, "the frog will continue to live, but with a very peculiarly modified activity." it does not appear to breathe, nor does it swallow. it will not move or croak, but if acid is placed upon the skin so as to irritate it, the legs make movements to push away and to clean off the irritating substance. the spinal cord is thus shown to be a center for defensive movements. if the cerebrum is separated from the rest of the nervous system, the frog seems to act a little differently from the normal animal. it jumps when touched, and swims when placed in water. it will croak when stroked, or swallow if food be placed in its mouth. but it manifests no hunger or fear, and is in every sense a machine which will perform certain actions after certain stimulations. its movements are automatic. if now we watch the movements of a frog which has the brain uninjured in any way, we find that it acts _spontaneously_. it tries to escape when caught. it feels hungry and seeks food. it is capable of voluntary action. it acts like a normal individual. [illustration: diagram to show the parts of the brain and action of the different parts of the brain.] functions of the cerebrum.--in general, the functions of the different parts of the brain in man agree with those functions we have already observed in the frog. the cerebrum has to do with conscious activity; that is, thought. it presides over what we call our thoughts, our will, and our sensations. a large part of the area of the outer layer of the cerebrum seems to be given over to some one of the different functions of speech, hearing, sight, touch, movements of bodily parts. the movement of the smallest part of the body appears to have its definite localized center in the cerebrum. experiments have been performed on monkeys, and these, together with observations made on persons who had lost the power of movement of certain parts of the body, and who, after death, were found to have had diseases localized in certain parts of the cerebrum, have given to us our knowledge on this subject. [illustration: diagram of the nerve path of a simple reflex action.] reflex actions; their meaning.--if through disease or for other reasons the cerebrum does not function, no will power is exerted, nor are intelligent acts performed. all acts performed in such a state are known as _reflex actions_. the involuntary brushing of a fly from the face, or the attempt to move away from the source of annoyance when tickled with a feather, are examples of reflexes. in a reflex act, a person does not think before acting. the nervous impulse comes from the outside to cells that are not in the cerebrum. the message is short-circuited back to the surface by motor nerves, without ever having reached the thinking centers. the nerve cells which take charge of such acts are located in the cerebellum or spinal cord. automatic acts.--some acts, however, are learned by conscious thought, as writing, walking, running, or swimming. later in life, however, these activities become automatic. the actual performance of the action is then taken up by the cerebellum, medulla, and spinal ganglia. thus the thinking portion of the brain is relieved of part of its work. bundles of habits.--it is surprising how little real thinking we do during a day, for most of our acts are habitual. habit takes care of our dressing, our bathing, our care of the body organs, our methods of eating; even our movements in walking and the kind of hand we write are matters of habit forming. we are bundles of habits, be they good ones or bad ones. habit formation.--the training of the different areas in the cerebrum to do their work well is the object of education. when we learned to write, we exerted conscious effort in order to make the letters. now the act of forming the letters is done without thought. by training, the act has become automatic. in the beginning, a process may take much thought and many trials before we are able to complete it. after a little practice, the same process may become almost automatic. we have formed a habit. habits are really acquired reflex actions. they are the result of nature's method of training. the conscious part of the brain has trained the cerebellum or spinal cord to do certain things that, at first, were taken charge of by the cerebrum. importance of forming right habits.--among the habits early to be acquired are the habits of studying properly, of concentrating the mind, of learning self-control, and, above all, of contentment. get the most out of the world about you. remember that the immediate effect in the study of some subjects in school may not be great, but the cultivation of correct methods of thinking may be of the greatest importance later in life. the man or woman who has learned how to concentrate on a problem, how to weigh all sides with an unbiased mind, and then to decide on what they believe to be best and right are the efficient and happy ones of their generation. "the hell to be endured hereafter, of which theology tells, is no worse than the hell we make for ourselves in this world by habitually fashioning our characters in the wrong way. could the young but realize how soon they will become mere walking bundles of habits, they would give more heed to their conduct while in the plastic state. we are spinning our own fates, good or evil, and never to be undone. every smallest stroke of virtue or of vice leaves its never-so-little scar. the drunken rip van winkle, in jefferson's play, excuses himself for every fresh dereliction by saying, 'i won't count this time!' well! he may not count it, and a kind heaven may not count it; but it is being counted none the less. down among his nerve cells and fibers the molecules are counting it, registering and storing it up to be used against him when the next temptation comes. nothing we ever do is, in strict scientific literalness, wiped out. of course this has its good side as well as its bad one. as we become permanent drunkards by so many separate drinks, so we become saints in the moral, and authorities in the practical and scientific, spheres by so many separate acts and hours of work. let no youth have any anxiety about the upshot of his education, whatever the line of it may be. if he keep faithfully busy each hour of the working day, he may safely leave the final result to itself. he can with perfect certainty count on waking up some fine morning, to find himself one of the competent ones of his generation, in whatever pursuit he may have singled out."--james, _psychology_. some rules for forming good habits.--professor horne gives several rules for making good or breaking bad habits. they are: "first, _act on every opportunity_. second, _make a strong start_. third, _allow no exception_. fourth, _for the bad habit establish a good one_. fifth, summoning all the man within, _use effort of will_." why not try these out in forming some good habit? you will find them effective. [illustration: the effect of fatigue on nerve cells. _a_, healthy brain cell; _b_, fatigued brain cell.] necessity of food, fresh air, and rest.--the nerve cells, like all other cells in the body, are continually wasting away and being rebuilt. oxidation of food material is more rapid when we do mental work. the cells of the brain, like muscle cells, are not only capable of fatigue, but show this in changes of form and of contents. _food_ brought to them in the blood, plenty of _fresh air_, especially when engaged in active brain work, and _rest_ at proper times, are essential in keeping the nervous system in condition. one of the best methods of resting the brain cells is a change of occupation. tennis, golf, baseball, and other outdoor sports combine muscular exercise with brain activity of a different sort from that of business or school work. but change of occupation will not rest exhausted neurones. for this, sleep is necessary. especially is sleep an important factor in the health of the nervous system of growing children. necessity of sleep.--most brain cells attain their growth early in life. changes occur, however, until some time after the school age. ten hours of sleep should be allowed for a child, and at least eight hours for an adult. at this time, only, do the brain cells have opportunity to rest and store food and energy for their working period. sleep is one way in which all cells in the body, and particularly those of the nervous system, get their rest. the nervous system, by far the most delicate and hardest-worked set of tissues in the body, needs rest more than do other tissues, for its work directing the body only ends with sleep or unconsciousness. the afternoon nap, snatched by the brain worker, gives him renewed energy for his evening's work. it is not hard application to a task that wearies the brain; it is _continuous_ work without rest. the senses touch.--in animals having a hard outside covering, such as certain worms, insects, and crustaceans, minute hairs, which are sensitive to touch, are found growing out from the body covering. at the base of these hairs are found neurones which send axons inward to the central nervous system. [illustration: nerves in the skin: _a_, nerve fiber; _b_, tactile papillæ, containing a tactile corpuscle; _c_, papillæ containing blood vessels. (after benda.)] organs of touch.--in man, the nervous mechanism which governs touch is located in the folds of the dermis or in the skin. special nerve endings, called the _tactile corpuscles_, are found there, each inclosed in a sheath or capsule of connective tissue. inside is a complicated nerve ending, and axons pass inward to the central nervous system. the number of tactile corpuscles present in a given area of the skin determines the accuracy and ease with which objects may be known by touch. if you test the different parts of the body, as the back of the hand, the neck, the skin of the arm, of the back, or the tip of the tongue, with a pair of open dividers, a vast difference in the accuracy with which the two points may be distinguished is noticed. on the tip of the tongue, the two points need only be separated by / of an inch to be so distinguished. in the small of the back, a distance of inches may be reached before the dividers feel like two points. temperature, pressure, pain.--the feeling of temperature, pressure, and pain is determined by different end organs in the skin. two kinds of nerve fibers exist in the skin, which give distinct sensations of heat and cold. these nerve endings can be located by careful experimentation. there are also areas of nerve endings which are sensitive to pressure, and still others, most numerous of all, sensitive to pain. taste organs.--the surface of the tongue is folded into a number of little projections known as papillæ. these may be more easily found on your own tongue if a drop of vinegar is placed on its broad surface. in the folds, between these projections on the top and back part of the tongue, are located the organs of taste. these organs are called _taste buds_. [illustration: _a_, isolated taste bud, from whose upper free end project the ends of the taste cells; _b_, supporting or protecting cell; _c_, sensory cell.] each taste bud consists of a collection of spindle-shaped neurones, each cell tipped at its outer end with a hairlike projection. these cells send inward fibers to other cells, the fibers from which ultimately reach the brain. the sensory cells are surrounded by a number of projecting cells which are arranged in layers about them. thus the organ in longitudinal section looks somewhat like an onion cut lengthwise. how we taste.--four kinds of substances may be distinguished by the sense of taste. these are sweet, sour, bitter, and salt. certain taste cells located near the back of the tongue are stimulated only by a bitter taste. sweet substances are perceived by cells near the tip of the tongue, sour substances along the sides, and salt about equally all over the surface. a substance must be dissolved in fluid in order to be tasted. many things which we believe we taste are in reality perceived by the sense of smell. such are spicy sauces and flavors of meats and vegetables. this may easily be proved by holding the nose and chewing, with closed eyes, several different substances, such as an apple, an onion, and a raw potato. smell.--the sense of smell is located in the membrane lining the upper part of the nose. here are found a large number of rod-shaped cells which are connected with the brain by means of the olfactory nerve. in order to perceive odors, it is necessary to have them diffused in the air; hence we sniff so as to draw in more air over the olfactory cells. the organ of hearing.--the organ of hearing is the ear. the outer ear consists of a funnel-like organ composed largely of cartilage which is of use in collecting sound waves. this part of the ear incloses the auditory canal, which is closed at the inner end by a tightly stretched membrane, the _tympanic membrane_ or ear drum. the function of the tympanic membrane is to receive sound waves, for all sound is caused by vibrations in the air, these vibrations being transmitted, by the means of a complicated apparatus found in the middle ear, to the real organ of hearing located in the inner ear. [illustration: section of ear: _e.m._, auditory canal; _ty.m._, tympanic membrane; _eu._, eustachian tube; _ty_, middle ear; _coc._, _a.s.c._, _e.s.c._, etc., internal ear.] middle ear.--the middle ear in man is a cavity inclosed by the temporal bone, and separated from the outer ear by the tympanic membrane. a little tube called the _eustachian tube_ connects the inner ear with the mouth cavity. by allowing air to enter from the mouth, the air pressure is equalized on the ear drum. for this reason, we open the mouth at the time of a heavy concussion and thus prevent the rupture of the delicate tympanic membrane. placed directly against the tympanic membrane and connecting it with the inner ear is a chain of three tiny bones, the smallest bones of the body. the outermost is called the _hammer_; the next the _anvil_; the third the _stirrup_. all three bones are so called from their resemblances in shape to the articles for which they are named. these bones are held in place by very small muscles which are delicately adjusted so as to tighten or relax the membranes guarding the middle and inner ear. the inner ear.--the inner ear is one of the most complicated, as well as one of the most delicate, organs of the body. deep within the temporal bone there are found two parts, one of which is called, collectively, the _semicircular canal region_, the other the _cochlea_, or organ of hearing. it has been discovered by experimenting with fish, in which the semicircular canal region forms the chief part of the ear, that this region has to do with the equilibrium or balancing of the body. we gain in part our knowledge of our position and movements in space by means of the _semicircular canals_. that part of the ear which receives sound waves is known as the _cochlea_, or snail shell, because of its shape. this very complicated organ is lined with sensory cells provided with cilia. the cavity of the cochlea is filled with a fluid. it is believed that somewhat as a stone thrown into water causes ripples to emanate from the spot where it strikes, so sound waves are transmitted by means of the fluid filling the cavity to the sensory cells of the cochlea (collectively known as the _organ of corti_) and thence to the brain by means of the auditory nerve. the character of sound.--when vibrations which are received by the ear follow each other at regular intervals, the sound is said to be musical. if the vibrations come irregularly, we call the sound a noise. if the vibrations come slowly, the pitch of the sound is low; if they come rapidly, the pitch is high. the ear is able to perceive as low as thirty vibrations per second and as high as almost thirty thousand. the ear can be trained to recognize sounds which are unnoticed in untrained ears. [illustration: longitudinal section through the eye.] the eye.--the eye or organ of vision is an almost spherical body which fits into a socket of bone, the _orbit_. a stalklike structure, the _optic nerve_, connects the eye with the brain. free movement is obtained by means of six little muscles which are attached to the outer coat, the _eyeball_, and to the bony socket around the eye. the wall of the eyeball is made up of three coats. an outer tough white coat, of connective tissue, is called the _sclerotic coat_. under the sclerotic coat, in front, the eye bulges outward a little. here the outer coat is continuous with a transparent tough layer called the _cornea_. a second coat, the _choroid_, is supplied with blood vessels and cells which bear pigments. it is a part of this coat which we see through the cornea as the colored part of the eye (the _iris_). in the center of the iris is a small circular hole (the _pupil_). the iris is under the control of muscles, and may be adjusted to varying amounts of light, the hole becoming larger in dim light, and smaller in bright light. the inmost layer of the eye is called the _retina_. this is, perhaps, the most delicate layer in the entire body. despite the fact that the retina is less than / of an inch in thickness, there are several layers of cells in its composition. the optic nerve enters the eye from behind and spreads out to form the surface of the retina. its finest fibers are ultimately connected with numerous elongated cells which are stimulated by light. the retina is dark purple in color, this color being caused by a layer of cells next to the choroid coat. this accounts for the black appearance of the pupil of the eye, when we look through the pupil into the darkened space within the eyeball. the retina acts as the sensitized plate in the camera, for on it are received the impressions which are transformed and sent to the brain as sensations of sight. the eye, like the camera, has a lens. this lens is formed of transparent, elastic material. it is found directly behind the iris and is attached to the choroid coat by means of delicate ligaments. in front of the lens is a small cavity filled with a watery fluid, the _aqueous humor_, while behind it is the main cavity of the eye, filled with a transparent, almost jellylike, _vitreous humor_. the lens itself is elastic. this circumstance permits of a change of form and, in consequence, a change of focus upon the retina of the lens. by means of this change in form, or _accommodation_, we are able to distinguish between near and distant objects. [illustration: how far away can you read these letters? measure the distance. twenty feet is a test for the normal eye.] defects in the eye.--in some eyes, the lens is in focus for near objects, but is not easily focused upon distant objects; such an eye is said to be nearsighted. other eyes which do not focus clearly on objects near at hand are said to be farsighted. still another eye defect is astigmatism, which causes images of lines in a certain direction to be indistinct, while images of lines transverse to the former are distinct. many nervous troubles, especially headaches, may be due to eye strain. we should have our eyes examined from time to time, especially if we are subject to headaches. the alcohol question.--it is agreed by investigators that in large or continued amounts alcohol has a narcotic effect; that it first dulls or paralyzes the nerve centers which control our judgment, and later acts upon the so-called motor centers, those which control our muscular activities. the reason, then, that a man in the first stages of intoxication talks rapidly and sometimes wittily, is because the centers of judgment are paralyzed. this frees the speech centers from control exercised by our judgment, with the resultant rapid and free flow of speech. in small amounts alcohol is believed by some physiologists to have always this same narcotic effect, while other physiologists think that alcohol does stimulate the brain centers, especially the higher centers, to increased activity. some scientific and professional men use alcohol in small amounts for this stimulation and report no seeming harm from the indulgence. others, and by far the larger number, agree that this stimulation from alcohol is only apparent and that even in the smallest amounts alcohol has a narcotic effect. the paralyzing effects of alcohol on the nervous system.--alcohol has the effect of temporarily paralyzing the nerve centers. the first effect is that of exhilaration. a man may do more work for a time under the stimulation of alcohol. this stimulation, however, is of short duration and is invariably followed by a period of depression and inertia. in this latter state, a man will do less work than before. in larger quantities, alcohol has the effect of completely paralyzing the nerve centers. this is seen in the case of a man "dead drunk." he falls in a stupor because all of the centers governing speech, sight, locomotion, etc., have been temporarily paralyzed. if a man takes a very large amount of alcohol, even the nerve centers governing respiration and circulation may become poisoned, and the victim will die. effect on the organs of special sense.--professor forel, one of the foremost european experts on the question of the effect of alcohol on the nervous system, says: "through all parts of nervous activity from the innervation of the muscles and the simplest sensation to the highest activity of the soul the paralyzing effect of alcohol can be demonstrated." several experimenters of undoubted ability have noted the paralyzing effect of alcohol even in small doses. by the use of delicate instruments of precision, ridge tested the effect of alcohol on the senses of smell, vision, and muscular sense of weight. he found that two drams of absolute alcohol produced a positive decrease in the sensitiveness of the nerves of feeling, that so small a quantity as one half dram of absolute alcohol diminished the power of vision and the muscular sense of weight. kraepelin and kurz by experiment determined that the acuteness of the special senses of sight, hearing, touch, taste, and smell was diminished by an ounce of alcohol, the power of vision being lost to one third of its extent and a similar effect being produced on the other special senses. other investigators have reached like conclusions. there is no doubt but that alcohol, even in small quantities, renders the organs of sense less sensitive and therefore less accurate. [illustration: table to show a comparison of chances of illness and death in drinkers and non-drinkers. solid black, drinkers. (from german sources.)] effect of alcohol on the ability to resist disease.--among certain classes of people the belief exists that alcohol in the form of brandy or some other drink or in patent medicines, malt tonics, and the like is of great importance in building up the body so as to resist disease or to cure it after disease has attacked it. nothing is further from the truth. in experiments on a large number of animals, including dogs, rabbits, guinea pigs, fowls, and pigeons, laitenen, of the university of helsingsfors, found that alcohol, without exception, made these animals more susceptible to disease than were the controls. one of the most serious effects of alcohol is the lowered resistance of the body to disease. it has been proved that a much larger proportion of hard drinkers die from infectious or contagious diseases than from special diseased conditions due to the direct action of alcohol on the organs of the body. this lowered resistance is shown in increased liability to contract disease and increased severity of the disease. we have already alluded to the findings of insurance companies with reference to the length of life--the abstainers from alcohol have a much better chance of a longer life and much less likelihood of infection by disease germs. use of alcohol in the treatment of disease.--in the london temperance hospital alcohol was prescribed seventy-five times in thirty-three years. the death rate in this hospital has been lower than that of most general hospitals. sir william collins, after serving nineteen years as surgeon in this hospital, said:-- "in my experience, speaking as a surgeon, the use of alcohol is not essential for successful surgery.... at the london temperance hospital, where alcohol is very rarely prescribed, the mortality in amputation cases and in operation cases generally is remarkably low. total abstainers are better subjects for operation, and recover more rapidly from accidents, than those who habitually take stimulants." in a paper read at the international congress on tuberculosis, in new york, , dr. crothers remarked that alcohol as a remedy or a preventive medicine in the treatment of tuberculosis is a most dangerous drug, and that all preparations of sirups containing spirits increase, rather than diminish, the disease. dr. kellogg says: "the paralyzing influence of alcohol upon the white cells of the blood--a fact which is attested by all investigators--is alone sufficient to condemn the use of this drug in acute or chronic infections of any sort." [illustration: effect of use of alcohol on memory.] the effect of alcohol upon intellectual ability.--with regard to the supposed quickening of the mental processes horsley and sturge, in their recent book, _alcohol and the human body_, say: "kraepelin found that the simple reaction period, by which is meant the time occupied in making a mere response to a signal, as, for instance, to the sudden appearance of a flag, was, after the ingestion of a small quantity of alcohol ( / to / ounce), slightly accelerated; that there was, in fact, a slight shortening of the time, as though the brain were enabled to operate more quickly than before. but he found that after a few minutes, in most cases, a slowing of mental action began, becoming more and more marked, and enduring as long as the alcohol was in active operation in the body, _i.e._ four to five hours.... kraepelin found that it was only more or less automatic work, such as reading aloud, which was quickened by alcohol, though even this was rendered less trustworthy and accurate." again: "kraepelin had always shared the popular belief that a small quantity of alcohol (one to two teaspoonfuls) had an accelerating effect on the activity of his mind, enabling him to perform test operations, as the adding and subtracting and learning of figures more quickly. but when he came to measure with his instruments the exact period and time occupied, he found, to his astonishment, that he had accomplished these mental operations, not more, but less, quickly than before.... numerous further experiments were carried out in order to test this matter, and these proved that _alcohol lengthens the time taken to perform complex mental processes_, while by a singular illusion the person experimented upon imagines that his psychical actions are rendered more rapid." [illustration: the effect of alcohol upon ability to do mental work.] _attention_--that is, the power of the mind to grasp and consider impressions obtained through the senses--is weakened by drink. the ability of the mind to associate or combine ideas, the faculty involved in sound _judgment_, showed that when the persons had taken the amounts of alcohol mentioned, the combinations of ideas or judgments expressed by them were confused, foggy, sentimental, and general. when the persons had taken no alcohol, their judgments were rational, specific, keen, showing closer observation. "the words of professor helmholtz at the celebration of his seventieth birthday are very interesting in this connection. he spoke of the ideas flashing up from the depths of the unknown soul, that lies at the foundation of every truly creative intellectual production, and closed his account of their origin with these words: 'the smallest quantity of an alcoholic beverage seemed to frighten these ideas away.'"--dr. g. sims woodhead, professor of pathology, cambridge university, england. professor von bunge (_textbook of physiological and pathological chemistry_) of switzerland says that: "the stimulating action which alcohol appears to exert on the brain functions is only a paralytic action. the cerebral functions which are first interfered with are _the power of clear judgment and reason_. no man ever became witty by aid of spirituous drinks. the lively gesticulations and useless exertions of intoxicated people are due to paralysis,--the restraining influences, which prevent a sober man from uselessly expending his strength, being removed." the drink habit.--the harmful effects of alcohol (aside from the purely physiological effect upon the tissues and organs of the body) are most terribly seen in the formation of the alcohol habit. the first effect of drinking alcoholic liquors is that of exhilaration. after the feeling of exhilaration is gone, for this is a temporary state, the subject feels depressed and less able to work than before he took the drink. to overcome this feeling, he takes another drink. the result is that before long he finds a habit formed from which he cannot escape. with body and mind weakened, he attempts to break off the habit. but meanwhile his will, too, has suffered from overindulgence. he has become a victim of the drink habit! "the capital argument against alcohol, that which must eventually condemn its use, is this, that _it takes away all the reserved control, the power of mastership, and therefore offends against the splendid pride in himself or herself, which is fundamental in every man or woman worth anything_."--dr. john johnson, quoting walt whitman. self-indulgence, be it in gratification of such a simple desire as that for candy or the more harmful indulgence in tobacco or alcoholic beverages, is dangerous--not only in its immediate effects on the tissues and organs, but in its more far-reaching effects on habit formation. each one of us is a bundle of appetites. if we gratify appetites of the wrong kind, we are surely laying the foundation for the habit of excess. self-denial is a good thing for each of us to practice at one time or another, if for no other purpose than to be ready to fight temptation when it comes. the economic effect of alcoholic poisoning.--in the struggle for existence, it is evident that the man whose intellect is the quickest and keenest, whose judgment is most sound, is the man who is most likely to succeed. the paralyzing effect of alcohol upon the nerve centers must place the drinker at a disadvantage. in a hundred ways, the drinker sooner or later feels the handicap that the habit of drink has imposed upon him. many corporations, notably several of our greatest railroads (the pennsylvania and the new york central railroad among them), refuse to employ any but abstainers in positions of trust. few persons know the number of railway accidents due to the uncertain eye of some engineer who mistook his signal, or the hazy inactivity of the brain of some train dispatcher who, because of drink, forgot to send the telegram that was to hold the train from wreck. in business and in the professions, the story is the same. the abstainer wins out over the drinking man. effect of alcohol on ability to do work.--in _physiological aspects of the liquor problem_, professor hodge, formerly of clark university, describes many of his own experiments showing the effect of alcohol on animals. he trained four selected puppies to recover a ball thrown across a gymnasium. to two of the dogs he gave food mixed with doses of alcohol, while the others were fed normally. the ball was thrown feet as rapidly as recovered. this was repeated times each day for fourteen successive days. out of times the dogs to which alcohol had been given brought back the ball only times, while the others secured it times. dr. parkes experimented with two gangs of men, selected to be as nearly similar as possible, in mowing. he found that with one gang abstaining from alcoholic drinks and the other not, the abstaining gang could accomplish more. on transposing the gangs, the same results were repeatedly obtained. similar results were obtained by professor aschaffenburg of heidelberg university, who found experimentally that men "were able to do per cent less work after taking alcohol." recently many experiments along the same lines have been made. in typewriting, in typesetting, in bricklaying, or in the highest type of mental work the result is the same. the quality and quantity of work done on days when alcohol is taken is less than on days when no alcohol is taken. the relation of alcohol to efficiency.--we have already seen that work is neither so well done nor is as much accomplished by drinkers as by non-drinkers. a massachusetts shoe manufacturer told a recent writer on temperance that in one year his firm lost over $ in shoes spoiled by drinking men, and that he had himself traced these spoiled shoes to the workmen who, through their use of alcoholic liquors, had thus rendered themselves incapable. this is a serious handicap to our modern factory system, and explains why so many factory towns and cities are strongly favoring a policy of "no license" in opposition to the saloons. "it is believed that the largest number of accidents in shops and mills takes place on monday, because the alcohol that is drunk on sunday takes away the skill and attentive care of the workman. to prove the truth of this opinion, the accidents of the building trades in zurich were studied during a period of six years, with the result shown by this table":-- [illustration: shaded, non-alcoholic; black, alcoholic, accidents. (from tolman, _hygiene for the worker_.)] another relation to efficiency is shown by the following chart. during the week the curve of working efficiency is highest on friday and lowest on monday. the number of accidents were also least on friday and greatest on monday. lastly the assaults were fewest in number on friday and greatest on sunday and monday. the moral is plain. workingmen are apt to spend their week's wages freely on saturday. much of this goes into drink, and as a result comes crime on sunday because of the deadened moral and mental condition of the drinker, and loss of efficiency on monday, because of the poisonous effects of the drug. [illustration: notice that the curve of efficiency is lowest on monday and that crimes and accidents are most frequent on sunday and monday. account for this.] effect of alcohol upon duration of life.--still more serious is the relation of alcohol as a direct cause of disease (see table). it is as yet quite impossible, in the united states at least, to tell just how many deaths are brought about, directly or indirectly, by alcohol. especially is this true in trying to determine the number of cases of deaths from disease promoted by alcohol. in switzerland provision is made for learning these facts, and the records of that country throw some light on the subject. dr. rudolph pfister made a study of the records of the city of basle for the years - , finding the percentage of deaths in which alcohol had been reported by the attending physician as one cause of death. he found that . per cent of all deaths of men between and years of age were caused, in part at least, by alcohol, and this at what should be the most active period in a man's life, the time when he is most needed by his family and community. taking all ages between and , he found that alcohol was one cause of death in one man in every ten who died. [illustration] another study was made by a certain doctor in sweden, from records of deaths occurring in his own practice and the local hospital. no case was counted as alcoholic of which there was the slightest doubt. of deaths of adult men, in every were due, directly or indirectly, to alcoholism. in middle life, between the ages of and , ; and between and years of age, . out of every deaths had alcohol as one cause, thus agreeing with other statistics we have been quoting.--from the _metropolitan_, vol. xxv, number . [illustration: the proportion of crime due to alcohol is shown in black.] the relation of alcohol to crime.--a recent study of more than habitual users of alcohol showed that over per cent had committed crime. usually the crimes had been done in saloons or as a result of quarrels after drinking. of another lot of , criminals questioned, , said that alcohol had led them to commit crime. the relation of alcohol to pauperism.--we have already spoken of the jukes family. these and many other families of a similar sort are more or less directly a burden upon the state. alcohol is in part at least responsible for the condition of such families. alcohol weakens the efficiency and moral courage, and thus leads to begging, pauperism, petty stealing or worse, and ultimately to life in some public institution. in massachusetts, of inmates of such institutions, per cent were alcoholics. the relation of alcohol to heredity.--perhaps the gravest side of the alcohol question lies here. if each one of us had only himself to think of, the question of alcohol might not be so serious. but drinkers may hand down to their unfortunate children tendencies toward drink as well as nervous diseases of various sorts; an alcoholic parent may beget children who are epileptic, neurotic, or even insane. in the state of new york there are at the present time some , insane persons in public and private hospitals. it is believed that about one fifth of them, or patients, owe their insanity to alcohol used either by themselves or by their parents. in the asylums of the united states there are , insane people. taking the same proportions as before, there are , persons in this country whom alcohol has made or has helped to make insane. this is the most terrible side of the alcohol problem. reference reading elementary hunter, _laboratory problems in civic biology_. american book company. overton, _general hygiene_. american book company. the gulick hygiene series, _emergencies, good health, the body at work, control of body and mind_. ginn and company. ritchie, _human physiology_. world book company. hough and sedgwick, _the human mechanism_. ginn and company. xxiv. man's improvement of his environment _problems.--how may we improve our home conditions of living?_ _how may we help improve our conditions at school?_ _how does the city care for the improvement of our environment?_ _(a) in inspection of buildings, etc._ _(b) in inspection of food supplies._ _(c) in inspection of milk._ _(d) in care of water supplies._ _(e) in disposal of wastes._ _(f) in care of public health._ laboratory suggestions _home exercise._--how to ventilate my bedroom. _demonstration._--effect of use of duster and damp cloth upon bacteria in schoolroom. _home exercise._--luncheon dietaries. _home exercise._--sanitary map of my own block. _demonstration._--the bacterial content of milk of various grades and from different sources. _demonstration._--bacterial content of distilled water, rain water, tap water, dilute sewage. _laboratory exercise._--study of board of health tables to plot curves of mortality from certain diseases during certain times of year. the purpose of this chapter.--in the preceding chapters we have traced the lives of both plants and animals within their own environment. we have seen that man, as well as plants and other animals, needs a favorable environment in order to live in comfort and health. it will be the purpose of the following pages first to show how we as individuals may better our home environment, and secondly, to see how we may aid the civic authorities in the betterment of conditions in the city in which we live. [illustration: how i should ventilate my bedroom.] home conditions.--the bedroom.--we spend about one third of our total time in our bedroom. this room, therefore, deserves more than passing attention. first of all, it should have good ventilation. two windows make an ideal condition, especially if the windows receive some sun. such a condition as this is manifestly impossible in a crowded city, where too often the apartment bedrooms open upon narrow and ill-ventilated courts. until comparatively recent time, tenement houses were built so that the bedrooms had practically no light or air; now, thanks to good tenement-house laws, wide airshafts and larger windows are required by statute. care of the bedroom.--since sunlight cannot always be obtained for a bedroom, we must so care for and furnish the room that it will be difficult for germs to grow there. bedroom furniture should be light and easy to clean, the bedstead of iron, the floors painted or of hardwood. no hangings should be allowed at the windows to collect dust, nor should carpets be allowed for the same reason. rugs on the floor may easily be removed when cleaning is done. the furniture and woodwork should be wiped with a damp cloth every day. why a _damp_ cloth? in certain tenements in new york city, tuberculosis is believed to have been spread by people occupying rooms in which a previous tenant has had tuberculosis. a new tenant should insist on a thorough cleaning of the bedrooms and removal of old wall paper before occupancy. sunlight important.--in choosing a house in the country we would take a location in which the sunlight was abundant. a shaded location might be too damp for health. sunlight should enter at least some of the rooms. in choosing an apartment we should have this matter in mind, for, as we know, germs cannot long exist in sunlight. [illustration: this map shows how cases of tuberculosis are found recurring in the same locality and in the same houses year after year. each black dot is one case of tuberculosis.] heating.--houses in the country are often heated by open fires, stoves or hot-air furnaces, all of which make use of heated currents of air to warm the rooms. but in the city apartments, usually pipes conduct steam or hot water from a central plant to our rooms. the difficulty with this system is that it does not give us fresh air, but warms over the stale air in a room. steam causes our rooms to be too warm part of the time, and not warm enough part of the time. thus we become overheated and then take cold by becoming chilled. steam heat is thus responsible for much sickness. lighting.--lighting our rooms is a matter of much importance. a student lamp, or shaded incandescent light, should be used for reading. shades must be provided so that the eyes are protected from direct light. gas is a dangerous servant, because it contains a very poisonous substance, carbon monoxide. "it is estimated that per cent of the total product of the gas plant leaks into the streets and houses of the cities supplied." this forms an unseen menace to the health in cities. gas pipes, and especially gas cocks, should be watched carefully for escaping gas. rubber tubing should not be used to conduct gas to movable gas lamps, because it becomes worn and allows gas to escape. [illustration: during the summer all food should be protected from flies. why?] insects and foods.--in the summer our houses should be provided with screens. all food should be carefully protected from flies. dirty dishes, scraps of food, and such garbage should be quickly cleaned up and disposed of after a meal. insect powder (pyrethrum) will help keep out "croton bugs" and other undesirable household pests, but cleanliness will do far more. most kitchen pests, as the roach, simply stay with us because they find dirt and food abundant. use of ice.--food should be properly cared for at all times, but especially during the summer. iceboxes are a necessity, especially where children live, in order to keep milk fresh. a dirty icebox is almost as bad as none at all, because food will decay or take on unpleasant odors from other foods. [illustration: the wrong and the right kind of garbage cans.] disposal of wastes.--in city houses the disposal of human wastes is provided for by a city system of sewers. the wastes from the kitchen, the garbage, should be disposed of each day. the garbage pail should be frequently sterilized by rinsing it with boiling water. plenty of lye or soap should be used. remember that flies frequent the uncovered garbage pail, and that they may next walk on your food. collection and disposal of garbage is the work of the municipality. [illustration: the culture (_a_) was exposed to the air of a dirty street in the crowded part of manhattan. (_b_) was exposed to the air of a well-cleaned and watered street in the uptown residence portion. which culture has the more colonies of bacteria? how do you account for this?] school surroundings.--how to improve them.--from five to six hours a day for forty weeks is spent by the average boy or girl in the schoolroom. it is part of our environment and should therefore be considered as worthy of our care. not only should a schoolroom be attractive, but it should be clean and sanitary. city schools, because of their locations, of the sometimes poor janitorial service, and especially because of the selfishness and carelessness of children who use them, may be very dirty and unsanitary. dirt and dust breed and carry bacteria. plate cultures show greatly increased numbers of bacteria to be in the air when pupils are moving about, for then dust, bearing bacteria, is stirred up and circulated through the air. sweeping and dusting with dry brooms or feather dusters only stirs up the dust, leaving it to settle in some other place with its load of bacteria. professor hodge tells of an experience in a school in worcester, mass. a health brigade was formed among the children, whose duty was to clean the rooms every morning by wiping all exposed surface with a damp cloth. in a school of pupils not a single case of contagious diseases appeared during the entire year. why not try this in your own school? unselfishness the motto.--pupils should be unselfish in the care of a school building. papers and scraps dropped by some careless boy or girl make unpleasant the surroundings for hundreds of others. chalk thrown by some mischievous boy and then tramped underfoot may irritate the lungs of a hundred innocent schoolmates. colds or worse diseases may be spread through the filthy habits of some boys who spit in the halls or on the stairways. lunch time and lunches.--if you bring your own lunch to school, it should be clean, tasty, and well balanced as a ration. in most large schools well-managed lunch rooms are part of the school equipment, and balanced lunches can be obtained at low cost. do not make a lunch entirely from cold food, if hot can be obtained. do not eat only sweets. ice cream is a good food, if taken with something else, but be sure of your ice cream. "hokey pokey" cream, tested in a new york school laboratory, showed the presence of many more colonies of bacteria than _good_ milk would show. above all, be sure the food you buy is clean. stands on the street, exposed to dust and germs, often sell food far from fit for human consumption. [illustration: a sensible lunch box, sanitary and compact.] if you eat your lunch on the street near your school, remember not to scatter refuse. paper, bits of lunch, and the like scattered on the streets around your school show lack of school spirit and lack of civic pride. let us learn above all other things to be good citizens. [illustration: dust exhausts on grinding wheels protect lungs of the workmen.] inspection of factories, public buildings, etc.--it is the duty of a city to inspect the condition of all public buildings and especially of factories. inspection should include, first, the supervision of the work undertaken. certain trades where grit, dirt, or poison fumes are given off are dangerous to human health, hence care for the workers becomes a necessity. factories should also be inspected as to cleanliness, the amount of air space per person employed, ventilation, toilet facilities, and proper fire protection. tenement inspection should be thorough and should aim to provide safe and sanitary homes. inspection of food supplies.--in a city certain regulations for the care of public supplies are necessary. foods, both fresh and preserved, must be inspected and rendered safe for the thousands of people who are to use them. all raw foods exposed on stands should be covered so as to prevent insects or dust laden with bacteria from coming in contact with them. meats must be inspected for diseases, such as tuberculosis in beef, or trichinosis in pork. cold storage plants must be inspected to prevent the keeping of food until it becomes unfit for use. inspection of sanitary conditions of factories where products are canned, or bakeries where foods are prepared, must be part of the work of a city in caring for its citizens. care of raw foods.--each one of us may coöperate with the city government by remembering that fruits and vegetables can be carriers of disease, especially if they are sold from exposed stalls or carts and handled by the passers-by. all vegetables, fruits, or raw foods should be carefully washed before using. spoiled or overripe fruit, as well as meat which is decayed, is swarming with bacteria and should not be used. an interesting exercise would be the inspection of conditions in your own home block. make a map showing the houses on the block. locate all stores, saloons, factories, etc. notice any cases of contagious disease, marking this fact on the map. mark all heaps of refuse in the street, all uncovered garbage pails, any street stands that sell uncovered fruit, and any stores with an excessive number of flies. in addition to food inspection, two very important supplies must be rendered safe by a city for its citizens. these are milk and water. [illustration: clean cows in clean barns with clean milkers and clean milk pails means clean milk in the city.] care in production of milk.--milk when drawn from a healthy cow should be free from bacteria. but immediately on reaching the air it may receive bacteria from the air, from the hands of the person who milks the cows, from the pail, or from the cow herself. cows should, therefore, be milked in surroundings that are sanitary, the milkers should wear clean garments, put on over their ordinary clothes at milking time, while pails and all utensils used should be kept clean. especially the surface exposed on the udder from which the milk is drawn should be cleansed before milking. most large cities now send inspectors to the farms from which milk is supplied. farms that do not accept certain standards of cleanliness are not allowed to have their milk become part of the city supply. tuberculosis and milk.--it is recognized that in some european countries from to per cent of all cattle have tuberculosis. many dairy herds in this country are also infected. it is also known that the tubercle bacillus of cattle and man are much alike in form and action and that _probably_ the germ from cattle would cause tuberculosis in man. fortunately, the tuberculosis germ does not _grow_ in milk, so that even if milk from tubercular cattle should get into our supply, it would be diluted with the milk of healthy cattle. in order to protect our milk supply from these germs it would be necessary to kill all tubercular cattle (almost an impossibility) or to pasteurize our milk so as to kill the germs in it. other disease germs in milk.--we have already shown how typhoid may be spread through milk. usually such outbreaks may be traced to a single case of typhoid, often a person who is a "typhoid carrier," _i.e._ one who may not suffer from the effects of the disease, but who carries the germs in his body, spreading them by contact. a recent epidemic of typhoid in new york city was traced to a single typhoid carrier on a farm far from the city. sometimes the milk cans may be washed in contaminated water or the cows may even get the germs on their udders by wading in a polluted stream. diphtheria, scarlet fever, and asiatic cholera are also undoubtedly spread through milk supplies. milk also plays a very important part in the high death rate from diarrhoeal diseases among young children in warm weather. why? [illustration: a diagram to show how typhoid may be spread in a city through an infected milk supply. the black spots in the blocks mean cases of typhoid. _a_, a farm where typhoid exists; the dashes in the streets represent the milk route. _b_ is a second farm which sends part of its milk to _a_; the milk cans from _b_ are washed at farm _a_ and sent back to _b_. a few cases of typhoid appear along _b_'s milk route. how do you account for that?] grades of milk in a city supply.--milk which comes to a city may be roughly placed in three different classes. the best milk, coming from farms where the highest sanitary standards exist, where the cows are all tubercular tested, where modern appliances for handling and cooling the milk exist, is known as certified or, in new york city, grade a milk. most of the milk sold, however, is not so pure nor is so much care taken in handling it. such milk, known in new york as grade b milk, is pasteurized before delivery, and is sold only in bottles. a still lower grade of milk (dipped milk) is sold direct from cans. it is evident that such milk, often exposed to dust and other dirt, is unfit for any purpose except for cooking. it should under no circumstances be used for children. a regulation recently made by the new york city department of health states that milk sold "loose" in restaurants, lunch-rooms, soda fountains, and hotels must be pasteurized. care of a city milk supply.--besides caring for milk in its production on the farm, proper transportation facilities must be provided. much of the milk used in new york city is forty-eight hours old before it reaches the consumer. during shipment it must be kept in refrigerator cars, and during transit to customers it should be iced. why? all but the highest grade milk should be pasteurized. why? milk should be bottled by machinery if possible so as to insure no personal contact; it should be kept in clean, cool places; and no milk should be sold by dipping from cans. why is this a method of dispensing impure milk? care of milk in the home.--finally, milk at home should receive the best of care. it should be kept on ice and in covered bottles, because it readily takes up the odors of other foods. if we are not certain of its purity or keeping qualities, it should be pasteurized at home. why? [illustration: new york city is spending $ , , to have a pure and abundant water supply. this is the tunnel which will bring the water from the catskill mountains to new york city.] water supplies.--one of the greatest assets to the health of a large city is pure water. by pure water we mean water free from all _organic_ impurities, including germs. water from springs and deep driven wells is the safest water, that from large reservoirs next best, while water that has drainage in it, river water for example, is very unsafe. the waters from deep wells or springs if properly protected will contain no bacteria. water taken from protected streams into which no sewage flows will have but few bacteria, and these will be destroyed if exposed to the action of the sun and the constant aëration (mixing with oxygen) which the surface water receives in a large lake or reservoir. but water taken from a river into which the sewage of other towns and cities flows must be filtered before it is fit for use. [illustration: the city of lowell in took its water _without filtering_, _i.e._ from the merrimack river at the point shown on the map. typhoid fever broke out in north chelmsford and about two weeks later cases began to appear in lowell until a great epidemic occurred. explain this outbreak. each black dot is a case of typhoid.] typhoid fever germs live in the food tube, hence the excreta of a typhoid patient will contain large numbers of germs. in a city with a system of sewage such germs might eventually pass from the sewers into a river. many cities take their water supply directly from rivers, sometimes not far below another large town. such cities must take many germs into their water supply. many cities, as cleveland and buffalo, take their water from lakes into which their sewage flows. others, as albany, pittsburgh, and philadelphia, take their drinking water directly from rivers into which sewage from cities above them on the river has flowed. filtering such water by means of passing the water through settling basins and sand filters removes about per cent of the germs. the result of drinking unfiltered and filtered water in certain large cities is shown graphically at right. in cities which drain their sewage into rivers and lakes, the question of sewage disposal is a large one, and many cities now have means of disposing of their sewage in some manner as to render it harmless to their neighbors. [illustration: filter beds at albany, n. y.] [illustration: cases of typhoid per , inhabitants before filtering water supply (solid) and after (shaded) in _a_, watertown, n. y.; _b_, albany, n. y.; _c_, lawrence, mass.; d, cincinnati, ohio. what is the effect of filtering the water supply?] railroads are often responsible for carrying typhoid and spreading it. it is said that a recent outbreak of typhoid in scranton, pa., was due to the fact that the excreta from a typhoid patient traveling in a sleeping car was washed by rain into a reservoir near which the train was passing. railroads are thus seen to be great open sewers. a sanitary car toilet is the only remedy. [illustration: this chart shows that during a cholera epidemic in there were hundreds of cases of cholera in hamburg, which used unfiltered water from the elbe, but in adjoining altona, where filtered water was used, the cases were very few.] [illustration: stone filter beds in a sewage disposal plant.] sewage disposal.--sewage disposal is an important sanitary problem for any city. some cities, like new york, pour their sewage directly into rivers which flow into the ocean. consequently much of the liquid which bathes the shores of manhattan island is dilute sewage. other cities, like buffalo or cleveland, send their sewage into the lakes from which they obtain their supply of drinking water. still other cities which are on rivers are forced to dispose of their sewage in various ways. some have a system of filter beds in which the solid wastes are acted upon by the bacteria of decay, so that they can be collected and used as fertilizer. others precipitate or condense the solid materials in the sewage and then dispose of it. another method is to flow the sewage over large areas of land, later using this land for the cultivation of crops. this method is used by many small european cities. [illustration: collecting ashes.] the work of the department of street cleaning.--in any city a menace to the health of its citizens exists in the refuse and garbage. the city streets, when dirty, contain countless millions of germs which have come from decaying material, or from people ill with disease. in most large cities a department of street cleaning not only cares for the removal of dust from the streets, but also has the removal of garbage, ashes, and other waste as a part of its work. the disposal of solid wastes is a tremendous task. in manhattan the dry wastes are estimated to be , , tons a year in addition to about , tons of garbage. prior to in the city of new york garbage was not separated from ashes; now the law requires that garbage be placed in separate receptacles from ashes. do you see why? the street-cleaning department should be aided by every citizen; rules for the separation of garbage, papers, and ashes should be kept. garbage and ash cans should be _covered_. the practice of upsetting ash or garbage cans is one which no young citizen should allow in his neighborhood, for sanitary reasons. the best results in summer street cleaning are obtained by washing or flushing the streets, for thus the dirt containing germs is prevented from getting into the air. the garbage is removed in carts, and part of it is burned in huge furnaces. the animal and plant refuse is cooked in great tanks; from this material the fats are extracted, and the solid matter is sold for fertilizer. ashes are used for filling marsh land. thus the removal of waste matter may pay for itself in a large city. [illustration: the upper picture shows the stables where millions of flies were bred; the lower picture, the disinfection of manure so as to prevent the breeding of flies.] an experiment in civic hygiene.--during the summer of an interesting experiment on the relation of flies and filth to disease was carried on in new york city by the bureau of public health and hygiene of the new york association for improving the condition of the poor. two adjoining blocks were chosen in a thickly populated part of the bronx near a number of stables which were the sources of great numbers of flies. in one block all houses were screened, garbage pails were furnished with covers, refuse was removed and the surroundings made as sanitary as possible. in the adjoining block conditions were left unchanged. during the summer as flies began to breed in the manure heaps near the stables all manure was disinfected. thus the breeding of flies was checked. the campaign of education was continued during the summer by means of moving pictures, nurses, boy scouts, and school children who became interested. at the end of the summer it was found that there had been a considerable decrease in the number of cases of fly-carried diseases and a still greater decrease in the total days of sickness (especially of children) in the screened and sanitary block. the table and pictures speak for themselves. if such a small experiment shows results like this, then what might a general clean-up of a city show? [illustration: in the upper picture a little girl can be seen dumping garbage from the fire escape. she was a foreigner and knew no better. the picture below shows the result of such garbage disposal.] public hygiene.--although it is absolutely necessary for each individual to obey the laws of health if he or she wishes to keep well, it has also become necessary, especially in large cities, to have general supervision over the health of people living in a community. this is done by means of a department or board of health. it is the function of this department to care for public health. in addition to such a body in cities, supervision over the health of its citizens is also exercised by state boards of health. but as yet the government of the united states has not established a bureau of health, important as such a bureau would be. the functions of a city board of health.--the administration of the board of health in new york city includes a number of divisions, each of which has a different work to do. each is in itself important, and, working together, the entire machine provides ways and means for making the great city a safe and sanitary place in which to live. let us take up the work of each division of the health board in order to find out how we may coöperate with them. [illustration: comparison of cases of illness during the summer of in two city blocks, one clean and the other dirty. what are your conclusions?] the division of infectious diseases.--infectious diseases are chiefly spread through _personal contact_. it is the duty of a government to prevent a person having such a disease from spreading it broadcast among his neighbors. this can be done by _quarantine_ or _isolation_ of the person having the disease. so the board of health at once isolates any case of disease which may be communicated from one person to another. no one save the doctor or nurse should enter the room of the person quarantined. after the disease has run its course, the clothing, bedding, etc., in the sick room is fumigated. this is usually done by the board of health. formaldehyde in the form of candles for burning or in a liquid form is a good disinfectant. in disinfecting the room should be tightly closed to prevent the escape of the gas used, as the object of the disinfection is to kill all the disease germs left in the room. in some cases of infectious disease, as scarlet fever, it is found best to isolate the patients in a hospital used for that purpose. examples of the most infectious diseases are measles, scarlet fever, whooping cough, and diphtheria. immunity.--in the prevention of germ diseases we must fight the germ by attacking the parasites directly with poisons that will kill them (such poisons are called _germicides_ or _disinfectants_), and we must strive to make the persons coming in contact with the disease unlikely to take it. this insusceptibility or _immunity_ may be either natural or acquired. natural immunity seems to be in the constitution of a person, and may be inherited. immunity may be acquired by means of such treatment as the antitoxin treatment for diphtheria. this treatment, as the name denotes, is a method of neutralizing the poison (toxin) caused by the bacteria in the system. it was discovered a few years ago by a german, von behring, that the serum of the blood of an animal immune to diphtheria is capable of neutralizing the poison produced by the diphtheria-causing bacteria. horses are rendered immune by giving them the diphtheria toxin in gradually increasing doses. the serum of the blood of these horses is then used to inoculate the patient suffering from or exposed to diphtheria, and thus the disease is checked or prevented altogether by the antitoxin injected into the blood. the laboratories of the board of health prepare this antitoxin and supply it fresh for public use. [illustration: antitoxin for diphtheria prepared by the new york board of health.] it has been found from experience in hospitals that deaths from diphtheria are largely preventable by _early use_ of antitoxin. when antitoxin was used on the first day of the disease no deaths took place. if not used until the second day, deaths occurred in every hundred cases, on the third day deaths, on the th day deaths, and on the th day deaths out of every hundred cases. it is therefore advisable, in a suspected case of diphtheria, to have antitoxin used at once to prevent serious results. vaccination.--smallpox was once the most feared disease in this country; per cent of all people suffered from it. as late as , over , persons lost their lives annually in russia from this disease. it is probably not caused by bacteria, but by a tiny animal parasite. smallpox has been brought under absolute control by vaccination,--the inoculation of man with the substance (called _virus_) which causes cowpox in a cow. cowpox is like a mild form of smallpox, and the introduction of this virus gives complete immunity to smallpox for several years after vaccination. this immunity is caused by the formation of a germicidal substance in the blood, due to the introduction of the virus. another function of the board of health is the preparation and distribution of vaccine (material containing the virus of cowpox). rabies (hydrophobia).--this disease, which is believed to be caused by a protozoan parasite, is communicated from one dog to another in the saliva by biting. in a similar manner it is transferred to man. the great french bacteriologist, louis pasteur, discovered a method of treating this disease so that when taken early at the time of the entry of the germ into the body of man, the disease can be prevented. in some large cities (among them new york) the board of health has established a laboratory where free treatment is given to all persons bitten by dogs suspected of having rabies. vaccination against typhoid.--typhoid fever has within the past five years received a new check from vaccination which has been introduced into our army and which is being used with good effect by the health departments of several large cities. the following figures show the differences between number of cases and mortality in the army in during the war with spain and in during the concentration of certain of our troops at san antonio, texas. -- nd division, th army corps, jacksonville, florida. june-october, mean strength, , . cases of typhoid certain and probable, . death from typhoid, . death from all diseases, . manoeuver division, san antonio, texas. march -july , . mean strength, , . cases of typhoid, . death from typhoid, . deaths all diseases, . [illustration: comparison of cases of and death from typhoid in and . what have we learned about combating typhoid since ?] during this period there were cases of typhoid and deaths in the near-by city of san antonio. but in camp, _where vaccination for typhoid was required_, all were practically immune. in the army at large, since typhoid vaccination has been practiced, - , the death rate from typhoid has dropped from . per to . per , a wonderful record when we remember that during the spanish-american war per cent of the deaths in the army were from typhoid fever. [illustration: the best cures for tuberculosis are rest, plenty of fresh out-of-door air, and wholesome food.] [illustration: a sanitarium for tuberculosis. notice the outdoor sleeping rooms.] how the board of health fights tuberculosis.--tuberculosis, which a few years ago killed fully one seventh of the people who died from disease in this country, now kills less than one tenth. this decrease has been largely brought about because of the treatment of the disease. since it has been proved that tuberculosis if taken early enough is curable, by quiet living, good food, and _plenty_ of fresh air and light, we find that numerous sanitaria have come into existence which are supported by private or public means. at these sanitaria the patients _live_ out of doors, especially sleep in the air, while they have plenty of nourishing food and little exercise. the department of health of new york city maintains a sanitarium at otisville in the catskill mountains. here people who are unable to provide means for getting away from the city are cared for at the city's expense and a large percentage of them are cured. in this way and by tenement house laws which require proper air shafts and window ventilation in dwellings, by laws against spitting in public places, and in other ways, the boards of health in our towns and cities are waging war on tuberculosis. ex-president roosevelt said, in one of his latest messages to congress:-- "there are about , , people seriously ill in the united states, of whom , are consumptives. _more than half of this illness is preventable._ if we count the value of each life lost at only $ and reckon the average earning lost by illness at $ a year for grown men, we find that the economic gain from mitigation of preventable disease in the united states would exceed $ , , , a year. this gain can be had through medical investigation and practice, school and factory hygiene, restriction of labor by women and children, the education of the people in both public and private hygiene, and through improving the efficiency of our health service, municipal, state, and national." work of the division of school and infant hygiene.--besides the work of the division of infectious disease, the division of sanitation, which regulates the general sanitary conditions of houses and their surroundings and the division of inspection, which looks after the purity and conditions of sale and delivery of milk and foods, there is another department which most vitally concerns school children. this is the division of school and infant hygiene. the work of this department is that of the care of the children of the city. during the year , , visits were made to the homes of school children of the city of new york by inspectors and nurses. besides this, thousands of children in school were cared for and aided by the city. adenoids.--many children suffer needlessly from adenoids,--growths in the back of the nose or mouth which prevent sufficient oxygen being admitted to the lungs. a child suffering from these growths is known as a "mouth breather" because the mouth is opened in order to get more air. the result to the child may be a handicap of deafness, chronic running of the nose, nervousness, and lack of power to think. his body cells are starving for oxygen. a very simple operation removes this growth. coöperation on the part of the children and parents with the doctors or nurses of the board of health will do much in removing this handicap from many young lives. eyestrain.--another handicap to a boy or girl is eyestrain. twenty-two per cent of the school children of massachusetts were recently found to have defects in vision. tests for defective eyesight may be made at school easily by competent doctors, and if the child or parent takes the advice given to correct this by procuring proper glasses, a handicap on future success will be removed. decayed teeth.--decayed teeth are another handicap, cared for by this division. free dental clinics have been established in many cities, and if children will do their share, the chances of their success in later life will be greatly aided. boys and girls, if handicapped with poor eyes or teeth, do not have a fair chance in life's competition. in a certain school in new york city there were pupils marked "c" in their school work. these children were examined, and were found to have bad teeth, defective vision, and other defects, as poor hearing, adenoids, enlarged tonsils, etc. of these children were treated for these various difficulties, and did not take treatment. during the following year's work of these pupils _improved_ from "c" to "b" or "a", while did not improve. if defects _are_ such a handicap in school, then what would be the chances of success in life outside. in conclusion: this department of school hygiene deserves the earnest aid of every young citizen, girl or boy. if each of us would honestly help by maintaining quarantine in the case of contagious disease, by observing the rules of the health department in fumigation, by acting upon advice given in case of eyestrain, bad teeth, or adenoids, and most of all by observing the rules of personal hygiene as laid down in this book, the city in which we live would, a generation hence, contain stronger, more prosperous, and more efficient citizens than it does to-day. reference books elementary hunter, _laboratory problems in civic biology_. american book company. davison, _the human body and health_. american book company. gulick hygiene series, _town and city_. ginn and company. hough and sedgwick, _the human mechanism_, part ii. ginn and company. overton, _general hygiene_. american book company. richards, _sanitation in daily life_. whitcomb and barrows. richmond and wallach, _good citizenship_. american book company. ritchie, _primer of sanitation_. world book company. sharpe, _laboratory manual of biology_, pages - . american book company. advanced allen, _civics and health_. ginn and company. chapin, _municipal sanitation in the united states_. snow and farnham. chapin, _sources and modes of infection_. wiley and sons. conn, _practical dairy bacteriology_. orange judd company. hough and sedgwick, _the human mechanism_. part ii. ginn and company. hutchinson, _preventable diseases_. the houghton, mifflin company. morse, _the collection and disposal of municipal waste_. municipal journal and engineer. overlock, _the working people, their health and how to protect it_. mass. health book publishing co. price, _handbook of sanitation_. wiley and sons. tolman, _hygiene for the worker_. american book company. reports, etc. _american health magazine._ annual report of department of health, city of new york (and other cities). bulletins and publications of committee of one hundred on national health. _school hygiene_, american school hygiene association. grinnell, _our army versus a bacillus_. national geographic magazine. xxv. some great names in biology if we were to attempt to group the names associated with the study of biology, we would find that in a general way they were connected either with discoveries of a purely scientific nature or with the benefiting of man's condition by the _application_ of the purely scientific discoveries. the first group are necessary in a science in order that the second group may apply their work. it was necessary for men like charles darwin or gregor mendel to prove their theories before men like luther burbank or any of the men now working in the department of agriculture could benefit mankind by growing new varieties of plants. the discovery of scientific truths must be achieved before the men of modern medicine can apply these great truths to the cure or prevention of disease. since we are most interested in discoveries which touch directly upon human life, the men of whom this chapter treats will be those who, directly or indirectly, have benefited mankind. the discoverers of living matter.--the names of a number of men living at different periods are associated with our first knowledge of cells. about the middle of the seventeenth century microscopes came into use. through their use plant cells were first described and pictured as hollow boxes or "cells." but it was not until that two german friends, schleiden and schwann by name, working on plants and animals, discovered that both of these forms of life contained a jellylike substance that later came to be called _protoplasm_. another german named max schultz in gave the name protoplasm to _all living matter_, and a little later still professor huxley, a famous englishman, friend and champion of charles darwin, called attention to the physical and chemical qualities of protoplasm so that it came to be known as the chemical and physical basis of life. [illustration: prof. tyndall's experiment to show that if air containing germs is kept from organic substances, such substances will not decay. the box is sterilized; likewise the tubes (_t_) containing nutrients. air is allowed to enter by the tubes (_u_), which are so made that dust is prevented from entering. a thermometer (_th_) records the temperature. the substances in the tubes do not decay, no matter how favorable the temperature.] life comes from life.--another group of men, after years of patient experimentation, worked out the fact that _life comes from other life_. in ancient times it was thought that life arose _spontaneously_; for example, that fish or frogs arose out of the mud of the river bottoms, and that insects came from the dew or rotting meat. it was believed that bacteria arose spontaneously in water, even as late as , when professor tyndall proved by experiment the contrary to be true. as early as william harvey, the court physician of charles i of england, showed that all life came from the egg. it was much later, however, that the part played by the sperm and egg cell in fertilization was carefully worked out. it is to harvey, too, that we owe the beginnings of our knowledge of the circulation of the blood. he showed that blood moved through tubes in the body and that the heart pumped it. he might be called the father of modern physiology as well as the father of embryology. a long list of names might be added to that of harvey to show how gradually our knowledge of the working of the human body has been added to. at the present time we are far from knowing all the functions of the various parts of the human engine, as is shown by the number of investigators in physiology at the present time. present-day problems have much to do with the care of the human mechanism and with its surroundings. the solution of these problems will come from applying the sciences of hygiene, preventive medicine, and sanitation. in the preceding chapters of this book we have learned something about our bodies and their care. we have found that man is able within limitations to control his environment so as to make it better to live in. all of the scientific facts that have been of use to man in the control of disease have been found out by men who have devoted their lives in the hope that their experiments and their sacrifices of time, energy, and sometimes life itself might make for the betterment of the human race. such men were harvey, jenner, lister, koch, and pasteur. [illustration: edward jenner, the discoverer of vaccination.] edward jenner and vaccination.--the civilized world owes much to edward jenner, the discoverer of vaccination against smallpox. born in berkeley, a little town of gloucestershire, england, in , as a boy he showed a strong liking for natural history. he studied medicine and also gave much time to the working out of biological problems. as early as he began to associate the disease called cowpox with that of smallpox, and gradually the idea of inoculation against this terrible scourge, which killed or disfigured hundreds of thousands every year in england alone, was worked out and applied. he believed that if the two diseases were similar, a person inoculated with the mild disease (cowpox) would after a slight attack of this disease be immune against the more deadly and loathsome smallpox. it was not until that he was able to prove his theory, as at first few people would submit to vaccination. war at this time was being waged between france and england, so that the former country, usually so quick to appreciate the value of scientific discoveries, was slow to give this method a trial. in spite of much opposition, however, by the year , vaccination was practiced in most of the civilized countries of the world. at the present time the death rate in great britain, the home of vaccination, is less than . to every , , living persons. this shows that the disease is practically wiped out in england. an interesting comparison with these figures might be made from the history of the disease in parts of russia where vaccination is not practiced. there, thousands of deaths from smallpox occur annually. during the winter of - an epidemic of smallpox with more than cases broke out in the city of niagara falls. this epidemic appears to be due to a campaign conducted by people who do not believe in vaccination. in cities and towns near by, where vaccination was practiced, no cases of smallpox occurred. naturally if opposition to vaccination is found nowadays, jenner had a much harder battle to fight in his day. he also had many failures, due to the imperfect methods of his time. the full worth of his discovery was not fully appreciated until long after his death, which occurred in . [illustration: louis pasteur.] louis pasteur.--the one man who, in biological science, did more than any other to directly benefit mankind was louis pasteur. born in , in the mountains near the border of northeastern france, he spent the early part of his life as a normal boy, fond of fishing and not very partial to study. he inherited from his father, however, a fine character and grim determination, so that when he became interested in scientific pursuits he settled down to work with enthusiasm and energy. at the age of twenty-five he became well known throughout france as a physicist. shortly after this he became interested in the tiny plants we call bacteria, and it was in the field of bacteriology that he became most famous. first as professor at strassburg and at lille, later as director of scientific studies in the École normale at paris, he showed his interest in the application of his discoveries to human welfare. in pasteur showed that fermentation was due to the presence of bacteria, it having been thought up to this time that it was a purely chemical process. this discovery led to very practical ends, for france was a great wine-producing country, and with a knowledge of the cause of fermentation many of the diseases which spoiled wine were checked. in - pasteur turned his attention to a silkworm disease which threatened to wipe out the silk industry of france and italy. he found that this disease was caused by bacteria. after a careful study of the case he made certain recommendations which, when carried out, resulted in the complete overthrow of the disease and the saving of millions of dollars to the poor people of france and italy. the greatest service to mankind came later in his life when he applied certain of his discoveries to the treatment of disease. first experimenting upon chickens and later with cattle, he proved that by making a virus (poison) from the germs which caused certain diseases he could reduce this virus to any desired strength. he then inoculated the animals with the virus of reduced strength, giving the inoculated animals a mild attack of the disease, and found that this made them _immune_ from future attacks. this discovery, first applied to chicken cholera, laid the foundation for all future work in the uses of serums, vaccines, and antitoxins. pasteur was perhaps the best known through his study of rabies. the great pasteur institute, founded by popular subscriptions from all over the world, has successfully treated over , cases of rabies with a death rate of less than per cent. but more than that it has been the place where roux, a fellow worker with pasteur, discovered the antitoxin for diphtheria which has resulted in the saving of thousands of human lives. here also have been established the principles of inoculation against bubonic plague, lockjaw, and other germ diseases. pasteur died in at the age of seventy-three, "the most perfect man in the realm of science," a man beloved by his countrymen and honored by the entire world. [illustration: robert koch.] robert koch.--another name associated with the battle against disease germs is that of robert koch. born in klausthal, hanover, in , he later became a practicing physician, and about was called to berlin to become a member of the sanitary commission and professor in the school of medicine. in he discovered the germ that causes tuberculosis and two years later the germ that causes asiatic cholera. his later work has been directed toward the discovery of a cure for tuberculosis and other germ diseases. as yet, however, no certain cure seems to have been found. lister and antiseptic treatment of wounds.--a third great benefactor of mankind was sir joseph lister, an englishman who was born in . as a professor of surgery he first applied antiseptics in the operating room. by means of the use of carbolic acid or other antiseptics on the surface of wounds, on instruments, and on the hands and clothing of the operating surgeons, disease germs were prevented from taking a foothold in the wounds. thus blood poisoning was prevented. this single discovery has done more to prevent death after operations than any other of recent time. modern workers on the blood.--at the present time several names stand out among investigators on the blood. paul ehrlich, a german born in , is justly famous for his work on the blood and its relation to immunity from certain diseases. his last great research has given to the world a specific against the dread disease syphilis. another name associated with the blood is that of elias metchnikoff, a russian. he was born in . metchnikoff first advanced the belief that the colorless blood corpuscles, or _phagocytes_, did service as the sanitary police of the body. he has found that there are several different kinds of colorless corpuscles, each having somewhat different work to do. much of the modern work done by physiologists on the blood are directly founded on the discoveries of metchnikoff. [illustration: charles darwin, the grand old man of biology.] heredity and evolution. charles darwin.--there is still another important line of investigation in biology that we have not mentioned. this is the doctrine of evolution and the allied discoveries along the line of heredity. the development or evolution of plants and animals from simpler forms to the many and present complex forms of life have a practical bearing on the betterment of plants and animals, including man himself. the one name indelibly associated with the word evolution is that of charles darwin. charles darwin was born on february , , a son of well-to-do parents, in the pretty english village of shrewsbury. as a boy he was very fond of out-of-door life, was a collector of birds' eggs, stamps, coins, shells, and minerals. he was an ardent fisherman, and as a young man became an expert shot. his studies, those of the english classical school, were not altogether to his liking. it is not strange, perhaps, that he was thought a very ordinary boy, because his interest in the out-of-doors led him to neglect his studies. later he was sent to edinburgh university to study medicine. here the dull lectures, coupled with his intense dislike for operations, made him determine never to become a physician. but all this time he showed his intense interest in natural history and took frequent part in the discussions at the meetings of one of the student zoölogical societies. in his father sent him to cambridge to study for the ministry. his three years at the university were wasted so far as preparation for the ministry were concerned, but they were invaluable in shaping his future. he made the acquaintance of one or two professors who were naturalists like himself, and in their company he spent many happy hours in roaming over the countryside collecting beetles and other insects. in an event occurred which changed his career and made darwin one of the world's greatest naturalists. he received word through one of his professional friends that the position of naturalist on her majesty's ship _beagle_ was open for a trip around the world. darwin applied for the position, was accepted, and shortly after started on an eventful five years' trip around the world. he returned to england a famous naturalist and spent the remainder of his long and busy life producing books which have done more than those of any other writer to account in a satisfactory way for the changes of form and habits of plants and animals on the earth. his theories established a foundation upon which plant and animal breeders were able to work. his wonderful discovery of the doctrine of evolution was due not only to his information and experimental evidence, but also to an iron determination and undaunted energy. in spite of almost constant illness brought about by eyestrain, he accomplished more than most well men have done. his life should mean to us not so much the association of his name with the _origin of species_ or _plants and animals under domestication_, two of his most famous books, but rather that of a patient, courteous, and brave gentleman who struggled with true english pluck against the odds of disease and the attacks of hostile critics. he gave to the world the proofs of the theory on which we to-day base the progress of the world. darwin lived long enough to see many of his critics turn about and come over to his beliefs. he died on the th of april, , at seventy-four years of age. associated with darwin's name we must place two other co-workers on heredity and evolution, alfred russel wallace, an englishman who independently and at about the same time reached many of the conclusions that darwin came to, and august weissman, a german. the latter showed that the protoplasm of the germ cells (eggs and sperms) is directly handed down from generation to generation, they being different from the other body cells from the very beginning. in a german named boveri discovered that the chromosomes of the egg and the sperm cell were at the time of fertilization just half in number of the other cells (see page ) so that a _fertilized_ egg was really a _whole cell_ made up of _two half cells_, one from each parent. the chromosomes within the nucleus, we remember, are believed to be the bearers of the hereditary qualities handed down from parent to child. this discovery shows us some of the mechanics of heredity. applications to plant and animal breeding.--turning to the practical applications of the scientific work on the method of heredity, the name of gregor mendel, an austrian monk, stands out most prominently. mendel lived from until . his work, of which we already have learned something (see page ), remained undiscovered until a few years ago. the application of his methods to plant and animal raising are of the utmost importance because the breeder is able to separate the qualities he desires and breed for those qualities only. another name we have mentioned with reference to plant breeding is hugo de vries, the dutchman who recently showed that in some cases plants arise as new species by sudden and great variations known as _mutations_. and lastly, in our own california, luther burbank, by careful hybridizing, is making lasting fame with his new and useful hybrid plants. references conn, _biology_. silver, burdett & co. darwin, _life and letters of charles darwin_. appletons. galton, _hereditary genius_. london ( ). thompson, _heredity_. john murray, london england. wasmann, _problem of evolution_. kegan paul, trench, trübner and co., london, e. c. appendix a suggested outline for biology beginning in the fall list of topics first term first week. why study biology? relation to human health, hygiene. relations existing between plants and animals. relation of bacteria to man. uses of plants and animals. conservation of plants and animals. relation to life of citizen in the city. plants and animals in relation to their environment. what is the environment; light, heat, water, soil, food, etc. what plants take out of the environment. what animals take out of the environment. dependence of plants and animals upon the factors of the environment. _laboratory_: study of a plant or an animal in the school or at home to determine what it takes from its environment. second week. some relations existing between plants (green) and animals. field trip planned to show that insects feed upon plants; make their homes upon plants. that flowers are pollinated by insects. insects lay eggs upon certain food plants. green plants make food for animals. other relations. (time allotment. one day trip, collecting, etc.; two days' discussion of trip in all its relations.) make a careful study of the locality you wish to visit, have a plan that the pupils know about beforehand. review and hygiene of pupil's environment, days. third week. study of a flower, parts essential to pollination named. adaptations for insect pollination worked out in laboratory. study of bee or butterfly as an insect carrier of pollen. names of parts of insect learned. elementary knowledge of groups of insects seen on field trip. bees, butterflies, grasshoppers, beetles, possibly flies and bugs. drawing of a flower, parts labeled. drawing of an insect, outline only, parts labeled. careful study of some fall flower fitted for insect pollination with an insect as pollinating agent. some examples of cross-pollination explained. practical value of cross-pollination. fourth week. living plants and animals compared. parts of plants, functions; organs, tissues, cells. demonstration cells of onion or elodea. how cells form others. what living matter can do. reproduction. growth of pollen tube, fertilization. development of ovule into seed. fruits, how formed. uses, to man. fifth week. what makes a seed grow. bean seed, a baby plant, and food supply. food, what is it? organic nutrients, tests for starch, protein, oil. show their presence in seeds. sixth week. need for foods. germination of bean due to (_a_) presence of foods, (_b_) outside factors. what is done with the food. release of energy. examples of engine, plants, human body. oxidation in body. proof by experiment. test for presence of co{ }. oxidation in growing plant, experiment. respiration a general need for both plants and animals. seventh week. need for digestion. the corn grain. parts, growth, food supply outside body of plant, how does it get inside. digestion, need for. test for grape sugar. enzymes, their function. action of diastase on starch. eighth week. what plants take from the soil, how they do this. use of root. influence of gravity and water. why? absorption a function. root hairs. demonstration. pocket gardens, optional home work, but each pupil must work on root hairs from actual specimen. how root absorbs. osmosis; what substances will osmose. experiments to demonstrate this. ninth week. composition of soil. what root hairs take out of soil. plant needs mineral matter to make living matter. why? nitrogen necessary. sources of nitrogen, the nitrogen-fixing bacteria. relation of this to man. rotation of crops. tenth week. how green plants make food. passage of liquids up stem. demonstration. structure of a green leaf. cellular structure demonstrated. microscopic demonstration of cells, stoma, air spaces, chlorophyll bodies. evaporation of water from green leaf, regulation of transpiration. eleventh week. _midterm examinations._ sun a source of energy. effect of light on green plants. experimental proof. starch made in green leaf. light and air necessary for starch making. proof. protein making in leaf. by-products in starch making. proof. respiration. twelfth week. the circulation and distribution of food in green plants. uses of bark, wood, what part of stem does food pass down. willow twig experiment. summary of functions of living matter in plant. forestry lecture. economic uses of green plants. reports. thirteenth week. plants without chlorophyll in their relation to man. saprophytic fungi. molds. growth on bread or other substances. conditions most favorable for growth. favorite foods. methods of prevention. economic importance. fourteenth week. yeasts in their relation to man. experiments to show fermentation is caused by yeasts. experiments to show conditions necessary for fermentation. the part played by yeasts in bread making, in wine making, in other industries. structure of yeast demonstrated. summary. fifteenth week. experiments to show where bacteria may be found and conditions necessary to growth begun. have cultures collected and placed in a warm room during the holidays. suggested experiments are exposure to air of quiet room and room with persons moving, dust of floor, knife blade, etc. sixteenth, seventeenth, and eighteenth weeks. the month of january should be devoted to the study of bacteria in their general relations to man. economically, both directly and indirectly. especial emphasis placed on the nature and necessity of decay. bacteria in relation to disease should also be emphasized. the experiments to be performed and the topics expected to be covered follow. conditions favorable and unfavorable for growth of bacteria. (use bouillon cultures.) effect of intense heat, sterile bouillon exposed to air, effect of boiling, effect of cold, effect of antiseptics (corrosive sublimate, carbolic acid, boric acid, formalin, etc.), effect of large amounts of sugar and salt and the relation of this to preserving, etc. bring out practical application of principles demonstrated. discuss sterilization in medicine and surgery, cold storage, canning, sterilization, _e.g._ laundries, etc., use of antiseptics, preserving by means of salt and sugar. microscopic demonstration of bacteria. methods of reproduction. importance in causing organic decay, fixation of nitrogen, various useful forms in cheese making, butter ripening, etc. harmfulness of bacteria as disease producers. specific diseases discussed: tuberculosis, typhoid, infective colds, blood poisoning, etc. vaccination. antitoxins begun--continued after knowledge of human body is gained. work of lister and pasteur. nineteenth and twentieth weeks. review and examinations. second term first week. the balanced aquarium. carbon and nitrogen cycles. balanced aquarium and hay infusion compared. second week. one protozoan, demonstration to show changes in shape, response to stimuli, summary of vital processes in cell. food getting, digestion, assimilation, oxidation, excretion, growth, reproduction. internal structure of protozoan. protozoa as cause of disease. third week. general survey of animal kingdom. survey introduced by museum trip if possible. protozoa, worm, insect, fish, mammal. distinction between vertebrate and invertebrate. character of mammalia. division of labor emphasized. man's place in nature. fourth week. study of the frog. relation to habitat, adaptations for locomotion, food getting, respiration, comparison of frog and fish on latter point. osmotic exchange of gases emphasized. cell respiration. fifth week. metamorphosis of frog. fertilization, cell division, and differentiation emphasized. touch on plant and animal breeding. function of chromosomes as bearers of heredity. comparison of bird's egg and mammal embryo. sixth week. factors in breeding. . variation. . selection. . heredity fixes variation. . hybridizing. . control of environment. eugenics in relation to (_a_) crime, (_b_) disease, (_c_) genius. continuity of germ plasm. work of darwin, mendel, de vries, burbank. seventh week. a brief study of the gross structure of the human body. skin, muscles, bones. removal of lime from bone by hcl to show other substances and need for lime. effect of posture, spinal curvature, fractures, sprains. eighth week. need for food. nutritive value of food. use of charts to show foods rich in carbohydrates, fats, proteins, minerals, water, refuse. the relation of age, sex, work, and environment to the food requirements. what is a cheap food. price list of common foods at present time. efforts of government to secure a cheap food supply for the people. digestibility of foods. ninth week. how the fuel value of food has been determined. meaning of calorie. the -caloric portion, its use in determining a daily or weekly dietary. standard dietary as determined by atwater. comparison of standards of chittenden and voit with those of atwater. tenth week. study of pupil's dietary. planning ideal meals. individual dietaries for one day required from each pupil. discussions and corrections. the family dietary. relation to cost. eleventh week. digestion. the digestive system in the frog and in man compared. drawings of each. glands and enzymes. internal secretions and their importance. demonstration of glandular tissues. experiment to show digestion of starch in mouth. twelfth week. digestion continued. digestion of white of egg by gastric juice. digestion of starch with pancreatic fluid. functions of pancreatic juice. microscopic examination of emulsion. reasons for digestion. part played by osmosis. demonstration of osmosis. non-osmosis of non-digested foods, comparison between osmosable qualities of starch and grape sugar. thirteenth week. absorption. where and how foods are absorbed. the structure of a villus explained. course taken by foods after absorption. function of liver. blood making the result of absorption. composition of blood, red and colorless corpuscles, plasma, blood plates, antibodies. microscopic drawing of corpuscles of frog's and man's blood. fourteenth week. circulation of blood. the heart and lungs of frog demonstrated. heart of man a force pump, explain with use of force pump. demonstration of beef's heart. circulation and changes of blood in various parts of body. work of cells with reference to blood made clear. capillary circulation (demonstration of circulation in tadpole's tail or web of frog's foot). fifteenth week. respiration and excretion. necessity for taking of oxygen to cells and removal of wastes from cells. part played by blood and lymph. mechanics of breathing (use of experiments). changes of air and blood in lungs (experiments). best methods of ventilation (experiments). elimination of wastes from blood by lungs, skin, and kidneys. cell respiration. sixteenth week. hygiene of organs of excretion, especially care of skin. the general structure and functions of the central nervous system. sensory and motor nerves. reflexes, instincts, habits. habit formation, importance of right habits. rules for habit formation. habit-forming drugs and other agents. lecture. seventeenth, eighteenth, nineteenth weeks. civic hygiene and sanitation. hygiene of special senses, eye and ear. a well citizen an efficient citizen. public health is purchasable. improvement of environment a means of obtaining this. civic hygiene and sanitation. cleaning up neighborhood, inquiry into home and street conditions. fighting the fly. conditions of milk and water supply. relation of above to disease. work of board of health, etc. review and examinations. suggested syllabus for course beginning february and ending the following january first term first week. why study biology? relation to human health, hygiene. relations existing between plants and animals. relation of bacteria to man. uses of plants and animals. conservation of plants and animals. relation to life of citizen in this city. needs of plants and animals: ( ) food, ( ) water, ( ) air, ( ) proper temperature. study of a single plant or animal in relation to its environment. problems of city government: (_a_) storage, preservation and distribution of foods, (_b_) water supply, (_c_) overcrowded tenements, (_d_) street cleaning, (_e_) clean schools. biological problems in city government. second week. interrelations between plants and animals. plants furnish food, clothing, shelter, and medicine. animals use food, shelter. man's use of plants as above. man's use of animals as above. plant and animal industries. use of balanced aquarium as illustrative material. third week. destruction of food and other things by mold. home experiment. conditions favorable to growth of mold. food, moisture, temperature. destruction of commodities by mold: food, leather, clothing. fourth week, fifth week. destruction of foods by bacteria. experiment. to show where bacteria are found. soil, dust, water, milk, hands, mouth. use and harm of decay. relation to agriculture. experiment. conditions favorable and unfavorable to growth of bacteria: boiling, cold, sugar, salt. bacteria in relation to disease briefly mentioned. bacteria in industries. sixth week. use of stored food by young green plant: (_a_) for energy, (_b_) for construction of tissue. experiment. structure of bean seed. draw to show outer coat, cotyledon, hypocotyl, and plumule. test for starch and sugar (grape). test for oil, protein, water, mineral matter. use of all nutrients to seedling. seventh week. other needs of young plants. home experiments to show (_a_) temperature, (_b_) amount of water most favorable to germination. experiment. to show need of oxygen. to show that germinating seeds give off carbon dioxide. proof of presence of carbon dioxide in breath. the needs of a young plant compared with those of a boy or girl. eighth week. digestion in seedling. structure of corn grain. experiment. to show that starch is digested in a growing seedling (corn). experiment. to show that diastase digests starch. discussion of experiments. ninth week. what plants take from the soil and how they do this. use of roots. proof that it holds plant in position, takes in water and mineral matter, and in some cases stores food. influence of gravity and water. labeled drawing of root hair. root hair as a _cell_ emphasized. osmosis demonstrated. tenth week. composition of the soil. demonstration of presence of mineral and organic substances in the soil. what root hairs take from the soil. mineral matter necessary and why. importance and sources of nitrogen. soil exhaustion and its prevention. nitrogen-fixing bacteria. review bacteria of decay. rotation of crops. eleventh week. upward course of materials in the stem. demonstration of pea seedlings with eosin to show above. demonstration of evaporation of water from a leaf. action of stomata in control of transpiration. cellular structure of leaf. demonstration of elodea to show cell. twelfth week. sun a source of energy. heliotropism. demonstration. necessity of sunlight for starch manufacture. necessity of air for starch manufacture. by-products in starch making. oil manufacture in leaf. protein manufacture in plant. respiration. thirteenth week. reproduction. necessity for (_a_) perpetuation, (_b_) regeneration. study of a typical flower to show sepals, petals, stamens, pistil. functions of each part. cross and longitudinal sections of ovary shown and drawn. emphasis on essential organs. pollination, self and cross. (note. at least one field trip must be planned for the month of may. this trip will take up the following topics: the relations between flowers and insects. the food and shelter relation between plants and animals. recognition of to common trees. need of conservation of forests. an extra trip could well be taken to give child a little knowledge and love for spring flowers and awakening nature.) fourteenth week. study of the bee or butterfly with reference to adaptations for insect pollination. study of an irregular flower to show adaptations for insect visitors. fertilization begun. growth of pollen tubes. fifteenth week. fertilization completed. use of chart to show part played by egg and sperm cell. ultimate result the formation of embryo and its growth under favorable conditions into young plant. relation of flower and fruit, pea, or bean used for this purpose. development of fleshy fruit. apple used for this purpose. sixteenth week. maturing of parts and storing of food in seed and fruit. the devices for scattering the seeds and relation to future plants. résumé of processes of nutrition to show how materials found in fruit and seed are obtained by the plant. seventeenth week. plant breeding. factors: (_a_) selective planting, (_b_) cross-pollination, (_c_) hybridizing. heredity and variation begun. darwin and burbank mentioned. eighteenth and nineteenth weeks. the natural resources of man: soil, water, plants, animals. the relation of plant life to the above factors of the environment. the relation of insects to plants (forage and other crops) and the relation of birds to insects. need for conservation of the helpful factors in the environment of plants. attention called to some native birds as insect and wood destroyers. twentieth week. review and examinations. second term first week. the balanced aquarium. study of conditions producing this. the rôle of green plants, the rôle of animals. what causes the balance. how the balance may be upset. the nitrogen cycle. what it means in the world outside the aquarium. symbiosis as opposed to parasitism. examples. second week. study of the paramoecium. study of a hay infusion to show how environment reacts upon animals. relation to environment. study of cell under microscope to show reactions. structure of cell. response to stimuli, function of cilia, gullet, nucleus, contractile vacuoles, food vacuoles, asexual reproduction. drawings to show how locomotion is performed, general structure. copy chart for fine structure. third week. a bird's-eye view of the animal kingdom. one day. development of a multicellular organism. (use models.) one day. physiological division of labor. tissues, organs. functions common to all animals. illustrative material. optional trip to museum for use of illustrative material to illustrate the principal characteristics of (_a_) a simple metazoan, sponge, or hydrazoan, (_b_) a segmented worm, (_c_) a crustacean (decapod), (_d_) an insect, (_e_) a mollusk and echinoderm, (_f_) vertebrates. (differences between vertebrates and invertebrates.) the characteristics of the vertebrates. distinguish between fishes, amphibia, reptiles, birds, mammals. two days for discussion. man's place in the animal series, elementary discussion of what evolution means. fourth week. the economic importance of animals. uses of animals: ( ) as food. directly: fish, shellfish, birds, domesticated mammals. ( ) indirectly as food: protozoa, crustacea. ( ) they destroy harmful animals and plants. snakes--birds; birds--insects; birds--weed seeds; herbivorous animals--weeds. ( ) furnish clothing, etc. pearl buttons, etc. ( ) animal industries, silkworm culture, etc. ( ) domesticated animals. animals do harm: ( ) to gardens. ( ) to crops. ( ) to stored food; examples, rats, insects, etc. ( ) to forest and shade trees. ( ) to human life. disease: parasitism and its results,--examples, from worms, etc.; disease carriers fly, etc. preventive measures. methods of extermination. references to toothaker's _commercial raw materials_. use one day for laboratory work from references. fifth week. the study of a water-breathing vertebrate. two days. the fish, adaptations in body, fins, for food getting, for breathing. structure of gills shown. laboratory demonstration to show how water gets to the gills. drawings. outline of fish, gills. required trip to aquarium. object, to see fish in environment. one day. home work at market. why are some fish more expensive than others. economic importance of fish. relation of habits of (_a_) food getting, (_b_) spawning to catching and extermination of fish. two days. means of preventing overfishing, stocking, fishing laws, artificial fertilization of eggs, methods. development of fish egg. comparison with that of frog and bird. sixth week. the factors underlying plant and animal breeding. study of pupils in class to show heredity and variation. conclusion. animals tend to vary and to be like their ancestors. heredity, rôle of sex cells, chromosomes. principles of plant breeding. selective planting, hybridizing, work of darwin, mendel, de vries, and burbank. methods and results. animal breeding, examples given, results. improvement of man: ( ) by control of environment, (_a_) example of clean-up campaign, ; ( ) by control of individual, personal hygiene, and control of heredity. eugenics. examples from davenport, goddard, etc. seventh week. the human machine. skin, bones and muscles, function of each. examples and demonstration with skeleton. organs of body cavity; show manikin. work done by cells in body. eighth week. study of foods to determine: (_a_) nutritive value. exercise with food charts to determine foods rich in water, starch, sugar, fats, proteins, mineral salts, refuse. one day. (_b_) nutritive value of foods as related to work, age, sex, environment, cost, and digestibility. foods compared to determine what is really a cheap food. ninth week. how the fuel value of food has been determined. the dietaries of atwater, chittenden, and voit. the -calorie portion table and its use. tenth week. the application of the -calorie portion to the making of the daily dietaries. luncheon dietaries. a balanced dietary for pupil for one day. family dietaries. relation to cost. reasons for this. eleventh week. food adulterations. tests. drugs and the alcohol question. twelfth week. digestion. the alimentary canal of frog and of man compared. drawings. (one day.) the work of glands. work of salivary gland. enzymes, internal secretions. experiments to show (_a_) digestion of starch by saliva, (_b_) digestion of proteins by gastric or pancreatic juice, (_c_) emulsification of fats in the presence of an alkaline medium. functions of other digestive glands. movements of stomach and intestine discussed and explained. thirteenth week. absorption. how it takes place, where it takes place. passage of foods into blood, function of liver, glycogen. fourteenth week. the blood and its circulation. composition and functions of plasma, red corpuscles, colorless corpuscles, blood plates, antibodies. the lymph and work of tissues. the blood and its method of distribution. heart a force pump. demonstration. arteries, capillaries (demonstration), veins. hygiene of exercise. fifteenth week. what respiration does for the body. the apparatus used. changes of blood within lungs, changes of air within lungs. demonstration. cell respiration. the mechanics of respiration. demonstration. ventilation, need for, explain proper ventilation. demonstration. hygiene of fresh air and proper breathing. dusting, sweeping, etc. sixteenth week. excretion, organs of. skin and kidneys, regulation of body heat. colds and fevers. proper care of skin, hygiene. summary of blood changes in body. explanation of same. seventeenth week. body control and habit formation. nervous system, nerve control. the neuron theory, brain psychology explained in brief. habits and habit formation. hygiene of sense organs. eighteenth and nineteenth weeks. civic hygiene and sanitation. the improvement of one's environment. civic conditions discussed. water, milk, food supplies. relation to disease. how safeguarded. how help improve conditions in city. twentieth week. review and examinations. hygiene outline (this outline may be introduced with plant biology, or, better, may come as application of the work in second-term biology.) the environment. changes for betterment under control. how a city boy may improve his environment: by proper clothing, proper food and preparation of food, by care in home life; by sanitary conditions in neighborhood and in home. review of activities of cell. irritability, food taking, assimilation, oxidation, excretion, reproduction. similarity of functions of plant and animal cells. all cells perform these functions. some cells perform functions especially well, _e.g._ contracting muscle cells. all cells need food and oxygen. some must have this carried to them. a system of tubes carries blood which carries food and oxygen. food must be prepared to get into the blood. digestive system: mouth, teeth, stomach, intestines, glands, and digestive juices. uses of above in preparing food to pass into the blood. absorption of food into the blood. how oxygen gets to the cells. nose, throat, windpipe, lungs; blood goes to lungs and carries away oxygen. excretion. cells give up wastes to blood and these wastes taken out of blood by kidneys and other glands and passed out of body. sweat, urine, carbon dioxide. certain kinds of work performed by certain kinds of cells. advantage of this. cells of movement. muscles, tissues. bones as levers necessary for some movements. this especially true for legs and arms. skeleton also necessary for protection of internal organs and support of body. making of special things in the body, _e.g._ digestive juices given to certain cells called gland cells. working together or coördination of different organs provided for by nervous system. this is composed of cells which are highly irritable or sensitive. collections of these nerve cells give us the power of feeling or sensation and of thinking. dietetics. diet influenced by age, weight, occupation, temperature or climate, cheapness of food, digestibility. nutrients. list of nutrients found in seeds and fruits, also other common foods. need of nutrients for human body. nitrogenous foods, examples. a mixed diet best. digestion and indigestion. what is digestion? where does it take place? _causes of indigestion._ eating too rapidly and not chewing food. eating foods hard to digest. overeating. eating between meals. hard exercise immediately before or after eating. constipation. a condition in which the bowels do not move at least once every day. dangers of constipation. poisonous materials may be absorbed, causing lack of inclination to work, headache. importance of regular habits of emptying the bowels. each one must try to get at the cause of constipation in his own case. _causes of constipation._ lack of exercise, improper food, not drinking enough water, lack of laxative food, as fruits; lack of sleep, lack of regular habits. _remedies._ avoid use of drugs. half hour before breakfast a glass of hot water, exercise of abdominal muscles, laxative foods, form habit of moving bowels after breakfast. hygiene of circulation and absorption. how digested foods get to the cells. absorption. definition. the passing of the digested food into the blood. how accomplished. blood vessels. in walls of stomach and food tube. membrane of cells separating food from blood. food passes by osmosis through the membrane and by osmosis through the thin walls of the blood vessels. circulation of foods. blood contains foods, oxygen, and waste materials. heart pumps the blood, blood vessels subdivide until very small and thin, so food, etc., passes from them to cells. hygiene of the heart. transpiration and excretion. skin, function in excretion. bathing. care of skin. hot baths. bathe at least twice a week. cold baths, how taken. bathtub not a necessity. effect of latter on educating skin to react. relation to catching cold. care of scalp and nails. scalp should be washed weekly. if dandruff present, wash often enough to keep clean. baldness often results from dandruff. finger nails cut even with end of fingers and cleaned daily with scrub brush. hygiene of respiration. definition of respiration. object of respiration. (connection between circulation and respiration.) necessity of oxygen. organs of respiration. lungs most important. deep breath, function. ventilation, reasons for. mouth breathing. results. lessened mental power, nasal catarrh, colds easily caught. plants harmful to man. poison ivy and mushrooms. treatment. poisoning. send for physician. cause vomiting by ( ) finger, ( ) mustard and water. (note. an unconscious person should not be given anything by the mouth unless he can swallow.) relation of yeasts and bacteria to man. fermentation a cause of indigestion. relation to candy, sirups, sour stomach, formation of gas causes pain. bacteria of mouth and alimentary canal. entrance of bacteria by mouth and nose. nose: "cold in the head," grippe, catarrh. mouth: decay of teeth, tonsillitis, diphtheria. germs pass from one person to another, no one originates germs in himself. precautions against receiving and transferring germs. common drinking cups, towels, coins, lead pencils, moistening fingers to turn pages in book or to count roll of bills. tuberculosis germs. entrance by mouth, lungs favorite place, may be any part of body. dust of air, sweeping streets, watering a necessity. spitting in streets and in public buildings. germs of typhoid fever. entrance: water, milk, fresh uncooked vegetables, oysters. thrive in small intestines. preventable. typhoid epidemics, methods of prevention of typhoid. conditions favorable for growth of specific disease germs. work of boards of health. home sanitary conditions, sunlight, air, curtains and blinds, open windows. live out of doors as much as possible. cleanliness. bare walls well scrubbed better than carpets and rugs. lace curtains, iron bedsteads, one thickness of paper on walls. open plumbing, dry cellars, all garbage promptly removed. this outline is largely the work of dr. l. j. mason and dr. c. h. morse of the department of biology of the de witt clinton high school. weights, measures, and temperatures as the metric system of weights and measures and the centigrade measurement of temperatures are employed in scientific work, the following tables showing the english equivalents of those in most frequent use are given for the convenience of those not already familiar with these standards. the values given are approximate only, but will answer for all practical purposes. weight =================================== kilogram | kg. | - / pounds ----------+-----+------------------ | | - / grains | | avoirdupois. | | / of an ounce gram | gm. | avoirdupois. ----------------------------------- capacity ----------------------------------- | | cubic inches, | | or a little more | | than quart, liter | l. | u. s. measure. ----------+-----+------------------ cubic | | cc. / of a centimeter| | cubic inch. =================================== measures of length ================================== metric |english equivalents ----------+---+------------------- kilometer |km.| / of a mile. | | ----------+---+------------------- meter | m.| inches. | | ----------+---+------------------- decimeter |dm.| inches. | | ----------+---+------------------- centimeter|cm.| / of an inch. | | ----------+---+------------------- millimeter|mm.| / of an inch. | | ================================== the next table gives the fahrenheit equivalent for every tenth degree centigrade from absolute zero to the boiling point of water. to find the corresponding f. for any degree c., multiply the given c. temperature by nine, divide by five, and add thirty-two. conversely, to change f. to c. equivalent, subtract thirty-two, multiply by five, and divide by nine. cent. fahr. ------------- - - - - - - - - - - - ------------- absolute zero - - laboratory equipment the following articles comprise a simple equipment for a laboratory class of ten. the equipment for larger classes is proportionately less in price. the following articles may be obtained from any reliable dealer in laboratory supplies, such as the bausch and lomb optical company of rochester, n.y., or the kny-scheerer company, , west th street, new york city:-- balance, harvard trip style, with weights on carrier. bell jar, about mm. high by mm. in diameter. wide mouth (salt mouth) bottles, with corks to fit. c.c. dropping bottles for iodine, etc. c.c. glass-stoppered bottles for stock solutions. test tubes, assorted sizes, principally " × / ". test tubes on base (excellent for demonstrations). graduated cylinders, one to c.c., one to c.c. package filter paper mm. in diameter. flasks, erlenmeyer form, c.c. capacity. glass funnels, one , one mm. in diameter. petri dishes, mm. in diameter, mm. in depth. feet glass tubing, soft, sizes , , , , , assorted. aquarium jar, liters capacity. specimen jars, glass tops, of about liter capacity. hand magnifiers, vulcanite or tripod form. compound demonstration microscopes or more expensive compound microscope. insect pins, klaeger, sizes assorted. feet rubber tubing to fit glass tubing, size / inch. chemical thermometer graduated to ° c. agate ware or tin trays about mm. long by wide. gal. per cent alcohol. (do not use denatured alcohol.) set gram weights, mg. to g. razor, for cutting sections. box rubber bands, assorted sizes. support stand with rings. books test paper, red and blue. syracuse watch glasses. steam sterilizer (tin will do). spool fine copper wire. test tube rack. test tube brushes. pairs scissors. pairs forceps. needles in handles. scalpels. mason jars, pints. mason jars, quarts. alcohol lamp. gross slides. cover slips no. . mortar and pestle. bulb pipettes. liter formol. oz. iodine cryst. oz. potassium iodide. oz. nitric acid. oz. ammonium hydrate. oz. benzole or xylol. oz. chloroform. / lb. copper sulphate. / lb. sodium hydroxide. / lb. rochelle salts. oz. glycerine. the materials for pasteur's solution and sach's nutrient solution can best be obtained from a druggist at the time needed and in very small and accurately measured quantities. the agar or gelatine cultures in petri dishes may be obtained from the local board of health or from any good druggist. these cultures are not difficult to make, but take a number of hours' consecutive work, often difficult for the average teacher to obtain. full directions how to prepare these cultures will be found in hunter's _laboratory problems in civic biology_. index (illustrations are indicated by page numerals in bold-faced type.) absorption, definition, ; of digested foods, , . accommodation of eye, . acetanilid, . action of the heart, . adaptations, ; in bee, ; in birds, | |; in fish, ; in frog, ; in mammalia, . adenoids, , . adulteration in foods, . air, and bacteria, ; composition of, | |; fresh, ; needed in germination, | |; necessary in starch making, ; passages in lungs, ; use to plants and animals, . albumin, . alcohol, a food, ; a poison, . and ability to resist disease, ; and ability to work, ; and body heat, ; and crime, , | |; and digestion, ; and duration of life, ; and efficiency, ; and heredity, ; and intellectual ability, ; and kidneys, ; and living matter, ; and memory, | |; and mental ability, | |; and nervous system, ; and organs of special sense, ; and pauperism, ; and resistance, ; and respiration, ; and the blood, ; and treatment of disease, ; effect on circulation, ; effect on eye, ; effect on liver, ; produces poisons, . algæ, . alfalfa plant, | |. alimentary canal, . alkali, . alkalinity, . alligator, | |. ambergris, . ammonium hydrate, . amoeba, | |, , . amphibia, , ; as food, . anal fin of fish, . angiosperms, . animals, as disease carriers, ; breeding of, ; domesticated, ; functions of, , ; need plants, ; oils of, ; parasitic, ; series, ; that prey upon man, ; use to man, ; use to plants, . annual rings, . anopheles, , | |. anosia plexippus, . anther, . antibodies, uses of, . antiseptics, . antitoxin, , | |. anura, . anvil, . aorta, . apoplexy, . appendages of the fish, . appendicular skeleton, . appendix, . apples, , | |. aqueous humor, . arachnida, . arteries, ; structure of, . arthropods, . artificial, cross-pollination, ; propagation of fishes, ; respiration, ; selection, . asexual reproduction, . assimilation in plants, . attention, effect of alcohol, . audubon, . auricle of human heart, ; of fish heart, . automatic activity, , . axial skeleton, . bacillus, . bacteria, ; and fermentation, ; cause decay, ; cause disease, ; effect on food, ; growth of, ; isolating a pure culture, ; nitrogen fixing, , | |, , | |; of decay, ; relation to man, ; size and form, , | |; useful, ; where found, , | |. bacteriology, . bad posture, | |. balanced, aquarium, , | |; diet, . barbels of fish, . barberry embryo, | |. bark, use of, . barrier, natural, | |. bast, . beans, as food, . beans, peas, . beans, seedlings, | |. bedroom, care of, . bee, adaptations, ; head of, | |; mouth parts, . beer and wine making, . benedict's test, . benzoic acid, . beverages and condiments, . biceps, | |. bichloride of mercury, . bile, functions of, , . biology, definition, ; relation to society, . birds, ; as food, ; classification, ; development, ; eat insects, ; eat weed seeds, ; embryo, | |, . bismuth, . bison, | |. black death, . blade of leaf, . blastula, . blood, amount and distribution, ; changes in lungs, ; circulation of man, ; clotting, ; composition, | |; effect of alcohol, ; function, ; plates, ; poisoning, ; temperature, ; vessel of skin, | |. blubber, . blue crab, | |. board of health, functions, . body, a machine, ; cavity, ; heat and alcohol, ; of fish, . bony fish, | |. boracic acid, . borax, . brain, of fish, ; of man, . bread, making, ; mold, | |. bream, | |. breathing, | |; and tight clothing, ; hygienic habits, ; in leaf, | |; of fish, ; of frog, ; of vertebrates, ; rate of, . breeding of animals, . bright's disease, . bronchi, . bronchial tubes, . bruises, . bryophytes, . bubonic plague, . budding, , | |. bumblebees, . burbank, luther, . burns, treatment of, . butter and eggs, , | |. calorie, portion, | |; requirement, . calyx, . cambium layer, . canning, . cannon, prof., . capillaries, , ; circulation in, | |; of fish, . carbohydrates, , . carbolic acid, . carbon and oxygen cycle, | |. carbon dioxide, test for, | |. care of milk supply, , . carnivorous, . caudal fin of fish, . cause of dyspepsia, . cells, | |; as units, ; division, | |; mucous, ; of pond scum, | |; reproduction of, ; respiration, ; tissue, | |; work of, . cephalothorax, . cerebellum, . cerebro-spinal nervous system, . cerebrum, . cestodes, . changes, of blood in lungs, ; of air in lungs, . characters, determiners of, . chelonia, . chemical, compounds, ; elements, ; of human body, | |. chestnut canker, | |. china, deforestation in, | |. chittenden table, | |. chloral, . chlorophyll bodies, , . chloroplasts, . chromosomes, ; and heredity, . chrysalis, . cilia, . circulation, effect of alcohol, ; effect of exercise, ; effect of tobacco, ; in fish, frog, man, | |, | |; in stem, , | |, | |; of blood of man, ; of fish, ; of frog, ; portal, ; pulmonary, ; systemic, . city's need for trees, . civic hygiene, . clams, . classification, of birds, ; of plants, . cloaca of frog, . clothing, . clotting of blood, . coal, . cobra, . cocaine, . coccus bacteria, . cochineal and lac, . cochlea, . codling moth, . coelenterates, . cold-blooded animals, ; effect of, | |. cold storage, . colds and fevers, . coleoptera, . collecting ashes, | |. colonies of bacteria, ; of trilliums, | |. colorless corpuscles, ; structure, ; function, | |. common foods contain nutrients, . comparison, of food tube of frog and man, | |; of mold, yeast and bacteria, | |; of starch making and milling, . complemental air, . complex one-celled animals, . composition, of milk, | |, | |; of plasma, ; of soil, . compound eyes of bumblebee, , | |. conservation, of food fish, ; of fur-bearing animals, ; of our natural resources, . constipation, . constrictor killing a mouse, | |. contagious diseases, . convolutions, . corn, , | |; germinated grain cut lengthwise, | |; long section of ear, | |; structure of grain, | |. cornfield, | |. corolla, . corpuscles, colorless and red, . cost of food and diet, , | |; of parasitism, . cotton, ; boll weevil, , | |, | |. cotyledons, ; food in, . crab, | |. crayfish, | |. crocodile, . crocodilia, . crustacea, . culex, , | |. culture medium, . cuts and bruises, treatment, , . daily calorie requirement, ; fuel needs of body, . dandelion, whorled leaves, | |. darwin, charles, , . darwin and natural selection, . deaths, table, | |. decay caused by bacteria, . decayed teeth, . defects in eye, . deforestation in china, | |. dendrites, . department of agriculture, work of, . department of street cleaning, . determiners, ; of character, . development, of apple, ; of bird, ; of egg, | |; of trout, | |; of mammal, ; of salmon, | |; of simple animal, . diagram of frog's tongue, | |; of gills of fish, | |; of neuron, | |; of wall small intestine, | |. diaphragm, , . diastase, , ; action on starch, . diet, and cost of food, ; and digestibility, ; balanced, ; relation of age, ; relation of environment to, ; relation to sex, ; relation of work to, ; the best, . dietary, the best, . digested food, absorption of, . digestibility and diet, . digestion, , , ; effect of alcohol, ; definition of, ; in stem, ; in stomach, ; of starch, ; purpose of, , . digestive system of fish, . digestive tract of frog and man, | |. diphtheria, . dipnoi, , . diptera, . discoverers of living matter, . disease, and alcohol, | |; and bacteria, ; carriers, animals, ; carriers, flies, ; carriers, insects, ; caused by bacteria, ; caused by protozoa, ; effect of alcohol, ; of nose and throat, ; protozoan, . disinfectants, . division of labor, , . dog, skeleton, | |. domesticated animals, , . dominant characters, . dormant, . dorsal, ; fin, . drugs, use and abuse, . duff, . dyspepsia, cause and prevention, . ear, section, | |. echinoderms, . economic value of green plants, ; importance of spawning habits of fishes, . ectoderm, . effect of light on leaves, . efficiency of a week, | |. egg, , . egg-laying habits of fishes, . ehrlich, paul, . elasmobranchs, . elements, chemical, , | |. elodea, | |, . embryo, , , | |; of bird, ; of mammal, | |. emulsion, . endoderm, . endoskeleton, definition, . endosperm, . enemies of forests, , | |. energy, ; of a tree, | |; source of, . english sparrow, . environment, | |, ; care and improvement of, ; changes in, ; determines kind of plants and animals, , | |, | |; normal, ; of man, , ; natural, ; relation to diet, ; what plants and animals take from, . enzymes, , , . epicotyl, . epidermis, . epithelial layer, . epithelium, . erosion, prevention of, | |, ; at sayre, pa., | |. essential organs, . esophagus, . eugenics, . eustachian tubes, , . euthenics, . evaporation, | |; of water, , , | |. evolution, , . excretion, , , ; organs of, ; in plants, . exercise and circulation, ; and health, . exoskeleton, , . extermination of birds, . eye, compound, | |; defects in, ; section of, | |. eyestrain, . factory inspection, . fallowing, . fatigue, ; and nerve cells, | |. fats and oils, , . fehling's solution, , . fermentation, , | |, . fertilization, of fish eggs, | |; of flower, . fibers, vegetable, . fibrin, . fibrinogen, . fig insect, . filament, . filter beds at albany, n. y., | |. fins, . fishes, ; artificial propagation, ; as food, ; body of, ; breathing, ; circulation, , | |; digestive system, ; egg-laying habits, ; food getting, ; food of, ; gills, | |; heart, ; migration, ; nervous system, ; skeleton, ; senses, ; swim bladder, . fission, . flagella of bacteria, . flatworms, . flax, | |. flea, | |. floral envelope, . flower, fertilization of, ; lengthwise section, | |; use and structure, . fluid, . fly, a disease carrier, ; foot of, | |; life history, | |; typhoid, . foods, absorption of, ; adulteration, ; amphibia as, ; birds as, ; cost of, | |; fish as, ; fruits and seeds, ; getting of fish, ; in cotyledons, ; inorganic, ; inspection, ; is alcohol a food, ; leaves, , | |; making in green leaf, | |; mammals as, ; of animal origin, | |; of bacteria, ; of fishes, ; of insects, ; of plant origin, | |; of starfish, ; reptiles as, ; roots as, ; stems as, ; taking, ; tube of frog, | |; values, tables, | |; waste in kitchen, ; why we need, . foraminifera, . forestry, . forest destruction, , | |; fires, ; of north carolina, | |; other uses, ; protecting, ; regions of united states, | |. formaldehyde, . formation of habits, . four o'clock embryo, | |. fresh air, | |. frog, adaptations for life, ; and man, digestive tract, | |; breathing, ; circulation, , ; development of, ; diagram of tongue, | |; food tube, ; glands, ; locomotion of, ; long section, | |; metamorphosis, | |; nervous system, ; sense organs, . fruit, a typical, . fruit of locust, | |. fruits and seeds as foods, . fruits, how scattered, . fuel, daily needs, . fuel values of nutrients, . functions, of all animals, ; of an animal, ; of bile, ; of blood, ; of cerebrum, ; of colorless corpuscle, | |; of lymph, ; of parts of plant, ; of red corpuscle, . fungi, , ; moldlike, ; of our homes, . fur-bearing animals, . gall bladder, ; insects, . gallflies, . ganoids, | |, . garbage cans, | |. garden fruits, . gastric glands, ; of frog, . gastric juice, . gastrula, , | |. genus, . geranium, | |. german forest, | |. germ cells, . germination, of bean, ; of pollen, | |. gills of fish, ; rakers, , . glands, , , ; gastric, ; lymph, ; of frog, ; salivary, . glomerulus, | |. glottis of frog, . glycogen, . gonorrhea, . grafting, | |. grains, . grape sugar, test for, | |. gravity, influence on root, . green plants, economic value, ; give off oxygen, ; harmful, ; make starch, , | |. groups of plants, . guano, . guard cells, . gullet, , , , , | |; of frog, . gymnosperms, . gypsy moth, . habits, . habitat of protozoa, . habit formation, . hæmoglobin, , . hammer, . hard palate, . harm done by insects, | |, . harmful green plants, ; preservatives, . hay infusion, , | |. head of a bee, | |. heart a force pump, | |; diagram, | |; in action, ; internal structure, ; of fish, ; size, position, . heat, and bacteria, ; effect of, ; output, . heating the house, . hemiptera, . hen's egg, | |. herbivorous animals, . heredity, and evolution, ; bearers of, ; definition, ; relation of alcohol to, . hervey, william, . hibernate, . hides, . hilum, . honey and wax, . hookworm, , , | |. horse, ancestor of, | |, | |. how food is swallowed, . human blood, | |. human body, a machine, ; composition of, | |. human physiology, definition, . humming bird, | |. humus, . hundred calorie portions, | |. huxley, . hybridizing, . hybrids, . hydra, | |. hydrochloric acid, . hydrogen of water, | |, . hydrophobia, . hygiene, ; of breathing, ; of skin, ; of mouth, ; of muscles and bones, ; outline, ; personal, . hypocotyl, . hymenoptera, . ichneumon fly, | |. illness of drinkers, | |. imperfect flowers, , | |. immunity, , . improvement, by selection, | |; of man, . impure water, . incisors, | |. infectious diseases, , , . infusoria, . inner ear, . inoculation, . inorganic soil, | |; foods, . insects, ; and foods, ; as disease carriers, ; as pollinating agents, ; damage done by, | |, ; diagram of, | |; food of, ; of the house, ; orders of, . inspection, of factories, ; of raw food, . instincts, . internal secretions, . intestinal fluid, ; glands, . intestine, large, . invertebrates, . iris, . isolation, . jenner, edward, | |. jimson weed, . jukes, . kidney bean, | |, | |. kidneys, ; human, | |; of frog, . kinetic energy, . knots, . koch, robert, | |. labor, division of, . laboratory equipment, . lacteals, , . lactic acid, . lactometer, . ladybug, . large intestine, ; of frog, . larva of milkweed butterfly, . latent energy, . lateral line, . leaves, as food, ; evaporation of water from, ; cell structure of, ; mosaic, ; respiration, ; section, | |; skeleton of, | |; structure, , | |. length measures, . leopard frog, | |. lepidoptera, . levers, | |. life comes from life, . life cycle, ; of plants, . life history of malarial parasite, . ligaments, . ligature, applying, | |. light, a condition of environment, , | |; and bacteria, ; effect of, | |; necessary for starch making, . lighting the home, . lily, narrow leaves, | |. limewater test, | |. lister, sir joseph, . liver, ; a storehouse, ; effect of alcohol on, ; of frog, . living matter and alcohol, ; plant and animal compared, ; things, needs of, ; things, varying sizes of, . lizard, | |. lobster, | |. locomotion, ; of frog, . lowell, typhoid area, | |. lumber transporting, | - |. lungs, air passages, ; changes of blood in, . lymph, function, ; glands and vessels, , | |. lysol, . macnichol, dr. t. alexander, . macronucleus, . malaria, cause, . malarial mosquito, | |. malarial parasite, life history, . mammal development, ; embryo, | |. mammals, ; adaptations, ; as food, ; classification, . mammary glands, . man, animals that prey upon, ; and his environment, ; circulation of blood, ; improvement of, ; in his environment, ; mouth cavity, ; place in nature, ; races of, ; stomach, . manufacture of fats, . measures, . mechanics of respiration, , | |. membrane, mucous, . mendel, gregor, , . mesenteric glands, . mesentery, . mesoderm, . metamorphosis of frog, , | |. metchnikoff, . methods, of cutting timber, ; of breathing in vertebrates, . micronucleus, . micropyle, . middle ear, | |. migration of fishes, . milk, and tuberculosis, ; composition of, | |, | |; germs in, ; grades of, ; under microscope, | |, | |. milkweed, butterfly, , | |. milling and starch making, . mink, . mixed diet, . moisture, | |, | |. mollusca, . mollusk, | |. mold, , , ; yeast and bacteria, | |. morning glory embryo, | |. mosquito, malarial, | |; yellow fever, . moss plant, | |. mother of pearl, | |. motor nerves, . mouth cavity in man, , | |. mouth parts of bee, . mucous membrane, . mucus cells, . muscles and bones, hygiene, . mutations, , . mutual aid between flowers and insects, . mycelium, . myriapoda, . natural environment, ; selection, . nectar, . need, of food, ; of sleep, ; of ventilation, . needs of living things, . nerve cells and fatigue, ; vasomotor, . nervous control, ; of heart, ; of respiration, ; of sweat glands, . nervous system, , | |; of frog, . neuron, diagram, | |. newt, | |. nicotine, . nictitating membrane of frog, . nitrates, . nitric acid, . nitrogen, ; cycle, | |; fixing bacteria, , | |, ; of air, | |. nodules, . normal heat output, . nose and throat, diseases, . nucleus, . nutrients, , ; fuel values, ; in common foods, . object of a field trip, . oils, test for, . operculum, . ophidia, . orbit of eye, . orchard fruits, . organic matter, . organic nutrients, . organisms, . organs, , , ; of corti, ; of excretion, ; of hearing, ; of respiration, ; of taste, ; of touch, . orthoptera, . osmosis, definition, ; experiment, | |; physiological importance, . ostrich, | |. outline of courses, - . ovaries of frog, . ovary, . ovules, . oxidation, ; in our bodies, . oxygen cycle, | |; given off by green plants, ; of air, ; of water, | |. oyster, | |, . packard (zoölogist), . palate, hard and soft, . palisade tissue, . pancreas, ; of frog, ; work of, . papillæ, . pappus, . paramoecium, , | |, | |; needs of, ; response to stimuli, . parasites, . parasitic animals cause disease, . parasitism, cost and remedy, . parotid, . pasteur, louis, | |. pasteurization, . pea pod, . pearls, . pectoral fin, . pelvic fin, . pepsin, . peptic gland, | |. perfumes, . pericardium, . peristaltic waves, | |. personal hygiene, . perspiration, . petals, . petri dishes, . phagocytes, . pharynx, . phenolphthalein, . phosphoric acid, . photosynthesis, | |, . physiology of mold, . pistil, . pith, . placentæ of mammal, . plankton, . plants, animals depend on, ; and animals, mutually helpful, ; classification, ; food for insects, ; as food makers, ; function of parts, ; groups, ; need minerals, ; need of nitrogen, , | |; processes, ; reproduction, . plasma, . plasmodium malariæ, , . pleura, . pleurococcus, | |. plumule, . pneumonia, . pocket garden, . poison, alcohol, ; ivy, | |; produced by alcohol, . polar bear, | |. pollen, ; germination of, , | |. pollination, , ; cross and self, ; wind, . pond scum, | |. pons, . porifera, . portal circulation, , . portions, hundred calorie, | |. potato beetle, . potato beetle, embryo, | |. premolars, . preservatives, . prevention of dyspepsia, ; of molds, . proboscis, . prolegs, . pronuba, | |, | |. protecting forests, . proteins, , ; making, ; test for, | |. protoplasm, ; what it can do, . protozoa, , , . protozoan diseases, . pteridophytes, . ptomaines, , . ptyalin, . public hygiene, . pulmonary circulation, . pulse, cause, . pupa of milkweed butterfly, | |. pupil of eye, . pure food laws, . purpose of digestion, , . pyloric cæca, . quarantine, , . rabies, . races of man, . radiolaria, . radiolarian skeleton, | |. recessive characters, . rectum, . red corpuscles, , . reflex actions, . regulation of heat of body, . relation, of age to diet, ; of alcohol to crime, ; of alcohol to heredity, ; of alcohol to pauperism, ; of animals to man, ; of bacteria to free nitrogen, ; of bacteria to man, ; of biology to society, ; of cost of food to diet, ; of digestibility to diet, ; of environment to diet, ; of green plants and animals, , , | |; of sex to diet, ; of work to diet, ; of yeasts to man, . rennin, . reproduction, , ; importance of, ; in seed plants, , ; of cells, ; of paramoecium, . reptiles, . reptilia, . reserve air, . residual air, . respiration, , ; and alcohol, ; and nervous control, ; and tobacco, ; mechanics of, , | |; necessity for, ; organs of, ; of cells, | |; of leaves, . retina, . rhizoids, . rhizopoda, . rice field, | |. ringworm, . roaches, . rock fern, | |. rockweed, | |. roots as food, ; as food storage, ; downward growth of, ; fine structure, ; give out acid, , | |; hairs, , | |; influence of gravity, ; influence of moisture, ; passage of soil water, ; pressure, ; system, primary, secondary, tertiary roots, ; uses of, . rotation of crops, . roundworms, , . rules of habit formation, . russian thistle, . saliva, , . salivary glands, ; glands of frog, . salmon, | |, | |. sand shark, | |. sandworm, | |. sanitarium for tuberculosis, | |. sanitation, . saprophytes, . scavengers, . schleiden and schwann, . schultz, max, . sclerotic coat, . sea anemones, | |. secretion, , . secretions, internal, . section, of ear, | |; of timber, | |. sedgwick, william t., . seed, ; how scattered, ; plants, reproduction, | |; why it grows, . seedlings of bean, | |. segmented worms, . selection, artificial, ; natural, . selective planting, . semicircular canal, . sensations, . sense organs, ; of fish, ; of frog, . senses, . sensory nerves, . sepals, . series, animal, . serum, . sewage disposal, | |. sex, relation to diet, . shelf fungi, | |. sieve tubes, . simple animal, development, . simplest plants, . skeleton, of dog, | |; of fish, ; of leaf, | |; of man, | |. skin, ; hygiene of, . skunk, . sleep, need of, . small intestine, | |, . smell, sense of, . snail, | |. snakes, | |; food of, . soft palate, . soil, composition of, ; how water is held in, , | |. sound, character of, . sour bread, . soy beans, | |. sparrow, | |. spawning habits, economic importance, . species, , . sperm, . spermaries of frog, . spermatophytes, . spinal cord of fish, . spiracles, . spirillum, . sponge, | |, , | |, . spore, , ; plants, . sporozoa, . sprengel, conrad, . squash bug, . stables, clean and filthy, | |. stamens, . starch, action of diastase, ; digestion, ; grains, | |; in bean, ; made by green leaves, , | |; test for, | |. starch making and milling, . starfish, | |; food of, . stegomyia, | |. stems, as food, ; passage of fluids up, | |; structure of, . sterilization, . sterilizer, | |. stigma, . stimulants, . stirrup, . stomata, , | |. stomach, ; digestive experiments, ; of frog, ; of man, . street cleaning department, . structure, colorless corpuscles, ; of leaf, ; of red corpuscle, ; of root, ; of root hairs, . sturgeon, | |. style, . sublingual glands, . submaxillary glands, . suffocation, . sulphur, . sun, source of energy, . sundew, . sunlight in home, . sweat glands, | |. sweeping and dusting, . swim bladder of fish, . symbiosis, . sympathetic nerves, ; nervous system, , . syphilis, , . systemic circulation, . table of cost of food, | |, | |. tactile corpuscles, . tænia solium, . tapeworm, | |. taproot, cross section, | |. taste buds, , . teeth, | |. teleosts, . temperature, ; of blood, . tern, | |. testa, . test, for carbon dioxide, ; nutrients, , . thallophytes, . thoracic duct, . tidal air, . timber, methods of cutting, . tissue cells, , | |. toad, use of, . tobacco and circulation, ; and respiration, ; users of, . tortoise, | |. touch, . tourniquet, . toxin, , . trachea, . transpiration, | |, . transportation of lumber, | |, | |. treatment of cuts and bruises, . trees, need of city, ; preventing erosion, ; regulate water supply, ; value of, . trichina, | |. trichinosis, . trillium, | |. trout, development, | |. trypanosomes, . tuberculosis, , | |; and milk, ; how to fight, , | |. tussock moth, | |. twig, section of, | |. tympanic membrane, . tympanum of frog, . tyndall box, | |. typhoid, | |, | |; and diarrhoea, | |. typhoid fever, , | |, | |. unit characters, . ureter, . urethra, . urine, . urodela, . uses, of animals, ; of antibodies, ; of green plants, ; of ice, ; of nutrients, ; of protozoa, . uterus of a mammal, | |. vaccination, , , . vacuoles, contracting, . value, of insects, ; of trees, . valves, , ; in vein, | |. variation, . vasomotor nerves, . vegetable fibers and oils, . veins, ; function and structure, ; valves, | |. venæ cavæ, . ventilation, , . ventricle, ; of fish heart, . venus fly trap, . vermiform appendix, . vertebral column, . vertebrates, breathing of, . villi, . virginia creeper, . virus, . vitreous humor, . vorticella, | |, . vries, hugo de, , . warner, chas. dudley, . waste of food, . water, ; composition of, | |; impure, ; supply, . weed, | |, . weights, . wheat crop, , | |. wild orchid, | |. windpipe, , . wood, uses of, . work of cells, ; of department of agriculture, ; relation to diet, . worms, . yeasts, , , ; relation to man, . yellow fever mosquito, . yucca, , | |. zygospore, | |. transcriber's notes: punctuation, use of hyphens, and accent marks were standardized. obsolete and alternative spellings were left unchanged. spelling of 'paramoecium' and 'paramoecia' were left unchanged. braces are used to indicate subscripted numbers in chemical formulas, e.g. co{ }. pipes (|) are used to designate bold numbers in the index. footnotes were moved to the end of the paragraph in which the anchor occurs. footnote anchors [ ] and [ ] refer to the same footnote, as do anchors [ ] and [ ]. greek letters in the chart in chapter xx are spelled within brackets: [alpha] and [beta]. the following changes were made for consistency within the text: 'zoological' to 'zoölogical' 'diarrhea' to 'diarrhoea' other changes: 'proteid' to 'protein' in the header of the table in chapter v 'do' added to header to chapter vii ... how do green plants make food?... 's arpe' to 'sharpe' in the reference books list at the end of chapter xiv 'yoke' to 'yolk' ... (which is the yolk or yellow portion) ... 'does' to 'do' ... both ... do the same thing ... 'page ' to 'page ,' in reference to lacteals. 'centers' to 'center's' ... the nervous center's controlling the blood ... 'scapels' to 'scalpels' in the appendix list of laboratory equipment 'and' added to '... pasteur's solution and sach's nutrient solution ...' 'scavangers' to 'scavengers' in the index richard prairie and pg distributed proofreaders discourses: biological & geological essays by thomas h. huxley preface the contents of the present volume, with three exceptions, are either popular lectures, or addresses delivered to scientific bodies with which i have been officially connected. i am not sure which gave me the more trouble. for i have not been one of those fortunate persons who are able to regard a popular lecture as a mere _hors d'oeuvre_, unworthy of being ranked among the serious efforts of a philosopher; and who keep their fame as scientific hierophants unsullied by attempts--at least of the successful sort--to be understanded of the people. on the contrary, i found that the task of putting the truths learned in the field, the laboratory and the museum, into language which, without bating a jot of scientific accuracy shall be generally intelligible, taxed such scientific and literary faculty as i possessed to the uttermost; indeed my experience has furnished me with no better corrective of the tendency to scholastic pedantry which besets all those who are absorbed in pursuits remote from the common ways of men, and become habituated to think and speak in the technical dialect of their own little world, as if there were no other. if the popular lecture thus, as i believe, finds one moiety of its justification in the self-discipline of the lecturer, it surely finds the other half in its effect on the auditory. for though various sadly comical experiences of the results of my own efforts have led me to entertain a very moderate estimate of the purely intellectual value of lectures; though i venture to doubt if more than one in ten of an average audience carries away an accurate notion of what the speaker has been driving at; yet is that not equally true of the oratory of the hustings, of the house of commons, and even of the pulpit? yet the children of this world are wise in their generation; and both the politician and the priest are justified by results. the living voice has an influence over human action altogether independent of the intellectual worth of that which it utters. many years ago, i was a guest at a great city dinner. a famous orator, endowed with a voice of rare flexibility and power; a born actor, ranging with ease through every part, from refined comedy to tragic unction, was called upon to reply to a toast. the orator was a very busy man, a charming conversationalist and by no means despised a good dinner; and, i imagine, rose without having given a thought to what he was going to say. the rhythmic roll of sound was admirable, the gestures perfect, the earnestness impressive; nothing was lacking save sense and, occasionally, grammar. when the speaker sat down the applause was terrific and one of my neighbours was especially enthusiastic. so when he had quieted down, i asked him what the orator had said. and he could not tell me. that sagacious person john wesley, is reported to have replied to some one who questioned the propriety of his adaptation of sacred words to extremely secular airs, that he did not see why the devil should be left in possession of all the best tunes. and i do not see why science should not turn to account the peculiarities of human nature thus exploited by other agencies: all the more because science, by the nature of its being, cannot desire to stir the passions, or profit by the weaknesses, of human nature. the most zealous of popular lecturers can aim at nothing more than the awakening of a sympathy for abstract truth, in those who do not really follow his arguments; and of a desire to know more and better in the few who do. at the same time it must be admitted that the popularization of science, whether by lecture or essay, has its drawbacks. success in this department has its perils for those who succeed. the "people who fail" take their revenge, as we have recently had occasion to observe, by ignoring all the rest of a man's work and glibly labelling him a more popularizer. if the falsehood were not too glaring, they would say the same of faraday and helmholtz and kelvin. on the other hand, of the affliction caused by persons who think that what they have picked up from popular exposition qualifies them for discussing the great problems of science, it may be said, as the radical toast said of the power of the crown in bygone days, that it "has increased, is increasing, and ought to be diminished." the oddities of "english as she is spoke" might be abundantly paralleled by those of "science as she is misunderstood" in the sermon, the novel, and the leading article; and a collection of the grotesque travesties of scientific conceptions, in the shape of essays on such trifles as "the nature of life" and the "origin of all things," which reach me, from time to time, might well be bound up with them. the tenth essay in this volume unfortunately brought me, i will not say into collision, but into a position of critical remonstrance with regard to some charges of physical heterodoxy, brought by my distinguished friend lord kelvin, against british geology. as president of the geological society of london at that time ( ), i thought i might venture to plead that we were not such heretics as we seemed to be; and that, even if we were, recantation would not affect the question of evolution. i am glad to see that lord kelvin has just reprinted his reply to my plea,[ ] and i refer the reader to it. i shall not presume to question anything, that on such ripe consideration, lord kelvin has to say upon the physical problems involved. but i may remark that no one can have asserted more strongly than i have done, the necessity of looking to physics and mathematics, for help in regard to the earliest history of the globe. (see pp. and of this volume.) [footnote : _popular lectures and addresses._ ii. macmillan and co. .] and i take the opportunity of repeating the opinion, that, whether what we call geological time has the lower limit assigned to it by lord kelvin, or the higher assumed by other philosophers; whether the germs of all living things have originated in the globe itself, or whether they have been imported on, or in, meteorites from without, the problem of the origin of those successive faunae and florae of the earth, the existence of which is fully demonstrated by paleontology remains exactly where it was. for i think it will be admitted, that the germs brought to us by meteorites, if any, were not ova of elephants, nor of crocodiles; not cocoa-nuts nor acorns; not even eggs of shell-fish and corals; but only those of the lowest forms of animal and vegetable life. therefore, since it is proved that, from a very remote epoch of geological time, the earth has been peopled by a continual succession of the higher forms of animals and plants, these either must have been created, or they have arisen by evolution. and in respect of certain groups of animals, the well- established facts of paleontology leave no rational doubt that they arose by the latter method. in the second place, there are no data whatever, which justify the biologist in assigning any, even approximately definite, period of time, either long or short, to the evolution of one species from another by the process of variation and selection. in the ninth of the following essays, i have taken pains to prove that the change of animals has gone on at very different rates in different groups of living beings; that some types have persisted with little change from the paleozoic epoch till now, while others have changed rapidly within the limits of an epoch. in (see below p. , ) in (vol. ii., p. ) and again in (ibid., p. - ) i argued, not as a matter of speculation, but, from paleontological facts, the bearing of which i believe, up to that time, had not been shown, that any adequate hypothesis of the causes of evolution must be consistent with progression, stationariness and retrogression, of the same type at different epochs; of different types in the same epoch; and that darwin's hypothesis fulfilled these conditions. according to that hypothesis, two factors are at work, variation and selection. next to nothing is known of the causes of the former process; nothing whatever of the time required for the production of a certain amount of deviation from the existing type. and, as respects selection, which operates by extinguishing all but a small minority of variations, we have not the slightest means of estimating the rapidity with which it does its work. all that we are justified in saying is that the rate at which it takes place may vary almost indefinitely. if the famous paint- root of florida, which kills white pigs but not black ones, were abundant and certain in its action, black pigs might be substituted for white in the course of two or three years. if, on the other hand, it was rare and uncertain in action, the white pigs might linger on for centuries. t.h. huxley. hodeslea, eastbourne, _april, ._ contents i on a piece of chalk [ ] (a lecture delivered to the working men of norwich during the meeting of the british association.) ii the problems of the deep sea [ ] iii on some of the results of the expedition of h.m.s. "challenger" [ ] iv yeast [ ] v on the formation of coal [ ] (a lecture delivered at the philosophical institute, bradford.) vi on the border territory between the animal and the vegetable kingdoms [ ] (a friday evening lecture delivered at the royal institution.) vii a lobster; or, the study of zoology [ ] (a lecture delivered at the south kensington museum.) viii biogenesis and abiogenesis [ ] (the presidential address to the meeting of the british association for the advancement of science at liverpool.) ix geological contemporaneity and persistent types of life [ ] (address to the geological society on behalf of the president by one of the secretaries.) x geological reform [ ] (presidential address to the geological society.) xi palaeontology and the doctrine of evolution [ ] (presidential address to the geological society.) i on a piece of chalk [ ] if a well were sunk at our feet in the midst of the city of norwich, the diggers would very soon find themselves at work in that white substance almost too soft to be called rock, with which we are all familiar as "chalk." not only here, but over the whole county of norfolk, the well-sinker might carry his shaft down many hundred feet without coming to the end of the chalk; and, on the sea-coast, where the waves have pared away the face of the land which breasts them, the scarped faces of the high cliffs are often wholly formed of the same material. northward, the chalk may be followed as far as yorkshire; on the south coast it appears abruptly in the picturesque western bays of dorset, and breaks into the needles of the isle of wight; while on the shores of kent it supplies that long line of white cliffs to which england owes her name of albion. were the thin soil which covers it all washed away, a curved band of white chalk, here broader, and there narrower, might be followed diagonally across england from lulworth in dorset, to flamborough head in yorkshire--a distance of over miles as the crow flies. from this band to the north sea, on the east, and the channel, on the south, the chalk is largely hidden by other deposits; but, except in the weald of kent and sussex, it enters into the very foundation of all the south-eastern counties. attaining, as it does in some places, a thickness of more than a thousand feet, the english chalk must be admitted to be a mass of considerable magnitude. nevertheless, it covers but an insignificant portion of the whole area occupied by the chalk formation of the globe, much of which has the same general characters as ours, and is found in detached patches, some less, and others more extensive, than the english. chalk occurs in north-west ireland; it stretches over a large part of france,-- the chalk which underlies paris being, in fact, a continuation of that of the london basin; it runs through denmark and central europe, and extends southward to north africa; while eastward, it appears in the crimea and in syria, and may be traced as far as the shores of the sea of aral, in central asia. if all the points at which true chalk occurs were circumscribed, they would lie within an irregular oval about , miles in long diameter--the area of which would be as great as that of europe, and would many times exceed that of the largest existing inland sea--the mediterranean. thus the chalk is no unimportant element in the masonry of the earth's crust, and it impresses a peculiar stamp, varying with the conditions to which it is exposed, on the scenery of the districts in which it occurs. the undulating downs and rounded coombs, covered with sweet-grassed turf, of our inland chalk country, have a peacefully domestic and mutton- suggesting prettiness, but can hardly be called either grand or beautiful. but on our southern coasts, the wall-sided cliffs, many hundred feet high, with vast needles and pinnacles standing out in the sea, sharp and solitary enough to serve as perches for the wary cormorant, confer a wonderful beauty and grandeur upon the chalk headlands. and, in the east, chalk has its share in the formation of some of the most venerable of mountain ranges, such as the lebanon. what is this wide-spread component of the surface of the earth? and whence did it come? you may think this no very hopeful inquiry. you may not unnaturally suppose that the attempt to solve such problems as these can lead to no result, save that of entangling the inquirer in vague speculations, incapable of refutation and of verification. if such were really the case, i should have selected some other subject than a "piece of chalk" for my discourse. but, in truth, after much deliberation, i have been unable to think of any topic which would so well enable me to lead you to see how solid is the foundation upon which some of the most startling conclusions of physical science rest. a great chapter of the history of the world is written in the chalk. few passages in the history of man can be supported by such an overwhelming mass of direct and indirect evidence as that which testifies to the truth of the fragment of the history of the globe, which i hope to enable you to read, with your own eyes, to-night. let me add, that few chapters of human history have a more profound significance for ourselves. i weigh my words well when i assert, that the man who should know the true history of the bit of chalk which every carpenter carries about in his breeches- pocket, though ignorant of all other history, is likely, if he will think his knowledge out to its ultimate results, to have a truer, and therefore a better, conception of this wonderful universe, and of man's relation to it, than the most learned student who is deep-read in the records of humanity and ignorant of those of nature. the language of the chalk is not hard to learn, not nearly so hard as latin, if you only want to get at the broad features of the story it has to tell; and i propose that we now set to work to spell that story out together. we all know that if we "burn" chalk the result is quicklime. chalk, in fact, is a compound of carbonic acid gas, and lime, and when you make it very hot the carbonic acid flies away and the lime is left. by this method of procedure we see the lime, but we do not see the carbonic acid. if, on the other hand, you were to powder a little chalk and drop it into a good deal of strong vinegar, there would be a great bubbling and fizzing, and, finally, a clear liquid, in which no sign of chalk would appear. here you see the carbonic acid in the bubbles; the lime, dissolved in the vinegar, vanishes from sight. there are a great many other ways of showing that chalk is essentially nothing but carbonic acid and quicklime. chemists enunciate the result of all the experiments which prove this, by stating that chalk is almost wholly composed of "carbonate of lime." it is desirable for us to start from the knowledge of this fact, though it may not seem to help us very far towards what we seek. for carbonate of lime is a widely-spread substance, and is met with under very various conditions. all sorts of limestones are composed of more or less pure carbonate of lime. the crust which is often deposited by waters which have drained through limestone rocks, in the form of what are called stalagmites and stalactites, is carbonate of lime. or, to take a more familiar example, the fur on the inside of a tea-kettle is carbonate of lime; and, for anything chemistry tells us to the contrary, the chalk might be a kind of gigantic fur upon the bottom of the earth-kettle, which is kept pretty hot below. let us try another method of making the chalk tell us its own history. to the unassisted eye chalk looks simply like a very loose and open kind of stone. but it is possible to grind a slice of chalk down so thin that you can see through it--until it is thin enough, in fact, to be examined with any magnifying power that may be thought desirable. a thin slice of the fur of a kettle might be made in the same way. if it were examined microscopically, it would show itself to be a more or less distinctly laminated mineral substance, and nothing more. but the slice of chalk presents a totally different appearance when placed under the microscope. the general mass of it is made up of very minute granules; but, imbedded in this matrix, are innumerable bodies, some smaller and some larger, but, on a rough average, not more than a hundredth of an inch in diameter, having a well-defined shape and structure. a cubic inch of some specimens of chalk may contain hundreds of thousands of these bodies, compacted together with incalculable millions of the granules. the examination of a transparent slice gives a good notion of the manner in which the components of the chalk are arranged, and of their relative proportions. but, by rubbing up some chalk with a brush in water and then pouring off the milky fluid, so as to obtain sediments of different degrees of fineness, the granules and the minute rounded bodies may be pretty well separated from one another, and submitted to microscopic examination, either as opaque or as transparent objects. by combining the views obtained in these various methods, each of the rounded bodies may be proved to be a beautifully-constructed calcareous fabric, made up of a number of chambers, communicating freely with one another. the chambered bodies are of various forms. one of the commonest is something like a badly-grown raspberry, being formed of a number of nearly globular chambers of different sizes congregated together. it is called _globigerina_, and some specimens of chalk consist of little else than _globigerinoe_ and granules. let us fix our attention upon the _globigerina_. it is the spoor of the game we are tracking. if we can learn what it is and what are the conditions of its existence, we shall see our way to the origin and past history of the chalk. a suggestion which may naturally enough present itself is, that these curious bodies are the result of some process of aggregation which has taken place in the carbonate of lime; that, just as in winter, the rime on our windows simulates the most delicate and elegantly arborescent foliage--proving that the mere mineral water may, under certain conditions, assume the outward form of organic bodies--so this mineral substance, carbonate of lime, hidden away in the bowels of the earth, has taken the shape of these chambered bodies. i am not raising a merely fanciful and unreal objection. very learned men, in former days, have even entertained the notion that all the formed things found in rocks are of this nature; and if no such conception is at present held to be admissible, it is because long and varied experience has now shown that mineral matter never does assume the form and structure we find in fossils. if any one were to try to persuade you that an oyster-shell (which is also chiefly composed of carbonate of lime) had crystallized out of sea-water, i suppose you would laugh at the absurdity. your laughter would be justified by the fact that all experience tends to show that oyster-shells are formed by the agency of oysters, and in no other way. and if there were no better reasons, we should be justified, on like grounds, in believing that _globigerina_ is not the product of anything but vital activity. happily, however, better evidence in proof of the organic nature of the _globigerinoe_ than that of analogy is forthcoming. it so happens that calcareous skeletons, exactly similar to the _globigerinoe_ of the chalk, are being formed, at the present moment, by minute living creatures, which flourish in multitudes, literally more numerous than the sands of the sea-shore, over a large extent of that part of the earth's surface which is covered by the ocean. the history of the discovery of these living _globigerinoe_, and of the part which they play in rock building, is singular enough. it is a discovery which, like others of no less scientific importance, has arisen, incidentally, out of work devoted to very different and exceedingly practical interests. when men first took to the sea, they speedily learned to look out for shoals and rocks; and the more the burthen of their ships increased, the more imperatively necessary it became for sailors to ascertain with precision the depth of the waters they traversed. out of this necessity grew the use of the lead and sounding line; and, ultimately, marine-surveying, which is the recording of the form of coasts and of the depth of the sea, as ascertained by the sounding-lead, upon charts. at the same time, it became desirable to ascertain and to indicate the nature of the sea-bottom, since this circumstance greatly affects its goodness as holding ground for anchors. some ingenious tar, whose name deserves a better fate than the oblivion into which it has fallen, attained this object by "arming" the bottom of the lead with a lump of grease, to which more or less of the sand or mud, or broken shells, as the case might be, adhered, and was brought to the surface. but, however well adapted such an apparatus might be for rough nautical purposes, scientific accuracy could not be expected from the armed lead, and to remedy its defects (especially when applied to sounding in great depths) lieut. brooke, of the american navy, some years ago invented a most ingenious machine, by which a considerable portion of the superficial layer of the sea-bottom can be scooped out and brought up from any depth to which the lead descends. in , lieut. brooke obtained mud from the bottom of the north atlantic, between newfoundland and the azores, at a depth of more than , feet, or two miles, by the help of this sounding apparatus. the specimens were sent for examination to ehrenberg of berlin, and to bailey of west point, and those able microscopists found that this deep-sea mud was almost entirely composed of the skeletons of living organisms--the greater proportion of these being just like the _globigerinoe_ already known to occur in the chalk. thus far, the work had been carried on simply in the interests of science, but lieut. brooke's method of sounding acquired a high commercial value, when the enterprise of laying down the telegraph-cable between this country and the united states was undertaken. for it became a matter of immense importance to know, not only the depth of the sea over the whole line along which the cable was to be laid, but the exact nature of the bottom, so as to guard against chances of cutting or fraying the strands of that costly rope. the admiralty consequently ordered captain dayman, an old friend and shipmate of mine, to ascertain the depth over the whole line of the cable, and to bring back specimens of the bottom. in former days, such a command as this might have sounded very much like one of the impossible things which the young prince in the fairy tales is ordered to do before he can obtain the hand of the princess. however, in the months of june and july, , my friend performed the task assigned to him with great expedition and precision, without, so far as i know, having met with any reward of that kind. the specimens or atlantic mud which he procured were sent to me to be examined and reported upon.[ ] [footnote : see appendix to captain dayman's _deep-sea soundings in the north atlantic ocean between ireland and newfoundland, made in h.m.s. "cyclops_." published by order of the lords commissioners of the admiralty, . they have since formed the subject of an elaborate memoir by messrs. parker and jones, published in the _philosophical transactions_ for .] the result of all these operations is, that we know the contours and the nature of the surface-soil covered by the north atlantic for a distance of , miles from east to west, as well as we know that of any part of the dry land. it is a prodigious plain--one of the widest and most even plains in the world. if the sea were drained off, you might drive a waggon all the way from valentia, on the west coast of ireland, to trinity bay, in newfoundland. and, except upon one sharp incline about miles from valentia, i am not quite sure that it would even be necessary to put the skid on, so gentle are the ascents and descents upon that long route. from valentia the road would lie down-hill for about miles to the point at which the bottom is now covered by , fathoms of sea-water. then would come the central plain, more than a thousand miles wide, the inequalities of the surface of which would be hardly perceptible, though the depth of water upon it now varies from , to , feet; and there are places in which mont blanc might be sunk without showing its peak above water. beyond this, the ascent on the american side commences, and gradually leads, for about miles, to the newfoundland shore. almost the whole of the bottom of this central plain (which extends for many hundred miles in a north and south direction) is covered by a fine mud, which, when brought to the surface, dries into a greyish white friable substance. you can write with this on a blackboard, if you are so inclined; and, to the eye, it is quite like very soft, grayish chalk. examined chemically, it proves to be composed almost wholly of carbonate of lime; and if you make a section of it, in the same way as that of the piece of chalk was made, and view it with the microscope, it presents innumerable _globigerinoe_ embedded in a granular matrix. thus this deep- sea mud is substantially chalk. i say substantially, because there are a good many minor differences; but as these have no bearing on the question immediately before us,--which is the nature of the _globigerinoe_ of the chalk,--it is unnecessary to speak of them. _globigerinoe_ of every size, from the smallest to the largest, are associated together in the atlantic mud, and the chambers of many are filled by a soft animal matter. this soft substance is, in fact, the remains of the creature to which the _globigerinoe_ shell, or rather skeleton, owes its existence--and which is an animal of the simplest imaginable description. it is, in fact, a mere particle of living jelly, without defined parts of any kind--without a mouth, nerves, muscles, or distinct organs, and only manifesting its vitality to ordinary observation by thrusting out and retracting from all parts of its surface, long filamentous processes, which serve for arms and legs. yet this amorphous particle, devoid of everything which, in the higher animals, we call organs, is capable of feeding, growing, and multiplying; of separating from the ocean the small proportion of carbonate of lime which is dissolved in sea-water; and of building up that substance into a skeleton for itself, according to a pattern which can be imitated by no other known agency. the notion that animals can live and flourish in the sea, at the vast depths from which apparently living _globigerinoe_; have been brought up, does not agree very well with our usual conceptions respecting the conditions of animal life; and it is not so absolutely impossible as it might at first sight appear to be, that the _globigcrinoe_ of the atlantic sea-bottom do not live and die where they are found. as i have mentioned, the soundings from the great atlantic plain are almost entirely made up of _globigerinoe_, with the granules which have been mentioned, and some few other calcareous shells; but a small percentage of the chalky mud--perhaps at most some five per cent. of it-- is of a different nature, and consists of shells and skeletons composed of silex, or pure flint. these silicious bodies belong partly to the lowly vegetable organisms which are called _diatomaceoe_, and partly to the minute, and extremely simple, animals, termed _radiolaria_. it is quite certain that these creatures do not live at the bottom of the ocean, but at its surface--where they may be obtained in prodigious numbers by the use of a properly constructed net. hence it follows that these silicious organisms, though they are not heavier than the lightest dust, must have fallen, in some cases, through fifteen thousand feet of water, before they reached their final resting-place on the ocean floor. and considering how large a surface these bodies expose in proportion to their weight, it is probable that they occupy a great length of time in making their burial journey from the surface of the atlantic to the bottom. but if the _radiolaria_ and diatoms are thus rained upon the bottom of the sea, from the superficial layer of its waters in which they pass their lives, it is obviously possible that the _globigerinoe_ may be similarly derived; and if they were so, it would be much more easy to understand how they obtain their supply of food than it is at present. nevertheless, the positive and negative evidence all points the other way. the skeletons of the full-grown, deep-sea _globigerinoe_ are so remarkably solid and heavy in proportion to their surface as to seem little fitted for floating; and, as a matter of fact, they are not to be found along with the diatoms and _radiolaria_ in the uppermost stratum of the open ocean. it has been observed, again, that the abundance of _globigerinoe_, in proportion to other organisms, of like kind, increases with the depth of the sea; and that deep-water _globigerinoe_ are larger than those which live in shallower parts of the sea; and such facts negative the supposition that these organisms have been swept by currents from the shallows into the deeps of the atlantic. it therefore seems to be hardly doubtful that these wonderful creatures live and die at the depths in which they are found.[ ] [footnote : during the cruise of h.m.s. _bulldog_, commanded by sir leopold m'clintock, in , living star-fish were brought up, clinging to the lowest part of the sounding-line, from a depth of , fathoms, midway between cape farewell, in greenland, and the rockall banks. dr. wallich ascertained that the sea-bottom at this point consisted of the ordinary _globigerina_ ooze, and that the stomachs of the star-fishes were full of _globigerinoe_. this discovery removes all objections to the existence of living _globigerinoe_ at great depths, which are based upon the supposed difficulty of maintaining animal life under such conditions; and it throws the burden of proof upon those who object to the supposition that the _globigerinoe_ live and die where they are found.] however, the important points for us are, that the living _globigerinoe_ are exclusively marine animals, the skeletons of which abound at the bottom of deep seas; and that there is not a shadow of reason for believing that the habits of the _globigerinoe_ of the chalk differed from those of the existing species. but if this be true, there is no escaping the conclusion that the chalk itself is the dried mud of an ancient deep sea. in working over the soundings collected by captain dayman, i was surprised to find that many of what i have called the "granules" of that mud were not, as one might have been tempted to think at first, the more powder and waste of _globigerinoe_, but that they had a definite form and size. i termed these bodies "_coccoliths_," and doubted their organic nature. dr. wallich verified my observation, and added the interesting discovery that, not unfrequently, bodies similar to these "coccoliths" were aggregated together into spheroids, which lie termed "_coccospheres_." so far as we knew, these bodies, the nature of which is extremely puzzling and problematical, were peculiar to the atlantic soundings. but, a few years ago, mr. sorby, in making a careful examination of the chalk by means of thin sections and otherwise, observed, as ehrenberg had done before him, that much of its granular basis possesses a definite form. comparing these formed particles with those in the atlantic soundings, he found the two to be identical; and thus proved that the chalk, like the surroundings, contains these mysterious coccoliths and coccospheres. here was a further and most interesting confirmation, from internal evidence, of the essential identity of the chalk with modern deep-sea mud. _globigerinoe_, coccoliths, and coccospheres are found as the chief constituents of both, and testify to the general similarity of the conditions under which both have been formed.[ ] [footnote : i have recently traced out the development of the "coccoliths" from a diameter of / th of an inch up to their largest size (which is about / th), and no longer doubt that they are produced by independent organisms, which, like the _globigerinoe_, live and die at the bottom of the sea.] the evidence furnished by the hewing, facing, and superposition of the stones of the pyramids, that these structures were built by men, has no greater weight than the evidence that the chalk was built by _globigerinoe_; and the belief that those ancient pyramid-builders were terrestrial and air-breathing creatures like ourselves, is not better based than the conviction that the chalk-makers lived in the sea. but as our belief in the building of the pyramids by men is not only grounded on the internal evidence afforded by these structures, but gathers strength from multitudinous collateral proofs, and is clinched by the total absence of any reason for a contrary belief; so the evidence drawn from the _globigerinoe_ that the chalk is an ancient sea-bottom, is fortified by innumerable independent lines of evidence; and our belief in the truth of the conclusion to which all positive testimony tends, receives the like negative justification from the fact that no other hypothesis has a shadow of foundation. it may be worth while briefly to consider a few of these collateral proofs that the chalk was deposited at the bottom of the sea. the great mass of the chalk is composed, as we have seen, of the skeletons of _globigerinoe_, and other simple organisms, imbedded in granular matter. here and there, however, this hardened mud of the ancient sea reveals the remains of higher animals which have lived and died, and left their hard parts in the mud, just as the oysters die and leave their shells behind them, in the mud of the present seas. there are, at the present day, certain groups of animals which are never found in fresh waters, being unable to live anywhere but in the sea. such are the corals; those corallines which are called _polyzoa_; those creatures which fabricate the lamp-shells, and are called _brachiopoda_; the pearly _nautilus_, and all animals allied to it; and all the forms of sea-urchins and star-fishes. not only are all these creatures confined to salt water at the present day; but, so far as our records of the past go, the conditions of their existence have been the same: hence, their occurrence in any deposit is as strong evidence as can be obtained, that that deposit was formed in the sea. now the remains of animals of all the kinds which have been enumerated, occur in the chalk, in greater or less abundance; while not one of those forms of shell-fish which are characteristic of fresh water has yet been observed in it. when we consider that the remains of more than three thousand distinct species of aquatic animals have been discovered among the fossils of the chalk, that the great majority of them are of such forms as are now met with only in the sea, and that there is no reason to believe that any one of them inhabited fresh water--the collateral evidence that the chalk represents an ancient sea-bottom acquires as great force as the proof derived from the nature of the chalk itself. i think you will now allow that i did not overstate my case when i asserted that we have as strong grounds for believing that all the vast area of dry land, at present occupied by the chalk, was once at the bottom of the sea, as we have for any matter of history whatever; while there is no justification for any other belief. no less certain it is that the time during which the countries we now call south-east england, france, germany, poland, russia, egypt, arabia, syria, were more or less completely covered by a deep sea, was of considerable duration. we have already seen that the chalk is, in places, more than a thousand feet thick. i think you will agree with me, that it must have taken some time for the skeletons of animalcules of a hundredth of an inch in diameter to heap up such a mass as that. i have said that throughout the thickness of the chalk the remains of other animals are scattered. these remains are often in the most exquisite state of preservation. the valves of the shell-fishes are commonly adherent; the long spines of some of the sea-urchins, which would be detached by the smallest jar, often remain in their places. in a word, it is certain that these animals have lived and died when the place which they now occupy was the surface of as much of the chalk as had then been deposited; and that each has been covered up by the layer of _globigerina_ mud, upon which the creatures imbedded a little higher up have, in like manner, lived and died. but some of these remains prove the existence of reptiles of vast size in the chalk sea. these lived their time, and had their ancestors and descendants, which assuredly implies time, reptiles being of slow growth. there is more curious evidence, again, that the process of covering up, or, in other words, the deposit of _globigerina_ skeletons, did not go on very fast. it is demonstrable that an animal of the cretaceous sea might die, that its skeleton might lie uncovered upon the sea-bottom long enough to lose all its outward coverings and appendages by putrefaction; and that, after this had happened, another animal might attach itself to the dead and naked skeleton, might grow to maturity, and might itself die before the calcareous mud had buried the whole. cases of this kind are admirably described by sir charles lyell. he speaks of the frequency with which geologists find in the chalk a fossilized sea-urchin, to which is attached the lower valve of a _crania_. this is a kind of shell-fish, with a shell composed of two pieces, of which, as in the oyster, one is fixed and the other free. "the upper valve is almost invariably wanting, though occasionally found in a perfect state of preservation in the white chalk at some distance. in this case, we see clearly that the sea-urchin first lived from youth to age, then died and lost its spines, which were carried away. then the young _crania_ adhered to the bared shell, grew and perished in its turn; after which, the upper valve was separated from the lower, before the echinus became enveloped in chalky mud."[ ] a specimen in the museum of practical geology, in london, still further prolongs the period which must have elapsed between the death of the sea- urchin, and its burial by the _globigerinoe_. for the outward face of the valve of a _crania_, which is attached to a sea-urchin, (_micraster_), is itself overrun by an incrusting coralline, which spreads thence over more or less of the surface of the sea-urchin. it follows that, after the upper valve of the _crania_ fell off, the surface of the attached valve must have remained exposed long enough to allow of the growth of the whole coralline, since corallines do not live embedded in mud.[ ] [footnote : _elements of geology_, by sir charles lyell, bart. f.b.s., p. .] the progress of knowledge may, one day, enable us to deduce from such facts as these the maximum rate at which the chalk can have accumulated, and thus to arrive at the minimum duration of the chalk period. suppose that the valve of the _cronia_ upon which a coralline has fixed itself in the way just described, is so attached to the sea-urchin that no part of it is more than an inch above the face upon which the sea-urchin rests. then, as the coralline could not have fixed itself, if the _crania_ had been covered up with chalk mud, and could not have lived had itself been so covered, it follows, that an inch of chalk mud could not have accumulated within the time between the death and decay of the soft parts of the sea-urchin and the growth of the coralline to the full size which it has attained. if the decay of the soft parts of the sea-urchin; the attachment, growth to maturity, and decay of the _crania_; and the subsequent attachment and growth of the coralline, took a year (which is a low estimate enough), the accumulation of the inch of chalk must have taken more than a year: and the deposit of a thousand feet of chalk must, consequently, have taken more than twelve thousand years. the foundation of all this calculation is, of course, a knowledge of the length of time the _crania_ and the coralline needed to attain their full size; and, on this head, precise knowledge is at present wanting. but there are circumstances which tend to show, that nothing like an inch of chalk has accumulated during the life of a _crania_; and, on any probable estimate of the length of that life, the chalk period must have had a much longer duration than that thus roughly assigned to it. thus, not only is it certain that the chalk is the mud of an ancient sea- bottom; but it is no less certain, that the chalk sea existed during an extremely long period, though we may not be prepared to give a precise estimate of the length of that period in years. the relative duration is clear, though the absolute duration may not be definable. the attempt to affix any precise date to the period at which the chalk sea began, or ended, its existence, is baffled by difficulties of the same kind. but the relative age of the cretaceous epoch may be determined with as great ease and certainty as the long duration of that epoch. you will have heard of the interesting discoveries recently made, in various parts of western europe, of flint implements, obviously worked into shape by human hands, under circumstances which show conclusively that man is a very ancient denizen of these regions. it has been proved that the whole populations of europe, whose existence has been revealed to us in this way, consisted of savages, such as the esquimaux are now; that, in the country which is now france, they hunted the reindeer, and were familiar with the ways of the mammoth and the bison. the physical geography of france was in those days different from what it is now--the river somme, for instance, having cut its bed a hundred feet deeper between that time and this; and, it is probable, that the climate was more like that of canada or siberia, than that of western europe. the existence of these people is forgotten even in the traditions of the oldest historical nations. the name and fame of them had utterly vanished until a few years back; and the amount of physical change which has been effected since their day renders it more than probable that, venerable as are some of the historical nations, the workers of the chipped flints of hoxne or of amiens are to them, as they are to us, in point of antiquity. but, if we assign to these hoar relics of long-vanished generations of men the greatest age that can possibly be claimed for them, they are not older than the drift, or boulder clay, which, in comparison with the chalk, is but a very juvenile deposit. you need go no further than your own sea-board for evidence of this fact. at one of the most charming spots on the coast of norfolk, cromer, you will see the boulder clay forming a vast mass, which lies upon the chalk, and must consequently have come into existence after it. huge boulders of chalk are, in fact, included in the clay, and have evidently been brought to the position they now occupy by the same agency as that which has planted blocks of syenite from norway side by side with them. the chalk, then, is certainly older than the boulder clay. if you ask how much, i will again take you no further than the same spot upon your own coasts for evidence. i have spoken of the boulder clay and drift as resting upon the chalk. that is not strictly true. interposed between the chalk and the drift is a comparatively insignificant layer, containing vegetable matter. but that layer tells a wonderful history. it is full of stumps of trees standing as they grew. fir-trees are there with their cones, and hazel-bushes with their nuts; there stand the stools of oak and yew trees, beeches and alders. hence this stratum is appropriately called the "forest-bed." it is obvious that the chalk must have been upheaved and converted into dry land, before the timber trees could grow upon it. as the bolls of some of these trees are from two to three feet in diameter, it is no less clear that the dry land thus formed remained in the same condition for long ages. and not only do the remains of stately oaks and well-grown firs testify to the duration of this condition of things, but additional evidence to the same effect is afforded by the abundant remains of elephants, rhinoceroses, hippopotamuses, and other great wild beasts, which it has yielded to the zealous search of such men as the rev. mr. gunn. when you look at such a collection as he has formed, and bethink you that these elephantine bones did veritably carry their owners about, and these great grinders crunch, in the dark woods of which the forest- bed is now the only trace, it is impossible not to feel that they are as good evidence of the lapse of time as the annual rings of the tree stumps. thus there is a writing upon the wall of cliffs at cromer, and whoso runs may read it. it tells us, with an authority which cannot be impeached, that the ancient sea-bed of the chalk sea was raised up, and remained dry land, until it was covered with forest, stocked with the great game the spoils of which have rejoiced your geologists. how long it remained in that condition cannot be said; but "the whirligig of time brought its revenges" in those days as in these. that dry land, with the bones and teeth of generations of long-lived elephants, hidden away among the gnarled roots and dry leaves of its ancient trees, sank gradually to the bottom of the icy sea, which covered it with huge masses of drift and boulder clay. sea-beasts, such as the walrus, now restricted to the extreme north, paddled about where birds had twittered among the topmost twigs of the fir-trees. how long this state of things endured we know not, but at length it came to an end. the upheaved glacial mud hardened into the soil of modern norfolk. forests grew once more, the wolf and the beaver replaced the reindeer and the elephant; and at length what we call the history of england dawned. thus you have, within the limits of your own county, proof that the chalk can justly claim a very much greater antiquity than even the oldest physical traces of mankind. but we may go further and demonstrate, by evidence of the same authority as that which testifies to the existence of the father of men, that the chalk is vastly older than adam himself. the book of genesis informs us that adam, immediately upon his creation, and before the appearance of eve, was placed in the garden of eden. the problem of the geographical position of eden has greatly vexed the spirits of the learned in such matters, but there is one point respecting which, so far as i know, no commentator has ever raised a doubt. this is, that of the four rivers which are said to run out of it, euphrates and hiddekel are identical with the rivers now known by the names of euphrates and tigris. but the whole country in which these mighty rivers take their origin, and through which they run, is composed of rocks which are either of the same age as the chalk, or of later date. so that the chalk must not only have been formed, but, after its formation, the time required for the deposit of these later rocks, and for their upheaval into dry land, must have elapsed, before the smallest brook which feeds the swift stream of "the great river, the river of babylon," began to flow. thus, evidence which cannot be rebutted, and which need not be strengthened, though if time permitted i might indefinitely increase its quantity, compels you to believe that the earth, from the time of the chalk to the present day, has been the theatre of a series of changes as vast in their amount, as they were slow in their progress. the area on which we stand has been first sea and then land, for at least four alternations; and has remained in each of these conditions for a period of great length. nor have these wonderful metamorphoses of sea into land, and of land into sea, been confined to one corner of england. during the chalk period, or "cretaceous epoch," not one of the present great physical features of the globe was in existence. our great mountain ranges, pyrenees, alps, himalayas, andes, have all been upheaved since the chalk was deposited, and the cretaceous sea flowed over the sites of sinai and ararat. all this is certain, because rocks of cretaceous, or still later, date have shared in the elevatory movements which gave rise to these mountain chains; and may be found perched up, in some cases, many thousand feet high upon their flanks. and evidence of equal cogency demonstrates that, though, in norfolk, the forest-bed rests directly upon the chalk, yet it does so, not because the period at which the forest grew immediately followed that at which the chalk was formed, but because an immense lapse of time, represented elsewhere by thousands of feet of rock, is not indicated at cromer. i must ask you to believe that there is no less conclusive proof that a still more prolonged succession of similar changes occurred, before the chalk was deposited. nor have we any reason to think that the first term in the series of these changes is known. the oldest sea-beds preserved to us are sands, and mud, and pebbles, the wear and tear of rocks which were formed in still older oceans. but, great as is the magnitude of these physical changes of the world, they have been accompanied by a no less striking series of modifications in its living inhabitants. all the great classes of animals, beasts of the field, fowls of the air, creeping things, and things which dwell in the waters, flourished upon the globe long ages before the chalk was deposited. very few, however, if any, of these ancient forms of animal life were identical with those which now live. certainly not one of the higher animals was of the same species as any of those now in existence. the beasts of the field, in the days before the chalk, were not our beasts of the field, nor the fowls of the air such as those which the eye of men has seen flying, unless his antiquity dates infinitely further back than we at present surmise. if we could be carried back into those times, we should be as one suddenly set down in australia before it was colonized. we should see mammals, birds, reptiles, fishes, insects, snails, and the like, clearly recognizable as such, and yet not one of them would be just the same as those with which we are familiar, and many would be extremely different. from that time to the present, the population of the world has undergone slow and gradual, but incessant, changes. there has been no grand catastrophe--no destroyer has swept away the forms of life of one period, and replaced them by a totally new creation: but one species has vanished and another has taken its place; creatures of one type of structure have diminished, those of another have increased, as time has passed on. and thus, while the differences between the living creatures of the time before the chalk and those of the present day appear startling, if placed side by side, we are led from one to the other by the most gradual progress, if we follow the course of nature through the whole series of those relics of her operations which she has left behind. it is by the population of the chalk sea that the ancient and the modern inhabitants of the world are most completely connected. the groups which are dying out flourish, side by side, with the groups which are now the dominant forms of life. thus the chalk contains remains of those strange flying and swimming reptiles, the pterodactyl, the ichthyosaurus, and the plesiosaurus, which are found in no later deposits, but abounded in preceding ages. the chambered shells called ammonites and belemnites, which are so characteristic of the period preceding the cretaceous, in like manner die with it. but, amongst these fading remainders of a previous state of things, are some very modern forms of life, looking like yankee pedlars among a tribe of red indians. crocodiles of modern type appear; bony fishes, many of them very similar to existing species, almost supplant the forms of fish which predominate in more ancient seas; and many kinds of living shell- fish first become known to us in the chalk. the vegetation acquires a modern aspect. a few living animals are not even distinguishable as species, from those which existed at that remote epoch. the _globigerina_ of the present day, for example, is not different specifically from that of the chalk; and the same maybe said of many other _foraminifera_. i think it probable that critical and unprejudiced examination will show that more than one species of much higher animals have had a similar longevity; but the only example which i can at present give confidently is the snake's-head lampshell (_terebratulina caput serpentis_), which lives in our english seas and abounded (as _terebratulina striata_ of authors) in the chalk. the longest line of human ancestry must hide its diminished head before the pedigree of this insignificant shell-fish. we englishmen are proud to have an ancestor who was present at the battle of hastings. the ancestors of _terebratulina caput serpentis_ may have been present at a battle of _ichthyosauria_ in that part of the sea which, when the chalk was forming, flowed over the site of hastings. while all around has changed, this _terebratulina_ has peacefully propagated its species from generation to generation, and stands to this day, as a living testimony to the continuity of the present with the past history of the globe. up to this moment i have stated, so far as i know, nothing but well- authenticated facts, and the immediate conclusions which they force upon the mind. but the mind is so constituted that it does not willingly rest in facts and immediate causes, but seeks always after a knowledge of the remoter links in the chain of causation. taking the many changes of any given spot of the earth's surface, from sea to land and from land to sea, as an established fact, we cannot refrain from asking ourselves how these changes have occurred. and when we have explained them--as they must be explained--by the alternate slow movements of elevation and depression which have affected the crust of the earth, we go still further back, and ask, why these movements? i am not certain that any one can give you a satisfactory answer to that question. assuredly i cannot. all that can be said, for certain, is, that such movements are part of the ordinary course of nature, inasmuch as they are going on at the present time. direct proof may be given, that some parts of the land of the northern hemisphere are at this moment insensibly rising and others insensibly sinking; and there is indirect, but perfectly satisfactory, proof, that an enormous area now covered by the pacific has been deepened thousands of feet, since the present inhabitants of that sea came into existence. thus there is not a shadow of a reason for believing that the physical changes of the globe, in past times, have been effected by other than natural causes. is there any more reason for believing that the concomitant modifications in the forms of the living inhabitants of the globe have been brought about in other ways? before attempting to answer this question, let us try to form a distinct mental picture of what has happened in some special case. the crocodiles are animals which, as a group, have a very vast antiquity. they abounded ages before the chalk was deposited; they throng the rivers in warm climates, at the present day. there is a difference in the form of the joints of the back-bone, and in some minor particulars, between the crocodiles of the present epoch and those which lived before the chalk; but, in the cretaceous epoch, as i have already mentioned, the crocodiles had assumed the modern type of structure. notwithstanding this, the crocodiles of the chalk are not identically the same as those which lived in the times called "older tertiary," which succeeded the cretaceous epoch; and the crocodiles of the older tertiaries are not identical with those of the newer tertiaries, nor are these identical with existing forms. i leave open the question whether particular species may have lived on from epoch to epoch. but each epoch has had its peculiar crocodiles; though all, since the chalk, have belonged to the modern type, and differ simply in their proportions, and in such structural particulars as are discernible only to trained eyes. how is the existence of this long succession of different species of crocodiles to be accounted for? only two suppositions seem to be open to us--either each species of crocodile has been specially created, or it has arisen out of some pre-existing form by the operation of natural causes. choose your hypothesis; i have chosen mine. i can find no warranty for believing in the distinct creation of a score of successive species of crocodiles in the course of countless ages of time. science gives no countenance to such a wild fancy; nor can even the perverse ingenuity of a commentator pretend to discover this sense, in the simple words in which the writer of genesis records the proceedings of the fifth and six days of the creation. on the other hand, i see no good reason for doubting the necessary alternative, that all these varied species have been evolved from pre- existing crocodilian forms, by the operation of causes as completely a part of the common order of nature as those which have effected the changes of the inorganic world. few will venture to affirm that the reasoning which applies to crocodiles loses its force among other animals, or among plants. if one series of species has come into existence by the operation of natural causes, it seems folly to deny that all may have arisen in the same way. a small beginning has led us to a great ending. if i were to put the bit of chalk with which we started into the hot but obscure flame of burning hydrogen, it would presently shine like the sun. it seems to me that this physical metamorphosis is no false image of what has been the result of our subjecting it to a jet of fervent, though nowise brilliant, thought to-night. it has become luminous, and its clear rays, penetrating the abyss of the remote past, have brought within our ken some stages of the evolution of the earth. and in the shifting "without haste, but without rest" of the land and sea, as in the endless variation of the forms assumed by living beings, we have observed nothing but the natural product of the forces originally possessed by the substance of the universe. ii the problems of the deep sea [ ] on the st of december, , h.m.s. _challenger_, an eighteen gun corvette, of , tons burden, sailed from portsmouth harbour for a three, or perhaps four, years' cruise. no man-of-war ever left that famous port before with so singular an equipment. two of the eighteen sixty-eight pounders of the _challenger's_ armament remained to enable her to speak with effect to sea-rovers, haply devoid of any respect for science, in the remote seas for which she is bound; but the main-deck was, for the most part, stripped of its war-like gear, and fitted up with physical, chemical, and biological laboratories; photography had its dark cabin; while apparatus for dredging, trawling, and sounding; for photometers and for thermometers, filled the space formerly occupied by guns and gun-tackle, pistols and cutlasses. the crew of the _challenger_ match her fittings. captain nares, his officers and men, are ready to look after the interests of hydrography, work the ship, and, if need be, fight her as seamen should; while there is a staff of scientific civilians, under the general direction of dr. wyville thomson, f.r.s. (professor of natural history in edinburgh university by rights, but at present detached for duty _in partibus_), whose business it is to turn all the wonderfully packed stores of appliances to account, and to accumulate, before the ship returns to england, such additions to natural knowledge as shall justify the labour and cost involved in the fitting out and maintenance of the expedition. under the able and zealous superintendence of the hydrographer, admiral richards, every precaution which experience and forethought could devise has been taken to provide the expedition with the material conditions of success; and it would seem as if nothing short of wreck or pestilence, both most improbable contingencies, could prevent the _challenger_ from doing splendid work, and opening up a new era in the history of scientific voyages. the dispatch of this expedition is the culmination of a series of such enterprises, gradually increasing in magnitude and importance, which the admiralty, greatly to its credit, has carried out for some years past; and the history of which is given by dr. wyville thomson in the beautifully illustrated volume entitled "the depths of the sea," published since his departure. "in the spring of the year , my friend dr. w.b. carpenter, at that time one of the vice-presidents of the royal society, was with me in ireland, where we were working out together the structure and development of the crinoids. i had long previously had a profound conviction that the land of promise for the naturalist, the only remaining region where there were endless novelties of extraordinary interest ready to the hand which had the means of gathering them, was the bottom of the deep sea. i had even had a glimpse of some of these treasures, for i had seen, the year before, with prof. sars, the forms which i have already mentioned dredged by his son at a depth of to fathoms off the loffoten islands. i propounded my views to my fellow-labourer, and we discussed the subject many times over our microscopes. i strongly urged dr. carpenter to use his influence at head-quarters to induce the admiralty, probably through the council of the royal society, to give us the use of a vessel properly fitted with dredging gear and all necessary scientific apparatus, that many heavy questions as to the state of things in the depths of the ocean, which were still in a state of uncertainty, might be definitely settled. after full consideration, dr. carpenter promised his hearty co- operation, and we agreed that i should write to him on his return to london, indicating generally the results which i anticipated, and sketching out what i conceived to be a promising line of inquiry. the council of the royal society warmly supported the proposal; and i give here in chronological order the short and eminently satisfactory correspondence which led to the admiralty placing at the disposal of dr. carpenter and myself the gunboat _lightninq_, under the command of staff- commander may, r.n., in the summer of , for a trial cruise to the north of scotland, and afterwards to the much wider surveys in h.m.s. _porcupine_, captain calver, r.n., which were made with the additional association of mr. gwyn jeffreys, in the summers of the years and ."[ ] [footnote : the depths of the sea, pp. - .] plain men may be puzzled to understand why dr. wyville thomson, not being a cynic, should relegate the "land of promise" to the bottom of the deep sea, they may still more wonder what manner of "milk and honey" the _challenger_ expects to find; and their perplexity may well rise to its maximum, when they seek to divine the manner in which that milk and honey are to be got out of so inaccessible a canaan. i will, therefore, endeavour to give some answer to these questions in an order the reverse of that in which i have stated them. apart from hooks, and lines, and ordinary nets, fishermen have, from time immemorial, made use of two kinds of implements for getting at sea- creatures which live beyond tide-marks--these are the "dredge" and the "trawl." the dredge is used by oyster-fishermen. imagine a large bag, the mouth of which has the shape of an elongated parallelogram, and is fastened to an iron frame of the same shape, the two long sides of this rim being fashioned into scrapers. chains attach the ends of the frame to a stout rope, so that when the bag is dragged along by the rope the edge of one of the scrapers rests on the ground, and scrapes whatever it touches into the bag. the oyster-dredger takes one of these machines in his boat, and when he has reached the oyster-bed the dredge is tossed overboard; as soon as it has sunk to the bottom the rope is paid out sufficiently to prevent it from pulling the dredge directly upwards, and is then made fast while the boat goes ahead. the dredge is thus dragged along and scrapes oysters and other sea-animals and plants, stones, and mud into the bag. when the dredger judges it to be full he hauls it up, picks out the oysters, throws the rest overboard, and begins again. dredging in shallow water, say ten to twenty fathoms, is an easy operation enough; but the deeper the dredger goes, the heavier must be his vessel, and the stouter his tackle, while the operation of hauling up becomes more and more laborious. dredging in fathoms is very hard work, if it has to be carried on by manual labour; but by the use of the donkey-engine to supply power,[ ] and of the contrivances known as "accumulators," to diminish the risk of snapping the dredge rope by the rolling and pitching of the vessel, the dredge has been worked deeper and deeper, until at last, on the nd of july, , h.m.s. _porcupine_ being in the bay of biscay, captain calver, her commander, performed the unprecedented feat of dredging in , fathoms, or , feet, a depth nearly equal to the height of mont blanc. the dredge "was rapidly hauled on deck at one o'clock in the morning of the rd, after an absence of - / hours, and a journey of upwards of eight statute miles," with a hundred weight and a half of solid contents. [footnote : the emotional side of the scientific nature has its singularities. many persons will call to mind a certain philosopher's tenderness over his watch--"the little creature"--which was so singularly lost and found again. but dr. wyville thomson surpasses the owner of the watch in his loving-kindness towards a donkey-engine. "this little engine was the comfort of our lives. once or twice it was overstrained, and then we pitied the willing little thing, panting like an overtaxed horse."] the trawl is a sort of net for catching those fish which habitually live at the bottom of the sea, such as soles, plaice, turbot, and gurnett. the mouth of the net may be thirty or forty feet wide, and one edge of its mouth is fastened to a beam of wood of the same length. the two ends of the beam are supported by curved pieces of iron, which raise the beam and the edge of the net which is fastened to it, for a short distance, while the other edge of the mouth of the net trails upon the ground. the closed end of the net has the form of a great pouch; and, as the beam is dragged along, the fish, roused from the bottom by the sweeping of the net, readily pass into its mouth and accumulate in the pouch at its end. after drifting with the tide for six or seven hours the trawl is hauled up, the marketable fish are picked out, the others thrown away, and the trawl sent overboard for another operation. more than a thousand sail of well-found trawlers are constantly engaged in sweeping the seas around our coast in this way, and it is to them that we owe a very large proportion of our supply of fish. the difficulty of trawling, like that of dredging, rapidly increases with the depth at which the operation is performed; and, until the other day, it is probable that trawling at so great a depth as fathoms was something unheard of. but the first news from the _challenger_ opens up new possibilities for the trawl. dr. wyville thomson writes ("nature," march , ):-- "for the first two or three hauls in very deep water off the coast of portugal, the dredge came up filled with the usual 'atlantic ooze,' tenacious and uniform throughout, and the work of hours, in sifting, gave the very smallest possible result. we were extremely anxious to get some idea of the general character of the fauna, and particularly of the distribution of the higher groups; and after various suggestions for modification of the dredge, it was proposed to try the ordinary trawl. we had a compact trawl, with a -feet beam, on board, and we sent it down off cape st. vincent at a depth of fathoms. the experiment looked hazardous, but, to our great satisfaction, the trawl came up all right and contained, with many of the larger invertebrate, several fishes.... after the first attempt we tried the trawl several times at depths of , , and, finally, fathoms, and always with success." to the coral-fishers of the mediterranean, who seek the precious red coral, which grows firmly fixed to rocks at a depth of sixty to eighty fathoms, both the dredge and the trawl would be useless. they, therefore, have recourse to a sort of frame, to which are fastened long bundles of loosely netted hempen cord, and which is lowered by a rope to the depth at which the hempen cords can sweep over the surface of the rocks and break off the coral, which is brought up entangled in the cords. a similar contrivance has arisen out of the necessities of deep-sea exploration. in the course of the dredging of the _porcupine_, it was frequently found that, while few objects of interest were brought up within the dredge, many living creatures came up sticking to the outside of the dredge-bag, and even to the first few fathoms of the dredge-rope. the mouth of the dredge doubtless rapidly filled with mud, and thus the things it should have brought up were shut out. to remedy this inconvenience captain calver devised an arrangement not unlike that employed by the coral- fishers. he fastened half a dozen swabs, such as are used for drying decks, to the dredge. a swab is something like what a birch-broom would be if its twigs were made of long, coarse, hempen yarns. these dragged along after the dredge over the surface of the mud, and entangled the creatures living there--multitudes of which, twisted up in the strands of the swabs, were brought to the surface with the dredge. a further improvement was made by attaching a long iron bar to the bottom of the dredge bag, and fastening large bunches of teased-out hemp to the end of this bar. these "tangles" bring up immense quantities of such animals as have long arms, or spines, or prominences which readily become caught in the hemp, but they are very destructive to the fragile organisms which they imprison; and, now that the trawl can be successfully worked at the greatest depths, it may be expected to supersede them; at least, wherever the ground is soft enough to permit of trawling. it is obvious that between the dredge, the trawl, and the tangles, there is little chance for any organism, except such as are able to burrow rapidly, to remain safely at the bottom of any part of the sea which the _challenger_ undertakes to explore. and, for the first time in the history of scientific exploration, we have a fair chance of learning what the population of the depths of the sea is like in the most widely different parts of the world. and now arises the next question. the means of exploration being fairly adequate, what forms of life may be looked for at these vast depths? the systematic study of the distribution of living beings is the most modern branch of biological science, and came into existence long after morphology and physiology had attained a considerable development. this naturally does not imply that, from the time men began to observe natural phenomena, they were ignorant of the fact that the animals and plants of one part of the world are different from those in other regions; or that those of the hills are different from those of the plains in the same region; or finally that some marine creatures are found only in the shallows, while others inhabit the deeps. nevertheless, it was only after the discovery of america that the attention of naturalists was powerfully drawn to the wonderful differences between the animal population of the central and southern parts of the new world and that of those parts of the old world which lie under the same parallels of latitude. so far back as abraham mylius, in his treatise "de animalium origine et migratione, populorum," argues that, since there are innumerable species of animals in america which do not exist elsewhere, they must have been made and placed there by the deity: buffon no less forcibly insists upon the difference between the faunae of the old and new world. but the first attempt to gather facts of this order into a whole, and to coordinate them into a series of generalizations, or laws of geographical distribution, is not a century old, and is contained in the "specimen zoologiae geographicae quadrupedum domicilia et migrationes sistens," published, in , by the learned brunswick professor, eberhard zimmermann, who illustrates his work by what he calls a "tabula zoographica," which is the oldest distributional map known to me. in regard to matters of fact, zimmermann's chief aim is to show that among terrestrial mammals, some occur all over the world, while others are restricted to particular areas of greater or smaller extent; and that the abundance of species follows temperature, being greatest in warm and least in cold climates. but marine animals, he thinks, obey no such law. the arctic and atlantic seas, he says, are as full of fishes and other animals as those of the tropics. it is, therefore, clear that cold does not affect the dwellers in the sea as it does land animals, and that this must be the case follows from the fact that sea water, "propter varias quas continet bituminis spiritusque particulas," freezes with much more difficulty than fresh water. on the other hand, the heat of the equatorial sun penetrates but a short distance below the surface of the ocean. moreover, according to zimmermann, the incessant disturbance of the mass of the sea by winds and tides, so mixes up the warm and the cold that life is evenly diffused and abundant throughout the ocean. in , risso, in his work on the ichthyology of nice, laid the foundation of what has since been termed "bathymetrical" distribution, or distribution in depth, by showing that regions of the sea bottom of different depths could be distinguished by the fishes which inhabit them. there was the _littoral region_ between tide marks with its sand-eels, pipe fishes, and blennies: the _seaweed region_, extending from low- water-mark to a depth of feet, with its wrasses, rays, and flat fish; and the _deep-sea region_, from feet to feet or more, with its file-fish, sharks, gurnards, cod, and sword-fish. more than twenty years later, m.m. audouin and milne edwards carried out the principle of distinguishing the faunae of different zones of depth much more minutely, in their "recherches pour servir à l'histoire naturelle du littoral de la france," published in . they divide the area included between highwater-mark and lowwater-mark of spring tides (which is very extensive, on account of the great rise and fall of the tide on the normandy coast about st. malo, where their observations were made) into four zones, each characterized by its peculiar invertebrate inhabitants. beyond the fourth region they distinguish a fifth, which is never uncovered, and is inhabited by oysters, scallops, and large starfishes and other animals. beyond this they seem to think that animal life is absent.[ ] [footnote : "enfin plus has encore, c'est-à-dire alors loin des côtes, le fond des eaux ne paraît plus être habité, du moms dans nos mers, par aucun de ces animaux" ( . c. tom. i. p. ). the "ces animaux" leaves the meaning of the authors doubtful.] audouin and milne edwards were the first to see the importance of the bearing of a knowledge of the manner in which marine animals are distributed in depth, on geology. they suggest that, by this means, it will be possible to judge whether a fossiliferous stratum was formed upon the shore of an ancient sea, and even to determine whether it was deposited in shallower or deeper water on that shore; the association of shells of animals which live in different zones of depth will prove that the shells have been transported into the position in which they are found; while, on the other hand, the absence of shells in a deposit will not justify the conclusion that the waters in which it was formed were devoid of animal inhabitants, inasmuch as they might have been only too deep for habitation. the new line of investigation thus opened by the french naturalists was followed up by the norwegian, sars, in , by edward forbes, in our own country, in ,[ ] and by oersted, in denmark, a few years later. the genius of forbes, combined with his extensive knowledge of botany, invertebrate zoology, and geology, enabled him to do more than any of his compeers, in bringing the importance of distribution in depth into notice; and his researches in the aegean sea, and still more his remarkable paper "on the geological relations of the existing fauna and flora of the british isles," published in , in the first volume of the "memoirs of the geological survey of great britain," attracted universal attention. [footnote : in the paper in the _memoirs of the survey_ cited further on, forbes writes:-- "in an essay 'on the association of mollusca on the british coasts, considered with reference to pleistocene geology,' printed in [the _edinburgh academic annual_ for] , i described the mollusca, as distributed on our shores and seas, in four great zones or regions, usually denominated 'the littoral zone,' 'the region of laminariae,' 'the region of coral-lines,' and 'the region of corals.' an extensive series of researches, chiefly conducted by the members of the committee appointed by the british association to investigate the marine geology of britain by means of the dredge, have not invalidated this classification, and the researches of professor lovén, in the norwegian and lapland seas, have borne out their correctness the first two of the regions above mentioned had been previously noticed by lamoureux, in his account of the distribution (vertically) of sea-weeds, by audouin and milne edwards in their _observations on the natural history of the coast of france_, and by sars in the preface to his _beskrivelser og jagttayelser_."] on the coasts of the british islands, forbes distinguishes four zones or regions, the littoral (between tide marks), the laminarian (between lowwater-mark and fathoms), the coralline (from to fathoms), and the deep sea or coral region (from fathoms to beyond fathoms). but, in the deeper waters of the aegean sea, between the shore and a depth of fathoms, forbes was able to make out no fewer than eight zones of life, in the course of which the number and variety of forms gradually diminished until, beyond fathoms, life disappeared altogether. hence it appeared as if descent in the sea had much the same effect on life, as ascent on land. recent investigations appear to show that forbes was right enough in his classification of the facts of distribution in depth as they are to be observed in the aegean; and though, at the time he wrote, one or two observations were extant which might have warned him not to generalize too extensively from his aegean experience, his own dredging work was so much more extensive and systematic than that of any other naturalist, that it is not wonderful he should have felt justified in building upon it. nevertheless, so far as the limit of the range of life in depth goes, forbes' conclusion has been completely negatived, and the greatest depths yet attained show not even an approach to a "zero of life":-- "during the several cruises of h.m. ships _lightning_ and _porcupine_ in the years , , and ," says dr. wyville thomson, "fifty-seven hauls of the dredge were taken in the atlantic at depths beyond fathoms, and sixteen at depths beyond , fathoms, and, in all cases, life was abundant. in , we took two casts in depths greater than , fathoms. in both of these life was abundant; and with the deepest cast, , fathoms, off the month of the bay of biscay, we took living, well-marked and characteristic examples of all the five invertebrate sub- kingdoms. and thus the question of the existence of abundant animal life at the bottom of the sea has been finally settled and for all depths, for there is no reason to suppose that the depth anywhere exceeds between three and four thousand fathoms; and if there be nothing in the conditions of a depth of , fathoms to prevent the full development of a varied fauna, it is impossible to suppose that even an additional thousand fathoms would make any great difference."[ ] [footnote : _the depths of the sea_, p. . results of a similar kind, obtained by previous observers, are stated at length in the sixth chapter, pp. - . the dredgings carried out by count pourtales, under the authority of professor peirce, the superintendent of the united states coast survey, in the years , , and , are particularly noteworthy, and it is probably not too much to say, in the words of professor agassiz, "that we owe to the coast survey the first broad and comprehensive basis for an exploration of the sea bottom on a large scale, opening a new era in zoological and geological research."] as dr. wyville thomson's recent letter, cited above, shows, the use of the trawl, at great depths, has brought to light a still greater diversity of life. fishes came up from a depth of to more than , fathoms, all in a peculiar condition from the expansion of the air contained in their bodies. on their relief from the extreme pressure, their eyes, especially, had a singular appearance, protruding like great globes from their heads. bivalve and univalve mollusca seem to be rare at the greatest depths; but starfishes, sea urchins and other echinoderms, zoophytes, sponges, and protozoa abound. it is obvious that the _challenger_ has the privilege of opening a new chapter in the history of the living world. she cannot send down her dredges and her trawls into these virgin depths of the great ocean without bringing up a discovery. even though the thing itself may be neither "rich nor rare," the fact that it came from that depth, in that particular latitude and longitude, will be a new fact in distribution, and, as such, have a certain importance. but it may be confidently assumed that the things brought up will very frequently be zoological novelties; or, better still, zoological antiquities, which, in the tranquil and little-changed depths of the ocean, have escaped the causes of destruction at work in the shallows, and represent the predominant population of a past age. it has been seen that audouin and milne edwards foresaw the general influence of the study of distribution in depth upon the interpretation of geological phenomena. forbes connected the two orders of inquiry still more closely; and in the thoughtful essay "on the connection between the distribution of the existing fauna and flora of the british isles, and the geological changes which have affected their area, especially during the epoch of the northern drift," to which reference has already been made, he put forth a most pregnant suggestion. in certain parts of the sea bottom in the immediate vicinity of the british islands, as in the clyde district, among the hebrides, in the moray firth, and in the german ocean, there are depressed areas, forming a kind of submarine valleys, the centres of which are from to fathoms, or more, deep. these depressions are inhabited by assemblages of marine animals, which differ from those found over the adjacent and shallower region, and resemble those which are met with much farther north, on the norwegian coast. forbes called these scandinavian detachments "northern outliers." how did these isolated patches of a northern population get into these deep places? to explain the mystery, forbes called to mind the fact that, in the epoch which immediately preceded the present, the climate was much colder (whence the name of "glacial epoch" applied to it); and that the shells which are found fossil, or sub-fossil, in deposits of that age are precisely such as are now to be met with only in the scandinavian, or still more arctic, regions. undoubtedly, during the glacial epoch, the general population of our seas had, universally, the northern aspect which is now presented only by the "northern outliers"; just as the vegetation of the land, down to the sea-level, had the northern character which is, at present, exhibited only by the plants which live on the tops of our mountains. but, as the glacial epoch passed away, and the present climatal conditions were developed, the northern plants were able to maintain themselves only on the bleak heights, on which southern forms could not compete with them. and, in like manner, forbes suggested that, after the glacial epoch, the northern animals then inhabiting the sea became restricted to the deeps in which they could hold their own against invaders from the south, better fitted than they to flourish in the warmer waters of the shallows. thus depth in the sea corresponded in its effect upon distribution to height on the land. the same idea is applied to the explanation of a similar anomaly in the fauna of the aegean:-- "in the deepest of the regions of depth of the aegean, the representation of a northern fauna is maintained, partly by identical and partly by representative forms.... the presence of the latter is essentially due to the law (of representation of parallels of latitude by zones of depth), whilst that of the former species depended on their transmission from their parent seas during a former epoch, and subsequent isolation. that epoch was doubtless the newer pliocene or glacial era, when the _mya truncata_ and other northern forms now extinct in the mediterranean, and found fossil in the sicilian tertiaries, ranged into that sea. the changes which there destroyed the _shallow water_ glacial forms, did not affect those living in the depths, and which still survive."[ ] [footnote : _memoirs of the geological survey of great britain_, vol. i. p. .] the conception that the inhabitants of local depressions of the sea bottom might be a remnant of the ancient population of the area, which had held their own in these deep fastnesses against an invading fauna, as britons and gaels have held out in wales and in scotland against encroaching teutons, thus broached by forbes, received a wider application than forbes had dreamed of when the sounding machine first brought up specimens of the mud of the deep sea. as i have pointed out elsewhere,[ ] it at once became obvious that the calcareous sticky mud of the atlantic was made up, in the main, of shells of _globigerina_ and other _foraminifera_, identical with those of which the true chalk is composed, and the identity extended even to the presence of those singular bodies, the coccoliths and coccospheres, the true nature of which is not yet made out. here then were organisms, as old as the cretaceous epoch, still alive, and doing their work of rock-making at the bottom of existing seas. what if _globigerina_ and the coccoliths should not be the only survivors of a world passed away, which are hidden beneath three miles of salt water? the letter which dr. wyville thomson wrote to dr. carpenter in may, , out of which all these expeditions have grown, shows that this query had become a practical problem in dr. thomson's mind at that time; and the desirableness of solving the problem is put in the foreground of his reasons for urging the government to undertake the work of exploration:-- [footnote : see above, "on a piece of chalk," p. .] "two years ago, m. sars, swedish government inspector of fisheries, had an opportunity, in his official capacity, of dredging off the loffoten islands at a depth of fathoms. i visited norway shortly after his return, and had an opportunity of studying with his father, professor sars, some of his results. animal forms were _abundant_; many of them were new to science; and among them was one of surpassing interest, the small crinoid, of which you have a specimen, and which we at once recognised as a degraded type of the _apiocrinidoe_, an order hitherto regarded as extinct, which attained its maximum in the pear encrinites of the jurassic period, and whose latest representative hitherto known was the _bourguettocrinus_ of the chalk. some years previously, mr. absjornsen, dredging in fathoms in the hardangerfjord, procured several examples of a starfish (_brisinga_), which seems to find its nearest ally in the fossil genus _protaster_. these observations place it beyond a doubt that animal life is abundant in the ocean at depths varying from to fathoms, that the forms at these great depths differ greatly from those met with in ordinary dredgings, and that, at all events in some cases, these animals are closely allied to, and would seem to be directly descended from, the fauna of the early tertiaries. "i think the latter result might almost have been anticipated; and, probably, further investigation will largely add to this class of data, and will give us an opportunity of testing our determinations of the zoological position of some fossil types by an examination of the soft parts of their recent representatives. the main cause of the destruction, the migration, and the extreme modification of animal types, appear to be change of climate, chiefly depending upon oscillations of the earth's crust. these oscillations do not appear to have ranged, in the northern portion of the northern hemisphere, much beyond , feet since the commencement of the tertiary epoch. the temperature of deep waters seems to be constant for all latitudes at °; so that an immense area of the north atlantic must have had its conditions unaffected by tertiary or post-tertiary oscillations."[ ] [footnote : the depths of the sea, pp. - .] as we shall see, the assumption that the temperature of the deep sea is everywhere ° f. ( ° cent.) is an error, which dr. wyville thomson adopted from eminent physical writers; but the general justice of the reasoning is not affected by this circumstance, and dr. thomson's expectation has been, to some extent, already verified. thus besides _globigerina_, there are eighteen species of deep-sea _foraminifera_ identical with species found in the chalk. imbedded in the chalky mud of the deep sea, in many localities, are innumerable cup- shaped sponges, provided with six-rayed silicious spicula, so disposed that the wall of the cup is formed of a lacework of flinty thread. not less abundant, in some parts of the chalk formation, are the fossils known as _ventriculites_, well described by dr. thomson as "elegant vases or cups, with branching root-like bases, or groups of regularly or irregularly spreading tubes delicately fretted on the surface with an impressed network like the finest lace"; and he adds, "when we compare such recent forms as _aphrocallistes, iphiteon, holtenia_, and _askonema_, with certain series of the chalk _ventriculites_, there cannot be the slightest doubt that they belong to the same family--in some cases to very nearly allied genera."[ ] [footnote : _the depths of the sea_, p. .] professor duncan finds "several corals from the coast of portugal more nearly allied to chalk forms than to any others." the stalked crinoids or feather stars, so abundant in ancient times, are now exclusively confined to the deep sea, and the late explorations have yielded forms of old affinity, the existence of which has hitherto been unsuspected. the general character of the group of star fishes imbedded in the white chalk is almost the same as in the modern fauna of the deep atlantic. the sea urchins of the deep sea, while none of them are specifically identical with any chalk form, belong to the same general groups, and some closely approach extinct cretaceous genera. taking these facts in conjunction with the positive evidence of the existence, during the cretaceous epoch, of a deep ocean where now lies the dry land of central and southern europe, northern africa, and western and southern asia; and of the gradual diminution of this ocean during the older tertiary epoch, until it is represented at the present day by such teacupfuls as the caspian, the black sea, and the mediterranean; the supposition of dr. thomson and dr. carpenter that what is now the deep atlantic, was the deep atlantic (though merged in a vast easterly extension) in the cretaceous epoch, and that the _globigerina_ mud has been accumulating there from that time to this, seems to me to have a great degree of probability. and i agree with dr. wyville thomson against sir charles lyell (it takes two of us to have any chance against his authority) in demurring to the assertion that "to talk of chalk having been uninterruptedly formed in the atlantic is as inadmissible in a geographical as in a geological sense." if the word "chalk" is to be used as a stratigraphical term and restricted to _globigerina_ mud deposited during the cretaceous epoch, of course it is improper to call the precisely similar mud of more recent date, chalk. if, on the other hand, it is to be used as a mineralogical term, i do not see how the modern and the ancient chalks are to be separated--and, looking at the matter geographically, i see no reason to doubt that a boring rod driven from the surface of the mud which forms the floor of the mid-atlantic would pass through one continuous mass of _globigerina_ mud, first of modern, then of tertiary, and then of mesozoic date; the "chalks" of different depths and ages being distinguished merely by the different forms of other organisms associated with the _globigerinoe_. on the other hand, i think it must be admitted that a belief in the continuity of the modern with the ancient chalk has nothing to do with the proposition that we can, in any sense whatever, be said to be still living in the cretaceous epoch. when the _challenger's_ trawl brings up an _ichthyosaurus_, along with a few living specimens of _belemnites_ and _turrilites_, it may be admitted that she has come upon a cretaceous "outlier." a geological period is characterized not only by the presence of those creatures which lived in it, but by the absence of those which have only come into existence later; and, however large a proportion of true cretaceous forms may be discovered in the deep sea, the modern types associated with them must be abolished before the fauna, as a whole, could, with any propriety, be termed cretaceous. i have now indicated some of the chief lines of biological inquiry, in which the _challenger_ has special opportunities for doing good service, and in following which she will be carrying out the work already commenced by the _lightning_ and _porcupine_ in their cruises of and subsequent years. but biology, in the long run, rests upon physics, and the first condition for arriving at a sound theory of distribution in the deep sea, is the precise ascertainment of the conditions of life; or, in other words, a full knowledge of all those phenomena which are embraced under the head of the physical geography of the ocean. excellent work has already been done in this direction, chiefly under the superintendence of dr. carpenter, by the _lightning_ and the _porcupine_,[ ] and some data of fundamental importance to the physical geography of the sea have been fixed beyond a doubt. [footnote : _proceedings of the royal society_, and ] thus, though it is true that sea-water steadily contracts as it cools down to its freezing point, instead of expanding before it reaches its freezing point as fresh water does, the truth has been steadily ignored by even the highest authorities in physical geography, and the erroneous conclusions deduced from their erroneous premises have been widely accepted as if they were ascertained facts. of course, if sea-water, like fresh water, were heaviest at a temperature of ° f. and got lighter as it approached ° f., the water of the bottom of the deep sea could not be colder than °. but one of the first results of the careful ascertainment of the temperature at different depths, by means of thermometers specially contrived for the avoidance of the errors produced by pressure, was the proof that, below fathoms in the atlantic, down to the greatest depths yet sounded, the water has a temperature always lower than ° fahr., whatever be the temperature of the water at the surface. and that this low temperature of the deepest water is probably the universal rule for the depths of the open ocean is shown, among others, by captain chimmo's recent observations in the indian ocean, between ceylon and sumatra, where, the surface water ranging from °- ° fahr., the temperature at the bottom, at a depth of to fathoms, was only from ° to ° fahr. as the mean temperature of the superficial layer of the crust of the earth may be taken at about ° fahr., it follows that the bottom layer of the deep sea in temperate and hot latitudes, is, on the average, much colder than either of the bodies with which it is in contact; for the temperature of the earth is constant, while that of the air rarely falls so low as that of the bottom water in the latitudes in question; and even when it does, has time to affect only a comparatively thin stratum of the surface water before the return of warm weather. how does this apparently anomalous state of things come about? if we suppose the globe to be covered with a universal ocean, it can hardly be doubted that the cold of the regions towards the poles must tend to cause the superficial water of those regions to contract and become specifically heavier. under these circumstances, it would have no alternative but to descend and spread over the sea bottom, while its place would be taken by warmer water drawn from the adjacent regions. thus, deep, cold, polar-equatorial currents, and superficial, warmer, equatorial-polar currents, would be set up; and as the former would have a less velocity of rotation from west to east than the regions towards which they travel, they would not be due southerly or northerly currents, but south-westerly in the northern hemisphere, and north-westerly in the southern; while, by a parity of reasoning, the equatorial-polar warm currents would be north-easterly in the northern hemisphere, and south- easterly in the southern. hence, as a north-easterly current has the same direction as a south-westerly wind, the direction of the northern equatorial-polar current in the extra-tropical part of its course would pretty nearly coincide with that of the anti-trade winds. the freezing of the surface of the polar sea would not interfere with the movement thus set up. for, however bad a conductor of heat ice may be, the unfrozen sea-water immediately in contact with the undersurface of the ice must needs be colder than that further off; and hence will constantly tend to descend through the subjacent warmer water. in this way, it would seem inevitable that the surface waters of the northern and southern frigid zones must, sooner or later, find their way to the bottom of the rest of the ocean; and there accumulate to a thickness dependent on the rate at which they absorb heat from the crust of the earth below, and from the surface water above. if this hypothesis be correct, it follows that, if any part of the ocean in warm latitudes is shut off from the influence of the cold polar underflow, the temperature of its deeps should be less cold than the temperature of corresponding depths in the open sea. now, in the mediterranean, nature offers a remarkable experimental proof of just the kind needed. it is a landlocked sea which runs nearly east and west, between the twenty-ninth and forty-fifth parallels of north latitude. roughly speaking, the average temperature of the air over it is ° fahr. in july and ° in january. this great expanse of water is divided by the peninsula of italy (including sicily), continuous with which is a submarine elevation carrying less than , feet of water, which extends from sicily to cape bon in africa, into two great pools--an eastern and a western. the eastern pool rapidly deepens to more than , feet, and sends off to the north its comparatively shallow branches, the adriatic and the aegean seas. the western pool is less deep, though it reaches some , feet. and, just as the western end of the eastern pool communicates by a shallow passage, not a sixth of its greatest depth, with the western pool, so the western pool is separated from the atlantic by a ridge which runs between capes trafalgar and spartel, on which there is hardly , feet of water. all the water of the mediterranean which lies deeper than about fathoms, therefore, is shut off from that of the atlantic, and there is no communication between the cold layer of the atlantic (below , fathoms) and the mediterranean. under these circumstances, what is the temperature of the mediterranean? everywhere below feet it is about ° fahr.; and consequently, at its greatest depths, it is some ° warmer than the corresponding depths of the atlantic. it seems extremely difficult to account for this difference in any other way, than by adopting the views so strongly and ably advocated by dr. carpenter, that, in the existing distribution of land and water, such a circulation of the water of the ocean does actually occur, as theoretically must occur, in the universal ocean, with which we started. it is quite another question, however, whether this theoretic circulation, true cause as it may be, is competent to give rise to such movements of sea-water, in mass, as those currents, which have commonly been regarded as northern extensions of the gulf-stream. i shall not venture to touch upon this complicated problem; but i may take occasion to remark that the cause of a much simpler phenomenon--the stream of atlantic water which sets through the straits of gibraltar, eastward, at the rate of two or three miles an hour or more, does not seem to be so clearly made out as is desirable. the facts appear to be that the water of the mediterranean is very slightly denser than that of the atlantic ( . to . ), and that the deep water of the mediterranean is slightly denser than that of the surface; while the deep water of the atlantic is, if anything, lighter than that of the surface. moreover, while a rapid superficial current is setting in (always, save in exceptionally violent easterly winds) through the straits of gibraltar, from the atlantic to the mediterranean, a deep undercurrent (together with variable side currents) is setting out through the straits, from the mediterranean to the atlantic. dr. carpenter adopts, without hesitation, the view that the cause of this indraught of atlantic water is to be sought in the much more rapid evaporation which takes place from the surface of the mediterranean than from that of the atlantic; and thus, by lowering the level of the former, gives rise to an indraught from the latter. but is there any sound foundation for the three assumptions involved here? firstly, that the evaporation from the mediterranean, as a whole, is much greater than that from the atlantic under corresponding parallels; secondly, that the rainfall over the mediterranean makes up for evaporation less than it does over the atlantic; and thirdly, supposing these two questions answered affirmatively: are not these sources of loss in the mediterranean fully covered by the prodigious quantity of fresh water which is poured into it by great rivers and submarine springs? consider that the water of the ebro, the rhine, the po, the danube, the don, the dnieper, and the nile, all flow directly or indirectly into the mediterranean; that the volume of fresh water which they pour into it is so enormous that fresh water may sometimes be baled up from the surface of the sea off the delta of the nile, while the land is not yet in sight; that the water of the black sea is half fresh, and that a current of three or four miles an hour constantly streams from it mediterraneanwards through the bosphorus;--consider, in addition, that no fewer than ten submarine springs of fresh water are known to burst up in the mediterranean, some of them so large that admiral smyth calls them "subterranean rivers of amazing volume and force"; and it would seem, on the face of the matter, that the sun must have enough to do to keep the level of the mediterranean down; and that, possibly, we may have to seek for the cause of the small superiority in saline contents of the mediterranean water in some condition other than solar evaporation. again, if the gibraltar indraught is the effect of evaporation, why does it go on in winter as well as in summer? all these are questions more easily asked than answered; but they must be answered before we can accept the gibraltar stream as an example of a current produced by indraught with any comfort. the mediterranean is not included in the _challenger's_ route, but she will visit one of the most promising and little explored of hydrographical regions--the north pacific, between polynesia and the asiatic and american shores; and doubtless the store of observations upon the currents of this region, which she will accumulate, when compared with what we know of the north atlantic, will throw a powerful light upon the present obscurity of the gulf-stream problem. iii on some of the results of the expedition of h.m.s. _challlenger_ [ ] in may, , i drew attention[ ] to the important problems connected with the physics and natural history of the sea, to the solution of which there was every reason to hope the cruise of h.m.s. _challenger_ would furnish important contributions. the expectation then expressed has not been disappointed. reports to the admiralty, papers communicated to the royal society, and large collections which have already been sent home, have shown that the _challenger's_ staff have made admirable use of their great opportunities; and that, on the return of the expedition in , their performance will be fully up to the level of their promise. indeed, i am disposed to go so far as to say, that if nothing more came of the _challengers_ expedition than has hitherto been yielded by her exploration of the nature of the sea bottom at great depths, a full scientific equivalent of the trouble and expense of her equipment would have been obtained. [footnote : see the preceding essay.] in order to justify this assertion, and yet, at the same time, not to claim more for professor wyville thomson and his colleagues than is their due, i must give a brief history of the observations which have preceded their exploration of this recondite field of research, and endeavour to make clear what was the state of knowledge in december, , and what new facts have been added by the scientific staff of the _challenger_. so far as i have been able to discover, the first successful attempt to bring up from great depths more of the sea bottom than would adhere to a sounding-lead, was made by sir john ross, in the voyage to the arctic regions which he undertook in . in the appendix to the narrative of that voyage, there will be found an account of a very ingenious apparatus called "clams"--a sort of double scoop--of his own contrivance, which sir john ross had made by the ship's armourer; and by which, being in baffin's bay, in ° ' n. and ° ' w., he succeeded in bringing up from , fathoms (or , feet), "several pounds" of a "fine green mud," which formed the bottom of the sea in this region. captain (now sir edward) sabine, who accompanied sir john ross on this cruise, says of this mud that it was "soft and greenish, and that the lead sunk several feet into it." a similar "fine green mud" was found to compose the sea bottom in davis straits by goodsir in . nothing is certainly known of the exact nature of the mud thus obtained, but we shall see that the mud of the bottom of the antarctic seas is described in curiously similar terms by dr. hooker, and there is no doubt as to the composition of this deposit. in , captain penny collected in assistance bay, in kingston bay, and in melville bay, which lie between ° ' and ° ' n., specimens of the residuum left by melted surface ice, and of the sea bottom in these localities. dr. dickie, of aberdeen, sent these materials to ehrenberg, who made out[ ] that the residuum of the melted ice consisted for the most part of the silicious cases of diatomaceous plants, and of the silicious spicula of sponges; while, mixed with these, were a certain number of the equally silicious skeletons of those low animal organisms, which were termed _polycistineoe_ by ehrenberg, but are now known as _radiolaria_. [footnote : _ueber neue anschauungen des kleinsten nördlichen polarlebens_.--monatsberichte d. k. akad. berlin, .] in , a very remarkable addition to our knowledge of the nature of the sea bottom in high northern latitudes was made by professor bailey of west point. lieutenant brooke, of the united states navy, who was employed in surveying the sea of kamschatka, had succeeded in obtaining specimens of the sea bottom from greater depths than any hitherto reached, namely from , fathoms ( , feet) in ° ' n., and ° ' e.; and from , fathoms ( , feet) in ° ' n. and ° ' e. on examining these microscopically, professor bailey found, as ehrenberg had done in the case of mud obtained on the opposite side of the arctic region, that the fine mud was made up of shells of _diatomacoe_, of spicula of sponges, and of _radiolaria_, with a small admixture of mineral matters, but without a trace of any calcareous organisms. still more complete information has been obtained concerning the nature of the sea bottom in the cold zone around the south pole. between the years and , sir james clark ross executed his famous antarctic expedition, in the course of which he penetrated, at two widely distant points of the antarctic zone, into the high latitudes of the shores of victoria land and of graham's land, and reached the parallel of ° s. sir james ross was himself a naturalist of no mean acquirements, and dr. hooker,[ ] the present president of the royal society, accompanied him as naturalist to the expedition, so that the observations upon the fauna and flora of the antarctic regions made during this cruise were sure to have a peculiar value and importance, even had not the attention of the voyagers been particularly directed to the importance of noting the occurrence of the minutest forms of animal and vegetable life in the ocean. [footnote : now sir joseph hooker. .] among the scientific instructions for the voyage drawn up by a committee of the royal society, however, there is a remarkable letter from von humboldt to lord minto, then first lord of the admiralty, in which, among other things, he dwells upon the significance of the researches into the microscopic composition of rocks, and the discovery of the great share which microscopic organisms take in the formation of the crust of the earth at the present day, made by ehrenberg in the years - . ehrenberg, in fact, had shown that the extensive beds of "rotten-stone" or "tripoli" which occur in various parts of the world, and notably at bilin in bohemia, consisted of accumulations of the silicious cases and skeletons of _diatomaceoe_, sponges, and _radiolaria_; he had proved that similar deposits were being formed by _diatomaceoe_, in the pools of the thiergarten in berlin and elsewhere, and had pointed out that, if it were commercially worth while, rotten-stone might be manufactured by a process of diatom-culture. observations conducted at cuxhaven in , had revealed the existence, at the surface of the waters of the baltic, of living diatoms and _radiolaria_ of the same species as those which, in a fossil state, constitute extensive rocks of tertiary age at caltanisetta, zante, and oran, on the shores of the mediterranean. moreover, in the fresh-water rotten-stone beds of bilin, ehrenberg had traced out the metamorphosis, effected apparently by the action of percolating water, of the primitively loose and friable deposit of organized particles, in which the silex exists in the hydrated or soluble condition. the silex, in fact, undergoes solution and slow redeposition, until, in ultimate result, the excessively fine-grained sand, each particle of which is a skeleton, becomes converted into a dense opaline stone, with only here and there an indication of an organism. from the consideration of these facts, ehrenberg, as early as the year , had arrived at the conclusion that rocks, altogether similar to those which constitute a large part of the crust of the earth, must be forming, at the present day, at the bottom of the sea; and he threw out the suggestion that even where no trace of organic structure is to be found in the older rocks, it may have been lost by metamorphosis.[ ] [footnote : _ueber die noch jetzt zahlreich lebende thierarten der kreidebildung und den organismus der polythalamien. abhandlungen der kön. akad. der wissenchaften._ . _berlin_. . i am afraid that this remarkable paper has been somewhat overlooked in the recent discussions of the relation of ancient rocks to modern deposits.] the results of the antarctic exploration, as stated by dr. hooker in the "botany of the antarctic voyage," and in a paper which he read before the british association in , are of the greatest importance in connection with these views, and they are so clearly stated in the former work, which is somewhat inaccessible, that i make no apology for quoting them at length-- "the waters and the ice of the south polar ocean were alike found to abound with microscopic vegetables belonging to the order _diatomaceoe_. though much too small to be discernible by the naked eye, they occurred in such countless myriads as to stain the berg and the pack ice wherever they were washed by the swell of the sea; and, when enclosed in the congealing surface of the water, they imparted to the brash and pancake ice a pale ochreous colour. in the open ocean, northward of the frozen zone, this order, though no doubt almost universally present, generally eludes the search of the naturalist; except when its species are congregated amongst that mucous scum which is sometimes seen floating on the waves, and of whose real nature we are ignorant; or when the coloured contents of the marine animals who feed on these algae are examined. to the south, however, of the belt of ice which encircles the globe, between the parallels of ° and ° s., and in the waters comprised between that belt and the highest latitude ever attained by man, this vegetation is very conspicuous, from the contrast between its colour and the white snow and ice in which it is imbedded. insomuch, that in the eightieth degree, all the surface ice carried along by the currents, the sides of every berg and the base of the great victoria barrier itself, within reach of the swell, were tinged brown, as if the polar waters were charged with oxide of iron. "as the majority of these plants consist of very simple vegetable cells, enclosed in indestructible silex (as other algae are in carbonate of lime), it is obvious that the death and decomposition of such multitudes must form sedimentary deposits, proportionate in their extent to the length and exposure of the coast against which they are washed, in thickness to the power of such agents as the winds, currents, and sea, which sweep them more energetically to certain positions, and in purity, to the depth of the water and nature of the bottom. hence we detected their remains along every icebound shore, in the depths of the adjacent ocean, between and fathoms. off victoria barrier (a perpendicular wall of ice between one and two hundred feet above the level of the sea) the bottom of the ocean was covered with a stratum of pure white or green mud, composed principally of the silicious shells of the _diatomaceoe_. these, on being put into water, rendered it cloudy like milk, and took many hours to subside. in the very deep water off victoria and graham's land, this mud was particularly pure and fine; but towards the shallow shores there existed a greater or less admixture of disintegrated rock and sand; so that the organic compounds of the bottom frequently bore but a small proportion to the inorganic." ... "the universal existence of such an invisible vegetation as that of the antarctic ocean, is a truly wonderful fact, and the more from its not being accompanied by plants of a high order. during the years we spent there, i had been accustomed to regard the phenomena of life as differing totally from what obtains throughout all other latitudes, for everything living appeared to be of animal origin. the ocean swarmed with _mollusca_, and particularly entomostracous _crustacea_, small whales, and porpoises; the sea abounded with penguins and seals, and the air with birds; the animal kingdom was ever present, the larger creatures preying on the smaller, and these again on smaller still; all seemed carnivorous. the herbivorous were not recognised, because feeding on a microscopic herbage, of whose true nature i had formed an erroneous impression. it is, therefore, with no little satisfaction that i now class the _diatomaceoe_ with plants, probably maintaining in the south polar ocean that balance between the vegetable and the animal kingdoms which prevails over the surface of our globe. nor is the sustenance and nutrition of the animal kingdom the only function these minute productions may perform; they may also be the purifiers of the vitiated atmosphere, and thus execute in the antarctic latitudes the office of our trees and grass turf in the temperate regions, and the broad leaves of the palm, &c., in the tropics." ... with respect to the distribution of the _diatomaceoe_, dr. hooker remarks:-- "there is probably no latitude between that of spitzbergen and victoria land, where some of the species of either country do not exist: iceland, britain, the mediterranean sea, north and south america, and the south sea islands, all possess antarctic _diatomaceoe_. the silicious coats of species only known living in the waters of the south polar ocean, have, during past ages, contributed to the formation of rocks; and thus they outlive several successive creations of organized beings. the phonolite stones of the rhine, and the tripoli stone, contain species identical with what are now contributing to form a sedimentary deposit (and perhaps, at some future period, a bed of rock) extending in one continuous stratum for measured miles. i allude to the shores of the victoria barrier, along whose coast the soundings examined were invariably charged with diatomaceous remains, constituting a bank which stretches miles north from the base of victoria barrier, while the average depth of water above it is fathoms, or , feet. again, some of the antarctic species have been detected floating in the atmosphere which overhangs the wide ocean between africa and america. the knowledge of this marvellous fact we owe to mr. darwin, who, when he was at sea off the cape de verd islands, collected an impalpable powder which fell on captain fitzroy's ship. he transmitted this dust to ehrenberg, who ascertained it to consist of the silicious coats, chiefly of american _diatomaceoe_, which were being wafted through the upper region of the air, when some meteorological phenomena checked them in their course and deposited them on the ship and surface of the ocean. "the existence of the remains of many species of this order (and amongst them some antarctic ones) in the volcanic ashes, pumice, and scoriae of active and extinct volcanoes (those of the mediterranean sea and ascension island, for instance) is a fact bearing immediately upon the present subject. mount erebus, a volcano , feet high, of the first class in dimensions and energetic action, rises at once from the ocean in the seventy-eighth degree of south latitude, and abreast of the _diatomaceoe_ bank, which reposes in part on its base. hence it may not appear preposterous to conclude that, as vesuvius receives the waters of the mediterranean, with its fish, to eject them by its crater, so the subterranean and subaqueous forces which maintain mount erebus in activity may occasionally receive organic matter from the bank, and disgorge it, together with those volcanic products, ashes and pumice. "along the shores of graham's land and the south shetland islands, we have a parallel combination of igneous and aqueous action, accompanied with an equally copious supply of _diatomaceoe_. in the gulf of erebus and terror, fifteen degrees north of victoria land, and placed on the opposite side of the globe, the soundings were of a similar nature with those of the victoria land and barrier, and the sea and ice as full of _diatomaceoe_. this was not only proved by the deep sea lead, but by the examination of bergs which, once stranded, had floated off and become reversed, exposing an accumulation of white friable mud frozen to their bases, which abounded with these vegetable remains." the _challenger_ has explored the antarctic seas in a region intermediate between those examined by sir james ross's expedition; and the observations made by dr. wyville thomson and his colleagues in every respect confirm those of dr. hooker:-- "on the th of february, lat. ° ' s., long. ° ' e., and march , lat. ° ' s., long. ° ' e., the sounding instrument came up filled with a very fine cream-coloured paste, which scarcely effervesced with acid, and dried into a very light, impalpable, white powder. this, when examined under the microscope, was found to consist almost entirely of the frustules of diatoms, some of them wonderfully perfect in all the details of their ornament, and many of them broken up. the species of diatoms entering into this deposit have not yet been worked up, but they appear to be referable chiefly to the genera _fragillaria, coscinodiscus, choetoceros, asteromphalus_, and _dictyocha_, with fragments of the separated rods of a singular silicious organism, with which we were unacquainted, and which made up a large proportion of the finer matter of this deposit. mixed with the diatoms there were a few small _globigerinoe_, some of the tests and spicules of radiolarians, and some sand particles; but these foreign bodies were in too small proportion to affect the formation as consisting practically of diatoms alone. on the th of february, in lat. °, ' s., long., ° " e., a little to the north of the heard islands, the tow-net, dragging a few fathoms below the surface, came up nearly filled with a pale yellow gelatinous mass. this was found to consist entirely of diatoms of the same species as those found at the bottom. by far the most abundant was the little bundle of silicious rods, fastened together loosely at one end, separating from one another at the other end, and the whole bundle loosely twisted into a spindle. the rods are hollow, and contain the characteristic endochrome of the _diatomaceoe_. like the _globigerina_ ooze, then, which it succeeds to the southward in a band apparently of no great width, the materials of this silicious deposit are derived entirely from the surface and intermediate depths. it is somewhat singular that diatoms did not appear to be in such large numbers on the surface over the diatom ooze as they were a little further north. this may perhaps be accounted for by our not having struck their belt of depth with the tow-net; or it is possible that when we found it on the th of february the bottom deposit was really shifted a little to the south by the warm current, the excessively fine flocculent _débris_ of the diatoms taking a certain time to sink. the belt of diatom ooze is certainly a little further to the southward in long. ° e., in the path of the reflux of the agulhas current, than in long. ° e. "all along the edge of the ice-pack--everywhere, in fact, to the south of the two stations--on the th of february on our southward voyage, and on the rd of march on our return, we brought up fine sand and grayish mud, with small pebbles of quartz and felspar, and small fragments of mica- slate, chlorite-slate, clay-slate, gneiss, and granite. this deposit, i have no doubt, was derived from the surface like the others, but in this case by the melting of icebergs and the precipitation of foreign matter contained in the ice. "we never saw any trace of gravel or sand, or any material necessarily derived from land, on an iceberg. several showed vertical or irregular fissures filled with discoloured ice or snow; but, when looked at closely, the discoloration proved usually to be very slight, and the effect at a distance was usually due to the foreign material filling the fissure reflecting light less perfectly than the general surface of the berg. i conceive that the upper surface of one of these great tabular southern icebergs, including by far the greater part of its bulk, and culminating in the portion exposed above the surface of the sea, was formed by the piling up of successive layers of snow during the period, amounting perhaps to several centuries, during which the ice-cap was slowly forcing itself over the low land and out to sea over a long extent of gentle slope, until it reached a depth considerably above fathoms, when the lower specific weight of the ice caused an upward strain which at length overcame the cohesion of the mass, and portions were rent off and floated away. if this be the true history of the formation of these icebergs, the absence of all land _débris_ in the portion exposed above the surface of the sea is readily understood. if any such exist, it must be confined to the lower part of the berg, to that part which has at one time or other moved on the floor of the ice-cap. "the icebergs, when they are first dispersed, float in from to fathoms. when, therefore, they have been drifted to latitudes of ° or ° s., the bottom of the berg just reaches the layer at which the temperature of the water is distinctly rising, and it is rapidly melted, and the mud and pebbles with which it is more or less charged are precipitated. that this precipitation takes place all over the area where the icebergs are breaking up, constantly, and to a considerable extent, is evident from the fact of the soundings being entirely composed of such deposits; for the diatoms, _globigerinoe_, and radiolarians are present on the surface in large numbers; and unless the deposit from the ice were abundant it would soon be covered and masked by a layer of the exuvia of surface organisms." the observations which have been detailed leave no doubt that the antarctic sea bottom, from a little to the south of the fiftieth parallel, as far as ° s., is being covered by a fine deposit of silicious mud, more or less mixed, in some parts, with the ice-borne _débris_ of polar lands and with the ejections of volcanoes. the silicious particles which constitute this mud, are derived, in part, from the diatomaceous plants and radiolarian animals which throng the surface, and, in part, from the spicula of sponges which live at the bottom. the evidence respecting the corresponding arctic area is less complete, but it is sufficient to justify the conclusion that an essentially similar silicious cap is being formed around the northern pole. there is no doubt that the constituent particles of this mud may agglomerate into a dense rock, such as that formed at oran on the shores of the mediterranean, which is made up of similar materials. moreover, in the case of freshwater deposits of this kind it is certain that the action of percolating water may convert the originally soft and friable, fine-grained sandstone into a dense, semi-transparent opaline stone, the silicious organized skeletons being dissolved, and the silex re-deposited in an amorphous state. whether such a metamorphosis as this occurs in submarine deposits, as well as in those formed in fresh water, does not appear; but there seems no reason to doubt that it may. and hence it may not be hazardous to conclude that very ordinary metamorphic agencies may convert these polar caps into a form of quartzite. in the great intermediate zone, occupying some ° of latitude, which separates the circumpolar arctic and antarctic areas of silicious deposit, the diatoms and _radiolaria_ of the surface water and the sponges of the bottom do not die out, and, so far as some forms are concerned, do not even appear to diminish in total number; though, on a rough estimate, it would appear that the proportion of _radiolaria_ to diatoms is much greater than in the colder seas. nevertheless the composition of the deep-sea mud of this intermediate zone is entirely different from that of the circumpolar regions. the first exact information respecting the nature of this mud at depths greater than , fathoms was given by ehrenberg, in the account which he published in the "monatsberichte" of the berlin academy for the year , of the soundings obtained by lieut. berryman, of the united states navy, in the north atlantic, between newfoundland and the azores. observations which confirm those of ehrenberg in all essential respects have been made by professor bailey, myself, dr. wallich, dr. carpenter, and professor wyville thomson, in their earlier cruises; and the continuation of the _globigerina_ ooze over the south pacific has been proved by the recent work of the _challenger_, by which it is also shown, for the first time, that, in passing from the equator to high southern latitudes, the number and variety of the _foraminifera_ diminishes, and even the _globigerinoe_ become dwarfed. and this result, it will be observed, is in entire accordance with the fact already mentioned that, in the sea of kamschatka, the deep-sea mud was found by bailey to contain no calcareous organisms. thus, in the whole of the "intermediate zone," the silicious deposit which is being formed there, as elsewhere, by the accumulation of sponge- spicula, _radiolaria_, and diatoms, is obscured and overpowered by the immensely greater amount of calcareous sediment, which arises from the aggregation of the skeletons of dead _foraminifera_. the similarity of the deposit, thus composed of a large percentage of carbonate of lime, and a small percentage of silex, to chalk, regarded merely as a kind of rock, which was first pointed out by ehrenberg,[ ] is now admitted on all hands; nor can it be reasonably doubted, that ordinary metamorphic agencies are competent to convert the "modern chalk" into hard limestone or even into crystalline marble. [footnote : the following passages in ehrenberg's memoir on _the organisms in the chalk which are still living_ ( ), are conclusive:-- " . the dawning period of the existing living organic creation, if such a period is distinguishable (which is doubtful), can only be supposed to have existed on the other side of, and below, the chalk formation; and thus, either the chalk, with its widespread and thick beds, must enter into the series of newer formations; or some of the accepted four great geological periods, the quaternary, tertiary, and secondary formations, contain organisms which still live. it is more probable, in the proportion of to , that the transition or primary period is not different, but that it is only more difficult to examine and understand, by reason of the gradual and prolonged chemical decomposition and metamorphosis of many of its organic constituents." " . by the mass-forming _infasoria_ and _polythalamia_, secondary are not distinguishable from tertiary formations; and, from what has been said, it is possible that, at this very day, rock masses are forming in the sea, and being raised by volcanic agencies, the constitution of which, on the whole, is altogether similar to that of the chalk. the chalk remains distinguishable by its organic remains as a formation, but not as a kind of rock."] ehrenberg appears to have taken it for granted that the _globigerinoe_ and other _foraminifera_ which are found in the deep-sea mud, live at the great depths in which their remains are found; and he supports this opinion by producing evidence that the soft parts of these organisms are preserved, and may be demonstrated by removing the calcareous matter with dilute acids. in , the evidence for and against this conclusion appeared to me to be insufficient to warrant a positive conclusion one way or the other, and i expressed myself in my report to the admiralty on captain dayman's soundings in the following terms:-- "when we consider the immense area over which this deposit is spread, the depth at which its formation is going on, and its similarity to chalk, and still more to such rocks as the marls of caltanisetta, the question, whence are all these organisms derived? becomes one of high scientific interest. "three answers have suggested themselves:-- "in accordance with the prevalent view of the limitation of life to comparatively small depths, it is imagined either: , that these organisms have drifted into their present position from shallower waters; or , that they habitually live at the surface of the ocean, and only fall down into their present position. " . i conceive that the first supposition is negatived by the extremely marked zoological peculiarity of the deep-sea fauna. "had the _globigerinoe_ been drifted into their present position from shallow water, we should find a very large proportion of the characteristic inhabitants of shallow waters mixed with them, and this would the more certainly be the case, as the large _globigerinoe_, so abundant in the deep-sea soundings, are, in proportion to their size, more solid and massive than almost any other _foraminifera_. but the fact is that the proportion of other _foraminifera_ is exceedingly small, nor have i found as yet, in the deep-sea deposits, any such matters as fragments of molluscous shells, of _echini_, &c., which abound in shallow waters, and are quite as likely to be drifted as the heavy _globigerinoe_. again, the relative proportions of young and fully formed _globigerinoe_ seem inconsistent with the notion that they have travelled far. and it seems difficult to imagine why, had the deposit been accumulated in this way, _coscinodisci_ should so almost entirely represent the _diatomaceoe_. " . the second hypothesis is far more feasible, and is strongly supported by the fact that many _polycistineoe [radiolaria]_ and _coscinodisci_ are well known to live at the surface of the ocean. mr. macdonald, assistant- surgeon of h.m.s. _herald_, now in the south-western pacific, has lately sent home some very valuable observations on living forms of this kind, met with in the stomachs of oceanic mollusks, and therefore certainly inhabitants of the superficial layer of the ocean. but it is a singular circumstance that only one of the forms figured by mr. macdonald is at all like a _globigerina_, and there are some peculiarities about even this which make me greatly doubt its affinity with that genus. the form, indeed, is not unlike that of a _globigerina_, but it is provided with long radiating processes, of which i have never seen any trace in _globigerina_. did they exist, they might explain what otherwise is a great objection to this view, viz., how is it conceivable that the heavy _globigerina_ should maintain itself at the surface of the water? "if the organic bodies in the deep-sea soundings have neither been drifted, nor have fallen from above, there remains but one alternative-- they must have lived and died where they are. "important objections, however, at once suggest themselves to this view. how can animal life be conceived to exist under such conditions of light, temperature, pressure, and aeration as must obtain at these vast depths? "to this one can only reply that we know for a certainty that even very highly-organized animals do continue to live at a depth of and fathoms, inasmuch as they have been dredged up thence; and that the difference in the amount of light and heat at and at , fathoms is probably, so to speak, very far less than the difference in complexity of organisation between these animals and the humbler _protozoa_ and _protophyta_ of the deep-sea soundings. "i confess, though as yet far from regarding it proved that the _globigerinoe_ live at these depths, the balance of probabilities seems to me to incline in that direction. and there is one circumstance which weighs strongly in my mind. it may be taken as a law that any genus of animals which is found far back in time is capable of living under a great variety of circumstances as regards light, temperature, and pressure. now, the genus _globigerina_ is abundantly represented in the cretaceous epoch, and perhaps earlier. "i abstain, however, at present from drawing any positive conclusions, preferring rather to await the result of more extended observations."[ ] [footnote : appendix to report on deep-sea soundings in the atlantic ocean, by lieut.-commander joseph dayman. .] dr. wallich, professor wyville thomson, and dr. carpenter concluded that the _globigerinoe_ live at the bottom. dr. wallich writes in --"by sinking very fine gauze nets to considerable depths, i have repeatedly satisfied myself that _globigerina_ does not occur in the superficial strata of the ocean."[ ] moreover, having obtained certain living star- fish from a depth of , fathoms, and found their stomachs full of "fresh-looking _globigerinoe_" and their _débris_--he adduces this fact in support of his belief that the _globigerinoe_ live at the bottom. [footnote : the _north atlantic sea-bed_, p. .] on the other hand, müller, haeckel, major owen, mr. gwyn jeffries, and other observers, found that _globigerinoe_, with the allied genera _orbulina_ and _pulvinulina_, sometimes occur abundantly at the surface of the sea, the shells of these pelagic forms being not unfrequently provided with the long spines noticed by macdonald; and in and , major owen more especially insisted on the importance of this fact. the recent work of the _challenger_ fully confirms major owen's statement. in the paper recently published in the proceedings of the royal society,[ ] from which a quotation has already been made, professor wyville thomson says:-- "i had formed and expressed a very strong opinion on the matter. it seemed to me that the evidence was conclusive that the _foraminifera_ which formed the _globigerina_ ooze lived on the bottom, and that the occurrence of individuals on the surface was accidental and exceptional; but after going into the thing carefully, and considering the mass of evidence which has been accumulated by mr. murray, i now admit that i was in error; and i agree with him that it may be taken as proved that all the materials of such deposits, with the exception, of course, of the remains of animals which we now know to live at the bottom at all depths, which occur in the deposit as foreign bodies, are derived from the surface. [footnote : "preliminary notes on the nature of the sea-bottom procured by the soundings of h.m.s. _challenger_ during her cruise in the southern seas, in the early part of the year ."--_proceedings of the royal society_, nov. , .] "mr. murray has combined with a careful examination of the soundings a constant use of the tow-net, usually at the surface, but also at depths of from ten to one hundred fathoms; and he finds the closest relation to exist between the surface fauna of any particular locality and the deposit which is taking place at the bottom. in all seas, from the equator to the polar ice, the tow-net contains _globigerinoe_. they are more abundant and of a larger size in warmer seas; several varieties, attaining a large size and presenting marked varietal characters, are found in the intertropical area of the atlantic. in the latitude of kerguelen they are less numerous and smaller, while further south they are still more dwarfed, and only one variety, the typical _globigerina bulloides_, is represented. the living _globigerinoe_ from the tow-net are singularly different in appearance from the dead shells we find at the bottom. the shell is clear and transparent, and each of the pores which penetrate it is surrounded by a raised crest, the crest round adjacent pores coalescing into a roughly hexagonal network, so that the pores appear to lie at the bottom of a hexagonal pit. at each angle of this hexagon the crest gives off a delicate flexible calcareous spine, which is sometimes four or five times the diameter of the shell in length. the spines radiate symmetrically from the direction of the centre of each chamber of the shell, and the sheaves of long transparent needles crossing one another in different directions have a very beautiful effect. the smaller inner chambers of the shell are entirely filled with an orange-yellow granular sarcode; and the large terminal chamber usually contains only a small irregular mass, or two or three small masses run together, of the same yellow sarcode stuck against one side, the remainder of the chamber being empty. no definite arrangement and no approach to structure was observed in the sarcode, and no differentiation, with the exception of round bright-yellow oil-globules, very much like those found in some of the radiolarians, which are scattered, apparently irregularly, in the sarcode. we never have been able to detect, in any of the large number of _globigerinoe_ which we have examined, the least trace of pseudopodia, or any extension, in any form, of the sarcode beyond the shell. * * * * * "in specimens taken with the tow-net the spines are very usually absent; but that is probably on account of their extreme tenuity; they are broken off by the slightest touch. in fresh examples from the surface, the dots indicating the origin of the lost spines may almost always be made out with a high power. there are never spines on the _globigerinoe_ from the bottom, even in the shallowest water." there can now be no doubt, therefore, that _globigerinoe_ live at the top of the sea; but the question may still be raised whether they do not also live at the bottom. in favour of this view, it has been urged that the shells of the _globigerinoe_ of the surface never possess such thick walls as those which are fouled at the bottom, but i confess that i doubt the accuracy of this statement. again, the occurrence of minute _globigerinoe_ in all stages of development, at the greatest depths, is brought forward as evidence that they live _in situ_. but considering the extent to which the surface organisms are devoured, without discrimination of young and old, by _salpoe_ and the like, it is not wonderful that shells of all ages should be among the rejectamenta. nor can the presence of the soft parts of the body in the shells which form the _globigerina_ ooze, and the fact, if it be one, that animals living at the bottom use them as food, be considered as conclusive evidence that the _globigerinoe_ live at the bottom. such as die at the surface, and even many of those which are swallowed by other animals, may retain much of their protoplasmic matter when they reach the depths at which the temperature sinks to ° or ° fahrenheit, where decomposition must become exceedingly slow. another consideration appears to me to be in favour of the view that the _globigerinoe_ and their allies are essentially surface animals. this is the fact brought out by the _challenger's_ work, that they have a southern limit of distribution, which can hardly depend upon anything but the temperature of the surface water. and it is to be remarked that this southern limit occurs at a lower latitude in the antarctic seas than it does in the north atlantic. according to dr. wallich ("the north atlantic sea bed," p. ) _globigerina_ is the prevailing form in the deposits between the faroe islands and iceland, and between iceland and east greenland--or, in other words, in a region of the sea-bottom which lies altogether north of the parallel of ° n.; while in the southern seas, the _globigerinoe_ become dwarfed and almost disappear between ° and ° s. on the other hand, in the sea of kamschatka, the _globigerinoe_ have vanished in ° n., so that the persistence of the _globigerina_ ooze in high latitudes, in the north atlantic, would seem to depend on the northward curve of the isothermals peculiar to this region; and it is difficult to understand how the formation of _globigerina_ ooze can be affected by this climatal peculiarity unless it be effected by surface animals. whatever may be the mode of life of the _foraminifera_, to which the calcareous element of the deep-sea "chalk" owes its existence, the fact that it is the chief and most widely spread material of the sea-bottom in the intermediate zone, throughout both the atlantic and pacific oceans, and the indian ocean, at depths from a few hundred to over two thousand fathoms, is established. but it is not the only extensive deposit which is now taking place. in , count pourtalès, an officer of the united states coast survey, which has done so much for scientific hydrography, observed, that the mud forming the sea-bottom at depths of one hundred and fifty fathoms, in ° ' n., ° ' w., off the coast of florida, was "a mixture, in about equal proportions, of _globigerinoe_ and black sand, probably greensand, as it makes a green mark when crushed on paper." professor bailey, examining these grains microscopically, found that they were casts of the interior cavities of _foraminifera_, consisting of a mineral known as _glauconite_, which is a silicate of iron and alumina. in these casts the minutest cavities and finest tubes in the foraminifer were sornetilnes reproduced in solid counterparts of the glassy mineral, while the calcareous original had been entirely dissolved away. contemporaneously with these observations, the indefatigable ehrenberg had discovered that the "greensands" of the geologist were largely made up of casts of a similar character, and proved the existence of _foraminifera_ at a very ancient geological epoch, by discovering such casts in a greensand of lower silurian age, which occurs near st. petersburg. subsequently, messrs. parker and jones discovered similar casts in process of formation, the original shell not having disappeared, in specimens of the sea-bottom of the australian seas, brought home by the late professor jukes. and the _challenger_ has observed a deposit of a similar character in the course of the agulhas current, near the cape of good hope, and in some other localities not yet defined. it would appear that this infiltration of _foraminifera_ shells with _glauconite_ does not take place at great depths, but rather in what may be termed a sublittoral region, ranging from a hundred to three hundred fathoms. it cannot be ascribed to any local cause, for it takes place, not only over large areas in the gulf of mexico and the coast of florida, but in the south atlantic and in the pacific. but what are the conditions which determine its occurrence, and whence the silex, the iron, and the alumina (with perhaps potash and some other ingredients in small quantity) of which the _glauconite_ is composed, proceed, is a point on which no light has yet been thrown. for the present we must be content with the fact that, in certain areas of the "intermediate zone," greensand is replacing and representing the primitively calcareo- silicious ooze. the investigation of the deposits which are now being formed in the basin of the mediterranean, by the late professor edward forbes, by professor williamson and more recently by dr. carpenter, and a comparison of the results thus obtained with what is known of the surface fauna, have brought to light the remarkable fact, that while the surface and the shallows abound with _foraminifera_ and other calcareous shelled organisms, the indications of life become scanty at depths beyond or fathoms, while almost all traces of it disappear at greater depths, and at , to , fathoms the bottom is covered with a fine clay. dr. carpenter has discussed the significance of this remarkable fact, and he is disposed to attribute the absence of life at great depths, partly to the absence of any circulation of the water of the mediterranean at such depths, and partly to the exhaustion of the oxygen of the water by the organic matter contained in the fine clay, which he conceives to be formed by the finest particles of the mud brought down by the rivers which flow into the mediterranean. however this may be, the explanation thus offered of the presence of the fine mud, and of the absence of organisms which ordinarily live at the bottom, does not account for the absence of the skeletons of the organisms which undoubtedly abound at the surface of the mediterranean; and it would seem to have no application to the remarkable fact discovered by the _challenger_, that in the open atlantic and pacific oceans, in the midst of the great intermediate zone, and thousands of miles away from the embouchure of any river, the sea-bottom, at depths approaching to and beyond , fathoms, no longer consists of _globigerina_ ooze, but of an excessively fine red clay. professor thomson gives the following account of this capital discovery:-- "according to our present experience, the deposit of _globigerina_ ooze is limited to water of a certain depth, the extreme limit of the pure characteristic formation being placed at a depth of somewhere about , fathoms. crossing from these shallower regions occupied by the ooze into deeper soundings, we find, universally, that the calcareous formation gradually passes into, and is finally replaced by, an extremely fine pure clay, which occupies, speaking generally, all depths below , fathoms, and consists almost entirely of a silicate of the red oxide of iron and alumina. the transition is very slow, and extends over several hundred fathoms of increasing depth; the shells gradually lose their sharpness of outline, and assume a kind of 'rotten' look and a brownish colour, and become more and more mixed with a fine amorphous red-brown powder, which increases steadily in proportion until the lime has almost entirely disappeared. this brown matter is in the finest possible state of subdivision, so fine that when, after sifting it to separate any organisms it might contain, we put it into jars to settle, it remained for days in suspension, giving the water very much the appearance and colour of chocolate. "in indicating the nature of the bottom on the charts, we came, from experience and without any theoretical considerations, to use three terms for soundings in deep water. two of these, gl. oz. and r. cl., were very definite, and indicated strongly-marked formations, with apparently but few characters in common; but we frequently got soundings which we could not exactly call '_globigerina_ ooze' or 'red clay,' and before we were fully aware of the nature of these, we were in the habit of indicating them as 'grey ooze' (gr. oz.) we now recognise the 'grey ooze' as an intermediate stage between the _globigerina_ ooze and the red clay; we find that on one side, as it were, of an ideal line, the red clay contains more and more of the material of the calcareous ooze, while on the other, the ooze is mixed with an increasing proportion of 'red clay.' "although we have met with the same phenomenon so frequently, that we were at length able to predict the nature of the bottom from the depth of the soundings with absolute certainty for the atlantic and the southern sea, we had, perhaps, the best opportunity of observing it in our first section across the atlantic, between teneriffe and st. thomas. the first four stations on this section, at depths from , to , fathoms, show _globigerina_ ooze. from the last of these, which is about miles from teneriffe, the depth gradually increases to , fathoms at , and , fathoms at miles from teneriffe. the bottom in these two soundings might have been called 'grey ooze,' for although its nature has altered entirely from the _globigerina_ ooze, the red clay into which it is rapidly passing still contains a considerable admixture of carbonate of lime. "the depth goes on increasing to a distance of , miles from teneriffe, when it reaches , fathoms; there the clay is pure and smooth, and contains scarcely a trace of lime. from this great depth the bottom gradually rises, and, with decreasing depth, the grey colour and the calcareous composition of the ooze return. three soundings in , , , , and , fathoms on the 'dolphin rise' gave highly characteristic examples of the _globigerina_ formation. passing from the middle plateau of the atlantic into the western trough, with depths a little over , fathoms, the red clay returned in all its purity; and our last sounding, in , fathoms, before reaching sombrero, restored the _globigerina_ ooze with its peculiar associated fauna. "this section shows also the wide extension and the vast geological importance of the red clay formation. the total distance from teneriffe to sombrero is about , miles. proceeding from east to west, we have-- about miles of volcanic mud and sand, " " _globigerina_ ooze, " , " red clay, " " _globigerina_ ooze, " " red clay, " " _globigerina_ ooze; giving a total of , miles of red clay to miles of _globigerina_ ooze. "the nature and origin of this vast deposit of clay is a question of the very greatest interest; and although i think there can be no doubt that it is in the main solved, yet some matters of detail are still involved in difficulty. my first impression was that it might be the most minutely divided material, the ultimate sediment produced by the disintegration of the land, by rivers and by the action of the sea on exposed coasts, and held in suspension and distributed by ocean currents, and only making itself manifest in places unoccupied by the _globigerina_ ooze. several circumstances seemed, however, to negative this mode of origin. the formation seemed too uniform: wherever we met with it, it had the same character, and it only varied in composition in containing less or more carbonate of lime. "again, the were gradually becoming more and more convinced that all the important elements of the _globigerina_ ooze lived on the surface, and it seemed evident that, so long as the condition on the surface remained the same, no alteration of contour at the bottom could possibly prevent its accumulation; and the surface conditions in the mid-atlantic were very uniform, a moderate current of a very equal temperature passing continuously over elevations and depressions, and everywhere yielding to the tow-net the ooze-forming _foraminifera_ in the same proportion. the mid-atlantic swarms with pelagic _mollusca_, and, in moderate depths, the shells of these are constantly mixed with the _globigerina_ ooze, sometimes in number sufficient to make up a considerable portion of its bulk. it is clear that these shells must fall in equal numbers upon the red clay, but scarcely a trace of one of them is ever brought up by the dredge on the red clay area. it might be possible to explain the absence of shell-secreting animals living on the bottom, on the supposition that the nature of the deposit was injurious to them; but then the idea of a current sufficiently strong to sweep them away is negatived by the extreme fineness of the sediment which is being laid down; the absence of surface shells appears to be intelligible only on the supposition that they are in some way removed. "we conclude, therefore, that the 'red clay' is not an additional substance introduced from without, and occupying certain depressed regions on account of some law regulating its deposition, but that it is produced by the removal, by some means or other, over these areas, of the carbonate of lime, which forms probably about per cent. of the material of the _globigerina_ ooze. we can trace, indeed, every successive stage in the removal of the carbonate of lime in descending the slope of the ridge or plateau where the _globigerina_ ooze is forming, to the region of the clay. we find, first, that the shells of pteropods and other surface _mollusca_ which are constantly falling on the bottom, are absent, or, if a few remain, they are brittle and yellow, and evidently decaying rapidly. these shells of _mollusca_ decompose more easily and disappear sooner than the smaller, and apparently more delicate, shells of rhizopods. the smaller _foraminifera_ now give way, and are found in lessening proportion to the larger; the coccoliths first lose their thin outer border and then disappear; and the clubs of the rhabdoliths get worn out of shape, and are last seen, under a high power, as infinitely minute cylinders scattered over the field. the larger _foraminifera_ are attacked, and instead of being vividly white and delicately sculptured, they become brown and worn, and finally they break up, each according to its fashion; the chamber-walls of _globigerina_ fall into wedge-shaped pieces, which quickly disappear, and a thick rough crust breaks away from the surface of _orbulina_, leaving a thin inner sphere, at first beautifully transparent, but soon becoming opaque and crumbling away. "in the meantime the proportion of the amorphous 'red clay' to the calcareous elements of all kinds increases, until the latter disappear, with the exception of a few scattered shells of the larger _foraminifera_, which are still found even in the most characteristic samples of the 'red clay.' "there seems to be no room left for doubt that the red clay is essentially the insoluble residue, the _ash_, as it were, of the calcareous organisms which form the _globigerina_ ooze, after the calcareous matter has been by some means removed. an ordinary mixture of calcareous _foraminifera_ with the shells of pteropods, forming a fair sample of _globigerina_ ooze from near st. thomas, was carefully washed, and subjected by mr. buchanan to the action of weak acid; and he found that there remained after the carbonate of lime had been removed, about per cent. of a reddish mud, consisting of silica, alumina, and the red oxide of iron. this experiment has been frequently repeated with different samples of _globigerina_ ooze, and always with the result that a small proportion of a red sediment remains, which possesses all the characters of the red clay." * * * * * "it seems evident from the observations here recorded, that _clay_, which we have hitherto looked upon as essentially the product of the disintegration of older rocks, may be, under certain circumstances, an organic formation like chalk; that, as a matter of fact, an area on the surface of the globe, which we have shown to be of vast extent, although we are still far from having ascertained its limits, is being covered by such a deposit at the present day. "it is impossible to avoid associating such a formation with the fine, smooth, homogeneous clays and schists, poor in fossils, but showing worm- tubes and tracks, and bunches of doubtful branching things, such as oldhamia, silicious sponges, and thin-shelled peculiar shrimps. such formations, more or less metamorphosed, are very familiar, especially to the student of palaeozoic geology, and they often attain a vast thickness. one is inclined, from the great resemblance between them in composition and in the general character of the included fauna, to suspect that these may be organic formations, like the modern red clay of the atlantic and southern sea, accumulations of the insoluble ashes of shelled creatures. "the dredging in the red clay on the th of march was usually rich. the bag contained examples, those with calcareous shells rather stunted, of most of the characteristic deep-water groups of the southern sea, including _umbellularia, euplectella, pterocrinus, brisinga, ophioglypha, pourtalesia_, and one or two _mollusca_. this is, however, very rarely the case. generally the red clay is barren, or contains only a very small number of forms." it must be admitted that it is very difficult, at present, to frame any satisfactory explanation of the mode of origin of this singular deposit of red clay. i cannot say that the theory put forward tentatively, and with much reservation by professor thomson, that the calcareous matter is dissolved out by the relatively fresh water of the deep currents from the antarctic regions, appears satisfactory to me. nor do i see my way to the acceptance of the suggestion of dr. carpenter, that the red clay is the result of the decomposition of previously-formed greensand. at present there is no evidence that greensand casts are ever formed at great depths; nor has it been proved that _glauconite_ is decomposable by the agency of water and carbonic acid. i think it probable that we shall have to wait some time for a sufficient explanation of the origin of the abyssal red clay, no less than for that of the sublittoral greensand in the intermediate zone. but the importance of the establishment of the fact that these various deposits are being formed in the ocean, at the present day, remains the same; whether its _rationale_ be understood or not. for, suppose the globe to be evenly covered with sea, to a depth say of a thousand fathoms--then, whatever might be the mineral matter composing the sea-bottom, little or no deposit would be formed upon it, the abrading and denuding action of water, at such a depth, being exceedingly slight. next, imagine sponges, _radiolaria, foraminifera_, and diatomaceous plants, such as those which now exist in the deep-sea, to be introduced: they would be distributed according to the same laws as at present, the sponges (and possibly some of the _foraminifera_), covering the bottom, while other _foraminifera_, with the _radiolaria_ and _diatomacea_, would increase and multiply in the surface waters. in accordance with the existing state of things, the _radiolaria_ and diatoms would have a universal distribution, the latter gathering most thickly in the polar regions, while the _foraminifera_ would be largely, if not exclusively, confined to the intermediate zone; and, as a consequence of this distribution, a bed of "chalk" would begin to form in the intermediate zone, while caps of silicious rock would accumulate on the circumpolar regions. suppose, further, that a part of the intermediate area were raised to within two or three hundred fathoms of the surface--for anything that we know to the contrary, the change of level might determine the substitution of greensand for the "chalk"; while, on the other hand, if part of the same area were depressed to three thousand fathoms, that change might determine the substitution of a different silicate of alumina and iron--namely, clay--for the "chalk" that would otherwise be formed. if the _challenger_ hypothesis, that the red clay is the residue left by dissolved _foraminiferous_ skeletons, is correct, then all these deposits alike would be directly, or indirectly, the product of living organisms. but just as a silicious deposit may be metamorphosed into opal or quartzite, and chalk into marble, so known metamorphic agencies may metamorphose clay into schist, clay-slate, slate, gneiss, or even granite. and thus, by the agency of the lowest and simplest of organisms, our imaginary globe might be covered with strata, of all the chief kinds of rock of which the known crust of the earth is composed, of indefinite thickness and extent. the bearing of the conclusions which are now either established, or highly probable, respecting the origin of silicious, calcareous, and clayey rocks, and their metamorphic derivatives, upon the archaeology of the earth, the elucidation of which is the ultimate object of the geologist, is of no small importance. a hundred years ago the singular insight of linnaeus enabled him to say that "fossils are not the children but the parents of rocks,"[ ] and the whole effect of the discoveries made since his time has been to compile a larger and larger commentary upon this text. it is, at present, a perfectly tenable hypothesis that all siliceous and calcareous rocks are either directly, or indirectly, derived from material which has, at one time or other, formed part of the organized framework of living organisms. whether the same generalization may be extended to aluminous rocks, depends upon the conclusion to be drawn from the facts respecting the red clay areas brought to light by the _challenger_. if we accept the view taken by wyville thomson and his colleagues--that the red clay is the residuum left after the calcareous matter of the _globigerinoe_ ooze has been dissolved away--then clay is as much a product of life as limestone, and all known derivatives of clay may have formed part of animal bodies. [footnote : "petrificata montium calcariorum non filii sed parentes sunt, cum omnis calx oriatur ab animalibus."--_systema naturae_, ed. xii., t. iii., p. . it must be recollected that linnaeus included silex, as well as limestone, under the name of "calx," and that he would probably have arranged diatoms among animals, as part of "chaos." ehrenberg quotes another even more pithy passage, which i have not been able to find in any edition of the _systema_ accessible to me: "sic lapides ab animalibus, nec vice versa. sic runes saxei non primaevi, sed temporis filiae."] so long as the _globigerinoe_;, actually collected at the surface, have not been demonstrated to contain the elements of clay, the _challenger_ hypothesis, as i may term it, must be accepted with reserve and provisionally, but, at present, i cannot but think that it is more probable than any other suggestion which has been made. accepting it provisionally, we arrive at the remarkable result that all the chief known constituents of the crust of the earth may have formed part of living bodies; that they may be the "ash" of protoplasm; that the "_rupes saxei_" are not only _"temporis,"_ but "_vitae filiae_"; and, consequently, that the time during which life has been active on the globe may be indefinitely greater than the period, the commencement of which is marked by the oldest known rocks, whether fossiliferous or unfossiliferous. and thus we are led to see where the solution of a great problem and apparent paradox of geology may lie. satisfactory evidence now exists that some animals in the existing world have been derived by a process of gradual modification from pre-existing forms. it is undeniable, for example, that the evidence in favour of the derivation of the horse from the later tertiary _hipparion_, and that of the _hipparion_ from _anchitherium_, is as complete and cogent as such evidence can reasonably be expected to be; and the further investigations into the history of the tertiary mammalia are pushed, the greater is the accumulation of evidence having the same tendency. so far from palaeontology lending no support to the doctrine of evolution--as one sees constantly asserted--that doctrine, if it had no other support, would have been irresistibly forced upon us by the palaeontological discoveries of the last twenty years. if, however, the diverse forms of life which now exist have been produced by the modification of previously-existing less divergent forms, the recent and extinct species, taken as a whole, must fall into series which must converge as we go back in time. hence, if the period represented by the rocks is greater than, or co-extensive with, that during which life has existed, we ought, somewhere among the ancient formations, to arrive at the point to which all these series converge, or from which, in other words, they have diverged--the primitive undifferentiated protoplasmic living things, whence the two great series of plants and animals have taken their departure. but, as a matter of fact, the amount of convergence of series, in relation to the time occupied by the deposition of geological formations, is extraordinarily small. of all animals the higher _vertebrata_ are the most complex; and among these the carnivores and hoofed animals (_ungulata_) are highly differentiated. nevertheless, although the different lines of modification of the _carnivora_ and those of the _ungulata_, respectively, approach one another, and, although each group is represented by less differentiated forms in the older tertiary rocks than at the present day, the oldest tertiary rocks do not bring us near the primitive form of either. if, in the same way, the convergence of the varied forms of reptiles is measured against the time during which their remains are preserved--which is represented by the whole of the tertiary and mesozoic formations--the amount of that convergence is far smaller than that of the lines of mammals between the present time and the beginning of the tertiary epoch. and it is a broad fact that, the lower we go in the scale of organization, the fewer signs are there of convergence towards the primitive form from whence all must have diverged, if evolution be a fact. nevertheless, that it is a fact in some cases, is proved, and i, for one, have not the courage to suppose that the mode in which some species have taken their origin is different from that in which the rest have originated. what, then, has become of all the marine animals which, on the hypothesis of evolution, must have existed in myriads in those seas, wherein the many thousand feet of cambrian and laurentian rocks now devoid, or almost devoid, of any trace of life were deposited? sir charles lyell long ago suggested that the azoic character of these ancient formations might be due to the fact that they had undergone extensive metamorphosis; and readers of the "principles of geology" will be familiar with the ingenious manner in which he contrasts the theory of the gnome, who is acquainted only with the interior of the earth, with those of ordinary philosophers, who know only its exterior. the metamorphism contemplated by the great modern champion of rational geology is, mainly, that brought about by the exposure of rocks to subterranean heat; and where no such heat could be shown to have operated, his opponents assumed that no metamorphosis could have taken place. but the formation of greensand, and still more that of the "red clay" (if the _challenger_ hypothesis be correct) affords an insight into a new kind of metamorphosis--not igneous, but aqueous--by which the primitive nature of a deposit may be masked as completely as it can be by the agency of heat. and, as wyville thomson suggests, in the passage i have quoted above (p. ), it further enables us to assign a new cause for the occurrence, so puzzling hitherto, of thousands of feet of unfossiliferous fine-grained schists and slates, in the midst of formations deposited in seas which certainly abounded in life. if the great deposit of "red clay" now forming in the eastern valley of the atlantic were metamorphosed into slate and then upheaved, it would constitute an "azoic" rock of enormous extent. and yet that rock is now forming in the midst of a sea which swarms with living beings, the great majority of which are provided with calcareous or silicious shells and skeletons; and, therefore, are such as, up to this time, we should have termed eminently preservable. thus the discoveries made by the _challenger_ expedition, like all recent advances in our knowledge of the phenomena of biology, or of the changes now being effected in the structure of the surface of the earth, are in accordance with and lend strong support to, that doctrine of uniformitarianism, which, fifty years ago, was held only by a small minority of english geologists--lyell, scrope, and de la beche--but now, thanks to the long-continued labours of the first two, and mainly to those of sir charles lyell, has gradually passed from the position of a heresy to that of catholic doctrine. applied within the limits of the time registered by the known fraction of the crust of the earth, i believe that uniformitarianism is unassailable. the evidence that, in the enormous lapse of time between the deposition of the lowest laurentian strata and the present day, the forces which have modified the surface of the crust of the earth were different in kind, or greater in the intensity of their action, than those which are now occupied in the same work, has yet to be produced. such evidence as we possess all tends in the contrary direction, and is in favour of the same slow and gradual changes occurring then as now. but this conclusion in nowise conflicts with the deductions of the physicist from his no less clear and certain data. it may be certain that this globe has cooled down from a condition in which life could not have existed; it may be certain that, in so cooling, its contracting crust must have undergone sudden convulsions, which were to our earthquakes as an earthquake is to the vibration caused by the periodical eruption of a geyser; but in that case, the earth must, like other respectable parents, have sowed her wild oats, and got through her turbulent youth, before we, her children, have any knowledge of her. so far as the evidence afforded by the superficial crust of the earth goes, the modern geologist can, _ex animo_, repeat the saying of hutton, "we find no vestige of a beginning--no prospect of an end." however, he will add, with hutton, "but in thus tracing back the natural operations which have succeeded each other, and mark to us the course of time past, we come to a period in which we cannot see any further." and if he seek to peer into the darkness of this period, he will welcome the light proffered by physics and mathematics. iv yeast [ ] it has been known, from time immemorial, that the sweet liquids which may be obtained by expressing the juices of the fruits and stems of various plants, or by steeping malted barley in hot water, or by mixing honey with water--are liable to undergo a series of very singular changes, if freely exposed to the air and left to themselves, in warm weather. however clear and pellucid the liquid may have been when first prepared, however carefully it may have been freed, by straining and filtration, from even the finest visible impurities, it will not remain clear. after a time it will become cloudy and turbid; little bubbles will be seen rising to the surface, and their abundance will increase until the liquid hisses as if it were simmering on the fire. by degrees, some of the solid particles which produce the turbidity of the liquid collect at its surface into a scum, which is blown up by the emerging air-bubbles into a thick, foamy froth. another moiety sinks to the bottom, and accumulates as a muddy sediment, or "lees." when this action has continued, with more or less violence, for a certain time, it gradually moderates. the evolution of bubbles slackens, and finally comes to an end; scum and lees alike settle at the bottom, and the fluid is once more clear and transparent. but it has acquired properties of which no trace existed in the original liquid. instead of being a mere sweet fluid, mainly composed of sugar and water, the sugar has more or less completely disappeared; and it has acquired that peculiar smell and taste which we call "spirituous." instead of being devoid of any obvious effect upon the animal economy, it has become possessed of a very wonderful influence on the nervous system; so that in small doses it exhilarates, while in larger it stupefies, and may even destroy life. moreover, if the original fluid is put into a still, and heated moderately, the first and last product of its distillation is simple water; while, when the altered fluid is subjected to the same process, the matter which is first condensed in the receiver is found to be a clear, volatile substance, which is lighter than water, has a pungent taste and smell, possesses the intoxicating powers of the fluid in an eminent degree, and takes fire the moment it is brought in contact with a flame. the alchemists called this volatile liquid, which they obtained from wine, "spirits of wine," just as they called hydrochloric acid "spirits of salt," and as we, to this day, call refined turpentine "spirits of turpentine." as the "spiritus," or breath, of a man was thought to be the most refined and subtle part of him, the intelligent essence of man was also conceived as a sort of breath, or spirit; and, by analogy, the most refined essence of anything was called its "spirit." and thus it has come about that we use the same word for the soul of man and for a glass of gin. at the present day, however, we even more commonly use another name for this peculiar liquid--namely, "alcohol," and its origin is not less singular. the dutch physician, van helmont, lived in the latter part of the sixteenth and the beginning of the seventeenth century--in the transition period between alchemy and chemistry--and was rather more alchemist than chemist. appended to his "opera omnia," published in , there is a very needful "clavis ad obscuriorum sensum referendum," in which the following passage occurs.-- "alcohol.--chymicis est liquor aut pulvis summé subtilisatus, vocabulo orientalibus quoque, cum primis habessinis, familiari, quibus _cohol_ speciatim pulverem impalpabilem ex antimonio pro oculis tingendis denotat ... hodie autem, ob analogiam, quivis pulvis tenerior ut pulvis oculorum cancri summé subtilisatus _alcohol_ audit, haud aliter ac spiritus rectificatissimi _alcolisati_ dicuntur." similarly, robert boyle speaks of a fine powder as "alcohol"; and, so late as the middle of the last century, the english lexicographer, nathan bailey, defines "alcohol" as "the pure substance of anything separated from the more gross, a very fine and impalpable powder, or a very pure, well-rectified spirit." but, by the time of the publication of lavoisier's "traité elémentaire de chimie," in , the term "alcohol," "alkohol," or "alkool" (for it is spelt in all three ways), which van helmont had applied primarily to a fine powder, and only secondarily to spirits of wine, had lost its primary meaning altogether; and, from the end of the last century until now, it has, i believe, been used exclusively as the denotation of spirits of wine, and bodies chemically allied to that substance. the process which gives rise to alcohol in a saccharine fluid is known tones as "fermentation"; a term based upon the apparent boiling up or "effervescence" of the fermenting liquid, and of latin origin. our teutonic cousins call the same process "gähren," "gäsen," "göschen," and "gischen"; but, oddly enough, we do not seem to have retained their verb or their substantive denoting the action itself, though we do use names identical with, or plainly derived from, theirs for the scum and lees. these are called, in low german, "gäscht" and "gischt"; in anglo- saxon, "gest," "gist," and "yst," whence our "yeast." again, in low german and in anglo-saxon there is another name for yeast, having the form "barm," or "beorm"; and, in the midland counties, "barm" is the name by which yeast is still best known. in high german, there is a third name for yeast, "hefe," which is not represented in english, so far as i know. all these words are said by philologers to be derived from roots expressive of the intestine motion of a fermenting substance. thus "hefe" is derived from "heben," to raise; "barm" from "beren" or "bären," to bear up; "yeast," "yst," and "gist," have all to do with seething and foam, with "yeasty" waves, and "gusty" breezes. the same reference to the swelling up of the fermenting substance is seen in the gallo-latin terms "levure" and "leaven." it is highly creditable to the ingenuity of our ancestors that the peculiar property of fermented liquids, in virtue of which they "make glad the heart of man," seems to have been known in the remotest periods of which we have any record. all savages take to alcoholic fluids as if they were to the manner born. our vedic forefathers intoxicated themselves with the juice of the "soma"; noah, by a not unnatural reaction against a superfluity of water, appears to have taken the earliest practicable opportunity of qualifying that which he was obliged to drink; and the ghosts of the ancient egyptians were solaced by pictures of banquets in which the wine-cup passes round, graven on the walls of their tombs. a knowledge of the process of fermentation, therefore, was in all probability possessed by the prehistoric populations of the globe; and it must have become a matter of great interest even to primaeval wine-bibbers to study the methods by which fermented liquids could be surely manufactured. no doubt it was soon discovered that the most certain, as well as the most expeditious, way of making a sweet juice ferment was to add to it a little of the scum, or lees, of another fermenting juice. and it can hardly be questioned that this singular excitation of fermentation in one fluid, by a sort of infection, or inoculation, of a little ferment taken from some other fluid, together with the strange swelling, foaming, and hissing of the fermented substance, must have always attracted attention from the more thoughtful. nevertheless, the commencement of the scientific analysis of the phenomena dates from a period not earlier than the first half of the seventeenth century. at this time, van helmont made a first step, by pointing out that the peculiar hissing and bubbling of a fermented liquid is due, not to the evolution of common air (which he, as the inventor of the term "gas," calls "gas ventosum"), but to that of a peculiar kind of air such as is occasionally met with in caves, mines, and wells, and which he calls "gas sylvestre." but a century elapsed before the nature of this "gas sylvestre," or, as it was afterwards called, "fixed air," was clearly determined, and it was found to be identical with that deadly "choke-damp" by which the lives of those who descend into old wells, or mines, or brewers' vats, are sometimes suddenly ended; and with the poisonous aëriform fluid which is produced by the combustion of charcoal, and now goes by the name of carbonic acid gas. during the same time it gradually became evident that the presence of sugar was essential to the production of alcohol and the evolution of carbonic acid gas, which are the two great and conspicuous products of fermentation. and finally, in , the italian chemist, fabroni, made the capital discovery that the yeast ferment, the presence of which is necessary to fermentation, is what he termed a "vegeto-animal" substance; that is, a body which gives of ammoniacal salts when it is burned, and is, in other ways, similar to the gluten of plants and the albumen and casein of animals. these discoveries prepared the way for the illustrious frenchman, lavoisier, who first approached the problem of fermentation with a complete conception of the nature of the work to be done. the words in which he expresses this conception, in the treatise on elementary chemistry to which reference has already been made, mark the year as the commencement of a revolution of not less moment in the world of science than that which simultaneously burst over the political world, and soon engulfed lavoisier himself in one of its mad eddies. "we may lay it down as an incontestable axiom that, in all the operations of art and nature, nothing is created; an equal quantity of matter exists both before, and after the experiment: the quality and quantity of the elements remain precisely the same, and nothing takes place beyond changes and modifications in the combinations of these elements. upon this principle the whole art of performing chemical experiments depends; we must always suppose an exact equality between the elements of the body examined and those of the products of its analysis. "hence, since from must of grapes we procure alcohol and carbonic acid, i have an undoubted right to suppose that must consists of carbonic acid and alcohol. from these premisses we have two modes of ascertaining what passes during vinous fermentation: either by determining the nature of, and the elements which compose, the fermentable substances; or by accurately examining the products resulting from fermentation; and it is evident that the knowledge of either of these must lead to accurate conclusions concerning the nature and composition of the other. from these considerations it became necessary accurately to determine the constituent elements of the fermentable substances; and for this purpose i did not make use of the compound juices of fruits, the rigorous analysis of which is perhaps impossible, but made choice of sugar, which is easily analysed, and the nature of which i have already explained. this substance is a true vegetable oxyd, with two bases, composed of hydrogen and carbon, brought to the state of an oxyd by means of a certain proportion of oxygen; and these three elements are combined in such a way that a very slight force is sufficient to destroy the equilibrium of their connection." after giving the details of his analysis of sugar and of the products of fermentation, lavoisier continues:-- "the effect of the vinous fermentation upon sugar is thus reduced to the mere separation of its elements into two portions; one part is oxygenated at the expense of the other, so as to form carbonic acid; while the other part, being disoxygenated in favour of the latter, is converted into the combustible substance called alkohol; therefore, if it were possible to re-unite alkohol and carbonic acid together, we ought to form sugar."[ ] [footnote : _elements of chemistry_. by m. lavoisier. translated by robert kerr. second edition, (pp. - ).] thus lavoisier thought he had demonstrated that the carbonic acid and the alcohol which are produced by the process of fermentation, are equal in weight to the sugar which disappears; but the application of the more refined methods of modern chemistry to the investigation of the products of fermentation by pasteur, in , proved that this is not exactly true, and that there is a deficit of from to per cent of the sugar which is not covered by the alcohol and carbonic acid evolved. the greater part of this deficit is accounted for by the discovery of two substances, glycerine and succinic acid, of the existence of which lavoisier was unaware, in the fermented liquid. but about - / per cent. still remains to be made good. according to pasteur, it has been appropriated by the yeast, but the fact that such appropriation takes place cannot be said to be actually proved. however this may be, there can be no doubt that the constituent elements of fully per cent. of the sugar which has vanished during fermentation have simply undergone rearrangement; like the soldiers of a brigade, who at the word of command divide themselves into the independent regiments to which they belong. the brigade is sugar, the regiments are carbonic acid, succinic acid, alcohol, and glycerine. from the time of fabroni, onwards, it has been admitted that the agent by which this surprising rearrangement of the particles of the sugar is effected is the yeast. but the first thoroughly conclusive evidence of the necessity of yeast for the fermentation of sugar was furnished by appert, whose method of preserving perishable articles of food excited so much attention in france at the beginning of this century. gay-lussac, in his "mémoire sur la fermentation,"[ ] alludes to appert's method of preserving beer-wort unfermented for an indefinite time, by simply boiling the wort and closing the vessel in which the boiling fluid is contained, in such a way as thoroughly to exclude air; and he shows that, if a little yeast be introduced into such wort, after it has cooled, the wort at once begins to ferment, even though every precaution be taken to exclude air. and this statement has since received full confirmation from pasteur. [footnote : _annales de chimie_, .] on the other hand, schwann, schroeder and dutch, and pasteur, have amply proved that air may be allowed to have free access to beer-wort, without exciting fermentation, if only efficient precautions are taken to prevent the entry of particles of yeast along with the air. thus, the truth that the fermentation of a simple solution of sugar in water depends upon the presence of yeast, rests upon an unassailable foundation; and the inquiry into the exact nature of the substance which possesses such a wonderful chemical influence becomes profoundly interesting. the first step towards the solution of this problem was made two centuries ago by the patient and painstaking dutch naturalist, leeuwenhoek, who in the year wrote thus:-- "saepissime examinavi fermnentum cerevisiae, semperque hoc ex globulis per materiam pellucidam fluitantibus, quarm cerevisiam esse censui, constare observavi: vidi etiam evidentissime, unumquemque hujus fermenti globulum denuo ex sex distinctis globulis constare, accurate eidem quantitate et formae, cui globulis sanguinis nostri, respondentibus. "verum talis mihi de horum origine et formatione conceptus formabam; globulis nempe ex quibus farina tritici, hordei, avenae, fagotritici, se constat aquae calore dissolvi et aquae commisceri; hac, vero aqua, quam cerevisiam vocare licet, refrigescente, multos ex minimis particulis in cerevisia coadunari, et hoc pacto efficere particulam sive globulum, quae sexta pars est globuli faecis, et iterum sex ex hisce globulis conjungi."[ ] [footnote : leeuwenhoek, _arcana naturae detecta._ ed. nov., .] thus leeuwenhoek discovered that yeast consists of globules floating in a fluid; but he thought that they were merely the starchy particles of the grain from which the wort was made, rearranged. he discovered the fact that yeast had a definite structure, but not the meaning of the fact. a century and a half elapsed, and the investigation of yeast was recommenced almost simultaneously by cagniard de la tour in france, and by schwann and kützing in germany. the french observer was the first to publish his results; and the subject received at his hands and at those of his colleague, the botanist turpin, full and satisfactory investigation. the main conclusions at which they arrived are these. the globular, or oval, corpuscles which float so thickly in the yeast as to make it muddy, though the largest are not more than one two-thousandth of an inch in diameter, and the smallest may measure less than one seven-thousandth of an inch, are living organisms. they multiply with great rapidity by giving off minute buds, which soon attain the size of their parent, and then either become detached or remain united, forming the compound globules of which leeuwenhoek speaks, though the constancy of their arrangement in sixes existed only in the worthy dutchman's imagination. it was very soon made out that these yeast organisms, to which turpin gave the name of _torula cerevisioe_, were more nearly allied to the lower fungi than to anything else. indeed turpin, and subsequently berkeley and hoffmann, believed that they had traced the development of the _torula_ into the well-known and very common mould--the _penicillium glaucum_. other observers have not succeeded in verifying these statements; and my own observations lead me to believe, that while the connection between _torula_ and the moulds is a very close one, it is of a different nature from that which has been supposed. i have never been able to trace the development of _torula_ into a true mould; but it is quite easy to prove that species of true mould, such as _penicillium_, when sown in an appropriate nidus, such as a solution of tartrate of ammonia and yeast-ash, in water, with or without sugar, give rise to _toruloe_, similar in all respects to _t. cerevisioe_, except that they are, on the average, smaller. moreover, bail has observed the development of a _torula_ larger than _t. cerevisioe_, from a _mucor_, a mould allied to _penicillium_. it follows, therefore, that the _toruloe_, or organisms of yeast, are veritable plants; and conclusive experiments have proved that the power which causes the rearrangement of the molecules of the sugar is intimately connected with the life and growth of the plant. in fact, whatever arrests the vital activity of the plant also prevents it from exciting fermentation. such being the facts with regard to the nature of yeast, and the changes which it effects in sugar, how are they to be accounted for? before modern chemistry had come into existence, stahl, stumbling, with the stride of genius, upon the conception which lies at the bottom of all modern views of the process, put forward the notion that the ferment, being in a state of internal motion, communicated that motion to the sugar, and thus caused its resolution into new substances. and lavoisier, as we have seen, adopts substantially the same view. but fabroni, full of the then novel conception of acids and bases and double decompositions, propounded the hypothesis that sugar is an oxide with two bases, and the ferment a carbonate with two bases; that the carbon of the ferment unites with the oxygen of the sugar, and gives rise to carbonic acid; while the sugar, uniting with the nitrogen of the ferment, produces a new substance analogous to opium. this is decomposed by distillation, and gives rise to alcohol. next, in , thénard propounded a hypothesis which partakes somewhat of the nature of both stahl's and fabroni's views. "i do not believe with lavoisier," he says, "that all the carbonic acid formed proceeds from the sugar. how, in that case, could we conceive the action of the ferment on it? i think that the first portions of the acid are due to a combination of the carbon of the ferment with the oxygen of the sugar, and that it is by carrying off a portion of oxygen from the last that the ferment causes the fermentation to commence--the equilibrium between the principles of the sugar being disturbed, they combine afresh to form carbonic acid and alcohol." the three views here before us may be familiarly exemplified by supposing the sugar to be a card-house. according to stahl, the ferment is somebody who knocks the table, and shakes the card-house down; according to fabroni, the ferment takes out some cards, but puts others in their places; according to thénard, the ferment simply takes a card out of the bottom story, the result of which is that all the others fall. as chemistry advanced, facts came to light which put a new face upon stahl's hypothesis, and gave it a safer foundation than it previously possessed. the general nature of these phenomena may be thus stated:--a body, a, without giving to, or taking from, another body b, any material particles, causes b to decompose into other substances, c, d, e, the sum of the weights of which is equal to the weight of b, which decomposes. thus, bitter almonds contain two substances, amygdalin and synaptase, which can be extracted, in a separate state, from the bitter almonds. the amygdalin thus obtained, if dissolved in water, undergoes no change; but if a little synaptase be added to the solution, the amygdalin splits up into bitter almond oil, prussic acid, and a kind of sugar. a short time after cagniard de la tour discovered the yeast plant, liebig, struck with the similarity between this and other such processes and the fermentation of sugar, put forward the hypothesis that yeast contains a substance which acts upon sugar, as synaptase acts upon amygdalin. and as the synaptase is certainly neither organized nor alive, but a mere chemical substance, liebig treated cagniard de la tour's discovery with no small contempt, and, from that time to the present, has steadily repudiated the notion that the decomposition of the sugar is, in any sense, the result of the vital activity of the _torula_. but, though the notion that the _torula_ is a creature which eats sugar and excretes carbonic acid and alcohol, which is not unjustly ridiculed in the most surprising paper that ever made its appearance in a grave scientific journal,[ ] may be untenable, the fact that the _toruloe_ are alive, and that yeast does not excite fermentation unless it contains living _toruloe_, stands fast. moreover, of late years, the essential participation of living organisms in fermentation other than the alcoholic, has been clearly made out by pasteur and other chemists. [footnote : "das enträthselte geheimniss der geistigen gährung (vorlänfige briefliche mittheilung)" is the title of an anonymous contribution to wöhler and liebig's _annalen der pharmacie_ for , in which a somewhat rabelaisian imaginary description of the organisation of the "yeast animals" and of the manner in which their functions are performed, is given with a circumstantiality worthy of the author of _gulliver's travels_. as a specimen of the writer's humour, his account of what happens when fermentation comes to an end may suffice. "sobald nämlich die thiere keinen zucker mehr vorfinden, so fressen sie sich gegenseitig selbst auf, was durch eine eigene manipulation geschieht; alles wird verdant bis auf die eier, welche unverändert durch den darmkanal hineingehen; man hat zuletzt wieder gährungsfähige hefe, nämlich den saamen der thiere, der übrig bleibt."] however, it may be asked, is there any necessary opposition between the so-called "vital" and the strictly physico-chemical views of fermentation? it is quite possible that the living _torula_ may excite fermentation in sugar, because it constantly produces, as an essential part of its vital manifestations, some substance which acts upon the sugar, just as the synaptase acts upon the amygdalin. or it may be, that, without the formation of any such special substance, the physical condition of the living tissue of the yeast plant is sufficient to effect that small disturbance of the equilibrium of the particles of the sugar, which lavoisier thought sufficient to effect its decomposition. platinum in a very fine state of division--known as platinum black, or _noir de platine_--has the very singular property of causing alcohol to change into acetic acid with great rapidity. the vinegar plant, which is closely allied to the yeast plant, has a similar effect upon dilute alcohol, causing it to absorb the oxygen of the air, and become converted into vinegar; and liebig's eminent opponent, pasteur, who has done so much for the theory and the practice of vinegar-making, himself suggests that in this case-- "la cause du phénomène physique qui accompagne la vie de la plante réside dans un état physique propre, analogue à celui du noir de platine. mais il est essentiel de remarquer que cet état physique de la plante est étroitement lié avec la vie de cette plante."[ ] [footnote : _etudes sur les mycodermes_, comptes-rendus, liv., .] now, if the vinegar plant gives rise to the oxidation of alcohol, on account of its merely physical constitution, it is at any rate possible that the physical constitution of the yeast plant may exert a decomposing influence on sugar. but, without presuming to discuss a question which leads us into the very arcana of chemistry, the present state of speculation upon the _modus operandi_ of the yeast plant in producing fermentation is represented, on the one hand, by the stahlian doctrine, supported by liebig, according to which the atoms of the sugar are shaken into new combinations either directly by the _toruloe_, or indirectly, by some substance formed by them; and, on the other hand, by the thénardian doctrine, supported by pasteur, according to which the yeast plant assimilates part of the sugar, and, in so doing, disturbs the rest, and determines its resolution into the products of fermentation. perhaps the two views are not so much opposed as they seem at first sight to be. but the interest which attaches to the influence of the yeast plants upon the medium in which they live and grow does not arise solely from its bearing upon the theory of fermentation. so long ago as , turpin compared the _toruloe_ to the ultimate elements of the tissues of animals and plants--"les organes élémentaires de leurs tissus, comparables aux petits végétaux des levures ordinaires, sont aussi les décompositeurs des substances qui les environnent." almost at the same time, and, probably, equally guided by his study of yeast, schwann was engaged in those remarkable investigations into the form and development of the ultimate structural elements of the tissues of animals, which led him to recognise their fundamental identity with the ultimate structural elements of vegetable organisms. the yeast plant is a mere sac, or "cell," containing a semi-fluid matter, and schwann's microscopic analysis resolved all living organisms, in the long run, into an aggregation of such sacs or cells, variously modified; and tended to show, that all, whatever their ultimate complication, begin their existence in the condition of such simple cells. in his famous "mikroskopische untersuchungen" schwann speaks of _torula_ as a "cell"; and, in a remarkable note to the passage in which he refers to the yeast plant, schwann says:-- "i have been unable to avoid mentioning fermentation, because it is the most fully and exactly known operation of cells, and represents, in the simplest fashion, the process which is repeated by every cell of the living body." in other words, schwann conceives that every cell of the living body exerts an influence on the matter which surrounds and permeates it, analogous to that which a _torula_ exerts on the saccharine solution by which it is bathed. a wonderfully suggestive thought, opening up views of the nature of the chemical processes of the living body, which have hardly yet received all the development of which they are capable. kant defined the special peculiarity of the living body to be that the parts exist for the sake of the whole and the whole for the sake of the parts. but when turpin and schwann resolved the living body into an aggregation of quasi-independent cells, each, like a _torula_, leading its own life and having its own laws of growth and development, the aggregation being dominated and kept working towards a definite end only by a certain harmony among these units, or by the superaddition of a controlling apparatus, such as a nervous system, this conception ceased to be tenable. the cell lives for its own sake, as well as for the sake of the whole organism; and the cells which float in the blood, live at its expense, and profoundly modify it, are almost as much independent organisms as the _toruloe_ which float in beer-wort. schwann burdened his enunciation of the "cell theory" with two false suppositions; the one, that the structures he called "nucleus"[ ] and "cell-wall" are essential to a cell; the other, that cells are usually formed independently of other cells; but, in , it was a vast and clear gain to arrive at the conception, that the vital functions of all the higher animals and plants are the resultant of the forces inherent in the innumerable minute cells of which they are composed, and that each of them is, itself, an equivalent of one of the lowest and simplest of independent living beings--the _torula_. [footnote : later investigations have thrown an entirely new light upon the structure and the functional importance of the nucleus; and have proved that schwann did not over-estimate its importance. .] from purely morphological investigations, turpin and schwann, as we have seen, arrived at the notion of the fundamental unity of structure of living beings. and, before long, the researches of chemists gradually led up to the conception of the fundamental unity of their composition. so far back as , thénard pointed out, in most distinct terms, the important fact that yeast contains a nitrogenous "animal" substance; and that such a substance is contained in all ferments. before him, fabroni and fourcroy speak of the "vegeto-animal" matter of yeast. in mulder endeavoured to demonstrate that a peculiar substance, which he called "protein," was essentially characteristic of living matter. in , payen writes:-- "enfin, une loi sans exception me semble apparaître dans les faits nombreux que j'ai observés et conduire à envisager sous un nouveau jour la vie végétale; si je ne m'abuse, tout ce que dans les tissus végétaux la vue directe où amplifiée nous permet de discerner sous la forme de cellules et de vaisseaux, ne représente autre chose que les enveloppes protectrices, les réservoirs et les conduits, à l'aide desquels les corps animés qui les secrètent et les façonnent, se logent, puisent et charrient leurs aliments, déposent et isolent les matières excrétées." and again:-- "afin de compléter aujourd'hui l'énoncé du fait général, je rappellerai que les corps, doué des fonctions accomplies dans les tissus des plantes, sont formés des éléments qui constituent, en proportion peu variable, les organismes animaux; qu'ainsi l'on est conduit à reconnaître une immense unité de composition élémentaire dans tous les corps vivants de la nature."[ ] [footnote : mém. sur les développements des végétaux, &c.--_mém. présentées_. ix. .] in the year ( ) in which these remarkable passages were published, the eminent german botanist, von mohl invented the word "protoplasm," as a name for one portion of those nitrogenous contents of the cells of living plants, the close chemical resemblance of which to the essential constituents of living animals is so strongly indicated by payen. and through the twenty-five years that have passed, since the matter of life was first called protoplasm, a host of investigators, among whom cohn, max schulze, and kühne must be named as leaders, have accumulated evidence, morphological, physiological, and chemical, in favour of that "immense unité de composition élémentaire dans tous les corps vivants de la nature," into which payen had, so early, a clear insight. as far back as , cohn wrote, apparently without any knowledge of what payen had said before him:-- "the protoplasm of the botanist, and the contractile substance and sarcode of the zoologist, must be, if not identical, yet in a high degree analogous substances. hence, from this point of view, the difference between animals and plants consists in this; that, in the latter, the contractile substance, as a primordial utricle, is enclosed within an inert cellulose membrane, which permits it only to exhibit an internal motion, expressed by the phenomena of rotation and circulation, while, in the former, it is not so enclosed. the protoplasm in the form of the primordial utricle is, as it were, the animal element in the plant, but which is imprisoned, and only becomes free in the animal; or, to strip off the metaphor which obscures simple thought, the energy of organic vitality which is manifested in movement is especially exhibited by a nitrogenous contractile substance, which in plants is limited and fettered by an inert membrane, in animals not so."[ ] [footnote : cohn, "ueber protococcus pluvialis," in the _nova acta_ for .] in , thinking that an untechnical statement of the views current among the leaders of biological science might be interesting to the general public, i gave a lecture embodying them in edinburgh. those who have not made the mistake of attempting to approach biology, either by the high _à priori_ road of mere philosophical speculation, or by the mere low _à posteriori_ lane offered by the tube of a microscope, but have taken the trouble to become acquainted with well-ascertained facts and with their history, will not need to be told that in what i had to say "as regards protoplasm" in my lecture "on the physical basis of life" (vol. i. of these essays, p. ), there was nothing new; and, as i hope, nothing that the present state of knowledge does not justify us in believing to be true. under these circumstances, my surprise may be imagined, when i found, that the mere statement of facts and of views, long familiar to me as part of the common scientific property of continental workers, raised a sort of storm in this country, not only by exciting the wrath of unscientific persons whose pet prejudices they seemed to touch, but by giving rise to quite superfluous explosions on the part of some who should have been better informed. dr. stirling, for example, made my essay the subject of a special critical lecture,[ ] which i have read with much interest, though, i confess, the meaning of much of it remains as dark to me as does the "secret of hegel" after dr. stirling's elaborate revelation of it. dr. stirling's method of dealing with the subject is peculiar. "protoplasm" is a question of history, so far as it is a name; of fact, so far as it is a thing. dr. stirling, has not taken the trouble to refer to the original authorities for his history, which is consequently a travesty; and still less has he concerned himself with looking at the facts, but contents himself with taking them also at second-hand. a most amusing example of this fashion of dealing with scientific statements is furnished by dr. stirling's remarks upon my account of the protoplasm of the nettle hair. that account was drawn up from careful and often- repeated observation of the facts. dr. stirling thinks he is offering a valid criticism, when he says that my valued friend professor stricker gives a somewhat different statement about protoplasm. but why in the world did not this distinguished hegelian look at a nettle hair for himself, before venturing to speak about the matter at all? why trouble himself about what either stricker or i say, when any tyro can see the facts for himself, if he is provided with those not rare articles, a nettle and a microscope? but i suppose this would have been "_aufklärung_"--a recurrence to the base common-sense philosophy of the eighteenth century, which liked to see before it believed, and to understand before it criticised dr. stirling winds up his paper with the following paragraph:-- [footnote : subsequently published under the title of "as regards protoplasm."] "in short, the whole position of mr. huxley, ( ) that all organisms consist alike of the same life-matter, ( ) which life-matter is, for its part, due only to chemistry, must be pronounced untenable--nor less untenable ( ) the materialism he would found on it." the paragraph contains three distinct assertions concerning my views, and just the same number of utter misrepresentations of them. that which i have numbered ( ) turns on the ambiguity of the word "same," for a discussion of which i would refer dr. stirling to a great hero of "_aufklärung_" archbishop whately; statement number ( ) is, in my judgment, absurd, and certainly i have never said anything resembling it; while, as to number ( ), one great object of my essay was to show that what is called "materialism" has no sound philosophical basis! as we have seen, the study of yeast has led investigators face to face with problems of immense interest in pure chemistry, and in animal and vegetable morphology. its physiology is not less rich in subjects for inquiry. take, for example, the singular fact that yeast will increase indefinitely when grown in the dark, in water containing only tartrate of ammonia a small percentage of mineral salts and sugar. out of these materials the _toruloe_ will manufacture nitrogenous protoplasm, cellulose, and fatty matters, in any quantity, although they are wholly deprived of those rays of the sun, the influence of which is essential to the growth of ordinary plants. there has been a great deal of speculation lately, as to how the living organisms buried beneath two or three thousand fathoms of water, and therefore in all probability almost deprived of light, live. if any of them possess the same powers as yeast (and the same capacity for living without light is exhibited by some other fungi) there would seem to be no difficulty about the matter. of the pathological bearings of the study of yeast, and other such organisms, i have spoken elsewhere. it is certain that, in some animals, devastating epidemics are caused by fungi of low order--similar to those of which _torula_ is a sort of offshoot. it is certain that such diseases are propagated by contagion and infection, in just the same way as ordinary contagious and infectious diseases are propagated. of course, it does not follow from this, that all contagious and infectious diseases are caused by organisms of as definite and independent a character as the _torula_; but, i think, it does follow that it is prudent and wise to satisfy one's self in each particular case, that the "germ theory" cannot and will not explain the facts, before having recourse to hypotheses which have no equal support from analogy. v on the formation of coal [ ] the lumps of coal in a coal-scuttle very often have a roughly cubical form. if one of them be picked out and examined with a little care, it will be found that its six sides are not exactly alike. two opposite sides are comparatively smooth and shining, while the other four are much rougher, and are marked by lines which run parallel with the smooth sides. the coal readily splits along these lines, and the split surfaces thus formed are parallel with the smooth faces. in other words, there is a sort of rough and incomplete stratification in the lump of coal, as if it were a book, the leaves of which had stuck together very closely. sometimes the faces along which the coal splits are not smooth, but exhibit a thin layer of dull, charred-looking substance, which is known as "mineral charcoal." occasionally one of the faces of a lump of coal will present impressions, which are obviously those of the stem, or leaves, of a plant; but though hard mineral masses of pyrites, and even fine mud, may occur here and there, neither sand nor pebbles are met with. when the coal burns, the chief ultimate products of its combustion are carbonic acid, water, and ammoniacal products, which escape up the chimney; and a greater or less amount of residual earthy salts, which take the form of ash. these products are, to a great extent, such as would result from the burning of so much wood. these properties of coal may be made out without any very refined appliances, but the microscope reveals something more. black and opaque as ordinary coal is, slices of it become transparent if they are cemented in canada balsam, and rubbed down very thin, in the ordinary way of making thin sections of non-transparent bodies. but as the thin slices, made in this way, are very apt to crack and break into fragments, it is better to employ marine glue as the cementing material. by the use of this substance, slices of considerable size and of extreme thinness and transparency may be obtained.[ ] [footnote : my assistant in the museum of practical geology, mr. newton, invented this excellent method of obtaining thin slices of coal.] now let us suppose two such slices to be prepared from our lump of coal-- one parallel with the bedding, the other perpendicular to it; and let us call the one the horizontal, and the other the vertical, section. the horizontal section will present more or less rounded yellow patches and streaks, scattered irregularly through the dark brown, or blackish, ground substance; while the vertical section will exhibit mere elongated bars and granules of the same yellow materials, disposed in lines which correspond, roughly, with the general direction of the bedding of the coal. this is the microscopic structure of an ordinary piece of coal. but if a great series of coals, from different localities and seams, or even from different parts of the same seam, be examined, this structure will be found to vary in two directions. in the anthracitic, or stone-coals, which burn like coke, the yellow matter diminishes, and the ground substance becomes more predominant, blacker, and more opaque, until it becomes impossible to grind a section thin enough to be translucent; while, on the other hand, in such as the "better-bed" coal of the neighbourhood of bradford, which burns with much flame, the coal is of a far lighter, colour and transparent sections are very easily obtained. in the browner parts of this coal, sharp eyes will readily detect multitudes of curious little coin-shaped bodies, of a yellowish brown colour, embedded in the dark brown ground substance. on the average, these little brown bodies may have a diameter of about one-twentieth of an inch. they lie with their flat surfaces nearly parallel with the two smooth faces of the block in which they are contained; and, on one side of each, there may be discerned a figure, consisting of three straight linear marks, which radiate from the centre of the disk, but do not quite reach its circumference. in the horizontal section these disks are often converted into more or less complete rings; while in the vertical sections they appear like thick hoops, the sides of which have been pressed together. the disks are, therefore, flattened bags; and favourable sections show that the three-rayed marking is the expression of three clefts, which penetrate one wall of the bag. the sides of the bags are sometimes closely approximated; but, when the bags are less flattened, their cavities are, usually, filled with numerous, irregularly rounded, hollow bodies, having the same kind of wall as the large ones, but not more than one seven-hundredth of an inch in diameter. in favourable specimens, again, almost the whole ground substance appears to be made up of similar bodies--more or less carbonized or blackened-- and, in these, there can be no doubt that, with the exception of patches of mineral charcoal, here and there, the whole mass of the coal is made up of an accumulation of the larger and of the smaller sacs. but, in one and the same slice, every transition can be observed from this structure to that which has been described as characteristic of ordinary coal. the latter appears to rise out of the former, by the breaking-up and increasing carbonization of the larger and the smaller sacs. and, in the anthracitic coals, this process appears to have gone to such a length, as to destroy the original structure altogether, and to replace it by a completely carbonized substance. thus coal may be said, speaking broadly, to be composed of two constituents: firstly, mineral charcoal; and, secondly, coal proper. the nature of the mineral charcoal has long since been determined. its structure shows it to consist of the remains of the stems and leaves of plants, reduced a little more than their carbon. again, some of the coal is made up of the crushed and flattened bark, or outer coat, of the stems of plants, the inner wood of which has completely decayed away. but what i may term the "saccular matter" of the coal, which, either in its primary or in its degraded form constitutes by far the greater part of all the bituminous coals i have examined, is certainly not mineral charcoal; nor is its structure that of any stem or leaf. hence its real nature is at first by no means apparent, and has been the subject of much discussion. the first person who threw any light upon the problem, as far as i have been able to discover, was the well-known geologist, professor morris. it is now thirty-four years since he carefully described and figured the coin-shaped bodies, or larger sacs, as i have called them, in a note appended to the famous paper "on the coalbrookdale coal-field," published at that time, by the present president of the geological society, mr. prestwich. with much sagacity, professor morris divined the real nature of these bodies, and boldly affirmed them to be the spore-cases of a plant allied to the living club-mosses. but discovery sometimes makes a long halt; and it is only a few years since mr. carruthers determined the plant (or rather one of the plants) which produces these spore-cases, by finding the discoidal sacs still adherent to the leaves of the fossilized cone which produced them. he gave the name of _flemingites gracilis_ to the plant of which the cones form a part. the branches and stem of this plant are not yet certainly known, but there is no sort of doubt that it was closely allied to the _lepidodendron_, the remains of which abound in the coal formation. the _lepidodendra_ were shrubs and trees which put one more in mind of an _araucaria_ than of any other familiar plant; and the ends of the fruiting branches were terminated by cones, or catkins, somewhat like the bodies so named in a fir, or a willow. these conical fruits, however, did not produce seeds; but the leaves of which they were composed bore upon their surfaces sacs full of spores or sporangia, such as those one sees on the under surface of a bracken leaf. now, it is these sporangia of the lepidodendroid plant _flemingites_ which were identified by mr. carruthers with the free sporangia described by professor morris, which are the same as the large sacs of which i have spoken. and, more than this, there is no doubt that the small sacs are the spores, which were originally contained in the sporangia. the living club-mosses are, for the most part, insignificant and creeping herbs, which, superficially, very closely resemble true mosses, and none of them reach more than two or three feet in height. but, in their essential structure, they very closely resemble the earliest lepidodendroid trees of the coal: their stems and leaves are similar; so are their cones; and no less like are the sporangia and spores; while even in their size, the spores of the _lepidodendron_ and those of the existing _lycopodium_, or club-moss, very closely approach one another. thus, the singular conclusion is forced upon us, that the greater and the smaller sacs of the "better-bed" and other coals, in which the primitive structure is well preserved, are simply the sporangia and spores of certain plants, many of which were closely allied to the existing club- mosses. and if, as i believe, it can be demonstrated that ordinary coal is nothing but "saccular" coal which has undergone a certain amount of that alteration which, if continued, would convert it into anthracite; then, the conclusion is obvious, that the great mass of the coal we burn is the result of the accumulation of the spores and spore-cases of plants, other parts of which have furnished the carbonized stems and the mineral charcoal, or have left their impressions on the surfaces of the layer. of the multitudinous speculations which, at various times, have been entertained respecting the origin and mode of formation of coal, several appear to be negatived, and put out of court, by the structural facts the significance of which i have endeavoured to explain. these facts, for example, do not permit us to suppose that coal is an accumulation of peaty matter, as some have held. again, the late professor quekett was one of the first observers who gave a correct description of what i have termed the "saccular" structure of coal; and, rightly perceiving that this structure was something quite different from that of any known plant, he imagined that it proceeded from some extinct vegetable organism which was peculiarly abundant amongst the coal-forming plants. but this explanation is at once shown to be untenable when the smaller and the larger sacs are proved to be spores or sporangia. some, once more, have imagined that coal was of submarine origin; and though the notion is amply and easily refuted by other considerations, it may be worth while to remark, that it is impossible to comprehend how a mass of light and resinous spores should have reached the bottom of the sea, or should have stopped in that position if they had got there. at the same time, it is proper to remark that i do not presume to suggest that all coal must needs have the same structure; or that there may not be coals in which the proportions of wood and spores, or spore-cases, are very different from those which i have examined. all i repeat is, that none of the coals which have come under my notice have enabled me to observe such a difference. but, according to principal dawson, who has so sedulously examined the fossil remains of plants in north america, it is otherwise with the vast accumulations of coal in that country. "the true coal," says dr. dawson, "consists principally of the flattened bark of sigillarioid and other trees, intermixed with leaves of ferns and _cordaites_, and other herbaceous _débris_, and with fragments of decayed wood, constituting 'mineral charcoal,' all these materials having manifestly alike grown and accumulated where we find them."[ ] [footnote : _acadian geology_, nd edition, p. .] when i had the pleasure of seeing principal dawson in london last summer, i showed him my sections of coal, and begged him to re-examine some of the american coals on his return to canada, with an eye to the presence of spores and sporangia, such as i was able to show him in our english and scotch coals. he has been good enough to do so; and in a letter dated september th, , he informs me that-- "indications of spore-cases are rare, except in certain coarse shaly coals and portions of coals, and in the roofs of the seams. the most marked case i have yet met with is the shaly coal referred to as containing _sporangites_ in my paper on the conditions of accumulation of coal ("journal of the geological society," vol. xxii. pp. , , and ). the purer coals certainly consist principally of cubical tissues with some true woody matter, and the spore-cases, &c., are chiefly in the coarse and shaly layers. this is my old doctrine in my two papers in the "journal of the geological society," and i see nothing to modify it. your observations, however, make it probable that the frequent _clear spots_ in the cannels are spore-cases." dr. dawson's results are the more remarkable, as the numerous specimens of british coal, from various localities, which i have examined, tell one tale as to the predominance of the spore and sporangium element in their composition; and as it is exactly in the finest and purest coals, such as the "better-bed" coal of lowmoor, that the spores and sporangia obviously constitute almost the entire mass of the deposit. coal, such as that which has been described, is always found in sheets, or "seams," varying from a fraction of an inch to many feet in thickness, enclosed in the substance of the earth at very various depths, between beds of rock of different kinds. as a rule, every seam of coal rests upon a thicker, or thinner, bed of clay, which is known as "under-clay." these alternations of beds of coal, clay, and rock may be repeated many times, and are known as the "coal-measures"; and in some regions, as in south wales and in nova scotia, the coal-measures attain a thickness of twelve or fourteen thousand feet, and enclose eighty or a hundred seams of coal, each with its under-clay, and separated from those above and below by beds of sandstone and shale. the position of the beds which constitute the coal-measures is infinitely diverse. sometimes they are tilted up vertically, sometimes they are horizontal, sometimes curved into great basins; sometimes they come to the surface, sometimes they are covered up by thousands of feet of rock. but, whatever their present position, there is abundant and conclusive evidence that every under-clay was once a surface soil. not only do carbonized root-fibres frequently abound in these under-clays; but the stools of trees, the trunks of which are broken off and confounded with the bed of coal, have been repeatedly found passing into radiating roots, still embedded in the under-clay. on many parts of the coast of england, what are commonly known as "submarine forests" are to be seen at low water. they consist, for the most part, of short stools of oak, beech, and fir-trees, still fixed by their long roots in the bed of blue clay in which they originally grew. if one of these submarine forest beds should be gradually depressed and covered up by new deposits, it would present just the same characters as an under-clay of the coal, if the _sigillaria_ and _lepidodendron_ of the ancient world were substituted for the oak, or the beech, of our own times. in a tropical forest, at the present day, the trunks of fallen trees, and the stools of such trees as may have been broken by the violence of storms, remain entire for but a short time. contrary to what might be expected, the dense wood of the tree decays, and suffers from the ravages of insects, more swiftly than the bark. and the traveller, setting his foot on a prostrate trunk, finds that it is a mere shell, which breaks under his weight, and lands his foot amidst the insects, or the reptiles, which have sought food or refuge within. the trees of the coal forests present parallel conditions. when the fallen trunks which have entered into the composition of the bed of coal are identifiable, they are mere double shells of bark, flattened together in consequence of the destruction of the woody core; and sir charles lyell and principal dawson discovered, in the hollow stools of coal trees of nova scotia, the remains of snails, millipedes, and salamander-like creatures, embedded in a deposit of a different character from that which surrounded the exterior of the trees. thus, in endeavouring to comprehend the formation of a seam of coal, we must try to picture to ourselves a thick forest, formed for the most part of trees like gigantic club- mosses, mares'-tails, and tree-ferns, with here and there some that had more resemblance to our existing yews and fir-trees. we must suppose that, as the seasons rolled by, the plants grew and developed their spores and seeds; that they shed these in enormous quantities, which accumulated on the ground beneath; and that, every now and then, they added a dead frond or leaf; or, at longer intervals, a rotten branch, or a dead trunk, to the mass. a certain proportion of the spores and seeds no doubt fulfilled their obvious function, and, carried by the wind to unoccupied regions, extended the limits of the forest; many might be washed away by rain into streams, and be lost; but a large portion must have remained, to accumulate like beech-mast, or acorns, beneath the trees of a modern forest. but, in this case it may be asked, why does not our english coal consist of stems and leaves to a much greater extent than it does? what is the reason of the predominance of the spores and spore-cases in it? a ready answer to this question is afforded by the study of a living full-grown club-moss. shake it upon a piece of paper, and it emits a cloud of fine dust, which falls over the paper, and is the well-known lycopodium powder. now this powder used to be, and i believe still is, employed for two objects which seem, at first sight, to have no particular connection with one another. it is, or was, employed in making lightning, and in making pills. the coats of the spores contain so much resinous matter, that a pinch of lycopodium powder, thrown through the flame of a candle, burns with an instantaneous flash, which has long done duty for lightning on the stage. and the same character makes it a capital coating for pills; for the resinous powder prevents the drug from being wetted by the saliva, and thus bars the nauseous flavour from the sensitive papilla; of the tongue. but this resinous matter, which lies in the walls of the spores and sporangia, is a substance not easily altered by air and water, and hence tends to preserve these bodies, just as the bituminized cerecloth preserves an egyptian mummy; while, on the other hand, the merely woody stem and leaves tend to rot, as fast as the wood of the mummy's coffin has rotted. thus the mixed heap of spores, leaves, and stems in the coal- forest would be persistently searched by the long-continued action of air and rain; the leaves and stems would gradually be reduced to little but their carbon, or, in other words, to the condition of mineral charcoal in which we find them; while the spores and sporangia remained as a comparatively unaltered and compact residuum. there is, indeed, tolerably clear evidence that the coal must, under some circumstances, have been converted into a substance hard enough to be rolled into pebbles, while it yet lay at the surface of the earth; for in some seams of coal, the courses of rivulets, which must have been living water, while the stratum in which their remains are found was still at the surface, have been observed to contain rolled pebbles of the very coal through which the stream has cut its way. the structural facts are such as to leave no alternative but to adopt the view of the origin of such coal as i have described, which has just been stated; but, happily, the process is not without analogy at the present day. i possess a specimen of what is called "white coal" from australia. it is an inflammable material, burning with a bright flame and having much the consistence and appearance of oat-cake, which, i am informed covers a considerable area. it consists, almost entirely, of a compacted mass of spores and spore-cases. but the fine particles of blown sand which are scattered through it, show that it must have accumulated, subaërially, upon the surface of a soil covered by a forest of cryptogamous plants, probably tree-ferns. as regards this important point of the subaërial region of coal, i am glad to find myself in entire accordance with principal dawson, who bases his conclusions upon other, but no less forcible, considerations. in a passage, which is the continuation of that already cited, he writes:-- "( ) the microscopical structure and chemical composition of the beds of cannel coal and earthy bitumen, and of the more highly bituminous and carbonaceous shale, show them to have been of the nature of the fine vegetable mud which accumulates in the ponds and shallow lakes of modern swamps. when such tine vegetable sediment is mixed, as is often the case, with clay, it becomes similar to the bituminous limestone and calcareo- bituminous shales of the coal-measures. ( ) a few of the under-clays, which support beds of coal, are of the nature of the vegetable mud above referred to; but the greater part are argillo-arenaceous in composition, with little vegetable matter, and bleached by the drainage from them of water containing the products of vegetable decay. they are, in short, loamy or clay soils, and must have been sufficiently above water to admit of drainage. the absence of sulphurets, and the occurrence of carbonate of iron in connection with them, prove that, when they existed as soils, rain-water, and not sea-water, percolated them. ( ) the coal and the fossil forests present many evidences of subaërial conditions. most of the erect and prostrate trees had become hollow shells of bark before they were finally embedded, and their wood had broken into cubical pieces of mineral charcoal. land-snails and galley-worms (_xylobius_) crept into them, and they became dens, or traps, for reptiles. large quantities of mineral charcoal occur on the surface of all the large beds of coal. none of these appearances could have been produced by subaqueous action. ( ) though the roots of the _sigillaria_ bear more resemblance to the rhizomes of certain aquatic plants; yet, structurally, they are absolutely identical with the roots of cycads, which the stems also resemble. further, the _sigillarioe_ grew on the same soils which supported conifers, _lepidodendra_, _cordaites_, and ferns-plants which could not have grown in water. again, with the exception perhaps of some _pinnularioe_, and _asterophyllites_, there is a remarkable absence from the coal measures of any form of properly aquatic vegetation. ( ) the occurrence of marine, or brackish-water animals, in the roofs of coal- beds, or even in the coal itself, affords no evidence of subaqueous accumulation, since the same thing occurs in the case of modern submarine forests. for these and other reasons, some of which are more fully stated in the papers already referred to, while i admit that the areas of coal accumulation were frequently submerged, i must maintain that the true coal is a subaërial accumulation by vegetable growth on soils, wet and swampy it is true, but not submerged." i am almost disposed to doubt whether it is necessary to make the concession of "wet and swampy"; otherwise, there is nothing that i know of to be said against this excellent conspectus of the reasons for believing in the subaërial origin of coal. but the coal accumulated upon the area covered by one of the great forests of the carboniferous epoch would in course of time, have been wasted away by the small, but constant, wear and tear of rain and streams had the land which supported it remained at the same level, or been gradually raised to a greater elevation. and, no doubt, as much coal as now exists has been destroyed, after its formation, in this way. what are now known as coal districts owe their importance to the fact that they were areas of slow depression, during a greater or less portion of the carboniferous epoch; and that, in virtue of this circumstance, mother earth was enabled to cover up her vegetable treasures, and preserve them from destruction. wherever a coal-field now exists, there must formerly have been free access for a great river, or for a shallow sea, bearing sediment in the shape of sand and mud. when the coal-forest area became slowly depressed, the waters must have spread over it, and have deposited their burden upon the surface of the bed of coal, in the form of layers, which are now converted into shale, or sandstone. then followed a period of rest, in which the superincumbent shallow waters became completely filled up, and finally replaced, by fine mud, which settled down into a new under-clay, and furnished the soil for a fresh forest growth. this flourished, and heaped up its spores and wood into coal, until the stage of slow depression recommenced. and, in some localities, as i have mentioned, the process was repeated until the first of the alternating beds had sunk to near three miles below its original level at the surface of the earth. in reflecting on the statement, thus briefly made, of the main facts connected with the origin of the coal formed during the carboniferous epoch, two or three considerations suggest themselves. in the first place, the great phantom of geological time rises before the student of this, as of all other, fragments of the history of our earth-- springing irrepressibly out of the facts, like the djin from the jar which the fishermen so incautiously opened; and like the djin again, being vaporous, shifting, and indefinable, but unmistakably gigantic. however modest the bases of one's calculation may be, the minimum of time assignable to the coal period remains something stupendous. principal dawson is the last person likely to be guilty of exaggeration in this matter, and it will be well to consider what he has to say about it:-- "the rate of accumulation of coal was very slow. the climate of the period, in the northern temperate zone, was of such a character that the true conifers show rings of growth, not larger, nor much less distinct, than those of many of their modern congeners. the _sigillarioe_ and _calamites_ were not, as often supposed, composed wholly, or even principally, of lax and soft tissues, or necessarily short-lived. the former had, it is true, a very thick inner bark; but their dense woody axis, their thick and nearly imperishable outer bark, and their scanty and rigid foliage, would indicate no very rapid growth or decay. in the case of the _sigillarioe_, the variations in the leaf-scars in different parts of the trunk, the intercalation of new ridges at the surface representing that of new woody wedges in the axis, the transverse marks left by the stages of upward growth, all indicate that several years must have been required for the growth of stems of moderate size. the enormous roots of these trees, and the condition of the coal-swamps, must have exempted them from the danger of being overthrown by violence. they probably fell in successive generations from natural decay; and making every allowance for other materials, we may safely assert that every foot of thickness of pure bituminous coal implies the quiet growth and fall of at least fifty generations of _sigillarioe_, and therefore an undisturbed condition of forest growth enduring through many centuries. further, there is evidence that an immense amount of loose parenchymatous tissue, and even of wood, perished by decay, and we do not know to what extent even the most durable tissues may have disappeared in this way; so that, in many coal-seams, we may have only a very small part of the vegetable matter produced." undoubtedly the force of these reflections is not diminished when the bituminous coal, as in britain, consists of accumulated spores and spore- cases, rather than of stems. but, suppose we adopt principal dawson's assumption, that one foot of coal represents fifty generations of coal plants; and, further, make the moderate supposition that each generation of coal plants took ten years to come to maturity--then, each foot- thickness of coal represents five hundred years. the superimposed beds of coal in one coal-field may amount to a thickness of fifty or sixty feet, and therefore the coal alone, in that field, represents x = , years. but the actual coal is but an insignificant portion of the total deposit, which, as has been seen, may amount to between two and three miles of vertical thickness. suppose it be , feet--which is times the thickness of the actual coal--is there any reason why we should believe it may not have taken times as long to form? i know of none. but, in this case, the time which the coal-field represents would be , x = , , years. as affording a definite chronology, of course such calculations as these are of no value; but they have much use in fixing one's attention upon a possible minimum. a man may be puzzled if he is asked how long rome took a-building; but he is proverbially safe if he affirms it not to have been built in a day; and our geological calculations are all, at present, pretty much on that footing. a second consideration which the study of the coal brings prominently before the mind of any one who is familiar with palaeontology is, that the coal flora, viewed in relation to the enormous period of time which it lasted, and to the still vaster period which has elapsed since it flourished, underwent little change while it endured, and in its peculiar characters, differs strangely little from that which at present exist. the same species of plants are to be met with throughout the whole thickness of a coal-field, and the youngest are not sensibly different from the oldest. but more than this. notwithstanding that the carboniferous period is separated from us by more than the whole time represented by the secondary and tertiary formations, the great types of vegetation were as distinct then as now. the structure of the modern club-moss furnishes a complete explanation of the fossil remains of the _lepidodendra_, and the fronds of some of the ancient ferns are hard to distinguish from existing ones. at the same time, it must be remembered, that there is nowhere in the world, at present, any _forest_ which bears more than a rough analogy with a coal-forest. the types may remain, but the details of their form, their relative proportions, their associates, are all altered. and the tree-fern forest of tasmania, or new zealand, gives one only a faint and remote image of the vegetation of the ancient world. once more, an invariably-recurring lesson of geological history, at whatever point its study is taken up: the lesson of the almost infinite slowness of the modification of living forms. the lines of the pedigrees of living things break off almost before they begin to converge. finally, yet another curious consideration. let us suppose that one of the stupid, salamander-like labyrinthodonts, which pottered, with much belly and little leg, like falstaff in his old age, among the coal- forests, could have had thinking power enough in his small brain to reflect upon the showers of spores which kept on falling through years and centuries, while perhaps not one in ten million fulfilled its apparent purpose, and reproduced the organism which gave it birth: surely he might have been excused for moralizing upon the thoughtless and wanton extravagance which nature displayed in her operations. but we have the advantage over our shovel-headed predecessor--or possibly ancestor--and can perceive that a certain vein of thrift runs through this apparent prodigality. nature is never in a hurry, and seems to have had always before her eyes the adage, "keep a thing long enough, and you will find a use for it." she has kept her beds of coal many millions of years without being able to find much use for them; she has sent them down beneath the sea, and the sea-beasts could make nothing of them; she has raised them up into dry land, and laid the black veins bare, and still, for ages and ages, there was no living thing on the face of the earth that could see any sort of value in them; and it was only the other day, so to speak, that she turned a new creature out of her workshop, who by degrees acquired sufficient wits to make a fire, and then to discover that the black rock would burn. i suppose that nineteen hundred years ago, when julius caesar was good enough to deal with britain as we have dealt with new zealand, the primaeval briton, blue with cold and woad, may have known that the strange black stone, of which he found lumps here and there in his wanderings, would burn, and so help to warm his body and cook his food. saxon, dane, and norman swarmed into the land. the english people grew into a powerful nation, and nature still waited for a full return of the capital she had invested in the ancient club-mosses. the eighteenth century arrived, and with it james watt. the brain of that man was the spore out of which was developed the modern steam-engine, and all the prodigious trees and branches of modern industry which have grown out of this. but coal is as much an essential condition of this growth and development as carbonic acid is for that of a club-moss. wanting coal, we could not have smelted the iron needed to make our engines, nor have worked our engines when we had got them. but take away the engines, and the great towns of yorkshire and lancashire vanish like a dream. manufactures give place to agriculture and pasture, and not ten men can live where now ten thousand are amply supported. thus, all this abundant wealth of money and of vivid life is nature's interest upon her investment in club-mosses, and the like, so long ago. but what becomes of the coal which is burnt in yielding this interest? heat comes out of it, light comes out of it; and if we could gather together all that goes up the chimney, and all that remains in the grate of a thoroughly-burnt coal-fire, we should find ourselves in possession of a quantity of carbonic acid, water, ammonia, and mineral matters, exactly equal in weight to the coal. but these are the very matters with which nature supplied the club-mosses which made the coal she is paid back principal and interest at the same time; and she straightway invests the carbonic acid, the water, and the ammonia in new forms of life, feeding with them the plants that now live. thrifty nature! surely no prodigal, but most notable of housekeepers! vi on the border territory between the animal and the vegetable kingdoms [ ] in the whole history of science there is nothing more remarkable than the rapidity of the growth of biological knowledge within the last half- century, and the extent of the modification which has thereby been effected in some of the fundamental conceptions of the naturalist. in the second edition of the "règne animal," published in , cuvier devotes a special section to the "division of organised beings into animals and vegetables," in which the question is treated with that comprehensiveness of knowledge and clear critical judgment which characterise his writings, and justify us in regarding them as representative expressions of the most extensive, if not the profoundest, knowledge of his time. he tells us that living beings have been subdivided from the earliest times into _animated beings_, which possess sense and motion, and _inanimated beings_, which are devoid of these functions and simply vegetate. although the roots of plants direct themselves towards moisture, and their leaves towards air and light,--although the parts of some plants exhibit oscillating movements without any perceptible cause, and the leaves of others retract when touched,--yet none of these movements justify the ascription to plants of perception or of will. from the mobility of animals, cuvier, with his characteristic partiality for teleological reasoning, deduces the necessity of the existence in them of an alimentary cavity, or reservoir of food, whence their nutrition may be drawn by the vessels, which are a sort of internal roots; and, in the presence of this alimentary cavity, he naturally sees the primary and the most important distinction between animals and plants. following out his teleological argument, cuvier remarks that the organisation of this cavity and its appurtenances must needs vary according to the nature of the aliment, and the operations which it has to undergo, before it can be converted into substances fitted for absorption; while the atmosphere and the earth supply plants with juices ready prepared, and which can be absorbed immediately. as the animal body required to be independent of heat and of the atmosphere, there were no means by which the motion of its fluids could be produced by internal causes. hence arose the second great distinctive character of animals, or the circulatory system, which is less important than the digestive, since it was unnecessary, and therefore is absent, in the more simple animals. animals further needed muscles for locomotion and nerves for sensibility. hence, says cuvier, it was necessary that the chemical composition of the animal body should be more complicated than that of the plant; and it is so, inasmuch as an additional substance, nitrogen, enters into it as an essential element; while, in plants, nitrogen is only accidentally joined with he three other fundamental constituents of organic beings--carbon, hydrogen, and oxygen. indeed, he afterwards affirms that nitrogen is peculiar to animals; and herein he places the third distinction between the animal and the plant. the soil and the atmosphere supply plants with water, composed of hydrogen and oxygen; air, consisting of nitrogen and oxygen; and carbonic acid, containing carbon and oxygen. they retain the hydrogen and the carbon, exhale the superfluous oxygen, and absorb little or no nitrogen. the essential character of vegetable life is the exhalation of oxygen, which is effected through the agency of light. animals, on the contrary, derive their nourishment either directly or indirectly from plants. they get rid of the superfluous hydrogen and carbon, and accumulate nitrogen. the relations of plants and animals to the atmosphere are therefore inverse. the plant withdraws water and carbonic acid from the atmosphere, the animal contributes both to it. respiration--that is, the absorption of oxygen and the exhalation of carbonic acid--is the specially animal function of animals, and constitutes their fourth distinctive character. thus wrote cuvier in . but, in the fourth and fifth decades of this century, the greatest and most rapid revolution which biological science has ever undergone was effected by the application of the modern microscope to the investigation of organic structure; by the introduction of exact and easily manageable methods of conducting the chemical analysis of organic compounds; and finally, by the employment of instruments of precision for the measurement of the physical forces which are at work in the living economy. that the semi-fluid contents (which we now term protoplasm) of the cells of certain plants, such as the _charoe_ are in constant and regular motion, was made out by bonaventura corti a century ago; but the fact, important as it was, fell into oblivion, and had to be rediscovered by treviranus in . robert brown noted the more complex motions of the protoplasm in the cells of _tradescantia_ in ; and now such movements of the living substance of plants are well known to be some of the most widely-prevalent phenomena of vegetable life. agardh, and other of the botanists of cuvier's generation, who occupied themselves with the lower plants, had observed that, under particular circumstances, the contents of the cells of certain water-weeds were set free, and moved about with considerable velocity, and with all the appearances of spontaneity, as locomotive bodies, which, from their similarity to animals of simple organisation, were called "zoospores." even as late as , however, a botanist of schleiden's eminence dealt very sceptically with these statements; and his scepticism was the more justified, since ehrenberg, in his elaborate and comprehensive work on the _infusoria_, had declared the greater number of what are now recognised as locomotive plants to be animals. at the present day, innumerable plants and free plant cells are known to pass the whole or part of their lives in an actively locomotive condition, in no wise distinguishable from that of one of the simpler animals; and, while in this condition, their movements are, to all appearance, as spontaneous--as much the product of volition--as those of such animals. hence the teleological argument for cuvier's first diagnostic character-- the presence in animals of an alimentary cavity, or internal pocket, in which they can carry about their nutriment--has broken down, so far, at least, as his mode of stating it goes. and, with the advance of microscopic anatomy, the universality of the fact itself among animals has ceased to be predicable. many animals of even complex structure, which live parasitically within others, are wholly devoid of an alimentary cavity. their food is provided for them, not only ready cooked, but ready digested, and the alimentary canal, become superfluous, has disappeared. again, the males of most rotifers have no digestive apparatus; as a german naturalist has remarked, they devote themselves entirely to the "minnedienst," and are to be reckoned among the few realisations of the byronic ideal of a lover. finally, amidst the lowest forms of animal life, the speck of gelatinous protoplasm, which constitutes the whole body, has no permanent digestive cavity or mouth, but takes in its food anywhere; and digests, so to speak, all over its body. but although cuvier's leading diagnosis of the animal from the plant will not stand a strict test, it remains one of the most constant of the distinctive characters of animals. and, if we substitute for the possession of an alimentary cavity, the power of taking solid nutriment into the body and there digesting it, the definition so changed will cover all animals except certain parasites, and the few and exceptional cases of non-parasitic animals which do not feed at all. on the other hand, the definition thus amended will exclude all ordinary vegetable organisms. cuvier himself practically gives up his second distinctive mark when he admits that it is wanting in the simpler animals. the third distinction is based on a completely erroneous conception of the chemical differences and resemblances between the constituents of animal and vegetable organisms, for which cuvier is not responsible, as it was current among contemporary chemists. it is now established that nitrogen is as essential a constituent of vegetable as of animal living matter; and that the latter is, chemically speaking, just as complicated as the former. starchy substances, cellulose and sugar, once supposed to be exclusively confined to plants, are now known to be regular and normal products of animals. amylaceous and saccharine substances are largely manufactured, even by the highest animals; cellulose is widespread as a constituent of the skeletons of the lower animals; and it is probable that amyloid substances are universally present in the animal organism, though not in the precise form of starch. moreover, although it remains true that there is an inverse relation between the green plant in sunshine and the animal, in so far as, under these circumstances, the green plant decomposes carbonic acid and exhales oxygen, while the animal absorbs oxygen and exhales carbonic acid; yet, the exact researches of the modern chemical investigators of the physiological processes of plants have clearly demonstrated the fallacy of attempting to draw any general distinction between animals and vegetables on this ground. in fact, the difference vanishes with the sunshine, even in the case of the green plant; which, in the dark, absorbs oxygen and gives out carbonic acid like any animal.[ ] on the other hand, those plants, such as the fungi, which contain no chlorophyll and are not green, are always, so far as respiration is concerned, in the exact position of animals. they absorb oxygen and give out carbonic acid. [footnote : there is every reason to believe that living plants, like living animals, always respire, and, in respiring, absorb oxygen and give off carbonic acid; but, that in green plants exposed to daylight or to the electric light, the quantity of oxygen evolved in consequence of the decomposition of carbonic acid by a special apparatus which green plants possess exceeds that absorbed in the concurrent respiratory process.] thus, by the progress of knowledge, cuvier's fourth distinction between the animal and the plant has been as completely invalidated as the third and second; and even the first can be retained only in a modified form and subject to exceptions. but has the advance of biology simply tended to break down old distinctions, without establishing new ones? with a qualification, to be considered presently, the answer to this question is undoubtedly in the affirmative. the famous researches of schwann and schleiden in and the following years, founded the modern science of histology, or that branch of anatomy which deals with the ultimate visible structure of organisms, as revealed by the microscope; and, from that day to this, the rapid improvement of methods of investigation, and the energy of a host of accurate observers, have given greater and greater breadth and firmness to schwann's great generalisation, that a fundamental unity of structure obtains in animals and plants; and that, however diverse may be the fabrics, or _tissues_, of which their bodies are composed, all these varied structures result from the metamorphosis of morphological units (termed _cells_, in a more general sense than that in which the word "cells" was at first employed), which are not only similar in animals and in plants respectively, but present a close resemblance, when those of animals and those of plants are compared together. the contractility which is the fundamental condition of locomotion, has not only been discovered to exist far more widely among plants than was formerly imagined; but, in plants, the act of contraction has been found to be accompanied, as dr. burdon sanderson's interesting investigations have shown, by a disturbance of the electrical state of the contractile substance, comparable to that which was found by du bois reymond to be a concomitant of the activity of ordinary muscle in animals. again, i know of no test by which the reaction of the leaves of the sundew and of other plants to stimuli, so fully and carefully studied by mr. darwin, can be distinguished from those acts of contraction following upon stimuli, which are called "reflex" in animals. on each lobe of the bilobed leaf of venus's fly-trap (_dionoea muscipula_) are three delicate filaments which stand out at right angle from the surface of the leaf. touch one of them with the end of a fine human hair and the lobes of the leaf instantly close together[ ] in virtue of an act of contraction of part of their substance, just as the body of a snail contracts into its shell when one of its "horns" is irritated. [footnote : darwin, _insectivorous plants_, p. .] the reflex action of the snail is the result of the presence of a nervous system in the animal. a molecular change takes place in the nerve of the tentacle, is propagated to the muscles by which the body is retracted, and causing them to contract, the act of retraction is brought about. of course the similarity of the acts does not necessarily involve the conclusion that the mechanism by which they are effected is the same; but it suggests a suspicion of their identity which needs careful testing. the results of recent inquiries into the structure of the nervous system of animals converge towards the conclusion that the nerve fibres, which we have hitherto regarded as ultimate elements of nervous tissue, are not such, but are simply the visible aggregations of vastly more attenuated filaments, the diameter of which dwindles down to the limits of our present microscopic vision, greatly as these have been extended by modern improvements of the microscope; and that a nerve is, in its essence, nothing but a linear tract of specially modified protoplasm between two points of an organism--one of which is able to affect the other by means of the communication so established. hence, it is conceivable that even the simplest living being may possess a nervous system. and the question whether plants are provided with a nervous system or not, thus acquires a new aspect, and presents the histologist and physiologist with a problem of extreme difficulty, which must be attacked from a new point of view and by the aid of methods which have yet to be invented. thus it must be admitted that plants may be contractile and locomotive; that, while locomotive, their movements may have as much appearance of spontaneity as those of the lowest animals; and that many exhibit actions, comparable to those which are brought about by the agency of a nervous system in animals. and it must be allowed to be possible that further research may reveal the existence of something comparable to a nervous system in plants. so that i know not where we can hope to find any absolute distinction between animals and plants, unless we return to their mode of nutrition, and inquire whether certain differences of a more occult character than those imagined to exist by cuvier, and which certainly hold good for the vast majority of animals and plants, are of universal application. a bean may be supplied with water in which salts of ammonia and certain other mineral salts are dissolved in due proportion; with atmospheric air containing its ordinary minute dose of carbonic acid; and with nothing else but sunlight and heat. under these circumstances, unnatural as they are, with proper management, the bean will thrust forth its radicle and its plumule; the former will grow down into roots, the latter grow up into the stem and leaves of a vigorous bean-plant; and this plant will, in due time, flower and produce its crop of beans, just as if it were grown in the garden or in the field. the weight of the nitrogenous protein compounds, of the oily, starchy, saccharine and woody substances contained in the full-grown plant and its seeds, will be vastly greater than the weight of the same substances contained in the bean from which it sprang. but nothing has been supplied to the bean save water, carbonic acid, ammonia, potash, lime, iron, and the like, in combination with phosphoric, sulphuric, and other acids. neither protein, nor fat, nor starch, nor sugar, nor any substance in the slightest degree resembling them, has formed part of the food of the bean. but the weights of the carbon, hydrogen, oxygen, nitrogen, phosphorus, sulphur, and other elementary bodies contained in the bean- plant, and in the seeds which it produces, are exactly equivalent to the weights of the same elements which have disappeared from the materials supplied to the bean during its growth. whence it follows that the bean has taken in only the raw materials of its fabric, and has manufactured them into bean-stuffs. the bean has been able to perform this great chemical feat by the help of its green colouring matter, or chlorophyll; for it is only the green parts of the plant which, under the influence of sunlight, have the marvellous power of decomposing carbonic acid, setting free the oxygen and laying hold of the carbon which it contains. in fact, the bean obtains two of the absolutely indispensable elements of its substance from two distinct sources; the watery solution, in which its roots are plunged, contains nitrogen but no carbon; the air, to which the leaves are exposed, contains carbon, but its nitrogen is in the state of a free gas, in which condition the bean can make no use of it;[ ] and the chlorophyll[ ] is the apparatus by which the carbon is extracted from the atmospheric carbonic acid--the leaves being the chief laboratories in which this operation is effected. [footnote : i purposely assume that the air with which the bean is supplied in the case stated contains no ammoniacal salts.] [footnote : the recent researches of pringsheim have raised a host of questions as to the exact share taken by chlorophyll in the chemical operations which are effected by the green parts of plants. it may be that the chlorophyll is only a constant concomitant of the actual deoxidising apparatus.] the great majority of conspicuous plants are, as everybody knows, green; and this arises from the abundance of their chlorophyll. the few which contain no chlorophyll and are colourless, are unable to extract the carbon which they require from atmospheric carbonic acid, and lead a parasitic existence upon other plants; but it by no means follows, often as the statement has been repeated, that the manufacturing power of plants depends on their chlorophyll, and its interaction with the rays of the sun. on the contrary, it is easily demonstrated, as pasteur first proved, that the lowest fungi, devoid of chlorophyll, or of any substitute for it, as they are, nevertheless possess the characteristic manufacturing powers of plants in a very high degree. only it is necessary that they should be supplied with a different kind of raw material; as they cannot extract carbon from carbonic acid, they must be furnished with something else that contains carbon. tartaric acid is such a substance; and if a single spore of the commonest and most troublesome of moulds--_penicillium_--be sown in a saucerful of water, in which tartrate of ammonia, with a small percentage of phosphates and sulphates is contained, and kept warm, whether in the dark or exposed to light, it will, in a short time, give rise to a thick crust of mould, which contains many million times the weight of the original spore, in protein compounds and cellulose. thus we have a very wide basis of fact for the generalisation that plants are essentially characterised by their manufacturing capacity--by their power of working up mere mineral matters into complex organic compounds. contrariwise, there is a no less wide foundation for the generalisation that animals, as cuvier puts it, depend directly or indirectly upon plants for the materials of their bodies; that is, either they are herbivorous, or they eat other animals which are herbivorous. but for what constituents of their bodies are animals thus dependent upon plants? certainly not for their horny matter; nor for chondrin, the proximate chemical element of cartilage; nor for gelatine; nor for syntonin, the constituent of muscle; nor for their nervous or biliary substances; nor for their amyloid matters; nor, necessarily, for their fats. it can be experimentally demonstrated that animals can make these for themselves. but that which they cannot make, but must, in all known cases, obtain directly or indirectly from plants, is the peculiar nitrogenous matter, protein. thus the plant is the ideal _prolétaire_ of the living world, the worker who produces; the animal, the ideal aristocrat, who mostly occupies himself in consuming, after the manner of that noble representative of the line of zähdarm, whose epitaph is written in "sartor resartus." here is our last hope of finding a sharp line of demarcation between plants and animals; for, as i have already hinted, there is a border territory between the two kingdoms, a sort of no-man's-land, the inhabitants of which certainly cannot be discriminated and brought to their proper allegiance in any other way. some months ago, professor tyndall asked me to examine a drop of infusion of hay, placed under an excellent and powerful microscope, and to tell him what i thought some organisms visible in it were. i looked and observed, in the first place, multitudes of _bacteria_ moving about with their ordinary intermittent spasmodic wriggles. as to the vegetable nature of these there is now no doubt. not only does the close resemblance of the _bacteria_ to unquestionable plants, such as the _oscillatorioe_ and the lower forms of _fungi_, justify this conclusion, but the manufacturing test settles the question at once. it is only needful to add a minute drop of fluid containing _bacteria_, to water in which tartrate, phosphate, and sulphate of ammonia are dissolved; and, in a very short space of time, the clear fluid becomes milky by reason of their prodigious multiplication, which, of course, implies the manufacture of living bacterium-stuff out of these merely saline matters. but other active organisms, very much larger than the _bacteria_, attaining in fact the comparatively gigantic dimensions of / of an inch or more, incessantly crossed the field of view. each of these had a body shaped like a pear, the small end being slightly incurved and produced into a long curved filament, or _cilium_, of extreme tenuity. behind this, from the concave side of the incurvation, proceeded another long cilium, so delicate as to be discernible only by the use of the highest powers and careful management of the light. in the centre of the pear-shaped body a clear round space could occasionally be discerned, but not always; and careful watching showed that this clear vacuity appeared gradually, and then shut up and disappeared suddenly, at regular intervals. such a structure is of common occurrence among the lowest plants and animals, and is known as a _contractile vacuole_. the little creature thus described sometimes propelled itself with great activity, with a curious rolling motion, by the lashing of the front cilium, while the second cilium trailed behind; sometimes it anchored itself by the hinder cilium and was spun round by the working of the other, its motions resembling those of an anchor buoy in a heavy sea. sometimes, when two were in full career towards one another, each would appear dexterously to get out of the other's way; sometimes a crowd would assemble and jostle one another, with as much semblance of individual effort as a spectator on the grands mulets might observe with a telescope among the specks representing men in the valley of chamounix. the spectacle, though always surprising, was not new to me. so my reply to the question put to me was, that these organisms were what biologists call _monads_, and though they might be animals, it was also possible that they might, like the _bacteria_, be plants. my friend received my verdict with an expression which showed a sad want of respect for authority. he would as soon believe that a sheep was a plant. naturally piqued by this want of faith, i have thought a good deal over the matter; and, as i still rest in the lame conclusion i originally expressed, and must even now confess that i cannot certainly say whether this creature is an animal or a plant, i think it may be well to state the grounds of my hesitation at length. but, in the first place, in order that i may conveniently distinguish this "monad" from the multitude of other things which go by the same designation, i must give it a name of its own. i think (though, for reasons which need not be stated at present, i am not quite sure) that it is identical with the species _monas lens_ as defined by the eminent french microscopist dujardin, though his magnifying power was probably insufficient to enable him to see that it is curiously like a much larger form of monad which he has named _heteromita_. i shall, therefore, call it not _monas_, but _heteromita lens_. i have been unable to devote to my _heteromita_ the prolonged study needful to work out its whole history, which would involve weeks, or it may be months, of unremitting attention. but i the less regret this circumstance, as some remarkable observations recently published by messrs. dallinger and drysdale[ ] on certain monads, relate, in part, to a form so similar to my _heteromita lens_, that the history of the one may be used to illustrate that of the other. these most patient and painstaking observers, who employed the highest attainable powers of the microscope and, relieving one another, kept watch day and night over the same individual monads, have been enabled to trace out the whole history of their _heteromita_; which they found in infusions of the heads of fishes of the cod tribe. [footnote : "researches in the life-history of a cercomonad: a lesson in biogenesis"; and "further researches in the life-history of the monads," --_monthly microscopical journal_, .] of the four monads described and figured by these investigators, one, as i have said, very closely resembles _heteromita lens_ in every particular, except that it has a separately distinguishable central particle or "nucleus," which is not certainly to be made out in _heteromita lens_; and that nothing is said by messrs. dallinger and drysdale of the existence of a contractile vacuole in this monad, though they describe it in another. their _heteromita_, however, multiplied rapidly by fission. sometimes a transverse constriction appeared; the hinder half developed a new cilium, and the hinder cilium gradually split from its base to its free end, until it was divided into two; a process which, considering the fact that this fine filament cannot be much more than / of an inch in diameter, is wonderful enough. the constriction of the body extended inwards until the two portions were united by a narrow isthmus; finally, they separated and each swam away by itself, a complete _heteromita_, provided with its two cilia. sometimes the constriction took a longitudinal direction, with the same ultimate result. in each case the process occupied not more than six or seven minutes. at this rate, a single _heteromita_ would give rise to a thousand like itself in the course of an hour, to about a million in two hours, and to a number greater than the generally assumed number of human beings now living in the world in three hours; or, if we give each _heteromita_ an hour's enjoyment of individual existence, the same result will be obtained in about a day. the apparent suddenness of the appearance of multitudes of such organisms as these in any nutritive fluid to which one obtains access is thus easily explained. during these processes of multiplication by fission, the _heteromita_ remains active; but sometimes another mode of fission occurs. the body becomes rounded and quiescent, or nearly so; and, while in this resting state, divides into two portions, each of which is rapidly converted into an active _heteromita_. a still more remarkable phenomenon is that kind of multiplication which is preceded by the union of two monads, by a process which is termed _conjugation_. two active _heteromitoe_ become applied to one another, and then slowly and gradually coalesce into one body. the two nuclei run into one; and the mass resulting from the conjugation of the two _heteromitoe_, thus fused together, has a triangular form. the two pairs of cilia are to be seen, for some time, at two of the angles, which answer to the small ends of the conjoined monads; but they ultimately vanish, and the twin organism, in which all visible traces of organisation have disappeared, falls into a state of rest. sudden wave- like movements of its substance next occur; and, in a short time, the apices of the triangular mass burst, and give exit to a dense yellowish, glairy fluid, filled with minute granules. this process, which, it will be observed, involves the actual confluence and mixture of the substance of two distinct organisms, is effected in the space of about two hours. the authors whom i quote say that they "cannot express" the excessive minuteness of the granules in question, and they estimate their diameter at less than / of an inch. under the highest powers of the microscope, at present applicable, such specks are hardly discernible. nevertheless, particles of this size are massive when compared to physical molecules; whence there is no reason to doubt that each, small as it is, may have a molecular structure sufficiently complex to give rise to the phenomena of life. and, as a matter of fact, by patient watching of the place at which these infinitesimal living particles were discharged, our observers assured themselves of their growth and development into new monads. in about four hours from their being set free, they had attained a sixth of the length of the parent, with the characteristic cilia, though at first they were quite motionless; and, in four hours more, they had attained the dimensions and exhibited all the activity of the adult. these inconceivably minute particles are therefore the germs of the _heteromita_; and from the dimensions of these germs it is easily shown that the body formed by conjugation may, at a low estimate, have given exit to thirty thousand of them; a result of a matrimonial process whereby the contracting parties, without a metaphor, "become one flesh," enough to make a malthusian despair of the future of the universe. i am not aware that the investigators from whom i have borrowed this history have endeavoured to ascertain whether their monads take solid nutriment or not; so that though they help us very much to fill up the blanks in the history of my _heteromita_, their observations throw no light on the problem we are trying to solve--is it an animal or is it a plant? undoubtedly it is possible to bring forward very strong arguments in favour of regarding _heteromita_ as a plant. for example, there is a fungus, an obscure and almost microscopic mould, termed _peronospora infestans_. like many other fungi, the _peronosporoe_ are parasitic upon other plants; and this particular _peronospora_ happens to have attained much notoriety and political importance, in a way not without a parallel in the career of notorious politicians, namely, by reason of the frightful mischief it has done to mankind. for it is this _fungus_ which is the cause of the potato disease; and, therefore, _peronospora infestans_ (doubtless of exclusively saxon origin, though not accurately known to be so) brought about the irish famine. the plants afflicted with the malady are found to be infested by a mould, consisting of fine tubular filaments, termed _hyphoe_, which burrow through the substance of the potato plant, and appropriate to themselves the substance of their host; while, at the same time, directly or indirectly, they set up chemical changes by which even its woody framework becomes blackened, sodden, and withered. in structure, however, the _peronospora_ is as much a mould as the common _penicillium_; and just as the _penicillium_ multiplies by the breaking up of its hyphoe into separate rounded bodies, the spores; so, in the _peronospora_, certain of the hyphoe grow out into the air through the interstices of the superficial cells of the potato plant, and develop spores. each of these hyphoe usually gives off several branches. the ends of the branches dilate and become closed sacs, which eventually drop off as spores. the spores falling on some part of the same potato plant, or carried by the wind to another, may at once germinate, throwing out tubular prolongations which become hyphoe, and burrow into the substance of the plant attacked. but, more commonly, the contents of the spore divide into six or eight separate portions. the coat of the spore gives way, and each portion then emerges as an independent organism, which has the shape of a bean, rather narrower at one end than the other, convex on one side, and depressed or concave on the opposite. from the depression, two long and delicate cilia proceed, one shorter than the other, and directed forwards. close to the origin of these cilia, in the substance of the body, is a regularly pulsating, contractile vacuole. the shorter cilium vibrates actively, and effects the locomotion of the organism, while the other trails behind; the whole body rolling on its axis with its pointed end forwards. the eminent botanist, de bary, who was not thinking of our problem, tells us, in describing the movements of these "zoospores," that, as they swim about, "foreign bodies are carefully avoided, and the whole movement has a deceptive likeness to the voluntary changes of place which are observed in microscopic animals." after swarming about in this way in the moisture on the surface of a leaf or stem (which, film though it may be, is an ocean to such a fish) for half an hour, more or less, the movement of the zoospore becomes slower, and is limited to a slow turning upon its axis, without change of place. it then becomes quite quiet, the cilia disappear, it assumes a spherical form, and surrounds itself with a distinct, though delicate, membranous coat. a protuberance then grows out from one side of the sphere, and rapidly increasing in length, assumes the character of a hypha. the latter penetrates into the substance of the potato plant, either by entering a stomate, or by boring through the wall of an epidermic cell, and ramifies, as a mycelium, in the substance of the plant, destroying the tissues with which it comes in contact. as these processes of multiplication take place very rapidly, millions of spores are soon set free from a single infested plant; and, from their minuteness, they are readily transported by the gentlest breeze. since, again, the zoospores set free from each spore, in virtue of their powers of locomotion, swiftly disperse themselves over the surface, it is no wonder that the infection, once started, soon spreads from field to field, and extends its ravages over a whole country. however, it does not enter into my present plan to treat of the potato disease, instructively as its history bears upon that of other epidemics; and i have selected the case of the _peroganspora_ simply because it affords an example of an organism, which, in one stage of its existence, is truly a "monad," indistinguishable by any important character from our _heteromita_, and extraordinarily like it in some respects. and yet this "monad" can be traced, step by step, through the series of metamorphoses which i have described, until it assumes the features of an organism, which is as much a plant as is an oak or an elm. moreover, it would be possible to pursue the analogy farther. under certain circumstances, a process of conjugation takes place in the _peronospora_. two separate portions of its protoplasm become fused together, surround themselves with a thick coat and give rise to a sort of vegetable egg called an _oospore_. after a period of rest, the contents of the oospore break up into a number of zoospores like those already described, each of which, after a period of activity, germinates in the ordinary way. this process obviously corresponds with the conjugation and subsequent setting free of germs in the _heteromita_. but it may be said that the _peronospora_ is, after all, a questionable sort of plant; that it seems to be wanting in the manufacturing power, selected as the main distinctive character of vegetable life; or, at any rate, that there is no proof that it does not get its protein matter ready made from the potato plant. let us, therefore, take a case which is not open to these objections. there are some small plants known to botanists as members of the genus _colcochaete_, which, without being truly parasitic, grow upon certain water-weeds, as lichens grow upon trees. the little plant has the form of an elegant green star, the branching arms of which are divided into cells. its greenness is due to its chlorophyll, and it undoubtedly has the manufacturing power in full degree, decomposing carbonic acid and setting oxygen free, under the influence of sunlight. but the protoplasmic contents of some of the cells of which the plant is made up occasionally divide, by a method similar to that which effects the division of the contents of the _peronospora_ spore; and the severed portions are then set free as active monad-like zoospores. each is oval and is provided at one extremity with two long active cilia. propelled by these, it swims about for a longer or shorter time, but at length comes to a state of rest and gradually grows into a _coleochaete_. moreover, as in the _peronospora_, conjugation may take place and result in an oospore; the contents of which divide and are set free as monadiform germs. if the whole history of the zoospores of _peronospora_ and of _coleochaete_ were unknown, they would undoubtedly be classed among "monads" with the same right as _heteromita_; why then may not _heteromita_ be a plant, even though the cycle of forms through which it passes shows no terms quite so complex as those which occur in _peronospora_ and _coleochaete_? and, in fact, there are some green organisms, in every respect characteristically plants, such as _chlamydomonas_, and the common _volvox_, or so-called "globe animalcule," which run through a cycle of forms of just the same simple character as those of _heteromita_. the name of _chlamydomonas_ is applied to certain microscopic green bodies, each of which consists of a protoplasmic central substance invested by a structureless sac. the latter contains cellulose, as in ordinary plants; and the chlorophyll which gives the green colour enables the _chlamydomonas_ to decompose carbonic acid and fix carbon as they do. two long cilia protrude through the cell-wall, and effect the rapid locomotion of this "monad," which, in all respects except its mobility, is characteristically a plant. under ordinary circumstances, the _chlamydomonas_ multiplies by simple fission, each splitting into two or into four parts, which separate and become independent organisms. sometimes, however, the _chlamydomonas_ divides into eight parts, each of which is provided with four instead of two cilia. these "zoospores" conjugate in pairs, and give rise to quiescent bodies, which multiply by division, find eventually pass into the active state. thus, so far as outward form and the general character of the cycle of modifications, through which the organism passes in the course of its life, are concerned, the resemblance between _chlamydomonas_ and _heteromita_ is of the closest description. and on the face of the matter there is no ground for refusing to admit that _heteromita_ may be related to _chlamydomonas_, as the colourless fungus is to the green alga. _volvox_ may be compared to a hollow sphere, the wall of which is made up of coherent chlamydomonads; and which progresses with a rotating motion effected by the paddling of the multitudinous pairs of cilia which project from its surface. each _volvox_-monad, moreover, possesses a red pigment spot, like the simplest form of eye known among animals. the methods of fissive multiplication and of conjugation observed in the monads of this locomotive globe are essentially similar to those observed in _chlamydomonas_; and, though a hard battle has been fought over it, _volvox_ is now finally surrendered to the botanists. thus there is really no reason why _heteromita_ may not be a plant; and this conclusion would be very satisfactory, if it were not equally easy to show that there is really no reason why it should not be an animal. for there are numerous organisms presenting the closest resemblance to _heteromita_, and, like it, grouped under the general name of "monads," which, nevertheless, can be observed to take in solid nutriment, and which, therefore, have a virtual, if not an actual, mouth and digestive cavity, and thus come under cuvier's definition of an animal. numerous forms of such animals have been described by ehrenberg, dujardin, h. james clark, and other writers on the _infusoria_. indeed, in another infusion of hay in which my _heteromita lens_ occurred, there were innumerable such infusorial animalcules belonging to the well-known species _colpoda cucullus_.[ ] [footnote : excellently described by stein, almost all of whose statements i have verified.] full-sized specimens of this animalcule attain a length of between / or / of an inch, so that it may have ten times the length and a thousand times the mass of a _heteromita_. in shape, it is not altogether unlike _heteromita_. the small end, however, is not produced into one long cilium, but the general surface of the body is covered with small actively vibrating ciliary organs, which are only longest at the small end. at the point which answers to that from which the two cilia arise in _heteromita_, there is a conical depression, the mouth; and, in young specimens, a tapering filament, which reminds one of the posterior cilium of _heteromita_, projects from this region. the body consists of a soft granular protoplasmic substance, the middle of which is occupied by a large oval mass called the "nucleus"; while, at its hinder end, is a "contractile vacuole," conspicuous by its regular rhythmic appearances and disappearances. obviously, although the _colpoda_ is not a monad, it differs from one only in subordinate details. moreover, under certain conditions, it becomes quiescent, incloses itself in a delicate case or _cyst_, and then divides into two, four, or more portions, which are eventually set free and swim about as active _colpodoe_. but this creature is an unmistakable animal, and full-sized _colpodoe_ may be fed as easily as one feeds chickens. it is only needful to diffuse very finely ground carmine through the water in which they live, and, in a very short time, the bodies of the _colpodoe_ are stuffed with the deeply-coloured granules of the pigment. and if this were not sufficient evidence of the animality of _colpoda_, there comes the fact that it is even more similar to another well-known animalcule, _paramoecium_, than it is to a monad. but _paramoecium_ is so huge a creature compared with those hitherto discussed--it reaches / of an inch or more in length--that there is no difficulty in making out its organisation in detail; and in proving that it is not only an animal, but that it is an animal which possesses a somewhat complicated organisation. for example, the surface layer of its body is different in structure from the deeper parts. there are two contractile vacuoles, from each of which radiates a system of vessel-like canals; and not only is there a conical depression continuous with a tube, which serve as mouth and gullet, but the food ingested takes a definite course, and refuse is rejected from a definite region. nothing is easier than to feed these animals, and to watch the particles of indigo or carmine accumulate at the lower end of the gullet. from this they gradually project, surrounded by a ball of water, which at length passes with a jerk, oddly simulating a gulp, into the pulpy central substance of the body, there to circulate up one side and down the other, until its contents are digested and assimilated. nevertheless, this complex animal multiplies by division, as the monad does, and, like the monad, undergoes conjugation. it stands in the same relation to _heteromita_ on the animal side, as _coleochaete_ does on the plant side. start from either, and such an insensible series of gradations leads to the monad that it is impossible to say at any stage of the progress where the line between the animal and the plant must be drawn. there is reason to think that certain organisms which pass through a monad stage of existence, such as the _myxomycetes_, are, at one time of their lives, dependent upon external sources for their protein matter, or are animals; and, at another period, manufacture it, or are plants. and seeing that the whole progress of modern investigation is in favour of the doctrine of continuity, it is a fair and probable speculation--though only a speculation--that, as there are some plants which can manufacture protein out of such apparently intractable mineral matters as carbonic acid, water, nitrate of ammonia, metallic and earthy salts; while others need to be supplied with their carbon and nitrogen in the somewhat less raw form of tartrate of ammonia and allied compounds; so there may be yet others, as is possibly the case with the true parasitic plants, which can only manage to put together materials still better prepared--still more nearly approximated to protein--until we arrive at such organisms as the _psorospermioe_ and the _panhistophyton_, which are as much animal as vegetable in structure, but are animal in their dependence on other organisms for their food. the singular circumstance observed by meyer, that the _torula_ of yeast, though an indubitable plant, still flourishes most vigorously when supplied with the complex nitrogenous substance, pepsin; the probability that the _peronospora_ is nourished directly by the protoplasm of the potato-plant; and the wonderful facts which have recently been brought to light respecting insectivorous plants, all favour this view; and tend to the conclusion that the difference between animal and plant is one of degree rather than of kind, and that the problem whether, in a given case, an organism is an animal or a plant, may be essentially insoluble. vii a lobster; or, the study of zoology [ ] natural history is the name familiarly applied to the study of the properties of such natural bodies as minerals, plants, and animals; the sciences which embody the knowledge man has acquired upon these subjects are commonly termed natural sciences, in contradistinction to other so- called "physical" sciences; and those who devote themselves especially to the pursuit of such sciences have been and are commonly termed "naturalists." linnaeus was a naturalist in this wide sense, and his "systema naturae" was a work upon natural history, in the broadest acceptation of the term; in it, that great methodising spirit embodied all that was known in his time of the distinctive characters of minerals, animals, and plants. but the enormous stimulus which linnaeus gave to the investigation of nature soon rendered it impossible that any one man should write another "systema naturae," and extremely difficult for any one to become even a naturalist such as linnaeus was. great as have been the advances made by all the three branches of science, of old included under the title of natural history, there can be no doubt that zoology and botany have grown in an enormously greater ratio than mineralogy; and hence, as i suppose, the name of "natural history" has gradually become more and more definitely attached to these prominent divisions of the subject, and by "naturalist" people have meant more and more distinctly to imply a student of the structure and function of living beings. however this may be, it is certain that the advance of knowledge has gradually widened the distance between mineralogy and its old associates, while it has drawn zoology and botany closer together; so that of late years it has been found convenient (and indeed necessary) to associate the sciences which deal with vitality and all its phenomena under the common head of "biology"; and the biologists have come to repudiate any blood-relationship with their foster-brothers, the mineralogists. certain broad laws have a general application throughout both the animal and the vegetable worlds, but the ground common to these kingdoms of nature is not of very wide extent, and the multiplicity of details is so great, that the student of living beings finds himself obliged to devote his attention exclusively either to the one or the other. if he elects to study plants, under any aspect, we know at once what to call him. he is a botanist, and his science is botany. but if the investigation of animal life be his choice, the name generally applied to him will vary according to the kind of animals he studies, or the particular phenomena of animal life to which he confines his attention. if the study of man is his object, he is called an anatomist, or a physiologist, or an ethnologist; but if he dissects animals, or examines into the mode in which their functions are performed, he is a comparative anatomist or comparative physiologist. if he turns his attention to fossil animals, he is a palaeontologist. if his mind is more particularly directed to the specific description, discrimination, classification, and distribution of animals, he is termed a zoologist. for the purpose of the present discourse, however, i shall recognise none of these titles save the last, which i shall employ as the equivalent of botanist, and i shall use the term zoology is denoting the whole doctrine of animal life, in contradistinction to botany, which signifies the whole doctrine of vegetable life. employed in this sense, zoology, like botany, is divisible into three great but subordinate sciences, morphology, physiology, and distribution, each of which may, to a very great extent, be studied independently of the other. zoological morphology is the doctrine of animal form or structure. anatomy is one of its branches; development is another; while classification is the expression of the relations which different animals bear to one another, in respect of their anatomy and their development. zoological distribution is the study of animals in relation to the terrestrial conditions which obtain now, or have obtained at any previous epoch of the earth's history. zoological physiology, lastly, is the doctrine of the functions or actions of animals. it regards animal bodies as machines impelled by certain forces, and performing an amount of work which can be expressed in terms of the ordinary forces of nature. the final object of physiology is to deduce the facts of morphology, on the one hand, and those of distribution on the other, from the laws of the molecular forces of matter. such is the scope of zoology. but if i were to content myself with the enunciation of these dry definitions, i should ill exemplify that method of teaching this branch of physical science, which it is my chief business to-night to recommend. let us turn away then from abstract definitions. let us take some concrete living thing, some animal, the commoner the better, and let us see how the application of common sense and common logic to the obvious facts it presents, inevitably leads us into all these branches of zoological science. i have before me a lobster. when i examine it, what appears to be the most striking character it presents? why, i observe that this part which we call the tail of the lobster, is made up of six distinct hard rings and a seventh terminal piece. if i separate one of the middle rings, say the third, i find it carries upon its under surface a pair of limbs or appendages, each of which consists of a stalk and two terminal pieces. so that i can represent a transverse section of the ring and its appendages upon the diagram board in this way. if i now take the fourth ring, i find it has the same structure, and so have the fifth and the second; so that, in each of these divisions of the tail, i find parts which correspond with one another, a ring and two appendages; and in each appendage a stalk and two end pieces. these corresponding parts are called, in the technical language of anatomy, "homologous parts." the ring of the third division is the "homologue" of the ring of the fifth, the appendage of the former is the homologue of the appendage of the latter. and, as each division exhibits corresponding parts in corresponding places, we say that all the divisions are constructed upon the same plan. but now let us consider the sixth division. it is similar to, and yet different from, the others. the ring is essentially the same as in the other divisions; but the appendages look at first as if they were very different; and yet when we regard them closely, what do we find? a stalk and two terminal divisions, exactly as in the others, but the stalk is very short and very thick, the terminal divisions are very broad and flat, and one of them is divided into two pieces. i may say, therefore, that the sixth segment is like the others in plan, but that it is modified in its details. the first segment is like the others, so far as its ring is concerned, and though its appendages differ from any of those yet examined in the simplicity of their structure, parts corresponding with the stem and one of the divisions of the appendages of the other segments can be readily discerned in them. thus it appears that the lobster's tail is composed of a series of segments which are fundamentally similar, though each presents peculiar modifications of the plan common to all. but when i turn to the forepart of the body i see, at first, nothing but a great shield-like shell, called technically the "carapace," ending in front in a sharp spine, on either side of which are the curious compound eyes, set upon the ends of stout movable stalks. behind these, on the under side of the body, are two pairs of long feelers, or antennae, followed by six pairs of jaws folded against one another over the mouth, and five pairs of legs, the foremost of these being the great pinchers, or claws, of the lobster. it looks, at first, a little hopeless to attempt to find in this complex mass a series of rings, each with its pair of appendages, such as i have shown you in the abdomen, and yet it is not difficult to demonstrate their existence. strip off the legs, and you will find that each pair is attached to a very definite segment of the under wall of the body; but these segments, instead of being the lower parts of free rings, as in the tail, are such parts of rings which are all solidly united and bound together; and the like is true of the jaws, the feelers, and the eye- stalks, every pair of which is borne upon its own special segment. thus the conclusion is gradually forced upon us, that the body of the lobster is composed of as many rings as there are pairs of appendages, namely, twenty in all, but that the six hindmost rings remain free and movable, while the fourteen front rings become firmly soldered together, their backs forming one continuous shield--the carapace. unity of plan, diversity in execution, is the lesson taught by the study of the rings of the body, and the same instruction is given still more emphatically by the appendages. if i examine the outermost jaw i find it consists of three distinct portions, an inner, a middle, and an outer, mounted upon a common stem; and if i compare this jaw with the legs behind it, or the jaws in front of it, i find it quite easy to see, that, in the legs, it is the part of the appendage which corresponds with the inner division, which becomes modified into what we know familiarly as the "leg," while the middle division disappears, and the outer division is hidden under the carapace. nor is it more difficult to discern that, in the appendages of the tail, the middle division appears again and the outer vanishes; while, on the other hand, in the foremost jaw, the so- called mandible, the inner division only is left; and, in the same way, the parts of the feelers and of the eye-stalks can be identified with those of the legs and jaws. but whither does all this tend? to the very remarkable conclusion that a unity of plan, of the same kind as that discoverable in the tail or abdomen of the lobster, pervades the whole organisation of its skeleton, so that i can return to the diagram representing any one of the rings of the tail, which i drew upon the board, and by adding a third division to each appendage, i can use it as a sort of scheme or plan of any ring of the body. i can give names to all the parts of that figure, and then if i take any segment of the body of the lobster, i can point out to you exactly, what modification the general plan has undergone in that particular segment; what part has remained movable, and what has become fixed to another; what has been excessively developed and metamorphosed and what has been suppressed. but i imagine i hear the question, how is all this to be tested? no doubt it is a pretty and ingenious way of looking at the structure of any animal; but is it anything more? does nature acknowledge, in any deeper way, this unity of plan we seem to trace? the objection suggested by these questions is a very valid and important one, and morphology was in an unsound state so long as it rested upon the mere perception of the analogies which obtain between fully formed parts. the unchecked ingenuity of speculative anatomists proved itself fully competent to spin any number of contradictory hypotheses out of the same facts, and endless morphological dreams threatened to supplant scientific theory. happily, however, there is a criterion of morphological truth, and a sure test of all homologies. our lobster has not always been what we see it; it was once an egg, a semifluid mass of yolk, not so big as a pin's head, contained in a transparent membrane, and exhibiting not the least trace of any one of those organs, the multiplicity and complexity of which, in the adult, are so surprising. after a time, a delicate patch of cellular membrane appeared upon one face of this yolk, and that patch was the foundation of the whole creature, the clay out of which it would be moulded. gradually investing the yolk, it became subdivided by transverse constrictions into segments, the forerunners of the rings of the body. upon the ventral surface of each of the rings thus sketched out, a pair of bud-like prominences made their appearance--the rudiments of the appendages of the ring. at first, all the appendages were alike, but, as they grew, most of them became distinguished into a stem and two terminal divisions, to which, in the middle part of the body, was added a third outer division; and it was only at a later period, that by the modification, or absorption, of certain of these primitive constituents, the limbs acquired their perfect form. thus the study of development proves that the doctrine of unity of plan is not merely a fancy, that it is not merely one way of looking at the matter, but that it is the expression of deep-seated natural facts. the legs and jaws of the lobster may not merely be regarded as modifications of a common type,--in fact and in nature they are so,--the leg and the jaw of the young animal being, at first, indistinguishable. these are wonderful truths, the more so because the zoologist finds them to be of universal application. the investigation of a polype, of a snail, of a fish, of a horse, or of a man, would have led us, though by a less easy path, perhaps, to exactly the same point. unity of plan everywhere lies hidden under the mask of diversity of structure--the complex is everywhere evolved out of the simple. every animal has at first the form of an egg, and every animal and every organic part, in reaching its adult state, passes through conditions common to other animals and other adult parts; and this leads me to another point. i have hitherto spoken as if the lobster were alone in the world, but, as i need hardly remind you, there are myriads of other animal organisms. of these, some, such as men, horses, birds, fishes, snails, slugs, oysters, corals, and sponges, are not in the least like the lobster. but other animals, though they may differ a good deal from the lobster, are yet either very like it, or are like something that is like it. the cray fish, the rock lobster, and the prawn, and the shrimp, for example, however different, are yet so like lobsters, that a child would group them as of the lobster kind, in contradistinction to snails and slugs; and these last again would form a kind by themselves, in contradistinction to cows, horses, and sheep, the cattle kind. but this spontaneous grouping into "kinds" is the first essay of the human mind at classification, or the calling by a common name of those things that are alike, and the arranging them in such a manner as best to suggest the sum of their likenesses and unlikenesses to other things. those kinds which include no other subdivisions than the sexes, or various breeds, are called, in technical language, species. the english lobster is a species, our cray fish is another, our prawn is another. in other countries, however, there are lobsters, cray fish, and prawns, very like ours, and yet presenting sufficient differences to deserve distinction. naturalists, therefore, express this resemblance and this diversity by grouping them as distinct species of the same "genus." but the lobster and the cray fish, though belonging to distinct genera, have many features in common, and hence are grouped together in an assemblage which is called a family. more distant resemblances connect the lobster with the prawn and the crab, which are expressed by putting all these into the same order. again, more remote, but still very definite, resemblances unite the lobster with the woodlouse, the king crab, the water flea, and the barnacle, and separate them from all other animals; whence they collectively constitute the larger group, or class, _crustacea_. but the _crustacea_ exhibit many peculiar features in common with insects, spiders, and centipedes, so that these are grouped into the still larger assemblage or "province" _articulata_; and, finally, the relations which these have to worms and other lower animals, are expressed by combining the whole vast aggregate into the sub-kingdom of _annulosa_. if i had worked my way from a sponge instead of a lobster, i should have found it associated, by like ties, with a great number of other animals into the sub-kingdom _protozoa_; if i had selected a fresh-water polype or a coral, the members of what naturalists term the sub-kingdom _coelenterata_, would have grouped themselves around my type; had a snail been chosen, the inhabitants of all univalve and bivalve, land and water, shells, the lamp shells, the squids, and the sea-mat would have gradually linked themselves on to it as members of the same sub-kingdom of _mollusca_; and finally, starting from man, i should have been compelled to admit first, the ape, the rat, the horse, the dog, into the same class; and then the bird, the crocodile, the turtle, the frog, and the fish, into the same sub-kingdom of _vertebrata_. and if i had followed out all these various lines of classification fully, i should discover in the end that there was no animal, either recent or fossil, which did not at once fall into one or other of these sub-kingdoms. in other words, every animal is organised upon one or other of the five, or more, plans, the existence of which renders our classification possible. and so definitely and precisely marked is the structure of each animal, that, in the present state of our knowledge, there is not the least evidence to prove that a form, in the slightest degree transitional between any of the two groups _vertebrata, annulosa, mollusca_, and _coelenterata_, either exists, or has existed, during that period of the earth's history which is recorded by the geologist.[ ] nevertheless, you must not for a moment suppose, because no such transitional forms are known, that the members of the sub-kingdoms are disconnected from, or independent of, one another. on the contrary, in their earliest condition they are all similar, and the primordial germs of a man, a dog, a bird, a fish, a beetle, a snail, and a polype are, in no essential structural respects, distinguishable. [footnote : the different grouping necessitated by later knowledge does not affect the principle of the argument.-- .] in this broad sense, it may with truth be said, that all living animals, and all those dead faunae which geology reveals, are bound together by an all-pervading unity of organisation, of the same character, though not equal in degree, to that which enables us to discern one and the same plan amidst the twenty different segments of a lobster's body. truly it has been said, that to a clear eye the smallest fact is a window through which the infinite may be seen. turning from these purely morphological considerations, let us now examine into the manner in which the attentive study of the lobster impels us into other lines of research. lobsters are found in all the european seas; but on the opposite shores of the atlantic and in the seas of the southern hemisphere they do not exist. they are, however, represented in these regions by very closely allied, but distinct forms--the _homarus americanus_ and the _homarus capensis:_ so that we may say that the european has one species of _homuarus_; the american, another; the african, another; and thus the remarkable facts of geographical distribution begin to dawn upon us. again, if we examine the contents of the earth's crust, we shall find in the latter of those deposits, which have served as the great burying grounds of past ages, numberless lobster-like animals, but none so similar to our living lobster as to make zoologists sure that they belonged even to the same genus. if we go still further back in time, we discover, in the oldest rocks of all, the remains of animals, constructed on the same general plan as the lobster, and belonging to the same great group of _crustacea_; but for the most part totally different from the lobster, and indeed from any other living form of crustacean; and thus we gain a notion of that successive change of the animal population of the globe, in past ages, which is the most striking fact revealed by geology. consider, now, where our inquiries have led us. we studied our type morphologically, when we determined its anatomy and its development, and when comparing it, in these respects, with other animals, we made out its place in a system of classification. if we were to examine every animal in a similar manner, we should establish a complete body of zoological morphology. again, we investigated the distribution of our type in space and in time, and, if the like had been done with every animal, the sciences of geographical and geological distribution would have attained their limit. but you will observe one remarkable circumstance, that, up to this point, the question of the life of these organisms has not come under consideration. morphology and distribution might be studied almost as well, if animals and plants were a peculiar kind of crystals, and possessed none of those functions which distinguish living beings so remarkably. but the facts of morphology and distribution have to be accounted for, and the science, the aim of which it is to account for them, is physiology. let us return to our lobster once more. if we watched the creature in its native element, we should see it climbing actively the submerged rocks, among which it delights to live, by means of its strong legs; or swimming by powerful strokes of its great tail, the appendages of the sixth joint of which are spread out into a broad fan-like propeller: seize it, and it will show you that its great claws are no mean weapons of offence; suspend a piece of carrion among its haunts, and it will greedily devour it, tearing and crushing the flesh by means of its multitudinous jaws. suppose that we had known nothing of the lobster but as an inert mass, an organic crystal, if i may use the phrase, and that we could suddenly see it exerting all these powers, what wonderful new ideas and new questions would arise in our minds! the great new question would be, "how does all this take place?" the chief new idea would be, the idea of adaptation to purpose,--the notion, that the constituents of animal bodies are not mere unconnected parts, but organs working together to an end. let us consider the tail of the lobster again from this point of view. morphology has taught us that it is a series of segments composed of homologous parts, which undergo various modifications--beneath and through which a common plan of formation is discernible. but if i look at the same part physiologically, i see that it is a most beautifully constructed organ of locomotion, by means of which the animal can swiftly propel itself either backwards or forwards. but how is this remarkable propulsive machine made to perform its functions? if i were suddenly to kill one of these animals and to take out all the soft parts, i should find the shell to be perfectly inert, to have no more power of moving itself than is possessed by the machinery of a mill when disconnected from its steam-engine or water-wheel. but if i were to open it, and take out the viscera only, leaving the white flesh, i should perceive that the lobster could bend and extend its tail as well as before. if i were to cut off the tail, i should cease to find any spontaneous motion in it; but on pinching any portion of the flesh, i should observe that it underwent a very curious change--each fibre becoming shorter and thicker. by this act of contraction, as it is termed, the parts to which the ends of the fibre are attached are, of course, approximated; and according to the relations of their points of attachment to the centres of motions of the different rings, the bending or the extension of the tail results. close observation of the newly- opened lobster would soon show that all its movements are due to the same cause--the shortening and thickening of these fleshy fibres, which are technically called muscles. here, then, is a capital fact. the movements of the lobster are due to muscular contractility. but why does a muscle contract at one time and not at another? why does one whole group of muscles contract when the lobster wishes to extend his tail, and another group when he desires to bend it? what is it originates, directs, and controls the motive power? experiment, the great instrument for the ascertainment of truth in physical science, answers this question for us. in the head of the lobster there lies a small mass of that peculiar tissue which is known as nervous substance. cords of similar matter connect his brain of the lobster, directly or indirectly, with the muscles. now, if these communicating cords are cut, the brain remaining entire, the power of exerting what we call voluntary motion in the parts below the section is destroyed; and, on the other hand, if, the cords remaining entire, the brain mass be destroyed, the same voluntary mobility is equally lost. whence the inevitable conclusion is, that the power of originating these motions resides in the brain and is propagated along the nervous cords. in the higher animals the phenomena which attend this transmission have been investigated, and the exertion of the peculiar energy which resides in the nerves has been found to be accompanied by a disturbance of the electrical state of their molecules. if we could exactly estimate the signification of this disturbance; if we could obtain the value of a given exertion of nerve force by determining the quantity of electricity, or of heat, of which it is the equivalent; if we could ascertain upon what arrangement, or other condition of the molecules of matter, the manifestation of the nervous and muscular energies depends (and doubtless science will some day or other ascertain these points), physiologists would have attained their ultimate goal in this direction; they would have determined the relation of the motive force of animals to the other forms of force found in nature; and if the same process had been successfully performed for all the operations which are carried on in, and by, the animal frame, physiology would be perfect, and the facts of morphology and distribution would be deducible from the laws which physiologists had established, combined with those determining the condition of the surrounding universe. there is not a fragment of the organism of this humble animal whose study would not lead us into regions of thought as large as those which i have briefly opened up to you; but what i have been saying, i trust, has not only enabled you to form a conception of the scope and purport of zoology, but has given you an imperfect example of the manner in which, in my opinion, that science, or indeed any physical science, may be best taught. the great matter is, to make teaching real and practical, by fixing the attention of the student on particular facts; but at the same time it should be rendered broad and comprehensive, by constant reference to the generalisations of which all particular facts are illustrations. the lobster has served as a type of the whole animal kingdom, and its anatomy and physiology have illustrated for us some of the greatest truths of biology. the student who has once seen for himself the facts which i have described, has had their relations explained to him, and has clearly comprehended them, has, so far, a knowledge of zoology, which is real and genuine, however limited it may be, and which is worth more than all the mere reading knowledge of the science he could ever acquire. his zoological information is, so far, knowledge and not mere hearsay. and if it were nay business to fit you for the certificate in zoological science granted by this department, i should pursue a course precisely similar in principle to that which i have taken to-night. i should select a fresh-water sponge, a fresh-water polype or a _cyanoea_, a fresh-water mussel, a lobster, a fowl, as types of the five primary divisions of the animal kingdom. i should explain their structure very fully, and show how each illustrated the great principles of zoology. having gone very carefully and fully over this ground, i should feel that you had a safe foundation, and i should then take you in the same way, but less minutely, over similarly selected illustrative types of the classes; and then i should direct your attention to the special forms enumerated under the head of types, in this syllabus, and to the other facts there mentioned. that would, speaking generally, be my plan. but i have undertaken to explain to you the best mode of acquiring and communicating a knowledge of zoology, and you may therefore fairly ask me for a more detailed and precise account of the manner in which i should propose to furnish you with the information i refer to. my own impression is, that the best model for all kinds of training in physical science is that afforded by the method of teaching anatomy, in use in the medical schools. this method consists of three elements-- lectures, demonstrations, and examinations. the object of lectures is, in the first place, to awaken the attention and excite the enthusiasm of the student; and this, i am sure, may be effected to a far greater extent by the oral discourse and by the personal influence of a respected teacher than in any other way. secondly, lectures have the double use of guiding the student to the salient points of a subject, and at the same time forcing him to attend to the whole of it, and not merely to that part which takes his fancy. and lastly, lectures afford the student the opportunity of seeking explanations of those difficulties which will, and indeed ought to, arise in the course of his studies. what books shall i read? is a question constantly put by the student to the teacher. my reply usually is, "none: write your notes out carefully and fully; strive to understand them thoroughly; come to me for the explanation of anything you cannot understand; and i would rather you did not distract your mind by reading." a properly composed course of lectures ought to contain fully as much matter as a student can assimilate in the time occupied by its delivery; and the teacher should always recollect that his business is to feed, and not to cram the intellect. indeed, i believe that a student who gains from a course of lectures the simple habit of concentrating his attention upon a definitely limited series of facts, until they are thoroughly mastered, has made a step of immeasurable importance. but, however good lectures may be, and however extensive the course of reading by which they are followed up, they are but accessories to the great instrument of scientific teaching--demonstration. if i insist unweariedly, nay fanatically, upon the importance of physical science as an educational agent, it is because the study of any branch of science, if properly conducted, appears to me to fill up a void left by all other means of education. i have the greatest respect and love for literature; nothing would grieve me more than to see literary training other than a very prominent branch of education: indeed, i wish that real literary discipline were far more attended to than it is; but i cannot shut my eyes to the fact, that there is a vast difference between men who have had a purely literary, and those who have had a sound scientific, training. seeking for the cause of this difference, i imagine i can find it in the fact that, in the world of letters, learning and knowledge are one, and books are the source of both; whereas in science, as in life, learning and knowledge are distinct, and the study of things, and not of books, is the source of the latter. all that literature has to bestow may be obtained by reading and by practical exercise in writing and in speaking; but i do not exaggerate when i say, that none of the best gifts of science are to be won by these means. on the contrary, the great benefit which a scientific education bestows, whether is training or as knowledge, is dependent upon the extent to which the mind of the student is brought into immediate contact with facts--upon the degree to which he learns the habit of appealing directly to nature, and of acquiring through his senses concrete images of those properties of things, which are, and always will be, but approximatively expressed in human language. our way of looking at nature, and of speaking about her, varies from year to year; but a fact once seen, a relation of cause and effect, once demonstratively apprehended, are possessions which neither change nor pass away, but, on the contrary, form fixed centres, about which other truths aggregate by natural affinity. therefore, the great business of the scientific teacher is, to imprint the fundamental, irrefragable facts of his science, not only by words upon the mind, but by sensible impressions upon the eye, and ear, and touch of the student, in so complete a manner, that every term used, or law enunciated, should afterwards call up vivid images of the particular structural, or other, facts which furnished the demonstration of the law, or the illustration of the term. now this important operation can only be achieved by constant demonstration, which may take place to a certain imperfect extent during a lecture, but which ought also to be carried on independently, and which should be addressed to each individual student, the teacher endeavouring, not so much to show a thing to the learner, as to make him see it for himself. i am well aware that there are great practical difficulties in the way of effectual zoological demonstrations. the dissection of animals is not altogether pleasant, and requires much time; nor is it easy to secure an adequate supply of the needful specimens. the botanist has here a great advantage; his specimens are easily obtained, are clean and wholesome, and can be dissected in a private house as well as anywhere else; and hence, i believe, the fact, that botany is so much more readily and better taught than its sister science. but, be it difficult or be it easy, if zoological science is to be properly studied, demonstration, and, consequently, dissection, must be had. without it, no man can have a really sound knowledge of animal organisation. a good deal may be done, however, without actual dissection on the student's part, by demonstration upon specimens and preparations; and in all probability it would not be very difficult, were the demand sufficient, to organise collections of such objects, sufficient for all the purposes of elementary teaching, at a comparatively cheap rate. even without these, much might be effected, if the zoological collections, which are open to the public, were arranged according to what has been termed the "typical principle"; that is to say, if the specimens exposed to public view were so selected that the public could learn something from them, instead of being, as at present, merely confused by their multiplicity. for example, the grand ornithological gallery at the british museum contains between two and three thousand species of birds, and sometimes five or six specimens of a species. they are very pretty to look at, and some of the cases are, indeed, splendid; but i will undertake to say, that no man but a professed ornithologist has ever gathered much information from the collection. certainly, no one of the tens of thousands of the general public who have walked through that gallery ever knew more about the essential peculiarities of birds when he left the gallery than when he entered it. but if, somewhere in that vast hall, there were a few preparations, exemplifying the leading structural peculiarities and the mode of development of a common fowl; if the types of the genera, the leading modifications in the skeleton, in the plumage at various ages, in the mode of nidification, and the like, among birds, were displayed; and if the other specimens were put away in a place where the men of science, to whom they are alone useful, could have free access to them, i can conceive that this collection might become a great instrument of scientific education. the last implement of the teacher to which i have adverted is examination--a means of education now so thoroughly understood that i need hardly enlarge upon it. i hold that both written and oral examinations are indispensable, and, by requiring the description of specimens, they may be made to supplement demonstration. such is the fullest reply the time at my disposal will allow me to give to the question--how may a knowledge of zoology be best acquired and communicated? but there is a previous question which may be moved, and which, in fact, i know many are inclined to move. it is the question, why should teachers be encouraged to acquire a knowledge of this, or any other branch of physical science? what is the use, it is said, of attempting to make physical science a branch of primary education? is it not probable that teachers, in pursuing such studies, will be led astray from the acquirement of more important but less attractive knowledge? and, even if they can learn something of science without prejudice to their usefulness, what is the good of their attempting to instil that knowledge into boys whose real business is the acquisition of reading, writing, and arithmetic? these questions are, and will be, very commonly asked, for they arise from that profound ignorance of the value and true position of physical science, which infests the minds of the most highly educated and intelligent classes of the community. but if i did not feel well assured that they are capable of being easily and satisfactorily answered; that they have been answered over and over again; and that the time will come when men of liberal education will blush to raise such questions--i should be ashamed of my position here to-night. without doubt, it is your great and very important function to carry out elementary education; without question, anything that should interfere with the faithful fulfilment of that duty on your part would be a great evil; and if i thought that your acquirement of the elements of physical science, and your communication of those elements to your pupils, involved any sort of interference with your proper duties, i should be the first person to protest against your being encouraged to do anything of the kind. but is it true that the acquisition of such a knowledge of science as is proposed, and the communication of that knowledge, are calculated to weaken your usefulness? or may i not rather ask, is it possible for you to discharge your functions properly without these aids? what is the purpose of primary intellectual education? i apprehend that its first object is to train the young in the use of those tools wherewith men extract knowledge from the ever-shifting succession of phenomena which pass before their eyes; and that its second object is to inform them of the fundamental laws which have been found by experience to govern the course of things, so that they may not be turned out into the world naked, defenceless, and a prey to the events they might control. a boy is taught to read his own and other languages, in order that he may have access to infinitely wider stores of knowledge than could ever be opened to him by oral intercourse with his fellow men; he learns to write, that his means of communication with the rest of mankind may be indefinitely enlarged, and that he may record and store up the knowledge he acquires. he is taught elementary mathematics, that he may understand all those relations of number and form, upon which the transactions of men, associated in complicated societies, are built, and that he may have some practice in deductive reasoning. all these operations of reading, writing, and ciphering, are intellectual tools, whose use should, before all things, be learned, and learned thoroughly; so that the youth may be enabled to make his life that which it ought to be, a continual progress in learning and in wisdom. but, in addition, primary education endeavours to fit a boy out with a certain equipment of positive knowledge. he is taught the great laws of morality; the religion of his sect; so much history and geography as will tell him where the great countries of the world are, what they are, and how they have become what they are. without doubt all these are most fitting and excellent things to teach a boy; i should be very sorry to omit any of them from any scheme of primary intellectual education. the system is excellent, so far as it goes. but if i regard it closely, a curious reflection arises. i suppose that, fifteen hundred years ago, the child of any well-to-do roman citizen was taught just these same things; reading and writing in his own, and, perhaps, the greek tongue; the elements of mathematics; and the religion, morality, history, and geography current in his time. furthermore, i do not think i err in affirming, that, if such a christian roman boy, who had finished his education, could be transplanted into one of our public schools, and pass through its course of instruction, he would not meet with a single unfamiliar line of thought; amidst all the new facts he would have to learn, not one would suggest a different mode of regarding the universe from that current in his own time. and yet surely there is some great difference between the civilisation of the fourth century and that of the nineteenth, and still more between the intellectual habits and tone of thought of that day and this? and what has made this difference? i answer fearlessly--the prodigious development of physical science within the last two centuries. modern civilisation rests upon physical science; take away her gifts to our own country, and our position among the leading nations of the world is gone to-morrow; for it is physical science only that makes intelligence and moral energy stronger than brute force. the whole of modern thought is steeped in science; it has made its way into the works of our best poets, and even the mere man of letters, who affects to ignore and despise science, is unconsciously impregnated with her spirit, and indebted for his best products to her methods. i believe that the greatest intellectual revolution mankind has yet seen is now slowly taking place by her agency. she is teaching the world that the ultimate court of appeal is observation and experiment, and not authority; she is teaching it to estimate the value of evidence; she is creating a firm and living faith in the existence of immutable moral and physical laws, perfect obedience to which is the highest possible aim of an intelligent being. but of all this your old stereotyped system of education takes no note. physical science, its methods, its problems, and its difficulties, will meet the poorest boy at every turn, and yet we educate him in such a manner that he shall enter the world as ignorant of the existence of the methods and facts of science as the day he was born. the modern world is full of artillery; and we turn out our children to do battle in it, equipped with the shield and sword of an ancient gladiator. posterity will cry shame on us if we do not remedy this deplorable state of things. nay, if we live twenty years longer, our own consciences will cry shame on us. it is my firm conviction that the only way to remedy it is to make the elements of physical science an integral part of primary education. i have endeavoured to show you how that may be done for that branch of science which it is my business to pursue; and i can but add, that i should look upon the day when every schoolmaster throughout this land was a centre of genuine, however rudimentary, scientific knowledge, as an epoch in the history of the country. but let me entreat you to remember my last words. addressing myself to you, as teachers, i would say, mere book learning in physical science is a sham and a delusion--what you teach, unless you wish to be impostors, that you must first know; and real knowledge in science means personal acquaintance with the facts, be they few or many.[ ] [footnote : it has been suggested to me that these words may be taken to imply a discouragement on my part of any sort of scientific instruction which does not give an acquaintance with the facts at first hand. but this is not my meaning. the ideal of scientific teaching is, no doubt, a system by which the scholar sees every fact for himself, and the teacher supplies only the explanations. circumstances, however, do not often allow of the attainment of that ideal, and we must put up with the next best system--one in which the scholar takes a good deal on trust from a teacher, who, knowing the facts by his own knowledge, can describe them with so much vividness as to enable his audience to form competent ideas concerning them. the system which i repudiate is that which allows teachers who have not come into direct contact with the leading facts of a science to pass their second-hand information on. the scientific virus, like vaccine lymph, if passed through too long a succession of organisms, will lose all its effect in protecting the young against the intellectual epidemics to which they are exposed. [the remarks on p. applied to the natural history collection of the british museum in . the visitor to the natural history museum in need go no further than the great hall to see the realisation of my hopes by the present director.]] viii biogenesis and abiogenesis (the presidential address to the british association for the advancement of science for ) it has long been the custom for the newly installed president of the british association for the advancement of science to take advantage of the elevation of the position in which the suffrages of his colleagues had, for the time, placed him, and, casting his eyes around the horizon of the scientific world, to report to them what could be seen from his watch-tower; in what directions the multitudinous divisions of the noble army of the improvers of natural knowledge were marching; what important strongholds of the great enemy of us all, ignorance, had been recently captured; and, also, with due impartiality, to mark where the advanced posts of science had been driven in, or a long-continued siege had made no progress. i propose to endeavour to follow this ancient precedent, in a manner suited to the limitations of my knowledge and of my capacity. i shall not presume to attempt a panoramic survey of the world of science, nor even to give a sketch of what is doing in the one great province of biology, with some portions of which my ordinary occupations render me familiar. but i shall endeavour to put before you the history of the rise and progress of a single biological doctrine; and i shall try to give some notion of the fruits, both intellectual and practical, which we owe, directly or indirectly, to the working out, by seven generations of patient and laborious investigators, of the thought which arose, more than two centuries ago, in the mind of a sagacious and observant italian naturalist. it is a matter of everyday experience that it is difficult to prevent many articles of food from becoming covered with mould; that fruit, sound enough to all appearance, often contains grubs at the core; that meat, left to itself in the air, is apt to putrefy and swarm with maggots. even ordinary water, if allowed to stand in an open vessel, sooner or later becomes turbid and full of living matter. the philosophers of antiquity, interrogated as to the cause of these phenomena, were provided with a ready and a plausible answer. it did not enter their minds even to doubt that these low forms of life were generated in the matters in which they made their appearance. lucretius, who had drunk deeper of the scientific spirit than any poet of ancient or modern times except goethe, intends to speak as a philosopher, rather than as a poet, when he writes that "with good reason the earth has gotten the name of mother, since all things are produced out of the earth. and many living creatures, even now, spring out of the earth, taking form by the rains and the heat of the sun."[ ] the axiom of ancient science, "that the corruption of one thing is the birth of another," had its popular embodiment in the notion that a seed dies before the young plant springs from it; a belief so widespread and so fixed, that saint paul appeals to it in one of the most splendid outbursts of his fervid eloquence:-- "thou fool, that which thou sowest is not quickened, except it die."[ ] [footnote : it is thus that mr. munro renders "linquitur, ut merito maternum nomen adepta terra sit, e terra quoniam sunt cuncta creata. multaque nunc etiam exsistant animalia terris imbribus et calido solis concreta vapore." _de rerum natura_, lib. v. - . but would not the meaning of the last line be better rendered "developed in rain-water and in the warm vapours raised by the sun"?] [footnote : corinthians xv. .] the proposition that life may, and does, proceed from that which has no life, then, was held alike by the philosophers, the poets, and the people, of the most enlightened nations, eighteen hundred years ago; and it remained the accepted doctrine of learned and unlearned europe, through the middle ages, down even to the seventeenth century. it is commonly counted among the many merits of our great countryman, harvey, that he was the first to declare the opposition of fact to venerable authority in this, as in other matters; but i can discover no justification for this widespread notion. after careful search through the "exercitationes de generatione," the most that appears clear to me is, that harvey believed all animals and plants to spring from what he terms a "_primordium vegetale_," a phrase which may nowadays be rendered "a vegetative germ"; and this, he says, is _"oviforme_," or "egg-like"; not, he is careful to add, that it necessarily has the shape of an egg, but because it has the constitution and nature of one. that this "_primordium oviforme_" must needs, in all cases, proceed from a living parent is nowhere expressly maintained by harvey, though such an opinion may be thought to be implied in one or two passages; while, on the other hand, he does, more than once, use language which is consistent only with a full belief in spontaneous or equivocal generation.[ ] in fact, the main concern of harvey's wonderful little treatise is not with generation, in the physiological sense, at all, but with development; and his great object is the establishment of the doctrine of epigenesis. [footnote : see the following passage in exercitatio i.:--"item _sponte nascentia_ dicuntur; non quod ex _putredine_ oriunda sint, sed quod casu, naturae sponte, et aequivocâ (ut aiunt) generatione, a parentibus sui dissimilibus proveniant." again, in _de uteri membranis:_--"in cunctorum viventium generatione (sicut diximus) hoc solenne est, ut ortum ducunt a _primordio_ aliquo, quod tum materiam tum elficiendi potestatem in se habet: sitque, adeo id, ex quo et a quo quicquid nascitur, ortum suum ducat. tale primordium in animalibus (_sive ab aliis generantibus proveniant, sive sponte, aut ex putredine nascentur_) est humor in tunicâ, aliquâaut putami ne conclusus." compare also what redi has to say respecting harvey's opinions, _esperienze_, p. .] the first distinct enunciation of the hypothesis that all living matter has sprung from pre-existing living matter, came from a contemporary, though a junior, of harvey, a native of that country, fertile in men great in all departments of human activity, which was to intellectual europe, in the sixteenth and seventeenth centuries, what germany is in the nineteenth. it was in italy, and from italian teachers, that harvey received the most important part of his scientific education. and it was a student trained in the same schools, francesco redi--a man of the widest knowledge and most versatile abilities, distinguished alike as scholar, poet, physician, and naturalist--who, just two hundred and two years ago, published his "esperienze intorno alla generazione degl' insetti," and gave to the world the idea, the growth of which it is my purpose to trace. redi's book went through five editions in twenty years; and the extreme simplicity of his experiments, and the clearness of his arguments, gained for his views, and for their consequences, almost universal acceptance. redi did not trouble himself much with speculative considerations, but attacked particular cases of what was supposed to be "spontaneous generation" experimentally. here are dead animals, or pieces of meat, says he; i expose them to the air in hot weather, and in a few days they swarm with maggots. you tell me that these are generated in the dead flesh; but if i put similar bodies, while quite fresh, into a jar, and tie some fine gauze over the top of the jar, not a maggot makes its appearance, while the dead substances, nevertheless, putrefy just in the same way as before. it is obvious, therefore, that the maggots are not generated by the corruption of the meat; and that the cause of their formation must be a something which is kept away by gauze. but gauze will not keep away aëriform bodies, or fluids. this something must, therefore, exist in the form of solid particles too big to get through the gauze. nor is one long left in doubt what these solid particles are; for the blowflies, attracted by the odour of the meat, swarm round the vessel, and, urged by a powerful but in this case misleading instinct, lay eggs out of which maggots are immediately hatched, upon the gauze. the conclusion, therefore, is unavoidable; the maggots are not generated by the meat, but the eggs which give rise to them are brought through the air by the flies. these experiments seem almost childishly simple, and one wonders how it was that no one ever thought of them before. simple as they are, however, they are worthy of the most careful study, for every piece of experimental work since done, in regard to this subject, has been shaped upon the model furnished by the italian philosopher. as the results of his experiments were the same, however varied the nature of the materials he used, it is not wonderful that there arose in redi's mind a presumption, that, in all such cases of the seeming production of life from dead matter, the real explanation was the introduction of living germs from without into that dead matter.[ ] and thus the hypothesis that living matter always arises by the agency of pre-existing living matter, took definite shape; and had, henceforward, a right to be considered and a claim to be refuted, in each particular case, before the production of living matter in any other way could be admitted by careful reasoners. it will be necessary for me to refer to this hypothesis so frequently, that, to save circumlocution, i shall call it the hypothesis of _biogenesis_; and i shall term the contrary doctrine--that living matter may be produced by not living matter--the hypothesis of _abiogenesis_. [footnote : "pure contentandomi sempre in questa ed in ciascuna altro cosa, da ciascuno più savio, là dove io difettuosamente parlassi, esser corretto; non tacero, che per molte osservazioni molti volti da me fatte, mi sento inclinato a credere che la terra, da quelle prime piante, e da quei primi animali in poi, che ella nei primi giorni del mondo produsse per comandemento del sovrano ed omnipotente fattore, non abbia mai più prodotto da se medesima nè erba nè albero, nè animale alcuno perfetto o imperfetto che ei se fosse; e che tutto quello, che ne' tempi trapassati è nato e che ora nascere in lei, o da lei veggiamo, venga tutto dalla semenza reale e vera delle piante, e degli animali stessi, i quali col mezzo del proprio seme la loro spezie conservano. e se bene tutto giorno scorghiamo da' cadaveri degli animali, e da tutte quante le maniere dell' erbe, e de' fiori, e dei frutti imputriditi, e corrotti nascere vermi infiniti-- 'nonne vides quaecunque mora, fluidoque calore corpora tabescunt in parva animalia verti'-- io mi sento, dico, inclinato, a credere che tutti quei vermi si generino dal seme paterno; e che le carni, e l' erbe, e l' altre cose tutte putrefatte, o putrefattibili non facciano altra parte, nè abbiano altro ufizio nella generazione degl' insetti, se non d'apprestare un luogo o un nido proporzionato, in cui dagli animali nel tempo della figliatura sieno portati, e partoriti i vermi, o l' uova o l' altre semenze dei vermi, i quali tosto che nati sono, trovano in esso nido un sufficiente alimento abilissimo per nutricarsi: e se in quello non son portate dalle madri queste suddette semenze, niente mai, e replicatamente niente, vi s' ingegneri e nasca."--redi, _esperienze_, pp. - .] in the seventeenth century, as i have said, the latter was the dominant view, sanctioned alike by antiquity and by authority; and it is interesting to observe that redi did not escape the customary tax upon a discoverer of having to defend himself against the charge of impugning the authority of the scriptures;[ ] for his adversaries declared that the generation of bees from the carcase of a dead lion is affirmed, in the book of judges, to have been the origin of the famous riddle with which samson perplexed the philistines:-- out of the eater came forth meat, and out of the strong came forth sweetness. [footnote : "molti, e molti altri ancora vi potrei annoverare, se non fossi chiamato a rispondere alle rampogne di alcuni, che bruscamente mi rammentano ciò, che si legge nel capitolo quattordicesimo del sacrosanto libro de' giudici ... "--redi, _loc. cit._ p. .] against all odds, however, redi, strong with the strength of demonstrable fact, did splendid battle for biogenesis; but it is remarkable that he held the doctrine in a sense which, if he lead lived in these times, would have infallibly caused him to be classed among the defenders of "spontaneous generation." "omne vivum ex vivo," "no life without antecedent life," aphoristically sums up redi's doctrine; but he went no further. it is most remarkable evidence of the philosophic caution and impartiality of his mind, that although he had speculatively anticipated the manner in which grubs really are deposited in fruits and in the galls of plants, he deliberately admits that the evidence is insufficient to bear him out; and he therefore prefers the supposition that they are generated by a modification of the living substance of the plants themselves. indeed, he regards these vegetable growths as organs, by means of which the plant gives rise to an animal, and looks upon this production of specific animals as the final cause of the galls and of, at any rate, some fruits. and he proposes to explain the occurrence of parasites within the animal body in the same way.[ ] [footnote : the passage (_esperienze_, p. ) is worth quoting in full:-- "se dovessi palesarvi il mio sentimento crederei che i frutti, i legumi, gli alberi e le foglie, in due maniere inverminassero. una, perchè venendo i bachi per dí fuora, e cercando l' alimento, col rodere ci aprono la strada, ed arrivano alla più interna midolla de' frutti e de' legni. l'altra maniera si è, che io per me stimerei, che non fosse gran fatto disdicevole il credere, che quell' anima o quella virtù, la quale genera i fiori ed i frutti nelle piante viventi, sia quella stessa che generi ancora i bachi di esse piante. e chi sà, forse, che molti frutti degli alberi non sieno prodotti, non per un fine primario e principale, ma bensi per un uffizio secondario e servile, destinato alla generazione di que' vermi, servendo a loro in vece di matrice, in cui dimorino un prefisso e determinato tempo; il quale arrivato escan fuora a godere il sole. "io m' immagino, che questo mio pensiero non vi parrà totalmento un paradosso; mentro farete riflessione a quelle tanto sorte di galle, di gallozzole, di coccole, di ricci, di calici, di cornetti ed i lappole, che son produtte dalle quercel, dalle farnie, da' cerri, da' sugheri, da' leeci e da altri simili alberi de ghianda; imperciocchè in quello gallozzole, e particolarmente nelle più grosse, che si chiamano coronati, ne' ricci capelluti, che ciuffoli da' nostri contadini son detti; nei ricci legnosi del cerro, ne' ricci stellati della quercia, nelle galluzze della foglia del leccio si vede evidentissimamente, che la prima e principale intenzione della natura è formare dentro di quelle un animale volante; vedendosi nel centro della gallozzola un uovo, che col crescere e col maturarsi di essa gallozzola va crescendo e maturando anch' egli, e cresce altresi a suo tempo quel verme, che nell' uovo si racchiude; il qual verme, quando la gallozzola è finita di maturare e che è venuto il termine destinato al suo nascimento, diventa, di verme che era, una mosca.... io vi confesso ingenuamente, che prima d'aver fatte queste mie esperienze intorno alla generazione degl' insetti mi dava a credere, o per dir meglio sospettava, che forse la gallozzola nascesse, perchè arrivando la mosca nel tempo della primavera, e facendo una piccolissima fessura ne' rami più teneri della quercia, in quella fessura nascondesse uno de suoi semi, il quale fosse cagione che sbocciasse fuora la gallozzola; e che mai non si vedessero galle o gallozzole o ricci o cornetti o calici o coccole, se non in que' rami, ne' quali le mosche avessero depositate le loro semenze; e mi dava ad intendere, che le gallozzole fossero una malattia cagionata nelle querce dalle punture delle mosche, in quella giusa stessa che dalle punture d'altri animaletti simiglievoli veggiamo crescere de' tumori ne' corpi degli animali."] it is of great importance to apprehend redi's position rightly; for the lines of thought he laid down for us are those upon which naturalists have been working ever since. clearly, he held _biogenesis_ as against _abiogenesis;_ and i shall immediately proceed, in the first place, to inquire how far subsequent investigation has borne him out in so doing. but redi also thought that there were two modes of biogenesis. by the one method, which is that of common and ordinary occurrence, the living parent gives rise to offspring which passes through the same cycle of changes as itself--like gives rise to like; and this has been termed _homogenesis_. by the other mode, the living parent was supposed to give rise to offspring which passed through a totally different series of states from those exhibited by the parent, and did not return into the cycle of the parent; this is what ought to be called _heterogenesis_, the offspring being altogether, and permanently, unlike the parent. the term heterogenesis, however, has unfortunately been used in a different sense, and m. milne-edwards has therefore substituted for it _xenogenesis_, which means the generation of something foreign. after discussing redi's hypothesis of universal biogenesis, then, i shall go on to ask how far the growth of science justifies his other hypothesis of xenogenesis. the progress of the hypothesis of biogenesis was triumphant and unchecked for nearly a century. the application of the microscope to anatomy in the hands of grew, leeuwenhoek, swammerdam, lyonnet, vallisnieri, réaurnur, and other illustrious investigators of nature of that day, displayed such a complexity of organisation in the lowest and minutest forms, and everywhere revealed such a prodigality of provision for their multiplication by germs of one sort or another, that the hypothesis of abiogenesis began to appear not only untrue, but absurd; and, in the middle of the eighteenth century, when needham and buffon took up the question, it was almost universally discredited.[ ] [footnote : needham, writing in , says:-- "les naturalistes modernes s'accordent unaninement à établir, comme une vérité certaine, que toute plante vient do sa sémence spécifique, tout animal d'un oeuf ou de quelque chose d'analogue préexistant dans la plante, ou dans l'animal de même espèce qui l'a produit."--_nouvelles observations_, p. . "les naturalistes out généralemente cru que les animaux microscopiques étaient engendrés par des oeufs transportés dans l'air, ou déposés dans des eaux dormantes par des insectes volans."--_ibid._ p. .] but the skill of the microscope makers of the eighteenth century soon reached its limit. a microscope magnifying diameters was a _chef d'oeuvre_ of the opticians of that day; and, at the same time, by no means trustworthy. but a magnifying power of diameters, even when definition reaches the exquisite perfection of our modern achromatic lenses, hardly suffices for the mere discernment of the smallest forms of life. a speck, only / th of an inch in diameter, has, at ten inches from the eye, the same apparent size as an object / th of an inch in diameter, when magnified times; but forms of living matter abound, the diameter of which is not more than / th of an inch. a filtered infusion of hay, allowed to stand for two days, will swarm with living things among which, any which reaches the diameter of a human red blood- corpuscle, or about / th of an inch, is a giant. it is only by bearing these facts in mind, that we can deal fairly with the remarkable statements and speculations put forward by buffon and needham in the middle of the eighteenth century. when a portion of any animal or vegetable body is infused in water, it gradually softens and disintegrates; and, as it does so, the water is found to swarm with minute active creatures, the so-called infusorial animalcules, none of which can be seen, except by the aid of the microscope; while a large proportion belong to the category of smallest things of which i have spoken, and which must have looked like mere dots and lines under the ordinary microscopes of the eighteenth century. led by various theoretical considerations which i cannot now discuss, but which looked promising enough in the lights of their time, buffon and needham doubted the applicability of redi's hypothesis to the infusorial animalcules, and needham very properly endeavoured to put the question to an experimental test. he said to himself, if these infusorial animalcules come from germs, their germs must exist either in the substance infused, or in the water with which the infusion is made, or in the superjacent air. now the vitality of all germs is destroyed by heat. therefore, if i boil the infusion, cork it up carefully, cementing the cork over with mastic, and then heat the whole vessel by heaping hot ashes over it, i must needs kill whatever germs are present. consequently, if redi's hypothesis hold good, when the infusion is taken away and allowed to cool, no animalcules ought to be developed in it; whereas, if the animalcules are not dependent on pre-existing germs, but are generated from the infused substance, they ought, by and by, to make their appearance. needham found that, under the circumstances in which he made his experiments, animalcules always did arise in the infusions, when a sufficient time had elapsed to allow for their development. in much of his work needham was associated with buffon, and the results of their experiments fitted in admirably with the great french naturalist's hypothesis of "organic molecules," according to which, life is the indefeasible property of certain indestructible molecules of matter, which exist in all living things, and have inherent activities by which they are distinguished from not living matter. each individual living organism is formed by their temporary combination. they stand to it in the relation of the particles of water to a cascade, or a whirlpool; or to a mould, into which the water is poured. the form of the organism is thus determined by the reaction between external conditions and the inherent activities of the organic molecules of which it is composed; and, as the stoppage of a whirlpool destroys nothing but a form, and leaves the molecules of the water, with all their inherent activities intact, so what we call the death and putrefaction of an animal, or of a plant, is merely the breaking up of the form, or manner of association, of its constituent organic molecules, which are then set free as infusorial animalcules. it will be perceived that this doctrine is by no means identical with _abiogenesis_, with which it is often confounded. on this hypothesis, a piece of beef, or a handful of hay, is dead only in a limited sense. the beef is dead ox, and the hay is dead grass; but the "organic molecules" of the beef or the hay are not dead, but are ready to manifest their vitality as soon as the bovine or herbaceous shrouds in which they are imprisoned are rent by the macerating action of water. the hypothesis therefore must be classified under xenogenesis, rather than under abiogenesis. such as it was, i think it will appear, to those who will be just enough to remember that it was propounded before the birth of modern chemistry, and of the modern optical arts, to be a most ingenious and suggestive speculation. but the great tragedy of science--the slaying of a beautiful hypothesis by an ugly fact--which is so constantly being enacted under the eyes of philosophers, was played, almost immediately, for the benefit of buffon and needham. once more, an italian, the abbé spallanzani, a worthy successor and representative of redi in his acuteness, his ingenuity, and his learning, subjected the experiments and the conclusions of needham to a searching criticism. it might be true that needham's experiments yielded results such as he had described, but did they bear out his arguments? was it not possible, in the first place, he had not completely excluded the air by his corks and mastic? and was it not possible, in the second place, that he had not sufficiently heated his infusions and the superjacent air? spallanzani joined issue with the english naturalist on both these pleas, and he showed that if, in the first place, the glass vessels in which the infusions were contained were hermetically sealed by fusing their necks, and if, in the second place, they were exposed to the temperature of boiling water for three-quarters of an hour,[ ] no animalcules ever made their appearance within them. it must be admitted that the experiments and arguments of spallanzani furnish a complete and a crushing reply to those of needham. but we all too often forget that it is one thing to refute a proposition, and another to prove the truth of a doctrine which, implicitly or explicitly, contradicts that proposition; and the advance of science soon showed that though needham might be quite wrong, it did not follow that spallanzani was quite right. [footnote : see spallanzani, _opere_, vi. pp. and .] modern chemistry, the birth of the latter half of the eighteenth century, grew apace, and soon found herself face to face with the great problems which biology had vainly tried to attack without her help. the discovery of oxygen led to the laying of the foundations of a scientific theory of respiration, and to an examination of the marvellous interactions of organic substances with oxygen. the presence of free oxygen appeared to be one of the conditions of the existence of life, and of those singular changes in organic matters which are known as fermentation and putrefaction. the question of the generation of the infusory animalcules thus passed into a new phase. for what might not have happened to the organic matter of the infusions, or to the oxygen of the air, in spallanzani's experiments? what security was there that the development of life which ought to have taken place had not been checked or prevented by these changes? the battle had to be fought again. it was needful to repeat the experiments under conditions which would make sure that neither the oxygen of the air, nor the composition of the organic matter, was altered in such a manner as to interfere with the existence of life. schulze and schwann took up the question from this point of view in and . the passage of air through red-hot glass tubes, or through strong sulphuric acid, does not alter the proportion of its oxygen, while it must needs arrest, or destroy, any organic matter which may be contained in the air. these experimenters, therefore, contrived arrangements by which the only air which should come into contact with a boiled infusion should be such as had either passed through red-hot tubes or through strong sulphuric acid. the result which they obtained was that an infusion so treated developed no living things, while, if the same infusion was afterwards exposed to the air, such things appeared rapidly and abundantly. the accuracy of these experiments has been alternately denied and affirmed. supposing then, to be accepted, however, all that they really proved was that the treatment to which the air was subjected destroyed _something_ that was essential to the development of life in the infusion. this "something" might be gaseous, fluid, or solid; that it consisted of germs remained only an hypothesis of greater or less probability. contemporaneously with these investigations a remarkable discovery was made by cagniard de la tour. he found that common yeast is composed of a vast accumulation of minute plants. the fermentation of must, or of wort, in the fabrication of wine and of beer, is always accompanied by the rapid growth and multiplication of these _toruloe_. thus, fermentation, in so far as it was accompanied by the development of microscopical organisms in enormous numbers, became assimilated to the decomposition of an infusion of ordinary animal or vegetable matter; and it was an obvious suggestion that the organisms were, in some way or other, the causes both of fermentation and of putrefaction. the chemists, with berzelius and liebig at their head, at first laughed this idea to scorn; but in , a man then very young, who has since performed the unexampled feat of attaining to high eminence alike in mathematics, physics, and physiology-- i speak of the illustrious helmholtz--reduced the matter to the test of experiment by a method alike elegant and conclusive. helmholtz separated a putrefying or a fermenting liquid from one which was simply putrescible or fermentable by a membrane which allowed the fluids to pass through and become intermixed, but stopped the passage of solids. the result was, that while the putrescible or the fermentable liquids became impregnated with the results of the putrescence or fermentation which was going on on the other side of the membrane, they neither putrefied (in the ordinary way) nor fermented; nor were any of the organisms which abounded in the fermenting or putrefying liquid generated in them. therefore the cause of the development of these organisms must lie in something which cannot pass through membranes; and as helmholtz's investigations were long antecedent to graham's researches upon colloids, his natural conclusion was that the agent thus intercepted must be a solid material. in point of fact, helmholtz's experiments narrowed the issue to this: that which excites fermentation and putrefaction, and at the same time gives rise to living forms in a fermentable or putrescible fluid, is not a gas and is not a diffusible fluid; therefore it is either a colloid, or it is matter divided into very minute solid particles. the researches of schroeder and dusch in , and of schroeder alone, in , cleared up this point by experiments which are simply refinements upon those of redi. a lump of cotton-wool is, physically speaking, a pile of many thicknesses of a very fine gauze, the fineness of the meshes of which depends upon the closeness of the compression of the wool. now, schroeder and dusch found, that, in the case of all the putrefiable materials which they used (except milk and yolk of egg), an infusion boiled, and then allowed to come into contact with no air but such as had been filtered through cotton-wool, neither putrefied, nor fermented, nor developed living forms. it is hard to imagine what the fine sieve formed by the cotton-wool could have stopped except minute solid particles. still the evidence was incomplete until it had been positively shown, first, that ordinary air does contain such particles; and, secondly, that filtration through cotton-wool arrests these particles and allows only physically pure air to pass. this demonstration has been furnished within the last year by the remarkable experiments of professor tyndall. it has been a common objection of abiogenists that, if the doctrine of biogeny is true, the air must be thick with germs; and they regard this as the height of absurdity. but nature occasionally is exceedingly unreasonable, and professor tyndall has proved that this particular absurdity may nevertheless be a reality. he has demonstrated that ordinary air is no better than a sort of stirabout of excessively minute solid particles; that these particles are almost wholly destructible by heat; and that they are strained off, and the air rendered optically pure, by its being passed through cotton-wool. it remains yet in the order of logic, though not of history, to show that among these solid destructible particles, there really do exist germs capable of giving rise to the development of living forms in suitable menstrua. this piece of work was done by m. pasteur in those beautiful researches which will ever render his name famous; and which, in spite of all attacks upon them, appear to me now, as they did seven years ago,[ ] to be models of accurate experimentation and logical reasoning. he strained air through cotton-wool, and found, as schroeder and dusch had done, that it contained nothing competent to give rise to the development of life in fluids highly fitted for that purpose. but the important further links in the chain of evidence added by pasteur are three. in the first place he subjected to microscopic examination the cotton-wool which had served as strainer, and found that sundry bodies clearly recognisable as germs, were among the solid particles strained off. secondly, he proved that these germs were competent to give rise to living forms by simply sowing them in a solution fitted for their development. and, thirdly, he showed that the incapacity of air strained through cotton- wool to give rise to life, was not due to any occult change effected in the constituents of the air by the wool, by proving that the cotton-wool might be dispensed with altogether, and perfectly free access left between the exterior air and that in the experimental flask. if the neck of the flask is drawn out into a tube and bent downwards; and if, after the contained fluid has been carefully boiled, the tube is heated sufficiently to destroy any germs which may be present in the air which enters as the fluid cools, the apparatus may be left to itself for any time and no life will appear in the fluid. the reason is plain. although there is free communication between the atmosphere laden with germs and the germless air in the flask, contact between the two takes place only in the tube; and as the germs cannot fall upwards, and there are no currents, they never reach the interior of the flask. but if the tube be broken short off where it proceeds from the flask, and free access be thus given to germs falling vertically out of the air, the fluid, which has remained clear and desert for months, becomes, in a few days, turbid and full of life. [footnote : _lectures to working men on the causes of the phenomena of organic nature_, . (see vol. ii. of these essays.)] these experiments have been repeated over and over again by independent observers with entire success; and there is one very simple mode of seeing the facts for one's self, which i may as well describe. prepare a solution (much used by m. pasteur, and often called "pasteur's solution") composed of water with tartrate of ammonia, sugar, and yeast- ash dissolved therein.[ ] divide it into three portions in as many flasks; boil all three for a quarter of an hour; and, while the steam is passing out, stop the neck of one with a large plug of cotton-wool, so that this also may be thoroughly steamed. now set the flasks aside to cool, and, when their contents are cold, add to one of the open ones a drop of filtered infusion of hay which has stood for twenty-four hours, and is consequently hill of the active and excessively minute organisms known as _bacteria_. in a couple of days of ordinary warm weather the contents of this flask will be milky from the enormous multiplication of _bacteria_. the other flask, open and exposed to the air, will, sooner or later, become milky with _bacteria_, and patches of mould may appear in it; while the liquid in the flask, the neck of which is plugged with cotton-wool, will remain clear for an indefinite time. i have sought in vain for any explanation of these facts, except the obvious one, that the air contains germs competent to give rise to _bacteria_, such as those with which the first solution has been knowingly and purposely inoculated, and to the mould-_fungi_. and i have not yet been able to meet with any advocate of abiogenesis who seriously maintains that the atoms of sugar, tartrate of ammonia, yeast-ash, and water, under no influence but that of free access of air and the ordinary temperature, re-arrange themselves and give rise to the protoplasm of _bacterium_. but the alternative is to admit that these _bacteria_ arise from germs in the air; and if they are thus propagated, the burden of proof that other like forms are generated in a different manner, must rest with the assertor of that proposition. [footnote : infusion of hay treated in the same way yields similar results; but as it contains organic matter, the argument which follows cannot be based upon it.] to sum up the effect of this long chain of evidence:-- it is demonstrable that a fluid eminently fit for the development of the lowest forms of life, but which contains neither germs, nor any protein compound, gives rise to living things in great abundance if it is exposed to ordinary air; while no such development takes place, if the air with which it is in contact is mechanically freed from the solid particles which ordinarily float in it, and which may be made visible by appropriate means. it is demonstrable that the great majority of these particles are destructible by heat, and that some of them are germs, or living particles, capable of giving rise to the same forms of life as those which appear when the fluid is exposed to unpurified air. it is demonstrable that inoculation of the experimental fluid with a drop of liquid known to contain living particles gives rise to the same phenomena as exposure to unpurified air. and it is further certain that these living particles are so minute that the assumption of their suspension in ordinary air presents not the slightest difficulty. on the contrary, considering their lightness and the wide diffusion of the organisms which produce them, it is impossible to conceive that they should not be suspended in the atmosphere in myriads. thus the evidence, direct and indirect, in favour of _biogenesis_ for all known forms of life must, i think, be admitted to be of great weight. on the other side, the sole assertions worthy of attention are that hermetically sealed fluids, which have been exposed to great and long- continued heat, have sometimes exhibited living forms of low organisation when they have been opened. the first reply that suggests itself is the probability that there must be some error about these experiments, because they are performed on an enormous scale every day with quite contrary results. meat, fruits, vegetables, the very materials of the most fermentable and putrescible infusions, are preserved to the extent, i suppose i may say, of thousands of tons every year, by a method which is a mere application of spallanzani's experiment. the matters to be preserved are well boiled in a tin case provided with a small hole, and this hole is soldered up when all the air in the case has been replaced by steam. by this method they may be kept for years without putrefying, fermenting, or getting mouldy. now this is not because oxygen is excluded, inasmuch as it is now proved that free oxygen is not necessary for either fermentation or putrefaction. it is not because the tins are exhausted of air, for _vibriones_ and _bacteria_ live, as pasteur has shown, without air or free oxygen. it is not because the boiled meats or vegetables are not putrescible or fermentable, as those who have had the misfortune to be in a ship supplied with unskilfully closed tins well know. what is it, therefore, but the exclusion of germs? i think that abiogenists are bound to answer this question before they ask us to consider new experiments of precisely the same order. and in the next place, if the results of the experiments i refer to are really trustworthy, it by no means follows that abiogenesis has taken place. the resistance of living matter to heat is known to vary within considerable limits, and to depend, to some extent, upon the chemical and physical qualities of the surrounding medium. but if, in the present state of science, the alternative is offered us,--either germs can stand a greater heat than has been supposed, or the molecules of dead matter, for no valid or intelligible reason that is assigned, are able to re- arrange themselves into living bodies, exactly such as can be demonstrated to be frequently produced in another way,--i cannot understand how choice can be, even for a moment, doubtful. but though i cannot express this conviction of mine too strongly, i must carefully guard myself against the supposition that i intend to suggest that no such thing as abiogenesis ever has taken place in the past, or ever will take place in the future. with organic chemistry, molecular physics, and physiology yet in their infancy, and every day making prodigious strides, i think it would be the height of presumption for any man to say that the conditions under which matter assumes the properties we call "vital" may not, some day, be artificially brought together. all i feel justified in affirming is, that i see no reason for believing that the feat has been performed yet. and looking back through the prodigious vista of the past, i find no record of the commencement of life, and therefore i am devoid of any means of forming a definite conclusion as to the conditions of its appearance. belief, in the scientific sense of the word, is a serious matter, and needs strong foundations. to say, therefore, in the admitted absence of evidence, that i have any belief as to the mode in which the existing forms of life have originated, would be using words in a wrong sense. but expectation is permissible where belief is not; and if it were given me to look beyond the abyss of geologically recorded time to the still more remote period when the earth was passing through physical and chemical conditions, which it can no more see again than a man can recall his infancy, i should expect to be a witness of the evolution of living protoplasm from not living matter. i should expect to see it appear under forms of great simplicity, endowed, like existing fungi, with the power of determining the formation of new protoplasm from such matters as ammonium carbonates, oxalates and tartrates, alkaline and earthy phosphates, and water, without the aid of light. that is the expectation to which analogical reasoning leads me; but i beg you once more to recollect that i have no right to call my opinion anything but an act of philosophical faith. so much for the history of the progress of redi's great doctrine of biogenesis, which appears to me, with the limitations i have expressed, to be victorious along the whole line at the present day. as regards the second problem offered to us by redi, whether xenogenesis obtains, side by side with homogenesis,--whether, that is, there exist not only the ordinary living things, giving rise to offspring which run through the same cycle as themselves, but also others, producing offspring which are of a totally different character from themselves,-- the researches of two centuries have led to a different result. that the grubs found in galls are no product of the plants on which the galls grow, but are the result of the introduction of the eggs of insects into the substance of these plants, was made out by vallisnieri, réaumur, and others, before the end of the first half of the eighteenth century. the tapeworms, bladderworms, and flukes continued to be a stronghold of the advocates of xenogenesis for a much longer period. indeed, it is only within the last thirty years that the splendid patience of von siebold, van beneden, leuckart, küchenmeister, and other helminthologists, has succeeded in tracing every such parasite, often through the strangest wanderings and metamorphoses, to an egg derived from a parent, actually or potentially like itself; and the tendency of inquiries elsewhere has all been in the same direction. a plant may throw off bulbs, but these, sooner or later, give rise to seeds or spores, which develop into the original form. a polype may give rise to medusae, or a pluteus to an echinoderm, but the medusa and the echinoderm give rise to eggs which produce polypes or glutei, and they are therefore only stages in the cycle of life of the species. but if we turn to pathology, it offers us some remarkable approximations to true xenogenesis. as i have already mentioned, it has been known since the time of vallisnieri and of réaumur, that galls in plants, and tumours in cattle, are caused by insects, which lay their eggs in those parts of the animal or vegetable frame of which these morbid structures are outgrowths. again, it is a matter of familiar experience to everybody that mere pressure on the skin will give rise to a corn. now the gall, the tumour, and the corn are parts of the living body, which have become, to a certain degree, independent and distinct organisms. under the influence of certain external conditions, elements of the body, which should have developed in due subordination to its general plan, set up for themselves and apply the nourishment which they receive to their own purposes. from such innocent productions as corns and warts, there are all gradations to the serious tumours which, by their mere size and the mechanical obstruction they cause, destroy the organism out of which they are developed; while, finally, in those terrible structures known as cancers, the abnormal growth has acquired powers of reproduction and multiplication, and is only morphologically distinguishable from the parasitic worm, the life of which is neither more nor less closely bound up with that of the infested organism. if there were a kind of diseased structure, the histological elements of which were capable of maintaining a separate and independent existence out of the body, it seems to me that the shadowy boundary between morbid growth and xenogenesis would be effaced. and i am inclined to think that the progress of discovery has almost brought us to this point already. i have been favoured by mr. simon with an early copy of the last published of the valuable "reports on the public health," which, in his capacity of their medical officer, he annually presents to the lords of the privy council. the appendix to this report contains an introductory essay "on the intimate pathology of contagion," by dr. burdon-sanderson, which is one of the clearest, most comprehensive, and well-reasoned discussions of a great question which has come under my notice for a long time. i refer you to it for details and for the authorities for the statements i am about to make. you are familiar with what happens in vaccination. a minute cut is made in the skin, and an infinitesimal quantity of vaccine matter is inserted into the wound. within a certain time a vesicle appears in the place of the wound, and the fluid which distends this vesicle is vaccine matter, in quantity a hundred or a thousandfold that which was originally inserted. now what has taken place in the course of this operation? has the vaccine matter, by its irritative property, produced a mere blister, the fluid of which has the same irritative property? or does the vaccine matter contain living particles, which have grown and multiplied where they have been planted? the observations of m. chauveau, extended and confirmed by dr. sanderson himself, appear to leave no doubt upon this head. experiments, similar in principle to those of helmholtz on fermentation and putrefaction, have proved that the active element in the vaccine lymph is non-diffusible, and consists of minute particles not exceeding / th of an inch in diameter, which are made visible in the lymph by the microscope. similar experiments have proved that two of the most destructive of epizootic diseases, sheep-pox and glanders, are also dependent for their existence and their propagation upon extremely small living solid particles, to which the title of _microzymes_ is applied. an animal suffering under either of these terrible diseases is a source of infection and contagion to others, for precisely the same reason as a tub of fermenting beer is capable of propagating its fermentation by "infection," or "contagion," to fresh wort. in both cases it is the solid living particles which are efficient; the liquid in which they float, and at the expense of which they live, being altogether passive. now arises the question, are these microzymes the results of _homogenesis_, or of _xenogenesis?_ are they capable, like the _toruloe_ of yeast, of arising only by the development of pre-existing germs? or may they be, like the constituents of a nut-gall, the results of a modification and individualisation of the tissues of the body in which they are found, resulting from the operation of certain conditions? are they parasites in the zoological sense, or are they merely what virchow has called "heterologous growths"? it is obvious that this question has the most profound importance, whether we look at it from a practical or from a theoretical point of view. a parasite may be stamped out by destroying its germs, but a pathological product can only be annihilated by removing the conditions which give rise to it. it appears to me that this great problem will have to be solved for each zymotic disease separately, for analogy cuts two ways. i have dwelt upon the analogy of pathological modification, which is in favour of the xenogenetic origin of microzymes; but i must now speak of the equally strong analogies in favour of the origin of such pestiferous particles by the ordinary process of the generation of like from like. it is, at present, a well-established fact that certain diseases, both of plants and of animals, which have all the characters of contagious and infectious epidemics, are caused by minute organisms. the smut of wheat is a well-known instance of such a disease, and it cannot be doubted that the grape-disease and the potato-disease fall under the same category. among animals, insects are wonderfully liable to the ravages of contagious and infectious diseases caused by microscopic _fungi_. in autumn, it is not uncommon to see flies motionless upon a window-pane, with a sort of magic circle, in white, drawn round them. on microscopic examination, the magic circle is found to consist of innumerable spores, which have been thrown off in all directions by a minute fungus called _empusa muscoe_, the spore-forming filaments of which stand out like a pile of velvet from the body of the fly. these spore-forming filaments are connected with others which fill the interior of the fly's body like so much fine wool, having eaten away and destroyed the creature's viscera. this is the full-grown condition of the _empusa_. if traced back to its earliest stages, in flies which are still active, and to all appearance healthy, it is found to exist in the form of minute corpuscles which float in the blood of the fly. these multiply and lengthen into filaments, at the expense of the fly's substance; and when they have at last killed the patient, they grow out of its body and give off spores. healthy flies shut up with diseased ones catch this mortal disease, and perish like the others. a most competent observer, m. cohn, who studied the development of the _empusa_ very carefully, was utterly unable to discover in what manner the smallest germs of the _empusa_ got into the fly. the spores could not be made to give rise to such germs by cultivation; nor were such germs discoverable in the air, or in the food of the fly. it looked exceedingly like a case of abiogenesis, or, at any rate, of xenogenesis; and it is only quite recently that the real course of events has been made out. it has been ascertained, that when one of the spores falls upon the body of a fly, it begins to germinate, and sends out a process which bores its way through the fly's skin; this, having reached the interior cavities of its body, gives off the minute floating corpuscles which are the earliest stage of the _empusa_. the disease is "contagious," because a healthy fly coming in contact with a diseased one, from which the spore-bearing filaments protrude, is pretty sure to carry off a spore or two. it is "infectious" because the spores become scattered about all sorts of matter in the neighbourhood of the slain flies. the silkworm has long been known to be subject to a very fatal and infectious disease called the _muscardine_. audouin transmitted it by inoculation. this disease is entirely due to the development of a fungus, _botrytis bassiana_, in the body of the caterpillar; and its contagiousness and infectiousness are accounted for in the same way as those of the fly-disease. but, of late years, a still more serious epizootic has appeared among the silkworms; and i may mention a few facts which will give you some conception of the gravity of the injury which it has inflicted on france alone. the production of silk has been for centuries an important branch of industry in southern france, and in the year it had attained such a magnitude that the annual produce of the french sericulture was estimated to amount to a tenth of that of the whole world, and represented a money- value of , , francs, or nearly five millions sterling. what may be the sum which would represent the money-value of all the industries connected with the working up of the raw silk thus produced, is more than i can pretend to estimate. suffice it to say, that the city of lyons is built upon french silk as much as manchester was upon american cotton before the civil war. silkworms are liable to many diseases; and, even before , a peculiar epizootic, frequently accompanied by the appearance of dark spots upon the skin (whence the name of "pébrine" which it has received), had been noted for its mortality. but in the years following this malady broke out with such extreme violence, that, in , the silk-crop was reduced to a third of the amount which it had reached in ; and, up till within the last year or two, it has never attained half the yield of . this means not only that the great number of people engaged in silk growing are some thirty millions sterling poorer than they might have been; it means not only that high prices have had to be paid for imported silkworm eggs, and that, after investing his money in them, in paying for mulberry-leaves and for attendance, the cultivator has constantly seen his silkworms perish and himself plunged in ruin; but it means that the looms of lyons have lacked employment, and that, for years, enforced idleness and misery have been the portion of a vast population which, in former days, was industrious and well-to-do. in the gravity of the situation caused the french academy of sciences to appoint commissioners, of whom a distinguished naturalist, m. de quatrefages, was one, to inquire into the nature of this disease, and, if possible, to devise some means of staying the plague. in reading the report[ ] made by m. de quatrefages in , it is exceedingly interesting to observe that his elaborate study of the pébrine forced the conviction upon his mind that, in its mode of occurrence and propagation, the disease of the silkworm is, in every respect, comparable to the cholera among mankind. but it differs from the cholera, and so far is a more formidable malady, in being hereditary, and in being, under some circumstances, contagious as well as infectious. [footnote : _Ã�tudes sur les maladies actuelles des vers à soie_, p. .] the italian naturalist, filippi, discovered in the blood of the silkworms affected by this strange disorder a multitude of cylindrical corpuscles, each about / th of an inch long. these have been carefully studied by lebert, and named by him _panhistophyton_; for the reason that in subjects in which the disease is strongly developed, the corpuscles swarm in every tissue and organ of the body, and even pass into the undeveloped eggs of the female moth. but are these corpuscles causes, or mere concomitants, of the disease? some naturalists took one view and some another; and it was not until the french government, alarmed by the continued ravages of the malady, and the inefficiency of the remedies which had been suggested, despatched m. pasteur to study it, that the question received its final settlement; at a great sacrifice, not only of the time and peace of mind of that eminent philosopher, but, i regret to have to add, of his health. but the sacrifice has not been in vain. it is now certain that this devastating, cholera-like, pébrine, is the effect of the growth and multiplication of the _panhistophyton_ in the silkworm. it is contagious and infectious, because the corpuscles of the _panhistophyton_ pass away from the bodies of the diseased caterpillars, directly or indirectly, to the alimentary canal of healthy silkworms in their neighbourhood; it is hereditary because the corpuscles enter into the eggs while they are being formed, and consequently are carried within them when they are laid; and for this reason, also, it presents the very singular peculiarity of being inherited only on the mother's side. there is not a single one of all the apparently capricious and unaccountable phenomena presented by the pébrine, but has received its explanation from the fact that the disease is the result of the presence of the microscopic organism, _panhistophyton_. such being the facts with respect to the pébrine, what are the indications as to the method of preventing it? it is obvious that this depends upon the way in which the _panhistophyton_ is generated. if it may be generated by abiogenesis, or by xenogenesis, within the silkworm or its moth, the extirpation of the disease must depend upon the prevention of the occurrence of the conditions under which this generation takes place. but if, on the other hand, the _panhistophyton_ is an independent organism, which is no more generated by the silkworm than the mistletoe is generated by the apple-tree or the oak on which it grows, though it may need the silkworm for its development in the same way as the mistletoe needs the tree, then the indications are totally different. the sole thing to be done is to get rid of and keep away the germs of the _panhistophyton_. as might be imagined, from the course of his previous investigations, m. pasteur was led to believe that the latter was the right theory; and, guided by that theory, he has devised a method of extirpating the disease, which has proved to be completely successful wherever it has been properly carried out. there can be no reason, then, for doubting that, among insects, contagious and infectious diseases, of great malignity, are caused by minute organisms which are produced from pre-existing germs, or by homogenesis; and there is no reason, that i know of, for believing that what happens in insects may not take place in the highest animals. indeed, there is already strong evidence that some diseases of an extremely malignant and fatal character to which man is subject, are as much the work of minute organisms as is the pébrine. i refer for this evidence to the very striking facts adduced by professor lister in his various well-known publications on the antiseptic method of treatment. it appears to me impossible to rise from the perusal of those publications without a strong conviction that the lamentable mortality which so frequently dogs the footsteps of the most skilful operator, and those deadly consequences of wounds and injuries which seem to haunt the very walls of great hospitals, and are, even now, destroying more men than die of bullet or bayonet, are due to the importation of minute organisms into wounds, and their increase and multiplication; and that the surgeon who saves most lives will be he who best works out the practical consequences of the hypothesis of redi. i commenced this address by asking you to follow me in an attempt to trace the path which has been followed by a scientific idea, in its long and slow progress from the position of a probable hypothesis to that of an established law of nature. our survey has not taken us into very attractive regions; it has lain, chiefly, in a land flowing with the abominable, and peopled with mere grubs and mouldiness. and it may be imagined with what smiles and shrugs, practical and serious contemporaries of redi and of spallanzani may have commented on the waste of their high abilities in toiling at the solution of problems which, though curious enough in themselves, could be of no conceivable utility to mankind. nevertheless, you will have observed that before we had travelled very far upon our road, there appeared, on the right hand and on the left, fields laden with a harvest of golden grain, immediately convertible into those things which the most solidly practical men will admit to have value--viz., money and life. the direct loss to france caused by the pébrine in seventeen years cannot be estimated at less than fifty millions sterling; and if we add to this what redi's idea, in pasteur's hands, has done for the wine-grower and for the vinegar-maker, and try to capitalise its value, we shall find that it will go a long way towards repairing the money losses caused by the frightful and calamitous war of this autumn. and as to the equivalent of redi's thought in life, how can we over-estimate the value of that knowledge of the nature of epidemic and epizootic diseases, and consequently of the means of checking, or eradicating them, the dawn of which has assuredly commenced? looking back no further than ten years, it is possible to select three ( , , and ) in which the total number of deaths from scarlet- fever alone amounted to ninety thousand. that is the return of killed, the maimed and disabled being left out of sight. why, it is to be hoped that the list of killed in the present bloodiest of all wars will not amount to more than this! but the facts which i have placed before you must leave the least sanguine without a doubt that the nature and the causes of this scourge will, one day, be as well understood as those of the pébrine are now; and that the long-suffered massacre of our innocents will come to an end. and thus mankind will have one more admonition that "the people perish for lack of knowledge"; and that the alleviation of the miseries, and the promotion of the welfare, of men must be sought, by those who will not lose their pains, in that diligent, patient, loving study of all the multitudinous aspects of nature, the results of which constitute exact knowledge, or science. it is the justification and the glory of this great meeting that it is gathered together for no other object than the advancement of the moiety of science which deals with those phenomena of nature which we call physical. may its endeavours be crowned with a full measure of success! ix geological contemporaneity and persistent types of life [ ] merchants occasionally go through a wholesome, though troublesome and not always satisfactory, process which they term "taking stock." after all the excitement of speculation, the pleasure of gain, and the pain of loss, the trader makes up his mind to face facts and to learn the exact quantity and quality of his solid and reliable possessions. the man of science does well sometimes to imitate this procedure; and, forgetting for the time the importance of his own small winnings, to re- examine the common stock in trade, so that he may make sure how far the stock of bullion in the cellar--on the faith of whose existence so much paper has been circulating--is really the solid gold of truth. the anniversary meeting of the geological society seems to be an occasion well suited for an undertaking of this kind--for an inquiry, in fact, into the nature and value of the present results of palaeontological investigation; and the more so, as all those who have paid close attention to the late multitudinous discussions in which palaeontology is implicated, must have felt the urgent necessity of some such scrutiny. first in order, as the most definite and unquestionable of all the results of palaeontology, must be mentioned the immense extension and impulse given to botany, zoology, and comparative anatomy, by the investigation of fossil remains. indeed, the mass of biological facts has been so greatly increased, and the range of biological speculation has been so vastly widened, by the researches of the geologist and palaeontologist, that it is to be feared there are naturalists in existence who look upon geology as brindley regarded rivers. "rivers," said the great engineer, "were made to feed canals;" and geology, some seem to think, was solely created to advance comparative anatomy. were such a thought justifiable, it could hardly expect to be received with favour by this assembly. but it is not justifiable. your favourite science has her own great aims independent of all others; and if, notwithstanding her steady devotion to her own progress, she can scatter such rich alms among her sisters, it should be remembered that her charity is of the sort that does not impoverish, but "blesseth him that gives and him that takes." regard the matter as we will, however, the facts remain. nearly , species of animals and plants have been added to the systema naturae by palaeontological research. this is a living population equivalent to that of a new continent in mere number; equivalent to that of a new hemisphere, if we take into account the small population of insects as yet found fossil, and the large proportion and peculiar organisation of many of the vertebrata. but, beyond this, it is perhaps not too much to say that, except for the necessity of interpreting palaeontological facts, the laws of distribution would have received less careful study; while few comparative anatomists (and those not of the first order) would have been induced by mere love of detail, as such, to study the minutiae of osteology, were it not that in such minutiae lie the only keys to the most interesting riddles offered by the extinct animal world. these assuredly are great and solid gains. surely it is matter for no small congratulation that in half a century (for palaeontology, though it dawned earlier, came into full day only with cuvier) a subordinate branch of biology should have doubled the value and the interest of the whole group of sciences to which it belongs. but this is not all. allied with geology, palaeontology has established two laws of inestimable importance: the first, that one and the same area of the earth's surface has been successively occupied by very different kinds of living beings; the second, that the order of succession established in one locality holds good, approximately, in all. the first of these laws is universal and irreversible; the second is an induction from a vast number of observations, though it may possibly, and even probably, have to admit of exceptions. as a consequence of the second law, it follows that a peculiar relation frequently subsists between series of strata containing organic remains, in different localities. the series resemble one another not only in virtue of a general resemblance of the organic remains in the two, but also in virtue of a resemblance in the order and character of the serial succession in each. there is a resemblance of arrangement; so that the separate terms of each series, as well as the whole series, exhibit a correspondence. succession implies time; the lower members of an undisturbed series of sedimentary rocks are certainly older than the upper; and when the notion of age was once introduced as the equivalent of succession, it was no wonder that correspondence in succession came to be looked upon as a correspondence in age, or "contemporaneity." and, indeed, so long as relative age only is spoken of, correspondence in succession _is_ correspondence in age; it is _relative_ contemporaneity. but it would have been very much better for geology if so loose and ambiguous a word as "contemporaneous" had been excluded from her terminology, and if, in its stead, some term expressing similarity of serial relation, and excluding the notion of time altogether, had been employed to denote correspondence in position in two or more series of strata. in anatomy, where such correspondence of position has constantly to be spoken of, it is denoted by the word "homology" and its derivatives; and for geology (which after all is only the anatomy and physiology of the earth) it might be well to invent some single word, such as "homotaxis" (similarity of order), in order to express an essentially similar idea. this, however, has not been done, and most probably the inquiry will at once be made--to what end burden science with a new and strange term in place of one old, familiar, and part of our common language? the reply to this question will become obvious as the inquiry into the results of palaeontology is pushed further. those whose business it is to acquaint themselves specially with the works of palaeontologists, in fact, will be fully aware that very few, if any, would rest satisfied with such a statement of the conclusions of their branch of biology as that which has just been given. our standard repertories of palaeontology profess to teach us far higher things--to disclose the entire succession of living forms upon the surface of the globe; to tell us of a wholly different distribution of climatic conditions in ancient times; to reveal the character of the first of all living existences; and to trace out the law of progress from them to us. it may not be unprofitable to bestow on these professions a somewhat more critical examination than they have hitherto received, in order to ascertain how far they rest on an irrefragable basis; or whether, after all, it might not be well for palaeontologists to learn a little more carefully that scientific "ars artium," the art of saying "i don't know." and to this end let us define somewhat more exactly the extent of these pretensions of palaeontology. every one is aware that professor bronn's "untersuchungen" and professor pictet's "traité de paléontologie" are works of standard authority, familiarly consulted by every working palaeontologist. it is desirable to speak of these excellent books, and of their distinguished authors, with the utmost respect, and in a tone as far as possible removed from carping criticism; indeed, if they are specially cited in this place, it is merely in justification of the assertion that the following propositions, which may be found implicitly, or explicitly, in the works in question, are regarded by the mass of palaeontologists and geologists, not only on the continent but in this country, as expressing some of the best- established results of palaeontology. thus:-- animals and plants began their existence together, not long after the commencement of the deposition of the sedimentary rocks; and then succeeded one another, in such a manner, that totally distinct faunae and florae occupied the whole surface of the earth, one after the other, and during distinct epochs of time. a geological formation is the sum of all the strata deposited over the whole surface of the earth during one of these epochs: a geological fauna or flora is the sum of all the species of animals or plants which occupied the whole surface of the globe, during one of these epochs. the population of the earth's surface was at first very similar in all parts, and only from the middle of the tertiary epoch onwards, began to show a distinct distribution in zones. the constitution of the original population, as well as the numerical proportions of its members, indicates a warmer and, on the whole, somewhat tropical climate, which remained tolerably equable throughout the year. the subsequent distribution of living beings in zones is the result of a gradual lowering of the general temperature, which first began to be felt at the poles. it is not now proposed to inquire whether these doctrines are true or false; but to direct your attention to a much simpler though very essential preliminary question--what is their logical basis? what are the fundamental assumptions upon which they all logically depend? and what is the evidence on which those fundamental propositions demand our assent? these assumptions are two: the first, that the commencement of the geological record is coëval with the commencement of life on the globe; the second, that geological contemporaneity is the same thing as chronological synchrony. without the first of these assumptions there would of course be no ground for any statement respecting the commencement of life; without the second, all the other statements cited, every one of which implies a knowledge of the state of different parts of the earth at one and the same time, will be no less devoid of demonstration. the first assumption obviously rests entirely on negative evidence. this is, of course, the only evidence that ever can be available to prove the commencement of any series of phenomena; but, at the same time, it must be recollected that the value of negative evidence depends entirely on the amount of positive corroboration it receives. if a.b. wishes to prove an _alibi_, it is of no use for him to get a thousand witnesses simply to swear that they did not see him in such and such a place, unless the witnesses are prepared to prove that they must have seen him had he been there. but the evidence that animal life commenced with the lingula- flags, _e.g._, would seem to be exactly of this unsatisfactory uncorroborated sort. the cambrian witnesses simply swear they "haven't seen anybody their way"; upon which the counsel for the other side immediately puts in ten or twelve thousand feet of devonian sandstones to make oath they never saw a fish or a mollusk, though all the world knows there were plenty in their time. but then it is urged that, though the devonian rocks in one part of the world exhibit no fossils, in another they do, while the lower cambrian rocks nowhere exhibit fossils, and hence no living being could have existed in their epoch. to this there are two replies: the first that the observational basis of the assertion that the lowest rocks are nowhere fossiliferous is an amazingly small one, seeing how very small an area, in comparison to that of the whole world, has yet been fully searched; the second, that the argument is good for nothing unless the unfossiliferous rocks in question were not only _contemporaneous_ in the geological sense, but _synchronous_ in the chronological sense. to use the _alibi_ illustration again. if a man wishes to prove he was in neither of two places, a and b, on a given day, his witnesses for each place must be prepared to answer for the whole day. if they can only prove that he was not at a in the morning, and not at b in the afternoon, the evidence of his absence from both is nil, because he might have been at b in the morning and at a in the afternoon. thus everything depends upon the validity of the second assumption. and we must proceed to inquire what is the real meaning of the word "contemporaneous" as employed by geologists. to this end a concrete example may be taken. the lias of england and the lias of germany, the cretaceous rocks of britain and the cretaceous rocks of southern india, are termed by geologists "contemporaneous" formations; but whenever any thoughtful geologist is asked whether he means to say that they were deposited synchronously, he says, "no,--only within the same great epoch." and if, in pursuing the inquiry, he is asked what may be the approximate value in time of a "great epoch"--whether it means a hundred years, or a thousand, or a million, or ten million years--his reply is, "i cannot tell." if the further question be put, whether physical geology is in possession of any method by which the actual synchrony (or the reverse) of any two distant deposits can be ascertained, no such method can be heard of; it being admitted by all the best authorities that neither similarity of mineral composition, nor of physical character, nor even direct continuity of stratum, are _absolute_ proofs of the synchronism of even approximated sedimentary strata: while, for distant deposits, there seems to be no kind of physical evidence attainable of a nature competent to decide whether such deposits were formed simultaneously, or whether they possess any given difference of antiquity. to return to an example already given: all competent authorities will probably assent to the proposition that physical geology does not enable us in any way to reply to this question--were the british cretaceous rocks deposited at the same time as those of india, or are they a million of years younger or a million of years older? is palaeontology able to succeed where physical geology fails? standard writers on palaeontology, as has been seen, assume that she can. they take it for granted, that deposits containing similar organic remains are synchronous--at any rate in a broad sense; and yet, those who will study the eleventh and twelfth chapters of sir henry de la beche's remarkable "researches in theoretical geology," published now nearly thirty years ago, and will carry out the arguments there most luminously stated, to their logical consequences, may very easily convince themselves that even absolute identity of organic contents is no proof of the synchrony of deposits, while absolute diversity is no proof of difference of date. sir henry de la beche goes even further, and adduces conclusive evidence to show that the different parts of one and the same stratum, having a similar composition throughout, containing the same organic remains, and having similar beds above and below it, may yet differ to any conceivable extent in age. edward forbes was in the habit of asserting that the similarity of the organic contents of distant formations was _prima facie_ evidence, not of their similarity, but of their difference of age; and holding as he did the doctrine of single specific centres, the conclusion was as legitimate as any other; for the two districts must have been occupied by migration from one of the two, or from an intermediate spot, and the chances against exact coincidence of migration and of imbedding are infinite. in point of fact, however, whether the hypothesis of single or of multiple specific centres be adopted, similarity of organic contents cannot possibly afford any proof of the synchrony of the deposits which contain them; on the contrary, it is demonstrably compatible with the lapse of the most prodigious intervals of time, and with the interposition of vast changes in the organic and inorganic worlds, between the epochs in which such deposits were formed. on what amount of similarity of their faunae is the doctrine of the contemporaneity of the european and of the north american silurians based? in the last edition of sir charles lyell's "elementary geology" it is stated, on the authority of a former president of this society, the late daniel sharpe, that between and per cent. of the species of silurian mollusca are common to both sides of the atlantic. by way of due allowance for further discovery, let us double the lesser number and suppose that per cent. of the species are common to the north american and the british silurians. sixty per cent. of species in common is, then, proof of contemporaneity. now suppose that, a million or two of years hence, when britain has made another dip beneath the sea and has come up again, some geologist applies this doctrine, in comparing the strata laid bare by the upheaval of the bottom, say, of st. george's channel with what may then remain of the suffolk crag. reasoning in the same way, he will at once decide the suffolk crag and the st. george's channel beds to be contemporaneous; although we happen to know that a vast period (even in the geological sense) of time, and physical changes of almost unprecedented extent, separate the two. but if it be a demonstrable fact that strata containing more than or per cent. of species of mollusca in common, and comparatively close together, may yet be separated by an amount of geological time sufficient to allow of some of the greatest physical changes the world has seen, what becomes of that sort of contemporaneity the sole evidence of which is a similarity of facies, or the identity of half a dozen species, or of a good many genera? and yet there is no better evidence for the contemporaneity assumed by all who adopt the hypothesis of universal faunae and florae, of a universally uniform climate, and of a sensible cooling of the globe during geological time. there seems, then, no escape from the admission that neither physical geology, nor palaeontology, possesses any method by which the absolute synchronism of two strata can be demonstrated. all that geology can prove is local order of succession. it is mathematically certain that, in any given vertical linear section of an undisturbed series of sedimentary deposits, the bed which lies lowest is the oldest. in many other vertical linear sections of the same series, of course, corresponding beds will occur in a similar order; but, however great may be the probability, no man can say with absolute certainty that the beds in the two sections were synchronously deposited. for areas of moderate extent, it is doubtless true that no practical evil is likely to result from assuming the corresponding beds to be synchronous or strictly contemporaneous; and there are multitudes of accessory circumstances which may fully justify the assumption of such synchrony. but the moment the geologist has to deal with large areas, or with completely separated deposits, the mischief of confounding that "homotaxis" or "similarity of arrangement," which _can_ be demonstrated, with "synchrony" or "identity of date," for which there is not a shadow of proof, under the one common term of "contemporaneity" becomes incalculable, and proves the constant source of gratuitous speculations. for anything that geology or palaeontology are able to show to the contrary, a devonian fauna and flora in the british islands may have been contemporaneous with silurian life in north america, and with a carboniferous fauna and flora in africa. geographical provinces and zones may have been as distinctly marked in the palaeozoic epoch as at present, and those seemingly sudden appearances of new genera and species, which we ascribe to new creation, may be simple results of migration. it may be so; it may be otherwise. in the present condition of our knowledge and of our methods, one verdict--"not proven, and not provable"--must be recorded against all the grand hypotheses of the palaeontologist respecting the general succession of life on the globe. the order and nature of terrestrial life, as a whole, are open questions. geology at present provides us with most valuable topographical records, but she has not the means of working them into a universal history. is such a universal history, then, to be regarded as unattainable? are all the grandest and most interesting problems which offer themselves to the geological student, essentially insoluble? is he in the position of a scientific tantalus--doomed always to thirst for a knowledge which he cannot obtain? the reverse is to be hoped; nay, it may not be impossible to indicate the source whence help will come. in commencing these remarks, mention was made of the great obligations under which the naturalist lies to the geologist and palaeontologist. assuredly the time will come when these obligations will be repaid tenfold, and when the maze of the world's past history, through which the pure geologist and the pure palaeontologist find no guidance, will be securely threaded by the clue furnished by the naturalist. all who are competent to express an opinion on the subject are, at present, agreed that the manifold varieties of animal and vegetable form have not either come into existence by chance, nor result from capricious exertions of creative power; but that they have taken place in a definite order, the statement of which order is what men of science term a natural law. whether such a law is to be regarded as an expression of the mode of operation of natural forces, or whether it is simply a statement of the manner in which a supernatural power has thought fit to act, is a secondary question, so long as the existence of the law and the possibility of its discovery by the human intellect are granted. but he must be a half-hearted philosopher who, believing in that possibility, and having watched the gigantic strides of the biological sciences during the last twenty years, doubts that science will sooner or later make this further step, so as to become possessed of the law of evolution of organic forms--of the unvarying order of that great chain of causes and effects of which all organic forms, ancient and modern, are the links. and then, if ever, we shall be able to begin to discuss, with profit, the questions respecting the commencement of life, and the nature of the successive populations of the globe, which so many seem to think are already answered. the preceding arguments make no particular claim to novelty; indeed they have been floating more or less distinctly before the minds of geologists for the last thirty years; and if, at the present time, it has seemed desirable to give them more definite and systematic expression, it is because palaeontology is every day assuming a greater importance, and now requires to rest on a basis the firmness of which is thoroughly well assured. among its fundamental conceptions, there must be no confusion between what is certain and what is more or less probable.[ ] but, pending the construction of a surer foundation than palaeontology now possesses, it may be instructive, assuming for the nonce the general correctness of the ordinary hypothesis of geological contemporaneity, to consider whether the deductions which are ordinarily drawn from the whole body of palaeontological facts are justifiable. [footnote : "le plus grand service qu'on puisse rendre à la science est d'y faire place nette avant d'y rien construire."--cuvier.] the evidence on which such conclusions are based is of two kinds, negative and positive. the value of negative evidence, in connection with this inquiry, has been so fully and clearly discussed in an address from the chair of this society,[ ] which none of us have forgotten, that nothing need at present be said about it; the more, as the considerations which have been laid before you have certainly not tended to increase your estimation of such evidence. it will be preferable to turn to the positive facts of palaeontology, and to inquire what they tell us. [footnote : anniversary address for , _quart. journ. geol. soc._ vol. vii.] we are all accustomed to speak of the number and the extent of the changes in the living population of the globe during geological time as something enormous: and indeed they are so, if we regard only the negative differences which separate the older rocks from the more modern, and if we look upon specific and generic changes as great changes, which from one point of view, they truly are. but leaving the negative differences out of consideration, and looking only at the positive data furnished by the fossil world from a broader point of view--from that of the comparative anatomist who has made the study of the greater modifications of animal form his chief business--a surprise of another kind dawns upon the mind; and under _this_ aspect the smallness of the total change becomes as astonishing as was its greatness under the other. there are two hundred known orders of plants; of these not one is certainly known to exist exclusively in the fossil state. the whole lapse of geological time has as yet yielded not a single new ordinal type of vegetable structure.[ ] [footnote : see hooker's _introductory essay to the flora of tasmania_, p. xxiii.] the positive change in passing from the recent to the ancient animal world is greater, but still singularly small. no fossil animal is so distinct from those now living as to require to be arranged even in a separate class from those which contain existing forms. it is only when we come to the orders, which may be roughly estimated at about a hundred and thirty, that we meet with fossil animals so distinct from those now living as to require orders for themselves; and these do not amount, on the most liberal estimate, to more than about per cent. of the whole. there is no certainly known extinct order of protozoa; there is but one among the coelenterata--that of the rugose corals; there is none among the mollusca; there are three, the cystidea, blastoidea, and edrioasterida, among the echinoderms; and two, the trilobita and eurypterida, among the crustacea; making altogether five for the great sub-kingdom of annulosa. among vertebrates there is no ordinally distinct fossil fish: there is only one extinct order of amphibia--the labyrinthodonts; but there are at least four distinct orders of reptilia, viz. the ichthyosauria, plesiosauria, pterosauria, dinosauria, and perhaps another or two. there is no known extinct order of birds, and no certainly known extinct order of mammals, the ordinal distinctness of the "toxodontia" being doubtful. the objection that broad statements of this kind, after all, rest largely on negative evidence is obvious, but it has less force than may at first be supposed; for, as might be expected from the circumstances of the case, we possess more abundant positive evidence regarding fishes and marine mollusks than respecting any other forms of animal life; and yet these offer us, through the whole range of geological time, no species ordinally distinct from those now living; while the far less numerous class of echinoderms presents three, and the crustacea two, such orders, though none of these come down later than the palaeozoic age. lastly, the reptilia present the extraordinary and exceptional phenomenon of as many extinct as existing orders, if not more; the four mentioned maintaining their existence from the lias to the chalk inclusive. some years ago one of your secretaries pointed out another kind of positive palaeontological evidence tending towards the same conclusion-- afforded by the existence of what he termed "persistent types" of vegetable and of animal life.[ ] he stated, on the authority of dr. hooker, that there are carboniferous plants which appear to be generically identical with some now living; that the cone of the oolitic _araucaria_ is hardly distinguishable from that of an existing species; that a true _pinus_ appears in the purbecks and a _juglans_ in the chalk; while, from the bagshot sands, a _banksia_, the wood of which is not distinguishable from that of species now living in australia, had been obtained. [footnote : see the abstract of a lecture "on the persistent types of animal life," in the _notices of the meetings of the royal institution of great britain_.--june , , vol. iii. p. .] turning to the animal kingdom, he affirmed the tabulate corals of the silurian rocks to be wonderfully like those which now exist; while even the families of the aporosa were all represented in the older mesozoic rocks. among the mollusca similar facts were adduced. let it be borne in mind that _avicula, mytilus, chiton, natica, patella, trochus, discina, orbicula, lingula, rhynchonclla_, and _nautilus_, all of which are existing _genera_, are given without a doubt as silurian in the last edition of "siluria"; while the highest forms of the highest cephalopods are represented in the lias by a genus _belemnoteuthis_, which presents the closest relation to the existing _loligo_. the two highest groups of the annulosa, the insecta and the arachnida, are represented in the coal, either by existing genera, or by forms differing from existing genera in quite minor peculiarities. turning to the vertebrata, the only palaeozoic elasmobranch fish of which we have any complete knowledge is the devonian and carboniferous _pleuracanthus_, which differs no more from existing sharks than these do from one another. again, vast as is the number of undoubtedly ganoid fossil fishes, and great as is their range in time, a large mass of evidence has recently been adduced to show that almost all those respecting which we possess sufficient information, are referable to the same sub-ordinal groups as the existing _lepidosteus, polypterus_, and sturgeon; and that a singular relation obtains between the older and the younger fishes; the former, the devonian ganoids, being almost all members of the same sub-order as _polypterus_, while the mesozoic ganoids are almost all similarly allied to _lepidosteus_.[ ] [footnote : "memoirs of the geological survey of the united kingdom.-- decade x. preliminary essay upon the systematic arrangement of the fishes of the devonian epoch."] again, what can be more remarkable than the singular constancy of structure preserved throughout a vast period of time by the family of the pycnodonts and by that of the true coelacanths; the former persisting, with but insignificant modifications, from the carboniferous to the tertiary rocks, inclusive; the latter existing, with still less change, from the carboniferous rocks to the chalk, inclusive? among reptiles, the highest living group, that of the crocodilia, is represented, at the early part of the mesozoic epoch, by species identical in the essential characters of their organisation with those now living, and differing from the latter only in such matters as the form of the articular facets of the vertebral centra, in the extent to which the nasal passages are separated from the cavity of the mouth by bone, and in the proportions of the limbs. and even as regards the mammalia, the scanty remains of triassic and oolitic species afford no foundation for the supposition that the organisation of the oldest forms differed nearly so much from some of those which now live as these differ from one another. it is needless to multiply these instances; enough has been said to justify the statement that, in view of the immense diversity of known animal and vegetable forms, and the enormous lapse of time indicated by the accumulation of fossiliferous strata, the only circumstance to be wondered at is, not that the changes of life, as exhibited by positive evidence, have been so great but that they have been so small. be they great or small, however, it is desirable to attempt to estimate them. let us, therefore, take each great division of the animal world in succession, and, whenever an order or a family can be shown to have had a prolonged existence, let us endeavour to ascertain how far the later members of the group differ from the earlier ones. if these later members, in all or in many cases, exhibit a certain amount of modification, the fact is, so far, evidence in favour of a general law of change; and, in a rough way, the rapidity of that change will be measured by the demonstrable amount of modification. on the other hand, it must be recollected that the absence of any modification, while it may leave the doctrine of the existence of a law of change without positive support, cannot possibly disprove all forms of that doctrine, though it may afford a sufficient refutation of many of them. the protozoa.--the protozoa are represented throughout the whole range of geological series, from the lower silurian formation to the present day. the most ancient forms recently made known by ehrenberg are exceedingly like those which now exist: no one has ever pretended that the difference between any ancient and any modern foraminifera is of more than generic value, nor are the oldest foraminifera either simpler, more embryonic, or less differentiated, than the existing forms. the coelenterata.--the tabulate corals have existed from the silurian epoch to the present day, but i am not aware that the ancient _heliolites_ possesses a single mark of a more embryonic or less differentiated character, or less high organisation, than the existing _heliopora_. as for the aporose corals, in what respect is the silurian _paloeocyclus_ less highly organised or more embryonic than the modern _fungia_, or the liassic aporosa than the existing members of the same families? the _mollusca_--in what sense is the living _waldheimia_ less embryonic, or more specialised, than the palaeozoic _spirifer_; or the existing _rhynchonelloe, cranioe, discinoe, linguloe_, than the silurian species of the same genera? in what sense can _loligo_ or _spirula_ be said to be more specialised, or less embryonic, than _belemnites_; or the modern species of lamellibranch and gasteropod genera, than the silurian species of the same genera? the annulosa.--the carboniferous insecta and arachnida are neither less specialised, nor more embryonic, than these that now live, nor are the liassic cirripedia and macrura; while several of the brachyura, which appear in the chalk, belong to existing genera; and none exhibit either an intermediate, or an embryonic, character. the vertebrata.--among fishes i have referred to the coelacanthini (comprising the genera _coelacanthus, holophagus, undina_, and _macropoma_) as affording an example of a persistent type; and it is most remarkable to note the smallness of the differences between any of these fishes (affecting at most the proportions of the body and fins, and the character and sculpture of the scales), notwithstanding their enormous range in time. in all the essentials of its very peculiar structure, the _macropoma_ of the chalk is identical with the _coelacanthus_ of the coal. look at the genus _lepidotus_, again, persisting without a modification of importance from the liassic to the eocene formations inclusively. or among the teleostei--in what respect is the _beryx_ of the chalk more embryonic, or less differentiated, than _beryx lineatus_ of king george's sound? or to turn to the higher vertebrata--in what sense are the liassic chelonia inferior to those which now exist? how are the cretaceous ichthyosauria, plesiosauria, or pterosauria less embryonic, or more differentiated, species than those of the lias? or lastly, in what circumstance is the _phascolotherium_ more embryonic, or of a more generalised type, than the modern opossum; or a _lophiodon_, or a _paloeotherium_, than a modern _tapirus_ or _hyrax_? these examples might be almost indefinitely multiplied, but surely they are sufficient to prove that the only safe and unquestionable testimony we can procure--positive evidence--fails to demonstrate any sort of progressive modification towards a less embryonic, or less generalised, type in a great many groups of animals of long-continued geological existence. in these groups there is abundant evidence of variation--none of what is ordinarily understood as progression; and, if the known geological record is to be regarded as even any considerable fragment of the whole, it is inconceivable that any theory of a necessarily progressive development can stand, for the numerous orders and families cited afford no trace of such a process. but it is a most remarkable fact, that, while the groups which have been mentioned, and many besides, exhibit no sign of progressive modification, there are others, co-existing with them, under the same conditions, in which more or less distinct indications of such a process seems to be traceable. among such indications i may remind you of the predominance of holostome gasteropoda in the older rocks as compared with that of siphonostone gasteropoda in the later. a case less open to the objection of negative evidence, however, is that afforded by the tetrabranchiate cephalopoda, the forms of the shells and of the septal sutures exhibiting a certain increase of complexity in the newer genera. here, however, one is met at once with the occurrence of _orthoceras_ and _baculites_ at the two ends of the series, and of the fact that one of the simplest genera, _nautilus_, is that which now exists. the crinoidea, in the abundance of stalked forms in the ancient formations as compared with their present rarity, seem to present us with a fair case of modification from a more embryonic towards a less embryonic condition. but then, on careful consideration of the facts, the objection arises that the stalk, calyx, and arms of the palaeozoic crinoid are exceedingly different from the corresponding organs of a larval _comatula_; and it might with perfect justice be argued that _actinocrinus_ and _eucalyptocrinus_, for example, depart to the full as widely, in one direction, from the stalked embryo of _comatula_, as _comatula_ itself does in the other. the echinidea, again, are frequently quoted as exhibiting a gradual passage from a more generalised to a more specialised type, seeing that the elongated, or oval, spatangoids appear after the spheroidal echinoids. but here it might be argued, on the other hand, that the spheroidal echinoids, in reality, depart further from the general plan and from the embryonic form than the elongated spatangoids do; and that the peculiar dental apparatus and the pedicellariae of the former are marks of at least as great differentiation as the petaloid ambulacra and semitae of the latter. once more, the prevalence of macrurous before brachyurous podophthalmia is, apparently, a fair piece of evidence in favour of progressive modification in the same order of crustacea; and yet the case will not stand much sifting, seeing that the macrurous podophthalmia depart as far in one direction from the common type of podophthalmia, or from any embryonic condition of the brachyura, as the brachyura do in the other; and that the middle terms between macrura and brachyura--the anomura--are little better represented in the older mesozoic rocks than the brachyura are. none of the cases of progressive modification which are cited from among the invertebrata appear to me to have a foundation less open to criticism than these; and if this be so, no careful reasoner would, i think, be inclined to lay very great stress upon them. among the vertebrata, however, there are a few examples which appear to be far less open to objection. it is, in fact, true of several groups of vertebrata which have lived through a considerable range of time, that the endoskeleton (more particularly the spinal column) of the older genera presents a less ossified, and, so far, less differentiated, condition than that of the younger genera. thus the devonian ganoids, though almost all members of the same sub-order as _polypterus_, and presenting numerous important resemblances to the existing genus, which possesses biconclave vertebrae, are, for the most part, wholly devoid of ossified vertebral centra. the mesozoic lepidosteidae, again, have, at most, biconcave vertebrae, while the existing _lepidosteus_ has salamandroid, opisthocoelous, vertebrae. so, none of the palaeozoic sharks have shown themselves to be possessed of ossified vertebrae, while the majority of modern sharks possess such vertebrae. again, the more ancient crocodilia and lacertilia have vertebrae with the articular facets of their centra flattened or biconcave, while the modern members of the same group have them procoelous. but the most remarkable examples of progressive modification of the vertebral column, in correspondence with geological age, are those afforded by the pycnodonts among fish, and the labyrinthodonts among amphibia. the late able ichthyologist heckel pointed out the fact, that, while the pycnodonts never possess true vertebral centra, they differ in the degree of expansion and extension of the ends of the bony arches of the vertebrae upon the sheath of the notochord; the carboniferous forms exhibiting hardly any such expansion, while the mesozoic genera present a greater and greater development, until, in the tertiary forms, the expanded ends become suturally united so as to form a sort of false vertebra. hermann von meyer, again, to whose luminous researches we are indebted for our present large knowledge of the organisation of the older labyrinthodonts, has proved that the carboniferous _archegosaurus_ had very imperfectly developed vertebral centra, while the triassic _mastodonsaurus_ had the same parts completely ossified.[ ] [footnote : as this address is passing through the press (march , ), evidence lies before me of the existence of a new labyrinthodont (_pholidogaster_), from the edinburgh coal-field with well-ossified vertebral centra.] the regularity and evenness of the dentition of the _anoplotherium_, as contrasted with that of existing artiodactyles, and the assumed nearer approach of the dentition of certain ancient carnivores to the typical arrangement, have also been cited as exemplifications of a law of progressive development, but i know of no other cases based on positive evidence which are worthy of particular notice. what then does an impartial survey of the positively ascertained truths of palaeontology testify in relation to the common doctrines of progressive modification, which suppose that modification to have taken place by a necessary progress from more to less embryonic forms, or from more to less generalised types, within the limits of the period represented by the fossiliferous rocks? it negatives those doctrines; for it either shows us no evidence of any such modification, or demonstrates it to have been very slight; and as to the nature of that modification, it yields no evidence whatsoever that the earlier members of any long-continued group were more generalised in structure than the later ones. to a certain extent, indeed, it may be said that imperfect ossification of the vertebral column is an embryonic character; but, on the other hand, it would be extremely incorrect to suppose that the vertebral columns of the older vertebrata are in any sense embryonic in their whole structure. obviously, if the earliest fossiliferous rocks now known are coëval with the commencement of life, and if their contents give us any just conception of the nature and the extent of the earliest fauna and flora, the insignificant amount of modification which can be demonstrated to have taken place in any one group of animals, or plants, is quite incompatible with the hypothesis that all living forms are the results of a necessary process of progressive development, entirely comprised within the time represented by the fossiliferous rocks. contrariwise, any admissible hypothesis of progressive modification must be compatible with persistence without progression, through indefinite periods. and should such an hypothesis eventually be proved to be true, in the only way in which it can be demonstrated, viz. by observation and experiment upon the existing forms of life, the conclusion will inevitably present itself, that the palaeozoic mesozoic, and cainozoic faunae and florae, taken together, bear somewhat the same proportion to the whole series of living beings which have occupied this globe, as the existing fauna and flora do to them. such are the results of palaeontology as they appear, and have for some years appeared, to the mind of an inquirer who regards that study simply as one of the applications of the great biological sciences, and who desires to see it placed upon the same sound basis as other branches of physical inquiry. if the arguments which have been brought forward are valid, probably no one, in view of the present state of opinion, will be inclined to think the time wasted which has been spent upon their elaboration. x geological reform [ ] "a great reform in geological speculation seems now to have become necessary." "it is quite certain that a great mistake has been made--that british popular geology at the present time is in direct opposition to the principles of natural philosophy."[ ] [footnote : on geological time. by sir w. thomson, ll.d. _transactions of the geological society of glasgow_, vol. iii.] in reviewing the course of geological thought during the past year, for the purpose of discovering those matters to which i might most fitly direct your attention in the address which it now becomes my duty to deliver from the presidential chair, the two somewhat alarming sentences which i have just read, and which occur in an able and interesting essay by an eminent natural philosopher, rose into such prominence before my mind that they eclipsed everything else. it surely is a matter of paramount importance for the british geologists (some of them very popular geologists too) here in solemn annual session assembled, to inquire whether the severe judgment thus passed upon them by so high an authority as sir william thomson is one to which they must plead guilty _sans phrase_, or whether they are prepared to say "not guilty," and appeal for a reversal of the sentence to that higher court of educated scientific opinion to which we are all amenable. as your attorney-general for the time being, i thought i could not do better than get up the case with a view of advising you. it is true that the charges brought forward by the other side involve the consideration of matters quite foreign to the pursuits with which i am ordinarily occupied; but, in that respect, i am only in the position which is, nine times out of ten, occupied by counsel, who nevertheless contrive to gain their causes, mainly by force of mother-wit and common-sense, aided by some training in other intellectual exercises. nerved by such precedents, i proceed to put my pleading before you. and the first question with which i propose to deal is, what is it to which sir w. thomson refers when he speaks of "geological speculation" and "british popular geology"? i find three, more or less contradictory, systems of geological thought, each of which might fairly enough claim these appellations, standing side by side in britain. i shall call one of them catastrophism, another uniformitarianism, the third evolutionism; and i shall try briefly to sketch the characters of each, that you may say whether the classification is, or is not, exhaustive. by catastrophism, i mean any form of geological speculation which, in order to account for the phenomena of geology, supposes the operation of forces different in their nature, or immeasurably different in power, from those which we at present see in action in the universe. the mosaic cosmogony is, in this sense, catastrophic, because it assumes the operation of extra-natural power. the doctrine of violent upheavals, _débâcles_, and cataclysms in general, is catastrophic, so far as it assumes that these were brought about by causes which have now no parallel. there was a time when catastrophism might, pre-eminently, have claimed the title of "british popular geology"; and assuredly it has yet many adherents, and reckons among its supporters some of the most honoured members of this society. by uniformitarianism, i mean especially, the teaching of hutton and of lyell. that great though incomplete work, "the theory of the earth," seems to me to be one of the most remarkable contributions to geology which is recorded in the annals of the science. so far as the not-living world is concerned, uniformitarianism lies there, not only in germ, but in blossom and fruit. if one asks how it is that hutton was led to entertain views so far in advance of those prevalent in his time, in some respects; while, in others, they seem almost curiously limited, the answer appears to me to be plain. hutton was in advance of the geological speculation of his time, because, in the first place, he had amassed a vast store of knowledge of the facts of geology, gathered by personal observation in travels of considerable extent; and because, in the second place, he was thoroughly trained in the physical and chemical science of his day, and thus possessed, as much as any one in his time could possess it, the knowledge which is requisite for the just interpretation of geological phenomena, and the habit of thought which fits a man for scientific inquiry. it is to this thorough scientific training that i ascribe hutton's steady and persistent refusal to look to other causes than those now in operation, for the explanation of geological phenomena. thus he writes:--"i do not pretend, as he [m. de luc] does in his theory, to describe the beginning of things. i take things such as i find them at present; and from these i reason with regard to that which must have been."[ ] [footnote : _the theory of the earth_, vol. i. p. , note.] and again:--"a theory of the earth, which has for object truth, can have no retrospect to that which had preceded the present order of the world; for this order alone is what we have to reason upon; and to reason without data is nothing but delusion. a theory, therefore, which is limited to the actual constitution of this earth cannot be allowed to proceed one step beyond the present order of things."[ ] [footnote : _ibid._, vol. i. p. .] and so clear is he, that no causes beside such as are now in operation are needed to account for the character and disposition of the components of the crust of the earth, that he says, broadly and boldly:--" ... there is no part of the earth which has not had the same origin, so far as this consists in that earth being collected at the bottom of the sea, and afterwards produced, as land, along with masses of melted substances, by the operation of mineral causes."[ ] [footnote : _ibid._. p. .] but other influences were at work upon hutton beside those of a mind logical by nature, and scientific by sound training; and the peculiar turn which his speculations took seems to me to be unintelligible, unless these be taken into account. the arguments of the french astronomers and mathematicians, which, at the end of the last century, were held to demonstrate the existence of a compensating arrangement among the celestial bodies, whereby all perturbations eventually reduced themselves to oscillations on each side of a mean position, and the stability of the solar system was secured, had evidently taken strong hold of hutton's mind. in those oddly constructed periods which seem to have prejudiced many persons against reading his works, but which are full of that peculiar, if unattractive, eloquence which flows from mastery of the subject, hutton says:-- "we have now got to the end of our reasoning; we have no data further to conclude immediately from that which actually is. but we have got enough; we have the satisfaction to find, that in nature there is wisdom, system, and consistency. for having, in the natural history of this earth, seen a succession of worlds, we may from this conclude that there is a system in nature; in like manner as, from seeing revolutions of the planets, it is concluded, that there is a system by which they are intended to continue those revolutions. but if the succession of worlds is established in the system of nature, it is in vain to look for anything higher in the origin of the earth. the result, therefore, of this physical inquiry is, that we find no vestige of a beginning,--no prospect of an end."[ ] [footnote : _ibid._, vol. i. p. .] yet another influence worked strongly upon hutton. like most philosophers of his age, he coquetted with those final causes which have been named barren virgins, but which might be more fitly termed the _hetairoe_ of philosophy, so constantly have they led men astray. the final cause of the existence of the world is, for hutton, the production of life and intelligence. "we have now considered the globe of this earth as a machine, constructed upon chemical as well as mechanical principles, by which its different parts are all adapted, in form, in quality, and in quantity, to a certain end; an end attained with certainty or success; and an end from which we may perceive wisdom, in contemplating the means employed. "but is this world to be considered thus merely as a machine, to last no longer than its parts retain their present position, their proper forms and qualities? or may it not be also considered as an organised body? such as has a constitution in which the necessary decay of the machine is naturally repaired, in the exertion of those productive powers by which it had been formed. "this is the view in which we are now to examine the globe; to see if there be, in the constitution of this world, a reproductive operation, by which a ruined constitution may be again repaired, and a duration or stability thus procured to the machine, considered as a world sustaining plants and animals."[ ] [footnote : _ibid._, vol. i. pp. , .] kirwan, and the other philistines of the day, accused hutton of declaring that his theory implied that the world never had a beginning, and never differed in condition from its present state. nothing could be more grossly unjust, as he expressly guards himself against any such conclusion in the following terms:-- "but in thus tracing back the natural operations which have succeeded each other, and mark to us the course of time past, we come to a period in which we cannot see any farther. this, however, is not the beginning of the operations which proceed in time and according to the wise economy of this world; nor is it the establishing of that which, in the course of time, had no beginning; it is only the limit of our retrospective view of those operations which have come to pass in time, and have been conducted by supreme intelligence."[ ] [footnote : _ibid._, vol. i. p. .] i have spoken of uniformitarianism as the doctrine of hutton and of lyell. if i have quoted the older writer rather than the newer, it is because his works are little known, and his claims on our veneration too frequently forgotten, not because i desire to dim the fame of his eminent successor. few of the present generation of geologists have read playfair's "illustrations," fewer still the original "theory of the earth"; the more is the pity; but which of us has not thumbed every page of the "principles of geology"? i think that he who writes fairly the history of his own progress in geological thought, will not be able to separate his debt to hutton from his obligations to lyell; and the history of the progress of individual geologists is the history of geology. no one can doubt that the influence of uniformitarian views has been enormous, and, in the main, most beneficial and favourable to the progress of sound geology. nor can it be questioned that uniformitarianism has even a stronger title than catastrophism to call itself the geological speculation of britain, or, if you will, british popular geology. for it is eminently a british doctrine, and has even now made comparatively little progress on the continent of europe. nevertheless, it seems to me to be open to serious criticism upon one of its aspects. i have shown how unjust was the insinuation that hutton denied a beginning to the world. but it would not be unjust to say that he persistently in practice, shut his eyes to the existence of that prior and different state of things which, in theory, he admitted; and, in this aversion to look beyond the veil of stratified rocks, lyell follows him. hutton and lyell alike agree in their indisposition to carry their speculations a step beyond the period recorded in the most ancient strata now open to observation in the crust of the earth. this is, for hutton, "the point in which we cannot see any farther"; while lyell tells us,-- "the astronomer may find good reasons for ascribing the earth's form to the original fluidity of the mass, in times long antecedent to the first introduction of living beings into the planet; but the geologist must be content to regard the earliest monuments which it is his task to interpret, as belonging to a period when the crust had already acquired great solidity and thickness, probably as great as it now possesses, and when volcanic rocks, not essentially differing from those now produced, were formed from time to time, the intensity of volcanic heat being neither greater nor less than it is now."[ ] [footnote : _principles of geology_, vol. ii. p. .] and again, "as geologists, we learn that it is not only the present condition of the globe which has been suited to the accommodation of myriads of living creatures, but that many former states also have been adapted to the organisation and habits of prior races of beings. the disposition of the seas, continents and islands, and the climates, have varied; the species likewise have been changed; and yet they have all been so modelled, on types analogous to those of existing plants and animals, as to indicate, throughout, a perfect harmony of design and unity of purpose. to assume that the evidence of the beginning, or end, of so vast a scheme lies within the reach of our philosophical inquiries, or even of our speculations, appears to be inconsistent with a just estimate of the relations which subsist between the finite powers of man and the attributes of an infinite and eternal being."[ ] [footnote : _ibid._, vol. ii. p. .] the limitations implied in these passages appear to me to constitute the weakness and the logical defect of uniformitarianism. no one will impute blame to hutton that, in face of the imperfect condition, in his day, of those physical sciences which furnish the keys to the riddles of geology, he should have thought it practical wisdom to limit his theory to an attempt to account for "the present order of things"; but i am at a loss to comprehend why, for all time, the geologist must be content to regard the oldest fossiliferous rocks as the _ultima thule_ of his science; or what there is inconsistent with the relations between the finite and the infinite mind, in the assumption, that we may discern somewhat of the beginning, or of the end, of this speck in space we call our earth. the finite mind is certainly competent to trace out the development of the fowl within the egg; and i know not on what ground it should find more difficulty in unravelling the complexities of the development of the earth. in fact, as kant has well remarked,[ ] the cosmical process is really simpler than the biological. [footnote : "man darf es sich also nicht befremden lassen, wenn ich mich unterstehe zu sagen, dass eher die bildung aller himmelskörper, die ursache ihrer bewegungen, kurz der ursprung der gantzen gegenwärtigen verfassung des weltbaues werden können eingesehen werden, ehe die erzeugung eines einzigen krautes oder einer raupe aus mechanischen gründen, deutlich und vollständig kund werden wird."--kant's _sämmtliche werke_, bd. i. p. .] this attempt to limit, at a particular point, the progress of inductive and deductive reasoning from the things which are, to those which were-- this faithlessness to its own logic, seems to me to have cost uniformitarianism the place, as the permanent form of geological speculation, which it might otherwise have held. it remains that i should put before you what i understand to be the third phase of geological speculation--namely, evolutionism. i shall not make what i have to say on this head clear, unless i diverge, or seem to diverge, for a while, from the direct path of my discourse, so far as to explain what i take to be the scope of geology itself. i conceive geology to be the history of the earth, in precisely the same sense as biology is the history of living beings; and i trust you will not think that i am overpowered by the influence of a dominant pursuit if i say that i trace a close analogy between these two histories. if i study a living being, under what heads does the knowledge i obtain fall? i can learn its structure, or what we call its anatomy; and its development, or the series of changes which it passes through to acquire its complete structure. then i find that the living being has certain powers resulting from its own activities, and the interaction of these with the activities of other things--the knowledge of which is physiology. beyond this the living being has a position in space and time, which is its distribution. all these form the body of ascertainable facts which constitute the _status quo_ of the living creature. but these facts have their causes; and the ascertainment of these causes is the doctrine of aetiology. if we consider what is knowable about the earth, we shall find that such earth-knowledge--if i may so translate the word geology--falls into the same categories. what is termed stratigraphical geology is neither more nor less than the anatomy of the earth; and the history of the succession of the formations is the history of a succession of such anatomies, or corresponds with development, as distinct from generation. the internal heat of the earth, the elevation and depression of its crust, its belchings forth of vapours, ashes, and lava, are its activities, in as strict a sense as are warmth and the movements and products of respiration the activities of an animal. the phenomena of the seasons, of the trade winds, of the gulf-stream, are as much the results of the reaction between these inner activities and outward forces, as are the budding of the leaves in spring and their falling in autumn the effects of the interaction between the organisation of a plant and the solar light and heat. and, as the study of the activities of the living being is called its physiology, so are these phenomena the subject-matter of an analogous telluric physiology, to which we sometimes give the name of meteorology, sometimes that of physical geography, sometimes that of geology. again, the earth has a place in space and in time, and relations to other bodies in both these respects, which constitute its distribution. this subject is usually left to the astronomer; but a knowledge of its broad outlines seems to me to be an essential constituent of the stock of geological ideas. all that can be ascertained concerning the structure, succession of conditions, actions, and position in space of the earth, is the matter of fact of its natural history. but, as in biology, there remains the matter of reasoning from these facts to their causes, which is just as much science as the other, and indeed more; and this constitutes geological aetiology. having regard to this general scheme of geological knowledge and thought, it is obvious that geological speculation may be, so to speak, anatomical and developmental speculation, so far as it relates to points of stratigraphical arrangement which are out of reach of direct observation; or, it may be physiological speculation so far as it relates to undetermined problems relative to the activities of the earth; or, it may be distributional speculation, if it deals with modifications of the earth's place in space; or, finally, it will be aetiological speculation if it attempts to deduce the history of the world, as a whole, from the known properties of the matter of the earth, in the conditions in which the earth has been placed. for the purposes of the present discourse i may take this last to be what is meant by "geological speculation." now uniformitarianism, as we have seen, tends to ignore geological speculation in this sense altogether. the one point the catastrophists and the uniformitarians agreed upon, when this society was founded, was to ignore it. and you will find, if you look back into our records, that our revered fathers in geology plumed themselves a good deal upon the practical sense and wisdom of this proceeding. as a temporary measure, i do not presume to challenge its wisdom; but in all organised bodies temporary changes are apt to produce permanent effects; and as time has slipped by, altering all the conditions which may have made such mortification of the scientific flesh desirable, i think the effect of the stream of cold water which has steadily flowed over geological speculation within these walls has been of doubtful beneficence. the sort of geological speculation to which i am now referring (geological aetiology, in short) was created, as a science, by that famous philosopher immanuel kant, when, in , he wrote his "general natural history and theory of the celestial bodies; or an attempt to account for the constitutional and the mechanical origin of the universe upon newtonian principles."[ ] [footnote : grant (_history of physical astronomy_, p. ) makes but the briefest reference to kant.] in this very remarkable but seemingly little-known treatise,[ ] kant expounds a complete cosmogony, in the shape of a theory of the causes which have led to the development of the universe from diffused atoms of matter endowed with simple attractive and repulsive forces. [footnote : "allgemeine naturgeschichte und theorie des himmels; oder versuch von der verfassung und dem mechanischen ursprunge des ganzen weltgebäudes nach newton'schen grundsatzen abgehandelt."--kant's _sämmtliche werke_, bd. i. p. .] "give me matter," says kant, "and i will build the world;" and he proceeds to deduce from the simple data from which he starts, a doctrine in all essential respects similar to the well-known "nebular hypothesis" of laplace.[ ] he accounts for the relation of the masses and the densities of the planets to their distances from the sun, for the eccentricities of their orbits, for their rotations, for their satellites, for the general agreement in the direction of rotation among the celestial bodies, for saturn's ring, and for the zodiacal light. he finds in each system of worlds, indications that the attractive force of the central mass will eventually destroy its organisation, by concentrating upon itself the matter of the whole system; but, as the result of this concentration, he argues for the development of an amount of heat which will dissipate the mass once more into a molecular chaos such as that in which it began. [footnote : _système du monde_, tome ii. chap. .] kant pictures to himself the universe as once an infinite expansion of formless and diffused matter. at one point of this he supposes a single centre of attraction set up; and, by strict deductions from admitted dynamical principles, shows how this must result in the development of a prodigious central body, surrounded by systems of solar and planetary worlds in all stages of development. in vivid language he depicts the great world-maelstrom, widening the margins of its prodigious eddy in the slow progress of millions of ages, gradually reclaiming more and more of the molecular waste, and converting chaos into cosmos. but what is gained at the margin is lost in the centre; the attractions of the central systems bring their constituents together, which then, by the heat evolved, are converted once more into molecular chaos. thus the worlds that are, lie between the ruins of the worlds that have been, and the chaotic materials of the worlds that shall be; and in spite of all waste and destruction, cosmos is extending his borders at the expense of chaos. kant's further application of his views to the earth itself is to be found in his "treatise on physical geography"[ ] (a term under which the then unknown science of geology was included), a subject which he had studied with very great care and on which he lectured for many years. the fourth section of the first part of this treatise is called "history of the great changes which the earth has formerly undergone and is still undergoing," and is, in fact, a brief and pregnant essay upon the principles of geology. kant gives an account first "of the gradual changes which are now taking place" under the heads of such as are caused by earthquakes, such as are brought about by rain and rivers, such as are effected by the sea, such as are produced by winds and frost; and, finally, such as result from the operations of man. [footnote : kant's _sämmtliche werke_, bd. viii. p. .] the second part is devoted to the "memorials of the changes which the earth has undergone in remote antiquity." these are enumerated as:--a. proofs that the sea formerly covered the whole earth. b. proofs that the sea has often been changed into dry land and then again into sea. c. a discussion of the various theories of the earth put forward by scheuchzer, moro, bonnet, woodward, white, leibnitz, linnaeus, and buffon. the third part contains an "attempt to give a sound explanation of the ancient history of the earth." i suppose that it would be very easy to pick holes in the details of kant's speculations, whether cosmological, or specially telluric, in their application. but for all that, he seems to me to have been the first person to frame a complete system of geological speculation by founding the doctrine of evolution. with as much truth as hutton, kant could say, "i take things just as i find them at present, and, from these, i reason with regard to that which must have been." like hutton, he is never tired of pointing out that "in nature there is wisdom, system, and consistency." and, as in these great principles, so in believing that the cosmos has a reproductive operation "by which a ruined constitution may be repaired," he forestalls hutton; while, on the other hand, kant is true to science. he knows no bounds to geological speculation but those of the intellect. he reasons back to a beginning of the present state of things; he admits the possibility of an end. i have said that the three schools of geological speculation which i have termed catastrophism, uniformitarianism, and evolutionism, are commonly supposed to be antagonistic to one another; and i presume it will have become obvious that in my belief, the last is destined to swallow up the other two. but it is proper to remark that each of the latter has kept alive the tradition of precious truths. catastrophism has insisted upon the existence of a practically unlimited bank of force, on which the theorist might draw; and it has cherished the idea of the development of the earth from a state in which its form, and the forces which it exerted, were very different from those we now know. that such difference of form and power once existed is a necessary part of the doctrine of evolution. uniformitarianism, on the other hand, has with equal justice insisted upon a practically unlimited bank of time, ready to discount any quantity of hypothetical paper. it has kept before our eyes the power of the infinitely little, time being granted, and has compelled us to exhaust known causes, before flying to the unknown. to my mind there appears to be no sort of necessary theoretical antagonism between catastrophism and uniformitarianism. on the contrary, it is very conceivable that catastrophes may be part and parcel of uniformity. let me illustrate my case by analogy. the working of a clock is a model of uniform action; good time-keeping means uniformity of action. but the striking of the clock is essentially a catastrophe; the hammer might be made to blow up a barrel of gunpowder, or turn on a deluge of water; and, by proper arrangement, the clock, instead of marking the hours, might strike at all sorts of irregular periods, never twice alike, in the intervals, force, or number of its blows. nevertheless, all these irregular, and apparently lawless, catastrophes would be the result of an absolutely uniformitarian action; and we might have two schools of clock-theorists, one studying the hammer and the other the pendulum. still less is there any necessary antagonists between either of these doctrines and that of evolution, which embraces all that is sound in both catastrophism and uniformitarianism, while it rejects the arbitrary assumptions of the one and the, as arbitrary, limitations of the other. nor is the value of the doctrine of evolution to the philosophic thinker diminished by the fact that it applies the same method to the living and the not-living world; and embraces, in one stupendous analogy, the growth of a solar system from molecular chaos, the shaping of the earth from the nebulous cub-hood of its youth, through innumerable changes and immeasurable ages, to its present form; and the development of a living being from the shapeless mass of protoplasm we term a germ. i do not know whether evolutionism can claim that amount of currency which would entitle it to be called british popular geology; but, more or less vaguely, it is assuredly present in the minds of most geologists. such being the three phases of geological speculation, we are now in position to inquire which of these it is that sir william thomson calls upon us to reform in the passages which i have cited. it is obviously uniformitarianism which the distinguished physicist takes to be the representative of geological speculation in general. and thus a first issue is raised, inasmuch as many persons (and those not the least thoughtful among the younger geologists) do not accept strict uniformitarianism as the final form of geological speculation. we should say, if hutton and playfair declare the course of the world to have been always the same, point out the fallacy by all means; but, in so doing, do not imagine that you are proving modern geology to be in opposition to natural philosophy. i do not suppose that, at the present day, any geologist would be found to maintain absolute uniformitarianism, to deny that the rapidity of the rotation of the earth _may_ be diminishing, that the sun _may_ be waxing dim, or that the earth itself _may_ be cooling. most of us, i suspect, are gallios, "who care for none of these things," being of opinion that, true or fictitious, they have made no practical difference to the earth, during the period of which a record is preserved in stratified deposits. the accusation that we have been running counter to the _principles_ of natural philosophy, therefore, is devoid of foundation. the only question which can arise is whether we have, or have not, been tacitly making assumptions which are in opposition to certain conclusions which may be drawn from those principles. and this question subdivides itself into two:--the first, are we really contravening such conclusions? the second, if we are, are those conclusions so firmly based that we may not contravene them? i reply in the negative to both these questions, and i will give you my reasons for so doing. sir william thomson believes that he is able to prove, by physical reasonings, "that the existing state of things on the earth, life on the earth--all geological history showing continuity of life--must be limited within some such period of time as one hundred million years" (_loc. cit._ p. ). the first inquiry which arises plainly is, has it ever been denied that this period _may_ be enough for the purposes of geology? the discussion of this question is greatly embarrassed by the vagueness with which the assumed limit is, i will not say defined, but indicated,-- "some such period of past time as one hundred million years." now does this mean that it may have been two, or three, or four hundred million years? because this really makes all the difference.[ ] [footnote : sir william thomson implies (_loc. cit_. p. ) that the precise time is of no consequence: "the principle is the same"; but, as the principle is admitted, the whole discussion turns on its practical results.] i presume that , feet may be taken as a full allowance for the total thickness of stratified rocks containing traces of life; , divided by , , = . . consequently, the deposit of , feet of stratified rock in , , years means that the deposit has taken place at the rate of / of a foot, or, say, / of an inch, per annum. well, i do not know that any one is prepared to maintain that, even making all needful allowances, the stratified rocks may not have been formed, on the average, at the rate of / of an inch per annum. i suppose that if such could be shown to be the limit of world-growth, we could put up with the allowance without feeling that our speculations had undergone any revolution. and perhaps, after all, the qualifying phrase "some such period" may not necessitate the assumption of more than / or / or / of an inch of deposit per year, which, of course, would give us still more ease and comfort. but, it may be said, that it is biology, and not geology, which asks for so much time--that the succession of life demands vast intervals; but this appears to me to be reasoning in a circle. biology takes her time from geology. the only reason we have for believing in the slow rate of the change in living forms is the fact that they persist through a series of deposits which, geology informs us, have taken a long while to make. if the geological clock is wrong, all the naturalist will have to do is to modify his notions of the rapidity of change accordingly. and i venture to point out that, when we are told that the limitation of the period during which living beings have inhabited this planet to one, two, or three hundred million years requires a complete revolution in geological speculation, the _onus probandi_ rests on the maker of the assertion, who brings forward not a shadow of evidence in its support. thus, if we accept the limitation of time placed before us by sir w. thomson, it is not obvious, on the face of the matter, that we shall have to alter, or reform, our ways in any appreciable degree; and we may therefore proceed with much calmness, and indeed much indifference, as to the result, to inquire whether that limitation is justified by the arguments employed in its support. these arguments are three in number.-- i. the first is based upon the undoubted fact that the tides tend to retard the rate of the earth's rotation upon its axis. that this must be so is obvious, if one considers, roughly, that the tides result from the pull which the sun and the moon exert upon the sea, causing it to act as a sort of break upon the rotating solid earth. kant, who was by no means a mere "abstract philosopher," but a good mathematician and well versed in the physical science of his time, not only proved this in an essay of exquisite clearness and intelligibility, now more than a century old,[ ] but deduced from it some of its more important consequences, such as the constant turning of one face of the moon towards the earth. [footnote : "untersuchung der frage oh die erde in ihrer umdrehung um die achse, wodurch sie die abwechselung des tages und der nacht hervorbringt, einige veränderung seit den ersten zeiten ihres ursprunges erlitten habe, &c."--kant's _sämmntliche werke_, bd. i. p. .] but there is a long step from the demonstration of a tendency to the estimation of the practical value of that tendency, which is all with which we are at present concerned. the facts bearing on this point appear to stand as follows:-- it is a matter of observation that the moon's mean motion is (and has for the last , years been) undergoing an acceleration, relatively to the rotation of the earth. of course this may result from one of two causes: the moon may really have been moving more swiftly in its orbit; or the earth may have been rotating more slowly on its axis. laplace believed he had accounted for this phenomenon by the fact that the eccentricity of the earth's orbit has been diminishing throughout these , years. this would produce a diminution of the mean attraction of the sun on the moon; or, in other words, an increase in the attraction of the earth on the moon; and, consequently, an increase in the rapidity of the orbital motion of the latter body. laplace, therefore, laid the responsibility of the acceleration upon the moon, and if his views were correct, the tidal retardation must either be insignificant in amount, or be counteracted by some other agency. our great astronomer, adams, however, appears to have found a flaw in laplace's calculation, and to have shown that only half the observed retardation could be accounted for in the way he had suggested. there remains, therefore, the other half to be accounted for; and here, in the absence of all positive knowledge, three sets of hypotheses have been suggested. (_a_.) m. delaunay suggests that the earth is at fault, in consequence of the tidal retardation. messrs. adams, thomson, and tait work out this suggestion, and, "on a certain assumption as to the proportion of retardations due to the sun and moon," find the earth may lose twenty-two seconds of time in a century from this cause.[ ] [footnote : sir w. thomson, _loc. cit_. p. .] (_b_.) but m. dufour suggests that the retardation of the earth (which is hypothetically assumed to exist) may be due in part, or wholly, to the increase of the moment of inertia of the earth by meteors falling upon its surface. this suggestion also meets with the entire approval of sir w. thomson, who shows that meteor-dust, accumulating at the rate of one foot in , years, would account for the remainder of retardation.[ ] [footnote : _ibid._ p. .] (_c_.) thirdly, sir w. thomson brings forward an hypothesis of his own with respect to the cause of the hypothetical retardation of the earth's rotation:-- "let us suppose ice to melt from the polar regions ( ° round each pole, we may say) to the extent of something more than a foot thick, enough to give . foot of water over those areas, or . of a foot of water if spread over the whole globe, which would, in reality, raise the sea-level by only some such undiscoverable difference as three-fourths of an inch or an inch. this, or the reverse, which we believe might happen any year, and could certainly not be detected without far more accurate observations and calculations for the mean sea-level than any hitherto made, would slacken or quicken the earth's rate as a timekeeper by one- tenth of a second per year."[ ] [footnote : _ibid._] i do not presume to throw the slightest doubt upon the accuracy of any of the calculations made by such distinguished mathematicians as those who have made the suggestions i have cited. on the contrary, it is necessary to my argument to assume that they are all correct. but i desire to point out that this seems to be one of the many cases in which the admitted accuracy of mathematical process is allowed to throw a wholly inadmissible appearance of authority over the results obtained by them. mathematics may be compared to a mill of exquisite workmanship, which grinds you stuff of any degree of fineness; but, nevertheless, what you get out depends upon what you put in; and as the grandest mill in the world will not extract wheat-flour from peascods, so pages of formulae will not get a definite result out of loose data. in the present instance it appears to be admitted:-- . that it is not absolutely certain, after all, whether the moon's mean motion is undergoing acceleration, or the earth's rotation retardation.[ ] and yet this is the key of the whole position. [footnote : it will be understood that i do not wish to deny that the earth's rotation _may be_ undergoing retardation.] . if the rapidity of the earth's rotation is diminishing, it is not certain how much of that retardation is due to tidal friction, how much to meteors, how much to possible excess of melting over accumulation of polar ice, during the period covered by observation, which amounts, at the outside, to not more than , years. . the effect of a different distribution of land and water in modifying the retardation caused by tidal friction, and of reducing it, under some circumstances, to a minimum, does not appear to be taken into account. . during the miocene epoch the polar ice was certainly many feet thinner than it has been during, or since, the glacial epoch. sir w. thomson tells us that the accumulation of something more than a foot of ice around the poles (which implies the withdrawal of, say, an inch of water from the general surface of the sea) will cause the earth to rotate quicker by one-tenth of a second per annum. it would appear, therefore, that the earth may have been rotating, throughout the whole period which has elapsed from the commencement of the glacial epoch down to the present time, one, or more, seconds per annum quicker than it rotated during the miocene epoch. but, according to sir w. thomson's calculation, tidal retardation will only account for a retardation of " in a century, or / (say / ) of a second per annum. thus, assuming that the accumulation of polar ice since the miocene epoch has only been sufficient to produce ten times the effect of a coat of ice one foot thick, we shall have an accelerating cause which covers all the loss from tidal action, and leaves a balance of / of a second per annum in the way of acceleration. if tidal retardation can be thus checked and overthrown by other temporary conditions, what becomes of the confident assertion, based upon the assumed uniformity of tidal retardation, that ten thousand million years ago the earth must have been rotating more than twice as fast as at present, and, therefore, that we geologists are "in direct opposition to the principles of natural philosophy" if we spread geological history over that time? ii. the second argument is thus stated by sir w. thomson:--"an article, by myself, published in 'macmillan's magazine' for march , on the age of the sun's heat, explains results of investigation into various questions as to possibilities regarding the amount of heat that the sun could have, dealing with it as you would with a stone, or a piece of matter, only taking into account the sun's dimensions, which showed it to be possible that the sun may have already illuminated the earth for as many as one hundred million years, but at the same time rendered it almost certain that he had not illuminated the earth for five hundred millions of years. the estimates here are necessarily very vague; but yet, vague as they are, i do not know that it is possible, upon any reasonable estimate founded on known properties of matter, to say that we can believe the sun has really illuminated the earth for five hundred million years."[ ] [footnote : _loc. cit._ p. .] i do not wish to "hansardise" sir william thomson by laying much stress on the fact that, only fifteen years ago he entertained a totally different view of the origin of the sun's heat, and believed that the energy radiated from year to year was supplied from year to year--a doctrine which would have suited hutton perfectly. but the fact that so eminent a physical philosopher has, thus recently, held views opposite to those which he now entertains, and that he confesses his own estimates to be "very vague," justly entitles us to disregard those estimates, if any distinct facts on our side go against them. however, i am not aware that such facts exist. as i have already said, for anything i know, one, two, or three hundred millions of years may serve the needs of geologists perfectly well. iii. the third line of argument is based upon the temperature of the interior of the earth. sir w. thomson refers to certain investigations which prove that the present thermal condition of the interior of the earth implies either a heating of the earth within the last , years of as much as ° f., or a greater heating all over the surface at some time further back than , years, and then proceeds thus:-- "now, are geologists prepared to admit that, at some time within the last , years, there has been all over the earth so high a temperature as that? i presume not; no geologist--no _modern_ geologist--would for a moment admit the hypothesis that the present state of underground heat is due to a heating of the surface at so late a period as , years ago. if that is not admitted we are driven to a greater heat at some time more than , years ago. a greater heating all over the surface than ° fahrenheit would kill nearly all existing plants and animals, i may safely say. are modern geologists prepared to say that all life was killed off the earth , , , , or , years ago? for the uniformity theory, the further back the time of high surface-temperature is put the better; but the further back the time of heating, the hotter it must have been. the best for those who draw most largely on time is that which puts it furthest back; and that is the theory that the heating was enough to melt the whole. but even if it was enough to melt the whole, we must still admit some limit, such as fifty million years, one hundred million years, or two or three hundred million years ago. beyond that we cannot go."[ ] [footnote : _loc. cit._ p. .] it will be observed that the "limit" is once again of the vaguest, ranging from , , years to , , . and the reply is, once more, that, for anything that can be proved to the contrary, one or two hundred million years might serve the purpose, even of a thoroughgoing huttonian uniformitarian, very well. but if, on the other hand, the , , or , , years appear to be insufficient for geological purposes, we must closely criticise the method by which the limit is reached. the argument is simple enough. _assuming_ the earth to be nothing but a cooling mass, the quantity of heat lost per year, _supposing_ the rate of cooling to have been uniform, multiplied by any given number of years, will be given the minimum temperature that number of years ago. but is the earth nothing but a cooling mass, "like a hot-water jar such as is used in carriages," or "a globe of sandstone," and has its cooling been uniform? an affirmative answer to both these questions seems to be necessary to the validity of the calculations on which sir w. thomson lays so much stress. nevertheless it surely may be urged that such affirmative answers are purely hypothetical, and that other suppositions have an equal right to consideration. for example, is it not possible that, at the prodigious temperature which would seem to exist at miles below the surface, all the metallic bases may behave as mercury does at a red heat, when it refuses to combine with oxygen; while, nearer the surface, and therefore at a lower temperature, they may enter into combination (as mercury does with oxygen a few degrees below its boiling-point), and so give rise to a heat totally distinct from that which they possess as cooling bodies? and has it not also been proved by recent researches that the quality of the atmosphere may immensely affect its permeability to heat; and, consequently, profoundly modify the rate of cooling the globe as a whole? i do not think it can be denied that such conditions may exist, and may so greatly affect the supply, and the loss, of terrestrial heat as to destroy the value of any calculations which leave them out of sight. my functions as your advocate are at an end. i speak with more than the sincerity of a mere advocate when i express the belief that the case against us has entirely broken down. the cry for reform which has been raised without, is superfluous, inasmuch as we have long been reforming from within, with all needful speed. and the critical examination of the grounds upon which the very grave charge of opposition to the principles of natural philosophy has been brought against us, rather shows that we have exercised a wise discrimination in declining, for the present, to meddle with our foundations. xi palaeontology and the doctrine of evolution [ ] it is now eight years since, in the absence of the late mr. leonard horner, who then presided over us, it fell to my lot, as one of the secretaries of this society, to draw up the customary annual address. i availed myself of the opportunity to endeavour to "take stock" of that portion of the science of biology which is commonly called "palaeontology," as it then existed; and, discussing one after another the doctrines held by palaeontologists, i put before you the results of my attempts to sift the well-established from the hypothetical or the doubtful. permit me briefly to recall to your minds what those results were:-- . the living population of all parts of the earth's surface which have yet been examined has undergone a succession of changes which, upon the whole, have been of a slow and gradual character. . when the fossil remains which are the evidences of these successive changes, as they have occurred in any two more or less distant parts of the surface of the earth, are compared, they exhibit a certain broad and general parallelism. in other words, certain forms of life in one locality occur in the same general order of succession as, or are _homotaxial_ with, similar forms in the other locality. . homotaxis is not to be held identical with synchronism without independent evidence. it is possible that similar, or even identical, faunae and florae in two different localities may be of extremely different ages, if the term "age" is used in its proper chronological sense. i stated that "geographical provinces, or zones, may have been as distinctly marked in the palaeozoic epoch as at present; and those seemingly sudden appearances of new genera and species which we ascribe to new creation, may be simple results of migration." . the opinion that the oldest known fossils are the earliest forms of life has no solid foundation. . if we confine ourselves to positively ascertained facts, the total amount of change in the forms of animal and vegetable life, since the existence of such forms is recorded, is small. when compared with the lapse of time since the first appearance of these forms, the amount of change is wonderfully small. moreover, in each great group of the animal and vegetable kingdoms, there are certain forms which i termed persistent types, which have remained, with but very little apparent change, from their first appearance to the present time. . in answer to the question "what, then, does an impartial survey of the positively ascertained truths of palaeontology testify in relation to the common doctrines of progressive modification, which suppose that modification to have taken place by a necessary progress from more to less embryonic forms, from more to less generalised types, within the limits of the period represented by the fossiliferous rocks?" i reply, "it negatives these doctrines; for it either shows us no evidence of such modification, or demonstrates such modification as has occurred to have been very slight; and, as to the nature of that modification, it yields no evidence whatsoever that the earlier members of any long-continued group were more generalised in structure than the later ones." i think that i cannot employ my last opportunity of addressing you, officially, more properly--i may say more dutifully--than in revising these old judgments with such help as further knowledge and reflection, and an extreme desire to get at the truth, may afford me. . with respect to the first proposition, i may remark that whatever may be the case among the physical geologists, catastrophic palaeontologists are practically extinct. it is now no part of recognised geological doctrine that the species of one formation all died out and were replaced by a brand-new set in the next formation. on the contrary, it is generally, if not universally, agreed that the succession of life has been the result of a slow and gradual replacement of species by species; and that all appearances of abruptness of change are due to breaks in the series of deposits, or other changes in physical conditions. the continuity of living forms has been unbroken from the earliest times to the present day. , . the use of the word "homotaxis" instead of "synchronism" has not, so far as i know, found much favour in the eyes of geologists. i hope, therefore, that it is a love for scientific caution, and not mere personal affection for a bantling of my own, which leads me still to think that the change of phrase is of importance, and that the sooner it is made, the sooner shall we get rid of a number of pitfalls which beset the reasoner upon the facts and theories of geology. one of the latest pieces of foreign intelligence which has reached us is the information that the austrian geologists have, at last, succumbed to the weighty evidence which m. barrande has accumulated, and have admitted the doctrine of colonies. but the admission of the doctrine of colonies implies the further admission that even identity of organic remains is no proof of the synchronism of the deposits which contain them. . the discussions touching the _eozoon,_ which commenced in , have abundantly justified the fourth proposition. in , the oldest record of life was in the cambrian rocks; but if the _eozoon_ be, as principal dawson and dr. carpenter have shown so much reason for believing, the remains of a living being, the discovery of its true nature carried life back to a period which, as sir william logan has observed, is as remote from that during which the cambrian rocks were deposited, as the cambrian epoch itself is from the tertiaries. in other words, the ascertained duration of life upon the globe was nearly doubled at a stroke. . the significance of persistent types, and of the small amount of change which has taken place even in those forms which can be shown to have been modified, becomes greater and greater in my eyes, the longer i occupy myself with the biology of the past. consider how long a time has elapsed since the miocene epoch. yet, at that time there is reason to believe that every important group in every order of the _mammalia_ was represented. even the comparatively scanty eocene fauna yields examples of the orders _cheiroptera, insectivora, rodentia_, and _perissodactyla_; of _artiodactyla_ under both the ruminant and the porcine modifications; of _caranivora, cetacea_, and _marsupialia_. or, if we go back to the older half of the mesozoic epoch, how truly surprising it is to find every order of the _reptilia_, except the _ophidia_, represented; while some groups, such as the _ornithoseclida_ and the _pterosauria_, more specialised than any which now exist, abounded. there is one division of the _amphibia_ which offers especially important evidence upon this point, inasmuch as it bridges over the gap between the mesozoic and the palaeozoic formations (often supposed to be of such prodigious magnitude), extending, as it does, from the bottom of the carboniferous series to the top of the trias, if not into the lias. i refer to the labyrinthodonts. as the address of was passing through the press, i was able to mention, in a note, the discovery of a large labyrinthodont, with well-ossified vertebrae, in the edinburgh coal-field. since that time eight or ten distinct genera of labyrinthodonts have been discovered in the carboniferous rocks of england, scotland, and ireland, not to mention the american forms described by principal dawson and professor cope. so that, at the present time, the labyrinthodont fauna of the carboniferous rocks is more extensive and diversified than that of the trias, while its chief types, so far as osteology enables us to judge, are quite as highly organised. thus it is certain that a comparatively highly organised vertebrate type, such as that of the labyrinthodonts, is capable of persisting, with no considerable change, through the period represented by the vast deposits which constitute the carboniferous, the permian, and the triassic formations. the very remarkable results which have been brought to light by the sounding and dredging operations, which have been carried on with such remarkable success by the expeditions sent out by our own, the american, and the swedish governments, under the supervision of able naturalists, have a bearing in the same direction. these investigations have demonstrated the existence, at great depths in the ocean, of living animals in some cases identical with, in others very similar to, those which are found fossilised in the white chalk. the _globigerinoe_, cyatholiths, coccospheres, discoliths in the one are absolutely identical with those in the other; there are identical, or closely analogous, species of sponges, echinoderms, and brachiopods. off the coast of portugal, there now lives a species of _beryx_, which, doubtless, leaves its bones and scales here and there in the atlantic ooze, as its predecessor left its spoils in the mud of the sea of the cretaceous epoch. many years ago[ ] i ventured to speak of the atlantic mud as "modern chalk," and i know of no fact inconsistent with the view which professor wyville thomson has advocated, that the modern chalk is not only the lineal descendant of the ancient chalk, but that it remains, so to speak, in the possession of the ancestral estate; and that from the cretaceous period (if not much earlier) to the present day, the deep sea has covered a large part of what is now the area of the atlantic. but if _globigerina_, and _terebratula caput-serpentis_ and _beryx_, not to mention other forms of animals and of plants, thus bridge over the interval between the present and the mesozoic periods, is it possible that the majority of other living things underwent a "sea-change into something new and strange" all at once? [footnote : see an article in the _saturday review_, for , on "chalk, ancient and modern."] . thus far i have endeavoured to expand and to enforce by fresh arguments, but not to modify in any important respect, the ideas submitted to you on a former occasion. but when i come to the propositions touching progressive modification, it appears to me, with the help of the new light which has broken from various quarters, that there is much ground for softening the somewhat brutus-like severity with which, in , i dealt with a doctrine, for the truth of which i should have been glad enough to be able to find a good foundation. so far, indeed, as the _invertebrata_ and the lower _vertebrata_ are concerned, the facts and the conclusions which are to be drawn from them appear to me to remain what they were. for anything that, as yet, appears to the contrary, the earliest known marsupials may have been as highly organised as their living congeners; the permian lizards show no signs of inferiority to those of the present day; the labyrinthodonts cannot be placed below the living salamander and triton; the devonian ganoids are closely related to _polypterus_ and to _lepidosiren_. but when we turn to the higher _vertebrata_, the results of recent investigations, however we may sift and criticise them, seem to me to leave a clear balance in favour of the doctrine of the evolution of living forms one from another. nevertheless, in discussing this question, it is very necessary to discriminate carefully between the different kinds of evidence from fossil remains which are brought forward in favour of evolution. every fossil which takes an intermediate place between forms of life already known, may be said, so far as it is intermediate, to be evidence in favour of evolution, inasmuch as it shows a possible road by which evolution may have taken place. but the mere discovery of such a form does not, in itself, prove that evolution took place by and through it, nor does it constitute more than presumptive evidence in favour of evolution in general. suppose a, b, c to be three forms, while b is intermediate in structure between a and c. then the doctrine of evolution offers four possible alternatives. a may have become c by way of b; or c may have become a by way of b; or a and c may be independent modifications of b; or a, b, and c may be independent modifications of some unknown d. take the case of the pigs, the _anoplothcridoe_, and the ruminants. the _anoplothcridoe_ are intermediate between the first and the last; but this does not tell us whether the ruminants have come from the pigs, or the pigs from ruminants, or both from _anoplothcridoe_, or whether pigs, ruminants, and _anoplotlicridoe_ alike may not have diverged from some common stock. but if it can be shown that a, b, and c exhibit successive stages in the degree of modification, or specialisation, of the same type; and if, further, it can be proved that they occur in successively newer deposits, a being in the oldest and c in the newest, then the intermediate character of b has quite another importance, and i should accept it, without hesitation, as a link in the genealogy of c. i should consider the burden of proof to be thrown upon any one who denied c to have been derived from a by way of b, or in some closely analogous fashion; for it is always probable that one may not hit upon the exact line of filiation, and, in dealing with fossils, may mistake uncles and nephews for fathers and sons. i think it necessary to distinguish between the former and the latter classes of intermediate forms, as _intercalary types_ and _linear types_. when i apply the former term, i merely mean to say that as a matter of fact, the form b, so named, is intermediate between the others, in the sense in which the _anoplotherium_ is intermediate between the pigs and the ruminants--without either affirming, or denying, any direct genetic relation between the three forms involved. when i apply the latter term, on the other hand, i mean to express the opinion that the forms a, b, and c constitute a line of descent, and that b is thus part of the lineage of c. from the time when cuvier's wonderful researches upon the extinct mammals of the paris gypsum first made intercalary types known, and caused them to be recognised as such, the number of such forms has steadily increased among the higher _mammalia_. not only do we now know numerous intercalary forins of _ungulata_, but m. gaudry's great monograph upon the fossils of pikermi (which strikes me as one of the most perfect pieces of palaeontological work i have seen for a long time) shows us, among the primates, _mesopithecus_ as an intercalary form between the _semnopitheci_ and the _macaci_; and among the _carnivora_, _hyoenictis_ and _ictitherium_ as intercalary, or, perhaps, linear types between the _viverridoe_ and the _hyoenidoe_. hardly any order of the higher _mammalia_ stands so apparently separate and isolated from the rest as that of the _cetacea_; though a careful consideration of the structure of the pinnipede _carnivora_, or seals, shows, in them, many an approximation towards the still more completely marine mammals. the extinct _zeuglodon_, however, presents us with an intercalary form between the type of the seals and that of the whales. the skull of this great eocene sea-monster, in fact, shows by the narrow and prolonged interorbital region; the extensive union of the parietal bones in a sagittal suture; the well-developed nasal bones; the distinct and large incisors implanted in premaxillary bones, which take a full share in bounding the fore part of the gape; the two-fanged molar teeth with triangular and serrated crowns, not exceeding five on each side in each jaw; and the existence of a deciduous dentition--its close relation with the seals. while, on the other hand, the produced rostral form of the snout, the long symphysis, and the low coronary process of the mandible are approximations to the cetacean form of those parts. the scapula resembles that of the cetacean _hyperoodon_, but the supra- spinous fossa is larger and more seal-like; as is the humerus, which differs from that of the _cetacea_ in presenting true articular surfaces for the free jointing of the bones of the fore-arm. in the apparently complete absence of hinder limbs, and in the characters of the vertebral column, the _zeuglodon_ lies on the cetacean side of the boundary line; so that upon the whole, the zeuglodonts, transitional as they are, are conveniently retained in the cetacean order. and the publication, in , of m. van beneden's memoir on the miocene and pliocene _squalodon_, furnished much better means than anatomists previously possessed of fitting in another link of the chain which connects the existing _cetacea_ with _zeuglodon_. the teeth are much more numerous, although the molars exhibit the zeuglodont double fang; the nasal bones are very short, and the upper surface of the rostrum presents the groove, filled up during life by the prolongation of the ethmoidal cartilage, which is so characteristic of the majority of the _cetacea_. it appears to me that, just as among the existing _carnivora_, the walruses and the eared seals are intercalary forms between the fissipede carnivora and the ordinary seals, so the zeuglodonts are intercalary between the _carnivora_, as a whole, and the _cetacea_. whether the zeuglodonts are also linear types in their relation to these two groups cannot be ascertained, until we have more definite knowledge than we possess at present, respecting the relations in time of the _carnivora_ and _cetacea_. thus far we have been concerned with the intercalary types which occupy the intervals between families or orders of the same class; but the investigations which have been carried on by professor gegenbaur, professor cope, and myself into the structure and relations of the extinct reptilian forms of the _ornithoscelida_ (or _dinosauria_ and _compsognatha_) have brought to light the existence of intercalary forms between what have hitherto been always regarded as very distinct classes of the vertebrate sub-kingdom, namely _reptilia_ and _aves_. whatever inferences may, or may not, be drawn from the fact, it is now an established truth that, in many of these _ornithoscelida_, the hind limbs and the pelvis are much more similar to those of birds than they are to those of reptiles, and that these bird-reptiles, or reptile-birds, were more or less completely bipedal. when i addressed you in , i should have been bold indeed had i suggested that palaeontology would before long show us the possibility of a direct transition from the type of the lizard to that of the ostrich. at the present moment, we have, in the _ornithoscelida_, the intercalary type, which proves that transition to be something more than a possibility; but it is very doubtful whether any of the genera of _ornithoscelida_ with which we are at present acquainted are the actual linear types by which the transition from the lizard to the bird was effected. these, very probably, are still hidden from us in the older formations. let us now endeavour to find some cases of true linear types, or forms which are intermediate between others because they stand in a direct genetic relation to them. it is no easy matter to find clear and unmistakable evidence of filiation among fossil animals; for, in order that such evidence should be quite satisfactory, it is necessary that we should be acquainted with all the most important features of the organisation of the animals which are supposed to be thus related, and not merely with the fragments upon which the genera and species of the palaeontologist are so often based. m. gaudry has arranged the species of _hyoenidoe, proboscidea, rhinocerotidoe_, and _equidoe_ in their order of filiation from their earliest appearance in the miocene epoch to the present time, and professor rütimeyer has drawn up similar schemes for the oxen and other _ungulata_--with what, i am disposed to think, is a fair and probable approximation to the order of nature. but, as no one is better aware than these two learned, acute, and philosophical biologists, all such arrangements must be regarded as provisional, except in those cases in which, by a fortunate accident, large series of remains are obtainable from a thick and widespread series of deposits. it is easy to accumulate probabilities--hard to make out some particular case in such a way that it will stand rigorous criticism. after much search, however, i think that such a case is to be made out in favour of the pedigree of the horses. the genus _equus_ is represented as far back as the latter part of the miocene epoch; but in deposits belonging to the middle of that epoch its place is taken by two other genera, _hipparion_ and _anchitherium_;[ ] and, in the lowest miocene and upper eocene, only the last genus occurs. a species of _anchitherium_ was referred by cuvier to the _paloeotheria_ under the name of _p. aurelianense_. the grinding-teeth are in fact very similar in shape and in pattern, and in the absence of any thick layer of cement, to those of some species of _paloeotherium_, especially cuvier's _paloeotherium minus_, which has been formed into a separate genus, _plagiolophus_, by pomel. but in the fact that there are only six full- sized grinders in the lower jaw, the first premolar being very small; that the anterior grinders are as large as, or rather larger than, the posterior ones; that the second premolar has an anterior prolongation; and that the posterior molar of the lower jaw has, as cuvier pointed out, a posterior lobe of much smaller size and different form, the dentition of _anchitherium_ departs from the type of the _paloeotherium_, and approaches that of the horse. [footnote : hermann von meyer gave the name of _anchitherium_ to _a. ezquerroe_; and in his paper on the subject he takes great pains to distinguish the latter as the type of a new genus, from cuvier's _paloeotherium d'orléans_. but it is precisely the _paloeotherium d'orléans_ which is the type of christol's genus _hipparitherium_; and thus, though _hipparitherium_ is of later date than _anchitherium_, it seemed to me to have a sort of equitable right to recognition when this address was written. on the whole, however, it seems most convenient to adopt _anchitherium_.] again, the skeleton of _anchitherium_ is extremely equine. m. christol goes so far as to say that the description of the bones of the horse, or the ass, current in veterinary works, would fit those of _anchitherium_. and, in a general way, this may be true enough; but there are some most important differences, which, indeed, are justly indicated by the same careful observer. thus the ulna is complete throughout, and its shaft is not a mere rudiment, fused into one bone with the radius. there are three toes, one large in the middle and one small on each side. the femur is quite like that of a horse, and has the characteristic fossa above the external condyle. in the british museum there is a most instructive specimen of the leg-bones, showing that the fibula was represented by the external malleolus and by a flat tongue of bone, which extends up from it on the outer side of the tibia, and is closely ankylosed with the latter bone.[ ] the hind toes are three, like those of the fore leg; and the middle metatarsal bone is much less compressed from side to side than that of the horse. [footnote : i am indebted to m. gervais for a specimen which indicates that the fibula was complete, at any rate, in some cases; and for a very interesting ramps of a mandible, which shows that, as in the _paloeotheria_, the hindermost milk-molar of the lower jaw was devoid of the posterior lobe which exists in the hindermost true molar.] in the _hipparion_, the teeth nearly resemble those of the horses, though the crowns of the grinders are not so long; like those of the horses, they are abundantly coated with cement. the shaft of the ulna is reduced to a mere style, ankylosed throughout nearly its whole length with the radius, and appearing to be little more than a ridge on the surface of the latter bone until it is carefully examined. the front toes are still three, but the outer ones are more slender than in _anchitherium_, and their hoofs smaller in proportion to that of the middle toe; they are, in fact, reduced to mere dew-claws, and do not touch the ground. in the leg, the distal end of the fibula is so completely united with the tibia that it appears to be a mere process of the latter bone, as in the horses. in _equus_, finally, the crowns of the grinding-teeth become longer, and their patterns are slightly modified; the middle of the shaft of the ulna usually vanishes, and its proximal and distal ends ankylose with the radius. the phalanges of the two outer toes in each foot disappear, their metacarpal and metatarsal bones being left as the "splints." the _hipparion_ has large depressions on the face in front of the orbits, like those for the "larmiers" of many ruminants; but traces of these are to be seen in some of the fossil horses from the sewalik hills; and, as leidy's recent researches show, they are preserved in _anchitherium_. when we consider these facts, and the further circumstance that the hipparions, the remains of which have been collected in immense numbers, were subject, as m. gaudry and others have pointed out, to a great range of variation, it appears to me impossible to resist the conclusion that the types of the _anchitherium_, of the _hipparion_, and of the ancient horses constitute the lineage of the modern horses, the _hipparion_ being the intermediate stage between the other two, and answering to b in my former illustration. the process by which the _anchitherium_ has been converted into _equus_ is one of specialisation, or of more and more complete deviation from what might be called the average form of an ungulate mammal. in the horses, the reduction of some parts of the limbs, together with the special modification of those which are left, is carried to a greater extent than in any other hoofed mammals. the reduction is less and the specialisation is less in the _hipparion_, and still less in the _anchitherium_; but yet, as compared with other mammals, the reduction and specialisation of parts in the _anchitherium_ remain great. is it not probable then, that, just as in the miocene epoch, we find an ancestral equine form less modified than _equus_, so, if we go back to the eocene epoch, we shall find some quadruped related to the _anchitherium_, as _hipparion_ is related to _equus_, and consequently departing less from the average form? i think that this desideratum is very nearly, if not quite, supplied by _plagiolophus_, remains of which occur abundantly in some parts of the upper and middle eocene formations. the patterns of the grinding-teeth of _plagiolophus_ are similar to those of _anchitherium_, and their crowns are as thinly covered with cement; but the grinders diminish in size forwards, and the last lower molar has a large hind lobe, convex outwards and concave inwards, as in _palueotherium_. the ulna is complete and much larger than in any of the _equidoe_, while it is more slender than in most of the true _paloeotheria_; it is fixedly united, but not ankylosed, with the radius. there are three toes in the fore limb, the outer ones being slender, but less attenuated than in the _equidoe_. the femur is more like that of the _paloeotheria_ than that of the horse, and has only a small depression above its outer condyle in the place of the great fossa which is so obvious in the _equidoe_. the fibula is distinct, but very slender, and its distal end is ankylosed with the tibia. there are three toes on the hind foot having similar proportions to those on the fore foot. the principal metacarpal and metatarsal bones are flatter than they are in any of the _equidoe_; and the metacarpal bones are longer than the metatarsals, as in the _paloeotheria_. in its general form, _plagiolophus_ resembles a very small and slender horse,[ ] and is totally unlike the reluctant, pig-like creature depicted in cuvier's restoration of his _paloeotherium minus_ in the "ossemens fossiles." [footnote : such, at least, is the conclusion suggested by the proportions of the skeleton figured by cuvier and de blainville; but perhaps something between a horse and an agouti would be nearest the mark.] it would be hazardous to say that _plagiolophus_ is the exact radical form of the equine quadrupeds; but i do not think there can be any reasonable doubt that the latter animals have resulted from the modification of some quadruped similar to _plagiolophus_. we have thus arrived at the middle eocene formation, and yet have traced back the horses only to a three-toed stock; but these three-toed forms, no less than the equine quadrupeds themselves, present rudiments of the two other toes which appertain to what i have termed the "average" quadruped. if the expectation raised by the splints of the horses that, in some ancestor of the horses, these splints would be found to be complete digits, has been verified, we are furnished with very strong reasons for looking for a no less complete verification of the expectation that the three-toed _plagiolophus_-like "avus" of the horse must have had a five-toed "atavus" at some earlier period. no such five-toed "atavus," however, has yet made its appearance among the few middle and older eocene _mammalia_ which are known. another series of closely affiliated forms, though the evidence they afford is perhaps less complete than that of the equine series, is presented to us by the _dichobune_ of the eocene epoch, the _cainotherium_ of the miocene, and the _tragulidoe_, or so-called "musk- deer," of the present day. the _tragulidoe_; have no incisors in the upper jaw, and only six grinding-teeth on each side of each jaw; while the canine is moved up to the outer incisor, and there is a diastema in the lower jaw. there are four complete toes on the hind foot, but the middle metatarsals usually become, sooner or later, ankylosed into a cannon bone. the navicular and the cuboid unite, and the distal end of the fibula is ankylosed with the tibia. in _cainotherium_ and _dichobune_ the upper incisors are fully developed. there are seven grinders; the teeth form a continuous series without a diastema. the metatarsals, the navicular and cuboid, and the distal end of the fibula, remain free. in the _cainotherium_, also, the second metacarpal is developed, but is much shorter than the third, while the fifth is absent or rudimentary. in this respect it resembles _anoplotherium secundarium_. this circumstance, and the peculiar pattern of the upper molars in _cainotherium_, lead me to hesitate in considering it as the actual ancestor of the modern _tragulidoe_. if _dichobune_ has a fore-toed fore foot (though i am inclined to suspect that it resembles _cainotherium_), it will be a better representative of the oldest forms of the traguline series; but _dichobune_ occurs in the middle eocene, and is, in fact, the oldest known artiodactyle mammal. where, then, must we look for its five-toed ancestor? if we follow down other lines of recent and tertiary _ungulata_, the same question presents itself. the pigs are traceable back through the miocene epoch to the upper eocene, where they appear in the two well-marked forms of _hyopopotamus_ and _choeropotamus_; but _hyopotamus_ appears to have had only two toes. again, all the great groups of the ruminants, the _bovidoe, antilopidoe, camelopardalidoe_, and _cervidoe_, are represented in the miocene epoch, and so are the camels. the upper eocene _anoplotherium_, which is intercalary between the pigs and the _tragulidoe_, has only two, or, at most, three toes. among the scanty mammals of the lower eocene formation we have the perissodactyle _ungulata_ represented by _coryphodon, hyracotherium_, and _pliolophus_. suppose for a moment, for the sake of following out the argument, that _pliolophus_ represents the primary stock of the perissodactyles, and _dichobune_ that of the artiodactyles (though i am far from saying that such is the case), then we find, in the earliest fauna of the eocene epoch to which our investigations carry us, the two divisions of the _ungulata_ completely differentiated, and no trace of any common stock of both, or of five-toed predecessors to either. with the case of the horses before us, justifying a belief in the production of new animal forms by modification of old ones, i see no escape from the necessity of seeking for these ancestors of the _ungulata_ beyond the limits of the tertiary formations. i could as soon admit special creation, at once, as suppose that the perissodactyles and artiodactyles had no five-toed ancestors. and when we consider how large a portion of the tertiary period elapsed before _anchitherium_ was converted into _equus_, it is difficult to escape the conclusion that a large proportion of time anterior to the tertiary period must have been expended in converting the common stock of the _ungulata_ into perissodactyles and artiodactyles. the same moral is inculcated by the study of every other order of tertiary monodelphous _mammalia_. each of these orders is represented in the miocene epoch: the eocene formation, as i have already said, contains _cheiroptera, insectivora, rodentia, ungulata, carnivora_, and _cetacea_. but the _cheiroptera_ are extreme modifications of the _insectivora_, just as the _cetacea_ are extreme modifications of the carnivorous type; and therefore it is to my mind incredible that monodelphous _insectivora_ and _carnivora_ should not have been abundantly developed, along with _ungulata_, in the mesozoic epoch. but if this be the case, how much further back must we go to find the common stock of the monodelphous _mammalia_? as to the _didelphia_, if we may trust the evidence which seems to be afforded by their very scanty remains, a hypsiprymnoid form existed at the epoch of the trias, contemporaneously with a carnivorous form. at the epoch of the trias, therefore, the _marsupialia_ must have already existed long enough to have become differentiated into carnivorous and herbivorous forms. but the _monotremata_ are lower forms than the _didelphia_ which last are intercalary between the _ornithodelphia_ and the _monodelphia_. to what point of the palaeozoic epoch, then, must we, upon any rational estimate, relegate the origin of the _monotremata?_ the investigation of the occurrence of the classes and of the orders of the _sauropsida_ in time points in exactly the same direction. if, as there is great reason to believe, true birds existed in the triassic epoch, the ornithoscelidous forms by which reptiles passed into birds must have preceded them. in fact there is, even at present, considerable ground for suspecting the existence of _dinosauria_ in the permian formations; but, in that case, lizards must be of still earlier date. and if the very small differences which are observable between the _crocodilia_ of the older mesozoic formations and those of the present day furnish any sort of approximation towards an estimate of the average rate of change among the _sauropsida_, it is almost appalling to reflect how far back in palaeozoic times we must go, before we can hope to arrive at that common stock from which the _crocodilia, lacertilia, ornithoscelida_, and _plesiosauria_, which had attained so great a development in the triassic epoch, must have been derived. the _amphibia_ and _pisces_ tell the same story. there is not a single class of vertebrated animals which, when it first appears, is represented by analogues of the lowest known members of the same class. therefore, if there is any truth in the doctrine of evolution, every class must be vastly older than the first record of its appearance upon the surface of the globe. but if considerations of this kind compel us to place the origin of vertebrated animals at a period sufficiently distant from the upper silurian, in which the first elasmobranchs and ganoids occur, to allow of the evolution of such fishes as these from a vertebrate as simple as the _amphioxus,_ i can only repeat that it is appalling to speculate upon the extent to which that origin must have preceded the epoch of the first recorded appearance of vertebrate life. such is the further commentary which i have to offer upon the statement of the chief results of palaeontology which i formerly ventured to lay before you. but the growth of knowledge in the interval makes me conscious of an omission of considerable moment in that statement, inasmuch as it contains no reference to the bearings of palaeontology upon the theory of the distribution of life; nor takes note of the remarkable manner in which the facts of distribution, in present and past times, accord with the doctrine of evolution, especially in regard to land animals. that connection between palaeontology and geology and the present distribution of terrestrial animals, which so strikingly impressed mr. darwin, thirty years ago, as to lead him to speak of a "law of succession of types," and of the wonderful relationship on the same continent between the dead and the living, has recently received much elucidation from the researches of gaudry, of rutimeyer, of leidy, and of alphonse milne-edwards, taken in connection with the earlier labours of our lamented colleague falconer; and it has been instructively discussed in the thoughtful and ingenious work of mr. andrew murray "on the geographical distribution of mammals."[ ] [footnote : the paper "on the form and distribution of the landtracts during the secondary and tertiary periods respectively; and on the effect upon animal life which great changes in geographical configuration have probably produced," by mr. searles v. wood, jun., which was published in the _philosophical magazine_, in , was unknown to me when this address was written. it is well worthy of the most careful study.] i propose to lay before you, as briefly as i can, the ideas to which a long consideration of the subject has given rise in my mind. if the doctrine of evolution is sound, one of its immediate consequences clearly is, that the present distribution of life upon the globe is the product of two factors, the one being the distribution which obtained in the immediately preceding epoch, and the other the character and the extent of the changes which have taken place in physical geography between the one epoch and the other; or, to put the matter in another way, the fauna and flora of any given area, in any given epoch, can consist only of such forms of life as are directly descended from those which constituted the fauna and flora of the same area in the immediately preceding epoch, unless the physical geography (under which i include climatal conditions) of the area has been so altered as to give rise to immigration of living forms from some other area. the evolutionist, therefore, is bound to grapple with the following problem whenever it is clearly put before him:--here are the faunae of the same area during successive epochs. show good cause for believing either that these faunae have been derived from one another by gradual modification, or that the faunae have reached the area in question by migration from some area in which they have undergone their development. i propose to attempt to deal with this problem, so far as it is exemplified by the distribution of the terrestrial _vertebrata_, and i shall endeavour to show you that it is capable of solution in a sense entirely favourable to the doctrine of evolution. i have elsewhere[ ] stated at length the reasons which lead me to recognise four primary distributional provinces for the terrestrial _vertebrata_ in the present world, namely,--first, the _novozelanian_, or new-zealand province; secondly, the _australian_ province, including australia, tasmania, and the negrito islands; thirdly, _austro-columbia_, or south america _plus_ north america as far as mexico; and fourthly, the rest of the world, or _arctogoea_, in which province america north of mexico constitutes one sub-province, africa south of the sahara a second, hindostan a third, and the remainder of the old world a fourth. [footnote : "on the classification and distribution of the alectoromorphoe;" _proceedings of the zoological society_, .] now the truth which mr. darwin perceived and promulgated as "the law of the succession of types" is, that, in all these provinces, the animals found in pliocene or later deposits are closely affined to those which now inhabit the same provinces; and that, conversely, the forms characteristic of other provinces are absent. north and south america, perhaps, present one or two exceptions to the last rule, but they are readily susceptible of explanation. thus, in australia, the later tertiary mammals are marsupials (possibly with the exception of the dog and a rodent or two, as at present). in austro-columbia, the later tertiary fauna exhibits numerous and varied forms of platyrrhine apes, rodents, cats, dogs, stags, _edentata_, and opossums; but, as at present, no catarrhine apes, no lemurs, no _insectivora_, oxen, antelopes, rhinoceroses, nor _didelphia_ other than opossums. and in the widespread arctogaeal province, the pliocene and later mammals belong to the same groups as those which now exist in the province. the law of succession of types, therefore, holds good for the present epoch as compared with its predecessor. does it equally well apply to the pliocene fauna when we compare it with that of the miocene epoch? by great good fortune, an extensive mammalian fauna of the latter epoch has now become known, in four very distant portions of the arctogaeal province which do not differ greatly in latitude. thus falconer and cautley have made known the fauna of the sub-himalayas and the perim islands; gaudry that of attica; many observers that of central europe and france; and leidy that of nebraska, on the eastern flank of the rocky mountains. the results are very striking. the total miocene fauna comprises many genera and species of catarrhine apes, of bats, of _insectivora_; of arctogaeal types of _rodentia_; of _proboscidea_; of equine, rhinocerotic, and tapirine quadrupeds; of cameline, bovine, antilopine, cervine, and traguline ruminants; of pigs and hippopotamuses; of _viverridoe_ and _hyoenidoe_ among other _carnivora_; with _edentata_ allied to the aretogaeal _oryeteropus_ and _manis_, and not to the austro-columbian edentates. the only type present in the miocene, but absent in the existing, fauna of eastern arctogaea, is that of the _didelphidoe_, which, however, remains in north america. but it is very remarkable that while the miocene fauna of the arctogaeal province, as a whole, is of the same character as the existing fauna of the same province, as a whole, the component elements of the fauna were differently associated. in the miocene epoch, north america possessed elephants, horses, rhinoceroses, and a great number and variety of ruminants and pigs, which are absent in the present indigenous fauna; europe had its apes, elephants, rhinoceroses, tapirs, musk-deer, giraffes, hyaenas, great cats, edentates, and opossum-like marsupials, which have equally vanished from its present fauna; and in northern india, the african types of hippopotamuses, giraffes, and elephants were mixed up with what are now the asiatic types of the latter, and with camels, and semnopithecine and pithecine apes of no less distinctly asiatic forms. in fact the miocene mammalian fauna of europe and the himalayan regions contains, associated together, the types which are at present separately located in the south-african and indian sub-provinces of arctogaea. now there is every reason to believe, on other grounds, that both hindostan, south of the ganges, and africa, south of the sahara, were separated by a wide sea from europe and north asia during the middle and upper eocene epochs. hence it becomes highly probable that the well-known similarities, and no less remarkable differences between the present faunae of india and south africa have arisen in some such fashion as the following. some time during the miocene epoch, possibly when the himalayan chain was elevated, the bottom of the nummulitic sea was upheaved and converted into dry land, in the direction of a line extending from abyssinia to the mouth of the ganges. by this means, the dekhan on the one hand, and south africa on the other, became connected with the miocene dry land and with one another. the miocene mammals spread gradually over this intermediate dry land; and if the condition of its eastern and western ends offered as wide contrasts as the valleys of the ganges and arabia do now, many forms which made their way into africa must have been different from those which reached the dekhan, while others might pass into both these sub-provinces. that there was a continuity of dry land between europe and north america during the miocene epoch, appears to me to be a necessary consequence of the fact that many genera of terrestrial mammals, such as _castor, hystrix, elephas, mastodon, equus, hipparion, anchitherium, rhinoceros, cervus, amphicyon, hyoenarctos_, and _machairodus_, are common to the miocene formations of the two areas, and have as yet been found (except perhaps _anchitherium_) in no deposit of earlier age. whether this connection took place by the east, or by the west, or by both sides of the old world, there is at present no certain evidence, and the question is immaterial to the present argument; but, as there are good grounds for the belief that the australian province and the indian and south-african sub-provinces were separated by sea from the rest of arctogaea before the miocene epoch, so it has been rendered no less probable, by the investigations of mr. carrick moore and professor duncan, that austro- columbia was separated by sea from north america during a large part of the miocene epoch. it is unfortunate that we have no knowledge of the miocene mammalian fauna of the australian and austro-columbian provinces; but, seeing that not a trace of a platyrrhine ape, of a procyonine carnivore, of a characteristically south-american rodent, of a sloth, an armadillo, or an ant-eater has yet been found in miocene deposits of arctogaea, i cannot doubt that they already existed in the miocene austro-columbian province. nor is it less probable that the characteristic types of australian mammalia were already developed in that region in miocene times. but austro-columbia presents difficulties from which australia is free; _cantelidoe_ and _tapirdoe_ are now indigenous in south america as they are in arctogaea; and, among the pliocene austro-columbian mammals, the arctogaeal genera _equus, mastodon,_ and _machairodus_ are numbered. are these postmiocene immigrants, or praemiocene natives? still more perplexing are the strange and interesting forms _toxodon, macrauchenia, typotherium_, and a new anoplotherioid mammal (_homalodotherhon_) which dr. cunningham sent over to me some time ago from patagonia. i confess i am strongly inclined to surmise that these last, at any rate, are remnants of the population of austro-columbia before the miocene epoch, and were not derived from arctogaea by way of the north and east. the fact that this immense fauna of miocene arctogaea is now fully and richly represented only in india and in south africa, while it is shrunk and depauperised in north asia, europe, and north america, becomes at once intelligible, if we suppose that india and south africa had but a scanty mammalian population before the miocene immigration, while the conditions were highly favourable to the new comers. it is to be supposed that these new regions offered themselves to the miocene ungulates, as south america and australia offered themselves to the cattle, sheep, and horses of modern colonists. but, after these great areas were thus peopled, came the glacial epoch, during which the excessive cold, to say nothing of depression and ice-covering, must have almost depopulated all the northern parts of arctogaea, destroying all the higher mammalian forms, except those which, like the elephant and rhinoceros, could adjust their coats to the altered conditions. even these must have been driven away from the greater part of the area; only those miocene mammals which had passed into hindostan and into south africa would escape decimation by such changes in the physical geography of arctogaea. and when the northern hemisphere passed into its present condition, these lost tribes of the miocene fauna were hemmed by the himalayas, the sahara, the red sea, and the arabian deserts, within their present boundaries. now, on the hypothesis of evolution, there is no sort of difficulty in admitting that the differences between the miocene forms of the mammalian fauna and those which exist at present are the results of gradual modification; and, since such differences in distribution as obtain are readily explained by the changes which have taken place in the physical geography of the world since the miocene epoch, it is clear that the result of the comparison of the miocene and present faunae is distinctly in favour of evolution. indeed i may go further. i may say that the hypothesis of evolution explains the facts of miocene, pliocene, and recent distribution, and that no other supposition even pretends to account for them. it is, indeed, a conceivable supposition that every species of rhinoceros and every species of hyaena, in the long succession of forms between the miocene and the present species, was separately constructed out of dust, or out of nothing, by supernatural power; but until i receive distinct evidence of the fact, i refuse to run the risk of insulting any sane man by supposing that he seriously holds such a notion. let us now take a step further back in time, and inquire into the relations between the miocene fauna and its predecessor of the upper eocene formation. here it is to be regretted that our materials for forming a judgment are nothing to be compared in point of extent or variety with those which are yielded by the miocene strata. however, what we do know of this upper eocene fauna of europe gives sufficient positive information to enable us to draw some tolerably safe inferences. it has yielded representatives of _insectivora_, of _cheiroptera_, of _rodentia_, of _carnivora_, of artiodactyle and perissodactyle _ungulata_, and of opossum-like marsupials. no australian type of marsupial has been discovered in the upper eocene strata, nor any edentate mammal. the genera (except perhaps in the case of some of the _insectivora, cheiroptera_, and _rodentia_) are different from those of the miocene epoch, but present a remarkable general similarity to the miocene and recent genera. in several cases, as i have already shown, it has now been clearly made out that the relation between the eocene and miocene forms is such that the eocene form is the less specialised; while its miocene ally is more so, and the specialisation reaches its maximum in the recent forms of the same type. so far as the upper eocene and the miocene mammalian faunae are comparable, their relations are such as in no way to oppose the hypothesis that the older are the progenitors of the more recent forms, while, in some cases, they distinctly favour that hypothesis. the period in tine and the changes in physical geography represented by the nummulitic deposits are undoubtedly very great, while the remains of middle eocene and older eocene mammals are comparatively few. the general facies of the middle eocene fauna, however, is quite that of the upper. the older eocene pre-nummulitic mammalian fauna contains bats, two genera of _carivora_, three genera of _ungulata_ (probably all perissodactyle), and a didelphid marsupial; all these forms, except perhaps the bat and the opossum, belong to genera which are not known to occur out of the lower eocene formation. the _coryphodon_ appears to have been allied to the miocene and later tapirs, while _pliolophus_, in its skull and dentition, curiously partakes of both artiodactyle and perissodactyle characters; the third trochanter upon its femur, and its three-toed hind foot, however, appear definitely to fix its position in the latter division. there is nothing, then, in what is known of the older eocene mammals of the arctogaeal province to forbid the supposition that they stood in an ancestral relation to those of the calcaire grossier and the gypsum of the paris basin, and that our present fauna, therefore, is directly derived from that which already existed in arctogaea at the commencement of the tertiary period. but if we now cross the frontier between the cainozoic and the mesozoic faunae, as they are preserved within the arctogaeal area, we meet with an astounding change, and what appears to be a complete and unmistakable break in the line of biological continuity. among the twelve or fourteen species of _mammalia_ which are said to have been found in the purbecks, not one is a member of the orders _cheiroptera, rodentia, ungulata_, or _carnivora_, which are so well represented in the tertiaries. no _insectivora_ are certainly known, nor any opossum-like marsupials. thus there is a vast negative difference between the cainozoic and the mesozoic mammalian faunae of europe. but there is a still more important positive difference, inasmuch as all these mammalia appear to be marsupials belonging to australian groups, and thus appertaining to a different distributional province from the eocene and miocene marsupials, which are austro-columbian. so far as the imperfect materials which exist enable a judgment to be formed, the same law appears to have held good for all the earlier mesozoic _mammalia_. of the stonesfield slate mammals, one, _amphitherium_, has a definitely australian character; one, _phascolotherium_, may be either dasyurid or didelphine; of a third, _stereognathus_, nothing can at present be said. the two mammals of the trias, also, appear to belong to australian groups. every one is aware of the many curious points of resemblance between the marine fauna of the european mesozoic rocks and that which now exists in australia. but if there was this australian facies about both the terrestrial and the marine faunae of mesozoic europe, and if there is this unaccountable and immense break between the fauna of mesozoic and that of tertiary europe, is it not a very obvious suggestion that, in the mesozoic epoch, the australian province included europe, and that the arctogaeal province was contained within other limits? the arctogaeal province is at present enormous, while the australian is relatively small. why should not these proportions have been different during the mesozoic epoch? thus i am led to think that by far the simplest and most rational mode of accounting for the great change which took place in the living inhabitants of the european area at the end of the mesozoic epoch, is the supposition that it arose from a vast alteration of the physical geography of the globe; whereby an area long tenanted by cainozoic forms was brought into such relations with the european area that migration from the one to the other became possible, and took place on a great scale. this supposition relieves us, at once, from the difficulty in which we were left, some time ago, by the arguments which i used to demonstrate the necessity of the existence of all the great types of the eocene epoch in some antecedent period. it is this mesozoic continent (which may well have lain in the neighbourhood of what are now the shores of the north pacific ocean) which i suppose to have been occupied by the mesozoic _monodelphia_; and it is in this region that i conceive they must have gone through the long series of changes by which they were specialised into the forms which we refer to different orders. i think it very probable that what is now south america may have received the characteristic elements of its mammalian fauna during the mesozoic epoch; and there can be little doubt that the general nature of the change which took place at the end of the mesozoic epoch in europe was the upheaval of the eastern and northern regions of the mesozoic sea-bottom into a westward extension of the mesozoic continent, over which the mammalian fauna, by which it was already peopled, gradually spread. this invasion of the land was prefaced by a previous invasion of the cretaceous sea by modern forms of mollusca and fish. it is easy to imagine how an analogous change might come about in the existing world. there is, at present, a great difference between the fauna of the polynesian islands and that of the west coast of america. the animals which are leaving their spoils in the deposits now forming in these localities are widely different. hence, if a gradual shifting of the deep sea, which at present bars migration between the easternmost of these islands and america, took place to the westward, while the american side of the sea-bottom was gradually upheaved, the palaeontologist of the future would find, over the pacific area, exactly such a change as i am supposing to have occurred in the north-atlantic area at the close of the mesozoic period. an australian fauna would be found underlying an american fauna, and the transition from the one to the other would be as abrupt as that between the chalk and lower tertiaries; and as the drainage-area of the newly formed extension of the american continent gave rise to rivers and lakes, the mammals mired in their mud would differ from those of like deposits on the australian side, just as the eocene mammals differ from those of the purbecks. how do similar reasonings apply to the other great change of life--that which took place at the end of the palaeozoic period? in the triassic epoch, the distribution of the dry land and of terrestrial vertebrate life appears to have been, generally, similar to that which existed in the mesozoic epoch; so that the triassic continents and their faunae seem to be related to the mesozoic lands and their faunae, just as those of the miocene epoch are related to those of the present day. in fact, as i have recently endeavoured to prove to the society, there was an arctogaeal continent and an arctogaeal province of distribution in triassic times as there is now; and the _sauropsida_ and _marsupialia_ which constituted that fauna were, i doubt not, the progenitors of the _sauropsida_ and _marsupialia_ of the whole mesozoic epoch. looking at the present terrestrial fauna of australia, it appears to me to be very probable that it is essentially a remnant of the fauna of the triassic, or even of an earlier, age[ ] in which case australia must at that time have been in continuity with the arctogaeal continent. [footnote : since this address was read, mr. krefft has sent us news of the discovery in australia of a freshwater fish of strangely palaeozoic aspect, and apparently a ganoid intermediate between _dipterus_ and _lepidosiren_. [the now well-known _ceratodus_. .]] but now comes the further inquiry, where was the highly differentiated sauropsidan fauna of the trias in palaeozoic times? the supposition that the dinosaurian, crocodilian, dicynodontian, and to plesiosaurian types were suddenly created at the end of the permian epoch may be dismissed, without further consideration, as a monstrous and unwarranted assumption. the supposition that all these types were rapidly differentiated out of _lacertilia_ in the time represented by the passage from the palaeozoic to the mesozoic formation, appears to me to be hardly more credible, to say nothing of the indications of the existence of dinosaurian forms in the permian rocks which have already been obtained. for my part, i entertain no sort of doubt that the reptiles, birds, and mammals of the trias are the direct descendants of reptiles, birds, and mammals which existed in the latter part of the palaeozoic epoch, but not in any area of the present dry land which has yet been explored by the geologist. this may seem a bold assumption, but it will not appear unwarrantable to those who reflect upon the very small extent of the earth's surface which has hitherto exhibited the remains of the great mammalian fauna of the eocene times. in this respect, the permian land vertebrate fauna appears to me to be related to the triassic much as the eocene is to the miocene. terrestrial reptiles have been found in permian rocks only in three localities; in some spots of france, and recently of england, and over a more extensive area in germany. who can suppose that the few fossils yet found in these regions give any sufficient representation of the permian fauna? it may be said that the carboniferous formations demonstrate the existence of a vast extent of dry land in the present dry-land area, and that the supposed terrestrial palaeozoic vertebrate fauna ought to have left its remains in the coal-measures, especially as there is now reason to believe that much of the coal was formed by the accumulation of spores and sporangia on dry land. but if we consider the matter more closely, i think that this apparent objection loses its force. it is clear that, during the carboniferous epoch, the vast area of land which is now covered by coal-measures must have been undergoing a gradual depression. the dry land thus depressed must, therefore, have existed, as such, before the carboniferous epoch--in other words, in devonian times--and its terrestrial population may never have been other than such as existed during the devonian, or some previous epoch, although much higher forms may have been developed elsewhere. again, let me say that i am making no gratuitous assumption of inconceivable changes. it is clear that the enormous area of polynesia is, on the whole, an area over which depression has taken place to an immense extent; consequently a great continent, or assemblage of subcontinental masses of land must have existed at some former time, and that at a recent period, geologically speaking, in the area of the pacific. but if that continent had contained mammals, some of them must have remained to tell the tale; and as it is well known that these islands have no indigenous _mammalia_, it is safe to assume that none existed. thus, midway between australia and south america, each of which possesses an abundant and diversified mammalian fauna, a mass of land, which may have been as large as both put together, must have existed without a mammalian inhabitant. suppose that the shores of this great land were fringed, as those of tropical australia are now, with belts of mangroves, which would extend landwards on the one side, and be buried beneath littoral deposits on the other side, as depression went on; and great beds of mangrove lignite might accumulate over the sinking land. let upheaval of the whole now take place, in such a manner as to bring the emerging land into continuity with the south-american or australian continent, and, in course of time, it would be peopled by an extension of the fauna of one of these two regions--just as i imagine the european permian dry land to have been peopled. i see nothing whatever against the supposition that distributional provinces of terrestrial life existed in the devonian epoch, inasmuch as m. barrande has proved that they existed much earlier. i am aware of no reason for doubting that, as regards the grades of terrestrial life contained in them, one of these may have been related to another as new zealand is to australia, or as australia is to india, at the present day. analogy seems to me to be rather in favour of, than against, the supposition that while only ganoid fishes inhabited the fresh waters of our devonian land, _amphibia_ and _reptilia_, or even higher forms, may have existed, though we have not yet found them. the earliest carboniferous _amphibia_ now known, such as _anthracosaurus_, are so highly specialised that i can by no means conceive that they have been developed out of piscine forms in the interval between the devonian and the carboniferous periods, considerable as that is. and i take refuge in one of two alternatives: either they existed in our own area during the devonian epoch and we have simply not yet found them; or they formed part of the population of some other distributional province of that day, and only entered our area by migration at the end of the devonian epoch. whether _reptilia_ and _mammalia_ existed along with them is to me, at present, a perfectly open question, which is just as likely to receive an affirmative as a negative answer from future inquirers. let me now gather together the threads of my argumentation into the form of a connected hypothetical view of the manner in which the distribution of living and extinct animals has been brought about. i conceive that distinct provinces of the distribution of terrestrial life have existed since the earliest period at which that life is recorded, and possibly much earlier; and i suppose, with mr. darwin, that the progress of modification of terrestrial forms is more rapid in areas of elevation than in areas of depression. i take it to be certain that labyrinthodont _amphibia_ existed in the distributional province which included the dry land depressed during the carboniferous epoch; and i conceive that, in some other distributional provinces of that day, which remained in the condition of stationary or of increasing dry land, the various types of the terrestrial _sauropsida_ and of the _mammalia_ were gradually developing. the permian epoch marks the commencement of a new movement of upheaval in our area, which dry land existed in north america, europe, asia, and africa, as it does now. into this great new continental area the mammals, birds, and reptiles developed during the palaeozoic epoch spread, and formed the great triassic arctogaeal province. but, at the end of the triassic period, the movement of depression recommenced in our area, though it was doubtless balanced by elevation elsewhere; modification and development, checked in the one province, went on in that "elsewhere"; and the chief forms of mammals, birds and reptiles, as we know them, were evolved and peopled the mesozoic continent. i conceive australia to have become separated from the continent as early as the end of the triassic epoch, or not much later. the mesozoic continent must, i conceive, have lain to the east, about the shores of the north pacific and indian oceans; and i am inclined to believe that it continued along the eastern side of the pacific area to what is now the province of austro-columbia, the characteristic fauna of which is probably a remnant of the population of the latter part of this period. towards the latter part of the mesozoic period the movement of upheaval around the shores of the atlantic once more recommenced, and was very probably accompanied by a depression around those of the pacific. the vertebrate fauna elaborated in the mesozoic continent moved westward and took possession of the new lands, which gradually increased in extent up to, and in some directions after, the miocene epoch. it is in favour of this hypothesis, i think, that it is consistent with the persistence of a general uniformity in the positions of the great masses of land and water. from the devonian period, or earlier, to the present day, the four great oceans, atlantic, pacific, arctic, and antarctic, may have occupied their present positions, and only their coasts and channels of communication have undergone an incessant alteration. and, finally, the hypothesis i have put before you requires no supposition that the rate of change in organic life has been either greater or less in ancient times than it is now; nor any assumption, either physical or biological, which has not its justification in analogous phenomena of existing nature. i have now only to discharge the last duty of my office, which is to thank you, not only for the patient attention with which you have listened to me so long to-day, but also for the uniform kindness with which, for the past two years, you have rendered my endeavours to perform the important, and often laborious, functions of your president a pleasure instead of a burden. biology and its makers _with portraits and other illustrations_ by william a. locy, ph.d., sc.d. _professor in northwestern university_ [illustration] new york henry holt and company copyright, , by henry holt and company published june, to my graduate students who have worked by my side in the laboratory inspired by the belief that those who seek shall find this account of the findings of some of the great men of biological science is dedicated by the author preface the writer is annually in receipt of letters from students, teachers, ministers, medical men, and others, asking for information on topics in general biology, and for references to the best reading on that subject. the increasing frequency of such inquiries, and the wide range of topics covered, have created the impression that an untechnical account of the rise and progress of biology would be of interest to a considerable audience. as might be surmised, the references most commonly asked for are those relating to different phases of the evolution theory; but the fact is usually overlooked by the inquirers that some knowledge of other features of biological research is essential even to an intelligent comprehension of that theory. in this sketch i have attempted to bring under one view the broad features of biological progress, and to increase the human interest by writing the story around the lives of the great leaders. the practical execution of the task resolved itself largely into the question of what to omit. the number of detailed researches upon which progress in biology rests made rigid selection necessary, and the difficulties of separating the essential from the less important, and of distinguishing between men of temporary notoriety and those of enduring fame, have given rise to no small perplexities. the aim has been kept in mind to give a picture sufficiently diagrammatic not to confuse the general reader, and it is hoped that the omissions which have seemed necessary will, in a measure, be compensated for by the clearness of the picture. references to selected books and articles have been given at the close of the volume, that will enable readers who wish fuller information to go to the best sources. the book is divided into two sections. in the first are considered the sources of the ideas--except those of organic evolution--that dominate biology, and the steps by which they have been molded into a unified science. the doctrine of organic evolution, on account of its importance, is reserved for special consideration in the second section. this is, of course, merely a division of convenience, since after its acceptance the doctrine of evolution has entered into all phases of biological progress. the portraits with which the text is illustrated embrace those of nearly all the founders of biology. some of the rarer ones are unfamiliar even to biologists, and have been discovered only after long search in the libraries of europe and america. an orderly account of the rise of biology can hardly fail to be of service to the class of inquirers mentioned in the opening paragraph. it is hoped that this sketch will also meet some of the needs of the increasing body of students who are doing practical work in biological laboratories. it is important that such students, in addition to the usual classroom instruction, should get a perspective view of the way in which biological science has come into its present form. the chief purpose of the book will have been met if i have succeeded in indicating the sources of biological ideas and the main currents along which they have advanced, and if i have succeeded, furthermore, in making readers acquainted with those men of noble purpose whose work has created the epochs of biological history, and in showing that there has been continuity of development in biological thought. of biologists who may examine this work with a critical purpose, i beg that they will think of it merely as an outline sketch which does not pretend to give a complete history of biological thought. the story has been developed almost entirely from the side of animal life; not that the botanical side has been underestimated, but that the story can be told from either side, and my first-hand acquaintance with botanical investigation is not sufficient to justify an attempt to estimate its particular achievements. the writer is keenly aware of the many imperfections in the book. it is inevitable that biologists with interests in special fields will miss familiar names and the mention of special pieces of notable work, but i am drawn to think that such omissions will be viewed leniently, by the consideration that those best able to judge the shortcomings of this sketch will also best understand the difficulties involved. the author wishes to acknowledge his indebtedness to several publishing houses and to individuals for permission to copy cuts and for assistance in obtaining portraits. he takes this opportunity to express his best thanks for these courtesies. the parties referred to are the director of the american museum of natural history; d. appleton & co.; p. blakiston's sons & co.; the macmillan company; the open court publishing company; the editor of the _popular science monthly_; charles scribner's sons; professors bateson, of cambridge, england; conklin, of philadelphia; joubin, of rennes, france; nierstrasz, of utrecht, holland; newcombe, of ann arbor, michigan; wheeler and e.b. wilson, of new york city. the editor of the _popular science monthly_ has also given permission to reprint the substance of chapters iv and x, which originally appeared in that publication. w.a.l. northwestern university, evanston, ill., april, . contents part i the sources of biological ideas except those of organic evolution chapter i page an outline of the rise of biology and of the epochs in its history, notable advances in natural science during the nineteenth century, . biology the central subject in the history of opinion regarding life, . it is of commanding importance in the world of science, . difficulties in making its progress clear, . notwithstanding its numerous details, there has been a relatively simple and orderly progress in biology, . many books about the facts of biology, many excellent laboratory manuals, but scarcely any attempt to trace the growth of biological ideas, . the growth of knowledge regarding organic nature a long story full of human interest, . the men of science, . the story of their aspirations and struggles an inspiring history, . the conditions under which science developed, . the ancient greeks studied nature by observation and experiment, but this method underwent eclipse, . aristotle the founder of natural history, . science before his day, , . aristotle's position in the development of science, . his extensive knowledge of animals, . his scientific writings, . personal appearance, . his influence, . pliny: his writings mark a decline in scientific method, . the arrest of inquiry and its effects, . a complete change in the mental interests of mankind, . men cease to observe and indulge in metaphysical speculation, . authority declared the source of knowledge, . the revolt of the intellect against these conditions, . the renewal of observation, . the beneficent results of this movement, . enumeration of the chief epochs in biological history: renewal of observation, ; the overthrow of authority in science, . harvey and experimental investigation, ; introduction of microscopes, ; linnæus, ; cuvier, ; bichat, ; von baer, ; the rise of physiology, ; the beginnings of evolutionary thought, ; the cell-theory, ; the discovery of protoplasm, . chapter ii vesalius and the overthrow of authority in science, vesalius, in a broad sense, one of the founders of biology, . a picture of the condition of anatomy before he took it up, . galen: his great influence as a scientific writer, . anatomy in the middle ages, . predecessors of vesalius: mundinus, berangarius, sylvius, . vesalius gifted and forceful, . his impetuous nature, . his reform in the teaching of anatomy, . his physiognomy, . his great book ( ), . a description of its illustrations, , . curious conceits of the artist, . opposition to vesalius: curved thigh bones due to wearing tight trousers, the resurrection bone, , . the court physician, . close of his life, . some of his successors: eustachius and fallopius, . the especial service of vesalius: he overthrew dependence on authority and reëstablished the scientific method of ascertaining truth, , . chapter iii william harvey and experimental observation, harvey's work complemental to that of vesalius, . their combined labors laid the foundations of the modern method of investigating nature, . harvey introduces experiments on living organisms, . harvey's education, . at padua, comes under the influence of fabricius, . return to england, . his personal qualities, - . harvey's writings, . his great classic on movement of the heart and blood ( ), . his demonstration of circulation of the blood based on cogent reasoning; he did not have ocular proof of its passage through capillaries, . views of his predecessors on the movement of the blood, . servetus, . realdus columbus, . cæsalpinus, . the originality of harvey's views, . harvey's argument, . harvey's influence, . a versatile student; work in other directions, . his discovery of the circulation created modern physiology, . his method of inquiry became a permanent part of biological science, . chapter iv the introduction of the microscope and the progress of independent observation, the pioneer microscopists: hooke and grew in england; malpighi in italy and swammerdam and leeuwenhoek in holland, . robert hooke, . his microscope and the micrographia ( ), . grew one of the founders of vegetable histology, . malpighi, - , . personal qualities, . education, . university positions, , . honors at home and abroad, . activity in research, . his principal writings: monograph on the silkworm, ; anatomy of plants, ; work in embryology, . jan swammerdam, - , . his temperament, . early interest in natural history, . studies medicine, . important observations, . devotes himself to minute anatomy, . method of working, . great intensity, . high quality of his work, . the _biblia naturæ_, . its publication delayed until fifty-seven years after his death, . illustrations of his anatomical work, - . antony van leeuwenhoek, - , . a composed and better-balanced man, . self-taught in science, the effect of this showing in the desultory character of his observations, , . physiognomy, . new biographical facts, . his love of microscopic observation, . his microscopes, . his scientific letters, . observes the capillary circulation in , . his other discoveries, . comparison of the three men: the two university-trained men left coherent pieces of work, that of leeuwenhoek was discursive, . the combined force of their labors marks an epoch, . the new intellectual movement now well under way, . chapter v the progress of minute anatomy, progress in minute anatomy a feature of the eighteenth century. attractiveness of insect anatomy. enthusiasm awakened by the delicacy and perfection of minute structure, . lyonet, - , . description of his remarkable monograph on the anatomy of the willow caterpillar, . selected illustrations, - . great detail-- , muscles, . extraordinary character of his drawings, . a model of detailed dissection, but lacking in comparison and insight, . the work of réaumur, roesel, and de geer on a higher plane as regards knowledge of insect life, . straus-dürckheim's monograph on insect anatomy, . rivals that of lyonet in detail and in the execution of the plates, . his general considerations now antiquated, . he attempted to make insect anatomy comparative, . dufour endeavors to found a broad science of insect anatomy, . newport, a very skilful dissector, with philosophical cast of mind, who recognizes the value of embryology in anatomical work, . leydig starts a new kind of insect anatomy embracing microscopic structure (histology), . this the beginning of modern work, . structural studies on other small animals, . the discovery of the simplest animals, . observations on the microscopic animalcula, . the protozoa discovered in by leeuwenhoek, . work of o.f. müller, , . of ehrenberg , . recent observations on protozoa, . chapter vi linnæus and scientific natural history, natural history had a parallel development with comparative anatomy, . the physiologus, or sacred natural history of the middle ages, , . the lowest level reached by zoölogy, . the return to the science of aristotle a real advance over the physiologus, . the advance due to wotton in , . gesner, - . high quality of his _historia animalium_, - . the scientific writings of jonson and aldrovandi, . john ray the forerunner of linnæus, . his writings, . ray's idea of species, . linnæus or linné, . a unique service to natural history. brings the binomial nomenclature into general use, . personal history, . quality of his mind, . his early struggles with poverty, . gets his degree in holland, . publication of the _systema naturæ_ in , . return to sweden, . success as a university professor in upsala, . personal appearance, . his influence on natural history, . his especial service, . his idea of species, . summary, . reform of the linnæan system, - . the necessity of reform, . the scale of being, . lamarck the first to use a genealogical tree, . cuvier's four branches, . alterations by von siebold and leuckart, - . tabular view of classifications, . general biological progress from linnæus to darwin. although details were multiplied, progress was by a series of steps, . analysis of animals proceeded from the organism to organs, from organs to tissues, from tissues to cells, the elementary parts, and finally to protoplasm, - . the physiological side had a parallel development, . chapter vii cuvier and the rise of comparative anatomy, the study of internal structure of living beings, at first merely descriptive, becomes comparative, . belon, . severinus writes the first book devoted to comparative anatomy in , . the anatomical studies of camper, . john hunter, . personal characteristics, . his contribution to progress, . vicq d'azyr the greatest comparative anatomist before cuvier, - . cuvier makes a comprehensive study of the structure of animals, . his birth and early education, . life at the sea shore, . six years of quiet study and contemplation lays the foundation of his scientific career, . goes to paris, . his physiognomy, . comprehensiveness of his mind, . founder of comparative anatomy, . his domestic life, . some shortcomings, . his break with early friends, . estimate of george bancroft, . cuvier's successors: milne-edwards, ; lacaze-duthiers, ; richard owen, ; oken, ; j. fr. meckel, ; rathke, ; j. müller, ; karl gegenbaur, ; e.d. cope, . comparative anatomy a rich subject, . it is now becoming experimental, . chapter viii bichat and the birth of histology, bichat one of the foremost men in biological history. he carried the analysis of animal organization to a deeper level than cuvier, . buckle's estimate, . bichat goes to paris, . attracts attention in desault's classes, . goes to live with desault, . his fidelity and phenomenal industry, . personal appearance, . begins to publish researches on tissues at the age of thirty, . his untimely death at thirty-one, . influence of his writings, . his more notable successors: schwann, ; koelliker, a striking figure in the development of biology, ; max schultze, ; rudolph virchow, ; leydig, ; ramon y cajal, . modern text-books on histology, . chapter ix the rise of physiology--harvey. haller. johannes müller, physiology had a parallel development with anatomy, . physiology of the ancients, . galen, . period of harvey, . his demonstration of circulation of the blood, . his method of experimental investigation, . period of haller, . physiology developed as an independent science, . haller's personal characteristics, . his idea of vital force, . his book on the elements of physiology a valuable work, . discovery of oxygen by priestley in , . charles bell's great discovery on the nervous system, . period of johannes müller, . a man of unusual gifts and personal attractiveness, . his personal appearance, . his great influence over students, . his especial service was to make physiology broadly comparative, . his monumental handbook of physiology, . unexampled accuracy in observation, . introduces the principles of psychology into physiology, . physiology after müller, - . ludwig, . du bois-reymond, . claude bernard, . two directions of growth in physiology--the chemical and the physical, . influence upon biology, . other great names in physiology, . chapter x von baer and the rise of embryology, romantic nature of embryology, . its importance, . rudimentary organs and their meaning, . the domain of embryology, . five historical periods, . the period of harvey and malpighi, - . the embryological work of these two men insufficiently recognized, . harvey's pioneer attempt critically to analyze the process of development, . his teaching regarding the nature of development, . his treatise on generation, . the frontispiece of the edition of , , . malpighi's papers on the formation of the chick within the egg, . quality of his pictures, . his belief in pre-formation, . malpighi's rank as embryologist, . the period of wolff, - . rise of the theory of pre-delineation, . sources of the idea that the embryo is pre-formed within the egg, . malpighi's observations quoted, . swammerdam's view, . leeuwenhoek and the discovery of the sperm, . bonnet's views on _emboîtement_, . wolff opposes the doctrine of pre-formation, . his famous theory of generation ( ), . sketches from this treatise, . his views on the directing force in development, . his highest grade of work, . opposition of haller and bonnet, . restoration of wolff's views by meckel, . personal characteristics of wolff, . the period of von baer, - . the greatest personality in embryology, . his monumental work on the development of animals a choice combination of observation and reflection, . von baer's especial service, . establishes the germ-layer theory, . consequences, . his influence on embryology, . the period from von baer to balfour, - . the process of development brought into a new light by the cell-theory, . rathke, remak, koelliker, huxley, kowalevsky, , . beginnings of the idea of germinal continuity, . influence of the doctrine of organic evolution, . the period of balfour, with an indication of present tendencies, - . the great influence of balfour's comparative embryology, . personality of balfour, . his tragic fate, . interpretation of the embryological record, . the recapitulation theory, . oskar hertwig, . wilhelm his, . recent tendencies; experimental embryology, ; cell-lineage, ; theoretical discussions, . chapter xi the cell-theory--schleiden. schwann. schultze, unifying power of the cell-theory, . vague foreshadowings, . the first pictures of cells from robert hooke's micrographia, . cells as depicted by malpighi, grew, and leeuwenhoek, , . wolff on cellular structure, , . oken, . the announcement of the cell-theory in - , . schleiden and schwann co-founders, . schleiden's work, . his acquaintance with schwann, . schwann's personal appearance, . influenced by johannes müller, . the cell-theory his most important work, . schleiden, his temperament and disposition, . schleiden's contribution to the cell-theory, . errors in his observations and conclusions, . schwann's treatise, . purpose of his researches, . quotations from his microscopical researches, . schwann's part in establishing the cell-theory more important than that of schleiden, . modification of the cell-theory, . necessity of modifications, . the discovery of protoplasm, and its effect on the cell-theory, . the cell-theory becomes harmonized with the protoplasm doctrine of max schultze, . further modifications of the cell-theory, . origin of cells in tissues, . structure of the nucleus, . chromosomes, . centrosome, . the principles of heredity as related to cellular studies, . verworn's definition, . vast importance of the cell-theory in advancing biology, . chapter xii protoplasm the physical basis of life, great influence of the protoplasm doctrine on biological progress, . protoplasm, . its properties as discovered by examination of the amoeba, . microscopic examination of a transparent leaf, . unceasing activity of its protoplasm, . the wonderful energies of protoplasm, . quotation from huxley, . the discovery of protoplasm and the essential steps in recognizing the part it plays in living beings, - . dujardin, . his personality, . education, . his contributions to science, . his discovery of "sarcode" in the simplest animals, in , . purkinje, in , uses the term protoplasma, . von mohl, in , brings the designation protoplasm into general use, . cohn, in , maintains the identity of sarcode and protoplasm, . work of de bary and virchow, . max schultze, in , shows that there is a broad likeness between the protoplasm of animals and plants, and establishes the protoplasm doctrine. the university life of schultze. his love of music and science. founds a famous biological periodical, - . the period from to an important one for biology, . chapter xiii the work of pasteur, koch, and others, the bacteria discovered by leeuwenhoek in , . the development of the science of bacteriology of great importance to the human race, . some general topics connected with the study of bacteria, . the spontaneous origin of life, - . biogenesis or abiogenesis, . historical development of the question, . i. from aristotle, b.c., to redi, , . the spontaneous origin of living forms universally believed in, . illustrations, . ii. from redi to schwann, - . redi, in , puts the question to experimental test and overthrows the belief in the spontaneous origin of forms visible to the unaided eye, . the problem narrowed to the origin of microscopic animalcula, . needham and buffon test the question by the use of tightly corked vials containing boiled organic solutions, . microscopic life appears in their infusions, . spallanzani, in , uses hermetically sealed glass flasks and gets opposite results, . the discovery of oxygen raises another question: does prolonged heat change its vitalizing properties? . experiments of schwann and schulze, - , . the question of the spontaneous origin of microscopic life regarded as disproved, . iii. pouchet reopens the question in , maintaining that he finds microscopic life produced in sterilized and hermetically sealed solutions, . the question put to rest by the brilliant researches of pasteur and of tyndall, , . description of tyndall's apparatus and his use of optically pure air, . weismann's theoretical speculations regarding the origin of biophors, . the germ-theory of disease, - . the idea of _contagium vivum_ revived in , . work of bassi, . demonstration, in , of the actual connection between anthrax and splenic fever, . veneration of pasteur, . his personal qualities, . filial devotion, . steps in his intellectual development, . his investigation of diseases of wine ( ), . of the silkworm plague ( - ), . his studies on the cause and prevention of disease constitute his chief service to humanity, . establishment of the pasteur institute in paris, . recent developments, . robert koch; his services in discovering many bacteria of disease, . sir joseph lister and antiseptic surgery, . bacteria in their relation to agriculture, soil inoculation, etc., . knowledge of bacteria as related to the growth of general biology, . chapter xiv heredity and germinal continuity--mendel. galton. weismann, the hereditary substance and the bearers of heredity, . the nature of inheritance, . darwin's theory of pangenesis, . the theory of pangens replaced by that of germinal continuity, . exposition of the theory of germinal continuity, . the law of cell-succession, . _omnis cellula e cellula_, . the continuity of hereditary substance, . early writers, . weismann, . germ-cells and body cells, . the hereditary substance is the germ-plasm, . it embodies all the past history of protoplasm, . the more precise investigation of the material basis of inheritance, . the nucleus of cells, . the chromosomes, . the fertilized ovum, the starting-point of new organisms, . behavior of the nucleus during division, . the mixture of parental qualities in the chromosomes, . prelocalized areas in the protoplasm of the egg, . the inheritance of acquired characteristics, . the application of statistical methods and experiments to the study of heredity, . mendel's important discovery of alternative inheritance, . francis galton, . carl pearson, . experiments on inheritance, . chapter xv the science of fossil life, extinct forms of life, . strange views regarding fossils, . freaks of nature, . mystical explanations, . large bones supposed to be those of giants, . determination of the nature of fossils by steno, . fossil deposits ascribed to the flood, . mosaic deluge regarded as of universal extent, . the comparison of fossil and living animals of great importance, . cuvier the founder of vertebrate palæontology, . lamarck founds invertebrate palæontology, . lamarck's conception of the meaning of fossils more scientific than cuvier's, . the arrangement of fossils in strata, . william smith, . summary of the growth of the science of fossil life, . fossil remains as an index to the past history of the earth, . epoch-making work of charles lyell, . effect of the doctrine of organic evolution on palæontology, . richard owen's studies on fossil animals, . agassiz and the parallelism between fossil forms of life and stages in the development of animals, . huxley's geological work, . leidy, . cope, . marsh, . carl zittel's writings and influence, . henry f. osborn, . method of collecting fossils, . fossil remains of man, . discoveries in the fayûm district of africa, . part ii the doctrine of organic evolution chapter xvi what evolution is: the evidence upon which it rests, etc., great vagueness regarding the meaning of evolution, . causes for this, . the confusion of darwinism with organic evolution, . the idea that the doctrine is losing ground, . scientific controversies on evolution relate to the factors, not to the fact, of evolution, . nature of the question: not metaphysical, not theological, but historical, . the historical method applied to the study of animal life, . the diversity of living forms, . are species fixed in nature? . wide variation among animals, . evolutionary series: the shells of slavonia and steinheim, - . evolution of the horse, . the collection of fossil horses at the american museum of natural history, new york, . the genealogy of the horse traced for more than two million years, . connecting forms: the archæopteryx and pterodactyls, . the embryological record and its connection with evolution, . clues to the past history of animals, . rudimentary organs, - . hereditary survivals in the human body, . remains of the scaffolding for its building, . antiquity of man, . pre-human types, . virtually three links: the java man; the neanderthal skull; the early neolithic man of engis, - . evidences of man's evolution based on palæontology, embryology, and archæology, . mental evolution, . sweep of the doctrine of organic evolution, - . chapter xvii theories of evolution--lamarck. darwin, the attempt to indicate the active factors of evolution is the source of the different theories, . the theories of lamarck, darwin, and weismann have attracted the widest attention, . lamarck, the man, - . his education, . leaves priestly studies for the army, . great bravery, . physical injury makes it necessary for him to give up military life, . portrait, . important work in botany, . pathetic poverty and neglect, . changes from botany to zoölogy at the age of fifty years, . profound influence of this change in shaping his ideas, . his theory of evolution, - . first public announcement in , . his _philosophie zoologique_ published in , . his two laws of evolution, . the first law embodies the principle of use and disuse of organs, the second that of heredity, . a simple exposition of his theory, . his employment of the word _besoin_, . lamarck's view of heredity, . his belief in the inheritance of acquired characters, . his attempt to account for variation, . time and favorable conditions the two principal means employed by nature, . salient points in lamarck's theory, . his definition of species, . neo-lamarckism, . darwin. his theory rests on three sets of facts. the central feature of his theory is natural selection. variation, . inheritance, . those variations will be inherited that are of advantage to the race, . illustrations of the meaning of natural selection, - . the struggle for existence and its consequences, . various aspects of natural selection, . it does not always operate toward increasing the efficiency of an organ--short-winged beetles, . color of animals, . mimicry, . sexual selection, . inadequacy of natural selection, . darwin the first to call attention to the inadequacy of this principle, . confusion between the theories of lamarck and darwin, . illustrations, . the origin of species published in , . other writings of darwin, . chapter xviii theories continued--weismann. de vries, weismann's views have passed through various stages of remodeling, . the evolution theory published in is the best exposition of his views, . his theory the field for much controversy. primarily a theory of heredity, . weismann's theory summarized, . continuity of the germ-plasm the central idea in weismann's theory, . complexity of the germ-plasm. illustrations, . the origin of variations, . the union of two complex germ-plasms gives rise to variations, . his extension of the principle of natural selection--germinal selection, . the inheritance of acquired characters, . weismann's analysis of the subject the best, . illustrations, . the question still open to experimental observation, . weismann's personality, . quotation from his autobiography, . the mutation theory of de vries, . an important contribution. his application of experiments commendable, . the mutation theory not a substitute for that of natural selection, . tendency toward a reconciliation of apparently conflicting views, . summary of the salient features of the theories of lamarck, of darwin, of weismann, and de vries, . causes for bewilderment in the popular mind regarding the different forms of the evolution theory, . chapter xix the rise of evolutionary thought, opinion before lamarck, . views of certain fathers of the church, . st. augustine, . st. thomas aquinas, . the rise of the doctrine of special creation, . suarez, . effect of john milton's writings, . forerunners of lamarck: buffon, erasmus darwin, goethe, . statement of buffon's views on evolution, . erasmus darwin the greatest of lamarck's predecessors, . his writings, . paley's natural theology directed against them, . goethe's connection with evolutionary thought, . causes for the neglect of lamarck's theoretical writings, . the temporary disappearance of the doctrine of organic evolution, . cuvier's opposition, . the debate between cuvier and st. hilaire, . its effect, . influence of lyell's principles of geology, . herbert spencer's analysis in , . darwin and wallace, . circumstances under which their work was laid before the linnæan society of london, . the letter of transmission signed by lyell and hooker, - . the personality of darwin, . appearance, . his charm of manner, . affectionate consideration at home, . unexampled industry and conscientiousness in the face of ill health, , . his early life and education, . voyage of the _beagle_, . the results of his five years' voyage, . life at downs, . parallelism in the thought of darwin and wallace, . darwin's account of how he arrived at the conception of natural selection, . wallace's narrative, . the darwin-wallace theory launched in , . darwin's book on the origin of species regarded by him as merely an outline, . the spread of the doctrine of organic evolution, . huxley one of its great popular exponents, . haeckel, . after darwin, the problem was to explain phenomena, . chapter xx retrospect and prospect. present tendencies in biology, biological thought shows continuity of development, . character of the progress--a crusade against superstition, . the first triumph of the scientific method was the overthrow of authority, . the three stages of progress--descriptive, comparative, experimental, . the notable books of biology and their authors, - . recent tendencies in biology: higher standards, ; improvement in the tools of science, ; advance in methods, ; experimental work, ; the growing interest in the study of processes, ; experiments applied to heredity and evolution, to fertilization of the egg, and to animal behavior, , . some tendencies in anatomical studies, . cell-lineage, . new work on the nervous system, . the application of biological facts to the benefit of mankind, . technical biology, . soil inoculation, . relation of insects to the transmission of diseases, . the food of fishes, . the establishment and maintenance of biological laboratories, . the station at naples, . other stations, . the establishment and maintenance of technical periodicals, . explorations of fossil records, . the reconstructive influence of biological progress, . reading list, i. general references, - . ii. special references, - . index, illustrations fig. page . aristotle, - b.c., . pliny, - a.d., . galen, - , . vesalius, - , . anatomical sketch from vesalius' _fabrica_ ( ), . the skeleton from vesalius' _fabrica_, . initial letters from the _fabrica_, . fallopius, - , . fabricius, harvey's teacher, - , . william harvey, - , . scheme of the portal circulation according to vesalius ( ), . hooke's microscope ( ), . malpighi, - , . from malpighi's _anatomy of the silkworm_ ( ), . swammerdam, - , . from swammerdam's _biblia naturæ_, . anatomy of an insect dissected and drawn by swammerdam, . leeuwenhoek, - , . leeuwenhoek's microscope, _a_. leeuwenhoek's mechanism for examining the circulation of the blood, _b_. the capillary circulation, after leeuwenhoek, . plant cells from leeuwenhoek's _arcana naturæ_, . lyonet, - , . larva of the willow moth, from lyonet's monograph ( ), . muscles of the larva of the willow moth, from lyonet's monograph, . central nervous system and nerves of the same animal, . dissection of the head of the larva of the willow moth, . the brain and head nerves of the same animal, . roesel von rosenhof, - , . réaumur, - , . nervous system of the cockchafer, from straus-dürckheim's monograph ( ), . ehrenberg, - , . gesner, - , . john ray, - , . linnæus at sixty ( - ), . karl th. von siebold, . rudolph leuckart, . severinus, - , . camper, - , . john hunter, - , . vicq d'azyr, - , . cuvier as a young man, - , . cuvier at the zenith of his power, . h. milne-edwards, - , . lacaze-duthiers, - , . lorenzo oken, - , . richard owen, - , . j. fr. meckel, - , . karl gegenbaur, - , . bichat, - , . von koelliker, - , . rudolph virchow, - , . franz leydig, - (april), . s. ramon y cajal, . albrecht haller, - , . charles bell, - , . johannes müller, - , . ludwig, - , . du bois-reymond, - , . claude bernard, - , . frontispiece of harvey's _generatione animalium_ ( ), . selected sketches from malpighi's works, . marcello malpighi, - , . plate from wolff's _theoria generationis_ ( ), . charles bonnet, - , . karl ernst von baer, - , . von baer at about seventy years of age, . sketches from von baer's embryological treatise ( ), . a. kowalevsky, - , . francis m. balfour, - , . oskar hertwig in , . wilhelm his, - , . the earliest known picture of cells, from hooke's _micrographia_ ( ), . sketch from malpighi's treatise on the anatomy of plants ( ), . theodor schwann, - , . m. schleiden, - , . the egg and early stages in its development (after gegenbaur), . an early stage in the development of the egg of a rock limpet (after conklin), . highly magnified tissue-cells from the skin of a salamander (after wilson), . diagram of the chief steps in cell-division (after parker), . diagram of a cell (modified after wilson), . (_a_) rotation of protoplasm in cells of nitella. (_b_) highly magnified cells of a tradescantia plant, showing circulation of protoplasm (after sedgwick and wilson), . félix dujardin, - , . purkinje, - , . carl nägeli, - , . hugo von mohl, - , . ferdinand cohn, - , . heinrich anton de bary, - , . max schultze, - , . francesco redi, - , . lazzaro spallanzani, - , . apparatus of tyndall for experimenting on spontaneous generation, . louis pasteur ( - ) and his granddaughter, . robert koch, born , . sir joseph lister, born , . gregor mendel, - , . francis galton, born , . charles lyell, - , . professor owen and the extinct fossil bird of new zealand, . louis agassiz, - , . e.d. cope, - , . o.c. marsh, - , . karl von zittel, - , . transmutations of paludina (after neumayer), . planorbis shells from steinheim (after hyatt), . bones of the foreleg of a horse, . bones of fossil ancestors of the horse, . representation of the ancestor of the horse drawn by charles r. knight under the direction of professor osborn. permission of the american museum of natural history, . fossil remains of a primitive bird (archæopteryx), . gill-clefts of a shark compared with those of the embryonic chick and rabbit, . jaws of an embryonic whale, showing rudimentary teeth, . profile reconstructions of the skulls of living and of fossil men, . lamarck, - , . charles darwin, - , . august weismann, born , . hugo de vries, . buffon, - , . erasmus darwin, - , . geoffroy saint hilaire, - , . charles darwin, - , . alfred russel wallace, born , . thomas henry huxley, - , . ernst haeckel, born , . the biological station at naples, part i the sources of biological ideas except those of organic evolution chapter i an outline of the rise of biology and of the epochs in its history "truth is the daughter of time." the nineteenth century will be for all time memorable for the great extension of the knowledge of organic nature. it was then that the results of the earlier efforts of mankind to interpret the mysteries of nature began to be fruitful; observers of organic nature began to see more deeply into the province of life, and, above all, began to see how to direct their future studies. it was in that century that the use of the microscope made known the similarity in cellular construction of all organized beings; that the substance, protoplasm, began to be recognized as the physical basis of life and the seat of all vital activities; then, most contagious diseases were traced to microscopic organisms, and as a consequence, medicine and surgery were reformed; then the belief in the spontaneous origin of life under present conditions was given up; and it was in that century that the doctrine of organic evolution gained general acceptance. these and other advances less generally known created an atmosphere in which biology--the great life-science--grew rapidly. in the same period also the remains of ancient life, long since extinct, and for countless ages embedded in the rocks, were brought to light, and their investigation assisted materially in understanding the living forms and in tracing their genealogy. as a result of these advances, animal organization began to have a different meaning to the more discerning naturalists, those whose discoveries began to influence the trend of thought, and finally, the idea which had been so often previously expressed became a settled conviction, that all the higher forms of life are derived from simpler ones by a gradual process of modification. besides great progress in biology, the nineteenth century was remarkable for similar advances in physics and chemistry. although these subjects purport to deal with inorganic or lifeless nature, they touch biology in an intimate way. the vital processes which take place in all animals and plants have been shown to be physico-chemical, and, as a consequence, one must go to both physics and chemistry in order to understand them. the study of organic chemistry in late years has greatly influenced biology; not only have living products been analyzed, but some of them have already been constructed in the chemical laboratory. the formation of living matter through chemical means is still far from the thought of most chemists, but very complex organic compounds, which were formerly known only as the result of the action of life, have been produced, and the possibilities of further advances in that direction are very alluring. it thus appears that the discoveries in various fields have worked together for a better comprehension of nature. the domain of biology.--the history of the transformation of opinion in reference to living organisms is an interesting part of the story of intellectual development. the central subject that embraces it all is biology. this is one of the fundamental sciences, since it embraces all questions relating to life in its different phases and manifestations. everything pertaining to the structure, the development, and the evolution of living organisms, as well as to their physiology, belongs to biology. it is now of commanding importance in the world of science, and it is coming more and more to be recognized that it occupies a field of compelling interest not only for medical men and scholars, but for all intelligent people. the discoveries and conquests of biology have wrought such a revolution in thought that they should be known to all persons of liberal culture. in addition to making acquaintance with the discoveries, one ought to learn something about the history of biology; for it is essential to know how it took its rise, in order to understand its present position and the nature of its influence upon expanding ideas regarding the world in which we live. in its modern sense, biology did not arise until about , when the nature of protoplasm was first clearly pointed out by max schultze, but the currents that united to form it had long been flowing, and we can never understand the subject without going back to its iatric condition, when what is now biology was in the germ and united with medicine. its separation from medicine, and its rise as an independent subject, was owing to the steady growth of that zest for exploration into unknown fields which began with the new birth of science in the sixteenth century, and has continued in fuller measure to the present. it was the outcome of applying observation and experiment to the winning of new truths. difficulties.--but biology is so comprehensive a field, and involves so many details, that it is fair to inquire: can its progress be made clear to the reader who is unacquainted with it as a laboratory study? the matter will be simplified by two general observations--first, that the growth of biology is owing to concurrent progress in three fields of research, concerned, respectively, with the structure or architecture of living beings, their development, and their physiology. we recognize also a parallel advance in the systematic classification of animals and plants, and we note, furthermore, that the idea of evolution permeates the whole. it will be necessary to consider the advances in these fields separately, and to indicate the union of the results into the main channel of progress. secondly, in attempting to trace the growth of ideas in this department of learning one sees that there has been a continuity of development. the growth of these notions has not been that of a chaotic assemblage of ideas, but a well-connected story in which the new is built upon the old in orderly succession. the old ideas have not been completely superseded by the new, but they have been molded into new forms to keep pace with the advance of investigation. in its early phases, the growth of biology was slow and discursive, but from the time of linnæus to darwin, although the details were greatly multiplied, there has been a relatively simple and orderly progress. facts and ideas.--there are many books about biology, with directions for laboratory observation and experiment, and also many of the leading facts of the science have been given to the public, but an account of the growth of the ideas, which are interpretations of the facts, has been rarely attempted. from the books referred to, it is almost impossible to get an idea of biology as a unit; this even the students in our universities acquire only through a coherent presentation of the subject in the classroom, on the basis of their work in the laboratory. the critical training in the laboratory is most important, but, after all, it is only a part, although an essential part, of a knowledge of biology. in general, too little attention is paid to interpretations and the drill is confined to a few facts. now, the facts are related to the ideas of the science as statistics to history--meaningless without interpretation. in the rise of biology the facts have accumulated constantly, through observation and experiment, but the general truths have emerged slowly and periodically, whenever there has been granted to some mind an insight into the meaning of the facts. the detached facts are sometimes tedious, the interpretations always interesting. the growth of the knowledge of organic nature is a long story, full of human interest. nature has been always the same, but the capacity of man as its interpreter has varied. he has had to pass through other forms of intellectual activity, and gradually to conquer other phases of natural phenomena, before entering upon that most difficult task of investigating the manifestations of life. it will be readily understood, therefore, that biology was delayed in its development until after considerable progress had been made in other sciences. it is an old saying that "truth is the daughter of time," and no better illustration of it can be given than the long upward struggle to establish even the elemental truths of nature. it took centuries to arrive at the conception of the uniformity of nature, and to reach any of those generalizations which are vaguely spoken of as the laws of nature. the men of science.--in the progress of science there is an army of observers and experimenters each contributing his share, but the rank and file supply mainly isolated facts, while the ideas take birth in the minds of a few gifted leaders, either endowed with unusual insight, or so favored by circumstances that they reach general conclusions of importance. these advance-guards of intellectual conquest we designate as founders. what were they like in appearance? under what conditions did they work, and what was their chief aim? these are interesting questions which will receive attention as our narrative proceeds. a study of the lives of the founders shows that the scientific mood is pre-eminently one of sincerity. the men who have added to the growth of science were animated by an unselfish devotion to truth, and their lasting influence has been in large measure a reflection of their individual characters. only those have produced permanent results who have interrogated nature in the spirit of devotion to truth and waited patiently for her replies. the work founded on selfish motives and vanity has sooner or later fallen by the wayside. we can recognize now that the work of scientific investigation, subjected to so much hostile criticism as it appeared from time to time, was undertaken in a reverent spirit, and was not iconoclastic, but remodelling in its influence. some of the glories of our race are exhibited in the lives of the pioneers in scientific progress, in their struggles to establish some great truth and to maintain intellectual integrity. the names of some of the men of biology, such as harvey, linnæus, cuvier, darwin, huxley, and pasteur, are widely known because their work came before the people, but others equally deserving of fame on account of their contributions to scientific progress will require an introduction to most of our readers. in recounting the story of the rise of biology, we shall have occasion to make the acquaintance of this goodly company. before beginning the narrative in detail, however, we shall look summarily at some general features of scientific progress and at the epochs of biology. the conditions under which science developed in a brief sketch of biology there is relatively little in the ancient world that requires notice except the work of aristotle and galen; but with the advent of vesalius, in , our interest begins to freshen, and, thereafter, through lean times and fat times there is always something to command our attention. the early conditions must be dealt with in order to appreciate what followed. we are to recollect that in the ancient world there was no science of biology as such; nevertheless, the germ of it was contained in the medicine and the natural history of those times. there is one matter upon which we should be clear: in the time of aristotle nature was studied by observation and experiment. this is the foundation of all scientific advancement. had conditions remained unchanged, there is reason to believe that science would have developed steadily on the basis of the greek foundation, but circumstances, to be spoken of later, arose which led not only to the complete arrest of inquiry, but also, the mind of man being turned away from nature, to the decay of science. aristotle the founder of natural history.--the greeks represented the fullest measure of culture in the ancient world, and, naturally, we find among them the best-developed science. all the knowledge of natural phenomena centered in aristotle ( - b.c.), and for twenty centuries he represented the highest level which that kind of knowledge had attained. it is uncertain how long it took the ancient observers to lift science to the level which it had at the beginning of aristotle's period, but it is obvious that he must have had a long line of predecessors, who had accumulated facts of observation and had molded them into a system before he perfected and developed that system. we are reminded that all things are relative when we find aristotle referring to the ancients; and well he might, for we have indubitable evidence that much of the scientific work of antiquity has been lost. one of the most striking discoveries pointing in that direction is the now famous papyrus which was found by georg ebers in egypt about . the recent translation of this ancient document shows that it was a treatise on medicine, dating from the fifteenth century b.c. at this time the science of medicine had attained an astonishingly high grade of development among that people. and since it is safe to assume that the formulation of a system of medicine in the early days of mankind required centuries of observation and practice, it becomes apparent that the manuscript in question was no vague, first attempt at reducing medicine to a system. it is built upon much scientific knowledge, and must have been preceded by writings both on medicine and on its allied sciences. it is not necessary that we should attempt to picture the crude beginnings of the observation of animated nature and the dawning of ideas relative to animals and plants; it is suitable to our purpose to commence with aristotle, and to designate him, in a relative sense, as the founder of natural history. that he was altogether dissatisfied with the state of knowledge in his time and that he had high ideals of the dignity of science is evidenced in his writings. although he refers to the views of the ancients, he regarded himself in a sense as a pioneer. "i found no basis prepared," he says, "no models to copy.... mine is the first step, and therefore a small one, though worked out with much thought and hard labor. it must be looked at as a first step and judged with indulgence." (from osborn's _from the greeks to darwin_.) there is general agreement that aristotle was a man of vast intellect and that he was one of the greatest philosophers of the ancient world. he has had his detractors as well as his partisan adherents. perhaps the just estimate of his attainments and his position in the history of science is between the enthusiastic appreciation of cuvier and the critical estimate of lewes. this great man was born in stagira in the year b.c., and lived until b.c. he is to be remembered as the most distinguished pupil of plato, and as the instructor of alexander the great. like other scholars of his time, he covered a wide range of subjects; we have mention, indeed, of about three hundred works of his composition, many of which are lost. he wrote on philosophy, metaphysics, psychology, politics, rhetoric, etc., but it was in the domain of natural history that he attained absolute pre-eminence. his position in the development of science.--it is manifestly unjust to measure aristotle by present standards; we must keep always in mind that he was a pioneer, and that he lived in an early day of science, when errors and crudities were to be expected. his greatest claim to eminence in the history of science is that he conceived the things of importance and that he adopted the right method in trying to advance the knowledge of the natural universe. in his program of studies he says: "first we must understand the phenomena of animals; then assign their causes; and, finally, speak of their generation." his position in natural history is frequently misunderstood. one of the most recent writers on the history of science, henry smith williams, pictures him entirely as a great classifier, and as the founder of systematic zoölogy. while it is true that he was the founder of systematic zoölogy, as such he did not do his greatest service to natural history, nor does the disposition to classify represent his dominant activity. in all his work classification is made incidental and subservient to more important considerations. his observations upon structure and development, and his anticipation of the idea of organic evolution, are the ones upon which his great fame rests. he is not to be remembered as a man of the type of linnæus; rather is he the forerunner of those men who looked deeper than linnæus into the structure and development of animal life--the morphologists. particular mention of his classification of animals will be found in the chapter on linnæus, while in what follows in this chapter attention will be confined to his observation of their structure and development and to the general influence of his work. his great strength was in a philosophical treatment of the structure and development of animals. professor osborn in his interesting book, _from the greeks to darwin_, shows that aristotle had thought out the essential features of evolution as a process in nature. he believed in a complete gradation from the lowest organisms to the highest, and that man is the highest point of one long and continuous ascent. his extensive knowledge of animals.--he made extensive studies of life histories. he knew that drone bees develop without previous fertilization of the eggs (by parthenogenesis); that in the squid the yolk sac of the embryo is carried in front of the mouth; that some sharks develop within the egg-tube of the mother, and in some species have a rudimentary blood-connection resembling the placenta of mammals. he had followed day by day the changes in the chick within the hen's egg, and observed the development of many other animals. in embryology also, he anticipated harvey in appreciating the true nature of development as a process of gradual building, and not as the mere expansion of a previously formed germ. this doctrine, which is known under the name of epigenesis, was, as we shall see later, hotly contested in the eighteenth century, and has a modified application at the present time. in reference to the structure of animals he had described the tissues, and in a rude way analyzed the organs into their component parts. it is known, furthermore, that he prepared plates of anatomical figures, but, unfortunately, these have been lost. in estimating the contributions of ancient writers to science, it must be remembered that we have but fragments of their works to examine. it is, moreover, doubtful whether the scientific writings ascribed to aristotle were all from his hand. the work is so uneven that huxley has suggested that, since the ancient philosophers taught _viva voce_, what we have of his zoölogical writings may possibly be the notes of some of his students. while this is not known to be the case, that hypothesis enables us to understand the intimate mixture of profound observation with trivial matter and obvious errors that occur in the writings ascribed to him. hertwig says: "it is a matter for great regret that there have been preserved only parts of his three most important zoölogical works, '_historia animalium_,' '_de partibus_,' and '_de generatione_,' works in which zoölogy is founded as a universal science, since anatomy and embryology, physiology and classification, find equal consideration." some errors.--dissections were little practised in his day, and it must be admitted that his observations embrace many errors. he supposed the brain to be bloodless, the arteries to carry air, etc., but he has been cleared by huxley of the mistake so often attributed to him of supposing the heart of mammals to have only three chambers. it is altogether probable that he is credited with a larger number of errors than is justified by the facts. he must have had unusual gifts in the exposition of these technical subjects; indeed, he made his researches appear so important to his royal patron, alexander, that he was aided in the preparation of his great natural history by a grant of talents (equivalent to $ , ) and by numerous assistants and collectors. thus in ancient times was anticipated the question that is being agitated to-day--that of the support and the endowment of research. personal appearance.--some idea of his looks may be gained from fig. . this is a copy of a bas-relief found in the collection of fulvius ursinus (d. ), and was originally published by j. faber. its authenticity as a portrait is attested ( ) by visconti, who says that it has a perfect resemblance to the head of a small bust upon the base of which the name of aristotle is engraved. portrait busts and statues of aristotle were common in ancient times. the picture of him most familiar to general readers is the copy of the head and shoulders of an ancient statue representing him with a draping over the left shoulder. this is an attractive portrait, showing a face of strong intellectuality. its authenticity, however, is not as well established as that of the picture shown here. other pictures, believed to be those of aristotle, represent him later in life with receding hair, and one exists in which his baldness is very extensive. he was described as short in stature, with spindling legs and small, penetrating eyes, and to have been, in his younger days, vain and showy in his dress. he was early left an orphan with a considerable fortune; and there are stories of early excesses after coming into his property. these charges, however, lack trustworthy support, and are usually regarded as due mainly to that undermining gossip which follows one holding prominent place and enviable recognition. his habits seem to have been those of a diligent student with a zest in his work; he was an omnivorous reader, and plato called him the mind of his school. his large private library and his manner of living bespeak the conserving of his property, rather than its waste in selfish indulgences. [illustration: fig. .--aristotle, - b.c.] his influence.--the influence of aristotle was in the right direction. he made a direct appeal to nature for his facts, and founded his natural history only on observation of the structure, physiology, and development of animals. unfortunately, the same cannot be said of his successors. galen, who is mentioned above in connection with aristotle, was a medical writer and the greatest anatomist of antiquity. on account of the relation of his work to the growth of anatomy, however, the consideration of it is reserved for the chapter on vesalius. soon after the period of aristotle the center of scientific investigation was transferred to alexandria, where ptolemy had erected a great museum and founded a large public library. here mathematics and geography flourished, but natural history was little cultivated. in order to find the next famous naturalist of antiquity, it is necessary to look to rome. rome, although great in political power, never became a true culture center, characterized by originality. all that remains of their thought shows us that the roman people were not creative. in the capital of the empire, the center of its life, there arose no great scientific investigator. [illustration: fig. .--pliny, - a.d.] pliny.--the situation is represented by pliny the elder ( - a.d.), roman general and littérateur (fig. ). his works on natural history, filling thirty-seven volumes, have been preserved with greater completeness than those of other ancient writers. their overwhelming bulk seems to have produced an impression upon those who, in the nineteenth century, heralded him as the greatest naturalist of antiquity. but an examination of his writings shows that he did nothing to deepen or broaden the knowledge of nature, and his natural history marks a distinct retrograde movement. he was, at best, merely a compiler--"a collector of anecdotes"--who, forsaking observation, indiscriminately mixed fable, fact, and fancy taken from the writings of others. he emphasized the feature of classification which aristotle had held in proper subordination, and he replaced the classification of aristotle, founded on plan of organization, by a highly artificial one, founded on the incidental circumstance of the abodes of animals--either in air, water, or on the earth. the arrest of inquiry and its effects.--thus, natural history, transferred from a greek to a roman center, was already on the decline in the time of pliny; but it was destined to sink still lower. it is an old, oft-repeated story how, with the overthrow of ancient civilization, the torch of learning was nearly extinguished. not only was there a complete political revolution; there was also a complete change in the mental interests of mankind. the situation is so complex that it is difficult to state it with clearness. so far as science is concerned, its extinction was due to a turning away from the external world, and a complete arrest of inquiry into the phenomena of nature. this was an important part of that somber change which came over all mental life. one of the causes that played a considerable part in the cessation of scientific investigation was the rise of the christian church and the dominance of the priesthood in all intellectual as well as in spiritual life. the world-shunning spirit, so scrupulously cultivated by the early christians, prompted a spirit which was hostile to observation. the behest to shun the world was acted upon too literally. the eyes were closed to nature and the mind was directed toward spiritual matters, which truly seemed of higher importance. presently, the observation of nature came to be looked upon as proceeding from a prying and impious curiosity. books were now scarcer than during the classical period; the schools of philosophy were reduced, and the dissemination of learning ceased. the priests who had access to the books assumed direction of intellectual life. but they were largely employed with the analysis of the supernatural, without the wholesome check of observation and experiment; mystical explanations were invented for natural phenomena, while metaphysical speculation became the dominant form of mental activity. authority declared the source of knowledge.--in this atmosphere controversies over trivial points were engendered, and the ancient writings were quoted as sustaining one side or the other. all this led to the referring of questions as to their truth or error to authority as the source of knowledge, and resulted in a complete eclipse of reason. amusing illustrations of the situation are abundant; as when, in the middle ages, the question of the number of teeth in the horse was debated with great heat in many contentious writings. apparently none of the contestants thought of the simple expedient of counting them, but tried only to sustain their position by reference to authority. again, one who noticed spots on the sun became convinced of the error of his eyes because aristotle had somewhere written "the face of the sun is immaculate." this was a barren period not only for science, but also for ecclesiastical advance. notwithstanding the fact that for more than a thousand years the only new works were written by professional theologians, there was no substantial advance in their field, and we cannot escape the reflection that the reciprocal action of free inquiry is essential to the growth of theology as of other departments of learning. in the period from the downfall of rome to the revival of learning, one eminent theologian, st. augustine, stands in relief for the openness of his mind to new truth and for his expressions upon the relation of revelation in the scriptures to the observation of nature. his position will be more clearly indicated in the chapter dealing with the rise of evolutionary thought. perhaps it has been the disposition of historians to paint the middle ages in too dark colors in order to provide a background on which fitly to portray the subsequent awakening. it was a remolding period through which it was necessary to pass after the overthrow of ancient civilization and the mixture of the less advanced people of the north with those of the south. the opportunities for advance were greatly circumscribed; the scarcity of books and the lack of facilities for travel prevented any general dissemination of learning, while the irresponsible method of the time, of appealing to authority on all questions, threw a barrier across the stream of progress. intellectuality was not, however, entirely crushed during the prevalence of these conditions. the medieval philosophers were masters of the metaphysical method of argument, and their mentality was by no means dull. while some branches of learning might make a little advance, the study of nature suffered the most, for the knowledge of natural phenomena necessitates a mind turned outward in direct observation of the phenomena of the natural and physical universe. renewal of observation.--it was an epoch of great importance, therefore, when men began again to observe, and to attempt, even in an unskilful way, hampered by intellectual inheritance and habit, to unravel the mysteries of nature and to trace the relation between causes and effects in the universe. this new movement was a revolt of the intellect against existing conditions. in it were locked up all the benefits that have accrued from the development of modern science. just as the decline had been due to many causes, so also the general revival was complex. the invention of printing, the voyages of mariners, the rise of universities, and the circulation of ideas consequent upon the crusades, all helped to disseminate the intellectual ferment. these generic influences aided in molding the environment, but, just as the pause in science had been due to the turning away from nature and to new mental interests, so the revival was a return to nature and to the method of science. the pioneers had to be men of determined independence; they labored against self-interest as well as opposition from the church and the priesthood, and they withstood the terrors of the inquisition and the loss of recognition and support. in this uncongenial atmosphere men like galileo, descartes, and vesalius established the new movement and overthrew the reign of authority. with the coming of vesalius the new era of biological progress was opened, but its growth was a slow one; a growth of which we are now to be concerned in tracing the main features. the epochs in biological history it will be helpful to outline the great epochs of biological progress before taking them up for fuller consideration. the foundation of progress was the renewal of observation in which, as already stated, all modern science was locked up. it was an epoch in biological history when vesalius overthrew the authority of galen, and studied at first hand the organization of the human body. it was an epoch when william harvey, by adding experiment to observation, demonstrated the circulation of the blood and created a new physiology. the two coördinate branches of biology were thus early outlined. the introduction of the microscope, mainly through the labors of grew, hooke, malpighi, and leeuwenhoek, opened a new world to the investigator, and the work of these men marks an epoch in the progress of independent inquiry. linnæus, by introducing short descriptions and uniform names for animals and plants, greatly advanced the subject of natural history. cuvier, by founding the school of comparative anatomy, so furthered the knowledge of the organization of animals that he created an epoch. bichat, his great contemporary, created another by laying the foundation of our knowledge of the structure of animal tissues. von baer, by his studies of the development of animal life, supplied what was lacking in the work of cuvier and bichat and originated modern embryology. haller, in the eighteenth, and johannes müller in the nineteenth century, so added to the ground work of harvey that physiology was made an independent subject and was established on modern lines. with buffon, erasmus darwin,, and lamarck began an epoch in evolutionary thought which had its culminating point in the work of charles darwin. after cuvier and bichat came the establishing of the cell-theory, which created an epoch and influenced all further progress. finally, through the discovery of protoplasm and the recognition that it is the seat of all vital activity, arrived the epoch which brought us to the threshold of the biology of the present day. step by step naturalists have been led from the obvious and superficial facts about living organisms to the deep-lying basis of all vital manifestations. chapter ii vesalius and the overthrow of authority in science vesalius, although an anatomist, is to be recognized in a broad sense as one of the founders of biology. when one is attempting to investigate animal and plant life, not only must he become acquainted with the external appearance of living organisms, but also must acquire early a knowledge of their structure, without which other facts relating to their lives can not be disclosed. anatomy, which is the science of the structure of organized beings, is therefore so fundamental that we find ourselves involved in tracing the history of its rise as one part of the story of biology. but it is not enough to know how animals and plants are constructed; we must also know something about the purpose of the structures and of the life that courses through them, and, accordingly, after considering the rise of anatomy, we must take a similar view of its counterpart, physiology. the great importance of vesalius in the history of science lies in the fact that he overthrew adherence to authority as the method of ascertaining truth, and substituted therefor observation and reason. several of his forerunners had tried to accomplish the same end, but they had failed. he was indebted to them as every man is indebted to his forebears, but at the same time we can not fail to see that vesalius was worthy of the victory. he was more resolute and forceful than any of his predecessors. he was one of those rare spirits who see new truth with clearness, and have the bravery to force their thoughts on an unsympathetic public. the beginning of anatomy.--in order to appreciate his service it is necessary to give a brief account of his predecessors, and of the condition of anatomy in his time. remembering that anatomy embraces a knowledge of the architecture of all animals and plants, we can, nevertheless, see why in early times its should have had more narrow boundaries. the medical men were the first to take an interest in the structure of the human body, because a knowledge of it is necessary for medicine and surgery. it thus happens that the earliest observations in anatomy were directed toward making known the structure of the human body and that of animals somewhat closely related to man in point of structure. anatomical studies, therefore, began with the more complex animals instead of the simpler ones, and, later, when comparative anatomy began to be studied, this led to many misunderstandings; since the structure of man became the type to which all others were referred, while, on account of his derivation, his structure presents the greatest modification of the vertebrate type. it was so difficult in the early days to get an opportunity to study the human body that the pioneer anatomists were obliged to gain their knowledge by dissections of animals, as the dog, and occasionally the monkey. in this way aristotle and his forerunners learned much about anatomy. about b.c., the dissection of the human body was legalized in the alexandrian school, the bodies of condemned criminals being devoted to that purpose. but this did not become general even for medical practitioners, and anatomy continued to be studied mainly from brute animals. galen.--the anatomist of antiquity who outshines all others was galen (claudius galenus, - a.d.), who lived some time in pergamos, and for five years in rome, during the second century of the christian era. he was a man of much talent, both as an observer and as a writer. his descriptions were clear and forceful, and for twelve centuries his works exerted the greatest influence of those of all scientific writers. in his writings was gathered all the anatomical knowledge of his predecessors, to which he had added observations of his own. he was a man of originality, but not having the human body for dissection, he erred in expounding its structure "on the faith of observations made on lower animals." he used the right method in arriving at his facts. huxley says: "no one can read galen's works without being impressed with the marvelous extent and diversity of his knowledge, and by his clear grasp of those experimental methods by which alone physiology can be advanced." anatomy in the middle ages.--but now we shall see how the arrest of inquiry already spoken of operated in the field of anatomy. the condition of anatomy in the middle ages was the condition of all science in the same period. from its practical importance anatomy had to be taught to medical men, while physics and chemistry, biology and comparative anatomy remained in an undeveloped state. the way in which this science was taught is a feature which characterizes the intellectual life of the middle ages. instead of having anatomy taught by observations, the writings of galen were expounded from the desk, frequently without demonstrations of any kind. thus his work came to be set up as the one unfailing authority on anatomical knowledge. this was in accord with the dominant ecclesiastical influence of the time. reference to authority was the method of the theologians, and by analogy it became the method of all learning. as the scriptures were accepted as the unfailing guide to spiritual truth, so galen and other ancient writers were made the guides to scientific truth and thought. the baneful effects of this in stifling inquiry and in reducing knowledge to parrot-like repetition of ancient formulas are so obvious that they need not be especially dwelt upon. [illustration: fig. .--galen, - . from _acta medicorum berolinensium_, .] predecessors of vesalius.--italy gave birth to the first anatomists who led a revolt against this slavery to authority in scientific matters. of the eminent anatomists who preceded vesalius it will be necessary to mention only three. mundinus, or mondino, professor at the university of bologna, who, in the early part of the fourteenth century, dissected three bodies, published in a work founded upon human dissection. he was a man of originality whose work created a sensation in the medical world, but did not supersede galen's. his influence, although exerted in the right direction, was not successful in establishing observation as the method of teaching anatomy. his book, however, was sometimes used as an introduction to galen's writings or in conjunction with them. the next man who requires notice is berengarius of carpi, who was a professor in the university of bologna in the early part of the sixteenth century. he is said to have dissected not less than one hundred human bodies; and although his opportunities for practical study were greater than those of mondino, his attempts to place the science of anatomy upon a higher level were also unsuccessful. we pass now from italy to france, where sylvius ( - ), one of the teachers of vesalius, made his mark. his name is preserved to-day in the _fissure of sylvius_ in the brain, but he was not an original investigator, and he succeeded only in "making a reputation to which his researches do not entitle him." he was a selfish, avaricious man whose adoption of anatomy was not due to scientific interest, but to a love of gain. at the age of fifty he forsook the teaching of the classics for the money to be made by teaching anatomy. he was a blind admirer of galen, and read his works to medical students without dissections, except that from time to time dogs were brought into the amphitheater and their structure exposed by unskilled barbers. vesalius.--vesalius now came upon the scene; and through his efforts, before he was thirty years of age, the idol of authority had been shattered, and, mainly through his persistence, the method of so great moment to future ages had been established. he was well fitted to do battle against tradition--strong in body, in mind, and in purpose, gifted and forceful; and, furthermore, his work was marked by concentration and by the high moral quality of fidelity to truth. vesalius was born in brussels on the last day of the year , of an ancestry of physicians and learned men, from whom he inherited his leaning toward scientific pursuits. early in life he exhibited a passion for anatomy; he dissected birds, rabbits, dogs, and other animals. although having a strong bent in this direction, he was not a man of single talent. he was schooled in all the learning of his time, and his earliest publication was a translation from the greek of the ninth book of _rhazes_. after his early training at brussels and at the university of louvain, in , at the age of , he went to paris to study medicine, where, in anatomy, he came under sylvius and günther. his force and independence.--his impetuous nature was shown in the amphitheatre of sylvius, where, at the third lecture, he pushed aside the clumsy surgeon barbers, and himself exposed the parts as they should be. he could not be satisfied with the exposition of the printed page; he must see with his own eyes, must grasp through his own experience the facts of anatomical structure. this demand of his nature shows not only how impatient he was with sham, but also how much more he was in touch with reality than were the men of his time. after three years at the french capital, owing to wars in belgium, he went back to louvain without obtaining his medical degree. after a short experience as surgeon on the field of battle, he went to padua, whither he was attracted by reports of the opportunities for practical dissection that he so much desired to undertake. there his talents were recognized, and just after receiving his degree of doctor of medicine in , he was given a post in surgery, with the care of anatomy, in the university. his reform of the teaching of anatomy.--the sympathetic and graphic description of this period of his career by sir michael foster is so good that i can not refrain from quoting it: "he at once began to teach anatomy in his own new way. not to unskilled, ignorant barbers would he entrust the task of laying bare before the students the secrets of the human frame; his own hand, and his own hand alone, was cunning enough to track out the pattern of the structures which day by day were becoming more clear to him. following venerated customs, he began his academic labors by 'reading' galen, as others had done before him, using his dissections to illustrate what galen had said. but, time after time, the body on the table said something different from that which galen had written. "he tried to do what others had done before him--he tried to believe galen rather than his own eyes, but his eyes were too strong for him; and in the end he cast galen and his writings to the winds, and taught only what he himself had seen and what he could make his students see, too. thus he brought into anatomy the new spirit of the time, and the men of the time, the young men of the time, answered the new voice. students flocked to his lectures; his hearers amounted, it is said, to some five hundred, and an enlightened senate recognized his worth by repeatedly raising his emoluments. [illustration: fig. .--vesalius, - .] "five years he thus spent in untiring labors at padua. five years he wrought, not weaving a web of fancied thought, but patiently disentangling the pattern of the texture of the human body, trusting to the words of no master, admitting nothing but that which he himself had seen; and at the end of the five years, in , while he was as yet not twenty-eight years of age, he was able to write the dedication to charles v of a folio work entitled the 'structure of the human body,' adorned with many plates and woodcuts which appeared at basel in the following year, ." his physiognomy.--this classic with the latin title, _de humani corporis fabrica_, requires some special notice; but first let us have a portrait of vesalius, the master. fig. shows a reproduction of the portrait with which his work is provided. he is represented in academic costume, probably that which he wore at lectures, in the act of demonstrating the muscles of the arm. the picture is reduced, and in the reduction loses something of the force of the original. we see a strong, independent, self-willed countenance; what his features lack in refinement they make up in force; not an artistic or poetic face, but the face of the man of action with scholarly training. his great book.--the book of vesalius laid the foundation of modern biological science. it is more than a landmark in the progress of science--it created an epoch. it is not only interesting historically, but on account of the highly artistic plates with which it is illustrated it is interesting to examine by one not an anatomist. for executing the plates vesalius secured the service of a fellow-countryman, john stephen de calcar, who was one of the most gifted pupils of titian. the drawings are of such high artistic quality that for a long time they were ascribed to titian. the artist has attempted to soften the necessarily prosaic nature of anatomical illustrations by introducing an artistic background of landscape of varied features, with bridges, roads, streams, buildings, etc. the employment of a background even in portrait-painting was not uncommon in the same century, as in leonardo da vinci's well-known mona lisa, with its suggestive perspective of water, rocks, etc. [illustration: fig. .--anatomical sketch from vesalius's _fabrica_. (photographed and reduced from the facsimile edition of .)] fig. will give an idea on a small scale of one of the plates illustrating the work of vesalius. the plates in the original are of folio size, and represent a colossal figure in the foreground, with a background showing between the limbs and at the sides of the figure. there is considerable variety as regards the background, no two plates being alike. also, in delineating the skeleton, the artist has given to it an artistic pose, as is shown in fig. , but nevertheless the bones are well drawn. no plates of equal merit had appeared before these; in fact, they are the earliest generally known drawings in anatomy, although woodcuts representing anatomical figures were published as early as by john ketham. ketham's figures showed only externals and preparations for opening the body, but rude woodcuts representing internal anatomy and the human skeleton had been published notably by magnus hundt, ; phrysen, ; and berengarius, and . leonardo da vinci and other artists had also executed anatomical drawings before the time of vesalius. previous to the publication of the complete work, vesalius, in , had published six tables of anatomy, and, in , he brought out a new edition of the _fabrica_, with slight additions, especially in reference to physiology, which will be adverted to in the chapter on harvey. [illustration: fig. .--the skeleton, from vesalius's _fabrica_.] in the original edition of the illustrations are not collected in the form of plates, but are distributed through the text, the larger ones making full-page (folio) illustrations. in this edition also the chapters are introduced with an initial letter showing curious anatomical figures in miniature, some of which are shown in fig. . [illustration: fig. .--initial letters from vesalius's _fabrica_ of .] the _fabrica_ of vesalius was a piece of careful, honest work, the moral influence of which must not be overlooked. at any moment in the world's history, work marked by sincerity exercises a wholesome influence, but at this particular stage of intellectual development such work was an innovation, and its significance for progress was wider and deeper than it might have been under different circumstances. opposition to vesalius.--the beneficent results of his efforts were to unfold afterward, since, at the time, his utterances were vigorously opposed from all sides. not only did the ecclesiastics contend that he was disseminating false and harmful doctrine, but the medical men from whom he might have expected sympathy and support violently opposed his teachings. many amusing arguments were brought forward to discredit vesalius, and to uphold the authority of galen. vesalius showed that in the human body the lower jaw is a single bone--that it is not divided as it is in the dog and other lower mammals, and, as galen had taught, also in the human subjects. he showed that the sternum, or breast bone, has three parts instead of eight; he showed that the thigh bones are straight and not curved, as they are in the dog. sylvius, his old teacher, was one of his bitterest opponents; he declared that the human body had undergone changes in structure since the time of galen, and, with the object of defending the ancient anatomist, "he asserted that the straight thigh bones, which, as every one saw, were not curved in accordance with the teaching of galen, were the result of the narrow trousers of his contemporaries, and that they must have been curved in their natural condition, when uninterfered with by art!" the theologians also found other points for contention. it was a widely accepted dogma that man should have one less rib on one side, because from the scriptural account eve was formed from one of adam's ribs. this, of course, vesalius did not find to be the case. it was also generally believed at this time that there was in the body an indestructible resurrection-bone which formed the nucleus of the resurrection-body. vesalius said that he would leave the question of the existence of such a bone to be decided by the theologians, as it did not appear to him to be an anatomical question. the court physician.--the hand of the church was heavy upon him, and the hatred shown in attacks from various quarters threw vesalius into a state of despondency and anger. in this frame of mind he destroyed manuscripts upon which he had expended much labor. his disappointment in the reception of his work probably had much to do in deciding him to relinquish his professorship and accept the post of court physician to charles v of the united kingdoms of spain and belgium. after the death of charles, he remained with philip ii, who succeeded to the throne. here he waxed rich and famous, but he was always under suspicion by the clerical powers, who from time to time found means of discrediting him. the circumstances of his leaving spain are not definitely known. one account has it that he made a _post-mortem_ examination of a body which showed signs of life during the operation, and that he was required to undertake a pilgrimage to the holy land to clear his soul of sacrilege. whether or not this was the reason is uncertain, but after nineteen years at the spanish court he left, in , and journeyed to jerusalem. on his return from palestine he suffered shipwreck and died from the effects of exposure on zanti, one of the ionian islands. it is also said that while on this pilgrimage he had been offered the position of professor of anatomy as successor to fallopius, who had died in , and that, had he lived, he would have come back honorably to his old post. eustachius and fallopius.--the work of two of his contemporaries, eustachius and fallopius, requires notice. cuvier says in his _histoire des sciences naturelles_ that those three men were the founders of modern anatomy. vesalius was a greater man than either of the other two, and his influence was more far-reaching. he reformed the entire field of anatomy, while the names of eustachius and fallopius are connected especially with a smaller part of the field. eustachius described the eustachian tube of the ear and gave especial attention to sense organs; fallopius made special investigations upon the viscera, and described the fallopian tube. fallopius was a suave, polite man, who became professor of anatomy at padua; he opposed vesalius, but his attacks were couched in respectful terms. eustachius, the professor of anatomy at rome, was of a different type, a harsh, violent man, who assailed vesalius with virulence. he corrected some mistakes of vesalius, and prepared new plates on anatomy, which, however, were not published until , and therefore did not exert the influence upon anatomical studies that those of vesalius did. [illustration: fig. .--fallopius, - .] the especial service of vesalius.--it should be remembered that both these men had the advantage of the sketches made under the direction of vesalius. pioneers and path-breakers are under special limitations of being in a new territory, and make more errors than they would in following another's survey of the same territory; it takes much less creative force to correct the errors of a first survey than to make the original discoveries. everything considered, vesalius is deserving of the position assigned to him. he was great in a larger sense, and it was his researches in particular which re-established scientific method and made further progress possible. his errors were corrected, not by an appeal to authority, but by the method which he founded. his great claim to renown is, not that his work outshone all other work (that of galen in particular) in accuracy and brilliancy, but that he overthrew dependence on authority and re-established the scientific method of ascertaining truth. it was the method of aristotle and galen given anew to the world. the spirit of progress was now released from bondage, but we have still a long way to go under its guidance to reach the gateway of modern biology. chapter iii william harvey and experimental observation after the splendid observations of vesalius, revealing in a new light the construction of the human body, harvey took the next general step by introducing experiment to determine the use or purpose of the structures that vesalius had so clearly exposed. thus the work of harvey was complemental to that of vesalius, and we may safely say that, taken together, the work of these two men laid the foundations of the modern method of investigating nature. the results they obtained, and the influence of their method, are of especial interest to us in the present connection, inasmuch as they stand at the beginning of biological science after the renaissance. although the observations of both were applied mainly to the human body, they served to open the entire field of structural studies and of experimental observations on living organisms. many of the experiments of harvey, notably those relating to the movements of the heart, were, of course, conducted upon the lower animals, as the frog, the dog, etc. his experiments on the living human body consisted mainly in applying ligatures to the arms and the legs. nevertheless, the results of all his experiments related to the phenomena of the circulation in the human body, and were primarily for the use of medical men. in what sense the observations of the two men were complemental will be better understood when we remember that there are two aspects in which living organisms should always be considered in biological studies; first, the structure, and, then, the use that the structures subserve. one view is essential to the other, and no investigation of animals and plants is complete in which the two ideas are not involved. just as a knowledge of the construction of a machine is necessary to understand its action, so the anatomical analysis of an organ must precede a knowledge of its office. the term "physiological anatomy of an organ," so commonly used in text-books on physiology, illustrates the point. we can not appreciate the work of such an organ as the liver without a knowledge of the arrangement of its working units. the work of the anatomist concerns the statics of the body, that of the physiologist the dynamics; properly combined, they give a complete picture of the living organism. it is to be remembered that the observations of vesalius were not confined exclusively to structure; he made some experiments and some comments on the use of parts of the body, but his work was mainly structural, while that which distinguishes harvey's research is inductions founded on experimental observation of the action of living tissues. the service of vesalius and harvey in opening the path to biological advance is very conspicuous, but they were not the only pioneers; their work was a part of the general revival of science in which galileo, descartes, and others had their part. while the birth of the experimental method was not due to the exertions of harvey alone, nevertheless it should stand to his credit that he established that method in biological lines. aristotle and galen both had employed experiments in their researches, and harvey's step was in the nature of a revival of the method of the old greeks. harvey's education.--harvey was fitted both by native talent and by his training for the part which he played in the intellectual awakening. he was born at folkestone, on the south coast of england, in , the son of a prosperous yeoman. the harvey family was well esteemed, and the father of william was at one time the mayor of folkestone. young harvey, after five years in the king's school at canterbury, went to cambridge, and in , at the age of sixteen, entered caius college. he had already shown a fondness for observations upon the organization of animals, but it is unlikely that he was able to cultivate this at the university. there his studies consisted mainly of latin and greek, with some training in debate and elementary instruction in the science of physics. at padua.--in , at the age of nineteen, he was graduated with the arts degree, and the following year he turned his steps toward italy in search of the best medical instruction that could be found at that time in all the world. he selected the great university of padua as his place of sojourn, being attracted thither by the fame of some of its medical teachers. he was particularly fortunate in receiving his instruction in anatomy and physiology from fabricius, one of the most learned and highly honored teachers in italy. the fame of this master of medicine, who, from his birthplace, is usually given the full name of fabricius _ab aquapendente_, had spread to the intellectual centers of the world, where his work as anatomist and surgeon was especially recognized. a fast friendship sprang up between the young medical student and this ripe anatomist, the influence of which must have been very great in shaping the future work of harvey. fabricius was already sixty-one years of age, and when harvey came to padua was perfecting his knowledge upon the valves of the veins. the young student was taken fully into his confidence, and here was laid that first familiarity with the circulatory system, the knowledge of which harvey was destined so much to advance and amplify. but it was the stimulus of his master's friendship, rather than what he taught about the circulation, that was of assistance to harvey. for the views of fabricius in reference to the circulation were those of galen; and his conception of the use of the valves of the veins was entirely wrong. a portrait of this great teacher of harvey is shown in fig. . at padua young harvey attracted notice as a student of originality and force, and seems to have been a favorite with the student body as well as with his teachers. his position in the university may be inferred from the fact that he belonged to one of the aristocratic-student organizations, and, further, that he was designated a "councilor" for england. the practice of having student councilors was then in vogue in padua; the students comprising the council met for deliberations, and very largely managed the university by their votes upon instructors and university measures. it is a favorable comment upon the professional education of his time that, after graduating at the university of cambridge, he studied four or more years (willis says five years) in scientific and medical lines to reach the degree of doctor of physic. on leaving padua, in , he returned to england and took the examinations for the degree of m.d. from cambridge, inasmuch as the medical degree from an english university advanced his prospects of receiving a position at home. he opened practice, was married in , and the same year began to give public lectures on anatomy. [illustration: fig. .--fabricius, - , harvey's teacher.] his personal qualities.--harvey had marked individuality, and seems to have produced a powerful impression upon those with whom he came in contact as one possessing unusual intellectual powers and independence of character. he inspired confidence in people, and it is significant that, in reference to the circulation of the blood, he won to his way of thinking his associates in the medical profession. this is important testimony as to his personal force, since his ideas were opposed to the belief of the time, and since also away from home they were vigorously assailed. although described as choleric and hasty, he had also winning qualities, so that he retained warm friendships throughout his life, and was at all times held in high respect. it must be said also that in his replies to his critics, he showed great moderation. [illustration: fig. .--william harvey, - .] the contemplative face of harvey is shown in fig. . this is taken from his picture in the national portrait gallery in london, and is usually regarded as the second-best portrait of harvey, since the one painted by jansen, now in possession of the royal college of physicians, is believed to be the best one extant. the picture reproduced here shows a countenance of composed intellectual strength, with a suggestion, in the forehead and outline of the face, of some of the portraits of shakespeare. an idea of his personal appearance may be had from the description of aubrey, who says: "harvey was not tall, but of the lowest stature; round faced, with a complexion like the wainscot; his eyes small, round, very black, and full of spirit; his hair black as a raven, but quite white twenty years before he died; rapid in his utterance, choleric, given to gesture," etc. he was less impetuous than vesalius, who had published his work at twenty-eight; harvey had demonstrated his ideas of the circulation in public anatomies and lectures for twelve years before publishing them, and when his great classic on the movement of the heart and blood first appeared in , he was already fifty years of age. this is a good example for young investigators of to-day who, in order to secure priority of announcement, so frequently rush into print with imperfect observations as preliminary communications. harvey's writings.--harvey's publications were all great; in embryology, as in physiology, he produced a memorable treatise. but his publications do not fully represent his activity as an investigator; it is known that through the fortunes of war, while connected with the sovereign charles i as court physician, he lost manuscripts and drawings upon the comparative anatomy and development of insects and other animals. his position in embryology will be dealt with in the chapter on the development of animals, and he will come up for consideration again in the chapter on the rise of physiology. here we are concerned chiefly with his general influence on the development of biology. his great classic on movement of the heart and blood.--since his book on the circulation of the blood is regarded as one of the greatest monuments along the highroad of biology, it is time to make mention of it in particular. although relatively small, it has a long title out of proportion to its size: _exercitatio anatomica de motu cordis et sanguinis in animalibus_, which maybe freely translated, "an anatomical disquisition on the movement of the heart and blood in animals." the book is usually spoken of under the shorter title, _de motu cordis et sanguinis_. the full title seems somewhat repellent, but the contents of the book will prove to be interesting to general readers. it is a clear, logical demonstration of the subject, proceeding with directness from one point to another until the culminating force of the argument grows complete and convincing. the book in its first edition was a quarto volume of seventy-eight pages, published in frankfort in . an interesting facsimile reprint of this work, translated into english, was privately reproduced in by dr. moreton and published in canterbury. as stated above, it is known that harvey had presented and demonstrated his views in his lectures since . in his book he showed for the first time ever in print, that all the blood in the body moves in a circuit, and that the beating of the heart supplies the propelling force. both ideas were new, and in order to appreciate in what sense they were original with harvey, we must inquire into the views of his forerunners. question as to harvey's originality.--the question of how near some of his predecessors came to anticipating his demonstration of the circulation has been much debated. it has been often maintained that servetus and realdus columbus held the conception of the circulation for which harvey has become so celebrated. of the various accounts of the views of harvey's predecessors, those of willis, huxley, and michael foster are among the most judicial; that of foster, indeed, inasmuch as it contains ample quotations from the original sources, is the most nearly complete and satisfactory. the discussion is too long to enter into fully here, but a brief outline is necessary to understand what he accomplished, and to put his discovery in the proper light. to say that he first discovered--or, more properly, demonstrated--the circulation of the blood carries the impression that he knew of the existence of capillaries connecting the arteries and the veins, and had ocular proof of the circulation through these connecting vessels. but he did not actually see the blood moving from veins to arteries, and he knew not of the capillaries. he understood clearly from his observations and experiments that all the blood passes from veins to arteries and moves in "a kind of circle"; still, he thought that it filters through the tissues in getting from one kind of vessel to the other. it was reserved for malpighi, in , and leeuwenhoek, in , to see, with the aid of lenses, the movement of the blood through the capillaries in the transparent parts of animal tissues. (see under leeuwenhoek, p. .) the demonstration by harvey of the movement of the blood in a circuit was a matter of cogent reasoning, based on experiments with ligatures, on the exposure of the heart in animals and the analysis of its movements. it has been commonly maintained (as by whewell) that he deduced the circulation from observations of the valves in the veins, but this is not at all the case. the central point of harvey's reasoning is that the quantity of blood which leaves the left cavity of the heart in a given space of time makes necessary its return to the heart, since in a half-hour (or less) the heart, by successive pulsations, throws into the great artery more than the total quantity of blood in the body. huxley points out that this is the first time that quantitative determinations were introduced into physiology. views of his predecessors on the movement of the blood.--galen's view of the movement of the blood was not completely replaced until the establishment of harvey's view. the greek anatomist thought that there was an ebb and flow of blood within both veins and arteries throughout the system. the left side of the heart was supposed to contain blood vitalized by a mixture of animal spirits within the lungs. the veins were thought to contain crude blood. he supposed, further, that there was a communication between the right and the left side of the heart through very minute pores in the septum, and that some blood from the right side passed through the pores into the left side and there became charged with animal spirits. it should also be pointed out that galen believed in the transference of some blood through the lungs from the right to the left side of the heart, and in this foreshadowed the views which were later developed by servetus and realdus columbus. [illustration: fig. .--scheme of the portal circulation according to vesalius, .] vesalius, in the first edition of his work ( ) expressed doubts upon the existence of pores in the partition-wall of the heart through which blood could pass; and in the second edition ( ) of the _fabrica_ he became more skeptical. in taking this position he attacked a fundamental part of the belief of galen. the careful structural studies of vesalius must have led him very near to an understanding of the connection between arteries and veins. fig. shows one of his sketches of the arrangement of arteries and veins. he saw that the minute terminals of arteries and veins came very close together in the tissues of the body, but he did not grasp the meaning of the observation, because his physiology was still that of galen; vesalius continued to believe that the arteries contained blood mixed with spirits, and the veins crude blood, and his idea of the movement was that of an ebb and flow. in reference to the anatomy of the blood-vessels, he goes so far as to say of the portal vein and the vena cava in the liver that "the extreme ramifications of these veins inosculate with each other, and in many places appear to unite and be continuous." all who followed him had the advantage of his drawings showing the parallel arrangement of arteries and veins, and their close approach to each other in their minute terminal twigs, but no one before harvey fully grasped the idea of the movement of the blood in a complete circuit. servetus, in his work on the restoration of christianity (_restitutio christianismi_, ), the work for which calvin accomplished his burning at the stake, expressed more clearly than galen had done the idea of a circuit of blood through the lungs. according to his view, some of the blood took this course, while he still admits that a part may exude through the wall of the ventricle from the right to the left side. this, however, was embodied in a theological treatise, and had little direct influence in bringing about an altered view of the circulation. nevertheless, there is some reason to think that it may have been the original source of the ideas of the anatomist columbus, as the studies into the character of that observer by michael foster seem to indicate. realdus columbus, professor of anatomy at rome, expressed a conception almost identical with that of servetus, and as this was in an important work on anatomy, published in , and well known to the medical men of the period, it lay in the direct line of anatomical thought and had greater influence. foster suggests that the devious methods of columbus, and his unblushing theft of intellectual property from other sources, give ground for the suspicion that he had appropriated this idea from servetus without acknowledgment. although calvin supposed that the complete edition of a thousand copies of the work of servetus had been burned with its author in , a few copies escaped, and possibly one of these had been examined by columbus. this assumption is strengthened by the circumstance that columbus gives no record of observations, but almost exactly repeats the words of servetus. cæsalpinus, the botanist and medical man, expressed in and similar ideas of the movement of the blood (probably as a matter of argument, since there is no record of either observations or experiments by him). he also laid hold of a still more important conception, viz., that some of the blood passes from the left side of the heart through the arteries of the body, and returns to the right side of the heart by the veins. but a fair consideration of the claims of these men as forerunners of harvey requires quotations from their works and a critical examination of the evidence thus adduced. this has been excellently done by michael foster in his _lectures on the history of physiology_. further considerations of this aspect of the question would lie beyond the purposes of this book. at most, before harvey, the circuit through the lungs had been vaguely defined by galen, servetus, columbus, and cæsalpinus, and the latter had supposed some blood to pass from the heart by the arteries and to return to it by the veins; but no one had arrived at an idea of a complete circulation of all the blood through the system, and no one had grasped the consequences involved in such a conception. harvey's idea of the movement of the heart (_de motu cordis_) was new; his notion of the circulation (_et sanguinis_) was new; and his method of demonstrating these was new. harvey's argument.--the gist of harvey's arguments is indicated in the following propositions quoted with slight modifications from hall's _physiology_: (i) the heart passively dilates and actively contracts; (ii) the auricles contract before the ventricles do; (iii) the contraction of the auricles forces the blood into the ventricles; (iv) the arteries have no "pulsific power," _i.e._, they dilate passively, since the pulsation of the arteries is nothing else than the impulse of the blood within them; (v) the heart is the organ of propulsion of the blood; (vi) in passing from the right ventricle to the left auricle the blood transudes through the parenchyma of the lungs; (vii) the quantity and rate of passage of the blood peripherally from the heart makes it a physical necessity that most of the blood return to the heart; (viii) the blood does return to the heart by way of the veins. it will be noticed that the proposition vii is the important one; in it is involved the idea of applying measurement to a physiological process. harvey's influence.--harvey was a versatile student. he was a comparative anatomist as well as a physiologist and embryologist; he had investigated the anatomy of about sixty animals and the embryology of insects as well as of vertebrates, and his general influence in promoting biological work was extensive. his work on the movement of the blood was more than a record of a series of careful investigations; it was a landmark in progress. when we reflect on the part played in the body by the blood, we readily see that a correct idea of how it carries nourishment to the tissues, and how it brings away from them the products of disintegrated protoplasm is of prime importance in physiology. it is the point from which spring all other ideas of the action of tissues, and until this was known the fine analysis of vital processes could not be made. the true idea of respiration, of the secretion by glands, the chemical changes in the tissues, in fact, of all the general activities of the body, hinge upon this conception. it was these consequences of his demonstration, rather than the fact that the blood moves in a circuit, which made it so important. this discovery created modern physiology, and as that branch of inquiry is one of the parts of general biology, the bearing of harvey's discovery upon biological thought can be readily surmised. those who wish to examine harvey's views at first hand, without the burden of translating them from the latin, will find an edition of his complete works translated into english by willis, and published by the ray society, of london. as is always the case with new truths, there was hostility to accepting his views. in england this hostility was slight on account of his great personal influence, but on the continent there was many a sharp criticism passed upon his work. his views were so illuminating that they were certain of triumph, and even in his lifetime were generally accepted. thus the new conception of vital activities, together with his method of inquiry, became permanent parts of biological science. chapter iv the introduction of the microscope and the progress of independent observation the introduction of the microscope greatly increased the ocular powers of observers, and, in the seventeenth century, led to many new departures. by its use the observations were carried from the plane of gross anatomy to that of minute structure; the anatomy of small forms of life, like insects, began to be studied, and also the smaller microscopic animalcula were for the first time made known. putting aside the disputed questions as to the time of the invention and the identity of the inventor of the microscope--whether to fontana, galileo, or the jenssens belongs the credit--we know that it was improved by the hollander drebbel in the early years of the seventeenth century, but was not seriously applied to anatomical studies till after the middle of that century. the pioneer microscopists the names especially associated with early microscopic observations are those of hooke and grew in england, malpighi in italy, and swammerdam and leeuwenhoek, both in holland. their microscopes were imperfect, and were of two kinds: simple lenses, and lenses in combination, forming what we now know as the compound microscope. some forms of these early microscopes will be described and illustrated later. although thus early introduced, microscopic observation did not produce its great results until the nineteenth century, just after magnifying-lenses had been greatly improved. [illustration: fig. .--hooke's microscope, . from carpenter's _the microscope and its revelations_. permission of p. blakiston's sons & co.] robert hooke ( - ), of london, published in a book of observations with the microscope entitled _micrographia_, which was embellished with eighty-three plates of figures. hooke was a man of fine mental endowment, who had received a good scientific training at the university of cambridge, but who lacked fixedness of purpose in the employment of his talents. he did good work in mathematics, made many models for experimenting with flying machines, and claimed to have discovered gravitation before newton, and also the use of a spring for regulating watches before huygens, etc. he gave his attention to microscopic study for a time and then dropped it; yet, although we can not accord to him a prominent place in the history of biology, he must receive mention as a pioneer worker with the microscope. his book gave a powerful stimulus to microscopy in england, and, partly through its influence, labor in this field was carried on more systematically by his fellow-countryman nehemiah grew. the form of the microscope used by hooke is known through a picture and a description which he gives of it in his _micrographia_. fig. is a copy of the illustration. his was a compound microscope consisting of a combination of lenses attached to a tube, one set near the eye of the observer and the other near the object to be examined. when we come to describe the microscopes of leeuwenhoek, with which so much good work was accomplished, we shall see that they stand in marked contrast, on account of their simplicity, to the somewhat elaborate instrument of hooke. grew ( - ) devoted long and continuous labor to microscopic observation, and, although he was less versatile and brilliant than hooke, his patient investigations give him just claim to a higher place in the history of natural science. grew applied the microscope especially to the structure of plants, and his books entitled _idea of a philosophical history of plants_ ( ) and _anatomy of vegetables_ ( ) helped to lay the foundations of vegetable histology. when we come to consider the work of malpighi, we shall see that he also produced a work upon the microscopic structure of plants which, although not more exact and painstaking than grew's, showed deeper comprehension. he is the co-founder with grew of the science of the microscopic anatomy of plants. it is not necessary to dwell long upon the work of either hooke or grew, since that of malpighi, swammerdam, and leeuwenhoek was more far-reaching in its influence. the publications of these three men were so important, both in reference to microscopic study and to the progress of independent investigation, that it will be necessary to deal with them in more detail. in the work of these men we come upon the first fruits of the application of the methods introduced by vesalius and harvey. of this triumvirate, one--malpighi--was an italian, and the other two were hollanders. their great service to intellectual progress consisted chiefly in this--that, following upon the foundations of vesalius and harvey, "they broke away from the thraldom of mere book-learning, and relying alone upon their own eyes and their own judgment, won for man that which had been quite lost--the blessings of independent and unbiased observation." it is natural that, working when they did, and independently as they did, their work overlapped in many ways. malpighi is noteworthy for many discoveries in anatomical science, for his monograph on the anatomy of the silkworm, for observations of the minute structure of plants, and of the development of the chick in the hen's egg. swammerdam did excellent and accurate work upon the anatomy and metamorphosis of insects, and the internal structure of mollusks, frogs, and other animals. leeuwenhoek is distinguished for much general microscopic work; he discovered various microscopic animalcula; he established, by direct observation, the fact of a connection between arteries and veins, and examined microscopically minerals, plants, and animals. to him, more than to the others, the general title of "microscopist" might be applied. since these men are so important in the growth of biology, let us, by taking them individually, look a little more closely into their lives and labors. marcello malpighi, - personal qualities.--there are several portraits of malpighi extant. these, together with the account of his personal appearance given by atti, one of his biographers, enable us to tell what manner of man he was. the portrait shown in fig. is a copy of the one painted by tabor and presented by malpighi to the royal society of london, in whose rooms it may still be seen. this shows him in the full attractiveness of his early manhood, with the earnest, intellectual look of a man of high ideals and scholarly tastes, sweet-tempered, and endowed with the insight that belongs to a sympathetic nature. some of his portraits taken later are less attractive, and the lines and wrinkles that show in his face give evidence of imperfect health. according to atti, he was of medium stature, with a brown skin, a delicate complexion, a serious countenance, and a melancholy look. accounts of his life show that he was modest, quiet, and of a pacific disposition, notwithstanding the fact that he lived in an atmosphere of acrimonious criticism, of jealousy and controversy. a family dispute in reference to the boundary-lines between his father's property and the adjoining land of the sbaraglia family gave rise to a feud, in which representatives of the latter family followed him all his life with efforts to injure both his scientific reputation and his good name. under all this he suffered acutely, and his removal from bologna to messina was partly to escape the harshness of his critics. some of his best qualities showed under these persecutions; he was dignified under abuse and considerate in his reply. in reference to the attacks upon his scientific standing, there were published after his death replies to his critics that were written while he was smarting under their injustice and severity, but these replies are free from bitterness and are written in a spirit of great moderation. the following picture, taken from ray's correspondence, shows the fine control of his spirit. under the date of april, , dr. tancred robinson writes: "just as i left bononia i had a lamentable spectacle of malpighi's house all in flames, occasioned by the negligence of his old wife. all his pictures, furniture, books, and manuscripts were burnt. i saw him in the very heat of the calamity, and methought i never beheld so much christian patience and philosophy in any man before; for he comforted his wife and condoled nothing but the loss of his papers." [illustration: fig. .--malpighi, - .] education.--malpighi was born at crevalcuore, near bologna, in . his parents were landed peasants, or farmers, enjoying an independence in financial matters. as their resources permitted it, they designed to give marcellus, their eldest child, the advantage of masters and schools. he began a life of study; and, before long, he showed a taste for belles-lettres and for philosophy, which he studied under natali. through the death of both parents, in , malpighi found himself orphaned at the age of twenty-one, and as he was the eldest of eight children, the management of domestic affairs devolved upon him. he had as yet made no choice of a profession; but, through the advice of natali, he resolved, in , to study medicine. this advice followed, in , at the age of twenty-five, he received from the university of bologna the degree of doctor of medicine. university positions.--in the course of a few years he married the sister of massari, one of his teachers in anatomy, and became a candidate for a chair in the university of bologna. this he did not immediately receive, but, about , he was appointed to a post in the university, and began his career as a teacher and investigator. he must have shown aptitude for this work, for he was soon called to the university of pisa, where, fortunately for his development, he became associated with borelli, who, as an older man, assisted him in many ways. they united in some work, and together they discovered the spiral character of the heart muscles. but the climate of pisa did not agree with him, and after three years he returned, in , to teach in the university of bologna, and applied himself assiduously to anatomy. here his fame was in the ascendant, notwithstanding the machinations of his enemies and detractors, led by sbaraglia. he was soon ( ) called to messina to follow the famous castelli. after a residence there of four years he again returned to bologna, and as he was now thirty-eight years of age, he thought it time to retire to his villa near the city in order to devote himself more fully to anatomical studies, but he continued his lectures in the university, and also his practice of medicine. honors at home and abroad.--malpighi's talents were appreciated even at home. the university of bologna honored him in with a latin _eulogium_; the city erected a monument to his memory; and after his death, in the city of rome, his body was brought to bologna and interred with great pomp and ceremony. at the three hundredth anniversary of his death, in , a festival was held in bologna, his monument was unveiled, and a book of addresses by eminent anatomists was published in his honor. during his lifetime he received recognition also from abroad, but that is less remarkable. in he was elected an honorary member of the royal society of london. he was very sensible of this honor; he kept in communication with the society; he presented them with his portrait, and deposited in their archives the original drawings illustrating the anatomy of the silkworm and the development of the chick. in he was taken to rome by the newly elected pope, innocent xii, as his personal physician, but under these new conditions he was not destined to live many years. he died there, in , of apoplexy. his wife, of whom it appears that he was very fond, had died a short time previously. among his posthumous works is a sort of personal psychology written down to the year , in which he shows the growth of his mind, and the way in which he came to take up the different subjects of investigation. in reference to his discoveries and the position he occupies in the history of natural science, it should be observed that he was an "original as well as a very profound observer." while the ideas of anatomy were still vague, "he applied himself with ardor and sagacity to the study of the fine structure of the different parts of the body," and he extended his investigations to the structure of plants and of different animals, and also to their development. entering, as he did, a new and unexplored territory, naturally he made many discoveries, but no man of mean talents could have done his work. activity in research.--during forty years of his life he was always busy with research. many of his discoveries had practical bearing on the advance of anatomy and physiology as related to medicine. in he demonstrated the structure of the lungs. previously these organs had been regarded as a sort of homogeneous parenchyma. he showed the presence of air-cells, and had a tolerably correct idea of how the air and the blood are brought together in the lungs, the two never actually in contact, but always separated by a membrane. these discoveries were first made on the frog, and applied by analogy to the interpretation of the lungs of the human body. he was a comparative anatomist, and the first to insist on analogies of structure between organs throughout the animal kingdom, and to make extensive practical use of the idea that discoveries on simpler animals can be utilized in interpreting the similar structures in the higher ones. it is very interesting to note that in connection with this work he actually observed the passage of blood through the capillaries of the transparent lungs of the frog, and also in the mesentery. although this antedates the similar observations of leeuwenhoek ( ), nevertheless the work of leeuwenhoek was much more complete, and he is usually recognized in physiology as the discoverer of the capillary connection between arteries and veins. at this same period malpighi also observed the blood corpuscles. soon after he demonstrated the mucous layer, or pigmentary layer of the skin, intermediate between the true and the scarf skin. he had separated this layer by boiling and maceration, and described it as a reticulated membrane. even its existence was for a long time controverted, but it remains in modern anatomy under the title of the malpighian layer. his observation of glands was extensive, and while it must be confessed that many of his conclusions in reference to glandular structure were erroneous, he left his name connected with the malpighian corpuscles of the kidney and of the spleen. he was also the first to indicate the nature of the papillæ on the tongue. the foregoing is a respectable list of discoveries, but much more stands to his credit. those which follow have a bearing on comparative anatomy, zoölogy, and botany. monograph on the structure and metamorphosis of the silkworm.--malpighi's work on the structure of the silkworm takes rank among the most famous monographs on the anatomy of a single animal. much skill was required to give to the world this picture of minute structure. the marvels of organic architecture were being made known in the human body and the higher animals, but "no insect--hardly, indeed, any animal--had then been carefully described, and all the methods of the work had to be discovered." he labored with such enthusiasm in this new territory as to throw himself into a fever and to set up an inflammation in the eyes. "nevertheless," says malpighi, "in performing these researches so many marvels of nature were spread before my eyes that i experienced an internal pleasure that my pen could not describe." he showed that the method of breathing was neither by lungs nor by gills, but through a system of air-tubes, communicating with the exterior through buttonhole shaped openings, and, internally, by an infinitude of branches reaching to the minutest parts of the body. malpighi showed an instinct for comparison; instead of confining his researches to the species in hand, he extended his observations to other insects, and has given sketches of the breathing-tubes, held open by their spiral thread, taken from several species. the nervous system he found to be a central white cord with swellings in each ring of the body, from which nerves are given off to all organs and tissues. the cord, which is, of course, the central nervous system, he found located mainly on the ventral surface of the body, but extending by a sort of collar of nervous matter around the oesophagus, and on the dorsal surface appearing as a more complex mass, or brain, from which nerves are given off to the eyes and other sense organs of the head. as illustrations from this monograph we have, in fig. , reduced sketches of the drawings of the nervous system and the food canal in the adult silkworm. the sketch at the right hand illustrates the central nerve cord with its ganglionic enlargement in each segment, the segments being indicated by the rows of spiracles at the sides. the original drawing is on a much larger scale, and reducing it takes away some of its coarseness. all of his drawings lack the finish and detail of swammerdam's work. he showed also the food canal and the tubules connected with the intestine, which retain his name in the insect anatomy of to-day, under the designation of malpighian tubes. the silk-forming apparatus was also figured and described. these structures are represented, as malpighi drew them, on the left of fig. . [illustration: fig. .--from malpighi's _anatomy of the silkworm_, .] this monograph, which was originally published in by the royal society of london, bears the latin title, _dissertatio epistolica de bombyce_. it has been several times republished, the best edition being that in french, which dates from montpellier, in , and which is prefaced by an account of the life and labors of malpighi. anatomy of plants.--malpighi's anatomy of plants constitutes one of his best, as well as one of his most extensive works. in the folio edition of his works, - , the _anatome plantarum_ occupies not less than pages and is illustrated by ninety-three plates of figures. it comprises an exposition of the structure of bark, stem, roots, seeds, the process of germination, and includes a treatise on galls, etc., etc. in this work the microscopic structure of plants is amply illustrated, and he anticipated to a certain degree the ideas on the cellular structure of plants. burnett says: "his observations appear to have been very accurate, and not only did he maintain the cellular structure of plants, but also declared that it was composed of separate cells, which he designated 'utricles.'" thus did he foreshadow the cell theory of plants as developed by schleiden in the nineteenth century. when it came to interpretations, he made several errors. applying his often-asserted principle of analogies, he concluded that the vessels of plants are organs of respiration and of circulation, from a certain resemblance that they bear to the breathing-tubes of insects. but his observations on structure are good, and if he had accomplished nothing more than this work on plants he would have a place in the history of botany. work in embryology.--difficult as was his task in insect anatomy and plant histology, a more difficult one remains to be mentioned, _viz._, his observations of the development of animals. he had pushed his researches into the finer structure of organisms, and now he attempted to answer this question: how does one of these organisms begin its life, and by what series of steps is its body built up? he turned to the chick, as the most available form in which to get an insight into this process, but he could not extend his observations successfully into periods earlier than about the twenty-four-hour stage of development. two memoirs were written on this subject, both in , which were published by the royal society of england under the titles _de formatione pulli in ovo_ and _de ovo incubato_. of all malpighi's work, this has received the least attention from reviewers, but it is, for his time, a very remarkable achievement. no one can look over the ten folio plates without being impressed with the extent and accuracy of his observations. his sketches are of interest, not only to students of embryology, but also to educated people, to see how far observations regarding the development of animals had progressed in . further consideration of his position in embryology will be found in the chapter on the rise of that subject. little is known regarding the form of microscope employed by malpighi. doubtless, much of his work was done with a simple lens, since he speaks of examining the dried lungs with a microscope of a single lens against the horizontal sun; but he is also known to have observed with an instrument consisting of two lenses. malpighi was a naturalist, but of a new type; he began to look below the surface, and essayed a deeper level of analysis in observing and describing the internal and minute structure of animals and plants, and when he took the further step of investigating their development he was anticipating the work of the nineteenth century. jan swammerdam ( - ) swammerdam was a different type of man--nervous, incisive, very intense, stubborn, and self-willed. much of his character shows in the portrait by rembrandt represented in fig. . although its authenticity has been questioned, it is the only known portrait of swammerdam. early interest in natural history.--he was born in , nine years after malpighi. his father, an apothecary of amsterdam, had a taste for collecting, which was shared by many of his fellow-townsmen. the dutch people of this time sent their ships into all parts of the world, and this vast commerce, together with their extensive colonial possessions, fostered the formation of private museums. the elder swammerdam had the finest and most celebrated collection in all amsterdam. this was stored, not only with treasures, showing the civilization of remote countries, but also with specimens of natural history, for which he had a decided liking. thus "from the earliest dawn of his understanding the young swammerdam was surrounded by zoölogical specimens, and from the joint influence, doubtless, of hereditary taste and early association, he became passionately devoted to the study of natural history." studies medicine.--his father intended him for the church, but he had no taste for theology, though he became a fanatic in religious matters toward the close of his life; at this period, however, he could brook no restraint in word or action. he consented to study medicine, but for some reason he was twenty-six years old before entering the university of leyden. this delay was very likely owing to his precarious health, but, in the mean time, he had not been idle; he had devoted himself to observation and study with great ardor, and had already become an expert in minute dissection. when he went to the university of leyden, therefore, he at once took high rank in anatomy. anything demanding fine manipulation and dexterity was directly in his line. he continued his studies in paris, and about took his degree of doctor of medicine. [illustration: fig. .--swammerdam, - .] during this period of medical study he made some rather important observations in human anatomy, and introduced the method of injection that was afterward claimed by ruysch. in he discovered the valves of lymphatic vessels by the use of slender glass tubes, and, three years later, first used a waxy material for injecting blood-vessels. it should be noted, in passing, that swammerdam was the first to observe and describe the blood corpuscles. as early as he described them in the blood of the frog, but not till fifty-seven years after his death were his observations published by boerhaave, and, therefore, he does not get the credit of this discovery. publication alone, not first observation, establishes priority, but there is conclusive evidence that he observed the blood corpuscles before either malpighi or leeuwenhoek had published his findings. love of minute anatomy.--after graduating in medicine he did not practice, but followed his strong inclination to devote himself to minute anatomy. this led to differences with his father, who insisted on his going into practice, but the self-willed stubbornness and firmness of the son now showed themselves. it was to gratify no love of ease that swammerdam thus held out against his father, but to be able to follow an irresistible leading toward minute anatomy. at last his father planned to stop supplies, in order to force him into the desired channel, but swammerdam made efforts, without success, to sell his own personal collection and preserve his independence. his father died, leaving him sufficient property to live on, and brought the controversy to a close soon after the son had consented to yield to his wishes. boerhaave, his fellow-countryman, gathered swammerdam's complete writings after his death and published them in under the title _biblia naturæ_. with them is included a life of swammerdam, in which a graphic account is given of his phenomenal industry, his intense application, his methods and instruments. most of the following passages are selected from that work. intensity as a worker.--he was a very intemperate worker, and in finishing his treatise on bees ( ) he broke himself down. "it was an undertaking too great for the strongest constitution to be continually employed by day in making observations and almost as constantly engaged by night in recording them by drawings and suitable explanations. this being summer work, his daily labors began at six in the morning, when the sun afforded him light enough to enable him to survey such minute objects; and from that time till twelve he continued without interruption, all the while exposed in the open air to the scorching heat of the sun, bareheaded, for fear of interrupting the light, and his head in a manner dissolving into sweat under the irresistible ardors of that powerful luminary. and if he desisted at noon, it was only because the strength of his eyes was too much weakened by the extraordinary efflux of light and the use of microscopes to continue any longer upon such small objects. "this fatigue our author submitted to for a whole month together, without any interruption, merely to examine, describe, and represent the intestines of bees, besides many months more bestowed upon the other parts; during which time he spent whole days in making observations, as long as there was sufficient light to make any, and whole nights in registering his observations, till at last he brought his treatise on bees to the wished-for perfection." method of work.--"for dissecting very minute objects, he had a brass table made on purpose by that ingenious artist, samuel musschenbroek. to this table were fastened two brass arms, movable at pleasure to any part of it, and the upper portion of these arms was likewise so contrived as to be susceptible of a very slow vertical motion, by which means the operator could readily alter their height as he saw most convenient to his purpose. the office of one of these arms was to hold the little corpuscles, and that of the other to apply the microscope. his microscopes were of various sizes and curvatures, his microscopical glasses being of various diameters and focuses, and, from the least to the greatest, the best that could be procured, in regard to the exactness of the workmanship and the transparency of the substance. "but the constructing of very fine scissors, and giving them an extreme sharpness, seems to have been his chief secret. these he made use of to cut very minute objects, because they dissected them equably, whereas knives and lancets, let them be ever so fine and sharp, are apt to disorder delicate substances. his knives, lancets, and styles were so fine that he could not see to sharpen them without the assistance of the microscope; but with them he could dissect the intestines of bees with the same accuracy and distinctness that others do those of large animals. "he was particularly dexterous in the management of small tubes of glass no thicker than a bristle, drawn to a very fine point at one end, but thicker at the other." these were used for inflating hollow structures, and also for making fine injections. he dissolved the fat of insects in turpentine and carried on dissections under water. an unbiased examination of his work will show that it is of a higher quality than malpighi's in regard to critical observation and richness of detail. he also worked with minuter objects and displayed a greater skill. the religious devotee.--the last part of his life was dimmed by fanaticism. he read the works of antoinette bourignon and fell under her influence; he began to subdue his warm and stubborn temper, and to give himself up to religious contemplation. she taught him to regard scientific research as worldly, and, following her advice, he gave up his passionate fondness for studying the works of the creator, to devote himself to the love and adoration of that same being. always extreme and intense in everything he undertook, he likewise overdid this, and yielded himself to a sort of fanatical worship until the end of his life, in . had he possessed a more vigorous constitution he would have been greater as a man. he lived, in all, but forty-three years; the last six or seven years were unproductive because of his mental distractions, and before that, much of his time had been lost through sickness. the biblia naturæ.--it is time to ask, what, with all his talents and prodigious application, did he leave to science? this is best answered by an examination of the _biblia naturæ_, under which title all his work was collected. his treatise on bees and mayflies and a few other articles were published during his lifetime, but a large part of his observations remained entirely unknown until they were published in this book fifty-seven years after his death. in the folio edition it embraces pages of text and fifty-three plates, replete with figures of original observations. it "contains about a dozen life-histories of insects worked out in more or less detail. of these, the mayfly is the most famous; that on the honey-bee the most elaborate." the greater amount of his work was in structural entomology. it is known that he had a collection of about three thousand different species of insects, which for that period was a very large one. there is, however, a considerable amount of work on other animals; the fine anatomy of the snail, the structure of the clam, the squid; observations on the structure and development of the frog; observations on the contraction of the muscles, etc., etc. it is to be remembered that swammerdam was extremely exact in all that he did. his descriptions are models of accuracy and completeness. fig. shows reduced sketches of his illustrations of the structure of the snail. the upper sketch shows the central nervous system and the nerve trunks connected therewith, and the lower figure shows the shell and the principal muscles. this is an exceptionally good piece of anatomization for that time, and is a fair sample of the fidelity with which he worked out details in the structure of small animals. besides showing this, these figures also serve the purpose of pointing out that swammerdam's fine anatomical work was by no means confined to insects. his determinations on the structure of the young frog were equally noteworthy. [illustration: fig. .--from swammerdam's _biblia naturæ_.] but we should have at least one illustration of his handling of insect anatomy to compare more directly with that of malpighi, already given. fig. is a reduced sketch of the anatomy of the larva of an ephemerus, showing, besides other structures, the central nervous system in its natural position. when compared with the drawings of malpighi, we see there is a more masterly hand at the task, and a more critical spirit back of the hand. the nervous system is very well done, and the greater detail in other features shows a disposition to go into the subject more deeply than malpighi. besides working on the structure and life-histories of animals, swammerdam showed, experimentally, the irritability of nerves and the response of muscles after their removal from the body. he not only illustrates this quite fully, but seems to have had a pretty good appreciation of the nature of the problem of the physiologist. he says: "it is evident from the foregoing observations that a great number of things concur in the contraction of the muscles, and that one should be thoroughly acquainted with that wonderful machine, our body, and the elements with which we are surrounded, to describe exactly one single muscle and explain its action. on this occasion it would be necessary for us to consider the atmosphere, the nature of our food, the blood, the brain, marrow, and nerves, that most subtle matter which instantaneously flows to the fibers, and many other things, before we could expect to attain a sight of the perfect and certain truth." in reference to the formation of animals within the egg, swammerdam was, as malpighi, a believer in the pre-formation theory. the basis for his position on this question will be set forth in the chapter on the rise of embryology. [illustration: fig. .--anatomy of an insect: dissected and drawn by swammerdam.] there was another question in his time upon which philosophers and scientific men were divided, which was in reference to the origin of living organisms: does lifeless matter, sometimes, when submitted to heat and moisture, spring into life? did the rats of egypt come, as the ancients believed, from the mud of the nile, and do frogs and toads have a similar origin? do insects spring from the dew on plants? etc., etc. the famous redi performed his noteworthy experiments when swammerdam was twenty-eight years old, but opinion was divided upon the question as to the possible spontaneous origin of life, especially among the smaller animals. upon this question swammerdam took a positive stand; he ranged himself on the side of the more scientific naturalists against the spontaneous formation of life. antony van leeuwenhoek ( - ) in leeuwenhoek we find a composed and better-balanced man. blessed with a vigorous constitution, he lived ninety-one years, and worked to the end of his life. he was born in , four years after malpighi, and five before swammerdam; they were, then, strictly speaking, contemporaries. he stands in contrast with the other men in being self-taught; he did not have the advantage of a university training, and apparently never had a master in scientific study. this lack of systematic training shows in the desultory character of his extensive observations. impelled by the same gift of genius that drove his confrères to study nature with such unexampled activity, he too followed the path of an independent and enthusiastic investigator. the portrait (fig. ) which forms a frontispiece to his _arcana naturæ_ represents him at the age of sixty-three, and shows the pleasing countenance of a firm man in vigorous health. richardson says: "in the face peering through the big wig there is the quiet force of cromwell and the delicate disdain of spinoza." "it is a mixed racial type, semitic and teutonic, a jewish-saxon; obstinate and yet imaginative; its very obstinacy a virtue, saving it from flying too far wild by its imagination." recent additions to his biography.--there was a singular scarcity of facts in reference to leeuwenhoek's life until , when dr. richardson published in _the asclepiad_[ ] the results of researches made by mr. a. wynter blyth in leeuwenhoek's native town of delft. i am indebted to that article for much that follows. his _arcana naturæ_ and other scientific letters contained a complete record of his scientific activity, but "about his parentage, his education, and his manner of making a living there was nothing but conjecture to go upon." the few scraps of personal history were contained in the encyclopædia articles by carpenter and others, and these were wrong in sustaining the hypothesis that leeuwenhoek was an optician or manufacturer of lenses for the market. although he ground lenses for his own use, there was no need on his part of increasing his financial resources by their sale. he held under the court a minor office designated 'chamberlain of the sheriff.' the duties of the office were those of a beadle, and were set forth in his commission, a document still extant. the requirements were light, as was also the salary, which amounted to about £ a year. he held this post for thirty-nine years, and the stipend was thereafter continued to him to the end of his life. van leeuwenhoek was derived from a good delft family. his grandfather and his great-grandfather were delft brewers, and his grandmother a brewer's daughter. the family were doubtless wealthy. his schooling seems to have been brought to a close at the age of sixteen, when he was "removed to a clothing business in amsterdam, where he filled the office of bookkeeper and cashier." after a few years he returned to delft, and at the age of twenty-two he married, and gave himself up largely to studies in natural history. six years after his marriage he obtained the appointment mentioned above. he was twice married, but left only one child, a daughter by his first wife. in the old church at delft is a monument erected by this daughter to the memory of her father. [illustration: fig. .--leeuwenhoek, - .] he led an easy, prosperous, but withal a busy life. the microscope had recently been invented, and for observation with that new instrument leeuwenhoek showed an avidity amounting to a passion. "that he was in comfortable, if not affluent, circumstances is clear from the character of his writings; that he was not troubled by any very anxious and responsible duties is certain from the continuity of his scientific work; that he could secure the services of persons of influence is discernible from the circumstances that, in , de graaf sent his first paper to the royal society of london; that in the same society admitted him as fellow; that the directors of the east india company sent him specimens of natural history, and that, in , peter the great paid him a call to inspect his microscopes and their revelations." leeuwenhoek seems to have been fascinated by the marvels of the microscopic world, but the extent and quality of his work lifted him above the level of the dilettante. he was not, like malpighi and swammerdam, a skilled dissector, but turned his microscope in all directions; to the mineral as well as to the vegetable and animal kingdoms. just when he began to use the microscope is not known; his first publication in reference to microscopic objects did not appear till , when he was forty-one years old. his microscopes.--he gave good descriptions and drawings of his instruments, and those still in existence have been described by carpenter and others, and in consequence we have a very good idea of his working equipment. during his lifetime he sent as a present to the royal society of london twenty-six microscopes, each provided with an object to examine. unfortunately, these were removed from the rooms of the society and lost during the eighteenth century. his lenses were of fine quality and were ground by himself. they were nearly all simple lenses, of small size but considerable curvature, and needed to be brought close to the object examined. he had different microscopes for different purposes, giving a range of magnifying powers from to diameters and possibly higher. the number of his lenses is surprising; he possessed not less than complete microscopes, two of which were provided with double lenses, and one with a triplet. in addition to the above, he had lenses set between plates of metal, which give a total of lenses used by him in his observations. three were of quartz, or rock crystal; the rest were of glass. more than one-half the lenses were mounted in silver; three were in gold. it is to be understood that all his microscopes were of simple construction; no tubes, no mirror; simple pieces of metal to hold the magnifying-glass and the objects to be examined, with screws to adjust the position and the focus. [illustration: fig. .--leeuwenhoek's microscope. natural size. from photographs by professor nierstrasz, of utrecht.] the three aspects of one of leeuwenhoek's microscopes shown in fig. will give a very good idea of how they were constructed. these pictures represent the actual size of the instrument. the photographs were made by professor nierstrasz from the specimen in possession of the university of utrecht. the instrument consists of a double copper plate in which the circular lens is inserted, and an object-holder--represented in the right-hand lower figure as thrown to one side. by a vertical screw the object could be elevated or depressed, and by a transverse screw it could be brought nearer or removed farther from the lens, and thus be brought into focus. fig. _a_ shows the way in which the microscope was arranged to examine the circulation of blood in the transparent tail of a small fish. the fish was placed in water in a slender glass tube, and the latter was held in a metallic frame, to which a plate (marked _d_) was joined, carrying the magnifying glass. the latter is indicated in the circle above the letter _d_, near the tail-fin of the fish. the eye was applied close to this circular magnifying-glass, which was brought into position and adjusted by means of screws. in some instances, he had a concave reflector with a hole in the center, in which his magnifying-glass was inserted; in this form of instrument the objects were illumined by reflected, and not by transmitted light. [illustration: fig. _a._--leeuwenhoek's mechanism for examining the circulation of the blood.] his scientific letters.--his microscopic observations were described and sent to learned societies in the form of letters. "all or nearly all that he did in a literary way was after the manner of an epistle," and his written communications were so numerous as to justify the cognomen, "the man of many letters." "the french academy of sciences, of which he was elected a corresponding member in , got twenty-seven; but the lion's share fell to the young royal society of london, which in fifty years-- - --received letters and papers." "the works themselves, except that they lie in the domain of natural history, are disconnected and appear in no order of systematized study. the philosopher was led by what transpired at any moment to lead him." [illustration: fig. _b_.--the capillary circulation. (after leeuwenhoek.)] the capillary circulation.--in he observed the minute circulation of the blood, and demonstrated the capillary connection between arteries and veins, thus forging the final link in the chain of observation showing the relation between these blood-vessels. this was perhaps his most important observation for its bearing on physiology. it must be remembered that harvey had not actually seen the circulation of the blood, which he announced in . he assumed on entirely sufficient grounds the existence of a complete circulation, but there was wanting in his scheme the direct ocular proof of the passage of blood from arteries to veins. this was supplied by leeuwenhoek. fig. _b_ shows one of his sketches of the capillary circulation. in his efforts to see the circulation he tried various animals; the comb of the young cock, the ears of white rabbits, the membraneous wing of the bat were progressively examined. the next advance came when he directed his microscope to the tail of the tadpole. upon examining this he exclaims: "a sight presented itself more delightful than any mine eyes had ever beheld; for here i discovered more than fifty circulations of the blood in different places, while the animal lay quiet in the water, and i could bring it before my microscope to my wish. for i saw not only that in many places the blood was conveyed through exceedingly minute vessels, from the middle of the tail toward the edges, but that each of the vessels had a curve or turning, and carried the blood back toward the middle of the tail, in order to be again conveyed to the heart. hereby it plainly appeared to me that the blood-vessels which i now saw in the animal, and which bear the names of arteries and veins are, in fact, one and the same; that is to say, that they are properly termed arteries so long as they convey the blood to the furtherest extremities of its vessels, and veins when they bring it back to the heart. and thus it appears that an artery and a vein are one and the same vessel prolonged or extended." this description shows that he fully appreciated the course of the minute vascular circulation and the nature of the communication between arteries and veins. he afterward extended his observations to the web of the frog's foot, the tail of young fishes and eels. in connection with this it should be remembered that malpighi, in , observed the flow of blood in the lungs and in the mesentery of the frog, but he made little of the discovery. leeuwenhoek did more with his, and gave the first clear idea of the capillary circulation. leeuwenhoek was anticipated also by malpighi in reference to the microscopic structure of the blood. (see also under swammerdam.) to malpighi the corpuscles appeared to be globules of fat, while leeuwenhoek noted that the blood disks of birds, frogs, and fishes were oval in outline, and those of mammals circular. he reserved the term 'globule' for those of the human body, erroneously believing them to be spheroidal. other discoveries.--among his other discoveries bearing on physiology and medicine may be mentioned: the branched character of heart muscles, the stripe in voluntary muscles, the structure of the crystalline lens, the description of spermatozoa after they had been pointed out to him in by hamen, a medical student in leyden, etc. richardson dignified him with the title 'the founder of histology,' but this, in view of the work of his great contemporary, malpighi, seems to me an overestimate. [illustration: fig. .--plant cells. (from leeuwenhoek's _arcana naturæ_.)] turning his microscope in all directions, he examined water and found it peopled with minute animalcules, those simple forms of animal life propelled through the water by innumerable hair-like cilia extending from the body like banks of oars from a galley, except that in many cases they extend from all surfaces. he saw not only the animalcules, but also the cilia that move their bodies. he also discovered the rotifers, those favorites of the amateur microscopists, made so familiar to the general public in works like gosse's _evenings at the microscope_. he observed that when water containing these animalcules evaporated they were reduced to fine dust, but became alive again, after great lapses of time, by the introduction of water. he made many observations on the microscopic structure of plants. fig. gives a fair sample of the extent to which he observed the cellular construction of vegetables and anticipated the cell theory. while malpighi's research in that field was more extensive, these sketches from leeuwenhoek represent very well the character of the work of the period on the minute structures of plants. his theoretical views.--it remains to say that on the two biological questions of the day he took a decisive stand. he was a believer in pre-formation or pre-delineation of the embryo in an extreme degree, seeing in fancy the complete outline of both maternal and paternal individuals in the spermatozoa, and going so far as to make sketches of the same. but on the question of the spontaneous origin of life he took the side that has been supported with such triumphant demonstration in this century; namely, the side opposing the theory of the occurrence of spontaneous generation under present conditions of life. comparison of the three men.--we see in these three gifted contemporaries different personal characteristics. leeuwenhoek, the composed and strong, attaining an age of ninety-one; malpighi, always in feeble health, but directing his energies with rare capacity, reaching the age of sixty-seven; while the great intensity of swammerdam stopped his scientific career at thirty-six and burned out his life at the age of forty-three. they were all original and accurate observers, but there is variation in the kind and quality of their intellectual product. the two university-trained men showed capacity for coherent observation; they were both better able to direct their efforts toward some definite end; leeuwenhoek, with the advantages of vigorous health and long working period, lacked the systematic training of the schools, and all his life wrought in discursive fashion; he left no coherent piece of work of any extent like malpighi's _anatome plantarum_ or swammerdam's _anatomy and metamorphosis of insects_. swammerdam was the most critical observer of the three, if we may judge by his labors in the same field as malpighi's on the silkworm. his descriptions are models of accuracy and completeness, and his anatomical work shows a higher grade of finish and completeness than malpighi's. malpighi, it seems to me, did more in the sum total than either of the others to advance the sciences of anatomy and physiology, and through them the interests of mankind. leeuwenhoek had larger opportunity; he devoted himself to microscopic observations, but he wandered over the whole field. while his observations lose all monographic character, nevertheless they were important in opening new fields and advancing the sciences of anatomy, physiology, botany, and zoölogy. the combined force of their labors marks an epoch characterized by the acceptance of the scientific method and the establishment of a new grade of intellectual life. through their efforts and that of their contemporaries of lesser note the new intellectual movement was now well under way. footnotes: [footnote : _leeuwenhoek and the rise of histology._ the asclepiad, vol. ii, .] chapter v the progress of minute anatomy. the work of malpighi, swammerdam, and leeuwenhoek stimulated investigations into the structure of minute animals, and researches in that field became a feature of the advance in the next century. considerable progress was made in the province of minute anatomy before comparative anatomy grew into an independent subject. the attractiveness of observations upon the life-histories and the structure of insects, as shown particularly in the publications of malpighi and swammerdam, made those animals the favorite objects of study. the observers were not long in recognizing that some of the greatest beauties of organic architecture are displayed in the internal structure of insects. the delicate tracery of the organs, their minuteness and perfection are well calculated to awaken surprise. well might those early anatomists be moved to enthusiasm over their researches. every excursion into this domain gave only beautiful pictures of a mechanism of exquisite delicacy, and their wonder grew into amazement. here began a new train of ideas, in the unexpected revelation that within the small compass of the body of an insect was embraced such a complex set of organs; a complete nervous system, fine breathing-tubes, organs of circulation, of digestion, etc., etc. lyonet.--the first piece of structural work after swammerdam's to which we must give attention is that of lyonet, who produced in the middle of the eighteenth century one of the most noteworthy monographs in the field of minute anatomy. this was a work like that of malpighi, upon the anatomy of a single form, but it was carried out in much greater detail. the figures on the plates are models of close observation and fine execution of drawings. [illustration: fig. .--lyonet, - .] lyonet (also written lyonnet) was a hollander, born in the hague in . he was a man of varied talents, a painter, a sculptor, an engraver, and a very gifted linguist. it is said that he was skilled in at least eight languages; and at one time he was the cipher secretary and confidential translator for the united provinces of holland. he was educated as a lawyer, but, from interest in the subject, devoted most of his time to engraving objects of natural history. among his earliest published drawings were the figures for lesser's _theology of insects_ ( ) and for trembley's famous treatise on _hydra_ ( ). his great monograph.--finally lyonet decided to branch out for himself, and produce a monograph on insect anatomy. after some preliminary work on the sheep-tick, he settled upon the caterpillar of the goat moth, which lives upon the willow-tree. his work, first published in , bore the title _traité anatomique de la chenille qui ronge le bois de saule_. in exploring the anatomy of the form chosen, he displayed not only patience, but great skill as a dissector, while his superiority as a draughtsman was continually shown in his sketches. he engraved his own figures on copper. the drawings are very remarkable for the amount of detail that they show. he dissected this form with the same thoroughness with which medical men have dissected the human body. the superficial muscles were carefully drawn and were then cut away in order to expose the next underlying layer which, in turn, was sketched and then removed. the amount of detail involved in this work may be in part realized from the circumstance that he distinguished , separate muscles. his sketches show these muscles accurately drawn, and the principal ones are lettered. when he came to expose the nerves, he followed the minute branches to individual small muscles and sketched them, not in a diagrammatic way, but as accurate drawings from the natural object. the breathing-tubes were followed in the same manner, and the other organs of the body were all dissected and drawn with remarkable thoroughness. lyonet was not trained in anatomy like malpighi and swammerdam, but being a man of unusual patience and manual dexterity, he accomplished notable results. his great quarto volume is, however, merely a description of the figures, and lacks the insight of a trained anatomist. his skill as a dissector is far ahead of his knowledge of anatomy, and he becomes lost in the details of his subject. extraordinary quality of the drawings.--a few figures will serve to illustrate the character of his work, but the reduced reproductions which follow can not do justice to the copper plates of the original. fig. gives a view of the external appearance of the caterpillar which was dissected. when the skin was removed from the outside the muscles came into view, as shown in fig. . this is a view from the ventral side of the animal. on the left side the more superficial muscles show, and on the right the next deeper layer. fig. shows his dissection of the nerves. in this figure the muscles are indicated in outline, and the distribution of nerves to particular muscles is shown. [illustration: fig. .--larva of the willow moth. (from lyonet's monograph, .)] lyonet's dissection of the head is an extraordinary feat. the entire head is not more than a quarter of an inch in diameter, but in a series of seven dissections he shows all of the internal organs in the head. fig. shows two sketches exhibiting the nervous ganglia, the air tubes, and muscles of the head in their natural position. fig. shows the nervous system of the head, including the extremely fine nervous masses which are designated the sympathetic nervous system. [illustration: fig. . fig. .--muscles of the larva of the willow moth. (from lyonet's monograph.)] [illustration: fig. . fig. .--central nervous system and nerves of the same.] the extraordinary character of the drawings in lyonet's monograph created a sensation. the existence of such complicated structures within the body of an insect was discredited, and, furthermore, some of his critics declared that even if such a fine organization existed, it would be beyond human possibilities to expose the details as shown in his sketches. accordingly, lyonet was accused of drawing on his imagination. in order to silence his critics he published in the second edition of his work, in , drawings of his instruments and a description of his methods. [illustration: fig. .--dissection of the head of the larva of the willow moth.] lyonet intended to work out the anatomy of the chrysalis and the adult form of the same animal. in pursuance of this plan, he made many dissections and drawings, but, at the age of sixty, on account of the condition of his eyes, he was obliged to stop all close work, and his project remained unfinished. the sketches which he had accumulated were published later, but they fall far short of those illustrating the _traité anatomique_. lyonet died in , at the age of eighty-one. [illustration: fig. .--the brain and head nerves of the same animal.] roesel, réaumur, and de geer on insect life.--we must also take note of the fact that, running parallel with this work on the anatomy of insects, observations and publications had gone forward on form, habits, and metamorphosis of insects, that did more to advance the knowledge of insect life than lyonet's researches. roesel, in germany, réaumur, in france, and de geer, in sweden, were all distinguished observers in this line. their works are voluminous and are well illustrated. those of réaumur and de geer took the current french title of _mémoires pour servir à l'histoire des insectes_. the plates with which the collected publications of each of the three men are provided show many sketches of external form and details of external anatomy, but very few illustrations of internal anatomy occur. the sketches of roesel in particular are worthy of examination at the present time. some of his masterly figures in color are fine examples of the art of painting in miniature. the name of roesel (fig. ) is connected also with the earliest observations of protoplasm and with a notable publication on the batrachians. réaumur (fig. ), who was distinguished for kindly and amiable personal qualities, was also an important man in his influence upon the progress of science. he was both physician and naturalist; he made experiments upon the physiology of digestion, which aided in the understanding of that process; he invented the thermometer which bears his name, and did other services for the advancement of science. [illustration: fig. .--roesel von rosenhof, - .] straus-dürckheim's monograph on insect anatomy.--insect anatomy continued to attract a number of observers, but we must go forward into the nineteenth century before we find the subject taking a new direction and merging into its modern phase. the remarkable monograph of straus-dürckheim represents the next step in the development of insect anatomy toward the position that it occupies to-day. his aim is clearly indicated in the opening sentence of his preface: "having been for a long time occupied with the study of articulated animals, i propose to publish a general work upon the comparative anatomy of that branch of the animal kingdom." he was working under the influence of cuvier, who, some years earlier, had founded the science of comparative anatomy and whom he recognized as his great exemplar. his work is dedicated to cuvier, and is accompanied by a letter to that great anatomist expressing his thanks for encouragement and assistance. [illustration: fig .--réaumur, - .] straus-dürckheim ( - ) intended that the general considerations should be the chief feature of his monograph, but they failed in this particular because, with the further developments in anatomy, including embryology and the cell-theory, his general discussions regarding the articulated animals became obsolete. the chief value of his work now lies in what he considered its secondary feature, _viz._, that of the detailed anatomy of the cockchafer, one of the common beetles of europe. owing to changed conditions, therefore, it takes rank with the work of malpighi and lyonet, as a monograph on a single form. originally he had intended to publish a series of monographs on the structure of insects typical of the different families, but that upon the cockchafer was the only one completed. comparison with the sketches of lyonet.--the quality of this work upon the anatomy of the cockchafer was excellent, and in it was accepted and crowned by the royal institute of france. the finely lithographed plates were prepared at the expense of the institute, and the book was published in with the following cumbersome title: _considérations générales sur l'anatomie comparée des animaux articulés auxquelles on a joint l'anatomie descriptive du melolontha vulgaris (hanneton) donnée comme example de l'organisation des coléoptères_. the sketches with which the plates are adorned are very beautiful, but one who compares his drawings, figure by figure, with those of lyonet can not fail to see that those of the latter are more detailed and represent a more careful dissection. one illustration from straus-dürckheim will suffice to bring the achievements of the two men into comparison. fig. shows his sketch of the anatomy of the central nervous system. he undertakes to show only the main branches of the nerves going to the different segments of the body, while lyonet brings to view the distribution of the minute terminals to particular muscles. comparison of other figures--notably that of the dissection of the head--will bring out the same point, _viz._, that lyonet was more detailed than straus-dürckheim in his explorations of the anatomy of insects, and fully as accurate in drawing what he had seen. nevertheless, the work of straus-dürckheim is conceived in a different spirit, and is the first serious attempt to make insect anatomy broadly comparative. comment.--such researches as those of swammerdam, lyonet, and straus-dürckheim represent a phase in the progress of the study of nature. perhaps their chief value lies in the fact that they embody the idea of critical observation. as examples of faithful, accurate observations the researches helped to bring about that close study which is our only means of getting at basal facts. these men were all enlisted in the crusade against superficial observation. this had to have its beginning, and when we witness it in its early stages, before the researches have become illuminated by great ideas, the prodigious effort involved in the detailed researches may seem to be poorly expended labor. nevertheless, though the writings of these pioneers have become obsolete, their work was of importance in helping to lift observations upon nature to a higher level. dufour.--léon dufour extended the work of straus-dürckheim by publishing, between and , researches upon the anatomy and physiology of different families of insects. his aim was to found a general science of insect anatomy. that he was unsuccessful in accomplishing this was owing partly to the absence of embryology and histology from his method of study. newport.--the thing most needed now was not greater devotion to details and a willingness to work, but a broadening of the horizon of ideas. this arrived in the englishman newport, who was remarkable not only for his skill as a dissector, but for his recognition of the importance of embryology in elucidating the problems of structure. his article "insecta" in todd's _cyclopædia of anatomy and physiology_, in , and his papers in the _philosophical transactions_ of the royal society contain this new kind of research. von baer had founded embryology by his great work on the development of animals in , before the investigations of dufour, but it was reserved for newport to recognize its great importance and to apply it to insect anatomy. he saw clearly that, in order to comprehend his problems, the anatomist must take into account the process of building the body, as well as the completed architecture of the adult. the introduction of this important idea made his achievement a distinct advance beyond that of his predecessors. [illustration: fig. .--nervous system of the cockchafer. (from straus-dürckheim's monograph, .)] leydig.--just as newport was publishing his conclusions the cell-theory was established (in - ); and this was destined to furnish the basis for a new advance. the influence of the doctrine that all tissues are composed of similar vital units, called cells, was far-reaching. investigators began to apply the idea in all directions, and there resulted a new department of anatomy, called histology. the subject of insect histology was an unworked field, but manifestly one of importance. franz leydig (for portrait see p. ) entered the new territory with enthusiasm, and through his extensive investigations all structural studies upon insects assumed a new aspect. in appeared his _vom bau des thierchen körpers_, which, together with his special articles, created a new kind of insect anatomy based upon the microscopic study of tissues. the application of this method of investigation is easy to see; just as it is impossible to understand the working of a machine without a knowledge of its construction, so a knowledge of the working units of an organ is necessary to comprehend its action. for illustration, it is perfectly evident that we can not understand what is taking place in an organ for receiving sensory impressions without first understanding its mechanism and the nature of the connections between it and the central part of the nervous system. the sensory organ is on the surface in order more readily to receive impressions from the outside world. the sensory cells are also modifications of surface cells, and, as a preliminary step to understanding their particular office, we must know the line along which they have become modified to fit them to receive stimulation. then, if we attempt to follow in the imagination the way by which the surface stimulations reach the central nervous system and affect it, we must investigate all the connections. it thus appears that we must know the intimate structure of an organ in order to understand its physiology. leydig supplied this kind of information for many organs of insects. in his investigations we see the foundation of that delicate work upon the microscopic structure of insects which is still going forward. summary.--in this brief sketch we have seen that the study of insect anatomy, beginning with that of malpighi and swammerdam, was lifted to a plane of greater exactitude by lyonet and straus-dürckheim. it was further broadened by the researches of dufour, and began to take on its modern aspects, first, through the labors of newport, who introduced embryology as a feature of investigation, and, finally, through leydig's step in introducing histology. in the combination of the work of these two observers, the subject for the first time reached its proper position. the studies of minute structure in the seventeenth and eighteenth centuries were by no means confined to insects; investigations were made upon a number of other forms. trembley, in the time of lyonet, produced his noteworthy memoirs upon the small fresh-water hydra (_mémoires pour servir à l'histoire des polypes d'eau douce_, ); the illustrations for which, as already stated, were prepared by lyonet. the structure of snails and other mollusks, of tadpoles, frogs, and other batrachia, was also investigated. we have seen that swammerdam, in the seventeenth century, had begun observations upon the anatomy of tadpoles, frogs, and snails, and also upon the minute crustacea commonly called water-fleas, which are just large enough to be distinguished by the unaided eye. we should remember also that in the same period the microscopic structure of plants began to be investigated, notably by grew, malpighi, and leeuwenhoek (see chapter iv). in addition to those essays into minute anatomy, in which scalpel and scissors were employed, an endeavor of more subtle difficulty made its appeal; there were forms of animal life of still smaller size and simpler organization that began to engage the attention of microscopists. a brief account of the discovery and subsequent observation of these microscopic animalcula will now occupy our attention. the discovery of the simplest animals and the progress of observations upon them these single-celled animals, since called protozoa, have become of unusual interest to biologists, because in them the processes of life are reduced to their simplest expression. the vital activities taking place in the bodies of higher animals are too complicated and too intricately mixed to admit of clear analysis, and, long ago, physiologists learned that the quest for explanations of living activities lay along the line of investigating them in their most rudimentary expression. the practical recognition of this is seen in our recent text-books upon human physiology, which commonly begin with discussions of the life of these simplest organisms. that greatest of all text-books on general physiology, written by max verworn, is devoted largely to experimental studies upon these simple organisms as containing the key to the similar activities (carried on in a higher degree) in higher animals. this group of animals is so important as a field of experimental observation that a brief account of their discovery and the progress of knowledge in reference to them will be in place in this chapter. discovery of the protozoa.--leeuwenhoek left so little unnoticed in the microscopic world that we are prepared to find that he made the first recorded observations upon these animalcula. his earliest observations were communicated by letter to the royal society of london, and were published in their _transactions_ in . it is very interesting to read his descriptions expressed in the archaic language of the time. the following quotation from a dutch letter turned into english will suffice to give the flavor of his writing: "in the year i discovered living creatures in rainwater which had stood but four days in a new earthen pot, glazed blew within. this invited me to view the water with great attention, especially those little animals appearing to me ten thousand times less than those represented by mons. swammerdam, and by him called water-fleas or water-lice, which may be perceived in the water with the naked eye. the first sorte by me discovered in the said water, i divers times observed to consist of five, six, seven or eight clear globules, without being able to discover any film that held them together or contained them. when these _animalcula_, or living atoms, did move they put forth two little horns, continually moving themselves; the place between these two horns was flat, though the rest of the body was roundish, sharpening a little towards the end, where they had a tayle, near four times the length of the whole body, of the thickness (by my microscope) of a spider's web; at the end of which appeared a globule, of the bigness of one of those which made up the body; which tayle i could not perceive even in very clear water to be mov'd by them. these little creatures, if they chanced to light upon the least filament or string, or other such particle, of which there are many in the water, especially after it has stood some days, they stook entangled therein, extending their body in a long round, and striving to dis-entangle their tayle; whereby it came to pass, that their whole body lept back towards the globule of the tayle, which then rolled together serpent-like, and after the manner of copper or iron wire, that having been wound around a stick, and unwound again, retains those windings and turnings," etc.[ ] any one who has examined under the microscope the well-known bell-animalcule will recognize in this first description of it, the stalk, and its form after contraction under the designation of a 'tayle which retains those windings and turnings.' there are many other descriptions, but the one given is typical of the others. he found the little animals in water, in infusions of pepper, and other vegetable substances, and on that account they came soon to be designated infusoria. his observations were not at first accompanied by sketches, but in he sent some drawings with further descriptions. o. fr. müller.--these animalcula became favorite objects of microscopic study. descriptions began to accumulate and drawings to be made until it became evident that there were many different kinds. it was, however, more than one hundred years after their discovery by leeuwenhoek that the first standard work devoted exclusively to these animalcula was published. this treatise by o. fr. müller was published in under the title of _animalcula infusoria_. the circumstance that this volume of quarto size had pages of description with plates of sketches will give some indication of the number of protozoa known at that time. ehrenberg.--observations in this domain kept accumulating, but the next publication necessary to mention is that of ehrenberg ( - ). this scientific traveler and eminent observer was the author of several works. he was one of the early observers of nerve fibres and of many other structures of the animal frame. his book of the protozoa is a beautifully illustrated monograph consisting of pages of letterpress and plates of folio size. it was published in under the german title of _die infusionsthierchen als vollkommene organismen_, or "the infusoria as perfect organisms." the animalcula which he so faithfully represented in his sketches have the habit, when feeding, of taking into the body collections of food-particles, aggregated into spherical globules or food vacuoles. these are distinctly separated, and slowly circulate around the single-celled body while they are undergoing digestion. in a fully fed animal these food-vacuoles occupy different positions, and are enclosed in globular spaces in the protoplasm, an adjustment that gave ehrenberg the notion that the animals possessed many stomachs. accordingly he gave to them the name "polygastrica," and assigned to them a much higher grade of organization than they really possess. these conclusions, based on the general arrangement of food globules, seem very curious to us to-day. his publication was almost simultaneous with the announcement of the cell-theory ( - ), the acceptance of which was destined to overthrow his conception of the protozoa, and to make it clear that tissues and organs can belong only to multicellular organisms. ehrenberg (fig. ) was a man of great scientific attainments, and notwithstanding the grotesqueness of some of his conclusions, was held in high esteem as a scientific investigator. his observations were accurate, and the beautiful figures with which his work on the protozoa is embellished were executed with such fidelity regarding fine points of microscopic detail that they are of value to-day. dujardin, whom we shall soon come to know as the discoverer of protoplasm, successfully combated the conclusions of ehrenberg regarding the organization of the protozoa. for a time the great german scientist tried to maintain his point, that the infusoria have many stomachs, but this was completely swept away, and finally the contention of von siebold was adopted to the effect that these animals are each composed of a single cell. [illustration: fig. .--ehrenberg, - .] in stein is engrossed in proposing names for the suborders of infusoria based upon the distribution of cilia upon their bodies. this simple method of classification, as well as the names introduced by stein, is still in use. from stein to bütschli, one of the present authorities on the group, there were many workers, but with the studies of bütschli on protozoa we enter the modern epoch. the importance of these animals in affording a field for experimentation on the simplest expressions of life has already been indicated. many interesting problems have arisen in connection with recent studies of them. the group embraces the very simplest manifestations of animal life, and the experiments upon the different forms light the way for studies of the vital activities of the higher animals. some of the protozoa are disease-producing; as the microbe of malaria, of the sleeping sickness, etc., while, as is well known, most diseases that have been traced to specific germs are caused by plants--the bacteria. many experiments of maupas, caulkins and others have a bearing upon the discussions regarding the immortality of the protozoa, an idea which was at one time a feature of weissmann's theory of heredity. binet and others have discussed the evidences of psychic life in these micro-organisms, and the daily activity of a protozoan became the field for observation and record in an american laboratory of psychology. the extensive studies of jennings on the nature of their responses to stimulations form a basis for some of the discussions on animal behavior. footnotes: [footnote : kent's manual of the infusoria, vol. i, p. . quotation from the _philosophical transactions_ for the year .] chapter vi linnÆus and scientific natural history we turn now from the purely anatomical side to consider the parallel development of the classification of animals and of plants. descriptive natural history reached a very low level in the early christian centuries, and remained there throughout the middle ages. the return to the writings of aristotle was the first influence tending to lift it to the position from which it had fallen. after the decline of ancient civilization there was a period in which the writers of classical antiquity were not read. not only were the writings of the ancient philosophers neglected, but so also were those of the literary men as well, the poets, the story-tellers, and the historians. as related in chapter i, there were no observations of animated nature, and the growing tendency of the educated classes to envelop themselves in metaphysical speculations was a feature of intellectual life. the physiologus or sacred natural history.--during this period of crude fancy, with a fog of mysticism obscuring all phenomena of nature, there existed a peculiar kind of natural history that was produced under theological influence. the manuscripts in which this sacred natural history was embodied exist in various forms and in about a dozen languages of eastern and western europe. the writings are known under the general title of the physiologus, or the bestiarius. this served for nearly a thousand years as the principal source of thought regarding natural history. it contains accounts of animals mentioned in the bible and others of a purely mythical character. these are made to be symbolical of religious beliefs, and are often accompanied by quotations of texts and by moral reflections. the phoenix rising from its ashes typifies the resurrection of christ. in reference to young lions, the _physiologus_ says: "the lioness giveth birth to cubs which remain three days without life. then cometh the lion, breatheth upon them, and bringeth them to life.... thus it is that jesus christ during three days was deprived of life, but god the father raised him gloriously." (quoted from white, p. .) besides forty or fifty common animals, the unicorn and the dragon of the scriptures, and the fabled basilisk and phoenix of secular writings are described, and morals are drawn from the stories about them. some of the accounts of animals, as the lion, the panther, the serpent, the weasel, etc., etc., are so curious that, if space permitted, it would be interesting to quote them; but that would keep us too long from following the rise of scientific natural history from this basis. for a long time the religious character of the contemplations of nature was emphasized and the prevalence of theological influence in natural history is shown in various titles, as lesser's _theology of insects_, swammerdam's _biblia naturæ_, spallanzani's _tracts_, etc. the zoölogy of the _physiologus_ was of a much lower grade than any we know about among the ancients, and it is a curious fact that progress was made by returning to the natural history of fifteen centuries in the past. the translation of aristotle's writings upon animals, and the disposition to read them, mark this advance. when, in the middle ages, the boundaries of interest began to be extended, it came like an entirely new discovery, to find in the writings of the ancients a storehouse of philosophic thought and a higher grade of learning than that of the period. the translation and recopying of the writers of classical antiquity was, therefore, an important step in the revival of learning. these writings were so much above the thought of the time that the belief was naturally created that the ancients had digested all learning, and they were pointed to as unfailing authorities in matters of science. the return to the science of the ancients.--the return to aristotle was wholesome, and under its influence men turned their attention once more to real animals. comments upon aristotle began to be made, and in course of time independent treatises upon animals began to appear. one of the first to modify aristotle to any purpose was edward wotton, the english physician, who published in a book on the distinguishing characteristics of animals (_de differentiis animalium_). this was a complete treatise on the zoölogy of the period, including an account of the different races of mankind. it was beautifully printed in paris, and was dedicated to edward vi. although embracing ten books, it was by no means so ponderous as were some of the treatises that followed it. the work was based upon aristotle, but the author introduced new matter, and also added the group of zoöphytes, or plant-like animals of the sea. gesner.--the next to reach a distinctly higher plane was conrad gesner ( - ), the swiss, who was a contemporary of vesalius. he was a practising physician who, in , was made professor of natural history in zurich. a man of extraordinary talent and learning, he turned out an astonishing quantity of work. besides accomplishing much in scientific lines, he translated from greek, arabic, and hebrew, and published in twenty volumes a universal catalogue of all works known in latin, greek, and hebrew, either printed or in manuscript form. in the domain of natural history he began to look critically at animals with a view to describing them, and to collect with zealous care new observations upon their habits. his great work on natural history (_historia animalium_) began to appear in , when he was thirty-five years of age, and four of the five volumes were published by . the fifth volume was not published until , twenty-two years after his death. the complete work consists of about " , folio pages," profusely illustrated with good figures. the edition which the writer has before him--that of - --embraces , pages of text and figures. brooks says: "one of gesner's greatest services to natural science is the introduction of good illustrations, which he gives his reader by hundreds." he was so exacting about the quality of his illustrations that his critical supervision of the work of artists and engravers had its influence upon contemporary art. some of the best woodcuts of the period are found in his work. his friend albrecht dürer supplied one of the originals--the drawing of the rhinoceros--and it is interesting to note that it is by no means the best, a circumstance which indicates how effectively gesner held his engraver and draughtsman up to fine work. he was also careful to mold his writing into graceful form, and this, combined with the illustrations, "made science attractive without sacrificing its dignity, and thus became a great educational influence." in preparing his work he sifted the writings of about two hundred and fifty authors, and while his book is largely a compilation, it is enriched with many observations of his own. his descriptions are verbose, but discriminating in separating facts and observations from fables and speculations. he could not entirely escape from old traditions. there are retained in his book pictures of the sea-serpent, the mermaids, and a few other fanciful and grotesque sketches, but for the most part the drawings are made from the natural objects. the descriptions are in several parts of his work alphabetically arranged, for convenience of reference, and thus animals that were closely related are often widely separated. gesner (fig. ) sacrificed his life to professional zeal during the prevalence of the plague in zurich in . having greatly overworked in the care of the sick, he was seized with the disease, and died at the age of forty-nine. considered from the standpoint of descriptions and illustrations, gesner's _historia animalium_ remained for a long time the best work in zoölogy. he was the best zoölogist between aristotle and john ray, the immediate predecessor of linnæus. [illustration: fig. .--gesner - .] jonston and aldrovandi.--at about the same period as gesner's work there appeared two other voluminous publications, which are well known--those of jonston, the scot (_historia animalium_, - ), and aldrovandi, the italian (_opera_, - ). the former consisted of four folio volumes, and the latter of thirteen, of ponderous size, to which was added a fourteenth on plants. jonston's works were translated, and were better known in england than those of gesner and aldrovandi. the wood-engravings in aldrovandi's volume are coarser than those of gesner, and are by no means so lifelike. in the institute at bologna are preserved twenty volumes of figures of animals in color, which were the originals from which the engravings were made. these are said to be much superior to the reproductions. the encyclopædic nature of the writings of gesner, aldrovandi, and jonston has given rise to the convenient and expressive title of the encyclopædists. ray.--john ray, the forerunner of linnæus, built upon the foundations of gesner and others, and raised the natural-history edifice a tier higher. he greatly reduced the bulk of publications on natural history, sifting from gesner and aldrovandi their irrelevancies, and thereby giving a more modern tone to scientific writings. he was the son of a blacksmith, and was born in southern england in . the original form of the family name was wray. he was graduated at the university of cambridge, and became a fellow of trinity college. here he formed a friendship with francis willughby, a young man of wealth whose tastes for natural history were like his own. this association proved a happy one for both parties. ray had taken orders in the church of england, and held his university position as a cleric; but, from conscientious scruples, he resigned his fellowship in . thereafter he received financial assistance from willughby, and the two men traveled extensively in great britain and on the continent, with the view of investigating the natural history of the places that they visited. on these excursions willughby gave particular attention to animals and ray to plants. of ray's several publications in botany, his _historia plantarum_ in three volumes ( - ) is the most extensive. in another work, as early as , he had proposed a new classification of plants, which in the next century was adopted by jussieu, and which gives ray a place in the history of botany. [illustration: fig. .--john ray, - .] willughby died in , at the age of thirty-eight, leaving an annuity to ray, and charging him with the education of his two sons, and the editing of his manuscripts. ray performed these duties as a faithful friend and in a generous spirit. he edited and published willughby's book on birds ( ) and fishes ( ) with important additions of his own, for which he sought no credit. after completing his tasks as the literary executor of willughby, he returned in to his birthplace and continued his studies in natural history. in he published "the wisdom of god manifested in the works of the creation," which was often reprinted, and became the forerunner of the works on natural theology like paley's, etc. this was an amplification of ideas he had embodied in a sermon thirty-one years earlier, and which at that time attracted much notice. he now devoted himself largely to the study of animals, and in published a work on the quadrupeds and serpents, a work which gave him high rank in the history of the classification of animals. he died in , but he had accomplished much good work, and was not forgotten. in there was founded, in london, in his memory, the ray society for the publication of rare books on botany and zoölogy. ray's idea of species.--one of the features of ray's work, in the light of subsequent development, is of special interest, and that is his limiting of species. he was the first to introduce into natural history an exact conception of species. before his time the word had been used in an indefinite sense to embrace groups of greater or less extent, but ray applied it to individuals derived from similar parents, thus making the term species stand for a particular kind of animal or plant. he noted some variations among species, and did not assign to them that unvarying and constant character ascribed to them by linnæus and his followers. ray also made use of anatomy as the foundation for zoölogical classification, and introduced great precision and clearness into his definitions of groups of animals and plants. in the particulars indicated above he represents a great advance beyond any of his precursors, and marks the parting of the ways between mediæval and modern natural history. in germany klein ( - ) elaborated a system of classification embracing the entire animal kingdom. his studies were numerous, and his system would have been of much wider influence in molding natural history had it not been overshadowed by that of linnæus. linnæus or linné.--the service of linnæus to natural history was unique. the large number of specimens of animals and plants, ever increasing through the collections of travelers and naturalists, were in a confused state, and there was great ambiguity arising from the lack of a methodical way of arranging and naming them. they were known by verbose descriptions and local names. no scheme had as yet been devised for securing uniformity in applying names to them. the same animal and plant had different names in the different sections of a country, and often different plants and animals had the same name. in different countries, also, their names were greatly diversified. what was especially needed was some great organizing mind to catalogue the animals and plants in a systematic way, and to give to natural science a common language. linnæus possessed this methodizing mind and supplied the need. while he did little to deepen the knowledge of the organization of animal and plant life, he did much to extend the number of known forms; he simplified the problem of cataloguing them, and he invented a simple method of naming them which was adopted throughout the world. by a happy stroke he gave to biology a new language that remains in use to-day. the tremendous influence of this may be realized when we remember that naturalists everywhere use identical names for the same animals and plants. the residents of japan, of italy, of spain, of all the world, in fact, as was just said, employ the same latin names in classifying organic forms. he also inspired many students with a love for natural history and gave an impulse to the advance of that science which was long felt. we can not gainsay that a higher class of service has been rendered by those of philosophic mind devoted to the pursuit of comparative anatomy, but the step of linnæus was a necessary one, and aided greatly in the progress of natural history. without this step the discoveries and observations of others would not have been so readily understood, and had it not been for his organizing force all natural science would have been held back for want of a common language. a close scrutiny of the practice among naturalists in the time of linnæus shows that he did not actually invent the binomial nomenclature, but by adopting the suggestions of others he elaborated the system of classification and brought the new language into common use. personal history.--leaving for the present the system of linnæus, we shall give attention to the personal history of the man. the great swedish naturalist was born in rashult in . his father was the pastor of the village, and intended his eldest son, carl, for the same high calling. the original family name was ignomarsen, but it had been changed to lindelius, from a tall linden-tree growing in that part of the country. in a patent of nobility was granted by the crown to linnæus, and thereafter he was styled carl von linné. his father's resources were very limited, but he managed to send his son to school, though it must be confessed that young linnæus showed little liking for the ordinary branches of instruction. his time was spent in collecting natural-history specimens, and his mind was engaged in thinking about them. the reports of his low scholarship and the statement of one of his teachers that he showed no aptitude for learning were so disappointing to his father that, in , he prepared to apprentice carl to a shoemaker, but was prevented from doing so through the encouragement of a doctor who, being able to appreciate the quality of mind possessed by the young linnæus, advised allowing him to study medicine instead of preparing for theology. accordingly, with a sum amounting to about $ , all his father could spare, he set off for the university of lund, to pursue the study of medicine. he soon transferred to the university of upsala, where the advantages were greater. his poverty placed him under the greatest straits for the necessities of life, and he enjoyed no luxuries. while in the university he mended his shoes, and the shoes which were given to him by some of his companions, with paper and birch-bark, to keep his feet from the damp earth. but his means did not permit of his taking his degree at upsala, and it was not until eight years later, in , that he received his degree in holland. at upsala he was relieved from his extreme poverty by obtaining an assistant's position, and so great was his knowledge of plants that he was delegated to read the lectures of the aged professor of botany, rudbeck. in he was chosen by the royal society of upsala to visit lapland as a collector and observer, and left the university without his degree. on returning to upsala, his lack of funds made itself again painfully felt, and he undertook to support himself by giving public lectures on botany, chemistry, and mineralogy. he secured hearers, but the continuance of his lectures was prevented by one of his rivals on the ground that linnæus had no degree, and was therefore legally disqualified from taking pay for instruction. presently he became tutor and traveling companion of a wealthy baron, the governor of the province of dalecarlia, but this employment was temporary. helped by his fiancée.--his friends advised him to secure his medical degree and settle as a practitioner. although he lacked the necessary funds, one circumstance contributed to bring about this end: he had formed an attachment for the daughter of a wealthy physician, named moré or moræus, and on applying for her hand in marriage, her father made it a condition of his consent that linnæus should take his medical degree and establish himself in the practice of medicine. the young lady, who was thrifty as well as handsome, offered her savings, amounting to one hundred dollars (swedish), to her lover. he succeeded in adding to this sum by his own exertions, and with thirty-six swedish ducats set off for holland to qualify for his degree. he had practically met the requirements for the medical degree by his previous studies, and after a month's residence at the university of hardewyk, his thesis was accepted and he was granted the degree in june, , in the twenty-eighth year of his age. instead of returning at once to sweden, he went to leyden, and made the acquaintance of several well-known scientific men. he continued his botanical studies with great energy, and now began to reap the benefits of his earlier devotion to natural history. his heart-breaking and harassing struggles were now over. the systema naturæ.--he had in his possession the manuscript of his _systema naturæ_, and with the encouragement of his new friends it was published in the same year. the first edition ( ) of that notable work, which was afterward to bring him so much fame, consisted of twelve printed folio pages. it was merely an outline of the arrangements of plants, animals, and minerals in a methodical catalogue. this work passed through twelve editions during his lifetime, the last one appearing in . after the first edition, the books were printed in octavo form, and in the later editions were greatly enlarged. a copy of the first edition was sent to boerhaave, the most distinguished professor in the university of leyden, and secured for linnæus an interview with that distinguished physician, who treated him with consideration and encouraged him in his work. boerhaave was already old, and had not long to live; and when linnæus was about to leave holland in , he admitted him to his sick-chamber and bade him a most affectionate adieu, and encouraged him to further work by most kindly and appreciative expressions. through the influence of boerhaave, linnæus became the medical attendant of cliffort, the burgomaster at amsterdam, who had a large botanic garden. cliffort, being desirous of extending his collections, sent linnæus to england, where he met sir hans sloane and other eminent scientific men of great britain. after a short period he returned to holland, and in brought out the _genera plantarum_, a very original work, containing an analysis of all the genera of plants. he had previously published, besides the _systema naturæ_, his _fundamenta botanica_, , and _bibliotheca botanica_, , and these works served to spread his fame as a botanist throughout europe. his wide recognition.--an illustration of his wide recognition is afforded by an anecdote of his first visit to paris in . "on his arrival he went first to the garden of plants, where bernard de jussieu was describing some exotics in latin. he entered without opportunity to introduce himself. there was one plant which the demonstrator had not yet determined, and which seemed to puzzle him. the swede looked on in silence, but observing the hesitation of the learned professor, cried out '_hæc planta faciem americanam habet_.' 'it has the appearance of an american plant.' jussieu, surprised, turned about quickly and exclaimed 'you are linnæus.' 'i am, sir,' was the reply. the lecture was stopped, and bernard gave the learned stranger an affectionate welcome." return to sweden.--after an absence of three and one-half years, linnæus returned to his native country in , and soon after was married to the young woman who had assisted him and had waited for him so loyally. he settled in stockholm and began the practice of medicine. in the period of his absence he had accomplished much: visited holland, england, and france, formed the acquaintance of many eminent naturalists, obtained his medical degree, published numerous works on botany, and extended his fame over all europe. in stockholm, however, he was for a time neglected, and he would have left his native country in disgust had it not been for the dissuasion of his wife. professor in upsala.--in he was elected professor of anatomy in the university of upsala, but by a happy stroke was able to exchange that position for the professorship of botany, materia medica, and natural history that had fallen to his former rival, rosen. linnæus was now in his proper element; he had opportunity to lecture on those subjects to which he had been devotedly attached all his life, and he entered upon the work with enthusiasm. he attracted numerous students by the power of his personal qualities and the excellence of his lectures. he became the most popular professor in the university of upsala, and, owing to his drawing power, the attendance at the university was greatly increased. in he had students devoted to studies in natural history. the number of students at the university had been about ; "whilst he occupied the chair of botany there it rose to , ." a part of this increase was due to other causes, but linnæus was the greatest single drawing force in the university. he was an eloquent as well as an enthusiastic lecturer, and he aroused great interest among his students, and he gave an astonishing impulse to the study of natural history in general, and to botany in particular. thus linnæus, after having passed through great privations in his earlier years, found himself, at the age of thirty-four, established in a position which brought him recognition, honor, and large emolument. [illustration: fig. .--linnæus at sixty, - .] in may, , the university of upsala celebrated the two hundredth anniversary of his birth with appropriate ceremonies. delegations of scientific men from all over the world were in attendance to do honor to the memory of the great founder of biological nomenclature. personal appearance.--the portrait of linnæus at the age of sixty is shown in fig. . he was described as of "medium height, with large limbs, brown, piercing eyes, and acute vision." his hair in early youth was nearly white, and changed in his manhood to brown, and became gray with the advance of age. although quick-tempered, he was naturally of a kindly disposition, and secured the affection of his students, with whom he associated and worked in the most informal way. his love of approbation was very marked, and he was so much praised that his desire for fame became his dominant passion. the criticism to which his work was subjected from time to time accordingly threw him into fits of despondency and rage. his influence upon natural history.--however much we may admire the industry and force of linnæus, we must admit that he gave to natural history a one-sided development, in which the more essential parts of the science received scant recognition. his students, like their master, were mainly collectors and classifiers. "in their zeal for naming and classifying, the higher goal of investigation, knowledge of the nature of animals and plants, was lost sight of and the interest in anatomy, physiology, and embryology lagged." r. hertwig says of him: "for while he in his _systema naturæ_ treated of an extraordinarily larger number of animals than any earlier naturalist, he brought about no deepening of our knowledge. the manner in which he divided the animal kingdom, in comparison with the aristotelian system, is to be called rather a retrogression than an advance. linnæus divided the animal kingdom into six classes--mammalia, aves, amphibia, pisces, insecta, vermes. the first four classes correspond to aristotle's four groups of animals with blood. in the division of the invertebrated animals into insecta and vermes linnæus stands undoubtedly behind aristotle, who attempted, and in part indeed successfully, to set up a larger number of groups. "but in his successors even more than in linnæus himself we see the damage wrought by the purely systematic method of consideration. the diagnoses of linnæus were for the most part models, which, _mutatis mutandis_, could be employed for new species with little trouble. there was needed only some exchanging of adjectives to express the differences. with the hundreds of thousands of different species of animals, there was no lack of material, and so the arena was opened for that spiritless zoölogy of species-making, which in the first half of the nineteenth century brought zoölogy into such discredit. zoölogy would have been in danger of growing into a tower of babel of species-description if a counterpoise had not been created in the strengthening of the physiologico-anatomical method of consideration." his especial service.--nevertheless, the work of linnæus made a lasting impression upon natural history, and we shall do well to get clearly in mind the nature of his particular service. in the first place, he brought into use the method of naming animals and plants which is employed to-day. in his _systema naturæ_ and in other publications he employed a means of naming every natural production in two words, and it is therefore called the binomial nomenclature. an illustration will make this clearer. those animals which had close resemblance, like the lion, tiger, leopard, the lynx, and the cat, he united under the common generic name of _felis_, and gave to each a particular trivial name, or specific name. thus the name of the lion became _felis leo_, of the tiger _felis tigris_, of the leopard _felis pardus_, of the cat felis catus; and to these the modern zoölogists have added, making the canada lynx _felis canadensis_, the domestic cat _felis domesticata_, etc. in a similar way, the dog-like animals were united into a genus designated _canis_, and the particular kinds or species became _canis lupus_, the wolf, _canis vulpes_, the fox, _canis familiaris_, the common dog. this simple method took the place of the varying names applied to the same animal in different countries and local names in the same country. it recognized at once their generic likeness and their specific individuality. all animals, plants, and minerals were named according to this method. thus there were introduced into nomenclature two groups, the genus and the species. the name of the genus was a noun, and that of the species an adjective agreeing with it. in the choice of these names linnæus sought to express some distinguishing feature that would be suggestive of the particular animal, plant, or mineral. the trivial, or specific, names were first employed by linnæus in , and were introduced into his _species plantarum_ in , and into the tenth edition of his _systema naturæ_ in . we recognize linnæus as the founder of nomenclature in natural history, and by the common consent of naturalists the date has come to be accepted as the starting-point for determining the generic and specific names of animals. the much vexed question of priority of names for animals is settled by going back to the tenth edition of his _systema naturæ_, while the botanists have adopted his _species plantarum_, , as their base-line for names. as to his larger divisions of animals and plants, he recognized classes and orders. then came genera and species. linnæus did not use the term family in his formulæ; this convenient designation was first used and introduced in by batch. the _systema naturæ_ is not a treatise on the organization of animals and plants; it is rather a catalogue of the productions of nature methodically arranged. his aim in fact was not to give full descriptions, but to make a methodical arrangement. to do justice, however, to the discernment of linnæus, it should be added that he was fully aware of the artificial nature of his classification. as kerner has said: "it is not the fault of this accomplished and renowned naturalist if a greater importance were attached to his system than he himself ever intended. linnæus never regarded his twenty-four classes as real and natural divisions of the vegetable kingdom, and specifically says so; it was constructed for convenience of reference and identification of species. a real natural system, founded on the true affinities of plants as indicated by the structural characters, he regarded as the highest aim of botanical endeavor. he never completed a natural system, leaving only a fragment (published in )." terseness of descriptions.--his descriptions were marked by extreme brevity, but by great clearness. this is a second feature of his work. in giving the diagnosis of a form he was very terse. he did not employ fully formed sentences containing a verb, but words concisely put together so as to bring out the chief things he wished to emphasize. as an illustration of this, we may take his characterization of the forest rose, "_rosa sylvestris vulgaris, flore odorata incarnato_." the common rose of the forest with a flesh-colored, sweet-smelling flower. in thus fixing the attention upon essential points he got rid of verbiage, a step that was of very great importance. his idea of species.--a third feature of his work was that of emphasizing the idea of species. in this he built upon the work of ray. we have already seen that ray was the first to define species and to bring the conception into natural history. ray had spoken of the variability of species, but linnæus, in his earlier publications, declared that they were constant and invariable. his conception of a species was that of individuals born from similar parents. it was assumed that at the original stocking of the earth, one pair of each kind of animals was created, and that existing species were the direct descendants without change of form or habit from the original pair. as to their number, he said: "_species tot sunt, quot formæ ab initio creatæ sunt_"--there are just so many species as there were forms created in the beginning; and his oft-quoted remark, "_nulla species nova_," indicates in terse language his position as to the formation of new species. linnæus took up this idea as expressing the current thought, without analysis of what was involved in it. he readily might have seen that if there were but a single pair of each kind, some of them must have been sacrificed to the hunger of the carnivorous kinds: but, better than making any theories, he might have looked for evidence in nature as to the fixity of species. while linnæus first pronounced upon the fixity of species, it is interesting to note that his extended observations upon nature led him to see that variation among animals and plants is common and extensive, and accordingly in the later editions of his _systema naturæ_ we find him receding from the position that species are fixed and constant. nevertheless, it was owing to his influence, more than to that of any other writer of the period, that the dogma of fixity of species was established. his great contemporary buffon looked upon species as not having a fixed reality in nature, but as being figments of the imagination; and we shall see in a later section of this book how the idea of linnæus in reference to the fixity of species gave way to accumulating evidence on the matter. summary.--the chief services of linnæus to natural science consisted of these three things: bringing into current use the binomial nomenclature, the introduction of terse formulæ for description, and fixing attention upon species. the first two were necessary steps; they introduced clearness and order into the management of the immense number of details, and they made it possible for the observations and discoveries of others to be understood and to take their place in the great system of which he was the originator. the effect of the last step was to direct the attention of naturalists to species, and thereby to pave the way for the coming consideration of their origin, a consideration which became such a burning question in the last half of the nineteenth century. reform of the linnæan system necessity of reform.--as indicated above, the classification established by linnæus had grave defects; it was not founded on a knowledge of the comparative structure of animals and plants, but in many instances upon superficial features that were not distinctive in determining their position and relationships. his system was essentially an artificial one, a convenient key for finding the names of animals and plants, but doing violence to the natural arrangement of those organisms. an illustration of this is seen in his classification of plants into classes, mainly on the basis of the number of stamens in the flower, and into orders according to the number of pistils. moreover, the true object of investigation was obscured by the linnæan system. the chief aim of biological study being to extend our knowledge of the structure, development, and physiology of animals and plants as a means of understanding more about their life, the arrangement of animals and plants into groups should be the outcome of such studies rather than an end in itself. it was necessary to follow different methods to bring natural history back into the line of true progress. the first modification of importance to the linnæan system was that of cuvier, who proposed a grouping of animals based upon a knowledge of their comparative anatomy. he declared that animals exhibit four types of organization, and his types were substituted for the primary groups of linnæus. the scale of being.--in order to understand the bearing of cuvier's conclusions we must take note of certain views regarding the animal kingdom that were generally accepted at the time of his writing. between linnæus and cuvier there had emerged the idea that all animals, from the lowest to the highest, form a graduated series. this grouping of animals into a linear arrangement was called exposing the scale of being, or the scale of nature (_scala naturæ_). buffon, lamarck, and bonnet were among the chief exponents of this idea. that lamarck's connection with it was temporary has been generally overlooked. it is the usual statement in the histories of natural science, as in the _encyclopædia britannica_, in the history of carus, and in thomson's _science of life_, that the idea of the scale of nature found its fullest expression in lamarck. thomson says: "his classification ( - ) represents the climax of the attempt to arrange the groups of animals in linear order from lower to higher, in what was called a _scala naturæ_" (p. ). even so careful a writer as richard hertwig has expressed the matter in a similar form. now, while lamarck at first adopted a linear classification, it is only a partial reading of his works that will support the conclusion that he held to it. in his _système des animaux sans vertèbres_, published in , he arranged animals in this way; but to do credit to his discernment, it should be observed that he was the first to employ a genealogical tree and to break up the serial arrangement of animal forms. in , in the second volume of his _philosophie zoologique_, as packard has pointed out, he arranged animals according to their relationships, in the form of a trunk with divergent branches. this was no vague suggestion on his part, but an actual pictorial representation of the relationship between different groups of animals, as conceived by him. although a crude attempt, it is interesting as being the first of its kind. this is so directly opposed to the idea of scale of being that we make note of the fact that lamarck forsook that view at least twenty years before the close of his life and substituted for it that of the genealogical tree. lamarck's position in science.--lamarck is coming into full recognition for his part in founding the evolution theory, but he is not generally, as yet, given due credit for his work in zoölogy. he was the most philosophical thinker engaged with zoölogy at the close of the eighteenth and the beginning of the nineteenth century. he was greater than cuvier in his reach of intellect and in his discernment of the true relationships among living organisms. we are to recollect that he forsook the dogma of fixity of species, to which cuvier held, and founded the first comprehensive theory of organic evolution. to-day we can recognize the superiority of his mental grasp over that of cuvier, but, owing to the personal magnetism of the latter and to his position, the ideas of lamarck, which cuvier combated, received but little attention when they were promulgated. we shall have occasion in a later chapter to speak more fully of lamarck's contribution to the progress of biological thought. cuvier's four branches.--we now return to the type-theory of cuvier. by extended studies in comparative anatomy, he came to the conclusion that animals are constructed upon four distinct plans or types: the vertebrate type; the molluscan type; the articulated type, embracing animals with joints or segments; and the radiated type, the latter with a radial arrangement of parts, like the starfish; etc. these types are distinct, but their representatives, instead of forming a linear series, overlap so that the lowest forms of one of the higher groups are simpler in organization than the higher forms of a lower group. this was very illuminating, and, being founded upon an analysis of structure, was important. it was directly at variance with the idea of scale of being, and overthrew that doctrine. cuvier first expressed these views in a pamphlet published in , and later in a better-known paper read before the french academy in , but for the full development of his type-theory we look to his great volume on the animal kingdom published in . the central idea of his arrangement is contained in the secondary title of his book, "the animal kingdom arranged according to its organization" (_le règne animal distribué d'après son organisation_, ). the expression "arranged according to its organization" embraces the feature in which this analysis of animals differs from all previous attempts. correlation of parts.--an important idea, first clearly expressed by cuvier, was that of correlation of parts. the view that the different parts of an animal are so correlated that a change in one, brought about through changes in use, involves a change in another. for illustration, the cleft hoof is always associated with certain forms of teeth and with the stomach of a ruminant. the sharp claws of flesh-eating animals are associated with sharp, cutting teeth for tearing the flesh of the victims, and with an alimentary tube adapted to the digestion of a fleshy diet. further account of cuvier is reserved for the chapter on the rise of comparative anatomy, of which he was the founder. von baer.--the next notable advance affecting natural history came through the work of von baer, who, in , founded the science of development of animal forms. he arrived at substantially the same conclusions as cuvier. thus the system founded upon comparative anatomy by cuvier came to have the support of von baer's studies in embryology. the contributions of these men proved to be a turning-point in natural history, and subsequent progress in systematic botany and zoölogy resulted from the application of the methods of cuvier and von baer, rather than from following that of linnæus. his nomenclature remained a permanent contribution of value, but the knowledge of the nature of living forms has been advanced chiefly by studies in comparative anatomy and embryology, and, also, in the application of experiments. the most significant advances in reference to the classification of animals was to come as a result of the acceptance of the doctrine of organic evolution, subsequent to . then the relationships between animals were made to depend upon community of descent, and a distinction was drawn between superficial or apparent relationships and those deep-seated characteristics that depend upon close genetic affinities. alterations by von siebold and leuckart.--but, in the mean time, naturalists were not long in discovering that the primary divisions established by cuvier were not well balanced, and, indeed, that they were not natural divisions of the animal kingdom. the group radiata was the least sharply defined, since cuvier had included in it not only those animals which exhibit a radial arrangement of parts, but also unicellular organisms that were asymmetrical, and some of the worms that showed bilateral symmetry. accordingly, karl th. von siebold, in , separated these animals and redistributed them. for the simplest unicellular animals he adopted the name protozoa, which they still retain, and the truly radiated forms, as starfish, sea-urchins, hydroid polyps, coral animals, etc., were united in the group zoöphyta. von siebold also changed cuvier's branch, articulata, separating those forms as crustacea, insects, spiders, and myriopods, which have jointed appendages, into a natural group called arthropoda, and uniting the segmented worms with those worms that cuvier has included in the radiate group, into another branch called vermes. this separation of the four original branches of cuvier was a movement in the right direction, and was destined to be carried still farther. [illustration: fig. .--karl th. von siebold, - .] von siebold (fig. ) was an important man in the progress of zoölogy, especially in reference to the comparative anatomy of the invertebrates. leuckart (fig. ), whose fame as a lecturer and teacher attracted many young men to the university of leipsic, is another conspicuous personality in zoölogical progress. this distinguished zoölogist, following the lead of von siebold, made further modifications. he split von siebold's group of zoöphytes into two distinct kinds of radiated animals; the star-fishes, sea-urchins, sea-cucumbers, etc., having a spiny skin, he designated echinoderma; the jelly-fishes, polyps, coral animals, etc., not possessing a true body cavity, were also united into a natural group, for which he proposed the name coelenterata. [illustration: fig. .--rudolph leuckart, - .] from all these changes there resulted the seven primary divisions--branches, subkingdoms, or phyla--which, with small modifications, are still in use. these are protozoa, coelenterata, echinoderma, vermes, arthropoda, mollusca, vertebrata. these seven phyla are not entirely satisfactory, and there is being carried on a redistribution of forms, as in the case of the brachiopods, the sponges, the tunicates, etc. while all this makes toward progress, the changes are of more narrow compass than those alterations due to von siebold and leuckart. summary.--in reviewing the rise of scientific natural history, we observe a steady development from the time of the _physiologus_, first through a return to aristotle, and through gradual additions to his observations, notably by gesner, and then the striking improvements due to ray and linnæus. we may speak of the latter two as the founders of systematic botany and zoölogy. but the system left by linnæus was artificial, and the greatest obvious need was to convert it into a natural system founded upon a knowledge of the structure and the development of living organisms. this was begun by cuvier and von baer, and was continued especially by von siebold and leuckart. to this has been added the study of habits, breeding, and adaptations of organisms, a study which has given to natural history much greater importance than if it stood merely for the systematic classification of animals and plants. tabular view of classifications.--a table showing the primary groups of linnæus, cuvier, von siebold, and leuckart will be helpful in picturing to the mind the modifications made in the classification of animals. such a table is given on the following page. l. agassiz, in his famous essay on classification, reviews in the most scholarly way the various systems of classification. one peculiar feature of agassiz's philosophy was his adherence to the dogma of the fixity of species. the same year that his essay referred to was published ( ) appeared darwin's _origin of species_. agassiz, however, was never able to accept the idea, of the transformations of species. linnæus cuvier von siebold leuckart mammalia vertebrata vertebrata vertebrata (embracing five (embracing five (five classes.) aves classes: mammalia, classes.) aves, reptilia, amphibia batrachia, pisces.) pisces insecta mollusca mollusca mollusca (including crustacea, {arthropoda etc.) articulata {vermes arthropoda vermes vermes {zoöphyta {echinoderma (including radiata {coelenterata mollusca and all {protozoa protozoa lower forms.) steps in biological progress from linnæus to darwin the period from linnæus to darwin is one full of important advances for biology in general. we have considered in this chapter only those features that related to changes in the system of classification, but in the mean time the morphological and the physiological sides of biology were being advanced not only by an accumulation of facts, but by their better analysis. it is an interesting fact that, although during this period the details of the subject were greatly multiplied, progress was relatively straightforward and by a series of steps that can be clearly indicated. it will be of advantage before the subject is taken up in its parts to give a brief forecast in which the steps of progress can be represented in outline without the confusion arising from the consideration of details. geddes, in , pointed out the steps in progress, and the account that follows is based upon his lucid analysis. the organism.--in the time of linnæus the attention of naturalists was mainly given to the organism as a whole. plants and animals were considered from the standpoint of the organism--the external features were largely dealt with, the habitat, the color, and the general appearance--features which characterize the organism as a whole. linnæus and jussieu represent this phase of the work, and buffon the higher type of it. modern studies in this line are like addition to the _systema naturæ_. organs.--the first distinct advance came in investigating animals and plants according to their structure. instead of the complete organism, the organs of which it is composed became the chief subject of analysis. the organism was dissected, the organs were examined broadly, and those of one kind of animal and plant compared with another. this kind of comparative study centered in cuvier, who, in the early part of the nineteenth century, founded the science of comparative anatomy of animals, and in hofmeister, who examined the structure of plants on a basis of broad comparison. tissues.--bichat, the famous contemporary of cuvier, essayed a deeper level of analysis in directing attention to the tissues that are combined to make up the organs. he distinguished twenty-one kinds of tissues by combinations of which the organs are composed. this step laid the foundation for the science of histology, or minute anatomy. bichat called it general anatomy (_anatomie générale_, ). cells.--before long it was shown that tissues are not the real units of structure, but that they are composed of microscopic elements called cells. this level of analysis was not reached until magnifying-lenses were greatly improved--it was a product of a closer scrutiny of nature with improved instruments. the foundation of the work, especially for plants, had been laid by leeuwenhoek, malpighi, and grew. but when the broad generalization, that all the tissues of animals and plants are composed of cells, was given to the world by schleiden and schwann, in - , the entire organization of living forms took on a new aspect. this was progress in understanding the morphology of animals and plants. protoplasm.--with improved microscopes and attention directed to cells, it was not long before the discovery was made that the cells as units of structure contain protoplasm. that this substance is similar in plants and animals and is the seat of all vital activity was determined chiefly by the researches of max schultze, published in . thus step by step, from , the date of the tenth edition of the _systema naturæ_, to , there was a progress on the morphological side, passing from the organism as a whole to organs, to tissues, to cells, and finally to protoplasm, the study of which in all its phases is the chief pursuit of biologists. the physiological side had a parallel development. in the period of linnæus, the physiology of the organism was investigated by haller and his school; following him the physiology of organs and tissues was advanced by j. müller, bichat, and others. later, virchow investigated the physiology of cells, and claude bernard the chemical activities of protoplasm. this set forth in outline will be amplified in the following chapters. chapter vii cuvier and the rise of comparative anatomy after observers like linnæus and his followers had attained a knowledge of the externals, it was natural that men should turn their attention to the organization or internal structure of living beings, and when the latter kind of investigation became broadly comparative, it blossomed into comparative anatomy. the materials out of which the science of comparative anatomy was constructed had been long accumulating before the advent of cuvier, but the mass of details had not been organized into a compact science. as indicated in previous chapters, there had been an increasing number of studies upon the structure of organisms, both plant and animal, and there had resulted some noteworthy monographs. all this work, however, was mainly descriptive, and not comparative. now and then, the comparing tendency had been shown in isolated writings such as those of harvey, malpighi, and others. as early as , belon had compared the skeleton of the bird with that of the human body "in the same posture and as nearly as possible bone for bone"; but this was merely a faint foreshadowing of what was to be done later in comparing the systems of the more important organs. we must keep in mind that the study of anatomy embraces not merely the bony framework of animals, but also the muscles, the nervous system, the sense organs, and all the other structures of both animals and plants. in the rise of comparative anatomy there gradually emerged naturalists who compared the structure of the higher animals with that of the simpler ones. these comparisons brought out so many resemblances and so many remarkable facts that anatomy, which seems at first a dry subject, became endued with great interest. [illustration: fig. .--severinus, - .] severinus.--the first book expressly devoted to comparative anatomy was that of severinus ( - ), designated _zootomia democritæ_. the title was derived from the roman naturalist democritæus, and the date of its publication, , places the treatise earlier than the works of malpighi, leeuwenhoek, and swammerdam. the book is illustrated by numerous coarse woodcuts, showing the internal organs of fishes, birds, and some mammals. there are also a few illustrations of stages in the development of these animals. the comparisons were superficial and incidental; nevertheless, as the first attempt, after the revival of anatomy, to make the subject comparative, it has some especial interest. severinus (fig. ) should be recognized as beginning the line of comparative anatomists which led up to cuvier. forerunners of cuvier.--anatomical studies began to take on broad features with the work of camper, john hunter, and vicq d'azyr. these three men paved the way for cuvier, but it must be said of the two former that their comparisons were limited and unsystematic. camper, whose portrait is shown in fig. , was born in leyden, in . he was a versatile man, having a taste for drawing, painting, and sculpture, as well as for scientific studies. he received his scientific training under boerhaave and other eminent men in leyden, and became a professor and, later, rector in the university of groningen. possessing an ample fortune, and also having married a rich wife, he was in position to follow his own tastes. he travelled extensively and gathered a large collection of skeletons. he showed considerable talent as an anatomist, and he made several discoveries, which, however, he did not develop, but left to others. perhaps the possession of riches was one of his limitations; at any rate, he lacked fixity of purpose. among his discoveries may be mentioned the semicircular canals in the ear of fishes, the fact that the bones of flying birds are permeated by air, the determination of some fossil bones, with the suggestion that they belonged to extinct forms. the latter point is of interest, as antedating the conclusions of cuvier regarding the nature of fossil bones. camper also made observations upon the facial angle as an index of intelligence in the different races of mankind, and in lower animals. he studied the anatomy of the elephant, the whale, the orang, etc. [illustration: fig. .--camper, - .] john hunter ( - ), the gifted scotchman whose museum in london has been so justly celebrated, was a man of extraordinary originality, who read few books but went directly to nature for his facts; and, although he made errors from which he would have been saved by a wider acquaintance with the writings of naturalists, his neglect of reading left his mind unprejudiced by the views of others. he was a wild, unruly spirit, who would not be forced into the conventional mold as regards either education or manners. his older brother, william, a man of more elegance and refinement, who well understood the value of polish in reference to worldly success, tried to improve john by arranging for him to go to the university of oxford, but john rebelled and would not have the classical education of the university, nor would he take on the refinements of taste and manner of which his brother was a good example. "why," the doughty john is reported to have said, "they wanted to make me study greek! they tried to make an old woman of me!" however much lack of appreciation this attitude indicated, it shows also the philistine independence of his spirit. this independence of mind is one of his striking characteristics. [illustration: fig. .--john hunter, - .] this is not the place to dwell upon the unfortunate controversy that arose between these two illustrious brothers regarding scientific discoveries claimed by each. the position of both is secure in the historical development of medicine and surgery. although the work of john hunter was largely medical and surgical, he also made extensive studies on the comparative anatomy of animals, and has a place as one of the most conspicuous predecessors of cuvier. he was very energetic both in making discoveries and in adding to his great museum. the original collections made by hunter are still open to inspection in the rooms of the royal college of surgeons, london. it was his object to preserve specimens to illustrate the phenomena of life in all organisms, whether in health or disease, and the extent of his museum may be divined from the circumstance that he expended upon it about three hundred and seventy-five thousand dollars. although he described and compared many types of animals, it was as much in bringing this collection together and leaving it to posterity that he advanced comparative anatomy as in what he wrote. after his death the house of commons purchased his museum for fifteen thousand pounds, and placed it under the care of the corporation of surgeons. hunter's portrait is shown in fig. . vicq d'azyr (fig. ), more than any other man, holds the chief rank as a comparative anatomist before the advent of cuvier into the same field. he was born in , the son of a physician, and went to paris at the age of seventeen to study medicine, remaining in the metropolis to the time of his death in . he was celebrated as a physician, became permanent secretary of the newly founded academy of medicine, consulting physician to the queen, and occupied other positions of trust and responsibility. he married the niece of daubenton, and, largely through his influence, was advanced to social place and recognition. on the death of buffon, in , he took the seat of that distinguished naturalist as a member of the french academy. [illustration: fig. .--vicq d'azyr, - .] he made extensive studies upon the organization particularly of birds and quadrupeds, making comparisons between their structure, and bringing out new points that were superior to anything yet published. his comparisons of the limbs of man and animals, showing a correspondence between the flexor and extensor muscles of the legs and arms, were made with great exactness, and they served to mark the beginning of a new kind of precise comparison. these were not merely fanciful comparisons, but exact ones--part for part; and his general considerations based upon these comparisons were of a brilliant character. as huxley has said, "he may be considered as the founder of the modern science of anatomy." his work on the structure of the brain was the most exact which had appeared up to that time, and in his studies on the brain he entered into broad comparisons as he had done in the study of the other parts of the animal organization. he died at the age of forty-six, without being able to complete a large work on human anatomy, illustrated with colored figures. this work had been announced and entered upon, but only that part relating to the brain had appeared at the time of his death. besides drawings of the exterior of the brain, he made sections; but he was not able to determine with any particular degree of accuracy the course of fiber tracks in the brain. this was left for other workers. he added many new facts to those of his predecessors, and by introducing exact comparisons in anatomy he opened the field for cuvier. cuvier.--when cuvier, near the close of the eighteenth century, committed himself definitely to the progress of natural science, he found vast accumulations of separate monographs to build upon, but he undertook to dissect representatives of all the groups of animals, and to found his comparative anatomy on personal observations. the work of vicq d'azyr marked the highest level of attainment, and afforded a good model of what comparisons should be; but cuvier had even larger ideas in reference to the scope of comparative anatomy than had his great predecessor. the particular feature of cuvier's service was that in his investigations he covered the whole field of animal organization from the lowest to the highest, and uniting his results with what had already been accomplished, he established comparative anatomy on broad lines as an independent branch of natural science. almost at the outset he conceived the idea of making a comprehensive study of the structure of the animal kingdom. it was fortunate that he began his investigations with thorough work upon the invertebrated animals; for from this view-point there was gradually unfolded to his great mind the plan of organization of the entire series of animals. not only is a knowledge of the structure of the simplest animals an essential in understanding that of the more modified ones, but the more delicate work required in dissecting them gives invaluable training for anatomizing those of more complex construction. the value attached to this part of his training by cuvier is illustrated by the advice that he gave to a young medical student who brought to his attention a supposed discovery in anatomy. "are you an entomologist?" inquired cuvier. "no," said the young man. "then," replied cuvier, "go first and anatomize an insect, and return to me; and if you still believe that your observations are discoveries i will then believe you." birth and early education.--cuvier was born in , at montbéliard, a village at that time belonging to württemberg, but now a part of the french jura. his father was a retired military officer of the swiss army, and the family, being protestants, had moved to montbéliard for freedom from religious persecution. cuvier was christened léopold-christian-frédéric-dagobert cuvier, but early in youth took the name of georges at the wish of his mother, who had lost an infant son by that name. he gave an early promise of intellectual leadership, and his mother, although not well educated, took the greatest pains in seeing that he formed habits of industry and continuous work, hearing him recite his lessons in latin and other branches, although she did not possess a knowledge of latin. he early showed a leaning toward natural history; having access to the works of gesner and buffon, he profited by reading these two writers. so great was his interest that he colored the plates in buffon's _natural history_ from descriptions in the text. it was at first contemplated by his family that he should prepare for theology, but failing, through the unfairness of one of his teachers, to get an appointment to the theological seminary, his education was continued in other directions. he was befriended by the sister of the duke of württemberg, who sent him as a pensioner to the famous carolinian academy at stuttgart. there he showed great application, and with the wonderful memory with which he was endowed, he took high rank as a student. here he met kielmeyer, a young instructor only four years older than himself, who shared his taste for natural history and, besides this, introduced him to anatomy. in after-years cuvier acknowledged the assistance of kielmeyer in determining his future work and in teaching him to dissect. life at the seashore.--in the resources of his family, which had always been slender, became further reduced by the inability of the government to pay his father's retiring stipend. as the way did not open for employment in other directions, young cuvier took the post of instructor of the only son in the family of count d'héricy, and went with the family to the sea-coast in normandy, near caen. for six years ( - ) he lived in this noble family, with much time at his disposal. for cuvier this period, from the age of nineteen to twenty-five, was one of constant research and reflection. while paris was disrupted by the reign of terror, cuvier, who, although of french descent, regarded himself as a german, was quietly carrying on his researches into the structure of the life at the seaside. these years of diligent study and freedom from distractions fixed his destiny. here at the sea-coast, without the assistance of books and the stimulus of intercourse with other naturalists, he was drawn directly to nature, and through his great industry he became an independent observer. here he laid the foundation of his extensive knowledge of comparative anatomy, and from this quiet spot he sent forth his earliest scientific writings, which served to carry his name to paris, the great center of scientific research in france. goes to paris.--his removal from these provincial surroundings was mainly owing to the warm support of tessier, who was spending the time of the reign of terror in retirement in an adjacent village, under an assumed name. he and cuvier met in a scientific society, where the identity of tessier was discovered by cuvier on account of his ease of speech and his great familiarity with the topics discussed. a friendship sprung up between them, and tessier addressed some of his scientific friends in paris in the interest of cuvier. by this powerful introduction, and also through the intervention of geoffroy saint-hilaire, he came to paris in and was welcomed into the group of working naturalists at the jardin des plantes, little dreaming at the time that he should be the leader of the group of men gathered around this scientific institution. he was modest, and so uncertain of his future that for a year he held to his post of instructor, bringing his young charge with him to paris. notwithstanding the doubt which he entertained regarding his abilities, his career proved successful from the beginning. in paris he entered upon a brilliant career, which was a succession of triumphs. his unmistakable talent, combined with industry and unusual opportunities, brought him rapidly to the front. the large amount of material already collected, and the stimulating companionship of other scientific workers, afforded an environment in which he grew rapidly. he responded to the stimulus, and developed not only into a great naturalist, but expanded into a finished gentleman of the world. circumstances shaped themselves so that he was called to occupy prominent offices under the government, and he came ultimately to be the head of the group of scientific men into which he had been welcomed as a young man from the provinces. [illustration: fig. .--cuvier as a young man, - .] his physiognomy.--it is very interesting to note in his portraits the change in his physiognomy accompanying his transformation from a young man of provincial appearance into an elegant personage. fig. shows his portrait in the early days when he was less mindful of his personal appearance. it is the face of an eager, strong, young man, still retaining traces of his provincial life. his long, light-colored hair is unkempt, but does not hide the magnificent proportions of his head. fig. shows the growing refinement of features which came with his advancement, and the aristocratic look of supremacy which set upon his countenance after his wide recognition passing by a gradation of steps from the position of head of the educational system, to that of baron and peer of france. [illustration: fig. .--cuvier at the zenith of his power.] cuvier was a man of commanding power and colossal attainments; he was a favorite of napoleon bonaparte, who elevated him to office and made him director of the higher educational institutions of the empire. but to whatever place of prominence he attained in the government, he never lost his love for natural science. with him this was an absorbing passion, and it may be said that he ranks higher as a zoölogist than as a legislator. comprehensiveness of mind.--soon after his arrival in paris he began to lecture upon comparative anatomy and to continue work in a most comprehensive way upon the subjects which he had cultivated at caen. he saw everything on a large scale. this led to his making extensive studies of whatever problems engaged his mind, and his studies were combined in such a manner as to give a broad view of the subject. indeed, comprehensiveness of mind seems to have been the characteristic which most impressed those who were acquainted with him. flourens says of him: "_ce qui caractérise partout m. cuvier, c'est l'esprit vaste._" his broad and comprehensive mind enabled him to map out on great lines the subject of comparative anatomy. his breadth was at times his undoing, for it must be confessed that when the details of the subject are considered, he was often inaccurate. this was possibly owing to the conditions under which he worked; having his mind diverted into many other channels, never neglecting his state duties, it is reasonable to suppose that he lacked the necessary time to prove his observations in anatomy, and we may in this way account for some of his inaccuracies. besides being at fault in some of his comparative anatomy, he adhered to a number of ideas that served to retard the progress of science. he was opposed to the ideas of his contemporary lamarck, on the evolution of animals. he is remembered as the author of the dogma of catastrophism in geology. he adhered to the old notion of the pre-formation of the embryo, and also to the theory of the spontaneous origin of life. founds comparative anatomy.--regardless of this qualification, he was a great and distinguished student, and founded comparative anatomy. from to appeared his _leçons d' anatomie comparée_, a systematic treatise on the comparative anatomy of animals, embracing both the invertebrates and the vertebrates. in was published his great work on the fossil bones about paris, an achievement which founded the science of vertebrate palæontology. his extensive examination of the structure of fishes also added to his already great reputation. his book on the animal kingdom (_le règne animal distribué d'après son organisation_, ), in which he expounded his type-theory, has been considered in a previous chapter. he was also deeply interested in the historical development of science, and his volumes on the rise of the natural sciences give us almost the best historical estimate of the progress of science that we have at the present day. his domestic life.--mrs. lee, in a chatty account of cuvier, shows one of his methods of work. he had the faculty of making others assist him in various ways. not only members of his family, but also guests in his household were pressed into service. they were invited to examine different editions of works and to indicate the differences in the plates and in the text. this practice resulted in saving much time for cuvier, since in the preparation of his historical lectures he undertook to examine all the original sources of the history with which he was engaged. in his lectures he summarized facts relating to different editions of books, etc. mrs. lee also gives a picture of his family life, which was, to all accounts, very beautiful. he was devoted to his wife and children, and in the midst of exacting cares he found time to bind his family in love and devotion. cuvier was called upon to suffer poignant grief in the loss of his children, and his direct family was not continued. he was especially broken by the death of his daughter who had grown to young womanhood and was about to be married. from the standpoint of a sincere admirer, mrs. lee writes of his generosity and nobility of temperament, declaring that his career demonstrated that his mind was great and free from both envy and smallness. some shortcomings.--nevertheless, there are certain things in the life of cuvier that we wish might not have been. his break with his old friends lamarck and saint-hilaire seems to show a domination of qualities that were not generous and kindly; those observations of lamarck showing a much profounder insight than any of which he himself was the author were laughed to scorn. his famous controversy with saint-hilaire marks a historical moment that will be dealt with in the chapter on evolution. george bancroft, the american historian, met him during a visit to paris in . he speaks of his magnificent eyes and his fine appearance, but on the whole cuvier seems to have impressed bancroft as a disagreeable man. some of his shortcomings that served to retard the progress of science have been mentioned. still, with all his faults, he dominated zoölogical science at the beginning of the nineteenth century, and so powerful was his influence and so undisputed was his authority among the french people that the rising young men in natural science sided with cuvier even when he was wrong. it is a noteworthy fact that france, under the influence of the traditions of cuvier, was the last country slowly and reluctantly to harbor as true the ideas regarding the evolution of animal life. cuvier's successors while cuvier's theoretical conclusions exercised a retarding influence upon the progress of biology, his practical studies more than compensated for this. it has been pointed out how his type-theory led to the reform of the linnæan system, but, besides this, the stimulus which his investigations gave to studies in comparative anatomy was even of more beneficent influence. as time passed the importance of comparative anatomy as one division of biological science impressed itself more and more upon naturalists. a large number of investigators in france, england, and germany entered the field and took up the work where cuvier had left it. the more notable of these successors of cuvier should come under consideration. [illustration: fig. .--h. milne-edwards, - .] his intellectual heirs in france were milne-edwards and lacaze-duthiers. milne-edwards.--h. milne-edwards ( - ) was a man of great industry and fine attainments; prominent alike in comparative anatomy, comparative physiology, and general zoölogy, professor for many years at the sorbonne in paris. in he introduced into biology the fruitful idea of the division of physiological labor. he completed and published excellent researches upon the structure and development of many animals, notably crustacea, corals, etc. his work on comparative anatomy took the form of explanations of the activities of animals, or comparative physiology. his comprehensive treatise _leçons sur la physiologie et l'anatomie comparée_, in fourteen volumes, - , is a mine of information regarding comparative anatomy as well as the physiology of organisms. [illustration: fig. .--lacaze-duthiers, - .] lacaze-duthiers.--henri de lacaze-duthiers ( - ), the man of comprehensive mind, stimulating as an instructor of young men, inspiring other workers, and producing a large amount of original research on his own account, director of the seaside stations at roscoff and banyuls, the founder of a noteworthy periodical of experimental zoölogy--this great man, whose portrait is shown in fig. , was one of the leading comparative anatomists in france. [illustration: fig. .--lorenzo oken, - .] r. owen.--in england richard owen ( - ) carried on the influence of cuvier. at the age of twenty-seven he went to paris and renewed acquaintance with the great cuvier, whom he had met the previous year in england. he spent some time at the jardin des plantes examining the extensive collections in the museum. although the idea was repudiated by owen and some of his friends, it is not unlikely that the collections of fossil animals and the researches upon them which engaged cuvier at that time had great influence upon the subsequent studies of owen. although he never studied under cuvier, in a sense he may be regarded as his disciple. owen introduced into anatomy the important conceptions of analogy and homology, the former being a likeness based upon the use to which organs are put, as the wing of a butterfly and the wing of a bat; while homology is a true relationship founded on likeness in structure and development, as the wing of a bat and the foreleg of a dog. analogy is a superficial, and often a deceiving relationship; homology is a true genetic relationship. it is obvious that this distinction is of great importance in comparing the different parts of animals. he made a large number of independent discoveries, and published a monumental work on the comparative anatomy of vertebrates ( - ). in much of his thought he was singular, and many of his general conclusions have not stood the test of time. he undertook to establish the idea of an archtype in vertebrate anatomy. he clung to the vertebral theory of the skull long after huxley had shown such a theory to be untenable. the idea that the skull is made up of modified vertebrae was propounded by goethe and oken. in the hands of oken it became one of the anatomical conclusions of the school of _naturphilosophie_. this school of transcendental philosophy was founded by schelling, and oken (fig. ) was one of its typical representatives. the vertebral theory of the skull was, therefore, not original with owen, but he adopted it, greatly elaborated it, and clung to it blindly long after the foundations upon which it rested were removed. [illustration: fig. .--richard owen, - .] richard owen (fig. ) was succeeded by huxley ( - ), whose exactness of observation and rare judgment as to the main facts of comparative anatomy mark him as one of the leaders in this field of research. the influence of huxley as a popular exponent of science is dealt with in a later chapter. meckel.--just as cuvier stands at the beginning of the school of comparative anatomy in france, so does j. fr. meckel in germany. meckel ( - ) was a man of rare talent, descended from a family of distinguished anatomists. from to he studied in paris under cuvier, and when he came to leave the french capital to become professor of anatomy at halle, he carried into germany the teachings and methods of his master. he was a strong force in the university, attracting students to his department by his excellent lectures and his ability to arouse enthusiasm. some of these students were stimulated to undertake researches in anatomy, and there came from his laboratory a number of investigations that were published in a periodical which he founded. meckel himself produced many scientific papers and works on comparative anatomy, which assisted materially in the advancement of that science. his portrait, which is rare, is shown in fig. . [illustration: fig. .--j. fr. meckel, - .] rathke.--martin henry rathke ( - ) greatly advanced the science of comparative anatomy by insisting upon the importance of elucidating anatomy with researches in development. this is such an important consideration that his influence upon the progress of comparative anatomy can not be overlooked. after being a professor in dorpat, he came, in , to occupy the position of professor of anatomy and zoölogy at königsberg, which had been vacated by von baer on the removal of the latter to st. petersburg. his writings are composed with great intelligence, and his facts are carefully coördinated. rathke belonged to the good old school of german writers whose researches were profound and extensive, and whose expression was clear, being based upon matured thought. his papers on the aortic arches and the wolffian body are those most commonly referred to at the present time. müller.--johannes müller ( - ), that phenomenal man, besides securing recognition as the greatest physiologist of the nineteenth century, also gave attention to comparative anatomy, and earned the title of the greatest morphologist of his time. his researches were so accurate, so complete, so discerning, that his influence upon the development of comparative anatomy was profound. although he is accorded, in history, the double distinction of being a great anatomist and a great physiologist, his teaching tended to physiology; and most of his distinguished students were physiologists of the broadest type, uniting comparative anatomy with their researches upon functional activities. (for müller's portrait see p. .) gegenbaur.--in karl gegenbaur ( - ) scientific anatomy reached its highest expression. his work was characterized by broad and masterly analysis of the facts of structure, to which were added the ideas derived from the study of the development of organs. he was endowed with an intensely keen insight, an insight which enabled him to separate from the vast mass of facts the important and essential features, so that they yielded results of great interest and of lasting importance. this gifted anatomist attracted many young men from the united states and from other countries to pursue under his direction the study of comparative anatomy. he died in heidelberg in , where he had been for many years professor of anatomy in the university. [illustration: fig. .--karl gegenbaur, - .] in the group of living german anatomists the names of fürbringer, waldeyer, and wiedersheim can not go unmentioned. e.d. cope.--in america the greatest comparative anatomist was e.d. cope ( - ), a man of the highest order of attainment, who dealt with the comparative anatomy not only of living forms, but of fossil life, and made contributions of a permanent character to this great science; a man whose title to distinction in the field of comparative anatomy will become clearer to later students with the passage of time. for cope's portrait see p. . of the successors of cuvier, we would designate meckel, owen, gegenbaur, and cope as the greatest. comparative anatomy is a very rich subject, and when elucidated by embryology, is one of the firm foundations of biology. if we regard anatomy as a science of statics, we recognize that it should be united with physiology, which represents the dynamical side of life. comparative anatomy and comparative physiology should go hand in hand in the attempt to interpret living forms. advances in these two subjects embrace nearly all our knowledge of living organisms. it is a cause for congratulation that comparative anatomy has now become experimental, and that gratifying progress is being made along the line of research designated as experimental morphology. already valuable results have been attained in this field, and the outlook of experimental morphology is most promising. chapter viii bichat and the birth of histology we must recognize bichat as one of the foremost men in biological history, although his name is not well known to the general public, nor constantly referred to by biologists as that of one of the chief luminaries of their science. in him was combined extraordinary talent with powers of intense and prolonged application; a combination which has always produced notable results in the world. he died at the age of thirty-one, but, within a productive period of not more than seven years, he made observations and published work that created an epoch and made a lasting impression on biological history. his researches supplemented those of cuvier, and carried the analysis of animal organization to a deeper level. cuvier laid the foundations of comparative anatomy by dissecting and arranging in a comprehensive system the organs of animals, but bichat went a step further and made a profound study of the tissues that unite to make up the organs. as we have already noted in a previous chapter, this was a step in reaching the conception of the real organization of living beings. buckle's estimate of bichat.--it is interesting to note the impression made by bichat upon one of the greatest students of the history of civilization. buckle says of him: "great, however, as is the name of cuvier, a greater still remains behind. i allude, of course, to bichat, whose reputation is steadily advancing as our knowledge advances; who, if we compare the shortness of his life with the reach and depth of his views, must be pronounced the most profound thinker and consummate observer by whom the organization of the animal frame has yet been studied. "we may except aristotle, but between aristotle and bichat i find no middle man." whether or not we agree fully with this panegyric of buckle, we must, i think, place bichat among the most illustrious men of biological history, as vesalius, j. müller, von baer, and balfour. marie françois xavier bichat was born in at thoirette, department of the ain. his father, who was a physician, directed the early education of his son and had the satisfaction of seeing him take kindly to intellectual pursuits. the young student was distinguished in latin and mathematics, and showed early a fondness for natural history. having elected to follow the calling of his father, he went to lyons to study medicine, and came under the instruction of petit in surgery. bichat in paris.--it was, on the whole, a fortunate circumstance for bichat that the turbulent events of the french revolution drove him from lyons to paris, where he could have the best training, the greatest stimulus for his growth, and at the same time the widest field for the exercise of his talents. we find him in paris in , studying under the great surgeon desault. he attracted attention to himself in the class of this distinguished teacher and operator by an extemporaneous report on one of the lectures. it was the custom in desault's classes to have the lectures of the professor reported upon before an assistant by some student especially appointed for the purpose. on one occasion the student who had been appointed to prepare and deliver the review was absent, and bichat, who was gifted with a powerful memory, volunteered without previous notice to take his place. the lecture was a long and difficult one on the fractures of the clavicle, but bichat's abstract was so clear, forceful, and complete that its delivery in well-chosen language produced a great sensation both upon the instructor and the students. this notable performance served to bring him directly to the attention of desault, who invited him to become his assistant and to live in his family. the association of bichat with the great surgeon was most happy. desault treated him as a son, and when he suddenly died in , the care of preparing his works for the printer was left to bichat. the fidelity with which bichat executed this trust was characteristic of his noble nature. he laid aside his own personal interests, and his researches in which he was already immersed, and by almost superhuman labor completed the fourth volume of desault's _journal of surgery_ and at the same time collected and published his scattered papers. to these he added observations of his own, making alterations to bring the work up to the highest plane. thus he paid the debt of gratitude which he felt he owed to desault for his friendship and assistance. in he was appointed professor of anatomy, at the age of twenty-six, and from then to the end of his life, in , he continued in his career of remarkable industry. the portrait of this very attractive man is shown in fig. . his face shows strong intellectuality. he is described as of "middling stature, with an agreeable face lighted by piercing and expressive eyes." he was much beloved by his students and associates, being "in all relations of life most amiable, a stranger to envy or other hateful passions, modest in demeanor and lively in his manners, which were open and free." his phenomenal industry.--his industry was phenomenal; besides doing the work of a professor, he attended to a considerable practice, and during a single winter he is said to have examined with care six hundred bodies in the pursuance of his researches upon pathological anatomy. [illustration: fig. .--bichat, - .] in the year , when he was thirty years old, began to appear the results of his matured researches. we speak of these as being matured, not on account of his age or the great number of years he had labored upon them, but from the intensity and completeness with which he had pursued his investigations, thus giving to his work a lasting quality. first came his treatise on the membranes (_traité des membranes_); followed quickly by his physiological researches into the phenomena of life and death (_recherches physiologiques sur la vie et la mort_); then appeared his general anatomy (_anatomie générale_) in , and his treatise upon descriptive anatomy, upon which he was working at the time of his death. his death occurred in , and was due partly to an accident. he slipped upon the stairs of the dissecting-room, and his fall was followed by gastric derangement, from which he died. results of his work.--the new science of the anatomy of the tissues which he founded is now known as histology, and the general anatomy, as he called it, has now become the study of minute anatomy of the tissues. bichat studied the membranes or tissues very profoundly, but he did not employ the microscope and make sketches of their cellular construction. the result of his work was to set the world studying the minute structure of the tissues, a consequence of which led to the modern study of histology. since this science was constructed directly upon his foundation, it is proper to recognize him as the founder of histology. carpenter says of him: "altogether bichat left an impress upon the science of life, the depth of which can scarcely be overrated; and this not so much by the facts which he collected and generalized, as by the method of inquiry which he developed, and by the systematic form which he gave to the study of general anatomy in its relations both to physiology and pathology." bichat's more notable successors.--his influence extended far, and after the establishment of the cell-theory took on a new phase. microscopic study of the tissues has now become a separate division of the science of anatomy, and engages the attention of a very large number of workers. while the men who built upon bichat's foundation are numerous, we shall select for especial mention only a few of the more notable, as schwann, koelliker, schultze, virchow, leydig, and ramon y cajal, whose researches stand in the direct line of development of the ideas promulgated by bichat. schwann.--schwann's cell-theory was the result of close attention to the microscopic structure of the tissues of animals. it was an extension of the knowledge of the tissues which bichat distinguished and so thoroughly investigated from other points of view. the cell-theory, which took rise in , was itself epoch-making, and the science of general anatomy was influenced by it as deeply as was the science of embryology. the leading founder of this theory was theodor schwann, whose portrait is shown on page , where there is also a more extended account of his labors in connection with the cell-theory. had not the life of bichat been cut off in his early manhood, he might well have lived to see this great discovery added to his own. koelliker.--albrecht von koelliker ( - ) was one of the greatest histologists of the nineteenth century. he is a striking figure in the development of biology in a general way, distinguished as an embryologist, as a histologist, and in other connections. during his long life, from to , he made an astounding number of additions to our knowledge of microscopic anatomy. in the early years of his scientific activity, "he helped in establishing the cell-theory, he traced the origin of tissues from the segmenting ovum through the developing embryo, he demonstrated the continuity between nerve-fibers and nerve-cells of vertebrates ( ), ... and much more." he is mentioned further, in connection with the rise of embryology, in chapter x. the strong features of this veteran of research are shown in the portrait, fig. , which represents him at the age of seventy. in he was called to the university of würzburg, where he remained to the time of his death. from to , scarcely a year passed without some important contribution from von koelliker extending the knowledge of histology. his famous text-book on the structure of the tissues (_handbuch der gewebelehre_) passed through six editions from to , the final edition of it being worked over and brought up to date by this extraordinary man after he had passed the age of seventy-five. by workers in biology this will be recognized as a colossal task. in the second volume of the last edition of this work, which appeared in , he went completely over the ground of the vast accumulation of information regarding the nervous system which an army of gifted and energetic workers had produced. this was all thoroughly digested, and his histological work brought down to date. schultze.--the fine observations of max schultze ( - ) may also be grouped with those of the histologists. we shall have occasion to speak of him more particularly in the chapter on protoplasm. he did memorable service for general biology in establishing the protoplasm doctrine, but many of his scientific memoirs are in the line of normal histology; as, those on the structure of the olfactory membrane, on the retina of the eye, the muscle elements, the nerves, etc., etc. [illustration: fig. .--von koelliker, - .] normal histology and pathology.--but histology has two phases: the investigation of the tissues in health, which is called normal histology; and the study of the tissues in disease and under abnormal conditions of development, which is designated pathological histology. the latter division, on account of its importance to the medical man, has been extensively cultivated, and the development of pathological study has greatly extended the knowledge of the tissues and has had its influence upon the progress of normal histology. goodsir, in england, and henle, in germany, entered the field of pathological histology, both doing work of historical importance. they were soon followed by virchow, whose eminence as a man and a scientist has made his name familiar to people in general. [illustration: fig. .--rudolph virchow, - .] virchow.--rudolph virchow ( - ), for many years a professor in the university of berlin, was a notable man in biological science and also as a member of the german parliament. he assisted in molding the cell-theory into better form, and in published a work on _cellular pathology_, which applied the cell-theory to diseased tissues. it is to be remembered that bichat was a medical man, intensely interested in pathological, or diseased, tissues, and we see in virchow the one who especially extended bichat's work on the side of abnormal histology. virchow's name is associated also with the beginning of the idea of germinal continuity, which is the basis of biological ideas regarding heredity (see, further, chapter xv). [illustration: fig. .--franz leydig, - (april). courtesy of dr. wm. m. wheeler.] leydig.--franz leydig (fig. ) was early in the field as a histologist with his handbook (_lehrbuch der histologie des menschen und der thiere_) published in . he applied histology especially to the tissues of insects in and subsequent years, an account of which has already been given in chapter v. [illustration: fig. .--s. ramon y cajal, -] cajal as histologist.--ramon y cajal, professor in the university of madrid, is a histologist whose work in a special field of research is of world-wide renown. his investigations into the microscopic texture of the nervous system and sense-organs have in large part cleared up the questions of the complicated relations between the nervous elements. in company with other european investigators he visited the united states in on the invitation of clark university, where his lectures were a feature of the celebration of the tenth anniversary of that university. besides receiving many honors in previous years, in he was awarded, in conjunction with the italian histologist golgi, one of the nobel prizes in recognition of his notable investigations. golgi invented the staining methods that ramon y cajal has applied so extensively and so successfully to the histology of the nervous system. these men in particular may be remembered as the investigators who expanded the work of bichat on the tissues: schwann, for disclosing the microscopic elements of animal tissues and founding the cell-theory; koelliker, as the typical histologist after the analysis of tissues into their elementary parts; virchow, as extending the cell-idea to abnormal histology; leydig, for applying histology to the lower animals; and ramon y cajal, for investigations into the histology of the nervous system. text-books of histology.--besides the works mentioned, the text-books of frey, stricker, ranvier, klein, schäfer, and others represent a period in the general introduction of histology to students between and . but these excellent text-books have been largely superseded by the more recent ones of stöhr, boem-davidoff, piersol, szymonowicz, and others. the number of living investigators in histology is enormous; and their work in the subject of cell-structure and in the department of embryology now overlaps. in pathological histology may be observed an illustration of the application of biological studies to medicine. while no attempt is made to give an account of these practical applications, they are of too great importance to go unmentioned. histological methods are in constant use in clinical diagnosis, as in blood counts, the study of inflammations, of the action of phagocytes, and of all manner of abnormal growths. in attempting to trace the beginning of a definite foundation for the work on the structure of tissues, we go back to bichat rather than to leeuwenhoek, as richardson has proposed. bichat was the first to give a scientific basis for histology founded on extensive observations, since all earlier observers gave only separated accounts of the structure of particular tissues. chapter ix the rise of physiology harvey haller johannes müller physiology had a parallel development with anatomy, but for convenience it will be considered separately. anatomy shows us that animals and plants are wonderfully constructed, but after we understand their architecture and even their minute structure, the questions remain, what are all the organs and tissues for? and what takes place within the parts that are actually alive? physiology attempts to answer questions of this nature. it stands, therefore, in contrast with anatomy, and is supplementary to it. the activities of living organisms are varied, and depend on life for their manifestations. these manifestations may be called vital activities. physiology embraces a study of them all. physiology of the ancients.--this subject began to attract the attention of ancient medical men who wished to fathom the activities of the body in order to heal its diseases, but it is such a difficult thing to begin to comprehend the activities of life that even the simpler relationships were imperfectly understood, and they resorted to mythical explanations. they spoke of spirits and humors in the body as causes of various changes; the arteries were supposed to carry air, the veins only blood; and nothing was known of the circulation. there arose among these early medical men the idea that the body was dominated by a subtle spirit. this went under the name _pneuma_, and the pneuma-theory held sway until the period of the revival of learning. among the ancient physiologists the great roman physician galen is the most noteworthy figure. as he was the greatest anatomist, so he was also the greatest physiologist of ancient times. all physiological knowledge of the time centered in his writings, and these were the standards of physiology for many centuries, as they were also for anatomy. in the early days anatomy, physiology, and medicine were all united into a poorly digested mass of facts and fancies. this state of affairs lasted until the sixteenth century, and then the awakening came, through the efforts of gifted men, endued with the spirit of independent investigation. the advances made depended upon the work or leadership of these men, and there are certain periods of especial importance for the advance of physiology that must be pointed out. period of harvey.--the first of these epochs to be especially noted here is the period of harvey ( - ). in his time the old idea of spirits and humors was giving way, but there was still much vagueness regarding the activities of the body. he helped to illuminate the subject by showing a connection between arteries and veins, and by demonstrating the circulation of the blood. as we have seen in an earlier chapter, harvey did not observe the blood passing through the capillaries from arteries to veins, but his reasoning was unassailable that such a connection must exist, and that the blood made a complete circulation. he gave his conclusions in his medical lectures as early as , but did not publish his views until . it was reserved for malpighi, in , actually to see the circulation through the capillaries under the microscope, and for leeuwenhoek, in and later years, to extend these observations. it was during harvey's life that the microscope was brought into use and was of such great assistance in advancing knowledge. harvey himself, however, made little use of this instrument. it was during his life also that the knowledge of development was greatly promoted, first through his own efforts, and later through those of malpighi. harvey is to be recognized, then, as the father of modern physiology. indeed, before his time physiology as such can hardly be spoken of as having come into existence. he introduced experimental work into physiology, and thus laid the foundation of modern investigation. it was the method of harvey that made definite progress in this line possible, and accordingly we honor him as one of the greatest as well as the earliest of physiologists. period of haller.--from harvey's time we pass to the period of haller ( - ), at the beginning of which physiology was still wrapped up with medicine and anatomy. the great work of haller was to create an independent science of physiology. he made it a subject to be studied for its own sake, and not merely as an adjunct to medicine. haller was a man of vast and varied learning, and to him was applied by unsympathetic critics the title of "that abyss of learning." his portrait, as shown in fig. , gives the impression of a somewhat pompous and overbearing personality. he was egotistical, self-complacent, and possessed of great self-esteem. the assurance in the inerrancy of his own conclusions was a marked characteristic of haller's mind. while he was a good observer, his own work showing conscientious care in observation, he was not a good interpreter, and we are to recollect that he vigorously opposed the idea of development set forth by wolff, and we must also recognize that his researches formed the chief starting-point of an erroneous conception of vitality. as verworn points out, haller's own experiments upon the phenomena of irritability were exact, but they were misinterpreted by his followers, and through the molding influence of others the attempted explanation of their meaning grew into the conception of a special vital force belonging to living organisms only. in its most complete form, this idea provided for a distinct dualism between living and lifeless matter, making all vital actions dependent upon the operation of a mystical supernatural agency. this assumption removed vital phenomena from the domain of clear scientific analysis, and for a long time exercised a retarding influence upon the progress of physiology. his chief service of permanent value was that he brought into one work all the facts and the chief theories of physiology carefully arranged and digested. this, as has been said, made physiology an independent branch of science, to be pursued for itself and not merely as an adjunct to the study of medicine. the work referred to is his elements of physiology (_elementa physiologiæ corporis humani_, ), one of the noteworthy books marking a distinct epoch in the progress of science. [illustration: fig. .--albrecht haller, - .] to the period of haller also belongs the discovery of oxygen, in , by priestley, a discovery which was destined to have profound influence upon the subsequent development of physiology, so that even now physiology consists largely in tracing the way in which oxygen enters the body, the manner in which it is distributed to the tissues, and the various phases of vital activity that it brings about within the living tissues. charles bell.--the period of haller may be considered as extending beyond his lifetime and as terminating when the influence of müller began to be felt. another discovery coming in the closing years of haller's period marks a capital advance in physiology. i refer to the discovery of charles bell ( - ) showing that the nerve fibers of the anterior roots of the spinal cord belong to the motor type, while those of the posterior roots belong to the sensory type. this great truth was arrived at theoretically, rather than as the result of experimental demonstration. it was first expounded by bell in in a small essay entitled _idea of a new anatomy of the brain_, which was printed for private distribution. it was expanded in his papers, beginning in , and published in the philosophical transactions of the royal society of london, and finally embodied in his work on the nervous system, published in . at this latter date johannes müller had reached the age of twenty-nine, and had already entered upon his career as the leading physiologist of germany. what bell had divined he demonstrated by experiments. charles bell (fig. ) was a surgeon of eminence; in private life he was distinguished by "unpretending amenity, and simplicity of manners and deportment." [illustration: fig. .--charles bell, - .] period of johannes müller.--the period that marks the beginning of modern physiology came next, and was due to the genius and force of johannes müller ( - ). verworn says of him: "he is one of those monumental figures that the history of every science brings forth but once. they change the whole aspect of the field in which they work, and all later growth is influenced by their labors." johannes müller was a man of very unusual talent and attainments, the possessor of a master mind. some have said, and not without reason, that there was something supernatural about müller, for his whole appearance bore the stamp of the uncommon. his portrait, with its massive head above the broad shoulders, is shown in fig. . in his lectures his manner and his gestures reminded one of a catholic priest. early in his life, before the disposition to devote himself to science became so overwhelming, he thought of entering the priesthood, and there clung to him all his life some marks of the holy profession. in his highly intellectual face we find "a trace of severity in his mouth and compressed lips, with the expression of most earnest thought on his brow and eyes, and with the remembrance of a finished work in every wrinkle of his countenance." this extraordinary man exercised a profound influence upon those who came into contact with him. he excited almost unbounded enthusiasm and great veneration among his students. they were allowed to work close by his side, and so magnetic was his personality that he stimulated them powerfully and succeeded in transmitting to them some of his own mental qualities. as professor of physiology in berlin, müller trained many gifted young men, among whom were ludwig ( - ), du bois-reymond ( - ), and helmholtz ( - ), who became distinguished scholars and professors in german universities. helmholtz, speaking of müller's influence on students, paid this tribute to the grandeur of his teacher: "whoever comes into contact with men of the first rank has an altered scale of values in life. such intellectual contact is the most interesting event that life can offer." the particular service of johannes müller to science was to make physiology broadly comparative. so comprehensive was his grasp upon the subject that he gained for himself the title of the greatest physiologist of modern times. he brought together in his great work on the physiology of man not only all that had been previously made known, carefully sifted and digested, but a great mass of new information, which was the result of his own investigations and of those of his students. so rigorous were his scientific standards that he did not admit into this treatise anything which had been untested either by himself or by some of his assistants or students. verworn says of this monumental work, which appeared in , under the title _handbuch der physiologie des menschen_: "this work stands to-day unsurpassed in the genuinely philosophical manner in which the material, swollen to vast proportions by innumerable special researches, was for the first time sifted and elaborated into a unitary picture of the mechanism within the living organism. in this respect the _handbuch_ is to-day not only unsurpassed, but unequalled." müller was the most accurate of observers; indeed, he is the most conspicuous example in the nineteenth century of a man who accomplished a prodigious amount of work all of which was of the highest quality. in physiology he stood on broader lines than had ever been used before. he employed every means at his command--experimenting, the observation of simple animals, the microscope, the discoveries in physics, in chemistry, and in psychology. he also introduced into physiology the principles of psychology, and it is from the period of johannes müller that we are to associate recognition of the close connection between the operations of the mind and the physiology of the brain that has come to occupy such a conspicuous position at the present time. [illustration: fig. .--johannes müller, - .] müller died in , having reached the age of fifty-seven, but his influence was prolonged through the teachings of his students. physiology after müller [illustration: fig. .--ludwig, - .] ludwig.--among the men who handed on the torch of müller there has already been mentioned ludwig (fig. ). for many years he lectured in the university of leipsic, attracting to that university high-minded, eager, and gifted young men, who received from this great luminary of physiology by expression what he himself had derived from contact with müller. there are to-day distributed through the universities a number of young physiologists who stand only one generation removed from johannes müller, and who still labor in the spirit that was introduced into this department of study by that great master. [illustration: fig. .--du bois-reymond, - .] du bois-reymond.--du bois-reymond (fig. ), another of his distinguished pupils, came to occupy the chair which müller himself had filled in the university of berlin, and during the period of his vigor was in physiology one of the lights of the world. it is no uncommon thing to find recently published physiologies dedicated either to the memory of johannes müller, as in the case of that remarkable _general physiology_ by verworn; or to ludwig, or to du bois-reymond, who were in part his intellectual product. from this disposition among physiologists to do homage to müller, we are able to estimate somewhat more closely the tremendous reach of his influence. bernard.--when müller was twelve years old there was born in saint-julien, department of the rhône, claude bernard, who attained an eminence as a physiologist, of which the french nation are justly proud. although he was little thought of as a student, nevertheless after he came under the influence of magendie, at the age of twenty-six, he developed rapidly and showed his true metal. he exhibited great manual dexterity in performing experiments, and also a luminous quality of mind in interpreting his observations. one of his greatest achievements in physiology was the discovery of the formation within the liver of glycogen, a substance chemically related to sugar. later he discovered the system of vaso-motor nerves that control and regulate the caliber of the blood-vessels. both of these discoveries assisted materially in understanding the wonderful changes that are going on within the human body. but besides his technical researches, any special consideration of which lies quite beyond the purpose of this book, he published in - a work upon the phenomena of life in animals and vegetables, a work that had general influence in extending the knowledge of vital activities. i refer to his now classic _leçons sur les phénomènes de la vie communs aux animaux et aux végétaux_. the thoughtful face of bernard is shown in his portrait, fig. . he was one of those retiring, silent men whose natures are difficult to fathom, and who are so frequently misunderstood. a domestic infelicity, that led to the separation of himself from his family, added to his isolation and loneliness. when touched by the social spirit he charmed people by his personality. he was admired by the emperor napoleon third, through whose influence bernard acquired two fine laboratories. in he was elected to the french academy, and became thereby one of the "forty immortals." [illustration: fig. .--claude bernard, - .] foster describes him thus: "tall in stature, with a fine presence, with a noble head, the eyes full at once of thought and kindness, he drew the look of observers upon him wherever he appeared. as he walked in the streets passers-by might be heard to say 'i wonder who that is; he must be some distinguished man.'" two directions of growth.--physiology, established on the broad foundations of müller, developed along two independent pathways, the physical and the chemical. we find a group of physiologists, among whom weber, ludwig, du bois-reymond, and helmholtz were noteworthy leaders, devoted to the investigations of physiological facts through the application of measurements and records made by machinery. with these men came into use the time-markers, the myographs, and the ingenious methods of recording blood-pressure, changes in respiration, the responses of muscle and nerve to various forms of stimulation, the rate of transmission of nerve-currents, etc. the investigation of vital activities by means of measurements and instrumental records has come to represent one especial phase of modern physiology. as might have been predicted, the discoveries and extensions of knowledge resulting from this kind of experimentation have been remarkable, since it is obvious that permanent records made by mechanical devices will rule out many errors; and, moreover, they afford an opportunity to study at leisure phenomena that occupy a very brief time. the other marked line of physiological investigation has been in the domain of chemistry, where wöhler, liebig, kühne, and others have, through the study of the chemical changes occurring in its body, observed the various activities that take place within the organism. they have reduced all tissues and all parts of the body to chemical analysis, studied the chemical changes in digestion, in respiration, etc. the more recent observers have also made a particular feature of the study of the chemical changes going on within the living matter. the union of these two chief tendencies into the physico-chemical aspects of physiology has established the modern way of looking upon vital activities. these vital activities are now regarded as being, in their ultimate analysis, due to physical and chemical changes taking place within the living substratum. all along, this physico-chemical idea has been in contest with that of a duality between the body and the life that is manifested in it. the vitalists, then, have had many controversies with those who make their interpretations along physico-chemical lines. we will recollect that vitalism in the hands of the immediate successors of haller became not only highly speculative, but highly mystical, tending to obscure any close analysis of vital activity and throwing explanations all back into the domain of mysticism. johannes müller was also a vitalist, but his vitalism was of a more acceptable form. he thought of changes in the body as being due to vitality--to a living force; but he did not deny the possibility of the transformation of this vital energy into other forms of energy; and upon the basis of müller's work there has been built up the modern conception that there is found in the human body a particular transformation-form of energy, not a mystical vital force that presides over all manifestations of life. the advances in physiology, beginning with those of william harvey, have had immense influence not only upon medicine, but upon all biology. we find now the successful and happy union between physiology and morphology in the work which is being so assiduously carried on to-day under the title of experimental morphology. the great names in physiology since müller are numerous, and perhaps it is invidious to mention particular ones; but, inasmuch as ludwig and du bois-reymond have been spoken of, we may associate with them the names of sir michael foster and burdon-sanderson, in england; and of brücke (one of müller's disciples) and verworn, in germany, as modern leaders whose investigations have promoted advance, and whose clear exposition of the facts and the theories of physiology have added much to the dignity of the science. chapter x von baer and the rise of embryology anatomy investigates the arrangement of organic tissues; embryology, or the science of development, shows how they are produced and arranged. there is no more fascinating division of biological study. as minot says: "indeed, the stories which embryology has to tell are the most romantic known to us, and the wildest imaginative creations of scott or dumas are less startling than the innumerable and almost incredible shifts of rôle and change of character which embryology has to entertain us with in her histories." embryology is one of the most important biological sciences in furnishing clues to the past history of animals. every organism above the very lowest, no matter how complex, begins its existence as a single microscopic cell, and between that simple state and the fully formed condition every gradation of structure is exhibited. every time an animal is developed these constructive changes are repeated in orderly sequence, and one who studies the series of steps in development is led to recognize that the process of building an animal's body is one of the most wonderful in all nature. rudimentary organs.--but, strangely enough, the course of development in any higher organism is not straightforward, but devious. instead of organs being produced in the most direct manner, unexpected by-paths are followed, as when all higher animals acquire gill-clefts and many other rudimentary organs not adapted to their condition of life. most of the rudimentary organs are transitory, and bear testimony, as hereditary survivals, to the line of ancestry. they are clues by means of which phases in the evolution of animal life may be deciphered. bearing in mind the continually shifting changes through which animals pass in their embryonic development, one begins to see why the adult structures of animals are so difficult to understand. they are not only complex; they are also greatly modified. the adult condition of any organ or tissue is the last step in a series of gradually acquired modifications, and is, therefore, the farthest departure from that which is ancestral and archetypal. but in the process of formation all the simpler conditions are exhibited. if, therefore, we wish to understand an organ or an animal, we must follow its development, and see it in simpler conditions, before the great modifications have been added. the tracing of the stages whereby cells merge into tissues, tissues into organs, and determining how the organs by combinations build up the body, is embryology. on account of the extended applications of this subject in biology, and the light which it throws on all structural studies, we shall be justified in giving its history at somewhat greater length than that adopted in treating of other topics. five historical periods.--the story of the rise of this interesting department of biology can, for convenience, be divided into five periods, each marked by an advance in general knowledge. these are: ( ) the period of harvey and malpighi; ( ) the period of wolff; ( ) the period of von baer; ( ) the period from von baer to balfour; and ( ) the period of balfour, with an indication of present tendencies. among all the leaders von baer stands as a monumental figure at the parting of the ways between the new and the old--the sane thinker, the great observer. the period of harvey and malpighi in general.--the usual account of the rise of embryology is derived from german writers. but there is reason to depart from their traditions, in which wolff is heralded as its founder, and the one central figure prior to pander and von baer. the embryological work of wolff's great predecessors, harvey and malpighi, has been passed over too lightly. although these men have received ample recognition in closely related fields of investigation, their insight into those mysterious events that culminate in the formation of a new animal has been rarely appreciated. now and then a few writers, as brooks and whitman, have pointed out the great worth of harvey's work in embryology, but fewer have spoken for malpighi in this connection. koelliker, it is true, in his address at the unveiling of the statue of malpighi, in his native town of crevalcuore, in , gives him well-merited recognition as the founder of embryology, and the late sir michael foster has written in a similar vein in his delightful _lectures on the history of physiology_. however great was harvey's work in embryology, i venture to say that malpighi's was greater when considered as a piece of observation. harvey's work is more philosophical; he discusses the nature of development, and shows unusual powers as an accurate reasoner. but that part of his treatise devoted to observation is far less extensive and exact than malpighi's, and throughout his lengthy discussions he has the flavor of the ancients. malpighi's work, on the contrary, flavors more of the moderns. in terse descriptions, and with many sketches, he shows the changes in the hen's egg from the close of the first day of development onward. it is a noteworthy fact that, at the period in which he lived, malpighi could so successfully curb the tendency to indulge in wordy disquisitions, and that he was satisfied to observe carefully, and tell his story in a simple way. this quality of mind is rare. as emerson has said: "i am impressed with the fact that the greatest thing a human soul ever does in this world is to see something, and tell what it saw in a plain way. hundreds of people can talk for one who can think, but thousands can think for one who can see. to see clearly is poetry, philosophy, and religion all in one." but "to see" here means, of course, to interpret as well as to observe. although there were observers in the field of embryology before harvey, little of substantial value had been produced. the earliest attempts were vague and uncritical, embracing only fragmentary views of the more obvious features of body-formation. nor, indeed, should we look for much advance in the field of embryology even in harvey's time. the reason for this will become obvious when we remember that the renewal of independent observation had just been brought about in the preceding century by vesalius, and that harvey himself was one of the pioneers in the intellectual awakening. studies on the development of the body are specialized, involving observations on minute structures and recondite processes, and must, therefore, wait upon considerable advances in anatomy and physiology. accordingly, the science of embryology was of late development. harvey.--harvey's was the first attempt to make a critical analysis of the process of development, and that he did not attain more was not owing to limitations of his powers of discernment, but to the necessity of building on the general level of the science of his time, and, further, to his lack of instruments of observation and technique. nevertheless, harvey may be considered as having made the first independent advance in embryology. by clearly teaching, on the basis of his own observations, the gradual formation of the body by aggregation of its parts, he anticipated wolff. this doctrine came to be known under the title of "epigenesis," but harvey's epigenesis[ ] was not, as wolff's was, directed against a theory of pre-delineation of the parts of the embryo, but against the ideas of the medical men of the time regarding the metamorphosis of germinal elements. it lacked, therefore, the dramatic setting which surrounded the work of wolff in the next century. had the doctrine of pre-formation been current in harvey's time, we are quite justified in assuming that he would have assailed it as vigorously as did wolff. his treatise on generation.--harvey's embryological work was published in under the title _exercitationes de generatione animalium_. it embraces not only observations on the development of the chick, but also on the deer and some other mammals. as he was the court physician of charles i, that sovereign had many deer killed in the park, at intervals, in order to give harvey the opportunity to study their development. as fruits of his observation on the chick, he showed the position in which the embryo arises within the egg, _viz._, in the white opaque spot or cicatricula; and he also corrected aristotle, fabricius, and his other predecessors in many particulars. harvey's greatest predecessor in this field, fabricius, was also his teacher. when, in search of the best training in medicine, harvey took his way from england to italy, as already recounted, he came under the instruction of fabricius in padua. in , fabricius published sketches showing the development of animals; and, again, in , six years after his death, appeared his illustrated treatise on the development of the chick. except the figures of coiter ( ), those of fabricius were the earliest published illustrations of the kind. altogether his figures show developmental stages of the cow, sheep, pig, galeus, serpent, rat, and chick. harvey's own treatise was not illustrated. with that singular independence of mind for which he was conspicuous, the vision of the pupil was not hampered by the authority of his teacher, and, trusting only to his own sure observation and reason, he described the stages of development as he saw them in the egg, and placed his own construction on the facts. one of the earliest activities to arrest his attention in the chick was a pulsating point, the heart, and, from this observation, he supposed that the heart and the blood were the first formations. he says: "but as soon as the egg, under the influence of the gentle warmth of the incubating hen, or of warmth derived from another source, begins to pullulate, this spot forthwith dilates, and expands like the pupil of the eye; and from thence, as the grand center of the egg, the latent plastic force breaks forth and germinates. this first commencement of the chick, however, so far as i am aware, has not yet been observed by any one." it is to be understood, however, that the descriptive part of his treatise is relatively brief (about pages out of in willis's translation), and that the bulk of the "exercises" into which his work is divided is devoted to comments on the older writers and to discussions of the nature of the process of development. the aphorism, "_omne vivum ex ovo_," though not invented by harvey, was brought into general use through his writings. as used in his day, however, it did not have its full modern significance. with harvey it meant simply that the embryos of all animals, the viviparous as well as the oviparous, originate in eggs, and it was directed against certain contrary medical theories of the time. [illustration: fig. .--frontispiece to harvey's _generatione animalium_ ( ).] the first edition of his _generatione animalium_, london, , is provided with an allegorical frontispiece embodying this idea. as shown in fig. , it represents jove on a pedestal, uncovering a round box, or ovum, bearing the inscription "_ex ovo omnia_," and from the box issue all forms of living creatures, including also man. malpighi.--the observer in embryology who looms into prominence between harvey and wolff is malpighi. he supplied what was greatly needed at the time--an illustrated account of the actual stages in the development of the chick from the end of the first day to hatching, shorn of verbose references and speculations. his observations on development are in two separate memoirs, both sent to the royal society in , and published by the society in latin, under the titles _de formatione pulli in ovo_ and _de ovo incubato_. the two taken together are illustrated by twelve plates containing eighty-six figures, and the twenty-two quarto pages of text are nearly all devoted to descriptions, a marked contrast to the pages of harvey unprovided with illustrations. his pictures, although not correct in all particulars, represent what he was able to see, and are very remarkable for the age in which they were made, and considering the instruments of observation at his command. they show successive stages from the time the embryo is first outlined, and, taken in their entirety, they cover a wide range of stages. his observations on the development of the heart, comprising twenty figures, are the most complete. he clearly illustrates the aortic arches, those transitory structures of such great interest as showing a phase in ancestral history. [illustration: fig. .--selected sketches from malpighi's works. showing stages in the development of the chick ( ).] he was also the first to show by pictures the formation of the head-fold and the neural groove, as well as the brain-vesicles and eye-pockets. his delineation of heart, brain, and eye-vesicles are far ahead of those illustrating wolff's _theoria generationis_, made nearly a hundred years later. fig. shows a few selected sketches from the various plates of his embryological treatises, to compare with those of wolff. (see fig. .) [illustration: fig. .--marcello malpighi, - .] the original drawings for _de ovo incubato_, still in possession of the royal society, are made in pencil and red chalk, and an examination of them shows that they far surpass the reproductions in finish and accuracy. while harvey taught the gradual formation of parts, malpighi, from his own observations, supposed the rudiments of the embryo to pre-exist within the egg. he thought that, possibly, the blood-vessels were in the form of tubes, closely wrapped together, which by becoming filled with blood were distended. nevertheless, in the treatises mentioned above he is very temperate in his expressions on the whole matter, and evidently believed in the new formation of many parts. the portrait of malpighi shown in fig. is taken from his life by atti. from descriptions of his personal appearance (see page ) one would think that this is probably a better likeness than the strikingly handsome portrait painted by tabor, and presented by malpighi to the royal society of london. for a reproduction of the latter see page . malpighi's rank.--on the whole, malpighi should rank above harvey as an embryologist, on account of his discoveries and fuller representation, by drawings and descriptions, of the process of development. as sir michael foster has said: "the first adequate description of the long series of changes by which, as they melt the one into the other, like dissolving views, the little white opaque spot in the egg is transformed into the feathered, living, active bird, was given by malpighi. and where he left it, so for the most part the matter remained until even the present century. for this reason we may speak of him as the founder of embryology." the period of wolff between harvey and wolff, embryology had become dominated by the theory that the embryo exists already pre-formed within the egg, and, as a result of the rise of this new doctrine, the publications of wolff had a different setting from that of any of his predecessors. it is only fair to say that to this circumstance is owing, in large part, the prominence of his name in connection with the theory of epigenesis. as we have already seen, harvey, more than a century before the publications of wolff, had clearly taught that development is a process of gradual becoming. nevertheless, wolff's work, as opposed to the new theory, was very important. while the facts fail to support the contention that he was the founder of epigenesis, it is to be remembered that he has claims in other directions to rank as the foremost student of embryology prior to von baer. as a preliminary to discussing wolff's position, we should bring under consideration the doctrine of pre-formation and encasement. rise of the theory of pre-delineation.--the idea of pre-formation in its first form is easily set forth. just as when we examine a seed we find within an embryo plantlet, so it was supposed that the various forms of animal life existed in miniature within the egg. the process of development was supposed to consist of the expansion or unfolding of this pre-formed embryo. the process was commonly illustrated by reference to flower-buds. "just as already in a small bud all the parts of the flower, such as stamens and colored petals, are enveloped by the green and still undeveloped sepals; just as the parts grow in concealment and then suddenly expand into a blossom, so also in the development of animals, it was thought that the already present, small but transparent parts grow, gradually expand, and become discernible." (hertwig.) from the feature of unfolding this was called in the eighteenth century the theory of _evolution_, giving to that term quite a different meaning from that attached to it at the present time. this theory, strange as it may seem to us now, was founded on a basis of actual observation--not entirely on speculation. although it was a product of the seventeenth century, from several printed accounts one is likely to gather the impression that it arose in the eighteenth century, and that bonnet, haller, and leibnitz were among its founders. this implication is in part fostered by the circumstance that swammerdam's _biblia naturæ_, which contains the germ of the theory, was not published until --more than half a century after his death--although the observations for it were completed before malpighi's first paper on embryology was published in . while it is well to bear in mind that date of publication, rather than date of observation, is accepted as establishing the period of emergence of ideas, there were other men, as malpighi and leeuwenhoek, contemporaries of swammerdam, who published in the seventeenth century the basis for this theory. malpighi supposed ( ) the rudiment of the embryo to pre-exist within the hen's egg, because he observed evidences of organization in the unincubated egg. this was in the heat of the italian summer (in july and august, as he himself records), and dareste suggests that the developmental changes had gone forward to a considerable degree before malpighi opened the eggs. be this as it may, the imperfection of his instruments and technique would have made it very difficult to see anything definitely in stages under twenty-four hours. in reference to his observations, he says that in the unincubated egg he saw a small embryo enclosed in a sac which he subjected to the rays of the sun. "frequently i opened the sac with the point of a needle, so that the animals contained within might be brought to the light, nevertheless to no purpose; for the individuals were so jelly-like and so very small that they were lacerated by a light stroke. therefore, it is right to confess that the beginnings of the chick pre-exist in the egg, and have reached a higher development in no other way than in the eggs of plants." ("quare _pulli stamina_ in ovo _præexistere_, altiorémque originem nacta esse fateri convenit, haud dispari ritu, ac in plantarum ovis.") swammerdam ( - ) supplied a somewhat better basis. he observed that the parts of the butterfly, and other insects as well, are discernible in the chrysalis stage. also, on observing caterpillars just before going into the pupa condition, he saw in outline the organs of the future stage, and very naturally concluded that development consists of an expansion of already formed parts. a new feature was introduced through the discovery, by leeuwenhoek, about ,[ ] of the fertilizing filaments of eggs. soon after, controversies began to arise as to whether the embryo pre-existed in the sperm or in the egg. by leeuwenhoek, hartsoeker, and others the egg was looked upon as simply a _nidus_ within which the sperm developed, and they asserted that the future animal existed in miniature in the sperm. these controversies gave rise to the schools of the animalculists, who believed the sperm to be the animal germ, and of the ovulists, who contended for the ovum in that rôle. it is interesting to follow the metaphysical speculations which led to another aspect of the doctrine of pre-formation. there were those, notably swammerdam, leibnitz, and bonnet, who did not hesitate to follow the idea to the logical consequence that, if the animal germ exists pre-formed, one generation after another must be encased within it. this gave rise to the fanciful idea of encasement or _emboîtement_, which was so greatly elaborated by bonnet and, by leibnitz, applied to the development of the soul. even swammerdam (who, by the way, though a masterly observer, was always a poor generalizer) conceived of the germs of all forthcoming generations as having been located in the common mother eve, all closely encased one within the other, like the boxes of a japanese juggler. the end of the human race was conceived of by him as a necessity, when the last germ of this wonderful series had been unfolded. [illustration: fig. .--plate from wolff's _theoria generationis_ ( ), showing stages in the development of the chick.] his successors, in efforts to compute the number of homunculi which must have been condensed in the ovary of eve, arrived at the amazing result of two hundred millions. work of wolff.--friedrich kaspar wolff, as a young man of twenty-six years, set himself against this grotesque doctrine of pre-formation and encasement in his _theoria generationis_, published in . this consists of three parts: one devoted to the development of plants, one to the development of animals, and one to theoretical considerations. he contended that the organs of animals make their appearance gradually, and that he could actually follow their successive stages of formation. the figures in it illustrating the development of the chick, some of which are shown in fig. , are not, on the whole, so good as malpighi's. wolff gives, in all, seventeen figures, while malpighi published eighty-six, and his twenty figures on the development of the heart are more detailed than any of wolff's. when the figures represent similar stages of development, a comparison of the two men's work is favorable to malpighi. the latter shows much better, in corresponding stages, the series of cerebral vesicles and their relation to the optic vesicles. moreover, in the wider range of his work, he shows many things--such as the formation of the neural groove, etc.--not included in wolff's observations. wolff, on the other hand, figures for the first time the primitive kidneys, or "wolffian bodies," of which he was the discoverer. although wolff was able to show that development consists of a gradual formation of parts, his theory of development was entirely mystical and unsatisfactory. the fruitful idea of germinal continuity had not yet emerged, and the thought that the egg has inherited an organization from the past was yet to be expressed. wolff was, therefore, in the same quandary as his predecessors when he undertook to explain development. since he assumed a total lack of organization in the beginning, he was obliged to make development "miraculous" through the action on the egg of a hyperphysical agent. from a total lack of organization, he conceived of its being lifted to the highly organized product through the action of a "_vis essentialis corporis_." he returned to the problem of development later, and, in - , published his best work in this field on the development of the intestine.[ ] this is a very original and strong piece of observational work. while his investigations for the _theoria generationis_ did not reach the level of malpighi's, those of the paper of surpassed them and held the position of the best piece of embryological work up to that of pander and von baer. this work was so highly appreciated by von baer that he said: "it is the greatest masterpiece of scientific observation which we possess." in it he clearly demonstrated that the development of the intestine and its appendages is a true process of becoming. still later, in , he published further theoretical considerations. opposition to wolff's views.--but all wolff's work was launched into an uncongenial atmosphere. the great physiologist haller could not accept the idea of epigenesis, but opposed it energetically, and so great was his authority that the views of wolff gained no currency. this retarded progress in the science of animal development for more than a half-century. bonnet was also a prolific writer in opposition to the ideas of wolff, and we should perhaps have a portrait of him (fig. ) as one of the philosophical naturalists of the time. his prominent connection with the theory of pre-delineation in its less grotesque form, his discovery of the development of the eggs of plant-lice without previous fertilization, his researches on regeneration of parts in polyps and worms, and other observations place him among the conspicuous naturalists of the period. his system of philosophy, which has been carefully analyzed by whitman, is designated by that writer as a system of negations. [illustration: fig. .--charles bonnet, - .] in , j. fr. meckel, recognizing the great value of wolff's researches on the development of the intestines, rescued the work from neglect and obscurity by publishing a german translation of the same, and bringing it to the attention of scholars. from that time onward wolff's labor was fruitful. his _de formatione intestinorum_ rather than his _theoria generationis_ embodies his greatest contribution to embryology. not only is it a more fitting model of observation, but in it he foreshadows the idea of germ-layers in the embryo, which, under pander and von baer, became the fundamental conception in structural embryology. throughout his researches both early and late, he likens the embryonic rudiments, which precede the formation of organs, to leaflets. in his work of he described in detail how the leaf-like layers give rise to the systems of organs; showing that the nervous system arises first from a leaf-like layer, and is followed, successively, by a flesh layer, the vascular system, and lastly, by the intestinal canal--all arising from original leaf-like layers. in these important generalizations, although they are verbally incorrect, he reached the truth as nearly as it was possible at the time, and laid the foundation of the germ-layer theory. wolff was a man of great power as an observer, and although his influence was for a long time retarded, he should be recognized as the foremost investigator in embryology before von baer. few biographical facts.--the little known of his life is gained through his correspondence and a letter by his amanuensis. through personal neglect, and hostility to his work, he could not secure a foothold in the universities of germany, and, in , on the invitation of catherine of russia, he went to the academy of sciences at st. petersburg, where he spent the last thirty years of his life. it has been impossible to discover a portrait of wolff, although i have sought one in various ways for several years. the secretary of the academy of sciences at st. petersburg writes that no portrait of wolff exists there, and that the academy will gratefully receive information from any source regarding the existence of a portrait of the great academician. his sincere and generous spirit is shown in his correspondence with haller, his great opponent. "and as to the matter of contention between us, i think thus: for me, no more than for you, glorious man, is truth of the very greatest concern. whether it chance that organic bodies emerge from an invisible into a visible condition, or form themselves out of the air, there is no reason why i should wish the one were truer than the other, or wish the one and not the other. and this is your view also, glorious man. we are investigating for truth only; we seek that which is true. why then should i contend with you?" (quoted from wheeler.) the period of von baer what johannes müller was for physiology, von baer was for embryology; all subsequent growth was influenced by his investigations. the greatest classic in embryology is his _development of animals_ (_entwicklungsgeschichte der tiere--beobachtung und reflexion_), the first part of which was published in , and the work on the second part completed in , although it was not published till . this second part was never finished according to the plan of von baer, but was issued by his publisher, after vainly waiting for the finished manuscript. the final portion, which von baer had withheld, in order to perfect in some particulars, was published in , after his death, but in the form in which he left it in . the observations for the first part began in , after he had received a copy of pander's researches, and covered a period of seven years of close devotion to the subject; and the observations for the last part were carried on at intervals for several years. it is significant of the character of his _reflexionen_ that, although published before the announcement of the cell-theory, and before the acceptance of the doctrine of organic evolution, they have exerted a molding influence upon embryology to the present time. the position of von baer in embryology is owing as much to his sagacity in speculation as to his powers as an observer. "never again have observation and thought been so successfully combined in embryological work" (minot). von baer was born in , and lived on to , but his enduring fame in embryology rests on work completed more than forty years before the end of his useful life. after his removal from königsberg to st. petersburg, in , he very largely devoted himself to anthropology in its widest sense, and thereby extended his scientific reputation into other fields. if space permitted, it would be interesting to give the biography[ ] of this extraordinary man, but here it will be necessary to content ourselves with an examination of his portraits and a brief account of his work. portraits.--several portraits of von baer showing him at different periods of his life have been published. a very attractive one, taken in his early manhood, appeared in _harper's magazine_ for . the expression of the face is poetical, and the picture is interesting to compare with the more matured, sage-like countenance forming the frontispiece of stieda's _life of von baer_ (see fig. ). this, perhaps the best of all his portraits, shows him in the full development of his powers. an examination of it impresses one with confidence in his balanced judgment and the thoroughness and profundity of his mental operations. [illustration: fig. .--karl ernst von baer, - .] the portrait of von baer at about seventy years of age, reproduced in fig. , is, however, destined to be the one by which he is commonly known to embryologists, since it forms the frontispiece of the great cooperative _handbook of embryology_ just published under the editorship of oskar hertwig. [illustration: fig. .--von baer at about seventy years of age.] von baer's especial service.--apart from special discoveries, von baer greatly enriched embryology in three directions: in the first place, he set a higher standard for all work in embryology, and thereby lifted the entire science to a higher level. activity in a great field of this kind is, with the rank and file of workers, so largely imitative that this feature of his influence should not be overlooked. in the second place, he established the germ-layer theory, and, in the third, he made embryology comparative. in reference to the germ-layer theory, it should be recalled that wolff had distinctly foreshadowed the idea by showing that the material out of which the embryo is constructed is, in an early stage of development, arranged in the form of leaf-like layers. he showed specifically that the alimentary canal is produced by one of these sheet-like expansions folding and rolling together. pander, by observations on the chick ( ), had extended the knowledge of these layers and elaborated the conception of wolff. he recognized the presence of three primary layers--an outer, a middle, and an inner--out of which the tissues of the body are formed. the germ-layers.--but it remained for von baer,[ ] by extending his observations into all the principal groups of animals, to raise this conception to the rank of a general law of development. he was able to show that in all animals except the very lowest there arise in the course of development leaf-like layers, which become converted into the "fundamental organs" of the body. now, these elementary layers are not definitive tissues of the body, but are embryonic, and therefore may appropriately be designated "germ-layers." the conception that these germ-layers are essentially similar in origin and fate in all animals was a fuller and later development of the germ-layer theory, a conception which dominated embryological study until a recent date. von baer recognized four such layers; the outer and inner ones being formed first, and subsequently budding off a middle layer composed of two sheets. a little later ( ) remak recognized the double middle layer of von baer as a unit, and thus arrived at the fundamental conception of three layers--the ecto-, endo-, and mesoderm--which has so long held sway. for a long time after von baer the aim of embryologists was to trace the history of these germ-layers, and so in a wider and much qualified sense it is to-day. it will ever stand to his credit, as a great achievement, that von baer was able to make a very complicated feature of development clear and relatively simple. given a leaf-like rudiment, with the layers held out by the yolk, as is the case in the hen's egg, it was no easy matter to conceive how they are transformed into the nervous system, the body-wall, the alimentary canal, and other parts, but von baer saw deeply and clearly that the fundamental anatomical features of the body are assumed by the leaf-like rudiments being rolled into tubes. fig. shows four sketches taken from the plates illustrating von baer's work. at _a_ is shown a stage in the formation of the embryonic envelope, or amnion, which surrounds the embryos of all animals above the class of amphibia. _b_, another figure of an ideal section, shows that, long before the day of microtomes, von baer made use of sections to represent the relationships of his four germ-layers. at _c_ and _d_ is represented diagrammatically the way in which these layers are rolled into tubes. he showed that the central nervous system arose in the form of a tube, from the outer layer; the body-wall in the form of a tube, composed of skin and muscle layers; and the alimentary tube from mucous and vascular layers. the generalization that embryos in development tend to recapitulate their ancestral history is frequently attributed to von baer, but the qualified way in which he suggests something of the sort will not justify one in attaching this conclusion to his work. von baer was the first to make embryology truly comparative, and to point out its great value in anatomy and zoölogy. by embryological studies he recognized four types of organization--as cuvier had done from the standpoint of comparative anatomy. but, since these types of organization have been greatly changed and subdivided, the importance of the distinction has faded away. as a distinct break, however, with the old idea of a linear scale of being it was of moment. among his especially noteworthy discoveries may be mentioned that of the egg of mammals ( ), and the notochord as occurring in all vertebrate animals. his discovery of the mammalian egg had been preceded by purkinje's observations upon the germinative spot in the bird's egg ( ). von baer's rank.--von baer has come to be dignified with the title of the "father of modern embryology." no man could have done more in his period, and it is owing to his superb intellect, and to his talents as an observer, that he accomplished what he did. as minot says: "he worked out, almost as fully as was possible at this time, the genesis of all the principal organs from the germ-layers, instinctively getting at the truth as only a great genius could have done." [illustration: fig. .--sketches from von baer's embryological treatise ( ).] after his masterly work, the science of embryology could never return to its former level; he had given it a new direction, and through his influence a period of great activity was introduced. the period from von baer to balfour in the period between von baer and balfour there were great general advances in the knowledge of organic structure that brought the whole process of development into a new light. among the most important advances are to be enumerated the announcement of the cell-theory, the discovery of protoplasm, the beginning of the recognition of germinal continuity, and the establishment of the doctrine of organic evolution. the cell-theory.--the generalization that the tissues of all animals and plants are structurally composed of similar units, called cells, was given to the world through the combined labors of schleiden and schwann. the history of this doctrine, together with an account of its being remodeled into the protoplasm doctrine, is given in chapter xii. the broad-reaching effects of the cell-theory may be easily imagined, since it united all animals on the broad place of likeness in microscopic structure. now for the first time the tissues of the body were analyzed into their units; now for the first time was comprehended the nature of the germ-layers of von baer. among the first questions to emerge in the light of the new researches were concerning the origin of cells in the organs, the tissues, and the germ-layers. the road to the investigation of these questions was already opened, and it was followed, step by step, until the egg and the sperm came to be recognized as modified cells. this position was reached, for the egg, about , when gegenbaur showed that the eggs of all vertebrate animals, regardless of size and condition, are in reality single cells. the sperm was put in the same category about . the rest was relatively easy: the egg, a single cell, by successive divisions produces many cells, and the arrangement of these into primary embryonic layers brings us to the starting-point of wolff and von baer. the cells, continuing to multiply by division, not only increase in number, but also undergo changes through division of physiological labor, whereby certain groups are set apart to perform a particular part of the work of the body. in this way arise the various tissues of the body, which are, in reality, similar cells performing a similar function. finally, from combinations of tissues, the organs are formed. but the egg, before entering on the process of development, must be stimulated by the union of the sperm with the nucleus of the egg, and thus the starting-point of every animal and plant, above the lowest group, proves to be a single cell with protoplasm derived from two parents. while questions regarding the origin of cells in the body were being answered, the foundation for the embryological study of heredity was also laid. advances were now more rapid and more sure; flashes of morphological insight began to illuminate the way, and the facts of isolated observations began to fit into a harmonized whole. apart from the general advances of this period, mentioned in other connections, the work of a few individuals requires notice. rathke and remak were engaged with the broader aspects of embryology, as well as with special investigations. from rathke's researches came great advances in the knowledge of the development of insects and other invertebrates, and remak is notable for similar work with the vertebrates. as already mentioned, he was the first to recognize the middle layer as a unit, through which the three germ-layers of later embryologists emerged into the literature of the subject. koelliker, - , the veteran embryologist, for so many years a professor in the university of würzburg, carried on investigations on the segmentation of the egg. besides work on the invertebrates, later he followed with care the development of the chick and the rabbit; he encompassed the whole field of embryology, and published, in and again in , a general treatise on vertebrate embryology, of high merit. the portrait of this distinguished man is shown in chapter viii, where also his services as a histologist are recorded. huxley took a great step toward unifying the idea of germ-layers throughout the animal kingdom, when he maintained, in , that the two cell-layers in animals like the hydra and oceanic hydrozoa correspond to the ectoderm and endoderm of higher animals. kowalevsky (fig. ) made interesting discoveries of a general bearing. in he showed the practical identity, in the early stages of development, between one of the lowest vertebrates (amphioxus) and a tunicate. the latter up to that time had been considered an invertebrate, and the effect of kowalevsky's observations was to break down the sharply limited line supposed to exist between the invertebrates and the vertebrates. this was of great influence in subsequent work. kowalevsky also founded the generalization that all animals in development pass through a gastrula stage--a doctrine associated, since , with the name of haeckel under the title of the gastræa theory. beginning of the doctrine of germinal continuity.--the conception that there is unbroken continuity of germinal substance between all living organisms, and that the egg and the sperm are endowed with an inherited organization of great complexity, has become the basis for all current theories of heredity and development. so much is involved in this conception that, in the present decade, it has been designated (whitman) "the central fact of modern biology." the first clear expression of it is found in virchow's _cellular pathology_, published in . it was not, however, until the period of balfour, and through the work of fol, van beneden (chromosomes, ), boveri, hertwig, and others, that the great importance of this conception began to be appreciated, and came to be woven into the fundamental ideas of development. [illustration: fig. .--a. kowalevsky, - .] influence of the doctrine of organic evolution.--this doctrine, although founded in its modern sense by lamarck in the early part of the nineteenth century, lay dormant until darwin, in , brought a new feature into its discussion by emphasizing the factor of natural selection. the general acceptance of the doctrine, which followed after fierce opposition, had, of course, a profound influence on embryology. the latter science is so intimately concerned with the genealogy of animals and plants, that the newly accepted doctrine, as affording an explanation of this genealogy, was the thing most needed. the development of organisms was now seen in the light of ancestral history, rudimentary organs began to have meaning as hereditary survivals, and the whole process of development assumed a different aspect. this doctrine supplied a new impulse to the interpretation of nature at large, and of the embryological record in particular. the meaning of the embryological record was so greatly emphasized in the period of balfour that it will be commented upon under the next division of our subject. the period between von baer and balfour proved to be one of great importance on account of the general advances in knowledge of all organic nature. observations were moving toward a better and more consistent conception of the structure of animals and plants. a new comparative anatomy, more profound and richer in meaning than cuvier's, was arising. the edifice on the foundation of von baer's work was now emerging into recognizable outlines. the period of balfour, with an indication of present tendencies balfour's masterly work.--the workers of this period inherited all the accumulations of previous efforts, and the time was ripe for a new step. observations on the development of different animals, vertebrates and invertebrates, had accumulated in great number, but they were scattered through technical periodicals, transactions of learned societies, monographs, etc., and there was no compact science of embryology with definite outlines. balfour reviewed all this mass of information, digested it, and molded it into an organized whole. the results were published in the form of two volumes with the title of _comparative embryology_. this book of "almost priceless value" was given to the world in - . it was a colossal undertaking, but balfour was a phenomenal worker. before his untimely death at the age of thirty-one, he had been able to complete this work and to produce, besides, a large number of technical researches. the period of balfour is taken arbitrarily in this volume as beginning about , when he published, with michael foster, _the elements of embryology_. [illustration: fig. .--francis m. balfour, - .] his university career.--balfour (fig. ) was born in . during his days of preparation for the university he was a good student, but did not exhibit in any marked way the powers for which later he became distinguished. at cambridge, his distinguished teacher, the late sir michael foster, recognized his great talents, and encouraged him to begin work in embryology. his labors in this field once begun, he threw himself into it with great intensity. he rose rapidly to a professorship in cambridge, and so great was his enthusiasm and earnestness as a lecturer that in seven years "voluntary attendance on his classes advanced from ten to ninety." he was also a stimulator of research, and at the time of his death there were twenty students engaged in his laboratory on problems of development. he was distinguished for personal attractiveness, and those who met him were impressed with his great sincerity, as well as his personal charm. he was welcomed as an addition to the select group of distinguished scientific men of england, and a great career was predicted for him. huxley, when he felt the call, at a great personal sacrifice, to lay aside the more rigorous pursuits of scientific research, and to devote himself to molding science into the lives of the people, said of balfour: "he is the only man who can carry out my work." his tragic fate.--but that was not destined to be. the story of his tragic end need be only referred to. after completing the prodigious labor on the _comparative embryology_ he went to switzerland for recuperation, and met his death, with that of his guide, by slipping from an alpine height into a chasm. his death occurred in july, . the memorial edition of his works fills four quarto volumes, but the "comparative embryology" is balfour's monument, and will give him enduring fame. it is not only a digest of the work of others, but contains also general considerations of a far-seeing quality. he saw developmental processes in the light of the hypothesis of organic evolution. his speculations were sufficiently reserved, and nearly always luminous. it is significant of the character of this work to say that the speculations contained in the papers of the rank and file of embryological workers for more than two decades, and often fondly believed to be novel, were for the most part anticipated by balfour, and were also better expressed, with better qualifications. the reading of ancestral history in the stages of development is such a characteristic feature of the embryological work of balfour's period that some observations concerning it will now be in place. interpretation of the embryological record.--perhaps the most impressive feature of animal development is the series of similar changes through which all pass in the embryo. the higher animals, especially, exhibit all stages of organization from the unicellular fertilized ovum to the fully formed animal so far removed from it. the intermediate changes constitute a long record, the possibility of interpreting which has been a stimulus to its careful examination. meckel, in , and later von baer, indicated the close similarity between embryonic stages of widely different animals; von baer, indeed, confessed that he was unable to distinguish positively between a reptile, a bird, and a mammalian embryo in certain early stages of growth. in addition to this similarity, which is a constant feature of the embryological record, there is another one that may be equally significant; _viz._, in the course of embryonic history, sets of rudimentary organs arise and disappear. rudimentary teeth make their appearance in the embryo of the whalebone whale, but they are transitory and soon disappear without having been of service to the animal. in the embryos of all higher vertebrates, as is well known, gill-clefts and gill-arches with an appropriate circulation, make their appearance, but disappear long before birth. these indications, and similar ones, must have some meaning. now whatever qualities an animal exhibits after birth are attributed to heredity. may it not be that all the intermediate stages are also inheritances, and, therefore, represent phases in ancestral history? if they be, indeed, clues to ancestral conditions, may we not, by patching together our observations, be able to interpret the record, just as the history of ancient peoples has been made out from fragments in the shape of coins, vases, implements, hieroglyphics, inscriptions, etc.? the recapitulation theory.--the results of reflection in this direction led to the foundation of the _recapitulation theory_, according to which animals are supposed, in their individual development, to recapitulate to a considerable degree phases of their ancestral history. this is one of the widest generalizations of embryology. it was suggested in the writings of von baer and louis agassiz, but received its first clear and complete expression in , in the writings of fritz müller. although the course of events in development is a record, it is, at best, only a fragmentary and imperfect one. many stages have been dropped out, others are unduly prolonged or abbreviated, or appear out of chronological order, and, besides this, some of the structures have arisen from adaptation of a particular organism to its conditions of development, and are, therefore, not ancestral at all, but, as it were, recent additions to the text. the interpretation becomes a difficult task, which requires much balance of judgment and profound analysis. the recapitulation theory was a dominant note in all balfour's speculations, and in that of his contemporary and fellow-student marshall. it has received its most sweeping application in the works of ernst haeckel. widely spread throughout recent literature is to be noted a reaction against the too wide and unreserved application of this doctrine. this is naturally to be expected, since it is the common tendency in all fields of scholarship to demand a more critical estimate of results, and to undergo a reaction from the earlier crude and sweeping conclusions. [illustration: fig. .--oskar hertwig in .] nearly all problems in anatomy and structural zoölogy are approached from the embryological side, and, as a consequence, the work of the great army of anatomists and zoölogists has been in a measure embryological. many of them have produced beautiful and important researches, but the work is too extended to admit of review in this connection. oskar hertwig, of berlin (fig. ), is one of the representative embryologists of europe, while, in this country, lights of the first magnitude are brooks, minot, whitman, e.b. wilson, and others. although no attempt is made to review the researches of the recent period, we cannot pass entirely without mention the discovery of chromosomes, and of their reduction in the ripening of the egg and in the formation of sperm. this has thrown a flood of light on the phenomena of fertilization, and has led to the recognition of chromosomes as probably the bearers of heredity. the nature of fertilization, investigated by fol, o. hertwig, and others, formed the starting-point for a series of brilliant discoveries. the embryological investigations of the late wilhelm his (fig. ) are also deserving of especial notice. his luminous researches on the development of the nervous system, the origin of nerve fibers, and his analysis of the development of the human embryo are all very important. recent tendencies. experimental embryology.--soon after the publication of balfour's great work on "comparative embryology," a new tendency in research began to appear which led onward to the establishment of experimental embryology. all previous work in this field had been concerned with the structure, or architecture, of organisms, but now the physiological side began to receive attention. whitman has stated with great aptness the interdependence of these two lines of work, as follows: "morphology raises the question, how came the organic mechanism into existence? has it had a history, reaching its present stage of perfection through a long series of gradations, the first term of which was a relatively simple stage? the embryological history is traced out, and the palæontological records are searched, until the evidence from both sources establishes the fact that the organ or organism under study is but the summation of modifications and elaborations of a relatively simple primordial. this point settled, physiology is called upon to complete the story. have the functions remained the same through the series? or have they undergone a series of modifications, differentiations, and improvements more or less parallel with the morphological series?" [illustration: fig. .--wilhelm his, - . at sixty-four years.] since physiology is an experimental science, all questions of this nature must be investigated with the help of experiments. organisms undergoing development have been subjected to changed conditions, and their responses to various forms of stimuli have been noted. in the rise of experimental embryology we have one of the most promising of the recent departures from the older aspects of the subject. the results already attained in this attractive and suggestive field make too long a story to justify its telling in this volume. roux, herbst, loeb, morgan, e.b. wilson, and many others have contributed to the growth of this new division of embryology. good reasons have been adduced for believing that qualitative changes take place in the protoplasm as development proceeds. and a curb has been put upon that "great fault of embryology, the tendency to explain any and every operation of development as merely the result of inheritance." it has been demonstrated that surrounding conditions have much to do with individual development, and that the course of events may depend largely upon stimuli coming from without, and not exclusively on an inherited tendency. cell-lineage.--investigations on the structural side have reached a high grade of perfection in studies on cell-lineage. the theoretical conclusions in the germ-layer theory are based upon the assumption of identity in origin of the different layers. but the lack of agreement among observers, especially in reference to the origin of the mesoderm, made it necessary to study more closely the early developmental stages before the establishment of the germ-layers. it is a great triumph of exact observation that, although continually changing, the consecutive history of the individual cells has been followed from the beginning of segmentation to the time when the germ-layers are established. some of the beautifully illustrated memoirs in this field are highly artistic. blochman ( ) was a pioneer in observations of this kind, and, following him, a number of american investigators have pursued studies on cell-lineage with great success. the researches of whitman, wilson, conklin, kofoid, lillie, mead, and castle have given us the history of the origin of the germ-layers, cell by cell, in a variety of animal forms. these studies have shown that there is a lack of uniformity in the origin of at least the middle layer, and therefore there can be no strict homology of its derivatives. this makes it apparent that the earlier generalizations of the germ-layer theory were too sweeping, and, as a result, the theory is retained in a much modified form. theoretical discussions.--certain theoretical discussions, based on embryological studies, have been rife in recent years. and it is to be recognized without question that discussions regarding heredity, regeneration, the nature of the developmental process, the question of inherited organization within the egg, of germinal continuity, etc., have done much to advance the subject of embryology. embryology is one of the three great departments of biology which, taken in combination, supply us with a knowledge of living forms along lines of structure, function, and development. the embryological method of study is of increasing importance to comparative anatomy and physiology. formerly it was entirely structural, but it is now becoming also experimental, and it will therefore be of more service to physiology. while it has a strictly technical side, the science of embryology must always remain of interest to intelligent people as embracing one of the most wonderful processes in nature--the development of a complex organism from the single-celled condition, with a panoramic representation of all the intermediate stages. footnotes: [footnote : as whitman has pointed out, aristotle taught epigenesis as clearly as harvey, and is, therefore, to be regarded as the founder of that conception.] [footnote : the discovery is also attributed to hamm, a medical student, and to hartsoeker, who claimed priority in the discovery.] [footnote : _de formatione intestinorum, nova commentar, ac. sci. petrop._, st. petersburg, xii., ; xiii., .] [footnote : besides biographical sketches by stieda, waldeyer, and others, we have a very entertaining autobiography of von baer, published in , for private circulation, but afterward ( ) reprinted and placed on sale.] [footnote : it is of more than passing interest to remember that pander and von baer were associated as friends and fellow-students, under döllinger at würzburg. it was partly through the influence of von baer that pander came to study with döllinger, and took up investigations on development. his ample private means made it possible for him to bear the expenses connected with the investigation, and to secure the services of a fine artist for making the illustrations. the result was a magnificently illustrated treatise. his unillustrated thesis in latin ( ) is more commonly known, but the illustrated treatise in german is rarer. von baer did not take up his researches seriously until pander's were published. it is significant of their continued harmonious relations that von baer's work is dedicated "an meinen jugendfreund, dr. christian pander."] chapter xi the cell theory--schleiden, schwann, schultze the recognition, in , of the fact that all the various tissues of animals and plants are constructed on a similar plan was an important step in the rise of biology. it was progress along the line of microscopical observation. one can readily understand that the structural analysis of organisms could not be completed until their elementary parts had been discovered. when these units of structure were discovered they were called cells--from a misconception of their nature--and, although the misconception has long since been corrected, they still retain this historical but misleading name. the doctrine that all tissues of animals and plants are composed of aggregations of these units, and the derivatives from the same, is known as the cell-theory. it is a generalization which unites all animals and plants on the broad plane of similitude of structure, and, when we consider it in the light of its consequences, it stands out as one of the great scientific achievements of the nineteenth century. there is little danger of overestimating the importance of this doctrine as tending to unify the knowledge of living organisms. vague foreshadowings of the cell-theory.--in attempting to trace the growth of this idea, as based on actual observations, we first encounter vague foreshadowings of it in the seventeenth and the eighteenth centuries. the cells were seen and sketched by many early observers, but were not understood. as long ago as robert hooke, the great english microscopist, observed the cellular construction of cork, and described it as made up of "little boxes or cells distinguished from one another." he made sketches of the appearance of this plant tissue; and, inasmuch as the drawings of hooke are the earliest ones made of cells, they possess especial interest and consequently are reproduced here. fig. , taken from the _micrographia_, shows this earliest drawing of hooke. he made thin sections with a sharp penknife; "and upon examination they were found to be all cellular or porous in the manner of a honeycomb, but not so regular." [illustration: fig. .--the earliest known picture of cells from hooke's _micrographia_ ( ). from the edition of .] we must not completely overlook the fact that aristotle ( - b.c.) and galen ( - a.d.), those profound thinkers on anatomical structure, had reached the theoretical position "that animals and plants, complex as they may appear, are yet composed of comparatively few elementary parts, frequently repeated"; but we are not especially concerned with the remote history of the idea, so much as with the principal steps in its development after the beginning of microscopical observations. [illustration: fig. .--sketch from malpighi's treatise on the anatomy of plants ( ).] pictures of cells in the seventeenth century.--the sketches illustrating the microscopic observations of malpighi, leeuwenhoek, and grew show so many pictures of the cellular construction of plants that one who views them for the first time is struck with surprise, and might readily exclaim: "here in the seventeenth century we have the foundation of the cell-theory." but these drawings were merely faithful representations of the appearance of the fabric of plants; the cells were not thought of as uniform elements of organic architecture, and no theory resulted. it is true that malpighi understood that the cells were separable "utricles," and that plant tissue was the result of their union, but this was only an initial step in the direction of the cell-theory, which, as we shall see later, was founded on the supposed identity in development of cells in animals and plants. fig. shows a sketch, made by malpighi about , illustrating the microscopic structure of a plant. this is similar to the many drawings of grew and leeuwenhoek illustrating the structure of plant tissues. wolff.--nearly a century after the work of malpighi, we find wolff, in , proposing a theory regarding the organization of animals and plants based upon observations of their mode of development. he was one of the most acute scientific observers of the period, and it is to be noted that his conclusions regarding structure were all founded upon what he was able to see; while he gives some theoretical conclusions of a purely speculative nature, wolff was careful to keep these separate from his observations. the purpose of his investigations was to show that there was no pre-formation in the embryo; but in getting at the basis of this question, he worked out the identity of structure of plants and animals as shown by their development. in his famous publication on the theory of development (_theoria generationis_) he used both plants and animals. huxley epitomizes wolff's views on the development of elementary parts as follows: "every organ, he says, is composed at first of a little mass of clear, viscous, nutritive fluid, which possesses no organization of any kind, but is at most composed of globules. in this semifluid mass cavities (_bläschen_, _zellen_) are now developed; these, if they remain round or polygonal, become the subsequent cells; if they elongate, the vessels; and the process is identically the same, whether it is examined in the vegetating point of a plant, or in the young budding organs of an animal." wolff was contending against the doctrine of pre-formation in the embryo (see further under the chapter on embryology), but on account of his acute analysis he should be regarded, perhaps, as the chief forerunner of the founders of the cell-theory. he contended for the same method of development that was afterward emphasized by schleiden and schwann. through the opposition of the illustrious physiologist haller his work remained unappreciated, and was finally forgotten, until it was revived again in . we can not show that wolff's researches had any direct influence in leading schleiden and schwann to their announcement of the cell-theory. nevertheless, it stands, intellectually, in the direct line of development of that idea, while the views of haller upon the construction of organized beings are a side-issue. haller declared that "the solid parts of animals and vegetables have this fabric in common, that their elements are either fibers or unorganized concrete." this formed the basis of the fiber-theory, which, on account of the great authority of haller in physiology, occupied in the accumulating writings of anatomists a greater place than the views of wolff. bichat, although he is recognized as the founder of histology, made no original observations on the microscopic units of the tissues. he described very minutely the membranes in the bodies of animals, but did not employ the microscope in his investigations. oken.--in the work of the dreamer oken ( - ), the great representative of the german school of "_naturphilosophie_," we find, about , a very noteworthy statement to the effect that "animals and plants are throughout nothing else than manifoldly divided or repeated vesicles, as i shall prove anatomically at the proper time." this is apparently a concise statement of the cell-idea prior to schleiden and schwann; but we know that it was not founded on observation. oken, as was his wont, gave rein to his imagination, and, on his part, the idea was entirely theoretical, and amounted to nothing more than a lucky guess. haller's fiber-theory gave place in the last part of the eighteenth century to the theory that animals and plants are composed of globules and formless material, and this globular theory was in force up to the time of the great generalization of schleiden and schwann. it was well expounded by milne-edwards in , and now we can recognize that at least some of the globules which he described were the nucleated cells of later writers. the announcement of the cell-theory.--we are now approaching the time when the cell-theory was to be launched. during the first third of the nineteenth century there had accumulated a great mass of separate observations on the microscopic structure of both animals and plants. for several years botanists, in particular, had been observing and writing about cells, and interest in these structures was increasing. "we must clearly recognize the fact that for some time prior to the cell had come to be quite universally recognized as a constantly recurring element in vegetable and animal tissues, though little importance was attached to it as an element of organization, nor had its character been clearly determined" (tyson). then, in , came the "master-stroke in generalization" due to the combined labors of two friends, schleiden and schwann. but, although these two men are recognized as co-founders, they do not share honors equally; the work of schwann was much more comprehensive, and it was he who first used the term cell-theory, and entered upon the theoretical considerations which placed the theory before the scientific world. schleiden was educated as a lawyer, and began the practice of that profession, but his taste for natural science was so pronounced that when he was twenty-seven years old he deserted law, and went back to the university to study medicine. after graduating in medicine, he devoted himself mainly to botany. he saw clearly that the greatest thing needed for the advancement of scientific botany was a study of plant organization from the standpoint of development. accordingly he entered upon this work, and, in , arrived at a new view regarding the origin of plant cells. it must be confessed that this new view was founded on erroneous observations and conclusions, but it was revolutionary, and served to provoke discussion and to awaken observation. this was a characteristic feature of schleiden's influence upon botany. his work acted as a ferment in bringing about new activity. the discovery of the nucleus in plant cells by robert brown in was an important preliminary step to the work of schleiden, since the latter seized upon the nucleus as the starting-point of new cells. he changed the name of the nucleus to cytoblast, and supposed that the new cell started as a small clear bubble on one side of the nucleus, and by continued expansion grew into the cell, the nucleus, or cytoblast, becoming encased in the cell-wall. all this was shown by nägeli and other botanists to be wrong; yet, curiously enough, it was through the help of these false observations that schwann arrived at his general conclusions. schleiden was acquainted with schwann, and in october, , while the two were dining together, he told schwann about his observations and theories. he mentioned in particular the nucleus and its relationship to the other parts of the cell. schwann was immediately struck with the similarity between the observations of schleiden and certain of his own upon _animal_ tissues. together they went to his laboratory and examined the sections of the dorsal cord, the particular structure upon which schwann had been working. schleiden at once recognized the nuclei in this structure as being similar to those which he had observed in plants, and thus aided schwann to come to the conclusion that the elements in animal tissues were practically identical with those in plant tissues. schwann.--the personalities of the co-founders of the cell-theory are interesting. schwann was a man of gentle, pacific disposition, who avoided all controversies aroused by his many scientific discoveries. in his portrait (fig. ) we see a man whose striking qualities are good-will and benignity. his friend henle gives this description of him: "he was a man of stature below the medium, with a beardless face, an almost infantile and always smiling expression, smooth, dark-brown hair, wearing a fur-trimmed dressing-gown, living in a poorly lighted room on the second floor of a restaurant which was not even of the second class. he would pass whole days there without going out, with a few rare books around him, and numerous glass vessels, retorts, vials, and tubes, simple apparatus which he made himself. or i go in imagination to the dark and fusty halls of the anatomical institute where we used to work till nightfall by the side of our excellent chief, johann müller. we took our dinner in the evening, after the english fashion, so that we might enjoy more of the advantages of daylight." schwann drew part of his stimulus from his great master, johannes müller. he was associated with him as a student, first in the university of würzburg, where müller, with rare discernment for recognizing genius, selected schwann for especial favors and for close personal friendship. the influence of his long association with müller, the greatest of all trainers of anatomists and physiologists of the nineteenth century, must have been very uplifting. a few years later, schwann found himself at the university of berlin, where müller had been called, and he became an assistant in the master's laboratory. there he gained the powerful stimulus of constant association with a great personality. [illustration: fig. .--theodor schwann, - .] in , just after the publication of his work on the cell-theory, schwann was called to a professorship in the university of louvain, and after remaining there nine years, was transferred to the university of liège. he was highly respected in the university, and led a useful life, although after going to belgium he published only one work--that on the uses of the bile. he was recognized as an adept experimenter and demonstrator, and "clearness, order, and method" are designated as the characteristic qualities of his teaching. [illustration: fig. .--m. schleiden, - .] his announcement of the cell-theory was his most important work. apart from that his best-known contributions to science are: experiments upon spontaneous generation, his discovery of the "sheath of schwann," in nerve fibers, and his theory of fermentation as produced by microbes. schleiden.--schleiden (fig. ) was quite different in temperament from schwann. he did not have the fine self-control of schwann, but was quick to take up the gauntlet and enter upon controversies. in his caustic replies to his critics, he indulged in sharp personalities, and one is at times inclined to suspect that his early experience as a lawyer had something to do with his method of handling opposition. with all this he had correct ideas of the object of scientific study and of the methods to be used in its pursuit. he insisted upon observation and experiment, and upon the necessity of studying the development of plants in order to understand their anatomy and physiology. he speaks scornfully of the botany of mere species-making as follows: "most people of the world, even the most enlightened, are still in the habit of regarding the botanist as a dealer in barbarous latin names, as a man who gathers flowers, names them, dries them, and wraps them in paper, and all of whose wisdom consists in determining and classifying this hay which he has collected with such great pains." although he insisted on correct methods, his ardent nature led him to champion conclusions of his own before they were thoroughly tested. his great influence in the development of scientific botany lay in his earnestness, his application of new methods, and his fearlessness in drawing conclusions, which, although frequently wrong, formed the starting-point of new researches. let us now examine the original publications upon which the cell-theory was founded. schleiden's contribution.--schleiden's paper was particularly directed to the question, how does the cell originate? and was published in müller's _archiv_, in , under the german title of _ueber phytogenesis_. as stated above, the cell had been recognized for some years, but the question of its origin had not been investigated. schleiden says: "i may omit all historical introduction, for, so far as i am acquainted, no direct observations exist at present upon the development of the cells of plants." he then goes on to define his view of the nucleus (cytoblast) and of the development of the cell around it, saying: "as soon as the cytoblasts have attained their full size, a delicate transparent vesicle arises upon their surface. this is the young cell." as to the position of the nucleus in the fully developed cell, he is very explicit: "it is evident," he says, "from the foregoing that the cytoblast can never lie free in the interior of the cell, but is always enclosed in the cell-wall," etc. schleiden fastened these errors upon the cell-theory, since schwann relied upon his observations. on another point of prime importance schleiden was wrong: he regarded all new cell-formation as the formation of "cells within cells," as distinguished from cell-division, as we now know it to take place. schleiden made no attempt to elaborate his views into a comprehensive cell-theory, and therefore his connection as a co-founder of this great generalization is chiefly in paving the way and giving the suggestion to schwann, which enabled the latter to establish the theory. schleiden's paper occupies some thirty-two pages, and is illustrated by two plates. he was thirty-four years old when this paper was published, and directly afterward was called to the post of adjunct professor of botany in the university of jena, a position which with promotion to the full professorship he occupied for twenty-three years. schwann's treatise.--in , schwann also announced his cell-theory in a concise form in a german scientific periodical, and, later, to the paris academy of sciences; but it was not till that the fully illustrated account was published. this treatise with the cumbersome title, "microscopical researches into the accordance in the structure and growth of animals and plants" (_mikroscopische untersuchungen über die uebereinstimmung in der structur und dem wachsthum der thiere und pflanzen_) takes rank as one of the great classics in biology. it fills octavo pages, and is illustrated with four plates. "the purpose of his researches was to prove the identity of structure, as shown by their development, between animals and plants." this is done by direct comparisons of the elementary parts in the two kingdoms of organic nature. his writing in the "microscopical researches" is clear and philosophical, and is divided into three sections, in the first two of which he confines himself strictly to descriptions of observations, and in the third part of which he enters upon a philosophical discussion of the significance of the observations. he comes to the conclusion that "the elementary parts of all tissues are formed of cells in an analogous, though very diversified manner, so that it may be asserted that there is one universal principle of development for the elementary parts of organisms, however different, and that this principle is the formation of cells." it was in this treatise also that he made use of the term cell-theory, as follows: "the development of the proposition that there exists one general principle for the formation of all organic productions, and that this principle is the formation of cells, as well as the conclusions which may be drawn from this proposition, may be comprised under the term _cell-theory_, using it in its more extended signification, while, in a more limited sense, by the theory of cells we understand whatever may be inferred from this proposition with respect to the powers from which these phenomena result." one comes from the reading of these two contributions to science with the feeling that it is really schwann's cell-theory, and that schleiden helped by lighting the way that his fellow-worker so successfully trod. modification of the cell-theory.--the form in which the cell-theory was given to the world by schleiden and schwann was very imperfect, and, as already pointed out, it contained fundamental errors. the founders of the theory attached too much importance to the cell-wall, and they described the cell as a hollow cavity bounded by walls that were formed around a nucleus. they were wrong as to the mode of the development of the cell, and as to its nature. nevertheless, the great truth that all parts of animals and plants are built of similar units or structures was well substantiated. this remained a permanent part of the theory, but all ideas regarding the nature of the units were profoundly altered. in order to perceive the line along which the chief modifications were made we must take account of another scientific advance of about the same period. this was the discovery of protoplasm, an achievement which takes rank with the advances of greatest importance in biology, and has proved to be one of the great events of the nineteenth century. the discovery of protoplasm and its effect on the cell-theory.--in , before the announcement of the cell-theory, living matter had been observed by dujardin. in lower animal forms he noticed a semifluid, jelly-like substance, which he designated sarcode, and which he described as being endowed with all the qualities of life. the same semifluid substance had previously caught the attention of some observers, but no one had as yet announced it as the actual living part of organisms. schleiden had seen it and called it gum. dujardin was far from appreciating the full importance of his discovery, and for a long time his description of sarcode remained separate; but in hugo von mohl, a botanist, observed a similar jelly-like substance in plants, which he called plant _schleim_, and to which he attached the name protoplasma. the scientific world was now in the position of recognizing living substance, which had been announced as sarcode in lower animals, and as protoplasm in plants; but there was as yet no clear indication that these two substances were practically identical. gradually there came stealing into the minds of observers the suspicion that the sarcode of the zoölogists and the protoplasm of the botanists were one and the same thing. this proposition was definitely maintained by cohn in , though with him it was mainly theoretical, since his observations were not sufficiently extensive and accurate to support such a conclusion. eleven years later, however, as the result of extended researches, max schultze promulgated, in , the protoplasm doctrine, to the effect that the units of organization consist of little masses of protoplasm surrounding a nucleus, and that this protoplasm, or living substance, is practically identical in both plants and animals. the effect of this conclusion upon the cell-theory was revolutionary. during the time protoplasm was being observed the cell had likewise come under close scrutiny, and naturalists had now an extensive collection of facts upon which to found a theory. it has been shown that many animal cells have no cell-wall, and the final conclusion was inevitable that the essential part of a cell is the semifluid living substance that resides within the cavity when a cell-wall is present. moreover, when the cell-wall is absent, the protoplasm is the "cell." the position of the nucleus was also determined to be within the living substance, and not, as schleiden had maintained, within the cell-wall. the definition of max schultze, that a cell is a globule of protoplasm surrounding a nucleus, marks a new era in the cell-theory, in which the original generalization became consolidated with the protoplasm doctrine. further modifications of the cell-theory.--the reformed cell-theory was, however, destined to undergo further modification, and to become greatly extended in its application. at first the cell was regarded merely as an element of structure; then, as a supplement to this restricted view, came the recognition that it is also a unit of physiology, _viz._, that all physiological activities take place within the cell. matters did not come to a rest, however, with the recognition of these two fundamental aspects of the cell. the importance of the cell in development also took firmer hold upon the minds of anatomists after it was made clear that both the egg and its fertilizing agents are modified cells of the parent's body. it was necessary to comprehend this fact in order to get a clear idea of the origin of cells within the body of a multicellular organism, and of the relation between the primordial element and the fully developed tissues. finally, when observers found within the nucleus the bearers of hereditary qualities, they began to realize that a careful study of the behavior of the cell elements during development is necessary for the investigation of hereditary transmissions. a statement of the cell-theory at the present time, then, must include these four conceptions: the cell as a unit of structure, the cell as a unit of physiological activity, the cell as embracing all hereditary qualities within its substance, and the cell in the historical development of the organism. some of these relations may now be more fully illustrated. origin of tissues.--the egg in which all organisms above the very lowest begin, is a single cell having, under the microscope, the appearance shown in fig. . after fertilization, this divides repeatedly, and many cohering cells result. the cells are at first similar, but as they increase in number, and as development proceeds, they grow different, and certain groups are set apart to perform particular duties. the division of physiological labor which arises at this time marks the beginning of separate tissues. it has been demonstrated over and over that all tissues are composed of cells and cell-products, though in some instances they are much modified. the living cells can be seen even in bone and cartilage, in which they are separated by a lifeless matrix, the latter being the product of cellular activity. [illustration: fig. .--the egg and early stages in its development. (after gegenbaur.)] fig. shows a stage in the development of one of the mollusks just as the differentiation of cells has commenced. the nucleus.--to the earlier observers the protoplasm appeared to be a structureless, jelly-like mass containing granules and vacuoles; but closer acquaintance with it has shown that it is in reality very complex in structure as well as in chemical composition. it is by no means homogeneous; adjacent parts are different in properties and aptitudes. the nucleus, which is more readily seen than other cell elements, was shown to be of great importance in cell-life--to be a structure which takes the lead in cell division, and in general dominates the rest of the protoplasm. chromosomes.--after dyes came into use for staining the protoplasm ( ), it became evident that certain parts of it stain deeply, while other parts stain faintly or not at all. this led to the recognition of protoplasm as made up of a densely staining portion called _chromatin_, and a faintly staining portion designated _achromatin_. this means of making different parts of protoplasm visible under the microscope led to important results, as when, in , it was discovered that the nucleus contains a definite number of small (usually rod-shaped) bodies, which become evident during nuclear division, and play a wonderful part in that process. these bodies take the stain more deeply than other components of the nucleus, and are designated _chromosomes_. [illustration: fig. .--an early stage in the development of the egg of a rock-limpet. (after conklin.)] attention having been directed to these little bodies, continued observations showed that, although they vary in number--commonly from two to twenty-four--in different parts of animals and plants, they are, nevertheless, of the same number in all the cells of any particular plant or animal. as a conclusion to this kind of observation, it needs to be said that the chromosomes are regarded as the actual bearers of hereditary qualities. the chromosomes do not show in resting-stages of the nucleus; their substance is present, but is not aggregated into the form of chromosomes. [illustration: fig. .--highly magnified tissue cells from the skin of a salamander in an active state of growth. dividing cells with chromosomes are shown at _a_, _b_, and _c_,. (after wilson.)] fig. shows tissue cells, some of which are in the dividing and others in the resting-stage. the nuclei in process of division exhibit the rod-like chromosomes, as shown at _a_, _b_, and _c_. [illustration: fig. .--diagram of the chief steps in cell-division. (after parker as altered from fleming.)] centrosome.--the discovery ( ) of a minute spot of deeply staining protoplasm, usually just outside the nuclear membrane, is another illustration of the complex structure of the cell. although the centrosome, as this spot is called, has been heralded as a dynamic agent, there is not complete agreement as to its purpose, but its presence makes it necessary to include it in the definition of a cell. the cell in heredity.--the problems of inheritance, in so far as they can be elucidated by structural studies, have come to be recognized as problems of cellular life. but we cannot understand what is implied by this conclusion without referring to the behavior of the chromosomes during cell-division. this is a very complex process, and varies somewhat in different tissues. we can, however, with the help of fig. , describe what takes place in a typical case. the nucleus does not divide directly, but the chromosomes congregate around the equator of a spindle (_d_) formed from the achromatin; they then undergo division lengthwise, and migrate to the poles (_e_, _f_, _g_), after which a partition wall is formed dividing the cell. this manner of division of the chromosomes secures an equable partition of the protoplasm. in the case of fertilized eggs, one-half of the chromosomes are derived from the sperm and one-half from the egg. each cell thus contains hereditary substance derived from both maternal and paternal nuclei. this is briefly the basis for regarding inheritance as a phenomenon of cell-life. [illustration: fig. .--diagram of a cell. (modified after wilson.)] a diagram of the cell as now understood (fig. ) will be helpful in showing how much the conception of the cell has changed since the time of schleiden and schwann. definition.--the definition of verworn, made in , may be combined with this diagram: a cell is "a body consisting essentially of protoplasm in its general form, including the unmodified cytoplasm, and the specialized nucleus and centrosome; while as unessential accompaniments may be enumerated: ( ) the cell membrane, ( ) starch grains, ( ) pigment granules, ( ) oil globules, and ( ) chlorophyll granules." no definition can include all variations, but the one quoted is excellent in directing attention to the essentials--to protoplasm in its general form, and the modified protoplasmic parts as distinguished from the unessential accompaniments, as cell membrane and cell contents. the definition of verworn was reached by a series of steps representing the historical advance of knowledge regarding the cell. schleiden and schwann looked upon the cell as a hollow chamber having a cell-wall which had been formed around the nucleus; it was a great step when schultze defined the cell in terms of living substance as "a globule of protoplasm surrounding a nucleus," and it is a still deeper level of analysis which gives us a discriminating definition like that of verworn. when we are brought to realize that, in large part, the questions that engage the mind of the biologist have their basis in the study of cells, we are ready to appreciate the force of the statement that the establishment of the cell-theory was one of the great events of the nineteenth century, and, further, that it stands second to no theory, with the single exception of that of organic evolution, in advancing biological science. chapter xii protoplasm, the physical basis of life the recognition of the rôle that protoplasm plays in the living world was so far-reaching in its results that we take up for separate consideration the history of its discovery. although it is not yet fifty years since max schultze established the protoplasm doctrine, it has already had the greatest influence upon the progress of biology. to the consideration of protoplasm in the previous chapter should be added an account of the conditions of its discovery, and of the personality and views of the men whose privilege it was to bring the protoplasm idea to its logical conclusion. before doing so, however, we shall look at the nature of protoplasm itself. protoplasm.--this substance, which is the seat of all vital activity, was designated by huxley "the physical basis of life," a graphic expression which brings before the mind the central fact that life is manifested in a material substratum by which it is conditioned. all that biologists have been able to discover regarding life has been derived from the observation of that material substratum. it is not difficult, with the help of a microscope, to get a view of protoplasmic activity, and that which was so laboriously made known about is now shown annually to students beginning biology. inasmuch as all living organisms contain protoplasm, one has a wide range of choice in selecting the plant or the animal upon which to make observations. we may, for illustration, take one of the simplest of animal organisms, the amoeba, and place it under the high powers of the microscope. this little animal consists almost entirely of a lump of living jelly. within the living substance of which its body is composed all the vital activities characteristic of higher animals are going on, but they are manifested in simpler form. these manifestations differ only in degree of development, not in kind, from those we see in bodies of higher organisms. we can watch the movements in this amoeba, determine at first hand its inherent qualities, and then draw up a sort of catalogue of its vital properties. we notice an almost continual flux of the viscid substance, by means of which it is able to alter its form and to change its position. this quality is called that of contractility. in its essential nature it is like the protoplasmic movement that takes place in a contracting muscle. we find also that the substance of the amoeba responds to stimulations--such as touching it with a bristle, or heating it, or sending through it a light electric shock. this response is quite independent of the contractility, and by physiologists is designated the property of being irritable. by further observations one may determine that the substance of the amoeba is receptive and assimilative, that it is respiratory, taking in oxygen and giving off carbonic dioxide, and that it is also secretory. if the amoeba be watched long enough, it may be seen to undergo division, thus producing another individual of its kind. we say, therefore, that it exhibits the power of reproduction. all these properties manifested in close association in the amoeba are exhibited in the bodies of higher organisms in a greater degree of perfection, and also in separation, particular organs often being set apart for the performance of one of these particular functions. we should, however, bear in mind that in the simple protoplasm of the amoeba is found the germ of all the activities of the higher animals. it will be convenient now to turn our attention to the microscopic examination of a plant that is sufficiently transparent to enable us to look within its living parts and observe the behavior of protoplasm. the first thing that strikes one is the continual activity of the living substance within the boundaries of a particular cell. this movement sometimes takes the form of rotation around the walls of the cell (fig. _a_). in other instances the protoplasm marks out for itself new paths, giving a more complicated motion, called circulation (fig. _b_). these movements are the result of chemical changes taking place within the protoplasm, and they are usually to be observed in any plant or animal organism. [illustration: fig. .--(_a_) rotation of protoplasm in the cells of nitella. (_b_) highly magnified cell of a tradescantia plant, showing circulation of protoplasm. (after sedgwick and wilson.)] under the most favorable conditions these movements, as seen under the microscope, make a perfect torrent of unceasing activity, and introduce us to one of the wonderful sights of which students of biology have so many. huxley (with slight verbal alterations) says: "the spectacle afforded by the wonderful energies imprisoned within the compass of the microscopic cell of a plant, which we commonly regard as a merely passive organism, is not easily forgotten by one who has watched its movement hour by hour without pause or sign of weakening. the possible complexity of many other organisms seemingly as simple as the protoplasm of the plant just mentioned dawns upon one, and the comparison of such activity to that of higher animals loses much of its startling character. currents similar to these have been observed in a great multitude of very different plants, and it is quite uniformly believed that they occur in more or less perfection in all young vegetable cells. if such be the case, the wonderful noonday silence of a tropical forest is due, after all, only to the dullness of our hearing, and could our ears catch the murmur of these tiny maelstroms as they whirl in the innumerable myriads of living cells that constitute each tree, we should be stunned as with the roar of a great city." the essential steps in recognizing the likeness of protoplasm in plants and animals dujardin.--this substance, of so much interest and importance to biologists, was first clearly described and distinguished from other viscid substance, as albumen, by félix dujardin in . both the substance and the movements therein had been seen and recorded by others: by rösel von rosenhof in in the proteus animalcule; again in by corti in chara; by mayen in in vallisnieria; and in by robert brown in tradescantia. one of these records was for the animal kingdom, and three were for plants. the observations of dujardin, however, were on a different plane from those of the earlier naturalists, and he is usually credited with being the discoverer of protoplasm. his researches, moreover, were closely connected with the development of the ideas regarding the rôle played in nature by this living substance. dujardin was a quiet modest man, whose attainments and service to the progress of biology have usually been under-rated. he was born in at tours, and died in at rennes. being descended from a race of watchmakers, he received in his youth a training in that craft which cultivated his natural manual dexterity, and, later, this assisted him in his manipulations of the microscope. he had a fondness for sketching, and produced some miniatures and other works of art that showed great merit. his use of colors was very effective, and in he went to paris for the purpose of perfecting himself in painting, and with the intention of becoming an artist. the small financial returns, however, "led him to accept work as an engineer directing the construction of hydraulic work in sédan." he had already shown a love for natural science, and this led him from engineering into work as a librarian and then as a teacher. he made field observations in geology and botany, and commenced publication in those departments of science. about he began to devote his chief efforts to microscopic work, toward which he had a strong inclination, and from that time on he became a zoölogist, with a steadily growing recognition for high-class observation. besides his technical scientific papers, he wrote in a popular vein to increase his income. among his writings of this type may be mentioned as occupying high rank his charmingly written "rambles of a naturalist" (_promenades d'un naturaliste_, ). by he had established such a good record as a scientific investigator that he was called to the newly founded university of rennes as dean of the faculty. he found himself in an atmosphere of jealous criticism, largely on account of his being elevated to the station of dean, and after two years of discomfort he resigned the deanship, but retained his position as a professor in the university. he secured a residence in a retired spot near a church, and lived there simply. in his leisure moments he talked frequently with the priests, and became a devout catholic. his contributions to science cover a wide range of subjects. in his microscopic work he discovered the rhizopods in , and the study of their structure gave him the key to that of the other protozoa. in he visited the mediterranean, where he studied the oceanic foraminifera, and demonstrated that they should be grouped with the protozoa, and not, as had been maintained up to that time, with the mollusca. it was during the prosecution of these researches that he made the observations upon sarcode that are of particular interest to us. his natural history of the infusoria ( ) makes a volume of pages, full of original observations and sketches. he also invented a means of illumination for the microscope, and wrote a manual of microscopic observation. among the ninety-six publications of dujardin listed by professor joubin there are seven general works, twenty relating to the protozoa, twenty-four to geology, three to botany, four to physics, twenty-five to arthropods, eight to worms, etc., etc. but as joubin says: "the great modesty of dujardin allowed him to see published by others, without credit to himself, numerous facts and observations which he had established." this failure to assert his claims accounts in part for the inadequate recognition that his work has received. [illustration: fig. .--félix dujardin, - .] no portrait of dujardin was obtainable prior to . somewhat earlier professor joubin, who succeeded other occupants of the chair which dujardin held in the university of rennes, found in the possession of his descendants a portrait, which he was permitted to copy. the earliest reproduction of this picture to reach this country came to the writer through the courtesy of professor joubin, and a copy of it is represented in fig. . his picture bespeaks his personality. the quiet refinement and sincerity of his face are evident. professor joubin published, in (_archives de parasitologie_), a biographical sketch of dujardin, with several illustrations, including this portrait and another one which is very interesting, showing him in academic costume. thanks to the spread of information of the kind contained in that article, dujardin is coming into wider recognition, and will occupy the historical position to which his researches entitle him. it was while studying the protozoa that he began to take particular notice of the substance of which their bodies are composed; and in he described it as a living jelly endowed with all the qualities of life. he had seen the same jelly-like substance exuding from the injured parts of worms, and recognized it as the same material that makes the body of protozoa. he observed it very carefully in the ciliated infusoria--in paramoecium, in vorticella, and other forms, but he was not satisfied with mere microscopic observation of its structure. he tested its solubility, he subjected it to the action of alcohol, nitric acid, potash, and other chemical substances, and thereby distinguished it from albumen, mucus, gelatin, etc. inasmuch as this substance manifestly was soft, dujardin proposed for it the name of sarcode, from the greek, meaning _soft_. thus we see that the substance protoplasm was for the first time brought very definitely to the attention of naturalists through the study of animal forms. for some time it occupied a position of isolation, but ultimately became recognized as being identical with a similar substance that occurs in plants. at the time of dujardin's discovery, sarcode was supposed to be peculiar to lower animals; it was not known that the same substance made the living part of all animals, and it was owing mainly to this circumstance that the full recognition of its importance in nature was delayed. the fact remains that the first careful studies upon sarcode were due to dujardin, and, therefore, we must include him among the founders of modern biology. [illustration: fig. .-purkinje, - .] purkinje.--the observations of the bohemian investigator purkinje ( - ) form a link in the chain of events leading up to the recognition of protoplasm. although purkinje is especially remembered for other scientific contributions, he was the first to make use of the name protoplasm for living matter, by applying it to the formative substance within the eggs of animals and within the cells of the embryo. his portrait is not frequently seen, and, therefore, is included here (fig. ), to give a more complete series of pictures of the men who were directly connected with the development of the protoplasm idea. purkinje was successively a professor in the universities of breslau and prague. his anatomical laboratory at breslau is notable as being one of the earliest ( ) open to students. he went to prague in as professor of physiology. [illustration: fig. .--carl nägeli, - .] von mohl.--in , eleven years after the discovery of dujardin, the eminent botanist hugo von mohl ( - ) designated a particular part of the living contents of the vegetable cell by the term protoplasma. the viscid, jelly-like substance in plants had in the mean time come to be known under the expressive term of plant "_schleim_." he distinguished the firmer mucilaginous and granular constituent, found just under the cell membrane, from the watery cell-sap that occupies the interior of the cell. it was to the former part that he gave the name protoplasma. previous to this, the botanist nägeli had studied this living substance, and perceived that it was nitrogenous matter. this was a distinct step in advance of the vague and indefinite idea of schleiden, who had in reality noticed protoplasm in , but thought of it merely as gum. the highly accomplished investigator nägeli (fig. ) made a great place for himself in botanical investigation, and his name is connected with several fundamental ideas of biology. to von mohl, however, belongs the credit of having brought the word protoplasm into general use. he stands in the direct line of development, while purkinje, who first employed the word protoplasm, stands somewhat aside, but his name, nevertheless, should be connected with the establishment of the protoplasm doctrine. [illustration: fig. .--hugo von mohl, - .] von mohl (fig. ) was an important man in botany. early in life he showed a great love for natural science, and as in his day medical instruction afforded the best opportunities for a man with scientific tastes, he entered upon that course of study in tübingen at the age of eighteen. he took his degree of doctor of medicine in , and spent several years in munich. he became professor of physiology in bern in , and three years later was transferred to tübingen as professor of botany. here he remained to the end of his life, refusing invitations to institutions elsewhere. he never married, and, without the cares and joys of a family, led a solitary and uneventful life, devoted to botanical investigation. cohn.--after von mohl's studies on "plant schleim" there was a general movement toward the conclusion that the sarcode of the zoölogists and the protoplasm of the botanists were one and the same substance. this notion was in the minds of more than one worker, but it is perhaps to ferdinand cohn ( - ) that the credit should be given for bringing the question to a head. after a study of the remarkable movements of the active spores of one of the simplest plants (protococcus), he said that vegetable protoplasm and animal sarcode, "if not identical, must be, at any rate, in the highest degree analogous substances" (geddes). cohn (fig. ) was for nearly forty years professor of botany in the university of breslau, and during his long life as an investigator greatly advanced the knowledge of bacteria. his statement referred to above was made when he was twenty-two years of age, and ran too far ahead of the evidence then accumulated; it merely anticipated the coming period of the acceptance of the conclusion in its full significance. [illustration: fig. .--ferdinand cohn, - .] de bary.--we find, then, in the middle years of the nineteenth century the idea launched that sarcode and protoplasm are identical, but it was not yet definitely established that the sarcode of lower animals is the same as the living substance of the higher ones, and there was, therefore, lacking an essential factor to the conclusion that there is only one general form of living matter in all organisms. it took another ten years of investigation to reach this end. the most important contributions from the botanical side during this period were the splendid researches of de bary (fig. ) on the myxomycetes, published in . here the resemblance between sarcode and protoplasm was brought out with great clearness. the myxomycetes are, in one condition, masses of vegetable protoplasm, the movements and other characteristics of which were shown to resemble strongly those of the protozoa. de bary's great fame as a botanist has made his name widely known. [illustration: fig. .--heinrich a. de bary, - .] in virchow also, by his extensive studies in the pathology of living cells, added one more link to the chain that was soon to be recognized as encircling the new domain of modern biology. [illustration: fig. .--max schultze, - .] schultze.--as the culmination of a long period of work, max schultze, in , placed the conception of the identity between animal sarcode and vegetable protoplasm upon an unassailable basis, and therefore he has received the title of "the father of modern biology." he showed that sarcode, which was supposed to be confined to the lower invertebrates, is also present in the tissues of higher animals, and there exhibits the same properties. the qualities of contractility and irritability were especially indicated. it was on physiological likeness, rather than on structural grounds, that he formed his sweeping conclusions. he showed also that sarcode agreed in physiological properties with protoplasm in plants, and that the two living substances were practically identical. his paper of considers the living substance in muscles (_ueber muskelkörperchen und das was man eine zelle zu nennen habe_), but in this he had been partly anticipated by ecker who, in , compared the "formed contractile substance" of muscles with the "unformed contractile substance" of the lower types of animal life (geddes). the clear-cut, intellectual face of schultze (fig. ) is that of an admirable man with a combination of the artistic and the scientific temperaments. he was greatly interested in music from his youth up, and by the side of his microscope was his well-beloved violin. he was some time professor in the university of halle, and in went to bonn as professor of anatomy and director of the anatomical institute. his service to histology has already been spoken of (chapter viii). this astute observer will have an enduring fame in biological science, not only for the part he played in the development of the protoplasm idea, but also on account of other extensive labors. in he founded the leading periodical in microscopic anatomy, the _archiv für mikroscopische anatomie_. this periodical was continued after the untimely death of schultze in , and to-day is one of the leading biological periodicals. it is easy, looking backward, to observe that the period between and was a very important one for modern biology. many new ideas were coming into existence, but through this period we can trace distinctly, step by step, the gradual approach to the idea that protoplasm, the living substance of organism, is practically the same in plants and in animals. let us picture to ourselves the consequences of the acceptance of this idea. now for the first time physiologists began to have their attention directed to the actually living substance; now for the first time they saw clearly that all future progress was to be made by studying this living substance--the seat of vital activity. this was the beginning of modern biology. protoplasm is the particular object of study for the biologist. to observe its properties, to determine how it behaves under different conditions, how it responds to stimuli and natural agencies, to discover the relation of the internal changes to the outside agencies: these, which constitute the fundamental ideas of biology, were for the first time brought directly to the attention of the naturalist, about the year --that epoch-making time when appeared darwin's _origin of species_ and spencer's _first principles_. chapter xiii the work of pasteur, koch, and others the knowledge of bacteria, those minutest forms of life, has exerted a profound influence upon the development of general biology. there are many questions relating to bacteria that are strictly medical, but other phases of their life and activities are broadly biological, and some of those broader aspects will next be brought under consideration. the bacteria were first described by leeuwenhoek in , twelve years after his discovery of the microscopic animalcula now called protozoa. they are so infinitesimal in size that under his microscope they appeared as mere specks, and, naturally, observation of these minute organisms was suspended until nearly the middle of the nineteenth century, after the improvement of microscope lenses. it is characteristic of the little knowledge of bacteria in linnæus's period that he grouped them into an order, with other microscopic forms, under the name _chaos_. at first sight, the bacteria appear too minute to figure largely in human affairs, but a great department of natural science--bacteriology--has been opened by the study of their activities, and it must be admitted that the development of the science of bacteriology has been of great practical importance. the knowledge derived from experimental studies of the bacteria has been the chief source of light in an obscure domain which profoundly affects the well-being of mankind. to the advance of such knowledge we owe the germ-theory of disease and the ability of medical men to cope with contagious diseases. the three greatest names connected with the rise of bacteriology are those of pasteur, koch, and lister, the results of whose labors will be considered later. among the general topics which have been clustered around the study of bacteria we take up, first, the question of the spontaneous origin of life. the spontaneous origin of life it will be readily understood that the question of the spontaneous generation of life is a fundamental one for the biologist. does life always arise from previously existing life, or under certain conditions is it developed spontaneously? is there, in the inorganic world, a happy concourse of atoms that become chained together through the action of the sun's rays and other natural forces, so that a molecule of living matter is constructed in nature's laboratory without contact or close association with living substance? this is a question of _biogenesis_--life from previous life--or of _abiogenesis_--life without preëxisting life or from inorganic matter alone. it is a question with a long history. its earliest phases do not involve any consideration of microscopic forms, since they were unknown, but its middle and its modern aspect are concerned especially with bacteria and other microscopic organisms. the historical development of the problem may be conveniently considered under three divisions: i. the period from aristotle, b.c., to the experiments of redi, in ; ii. from the experiments of redi to those of schulze and schwann in and ; iii. the modern phase, extending from pouchet's observations in to the present. i. from aristotle to redi.--during the first period, the notion of spontaneous generation was universally accepted, and the whole question of spontaneous origin of life was in a crude and grotesque condition. it was thought that frogs and toads and other animals arose from the mud of ponds and streams through the vivifying action of the sun's rays. rats were supposed to come from the river nile, the dew was supposed to give origin to insects, etc. the scientific writers of this period had little openness of mind, and they indulged in scornful and sarcastic comments at the expense of those who doubted the occurrence of spontaneous generation. in the seventeenth century alexander ross, commenting on sir thomas brown's doubt as to whether mice may be bred by putrefaction, flays his antagonist in the following words: "so may we doubt whether in cheese and timber worms are generated, or if beetles and wasps in cow-dung, or if butterflies, locusts, shell-fish, snails, eels, and such life be procreated of putrefied matter, which is to receive the form of that creature to which it is by formative power disposed. to question this is to question reason, sense, and experience. if he doubts this, let him go to egypt, and there he will find the fields swarming with mice begot of the mud of nylus, to the great calamity of the inhabitants." ii. from redi to schwann.--the second period embraces the experimental tests of redi ( ), spallanzani ( ), and schwann ( )--notable achievements that resulted in a verdict for the adherents to the doctrine of biogenesis. here the question might have rested had it not been opened upon theoretical ground by pouchet in . the first experiments.--the belief in spontaneous generation, which was so firmly implanted in the minds of naturalists, was subjected to an experimental test in by the italian redi. it is a curious circumstance, but one that throws great light upon the condition of intellectual development of the period, that no one previous to redi had attempted to test the truth or falsity of the theory of spontaneous generation. to approach this question from the experimental side was to do a great service to science. the experiments of redi were simple and homely. he exposed meat in jars, some of which were left uncovered, some covered with parchment, and others with fine wire gauze. the meat in all these vessels became spoiled, and flies, being attracted by the smell of decaying meat, laid eggs in that which was exposed, and there came from it a large crop of maggots. the meat which was covered by parchment also decayed in a similar manner, without the appearance of maggots within it; and in those vessels covered by wire netting the flies laid their eggs upon the wire netting. there they hatched, and the maggots, instead of appearing in the meat, appeared on the surface of the wire gauze. from this redi concluded that maggots arise in decaying meat from the hatching of the eggs of insects, but inasmuch as these animals had been supposed to arise spontaneously within the decaying meat, the experiment took the ground from under that hypothesis. he made other observations on the generation of insects, but with acute scientific analysis never allowed his conclusions to run ahead of his observations. he suggested, however, the probability that all cases of the supposed production of life from dead matter were due to the introduction of living germs from without. the good work begun by redi was confirmed and extended by swammerdam ( - ) and vallisnieri ( - ), until the notion of the spontaneous origin of any forms of life visible to the unaided eye was banished from the minds of scientific men. [illustration: fig. .--francesco redi, - .] redi (fig. ) was an italian physician living in arentino, distinguished alike for his attainments in literature and for his achievements in natural science. he was medical adviser to two of the grand dukes of tuscany, and a member of the academy of crusca. poetry as well as other literary compositions shared his time with scientific occupations. his collected works, literary, scientific, and medical, were published in nine octavo volumes in milan, - . this collection includes his life and letters, and embraces one volume of sonnets. the book that has been referred to as containing his experiments was entitled _esperienze intorno alla generazione degl'insetti_, and first saw the light in quarto form in florence in . it went through five editions in twenty years. some of the volumes were translated into latin, and were published in miniature, making books not more than four inches high. huxley says: "the extreme simplicity of his experiments, and the clearness of his arguments, gained for his views and for their consequences almost universal acceptance." new form of the question.--the question of the spontaneous generation of life was soon to take on a new aspect. seven years after the experiments of redi, leeuwenhoek made known a new world of microscopic organisms--the infusoria--and, as we have seen, he discovered, in , those still minuter forms, the bacteria. strictly speaking, the bacteria, on account of their extreme minuteness, were lost sight of, but spontaneous generation was evoked to account for the birth of all microscopic organisms, and the question circled mainly around the infusorial animalcula. while the belief in the spontaneous generation of life among forms visible to the unaided eye had been surrendered, nevertheless doubts were entertained as to the origin of microscopic organisms, and it was now asserted that here were found the beginnings of life--the place where inorganic material was changed through natural agencies into organized beings microscopic in size. more than seventy years elapsed before the matter was again subjected to experimental tests. then needham, using the method of redi, began to experiment on the production of microscopic animalcula. in many of his experiments he was associated with buffon, the great french naturalist, who had a theory of organic molecules that he wished to sustain. needham ( - ), a priest of the catholic faith, was an englishman living on the continent; he was for many years director of the academy of maria theresa at brussels. he engaged in scientific investigations in connection with his work of teaching. the results of needham's first experiments were published in . these experiments were conducted by extracting the juices of meat by boiling; by then enclosing the juices in vials, the latter being carefully corked and sealed with mastic; by subjecting the sealed bottles, finally, to heat, and setting them away to cool. in due course of time, the fluids thus treated became infected with microscopic life, and, inasmuch as needham believed that he had killed all living germs by repeated heating, he concluded that the living forms had been produced by spontaneous generation. spallanzani.--the epoch-making researches of spallanzani, a fellow-countryman of redi, were needed to point out the error in needham's conclusions. spallanzani (fig. ) was one of the most eminent men of his time. he was educated for the church, and, therefore, he is usually known under the title of abbé spallanzani. he did not, however, actively engage in his churchly offices, but, following an innate love of natural science and of investigation, devoted himself to experiments and researches and to teaching. he was first a professor at bologna, and afterward at the university of pavia. he made many additions to knowledge of the development and the physiology of organisms, and he was the first to make use of glass flasks in the experimental study of the question of the spontaneous generation of life. spallanzani thought that the experiments of needham had not been conducted with sufficient care and precision; accordingly, he made use of glass flasks with slender necks which could be hermetically sealed after the nutrient fluids had been introduced. the vials which needham used as containers were simply corked and sealed with mastic, and it was by no means certain that the entrance of air after heating had been prevented; moreover, no record was made by needham of the temperature and the time of heating to which his bottles and fluids had been subjected. [illustration: fig. .--lazzaro spallanzani, - .] spallanzani took nutrient fluids, such as the juices of vegetables and meats which had been extracted by boiling, placed them in clear flasks, the necks of which were hermetically sealed in flame, and afterward immersed them in boiling water for three-quarters of an hour, in order to destroy all germs that might be contained in them. the organic infusions of spallanzani remained free from change. it was then, as now, a well-known fact that organic fluids, when exposed to air, quickly decompose and acquire a bad smell; they soon become turbid, and in a little time a scum is formed upon their surface. the fluids in the flasks of spallanzani remained of the same appearance and consistency as when they were first introduced into the vessel, and the obvious conclusion was drawn that microscopic life is not spontaneously formed within nutrient fluids. "but needham was not satisfied with these results, and with a show of reason maintained that such a prolonged boiling would destroy not only germs, but the germinative, or, as he called it, the 'vegetative force' of the infusion itself. spallanzani easily disposed of this objection by showing that when the infusions were again exposed to the air, no matter how severe or prolonged the boiling to which they had been subjected, the infusoria reappeared. his experiments were made in great numbers, with different infusions, and were conducted with the utmost care and precision" (dunster). it must be confessed, however, that the success of his experiments was owing largely to the purity of the air in which he worked, the more resistant atmospheric germs were not present: as wyman showed, long afterward, that germs may retain their vitality after being subjected for several hours to the temperature of boiling water. schulze and schwann.--the results of spallanzani's experiments were published in , and were generally regarded by the naturalists of that period as answering in the negative the question of the spontaneous generation of life. doubts began to arise as to the conclusive nature of spallanzani's experiments, on account of the discovery of the part which oxygen plays in reference to life. the discovery of oxygen, one of the greatest scientific events of the eighteenth century, was made by priestley in . it was soon shown that oxygen is necessary to all forms of life, and the question was raised: had not the boiling of the closed flasks changed the oxygen so that through the heating process it had lost its life-giving properties? this doubt grew until a reëxamination of the question of spontaneous generation became necessary under conditions in which the nutrient fluids were made accessible to the outside air. in franz schulze, and, in the following year, theodor schwann, devised experiments to test the question on this new basis. schwann is known to us as the founder of the cell-theory, but we must not confuse schulze with max schultze, who established the protoplasm doctrine. in the experiments of schulze, a flask was arranged containing nutrient fluids, with a large cork perforated and closely fitted with bent glass tubes connected on one side with a series of bulbs in which were placed sulphuric acid and other chemical substances. an aspirator was attached to the other end of this system, and air from the outside was sucked into the flask, passing on its way through the bulbs containing the chemical substances. the purpose of this was to remove the floating germs that exist in the air, while the air itself was shown, through other experiments by schwann, to remain unchanged. tyndall says in reference to these experiments: "here again the success of schulze was due to his working in comparatively pure air, but even in such air his experiment is a risky one. germs will pass unwetted and unscathed through sulphuric acid unless the most special care is taken to detain them. i have repeatedly failed, by repeating schulze's experiments, to obtain his results. others have failed likewise. the air passes in bubbles through the bulbs, and to render the method secure, the passage of the air must be so slow as to cause the whole of its floating matter, even to the very core of each bubble, to touch the surrounding fluid. but if this precaution be observed _water will be found quite as effectual as sulphuric acid_." schwann's apparatus was similar in construction, except that the bent tube on one side was surrounded by a jacket of metal and was subjected to a very high temperature while the air was being drawn through it, the effect being to kill any floating germs that might exist in the air. great care was taken by both experimenters to have their flasks and fluids thoroughly sterilized, and the results of their experiments were to show that the nutrient fluids remained uncontaminated. these experiments proved that there is something in the atmosphere which, unless it be removed or rendered inactive, produces life within nutrient fluids, but whether this something is solid, fluid, or gaseous did not appear from the experiments. it remained for helmholtz to show, as he did in , that this something will not pass through a moist animal membrane, and is therefore a solid. the results so far reached satisfied the minds of scientific men, and the question of the spontaneous origin of life was regarded as having been finally set at rest. iii. the third period. pouchet.--we come now to consider the third historical phase of this question. although it had apparently been set at rest, the question was unexpectedly opened again in by the frenchman pouchet, the director of the natural history museum of rouen. the frame of mind which pouchet brought to his experimental investigations was fatal to unbiased conclusions: "when, _by meditation_," he says, in the opening paragraph of his book on _heterogenesis_, "it was evident to me that spontaneous generation was one of the means employed by nature for the production of living beings, i applied myself to discover by what means one could place these phenomena in evidence." although he experimented, his case was prejudiced by metaphysical considerations. he repeated the experiments of previous observers with opposite results, and therefore he declared his belief in the falsity of the conclusions of spallanzani, schulze, and schwann. he planned and executed one experiment which he supposed was conclusive. in introducing it he said: "the opponents of spontaneous generation assert that the germs of microscopic organisms exist in the air, which transports them to a distance. what, then, will these opponents say if i succeed in introducing the generation of living organisms, while substituting artificial air for that of the atmosphere?" he filled a flask with boiling water and sealed it with great care. this he inverted over a bath of mercury, thrusting the neck of the bottle into the mercury. when the water was cooled, he opened the neck of the bottle, still under the mercury, and connected it with a chemical retort containing the constituents for the liberation of oxygen. by heating the retort, oxygen was driven off from the chemical salts contained in it, and being a gas, the oxygen passed through the connecting tube and bubbled up through the water of the bottle, accumulating at the upper surface, and by pressure forcing water out of the bottle. after the bottle was about half filled with oxygen imprisoned above the water, pouchet took a pinch of hay that had been heated to a high temperature in an oven, and with a pair of sterilized forceps pushed it underneath the mercury and into the mouth of the bottle, where the hay floated into the water and distributed itself. he thus produced a hay infusion in contact with pure oxygen, and after a few days this hay infusion was seen to be cloudy and turbid. it was, in fact, swarming with micro-organisms. pouchet pointed with triumphant spirit to the apparently rigorous way in which his experiment had been carried on: "where," said he, "does this life come from? it can not come from the water which had been boiled, destroying all living germs that may have existed in it. it can not come from the oxygen which was produced at the temperature of incandescence. it can not have been carried in the hay, which had been heated for a long period before being introduced into the water." he declared that this life was, therefore, of spontaneous origin. the controversy now revived, and waxed warm under the insistence of pouchet and his adherents. finally the academy of sciences, in the hope of bringing it to a conclusion, appointed a committee to decide upon conflicting claims. pasteur.--pasteur had entered into the investigation of the subject about , and, with wonderful skill and acumen, was removing all possible grounds for the conclusions of pouchet and his followers. in , before a brilliant audience at the sorbonne, he repeated the experiment outlined above and showed the source of error. in a darkened room he directed a bright beam of light upon the apparatus, and his auditors could see in the intense illumination that the surface of the mercury was covered with dust particles. pasteur then showed that when a body was plunged beneath the mercury, some of these surface granules were carried with it. in this striking manner pasteur demonstrated that particles from the outside had been introduced into the bottle of water by pouchet. this, however, is probably not the only source of the organisms which were developed in pouchet's infusions. it is now known that a hay infusion is very difficult to sterilize by heat, and it is altogether likely that the infusions used by pouchet were not completely sterilized. the investigation of the question requires more critical methods than was at first supposed, and more factors enter into its solution than were realized by spallanzani and schwann. pasteur demonstrated that the floating particles of the air contained living germs, by catching them in the meshes of gun cotton, and then dissolving the cotton with ether and examining the residue. he also showed that sterilized organic fluids could be protected by a plug of cotton sufficiently porous to admit of exchange of air, but matted closely enough to entangle the floating particles. he showed also that many of the minute organisms do not require free oxygen for their life processes, but are able to take the oxygen by chemical decomposition which they themselves produce from the nutrient fluids. jeffries wyman, of harvard college, demonstrated that some germs are so resistant to heat that they retain their vitality after several hours of boiling. this fact probably accounts for the difference in the results that have been obtained by experimenters. the germs in a resting-stage are surrounded by a thick protective coat of cellulose, which becomes softened and broken when they germinate. on this account more recent experimenters have adopted a method of discontinuous heating of the nutrient fluid that is being tested. the fluids are boiled at intervals, so that the unusually resistant germs are killed after the coating has been rendered soft, and when they are about to germinate. after the brilliant researches of pasteur, the question of spontaneous germination was once again regarded as having been answered in the negative; and so it is regarded to-day by the scientific world. nevertheless, attempts have been made from time to time, as by bastian, of england, in , to revive it on the old lines. [illustration: fig. .--apparatus of tyndall for experimenting on spontaneous generation.] tyndall.--john tyndall ( - ), the distinguished physicist, of london, published, in , the results of his experiments on this question, which, for clearness and ingenuity, have never been surpassed. for some time he had been experimenting in the domain of physics with what he called optically pure air. it was necessary for him to have air from which the floating particles had been sifted, and it occurred to him that he might expose nutrient fluids to this optically pure air, and thus very nicely test the question of the spontaneous origin of life within them. he devised a box, or chamber, as shown in fig. , having in front a large glass window, two small glass windows on the ends, and in the back a little air-tight trap-door. through the bottom of this box he had fitted ordinary test tubes of the chemist, with an air-tight surrounding, and on the top he had inserted some coiled glass tubes, which were open at both ends and allowed the passage of air in and out of the box through the tortuous passage. in the middle of the top of the box was a round piece of rubber. when he perforated this with a pinhole the elasticity of the rubber would close the hole again, but it would also admit of the passage through it of a small glass tube, such as is called by chemists a "thistle tube." the interior of this box was painted with a sticky substance like glycerin, in order to retain the floating particles of the air when they had once settled upon its sides and bottom. the apparatus having been prepared in this way, was allowed to stand, and the floating particles settled by their own weight upon the bottom and sides of the box, so that day by day the number of floating particles became reduced, and finally all of them came to rest. the air now differed from the outside air in having been purified of all of its floating particles. in order to test the complete disappearance of all particles. tyndall threw a beam of light into the air chamber. he kept his eye in the darkness for some time in order to increase its sensitiveness; then, looking from the front through the glass into the box, he was able to see any particles that might be floating there. the floating particles would be brightly illuminated by the condensed light that he directed into the chamber, and would become visible. when there was complete darkness within the chamber, the course of the beam of light was apparent in the room as it came up to the box and as it left the box, being seen on account of the reflection from the floating particles in the air, but it could not be seen at all within the box. when this condition was reached, tyndall had what he called optically pure air, and he was now ready to introduce the nutrient fluids into his test tubes. through a thistle tube, thrust into the rubber diaphragm above, he was able to bring the mouth of the tube successively over the different test tubes, and, by pouring different kinds of fluids from above, he was able to introduce these into different test tubes. these fluids consisted of mutton broth, of turnip-broth, and other decoctions of animal and vegetable matter. it is to be noted that the test tubes were not corked and consequently that the fluids contained within them were freely exposed to the optically pure air within the chamber. the box was now lifted, and the ends of the tubes extending below it were thrust into a bath of boiling oil. this set the fluids into a state of boiling, the purpose being to kill any germs of life that might be accidentally introduced into them in the course of their conveyance to the test tubes. these fluids, exposed freely to the optically pure air within this chamber, then remained indefinitely free from micro-organisms, thus demonstrating that putrescible fluids may be freely exposed to air from which the floating particles have been removed, and not show a trace either of spoiling or of organic life within them. it might be objected that the continued boiling of the fluids had produced chemical changes inimical to life, or in some way destroyed their life-supporting properties; but after they had remained for months in a perfectly clear state, tyndall opened the little door in the back of the box and closed it at once, thereby admitting some of the floating particles from the outside air. within a few days' time the fluids which previously had remained uncontaminated were spoiling and teeming with living organisms. these experiments showed that under the conditions of the experiments no spontaneous origin of life takes place. but while we must regard the hypothesis of spontaneous generation as thus having been disproved on an experimental basis, it is still adhered to from the theoretical standpoint by many naturalists; and there are also many who think that life arises spontaneously at the present time in ultra-microscopic particles. weismann's hypothetical "biophors," too minute for microscopic observation, are supposed to arise by spontaneous generation. this phase of the question, however, not being amenable to scientific tests, is theoretical, and therefore, so far as the evidence goes, we may safely say that the spontaneous origin of life under present conditions is unknown. practical applications.--there are, of course, numerous practical applications of the discovery that the spoiling of putrescible fluids is due to floating germs that have been introduced from the air. one illustration is the canning of meats and fruits, where the object is, by heating, to destroy all living germs that are distributed through the substance, and then, by canning, to keep them out. when this is entirely successful, the preserved vegetables and meats go uncontaminated. one of the most important and practical applications came in the recognition ( ) by the english surgeon lister that wounds during surgical operations are poisoned by floating particles in the air or by germs clinging to instruments or the skin of the operator, and that to render all appliances sterile and, by antiseptic dressings, completely to prevent the entrance of these bacteria into surgical wounds, insures their being clean and healthy. this led to antiseptic surgery, with which the name of lister is indissolubly connected. the germ-theory of disease the germ-theory of disease is another question of general bearing, and it will be dealt with briefly here. after the discovery of bacteria by leeuwenhoek, in , some medical men of the time suggested the theory that contagious diseases were due to microscopic forms of life that passed from the sick to the well. this doctrine of _contagium vivum_, when first promulgated, took no firm root, and gradually disappeared. it was not revived until about . if we attempt briefly to sketch the rise of the germ-theory of disease, we come, then, first to the year , when the italian bassi investigated the disease of silkworms, and showed that the transmission of that disease was the result of the passing of minute glittering particles from the sick to the healthy. upon the basis of bassi's observation, the distinguished anatomist henle, in , expounded the theory that all contagious diseases are due to microscopic germs. the matter, however, did not receive experimental proof until , when pasteur and robert koch showed the direct connection between certain microscopic filaments and the disease of splenic fever, which attacks sheep and other cattle. koch was able to get some of these minute filaments under the microscope, and to trace upon a warm stage the different steps in their germination. he saw the spores bud and produce filamentous forms. he was able to cultivate these upon a nutrient substance, gelatin, and in this way to obtain a pure culture of the organism, which is designated under the term anthrax. he inoculated mice with the pure culture of anthrax germs, and produced splenic fever in the inoculated forms. he was able to do this through several generations of mice. in the same year pasteur showed a similar connection between splenic fever and the anthrax. this demonstration of the actual connection between anthrax and splenic fever formed the first secure foundation of the germ-theory of disease, and this department of investigation became an important one in general biology. the pioneer workers who reached the highest position in the development of this knowledge are pasteur, koch, and lister. [illustration: fig. .--louis pasteur ( - ) and his granddaughter.] veneration of pasteur.--pasteur is one of the most conspicuous figures of the nineteenth century. the veneration in which he is held by the french people is shown in the result of a popular vote, taken in , by which he was placed at the head of all their notable men. one of the most widely circulated of the french journals--the _petit parisien_--appealed to its readers all over the country to vote upon the relative prominence of great frenchmen of the last century. pasteur was the winner of this interesting contest, having received , , votes of the fifteen millions cast, and ranking above victor hugo, who stood second in popular estimation, by more than one hundred thousand votes. this enviable recognition was won, not by spectacular achievements in arms or in politics, but by indefatigable industry in the quiet pursuit of those scientific researches that have resulted in so much good to the human race. personal qualities.--he should be known also from the side of his human qualities. he was devotedly attached to his family, enjoying the close sympathy and assistance of his wife and his daughter in his scientific struggles, a circumstance that aided much in ameliorating the severity of his labors. his labors, indeed, overstrained his powers, so that he was smitten by paralysis in , at the age of forty-six, but with splendid courage he overcame this handicap, and continued his unremitting work until his death in . the portrait of pasteur with his granddaughter (fig. ) gives a touch of personal interest to the investigator and the contestant upon the field of science. his strong face shows dignity of purpose and the grim determination which led to colossal attainments; at the same time it is mellowed by gentle affection, and contrasts finely with the trusting expression of the younger face. pasteur was born of humble parents in dôle in the jura, on december the th, . his father was a tanner, but withal, a man of fine character and stern experience, as is "shown by the fact that he had fought in the legions of the first empire and been decorated on the field of battle by napoleon." the filial devotion of pasteur and his justifiable pride in his father's military service are shown in the dedication of his book, _studies on fermentation_, published in : "to the memory of my father, formerly a soldier under the first empire, and knight of the legion of honor. the longer i live, the better do i understand the kindness of thy heart and the superiority of thy judgment. the efforts which i have devoted to these studies and to those which have preceded them are the fruits of thy example and of thy counsel. desiring to honor these precious recollections, i dedicate this book to thy memory." when pasteur was an infant of two years his parents removed to the town of arbois, and here he spent his youth and received his early education. after a period of indifference to study, during which he employed his time chiefly in fishing and sketching, he settled down to work, and, thereafter, showed boundless energy and enthusiasm. pasteur, whom we are to consider as a biologist, won his first scientific recognition at the age of twenty-five, in chemistry and molecular physics. he showed that crystals of certain tartrates, identical in chemical composition, acted differently upon polarized light transmitted through them. he concluded that the differences in optical properties depended upon a different arrangement of the molecules; and these studies opened the fascinating field of molecular physics and physical chemistry. pasteur might have remained in this field of investigation, but his destiny was different. as tyndall remarked, "in the investigation of microscopic organisms--the 'infinitely little,' as pasteur loved to call them--and their doings in this, our world, pasteur found his true vocation. in this broad field it has been his good fortune to alight upon a crowd of connected problems of the highest public and scientific interest, ripe for solution, and requiring for their successful treatment the precise culture and capacities which he has brought to bear upon them." in pasteur went to paris as director of scientific studies in the École normale, having previously been a professor in strasburg and in lille. from this time on his energies became more and more absorbed in problems of a biological nature. it was a momentous year ( ) in the annals of bacteriology when pasteur brought convincing proof that fermentation (then considered chemical in its nature) was due to the growth of organic life. again in he demonstrated that both lactic (the souring of milk) and alcoholic fermentation are due to the growth of microscopic organisms, and by these researches he developed the province of biology that has expanded into the science of bacteriology. after pasteur entered the path of investigation of microbes his progress was by ascending steps; each new problem the solution of which he undertook seemed of greater importance than the one just conquered. he was led from the discovery of microbe action to the application of his knowledge to the production of antitoxins. in all this he did not follow his own inclinations so much as his sense of a call to service. in fact, he always retained a regret that he was not permitted to perfect his researches on crystallography. at the age of seventy he said of himself: "if i have a regret, it is that i did not follow that route, less rude it seems to me, and which would have led, i am convinced, to wonderful discoveries. a sudden turn threw me into the study of fermentation, fermentations set me at diseases, but i am still inconsolable to think that i have never had the time to go back to my old subject" (tarbell). although the results of his combined researches form a succession of triumphs, every point of his doctrines was the subject of fierce controversy; no investigations ever met with more determined opposition, no investigator ever fought more strenuously for the establishment of each new truth. he went from the study of the diseases of wines ( ) to the investigation ( - ) of the silkworm plague which had well-nigh crushed the silk industry of his country. the result was the saving of millions of francs annually to the people of france. his supreme service.--he then entered upon his chief services to humanity--the application of his discoveries to the cure and prevention of diseases. by making a succession of pure cultures of a disease-producing virus, he was able to attenuate it to any desired degree, and thereby to create a vaccinating form of the virus capable of causing a mild affection of the disease. the injection of this attenuated virus secured immunity from future attacks. the efficacy of this form of inoculation was first proved for the disease of fowl cholera, and then came the clear demonstration ( ) that the vaccine was effective against the splenic fever of cattle. crowning this series of discoveries came the use of inoculation ( ) to prevent the development of hydrophobia in one bitten by a mad dog. the pasteur institute.--the time had now come for the establishment of an institute, not alone for the treatment of hydrophobia, but also for the scientific study of means to control other diseases, as diphtheria, typhoid, tuberculosis, etc. a movement was set on foot for a popular subscription to meet this need. the response to this call on the part of the common people was gratifying. "the extraordinary enthusiasm which accompanied the foundation of this great institution has certainly not been equaled in our time. considerable sums of money were subscribed in foreign countries, while contributions poured in from every part of france. even the inhabitants of obscure little towns and villages organized fêtes, and clubbed together to send their small gifts" (franckland). the total sum subscribed on the date of the opening ceremony amounted to , , francs. the institute was formally opened on november th, , with impressive ceremonies presided over by the president of the republic of france. the establishment of this institute was an event of great scientific importance. here, within the first decade of its existence, were successfully treated more than twenty thousand cases of hydrophobia. here has been discovered by roux the antitoxin for diphtheria, and here have been established the principles of inoculation against the bubonic plague, against lockjaw, against tuberculosis and other maladies, and of the recent microbe inoculations of wright of london. more than thirty "pasteur institutes," with aims similar to the parent institution, have been established in different parts of the civilized world. pasteur died in , greatly honored by the whole world. on saturday, october th of that year, a national funeral was conducted in the church of notre-dame, which was attended by the representatives of the state and of numerous scientific bodies and learned societies. koch.--robert koch (fig. ) was born in , and is still living, engaged actively in work in the university of berlin. his studies have been mainly those of a medical man, and have been crowned with remarkable success. in he discovered the germ of tuberculosis, in the germ that produces asiatic cholera, and since that time his name has been connected with a number of remarkable discoveries that are of continuous practical application in the science of medicine. [illustration: fig. .--robert koch, born .] koch, with the rigorous scientific spirit for which he is noteworthy, established four necessary links in the chain of evidence to show that a particular organism is connected with a particular disease. these four postulates of koch are: first, that a microscopic organism of a particular type should be found in great abundance in the blood and the tissue of the sick animal; second, that a pure culture should be made of the suspected organism; third, that this pure culture, when introduced into the body of another animal, should produce the disease; and, fourth, that in the blood and tissues of that animal there should be found quantities of the particular organism that is suspected of producing the disease. in the case of some diseases this entire chain of evidence has been established; but in others, such as cholera and typhoid fever, the last steps have not been completed, for the reason that the animals experimented upon, namely, guinea-pigs, rabbits, and mice, are not susceptible to these diseases. [illustration: fig. .--sir joseph lister, born .] lister.--the other member of the great triumvirate of bacteriology is sir joseph lister (fig. ); born in , he has been successively professor of surgery in the universities of glasgow ( ) and of edinburgh ( ), and in king's college, london ( ). his practical application of the germ-theory introduced aseptic methods into surgery and completely revolutionized that field. this was in . in an address given that year before the british medical association in dublin, he said: "when it had been shown by the researches of pasteur that the septic property of the atmosphere depended, not on oxygen or any gaseous constituent, but on minute organisms suspended in it, which owed their energy to their vitality, it occurred to me that decomposition in the injured part might be avoided without excluding the air, by applying as a dressing some material capable of destroying the life of the floating particles." at first he used carbolic acid for this purpose. "the wards of which he had charge in the glasgow infirmary were especially affected by gangrene, but in a short time became the healthiest in the world; while other wards separated by a passageway retained their infection." the method of lister has been universally adopted, and at the same time has been greatly extended and improved. the question of immunity, _i.e._, the reason why after having had certain contagious diseases one is rendered immune, is of very great interest, but is of medical bearing, and therefore is not dealt with here. bacteria and nitrates.--one further illustration of the connection between bacteria and practical affairs may be mentioned. it is well known that animals are dependent upon plants, and that plants in the manufacture of protoplasm make use of certain nitrites and nitrates which they obtain from the soil. now, the source of these nitrites and nitrates is very interesting. in animals the final products of broken-down protoplasm are carbon dioxide, water, and a nitrogenous substance called urea. these products are called excretory products. the animal machine is unable to utilize the energy which exists in the form of potential energy in these substances, and they are removed from the body. the history of nitrogenous substance is the one which at present interests us the most. entering the soil, it is there acted upon by bacteria residing in the soil, these bacteria possessing the power of making use of the lowest residuum of energy left in the nitrogenous substance. they cause the nitrogen and the hydrogen to unite with oxygen in such a way that there are produced nitrous and nitric acids, and from these two acids, through chemical action, result the nitrites and the nitrates. these substances are then utilized by the plant in the manufacture of protoplasm, and the plant is fed upon by animal organisms, so that a direct relationship is established between these lower forms of life and the higher plant and animal series; a relationship that is not only interesting, but that helps to throw an important side-light upon the general nature of vital activities, their kind and their reach. in addition to the soil bacteria mentioned above, there are others that form association with the rootlets of certain plants and possess the power of fixing free nitrogen from the air. the nitrifying bacteria, are, of course, of great importance to the farmer and the agriculturist. it is not our purpose, however, to trace the different phases of the subject of bacteriology to their conclusions, but rather to give a picture of the historical development of this subject as related to the broader one of general biology. chapter xiv heredity and germinal continuity--mendel, galton, weismann it is a matter of common observation that in the living world like tends to produce like. the offspring of plants, as well as of animals, resembles the parent, and among all organisms endowed with mind, the mental as well as the physical qualities are inherited. this is a simple statement of the fact of heredity, but the scientific study of inheritance involves deep-seated biological questions that emerged late in the nineteenth century, and the subject is still in its infancy. in investigating this question, we need first, if possible, to locate the bearers of hereditary qualities within the physical substance that connects one generation with the next; then, to study their behavior during the transmission of life in order to account for the inheritance of both maternal and paternal qualities; and, lastly, to determine whether or not transiently acquired characteristics are inherited. hereditary qualities in the germinal elements.--when we take into consideration the fact established for all animals and plants (setting aside cases of budding and the division of unicellular organisms), that the only substance that passes from one generation to another is the egg and the sperm in animals, and their representatives in plants, we see that the first question is narrowed to these bodies. if all hereditary qualities are carried in the egg and the sperm--as it seems they must be--then it follows that these germinal elements, although microscopic in size, have a very complex organization. the discovery of this organization must depend upon microscopic examination. knowledge regarding the physical basis of heredity has been greatly advanced by critical studies of cells under the microscope and by the application of experimental methods, while other phases of the problems of inheritance have been elucidated by the analysis of statistics regarding hereditary transmissions. the whole question, however, is so recent that a clear formulation of the direction of the main currents of progress will be more helpful than any attempt to estimate critically the underlying principles. early theories.--there were speculations regarding the nature of inheritance in ancient and mediæval times. to mention any of them prior to the eighteenth century would serve no useful purpose, since they were vague and did not form the foundation upon which the modern theories were built. the controversies over pre-formation and epigenesis (see chapter x) of the eighteenth century embodied some ideas that have been revived. the recent conclusion that there is in the germinal elements an inherited organization of great complexity which conditions inheritance seems, at first, to be a return to the doctrine of pre-formation, but closer examination shows that there is merely a general resemblance between the ideas expressed by haller, bonnet, and philosophers of their time and those current at the present time. inherited organization, as now understood, is founded on the idea of germinal continuity and is vastly different from the old theory of pre-formation. the meaning of epigenesis, as expressed by wolff, has also been modified to include the conception of pre-localization of hereditary qualities within particular parts of the egg. it has come now to mean that development is a process of differentiation of certain qualities already laid down in the germinal elements. darwin's theory of pangenesis.--in attempting to account for heredity, darwin saw clearly the necessity of providing some means of getting all hereditary qualities combined within the egg and the sperm. accordingly he originated his provisional theory of pangenesis. keeping in mind the fact that all organisms begin their lives in the condition of single cells, the idea of inheritance through these microscopic particles becomes difficult to understand. how is it possible to conceive of all the hereditary qualities being contained within the microscopic germ of the future being? darwin supposed that very minute particles, which he called gemmules, were set free from all the cells in the body, those of the muscular system, of the nervous system, of the bony tissues, and of all other tissues contributing their part. these liberated gemmules were supposed to be carried by the circulation and ultimately to be aggregated within the germinal elements (ovum and sperm). thus the germinal elements would be a composite of substances derived from all organs and all tissues. with this conception of the blending of the parental qualities within the germinal elements we can conceive how inheritance would be possible and how there might be included in the egg and the sperm a representative in material substance of all the qualities of the parents. since development begins in a fertilized ovum, this complex would contain minute particles derived from every part of the bodies of both parents, which by growth would give rise to new tissues, all of them containing representatives of the tissues of the parent form. theory of pangenesis replaced by that of germinal continuity.--this theory of darwin served as the basis for other theories founded upon the conception of the existence of pangens; and although the modifications of spencer, brooks, and others were important, it is not necessary to indicate them in detail in order to understand what is to follow. the various theories founded upon the idea of pangens were destined to be replaced by others founded on the conception of germinal continuity--the central idea in nineteenth-century biology. the four chief steps which have led to the advancement of the knowledge of heredity, as suggested by thomson, are as follows: "(a) the exposition of the doctrine of germinal continuity, (b) more precise investigation of the material basis of inheritance, (c) suspicions regarding the inheritance of acquired characteristics, (d) application of statistical methods which have led to the formulation of the law of ancestral heredity." we shall take these up in order. exposition of the doctrine of germinal continuity.--from parent to offspring there passes some hereditary substance; although small in amount, it is the only living thread that connects one generation with another. it thus appears that there enters into the building of the body of a new organism some of the actual substance of both parents, and that this transmitted substance must be the bearer of hereditary qualities. does it also contain some characteristics inherited from grandparents and previous generations? if so, how far back in the history of the race does unbroken continuity extend? briefly stated, genetic continuity means that the ovum and its fertilizing agent are derived by continuous cell-lineage from the fertilized ovum of previous generations, extending back to the beginning of life. the first clear exposition of this theory occurs in the classical work of virchow on _cellular pathology_, published in . virchow ( - ), the distinguished professor of the university of berlin, has already been spoken of in connection with the development of histology. he took the step of overthrowing the theory of free cell-formation, and replacing it by the doctrine of cell-succession. according to the theory of schleiden and schwann, cells arose from a blastema by a condensation of matter around a nucleus, and the medical men prior to believed in free cell-formation within a matrix of secreted or excreted substance. this doctrine was held with tenacity especially for pathological growths. virchow demonstrated, however, that there is a continuity of living substance in all growths--that cells, both in health and in disease, arise only by the growth and division of previously existing living cells; and to express this truth he coined the formula "_omnis cellula e cellula_." manifestly it was necessary to establish this law of cell-succession before any idea of germinal continuity could prevail. virchow's work in this connection is of undying value. when applied to inheritance the idea of the continuity of living substance leads to making a distinction between germ-cells and body-cells. this had been done before the observations of virchow made their separation of great theoretical value. richard owen, in , pointed out certain differences between the body-cells and the germinal elements, but he did not follow up the distinction which he made. haeckel's _general morphology_, published in , forecasts the idea also, and in jaeger made use of the phrase "continuity of the germ protoplasm." other suggestions and modifications led to the clear expression by nussbaum, about , that the germinal substance was continued by unbroken generations from the past, and is the particular substance in which all hereditary qualities are included. but the conception finds its fullest expression in the work of weismann. weismann's explanation of heredity is at first sight relatively simple. in reply to the question, "why is the offspring like the parent?" he says, "because it is composed of some of the same stuff." in other words, there has been unbroken germinal continuity between generations. his idea of germinal continuity, _i.e._, unbroken continuity, through all time, of the germinal substance, is a conception of very great extent, and now underlies all discussion of heredity. in order to comprehend it, we must first distinguish between the germ-cells and the body-cells. weismann regards the body, composed of its many cells, as a derivative that becomes simply a vehicle for the germ-cells. owen's distinction between germ-cells and body-cells, made in , was not of much importance, but in the theory of weismann it is of vital significance. the germ-cells are the particular ones which carry forward from generation to generation the life of the individual. the body-cells are not inherited directly, but in the transmission of life the germ-cells pass to the succeeding generation, and they in turn have been inherited from the previous generation, and, therefore, we have the phenomenon of an unbroken connection with all previous generations. when the full significance of this conception comes to us, we see why the germ-cells have an inherited organization of remarkable complexity. this germinal substance embodies all the past history of the living, impressionable protoplasm, which has had an unbroken series of generations. during all time it has been subjected to the molding influence of external circumstances to which it has responded, so that the summation of its experiences becomes in some way embedded within its material substance. thus we have the germinal elements possessing an inherited organization made up of all the previous experiences of the protoplasm, some of which naturally are much more dominant than the others. we have seen that this idea was not first expressed by weismann; it was a modification of the views of nussbaum and hertwig. while it was not his individually, his conclusions were apparently reached independently. this idea was in the intellectual atmosphere of the times. several investigators reached their conclusions independently, although there is great similarity between them. although the credit for the first formulation of the law of germinal continuity does not belong to weismann, that of the greatest elaboration of it does. this doctrine of germinal continuity is now so firmly embedded in biological ideas of inheritance and the evolution of animal life that we may say it has become the corner-stone of modern biology. the conclusion reached--that the hereditary substance is the germ-plasm--is merely preliminary; the question remains, is the germ-plasm homogeneous and endowed equally in all parts with a mixture of hereditary qualities? this leads to the second step. the more precise investigation of the material basis of inheritance.--the application of the microscope to critical studies of the structure of the germ-plasm has brought important results which merge with the development of the idea of germinal continuity. can we by actual observation determine the particular part of the protoplasmic substance that carries the hereditary qualities? the earliest answer to this question was that the protoplasm, being the living substance, was the bearer of heredity. but close analysis of the behavior of the nucleus during development led, about , to the idea that the hereditary qualities are located within the nucleus of the cell. this idea, promulgated by fol, koelliker, and oskar hertwig, narrowed the attention of students of heredity from the general protoplasmic contents of the cell to the nucleus. later investigations show that this restriction was, in a measure, right. the nucleus takes an active part during cell-division, and it was very natural to reach the conclusion that it is the particular bearer of hereditary substance. but, in , van beneden and boveri made the discovery that within the nucleus are certain distinct little rod-like bodies which make their appearance during cell-division. these little bodies, inasmuch as they stain very deeply with the dyes used in microscopic research, are called chromosomes. and continued investigation brought out the astounding fact that, although the number of chromosomes vary in different animals (commonly from two to twenty-four), they are of the same number in all the cells of any particular animal or plant. these chromosomes are regarded as the bearers of heredity, and their behavior during fertilization and development has been followed with great care. brilliant studies of the formation of the egg have shown that the egg nucleus, in the process of becoming mature, surrenders one-half its number of chromosomes; it approaches the surface of the egg and undergoes division, squeezing out one-half of its substance in the form of a polar globule; and this process is once repeated.[ ] the formation of polar globules is accompanied by a noteworthy process of reduction in the number of chromosomes, so that when the egg nucleus has reached its mature condition it contains only one-half the number of chromosomes characteristic of the species, and will not ordinarily undergo development without fertilization. the precise steps in the formation of the sperm have also been studied, and it has been determined that a parallel series of changes occur. the sperm, when it is fully formed, contains also one-half the number of chromosomes characteristic of the species. now, egg and sperm are the two germinal elements which unite in development. fertilization takes place by the union of sperm and egg, and inasmuch as the nuclei of each of these structures contain one-half of the number of chromosomes characteristic of the species, their union in fertilization results in the restoration of the original number of chromosomes. the fertilized ovum is the starting-point of a new organism, and from the method of its fertilization it appears that the parental qualities are passed along to the cells of every tissue. the complex mechanism exhibited in the nucleus during segmentation is very wonderful. the fertilized ovum begins to divide, the nucleus passing through a series of complicated changes whereby its chromosomes undergo a lengthwise division--a division that secures an equable partition of the substance of which they are composed. with each successive division, this complicated process is repeated, and the many cells, arising from continued segmentation of the original cell, contain nuclei in which are embedded descendants of the chromosomes in unbroken succession. moreover, since these chromosomes are bi-parental, we can readily understand that every cell in the body carries both maternal and paternal qualities. the careful analysis of the various changes within the nuclei of the egg proves to be the key to some of the central questions of heredity. we see the force of the point which was made in a previous chapter, that inheritance is in the long run a cellular study, and we see in a new light the importance of the doctrine of germinal continuity. this conception, in fact, elucidates the general problem of inheritance in a way in which it has never been elucidated by any other means. for some time the attention of investigators was concentrated upon the nucleus and the chromosomes, but it is now necessary to admit that the basis of some structures is discoverable within the cytoplasm that surrounds the nucleus. experimental observations (conklin, lillie, wilson) have shown the existence of particular areas within the apparently simple substance of the egg, areas which are definitely related to the development of particular parts of the embryo. the removal of any one of these pre-localized areas prevents the development of the part with which it is genetically related. researches of this kind, necessitating great ingenuity in method and great talents in the observers, are widening the field of observation upon the phenomena of heredity. the inheritance of acquired characteristics.--the belief in the inheritance of acquired characteristics was generally accepted up to the middle of the nineteenth century, but the reaction against it started by galton and others has assumed great proportions. discussions in this line have been carried on extensively, and frequently in the spirit of great partizanship. these discussions cluster very much about the name and the work of weismann, the man who has consistently stood against the idea of acquired characteristics. more in reference to this phase of the question is given in the chapter dealing with weismann's theory of evolution (see p. ). wherever the truth may lie, the discussions regarding the inheritance of acquired characteristics provoked by weismann's theoretical considerations, have resulted in stimulating experiment and research, and have, therefore, been beneficial to the advance of science. the application of statistical methods and experiments to the ideas of heredity. mendel.--this feature of investigating questions of heredity is of growing importance. the first to complete experiments and to investigate heredity to any purpose was the austrian monk mendel ( - ) (fig. ), the abbot of a monastery at brünn. in his garden he made many experiments upon the inheritance, particularly in peas, of color and of form; and through these experiments he demonstrated a law of inheritance which bids fair to be one of the great biological discoveries of the nineteenth century. he published his papers in and , but since the minds of naturalists at that time were very much occupied with the questions of organic evolution, raised through the publications of darwin, the ideas of mendel attracted very little attention. the principles that he established were re-discovered in by de vries and other botanists, and thus naturalists were led to look up the work of mendel. [illustration: fig. .--gregor mendel, - . permission of professor bateson.] the great discovery of mendel may be called that of the purity of the germ-cells. by cross-fertilization of pure breeds of peas of different colors and shapes he obtained hybrids. the hybrid embodied the characteristics of the crossed peas; one of the characteristics appearing, and the other being held in abeyance--present within the organization of the pea, but not visible. when peas of different color were cross-fertilized, one color would be stronger apparently than the other, and would stand out in the hybrids. this was called the dominant color. the other, which was held in abeyance, was called recessive; for, though unseen, it was still present within the young seeds. that the recessive color was not blotted out was clearly shown by raising a crop from the hybrid, a condition under which they would produce seeds like those of the two original forms, and in equal number; and thereafter the descendants of these peas would breed true. this so-called purity of the germ-cells, then, may be expressed in this way: "the hybrid, whatever its own character, produces ripe germ-cells, which produce only the pure character of one parent or of the other" (castle). although mendel's discovery was for a long time overlooked, happily the facts were re-discovered, and at the present time extensive experiments are being made with animals to test this law: experiments in the inheritance of poultry, the inheritance of fur in guinea-pigs, of erectness in the ears of rabbits, etc., etc. in this country the experiments of castle, davenport, and others with animals tend to support mendel's conclusion and lift it to the position of a law. rank of mendel's discovery.--the discovery by mendel of alternative inheritance will rank as one of the greatest discoveries in the study of heredity. the fact that in cross-breeding the parental qualities are not blended, but that they retain their individuality in the offspring, has many possible practical applications both in horticulture and in the breeding of animals. the germ-cells of the hybrids have the dominant and the recessive characters about equally divided; this will appear in the progeny of the second generation, and the races, when once separated, may be made to breed true. mendel's name was not recognized as a prominent one in the annals of biological history until the re-discovery of his law in ; but now he is accorded high rank. it may be remarked in passing that the three leading names in the development of the theories of heredity are those of mendel, galton, and weismann. [illustration: fig. .--francis galton, born .] galton.--the application of statistical methods is well illustrated in the theories of francis galton (fig. ). this distinguished english statistician was born in , and is still living. he is the grandson of dr. erasmus darwin and the cousin of charles. after publishing books on his travels in africa, he began the experimental study of heredity and, in , he read before the royal society of london a paper on pangenesis, in which he departed from that theory as developed by darwin. the observations upon which he based his conclusions were made upon the transfusion of blood in rabbits and their after-breeding. he studied the inheritance of stature, and other characteristics, in human families, and the inheritance of spots on the coat of certain hounds, and was led to formulate a law of ancestral inheritance which received its clearest expression in his book, _natural inheritance_, published in . he undertook to determine the proportion of heritage that is, on the average, contributed by each parent, grandparent, etc., and arrived at the following conclusions: "the parents together contribute one-half the total heritage, the four grandparents together one-fourth, the eight great-grandparents one-sixteenth, and all the remainder of the ancestry one-sixteenth." carl pearson has investigated this law of ancestral inheritance. he substantiates the law in its principle, but modifies slightly the mathematical expression of it. this field of research, which involves measurements and mathematics and the handling of large bodies of statistics, has been considerably cultivated, so that there is in existence in england a journal devoted exclusively to biometrics, which is edited by carl pearson, and is entitled _biometrika_. the whole subject of heredity is undergoing a thorough revision. what seems to be most needed at the present time is more exact experimentation, carried through several generations, together with more searching investigations into the microscopical constitution of egg and sperm, and close analysis of just what takes place during fertilization and the early stages of the development of the individual. experiments are being conducted on an extended scale in endowed institutions. there is notably in this country, established under the carnegie institution, a station for experimental evolution, at cold spring harbor, new york, of which c.b. davenport is director. other experimental stations in england and on the continent have been established, and we are to expect as the result of coördinated and continuous experimental work many substantial contributions to the knowledge of inheritance. footnotes: [footnote : there are a few exceptions to this rule, as in the eggs of plant-lice, etc., in which a single polar globule is produced.] chapter xv the science of fossil life it gradually dawned on the minds of men that the crust of the earth is like a gigantic mausoleum, containing within it the remains of numerous and varied forms of life that formerly existed upon the surface of the earth. the evidence is clear that untold generations of living forms, now preserved as fossils, inhabited the earth, disported themselves, and passed away long before the advent of man. the knowledge of this fossil life, on account of its great diversity, is an essential part of biology, and all the more so from the circumstance that many forms of life, remains of which are exhibited in the rocks, have long since become extinct. no history of biology would be complete without an account of the rise and progress of that department of biology which deals with fossil life. it has been determined by collecting and systematically studying the remains of this ancient life that they bear testimony to a long, unbroken history in which the forms of both animals and plants have been greatly altered. the more ancient remains are simple in structure, and form with the later ones, a series that exhibits a gradually increasing complexity of structure. the study of the fossil series has brought about a very great extension of our knowledge regarding the age of the world and of the conditions under which life was evolved. strange views regarding fossils.--but this state of our knowledge was a long time coming, and in the development of the subject we can recognize several distinct epochs, "well-marked by prominent features, but like all stages of intellectual growth, without definite boundaries." fossils were known to the ancients, and by some of the foremost philosophers of greece were understood to be the remains of animals and plants. after the revival of learning, however, lively controversies arose as to their nature and their meaning. some of the fantastic ideas that were entertained regarding the nature of fossil remains may be indicated. the fossils were declared by many to be freaks of nature; others maintained that they were the results of spontaneous generation, and were produced by the plastic forces of nature within the rocks in which they were found embedded. another opinion expressed was that they were generated by fermentations. as the history of intellectual development shows, the mind has ever seemed benumbed in the face of phenomena that are completely misconceived; mystical explanations have accordingly been devised to account for them. some of the pious persons of that period declared that fossils had been made and distributed by the creator in pursuance of a plan beyond our comprehension. another droll opinion expressed was that the creator in his wisdom had introduced fossil forms into the rocks in order that they should be a source of confusion to the race of geologists that was later to arise. and still another fantastic conception suggested that the fossils were the original molds used by the creator in forming different varieties of animals and plants, some of which had been used and others discarded. it was supposed that in preparing for the creation of life he experimented and discarded some of his earliest attempts; and that fossils represented these discarded molds and also, perhaps, some that had been used in fashioning the created forms. when large bones, as of fossil elephants, began to be exhumed, they became for the most part the objects of stupid wonder. the passage in the scriptures was pointed out, that "there were giants in those days," and the bones were taken to be evidences of the former existence of giants. the opinions expressed regarding the fossil bones were varied and fantastic, "some saying that they were rained from heaven, others saying that they were the gigantic limbs of the ancient patriarchs, men who were believed to be tall because they were known to be old." following out this idea, "henrion in published a work in which he assigned to adam a height of feet inches, noah being feet shorter, and so on." determination of the nature of fossils.--in due course it came to be recognized that fossils were the remains of forms that had been alive during earlier periods of time; but in reaching this position there was continual controversy. objections were especially vigorous from theological quarters, since such a conclusion was deemed to be contradictory to the scriptures. the true nature of fossils had been clearly perceived by leonardo da vinci ( - ) and certain others in the sixteenth century. the work, however, that approached more nearly to scientific demonstration was that of steno ( - ), a dane who migrated to italy and became the court physician to the dukes of tuscany. he was a versatile man who had laid fast hold upon the new learning of his day. eminent as anatomist, physiologist, and physician, with his ever active mind he undertook to encompass all learning. it is interesting that steno--or stensen--after being passionately devoted to science, became equally devoted to religion and theology, and, forsaking all scientific pursuits, took orders and returned to his native country with the title of bishop. here he worked in the service of humanity and religion to the end of his life. in reference to his work in geology, his conclusions regarding fossils ( ) were based on the dissection of the head of a shark, by which means he showed an almost exact correspondence between certain glossy fossils and the teeth of living sharks. he applied his reasoning, that like effects imply like causes, to all manner of fossils, and clearly established the point that they should be regarded as the remains of animals and plants. the method of investigation practiced by steno was that "which has consciously or unconsciously guided the researches of palæontologists ever since." although his conclusions were well supported, they did not completely overthrow the opposing views, and become a fixed basis in geology. when, at the close of the eighteenth century and the beginning of the nineteenth, fossil remains were being exhumed in great quantities in the paris basin, cuvier, the great french naturalist, reëstablished the doctrine that fossils are the remains of ancient life. an account of this will be given presently, and in the mean time we shall go on with the consideration of a question raised by the conclusions of steno. fossil deposits ascribed to the flood.--after it began to be reluctantly conceded that fossils might possibly be the remains of former generations of animals and plants, there followed a period characterized by the general belief that these entombed forms had been deposited at the time of the mosaic deluge. this was the prevailing view in the eighteenth century. as observation increased and the extent and variety of fossil life became known, as well as the positions in which fossils were found, it became more difficult to hold this view with any appearance of reason. large forms were found on the tops of mountains, and also lighter forms were found near the bottom. miles upon miles of superimposed rocks were discovered, all of them bearing quantities of animal forms, and the interpretation that these had been killed and distributed by a deluge became very strained. but to the reasoners who gave free play to their fancies the facts of observation afforded little difficulty. some declared that the entire surface of the earth had been reduced to the condition of a pasty mass, and that the animals drowned by the deluge had been deposited within this pasty mass which, on the receding of the waters, hardened into rocks. the belief that fossil deposits were due to the deluge sensibly declined, however, near the close of the eighteenth century, but was still warmly debated in the early part of the nineteenth century. fossil bones of large tropical animals having been discovered about , embedded in the stalagmite-covered floor of a cavern in yorkshire, england, some of the ingenious supporters of the flood-theory maintained that caves were produced by gases proceeding from the bodies of decaying animals of large size; that they were like large bubbles in the crust of the earth, and, furthermore, that bones found in caverns were either those from the decayed carcasses or others that had been deposited during the occurrence of the flood. even the utterances of cuvier, in his theory of catastrophism to which we shall presently return, gave countenance to the conclusion that the deluge was of universal extent. as late as , william buckland, reader in geology in oxford, and later canon ( ) of christ church, and dean ( ) of westminster, published his _reliquiæ diluvianæ_, or _observations on the organic remains attesting the action of a universal deluge_. the theory that the mosaic deluge had any part in the deposit of organic fossils was finally surrendered through the advance of knowledge, owing mainly to the labors of lyell and his followers. the comparison of fossil and living animals.--the very great interest connected with the reëstablishment of the conclusion of steno, that fossils were once alive, leads us to speak more at length of the discoveries upon which cuvier passed his opinion. in the gypsum rocks about paris the workmen had been turning up to the light bones of enormous size. while the workmen could recognize that they were bones of some monsters, they were entirely at loss to imagine to what kind of animals they had belonged, but the opinion was frequently expressed that they were the bones of human giants. cuvier, with his extensive preparation in comparative anatomy, was the best fitted man perhaps in all the world to pass judgment upon these particular bones. he went to the quarries and, after observing the remains, he saw very clearly that they were different from the bones of any animals now existing. his great knowledge of comparative anatomy was founded on a comprehensive study of the bony system as well as the other structures of all classes of living animals. he was familiar with the anatomy of elephants, and when he examined the large bones brought to light in the quarries of montmartre, he saw that he was confronted with the bones of elephant-like animals, but animals differing in their anatomy from those at present living on the earth. the great feature of cuvier's investigations was that he instituted comparisons on a broad scale between fossil remains and living animals. it was not merely that he followed the method of investigation employed by steno; he went much further and reached a new conclusion of great importance. not only was the nature of fossil remains determined, but by comparing their structure with that of living animals the astounding inference was drawn that the fossil remains examined belonged to forms that were truly extinct. this discovery marks an epoch in the development of the knowledge of extinct animals. cuvier the founder of vertebrate palæontology.--the interesting discovery that the fossil relics in the eocene rocks about paris embraced extinct species was announced to the institute by cuvier in january, ; and thereafter he continued for a quarter of a century to devote much attention to the systematic study of collections made in that district. these observations were, however, shared with other labors upon comparative anatomy and zoölogy, which indicates the prodigious industry for which he was notable. in - he published a monumental work, profusely illustrated, under the title _ossemens fossiles_. this standard publication entitles him to recognition as the founder of vertebrate palæontology. in examining the records of fossil life, cuvier and others saw that the evidence indicated a succession of animal populations that had become extinct, and also that myriads of new forms of life appeared in the rocks of succeeding ages. here cuvier, who believed that species were fixed and unalterable, was confronted with a puzzling problem. in attempting to account for the extinction of life, and what seemed to him the creation of new forms, he could see no way out consistent with his theoretical views except to assume that the earth had periodically been the scene of great catastrophes, of which the mosaic deluge was the most recent, but possibly not the last. he supposed that these cataclysms of nature resulted in the extinction of all life, and that after each catastrophe the salubrious condition of the earth was restored, and that it was re-peopled by a new creation of living beings. this conception, known as the theory of catastrophism, was an obstacle to the progress of science. it is to be regretted that cuvier was not able to accept the views of his illustrious contemporary lamarck, who believed that the variations in fossil life, as well as those of living forms, were owing to gradual transformations. lamarck founds invertebrate palæontology.--the credit of founding the science of palæontology does not belong exclusively to cuvier. associated with his name as co-founders are those of lamarck and william smith. lamarck, that quiet, forceful thinker who for so many years worked by the side of cuvier, founded the science of invertebrate palæontology. the large bones with which cuvier worked were more easy to be recognized as unique or as belonging to extinct animals than the shells which occurred in abundance in the rocks about paris. the latter were more difficult to place in their true position because the number of forms of life in the sea is very extended and very diverse. just as cuvier was a complete master of knowledge regarding vertebrate organization, so lamarck was equally a master of that vast domain of animal forms which are of a lower grade of organization--the invertebrates. from his study of the collections of shells and other invertebrate forms from the rocks, lamarck created invertebrate palæontology and this, coupled with the work of cuvier, formed the foundations of the entire field. lamarck's study of the extinct invertebrates led him to conclusions widely at variance with those of cuvier. instead of thinking of a series of catastrophes, he saw that not all of the forms of life belonging to one geological period became extinct, but that some of them were continued into the succeeding period. he saw, therefore, that the succession of life in the rocks bore testimony to a long series of gradual changes upon the earth's surface, and did not in any way indicate the occurrence of catastrophes. the changes, according to the views of lamarck, were all knit together into a continuous process, and his conception of the origin of life upon the earth grew and expanded until it culminated in the elaboration of the first consistent theory of evolution. these two men, lamarck and cuvier, form a contrast as to the favors distributed by fortune: cuvier, picturesque, highly honored, the favorite of princes, advanced to the highest places of recognition in the government, acclaimed as the jove of natural science; lamarck, hard-working, harassed by poverty, insufficiently recognized, and, although more gifted than his confrère, overlooked by the scientific men of the time. the judgment of the relative position of these two men in natural science is now being reversed, and on the basis of intellectual supremacy lamarck is coming into general recognition as the better man of the two. in the chapters dealing with organic evolution some events in the life of this remarkable man will be given. the arrangement of fossils in strata.--the other name associated with lamarck and cuvier is that of william smith, the english surveyor. both lamarck and cuvier were men of extended scientific training, but william smith had a moderate education as a surveyor. while the two former were able to express scientific opinions upon the nature of the fossil forms discovered, william smith went at his task as an observer with a clear and unprejudiced mind, an observer who walked about over the fields, noticing the conditions of rocks and of fossil forms embedded therein. he noted that the organic remains were distributed in strata, and that particular forms of fossil life characterized particular strata and occupied the same relative position to one another. he found, for illustration, that certain particular forms would be found underlying certain other forms in one mass of rocks in a certain part of the country. wherever he traveled, and whatever rocks he examined, he found these forms occupying the same relative positions, and thus he came to the conclusion that the living forms within the rocks constitute a stratified series, having definite and unvarying arrangement with reference to one another. in short, the work of these three men--cuvier, lamarck, and william smith--placed the new science of palæontology upon a secure basis at the beginning of the nineteenth century. summary.--the chief steps up to this time in the growth of the science of fossil life may now be set forth in categories, though we must remember that the advances proceeded concurrently and were much intermingled, so that, whatever arrangement we may adopt, it does not represent a strict chronological order of events: i. the determination of the nature of fossils. owing to the labors of da vinci, steno, and cuvier, the truth was established that fossils are the remains of former generations of animals and plants. ii. the comparison of organic fossils with living forms that was instituted on a broad scale by cuvier resulted in the conclusion that some of the fossils belong to extinct races. the belief of cuvier that entire populations became extinct simultaneously, led him to the theory of catastrophism. the observations of lamarck, that, while some species disappear, others are continued and pass through transmutations, were contrary to that theory. iii. the recognition that the stratified rocks in which fossils are distributed are sedimentary deposits of gradual formation. this observation and the following took the ground from under the theory that fossils had been deposited during the mosaic deluge. iv. the discovery by william smith that the arrangement of fossils within rocks is always the same, and the relative age of rocks may be determined by an examination of their fossil contents. upon the basis of the foregoing, we come to the next advance, _viz._: v. the application of this knowledge to the determination of the history of the earth. fossil remains as an index to the past history of the earth.--the most advanced and enlightened position that had been taken in reference to the fossil series during the first third of the nineteenth century was that taken by lamarck, he being the first to read in the series the history of life upon the globe, weaving it into a connected story, and establishing thereon a doctrine of organic evolution. it was not until after , however, that the truth of this conclusion was generally admitted, and when it was accepted it was not through the earlier publications of lamarck, but through the arguments of later observers, founded primarily upon the hypothesis set forth by darwin. there were several gradations of scientific opinion in the period, short as it was, between the time of cuvier and of darwin; and this intermediate period was one of contention and warfare between the theologians and the geologists. cuvier had championed the theory of a succession of catastrophes, and since this hypothesis did not come into such marked conflict with the prevailing theological opinion as did the views of lamarck, the theologians were ready to accept the notion of cuvier, and to point with considerable satisfaction to his unique position as an authority. lyell.--in there was published an epoch-making work in geology by charles lyell (fig. ), afterward sir charles, one of the most brilliant geologists of all the world. this british leader of scientific thought showed the prevalence of a uniform law of development in reference to the earth's surface. he pointed out the fact that had been maintained by hutton, that changes in the past were to be interpreted in the light of what is occurring in the present. by making a careful study of the work performed by the waters in cutting down the continents and in transferring the eroded material to other places, and distributing it in the form of deltas; by observing also the action of frost and wind and wave; by noting, furthermore, the conditions under which animals die and are subsequently covered up in the matrix of detritus--by all this he showed evidences of a series of slow, continuous changes that have occurred in the past and have molded the earth's crust into its present condition. [illustration: fig. .--charles lyell, - .] he showed, further, that organic fossils are no exception to this law of uniform change. he pointed to the evidences that ages of time had been required for the formation of the rocks bearing fossils; and that the regular succession of animal forms indicates a continual process of development of animal life; and that the disappearance of some forms, that is, their becoming extinct, was not owing to sudden changes, but to gradual changes. when this view was accepted, it overthrew the theory of catastrophism and replaced it by one designated uniformatism, based on the prevalence of uniform natural laws. this new conception, with all of its logical inferences, was scouted by those of theological bias, but it won its way in the scientific world and became an important feature in preparing for the reception of darwin's great book upon the descent of animal life. we step forward now to the year , to consider the effect upon the science of palæontology of the publication of darwin's _origin of species_. its influence was tremendous. the geological theories that had provoked so much controversy were concerned not merely with the disappearance of organic forms, but also with the introduction of new species. the _origin of species_ made it clear that the only rational point of view in reference to fossil life was that it had been gradually developed, that it gave us a picture of the conditions of life upon the globe in past ages, that the succession of forms within the rocks represented in outline the successive steps in the formation of different kinds of animals and plants. owen.--both before and after darwin's hypothesis was given to science, notable anatomists, a few of whom must be mentioned, gave attention to fossil remains. richard owen ( - ) had his interest in fossil life stimulated by a visit to cuvier in , and for more than forty years thereafter he published studies on the structure of fossil animals. his studies on the fossil remains of australia and new zealand brought to light some interesting forms. the extinct giant bird of new zealand (fig. ) was a spectacular demonstration of the enormous size to which birds had attained during the eocene period. owen's monograph ( ) on the oldest known bird--the archæopteryx--described an interesting form uniting both bird-like and reptilian characteristics. [illustration: fig. .--professor owen and the extinct fossil bird (dinornis) of new zealand. permission of d. appleton & co.] agassiz.--louis agassiz ( - ) (fig. ) also came into close personal contact with cuvier, and produced his first great work partly under the stimulus of the latter. when agassiz visited paris, cuvier placed his collections at agassiz's disposal, together with numerous drawings of fossil fishes. the profusely illustrated monograph of agassiz on the fossil fishes ( - ) began to appear in , the year after cuvier's death, and was carried on eleven years before it was completed. [illustration: fig. .--louis agassiz, - .] agassiz, with his extensive knowledge of the developmental stages of animals, came to see a marked parallelism between the stages in development of the embryo and the successive forms in the geological series. this remarkable parallelism between the fossil forms of life and the stages in the development of higher forms of recent animals is very interesting and very significant, and helps materially in elucidating the idea that the fossil series represent roughly the successive stages through which animal forms have passed in their upward course of development from the simplest to the highest, through long ages of time. curiously enough, however, agassiz failed to grasp the meaning of the principle that he had worked out. after illustrating so nicely the process of organic evolution, he remained to the end of his life an opponent of that theory. huxley.--thomas henry huxley ( - ) was led to study fossil life on an extended scale, and he shed light in this province as in others upon which he touched. with critical analysis and impartial mind he applied the principles of evolution to the study of fossil remains. his first conclusion was that the evidence of evolution derived from palæontology was negative, but with the advances in discovery he grew gradually to recognize that palæontologists, in bringing to light complete evolutionary series, had supplied some of the strongest supporting evidence of organic evolution. by many geologists fossils have been used as time-markers for the determination of the age of various deposits; but, with huxley, the study of them was always biological. it is to the latter point of view that palæontology owes its great importance and its great development. the statement of huxley, that the only difference between a fossil and a recent animal is that one has been dead longer than the other, represents the spirit in which the study is being carried forward. [illustration: fig. .--e.d. cope, - .] with the establishment of the doctrine of organic evolution palæontology entered upon its modern phase of growth; upon this basis there is being reared a worthy structure through the efforts of the recent votaries to the science. it is neither essential nor desirable that the present history of the subject should be followed here in detail. the collections of material upon which palæontologists are working have been enormously increased, and there is perhaps no place where activity has been greater than in the united states. the rocks of the western states and territories embrace a very rich collection of fossil forms, and, through the generosity of several wealthy men, exploring parties have been provided for and immense collections have been brought back to be preserved in the museums, especially of new haven, conn., and in the american museum of natural history in new york city. leidy, cope, and marsh.--among the early explorers of the fossils of the west must be named joseph leidy, e.d. cope (fig. ), and o.c. marsh. these gentlemen all had access to rich material, and all of them made notable contributions to the science of palæontology. the work of cope ( - ) is very noteworthy. he was a comparative anatomist equal to cuvier in the extent of his knowledge, and of larger philosophical views. his extended publications under the direction of the united states government have very greatly extended the knowledge of fossil vertebrate life in america. o.c. marsh (fig. ) is noteworthy for similar explorations; his discovery of toothed birds in the western rocks and his collection of fossil horses, until recently the most complete one in existence, are all very well known. throughout his long life he contributed from his own private fortune, and intellectually through his indefatigable labors, to the progress of palæontology. [illustration: fig. .--o.c. marsh, - .] zittel.--the name most widely known in palæontology is that of the late karl von zittel ( - ), who devoted all his working life to the advancement of the science of fossils. in his great work, _handbuch der palaeontologie_ ( - ), he brought under one view the entire range of fossils from the protozoa up to the mammals. osborn says: "it is probably not an exaggeration to say that he did more for the promotion and diffusion of palæontology than any other single man who lived during the nineteenth century. while not gifted with genius, he possessed extraordinary judgment, critical capacity, and untiring industry." his portrait (fig. ) shows a face "full of keen intelligence and enthusiasm." zittel's influence was exerted not only through his writings, but also through his lectures and the stimulus imparted to the large number of young men who were attracted to munich to study under his direction. these disciples are now distributed in various universities in europe and the united states, and are there carrying forward the work begun by zittel. the great collection of fossils which he left at munich contains illustrations of the whole story of the evolution of life through geological ages. recent developments.--the greatest advance now being made in the study of fossil vertebrate life consists in establishing the lineage of families, orders, and classes. investigators have been especially fortunate in working out the direct line of descent of a number of living mammals. fossils have been collected which supply a panoramic view of the line of descent of horses, of camels, of rhinoceroses, and of other animals. the most fruitful worker in this field at the present time is perhaps henry f. osborn, of the american museum of natural history, new york city. his profound and important investigations in the ancestry of animal life are now nearing the time of their publication in elaborated form. palæontology, by treating fossil life and recent life in the same category, has come to be one of the important lines of investigation in biology. it is, of course, especially rich in giving us a knowledge of the hard parts of animals, but by ingenious methods we can arrive at an idea of some of the soft parts that have completely disappeared. molds of the interior of the cranium can be made, and thus one may form a notion of the relative size and development of the brain in different vertebrated animals. this method of making molds and studying them has shown that one characteristic of the geological time of the tertiary period was a marked development in regard to the brain size of the different animals. there was apparently, just prior to the quaternary epoch, a need on the part of animals to have an increased brain-growth; and one can not doubt that this feature which is demonstrated by fossil life had a great influence in the development of higher animal forms. [illustration: fig. .--karl von zittel, - .] the methods of collecting fossils in the field have been greatly developed. by means of spreading mucilage and tissue paper over delicate bones that crumble on exposure to the air, and of wrapping fossils in plaster casts for transportation, it has been made possible to uncover and preserve many structures which with a rougher method of handling would have been lost to science. fossil man.--one extremely interesting section of palæontology deals with the fossil remains of the supposed ancestors of the present human race. geological evidence establishes the great antiquity of man, but up to the present time little systematic exploration has been carried on with a view to discover all possible traces of fossil man. from time to time since there have been discovered in caverns and river-gravels bones which, taken together, constitute an interesting series. the parts of the skull are of especial importance in this kind of study, and there now exists in different collections a series containing the neanderthal skull, the skulls of spy and engis, and the java skull described in by dubois. there have also been found recently (november, ) in deposits near lincoln, neb., some fossil human remains that occupy an intermediate position between the neanderthal skull and the skulls of the lower representatives of living races of mankind. we shall have occasion to revert to this question in considering the evidences of organic evolution. (see page .) the name palæontology was brought into use about . the science affords, in some particulars, the most interesting field for biological research, and the feature of the reconstruction of ancient life and the determination of the lineage of living forms has taken a strong hold on the popular imagination. according to osborn, the most important palæontological event of recent times was the discovery, in , of fossil beds of mammals in the fayûm lake-province of egypt, about forty-seven miles south of cairo. here are embedded fossil forms, some of which have been already described in a volume by charles w. andrews, which osborn says "marks a turning-point in the history of mammalia of the world." it is now established that "africa was a very important center in the evolution of mammalian life." it is expected that the lineage of several orders of mammalia will be cleared up through the further study of fossils from this district. part ii the doctrine of organic evolution chapter xvi what evolution is: the evidence upon which it rests, etc. the preceding pages have been devoted mainly to an account of the shaping of ideas in reference to the architecture, the physiology, and the development of animal life. we come now to consider a central theme into which all these ideas have been merged in a unified system; _viz._, the process by which the diverse forms of animals and plants have been produced. crude speculations regarding the derivation of living forms are very ancient, and we may say that the doctrine of organic evolution was foreshadowed in greek thought. the serious discussion of the question, however, was reserved for the nineteenth century. the earlier naturalists accepted animated nature as they found it, and for a long time were engaged in becoming acquainted merely, with the different kinds of animals and plants, in working out their anatomy and development; but after some progress had been made in this direction there came swinging into their horizon deeper questions, such as that of the derivation of living forms. the idea that the higher forms of life are derived from simpler ones by a process of gradual evolution received general acceptance, as we have said before, only in the last part of the nineteenth century, after the work of charles darwin; but we shall presently see how the theory of organic development was thought out in completeness by lamarck in the last years of the eighteenth century, and was further molded by others before darwin touched it. vagueness regarding evolution.--although "evolution" is to-day a word in constant use, there is still great vagueness in the minds of most people as to what it stands for; and, what is more, there is very little general information disseminated regarding the evidence by which it is supported, and regarding the present status of the doctrine in the scientific world. in its broad sense, evolution has come to mean the development of all nature from the past. we may, if we wish, think of the long train of events in the formation of the world, and in supplying it with life as a story inscribed upon a scroll that is being gradually unrolled. everything which has come to pass is on that part so far exposed, and everything in the future is still covered, but will appear in due course of time; thus the designation of evolution as "the unrolling of the scroll of the universe" becomes picturesquely suggestive. in its wide meaning, it includes the formation of the stars, solar systems, the elements of the inorganic world, as well as all living nature--this is general evolution; but the word as commonly employed is limited to organic evolution, or the formation of life upon our planet. it will be used hereafter in this restricted sense. the vagueness regarding the theory of organic evolution arises chiefly from not understanding the points at issue. one of the commonest mistakes is to confuse darwinism with organic evolution. it is known, for illustration, that controversies are current among scientific workers regarding darwinism and certain phases of evolution, and from this circumstance it is assumed that the doctrine of organic evolution as a whole is losing ground. the discussions of de vries and others--all believers in organic evolution--at the scientific congress in st. louis in , led to the statement in the public press that the scientific world was haggling over the evolution-theory, and that it was beginning to surrender it. such statements are misleading and tend to perpetuate the confusion regarding its present status. furthermore, the matter as set forth in writings like the grotesque little book, _at the deathbed of darwinism_ tends to becloud rather than to clear the atmosphere. the theory of organic evolution relates to the history of animal and plant life, while darwin's theory of natural selection is only one of the various attempts to point out the causes for that history's being what it is. an attack upon darwinism is not, in itself, an attack upon the general theory, but upon the adequacy of his explanation of the way in which nature has brought about the diversity of animal and plant life. natural selection is the particular factor which darwin has emphasized, and the discussion of the part played by other factors tends only to extend the knowledge of the evolutionary process, without detracting from it as a general theory. while the controversies among scientific men relate for the most part to the influences that have been operative in bringing about organic evolution, nevertheless there are a few in the scientific camp who repudiate the doctrine. fleischmann, of erlangen, is perhaps the most conspicuous of those who are directing criticism against the general doctrine, maintaining that it is untenable. working biologists will be the first to admit that it is not demonstrated by indubitable evidence, but the weight of evidence is so compelling that scientific men as a body regard the doctrine of organic evolution as merely expressing a fact of nature, and we can not in truth speak of any considerable opposition to it. since fleischmann speaks as an anatomist, his suppression of anatomical facts with which he is acquainted and his form of special pleading have impressed the biological world as lacking in sincerity. this is not the place, however, to deal with the technical aspects of the discussion of the factors of organic evolution; it is rather our purpose here to give a descriptive account of the theory and its various explanations. first we should aim to arrive at a clear idea of what the doctrine of evolution is, and the basis upon which it rests; then of the factors which have been emphasized in attempted explanations of it; and, finally, of the rise of evolutionary thought, especially in the nineteenth century. the bringing forward of these points will be the aim of the following pages. nature of the question.--it is essential at the outset to perceive the nature of the question involved in the theories of organic evolution. it is not a metaphysical question, capable of solution by reflection and reasoning with symbols; the data for it must rest upon observation of what has taken place in the past in so far as the records are accessible. it is not a theological question, as so many have been disposed to argue, depending upon theological methods of interpretation. it is not a question of creation through divine agencies, or of non-creation, but a question of method of creation. evolution as used in biology is merely a history of the steps by which animals and plants came to be what they are. it is, therefore, a historical question, and must be investigated by historical methods. fragments of the story of creation are found in the strata of the earth's crust and in the stages of embryonic development. these clues must be brought together; and the reconstruction of the story is mainly a matter of getting at the records. drummond says that evolution is "the story of creation as told by those who know it best." the historical method.--the historical method as applied to searching out the early history of mankind finds a parallel in the investigations into the question of organic evolution. in the buried cities of palestine explorers have uncovered traces of ancient races and have in a measure reconstructed their history from fragments, such as coins, various objects of art and of household use, together with inscriptions on tombs and columns and on those curious little bricks which were used for public records and correspondence. one city having been uncovered, it is found by lifting the floors of temples and other buildings, and the pavement of public squares, that this city, although very ancient, is built upon the ruins of a more ancient one, which in turn covers the ruins of one still older. in this way, as many as seven successive cities have been found, built one on top of the other, and new and unexpected facts regarding ancient civilization have been brought to light. we must admit that this gives us an imperfect history, with many gaps; but it is one that commands our confidence, as being based on facts of observation, and not on speculation. in like manner the knowledge of the past history of animal life is the result of explorations by trained scholars into the records of the past. we have remains of ancient life in the rocks, and also traces of past conditions in the developing stages of animals. these are all more ancient than the inscriptions left by the hand of man upon his tombs, his temples, and his columns, but nevertheless full of meaning if we can only understand them. this historical method of investigation applied to the organic world has brought new and unexpected views regarding the antiquity of life. the diversity of living forms.--sooner or later the question of the derivation of the animals and plants is bound to come to the mind of the observer of nature. there exist at present more than a million different kinds of animals. the waters, the earth, the air teem with life. the fishes of the sea are almost innumerable, and in a single order of the insect-world, the beetles, more than , species are known and described. in addition to living animals, there is entombed in the rocks a great multitude of fossil forms which lived centuries ago, and many of which have become entirely extinct. how shall this great diversity of life be accounted for? has the great variety of forms existed unchanged from the days of their creation to the present? or have they, perchance, undergone modifications so that one original form, or at least a few original types, may have through transformations merged into different kinds? this is not merely an idle question, insoluble from the very nature of the case; for the present races of animals have a lineage reaching far into the past, and the question of fixity of form as against alteration of type is a historical question, to be answered by getting evidence as to their line of descent. are species fixed in nature?--the aspect of the matter which presses first upon our attention is this: are the species (or different kinds of animals and plants) fixed, and, within narrow limits, permanent, as linnæus supposed? have they preserved their identity through all time, or have they undergone changes? this is the heart of the question of organic evolution. if observation shows species to be constant at the present time, and also to have been continuous so far as we can trace their parentage, we must conclude that they have not been formed by evolution; but if we find evidence of their transmutation into other species, then there has been evolution. it is well established that there are wide ranges of variation among animals and plants, both in a wild state and under domestication. great changes in flowers and vegetables are brought about through cultivation, while breeders produce different kinds of pigeons, fowls, and stock. we know, therefore, that living beings may change through modification of the circumstances and conditions that affect their lives. but general observations extending over a few decades are not sufficient. we must, if possible, bring the history of past ages to bear upon the matter, and determine whether or not there had been, with the lapse of time, any considerable alteration in living forms. evolutionary series.--fortunately, there are preserved in the rocks the petrified remains of animals, showing their history for many thousands of years, and we may use them to test the question. it is plain that rocks of a lower level were deposited before those that cover them, and we may safely assume that the fossils have been preserved in their proper chronological order. now, we have in slavonia some fresh-water lakes that have been drying up from the tertiary period. throughout the ages, these waters were inhabited by snails, and naturally the more ancient ones were the parents of the later broods. as the animals died their shells sank to the bottom and were covered by mud and débris, and held there like currants in a pudding. in the course of ages, by successive accumulations, these layers thickened and were changed into rock, and by this means shells have been preserved in their proper order of birth and life, the most ancient at the bottom and the newest at the top. we can sink a shaft or dig a trench, and collect the shells and arrange them in proper order. although the shells in the upper strata are descended from those near the bottom, they are very different in appearance. no one would hesitate to name them different species; in fact, when collections were first made, naturalists classified these shells into six or eight different species. if, however, a collection embracing shells from all levels is arranged in a long row in proper order, a different light is thrown on the matter; while those at the ends are unlike, yet if we begin at one end and pass to the other we observe that the shells all grade into one another by such slight changes that there is no line showing where one kind leaves off and another begins. thus their history for thousands of years bears testimony to the fact that the species have not remained constant, but have changed into other species. fig. will give an idea of the varieties and gradations. it represents shells of a genus, paludina, which is still abundant in most of the fresh waters of our globe. [illustration: fig. .--transmutations of paludina. (after neumayer.)] a similar series of shells has been brought to light in württemberg in which the variations pass through wider limits, so that not only different species may be observed, but different genera connected by almost insensible gradations. these transformations are found in a little flattened pond-shell similar to the planorbis, which is so common at the present time. [illustration: fig. .--planorbis shells from steinheim. (after hyatt.)] fig. shows some of these transformations, the finer gradations being omitted. the shells from these two sources bear directly upon the question of whether or not species have held rigidly to their original form. after this kind of revelation in reference to lower animals, we turn with awakened interest to the fossil bones of the higher animals. evolution of the horse.--when we take into account the way in which fossils have been produced we see clearly that it is the hard parts, such as the shells and the bones, that will be preserved, while the soft parts of animals will disappear. is it not possible that we may find the fossil bones of higher animals arranged in chronological order and in sufficient number to supplement the testimony of the shells? there has been preserved in the rocks of our western states a very complete history of the evolution of the horse family, written, as it were, on tablets of stone, and extending over a period of more than two million years, as the geologists estimate time. geologists can, of course, measure the thickness of rocks and form some estimate of the rate at which they were deposited by observing the character of the material and comparing the formation with similar water deposits of the present time. near the surface, in the deposits of the quarternary period, are found remains of the immediate ancestors of the horse, which are recognized as belonging to the same genus, equus, but to a different species; thence, back to the lowest beds of the tertiary period we come upon the successive ancestral forms, embracing several distinct genera and exhibiting an interesting series of transformations. if in this way we go into the past a half-million years, we find the ancestors of the horse reduced in size and with three toes each on the fore and hind feet. the living horse now has only a single toe on each foot, but it has small splint-like bones that represent the rudiments of two more. if we go back a million years, we find three toes and the rudiments of a fourth; and going back two million years, we find four fully developed toes, and bones in the feet to support them. it is believed that in still older rocks a five-toed form will be discovered, which was the parent of the four-toed form. in the collections at yale college there are preserved upward of thirty steps or stages in the history of the horse family, showing that it arose by evolution or gradual change from a four-or five-toed ancestor of about the size of a fox, and that it passed through many changes, besides increase in size, in the two million years in which we can get facts as to its history. remarkable as is this feature of the marsh collection at new haven, it is now surpassed by that in the museum of natural history in new york city. here, through the munificent gifts of the late w.c. whitney, there has been accumulated the most complete and extensive collection of fossil horses in the world. this embraced, in , some portions of fossil horses, having been derived from explorations under the whitney fund. the extraordinary character of the collection is shown from the fact that it contains five complete skeletons of fossil horses--more than existed at that time in all other museums of the world. the specimens in this remarkable collection show phases in the parallel development of three or four distinct races of horse-like animals, and this opens a fine problem in comparative anatomy; _viz._, to separate those in the direct line of ancestry of our modern horse from all the others. this has been accomplished by osborn, and through his critical analysis we have become aware of the fact that the races of fossil horses had not been distinguished in any earlier studies. as a result of these studies, a new ancestry of the horse, differing in details from that given by huxley and marsh, is forthcoming. fig. shows the bones of the foreleg of the modern horse, and fig. some of the modifications through which it has passed. fig. shows a reconstruction of the ancestor of the horse made by charles r. knight, the animal painter, under the direction of professor osborn. [illustration: fig. .--bones of the foreleg of a horse.] while the limbs were undergoing the changes indicated, other parts of the organism were also being transformed and adapted to the changing conditions of its life. the evolution of the grinding teeth of the horse is fully exhibited in the fossil remains. all the facts bear testimony that the horse was not originally created as known to-day, but that his ancestors existed in different forms, and in evolution have transcended several genera and a considerable number of species. the highly specialized limb of the horse adapted for speed was the product of a long series of changes, of which the record is fairly well preserved. moreover, the records show that the atavus of the horse began in north america, and that by migration the primitive horses spread from this continent to europe, asia, and africa. [illustration: fig. .--bones of the foreleg and molar teeth of fossil ancestors of the horse. european forms. (after kayser.)] so far we have treated the question of fixity of species as a historical one, and have gone searching for clues of past conditions just as an archæologist explores the past in buried cities. the facts we have encountered, taken in connection with a multitude of others pointing in the same direction, begin to answer the initial question, were the immense numbers of living forms created just as we find them, or were they evolved by a process of transformation? the geological record of other families of mammals has also been made out, but none so completely as that of the horse family. the records show that the camels were native in north america, and that they spread by migration from the land of their birth to asia and africa, probably crossing by means of land-connections which have long since become submerged. the geological record, considered as a whole, shows that the earlier formed animals were representatives of the lower groups, and that when vertebrate animals were formed, for a very long time only fishes were living, then amphibians, reptiles, birds, and finally, after immense reaches of time, mammals began to appear. connecting forms.--interesting connecting forms between large groups sometimes are found, or, if not connecting forms, generalized ones embracing the structural characteristics of two separate groups. such a form is the archæopteryx (fig. ), a primitive bird with reptilian anatomy, with teeth in its jaws, and a long, lizard-like tail covered with feathers, which seems to show connection between birds and reptiles. the wing also shows the supernumerary fingers, which have been suppressed in modern birds. another suggestive type of this kind is the flying reptile or pterodactyl, of which a considerable number have been discovered. illustrations indicating that animals have had a common line of descent might be greatly multiplied. [illustration: fig. .--reconstruction of the ancestor of the horse by charles r. knight, under the direction of professor osborn. permission american museum natural history.] the embryological record and its connection with evolution.--the most interesting, as well as the most comprehensive clues bearing on the evolution of animal life are found in the various stages through which animals pass on their way from the egg to the fully formed animal. all animals above the protozoa begin their lives as single cells, and between that rudimentary condition and the adult stage every gradation of structure is exhibited. as animals develop they become successively more and more complex, and in their shifting history many rudimentary organs arise and disappear. for illustration, in the young chick, developing within the hen's egg, there appear, after three or four days of incubation, gill-slits, or openings into the throat, like the gill-openings of lower fishes. these organs belong primarily to water life, and are not of direct use to the chick. the heart and the blood-vessels at this stage are also of the fish-like type, but this condition does not last long; the gill-slits, or gill-clefts, fade away within a few days, and the arteries of the head and the neck undergo great changes long before the chick is hatched. similar gill-clefts and similar arrangements of blood-vessels appear also very early in the development of the young rabbit, and in the development of all higher life. except for the theory of descent, such things would remain a lasting enigma. the universal presence of gill-clefts is not to be looked on as a haphazard occurrence. they must have some meaning, and the best suggestion so far offered is that they are survivals inherited from remote ancestors. the higher animals have sprung from simpler ones, and the gill-slits, along with other rudimentary organs, have been retained in their history. it is not necessary to assume that they are inherited from adult ancestors; they are, more likely, embryonic structures still retained in the developmental history of higher animals. such traces are like inscriptions on ancient columns--they are clues to former conditions, and, occurring in the animal series, they weigh heavily on the side of evolution. [illustration: fig. .--fossil remains of a primitive bird (archeopteryx). from the specimen in the berlin museum. (after kayser.)] an idea of the appearance of gill-clefts may be obtained from fig. showing the gill-clefts in a shark and those in the embryo of a chick and a rabbit. [illustration: fig. .--the gill-clefts of a shark (upper fig.) compared with those of the embryonic chick (to the left) and rabbit.] of a similar nature are the rudimentary teeth in the jaws of the embryo of the whalebone whale (fig. ). the adults have no teeth, these appearing only as transitory rudiments in the embryo. it is to be assumed that the teeth are inheritances, and that the toothless baleen whale is derived from toothed ancestors. [illustration: fig. .--the jaws of an embryonic whale, showing rudimentary teeth.] if we now turn to comparative anatomy, to classification, and to the geographical distribution of animals, we find that it is necessary to assume the doctrine of descent in order to explain the observed facts; the evidence for evolution, indeed, becomes cumulative. but it is not necessary, nor will space permit, to give extended illustrations from these various departments of biological researches. the human body.--although the broad doctrine of evolution rests largely upon the observation of animals and plants, there is naturally unusual interest as to its teaching in reference to the development of the human body. that the human body belongs to the animal series has long been admitted, and that it has arisen through a long series of changes is shown from a study of its structure and development. it retains marks of the scaffolding in its building. the human body has the same devious course of embryonic development as that of other mammals. in the course of its formation gill-clefts make their appearance; the circulation is successively that of a single-, a double-, and a four-chambered heart, with blood-vessels for the gill-clefts. time and energy are consumed in building up rudimentary structures which are evanescent and whose presence can be best explained on the assumption that they are, as in other animals, hereditary survivals. wiedersheim has pointed out more than one hundred and eighty rudimentary or vestigial structures belonging to the human body, which indicate an evolutionary relationship with lower vertebrates. it would require a considerable treatise to present the discoveries in reference to man's organization, as wiedersheim has done in his _structure of man_. as passing illustrations of the nature of some of these suggestive things bearing on the question of man's origin may be mentioned: the strange grasping power of the newly born human infant, retained for a short time, and enabling the babe to sustain its weight; the presence of a tail and rudimentary tail muscles; of rudimentary ear muscles; of gill-clefts, etc. antiquity of man.--the geological history of man is imperfectly known, although sporadic explorations have already accumulated an interesting series, especially as regards the shape and capacity of skulls. the remains of early quarternary man have been unearthed in various parts of europe, and the probable existence of man in the tertiary period is generally admitted. as osborn says, "virtually three links have been found in the chain of human ancestry." the most primitive pre-human species is represented by portions of the skull and of the leg bones found in java by the dutch surgeon dubois in the year . these remains were found in tertiary deposits, and were baptized under the name of _pithecanthropus erectus_. the structural position of this fossil is between the chimpanzee, the highest of anthropoid apes, and the "neanderthal man." with characteristic scientific caution osborn says that the _pithecanthropus_ "belongs in the line of none of the existing anthropoid apes, and falls very near, but not directly, in the line of human ancestry." the second link is supplied by the famous neanderthal skull found in the valley of the neander, near düsseldorf, in . the discovery of this skull, with its receding forehead and prominent ridges above the orbits of the eyes, and its small cranial capacity, created a sensation, for it was soon seen that it was intermediate between the skulls of the lowest human races and those of the anthropoid apes. virchow declared that if the skull was pre-human its structural characteristics were abnormal. this conclusion, however, was rendered untenable by the discovery in of similar skulls and the skeletons of two persons, in a cave near spy in belgium. the "spy man" and the "neanderthal man" belong to the same type and are estimated to have been living in the middle of the palæolithic age. [illustration: fig. .--profile reconstructions of the skulls of living and fossil men: . brachycephalic european; . the more ancient of the nebraska skulls; . the neanderthal man; . one of the spy skulls; . skull of the java man. (altered from schwalbe and osborn.)] the third link is in the early neolithic man of engis. and now to this interesting series of gradations has been added another by the discovery in of a supposed primitive race of men in nebraska. the two skulls unearthed in douglass county in that state indicate a cranial capacity falling below that of the "australian negro, the lowest existing type of mankind known at present." fig. shows in outline profile reconstructions of the skulls of some of the fossil types as compared with the short-headed type of europe. palæontological discoveries are thus coming to support the evidences of man's evolution derived from embryology and archæology. while we must admit that the geological evidences are at present fragmentary, there is, nevertheless, reasonable ground for the expectation that they will be extended by more systematic explorations of caverns and deposits of the quarternary and late tertiary periods. mental evolution.--already the horizon is being widened, and new problems in human evolution have been opened. the evidences in reference to the evolution of the human body are so compelling as to be already generally accepted, and we have now the question of evolution of mentality to deal with. the progressive intelligence of animals is shown to depend upon the structure of the brain and the nervous system, and there exists such a finely graded series in this respect that there is strong evidence of the derivation of human faculties from brute faculties. sweep of the doctrine of evolution.--the great sweep of the doctrine of evolution makes it "one of the greatest acquisitions of human knowledge." there has been no point of intellectual vantage reached which is more inspiring. it is so comprehensive that it enters into all realms of thought. weismann expresses the opinion that "the theory of descent is the most progressive step that has been taken in the development of human knowledge," and says that this position "is justified, it seems to me, even by this fact alone: that the evolution idea is not merely a new light on the special region of biological sciences, zoölogy and botany, but is of quite general importance. the conception of an evolution of life upon the earth reaches far beyond the bounds of any single science, and influences our whole realm of thought. it means nothing less than the elimination of the miraculous from our knowledge of nature, and the placing of the phenomena of life on the same plane as the other natural processes, that is, as having been brought about by the same forces and being subject to the same laws." one feature of the doctrine is very interesting; it has enabled anatomists to predict that traces of certain structures not present in the adult will be found in the embryonic condition of higher animals, and by the verification of these predictions, it receives a high degree of plausibility. the presence of an _os centrale_ in the human wrist was predicted, and afterward found, as also the presence of a rudimentary thirteenth rib in early stages of the human body. the predictions, of course, are chiefly technical, but they are based on the idea of common descent and adaptation. it took a long time even for scientific men to arrive at a belief in the continuity of nature, and having arrived there, it is not easy to surrender it. there is no reason to think that the continuity is broken in the case of man's development. naturalists have now come to accept as a mere statement of a fact of nature that the vast variety of forms of life upon our globe has been produced by a process of evolution. if this position be admitted, the next question would be, what are the factors which have been operative to bring this about? this brings us naturally to discuss the theories of evolution. chapter xvii theories of evolution: lamarck, darwin the impression so generally entertained that the doctrine of organic evolution is a vague hypothesis, requiring for its support great stretches of the imagination, gives way to an examination of the facts, and we come to recognize that it is a well-founded theory, resting upon great accumulations of evidence. if the matter could rest here, it would be relatively simple; but it is necessary to examine into the causes of the evolutionary process. while scientific observation has shown that species are not fixed, but undergo transformations of considerable extent, there still remains to be accounted for the way in which these changes have been produced. one may assume that the changes in animal life are the result of the interaction of protoplasm and certain natural agencies in its surroundings, but it is evidently a very difficult matter to designate the particular agencies or factors of evolution that have operated to bring about changes in species. the attempts to indicate these factors give rise to different theories of evolution, and it is just here that the controversies concerning the subject come in. we must remember, however, that to-day the controversies about evolution are not as to whether it was or was not the method of creation, but as to the factors by which the evolution of different forms was accomplished. says packard: "we are all evolutionists, though we may differ as to the nature of the efficient causes." of the various theories which had been advanced to account for evolution, up to the announcement of the mutation-theory of de vries in , three in particular had commanded the greatest amount of attention and been the field for varied and extensive discussion. these are the theories of lamarck, darwin, and weismann. they are comprehensive theories, dealing with the process as a whole. most of the others are concerned with details, and emphasize certain phases of the process. doubtless the factors that have played a part in molding the forms that have appeared in the procession of life upon our globe have been numerous, and, in addition to those that have been indicated, osborn very aptly suggests that there may be undiscovered factors of evolution. within a few years de vries has brought into prominence the idea of sudden transformations leading to new species, and has accounted for organic evolution on that basis. further consideration of this theory, however, will be postponed, while in the present chapter we shall endeavor to bring out the salient features of the theories of lamarck and darwin, without going into much detail regarding them. lamarck lamarck was the first to give a theory of evolution that has retained a place in the intellectual world up to the present time, and he may justly be regarded as the founder of that doctrine in the modern sense. the earlier theories were more restricted in their reach than that of lamarck. erasmus darwin, his greatest predecessor in this field of thought, announced a comprehensive theory, which, while suggestive and forceful in originality, was diffuse, and is now only of historical importance. the more prominent writers on evolution in the period prior to lamarck will be dealt with in the chapter on the rise of evolutionary thought. lamarck was born in , and led a quiet, monotonous life, almost pathetic on account of his struggles with poverty, and the lack of encouragement and proper recognition by his contemporaries. his life was rendered more bearable, however, even after he was overtaken by complete blindness, by the intellectual atmosphere that he created for himself, and by the superb confidence and affection of his devoted daughter cornélie, who sustained him and made the truthful prediction that he would be recognized by posterity ("_la postérité vous honorera_"). his family.--he came of a military family possessing some claims to distinction. the older name of the family had been de monet, but in the branch to which lamarck belonged the name had been changed to de lamarque, and in the days of the first republic was signed plain lamarck by the subject of this sketch. jean baptiste lamarck was the eleventh and last child of his parents. the other male members of the family having been provided with military occupations, jean was selected by his father, although against the lad's own wish, for the clerical profession, and accordingly was placed in the college of the jesuits at amiens. he did not, however, develop a taste for theological studies, and after the death of his father in "nothing could induce the incipient abbé, then seventeen years of age, longer to wear his bands." his ancestry asserted itself, and he forsook the college to follow the french army that was then campaigning in germany. mounted on a broken-down horse which he had succeeded in buying with his scanty means, he arrived on the scene of action, a veritable raw recruit, appearing before colonel lastic, to whom he had brought a letter of recommendation. military experience.--the colonel would have liked to be rid of him, but owing to lamarck's persistence, assigned him to a company; and, being mounted, lamarck took rank as a sergeant. during his first engagement his company was exposed to the direct fire of the enemy, and the officers one after another were shot until lamarck by order of succession was in command of the fourteen remaining grenadiers. although the french army retreated, lamarck refused to move with his squad until he received directions from headquarters to retire. in this his first battle he showed the courage and the independence that characterized him in later years. adopts natural science.--an injury to the glands of the neck, resulting from being lifted by the head in sport by one of his comrades, unfitted him for military life, and he went to paris and began the study of medicine, supporting himself in the mean time by working as a bank clerk. it was in his medical course of four years' severe study that lamarck received the exact training that was needed to convert his enthusiastic love for science into the working powers of an investigator. he became especially interested in botany, and, after a chance interview with rousseau, he determined to follow the ruling passion of his nature and devote himself to natural science. after about nine years' work he published, in , his _flora of france_, and in due course was appointed to a post in botany in the academy of sciences. he did not hold this position long, but left it to travel with the sons of buffon as their instructor. this agreeable occupation extended over two years, and he then returned to paris, and soon after was made keeper of the herbarium in the royal garden, a subordinate position entirely beneath his merits. lamarck held this poorly paid position for several years, and was finally relieved by being appointed a professor in the newly established _jardin des plantes_. he took an active part in the reorganization of the royal garden (_jardin du roi_) into the _jardin des plantes_. when, during the french revolution, everything that was suggestive of royalty became obnoxious to the people, it was lamarck who suggested in that the name of the king's garden be changed to that of the botanical garden (_jardin des plantes_). the royal garden and the cabinet of natural history were combined, and in the name jardin des plantes proposed by lamarck was adopted for the institution. it was through the endorsements of lamarck and geoffroy saint-hilaire that cuvier was brought into this great scientific institution; cuvier, who was later to be advanced above him in the jardin and in public favor, and who was to break friendship with lamarck and become the opponent of his views, and who also was to engage in a memorable debate with his other supporter, saint-hilaire. the portrait of lamarck shown in fig. is one not generally known. its date is undetermined, but since it was published in thornton's _british plants_ in , we know that it was painted before the publication of lamarck's _philosophie zoologique_, and before the full force of the coldness and heartless neglect of the world had been experienced. in his features we read supremacy of the intellect, and the unflinching moral courage for which he was notable. lamarck has a more hopeful expression in this portrait than in those of his later years. [illustration: fig. .--lamarck, - . from thornton's _british plants_, .] lamarck changes from botany to zoölogy.--until , when he was fifty years of age, lamarck was devoted to botany, but on being urged, after the reorganization of the _jardin du roi_, to take charge of the department of invertebrates, he finally consented and changed from the study of plants to that of animals. this change had profound influence in shaping his ideas. he found the invertebrates in great confusion, and set about to bring order out of chaos, an undertaking in which, to his credit be it acknowledged, he succeeded. the fruit of his labors, the natural history of invertebrated animals (_historie naturelle des animaux sans vertèbres_, - ), became a work of great importance. he took hold of this work, it should be remembered, as an expert observer, trained to rigid analysis by his previous critical studies in botany. in the progress of the work he was impressed with the differences in animals and the difficulty of separating one species from another. he had occasion to observe the variations produced in animals through the influence of climate, temperature, moisture, elevation above the sea-level, etc. he observed also the effects of use and disuse upon the development of organs: the exercise of an organ leading to its greater development, and the disuse to its degeneration. numerous illustrations are cited by lamarck which serve to make his meaning clear. the long legs of wading birds are produced and extended by stretching to keep above the water; the long neck and bill of storks are produced by their habit of life; the long neck of the giraffe is due to reaching for foliage on trees; the web-footed birds, by spreading the toes when they strike the water, have stimulated the development of a membrane between the toes, etc. in the reverse direction, the loss of the power of flight in the "wingless" bird of new zealand is due to disuse of the wings; while the loss of sight in the mole and in blind cave animals has arisen from lack of use of eyes. the changes produced in animal organization in this way were believed to be continued by direct inheritance and improved in succeeding generations. he believed also in a perfecting principle, tending to improve animals--a sort of conscious endeavor on the part of the animal playing a part in its better development. finally, he came to believe that the agencies indicated above were the factors of the evolution of life. his theory of evolution.--all that lamarck had written before he changed from botany to zoölogy ( ) indicates his belief in the fixity of species, which was the prevailing notion among naturalists of the period. then, in , we find him apparently all at once expressing a contrary opinion, and an opinion to which he held unwaveringly to the close of his life. it would be of great interest to determine when lamarck changed his views, and upon what this radical reversal of opinion was based; but we have no sure record to depend upon. since his theory is developed chiefly upon considerations of animal life, it is reasonable to assume that his evolutionary ideas took form in his mind after he began the serious study of animals. doubtless, his mind having been prepared and his insight sharpened by his earlier studies, his observations in a new field supplied the data which led him directly to the conviction that species are unstable. as packard, one of his recent biographers, points out, the first expression of his new views of which we have any record occurred in the spring of , on the occasion of his opening lecture to his course on the invertebrates. this avowal of belief in the extensive alteration of species was published in as the preface to his _système des animaux sans vertèbres_. here also he foreshadowed his theory of evolution, saying that nature, having formed the simplest organisms, "then with the aid of much time and favorable circumstances ... formed all the others." it has been generally believed that lamarck's first public expression of his views on evolution was published in in his _recherches sur l'organisation des corps vivans_, but the researches of packard and others have established the earlier date. lamarck continued for several years to modify and amplify the expression of his views. it is not necessary, however, to follow the molding of his ideas on evolution as expressed in the opening lectures to his course in the years , , , and , since we find them fully elaborated in his _philosophie zoologique_, published in , and this may be accepted as the standard source for the study of his theory. in this work he states two propositions under the name of laws, which have been translated by packard as follows: "_first law_: in every animal which has not exceeded the term of its development, the more frequent and sustained use of any organ gradually strengthens this organ, develops and enlarges it, and gives it a strength proportioned to the length of time of such use; while the constant lack of use of such an organ imperceptibly weakens it, causing it to become reduced, progressively diminishes its faculties, and ends in its disappearance. "_second law_: everything which nature has caused individuals to acquire or lose by the influence of the circumstances to which their race may be for a long time exposed, and consequently by the influence of the predominant use of such an organ, or by that of the constant lack of use of such part, it preserves by heredity and passes on to the new individuals which descend from it, provided that the changes thus acquired are common to both sexes, or to those which have given origin to these new individuals. "these are the two fundamental truths which can be misunderstood only by those who have never observed or followed nature in its operations," etc. the first law embodies the principle of use and disuse, the second law that of heredity. in his theory received some extensions of minor importance. the only points to which attention need be called are that he gives four laws instead of two, and that a new feature occurs in the second law in the statement that the production of a new organ is the result of a new need (_besoin_) which continues to make itself felt. simplified statement of lamarck's views.--for practical exposition the theory maybe simplified into two sets of facts: first, those to be classed under variation; and, second, those under heredity. variations of organs, according to lamarck, arise in animals mainly through use and disuse, and new organs have their origin in a physiological need. a new need felt by the animal expresses itself on the organism, stimulating growth and adaptations in a particular direction. this part of lamarck's theory has been subjected to much ridicule. the sense in which he employs the word _besoin_ has been much misunderstood; when, however, we take into account that he uses it, not merely as expressing a wish or desire on the part of the animal, but as the reflex action arising from new conditions, his statement loses its alleged grotesqueness and seems to be founded on sound physiology. inheritance.--lamarck's view of heredity was uncritical; according to his conception, inheritance was a simple, direct transmission of those superficial changes that arise in organs within the lifetime of an individual owing to use and disuse. it is on this question of the direct inheritance of variations acquired in the lifetime of an individual that his theory has been the most assailed. the belief in the inheritance of acquired characteristics has been so undermined by experimental evidence that at the present time we can not point to a single unchallenged instance of such inheritance. but, while lamarck's theory has shown weakness on that side, his ideas regarding the production of variations have been revived and extended. variation.--the more commendable part of his theory is the attempt to account for variation. darwin assumed variation, but lamarck attempted to account for it, and in this feature many discerning students maintain that the theory of lamarck is more philosophical in its foundation than that of darwin. in any theory of evolution we must deal with the variation of organisms and heredity, and thus we observe that the two factors discussed by lamarck are basal. although it must be admitted that even to-day we know little about either variation or heredity, they remain basal factors in any theory of evolution. time and favorable conditions.--lamarck supposed a very long time was necessary to bring about the changes which have taken place in animals. the central thought of time and favorable conditions occurs again and again in his writings. the following quotation is interesting as coming from the first announcement of his views in : "it appears, as i have already said, that _time_ and _favorable conditions_ are the two principal means which nature has employed in giving existence to all her productions. we know that for her time has no limit, and that consequently she has it always at her disposal. "as to the circumstances of which she has had need and of which she makes use every day in order to cause her productions to vary, we can say that in a manner they are inexhaustible. "the essential ones arising from the influence and from all the environing media, from the diversity of local causes, of habits, of movements, of action, finally of means of living, of preserving their lives, of defending themselves, of multiplying themselves, etc. moreover, as the result of these different influences, the faculties, developed and strengthened by use, become diversified by the new habits maintained for long ages, and by slow degrees the structure, the consistence--in a word, the nature, the condition of the parts and of the organs consequently participating in all these influences, became preserved and were propagated by heredity (génération)." (packard's translation.) salient points.--the salient points in lamarck's theory may be compacted into a single sentence: it is a theory of the evolution of animal life, depending upon variations brought about mainly through use and disuse of parts, and also by responses to external stimuli, and the direct inheritance of the same. his theory is comprehensive, so much so that he includes mankind in his general conclusions. lamarck supposed that an animal having become adapted to its surroundings would remain relatively stable as to its structure. to the objection raised by cuvier that animals from egypt had not changed since the days when they were preserved as mummies, he replied that the climate of egypt had remained constant for centuries, and therefore no change in its fauna was to be expected. species.--since the question of the fixity of species is the central one in theories of evolution, it will be worth while to quote lamarck's definition of species: "all those who have had much to do with the study of natural history know that naturalists at the present day are extremely embarrassed in defining what they mean by the word species.... we call _species_ every collection of individuals which are alike or almost so, and we remark that the regeneration of these individuals conserves the species and propagates it in continuing successively to reproduce similar individuals." he then goes on with a long discussion to show that large collections of animals exhibit a great variation in species, and that they have no absolute stability, but "enjoy only a relative stability." herbert spencer adopted and elaborated the theory of lamarck. he freed it from some of its chief crudities, such as the idea of an innate tendency toward perfection. in many controversies mr. spencer defended the idea of the transmission of acquired characters. the ideas of lamarck have, therefore, been transmitted to us largely in the spencerian mold and in the characteristic language of that great philosopher. there has been but little tendency to go to lamarck's original writings. packard, whose biography of lamarck appeared in , has made a thorough analysis of his, writings and had incidentally corrected several erroneous conception. neo-lamarckism.--the ideas of lamarck regarding the beginning of variations have been revived and accorded much respect under the designation of neo-lamarckism. the revival of lamarckism is especially owing to the palæontological investigations of cope and hyatt. the work of e.d. cope in particular led him to attach importance to the effect of mechanical and other external causes in producing variation, and he points out many instances of use-inheritance. neo-lamarckism has a considerable following; it is a revival of the fundamental ideas of lamarck. darwin's theory while lamarck's theory rests upon two sets of facts, darwin's is founded on three: _viz._, the facts of variation, of inheritance, and of natural selection. the central feature of his theory is the idea of natural selection. no one else save wallace had seized upon this feature when darwin made it the center of his system. on account of the part taken by wallace simultaneously with darwin in announcing natural selection as the chief factor of evolution, it is appropriate to designate this contribution as the darwin-wallace principle of natural selection. the interesting connection between the original conclusions of darwin and wallace is set forth in chapter xix. variation.--it will be noticed that two of the causes assigned by darwin are the same as those designated by lamarck, but their treatment is quite different. darwin (fig. ) assumed variation among animals and plants without attempting to account for it, while lamarck undertook to state the particular influences which produce variation, and although we must admit that lamarck was not entirely successful in this attempt, the fact that he undertook the task places his contribution at the outset on a very high plane. [illustration: fig. .--charles darwin, - .] the existence of variation as established by observation is unquestioned. no two living organisms are exactly alike at the time of their birth, and even if they are brought up together under identical surroundings they vary. the variation of plants and animals under domestication is so conspicuous and well known that this kind of variation was the first to attract attention. it was asserted that these variations were perpetuated because the forms had been protected by man, and it was doubted that animals varied to any considerable extent in a state of nature. extended collections and observations in field and forest have, however, set this question at rest. if crows or robins or other birds are collected on an extensive scale, the variability of the same species will be evident. many examples show that the so-called species differ greatly in widely separated geographical areas, but collections from the intermediate territory demonstrate that the variations are connected by a series of fine gradations. if, for illustration, one should pass across the united states from the atlantic to the pacific coast, collecting one species of bird, the entire collection would exhibit wide variations, but the extremes would be connected by intermediate forms. the amount of variation in a state of nature is much greater than was at first supposed, because extensive collections were lacking, but the existence of wide variation is now established on the basis of observation. this fact of variation among animals and plants in the state of nature is unchallenged, and affords a good point to start from in considering darwinism. inheritance.--the idea that these variations are inherited is the second point. but what particular variations will be preserved and fostered by inheritance, and on what principle they will be selected, is another question--and a notable one. darwin's reply was that those variations which are of advantage to the individual will be the particular ones selected by nature for inheritance. while darwin implies the inheritance of acquired characteristics, his theory of heredity was widely different from that of lamarck. darwin's theory of heredity, designated the provisional theory of pangenesis, has been already considered (see chapter xiv). natural selection.--since natural selection is the main feature of darwin's doctrine, we must devote more time to it. darwin frequently complained that very few of his critics took the trouble to find out what he meant by the term natural selection. a few illustrations will make his meaning clear. let us first think of artificial selection as it is applied by breeders of cattle, fanciers of pigeons and of other fowls, etc. it is well known that by selecting particular variations in animals and plants, even when the variations are slight, the breeder or the horticulturalist will be able in a short time to produce new races of organic forms. this artificial selection on the part of man has given rise to the various breeds of dogs, the different kinds of pigeons, etc., all of which breed true. the critical question is, have these all an individual ancestral form in nature? observation shows that many different kinds--as pigeons--may be traced back to a single ancestral form, and thus the doctrine of the fixity of species is overthrown. now, since it is demonstrated by observation that variations occur, if there be a selective principle at work in nature, effects similar to those caused by artificial selection will be produced. the selection by nature of the forms fittest to survive is what darwin meant by natural selection. we can never understand the application, however, unless we take into account the fact that while animals tend to multiply in geometrical progression, as a matter of fact the number of any one kind remains practically constant. although the face of nature seems undisturbed, there is nevertheless a struggle for existence among all animals. this is easily illustrated when we take into account the breeding of fishes. the trout, for illustration, lays from , to , eggs. if the majority of these arrived at maturity and gave rise to progeny, the next generation would represent a prodigious number, and the numbers in the succeeding generations would increase so rapidly that soon there would not be room in the fresh waters of the earth to contain their descendants. what becomes of the immense number of fishes that die? they fall a prey to others, or they are not able to get food in competition with other more hardy relatives, so that it is not a matter of chance that determines which ones shall survive; those which are the strongest, the better fitted to their surroundings, are the ones which will be perpetuated. the recognition of this struggle for existence in nature, and the consequent survival of the fittest, shows us more clearly what is meant by natural selection. instead of man making the selection of those particular forms that are to survive, it is accomplished in the course of nature. this is natural selection. various aspects of natural selection.--further illustrations are needed to give some idea of the various phases of natural selection. speed in such animals as antelopes may be the particular thing which leads to their protection. it stands to reason that those with the greatest speed would escape more readily from their enemies, and would be the particular ones to survive, while the weaker and slower ones would fall victims to their prey. in all kinds of strain due to scarcity of food, inclemency of weather, and other untoward circumstances, the forms which are the strongest, physiologically speaking, will have the best chance to weather the strain and to survive. as another illustration, darwin pointed out that natural selection had produced a long-legged race of prairie wolves, while the timber wolves, which have less occasion for running, are short-legged. we can also see the operation of natural selection in the production of the sharp eyes of birds of prey. let us consider the way in which the eyes of the hawk have been perfected by evolution. natural selection compels the eye to come up to a certain standard. those hawks that are born with weak or defective vision cannot cope with the conditions under which they get their food. the sharp-eyed forms would be the first to discern their prey, and the most sure in seizing upon it. therefore, those with defective vision or with vision that falls below the standard will be at a very great disadvantage. the sharp-eyed forms will be preserved by a selective process. nature selects, we may say, the keener-eyed birds of prey for survival, and it is easy to see that this process of natural selection would establish and maintain a standard of vision. but natural selection tends merely to adapt animals to their surroundings, and does not always operate in the direction of increasing the efficiency of the organ. we take another illustration to show how darwin explains the origin of races of short-winged beetles on certain oceanic islands. madeira and other islands, as kerguelen island of the indian ocean, are among the most windy places in the world. the strong-winged beetles, being accustomed to disport themselves in the air, would be carried out to sea by the sudden and violent gales which sweep over those islands, while the weaker-winged forms would be left to perpetuate their kind. thus, generation after generation, the strong-winged beetles would be eliminated by a process of natural selection, and there would be left a race of short-winged beetles derived from long-winged ancestors. in this case the organs are reduced in their development, rather than increased; but manifestly the short-winged race of beetles is better adapted to live under the particular conditions that surround their life in these islands. while this is not a case of increase in the particular organ, it illustrates a progressive series of steps whereby the organism becomes better adapted to its surroundings. a similar instance is found in the suppression of certain sets of organs in internal parasites. for illustration, the tapeworm loses particular organs of digestion for which it does not have continued use; but the reproductive organs, upon which the continuance of its life depends, are greatly increased. such cases as the formation of short-winged beetles show us that the action of natural selection is not always to preserve what we should call the best, but simply to preserve the fittest. development, therefore, under the guidance of natural selection is not always progressive. selection by nature does not mean the formation and preservation of the ideally perfect, but merely the survival of those best fitted to their environment. color.--the various ways in which natural selection acts are exceedingly diversified. the colors of animals may be a factor in their preservation, as the stripes on the zebra tending to make it inconspicuous in its surroundings. the stripes upon the sides of tigers simulate the shadows cast by the jungle grass in which the animals live, and serves to conceal them from their prey as well as from enemies. those animals that assume a white color in winter become thereby less conspicuous, and they are protected by their coloration. as further illustrating color as a factor in the preservation of animals, we may cite a story originally told by professor e.s. morse. when he was collecting shells on the white sand of the japanese coast, he noticed numerous white tiger-beetles, which could scarcely be seen against the white background. they could be detected chiefly by their shadows when the sun was shining. as he walked along the coast he came to a wide band of lava which had flowed from a crater across the intervening country and plunged into the sea, leaving a broad dark band some miles in breadth across the white sandy beach. as he passed from the white sand to the dark lava, his attention was attracted to a tiger-beetle almost identical with the white one except as to color. instead of being white, it was black. he found this broad, black band of lava inhabited by the black tiger beetle, and found very few, if any, of the white kind. this is a striking illustration of what has occurred in nature. these two beetles are of the same species, and in examining the conditions under which they grow, it is discovered that out of the eggs laid by the original white forms, there now and then appears one of a dusky or black color. consider how conspicuous this dark object would be against the white background of sand. it would be an easy mark for the birds of prey that fly about, and therefore on the white surface the black beetles would be destroyed, while the white ones would be left. but on the black background of lava the conditions are reversed. there the white forms would be the conspicuous ones; as they wandered upon the black surface, they would be picked up by birds of prey and the black ones would be left. thus we see another instance of the operation of natural selection. mimicry.--we have, likewise, in nature a great number of cases that are designated mimicry. for illustration, certain caterpillars assume a stiff position, resembling a twig from a branch. we have also leaf-like butterflies. the kallima of india is a conspicuous illustration of a butterfly having the upper surface of its wings bright-colored, and the lower surface dull. when it settles upon a twig the wings are closed and the under-sides have a mark across them resembling the mid-rib of a leaf, so that the whole butterfly in the resting position becomes inconspicuous, being protected by mimicry. one can readily see how natural selection would be evoked in order to explain this condition of affairs. those forms that varied in the direction of looking like a leaf would be the most perfectly protected, and this feature being fostered by natural selection, would, in the course of time, produce a race of butterflies the resemblance of whose folded wings to a leaf would serve as a protection from enemies. it may not be out of place to remind the reader that the illustrations cited are introduced merely to elucidate darwin's theory and the writer is not committed to accepting them as explanations of the phenomena involved. he is not unmindful of the force of the criticisms against the adequacy of natural selection to explain the evolution of all kinds of organic structures. many other instances of the action of color might be added, such as the wearing of warning colors, those colors which belong to butterflies, grubs, and other animals that have a noxious taste. these warning colors have taught birds to leave alone the forms possessing those colors. sometimes forms which do not possess a disagreeable taste secure protection by mimicking the colors of the noxious varieties. sexual selection.--there is an entirely different set of cases which at first sight would seem difficult to explain on the principle of selection. how, for instance, could we explain the feathers in the tails of the birds of paradise, or that peculiar arrangement of feathers in the tail of the lyre-bird, or the gorgeous display of tail-feathers of the male peacock? here mr. darwin seized upon a selective principle arising from the influence of mating. the male birds in becoming suitors for a particular female have been accustomed to display their tail-feathers; the one with the most attractive display excites the pairing instinct in the highest degree, and becomes the selected suitor. in this way, through the operation of a form of selection which darwin designates sexual selection, possibly such curious adaptations as the peacock's tail may be accounted for. it should be pointed out that this part of the theory is almost wholly discredited by biologists. experimental evidence is against it. nevertheless in a descriptive account of darwin's theory it may be allowed to stand without critical comment. inadequacy of natural selection.--in nature, under the struggle for existence, the fittest will be preserved; and natural selection will operate toward the elaboration or the suppression of certain organs or certain characteristics when the elaboration or the suppression is of advantage to the animal form. much has been said of late as to the inadequacy of natural selection. herbert spencer and huxley, both accepting natural selection as one of the factors, doubted its complete adequacy. one point is often overlooked, and should be brought out with clearness; _viz._, that darwin himself was the first to point out clearly the inadequacy of natural selection as a universal law for the production of the great variety of animals and plants. in the second edition of the _origin of species_ he says: "but, as my conclusions have lately been much misrepresented, and it has been stated that i attribute the modification of species exclusively to natural selection, i may be permitted to remark that in the first edition of this work and subsequently i placed in a most conspicuous position,--namely, at the close of the introduction--the following words: 'i am convinced that natural selection has been the main, but not the exclusive means of modification.' this has been of no avail. great is the power of steady misrepresentation. but the history of science shows that fortunately this power does not long endure." the reaction against the all-sufficiency of natural selection, therefore, is something which was anticipated by darwin, and the quotation made above will be a novelty to many of our readers who supposed that they understood darwin's position. confusion between lamarck's and darwin's theories.--besides the failure to understand what darwin has written, there is great confusion, both in pictures and in writings, in reference to the theories of darwin and lamarck. poulton illustrated a state of confusion in one of his lectures on the theory of organic evolution, and the following instances are quoted from memory. we are most of us familiar with such pictures as the following: a man standing and waving his arms; in the next picture these arms and hands become enlarged, and in the successive pictures they undergo transformations into wings, and the transference is made into a flying animal. such pictures are designated "the origin of flight after darwin." the interesting circumstance is this, that the illustration does not apply to darwin's idea of natural selection at all, but is pure lamarckism. lamarck contended for the production of new organs through the influence of use and disuse, and this particular illustration refers to that, and not to natural selection at all. among the examples of ridicule to which darwin's ideas have been exposed, we cite one verse from the song of lord neaves. his lordship wrote a song with a large number of verses hitting off in jocular vein many of the claims and foibles of his time. in attempting to make fun of darwin's idea he misses completely the idea of natural selection, but hits upon the principle enunciated by lamarck, instead. he says: "a deer with a neck which was longer by half than the rest of his family's--try not to laugh-- by stretching and stretching became a giraffe, which nobody can deny." the clever young woman, miss kendall, however, in her _song of the ichthyosaurus_, showed clearness in grasping darwin's idea when she wrote: "ere man was developed, our brother, we swam, we ducked, and we dived, and we dined, as a rule, on each other. what matter? the toughest survived." this hits the idea of natural selection. the other two illustrations miss it, but strike the principle which was enunciated by lamarck. this confusion between lamarckism and darwinism is very wide-spread. darwin's book on the _origin of species_, published in , was epoch-making. if a group of scholars were asked to designate the greatest book of the nineteenth century--that is, the book which created the greatest intellectual stir--it is likely that a large proportion of them would reply that it is darwin's _origin of species_. its influence was so great in the different domains of thought that we may observe a natural cleavage between the thought in reference to nature between and all preceding time. his other less widely known books on _animals and plants under domestication_, the _descent of man_, etc., etc., are also important contributions to the discussion of his theory. a brief account of darwin, the man, will be found in chapter xix. chapter xviii theories of evolution continued: weismann, de vries weismann's views have passed through various stages of remodeling since his first public championship of the theory of descent on assuming, in , the position of professor of zoölogy in the university of freiburg. some time after that date he originated his now famous theory of heredity, which has been retouched, from time to time, as the result of aggressive criticism from others, and the expansion of his own mental horizon. as he said in , regarding his lectures on evolution which have been delivered almost regularly every year since , they "were gradually modified in accordance with the state of my knowledge at the time, so that they have been, i may say, a mirror of my own intellectual evolution." passing over his book, _the germ plasm_, published in english in , we may fairly take his last book, _the evolution theory_, , as the best exposition of his conclusions. the theoretical views of weismann have been the field of so much strenuous controversy that it will be well perhaps to take note of the spirit in which they have been presented. in the preface of his book just mentioned, he says: "i make this attempt to sum up and present as a harmonious whole the theories which for forty years i have been gradually building up on the basis of the legacy of the great workers of the past, and on the results of my own investigations and those of my fellow-workers, not because i regard the picture as incomplete or incapable of improvement, but because i believe its essential features to be correct, and because an eye-trouble which has hindered my work for many years makes it uncertain whether i shall have much more time and strength granted to me for its further elaboration." the germ-plasm theory is primarily a theory of heredity, and only when connected with other considerations does it become the full-fledged theory of evolution known as weismannism. the theory as a whole involves so many intricate details that it is difficult to make a clear statement of it for general readers. if in considering the theories of lamarck and darwin it was found advantageous to confine attention to salient points and to omit details, it is all the more essential to do so in the discussion of weismann's theory. in his prefatory note to the english edition of _the evolution theory_ thomson, the translator, summarizes weismann's especial contributions as: "( ) the illumination of the evolution process with a wealth of fresh illustrations; ( ) the vindication of the 'germ-plasm' concept as a valuable working hypothesis; ( ) the final abandonment of any assumption of transmissible acquired characters; ( ) a further analysis of the nature and origin of variations; and ( ), above all, an extension of the selection principle of darwin and wallace, which finds its logical outcome in the suggestive theory of germinal selection." continuity of the germ-plasm.--weismann's theory is designated that of continuity of the germ-plasm, and in considering it we must first give attention to his conception of the germ-plasm. as is well known, animals and plants spring from germinal elements of microscopic size; these are, in plants, the spores, the seeds, and their fertilizing agents; and, in animals, the eggs and the sperms. now, since all animals, even the highest developed, begin in a fertilized egg, that structure, minute as it is, must contain all hereditary qualities, since it is the only material substance that passes from one generation to another. this hereditary substance is the germ-plasm. it is the living, vital substance of organisms that takes part in the development of new generations. naturalists are agreed on this point, that the more complex animals and plants have been derived from the simpler ones; and, this being accepted, the attention should be fixed on the nature of the connection between generations during their long line of descent. in the reproduction of single-celled organisms, the substance of the entire body is divided during the transmission of life, and the problem both of heredity and origin is relatively simple. it is clear that in these single-celled creatures there is unbroken continuity of body-substance from generation to generation. but in the higher animals only a minute portion of the organism is passed along. weismann points out that the many-celled body was gradually produced by evolution; and that in the transmission of life by the higher animals the continuity is not between body-cells and their like, but only between germinal elements around which in due course new body-cells are developed. thus he regards the body-cells as constituting a sort of vehicle within which the germ-cells are carried. the germinal elements represent the primordial substance around which the body has been developed, and since in all the long process of evolution the germinal elements have been the only form of connection between different generations, they have an unbroken continuity. this conception of the continuity of the germ-plasm is the foundation of weismann's doctrine. as indicated before, the general way in which he accounts for heredity is that the offspring is like the parent because it is composed of some of the same stuff. the rise of the idea of germinal continuity has been indicated in chapter xiv, where it was pointed out that weismann was not the originator of the idea, but he is nevertheless the one who has developed it the most extensively. complexity of the germ-plasm.--the germ-plasm has been molded for so many centuries by external circumstances that it has acquired an organization of great complexity. this appears from the following considerations: protoplasm is impressionable; in fact, its most characteristic feature is that it responds to stimulation and modifies itself accordingly. these subtle changes occurring within the protoplasm affect its organization, and in the long run it is the summation of experiences that determines what the protoplasm shall be and how it will behave in development. two masses of protoplasm differ in capabilities and potentialities according to the experiences through which they have passed, and no two will be absolutely identical. all the time the body was being evolved the protoplasm of the germinal elements was being molded and changed, and these elements therefore possess an inherited organization of great complexity. when the body is built anew from the germinal elements, the derived qualities come into play, and the whole process is a succession of responses to stimulation. this is in a sense, on the part of the protoplasm, a repeating of its historical experience. in building the organism it does not go over the ground for the first time, but repeats the activities which it took centuries to acquire. the evident complexity of the germ-plasm made it necessary for weismann, in attempting to explain inheritance in detail, to assume the existence of distinct vital units within the protoplasm of the germinal elements. he has invented names for these particular units as biophors, the elementary vital units, and their combination into determinants, the latter being united into ids, idants, etc. the way in which he assumes the interactions of these units gives to his theory a highly speculative character. the conception of the complex organization of the germ-plasm which weismann reached on theoretical grounds is now being established on the basis of observation (see chapter xiv, p. ). the origin of variations.--the way in which weismann accounts for the origin of variation among higher animals is both ingenious and interesting. in all higher organisms the sexes are separate, and the reproduction of their kind is a sexual process. the germinal elements involved are seeds and pollen, eggs and sperms. in animals the egg bears all the hereditary qualities from the maternal side, and the sperm those from the paternal side. the intimate mixture of these in fertilization gives great possibilities of variations arising from the different combinations and permutations of the vital units within the germ-plasm. this union of two germ-plasms weismann calls amphimixis, and for a long time he maintained that the purpose of sexual reproduction in nature is to give origin to variations. later he extended his idea to include a selection, mainly on the basis of nutrition, among the vital elements composing the germ-plasm. this is germinal selection, which aids in the production of variations. in _the evolution theory_, volume ii, page , he says: "now that i understand these processes more clearly, my opinion is that the roots of all heritable variation lie in the germ-plasm; and, furthermore, that the determinants are continually oscillating hither and thither in response to very minute nutritive changes and are readily compelled to _variation in a definite direction_, which may ultimately lead to considerable variations in the structure of the species, if they are favored by personal selection, or at least if they are not suppressed by it as prejudicial." but while sexual reproduction may be evoked to explain the origin of variation in higher animals, weismann thought it was not applicable to the lower ones, and he found himself driven to assume that variation in single-celled organisms is owing to the direct influence of environment upon them, and thus he had an awkward assumption of variations arising in a different manner in the higher and in the simplest organisms. if i correctly understand his present position, the conception of variation as due to the direct influence of environment is being surrendered in favor of the action of germinal selection among the simplest organisms. extension of the principle of natural selection.--these variations, once started, will be fostered by natural selection provided they are of advantage to the organism in its struggle for existence. it should be pointed out that weismann is a consistent darwinian; he not only adopts the principle of natural selection, but he extends the field of its operation from externals to the internal parts of the germinal elements. "roux and others have elaborated the idea of a struggle of the parts within the organism, and of a corresponding intra-selection; ... but weismann, after his manner, has carried the selection-idea a step farther, and has pictured the struggle among the determining elements of the germ-cell's organization. it is at least conceivable that the stronger 'determinants,' _i.e._, the particles embodying the rudiments of certain qualities, will make more of the food-supply than those which are weaker, and that a selective process will ensue" (thomson). this is the conception of germinal selection. he has also extended the application of the general doctrine of natural selection by supplying a great number of new illustrations. the whole theory of weismann is so well constructed that it is very alluring. each successive position is worked out with such detail and apt illustration that if one follows him step by step without dissent on some fundamental principle, his conclusion seems justified. as a system it has been elaborated until it makes a coherent appeal to the intellect. inheritance of acquired characters.--another fundamental point in weismann's theory is the denial that acquired characters are transmitted from parent to offspring. probably the best single discussion of this subject is contained in his book on _the evolution theory_, , to which readers are referred. a few illustrations will be in place. acquired characters are any acquisitions made by the body-cells during the lifetime of an individual. they may be obvious, as skill in piano-playing, bicycle-riding, etc.; or they may be very recondite, as turns of the intellect, acquired beliefs, etc. acquired bodily characters may be forcibly impressed upon the organism, as the facial mutilations practiced by certain savage tribes, the docking of the tails of horses, of dogs, etc. the question is, are any acquired characters, physical or mental, transmitted by inheritance? manifestly, it will be difficult to determine on a scientific basis whether or not such qualities are inheritable. one would naturally think first of applying the test of experiment to supposed cases of such inheritances, and this is the best ground to proceed on. it has been maintained on the basis of the classical experiments of brown-séquard on guinea-pigs that induced epilepsy is transmitted to offspring; and, also, on the basis of general observations, that certain bodily mutilations are inherited. weismann's analysis of the whole situation is very incisive. he experimented by cutting off the tails of both parents of breeding mice. the experiments were carried through twenty-two generations, both parents being deprived of their tails, without yielding any evidence that the mutilations were inheritable. to take one other case that is less superficial, it is generally believed that the thirst for alcoholic liquors has been transmitted to the children of drunkards, and while weismann admits the possibility of this, he maintains that it is owing to the germinal elements being exposed to the influence of the alcohol circulating in the blood of the parent or parents; and if this be the case it would not be the inheritance of an acquired character, but the response of the organism to a drug producing directly a variation in the germ-plasm. notwithstanding the well-defined opposition of weismann, the inheritance of acquired characters is still a mooted question. herbert spencer argued in favor of it, and during his lifetime had many a pointed controversy with weismann. eimer stands unalterably against weismann's position, and the neo-lamarckians stand for the direct inheritance of useful variations in bodily structure. the question is still undetermined and is open to experimental observation. in its present state there are competent observers maintaining both sides, but it must be confessed that there is not a single case in which the supposed inheritance of an acquired character has stood the test of critical examination. the basis of weismann's argument is not difficult to understand. acquired characters affect the body-cells, and according to his view the latter are simply a vehicle for the germinal elements, which are the only things concerned in the transmission of hereditary qualities. inheritance, therefore, must come through alterations in the germ-plasm, and not directly through changes in the body-cells. [illustration: fig. .--august weismann, born . permission of charles scribner's sons.] weismann, the man.--the man who for more than forty years has been elaborating this theory (fig. ) is still living and actively at work in the university of freiburg. august weismann was born at frankfort-on-the-main in . he was graduated at göttingen in , and for a short time thereafter engaged in the practice of medicine. this line of activity did not, however, satisfy his nature, and he turned to the pursuit of microscopic investigations in embryology and morphology, being encouraged in this work by leuckart, whose name we have already met in this history. in he settled in freiburg as _privat-docent_, and has remained connected with the university ever since. from onward he has occupied the chair of zoölogy in that institution. he has made his department famous, especially by his lectures on the theory of descent. he is a forceful and interesting lecturer. one of his hearers in wrote: "his lecture-room is always full, and his popularity among his students fully equals his fame among scientists." it is quite generally known that weismann since he reached the age of thirty has been afflicted with an eye-trouble, but the inference sometimes made by those unacquainted with his work as an investigator, that he has been obliged to forego practical work in the field in which he has speculated, is wrong. at intervals his eyes have strengthened so that he has been able to apply himself to microscopic observations, and he has a distinguished record as an observer. in embryology his studies on the development of the diptera, and of the eggs of daphnid crustacea, are well known, as are also his observations on variations in butterflies and other arthropods. he is an accomplished musician, and during the period of his enforced inactivity in scientific work he found much solace in playing "a good deal of music." "his continuous eye trouble must have been a terrible obstacle, but may have been the prime cause of turning him to the theories with which his name is connected." in a short autobiography published in _the lamp_ in , although written several years earlier, he gives a glimpse of his family life. "during the ten years ( - ) of my enforced inactivity and rest occurred my marriage with fräulein marie gruber, who became the mother of my children and was my true companion for twenty years, until her death. of her now i think only with love and gratitude. she was the one who, more than any one else, helped me through the gloom of this period. she read much to me at this time, for she read aloud excellently, and she not only took an interest in my theoretical and experimental work, but she also gave practical assistance in it." in he published _the germ-plasm, a theory of heredity_, a treatise which elicited much discussion. from that time on he has been actively engaged in replying to his critics and in perfecting his system of thought. the mutation-theory of de vries.--hugo de vries (fig. ), director of the botanical garden in amsterdam, has experimented widely with the growth of plants, especially the evening primrose, and has shown that different species appear to rise suddenly. the sudden variations that breed true, and thus give rise to new forms, he calls mutations, and this indicates the source of the name applied to his theory. in his _die mutationstheorie_, published in , he argues for the recognition of mutations as the universal source of the origin of species. although he evokes natural selection for the perpetuation and improvement of variations, and points out that his theory is not antagonistic to that of natural selection, it is nevertheless directly at variance with darwin's fundamental conception--that slight individual variations "are probably the sole differences which are effective in the production of new species" and that "as natural selection acts solely by accumulating slight, successive, favorable variations, it can produce no great or sudden modifications." the foundation of de vries's theory is that "species have not arisen through gradual selection, continued for hundreds or thousands of years, but by jumps through sudden, through small transformations." (whitman's translation.) the work of de vries is a most important contribution to the study of the origin of species, and is indicative of the fact that many factors must be taken into consideration when one attempts to analyze the process of organic evolution. one great value of his work is that it is based on experiments, and that it has given a great stimulus to experimental studies. experiment was likewise a dominant feature in darwin's work, but that seems to have been almost overlooked in the discussions aroused by his conclusions; de vries, by building upon experimental evidence, has led naturalists to realize that the method of evolution is not a subject for argumentative discussion, but for experimental investigation. this is most commendable. [illustration: fig. .--hugo de vries.] de vries's theory tends also to widen the field of exploration. davenport, tower, and others have made it clear that species may arise by slow accumulations of trivial variations, and that, while the formation of species by mutation may be admitted, there is still abundant evidence of evolution without mutation. reconciliation of different theories.--all this is leading to a clearer appreciation of the points involved in the discussion of the theories of evolution; the tendency is not for the breach between the different theories to be widened, but for evolutionists to realize more fully the great complexity of the process they are trying to explain, and to see that no single factor can carry the burden of an explanation. mutation is not a substitute for natural selection, but a coöperating factor; and neither mutation nor natural selection is a substitute for the doctrine of the continuity of the germ-plasm. thus we may look forward to a reconciliation between apparently conflicting views, when naturalists by sifting shall have determined the truth embodied in the various theories. one conviction that is looming into prominence is that this will be promoted by less argument and more experimental observation. that the solution of the underlying question in evolution will still require a long time is evident; as whitman said in his address before the congress of arts and science in st. louis in : "the problem of problems in biology to-day, the problem which promises to sweep through the present century as it has the past one, with cumulative interest and correspondingly important results, is the one which became the life-work of charles darwin, and which can not be better or more simply expressed than in the title of his epoch-making book, _the origin of species_." summary.--the number of points involved in the four theories considered above is likely to be rather confusing, and we may now bring them into close juxtaposition. the salient features of these theories are as follows: i. lamarck's theory of evolution. . variation is explained on the principle of use and disuse. . heredity: the variations are inherited directly and improved in succeeding generations. a long time and favorable conditions are required for the production of new species. ii. darwin's theory of natural selection. . variations assumed. . heredity: those slight variations which are of use to the organism will be perpetuated by inheritance. . natural selection is the distinguishing feature of the theory. through the struggle for existence nature selects those best fitted to survive. the selection of trivial variations that are of advantage to the organism, and their gradual improvement, leads to the production of new species. iii. weismann's theory of continuity of the germ-plasm. . the germ-plasm has had unbroken continuity from the beginning of life. owing to its impressionable nature, it has an inherited organization of great complexity. . heredity is accounted for on the principle that the offspring is composed of some of the same stuff as its parents. the body-cells are not inherited, _i.e._, . there is no inheritance of acquired characters. . variations arise from the union of the germinal elements, giving rise to varied combinations and permutations of the qualities of the germ-plasm. the purpose of amphimixis is to give rise to variations. the direct influence of environment has produced variations in unicellular organisms. . weismann adopts and extends the principle of natural selection. germinal selection is exhibited in the germ-plasm. iv. de vries's theory of mutations. . the formation of species is due not to gradual changes, but to sudden mutations. . natural selection presides over and improves variations arising from mutation. among the other theories of evolution that of eimer is the most notable. he maintains that variations in organisms take place not fortuitously or accidentally, but follow a perfectly determinate direction. this definitely directed evolution is called orthogenesis. he insists that there is continuous inheritance of acquired characters, and he is radically opposed to the belief that natural selection plays an important part in evolution. the title of his pamphlet published in , _on orthogenesis and the impotence of natural selection in species-formation_, gives an indication of his position in reference to natural selection. a consideration of eimer's argument would be beyond the purpose of this book. the cause for the general confusion in the popular mind regarding any distinction between organic evolution and darwinism is not far to seek. as has been shown, lamarck launched the doctrine of organic evolution, but his views did not even get a public hearing. then, after a period of temporary disappearance, the doctrine of evolution emerged again in . and this time the discussion of the general theory centered around darwin's hypothesis of natural selection. it is quite natural, therefore, that people should think that darwinism and organic evolution are synonymous terms. the distinction between the general theory and any particular explanation of it has, i trust, been made sufficiently clear in the preceding pages. chapter xix the rise of evolutionary thought a current of evolutionary thought can be traced through the literature dealing with organic nature from ancient times. it began as a small rill among the greek philosophers and dwindles to a mere thread in the middle ages, sometimes almost disappearing, but is never completely broken off. near the close of the eighteenth century it suddenly expands, and becomes a broad and prevailing influence in the nineteenth century. osborn, in his book, _from the greeks to darwin_, traces the continuity of evolutionary thought from the time of the greek philosophers to darwin. the ancient phase, although interesting, was vague and general, and may be dismissed without much consideration. after the renaissance naturalists were occupied with other aspects of nature-study. they were at first attempting to get a knowledge of animals and plants as a whole, and later of their structure, their developments, and their physiology, before questions of their origin were brought under consideration. opinion before lamarck.--the period just prior to lamarck is of particular interest. since lamarck was the first to give a comprehensive and consistent theory of evolution, it will be interesting to determine what was the state of opinion just prior to the appearance of his writings. studies of nature were in such shape at that time that the question of the origin of species arose, and thereafter it would not recede. this was owing mainly to the fact that ray and linnæus by defining a species had fixed the attention of naturalists upon the distinguishing features of the particular kinds of animals and plants. are species realities in nature? the consideration of this apparently simple question soon led to divergent views, and then to warm controversies that extended over several decades of time. the view first adopted without much thought and as a matter of course was that species are fixed and constant; _i.e._, that the existing forms of animals and plants are the descendants of entirely similar parents that were originally created in pairs. this idea of the fixity of species was elevated to the position of a dogma in science as well as in theology. the opposing view, that species are changeable, arose in the minds of a few independent observers and thinkers, and, as has already been pointed out, the discussion of this question resulted ultimately in a complete change of view regarding nature and man's relation to it. when the conception of evolution came upon the scene, it was violently combated. it came into conflict with the theory designated special creation. views of certain fathers of the church.--and now it is essential that we should be clear as to the sources of this dogma of special creation. it is perhaps natural to assume that there was a conflict existing between natural science and the views of the theologians from the earliest times; that is, between the scientific method and the method of the theologians, the latter being based on authority, and the former upon observation and experiment. although there is a conflict between these two methods, there nevertheless was a long period in which many of the leading theological thinkers were in harmony with the men of science with reference to their general conclusions regarding creation. some of the early fathers of the church exhibited a broader and more scientific spirit than their successors. st. augustine ( - ), in the fifth century, was the first of the great theologians to discuss specifically the question of creation. his position is an enlightened one. he says: "it very often happens that there is some question as to the earth or the sky, or the other elements of this world ... respecting which one who is not a christian has knowledge derived from most certain reasoning or observation" (that is, a scientific man); "and it is very disgraceful and mischievous and of all things to be carefully avoided, that a christian speaking of such matters as being according to the christian scriptures, should be heard by an unbeliever talking such nonsense that the unbeliever, perceiving him to be as wide from the mark as east from west, can hardly restrain himself from laughing." (quoted from osborn.) augustine's view of the method of creation was that of derivative creation or creation _causaliter_. his was a naturalistic interpretation of the mosaic record, and a theory of gradual creation. he held that in the beginning the earth and the waters of the earth were endowed with power to produce plants and animals, and that it was not necessary to assume that all creation was formed at once. he cautions his readers against looking to the scriptures for scientific truths. he said in reference to the creation that the days spoken of in the first chapter of genesis could not be solar days of twenty-four hours each, but that they must stand for longer periods of time. this view of st. augustine is interesting as being less narrow and dogmatic than the position assumed by many theologians of the nineteenth century. the next theologian to take up the question of creation was st. thomas aquinas ( - ) in the thirteenth century. he quotes st. augustine's view with approval, but does not contribute anything of his own. one should not hastily conclude, however, because these views were held by leaders of theological thought, that they were universally accepted. "the truth is that all classes of theologians departed from the original philosophical and scientific standards of some of the fathers of the church, and that special creation became the universal teaching from the middle of the sixteenth to the middle of the nineteenth centuries." the doctrine of special creation.--about the seventeenth century a change came about which was largely owing to the writings and influence of a spanish theologian named suarez ( - ). although suarez is not the sole founder of this conception, it is certain, as huxley has shown, that he engaged himself with the questions raised by the biblical account of creation; and, furthermore, that he opposed the views that had been expressed by augustine. in his tract upon the work of the six days (_tractatus de opere sex dierum_) he takes exception to the views expressed by st. augustine; he insisted that in the scriptural account of creation a day of twenty-four hours was meant, and in all other cases he insists upon a literal interpretation of the scriptures. thus he introduced into theological thought the doctrine which goes under the name of special creation. the interesting feature in all this is that from the time of st. augustine, in the fifth century, to the time when the ideas of suarez began to prevail, in the seventeenth, there had been a harmonious relation between some of the leading theologians and scientific men in their outlook upon creation. the opinion of augustine and other theologians was largely owing to the influence of aristotle. "we know," says osborn, "that greek philosophy tinctured early christian theology; what is not so generally realized is that the aristotelian notion of the development of life led to the true interpretation of the mosaic account of the creation. "there was in fact a long greek period in the history of the evolutionary idea extending among the fathers of the church and later among some of the schoolmen, in their commentaries upon creation, which accord very closely with the modern theistic conception of evolution. if the orthodoxy of augustine had remained the teaching of the church, the final establishment of evolution would have come far earlier than it did, certainly during the eighteenth century instead of the nineteenth century, and the bitter controversy over this truth of nature would never have arisen." the conception of special creation brought into especial prominence upon the continent by suarez was taken up by john milton in his great epic _paradise lost_, in which he gave a picture of creation that molded into specific form the opinion of the english-speaking clergy and of the masses who read his book. when the doctrine of organic evolution was announced, it came into conflict with this particular idea; and, as huxley has very pointedly remarked, the new theory of organic evolution found itself in conflict with the miltonic, rather than the mosaic cosmology. all this represents an interesting phase in intellectual development. forerunners of lamarck.--we now take up the immediate predecessors of lamarck. those to be mentioned are buffon, erasmus darwin, and goethe. buffon ( - ) (fig. ), although of a more philosophical mind than many of his contemporaries, was not a true investigator. that is, he left no technical papers or contributions to science. from to the time of his death he was the superintendent of the _jardin du roi_. he was a man of elegance, with an assured position in society. he was a delightful writer, a circumstance that enabled him to make natural history popular. it is said that the advance sheets of buffon's _histoire naturelle_ were to be found on the tables of the boudoirs of ladies of fashion. in that work he suggested the idea that the different forms of life were gradually produced, but his timidity and his prudence led him to be obscure in what he said. [illustration: fig. .--buffon, - .] packard, who has studied his writings with care, says that he was an evolutionist through all periods of his life, not, as is commonly maintained, believing first in the fixity of species, later in their changeability, and lastly returning to his earlier position. "the impression left on the mind after reading buffon is that even if he threw out these suggestions and then retracted them, from fear of annoyance or even persecution from the bigots of his time, he did not himself always take them seriously, but rather jotted them down as passing thoughts. certainly he did not present them in the formal, forcible, and scientific way that erasmus darwin did. the result is that the tentative views of buffon, which have to be with much research extracted from the forty-four volumes of his works, would now be regarded as in a degree superficial and valueless. but they appeared thirty-four years before lamarck's theory, and though not epoch-making, they are such as will render the name of buffon memorable for all time." (packard.) [illustration: fig. .--erasmus darwin, - .] erasmus darwin (fig. ) was the greatest of lamarck's predecessors. in he published the _zoönomia_. in this work he stated ten principles; among them he vaguely suggested the transmission of acquired characteristics, the law of sexual selection--or the law of battle, as he called it--protective coloration, etc. his work received some notice from scholars. paley's _natural theology_, for illustration, was written against it, although paley is careful not to mention darwin or his work. the success of paley's book is probably one of the chief causes for the neglect into which the views of buffon and erasmus darwin fell. inasmuch as darwin's conclusions were published before lamarck's book, it would be interesting to determine whether or not lamarck was influenced by him. the careful consideration of this matter leads to the conclusion that lamarck drew his inspiration directly from nature, and that points of similarity between his views and those of erasmus darwin are to be looked upon as an example of parallelism in thought. it is altogether likely that lamarck was wholly unacquainted with darwin's work, which had been published in england. goethe's connection with the rise of evolutionary thought is in a measure incidental. in he published his _metamorphosis of plants_, showing that flowers are modified leaves. this doctrine of metamorphosis of parts he presently applied to the animal kingdom, and brought forward his famous, but erroneous, vertebrate theory of the skull. as he meditated on the extent of modifications there arose in his mind the conviction that all plants and animals have been evolved from the modification of a few parental types. accordingly he should be accorded a place in the history of evolutionary thought. opposition to lamarck's views.--lamarck's doctrine, which was published in definite form in , has been already outlined. we may well inquire, why did not his views take hold? in the first place, they were not accepted by cuvier. cuvier's opposition was strong and vigorous, and succeeded in causing the theory of lamarck to be completely neglected by the french people. again, we must recognize that the time was not ripe for the acceptance of such truths; and, finally, that there was no great principle enunciated by lamarck which could be readily understood as there was in darwin's book on the doctrine of natural selection. the temporary disappearance of the doctrine of organic evolution which occurred after lamarck expounded his theory was also owing to the reaction against the speculations of the school of _natur-philosophie_. the extravagant speculation of oken and the other representatives of this school completely disgusted men who were engaged in research by observation and experiment. the reaction against that school was so strong that it was difficult to get a hearing for any theoretical speculation; but cuvier's influence must be looked upon as the chief one in causing disregard for lamarck's writings. the work of cuvier has been already considered in connection both with comparative anatomy and zoölogy, but a few points must still be held under consideration. cuvier brought forward the idea of catastrophism in order to explain the disappearance of the groups of fossil animals. he believed in the doctrine of spontaneous generation. he held to the doctrine of pre-delineation, so that it must be admitted that whenever he forsook observation for speculation he was singularly unhappy, and it is undeniable that his position of hostility in reference to the speculation of lamarck retarded the progress of science for nearly half a century. cuvier and saint-hilaire.--in there occurred a memorable controversy between cuvier and saint-hilaire. the latter (fig. ) was in early life closely associated with lamarck, and shared his views in reference to the origin of animals and plants; though in certain points saint-hilaire was more a follower of buffon than of lamarck. strangely enough, saint-hilaire was regarded as the stronger man of the two. he was more in the public eye, but was not a man of such deep intellectuality as lamarck. his scientific reputation rests mainly upon his _philosophie anatomique_. the controversy between him and cuvier was on the subject of unity of type; but it involved the question of the fixity or mutability of species, and therefore it involved the foundation of the question of organic evolution. fig. .--geoffroy saint-hilaire, - . this debate stirred all intellectual europe. cuvier won as being the better debater and the better manager of his case. he pointed triumphantly to the four branches of the animal kingdom which he had established, maintaining that these four branches represented four distinct types of organization; and, furthermore, that fixity of species and fixity of type were necessary for the existence of a scientific natural history. we can see now that his contention was wrong, but at the time he won the debate. the young men of the period, that is, the rising biologists of france, were nearly all adherents of cuvier, so that the effect of the debate was, as previously stated, to retard the progress of science. this noteworthy debate occurred in february, . the wide and lively interest with which the debate was followed may be inferred from the excitement manifested by goethe. of the great poet-naturalist, who was then in his eighty-first year, the following incident is told by soret: "monday, aug. d, .--the news of the outbreak of the revolution of july arrived in weimar to-day, and has caused general excitement. in the course of the afternoon i went to goethe. 'well,' he exclaimed as i entered, 'what do you think of this great event? the volcano has burst forth, all is in flames, and there are no more negotiations behind closed doors.' 'a dreadful affair,' i answered; 'but what else could be expected under the circumstances, and with such a ministry, except that it would end in the expulsion of the present royal family?' 'we do not seem to understand each other, my dear friend,' replied goethe. 'i am not speaking of those people at all; i am interested in something very different. i mean the dispute between cuvier and geoffroy de saint-hilaire, which has broken out in the academy, and which is of such great importance to science.' this remark of goethe came upon me so unexpectedly that i did not know what to say, and my thoughts for some minutes seemed to have come to a complete standstill. 'the affair is of the utmost importance,' he continued, 'and you can not form any idea of what i felt on receiving the news of the meeting on the th. in geoffroy de saint-hilaire we have now a mighty ally for a long time to come. but i see also how great the sympathy of the french scientific world must be in this affair, for, in spite of the terrible political excitement, the meeting on the th was attended by a full house. the best of it is, however, that the synthetic treatment of nature, introduced into france by geoffroy, can now no longer be stopped. this matter has now become public through the discussions in the academy, carried on in the presence of a large audience; it can no longer be referred to secret committees, or be settled or suppressed behind closed doors.'" influence of lyell's principles of geology.--but just as cuvier was triumphing over saint-hilaire a work was being published in england which was destined to overthrow the position of cuvier and to bring again a sufficient foundation for the basis of mutability of species. i refer to lyell's _principles of geology_, the influence of which has already been spoken of in chapter xv. lyell laid down the principle that we are to interpret occurrences in the past in the terms of what is occurring in the present. he demonstrated that observations upon the present show that the surface of the earth is undergoing gradually slow changes through the action of various agents, and he pointed out that we must view the occurrences in the past in the light of occurrences in the present. once this was applied to animal forms it became evident that the observations upon animals and plants in the present must be applied to the life of the fossil series. these ideas, then, paved the way for the conception of changes in nature as being one continuous series. h. spencer.--in came the publication of herbert spencer in the _leader_, in which he came very near anticipating the doctrine of natural selection. he advanced the developmental hypothesis, saying that even if its supporters could "merely show that the production of species by the process of modification is conceivable, they would be in a better position than their opponents. but they can do much more than this; they can show that the process of modification has affected and is affecting great changes in all organisms subject to modifying influences.... they can show that any existing species, animal or vegetable, when placed under conditions different from its previous ones, immediately begins to undergo certain changes of structure fitting it for the new conditions. they can show that in successive generations these changes continue, until ultimately the new conditions become the natural ones. they can show that in cultivated plants and domesticated animals, and in the several races of men, these changes have uniformly taken place. they can show that the degrees of difference so produced are often, as in dogs, greater than those on which distinctions of species are in other cases founded. they can show that it is a matter of dispute whether some of these modified forms _are_ varieties or modified species. and thus they can show that throughout all organic nature there is at work a modifying influence of the kind they assign as the cause of these specific differences; an influence which, though slow in its action, does in time, if the circumstances demand it, produce marked changes; an influence which, to all appearance, would produce in the millions of years, and under the great varieties of conditions which geological records imply, any amount of change." "it is impossible," says marshall, "to depict better than this the condition prior to darwin. in this essay there is full recognition of the fact of transition, and of its being due to natural influences or causes, acting now and at all times. yet it remained comparatively unnoticed, because spencer, like his contemporaries and predecessors, while advocating evolution, was unable to state explicitly what these causes were." darwin and wallace.--in we come to the crowning event in the rise of evolutionary thought, when alfred russel wallace sent a communication to mr. darwin, begging him to look it over and give him his opinion of it. darwin, who had been working upon his theory for more than twenty years, patiently gathering facts and testing the same by experiment, was greatly surprised to find that mr. wallace had independently hit upon the same principle of explaining the formation of species. in his generosity, he was at first disposed to withdraw from the field and publish the essay of wallace without saying anything about his own work. he decided, however, to abide by the decision of two of his friends, to whom he had submitted the matter, and the result was that the paper of wallace, accompanied by earlier communications of darwin, were laid before the linnæan society of london. this was such an important event in the history of science that its consideration is extended by quoting the following letter: "london, june th, . "my dear sir: the accompanying papers, which we have the honor of communicating to the linnæan society, and which all relate to the same subject; _viz_., the laws which affect the production of varieties, races, and species, contain the results of the investigations of two indefatigable naturalists, mr. charles darwin and mr. alfred wallace. "these gentlemen having, independently and unknown to one another, conceived the same very ingenious theory to account for the appearance and perpetuation of varieties and of specific forms on our planet, may both fairly claim the merit of being original thinkers in this important line of inquiry; but neither of them having published his views, though mr. darwin has for many years past been repeatedly urged by us to do so, and both authors having now unreservedly placed their papers in our hands, we think it would best promote the interests of science that a selection from them should be laid before the linnæan society. "taken in the order of their dates, they consist of: " . extracts from a ms. work on species, by mr. darwin, which was sketched in and copied in , when the copy was read by dr. hooker, and its contents afterward communicated to sir charles lyell. the first part is devoted to _the variation of organic beings under domestication and in their natural state_; and the second chapter of that part, from which we propose to read to the society the extracts referred to, is headed _on the variation of organic beings in a state of nature; on the natural means of selection; on the comparison of domestic races and true species_. " . an abstract of a private letter addressed to professor asa gray, of boston, u.s., in october, , by mr. darwin, in which he repeats his views, and which shows that these remained unaltered from to . " . an essay by mr. wallace, entitled _on the tendency of varieties to depart indefinitely from the original type_. this was written at ternate in february, , for the perusal of his friend and correspondent, mr. darwin, and sent to him with the expressed wish that it should be forwarded to sir charles lyell, if mr. darwin thought it sufficiently novel and interesting. so highly did mr. darwin appreciate the value of the views therein set forth that he proposed, in a letter to sir charles lyell, to obtain mr. wallace's consent to allow the essay to be published as soon as possible. of this step we highly approved, provided mr. darwin did not withhold from the public, as he was strongly inclined to do (in favor of mr. wallace), the memoir which he had himself written on the same subject, and which, as before stated, one of us had perused in , and the contents of which we had both of us been privy to for many years. "on representing this to mr. darwin, he gave us permission to make what use we thought proper of his memoir, etc.; and in adopting our present course, of presenting it to the linnæan society, we have explained to him that we are not solely considering the relative claims to priority of himself and his friend, but the interests of science generally; for we feel it to be desirable that views founded on a wide deduction from facts, and matured by years of reflecting, should constitute at once a goal from which others may start; and that, while the scientific world is waiting for the appearance of mr. darwin's complete work, some of the leading results of his labours, as well as those of his able correspondent, should together be laid before the public. "we have the honour to be yours very obediently, charles lyell, jos. d. hooker." personality of darwin.--the personality of darwin is extremely interesting. of his numerous portraits, the one shown in fig. is less commonly known than those showing him with a beard and a much furrowed forehead. this portrait represents him in middle life, about the time of the publication of his _origin of species_. it shows a rather typical british face, of marked individuality. steadiness, sincerity, and urbanity are all depicted here. his bluish-gray eyes were overshadowed by a projecting ridge and very prominent, bushy eyebrows that make his portrait, once seen, easily recognized thereafter. in the full-length portraits representing him seated, every line in his body shows the quiet, philosophical temper for which he was notable. an intimate account of his life is contained in the _life and letters of charles darwin_ ( ) and in _more letters of darwin_ ( ), both of which are illustrated by portraits and other pictures. the books about darwin and his work are numerous, but the reader is referred in particular to the two mentioned as giving the best conception of the great naturalist and of his personal characteristics. [illustration: fig. .--charles darwin, - .] he is described as being about six feet high, but with a stoop of the shoulders which diminished his apparent height; "of active habits, but with no natural grace or neatness of movement." "in manner he was bright, animated, and cheerful; a delightfully considerate host, a man of never-failing courtesy, leading him to reply at length to letters from anybody, and sometimes of a most foolish kind." his home life.--"darwin was a man greatly loved and respected by all who knew him. there was a peculiar charm about his manner, a constant deference to others, and a faculty for seeing the best side of everything and everybody." he was most affectionate and considerate at home. the picture of darwin's life with his children gives a glimpse of the tenderness and deep affection of his nature, and the reverent regard with which he was held in the family circle is very touching. one of his daughters writes: "my first remembrances of my father are of the delights of his playing with us. he was passionately attached to his own children, although he was not an indiscriminate child-lover. to all of us he was the most delightful playfellow, and the most perfect sympathizer. indeed, it is impossible adequately to describe how delightful a relation his was to his family, whether as children or in their later life. "it is a proof of the terms on which we were, and also of how much he was valued as a playfellow, that one of his sons, when about four years old, tried to bribe him with a sixpence to come and play in working hours. we all knew the sacredness of working time, but that any one should resist sixpence seemed an impossibility." method of work.--darwin's life, as might be inferred from the enduring quality of his researches, shows an unswerving purpose. his theory was not the result of a sudden flash of insight, nor was it struck out in the heat of inspiration, but was the product of almost unexampled industry and conscientious endeavor in the face of unfavorable circumstances. although strikingly original and independent as a thinker, he was slow to arrive at conclusions, examining with the most minute and scrupulous care the ground for every conclusion. "one quality of mind that seemed to be of especial advantage in leading him to make discoveries was the habit of never letting exceptions pass unnoticed." he enjoyed experimenting much more than work which only entailed reasoning. of course, he was a great reader, but for books as books he had no respect, often cutting large ones in two in order to make them easier to hold while in use. darwin's early life.--charles darwin was born in at shrewsbury, england, of distinguished ancestry, his grandfather being the famous dr. erasmus darwin, the founder, as we have seen, of a theory of evolution. in his youth he gave no indication of future greatness. he was sent to edinburgh to study medicine, but left there after two sessions, at the suggestion of his father, to study for the church. he then went to the university of cambridge, where he remained three years, listening to "incredibly dull lectures." after taking his baccalaureate degree, came the event which proved, as darwin says, "the turning-point of my life." this was his appointment as naturalist on the surveying expedition about to be entered upon by the ship _beagle_. in cambridge he had manifested an interest in scientific study, and had been encouraged by professor henslow, to whom he was also indebted for the recommendation to the post on the _beagle_. an amusing circumstance connected with his appointment is that he was nearly rejected by captain fitz-roy, who doubted "whether a man with such a shaped nose could possess sufficient energy and determination for the voyage." voyage of the beagle.--the voyage of the _beagle_ extended over five years ( - ), mainly along the west coast of south america. it was on this voyage that darwin acquired the habit of constant industry. he had also opportunity to take long trips on shore, engaged in observation and in making extensive collections. he observed nature in the field under exceptional circumstances. as he traveled he noted fossil forms in rocks as well as the living forms in field and forest. he observed the correspondence in type between certain extinct forms and recent animals in south america. he noticed in the galapagos islands a fauna similar in general characteristics to that of the mainland, five or six hundred miles distant, and yet totally different as to species. moreover, certain species were found to be confined to particular islands. these observations awakened in his mind, a mind naturally given to inquiring into the causes of things, questions that led to the formulation of his theory. it was not, however, until that he commenced his first note-book for containing his observations upon the transmutations of animals. he started as a firm believer in the fixity of species, and spent several years collecting and considering data before he changed his views. at downs.--on his return to england, after spending some time in london, he purchased a country-place at downs, and, as his inheritance made it possible, he devoted himself entirely to his researches. but, as is well known, he found in his illness a great obstacle to steady work. he had been a vigorous youth and young man, fond of outdoor sports, as fishing, shooting, and the like. after returning from his long voyage, he was affected by a form of constant illness, involving a giddiness in the head, and "for nearly forty years he never knew one day of the health of an ordinary man, and thus his life was one long struggle against the weariness and strain of sickness." gould in his _biographical clinics_ attributes his illness to eye-strain. "under such conditions absolute regularity of routine was essential, and the day's work was carefully planned out. at his best, he had three periods of work: from . to . ; from . to . ; and from . to . , each period being under two hours' duration." the patient thoroughness of his experimental work and of his observation is shown by the fact that he did not publish his book on the _origin of species_ until he had worked on his theory twenty-two years. the circumstances that led to his publishing it when he did have already been indicated. parallelism in the thought of darwin and wallace.--no one can read the letters of darwin and wallace explaining how they arrived at their idea of natural selection without marveling at the remarkable parallelism in the thought of the two. it is a noteworthy circumstance that the idea of natural selection came to both by the reading of the same book, _malthus on population_. darwin's statement of how he arrived at the conception of natural selection is as follows: "in october, , that is, fifteen months after i had begun my systematic inquiry, i happened to read for amusement _malthus on population_, and being well prepared to appreciate the struggle for existence which everywhere goes on from long-continued observations of the habits of animals and plants, it at once struck me that under these circumstances favourable variations would tend to be preserved and unfavourable ones to be destroyed. _the result of this would be the formation of new species._ here then i had at last got a theory by which to work, but i was so anxious to avoid prejudice that i determined not for some time to write even the briefest sketch of it. in june, , i first allowed myself the satisfaction of writing a very brief abstract of my theory in pencil, in thirty-five pages, and this was enlarged during the summer of into one of pages." [illustration: fig. .--alfred russel wallace, born .] and wallace gives this account: "in february, , i was suffering from a rather severe attack of intermittent fever at ternate, in the moluccas; and one day, while lying on my bed during the cold fit, wrapped in blankets, though the thermometer was at ° fahr., the problem again presented itself to me, and something led me to think of the 'positive checks' described by malthus in his _essay on population_, a work i had read several years before, and which had made a deep and permanent impression on my mind. these checks--war, disease, famine, and the like--must, it occurred to me, act on animals as well as man. then i thought of the enormously rapid multiplication of animals, causing these checks to be much more effective in them than in the case of man; and while pondering vaguely on this fact, there suddenly flashed upon me the _idea_ of the survival of the fittest--that the individuals removed by these checks must be on the whole inferior to those that survived. in the two hours that elapsed before my ague fit was over, i had thought out almost the whole of the theory; and the same evening i sketched the draught of my paper, and in the two succeeding evenings wrote it out in full, and sent it by the next post to mr. darwin." it thus appears that the announcement of the darwin-wallace theory of natural selection was made in , and in the following year was published the book, the famous _origin of species_, upon which darwin had been working when he received mr. wallace's essay. darwin spoke of this work as an outline, a sort of introduction to other works that were in the course of preparation. his subsequent works upon _animals and plants under domestication_, _the descent of man_, etc., etc., expanded his theory, but none of them effected so much stir in the intellectual world as the _origin of species_. this skeleton outline should be filled out by reading _darwin's life and letters_, by his son, and the complete papers of darwin and wallace, as originally published in the _journal of the linnæan society_. the original papers are reproduced in the _popular science monthly_ for november, . wallace was born in , and is still living. he shares with darwin the credit of propounding the theory of natural selection, and he is notable also for the publication of important books, as the _malay archipelago_, _the geographical distribution of animals_, _the wonderful century_, etc. the spread of the doctrine of organic evolution. huxley.--darwin was of a quiet habit, not aggressive in the defense of his views. his theory provoked so much opposition that it needed some defenders of the pugnacious type. in england such a man was found in thomas henry huxley ( - ). he was one of the greatest popular exponents of science of the nineteenth century; a man of most thorough and exact scholarship, with a keen, analytical mind that went directly to the center of questions under consideration, and powers as a writer that gave him a wide circle of readers. he was magnificently sincere in his fight for the prevalence of intellectual honesty. doubtless he will be longer remembered for this service than for anything else. [illustration: fig. .--thomas henry huxley, - .] he defended the doctrine of evolution, not only against oratorical attacks like that of bishop wilberforce, but against well-considered arguments and more worthy opponents. he advanced the standing of the theory in a less direct way by urging the pursuit of scientific studies by high-school and university students, and by bringing science closer to the people. he was a pioneer in the laboratory teaching of biology, and his _manual_ has been, ever since its publication in , the inspiration and the model for writers of directions for practical work in that field. it is not so generally known that he was also a great investigator, producing a large amount of purely technical researches. after his death a memorial edition of his scientific memoirs was published in four large quarto volumes. the extent of his scientific output when thus assembled was a surprise to many of his co-workers in the field of science. his other writings of a more general character have been collected in fourteen quarto volumes. some of the essays in this collection are models of clear and vigorous english style. mr. huxley did an astonishing amount of scientific work, especially in morphology and palæontology. those who have been privileged to look over his manuscripts and unpublished drawings in his old room at south kensington could not fail to have been impressed, not only with the extent, but also with the accuracy of his work. taking johannes müller as his exemplar, he investigated animal organisms with a completeness and an exactness that have rarely been equaled. an intimate account of his life will be found in _the life and letters of thomas henry huxley_, by his son. haeckel.--ernst haeckel, of jena, born in (fig. ), was one of the earliest in germany to take up the defense of darwin's hypothesis. as early as he applied the doctrine of evolution to all organisms in his _generelle morphologie_. this work, which has been long out of print, represents his best contribution to evolutionary thought. he has written widely for general readers, and although his writings are popularly believed to represent the best scientific thought on the matter, those written for the general public are not regarded by most biologists as strictly representative. as a thinker he is more careless than huxley, and as a result less critical and exact as a writer. [illustration: fig. .--ernst haeckel, born .] there can be no doubt that the germs of evolutionary thought existed in greek philosophy, and that they were retained in a state of low vitality among the mediæval thinkers who reflected upon the problem of creation. it was not, however, until the beginning of the nineteenth century that, under the nurture of lamarck, they grew into what we may speak of as the modern theory of evolution. after various vicissitudes this doctrine was made fertile by darwin, who supplied it with a new principle, that of natural selection. the fruits of this long growth are now being gathered. after darwin the problem of biology became not merely to describe phenomena, but to explain them. this is the outcome of the rise and progress of biology: first, crude and uncritical observations of the forms of animated nature; then descriptive analysis of their structure and development; and, finally, experimental studies, the effort to explain vital phenomena, an effort in which biologists are at present engaged. chapter xx retrospect and prospect. recent tendencies in biology when one views the progress of biology in retrospect, the broad truth stands out that there has been a continuity of development in biological thought and interpretation. the new proceeds out of the old, but is genetically related to it. a good illustration of this is seen in the modified sense in which the theories of epigenesis and pre-formation have been retained in the biological philosophy of the nineteenth century. the same kind of question that divided the philosophers of the seventeenth and eighteenth centuries has remained to vex those of the nineteenth; and, although both processes have assumed a different aspect in the light of germinal continuity, the theorists of the last part of the nineteenth century were divided in their outlook upon biological processes into those of the epigenetic school and those who are persuaded of a pre-organization in the germinal elements of organisms. leading biological questions were warmly discussed from these different points of view. in its general character the progress of natural science has been, and still is, a crusade against superstition; and it may be remarked in passing that "the nature of superstition consists in a gross misunderstanding of the causes of natural phenomena." the struggle has been more marked in biology than in other departments of science because biology involves the consideration of living organisms and undertakes to establish the same basis for thinking about the organization of the human body as about the rest of the animal series. the first triumph of the scientific method was the overthrow of authority as a means of ascertaining truth and substituting therefor the method of observation and experiment. this carries us back to the days of vesalius and harvey, before the framework of biology was reared. but the scientific method, once established, led on gradually to a belief in the constancy of nature and in the prevalence of universal laws in the production of all phenomena. in its progress biology has exhibited three phases which more or less overlap: the first was the descriptive phase, in which the obvious features of animals and plants were merely described; the descriptive was supplemented by the comparative method; this in due course by the experimental method, or the study of the processes that take place in organisms. thus, description, comparison, and experiment represent the great phases of biological development. the notable books of biology and their authors.--the progress of biology has been owing to the efforts of men of very human qualities, yet each with some special distinguishing feature of eminence. certain of their publications are the mile-stones of the way. it may be worth while, therefore, in a brief recapitulation to name the books of widest general influence in the progress of biology. only those publications will be mentioned that have formed the starting-point of some new movement, or have laid the foundation of some new theory. beginning with the revival of learning, the books of vesalius, _de corporis humani fabrica_ ( ), and harvey, _de motu cordis et sanguinis_ ( ), laid the foundations of scientific method in biology. the pioneer researches of malpighi on the minute anatomy of plants and animals, and on the development of the chick, best represent the progress of investigation between harvey and linnæus. the three contributions referred to are those on the _anatomy of plants_ (_anatome plantarum_, - ); on the _anatomy of the silkworm_ (_de bombyce_, ); and on the _development of the chick_ (_de formatione pulli in ovo_ and _de ovo incubato_, both ). we then pass to the _systema naturæ_ (twelve editions, - ) of linnæus, a work that had such wide influence in stimulating activity in systematic botany and zoölogy. wolff's _theoria generationis_, , and his _de formatione intestinorum_, , especially the latter, were pieces of observation marking the highest level of investigation of development prior to that of pander and von baer. cuvier, in _le règne animal_, , applied the principles of comparative anatomy to the entire animal kingdom. the publication in of bichat's _traité des membranes_ created a new department of anatomy, called histology. lamarck's book, _la philosophie zoologique_, , must have a place among the great works in biology. its influence was delayed for more than fifty years after its publication. the monumental work of von baer on _development_ (_ueber entwicklungsgeschichte der thiere_), , is an almost ideal combination of observation and conclusion in embryology. the _microscopische untersuchungen_, , of schwann marks the foundation of the cell-theory. the _handbook_ of johannes müller (_handbuch der physiologie des menschen_), , remains unsurpassed as to its plan and its execution. max schultze in his treatise _ueber muskelkörperchen und das was man eine zelle zu nennen habe_, , established one of the most important conceptions with which biology has been enriched, viz., the protoplasm doctrine. darwin's _origin of species_, , is, from our present outlook, the greatest classic in biology. pasteur's _studies on fermentation_, , is typical of the quality of his work, though his later investigations on inoculations for the prevention of hydrophobia and other maladies are of greater importance to mankind. it is somewhat puzzling to select a man to represent the study of fossil life, one is tempted to name e.d. cope, whose researches were conceived on the highest plane. zittel, however, covered the entire field of fossil life, and his _handbook of palæontology_ is designated as a mile-post in the development of that science. before the renaissance the works of aristotle and galen should be included. from the view-point suggested, the more notable figures in the development of biology are: aristotle, galen, vesalius, harvey, malpighi, linnæus, wolff, cuvier, bichat, lamarck, von baer, j. müller, schwann, schultze, darwin, pasteur, and cope. such a list is, as a matter of course, arbitrary, and can serve no useful purpose except that of bringing into combination in a single group the names of the most illustrious founders of biological science. the individuals mentioned are not all of the same relative rank, and the list should be extended rather than contracted. schwann, when the entire output of the two is considered, would rank lower as a scientific man than koelliker, who is not mentioned, but the former must stand in the list on account of his connection with the cell-theory. virchow, the presumptive founder of pathology, is omitted, as are also investigators like koch, whose line of activity has been chiefly medical. recent tendencies in biology. higher standards.--in attempting to indicate some of the more evident influences that dominate biological investigation at the present time, nothing more than an enumeration of tendencies with a running commentary is possible. one notes first a wholesome influence in the establishment of higher standards, both of research and of scientific publication. investigations as a whole have become more intensive and more critical. much of the work that would have passed muster for publication two decades ago is now regarded by the editors of the best biological periodicals as too general and too superficial. the requisites for the recognition of creditable work being higher, tends to elevate the whole level of biological science. improvement in tools and methods.--this has come about partly through improvement in the tools and in the methods of the investigators. it can hardly be said, however, that thinking and discernment have been advanced at the same rate as the mechanical helps to research. in becoming more intensive, the investigation of biological problems has lost something in comprehensiveness. that which some of the earlier investigators lacked in technique was compensated for in the breadth of their preliminary training and in their splendid appreciation of the relations of the facts at their disposal. the great improvement in the mechanical adjustments and in the optical powers of microscopes has made it possible to see more regarding the physical structure and the activities of organisms than ever before. microtomes of the best workmanship have placed in the hands of histologists the means of making serial sections of remarkable thinness and regularity. the great development of micro-chemical technique also has had the widest influence in promoting exact researches in biology. special staining methods, as those of golgi and bethe, by means of which the wonderful fabric of the nervous system has been revealed, are illustrations. the separation by maceration and smear preparation of entire histological elements so that they may be viewed as solids has come to supplement the study of sections. reconstruction, by carving wax plates of known thickness into the form of magnified sections drawn upon their surfaces to a scale, and then fitting the plates together, has been very helpful in picturing complicated anatomical relations. this method has made it possible to produce permanent wax models of minute structures magnified to any desired degree. minute dissections, although not yet sufficiently practiced, are nevertheless better than the wax models for making accurate drawings of minute structures as seen in relief. the injection of the blood-vessels of extremely small embryos has made it possible to study advantageously the circulatory system. the softening of bones by acid after the tissues are already embedded in celloidin has offered a means of investigating the structure of the internal ear by sections, and is widely applicable to other tissues. with the advantage of the new appliances and the new methods, the old problems of anatomy are being worked over on a higher level of requirement. still, it is doubtful whether even the old problems will be solved in more than a relative way. it is characteristic of the progress of research that as one proceeds the horizon broadens and new questions spring up in the pathway of the investigator. he does not solve the problems he sets out to solve, but opens a lot of new ones. this is one of the features of scientific research that make its votaries characteristically optimistic. experimental work.--among the recent influences tending to advance biology, none is more important than the application of experiments to biological studies. the experimental method is in reality applicable to diverse fields of biological research, and its extensive use at present indicates a movement in the right direction; that is, a growing interest in the study of processes. one of the earliest problems of the biologist is to investigate the architecture of living beings; then there arise questions as to the processes that occur within the organism, and the study of processes involves the employment of experiments. in the pursuit of physiology experiments have been in use since the time of harvey, but even in that science, where they are indispensable, experiments did not become comparative until the nineteenth century. it now appears that various forms of experiment give also a better insight into the structure of organisms, and the practice of applying experiments to structural studies has given rise to the new department of experimental morphology. for the purpose of indicating some of the directions in which biology has been furthered by the experimental method of investigation, we designate the fields of heredity and evolution, changes in the environment of organisms, studies on fertilization and on animal behavior. the recognition that both heredity and the process of evolution can be subjected to experimental tests was a revelation. darwin and the early evolutionists thought the evolutionary changes too slow to be appreciated, but now we know that many of the changes can be investigated by experiment. numerous experiments on heredity in poultry (davenport), in rats, in rabbits, and in guinea-pigs (castle) have been carried out--experiments that test the laws of ancestral inheritance and throw great light upon the questions introduced by the investigations of mendel and de vries. the investigations of de vries on the evolution of plant-life occupy a notable position among the experimental studies. a large number of experiments on the effects produced by changes in the external conditions of life have been made. to this class of investigations belong studies on the regulation of form and function in organisms (loeb, child), the effects produced by altering mechanical conditions of growth, by changing the chemical environment, etc. there is some internal mechanism in living matter that is influenced by changes in external conditions, and the study of the regulation of the internal processes that produce form and structure have given rise to a variety of interesting problems. the regeneration of lost parts and regeneration after intentionally-imposed injury has received much attention (morgan). marine animals are especially amenable to manipulations of this nature, as well as to alterations in their surroundings, on account of the ease in altering the chemical environment in which they live. the latter may be accomplished by dissolving harmless chemical salts in the sea-water, and observing the changes produced by the alterations of the surrounding conditions. by this means herbst and others have produced very interesting results. in the field of artificial fertilization, free swimming larvæ have been raised from eggs artificially fertilized by changes in osmotic pressure, and also by treating them with both organic and inorganic acids; and these studies have greatly altered opinion regarding the nature of fertilization, and of certain other phenomena of development. animal behavior.--the study of animal behavior (jennings) is a very characteristic activity of the present, in which certain psychological processes are investigated. these investigations have given rise to a distinct line of research participated in by psychologists and biologists. the study of the way in which animals will react toward light of different colors, to variations in the intensity of light, to alterations in temperature, and to various other forms of stimuli are yielding very important results, that enable investigators to look beneath the surface and to make important deductions regarding the nature of psychological processes. a line closely allied to experimentation is the application of statistics to biological processes, such as those of growth, stature, the law of ancestral inheritance, the statistical study of variations in spines, markings on shells, etc., etc., (galton, pearson, davenport). other branches of biology that have been greatly developed by the experimental method are those of bacteriology and physiological chemistry. the advances in the latter have greatly widened the horizon of our view regarding the nature of vital activities, and they compose one of the leading features of current biological investigation. some tendencies in anatomical studies. cell-lineage.--while experimental work occupies the center of the stage, at the same time great improvements in morphological studies are evident. it will be only possible, however, to indicate in a general way the direction in which investigations are moving. we note, first, as in a previous paragraph, that the improvement in morphology is generic as well as specific. anatomical analysis is being carried to its limits in a number of directions. the investigations that are connected with the study of cells afford a conspicuous illustration of this fact. studies in cell-lineage have led to an exact determination of cell-succession in the development of certain animals, and such studies are still in progress. great progress also has been made in the study of physical structure of living matter. the tracing of cell-lineage is a feat of remarkably accurate and patient work. but, however much this may command our admiration, it has been surpassed (as related in chapter xi) by investigations regarding the organization of the egg and the analysis of chromosomes. boveri, conklin, wilson, and others have shown that there are recognizable areas within the protoplasm of the egg that have a definite historical relationship to certain structures in process of development. this is the basis upon which rests the doctrine of pre-localization of tissue-forming substances within the protoplasm of the egg. anatomy of the nervous system.--in another direction the progress of anatomical studies is very evident, that is, investigations of the nervous system and the sense-organs. the wonderfully complicated relations of nerve elements have been worked out by ramon y cajal. the studies of hodge and others upon optical changes occurring within the cells of the nervous system owing to their functional activity have opened a great field for investigation. the studies of strong, herrick, and others upon the distribution of nerve-components in the nerves of the head and the investigations of harrison on the growth and the regeneration of nerve-fibers give illustrations of current tendencies in biological investigation. the analysis of the central nervous system into segmental divisions on the basis of functional activity (johnston) is still another illustration. the application of biological facts to the benefit of mankind.--the practical application of biology to the benefit of mankind is a striking feature of present-day tendencies. the activity set on foot by the researches of pasteur, koch, and others has created a department of technical biology of the greatest importance to the human race. under the general heading should be included the demonstration of the connection between insects and the propagation of yellow fever, malaria, and other disorders; and as an illustration of activity in , we think of the commission recently appointed to investigate the terrible scourge of the sleeping-sickness which has been prevalent in africa. here also we would group studies of a pathological character on blood-immunity, toxin and antitoxin, also studies on the inoculation for the prevention of various diseases that affect animals and mankind. very much benefit has already accrued from the practical application of biological researches of this nature, which, in reality, are still in their infancy. we find the application of biological facts to agriculture in the form of soil-inoculation, in the tracing of the sources of nitrates in the soil, and studies of the insects injurious to vegetation; their further application to practical forestry, and in sanitary sciences. this kind of research is also applied to the study of food-supply for fishes, as in the case of plankton studies. the establishment and maintenance of biological laboratories.--the establishment of seaside biological observatories and various other stations for research have had a great influence on the development of biology. the most famous biological station is that founded at naples (fig. ) in by anton dohrn, and it is a gratification to biologists to know that he still remains its director. this international station for research has stimulated, and is at present stimulating, the growth of biology by providing the best conditions for carrying on researches and by the distribution of material which has been put up at the sea-coast by the most skilled preservators. there are many stations modeled after that at naples. the marine biological laboratory at woods holl, mass., is of especial prominence, and the recently reorganized wistar institute of anatomy at philadelphia is making a feature of the promotion of anatomical researches, especially those connected with the anatomy of the nervous system. laboratories similar to those at the seaside have been established on several fresh-water lakes. the studies carried on in those places of the complete biology of lakes, taking into account the entire surroundings of organisms, are very interesting and important. [illustration: fig. .--the biological station at naples.] under this general head should be mentioned stations under the control of the carnegie institution, the various scientific surveys under the government, and the united states fish commission, which carries on investigations in the biology of fishes as well as observations that affect their use as articles of diet. the combined output of the various laboratories and stations of this nature is very considerable, and their influence upon the progress of biology is properly included under the head of present tendencies. the organization of laboratories in our great universities and their product exercise a wide influence on the progress of biology, that science having within twenty-five years come to occupy a position of great importance among the subjects of general education. establishment and maintenance of technical periodicals.--it is manifestly very important to provide means for the publication of results and, as needed, to have technical periodicals established and properly maintained. their maintenance can not be effected on a purely commercial basis, and the result is that some of our best periodicals require financial assistance in order to exist at all. the subsidizing and support of these periodicals aid materially in the biological advance. a typical technical periodical is schultze's famous _archiv für mikroscopische anatomie_, founded in by schultze and continued to the present time. into its pages go the highest grade of investigations, and its continued existence has a salutary influence upon the progress of biology. the list of technical periodicals would be too long to name, but among others the _morphologisches jahrbuch_ of gegenbaur, and koelliker's _zeitschrift für wissenschaftliche zoologie_ have had wide influence. in england the _quarterly journal of microscopical science_ is devoted to morphological investigations, while physiology is provided for in other journals, as it is also in germany and other countries. in the united states the _journal of morphology_, edited by c.o. whitman, passed through seventeen volumes and was maintained on the highest plane of scholarship. the fine execution of the plates and the high grade of typographical work made this journal conspicuous. it represents in every way an enterprise of which americans can be justly proud. the _american journal of anatomy_ is now filling the field left unoccupied by the cessation of the _journal of morphology_.[ ] in the department of experimental work many journals have sprung up, as _biometrica_, edited by carl pearson, roux's _archiv für entwicklungsmechanik_, the _journal of experimental zoology_ recently established in the united states, etc., etc. exploration of the fossil records.--explorations of the fossil records have been recently carried out on a scale never before attempted, involving the expenditure of large sums, but bringing results of great importance. the american museum of natural history, in new york city, has carried on an extensive survey, which has enriched it with wonderful collections of fossil animals. besides explorations of the fossil-bearing rocks of the western states and territories, operations in another locality of great importance are conducted in the fayûm district of egypt. the result of the studies of these fossil animals is to make us acquainted not only with the forms of ancient life, but with the actual line of ancestry of many living animals. the advances in this direction are most interesting and most important. this extensive investigation of the fossil records is one of the present tendencies in biology. conclusion.--in brief, the chief tendencies in current biological researches are mainly included under the following headings: experimental studies in heredity, evolution, and animal behavior; more exact anatomical investigations, especially in cytology and neurology, the promotion and dissemination of knowledge through biological periodicals; the provision of better facilities in specially equipped laboratories, in the application of results to the benefit of mankind, and in the investigation of the fossil records. the atmosphere of thought engendered by the progress of biology is beneficial in every way. while its progress has dealt the death-blow to many superstitions and changed materially views regarding the universe, it is gratifying to think that it has not been iconoclastic in its influence, but that it has substituted something better for that which was taken away. it has given a broader and more wholesome basis for religion and theories of ethics; it has taught greater respect for truth and morality. however beneficial this progress has been in the past, who can doubt that the mission of biology to the twentieth century will be more important than to the past, and that there will be embraced in its progress greater benefits than any we have yet known? footnotes: [footnote : it is a source of gratification to biologists that--thanks to the wistar institute of anatomy--the publication of the _journal of morphology_ is to be continued.] reading list the books and articles relating to the history of biology are numerous. those designated below embrace some of the more readily accessible ones. while some attention has been given to selecting the best sources, no attempt has been made to give a comprehensive list. i. general references cuvier. histoire des sciences naturelles. vols., - . excellent. written from examination of the original documents. carus. geschichte der zoologie, . also histoire de la zoologie, . a work of scholarship. contains excellent account of the physiologus. sachs. history of botany, . excellent. articles in the _botanical gazette_ for supplement his account by giving the more recent development of botany. white. a history of the warfare of science with theology in christendom, vols., . good account of vesalius and the overthrow of authority in science. whewell. history of the inductive sciences, vol. ii, . lacks insight into the nature of biology and the steps in its progress. mentioned because so generally known. williams. a history of science, vols., . finely illustrated. contains many defects in the biological part as to the relative rank of the founders: vesalius diminished, paracelsus magnified, etc. also, the story of nineteenth century science, . collected articles from _harper's magazine_. good portraits. uncritical on biological matters. thomson. the science of life, . an excellent brief history of biology. foster. lectures on the history of physiology, . fascinatingly written. notable for poise and correct estimates, based on the use of the original documents. geddes. a synthetic outline of the history of biology. _proc. roy. soc. edinb._, - . good. richardson. disciples of Æsculapius, vols., . collected papers from _the asclepiad_. sympathetic accounts of vesalius, malpighi, j. hunter, and others. good illustrations. lankester. the history and scope of zoology, in the advancement of science, . good. same article in ency. brit. under the title of zoology. spencer. principles of biology, vols., . hertwig. the growth of biology in the nineteenth century, _ann. rept. smithson. inst._, . buckle. history of civilization, vol. i, second edition, . macgilivray. lives of eminent zoölogists from aristotle to linnæus. merz. a history of european thought in the nineteenth century, vol. ii, scientific thought, . routledge. a popular history of science. general and uncritical as to biology. hoefer. histoire de la zoologie, . not very good. encyclopædia britannica. among the more excellent articles are: biology by huxley; protoplasm by geddes; history of anatomy by turner. chambers's encyclopædia. new edition. discerning articles by thomson on the cell-theory, by geddes on biology, evolution. nouvelle biographie générale. good articles on the older writers. often unreliable as to dates. haeckel. the historical chapters in the evolution of man, , and anthropogenie, fifth edition, . good. haeckel. the history of creation, vol. i, . hertwig. the general survey of the history of zoölogy in his manual of zoölogy, . brief but excellent. parker and haswell. text-book of zoölogy, . historical chapter in vol. ii. nicholson. natural history, its rise and progress in britain, . also biology. pettigrew. gallery of medical portraits, vols. contains many portraits and biographical sketches of men of general influence, as bichat, galen, malpighi, etc. puschmann. handbuch der geschichte der medizin, vols. good for topics in anatomy and physiology. baas. the history of medicine, . radl. geschichte der biologischen theorien seit dem ende des siebzehnten jahrhundert, . janus. a periodical devoted to the history of medicine and natural science, founded in . zoologische annalen. founded by max braun in in the interests of the history of zoölogy. mitteilungen zur geschichte der medizin und naturwissenschaften, founded . surgeon general's library. the catalogue should be consulted for its many biographical references to biologists. the library is especially rich in historical documents, as old anatomies, physiologies, zoölogies, etc. evolution. the bibliography of evolution is given below under the chapters dealing with the evolution theory. ii. special references chapter i ancient biological science: carus; botany after , sachs. aristotle: cuvier, a panegyric; lewes, aristotle--a chapter from the history of science, , a critical study; huxley, on some mistakes attributed to aristotle; macgilivray; aristotle's history of animals translated in bohn's classical library, . pliny: macgilivray; thorndike, the place of magic in the intellectual history of europe, , chap. iii. the renaissance: symonds. epochs in biological history: geddes (see general list). chapter ii vesalius: roth, andreas vesalius bruxellensis, the edition of , the standard source of knowledge of vesalius and his times, contains bibliography, references to his different portraits, the resurrection bone, etc., etc.; foster (see general list), lecture i, excellent; richardson in disciples of Æsculapius, vol. i, contains pictures, his signature, etc.; pettigrew; white, vol. ii, pp. - ; the practitioner, , vol. ; the asclepiad, , vol. ii; de humani corporis fabrica, editions of and ; opera omnia, edited by boerhaave, vols., . galen: pettigrew; huxley in his essay on william harvey. chapter iii harvey: foster, lecture ii, with quotations, excellent; dalton, history of the circulation; huxley, william harvey, a critical essay; harvey's works translated by willis, with biography, sydenham society, ; life of harvey by d'arcy power, ; brooks, harvey as embryologist, bull. johns hop. hospit., vol. viii, , good. an anatomical dissertation upon the movement of the heart and blood in animals, a facsimile reproduction of the first edition of the famous de motu cordis et sanguinis, . privately reproduced by dr. moreton in . very interesting. chapter iv hooke: biography in encyclopædias, his microscope in carpenter, the microscope and its revelations, th ed., . malpighi: richardson, vol. ii; same article in _the asclepiad_, vol. x, ; atti, life and work, in italian, , portrait; pettigrew, vol. ii; marcello malpighi e l'opera sua, , a collection of addresses at the unveiling of malpighi's monument at crevalcuore, that by koelliker excellent; locy, malpighi, swammerdam, and leeuwenhoek, _pop. sci. mo._, --portrait and pictures from his works; maccallum, _j. hop. univ. hospit. bull_. malpighi's writings: opera omnia, difficult to obtain, the robt. littlebury edition, lond., , contains posthumous papers and biography; separate works not uncommon; traité du ver à soie, montpellier, , contains his life and works. swammerdam: life by boerhaave in biblia naturæ, ; also bibel der natur, ; also the book of nature, ; von baer, johann swammerdam's leben und verdienste um die wissenschaft, , in _reden_, vol. i; locy, _loc. cit._--portrait. leeuwenhoek: new biographical facts in richardson, vol. i, p. ; same article in _the asclepiad_, vol. ii, , portrait, signature, and other illustrations; arcana naturæ; selected works in english, ; locy, _pop. sci. mo._, april, . chapter v lyonet: _the gentleman's magazine_, lix, ; the famous traité anatomique, etc., , , not rare. réaumur: portrait and life in _les savants modernes_, p. . roesel: portrait and biography in _der monatlich herausgegebenen insecten belustigung_, part iv, ; zeigler in _natur und haus_, --nine figs. straus-dürckheim: his monograph on anatomy of the cockchafer, rather rare. the minute anatomists: straus-dürckheim, dufour, newport, leidig, etc., in miall and denney's the cockroach, . discovery of the protozoa: leeuwenhoek, müller, ehrenberg, dujardin, etc., kent's manual of the infusoria, vol. i. ehrenberg: life by laue, . chapter vi the physiologus: carus, white (for titles see general list). gesner: brooks in _pop. sci. mo._, --illustrations; cuvier, _loc. cit._; jardine's naturalist's library, vol. vi; gesner's historia animalium, - . aldrovandi: naturalist's library, vol. iii; macgilivray, _loc. cit._ jonston: macgilivray. ray: macgilivray; nicholson; memorial of, in the ray society, ; correspondence of, ray soc., . linnæus: macgilivray; _janus_, vol. , ; cuvier, _loc. cit._; agassiz, essay on classification, ; jubilee at upsala, _science_, apl. , ; caddy, through the fields with linnæus, ; the systema naturæ, especially the tenth edition, . leuckart: archives de parasit., vol. i, no. ; _nature_, . general biological progress from linnæus to darwin: geddes, proc. roy. soc. edinb., vol. , - . chapter vii camper: naturalist's library, vol. vii; vorlesungen, by his son, with short sketch of his life, ; cuvier, _loc. cit._; _kleinere schriften_, vols. with copper plates illustrating brain and ear of fishes, etc., - . john hunter: the scientific works of, vols., ; the _asclepiad_, vol. viii, ; the same article with illustrations in richardson, _loc. cit._; pettigrew, _loc. cit._ vicq d'azyr: cuvier, _loc. cit._; huxley in life of owen, p. ; his works in vols., . cuvier: life by flourens; memoirs by mrs. lee, ; buckle, hist. civ., vol. i, p. et seq.; lettres de geo. cuvier à c.m. paff, - , translated from the german, . cuvier's numerous writings--the animal kingdom, leçons d'anat. comparée, etc.--are readily accessible. h. milne-edwards: biographical sketch in _ann. rept. smithson. inst_, for . lacaze-duthiers: life with portraits in _archives de zool. expériment._, vol. , . richard owen: life and letters, vols., ; clark, old friends at cambridge and elsewhere, p. et seq. j. fr. meckel: carus, _loc. cit._ gegenbaur: erlebtes und erstrebtes, portrait, ; anat. anz., vol. , ; _ann. rept. smithson. inst._, . cope: osborn in _the century_, vol. , ; gill, edward drinker cope, naturalist, a chapter in the history of science, _am. naturalist_, ; obituary notice, with portraits, _am. naturalist_, ; _pop. sci. mo_., vol. , . chapter viii bichat: pettigrew; buckle, hist. civ., vol. i, p. ; the hundred greatest men; _les savants modernes_, p. ; _the practitioner_, vol. , . koelliker: his autobiography, erinnerungen aus meinem leben, , several portraits, interesting; weldon, life and works in _nature_, vol. , with fine portrait; sterling, _ann. rept. smithson. inst._, . schultze: portrait and necrology by schwalbe in _archiv für mikroscop. anat._, vol. , ; see further under chapter xii. virchow: _j. hop. univ. circulars_, vol. xi, , celebration of seventieth birthday of virchow, addresses by osler, welch, and others; jacobi, _medical record_, n.y., vol. xx, , good; israel, in _ann. rept. smithson. inst._, . leydig: brief sketch in his horæ zoologicæ, . ramon y cajal: portrait in tenth anniversary of clark university, . chapter ix the best brief account of the rise of physiology in verworn's general physiology, . more recent german editions of the same work. historical outline in rutherford's text-book of physiology, . galen's physiology: verworn. harvey: see references under chapter iii; the analysis of his writings by willis in the works of harvey, translated into english, sydenham soc., ; see also dr. moreton's facsimile reproduction of the first edition ( ) of de motu cordis et sanguinis, . haller: fine portrait in his elementa physiologiæ, ; english translations of the elementa. charles bell: pettigrew; good summary in foster's life of claude bernard, p. et seq. johannes müller: his life, complete list of works, etc., in gedächtnissrede auf johannes müller by du bois-reymond, ; _eloge_ by virchow in _edinburgh med. journ._, vol. ; picture of his monument in coblenz, _archiv f. mik. anat._, vol. ; briefe von j. müller and anders retzius ( - ), ; his famous handbuch der physiologie and english translations should be inspected. ludwig: burdon-sanderson, ludwig and modern physiology, _sci. progress_, vol. v, ; the same article in _ann. rept. smithson. inst._, . claude bernard: life by m. foster, , excellent. chapter x good general account of the rise of embryology in koelliker's embryologie, ; minot, embryology and medical progress, _pop. sci. mo._, vol. , ; eycleshymer, a sketch of the past and future of embryology, _st. louis med. rev._, . harvey: as embryologist, brooks in _j. hop. univ. hospit. bull._, vol. viii, . see above, chaps. iii and ix for further references to harvey. malpighi: in embryology, locy in _pop. sci. mo._, --portrait and selected sketches from his embryological treatises. wolff: wheeler, wolff and the theoria generationis, in woods holl biological lectures, ; kirchoff in _jenaische zeitschr._, vol. , ; waldeyer, festrede in sitzbr. d. k. preus. akad. d. wissenschaft., ; haeckel in evolution of man, vol. i, . bonnet and pre-delineation: whitman, bonnet's theory of evolution, also evolution and epigenesis, both in woods holl biological lectures, . von baer: leben und schriften, his autobiography ( ), d edition, ; life by steida, ; obituary, _proc. roy. soc._, ; waldeyer in _allg. wien. med. ztg._, ; _nature_, vol. ; life by stölzle, ; haeckel, _loc. cit._, vol. i; locy, v. baer and the rise of embryology, _pop. sci. mo._, ; fine portrait as young man in _harper's mag_. for ; _rev. scient._, . kowalevsky: lankester in _nature_, vol. , ; portrait and biog. in _ann. mus. hist. nat. marseille_, vol. , . balfour: m. foster in _nature_, vol. , ; also life with portrait in the memorial edition of balfour's works; waldeyer in _arch. f. mik. anat._, vol. , ; osborn recollections, with portrait, _science_, vol. , . his: mall in _am. journ. anat._, vol. , ; biography in _anat. anz._, vol. , . chapter xi the cell-doctrine by tyson, . the cell-theory, huxley, _medico-chir. review_, , also in scientific memoirs, vol. i, ; the modern cell-theory, m'kendrick, _proc. phil. soc. glasgow_, vol. xix, ; the cell-theory, past and present, turner, _nature_, vol. , ; the cell-doctrine, burnett, _trans. am. med. assn._, vol. vi, ; first illustration of cells in rob't hooke's micrographia, , , etc.; the cell in development and inheritance, wilson, ; article cell, in chambers's (new) cyclopædia, by thomson. schleiden: sketch of, _pop. sci. mo._, vol. , - ; sachs' hist. of botany ; translation of his original paper of (ueber phytogenesis)--illustrations--sydenham soc., . schwann: life, _pop. sci. mo._, vol. , ; sa vie et ses travaux, frédéricq, ; nachruf, henle, _archiv f. mik. anat._, vol. , ; lankester, _nature_, vol. xxv, ; _the practitioner_, vol. , ; _the catholic world_, vol. , . translation of his contribution of (mikroscopische untersuchungen ueber die uebereinstimmung in der structur und dem wachstum der thiere und pflanzen), sydenham soc., . chapter xii on the physical basis of life, huxley, ; reprint in methods and results, . article protoplasm in ency. brit, by geddes. dujardin: _notice biographique_, with portraits and other illustrations, joubin, _archives de parasitol._, vol. , ; portrait of dujardin hitherto unpublished. dujardin's original description of sarcode, _ann. des sci. nat._ (_botanique_), vol. , p. , . von mohl: sachs' history of botany, . translation of his researches, sydenham soc., . cohn: blätter der erinnerung, , with portrait. schultze: necrology, by schwalbe in _archiv f. mik. anat._, vol. , , with portrait. schultze's paper founding the protoplasm doctrine in _archiv f. anat. und phys._, , entitled ueber muskelkörperchen und das was man eine zelle zu nennen habe. chapter xiii spontaneous generation: tyndall, _pop. sci. mo._, vol. , ; also in floating matter of the air, ; j.c. dalton in _n.y. med. journ._, ; dunster, good account in _proc. ann arbor sci. assn._, ; huxley, _rept. brit. assn. for adv. sci._, , republished in many journals, reprint in scientif. memoirs, vol. iv, . redi: works in vols., - , with life and letters and portraits; good biographical sketch in _archives de parasitol._, vol. i, ; redi's esperienze intorno alla generazione degl'insetti, plates, first edition, , in florence, ; reprinted at various dates, not uncommon. spallanzani: foster, lects. on physiol.; huxley, _loc. cit._; dunster, _loc. cit._; l'abbato spallanzani, by pavesi, , portrait. pouchet: his treatise of historical importance--hétérogénie; ou traité de la génération spontanée, basé sur des nouvelles expériences, . pasteur: life by rené vallery-radot, vols., ; percy and g. frankland, ; pasteur at home, illustrated, tarbell in _mcclure's mag._, vol. i, ; also _mcclure's_, vol. , , review of vallery-radot's life of pasteur; _nature_, vol. , ; _les savants modernes_, p. ; life by his son-in-law, translated by lady hamilton, ; sketches of pasteur, very numerous. bacteriology: woodhead, bacteria and their products, ; fraenkel, text-book of bacteriology, ; prudden, the story of bacteria, etc., . germ-theory of disease: crookshank's bacteriology, d edition, . koch: _pop. sci. mo._, vol. , ; _review of reviews_, vol. , ; sketches and references to his discoveries numerous. lister: _pop. sci. mo._, vol. , ; _review of reviews_, vol. , ; celebration of lister's th birthday, _pop. sci. mo._, june, ; _janus_, vol. , . the new microbe inoculation of wright, _harper's mag._, july, . chapter xiv the history and theory of heredity, j.a. thomson, _proc. roy. soc. edinb._, vol. xvi, ; chapter on heredity in thomson's science of life, ; also in his study of animal life, . mendel: mendel's principles of heredity, with translations of his original papers on hybridization, bateson, ; mendel's versuche über pflanzenhybriden, two papers ( and ), edited by tschermak, ; _ann. rept. smithson. inst._, - ; _pop. sci. mo._, vol. , ; vol. , ; _science_, vol. , . galton: _pop. sci. mo._, vol. , ; _nature_, vol. , ; galton's natural inheritance, . weismann: brief autobiography, with portrait, in _the lamp_, vol. , ; solomonsen, bericht über die feier des geburtstages von august weismann, ; weismann's the germ-plasm, , and the evolution theory, . chapter xv history of geology and paleontology, zittel, . the founders of geology, geikie, d edition, . history and methods of paleontological discovery, marsh, _proceed. am. adv. sci._, . same article in _pop. sci. mo._, vol. , - . the rise and progress of paleontology, huxley, _pop. sci. mo._, vol. , . lyell: charles lyell and modern geology, bonney, ; sketch in _pop. sci. mo._, vol. i, , also vol. , - . owen: life of, by his grandson, vols., ; see also above under chapter vii. agassiz: life and correspondence, by his wife, vols., ; life, letters and works, marcou, vols., ; what we owe to agassiz, wilder, _pop. sci. mo._, july, ; agassiz at penikese, _am. nat._, . cope: a great naturalist, osborn in _the century_, ; see above, under chapter vii, for further references. marsh: _pop. sci. mo._, vol. , ; sketches of, _nature_, vol. , - ; _science_, vol. , ; _am. j. sci._, vol. , . zittel: biographical sketch with portrait, schuchert, _ann. rept. smithson. inst._, - . osborn, papers on paleontological discovery in science from onward. the fayûm expedition of the am. museum of nat. history, _science_, march , . * * * * * note. since the four succeeding chapters deal with the evolution theory, it maybe worth while to make a few general comments on the literature pertaining to organic evolution. the number of books and articles is very extensive, and i have undertaken to sift from the great number a limited list of the more meritorious. owing to the prevalent vagueness regarding evolution theories, one is likely to read only about darwin and darwinism. this should be avoided by reading as a minimum some good reference on lamarck, weismann, and de vries, as well as on darwin. it is well enough to begin with darwin's theory, but it is not best to take his origin of species as the first book. to do this is to place oneself fifty years in the past. the evidences of organic evolution have greatly multiplied since , and a better conception of darwin's theory can be obtained by reading first romanes's darwin and after darwin, vol. i. this to be followed by wallace's darwinism, and, thereafter, the origin of species may be taken up. these will give a good conception of darwin's theory, and they should be followed by reading in the order named: packard's lamarck; weismann's the evolution theory; and de vries's the origin of species and varieties by mutation. simultaneously one may read with great profit osborn's from the greeks to darwin. chapter xvi general: romanes, darwin and after darwin, , vol. i, chaps. i-v; same author, the scientific evidences of organic evolution; weismann, introduction to the evolution theory, ; osborn, alte und neue probleme der phylogenese, _ergebnisse der anat. u. entwickel._, vol. iii, ; ziegler, ueber den derzeitigen stand der descendenzlehre in der zoologie, ; jordan and kellogg, evolution and animal life, , chaps. i and xiv. evolutionary series--shells: romanes, _loc. cit._; hyatt, transformations of planorbis at steinheim, _proc. am. ass. adv. sci._, vol. , . horse: lucas, the ancestry of the horse, _mcclure's mag._, oct., ; huxley, three lectures on evolution, in amer. addresses. embryology--recapitulation theory: marshall, biolog. lectures and addresses, ; vertebrate embryology, ; haeckel, evolution of man, . primitive man: osborn, discovery of a supposed primitive race of men in nebraska, _century mag._, jan., ; haeckel, the last link, . huxley, man's place in nature, collected essays, ; published in many forms. romanes, mental evolution in man and animals. chapter xvii lamarck: packard, lamarck, the founder of evolution, his life and work, with translations of his writings on organic evolution, ; lamarck's philosophie zoologique, . recherches sur l'organisation des corps vivans, , contains an early, not however the first statement of lamarck's views. for the first published account of lamarck's theory see the introduction to his système des animaux sans vertèbres, . neo-lamarckism: packard, _loc. cit._; also in the introduction to the standard natural history, ; spencer, the principles of biology, --based on the lamarckian principle. cope, the origin of genera, ; origin of the fittest, ; primary factors of organic evolution, , the latter a very notable book. hyatt, jurassic ammonites, _proced. bost. sci. nat. hist._, . osborn, _trans. am. phil. soc._, vol. , . eigenmann, the eyes of the blind vertebrates of north america, _archiv f. entwicklungsmechanik_, vol. , . darwin's theory (for biographical references to darwin see below under chapter xix): wallace, darwinism, ; romanes, darwin and after darwin, vol. i, ; metcalf, an outline of the theory of organic evolution, , good for illustrations. color: poulton, the colors of animals; chapters in weismann's the evolution theory, . mimicry: weismann, _loc. cit._ sexual selection: darwin, the descent of man, new ed., . inadequacy of nat. selection: spencer, the inadequacy of natural selection, ; morgan, evolution and adaptation, . kellogg, darwinism to-day, , contains a good account of criticisms against darwinism. chapter xviii weismann's the evolution theory, translated by j.a. and margaret thomson, vols., , contains the best statement of weismann's views. it is remarkably clear in its exposition of a complicated theory. the germ-plasm, ; romanes's an examination of weismannism, . inheritance of acquired characters: weismann's discussion, _loc. cit._, vol. ii, very good. romanes's darwin and after darwin, vol. ii. personality of weismann: sketch and brief autobiography, in _the lamp_, vol. , , portrait; solomonsen, bericht über die feier des geburtstages von august weismann, , portraits. mutation-theory of de vries: die mutations-theorie, ; species and varieties, their origin by mutation, ; morgan, evolution and adaptation, , gives a good statement of the mutation theory, which is favored by the author; whitman, the problem of the origin of species, _congress of arts and science, universal exposition, st. louis_, ; davenport, evolution without mutation, _journ. exp. zool._, april, . chapter xix for early phases of evolutionary thought consult osborn, from the greeks to darwin, , and clodd, pioneers of evolution, . suarez and the doctrine of special creation: huxley, in mr. darwin's critics, _cont. rev._, p. , reprinted in critiques and addresses, . buffon: in packard's life of lamarck, chapter . e. darwin: krause's life of e. darwin translated into english, ; packard, _loc. cit._ goethe: die idee der pflanzenmetamorphose bei wolff und bei goethe, kirchoff, ; goethe's die metamorphose der pflanzen, . oken: his elements of physiophilosophy, ray soc., . cuvier and st. hilaire: perrier, la philosophie zoologique avant darwin, ; osborn, _loc. cit._ darwin and wallace: the original communications of darwin and wallace, with a letter of transmissal signed by hooker and lyell, published in the _trans. linnæan soc._ for , were reprinted in the _pop. sci. mo._, vol. , . darwin: personality and biography (for references to his theory see under chapter xvii); life and letters by his son, vols., , new ed., ; more letters of charles darwin, vols., ; chapter in marshall's lectures on the darwinian theory; darwin, naturalist's voyage around the world, ; gould, biographical clinics, for darwin's illness due to eye-strain; poulton, chas. darwin and the theory of natural selection, . wallace: my life, vols., ; the critic, oct., . huxley: life and letters by his son, ; numerous sketches at the time of his death, , in _nature_, _nineteenth century_, _pop. sci. mo._, etc., etc. haeckel: his life and work by bölsche, . chapter xx it is deemed best to omit the references to technical papers upon which the summaries of recent tendencies are based. morgan's experimental zoology, . jennings, behavior of the lower organisms, . mosquitoes and other insects in connection with the transmission of disease, see folsom, entomology, , chapter ix, p. . biological laboratories: dean, the marine biological stations of europe, _ann. rept. smithson. inst._, ; marine biolog. station at naples, _harper's mag._, ; the _century_, vol. (emily nunn whitman); williams, a history of science, vol. v, chapter v, ; _am. nat._, vol. , ; _pop. sci. mo._, vol. , ; _ibid._, vol. , . woods hole station--a marine university, _ann. rept. smithson. inst._, . index a abiogenesis, acquired characters, inheritance of, ; weismann on, agassiz, essay on classification, ; agreement of embryological stages and the fossil record, ; fossil fishes, ; portrait, aldrovandi, alternative inheritance, amphimixis, the source of variations, anatomical sketches, the earliest, ; from vesalius, , anatomical studies, recent tendencies of, anatomy, of aristotle, ; beginnings of, ; earliest known illustrations, ; of galen, ; of the middle ages, ; comparative, rise of, - ; of insects, dufour, ; lyonet, ; malpighi, ; newport, ; réaumur, ; roesel, ; straus-dürckheim, ; swammerdam, , - ; minute, progress of, - ; of plants, grew, ; malpighi, ancients, return to the science of, animal behavior, studies of, animal kingdom of cuvier, aquinas, st. thomas, on creation, arcana naturæ, of leeuwenhoek, aristotle, - ; books of, ; errors of, ; estimate of, ; extensive knowledge of animals, ; the founder of natural history, ; influence of, ; personal appearance, , ; portrait, ; position in the development of science, arrest of inquiry, effect of, augustine, st., on creation, authority declared the source of knowledge, b bacteria, discovery of, ; disease-producing, ; and antiseptic surgery, ; nitrifying, of the soil, bacteriology, development of, baer, von, and the rise of embryology, - ; his great classic on development of animals, ; and germ-layers, ; makes embryology comparative, ; and pander ; period in embryology, - ; portraits, , ; his rank in embryology, ; his especial service, ; sketches from his embryological treatise, balfour, masterly work of, ; his period in embryology, - ; personality, ; portrait, ; tragic fate, ; university career, bary, h.a. de, ; portrait, bassi, and the germ-theory of disease, bell, charles, discoveries on the nervous system, ; portrait, berengarius, bernard, claude, in physiology, ; personality, ; portrait, biblia naturæ of swammerdam, bichat, and the birth of histology, - ; buckle's estimate of, , ; education, ; in paris, ; personality, ; phenomenal industry, ; portrait, ; results of his work, ; writings, ; successes of, binomial nomenclature of linnæus, biological facts, application of, biological laboratories, establishment and maintenance of, ; the station at naples, ; picture of, ; the woods hole station, biological periodicals, biological progress, continuity of, ; atmosphere engendered by, ; from linnæus to darwin, - biology, defined, ; domain of, , ; epochs of, ; progress of, , ; applied, boerhaave, quoted, , ; and linnæus, bois-reymond, du, ; portrait, bones, fossil, , bonnet, and emboîtement, ; opposition to wolff, ; portrait, books, the notable, of biology, brown, robert, discovers the nucleus in plant-cells, buckland, buckle, on bichat, , buffon, , ; portrait, ; position in evolution, c cæsalpinus, on the circulation, cajal, ramon y, ; portrait, camper, anatomical work of, ; portrait, carpenter, quoted, carpi, the anatomist, castle, experiments on inheritance, catastrophism, theory of, cuvier, ; lyell on, caulkins, on protozoa, cell, definition of, ; diagram of, ; earliest known pictures of, , ; in heredity, cell-lineage, , cell-theory, announcement of, ; effect on embryology, , ; founded by schleiden and schwann, ; schleiden's contribution, ; schwann's treatise, ; modifications of, ; vague foreshadowings of, child, studies on regulation, chromosomes, , circulation of the blood, harvey, , ; servetus, ; columbus, ; cæsalpinus, ; in the capillaries, ; leeuwenhoek's sketch of, ; vesalius on, with illustration, classification of animals, tabular view of, - cohn, portrait, color, in evolution, columbus, on the circulation, comparative anatomy, rise of, - ; becomes experimental, cope, in comparative anatomy, ; portrait, ; important work in palæontology, , creation, aquinas on, ; st. augustine on, ; special, ; evolution the method of, cuvier, birth and early education, ; and catastrophism, ; comprehensiveness of mind, ; correlation of parts, ; debate with st. hilaire, ; domestic life, ; forerunners of, ; founds comparative anatomy, ; founder of vertebrate palæontology, ; his four branches of the animal kingdom, ; goes to paris, ; life at the seashore, ; opposition to lamarck, ; portraits, , ; physiognomy, ; and the rise of comparative anatomy, - ; shortcomings of, ; successors of, ; type-theory of, d darwin, charles, his account of the way his theory arose, ; factors of evolution, ; habits of work, ; home life, ; at downs, ; ill health, ; naturalist on the beagle, ; natural selection, ; opens note-book on the origin of species, ; personality, ; portraits, , ; parallelism in thought with wallace, ; publication of the origin of species, ; his other works, , ; theory of pangenesis, ; variation in nature, ; the original drafts of his theory sent by hooker and lyell to the linnæan society, - ; working hours, ; summary of his theory, darwin, erasmus, ; portrait, darwinism and lamarckism confused, ; not the same as organic evolution, davenport, experiments, deluge, and the deposit of fossils, de vries, mutation theory of, ; portrait, ; summary, dufour, léon, on insect anatomy, dujardin, , ; discovers sarcode, , ; portrait, ; writings, e edwards, h. milne-, ; portrait, ehrenberg, , ; portrait, embryological record, interpretation of, embryology, von baer and the rise of, - ; experimental, ; gill-clefts and other rudimentary organs in embryos, ; theoretical, epochs in biological history, evolution, doctrine of, generalities regarding, ; controversies regarding the factors, , ; factors of, ; effect on embryology, ; on palæontology, ; nature of the question regarding, ; a historical question, ; the historical method in, ; sweep of, ; one of the greatest acquisitions of human knowledge, ; predictions verified, ; theories of, ; lamarck, ; darwin, ; weismann, ; de vries, ; summary of evolution theories, ; vagueness regarding, evolutionary series, ; shells, ; horses, evolutionary thought, rise of, - ; views of certain fathers of the church, experimental observation, introduced by harvey, - experimental work in biology, f fabrica, of vesalius, fabricius, harvey's teacher, ; portrait, factors of evolution, fallopius, ; portrait, flood, fossils ascribed to, fossil life, the science of, - ; bones, , ; horses in america, ; collections in new haven, ; in new york, ; man, , ; neanderthal skull, ; ape-like man, fossil remains an index to past history, fossils, arrangement in strata, ; ascribed to the flood, ; their comparison with living animals, ; from the fayûm district, ; method of collecting, ; nature of, ; determination of, by cuvier, ; da vinci, ; steno, ; strange views regarding, g galen, , ; portrait, galton, law of ancestral inheritance, ; portrait, geer, de, on insects, gegenbaur, ; portrait, generation, wolff's theory of, germ-cells, organization of, germ-layers, germ-plasm, continuity of, ; complexity of, ; the hereditary substance, ; union of germ-plasms the source of variations, germ-theory of disease, germinal continuity, , ; doctrine of, , , germinal elements, germinal selection, germinal substance, gesner, ; personality, ; portrait, ; natural history of, gill-clefts in embryos, goodsir, grew, work of, h haeckel, ; portrait, haller, fiber-theory, ; opposition to wolff, ; in physiology, ; portrait, harvey, and experimental observation, - ; his argument for the circulation, ; discovery of the circulation, ; his great classic, ; education, ; in embryology, ; embryological treatise, , ; frontispiece from his generation of animals ( ), ; influence of, ; introduces experimental method, ; at padua, ; period in physiology, ; personal appearance and qualities, , , ; portrait, ; predecessors of, ; question as to his originality, ; his teacher, ; writings, heredity, ; a cellular study, ; according to darwin, ; weismann, ; application of statistics to, ; inheritance of acquired characters, ; steps in advance of knowledge of, hertwig, oskar, portrait, ; service in embryology, ; richard, quoted, hilaire, st., portrait, ; see st. hilaire his, wilhelm, ; portrait, histology, birth of, - ; bichat its founder, ; normal and pathological, ; text-books of, hooke, robert, ; his microscope illustrated, hooker, letter on the work of darwin and wallace, - horse, evolution of, human ancestry, links in, , human body, evolution of, human fossils, , hunter, john, ; portrait, huxley, in comparative anatomy, ; influence on biology, ; in palæontology, ; portrait, i inheritance, alternative, mendel, ; ancestral, ; darwin's theory of, ; material basis of, - ; nature of, inheritance of acquired characters, ; lamarck on, ; weismann on, inquiry, the arrest of, insects, anatomy of, dufour, ; malpighi, ; illustration, ; newport, ; leydig, ; straus-dürckheim, ; swammerdam, , ; illustration, ; theology of, j jardin du roi changed to jardin des plantes, jennings, on animal behavior, , jonston, k klein, koch, robert, discoveries of, ; portrait, koelliker, in embryology, ; in histology, ; portrait, kowalevsky, in embryology, ; portrait, l lacaze-duthiers, ; portrait, lamarck, changes from botany to zoölogy, ; compared with cuvier, ; education, ; first announcement of his evolutionary views, ; forerunners of, ; first use of a genealogical tree, ; founds invertebrate palæontology, ; on heredity, ; laws of evolution, ; military experience, ; opposition to, ; philosophie zoologique, ; portrait, ; position in science, ; salient points in his theory, ; his theory of evolution, ; compared with that of darwin, , ; time and favorable conditions, ; use and disuse, leeuwenhoek, - ; new biographical facts, ; capillary circulation, , ; sketch of, ; comparison with malpighi and swammerdam, ; discovery of the protozoa, ; other discoveries, ; and histology, ; his microscopes, ; pictures of, , ; occupation of, ; portrait, ; scientific letters, ; theoretical views, leibnitz, leidy in palæontology, lesser's theology of insects, leuckart, ; portrait, leydig, ; anatomy of insects, ; in histology, ; portrait, linnæan system, reform of, - linnæus, - ; binomial nomenclature, ; his especial service, ; features of his work, , ; his idea of species, , ; influence on natural history, ; personal appearance, ; personal history, ; portrait, ; helped by his fiancée, ; return to sweden, ; and the rise of natural history, - ; the systema naturæ, , , ; professor in upsala, ; celebration of two hundredth anniversary of his birth, ; as university lecturer, ; wide recognition, ; summary on, - lister, sir joseph, and antiseptic surgery, ; portrait, loeb, ; on artificial fertilization, ; on regulation, ludwig, in physiology, ; portrait, lyell, epoch-making work in geology, ; letter on darwin and wallace, - ; portrait, lyonet, ; portrait and personality, ; great monograph on insect anatomy, ; illustrations from, , , , ; extraordinary quality of his sketches, m malpighi, - ; activity in research, ; anatomy of plants, ; anatomy of the silkworm, ; compared with leeuwenhoek and swammerdam, ; work in embryology, , ; rank as embryologist, ; honors at home and abroad, ; personal appearance, ; portraits, , ; sketches from his embryological treatises, ; and the theory of pre-delineation, man, antiquity of, ; evolution of, ; fossil, , marsh, o.c., portrait, meckel, j. fr., ; portrait, men, of biology, , ; the foremost, ; of science, mendel, ; alternative inheritance, ; law of, ; purity of the germ-cells, ; portrait, ; rank of mendel's discovery, , microscope, hooke's, fig. of, ; leeuwenhoek's, , figs. of, , microscopic observation, introduction of, ; of hooke, ; grew, ; ehrenberg, ; malpighi, , ; leeuwenhoek, , , , microscopists, the pioneer, middle ages, a remolding period, ; anatomy in, milne-edwards, portrait, mimicry, mohl, von, ; portrait, müller, fritz, ; o. fr., müller, johannes, as anatomist, ; general influence, ; influence on physiology, ; as a teacher, ; his period in physiology, ; personality, ; portrait, ; physiology after müller, n nägeli, portrait, naples, biological station at, ; picture of, natural history, of gesner, , , ; of ray, - ; of linnæus, - ; sacred, ; rise of scientific, - natural selection, ; discovery of, ; darwin and wallace on, ; extension of, by weismann, ; illustrations of, ; inadequacy of, nature, continuity of, ; return to, ; renewal of observation, naturphilosophie, school of, neanderthal skull, needham, experiments on spontaneous generation, neo-lamarckism, newport, on insect anatomy, nineteenth century, summary of discoveries in, nomenclature of biology, , nucleus, discovery of, by brown, ; division of, , o observation, arrest of, ; renewal of, ; in anatomy, ; and experiment the method of science, , oken, on cells, ; portrait, omne vivum ex ovo, omnis cellula e cellula, organic evolution, doctrine of, - ; influence of, on embryology, ; theories of, - ; rise of evolutionary thought, - ; sweep of the doctrine of, osborn, quoted, , , ; in palæontology, p palæontology, cuvier founds vertebrate, ; of the fayûm district, ; lamarck founder of invertebrate, ; agassiz, ; cope, ; huxley, ; lyell, ; marsh, ; osborn, ; owen, ; william smith, ; steps in the rise of, pander, and the germ-layer theory, pangenesis, darwin's theory of, pasteur, on fermentation, ; spontaneous generation, ; inoculation for hydrophobia, ; investigation of microbes, ; personality, ; portrait, ; his supreme service, ; veneration of, pasteur institute, foundation of, ; work of, pearson, carl, and ancestral inheritance, philosophie anatomique of st. hilaire, philosophie zoologique of lamarck, physiologus, the sacred natural history, - physiology, of the ancients, ; rise of, - ; period of harvey, ; of haller, ; of j. müller, ; great influence of müller, ; after müller, pithecanthropus erectus, , pliny, portrait, pouchet, on spontaneous generation, pre-delineation, theory of, ; rise of, malpighi, ; swammerdam, ; wolff, pre-formation. see pre-delineation primitive race of men, protoplasm, ; discovery of, , ; doctrine and sarcode, , ; its movements, ; naming of, ; its powers, protozoa, discovery of, ; growth of knowledge concerning, - purkinje, portrait, r rathke, in comparative anatomy, ; in embryology, ray, john, ; portrait, ; and species, réaumur, ; portrait, recapitulation theory, recent tendencies, in biology, ; in embryology, redi, earliest experiments on the generation of life, ; portrait, remak, in embryology, roesel, on insects, ; portrait, s sarcode and protoplasm, , scala naturæ, scale of being, schleiden, ; contribution to the cell-theory, ; personality, ; portrait, schultze, max, establishes the protoplasm doctrine, ; in histology, ; portrait, schulze, franz, on spontaneous generation, schwann, and the cell-theory, , , , ; in histology, ; and spontaneous generation, science, of the ancients, return to, ; conditions under which it developed, ; biological, servetus, on circulation of the blood, severinus, in comparative anatomy, ; portrait, sexual selection, shells, evolution of, , siebold, von, , ; portrait, silkworm, malpighi on, ; pasteur on, smith, wm., in geology, spallanzani, experiments on generation, ; portrait, special creation, theory of, species, ray, ; linnæus, ; are they fixed in nature, ; origin of, - spencer, ; his views on evolution in , spontaneous generation, belief in, ; disproved, ; first experiments on, ; new form of the question, ; redi, ; pasteur, ; pouchet, ; spallanzani, ; tyndall, steno, on fossils, straus-dürckheim, his monograph, ; illustrations from, suarez, and the theory of special creation, swammerdam, his biblia naturæ, ; illustrations from, , ; early interest in natural history, ; life and works, - ; love of minute anatomy, ; method of work, ; personality, ; portrait, ; compared with malpighi and leeuwenhoek, system, linnæan, reform of, - systema naturæ, of linnæus, , t theory, the cell-, ; the protoplasm, ; of organic evolution, - ; of special creation, tyndall, on spontaneous generation, ; his apparatus for getting optically pure air, type-theory, of cuvier, u uniformatism, and catastrophism, v variation, of animals, in a state of nature, ; origin of, according to weismann, vesalius, and the overthrow of authority, in science, - ; great book of, ; as court physician, ; death, ; force and independence, ; method of teaching anatomy, , ; opposition to, ; personality, , , ; physiognomy, ; portrait, ; predecessors of, ; especial service of, ; sketches from his works, , , , vicq d'azyr, ; portrait, vinci, leonardo da, and fossils, virchow, and germinal continuity, ; in histology, ; portrait, vries, hugo de, his mutation theory, ; portrait, ; summary of theory, w wallace, and darwin, ; his account of the conditions under which his theory originated, ; portrait, ; writings, weismann, the man, ; quotation from autobiography, ; personal qualities, ; portrait, ; his theory of the germ-plasm, - ; summary of his theory, whitney collection of fossil horses, willoughby, his connection with ray, wolff, on cells, ; his best work, ; and epigenesis, ; and haller, , ; opposed by bonnet and haller, ; his period in embryology, - ; personality, ; plate from his theory of generation, ; the theoria generationis, wyman, jeffries, on spontaneous generation, z zittel, in palæontology, ; portrait, advertisements darwinism to-day by prof. vernon l. kellogg, of leland stanford university author of "american insects," etc. pp. and index. vo. $ . net; by mail, $ . . a simple and concise discussion for the educated layman of present-day scientific criticism of the darwinian selection theories, together with concise accounts of the other more important proposed auxiliary and alternative theories of species-forming. with special notes and exact references to original sources and to the author's own observations and experiments. "its value cannot be overestimated. a book the student must have at hand at all times, and it takes the place of a whole library. no other writer has attempted to gather together the scattered literature of this vast subject, and none has subjected this literature to such uniformly trenchant and uniformly kindly criticism. pledged to no theory of his own, and an investigator of the first rank, and master of a clear and forceful literary style, professor kellogg is especially well fitted to do justice to the many phases of present-day darwinism."--david starr jordan in _the dial_. "may be unhesitatingly recommended to the student of biology as well as to the non-professional or even non-biological reader of intelligence ... gives a full, concise, fair and very readable exposition of the present status of evolution."--_the independent._ "can write in english as brightly and as clearly as the old-time frenchmen ... a book that the ordinary reader can read with thorough enjoyment and understanding and that the specialist can turn to with profit as well ... in his text he explains the controversy so that the plain man may understand it, while in the notes he adduces the evidence that the specialist requires. the whole matter is thoroughly digested and put in an absolutely intelligible manner ... a brilliant book that deserves general attention."--_new york sun._ "the balance-sheet of darwinism is struck in this work ... the attack and the defense of darwinism, well summarized ... the value of this book lies in its summing up of the darwinian doctrines as they have been modified or verified down to date."--_literary digest._ if the reader will send his name and address, the publishers will send, from time to time, information regarding their new books. henry holt and company publishers new york american science series the two principal objects of the series are to supply authoritative books whose principles are, so far as practicable, illustrated by american facts, and also to supply the lack that the advance of science perennially creates, of text-books which at least do not contradict the latest generalizations. physics. by a.l. kimball, professor in amherst college. (_in preparation._) physics. by george f. barker, x + pp. $ . . chemistry. by ira remsen, president of the johns hopkins university. advanced course. xxii + pp. $ . . college chemistry. xx + pp. $ . . briefer course. xxiv + pp. $ . . elementary course. x + pp. cents. astronomy. by simon newcomb and edward s. holden. advanced course. xii + pp. $ . . briefer course. x + pp. $ . . elementary course. xv + pp. $ . . geology. by thomas c. chamberlin and rollin d. salisbury, professors in the university of chicago. vols. vo. _vol. i. geological processes and their results_. xix + pp. $ . . _vols. ii and iii. earth history_. xxxvii + pp. (_not sold separately._) $ . . physiography. by rollin d. salisbury, professor in chicago university. advanced course. xx + pp. $ . . briefer course. viii + pp. $ . . general biology. by william t. sedgwick, professor in the mass. institute, and edmund b. wilson, professor in columbia university. xii + pp. $ . . botany. by charles e. bessey, professor in the university of nebraska. advanced course. x + pp. $ . . briefer course. vii + pp. $ . . zoology. by a.s. packard. advanced course. viii + pp. $ . . briefer course. viii + pp. $ . . elementary course. viii + pp. cents. the human body. by h. newell martin. advanced course. xvi + pp. $ . . briefer course. xiv + pp. $ . . elementary course. vi + pp. cents. psychology. by william james, professor in harvard university. advanced course. volumes. $ . . briefer course. xiii + pp. $ . . ethics. by john dewey, professor in columbia university, and james h. tufts, professor in the university of chicago. (_in press._) political economy. by francis a. walker. advanced course. viii + pp. $ . . briefer course. viii + pp. $ . . elementary course. x + pp. $ . . finance. by henry c. adams, professor in the university of michigan. xiv + pp. $ . . henry holt & co. west d st., new york wabash ave., chicago the american nature series in the hope of doing something toward furnishing a series where the nature-lover can surely find a readable book of high authority, the publishers of the american science series have begun the publication of the american nature series. it is the intention that in its own way, the new series shall stand on a par with its famous predecessor. the primary object of the new series is to answer questions which the contemplation of nature is constantly arousing in the mind of the unscientific intelligent person. but a collateral object will be to give some intelligent notion of the "causes of things." while the coöperation of foreign scholars will not be declined, the books will be under the guarantee of american experts, and generally from the american point of view; and where material crowds space, preference will be given to american facts over others of not more than equal interest. the series will be in six divisions: i. natural history this division will consist of two sections. section a. a large popular natural history in several volumes, with the topics treated in due proportion, by authors of unquestioned authority. vo. - / × - / in. the books so far publisht in this section are: fishes, by david starr jordan, president of the leland stanford junior university. $ . net; carriage extra. american insects, by vernon l. kellogg, professor in the leland stanford junior university. $ . net; carriage extra. arranged for are: seedless plants, by george t. moore, head of department of botany, marine biological laboratory, assisted by other specialists. wild mammals of north america, by c. hart merriam, chief of the united states biological survey. birds of the world. a popular account by frank h. knowlton, m.s., ph.d., member american ornithologists union, president biological society of washington, etc., etc., with chapter on anatomy of birds by frederic a. lucas, chief curator brooklyn museum of arts and sciences, and edited by robert ridgway, curator of birds, u.s. national museum. reptiles and batrachians, by leonhard steineger, curator of reptiles, u.s. national museum. section b. a shorter natural history, mainly by the authors of section a, preserving its popular character, its proportional treatment, and its authority so far as that can be preserved without its fullness. size not yet determined. ii. classification of nature section a. realms of nature. detailed treatment of various departments in a literary and popular way. vo. - / × - / in. already publisht: ferns, by campbell e. waters, of johns hopkins university. vo, pp. xi + . $ . net; by mail, $ . . section b. identification books-- . library series, very full descriptions. vo. - / × - / in. already publisht: north american trees, by n.l. britton, director of the new york botanical garden. $ . net; carriage extra. . pocket series, "how to know," brief and in portable shape. iii. functions of nature these books will treat of the relation of facts to causes and effects--of heredity in organic nature, and of the environment in all nature. vo. - / × - / in. already publisht: the bird: its form and function, by c.w. beebe, curator of birds in the new york zoological park. vo, pp. $ . net; by mail, $ . . arranged for: the insect: its form and function, by vernon l. kellogg, professor in the leland stanford junior university. the fish: its form and function, by h.m. smith, of the u.s. bureau of fisheries. iv. working with nature how to propagate, develop and care for the plants and animals. the volumes in this group cover such a range of subjects that it is impracticable to make them of uniform size. already publisht: nature and health, by edward curtis, professor emeritus in the college of physicians and surgeons. mo. $ . net; by mail, $ . . arranged for: photographing nature, by e.r. sanborn, photographer of the new york zoological park. the shellfish industries, by james l. kellogg, professor in williams college. chemistry of daily life, by henry p. talbot, professor of chemistry in the massachusetts institute of technology. domestic animals, by william h. brewer, professor emeritus in yale university. the care of trees in lawn, street and park, by b.e. fernow, professor of forestry in the university of toronto. v. diversions from nature this division will include a wide range of writings not rigidly systematic or formal, but written only by authorities of standing. large mo. - / × - / in. fish stories, by david starr jordan and charles f. holder. horse talk, by william h. brewer. bird notes, by c.w. beebe. insect stories, by vernon l. kellogg. vi. the philosophy of nature a series of volumes by president jordan, of stanford university, and professors brooks of johns hopkins, lull of yale, thomson of aberdeen, przibram of austria, zur strassen of germany, and others. edited by professor kellogg of leland stanford. mo. - / × - / in. henry holt and company, new york june, ' . vertebrata*** e-text prepared by "teary eyes" anderson and dedicated to destanie; with hopes her dream of becoming a veterinarian comes true special thanks to deborah furness of the university college london for her help, and research, in learning about this book, and helping me understand it better. spellchecked with www.thesolutioncafe.com note: project gutenberg also has an html version of this file which includes the original illustrations. see -h.htm or -h.zip: (http://www.gutenberg.net/dirs/ / / / / / -h/ -h.htm) or (http://www.gutenberg.net/dirs/ / / / / / -h.zip) transcriber's note: i try to edit my e-texts so they can easily be used with voice speech programs, i believe blind people and children should also be able to enjoy the many books now available electronically. i use the -- for an em-dash, with a space either before or after it depending on its usage. this helps to keep certain programs from squishing the words together, such as down-stairs. also to help voice speech programs i've enclosed upper case text between - and _ (-upper case text_), and used underscores to show chapter and section headers. i also added a second contents that shows the other sections of this e-text. this e-text was made with a "top can" text scanner, with a bit of correcting here and there. this book is volume one of two. it was later reworked by a. m. davies in under the title "text-book of zoology", then revised and rewritten by j. t. cunningham about and w. h. leigh-sharpe around . although these editions gave wells the main credit, most of wells' writing and all his drawings were removed; only his rough outline seems to have been used. it was re-published by university tutorial press. the first edition, as well as the second and revised edition (with dissections redrawn by miss a. c. robbins) are used in this e-text. the first edition had some small minor errors, as well as dissection abbreviations that are shown on the dissection sheets, but no mention of them was listed in the text. certain figures on the dissections sheets are missing (such as figures , , , with no mention to a , as if mr. wells drew a figure but found it was not needed and removed it from the book). rather then leaving it as is, i put {} marks around my notes saying things like {no figure }. for the "second and revised edition" wells was able to change some of these errors and missing parts, but many of the same printing tablets were used and with almost each addition other things were removed, (in one instance one entire section from a chapter), and many of the helpful suggestions were shortened or removed so other things could be explained more. in an ideal version of the book both could have been used, but with reprinting the entire book from the first to the second editions almost as many things were lost as were gained, so i've tried to indicate where both text go separate paths with the following; [second edition only text] and -first edition only text,- and also {lines from second edition only.} and {lines from first edition only.} were more then just a sentence is added or removed. other things to notice is how some words are spelt or punctuated differently throughout the book, such as; blood vessels blood-vessels bloodvessels i've tried to keep these as close to the original book as possible. university correspondence college tutorial series. -text-book of biology._ by h. g. wells, bachelor of science, london., fellow of the zoological society. lecturer in biology at university tutorial college. with an introduction by g. b. howes, fellow of the linnean society, fellow of the zoological society. assistant professor of zoology, royal college of science, london. part .-- vertebrata. contents introduction preface the rabbit-- . external form and general considerations . the alimentary canal of the rabbit . the circulation . the amoeba, cells and tissue . the skeleton . muscle and nerve . the nervous system . renal and reproductive organs . classificatory points . questions and exercises the frog-- . general anatomy . the skull of the frog (and the vertebrate skull generally) . questions on the frog the dog-fish-- . general anatomy . questions on the dog-fish amphioxus-- . anatomy . the development of amphioxus . questions on amphioxus development-- the development of the frog the development of the fowl the development of the rabbit the theory of evolution questions on embryology miscellaneous questions note on making comparisons syllabus of practical work {contents part } key for dissection sheets, and abbreviations -introduction_ in the year i was invited to give tuition by correspondence, in biology. although disposed at the time to ridicule the idea of imparting instruction in natural science by letter, i gladly accepted the opportunity thus afforded me of ascertaining for myself what could and could not be accomplished in that direction. anyone familiar with the scope of biological enquiry, and the methods of biological instruction, will not need to be reminded that it is only by the most rigorous employment of precise directions for observation, that any good results are to be looked for at the hand of the elementary student. true to this principle, i determined to issue to my correspondence pupils rigid instructions, and to demand in return faithful annotated drawings of facts observed in their usage. in the case of two among the few students who passed through my hands, the result far exceeded my most sanguine anticipations. the notes sent in by one of them-- a man working at a distance, alone and unaided-- far excelled those wrung from many a student placed under the most favourable surroundings; and their promise for the future has been fulfilled to the utmost, the individual in question being now a recognised investigator. it thus became clear that, not-with-standing the complex conditions of work in the biological field, tuition by correspondence would suffice to awaken the latent abilities of a naturally qualified enquirer. the average members of a university correspondence class will be found neither better nor worse than those of any other, and they may therefore pass unnoticed; if however, the correspondence system of tuition may furnish the means of arousing a latent aptitude, when the possibilities of other methods of approach are excluded-- and in so doing, of elevating the individual to that position for which he was by nature qualified, ensuring him the introduction to the one sphere of labour for which he was born-- it will have created its own defence, and have merited the confidence of all right-thinking people. the plucking of one such brand from the burning is ample compensation for the energy expended on any number of average dullards, who but require to be left alone to find their natural level. mr. wells' little book is avowedly written for examination purposes, and in conformity with the requirements of the now familiar "type system" of teaching. recent attempts have been made to depreciate this. while affording a discipline in detailed observation and manipulation second to that of no other branch of learning, it provides for that "deduction" and "verification" by which all science has been built up; and this appears to me ample justification for its retention, as the most rational system which can be to-day adopted. evidence that its alleged shortcomings are due rather to defective handling than to any inherent weakness of its own, would not be difficult to produce. although rigid in its discipline, it admits of commentatorial treatment which, while heightening the interest of the student, is calculated to stimulate alike his ambition and his imagination. that the sister sciences of botany and zoology fall under one discipline, is expressed in the english usage of the term "biology." experience has shown that the best work in either department has been produced by those who have acquired on all-round knowledge of at least the elementary stages of both; and, that the advanced morphologist and physiologist are alike the better for a familiarity with the principles-- not to say with the progressive advancement-- of each other's domain, is to-day undeniable. these and other allied considerations, render it advisable that the elementary facts of morphology and physiology should be presented to the beginner side by side-- a principle too frequently neglected in books which, like this one, are specially written for the biological neophyte. although the student is the wiser for the actual observation of the fact of nature, he becomes the better only when able to apply them, as for example, by the judicious construction of elementary generalizations, such as are introduced into the pages of this work. so long as these generalizations, regarded as first attempts to deduce "laws" in the form of "generalized statement of facts based observation," are properly introduced into an elementary text-book, intended for the isolated worker cut off from the lecture room, their intercalation is both healthy and desirable. mr. wells has kept these precepts constantly in mind in the preparation of his work, and in the formulation of his plans for its future extension, thereby enhancing the value of the book itself, and at the same time, discouraging the system of pure cram, which is alien to the discipline of biological science. g. b. howes royal college of science, south kensington; november , . -preface_ no method of studying-- more especially when the objects of study are tangible things-- can rival that prosecuted under the direction and in the constant presence of a teacher who has also a living and vivid knowledge of the matter which he handles with the student. in the ideal world there is a plentiful supply of such teachers, and easy access to their teaching, but in this real world only a favoured few enjoy these advantages. through causes that cannot be discussed here, a vast number of solitary workers are scattered through the country, to whom sustained help in this form is impossible, or possible only in days stolen from a needed vacation; and to such students especially does this book appeal, as well as to those more fortunate learners who are within reach of orderly instruction, but anxious to save their teachers' patience and their own time by some preliminary work. one of the most manifest disadvantages of book-work, under the conditions of the solitary worker, is the rigidity of its expressions; if the exact meaning is doubtful, he can not ask a question. this has been kept in view throughout; the writer has, above all, sought to be explicit-- has, saving over-sights, used no uncommon or technical term without a definition or a clear indication of its meaning. in this study of biology, the perception and memory of form is a very important factor indeed. every student should draw sketches of his dissections, and accustom himself to copying book diagrams, in order to train his eye to perception of details he might otherwise disregard. the drawing required is within the reach of all; but for those who are very inexperienced, tracing figures is a useful preliminary exercise. by the time the student has read the "circulation of the rabbit" (sections to ), he will be ready to begin dissection. it is possible to hunt to death even such a sound educational maxim as the "thing before the name," and we are persuaded, by a considerable experience, that dissection before some such preparatory reading is altogether a mistake. at the end of the book is a syllabus (with suggestions) for practical work, originally drawn up by the writer for his own private use with the evening classes of the university tutorial college-- classes of students working mainly in their spare time for the london examination, and at an enormous disadvantage, as regards the number of hours available, in comparison with the leisurely students of a university laboratory. this syllabus may, perhaps by itself, serve a useful purpose in some cases, but in this essential part of the study the presence of some experienced overlooker to advise, warn, and correct, is at first almost indispensable. a few words may, perhaps be said with respect to the design of this volume. it is manifestly modelled upon the syllabus of the intermediate examination in science of london university. that syllabus, as at present constituted, appears to me to afford considerable scope for fairly efficient biological study. the four types dealt with in this book are extremely convenient for developing the methods of comparative anatomy and morphological embryology. without any extensive reference to related organisms, these four forms, and especially the three vertebrata, may be made to explain and illustrate one another in a way that cannot fail to be educational in the truest sense. after dealing with the rabbit, therefore, as an organic mechanism, our sections upon the frog and dog-fish, and upon development, are simply statements of differences, and a commentary, as it were, upon the anatomy of the mammalian type. in the concluding chapter, a few suggestions of the most elementary ideas of it is hoped to make this first part of our biological course complete in itself, and of some real and permanent value to the student. and the writer is convinced that not only is a constant insistence upon resemblances and differences, and their import, intellectually the most valuable, but also the most interesting, and therefore the easiest, way of studying animal anatomy. that chaotic and breathless cramming of terms misunderstood, tabulated statements, formulated "tips," and lists of names, in which so many students, in spite of advice, waste their youth is, i sincerely hope, as impossible with this book as it is useless for the purposes of a london candidate. on the other hand, our chief endeavour has been to render the matter of the book clear, connected, progressive, and easily assimilable. in the second part plants, unicellular organisms, and invertebrata will be dealt with, in a wider and less detailed view of the entire biological province. {lines from first edition only.} -in this volume, we study four organisms, and chiefly in their relation to each other; in the next, we shall study a number of organisms largely in relation to their environment. in this part our key note is the evidence of inheritance; in our second part it will be of adaptation to circumstances.- this book will speedily, under the scrutiny of the critical reader, reveal abundant weakness. for these the author claims the full credit. for whatever merit it may posses, he must however, acknowledge his profound indebtedness to his former teacher, professor howes. not only has the writer enjoyed in the past the privilege of professor howes' instruction and example, but he has, during the preparation of this work, received the readiest help, advise, and encouragement from him-- assistance as generous as it was unmerited, and as unaffected as it was valuable. {lines from second edition only.} [the publication of a second and revised edition of this part affords the author an opportunity of expressing his sense of the general kindliness of his reviewers, and the help they have him in improving this maiden effort. to no one is there vouchsafed such a facility in the discovery of errors in a book as to its author, so soon as it has passed beyond his power of correction. hence the general tone of encouragement (and in some cases the decided approval) of the members of this termination to a period of considerable remorse and apprehension.] i have been able through their counsel, and the experience i have had while using this book in teaching, to correct several printer's errors and to alter various ambiguous or misleading expressions, as well as to bring the book up to date again in one or two particulars. my thanks are particularly due to my friend miss robbins, who has very kindly redrawn the occasionally rather blottesque figures of the first edition. not only have these plates gained immensely in grace and accuracy, but the lettering is now distinct-- an improvement that any student who has had to hunt my reference letters in the first edition will at once appreciate. h. g. wells november, . {first edition.} december, . {second edition.} -the rabbit._ . _external form and general considerations._ section . it is unnecessary to enter upon a description of the appearance of this familiar type, but it is not perhaps superfluous, as we proceed to consider its anatomy, to call attention to one or two points in its external, or externally apparent structure. most of our readers know that it belongs to that one of two primary animal divisions which is called the vertebrata, and that the distinctive feature which place it in this division is the possession of a spinal column or backbone, really a series of small ring-like bones, the vertebrae (figure v.b.) strung together, as it were, on the main nerve axis, the spinal cord (figure s.c.). this spinal column can be felt along the neck and back to the tail. this tail is small, tilted up, and conspicuously white beneath, and it serves as a "recognition mark" to guide the young when, during feeding, an alarm is given and a bolt is made for the burrows. in those more primitive (older and simpler-fashioned) vertebrata, the fishes, the tail is much large and far more important, as compared with the rest of the body, than it is in most of the air-inhabiting vertebrates. in the former it is invariably a great muscular mass to propel the body forward; in the latter it may disappear, as in the frog, be simply a feather-bearing stump, as in the pigeon, a fly flicker, as in the cow or horse, a fur cape in squirrel, or be otherwise reduced and modified to meet special requirements. section . at the fore end, or as english zoologists prefer to say, anterior end, of the vertebral column of the rabbit, is of course the skull, containing the anterior portion of the nerve axis, the brain (figure br.). between the head and what is called "the body," in the more restricted sense of the word, is the neck. the neck gives freedom of movement to the head, enables the animal to look this way and that, to turn its ears about to determine the direction of a sound, and to perform endless motions in connexion with biting and so forth easily. we may note that in types which swim through the water, the neck dose not appear-- in the fish and frog, for instance-- and the head simply widens out as one passes back to the body. the high resistance offered by water necessitates this tendency to a cigar or ship outline, just as it has determined the cigar shape of the ordinary fish torpedo. section . in the body of the rabbit, as examined from the outside, we can make out by feeling two distinct regions, just as we might in the body of a man; anteriorily a bony cage, having the ribs at the sides, a rod-like bone in the front, the sternum (figure -st.-, [stm.]), and the backbone behind, and called the chest or thorax; and posteriorily a part called the abdomen, which has no bony protection over its belly, or ventral surface. these parts together with the neck constitute the trunk. as a consequence of these things, in the backbone of the rabbit there are four regions: the neck, or cervical part, consisting of seven vertebrae, the thoracic part of twelve joined to ribs, the abdominal (also called the lumbar) region of seven without ribs, and the tail or caudal of about fifteen. between the lumbar and caudal come four vertebrae, the sacral, which tend to run together into a bony mass as the animal grows old, and which form a firm attachment for the base of the hind limb. section . the thorax and abdomen are separated by a partition, the diaphragm (figure dia.). this structure is distinctive of that class of the vertebrata called mammals, and which includes man, most of the larger and commoner land animals, and whales and manatee. we shall find later that it is essentially connected with the perfection of the air breathing to which this group has attained. another characteristic shared by all mammals, and by no other creature, is the presence of hair. in birds we have an equally characteristic cover in the feathers, the frog is naked, and the fishes we find either naked skins or scales. section . the short strong fore limbs are adapted to the burrowing habit, and have five digits; the hind limbs are very much longer and muscular, enable the animal to progress rapidly by short leaps, and they have four toes. if the student thinks it worth while to attempt to remember the number of digits-- it is the fault of examiners if any value dose attach to such intrinsically valueless facts-- he should associate the number ( in front, behind) with the rabbit, and observe that with the frog the reverse is the case. section . we may note here the meaning of certain terms we shall be constantly employing. the head end of the rabbit is anterior, the tail end posterior, the backbone side of the body-- the upper side in life-- is dorsal, the breast and belly side, the lower side of the animal, is ventral. if we imagine the rabbit sawn asunder, as it were, by a plane passing through the head and tail, that would be the median plane, and parts on either side of it are lateral, and left or right according as they lie to the animal's left or right. in a limb, or in the internal organs, the part nearest the central organ, or axis, is proximal, the more remote or terminal parts are distal. for instance, the mouth is anteriorly placed, the tongue on its ventral wall; the tongue is median, the eyes are lateral, and the fingers are distal to the elbow. the student must accustom himself to these words, and avoid, in his descriptions, the use of such terms as "above," "below," "outside," which vary with the position in which we conceive the animal placed. section . so much for the general form; we may note a few facts of general knowledge, in connection with the rabbit's life-activity. in a day of the rabbit's life a considerable amount of work is done-- the animal runs hither and thither, for instance; in other words, a certain mass of matter is moved through space, and for that we know force must be exerted. whence comes the force? section . we find the rabbit occupies a considerable amount of its time in taking in vegetable matter, consisting chiefly of more or less complex combustible and unstable organic compounds. it is a pure vegetarian, and a remarkably moderate drinker. some but only a small proportion, of the vegetable matter it eats, leaves its body comparatively unchanged, in little pellets, the faeces, in the process of defaecation. for the rest we have to account. section . we find, also, that the rabbit breathes air into its lungs, which is returned to the atmosphere with a lessened amount of oxygen, and the addition of a perceptible amount of carbon dioxide. the rabbit also throws off, or excretes, a fluid, the urine, which consists of water with a certain partially oxydised substance containing nitrogen, and called urea, and other less important salts. the organs within the body, by which the urine is separated, are called the kidneys. section . repeating these facts in other words, the rabbit takes into its body complex and unstable organic compounds containing nitrogen, carbon, hydrogen, a certain amount of oxygen, a small quantity of sulphur, and still smaller amounts of other elements. it also breathes in oxygen. section . it returns a certain rejected part of its food comparatively unchanged. besides this, it returns carbon dioxide and water, which are completely oxydised, and very simple and stable bodies, and urea-- a less completely oxydised compound, but a very simple one compared with the food constituents. section . now the chemist tells us that when a stable body is formed, or when an unstable compound decomposes into simpler stable ones, force is evolved. the oxydation of carbon, for instance, in the fireplace, is the formation of the stable compound called carbon dioxide, and light and heat are evolved. the explosion of dynamite, again is the decomposition of an unstable compound. hence, we begin to perceive that force-- the vital force-- which keeps the rabbit moving, is supplied by the decomposition and partial oxydation of compounds continued in its food, to carbon dioxide, water, urea, and smaller quantities of other substances. section . this is the roughest statement of the case possible, but it will give the general idea underlying our next chapters. we shall consider how the food enters the body and is taken up into the system, how it is conveyed to the muscles in the limbs, to the nerve centres, and to wherever work is done, to be there decomposed and partially oxydised, and finally how the products of its activity-- the katastases, of which the three principal are carbon dioxide, water, and urea-- are removed from the body. section . there are one or two comparatively modern terms that we may note here. this decomposition of unstable chemical compounds, releasing energy, is called kataboly. a reverse process, which has a less conspicuous part in our first view of the animal's life action, by which unstable compounds are built up and energy stored, is called anaboly. the katastases are the products of kataboly. section . in an ordinary animal, locomotion and other activity predominate over nutritive processes, which fact we may express, in the terms just given, by saying that kataboly prevails over anaboly. an animal, as we have just explained, is an apparatus for the decomposition and partial oxydation of certain compounds, and these are obtained either directly or indirectly-- through other animals, in the case of meat-eaters-- from the vegetable kingdom. as the student will learn early in his botanical reading, the typical plant has, in its green colouring matter, chlorophyll, a trap to catch the radiating energy of the sun, and to accomplish, by the absorption of that energy, the synthesis (building up) of those organic compounds which the animal destroys. the typical plant is, on whole, passive and synthetic, or anabolic; the typical animal, active and katabolic; and the excess of kataboly over anaboly in the animal is compensated for by the anabolic work stored up, as it were, by the plant, which is, directly or indirectly, the animal's food. . _the alimentary canal of the rabbit_ section . figure represents the general anatomy of the rabbit, but is especially intended to show the alimentary (= food) canal, shortened to a certain extent, and with the proportions altered, in order to avoid any confusing complications. it is evidently simply a coiled tube-- coiled for the sake of packing-- with occasional dilatations, and with one side-shunt, the caecum (cae.), into which the food enters, and is returned to the main line, after probably absorbent action, imperfectly understood at present. a spiral fold in this cul-de-sac {bottom-of-sack}, which is marked externally by constrictions, has a directive influence on the circulation of its contents. the student should sketch figure once or twice, and make himself familiar with the order and names of the parts before proceeding. we have, in succession, the mouth (m.), separated from the nasal passage (na.) above the palate; the pharynx (ph.), where the right and left nasal passages open by the posterior nares into the mouth; the oesophagus (oes.); the bag-like stomach, its left (section ) end being called the cardiac (cd.st.), and its right the pyloric end (py.); the u-shaped duodenum (ddnm.) and the very long and greatly coiled ileum (il.). the duodenum and ileum together form the small intestine; and the ileum is dilated at its distal end into a thick-walled sacculus rotundus (s.r.), beyond which point comes the large intestine. the colon (co.) and rectum (r.) continue the main line of the alimentary canal; but, at the beginning of the large intestine, there is also inserted a great side-shunt, the caecum (cae.), ending blindly in a fleshy vermiform appendix (v.ap.). the figure will indicate how the parts are related better than any verbal description can. between the coiling alimentary tube and the body walls is a space, into which the student cuts when he begins dissecting; this is the peritoneal cavity (pt.). a thin, transparent membrane, the mesentery, holds the intestines in place, and binds them to the dorsal wall of this peritoneal space. section . the food stuffs of an animal, the unstable compounds destined ultimately to be worked into its life, and to leave it again in the form of katastases (section ), fall into two main divisions. the first of these includes the non-nitrogenous food stuffs, containing either carbon together with hydrogen and oxygen in the proportion of h o (the carbo-hydrates), or carbon and hydrogen without oxygen (the hydrocarbons). the second division consists of the nitrogenous materials, containing also carbon, hydrogen, a certain amount of oxygen, sulphur, and possibly other elements. among the carbohydrates, the commonest are starch and cellulose, which are insoluble bodies, and sugar, which is soluble. the hydrocarbons, fats, oils, and so on, form a comparatively small proportion of the rabbit's diet; the proverb of "oil and water" will remind the student that these are insoluble. the nitrogenous bodies have their type in the albumen of an egg; and muscle substance and the less modified living "protoplasm" of plants, a considerable proportion of the substance of seeds, bulbs, and so on, are albuminous bodies, or proteids. these also are insoluble bodies, or when soluble, will not diffuse easily through animal membranes. section . now the essential problem which the digestive canal of the rabbit solves is to get these insoluble, or quasi-insoluble, bodies into its blood and system. they have to pass somehow into the circulation through the walls of the alimentary canal. in order that a compound should diffuse through a membrane, it must be both soluble and diffusible, and therefore an essential preliminary to the absorption of nutritive matter is its conversion into a diffusible soluble form. this is effected by certain fluids, formed either by the walls of the alimentary canal or by certain organs called glands, which open by ducts into it; all these fluids contain small quantities of organic compounds of the class called ferments, and these are the active agents in the change. the soluble form of the carbohydrates is sugar; proteids can be changed into the, of course, chemically equivalent but soluble and diffusible the peptones; and fats and oils undergo a more complicated, but finally similar change. section . we shall discuss the structure and action of -a gland- [glands] a little more fully in a subsequent chapter. here we will simply say that they are organs forming each its characteristic fluid or secretion, and sending it by a conduit, the duct, to the point where its presence is required. the saliva in our mouths, tears, and perspiration, are examples of the secretions of glands. section . in the month of the rabbit the food is acted upon by the teeth and saliva. the saliva contains ptyalin, a ferment converting starch into sugar, and it also serves to moisten the food as it is ground up by the cheek teeth. it does not act on fat to any appreciable extent. the teeth of the rabbit are shown in figure xviii., sheet . the incisor teeth in front, two pairs above and one pair below (i.), are simply employed in grasping the food; the cheek teeth-- the premolars (pm.) and molars (m.) behind-- triturate the food by a complicated motion over each. their crowns are flat for this purpose, with harder ridges running across them. section . this grinding up of the food in the mouth invariably occurs in herbivorous animals, where there is a considerable amount of starch and comparatively little hydrocarbon in the food. by finely dividing the food, it ensures its intimate contact with the digestive ferment, ptyalin. in such meat-eaters as the cat and dog, where little starchy matter and much fat is taken, the saliva is, of course, of less importance, and this mastication does not occur. the cheek teeth of a dog ({section }), and more so of a cat, are sharp, and used for gnawing off fragments of food, which are swallowed at once. between the incisors and premolars of a dog come the characteristic biting teeth, or canines, absent in the rabbit. section . the student will probably ask why the cheek teeth, which are all similar in appearance, are divided into premolars and molars. the rabbit has a set of milk molars-- a milk dentition-- which are followed by the permanent teeth, just as in man. those cheek teeth of the second set, which have predecessors in the first series, are called premolars; the ones posterior to these are the molars. section . after mastication, the food is worked by the tongue and cheeks into a saliva-soaked "bolus" and swallowed. the passage down the oesophagus is called deglutition. in the stomach it comes under the influence of the gastric juice, formed in little glandular pits in the stomach wall-- the gastric (figure viii. sheet ) and pyloric glands. this fluid is distinctly acid, its acidity being due to about one-tenth per cent {of a hundred} of hydrochloric acid, and it therefore stops any further action of the ptyalin, which can act only on neutral or slightly alkaline fluids. the gastric juice does not act on carbo-hydrates or hydrocarbons to any very noticeable degree. its essential property is the conversion of proteids into peptones, and the ferment by which this is effected is called pepsin. milk contains a peculiar soluble proteid, called casein, which is precipitated by a special ferment, the rennet-ferment, and the insoluble proteid, the curd, thus obtained is then acted on by the pepsin. in the manufacture of cheese, the rennetferment obtained, from the stomach of a calf is used to curdle the milk. section . after the food has undergone digestion in the stomach it passes into the duodenum, the u-shaped loop of intestine immediately succeeding the stomach. the duodenum is separated from the stomach by a ring-like muscular valve, the pylorus; this valve belongs to the class of muscles called sphincters, which, under ordinary circumstances, are closed, but which relax to open the circular central aperture. the valve at the anus, which retains the faeces, is another instance of a sphincter. section . the food at this stage is called chyme; it is an acid and soup-like fluid-- acid through the influence of the gastric juice. the temperature of the animal's body is sufficiently high to keep most of the fat in the food melted and floating in oily drops; much of the starch, has been changed to sugar, and the solid proteids to soluble peptones, but many fragments of material still float unchanged. section . it meets now with the bile, a greenish fluid secreted by that large and conspicuous gland the liver. the bile is not simply a digestive secretion, like the saliva or the gastric juice; it contains matters destined to mix in, and after a certain amount of change to be passed out of the body with, the faeces; among these substances, of which some portion is doubtless excretory, are compounds containing sulphur-- the bile salts. there is also a colouring matter, bili verdin, which may possibly also be excretory. if the student will compare sections and , he will notice that in those paragraphs no account is taken of the sulphur among the katastases, the account does not balance, and he will at once see that here probably is the missing item on the outgoing side. the bile, through the presence of these salts, is strongly alkaline, and so stops the action of the gastric juice, and prepares for that of the pancreas, which can act only in an alkaline medium. the fermentive action of the bile is trifling; it dissolves fats, to a certain extent, and is antiseptic, that is, it prevents putrefaction to which the chyme might be liable; it also seems to act as a natural purgative. section . the bile, as we shall see later, is by no means the sole product of the liver. section . the pancreatic juice, the secretion of the pancreas is remarkable as acting on all the food stuffs that have not already become soluble. it emulsifies fats, that is, it breaks, the drops up into extremely small globules, forming a milky fluid, and it furthermore has a fermentive action upon them; it splits them up into fatty acids, and the soluble body glycerine. the fatty acids combine with alkaline substances (section ) to form bodies which belong to the chemical group of soaps, and which are soluble also. the pancreatic juice also attacks any proteids that have escaped the gastric juice, and converts them into peptones, and any residual starch into sugar. hence by this stage, in the duodenum, all the food constituents noticed in section are changed into soluble forms. there are probably, three distinct ferments in the pancreatic juice acting respectively on starch, fat, and proteid, but they have not been isolated, and the term pancreatin is sometimes used to suggest the three together. section . a succus entericus, a saliva-like fluid secreted by numerous small glands in the intestine wall (brunner's glands, lieberkuhnian follicles), probably aids, to an unknown but comparatively small extent, in the digestive processes. section . the walls of the whole of the small intestine are engaged in the absorption of the soluble results of digestion. in the duodenum, especially, small processes, the villi project into the cavity, and being, like the small hairs of velvet pile, and as thickly set, give its inner coat a velvety appearance. in a villus we find (figure ix., sheet ) a series of small blood-vessels and with it another vessel called a lacteal. the lacteals run together into larger and larger branches until they form a main trunk, the thoracic duct, which opens into the blood circulation at a point near the heart; but of this we shall speak further later. they contain, after a meal, a fluid called chyle. section . emulsified fats pass into the chyle. water and diffusible salts certainly pass into the vein. the course taken by the peptones is uncertain, but professor foster favours the chyle in the case of the rabbit-- the student should read his text-book of physiology, part , chapter , section , if interested in the further discussion of this question. section . the processes that occur in the remaining portions of the alimentary canal are imperfectly understood. the caecum is so large in the rabbit that it must almost certainly be of considerable importance. in carnivorous animals it may be so much reduced as to be practically absent. an important factor in the diet of the herbivorous animals, and one absent from the food of the carnivora, is that carbohydrate, the building material of all green-meat- [food], cellulose, and there is some ground for thinking that the caecum is probably a region of special fermentive action upon it. the pancreatic juice, it may be noted, exercises a slight digestive activity upon this substance. section . water is most largely absorbed in the large intestine, and in it the rejected (mainly insoluble) portion of the food gradually acquires its dark colour and other faecal characteristics. . _the circulation_ section . the next thing to consider is the distribution of the food material absorbed through the walls of the alimentary canal to the living and active parts of the body. this is one of the functions of the series of structures-- heart and blood-vessels, called the circulation, circulatory system, or vascular system. it is not the only function. the blood also carries the oxygen from the lungs to the various parts where work is done and kataboly occurs, and it carries away the katastases to the points where they are excreted-- the carbon dioxide and some water to the lungs, water and urea to the kidneys, sulphur compounds of some kind to the liver. section . the blood (figure , sheet ) is not homogeneous; under the low power of the microscope it may be seen to consist of-- ( .) a clear fluid, the plasma, in which float-- ( .) a few transparent colourless bodies of indefinite and changing shape, and having a central brighter portion, the nucleus with a still brighter dot therein the nucleolus-- the white corpuscles (w.c.), and ( .) flat round discs, without a nucleus, the red corpuscles (r.c.), greatly more numerous than the white. section . the chyle of the lacteals passes, as we have said, by the thoracic duct directly into the circulation. it enters the left vena cava superior (l.v.c.s.) near where this joins the jugular vein (ex.j.) (see figure , sheet , th.d.) and goes on at once with the rest of the blood to the heart. the small veins of the villi, however, which also help suck up the soluble nutritive material, are not directly continuous with the other body veins, the systemic veins; they belong to a special system, and, running together into larger and larger branches, form the lieno gastric (l.g.v.) and mesenteric (m.v.) veins, which unite to form the portal vein (p.v.) which enters the liver (l.v.) and there breaks up again into smaller and smaller branches. the very finest ramifications of this spreading network are called the (liver) capillaries, and these again unite to form at last the hepatic vein (h.v.) which enters the vena cava inferior (v.c.i.), a median vessel, running directly to the heart. this capillary network in the liver is probably connected with changes requisite before the recently absorbed materials can enter the general blood current. section . the student has probably already heard the terms vein and artery employed. in the rabbit a vein is a vessel bringing blood towards the heart, while an artery is a vessel conducting it away. veins are thin-walled, and therefore flabby, a conspicuous purple when full of blood, and when empty through bleeding and collapsed sometimes difficult to make out in dissection. they are formed by the union of lesser factors. the portal breaks up into lesser branches within the liver. arteries have thick muscular and elastic walls, thick enough to prevent the blood showing through, and are therefore pale pink or white and keep their round shape. section . the heart of the rabbit is divided by partitions into four chambers: two upper thin-walled ones, the auricles (au.), and two lower ones, both, and especially the left, with very muscular walls, the ventricles (vn.). the right ventricle (r.vn.) and auricle (r.au.) communicate, and the left ventricle (l.vn.) and auricle (l.au.). section . the blood coming from all parts of the body, partly robbed of its oxygen and containing much carbon dioxide and other katastases, enters the right auricle of the heart through three great veins, the median vena cava inferior from the posterior parts of the body, and the paired venae cavae superiores from the anterior. with the beating of the heart, described below, it is forced into the right ventricle and from there through the pulmonary artery (p.a.) seen in the figure passing under the loop of the aorta (ao.) to the lungs. section . the lungs (lg. figure , sheet ) are moulded to the shape of the thoracic cavity and heart; they communicate with the pharynx by the trachea (tr. in figure , sheet ) or windpipe, and are made up of a tissue of continually branching and diminishing air-tubes, which end at last in small air-sacs, the alveoli. the final branches of the pulmonary arteries, the lung capillaries, lie in the walls of these air-sacs, and are separated from the air by an extremely thin membrane through which the oxygen diffuses into, and the carbon dioxide escapes from, the blood. section . the mechanism of respiration will be understood by reference to figure , sheet . it will be noted, in dissecting that the lungs have shrunk away from the walls of the thorax; this collapse occurs directly an aperture is made in the thorax wall, and is in part due to their extreme elasticity. in life the cavity of the thorax forms an air-tight box, between which and the lungs is a slight space, the pleural cavity (pl.c.) lined by a moist membrane, which is also reflected, over the lungs. the thorax wall is muscular and bony, and resists the atmospheric pressure on its outer side, so that the lungs before this is cut through are kept distended to the size of the thoracic cavity by the pressure of the air within them. in inspiration (or breathing-in) the ribs are raised by the external intercostal (anglice, between-ribs, e.i.c.m.) and other allied muscles, and the diaphragm (dia.) contracts and becomes flatter; the air is consequently sucked, in as the lungs follow the movement of the thorax wall. in expiration the intercostals and diaphragm relax and allow the elastic recoil of the lungs to come into play. the thoracic wall is simultaneously depressed by the muscles of the abdominal area, the diaphragm thrust forwards, as the result of the displacement and compression of the alimentary viscera thus brought about. (r.r.r. in the figure mark ribs.) section . the oxygen and carbon dioxide are not carried in exactly the same way by the blood. the student will know from his chemical reading that neither of these gases is very soluble, but carbon dioxide is sufficiently so in an alkaline fluid to be conveyed by the liquid plasma. the oxygen however, needs a special portative mechanism in the colouring matter of the red corpuscles, the haemoglobin, with which it combines weakly to form oxy-haemoglobin of a bright red colour, and decomposing easily in the capillaries (the finest vessels between the arteries and veins), to release the oxygen again. the same compound occurs in all true vertebrata, and in the blood-fluid of the worm; in the crayfish a similar substance, haemocyanin, which when oxygenated is blue, and when deoxydized colourless, discharges the same function. section . the blood returns from the lungs to the left auricle (l.au.) by the pulmonary veins, hidden in the figure by the heart, passes thence to the thick-walled left ventricle (l.vn.), and on into the aorta (ao.). section . the beating of the heart is, of course, a succession of contractions and expansions of its muscular wall. the contraction, or systole, commences at the base of the venae cavae and passes to the auricles, driving the blood before it into the ventricles, which then contract sharply and drive it on into the aorta or pulmonary artery; a pause and then a dilatation, the diastole follows. the flow of the blood is determined in one direction by the various valves of the heart. no valves occur in the opening of the superior cavae but an imperfect one, the eustachian valve, protects the inferior cava; the direction of the heart's contraction prevents any excessive back-flow into the veins, and the onward, tendency is encouraged by the suck of the diastole of the ventricles. between the left ventricle and auricle is a valve made up of two flaps of skin, the mitral valve, the edges of the flaps being connected with the walls of the ventricle through the intermediation of small muscular threads, the chordae tendinae, which stretch across its cavity to little muscular pillars, the papillary muscles; these attachments prevent the mitral valve from flapping back into the auricle, and as the blood flows into and accumulates in the ventricle it gets behind the flaps of the valve and presses its edges together. when the systole of the ventricle occurs, the increased, tension of the blood only closes the aperture the tighter, and the current passes on into the aorta, where we find three watch-pocket valves, with the pocket turned away from the heart, which are also closed and tightened by any attempt at regurgitation (back-flow). a similar process occurs on the right side of the heart, but here, instead of a mitral valve of two flaps between auricle and ventricle, we have a tricuspid valve with three. the thickness of the muscular walls, in view of the lesser distance through which it has to force the blood, -are- [is] less for the right ventricle than the left. section . the following are the chief branches of the aorta. the student should be able to follow them with certainty in dissection; they are all displayed in the figure; but it must not be imagined for a moment that familiarity with this diagram will obviate the necessity for the practical work; (in.) is the innominate artery; it forks into (s.cl.a.) the right subclavian, and (r.c.c.) the right common carotid. each carotid splits at the angle of the jaw into an internal and an external branch. the left common carotid, (l.c.c.) arises from the base of the innominate,* (l.s.cl.a.) the left subclavian, directly from the aorta. the aorta now curves round to the dorsal middle line, and runs down as seen in figure , sheet (d.ao.) and figure , sheet (d.ao.). small branches are given off to the ribs, and then comes the median coeliac (coe.a.) to the stomach and spleen, the median superior mesenteric (s.mes.a.) to the main portion of the intestine, and the inferior mesenteric (p.m.a.) to the rectum. note that no veins to the inferior vena cava correspond to these arteries-- the blood they supply going back by the portal vein (p.v.). the paired renal arteries (r.a.) supply the kidneys, and the common iliacs (c.il.a.) the hind legs, splitting into the internal iliacs (i.il.a.) and the femoral (f.). {lines from second edition only.} [the student should note that the only arteries in the middle line are those supplying the alimentary canal.] {lines from first edition only.} * -the figure is inaccurate, and represents the left common carotid as arising from the aortic arch.- section . the distribution of the veins of the rabbit has only a superficial parallelism with arteries. the chief factors of vena cava inferior are the hepatic vein (h.v.), which receives the liver blood, the renal veins (r.v.), from the kidneys, the ilaeo-lumbar, from the abdominal wall, and the external (e.il.v.) and internal ilias (i.il.v.); with the exception of the renal veins none of these run side by side with arteries. the superior cavae (r. and l.v.c.s.) are formed by the union of internal (i.j.) and external jugular (e.j.) veins with a subclavian (s.cl.v.) from the fore limb. the term pre-caval vein is sometimes used for superior cava. the attention, of the student is called to the small azygos vein (az.) running into the right vena cava superior, and forming the only asymmetrical (not-balancing) feature of the veins in front of the heart; it brings blood back from the ribs of the thorax wall, and is of interest mainly because it answers to an enormous main vessel, the right post-cardinal sinus, in fishes. there are spermatic arteries and veins (s.v. and a.) to the genital organs. all these vessels should be patiently dissected out by the student, and drawn. section . between the final branches of the arteries and the first fine factors of the veins, and joining them, come the systemic capillaries. these smallest and ultimate ramifications of the circulation penetrate every living part of the animal, so that if we could isolate the vascular system we should have the complete form of the rabbit in a closely-meshed network. it is in the capillaries that the exchange of gases occurs and that nutritive material passes out to the tissues and katastases in from them; they are the essential factor in the circulatory system of the mammal-- veins, arteries, and heart simply exist to remove and replace their contents. the details of the branching of the pulmonary artery and the pulmonary veins need not detain us now. section . summarising the course of the circulation, starting from the right ventricle, we have-- pulmonary artery, pulmonary capillaries, pulmonary vein, left auricle, left ventricle, aorta, arteries, and systemic capillaries. after this, from all parts except the spleen and alimentary canal, the blood returns to systemic veins, superior or inferior cavae, right auricle, and right ventricle. the blood from the stomach spleen, and intestines however, passes via {through} the portal vein to the liver capillaries and then through the hepatic vein to inferior cava, and so on. material leaves the blood to be excreted in lungs, kidneys, by the skin (as perspiration), and elsewhere. new material enters most conspicuously; (a) by the portal veins portal veins and (b) by the thoracic duct and left superior cava. section . the following table summarises what we have learnt up to the present of the physiology of the rabbit, considered as a mechanism using up food and oxygen and disengaging energy:-- -air_ {nitrogen... returned unchanged.} {oxygen... through pulmonary vein to--} {see .} -food_ {carbo-hydrates (starch, sugar, cellulose.)} sugar. {protein.} {peptones.} {fat (little in rabbit.)} {glycerine, and fatty acids in soups.} {rejected matter got rid of in defaecation.} a. {chyle in lacteals going via {through} thoracic duct and left superior cava to--} {see .} b. {veins of villi--} {portal vein--} {liver--} {hepatic vein and inferior cava to--} {see .} . {right side of heart; then to lungs, and then to--} {see .} . {left side of heart; whence to systemic arteries and capillaries.} . {the tissues and -kataboly_.} . {urea (?liver) kidney and sweat glands} {co } {lungs} {h o} {lungs, kidney, sweat glands} {other substances} {mainly by [kidney,] liver and alimentary canal} . _the amoeba. cells, and tissue_ section . we have thus seen how the nutritive material is taken into the animal's system and distributed over its body, and incidentally, we have noted how the resultant products of the creature's activity are removed. the essence of the whole process, as we have already stated, is the decomposition and partial oxydation of certain complex chemical compounds to water, carbon dioxide, a low nitrogenous body, which finally takes the form of urea, and other substances. we may now go on to a more detailed study, the microscopic study, or histology, of the tissues in which metaboly and kataboly occur, but before we do this it will be convenient to glance for a moment at another of our animal types-- the amoeba, the lowest as the rabbit is the highest, in our series. section . this is shown in figure iii., sheet , as it would appear under the low power of the microscope. we have a mass of a clear, transparent, greyish substance called protoplasm, granular in places, and with a clearer border; within this is a denser portion called the nucleus, or endoplast (n.), which, under the microscope, by transmitted light, appear brighter, and within that a still denser spot, the nucleolus (ns.) or endoplastule. the protoplasm is more or less extensively excavated by fluid spaces, vacuoles; one clearer circular space or vacuole, which is invariably present, appears at intervals, enlarges gradually, and then vanishes abruptly, to reappear after a brief interval; this is called the contractile vacuole (c.v.). the amoeba is constantly changing its shape, whence its older name of the proteus animalcule, thrusting out masses of its substance in one direction, and withdrawing from another, and hence slowly creeping about. these thrust-out parts, in its outline, are called pseudopodia (ps.). by means of them it gradually creeps round and encloses its food. little particles of nutritive matter are usually to be detected in the homogeneous protoplasm of its body; commonly these are surrounded by a drop of water taken in with them, and the drop of water is then called a food vacuole. the process of taking in food is called ingestion. the amoeba, in all probability, performs essentially the same chemical process as we have summarised in sections , , ; it ingests food, digests it in the food vacuoles and builds it up into its body protoplasm, to undergo kataboly and furnish the force of its motion-- the contractile vacuole, is probably respiratory and perhaps excretory, accumulating and then, by its "systole" (compare section ), forcing out of its body, the water, carbon dioxide, urea, and other katastases, which are formed concomitantly with its activity. the amoeba reproduces itself in the simplest way; the nucleus occasionally divides into two portions and a widening fissure in the protoplasm of the animal's body separates one from the other. it is impossible to say that one is the parent cell, and the other the offspring; the amoeba we merely perceive, was one and is now two. it is curious to note, therefore, that the amoeba is, in a sense, immortal-- that the living nucleus of one of these minute creatures that we examine to-day under a microscope may have conceivably drawn, out an unbroken thread of life since the remotest epochs of the world's history. although no sexual intercourse can be observed, there is reason to believe that a process of supposed "cannabalism," in which a larger amoeba may occasionally engulph a smaller one, is really a conjugative reproductive process, and followed by increased vitality and division. section . now if the student will compare section , he will see that in the white blood corpuscles we have a very remarkable resemblance to the amoeba; the contractile vacuole is absent, but we have the protoplasmic body, the nucleus and nucleolus, and those creeping fluctuations of shape through the thrusting out and withdrawal of pseudopodia, which constitute "amoeboid" motion. they also multiply, in the same way, by division. section . it is not only in the white corpuscle of the blood that we find this resemblance; in all the firmer parts of the body we find, on microscopic examination, similar little blebs of protoplasm, and at an early stage of development the young rabbit is simply one mass of these protoplasmic bodies. their division and multiplication is an essential condition, of growth. through an unfortunate accident, these protoplasmic blebs, which constitute the living basis of the animal body, have come to be styled "cells," though the term "corpuscles" is far more appropriate. section . the word is "cell" suggests something enclosed by firm and definite walls, and it was first employed in vegetable histology. unlike the typical cells of animals, the cells of most plants are not naked protoplasm, but protoplasm enclosed in a wall of substance (cell wall) called cellulose. the presence of this cellulose cell wall, and the consequent necessity of feeding entirely upon liquids and gases that soak through it instead of being able to ingest a portion of solid food is indeed, the primary distinction between the vegetable and the animal kingdoms, as ordinarily considered. section . throughout life, millions of these cells retain their primary characters, and constitute the white corpuscles of blood, "phagocytes," and connective tissue corpuscles; others again, engage in the formation of material round themselves, and lie, in such cases, as gristle and bone, embedded in the substance they have formed; others again, undergo great changes in form and internal structure, and become permanently modified into, for instance, nerve fibres and muscle substance. the various substances arising in this way through the activity of cells are called tissues, the building materials of that complex thing, the animal body. since such a creature as the rabbit is formed through the co-operation of a vast multitude of cells, it is called multicellular; the amoeba, on the other hand, is unicellular. the rabbit may be thus regarded as a vast community of amoeboid creatures and their products. section . figure iv., sheet represents, diagrammatically, embryonic tissue, of which, to begin with, the whole animal consists. the cells are all living, capable of dividing and similar, but as development proceeds, they differentiate, some take on one kind of duty (function), and some another, like boys taking to different trades on leaving school, and wide differences in structure and interdependence become apparent. section . it is convenient to divide tissues into three classes, though the divisions are by no means clearly marked, nor have they any scientific value. the first of these comprises tissues composed wholly, or with the exception of an almost imperceptible cementing substance, of cells; the second division includes the skeletal tissues, the tissue of mesentery, and the connective and basement tissue of most of the organs, tissues which, generally speaking, consist of a matrix or embedding substance, formed by the cells and outside of them, as well as the cells themselves; and, thirdly, muscular and nervous tissue. we shall study the former two in this chapter, and defer the third division until later. section . the outer layer of the skin (the epidermis), the inmost lining of the alimentary canal, the lining of the body cavity, and the inner linings of blood-vessels, glands, and various ducts constitute our first division. the general name for such tissues is epithelium. when the cells are more or less flattened, they form squamous epithelium (figure vi.) such as we find lining the inside of a man's cheek (from which the cells sq.ep. were taken) or covering the mesentery of various types-- sq.end. are from the mesentery (section ) of a frog. a short cylindroidal form of cell makes up columnar epithelium, seen typically in the cells covering the villi of the duodenum (figure v.). this epithelium of the villi has the outer border curiously striated, and this is usually spoken of as leading towards "ciliated" epithelium, to be described immediately. the epithelium of the epididermis is stratified-- that is to say, has many thicknesses of cells; the deeper layers are alive and dividing (stratum mucosum), while the more superficial are increasingly flattened and drier as the surface is approached (stratum corneum) and are continually being rubbed off and replaced from below. section . in the branching air-tubes of the lung, the central canal of the spinal cord, and in the ureters of the rabbit, and in most other types, in various organs, we find ciliated epithelium (figure vii.). this is columnar or cubical in form, and with the free edge curiously modified and beset with a number of hair-like processes, the cilia, by which, during the life of the cell, a waving motion is sustained in one direction. this motion assists in maintaining a current in the contents of ducts which are lined with this tissue. the motion is independent of the general life of the animal, so long as the constituent cell still lives, and so it is easy for the student to witness it himself with a microscope having a / -inch or / -inch objective. very fine cilia may be seen by gently scraping the roof of a frog's mouth (the cells figured are from this source), or the gill of a recently killed mussel, and mounting at once in water, or, better, in a very weak solution of common salt. section . the lining of glands is secretory epithelium; the cells are usually cubical or polygonal ( , g.ep.), and they display in the most characteristic form what is called metabolism. anaboly (see section ) we have defined, as a chemical change in an upward direction-- less stable and more complex compounds are built up in the processes of vegetable and animal activity towards protoplasm; kataboly is a chemical running down; metaboly is a more general term, covering all vital chemical changes. the products of the action of a glandular epithelium are metabolic products, material derived from the blood is worked, up within the cell, not necessarily with conspicuous gain or loss of energy, and discharged into the gland space. the most striking case of this action is in the "goblet cells" that are found among the villi; these are simply glands of one cell, unicellular glands, and in figure v. we see three stages in their action: at g.c. material (secretion) is seen forming in the cell, at g.c. it approaches the outer border, and at g.c. it has been discharged, leaving a hollowed cell. usually however, the escape of secreted matter is not so conspicuous, and the gland-cells are collected as the lining of pits, simple, as in the gastric, pyloric, and lieberkuhnian glands (figure viii., sections , ), or branching like a tree or a bunch of grapes (figure r.g.), as in brunner's glands (section ) the pancreas, and the salivary glands. the salivary glands, we may mention, are a pair internal to the posterior ventral angle of the jaw, the sub-maxillary; a pair anterior to these, the sub-lingual; a pair posterior to the jaw beneath the ear, the parotid, and a pair beneath the eye, the infra orbital. section . the liver is the most complicated gland in the body (figure x.). the bile duct (b.d.) branches again and again, and ends at last in the final pits, the lobuli (lb.), which are lined with secretory epithelium, and tightly packed, and squeeze each other into polygonal forms. the blood supply from which the bile would appear to be mainly extracted, is brought by the portal vein, but this blood is altogether unfit for the nutrition of the liver tissue; for this latter purpose a branch of the coeliac artery, the hepatic serves. hence in the tissue of the liver we have, branching and interweaving among the lobuli, the small branches of the bile duct (b.d.), which carries away the bile formed, the portal vein (p.v.), the hepatic artery (h.a.), and the hepatic vein (h.v.). (compare section .) figure x.b shows a lobule; the portal vein and the artery ramify round the lobules-- are inter-lobular, that is (inter, between); the hepatic vein begins in the middle of the lobules (intra-lobular), and receives their blood. (compare x.a.) besides its function in the manufacture of the excretory, digestive, and auxiliary bile, the liver performs other duties. it appears to act as an inspector of the assimilation material brought in by the portal vein. the villi, for instance, will absorb arsenic, but this is arrested and thrown down in the liver. a third function is the formation of what would seem to be a store of carbo-hydrate, glycogen, mainly it would appear, from the sugar in the portal vein, though also, very probably, from nitrogenous material, though this may occur only under exceptional conditions. finally, the nitrogenous katastates, formed in the working of muscle and nerve, and returned by them to the blood for excretion, are not at that stage in the form of urea. whatever form they assume, they undergo a further metabolism into urea before leaving the body, and the presence of considerable quantities of this latter substance in the liver suggests this as a fourth function of this organ-- the elaboration of urea. section . similar from a physiological point of view, to the secretory glands which form the digestive fluids are those which furnish lubricating fluids, the lachrymal gland, and harderian glands in the orbit internally to the eye, and posterior and anterior to it respectively, the sebaceous glands (oil glands) connected with the hair, and the anal and perineal glands. the secretions of excretory glands are removed from the body; chief among them are the sweat glands and kidneys. the sweat glands are microscopic tubular glands, terminating internally in a small coil (figure viii. s.g.) and are scattered thickly over the body, the water of their secretion being constantly removed by evaporation, and the small percentage of salt and urea remaining to accumulate as dirt, and the chief reasonable excuse for washing. the kidney structure is shown diagrammatically in figure , sheet . a great number of branching and straight looped, tubuli (little tubes) converge on an open space, the pelvis. towards the outer layers (cortex) of the kidney, these tubuli terminate in little dilatations into which tangled knots of blood-vessels project: the dilatations are called bowman's capsules (b.c.), and each coil of bloodvessel a glomerulus (gl.). in the capsules, water is drained from the blood; in the tubuli, urea and other salts in the urine are secreted from a branching network of vessels. section . in all the epithelial tissues that we have considered we have one feature in common: they are cells, each equivalent to the amoeba, that have taken on special duties-- in a word, they are specialists. the amoeba is jack of all trades and a free lance; the protective epidermal cell, the current-making ciliated cell, the bile or urea-making secretory cell, is master of one trade, and a soldier in a vast and wonderfully organized host. we will now consider our second kind of cell in this organization, the cell of which the especial aim is the building round it of a tissue. section . the simplest variety in this group is hyaline (i.e. glassy) cartilage (gristle). in this the formative cells (the cartilage corpuscles) are enjellied in a clear structureless matrix (figure xii.), consisting entirely of organic compounds accumulated by their activity. immediately round the cell lies a capsule of newer material. some of the cells have recently divided ( ); others have done so less recently, and there has been time for the interpolation of matrix, as at . in this way the tissue grows and is repaired. a thin layer of connective tissue (see below), the perichondrium, clothes the cartilaginous structure. section . connective tissue (figure xiii) is a general name for a group of tissues of very variable character. it is usually described as consisting typically in the mammals of three chief elements felted together; of comparatively unmodified corpuscles (c.c.), more or less amoeboid, and of fibres which are elongated, altered, and distorted cells. the fibres are of two kinds: yellow, branching, and highly elastic (y.e.f.), in consequence of which they fall into sinuous lines in a preparation, and white and inelastic ones (w.i.f.), lying in parallel bundles. where the latter element is entirely dominant, the connective tissue is tendon, found especially at the point of attachment of muscles to the parts they work. some elastic ligaments are almost purely yellow fibrous tissue. a loose interweaving of the three elements is areolar tissue, the chief fabric of mesentery, membrane, and the dermis (beneath the epidermis). with muscle it is the material of the walls of the alimentary canal and bloodvessels, and generally it enters into, binds together, and holds in place other tissue. the connective tissue of fishes displays the differentiation of fibres in a far less distinct manner. section . through connective tissues wander the phagocytes, cells that are difficult to distinguish, if really distinct, from the white blood corpuscles. these cells possess a remarkable freedom; they show an initiative of their own, and seem endowed with a subordinate individuality. they occur in great numbers in a tissue called, botryoidal tissue (figure xiv.), which occurs especially in masses and patches along the course of the alimentary canal, in its walls. the tonsils, swellings on either side of the throat, are such masses, and aggregates occur as visible patches, the peyer's patches, on the ileum. it also constitutes the mass of the vermiform appendix and the wall of the sacculus rotundus; and in the young animal the "thymus gland," ventral to the heart, and less entirely, the "thyroid gland," ventral to the larynx, are similar structures, which are reduced or disappear as development proceeds. it is evident that in these two latter cases the term "gland" is somewhat of a misnomer. the matrix of botryoidal tissue is a network of stretched and hollowed connective tissue cells-- it is not a secretion, as cartilage matrix appears to be. during digestion, the phagocytes prowl into the intestine, and ingest and devour bacteria, that might otherwise give rise to disease. in inflammation, we may note here, they converge from all directions upon the point wounded or irritated. they appear to be the active agents in all processes of absorption (see osteoclasts under bone), and for instance, migrate into and devour the tissue of the tadpole's tail, during its metamorphosis to the adult frog. section . within the connective tissue cells fat drops may be formed, as in figure xv. adipose tissue is simply connective tissue loaded with fat-distended cells. the tissue is, of course, a store form of hydro-carbon (section ) provided against the possible misadventure of starvation. with the exception of some hybernating animals, such store forms would seem to be of accidental importance only among animals, whereas among plants they are of invariable and necessary occurrence. section . we now come to bone, a tissue confined to the vertebrata, and typically shown only in the higher types. as we descend in the scale from birds and mammals to lizards, amphibia (frogs and toads) and fish, we find cartilage continually more important, and the bony constituent of the skeleton correspondingly less so. in such a type as the dog-fish, the skeleton is entirely cartilaginous, bone only occurs in connection with the animal's scales; it must have been in connection with scales that bone first appeared in the vertebrate sub-kingdom. in the frog we have a cartilaginous skeleton overlaid by numerous bony scutes (shield-like plates) which, when the student comes to study that type, he will perceive are equivalent to the bony parts of such scales as occur in the dog-fish, sunk inward, and plating over the cartilage; and in the frog the cartilage also is itself, in a few places, replaced by bony tissue. in the adult rabbit these two kinds of bone, the bone overlying what was originally cartilage (membrane bone), and the bone replacing the cartilage (cartilage bone) have, between them, practically superseded the cartilage altogether. the structure of the most characteristic kind of bone will be understood by reference to figure xvi. it is a simplified diagram of the transverse section of such a bone as the thigh bone. m.c. is the central marrow cavity, h.v., h.v. are cross sections of small bloodvessels, the haversian vessels running more or less longitudinally through, the bone in canals, the haversian canals. arranged round these vessels are circles of the formative elements, the bone corpuscles or osteoblasts (b.c.) each embedded in bony matrix in a little bed, the lacuna, and communicating one with another by fine processes through canaliculi in the matrix, which processes are only to be seen clearly in decalcified bone (see section ). the osteoblasts are arranged in concentric series, and the matrix is therefore in concentric layers, or lamellae (c.l.). without and within the zone of haversian systems are (o.l. and i.l.), the outer and inner lamellae. the bone is surrounded by connective tissue, the periosteum. in addition to this compact bone, there is a lighter and looser variety in which spicules and bars of bony tissue are loosely interwoven. many flat bones, the bones of the skull, for instance, consist of this spongy bone, plated (as an electro spoon is plated) with compact bone. section . among the bony bars and spicules of spongy bone occurs the red marrow-- which must not be confused with the yellow marrow, the fatty substance in the central cavity of long bones. in this red marrow are numerous large colourless cells, which appear to form within their substance and then liberate red blood corpuscles. this occurs especially in the spongy bone within the ribs. section . the matrix of bone differs from that of cartilage or of most other tissues in consisting chiefly of inorganic salts. the chief of these is calcium phosphate, with which much smaller quantities of calcium carbonate, and magnesium phosphate and carbonate occur. these inorganic salts can be removed by immersion of the bone in weak hydrochloric acid, and a flexible network of connecting tissue, haversian vessels, bone corpuscles, and their processes remains. this is decalcified bone alluded to above. section . in the very young rabbit, the limb bones, vertebral column, and many of the skull bones are simply plates and bars of cartilage; the future membrane bones, however are planned out in connective tissue. the development of the latter is simple, the connective tissue corpuscles functioning by a simple change of product as osteoblast. the development of the cartilage bones, however, is more complicated. figure xvii., represents, in a diagrammatic way, the stages in the conversion of a cartilaginous bar to bone. to begin with, the previously sporadically-arranged (scattered anyhow) corpuscles (u.c.c.) are gathered into groups in single file, or in other words, into "columnar" groups (as at c.c.). the matrix becomes clouded with inorganic salts of lime, and it is then said to be calcified. this calcified cartilage then undergoes absorption-- it must not be imagined for a moment that bone is calcified cartilage. simultaneous with the formation of the cavities (s.) due to this absorption, connective tissue (p.c.i.) from the surrounding perichondrium (p.c.) grows into the ossifying* bar. it is from this connective tissue that the osteoblasts (o.b.) arise, and bone is built up. throughout life a bone is continually being absorbed and reformed by the activity of the osteoblasts. an osteoblast engaged in the absorption instead of the formation of bone is called an osteoclast. * the formation of bone is called ossification. to ossify is to become bony. section . the great thing to notice about this is that cartilage does not become bone, but is eaten into and ousted by it; the osteoblasts and osteoclasts replace entirely the cartilage corpuscles, and are not derived from them. section . we may mention here the structure of the spleen (figure , sheet ). it consists of a connective tissue and muscular coating, with an internal soft matrix much resembling botryoidal tissue, traversed by fibrous trabeculne (= beams, planks) containing blood-vessels, and the whole organ is gorged with blood, particularly after meals. the consideration of its function the student may conveniently defer for the present. section . here also, we may notice the lymphatics, a series of small vessels which return the overflow of the blood serum from the capillaries, in the nutrition of the tissues in all parts of the body, to the thoracic duct (see section ), and the general circulation. at intervals their course is interrupted by gland-like dilatations, the lymphatic glands, in which masses of rapidly dividing and growing (proliferating) cells occur, of which, doubtless, many are detached and become, first "lymph corpuscles," and, when they reach the veins, white blood corpuscles. . _the skeleton_ section . we are now in a position to study the rabbit's skeleton. we strongly recommend the student to do this with the actual bones at hand-- they may be cleared very easily in a well-boiled rabbit. this recommendation may appear superfluous to some readers, but, as a matter of fact, the marked proclivity of the average schoolmaster for mere book-work has put such a stamp on study, that, in nine cases out of ten, a student, unless he is expressly instructed to the contrary, will go to the tortuous, and possibly inexact, descriptions of a book for a knowledge of things that lie at his very finger-tips. we have not written, this chapter to give a complete knowledge of the skeleton, but simply as an aid in the actual examination of the bones. section . we may take the skeleton under five headings. there is the central axis, the chain of little bones, the vertebrae, threaded on the spinal cord (see figure and section ); the thorax, the box enclosed by ribs and sternum; the fore-limb and bones connected with it (pectoral girdle and limb), and the hind-limb and bones connected with it (pelvic girdle). finally there is the skull, but following the london university syllabus, we shall substitute the skull of the dog for of that of the rabbit, as more typically mammalian (section ). section . in section (which the student should refer to) we have a division of the vertebrae into four varieties. of these most representative is the thoracic. a thoracic vertebra (figure , sheet , t.v.) consists of a central bony mass, the body or centrum (b.), from which there arises dorsally an arch, the neural arch (n.a.), completed by a keystone, the neural spine (n.s.); and coming off laterally from the arch is the transverse process (tr.p.). looking at the vertebra sideways, we see that the arch is notched, for the exit of nerves. jointed to the thoracic vertebrae on either side are the ribs (r.). each rib has a process, the tuberculum, going up to articulate with the transverse process, and one, the capitulum articulating between the bodies of two contiguous vertebrae. the facets for the articulation of the capitulum are indicated in the side view by shading. at either end of the body of a vertebra of a young rabbit are bony caps, the epiphyses (ep.), separated from the body by a plane of unossified cartilage (indicated, by the dots). these epiphyses to the vertebral bodies occur only among mammals, and are even absent in some cases within the class. in the adult rabbit they have ossified continuously with the rest of the body. section . a cervical vertebra (c.v.) seems, upon cursory inspection, to have no rib. the transverse processes differ from those of thoracic series in having a perforation, the vertebrarterial canal, through which the vertebral artery runs up the neck. a study of the development of these bones shows that the part marked f.r. ossifies separately from the rest of the transverse process; and the form of the equivalent structures in certain peculiar lower mammals and in reptiles leaves no doubt that f.r. is really an abbreviated rib; fused up with the transverse process and body. the two anterior cervical vertebrae are peculiar. the first (at.) is called the atlas-- the figure shows the anterior view-- and has great articular faces for the condyles (section ) of the skull, and a deficient centrum. the next is the axis, and it is distinguished by an odontoid peg (od.p.), which fits into the space where the body of the atlas is deficient. in development the centrum of the axis ossifies from one centre, and the odontoid, peg from another, which at that time occupies the position of centrum of the atlas. so that it would seem that the atlas is a vertebra minus a centrum, and the axis is a vertebra plus a centrum, added at the expense of the atlas. section . the lumbar vertebrae (l.v.) are larger, and have cleft transverse processes, each giving rise to an ascending limb, the metapophyses, and a descending one. the latter (generally spoken of as the transverse processes) point steeply downward, and are considerably longer than those of thoracic series. the sacral vertebrae (s.v.) have great flattened transverse propcesses for articulation with the ilia. the caudal vertebrae (c.v.) are gradually reduced to the mere elongated centra, as we proceed, towards the tip of the tail. section . all the vertebrae join with their adjacent fellows through the intermediation of certain intervertebral pads, and also articulate by small processes at either end at the upper side of the arch, the zygapophyses. the normals to the polished facets of these point, in the case of the anterior zygapophyses, up and in (mnemonic: ant-up-in), and in the case of the posterior, down and out. the student should make this, and the other features of vertebrae, out upon actual specimens. section . the thorax is bounded dorsally by the vertebral column, and ventrally by the sternum. the sternum consists of segments, the sternebrae (st.); anteriorly there is a bony manubrium (mb.), posteriorly a thin cartilaginous plate, the xiphisternum (xi.). seven pairs of ribs articulate by cartilaginous ends (sternal ribs) with the sternum directly, as indicated in the figure; five (false) ribs are joined, to each other and to the seventh, and not to the sternum directly. the last four ribs have no tuberculum (section ). section . the fore-limb (pectoral limb) consists of an upper arm bone, the humerus (hum.) the distal end of which is deeply excavated by the olecranon fossa (o.f.) as indicated by the dotted lines; of two bones, the ulna (u.) and radius (r.) which are firmly bound by ligament in the position of the figure (i.e., with the palm of the hand downward, "prone"); of a number of small bones (carpalia), the carpus (c.); of a series of metacarpals (mc.); and of three digits (= fingers) each, except the first, or pollex, of three small bones-- the phalanges, only the proximal of which appear in the figure. the ulna has a hook-like head, the olecranon (o.) which distinguishes it easily from the distally thickened radius. the limb is attached to the body through the intermediation of the shoulder-blade (scapula, sc.) a flattened bone with a median external ridge with a hook-like termination, the acromion (acr.). there is also a process overhanging the glenoid cavity (g.) wherein the humerus articulates, which process is called coracoid (co.); it is ossified from two separate centres, and represents a very considerable bone in the bird, reptile, and frog. along the dorsal edge of the scapula of the rabbit is unossified cartilage, which is called the supra-scapula (s.sc.). in man there runs from the acromion to the manubrium of the sternum a bone, the collar-bone or clavicle. this is represented by a very imperfectly ossified rudiment in the rabbit. the scapula and clavicle, the bones of the body connected with the fore-limb, are frequently styled the pectoral girdle, or shoulder-girdle; this name of girdle will appear less of a misnomer when lower vertebrate types are studied. section . the hind limb and its body bones-- pelvic limb and girdle-- are shown in figure . the limb skeleton corresponds closely with that of the fore-limb. the femur (fe.) answers to the humerus, and is to be distinguished from it by the greater distinctness of its proximal head (hd.) and by the absence of an olecranon fossa from its distal end. the tibia (ti = the radius) is fused for the distal half of its length with the fibula (fb. = ulna). a tarsus (tarsalia) equals the carpus.* two of the proximal tarsalia may be noted: one working like a pulley under the tibia, is the astragalus (as.); one forming the bony support of the heel, is the calcaneum (ca.). there is a series of metatarsals, and then come four digits of three phalanges each. * such a resemblance as exists between one vertebra and another in the rabbit, or between the humerus and the femur, is called serial homology; the two things correspond with each other to the extent of imperfect reduplication. "homology" simply is commonly used to indicate the resemblance between any two structures in different animals, in origin and position as regards other parts. thus the heart of the rabbit and of the frog are homologous structures, corresponding in position, and resembling each other much as two memory sketches of one picture might do. section . the pelvic girdle differs from the pectoral in most land vertebrata in being articulated with the vertebral column. this difference does not exist in fishes. it consist in the rabbit of four bones; the ilium (i.), the ischium (is.), the pubis (pb.), and the small cotyloid bone-- the first two and the latter one meeting in the acetabular fossa (ac.) in which the head of the femur works. the pubes and ischia are fused along the mid-ventral line. many morphologists regard, the ilium as equivalent to, that is, strictly corresponding in its relation, to the scapula, the pubis to the cartilaginous substratum of the clavicle, and the ischium to the coracoid. section . these bones will be studied at the greatest advantage when dissected out from a boiled rabbit. prepared and wired skeletons, disarticulated skeletons, plates of figures, and written descriptions are in succession more tedious and less satisfactory ways to a real comprehension, of this matter. this chapter directs the student's attention to the most important points in the study of the skeleton, but it is in no way intended to mitigate the necessity of practical work. it is a guide simply. section . the mammalian skull will be better understood after the study of that of some lower vertebrate. we shall describe its main features now, but their meaning will be much clearer after the lower type is read. our figures are of canis. in section (figure vi., sheet ), we perceive a brain case (cranium) opening behind by a large aperture, the foramen magnum (f.m.). in front of this is an extensive passage, the nasal passage (e.n. to p.n.) which is divided from the mouth by a bony floor, the palate, and which opens into the pharynx behind at the posterior nares (p.n.) and to the exterior by the anterior or external nares (e.n.). it is divided into right and left passages by a middle partition, the nasal septum. outside the skull, on its wings, is a flask-like bone, the bulla tympani (b. in figures and ), protecting the middle ear, and from above this there passes an arch, the cheek bone (ju. in figures , , and ), to the upper jaw, forming in front the bony lower protection of the cavity containing the eye, the orbit. the cheek arch, nasal passage, and jaws, form collectively the "facial apparatus," as distinguished from the cranium, and the whole skull is sometimes referred to as, the "cranio-facial apparatus." two eminences for articulation with the atlas vertebra, the condyles (c.), lie one on each side of the lower boundary of the foramen magnum. section . the floor of the cranium consists of a series of cartilage bones, the basi-occipital (b.o.), basi-sphenoid (b.sp.), pre-sphenoid (p.sp.), and in front, the ethmoid (eth.), which sends down a median plate, not shown, in the figure, to form the nasal septum between right and left nasal passages. like extended wings on either side of the basi-occipital are the ex-occipital (e.o.) (the bone is marked in figure , but the letters are a little obscured by shading). similarly the ali-sphenoids (a.s.), are wings to the basi-, and the orbito-sphenoids (o.s.), to the pre-sphenoid bone (p.sp.). between the ex-occipital and ali-sphenoid there is wedged in a bone, the periotic (p.o.) containing the internal ear (section ). above the foramen magnum the median supra-occipital bone completes what is called the occipital arch. a pair of parietals (pa.) come above the ali-sphenoids, and a pair of frontals (f.) above the orbito-sphenoids. at the side the brain case is still incomplete, and here the aquamosal (sq.) enters into its wall. in the external view (figure ) the bulla hides the periotic bone from without. the student should examine all four figures for these bones before proceeding. section . the outer edge of the upper jaw and the cheek arch are made up of three paired bones. first comes the premaxilla (p.m.) (not p.m. or p.m. ), containing in the dog, the three incisors of either side. then comes the maxilla, bearing the rest of the teeth.* the jugal or malar (ju.) reaches over from the maxilla to meet a zygomatic process (= connecting outgrowth) (z.p.) of the squamosal bone. * in the dog a sabre-like canine (c.), four premolars (p.m. and p.m. ) and two molars (m. and m. ). section . in the under view of the skull (figure ) it will be seen that the maxilla sends in a plate to form the front part of the hard palate. behind, the hard palate is completed by the pair of palatine bones (pal.), which conceal much of the pre- and orbito-sphenoid in the ventral view, and which run back as ridges to terminate in two small angular bones, the pterygoids (pt.) which we shall find represent much more important structures in the lower vertebrata. section . the pre-maxillae and maxillae bound the sides of the nasal passage, and it is completed above by a pair of splints, the nasals. along the floor of the nasal passage, on the middle line, lies a splint of bone formed by the coalescence of two halves. it embraces in a v-like groove the mesethmoid (nasal septum) above, and lies on the palate. {lines from first edition only.} -its position is indicated by a heavy black line in , and it is called, the vomer bone (vo.).- {lines from second edition only.} [in the frog it is represented by two laterally situated bones. this is the vomer bone (vo.).] the nasal passages are partially blocked by foliated bony outgrowths, from the inner aspect of their walls, which in life are covered with mucous membrane, and increase the surface sensitive to smell. the ethmoid ends in the ethmo-turbinal (e.t.); the nasal, the naso-turbinal (n.t.); and the maxilla, the maxillo-turbinal (m.t.). in the anterior corner of the orbit there is a bone, the lachrymal (lc. figure ), which is hidden by the maxilla in the side view of the skull. section . the lower jaw (mandible) is one continuous bone in the mammal. three incisors bite against the three of the upper jaw. then comes a canine, four premolars, and three molars, the first of which is blade-like (sectorial tooth), and bites against the similar sectorial tooth (last premolar) of the upper jaw. the third molar is small. the arrangement of tooth is indicated in the following dental formula:-- i. . / . , c. . / . , p.m. . / . , m. . / . section . attached just behind the bulla above, and passing round on either side of the throat to meet at the base of the tongue, is the hyoid apparatus (figure ). the stylohyal (s.h.), epihyal (e.h.), and ceratohyal (c.h.) form the anterior cornu of the hyoid. the body of the hyoid (b.h.) forms a basis for the tongue. the posterior coruna (t.h.) of the hyoid are also called the thyrohyals. section . the following table presents these bones in something like their relative positions. a closer approximation to the state of the case will be reached if the student will imagine the maxilla raised up so as to overlie and hide the palatine and presphenoid, the squamosal similarly overlying the periotic bone, and the jugal reaching between them. membrane bones are distinguished by capital letters. -cranium_ -nasal_ (paired), ethmoid bone (median), -vomer_ -frontal_ (paired), -lachrymal_ (paired), orbito-sphenoid (paired), pre-sphenoid (median), eye -parietal_ (paired), ali-sphenoid (paired), basi-sphenoid (median)*, periotic bone (paired) -bulla_ (paired) supra-occipital (median), ex-occipital (paired), basi-occipital (median) -upper jaw_ -pre-maxilla_ (paired) palatine (paired) pterygoid (paired) -lower jaw_ -maxilla_ (paired) -jugal_ (paired) -squamosal_ (paired) *in this table the small bones of the ear are simply indicated by an asterisk. section . hidden by the bulla, and just external to the periotic bone, are the auditory ossicles, the incus, malleus, os orbiculare, and stapes. these will be more explicitly treated when we discuss the ear. section . when we come to the study of the nerves, we shall revert to the skull, and treat of its perforations. the student should not fail, before proceeding, to copy and recopy our figures, and to make himself quite familiar with them, and he should also obtain and handle an actual skull. for all practical purposes the skull of a sheep or cat will be almost as useful as that of the dog. . _muscle and nerve_ section . we have, in the skeleton, a complicated apparatus of parts hinged and movable upon one another; the agent moving these parts is the same agent that we find in the heart walls propelling the blood through the circulation, in the alimentary canal squeezing the food along its course, and universally in the body where motion occurs, except in the case of the creeping phagocytes, and the ciliary waving of ciliated epithelium. this agent is muscle. we have, in muscular tissue, a very wide departure from the structure of the primordial cell; to use a common biological expression, a very great amount of modification (= differentiation). sheet represents the simpler kind of muscular tissue, unstriated muscle, in which the cell character is still fairly obvious. the cells are fusiform (spindle-shaped), have a distinct nucleus and faint longitudinal striations (striations along their length), but no transverse striations. section . in striated muscle extensive modifications mask the cell character. under a / inch objective, transverse striations of the fibres are also distinctly visible, and under a much higher power we discern in a fibre (sheet ) transverse columns of rod-like sarcous elements (s.e.), the columns separated by lines of dots, the membranes of krause (k.m.), and nuclei (n.), flattened and separated into portions, and lying, in some cases, close to the sarcolemma (sc.) the connective tissue enclosing the fibre, in others scattered throughout the substance of the fibre. the figure shows the fibre ruptured, in order to display the sarcolemma; e.p. is the end plate of a nerve (n.v.), and fb. are the fibrillae into which a fibre may be teased. section . in the heart we have an intermediate kind of muscle cardiac muscle (figure ), in which the muscle fibres branch; there is apparently no sarcolemma, and the undivided nuclei lie in the centre of the cell. section . unstriated muscle is sometimes called involuntary, and striated, voluntary muscle; but there is really not the connexion with the will that these terms suggest. we have just mentioned that the heart-muscle is striated, but who can alter the beating of the heart by force of will? and the striated muscles of the limbs perform, endless involuntary acts. it would seem that unstriated muscle contracts slowly, and we find it especially among the viscera; in the intestine for instance, where it controls that "peristaltic" movement which pushes the food forward. voluntary muscle, on the other hand, has a sharp contraction. the muscle of the slow-moving snails, slugs, and mussels is unstriated; all the muscle of the active insects and crustacea (crabs, lobsters, and crayfish) is striated. still if the student bears the exception of the heart in mind, and considers muscles as "voluntary" that his will can reach, the terms voluntary and involuntary will serve to give him an idea of the distribution of these two types of muscle in his own body, and in that of the rabbit. section . muscular contraction, and generally all activity in the body is accompanied by kataboly. the medium by which these katabolic changes are set going and controlled is the nervous system. the nervous system holds the whole body together in one harmonious whole; it is the governing organization of the multicellular community (section ), and the supreme head of the government resides in the brain, and is called the mind. but just as in a political state only the most important and most exceptional duties are performed by the imperial body, and minor matters and questions of routine are referred to boards and local authorities, so the mind takes cognisance only of a few of the higher concerns of the animal, and a large amount of the work of the nervous system goes on insensibly, in a perfectly automatic way-- even much that occurs in the brain. section . the primary elements in the tissue of the nervous system are three; nerve fibres, which are simply conducting threads, telegraph wires; ganglion cells, which are the officials of the system; and neuroglia, a fine variety of connective tissue which holds these other elements together, and may also possibly exercise a function in affecting impressions. a message along a nerve to a ganglion cell is an afferent impression, from a cell to a muscle or other external end is an efferent impression. the passage of an impression may be defined as a flash of kataboly along the nerve, and so every feeling, thought, and determination involves the formation of a certain quantity of katastases, and the necessity for air and nutrition. section . unlike telegraph wires, to which they are often compared, nervous fibres usually convey impressions only in one direction, either centrally (afferent or sensory nerve fibres), or outwardly (efferent or motor nerve fibres). but the so-called motor nerve fibres include not only those that set muscles in motion, but those that excite secretion, check impulsive movements, and govern nutrition. section . figure , sheet , shows the typical structure of nervous tissues. the nerve fibres there figured are bound together by endoneurium into small ropes, the nerves, encased in perineurium. there is always a grey axis cylinder (a.c.), which may (in medullated nerves), or may not (in non-medullated or grey nerves) have a medullary sheath (s.s.) interrupted at intervals by the nodes of ranvier (n.r.). nuclei (n.) at intervals under the sheath indicate the cells from which nerve fibres are derived by a process of elongation. the nerves of invertebrata, where they possess nerves, are mostly grey, and so are those of the sympathetic system of vertebrata, to be presently described, g.c., g.c. are ganglion cells; they may have many hair-like processes, usually running into continuity with the axis cylinders of nerve fibres, in which case they are called multi-polar cells, or they may be uni- or bi-polar. section . the simplest example of the action of the nervous system is reflex action. for instance, when the foot of a frog, or the hand of a soundly sleeping person, is tickled very gently, the limb is moved away from the irritation, without any mental action, and entirely without will being exercised. and when we go from light into darkness, the pupil of the eye enlarges, without any direct consciousness of the change of its shape on our part. similarly, the presence or food in the pharynx initiates a series of movements-- swallowing, the digestive movements, and so on-- which in health are entirely beyond our mental scope. section . a vast amount of our activities are reflex, and in such action an efferent stimulus follows an afferent promptly and quite mechanically. it is only where efferent stimuli do not immediately become entirely transmuted into outwardly moving impulses that mental action comes in and an animal feels. there appears to be a direct relation between sensation and motion. for instance, the shrieks and other instinctive violent motions produced by pain, "shunt off" a certain amount of nervous impression that would otherwise register itself as additional painful sensation. similarly most women and children understand the comfort of a "good cry," and its benefit in shifting off a disagreeable mental state. section . the mind receives and stores impressions, and these accumulated experiences are the basis of memory, comparison, imagination, thought, and apparently spontaneous will. voluntary actions differ from reflex by the interposition of this previously stored factor. for instance, when a frog sees a small object in front of him, that may or may not be an edible insect, the direct visual impression does not directly determine his subsequent action. it revives a number of previous experiences, an image already stored of similar insects and associated with painful or pleasurable gustatory experiences. with these arise an emotional effect of desire or repulsion which, passes into action of capture or the reverse. section . voluntary actions may, by constant repetition, become quasi-reflex in character. the intellectual phase is abbreviated away. habits are once voluntary and deliberated actions becoming mechanical in this way, and slipping out of the sphere of mind. for instance, many of the detailed movements of writing and walking are performed without any attention to the details. an excessive concentration of the attention upon one thing leads to absent-mindedness, and to its consequent absurdities of inappropriate, because imperfectly acquired, reflexes. section . this fluctuating scope of mind should be remembered, more especially when we are considering the probable mental states of the lower animals. an habitual or reflex action may have all the outward appearance of deliberate adjustment. we cannot tell in any particular case how far the mental comes in, or whether it comes in at all. seeing that in our own case consciousness does not enter into our commonest and most necessary actions, into breathing and digestion, for instance, and scarcely at all in the details of such acts as walking and talking we might infer that nature was economical in its use, and that in the case of such an animal as the rabbit, which follows a very limited routine, and in which scarcely any versatility in emergencies is evident, it must be relatively inconsiderable. perhaps after all, pain is not scattered so needlessly and lavishly throughout the world as the enemies of the vivisectionist would have us believe. . _the nervous system_ section . a little more attention must now be given to the detailed anatomy of the peripheral and central nerve ends. a nerve, as we have pointed out, terminates centrally in some ganglion cell, either in a ganglion or in the spinal cord or brain; peripherally there is a much greater variety of ending. we may have tactile (touch) ends of various kinds, and the similar olfactory and gustatory end organs; or the nerve may conduct efferent impressions, and terminate in a gland which it excites to secretion, in a muscle end-plate, or in fact, anywhere, where kataboly can be set going and energy disengaged. we may now briefly advert to the receptive nerve ends. section . many sensory nerves, doubtless, terminate in fine ends among the tissues. there are also special touch corpuscles, ovoid bodies, around which a nerve twines, or within which it terminates. section . the eye (figure ) has a tough, dense, outer coat, the sclerotic (sc.), within which is a highly vascular and internally pigmented layer, the choroid, upon which the percipient nervous layer, the retina (r.) rests. the chief chamber of the eye is filled with a transparent jelly, the vitreous humour (v.h.). in front of the eye, the white sclerotic passes into the transparent cornea (c.). the epidermis is continued over the outer face of this as a thin, transparent epithelium. the choroid coat is continued in front by a ring-shaped muscle, the iris (ir.) the coloured portion of the eyes. this iris enlarges or contracts its central aperture (the black pupil) by reflex action, as the amount of light diminishes or increases. immediately behind this curtain is the crystalline lens (l.), the curvature of the anterior face or which is controlled by the ciliary muscle (c.m.). in front of the lens is the aqueous humour (a.h.). the description of the action of this apparatus involves the explanation of several of the elementary principles of optics, and will be found by the student in any text-book of that subject. here it would have no very instructive bearing, either on general physiological considerations or upon anatomical fact. section . the structure of the retina demands fuller notice. figure shows an enlarged, diagram of a small portion of this, the percipient part of the eye. the optic nerve (o.n. in figure ) enters the eye at a spot called the blind spot (b.s.), and the nerve fibres spread thence over the inner retinal surface. from this layer of nerve fibres (o.n. in figure ) threads run outward, through certain clear and granular layers, to an outermost stratum of little rods (r.) and fusiform bodies called cones (c.), lying side by side. the whole of the retina consists of quite transparent matter, and it is this outermost layer of rods and cones (r. and c.) that receives and records the visual impression. this turning of the recipient ends away from the light is hardly what one would at first expect-- it seems such a roundabout arrangement-- but it obtains in all vertebrata, and it is a striking point of comparison with the ordinary invertebrate eye. section . we may pause to call the student's attention to a little point in the physiology of nerves, very happily illustrated here. the function of a nerve fibre is the conduction of impressions pure and simple; the light radiates through the fibrous layer of the retina without producing the slightest impression, and at the blind spot, where the rods and cones are absent, and the nerve fibres are gathered together, no visual impressions are recorded. if there is any doubt as to the existence of a blind spot in the retinal picture, the proof is easy. let the reader shut his left eye, and regard these two asterisks, fixing his gaze intently upon the left-hand one of them. * * at a distance of three or four inches from the paper, both spots will be focussed on his retina, the left one in the centre of vision, and the right one at some spot internal to this, and he will see them both distinctly. now, if he withdraws his head slowly, the right spot will of course appear to approach the left, and at a distance of ten or twelve inches it will, in its approach, pass over the blind spot and vanish, to reappear as he continues to move his head away from the paper. the function of nerve fibres is simply conduction, and the nature of the impressions they convey is entirely determined by the nature of their distal and proximal terminations. section . certain small muscles in the orbit (eye-socket) move the eye, and by their action contribute to our perception of the relative position of objects. there is a leash of four muscles rising from a spot behind the exit of the optic nerve from the cranium to the upper, under, anterior, and posterior sides of the eyeball. these are the superior, inferior, anterior, and posterior recti. running from the front of the orbit obliquely to the underside of the eyeball is the inferior oblique muscle. corresponding to it above is a superior oblique. a lachrymal gland lies in the postero-inferior angle of the orbit, and a handerian gland in the corresponding position in front. in addition to the upper and lower eyelids of the human subject, the rabbit has a third, the nictitating lid, in the anterior corner of the eye. section . the ear (sheet vii.) consists of an essential organ of hearing, and of certain superadded parts. the essential part is called the internal ear, and is represented in all the true vertebrata (i.e., excluding the lancelet and its allies). in the lower forms it is a hollow membranous structure, embedded in a mass of cartilage, the otic capsule; in the mammal the latter is entirely ossified, to form the periotic bone. the internal ear consists of a central sac, from which three semicircular canals spring. the planes of the three canals are mutually at right angles; two are vertical, the anterior and posterior (p.v.c.) vertical canals, and one is horizontal, the horizontal canal (h.c.). there are dilatations, called ampullae, at the anterior base of the anterior, and at the posterior base of the posterior and horizontal canals. indirectly connected with the main sac is a spirally-twisted portion, resembling a snail shell in form, the cochlea. this last part is distinctive of the mammalia, but the rest of the internal ear is represented in all vertebrata, with one or two exceptions. the whole of the labyrinth is membranous, and contains a fluid, the endolymph; between the membranous wall of the labyrinth and the enclosing bone is a space containing the perilymph. strange as it may appear at first, the entire lining of the internal ear is, at an early stage, continuous with the general epidermis of the animal. it grows in just as a gland might grow in, and is finally cut off from the exterior; but a considerable relic of this former communication remains as a thin, vertical blind tube (not shown in the figure), the ductus endolymphaticus. section . the eighth nerve runs from the brain case (cr.), into the periotic bone, and is distributed to the several portions of this labyrinth. in an ordinary fish this internal ear is the sole auditory organ we should find; the sound-waves would travel through the water to the elastic cranium and so reach and affect the nerves. but in all air-frequenting animals this original plan of an ear has to be added to, to fit it to the much fainter sound vibrations of the compressible and far less elastic air. a "receiving apparatus" is needed, and is supplied by the ear-drum, middle ear, or tympanic cavity (t.). in the mammal there is also a collecting ear trumpet (the ear commonly so-called), the external ear, and external auditory meatus (e.a.m.). a tightly stretched membrane, the tympanic membrane, separates this from the drum. a chain of small bones, the malleus (m.), the incus (i.), the os orbiculare (o.or.), a very small bone, and a stirrup-shaped stapes, swing across the tympanum, from the tympanic membrane to the internal ear. at two points the bony investment of this last is incomplete-- at the fenestra rotunda (f.r.), and at the fenestra ovalis, (f.o.), into which latter the end of the stapes fits, and so communicates the sound vibrations of the tympanic membrane to the endolymph. a passage, the eustachian tube, communicates between the tympanic cavity and the pharynx (ph.), and serves to equalize the pressure on either side of the drum-head. a comparative study of the ears of the vertebrata brings to light the fact that, as we descend in the animal scale, the four ear ossicles are replaced by large bones and cartilages connected with the jaw, and the drum and eustachian tube by a gill slit. we have, in fact, in the ear, as the student will perceive in the sequel, an essentially aquatic auditory organ, added to and patched up to fit the new needs of a life out of water. section . the impressions of smell are conducted through the first nerve to the brain, and are first received by special hair-bearing cells in the olfactory mucous membrane of the upper part of the nasal passage. the sense of taste has a special nerve in the ninth, the fibres of which terminate in special cells and cell aggregates in the little papillae (velvet pile-like processes) that cover the tongue. section . at an early stage in development, the brain of a mammal consists of a linear arrangement of three hollow vesicles (figure , sheet viii., , , and ), which are the fore-, mid-, and hind-brain respectively. the cavities in these in these vesicles are continuous with a hollow running through the spinal cord. on the dorsal side of the fore-brain is a structure to be dealt with more fully later, the pineal gland (p.g.), while on its under surface is the pituitary body (pt.). section . the lower figure of ( ) shows, in a diagrammatic manner, the derivation of the adult brain from this primitive state. from the fore-brain vesicle, a hollow outgrowth on either side gives rises to the (paired) cerebral hemisphere (c.h.), which is prolonged forward as the olfactory lobe (o.l.). from the fore-brain the retina of the eye and the optic nerve also originate as an, at first, hollow outgrowth (op.). the roof of the mid-brain is also thickened, and bulges up to form two pairs of thickenings, the corpora quadrigemina, (c.q.). the hind-brain sends up in front a median outgrowth, which develops lateral wings, the cerebellum (cbm.), behind which the remainder of the hind-brain is called the medulla oblongata, and passes without any very definite demarcation into the spinal cord. section . figure is a corresponding figure of the actual state of affairs in the adult. the brain is seen in median vertical section. (ch.) is the right cerebral hemisphere, an inflated vesicle, which, in the mammal-- but not in our lower types-- reaches back over the rest of the fore-brain, and also over the mid-brain, and hides these and the pineal gland in the dorsal view of the brain (figure ). the hollow of the hemisphere on either side communicates with the third ventricle, the original cavity of the fore-brain ( in figure ), by an aperture (the foramen of monro), indicated by a black arrow (f.m.). besides their original communication through the intermediation of the fore-brain, the hemispheres are also united above its roof by a broad bridge of fibre, the corpus callosum (c.c.), which is distinctive of the mammalian animals. the original fore-brain vesicle has its lateral walls thickened to form the optic thalami (o.th.), between which a middle commissure, (m.c.), absent in lower types, stretches like a great beam across the third ventricle. the original fore-brain is often called the thalamencephalon, the hemisphere, the prosencephalon, the olfactory lobes, the rhinencephalon. section . the parts of mid-brain (mesencephalon) will be easily recognised. its cavity is in the adult mammal called the iter; its floor is differentiated into bundles of fibres, the crura cerebri (c.cb.), figured also in figure . section . the cerebellum (metencephalon) consists of a central mass, the vermis (v.cbm.), and it also has lateral lobes (l.l.), prolonged into flocculi (f.cbm.), which lastare -em-bedded in pits, [in] the periotic bone, and on that account render the extraction of the brain from the cranium far more difficult than it would otherwise be. the roof of the hind-brain, before and behind the cerebellum, consists of extremely thin plates of nervous matter. its floor is greatly thickened to form the mass of the medulla, and in front a great transverse track of fibres is specialized, the pons varolii (p.v.). its cavity is called, the fourth ventricle. section . figure gives a dorsal view of the rabbit's brain; a horizontal slice has been taken at the level of the corpus callosum. the lateral ventricle (i.e., the hollows of the hemisphere) is not yet opened. a lower cut (figure ) exposes this (v.l.). the level of these slices is approximately indicated in figure by the lines a and b. this latter figure will repay careful examination. the arrow, ar., plunges into the third ventricle, behind the great middle commissure (m.c.), and the barb is supposed to lie under the roof of the mid-brain, the corpora quadrigemina (c.q.). the position of ar. is also indicated in figure . before reading on, the beginner should stop a while here; he should carefully copy or trace our figures and, putting the book aside, name the parts, and he should then recopy, on an enlarged scale, and finally draw from memory, correct, and again draw. by doing this before the brain is dissected a considerable saving of time is possible. section . proceeding from the brain are twelve pairs of cranial nerves. from the fore-brain spring two pairs, which differ from the rest of the cranial nerves in being, first of all, hollow outgrowths of the brain-- the others are from the beginning solid. the first nerve is the olfactory lobe, which sends numerous filaments through the ethmoid bone to the olfactory organ. the second is the optic nerve, the visual sensory nerve. section . the mid-brain gives rise to only one nerve, the third, which supplies all the small muscles of the eye (see section ), except the superior oblique and external rectus. section . the remainder of the nerves spring from the hind-brain. the fourth pair supply the superior obliques, and the sixth the external recti; so that iii., iv., and vi. are alike purely motor nerves, small and distributed, to the orbit. the fifth nerve, the trigeminal, is a much larger and more important one; it is a mixed nerve, having three main branches, of which the first two are chiefly sensory, the third almost entirely motor; it lies deeply in the orbit. v (see sheet ) runs up over the recti behind the eyeball, it is the ophthalmic branch; v , the maxillary branch, runs deeply under the eyeball and emerges in front of the malar, and v , the mandibular branch, runs down on the inner side of the jaw-bone to the jaw muscles and tongue. section . if the student will now recur to the figures of the dog's skull (sheet ), he will see certain apertures indicated in the cranial wall. of these, o.f. is the optic foramen for the exit of nerve ii., perforating the orbito-sphenoid. behind this there comes an irregular aperture, (f.l.a.), the foramen lacerum anterius, giving exit to iii., iv., vi., and v . v emerges from the foramen rotundum, and v from the foramen ovale, two apertures uniting behind a bony screen.* just in front of the bulla is a foramen lacerum medium (f.l.m.), through which no nerve passes. * in the rabbit's skull f.l. anterius, the foramen rotundum, and foramen ovale are not distinct, and there are two condylar foramina instead of one, through each of which, a moiety of xii. passes. section . the eighth nerve (auditory) is purely sensory, the nerve of the special sense of hearing; it runs into the periotic bone, and breaks up on the labyrinth. the seventh nerve (facial) is almost entirely motor; it passes through the periotic anterior to viii., and emerges by the stylo-mastoid foramen (s.m.f.) behind the bulla, to run outside the great jaw muscle across the cheek immediately under the skin (figure ). section . the ninth (glossopharyngeal) nerve is chiefly sensory; it is the special nerve of taste, and is distributed to the tongue. the tenth nerve (vagus) arises by a number of roots, and passes out of the skull, together with ix and xi, by the foramen lacerum -posterium- [posterius] (f.l.p.). it is a conspicuous white nerve, and runs down the neck by the side of the common carotid artery. it sends a superior laryngeal branch (xa) to the larynx. the left vagus passes ventral to the aortic arch, and sends a branch (l.x.b.) under this along the trachea to the larynx-- the recurrent laryngeal nerve. the corresponding nerve on the right (r.x.b.) loops under the subclavian artery. the main vagus, after this branching, passes behind the heart to the oesophagus and along it to the stomach. xi., the spinal accessory, supplies certain of the neck nerves. xii., the hypoglossal, runs out of the skull by the condylar foramen (c.f.), is motor, crosses the roots of xi., x., and ix., passes ventral to the carotid, and breaks up among the muscles of the tongue and neck. section . of the functions of the several parts of the brain there is still very considerable doubt. with disease or willful destruction of the cerebral tissue the personal initiative is affected-- the animal becomes more distinctly a mechanism; the cerebellum is probably concerned in the coordination of muscular movements; and the medulla is a centre for the higher and more complicated respiratory reflexes, yawning, coughing, and so on. the great majority of reflex actions centre, however, in the spinal cord, and do not affect the brain. section . a cross section of the spinal cord is shown in figure , sheet . it is a cylinder, almost bisected by a dorsal (d.f.) and a ventral (v.f.) fissure. through its centre runs a central canal (c.c.), continuous with the brain ventricles, and lined by ciliated epithelium. the spinal cord consists of an outer portion, mainly of nervous fibres, the white matter, and of inner, ganglionated, and more highly vascular grey matter. (in the cerebrum the grey matter is external, and the white internal.) the cord, like the brain, is surrounded by a vascular fibrous investment, and protected from concussion by a serous fluid. the nerves which emerge from the vertebral column between the vertebrae, arise, unlike the cranial nerves, by two roots. the dorsal of these, the sensory root (d.n.), has a swelling upon it, the dorsal ganglion, and-- by experiments upon living animals-- has been shown to contain only afferent fibres; the ventral, the motor root, is without a ganglion, and entirely or mainly motor. the two unite outside the cord, and thereafter the spinal nerves are both sensory and motor. section . besides the great mass of brain and spinal cord (cerebro-spinal axis), there is, on either side of the dorsal wall of the body cavity, a sympathetic nervous chain. the nerve fibres of this system, like the nerve fibres of invertebrates, are non-medullated. it may be seen as a greyish thread running close by the common carotid in the neck (sym., figure ); it then runs over the heads of the ribs in the thorax and close beside the dorsal aorta in the abdominal region. in the anterior region of the neck it dilates to form a superior cervical ganglion, and opposite the first rib it forms an inferior cervical ganglion. thence, backwards, there is a ganglion on each sympathetic chain opposite each spinal nerve, and the two exchange fibres through a thread, the ramus communicans. to the sympathetic chain is delegated much of the routine work of reflex control of the bloodvessels and other viscera, which would otherwise fall upon the spinal cord. section . there are eight cervical (spinal) nerves, one in front of the atlas, and one behind each of the cervical vertebrae. the last four and the first thoracic (spinal) contribute to a leash of nerves running out to the fore limb, the brachial plexus (plexus, literally network, but here meaning a plaited cord). the fourth cervical also sends down a phrenic nerve (p.n., figure ), along by the external jugular vein and the superior caval vein to the diaphragm. the last three lumbar and the sacral nerves form a sacral plexus, supplying the hind limb. section . from the sympathetic in the hinder region of the thorax a nerve, the great splanchnic nerve, arises, and runs, back to a ganglionated nervous network, just behind the coeliac artery, into which the vagus also enters; this is the coeliac ganglion, and together with a similar superior mesenteric ganglion around the corresponding artery, makes up a subsidiary visceral nervous network, the solar plexus. a similar and smaller nervous tangle, bearing an inferior mesenteric ganglion, lies near the inferior mesenteric artery. section . finally, we may note the pineal gland and the pituitary body, as remarkable appendages above and below the thalamencephalon. their function, if they have a function, is altogether unknown. probably, they are inherited from ancestors to whom they were of value. such structures are called reduced or vestigial structures, and among other instances are the clavicles of the rabbit, the hair on human limbs, the little pulpy nodule in the corner of the human eye, representing the rabbit's third eyelid, and the caudal vertebrae at the end of the human spinal column. in certain lowly reptiles, in the lampreys, and especially in a peculiar new zealand lizard, the pineal gland has the most convincing resemblance to an eye, both in its general build and in the microscopic structure of its elements; and it seems now more than probable that this little vascular pimple in our brains is a relic of a third and median eye possessed by ancestral vertebrata. the pituitary body is probably equivalent to a ciliated pit we shall describe in the lacelet (amphioxus). . _renal and reproductive organs_ section . we have now really completed our survey of the individual animal's mechanism. but no animal that was merely complete in itself would be long sanctioned by nature. for an animal species to survive, there must evidently, also, be proper provision for the production of young, and the preservation of the species as well as of the individual. hence in an animal's physiology and psychology we meet with a vast amount of unselfish provision, and its structure and happiness are more essentially dependent on the good of its kind than on its narrow personal advantage. the mammalia probably owe their present dominant position in the animal kingdom to the exceptional sacrifices made by them for their young. instead of laying eggs and abandoning them before or soon after hatching, the females retain the eggs within their bodies until the development of the young is complete, and thereafter associate with them for the purposes of nourishment, protection, and education. in the matter of the tail, for instance, already noted, the individual rabbit incurs the disadvantage of conspicuousness for the rear, in order to further the safety of the young. section . the female organs of reproduction are shown in sheet . the essential organ is the ovary (ov.), in which the ova (eggs) are formed. figure gives an enlarged and still more diagrammatic rendering of the ovary. there is a supporting ground mass, or stroma, into which numerous bloodvessels and nerves enter and break up. the ova appear first as small cells in the external substance of the ovary (as at ), and move inward ( and ), surrounded by a number of sister cells, which afford them nourishment. at ( ) an ovum with its surrounding group of cells is more distinct and near the centre of the ovary; a fluid is appearing within the ovisac as the development proceeds. ( ) is a much more mature ovisac or graafian follicle. section . the ovum (ov.), is now large, and its nucleus and nucleolus (the germinal vesicle and spot) are very distinct. the wall of the follicle consists, in the mammal, of several layers of cells, the membrana granulosa (or "granulosa" simply); the ovum lies on its outer side embedded in a mass of cells, discus proligerus, separated from actual contact with the ovum by a zona pellucida. the ripening follicle moves to the surface of the ovary and bursts, the ovum falls into the body cavity. in figure , a ripe graafian follicle (g.f.), projects upon the ovary. section . the liberated ovum is caught up by the funnel-shaped opening of the fallopian tube, which passes without any very conspicuous demarcation into the cornu uteri (c.ut.) of its side; the two uterine cornua meeting together in the middle line form the vagina (v.), which runs out into a vestibule (vb.) opening between tumid lips to the exterior. the urinary bladder (ur.b.) also opens into the vestibule, and receives the two ureters from the kidney. section . in the male we find, in the position of the female uterus, a uterus masculinus (u.m.). the essential sexual organ is the testis (t.), a compact mass of coiling tubuli, which opens by a number of ducts, the vasa efferentia, into a looser and softer epididymis (ep.), which sends the sexual product onward through a vas deferens (v.d.), to open at the base of the uterus masculinus. the urinary bladder and ureters correspond with those of the female, and the common urogenital duct (= vestibule), the urethra, is prolonged into an erectile penis (p.) surrounded by a fold of skin, the prepuce. a prostate gland (pr.), contributes to the male sexual fluid. the character of the essential male element, the spermatozoon, the general nature of the reproductive process, will be conveniently deferred until the chapters upon development are reached. . _classificatory points_ section . the following facts of classificatory importance may now be considered, but their full force will be better appreciated after the study of other vertebrate types. they are such as come prominently forward in the comparison of the rabbit with other organisms. section . in the first place, the rabbit is a metazoon, one of the metazoa, i.e., a multicellular organism, as compared with the amoeba, which belongs to the protozoa or one-cell animals (section ). in the next place, it is externally bilaterally symmetrical, its parts balance, and where, in its internal anatomy, it departs from this symmetry (as in the case of the aorta, the stomach and intestines, and the kidneys), the departure has an appearance of being the results of partial reductions and distortions of an originally quite symmetrical plan. and the facts of development strengthen this idea; in the very earliest stages we have paired aortic arches, of which, the left only remains, a straight alimentary canal, and less asymmetrical kidneys. in the vast majority of animals the same bilateral symmetry is to be seen, but in the star-fish and sea-urchins, and in the jelly-fish, corals, sea anemones, and hydra, the general form of the animal is, instead, arranged round a centre, like a star and its rays, and the symmetry is called radial. section . we also see in various organs of the rabbit, and especially in the case of the limbs and vertebral column, what is called metameric segmentation, that is, a repetition of parts, one behind the other, along the axis of the body. thus the bodies and arches of the vertebrae repeat each other, and so do the spinal nerves. the renal organ of the rabbit, some time before birth, displays a metameric arrangement of its parts; but this disappears, as development proceeds, into the compact kidney of the adult. but the metameric segmentation in the rabbit's organism is not nearly so marked as that of an earthworm, for instance, which is visibly a chain of rings. if the student wants a perfect figure of metameric segmentation he should think of a train of precisely similar carriages, or a string of beads. one bead, one carriage, one vertebra, would be a metamere. section . in contrast to metameric segmentation is the antimeric repetition of radial symmetry (section ), in which each ray of the star is called an antimere. it is possible to have bilateral symmetry without a metameric arrangement of parts, as in the mussel and the cuttle-fish; but metameric segmentation without complete or reduced bilateral symmetry does not occur. section . we are now in a position to appreciate the fact that the old and more popularly know division of animals into vertebrata and invertebrata scarcely represents the facts of the case, that the primary division should be into protozoa and metazoa, and that the vertebrata are one of several groups of metazoa with a fundamental bilateral symmetry and imperfect metameric segmentation. the rabbit is one of the vertebrata, and, in common with all the other animals collected under this head, it has-- (a) a skeletal axis (the vertebral column) between its central nervous system and its body cavity. in the adult rabbit this consists of a chain of vertebrae, but in the embryo (i.e., the young rabbit before birth) it is represented by a continuous chord, the notochord, and it remains as such in some of the lowest vertebrata throughout life. in other words, in these lower vertebrata, the vertebral axis is not metameric. (b) a dorsal and -tubular_ nervous axis. (section , the central canal) (c) it has, though in the embryo only, certain slits between the throat and the exterior, like the gill slits of a fish. such slits are-- with one or two remarkable exceptions outside the sub-kingdom-- distinctly vertebrate features, and remain, of course, in fishes throughout life. the presence of true cartilage and bone mark a vertebrate, but vertebrata occur in which -these tissues- [bone] -are- [is] absent. section . the rabbit shares the following features with all the vertebrata, except the true fishes, which do not possess any of them-- (a) lungs (but many fish have a swimming bladder which answers to the lungs in its anatomical relations.) (b) limbs which consist of a proximal joint of one bone an intermediate part of two, and a distal portion which has five digits, or is evidently a reduced form of the five-digit limb.* (c) the absence of a median fin supported by fin rays.** * the frog shows indications of a sixth digit. ** the frog's tadpole has a median fin, but no fin rays. section . the rabbit shares the following features with all the vertebrata above the fishes and amphibia (= frogs, toads, newts, and etc.)-- (a) absence of gills (not gill slits, note) at any stage in development. (b) an amnion, and (c) an allantois in development. the meaning of (b) and (c) we shall explain to the student in the chapters on embryology. we simply mention them here to render our table complete. section . the rabbit shares with all mammals, and differs from all other vertebrata (i.e., birds, reptiles, amphibia, and fishes), in having-- (a) hair. (b) a diaphragm. (c) only one aortic arch, and that on the left side of the body. (d) its young born alive. (but two very reptile-like mammals of australia, the duck-billed platypus and the echidna, lay eggs, and certain fish and reptiles bear living young.) (e) epiphyses to its vertebral -centre- [centra].* (f) the cerebral hemispheres covering the mid-brain. (g) corpora quadrigemina instead of bigemina. [(h) a corpus callosum.] [(i) a spirally coiled cochlea to the internal ear.] [(in respect to h and i also, the echidna and platypus are scarcely mammalinan.)] * but certain mammals have no such epiphyses. section . the rabbit, together with the hares and conies, rats and mice, voles, squirrels, beavers, cavies, guineapigs is included in that order of the class of mammals which is called the rodentia, and is distinguished by the character of the incisor teeth from other orders of the class. . _questions and exercises_ . describe the venous circulation of the rabbit (with diagrams). compare a vein and artery. compare the distribution of the great venous trunks with that of the arterial system. . construct a general diagram of the circulation of the rabbit, to show especially the relation of the portal system, the lymphatics and lacteals, and the renal circulation to the main blood current. . draw the alimentary canal of the rabbit from memory. . what is a villus? describe its epithelium, and the vessels within it. write as explicit an account as you can of the absorbent action of a villus. . tabulate the alimentary secretions, and their action on the food. . what is botryoidal tissue? where does it occur? what is known of its functions? . copy diagram i. (enlarged), and insert upon it the visceral nerves as far as you can. . what are the most characteristic points in the mammalian vertebral column? . describe cartilage and bone, and compare them with one another. . give an account of the amoeba, and compare it with a typical tissue cell in a metazoon (e.g., the rabbit). . give a general account of connective tissue. what is tendon? . trace, briefly, the increased modification of tissues in the vertebrata. . describe, with diagrams, the structure of blood. state the function of each factor you describe. . compare the pectoral with the pelvic limb and girdle. what other structures of the adult rabbit display a similar repetition of similar parts? . draw from memory typical vertebrae from each region of the vertebral column. . what are bilateral symmetry and metameric segmentation? . give a schedule of distinctive mammalian features. . describe the rabbit's brain (with diagrams). . give a list of the cranial nerves of the rabbit, and note their origin in the brain. . give a list of the nerve apertures of the dog's skull. . what are the chief anatomical differences between a typical cranial, a spinal, and a sympathetic nerve? . describe and figure the distribution of nerves v., vii., ix., and x. . describe the muscles, glands, and nerves of the orbit of the rabbit. . describe, with figures, the eye of the rabbit. . give a diagram of the rabbit's internal ear. . draw and describe the ear ossicles. what is their function? . draw and state the precise position of the hyoid bone, the clavicle, the calcaneum, and the olecranon process. . describe, as accurately as possible, the position of palatine bones, pterygoids, the ethmoid bone, the pre- and basi-sphenoids, in the dog's skull. . what is membrane bone? what is cartilage bone? discuss their mutual relationship. . what is an excretion? what are the chief excretory products of an animal? how are they removed? . describe the minute anatomy of the liver. give a general account of its functions. . describe the minute anatomy of the kidney, and the functions of the several parts. . what is ciliated epithelium? where does it occur in the rabbit? . describe the mechanism of respiration. what is the relation of respiration to the general life of the animal? . what are the functions of the skin? describe its structure. . what is a secretion? tabulate and classify secretary organs. what is a goblet cell? . draw, from memory, the dorsal and ventral aspects of, and a median section through, a dog's skull. . name any structures that appear to you to be vestiges or rudiments, i.e., structures without adequate physiological reason, in the rabbit's anatomy. . how are such structures interpreted? . describe the structure of striated muscular fibre. describe its functions, and the various means by which they may be called into activity. . describe the characters and structure of the blood of the rabbit. what is the lymphatic system? describe its relation to the blood system in a mammal. . describe the structure of (a) blood, (b) hyaline cartilage, (c) bone, in the rabbit; (d) point out the most important resemblances and differences between these tissues; (e) state what you know of the development of the same tissues. . draw diagrams, with the parts named, of the male and female generative organs of the rabbit. . in the rabbit provided dissect on one side and demonstrate by means of flag-labels the main trunk of the vagus nerve, the phrenic nerve, and the recurrent laryngeal nerve. . dissect the rabbit provided so as to expose the abdominal viscera. mark with flag-labels the duct of the pancreas, the ureters, and the oviducts or the sperm ducts (as the case may be). [many of the above questions were actually set at london university examinations in biology.] {in both editions.} -the frog_ . _general anatomy_ section . we will now study the adult anatomy of the frog, and throughout we shall make constant comparisons with that of the rabbit. in the rabbit we have a distinctly land-loving, burrowing animal; it eats purely vegetable food, and drinks but little. in the frog we have a mainly insectivorous type, living much in the water. this involves the moister skin, the shorter alimentary canal, and the abbreviated neck (rabbit, section ) of the frog; the tail is absent-- in a fish it would do the work the frog accomplishes with his hind legs-- and the apertures which are posterior in the rabbit, run together into one dorsal opening, the cloaca. there is, of course (rabbit, section ), no hair the skin is smooth, and an external ear is also absent. the remarkable looseness of the frog's skin is due to great lymph spaces between it and the body wall. section . if we now compare the general anatomy of the frog (vide sheet ) with that of the rabbit, we notice that the diaphragm is absent (rabbit, section ), and the body cavity, or coelom, is, with the exception of the small bag of the pericardium round the heart, one continuous space. the forked tongue is attached in front of the lower jaw, and can be flicked out and back with great rapidity in the capture of the small insects upon which the frog lives. the posterior nares open into the front of the mouth-- there is no long nasal chamber, and no palate, and there is no long trachea between the epiglottis and the lungs. the oesophagus is less distinct, and passes gradually, so far as external appearances go, into the bag-like stomach, which is much less inflated and transverse than that of the rabbit. the duodenum is not a u-shaped loop, but makes one together with the stomach; the pancreas lies between it and the stomach, and is more compact than the rabbit's. there is no separate pancreatic duct, but the bile duct runs through the pancreas, and receives a series of ducts from that gland as it does so. the ileum is shorter, there is no sacculus rotundus, and the large intestine has no caecum, none of the characteristic sacculations of the rabbit's colon, and does not loop back to the stomach before the rectum section commences. the anus opens not upon the exterior, but into a cloacal chamber. the urinary and genital ducts open separately into this cloaca, and dorsally and posteriorly to the anus. the so-called urinary bladder is ventral to the intestine, in a position answering to that of the rabbit, but it has no connection with the ureters, and it is two-horned. section . the spleen is a small, round body, not so intimately bound to the stomach as in the rabbit, but in essentially the same position. section . much that we knew of the physiology of the frog is arrived at mainly by inferences from our mammalian knowledge. its histology is essentially similar. ciliated epithelium is commoner and occurs more abundantly than in the rabbit, in the roof of the mouth for instance, and its red blood corpuscles are much larger, oval, and nucleated. section . the lungs of the frog are bag-like; shelves and spongy partitions project into their cavities, but this structure is much simpler than that of the rabbit's lung, in which the branching bronchi, the imperfect cartilaginous rings supporting them, alveoli, arteries and veins, form together a quasi-solid mass. section . the mechanism of respiration is fundamentally different from that of the mammal. the method is as follows:-- the frog opens its anterior nares, and depresses the floor of the mouth, which therefore fills with air. the anterior nares are then closed, and the floor mouth rises and forces the air into the lungs-- the frog, therefore, swallows its air rather than inhales it. the respiratory instrument of the rabbit is a suction pump, while that of the frog is a "buccal force pump." section . the heart is not quadrilocular (i.e., of four chambers), but trilocular (of three), and two structures, not seen in lepus, the truncus arteriosus and the sinus venosus, into the latter of which the venous blood runs before entering the right auricle, are to be noted. the single ventricle is blocked with bars of tissue that render its interior, not an open cavity, but a spongy mass. figure , sheet , shows the heart opened; l.au. and r.au. are the left and right auricles respectively; the truncus arteriosus is seen to be imperfectly divided by a great longitudino-spiral valve (l.s.v.); p.c. is the pulmo-cutaneous artery -going to the lungs- [supplying skin and lungs]; d.ao., the dorsal aorta [furnishing the supply of the body and limbs]; and c.a. the carotid artery going to the head; all of which vessels (compare figure ) are paired. section . it might be inferred from this that pure and impure blood mix in the ventricle, and that a blood of uniform quality flows to lungs, head, and extremities; but this is not so. the spongy nature of the ventricle sufficiently retards this mixing. it will be noted that the opening of pulmonary arteries lies nearest to the heart, next come the aortic and carotid arches, which have a common opening at a. furthermore, at c.g.l. [the carotid artery, repeatedly divides to form a close meshwork of arterioles, the carotid gland, forming a sponge-like plug in this vessel.] is a spongy mass of matter, the carotid gland inserted upon the carotid. hence the pulmonary arteries yawn nearest for the blood, and, being short, wide vessels, present the least resistance to the first rush of blood-- mainly venous blood for the right auricle. as they fill up, the back resistance in them becomes equal, and then greater, than the resistance at a, and the rush of blood, now of a mixed quality passes through that aperture. it selects the dorsal aorta, because the carotid arch, plugged by the carotid gland, offers the greater resistance. presently, however, the back resistance of the filled dorsal aorta rises above this, and the last flow of blood, from the ventricular systole-- almost purely oxygenated blood for the left auricle-- goes on towards the head. section . at the carotid gland the carotid artery splits into -an- [a] -external carotid- [lingual] (e.c.), and a deeper internal carotid. the dorsal aorta passes round on each side of the oesophagus, as indicated by the dotted lines in figure , sheet , and meets its fellow dorsal to the liver. each arch gives off subclavian arteries to the limbs, and the left, immediately before meeting the right, gives off the coeliaco-mesenteric artery [to the alimentary canal]. this origin of the coeliaco-mesenteric artery a little to the left, is the only asymmetry (want of balance) in the arterial system of the frog, as contrasted with the very extensive asymmetry of the great vessels near the heart of the rabbit. [posteriorly the dorsal aorta forks into two common iliac arteries (right and left) supplying the hind limbs.] section . figure gives a side view of the frog, to display the circulation. {lines from second edition only.} [the venous return to the heart, as in the rabbit, is by paired venae cavae anteriores and by a single vena cava inferior. the factors of the anterior cava on either side are an external jugular (ex.j.) an innominate vein (in.v.) and subclavian (scl.v.). the last receives not only the brachial vein (b.v.) from the fore limb, but also a large vein bringing blood for the skin, the cutaneous (p.v.). the innominate vein has also two chief factors, the internal jugular (l.i.j.v.) and the subscapular (s.s.v.). the blood returns from each hind limb by a sciatic (l.sc.) or femoral (f.m.) vein, and either passes to a renal portal vein (l.r.p.), which breaks into capillaries in the kidney, or by a paired pelvic vein (l.p.v. in figures and ) which meets its fellow in the middle line to form the anterior abdominal vein (a.ab.v.) going forward and uniting with the (median) portal vein (p.v.) to enter the liver.] -the vessels are named in the references to the figure, which should be carefully copied and mastered. here we need only- [comparing with the rabbit, we would especially] call attention to the fact that the vena cava inferior extends posteriorly only to the kidney, and that there is a renal portal system. the blood from the hind limbs either flows by the anterior abdominal vein to the portal vein and liver, or it passes by the renal portal vein to the kidney. there the vein breaks up, and we find in the frog's kidney, just as we find in the frog's and rabbit's liver, a triple system of (a) nutritive arterial, (b) afferent* venous and (c) efferent** venous vessels. * a, ad = to; ** e, ex = out of. {this section missing from second edition.} -section . it is not very improbable that the kidney of the frog shares, or performs, some of the functions of the rabbit's liver, or parallel duties, in addition to the simply excretory function. since specialization of cells must be mainly the relatively excessive exaggeration of some one of the general properties of the undifferentiated cell, it is not a difficult thing to imagine a gradual transition, as we move from one organism to another, of the functions of glands and other cellular organs. it is probable that the mammalian kidney is, physiologically, a much less important (though still quite essential) organ than the structures which correspond to it in position and development in the lower vertebrate types.- section . the lymphatic system is extensively developed in the frog, but, in the place of a complete system of distinctly organized vessels, there are great lymph sinuses (compare section ). in figure , sheet , the position of two lymph hearts (l.h., l.h.) which pump lymph into the adjacent veins, is shown. section . the skull of the frog will repay a full treatment, and will be dealt with by itself later. the vertebral column (sheet ) consists of nine vertebrae, the centra of which have faces, not flat, but hollow in front (pro-coelous), and evidently without epiphyses (compare the rabbit). the anterior is sometimes called the atlas, but it is evidently not the homologue of the atlas of the rabbit, since the first spinal nerve has a corresponding distribution to the twelfth cranial of the mammal, and since, therefore, it is probable that the mammalian skull = the frog's skull + one (or more) vertebrae incorporated with it. posteriorly the vertebral column terminates in the urostyle, a calcified unsegmented rod. the vertebrae have transverse processes, but no ribs. section . the fore-limb (figure , sheet ) consists of an upper segment of one bone, the humerus, as in the rabbit; a middle section, the radius and ulna, fused here into one bone, and not, as in the mammalian type, separable; of a carpus, and of five digits, of which the fourth is the longest. the shoulder girdle is more important and complete than that of the higher type. there is a scapula (sc.) with an unossified cartilaginous supra-scapula (s.sc.); the anterior border of the scapula answers to the acromion. on the ventral side a cartilaginous rod, embraced by the clavicle (cl.) (a membrane bone in this type), runs to the sternum, and answers to the clavicle of the rabbit. in the place of the rabbit's coracoid process, is a coracoid bone (co.), which reaches from the glenoid cavity to the sternum; it is hidden on the right side of figure , which is a dorsal view of the shoulder girdle. there is a pre-omosternum (o.st.) and a post-omosternum, sometimes termed a xiphisternum (x.). section . figure shows the pelvic girdle and limb of the frog. there is a femur (f.); tibia and fibula (t. and f.) are completely fused; the proximal bones of the tarsus, the astragalus (as.), and calcaneum (cal.) are elongated, there are five long digits, and in the calcar (c.) an indication of a sixth. with considerable modifications of form, the three leading constituents of the rabbit's pelvic girdle occur in relatively identical positions. the greatly elongated ilium (il.) articulates with the single (compare rabbit) sacral vertebra (s.v. in figure ). the ischium (is.) is relatively smaller than in the rabbit, and the pubis (pu.) is a ventral wedge of unossified cartilage. the shape of the pelvic girdle of the frog is a wide departure from that found among related forms. in connection with the leaping habit, the ilia are greatly elongated, and the pubes and ischia much reduced. generally throughout the air-frequenting vertebrata, we find the same arrangement of these three bones, usually in the form of an inverted. y-- the ilium above, the ischium and pubis below, and the acetabulum at the junction of the three. section . the uro-genital organs of the frog, and especially those of the male, correspond with embryonic stages of the rabbit. in this sex the testes (t., sheet ) lie in the body cavity, and are white bodies usually dappled with black pigment. vasa efferentia (v.e.) run to the internal border of the anterior part of the kidney, which answers, therefore, to the rabbit's epididymis. the hinder part of the kidney is the predominant renal organ. there is a common uro-genital duct, into which a seminal vesicle, which is especially large in early spring, opens. this is the permanent condition of the frog. in the rabbit, for urogenital duct, we have ureter and vas deferens; the testes and that anterior part of the primitive kidney, the epididymis, shift back into the scrotal sacs, and the ureters shift round the rectum and establish a direct connection with the bladder, carrying the genital ducts looped over them. the oviducts of the female do not fuse distally to form a median vagina as they do in the rabbit. in front of the genital organ in both sexes is a corpus adiposum (c.ad.), which acts as a fat store, and is peculiar to the frogs and toads. the distal end of the oviduct of the female is in the breeding season (early march) enormously distended with ova, and the ovaries become then the mere vestiges of their former selves. the distal end of the oviduct is, therefore, not unfrequently styled the uterus. there is no penis in the male, fertilisation of the ova occurring as they are squeezed out of the female by the embracing fore limbs of the male. the male has a pad, black in winter, shown in figure , which is closely pressed against the ventral surface of the female in copulation, and which serves as a ready means of distinguishing the sex. section . the spinal cord has a general similarity to that of the rabbit; the ratio of its size to that of the brain is larger, and the nerves number ten pairs altogether. the first of these (sp. , in figure , sheet - - ) {first edition error.} [ ] corresponds in distribution with the rabbit's hypoglossal nerve, a point we shall refer to again when we speak of the skull. the second and third constitute the brachial plexus. the last three form the sciatic plexus going to the hind limb. section . the same essential parts are to be found in the brain of both frog and rabbit, but in the former the adult is not so widely modified from the primitive condition as in the latter. the fore-brain consists of a thalamencephalon (th.c. and ), which is exposed in the dorsal view of the brain, and which has no middle commissure. the cerebral hemispheres (c.h.) are not convoluted, do not extend back to cover parts behind them, as they do in the rabbit, and are not connected above the roof of the thalamencephalon by a corpus callosum. moreover, the parts usually regarded, as the olfactory lobes (rh.) fuse in the middle line. the mid-brain gives rise to the third nerve, and has the optic lobes on its dorsal side, but these are hollow, and they are not subdivided by a transverse groove into corpora quadrigemina, as in the rabbit. in the hind-brain the cerebellum is a mere band of tissue without lateral lobes or flocculi, and the medulla gives origin only to nerves four to ten; there is no eleventh nerve, and the hypoglossal is the first spinal-- from which it has been assumed that the rabbit's medulla equals that of the frog, plus a portion of the spinal cord incorporated with it. the hypoglossal is very distinctly seen on opening the skin beneath the hyoid plate. section . the first, second, third, and fourth cranial nerves of the frog correspond with those of the rabbit in origin and distribution. so do five, six and eight. the seventh nerve forks over the ear-drum-- the larger branch emerging behind it and running superficially, as shown in figure . there is also a deeper palatine branch of vii. (p.) running under v and v below the orbit, and to be seen together with v and v after removal of the eyeball. the ninth nerve similarly forks over the first branchial slit of the tadpole, and evidence of the fork remains in the frog. it is seen curving round anterior to the hypoglossal nerve, and lying rather deeper in dissection. the vagus (tenth) nerve is distributed to heart, lungs, and viscera-- in the tadpole it also sends for forking branches over the second, third, and fourth branchial slits. it lies deeper than ix., and internal to the veins, and runs close beside the cutaneous artery. most of these nerves are easily dissected and no student should rest satisfied until he has actually seen them. section . the sympathetic chain is closely connected with the aorta. it is, of course, paired, and is easily found in dissection by lifting the dorsal aorta and looking at its mesentery. in the presence of ganglia corresponding to the spinal nerves, and of rami communicantes, it resembles that of the rabbit. section . the whole of this chapter is simply a concise comparison, of frog and rabbit. in addition to reading it, the student should very carefully follow the annotations to the figures, and should copy and recopy these side by side with the corresponding diagrams of the other types. . _the skull of the frog (and the vertebrate skull generally)_ section . we have already given a description of the mammalian skull, and we have stated where the origin of the several bones was in membrane, and where in cartilage; but a more complete comprehension of the mammalian skull becomes possible with the handling of a lower type. we propose now, first to give some short account of the development and structure of the skull of the frog, and then to show briefly how its development and adult arrangement demonstrate the mammalian skull to be a fundamentally similar structure, complicated and disguised by further development and re-adjustment. section . figure ,i. sheet , shows a dorsal view of a young tadpole cranium; the brain has been removed, and it is seen that it was supported simply upon two cartilaginous rods, the trabeculae cranii (tr.c.). behind these trabeculae comes the notochord (n.c.), and around its anterior extremity is a paired tract of cartilage, the parachordals (p.c.). these structures, underlying the skull, are all that appear[s] at first of the brain box. in front, and separate from the cranium, are the nasal organs (n.c.); the eyes lie laterally to the trabeculae, and laterally to the parachordals are two tracts of cartilage enclosing the internal ear, the otic capsules. section . figure , ii., is a more advanced, phase of the same structures. the trabeculae have met in front and sent forward a median (c.t.) and lateral parts (a.o.) to support the nasal organs. they have also flattened, out very considerably, and have sent up walls on either side of the brain to meet above it and form an incomplete roof (t.) over it. the parachordals have similarly grown up round, the hind-brain and formed a complete ring, the roof of which is indicated, by b. further, the otic capsules are fusing with the brain-case. with certain differences of form these elements-- the trabeculae, the parachordals, and the otic capsules, are also the first formed structures of the mammalian cranium. section . in figures ,i. and ii., there appears beneath the eye a bar of cartilage (p.p.), the palato-pterygoid cartilage, which is also to be seen from the side in figures ,i. and iii. it will be learnt from these latter that this bar is joined in front to the cranium behind the nasal organ, and behind to the otic capsule. the cartilaginous bar from the palato-pterygoid to the otic capsule is called the quadrate, and at the point of junction, at the postero-ventral angle of the palato-pterygoid, articulates with the cartilaginous bar which is destined to form the substratum of the lower jaw-- meckel's cartilage (m.c. in figure ,i.). section . figure shows a dorsal view of these structures in a young frog. the parts corresponding to these in ,ii. will be easily made out, but now ossification has set in at various points of this cartilaginous cranium. in front of the otic capsule is the paired pro-otic bone (p.o.); behind it at the sides of the parachordal ring is the paired ex-occipital (e.o.); in front of the cranium box, and behind the nasal capsules, is a ring of bone, the (median, but originally paired) sphenethmoid (s.e.). -a paired ossification appears in the palato-pterygoid cartilage the pterygoid bone (pt.), while- a splint of bone, the quadrato-jugal, appears at the angle of articulation with the lower jaw. these are all the cartilage bones that appear in the cranium and upper jaw of the frog. section . but another series of bones, developed first chiefly in dermal connective tissue, and coming to plate over the cranium of cartilage, are not shown in figure . they are, however, in figure . these membrane bones are: along the dorsal middle line, the parieto-frontals (p.f.), originally two pairs of bones which fuse in development, and the nasals (na.). round the edge of the jaw, and bearing the teeth, are pre-maxillae (p.m.), and maxillae (mx.), and overlying the quadrate cartilage and lateral to the otic capsules are the t-shaped squamosal bones (sq.). in the ventral view of the skull (figure ) we see a pair of vomers (vo.) bearing teeth, a pair of palatines (pal.), [and a pair of pterygoids (pt.)] (which [palatines and pterygoids, we may note,] unlike those of the rabbit, are -stated to be- membrane bones), and a great median dagger-shaped para-sphenoid (p.sp.). these two figures, and , which shows the same bones in side view, should be carefully mastered before the student proceeds with this chapter. the cartilage bones are distinguished from membrane bones by cross-shading. section . turning now to figure ,i., we have a side view of a tadpole's skull. on the ventral side of the head is a series of vertical cartilaginous bars, the visceral arches supporting the walls of the tadpole's gill slits. the first of these is called the hyoid arch (c.h.), and the four following this, the first (br. ), second, third, and fourth (br. ), branchial arches. altogether there are four gill slits and between the hyoid arch and the jaw arch, as it is called (= meckel's cartilage + the palato-pterygoid), is "an imperforate slit," which becomes the ear-drum.* the frog no longer breathes by gills, but by lungs, and the gills are lost, the gill slits closed, and the branchial arches consequently much reduced. figures , ii., and , iii., show stages in this reduction. the hyoid arch becomes attached, to the otic capsule, and its median ventral plate, including also the vestiges of the first, second, and fourth branchial arches, is called the hyoid apparatus. in figure , the apparatus is seen from the side; c.h. is called the (right) anterior cornu** of the hyoid. the function of the hyoid apparatus in the frog is to furnish, a basis of attachment to the tongue muscles; it remains cartilaginous, with the exception of the relic of one branchial arch, which ossifies as the thyro-hyal (figure th.h.). it will be noted that, as development proceeds, the angle of the jaw swings backward, and the hyoid apparatus, shifts relatively forward. these changes of position are indicated in figure , iii., by little arrow-heads. * we may note here that, comparing the ear of the frog with that of the rabbit, there is no external ear. there is, moreover, no bulla supporting the middle ear, and the tympanic membrane stretches between the squamosal in front and the anterior cornu of the hyoid behind. a rod-like columella auris replaces the chain of ear ossicles, and may, or may not, answer to the stapes alone, or even possibly to the entire series. in the internal ear there is no cochlea, and the otic mass is largely cartilaginous instead of entirely bony. ** plural cornua. section . before proceeding to the comparison of the mammalian skull with this, we would strongly recommend the student thoroughly to master this portion of the work, and in no way can he do this more thoroughly and quickly than by taking a parboiled frog, picking off the skin, muscle, and connective tissue from its skull, and making out the various bones with the help of our diagrams. section . figure represents, in the most diagrammatic way, the main changes in form of the essential constituents of the cranio-facial apparatus, as we pass from the amphibian to the mammalian skull. f. is the frog from the side and behind; b.c. is the brain-case, o.c. the otic capsule, e. the eye, n.c. the nasal capsule, p.p. the palato-pterygoid cartilage, mx. the maxillary membrane bones, sq. the squamosal, and mb. the mandible. the student should compare with figure , and convince himself that he appreciates the diagrammatic rendering of these parts. now all the distinctive differences in form, from this of the dog's skull (d.), are reducible to two primary causes-- ( ) the brain is enormously larger, and the brain-case is vastly inflated, so that-- (a) the otic capsule becomes embedded in the brain-case wall; (b) the palato-pterygoid rod lies completely underneath the brain-case instead of laterally to it; (c) the squamosal tilts down and in, instead of down and out, and the lower jaw articulates with its outer surface instead of below its inner, and, moreover, with the enormous distention of the brain-case it comes about that the squamosal is incorporated with its wall. ( ) the maxilla anteriorly and the palatine posteriorly send down palatine plates that grow in to form the bony palate, cutting off a nasal passage (n.p.) from the mouth cavity (m.p.), and carrying the posterior nares from the front part of the mouth, as they are in the frog, to the pharynx. hence the vomers of the dog lie, not in the ceiling of the mouth, but in the floor of this nasal passage. section . the quadrate cartilage of the frog is superseded by the squamosal as the suspensorium of the lower jaw. it is greatly reduced, therefore; but it is not entirely absent. in the young mammal, a quadrate cartilage can be traced, connected with the palato-pterygoid cartilage, and articulating with meckel's cartilage. its position is, of course, beneath the squamosal, and just outside the otic capsule. as development proceeds, the increase in size of the quadrate, does not keep pace with that of the skull structures. it loses its connection with the palato-pterygoid, and apparently ossifies as a small ossicle-- the incus of the middle ear. a small nodule of cartilage, cut off from the proximal end of meckel's cartilage, becomes the malleus. the stapes would appear to be derived from the hyoid arch. hence these small bones seem to be the relics of the discarded jaw suspensorium of the frog utilized in a new function. considerable doubt, however, attaches to this interpretation-- doubt that, if anything, is gaining ground. section . the tympanic bulla of the dog is not indicated in diagram , and it would appear to be a new structure (neomorph), not represented in the frog. section . besides these great differences in form, there are important differences in the amount and distribution of centres of ossification of the skull of frog and mammal. there is no parasphenoid in the mammal*; and, instead, a complete series of ossifications, the median-, basi-, and pre-sphenoids, and the lateral ali- and orbito-sphenoids occur. the points can be rendered much more luminously in a diagram than in the text, and we would counsel the student to compare this very carefully with that of the rabbit. * faint vestigeal indications occur in the developing skulls of some insectivora. section . -cranium_ -nasal_ (paired), -vomer_ (paired) -fronto-parietal_, sphenethmoid bone (median), eye, pro-otic bone, otic cartilage, ex-occipital (paired) -para-sphenoid bone_ -upper jaw_ -pre-maxilla_ (paired), -palatine_ (paired), pterygoid (paired), -squamosal_, quadrate cartilage {to .} -maxilla_ . quadrato-jugal -lower jaw_ mento-meckelian, -dentary_, -articulare- [-angulo splenial_] section . -points especially- [additional points] to be noticed are: ( ) the otic capsule (= periotic bone) of the dog ossifies from a number of centres, one of which is equivalent to the frog's prootic. ( ) the several constituents of the lower jaw are not to be distinguished in the adult mammal. ( ) the frog has no lachrymal bone. section . we are now in a position to notice, without any danger of misconception, what is called the segmental theory of the skull. older anatomists, working from adult structure only, conceived the idea that the brain-case of the mammal represented three inflated vertebrae. the most anterior had the pre-sphenoid for its body, the orbito-sphenoids for its neural processes, and the arch was completed above by the frontals (frontal segment). similarly, the basi-sphenoids, ali-sphenoids, and parietals formed a second arch (parietal segment), and the ex-, basi-, and supra-occipitals a third (occipital segment). if this were correct, in the frog, which is a more primitive rendering of the vertebrate plan, we should find the vertebral characters more distinct. but, as a matter of fact, as the student will perceive, frontal segment, parietal segment, and occipital segment, can no longer be traced; and the mode of origin from trabeculae and para-chordals show very clearly the falsity of this view. the vertebrate cranium is entirely different in nature from vertebrae. the origin of the parietals and frontals as paired bones in membrane reinforces this conclusion. section . but as certainly as we have no such metameric segmentation, as this older view implies, in the brain-case of the frog, so quite as certainly is metameric segmentation evident in its branchial arches. we have the four gill slits of the tadpole and their bars repeating one another; the hyoid bar in front of these is evidently of a similar nature; and that the ear drum is derived from an imperforate gill slit is enforced by the presence of an open slit (the spiracle) in the rays and dog-fish in an entirely equivalent position. does the mouth answer to a further pair of gill slits, and is the jaw arch (palato-pterygoid + meckel's cartilage) equivalent to the arches that come behind it? this question has been asked, and answered in the affirmative, by many morphologists, but not by any means by all. the cranial nerves have a curious similarity of arrangement with regard to the gill slits and the mouth; the fifth nerve forks over the mouth, the seventh forks over the ear drum, the ninth, in the tadpole and fish, forks over the first branchial slit, and the tenth is, as it were, a leash of nerves, each forking over one of the remaining gill slits. but this matter will be more intelligible when the student has worked over a fish type, and need not detain us any further now. section . see also section again, in which is the suggestion that the occipital part of the skull is possibly a fusion of vertebrae, a new view with much in its favour, and obviously an entirely different one from the old "segmental" view of the entire skull, discussed in section . _questions on the frog_ [all these questions were actually set at london university examinations.] {in both editions.} . give an account, with illustrative sketches, of the digestive organs of the common frog, specifying particularly the different forms of epithelium met with in the several regions thereof. . describe the heart of a frog, and compare it with that of a fish and of a mammal, mentioning in each case the great vessels which open into each cavity. . compare with one another the breathing organs and the mechanism of respiration in a frog and in a rabbit. give figures showing the condition of the heart and great arteries in these animals, and indicate in each case the nature of the blood in the several cavities of the heart. . draw diagrams, with the parts named, illustrating the arrangement of the chief arteries of (a) the frog, (b) the rabbit. (c) compare briefly the arrangements thus described. (d) in what important respects does the vascular mechanism of the frog differ from that of the fish, in correlation with the presence of lungs? . in the frog provided, free the heart, both aortic arches, dorsal aorta as far as its terminal bifurcation, and both chains of sympathetic ganglia from surrounding structures; and remove them, in their natural connection, from the animal into a watch-glass. . describe the male and female reproductive organs of the common frog, and give some account of their development. . describe, with figures, the bones of the limbs and limb-girdles of a frog. . remove the brain from the frog provided, and place it in spirit. make a lettered drawing of its ventral and dorsal surfaces. . point out the corresponding regions in the brain of a frog and a mammal, and state what are the relations of the three primary brain-vesicles to these regions. . (a) give an account, with diagrams, of the brain of the frog; (b) point out the most important differences between it and the brain of the rabbit. (c) describe the superficial origin and the distribution of the third, (d) of the fifth, (e) of the seventh., (f) of the ninth, and (g) of the tenth cranial nerves of the frog. . describe, with figures, the brain of a frog, and compare it with that of a rabbit. what do you know concerning the functions of the several parts of the brain in the frog? . describe briefly the fundamental properties of the spinal cord in the frog. by what means would you determine whether a given nerve is motor or sensory? . prepare the skull of the frog provided. remove from it and place in glycerine on a glass slip the fronto-parietal and parasphenoid bones. label them. mark on the skull with long needles and flag-labels the sphenethmoid and the pro-otic bones. . compare the skull of the rabbit and the frog; especially in regard to the attachment of the jaw apparatus to the cranium, and other points which distinctly characterize the higher as contrasted with the lower vertebrata. . describe the skeleton of the upper and lower jaw (a) in the frog, (b) in the rabbit. point out exactly what parts correspond with one another in the two animals compared. (c) what bone in the rabbit is generally regarded as corresponding to the quadrate cartilage of the frog? -the dog-fish_ . _general anatomy_ section . in the dog-fish we have a far more antique type of structure than in any of the forms we have hitherto considered. forms closely related to it occur among the earliest remains of vertebrata that are to be found in the geological record. since the immeasurably remote silurian period, sharks and dog-fish have probably remained without any essential changes of condition, and consequently without any essential changes of structure, down to the present day. then, as now, they dominated the seas. they probably branched off from the other vertebrata before bone had become abundant in the inner skeleton, which is consequently in their case cartilaginous, with occasional "calcification" and no distinct bones at all. unlike the majority of fish, they possess no swimming bladder-- the precursor of the lungs; but in many other respects, notably in the uro-genital organs, they have, in common with the higher vertebrata, preserved features which may have been disguised or lost in the perfecting of such modern and specialized fish as, for instance, the cod, salmon, or herring. section . comparing the general build, of a dog-fish with that of a rabbit, we notice the absence of a distinct neck, and the general conical form; the presence of a large tail, as considerable, at first, in diameter as the hind portion of the body, and of the first importance in progression, in which function the four paddle-shaped limbs, the lateral fins, simply co-operate with the median fin along the back for the purpose of steering; and, as a consequence of the size of the tail, we note also the ventral position of the apertures of the body. the anus, and urinary and genital ducts unite in one common chamber, the cloaca. behind the head, and in front of the fore fin (pectoral fin), are five gill slits (g.s.) leading from the pharynx to the exterior. just behind the eye is a smaller and more dorsal opening of the same kind, the spiracle (sp.). on the under side of the head, in front of the mouth, is the nasal aperture (olf.), the opening of the nasal sac, which, unlike the corresponding organ of the air-frequenting vertebrata, has no internal narial opening. there is, however, a groove running from olf. to the corner of the mouth, and this, closing, in the vertebrate types that live in air and are exposed to incessant evaporation of their lubricating secretions, constitutes the primitive nasal passage. the limbs are undifferentiated into upper, lower, and digital portions, and are simply jointed, flattened expansions. section . the skin of the dog-fish is closely set with pointed tooth-like scales, the placoid scales, and these are continued over the lips into the mouth as teeth. each scale consists of a base of true bone, with a little tubercle of a harder substance, dentine, capped by a still denser covering, the enamel. the enamel is derived from the outer layer of the embryonic dog-fish, the epiblast, which also gives rise to the epidermis; while the dentine and bony base arise in the underlying mesoblast, the dermis. a mammalian tooth has essentially the same structure: an outer coat of enamel, derived from epiblast, overlies a mass of dentine, resting on bone, but the dentine is excavated internally, to form a pulpcavity containing blood-vessels and nerves. most land animals, however, have teeth only in their mouths, and have lost altogether the external teeth which constitute the armour of the dog-fish. besides the teeth there perhaps remain relics of the placoid scales in the anatomy of the higher vertebrata, in the membrane bones. how placoid scales may have given rise to these structures will be understood by considering such a bone as the vomer of the frog. this bone lies on the roof of the frog's mouth, and bears a number of denticles, and altogether there is a very strong resemblance in it to a number of placoid scales the bony bases of which have become confluent. in the salamander, behind the teeth-bearing vomers comes a similar toothed parasphenoid bone. the same bone occurs in a corresponding position in the frog, but without teeth. in some tailed amphibians the vomers and splenials are known to arise by the fusion of small denticles. these facts seem to point to stages in the fusion of placoid bases, and their withdrawal from the surface to become incorporated with the cranial apparatus as membrane bones, a process entirely completed in the mammalian type. section . the alimentary canal of the dog-fish, is a simple tube thrown into a z shape. the mouth is rough with denticles, and has a fleshy immovable tongue on its floor. in the position of the eustachian tube there is a passage, the spiracle (sp.), running out to the exterior just external to the cartilage containing the ear. the pharynx communicates with the exterior through five gill slits (g.s.), and has, of course, no glottis or other lung opening. there is a wide oesophagus passing into a u-shaped stomach (st.), having, like the rabbit's, the spleen (sp.) on its outer curvature. there is no coiling small intestine, but the short portion, receiving the bile duct (b.d.) and duct of the pancreas (pan.), is called the duodenum (d'dum.). the liver has large left (l.lv.) and right lobes, and a median lobe (m.lv.), in which the gall bladder (g.bl.) is embedded. the next segment of the intestine is fusiform, containing a spiral valve (figure ), the shelf of which points steeply forward; it is sometimes called the colon (co.). it is absorptive in function and probably represents morphologically, as it does physiologically, the greater portion of the small intestine. a rectal gland (r.g.) opens from the dorsal side into the final portion of the canal (rectum). section . the circulation presents, in many respects, an approximation to the state of affairs in the developing embryos of the higher types. the heart (figure , sheet - - {error in first edition} [ ]) is roughly, z shape, and transmits only venous blood. it lies in a cavity, the pericardial cavity (p.c.c.), cut off by a partition from the general coelome. at one point this partition is imperfect, and the two spaces communicate through a pericardio-peritoneal canal (p.p.c.), which is also indicated by an arrow (p.p.) in the position and direction in which the student, when dissecting, should thrust his "seeker," in figure sheet . a sinus venosus (s.v. in figure , sheet ) receives the venous trunks, and carries the blood through a valve into the baggy and transversely extended -auricle- [atrium] (au.), whence it passes into the muscular ventricle (vn.), and thence into the truncus arteriosus. this truncus consists of two parts: the first, the conus or pylangium (c.a.), muscular, contractile, and containing a series of valves; the second, the bulbus or synangium (b.a.), without valves and pulsatile. in the rabbit both sinus and truncus are absent, or merged in the adjacent parts of the heart. section . from the bulbus there branch, on either side, four arterial trunks, the first of which forks, so that altogether there are five afferent branchials (a.br.) taking blood to be aerated in the gills, here highly vascular filamentary outgrowths of the internal walls of the gill slits. {lines from second edition only.} [there are altogether nine vascular outgrowths (demi-branchs), one on each wall of each gill slit except the last, on the hind wall of which there is none. (in the spiracle is a miniature demibranch, the pseudo-branch. this suggests that the spiracle is really a somewhat modified gill slit.)] four efferent branchials (e.br.) carry the aerated blood on to the dorsal aorta (d.ao.). a carotid artery runs forward to the head, and a hypo-branchial artery supplies the ventral side of the pharyngeal region. there are sub-clavian, coeliac, mesenteric, and pelvic arteries, and the dorsal aorta is continued through the length of the tail as the caudal artery (cd.a.). section . a caudal vein (cd.v.), bringing blood back from the tail, splits behind the kidneys (k.), and forms the paired renal portal veins (r.p.v.), breaking up into a capillary system in the renal organ. a portal vein brings blood from the intestines to the liver. section . instead of being tubular vessels, the chief veins of the dog-fish are, in many cases, irregular baggy sinuses. three main venous trunks flow into the sinus venosus. in the median line from behind comes the hepatic sinus (h.s.); and laterally, from a dorsal direction, the cuvierian sinuses (c.s.) enter it. these, as the student will presently perceive, are the equivalents of the rabbit's superior cavae. they receive, near their confluence with the sinus venosus, the inferior jugular vein (i.j.v.). at their dorsal origin, they are formed by the meeting of the anterior (a.c.s.) and posterior (p.c.s.) cardinal sinuses. the anterior cardinal sinus -is, roughly, the equivalent of the internal jugular vein-, lies along dorsal to the gill slits (g.s.), and receives an orbital sinus from the eye. the posterior cardinal sinus receives a sub-clavian vein (s.c.v.) and a lateral vein (l.v.), and fuses posteriorly with its fellow in the middle line. this median fusion is a departure from the normal fish type. it must not be confused with the inferior cava, which is not found in the dog-fish, the [right] posterior cardinals representing the rabbit's azygos vein. a simplified diagram of the circulation of a fish is given in figure , sheet , and this should be carefully compared with the corresponding small figure given of the vascular system of our other types. {lines from second edition only.} [the blood of the dog-fish resembles that of the frog.] section . the internal skeleton, as we have said, is entirely cartilaginous, and only those parts which are pre-formed in cartilage in the skeletons of the higher types are represented here. the spinal column consists of two types of vertebrae, the trunk, bearing short, distinct, horizontally-projecting ribs (r.), and the caudal. the diagrams of figure [(sheet )] are to illustrate the structure of the centrum of a dog-fish vertebra; c is a side view, d a horizontal median section, a and b are transverse sections at the points indicated by -b and a- [a and b] respectively in figure c. -(by an unfortunate slip of the pen in the figure, a was substituted for b; section a corresponds to line b, and vice versa.)- the vertebrae are hollowed out both anteriorly and posteriorly (amphi-coelous), and a jelly-like notochord runs through the entire length of the vertebral column, being constricted at the centres of the centra, and dilated between them. the neural arch above the centrum, and containing the spinal cord, is made up of neural plates (n.p.), and interneural plates (i.n.p.), completed above by a median neural spine (n.s.). in the caudal region, instead of ribs projecting outwardly, there are haemal processes, inclined downwards and meeting below, forming an arch, the haemal arch, containing the caudal artery and vein-- the vein ventral to the artery-- and resembling the neural arch, which contains the spinal cord above, in shape and size. section . the pectoral limb and girdle (figure , sheet ) have only a very vague resemblance to the corresponding structures in the rabbit. the girdle (g.) is a transverse bar lying ventral to the pericardial wall, and sending up a portion (sc.), dorsal to the attachment of the limb, which answers to the scapula and supra-scapula of the forms above the fish. three main cartilages, named respectively the pro- (p.p.), meso- (m.p.), and meta-pterygium, form the base of the limb. with these, smaller cartilaginous plates, rods, and nodules articulate, and form a flattened skeletal support for the fin. section . the pelvic girdle and limb (figure , sheet ) are similar in structure, but the pro-pterygium and meso-pterygium are absent, and the cartilage answering to the meta-pterygium goes by the name of the basi-pterygium. in the male, but not in the female, the pelvic fins are united behind the cloaca, and there are two stiff grooved copulatory organs, the claspers (cl. in figure ), which have a cartilaginous support (cl.c.). these claspers form the readiest means of determining the sex of a specimen before dissection. section . the skull consists of a cartilaginous cranium, and of jaw and visceral arches. the cranium persists throughout life, in what closely resembles a transitory embryonic condition of the higher types. there is a nasal capsule (na.c.), a brain case proper, and lateral otic (auditory) capsules (ot.c.) containing the internal ear. (this should be compared with the frog's embryonic skull.) the upper jaw has a great bar of cartilage, the palato-pterygoid, as its sole support; the arch of premaxilla, maxilla, jugal, and squamosal-- all membrane bones-- is, of course, not represented. in the frog this bar of cartilage is joined directly to the otic capsule by a quadrate portion, but this is only doubtfully represented in the dog-fish by a nodule of cartilage in the pre-spiracular ligament (p.s.). the lower jaw is supported, by meckel's cartilage (m.c.). the hyoid arch consists of two main masses of cartilage, the hyomandibular (h.m.), and the ceratohyal (c.h.); the former of these is tilted slightly forward, so that the gill slit between it and the jaw arch is obliterated below, and the cartilage comes to serve as the intermediary in the suspension of the jaw from the otic mass. there are five branchia[l] arches, made up pharyngo-, epi- and cerato-branchials, and the ventral elements fuse in the middle line to form a common plate of cartilage. outside these arches are certain small cartilages, the extra branchials (ex.b.) which, together with certain small labials by the nostrils and at the sides of the gape, probably represent structures of considerably greater importance in that still more primitive fish, the lamprey. the deep groove figured lateral to the otic capsule is the connecting line of the orbital and anterior cardinal sinuses; the outline of the anterior cardinal sinus in this figure and in figure is roughly indicated by a dotted line. section . figure a is a rough diagram of the internal ear-- the only auditory structure of our type (compare rabbit, sheet ). to dissect out the auditory labyrinth without injury is a difficult performance, but its structure may be made out very satisfactorily by paring away successive slices of the otic mass. such a section is shown by figure b; through the translucent hyaline cartilage the utriculus and horizontal canal can be darkly seen. the ductus endolymphaticus (vide rabbit) is indicated by a dotted line in our figure. it is situated internal to the right-angle between the two vertical canals, and reaches to the surface of the otic capsule. section . the brain shows the three primary vesicles much more distinctly than do our higher types. the fore-brain has large laterally separated olfactory lobes (rh.), there are relatively small "hemispheres" (pr.c.), the stalk of the pineal gland tilts forward, and the gland itself is much nearer the surface, being embedded in the cartilage of the brain case, and the pituitary body is relatively very large, and has lateral vascular lobes on either side. following the usual interpretation of the parts, we find optic lobes (op.l.) as the roof of the mid-brain, and behind a very large, median, hollow, tongue-shaped cerebellum (c.b.). the medulla is large, and certain lateral restiform tracts (r.t.) therein, which also occur in the higher types, are here exceptionally conspicuous. section . the dog-fish has ten pairs of cranial nerves, corresponding to the anterior ten of the rabbit very closely, when we allow for the modification the latter has suffered through the conversion of some part of the spiracular cleft to an eardrum, and the obliteration of the post-hyoid branchial slits. the first and second nerves are really brain lobes, and nerves of the special senses of smell and sight respectively. the third (oculomotor), the fourth (patheticus), and the sixth (abducens) are distributed to exactly the same muscles of the eyeball as they are in the rabbit. the fifth nerve, has, in the dog-fish, as in the rabbit, three chief branches. v. and v. fork over the mouth just as they do in the mammal; v. passes out of the cranium by a separate and more dorsal opening, and runs along a groove along the dorsal internal wall of the orbit, immediately beneath a similar branch of vii., which is not distinct in the rabbit. the grooves are shown in the figure of the cranium, sheet ; the joint nerve thus compounded of v. and vii. is called the ophthalmic (oph.). it is distributed to the skin above the nose and orbit. when the student commences to dissect the head of a dog-fish he notices over the dorsal surface of the snout an exudation of a yellowish jelly-like substance, and on removing the tough skin over this region and over the centre of the skull he finds, lying beneath it, a quantity of coiling simple tubuli full of such yellowish matter. these tubuli open on the surface by small pores, and the nerves terminate in hair-like extremities in their lining. these sense tubes are peculiar to aquatic forms; allied structures are found over the head and along a lateral line (see below) in the tadpole, but when the frog emerges from the water they are lost. they, doubtless, indicate some unknown sense entirely beyond our experience, and either only possible or only necessary when the animal is submerged. in addition to the ophthalmic moiety mentioned above, the seventh nerve has a vidian branch (vid.) running over the roof of the mouth, and besides this its main branches fork over the spiracle, just as v. forks over the mouth, and as ix. and x. fork over gill clefts. this nerve in the rabbit is evidently considerably modified from this more primitive condition. the eighth is the auditory nerve, as in the rabbit. the ninth nerve forks over the first branchial cleft. the tenth nerve is easily exposed by cutting down through the body wall muscles over the gill clefts, into the anterior cardinal sinus (a.c.s.). it gives off (a) branches forking over the posterior four gill slits, (b) a great lateral nerve running inward, and back through the body-wall muscle, and connected with a line of sense organs similar to those in the head, the lateral line, and (c) a visceral nerve curving round to the oesophagus and stomach. in dissection it becomes very evident that the tenth nerve is really a leash of nerves, each one equivalent to the ninth. we may here call the attention of the reader to the fact of the singular resemblance of v., vii., ix., and the factors of x. that each has a ventral fork, we have already noticed. each also (?ix.) has a dorsal constituent connected with the sense organs of the skin. the vidian branch of vii., however, is not evidently represented in the others. section . the coelom of the dog-fish is peculiar-- among the types we treat of-- in the possession of two direct communications with the exterior, in addition to the customary indirect way through the oviduct. these are the abdominal pores (a.p.) on either side of the cloaca in either sex. they can always be readily demonstrated by probing out from the body cavity, in the direction indicated by the arrow (a.p.) in figure , sheet . they probably serve to equalize the internal and external pressure of the fish as it changes its depth in the water, just as the eustachian tubes equalize the pressure on either side of the mammal's tympanic membrane. section . the musculature of the dog-fish body is cut into v-shaped segments, the point of the v being directed forward. the segments alternate with the vertebrae, and are called myomeres. such a segmentation is evident, though less marked, in the body wall muscles of the frog, and in the abdominal musculature of the rabbit and other mammals it is still to be traced. section . the uro-genital organs of the female dog-fish (figure , sheet ) consist of an unpaired ovary (ov.), paired oviducts (o.d.), enlarged at one point to form an oviducal gland (o.d.g.), kidneys (k.), with ureters (ur.) uniting to form a urinary sinus (u.s.) opening into the cloaca by a median urinary papilla separate from the oviducal openings. the eggs contain much yolk, and, like those of the fowl, are very large; like the fowl, too, one of the ovaries is suppressed, and it is the right ovary that alone remains. the two oviducts meet in front of the liver ventral to the oesophagus, and have there a common opening by which the ova are received after being shed into the body cavity. the eggs receive an oblong horny case in the oviduct; in the figure such a case is figured as distending the duct at e. the testes of the male (t. in figure ) are partially confluent in the middle line. they communicate through vasa efferentia (v.e.) with the modified anterior part of the kidney, the epididymis (ep.), from which the vas deferens (v.) runs to the median uro-genital sinus (u.g.s.), into which the ureters (ur.) also open. the silvery peritoneum (lining of the body cavity) covers over the reddish kidneys, and hides them in dissection. section . figure , sheet , is a generalized diagram of the uro-genital organs in the vertebrata; m.l. is the middle line of the body, g. is the genital organ, pr. is the pronephros, or fore kidney, a structure which is never developed in the dog-fish, but which has functional importance in the tadpole and cod, and appears as a transitory rudiment in the chick. a duct, which is often spoken of as the pronephric duct (p.d.), and which we have figured under that name, is always developed. anteriorly it opens into the body cavity. it is also called the mullerian duct, and in the great majority of vertebrata it becomes the oviduct, uniting with its fellow, in the case of the dog-fish, ventral to the oesophagus. in the male it usually disappears; the uterus masculinus of the rabbit is still very generally regarded as a vestige of it. kolliker has shown, however, that this interpretation is improbable. ms. is the mesonephros, some or all of which becomes the epididymis in the male of types possessing that organ, and is connected with g. by the vasa efferentia. mt., the metanephros, is, in -actual fact- [the frog], indistinguishably continuous with ms., and is the functional kidney, its duct (metanephric duct) being either undifferentiated from the mesonephric (as is the case with the frog) or largely split off from it, as in the dog-fish, to form the ureter. section . the correspondence of the male organs of the dog-fish with those of the rabbit, will be more evident if the student imagine-- (a) the testes, vasa efferentia, and epididymis of each side to shift posteriorly until they reach a position on either side of the cloaca; and (b) the uro-genital apertures, instead of meeting dorsally and posteriorly to the anus, to shift round that opening and meet anteriorly and ventrically to it. section . this completes our survey of this type. except where we have specified differences, the general plan of its anatomy follows the lines of the other vertebrate types described. _questions on the dog-fish_ . describe the alimentary canal of the dog-fish, and compare it with that of the rabbit in detail. . compare the coelom of the dog-fish and rabbit. . draw diagrams to illustrate the course of the circulation in the dog-fish. . (a) describe fully the heart of a dog-fish. (b) compare it with that of a rabbit. . give an account of the respiratory apparatus of the dog-fish. . draw diagrams of a dog-fish vertebra, and compare the centrum with that of a rabbit. . compare the vertebral column of the dog-fish and rabbit. . draw diagrams of the limbs and limb-girdles of the dog-fish. compare the pectoral with the pelvic fin. . draw diagrams of (a) the male and (b) the female urogenital organs of the dog-fish. (c) compare them carefully with those of the rabbit. . compare the circulation in the kidney of dog-fish and rabbit. . give an account of the cranio-facial apparatus of the dog-fish. state clearly what representation of this occurs in the frog and in the rabbit. . give drawing (a) from above, (b) from the side, of the dog-fish brain. . state the origin and the distribution of the fifth, seventh, ninth, and tenth cranial nerves in the dog-fish. . compare, one by one, the cranial nerves of the dog-fish with those of any higher vertebrate, as regards their origin and their distribution. . describe the auditory organ of the dog-fish. what parts are added to this in the higher type? . draw the cloaca (a) of a male, (b) a female dog-fish. . (practical.) demonstrate in a dog-fish the pathetic nerve, the opening between pericardium and coelom. the abdominal pores, and the ureter. -amphioxus_ . _anatomy_ section . we find in amphioxus the essential vertebrate features reduced to their simplest expression and, in addition, somewhat distorted. there are wide differences from that vertebrate plan with which the reader may now be considered familiar. there are no limbs. there is an unbroken fin along the median dorsal line and coming round along the ventral middle line for about half the animal's length. but two lowly vertebrates, the hag-fish and lamprey, have no limbs and a continuous fin. there is, as we shall see more clearly, a structure, the respiratory atrium, not apparently represented in the true vertebrate types, at least in their adult stages. there is no distinct heart, only a debateable brain, quite without the typical division into three primary vesicles, no skull, no structures whatever of cartilage or bone, no genital ducts, no kidneys at all resembling those of the vertebrata, no pancreas, no spleen; apparently no sympathetic chain, no paired sense organs, eyes, ears, or nasal sacs, in all of which points we have striking differences from all true vertebrata; and such a characteristic vertebrate peculiarity as the pineal gland we can only say is represented very doubtfully by the eye spot. section . the vertebral column is devoid of vertebrae; it is throughout life a rod of gelatinous tissue, the notochord (figure , n.c.), surrounded by a cellular sheath. such a rod is precursor to the vertebral column in the true vertebrates, but, except in such lowly forms as the lamprey, is usually replaced, partially (e.g., dog-fish) or wholly (as in the rabbit) by at first cartilaginous vertebrae whose bodies are derived from its sheath. further, while in all true vertebrata the notochord of the developing young reaches anteriorly at most to the mid-brain, and is there at its termination enclosed by the middle portion of the skull, in amphioxus it reaches far in front of the anterior extremity of the nervous system, to the end of the animal's body.* on this account the following classification is sometimes made of those animals which have a notochord:-- -chordata_ (= vertebrata, as used by lankester). . having the notochord reaching in front of the brain. cephalochorda = amphioxus. . having the notochord reaching anteriorly to the mid-brain, a brain of three primary vesicles and a skull. craniata = all "true vertebrata": fishes, amphibia, reptiles, birds, and mammals (vertebrata of balfour). . having the notochord confined to the tail. urochorda = the ascidians, or sea-squirts, certain forms of life only recently recognised as relatives of the vertebrata. * the anterior end of the notochord in the developing rabbit or dog lies where the middle of the basisphenoid bone is destined to be. section . figure , sheet , shows the general anatomy of amphioxus. we recognise four important points of resemblance to the earlier phases of the higher and the permanent structure of the lower members of the vertebrata, and it is these that justify the inclusion of amphioxus in this volume. in the first place there is the-- -notochord_. in the next, just above it (at s.c.) we find-- -a dorsal tubular nervous axis_, the spinal cord. thirdly, the pharynx (ph.) is perforated by-- -respiratory slits_, though these, instead of being straight slashes, are modified from a u-shape [slant very much forward and are much more numerous than in any true vertebrate.]. -and-, fourthly, there is, as we shall see, a-- -vertebrate type of circulation_. [and finally the body-wall muscles are divided into--] [-myomers_.] section . the alimentary canal of amphioxus commences with an "oral cavity," not represented in our vertebrata, surrounded by a number of cirri, or tentacles, supported by a horny substance which seems to be chitin, a common skeletal material among invertebrates. a velum (v.) forms a curtain, perforated by the mouth and by two smaller hyoidean apertures, between the oral cavity and the pharynx (ph.). "pharyux" is here used in a wider sense than in the true vertebrata; it reaches back close to the liver, and is therefore equivalent to pharynx + oesophagus + a portion or all of the stomach. the [so-called] hyoidean apertures are not equivalent to the similarly-named parts of the vertebrata. behind the pharynx the intestine (int.) runs straight out to the anus (an.), which opens not in the middle line, as one might expect, but in the left side! the liver lies usually on the creature's right, and instead of being a compact gland, is simply bag-like. section . the circulation is peculiarly reduced (figure ). the cardiac aorta (c.ao.) lies along the ventral side of the pharynx, and sends branches up along the complete bars between the gill slits. there is no -distinct- heart, but the whole of the cardiac aorta is contractile, and at the bases of the aortic arches that run up the bars there are contractile dilatations that assist in the propulsion of the blood. dorsal to the pharynx, as in fishes, there is a pair of dorsal aorta (d.ao.) that unite above the liver (compare the frog, for instance), and thence run backward as a median dorsal aorta (d.ao.'). a portal vein (p.v.) bring blood back from the intestine (and apparently from the whole posterior portion of the animal) to the liver. thence hepatic veins (hep.) take it to the cardiac aorta. {lines from first edition only.} -when we remember that in the embryonic vertebrate the heart is at first a straight tube, this circulation appears even more strikingly vertebrate in its character than before.- section . the coelom, or body cavity, of amphioxus lies, of course, as in the vertebrata, between the intestinal wall and the body walls, and, just as in the vertebrata, it is largely reduced where gill slits occur. but matters are rather complicated by the presence of an atrial cavity round the pharynx, which is not certainly represented in the vertebrata, and which the student is at first apt to call the body cavity, although it is entirely distinct and different from that space. the mutual relation of the two will become apparent after a study of figures , , (sheet ). figure gives diagrammatically a section of a very young stage of amphioxus; p is the pharynx portion of the alimentary canal, coe. is the coelom surrounding it at this stage here as elsewhere; mt.c. are certain lymph spaces, the metapleural canals, between which a small invagination (i.e., a pushing-in), at., of the outer epidermis occurs; n.c. is the notochord, and s.c. the spinal cord. the gill slits, by which p. communicates with the exterior, are not shown. next figure shows the invagination (at.) pushing its way in, and cut off from the exterior by a meeting of the body wall below. note that at. is a portion of the animal's exterior thus embraced by its body, and that its lining is therefore of the same material as the external integument. in figure , at. is developing upward, so that the true body hangs into it. now imagine the gill slits perforated, as shown by the double-headed arrow in figure . figure , on sheet , is a less diagrammatic representation of a cross-section of the pharyngeal region (vide figure , sheet ). the student should compare figure , sheet , and figure , sheet . the atrium and metapleural canals are easily recognised in both. in figure the coelom is much cut up by the gill slits, and we have remaining of it (a) the dorsal coelomic canals (d.c.c.) and (b) the branchial canals (br.c.) in the bars between the slits. the atrial cavity remains open to the exterior at one point, the atrial pore (at.p.). section . the method of examining cross-sections is an extremely convenient one in the study of such a type as amphioxus. the student should very carefully go over and copy the six sections on sheet , comparing figure as he goes. he should do this before reading what follows. one little matter must be borne in mind. these figures are merely intended to convey the great structural ideas, and they are considerably simplified; they must not be regarded as a substitute for the examination of microscopic sections. [he will notice a number of rounded masses from the body wall. the] -for instance, the body-wall- muscles of amphioxus are arranged in bundles bent sharply in an arrow shape, the point forward. -a number of these bundles are cut in any one section, and so the even shading of our diagrams, if they professed to be anything more than diagrams, should be broken up into masses.- these -bundles, we may mention-, are called myomeres, and they are indicated in figure by lines pointing acutely forward. [several are consequently cut in any transverse section (sheet ), and these are the rounded masses he sees.] similar myomeres, similarly situated, are found in fish, behind the head, and, less obviously, they occur with diminishing importance as the scale of the vertebrata is ascended. section . if we compare the nervous system of amphioxus with that of any vertebrate, we find at once a number of striking differences. in the first place, the skeletal covering of it, the cranium and the neural arches of vertebrae, are represented only by a greatly simplified connective tissue. in the next, a simple and slight anterior dilatation alone represents the brain. a patch of black pigment anterior to this (e.s.) may or may not be what its name implies an eye-spot. there is a ciliated funnel, c.f. (figure , sheet ), opening on the left side, which has been assumed to be olfactory in its functions, and in the mouth chamber a ciliated pit (c.p.), which may, or may not, be an organ of taste. the ventral fissure of the spinal cord is absent. the dorsal nerves are without ganglia, and do not come off in pairs, but alternately, one to the left, then one to the right, one to the left, one to the right, and so on. the ventral nerves are very short, more numerous than the dorsal, and never unite with these latter to form mixed nerves. the student will observe that here, just as in the case of the ciliated funnel and anus, the amphioxus is not strictly symmetrical, but twisted, as it were, and so departs from the general rule of at least external bilateral symmetry obtaining among the vertebrates. it habitually lies on one side in the mud of the sea bottom, and it is probable that this external asymmetry is due to this habit, so that too much classificatory importance must not be attached to it. the soles and other related fish, for instance, are twisted and asymmetrical, through a similar specific habit, to such an extent that both eyes lie on one side of the animal. section . no kidney on the vertebrate pattern is found, but the following structures have, among others, been suggested as renal organs:-- (a) certain canals, the brown tubes of lankester (b.t.l., figure , sheet ), a pair of pigmented tubes opening into the atrium at the hind end of the pharynx, lying forward along by the dorsal coelomic canals, and having an internal opening also. (b) certain tubuli described by weiss as situated in a series along the upper corners of the atrial cavity, and communicating, after the fashion, of the "nephridia" of the earthworm, with the coelom and with the exterior (or, rather, with that portion of the animal's exterior enclosed in by the atrial wall; compare section ). (c) the general epithelial lining of the atrium. the reproductive organs (figure , sheet , g.) are masses of cells situated in an isolated part of the coelom in the atrial folds, and, having no ducts, their contents must escape into the atrium by rupture of the body-wall. thence they escape either by gill-slits, pharynx and mouth, or, more generally, through the atrial pore. the animals, like all the vertebrata, are dioecious, i.e., male or female. section . the endostyle (end.), in figures and , is a ciliated path or groove on the under side of the pharynx, which is generally supposed to represent the thyroid gland of vertebrates. the vertebrate thyroid, early in development, is certainly an open and long narrow groove in the ventral side of the pharynx. the hyper-pharyngeal groove (h.p.) has been in the past compared to the pituitary body, but there is little doubt now that this structure is represented by the ciliated pit. section . the student is advised to revise this chapter before proceeding, and to schedule carefully the anatomical features under the headings of ( .) distinctly vertebrate characters, ( .) characters contrasting with the normal vertebrate structure, ( .) facts of doubtful import, with the suggestions given in the text written against them. . _the development of amphioxus_ section . the development of amphioxus, studied completely, is at once one of the most alluring and difficult tasks in the way of the zoologist; but certain of its earlier and most obvious fasts may very conveniently be taken into consideration now. section . the phenomena of the extrusion of polar bodies and fertilization are treated of later, and will, therefore, not be considered now. we will start our description with an egg-cell, which has escaped, of course, since there are no genital ducts, by rupture of the parent, has been fertilized by the male element, and is about to develop into a young amphioxus. it is simply a single cell, with some power of amoeboid motion, a single nucleus and nucleolus; and in amphioxus its protoplasm is clear and transparent. frequently ova are loaded with granules of food store (yolk), which enable the young animal to go far with its development before it is hatched and has to begin fending for itself. such an ovum as that of our present type, however being devoid of such yolk (alecithal = without yolk), necessitates a very early start in life, and, for reasons too complicated to state fully here, the development in such a case is considered particularly instructive and primitive by zoologists. section . the first thing to be seen in the developing cell is a deepening circular groove (figure , sheet ), which divides the ovum into two parts. another groove then cuts at right angles to this subdividing the two into four (figure ). another groove, at right angles to both the former, follows, making the four eight (figure ). and so subdivision goes on. the whole process is called segmentation or cleavage. section . at the end of segmentation we get a hollow sphere of small cells, the cells separating from one another centrally and enclosing a cavity as the process proceeds. this is the blastosphere, shown diagrammatically in figure , and of which an internal view, rather truer to the facts of the case as regards shape, is given as figure . the central cavity is the segmentation cavity (s.c.). section . invagination follows (figure ). in this process a portion of the blastosphere wall is the tucked into the rest, as indicated by the arrow, so that a two-layered sack is formed. the space ar. is the archenteron, the primordial intestine, and its mouth is called, the blastopore (bp.). the outer layer of this double-walled sac is called the epiblast. for the present we will give the inner lining no special term. the young amphioxus has, at this stage, which is called the gastrula stage, a curious parallelism with such a lowly form as the hydra of our ditches. this latter creature, like the gastrula, consists essentially of two layers of cells, an outer protective and sensory layer, and an inner digestive one; it has a primordial intestine, or archenteron, and its mouth is sometimes regarded as being a blastopore. all animals that have little yolk, and start early in life for themselves, pass through a gastrula stage, substantially the same as this of amphioxus. section . the anus is perforated later near the region occupied at this stage by the blastopore. hence the anterior end of the future amphioxus, the head end, is pointing towards the figure , and the letters ep. are marked on the side which will be dorsal. section . figure i. is a dorsal view of the gastrula at a somewhat later stage, and here indications of distinctly vertebrate relationships already appear. figure ii. is a cross-section, its position, being shown by cross-lines in i. and . note first that the epiblast along the mid-dorsal line is sinking in to form what is called the neural plate (n.p.), and simultaneously on either side of it rise the neural folds (n.f.). now, at figure , a slightly later stage is represented, and at i. the inturned part is separated from the general external epiblast as the spinal cord. the remainder of the epiblast constitutes the epidermis. section . reverting to figure ii., along the dorsal side of the archenteron a thickening of its wall appears, and is gradually pinched off from it to form a cellular rod, lying along under the nervous axis and above the intestine. this is the notochord (compare figures and ). section . finally, we note two series of buds of cells, one on either side of the archenteron in figure ii. in these buds have become hollow vesicles, growing out from it, the coelomic pouches. they are further developed in ; and in ii., which is a diagrammatic figure, they are indicated by dotted lines. they finally appear to (? entirely) obliterate the segmentation cavity-- they certainly do so throughout the body; and their cavities are in time cut off from the mesenteron, by the gradual constriction of their openings. in this way the coelom (body cavity) arises as a series of hollow "archenteric" outgrowths, and ms. becomes the alimentary canal. mt.c., the metapleural canals, probably arise subsequently to, and independently of, the general coelomic space, by a splitting in the body-wall substance. section . hence, in considering the structure of amphioxus, we have three series of cells from which its tissues are developed:-- . the epiblast. . walls of the coelomic pouches, which form (a) an inner lining to the epiblast, (b) an outer coating to the hypoblast, and (c) the mesentery (m.), by which the intestine is supported. this is the mesoblast. . the lining of the mesenteron, or hypoblast. from the epiblast the epidermis (not the dermis), the nervous system (including the nerves), and the sensory part of all sense organs are derived. from the mesoblast the muscles, the dermis genital and excretory organs, circulatory fluid and apparatus, any skeletal structures; and all connective tissue are derived. the mass of the body is thus evidently made of mesoblast. the hypoblast is the lining of the intestine and of the glands which open into it; and the material of the notochord is also regarded, as hypoblast. section . figure ii. shows all the essential points of the structure of amphioxus. epiblast is indicated by a line of dashes, mesoblast by dots, and hypoblast, dark or black. the true mouth is formed late by a tucking-in of epiblast, the stomodaeum (s.d.), which meets and fuses with the hypoblast, and is then perforated. the position of this mouth is at the velum. the formation of the atrium has been described. the metapleural folds run forward in front of the velum, as the epipleurs (ep. in sections and ), and form an oral hood (b.c.), around which the tentacles appear, and which is evidently not equivalent to the vertebrate mouth cavity, but in front of and outside it. the anus is formed by a tucking in, the proctodaeum, similar to the stomodaeum. section . the formation of the respiratory slits is complicated, and difficult to describe, but, since investigators have still to render its meaning apparent, it need not detain the elementary student.* * see balfour's embryology, volume , and quarterly journal of microscopical science march, . _questions on amphioxus_ . draw diagrams, with the parts named, of the alimentary canal of (a) amphioxus, (b) any craniate; (c) indicate very shortly the principal structural differences between the two. . describe, with a diagram, the circulation of amphioxus. compare it with that of the craniata. . draw from memory transverse sections, of amphioxus (a) in the oral region, (b) through the pharynx, (c) just anterior, and (d) just posterior to atrial pore. . describe fully the coelom of amphioxus, and compare it with that of the frog in regard to (a) development, (b) its relation to other organs in the adult. . compare the atrial cavity and coelom of amphioxus. to what series of cavities in the frog are the metapleural canals to be compared? . describe the notochord of amphioxus, and point out its differences from the vertebrate notochord. . describe, with diagrams, the nervous system of amphioxus, and compare its nervous axis, in detail, with that of a vertebrate. . compare the genital organs of amphioxus with those of a higher vertebrate. . what structures have been regarded, as renal organs in amphioxus? . what is a gastrula? with what lower type has the gastrula been compared? discuss the comparison. -development_ _the development of the frog_ section . we have now to consider how the body of the frog is built up out of the egg cell, but previously to doing so we must revert to the reproductive organs of our type. section . in the testes of the male is found an intricate network of tubuli, the lining of which is, of course, an epithelium. the cells of this epithelium have their internal borders differentiated into spermatozoa, which, at a subsequent stage, are liberated. a spermatozoon (figure , sheet , sp.) is a rod-shaped cell containing a nucleus; in fact, consisting chiefly of nucleus, with a tail, the flagellum, which is vibratile, and forces the spermatozoon, forward by its lashing. the spermatozoa float in a fluid which is the joint product of the testes, anterior part of the kidney, and perhaps the prostate glands. section . in the ovary, the ova are formed, and grow to a considerable size. they are nucleated cells, the nucleus going by the special name of the germinal vesicle and the nucleolus the germinal spot. the ova prey upon the adjacent cells as they develop. the protoplasm of the ovum, except at that part of the surface where the germinal vesicle lies, is packed with a great amount of food material, the yolk granules. this yolk is non-living inert matter. an ovum such as this, in which the protoplasm is concentrated towards one pole, is called telolecithal. section . after the ovum has finished its growth, and elaborated the yolk within itself, a peculiar change occurs in the small area free from yolk-- the animal pole, in which the germinal vesicle lies. this germinal vesicle divides, and one moiety is budded off from the ovum. the ovum has, in fact, undergone cell division into a very large cell containing most of its substance, and a small protoplasmic pimple surrounding half of its nucleus. the disproportion is so great between the two cells, that the phenomenon does not at first suggest the idea of cell division, and it is usually described as the extrusion of the first polar body. there follows a second and similar small cell, behind the first, the second polar body. since the nucleus of the ovum has divided twice, it is evident that the nucleus remaining now in the ovum is a quarter of the original nucleus. very little protoplasm is given off with the polar bodies; they play no further part in development, but simply drop off and disappear. not only in the frog's ovum, but in all vertebrata, two polar bodies are given off in this way before the sexual process occurs. their exact meaning has been widely discussed. it is fairly evident that some material is removed from the nucleus, which would be detrimental to further developments, and the point debated is what is the precise nature of this excreted material. this burning question we can scarcely deal with here. section . but here we may point out that in all cells the function of the nucleus appears to be to determine growth and division. it is the centre of directive energy in the cell. section . fertilization is effected by a spermatozoon meeting with the ovum. it fuses with it, its nucleus becoming the male pro-nucleus. this and the female pro-nucleus, left after the extrusion of the polar cells, move towards each other, and unite to form the first segmentation nucleus. section . the ovum next begins to divide. a furrow cutting deeper and deeper divides it into two; another follows at right angles to this, making the two four, and another equatorial furrow cuts off the animal pole from the yolk or vegetative pole. (see sheet , figures , , and .) and so segmentation (= cleavage) proceeds, and, at last, a hollow sphere, the blastosphere (figure ) is formed, with a segmentation cavity (s.c.). but, because of the presence of the yolk at the vegetative pole of ovum, and of the mechanical resistance it offers to the force of segmentation, the protoplasm there is not nearly so finely divided-- the cells, that is to say, are much larger than at the animal pole. the blastosphere of the frog is like what the blastosphere of amphioxus would be, if the future hypoblast cells were enormously larger through their protoplasm being diluted with yolk. section . the next phase of development has an equally curious resemblance to and difference from what occurs in the case of the ova of animals which do not contain yolk. in such types (e.g., amphioxus) a part of the blastosphere wall is tucked into the rest, and a gastrula formed by this process of invagination. in the frog (figure ) there is a tucking-in, but the part that should lie within the gastrula, the yolk-containing cells, are far larger than the epiblast (ep.) which should, form the outer layer of cells. hence the epiblast can only by continual growth accommodate what it must embrace, and the process of tucking-in is accompanied by one of growth of the epiblast, as shown by the unbarbed arrow, over the yolk. this stage is called the gastrula stage; ar. is the cavity of the gastrula, the archenteron; b.p. is its opening or blastopore. such a gastrula, formed mainly by overgrowth of the epiblast, is called an epibolic gastrula, as distinguished from the invaginate gastrula of amphioxus. the difference is evidently entirely due to the presence of yolk, and the consequent modification of invagination in the former case. section . comparing the two gastrulas, it is not difficult to see that if we imagine the ventral wall of the archenteron of amphioxus to have its cells enormously enlarged through the mixing of yolk with their protoplasm, we should have a gastrula essentially like that of the frog. section . figure shows a slightly later ovum than figure , seen from the dorsal side. b.p. is the blastopore. in front of that appears a groove, the neural groove, bordered on either side by a ridge, the neural fold (n.f.). this is seen in section in figure ; s.c. is the neural groove; n.f., as before, the neural fold. the neural folds ultimately bend over and meet above, so that s.c. becomes a canal, and is finally separated from the epiblast to form the spinal cord. below the neural groove a thickening of the dorsal wall of the archenteron appears, and is pinched off to form a longitudinal rod, the precursor of the vertebral column, the notochord, shown in figure (n.c.), as imperfectly pinched off. section . simultaneously, on either side of the notochord appear a series of solid masses of cells, derived mainly by cell division from the cells of the wall of the archenteron, and filling up and obliterating the segmentation cavity. these masses increase in number by the addition of fresh ones behind, during development, and are visible in the dorsal view as brick-like masses, the mesoblastic somites or proto-vertebrae (figure , i., ii., iii.). in figure , these masses are indicated by dotting. in such a primitive type as amphioxus these mesoblastic -somites- [masses] contain a cavity, destined to be the future body cavity, from the first. in the frog, the cavity is not at first apparent; the mesoblast at first seems quite solid, but subsequently what is called the splitting of the mesoblast occurs, and the body cavity (b.c. in figure ) appears. the outer mesoblast, lying immediately under the epiblast, constitutes the substance of the somatopleur, and from it will be formed the dermis, the muscles of the body wall, almost all the cartilage and bone of the skeleton, the substance of the limbs, the kidneys, genital organs, heart and bloodvessels, and, in short, everything between the dermis and the coelom, except the nervous system and nerves, and the notochord. the inner mesoblast, the mass of the splanchnopleur, will form the muscle and connective tissue of the wall of the alimentary canal, and the binding substance of the liver and other glands that open into the canal. section . figure is one which we reproduce, with the necessary changes in each plate of embryological figures given in this book, so that the student will find it a convenient, one for the purpose of comparison. the lines of dashes, in all cases, signify -epiblast- [hypoblast] , the unbroken black line is -hypoblast-, [epiblast] dotting shows mesoblast, and the shaded rod (n.c.) is the notochord. c.s. is the spinal cord; br. , br. , br. are the three primary vesicles which constitute the brain, and which form fore, mid, and hind brain respectively. i. is the intestine and y. the yolk cells that at this early stage constitute its ventral wall. section . figure gives a similar diagram of a later stage, but here the blastopore is closed. an epiblastic tucking-in at st., the stomodaeum pre-figures the mouth; pr., the proctodaeum, is a similar posterior invagination which will become the anus. y., the yolk, is evidently much absorbed. figure is a young tadpole, seen from the side. the still unabsorbed yolk in the ventral wall of the mesentery gives the creature a big belly. its mouth is suctorial at this stage, and behind it is a sucker (s.) by which the larvae attach themselves to floating reeds and wood, as shown in the three black figures below. section . we may now consider the development of the different organs slightly more in detail, though much of this has already been approached. the nervous system, before the closure of the neural groove, has three anterior dilatations, the fore-, mid-, and hind-brains, the first of which gives rise by hollow outgrowths to two pairs of lateral structures, the hemispheres and the optic vesicles. the latter give rise to the retina and optic nerve as described in {development} section . section . the hypoblastic notochord is early embraced by a mesoblastic sheath derived from the protovertebrae. this becomes truly cartilaginous, and at regular intervals is alternately thicker and thinner, compressing the notochord at the thicker parts. hence the notochord has a beaded form within this, at first, continuous cartilaginous sheath. this sheath is soon cut into a series of vertebral bodies by jointings appearing through the points where the cartilage is thickest and the notochord most constricted. hence what remains of the notochord lies within the vertebral bodies in the frog; while in a cartilaginous fish, such as the dog-fish, or in the embryonic rabbit, the lines of separation appear where the notochord is thickest, and it comes to lie between hollow-faced vertebrae. cartilaginous neural arches and spines, formed outside the notochordal sheath, enclose the spinal cord in an arcade. the final phase is ossification. as the tadpole approaches the frog stage the vertebral column in the tail is rapidly absorbed, and its vestiges appear in the adult as the urostyle. section . the development of the skull is entirely dissimilar to that of the vertebral column. it is shown on figures and , sheet ; and in the section devoted to the frog's skull a very complete account of the process is given. the process of ossification is described under the histology of the rabbit. section . the origin of the circulatory and respiratory organs is of especial interest in the frog. in the tadpole we have essentially the necessities and organization of the fish; in the adult frog we have a clear exposition of the structure of pigeon and rabbit. the tadpole has, at first, a straight tubular heart, burrowed out in somatic mesoblast, and produced forward into a truncus arteriosus. from this arise four afferent branchial arteries, running up along the sides of the four branchial arches, and supplying gills. they unite above on either side in paired hyper-branchial arteries, which meet behind dorsal to the liver, to form a median dorsal aorta. internal and external carotid arteries supply the head. these four afferent branchial arches are equivalent to the first four of the five vessels of the dog-fish. at first, the paired gills are three in number, external, and tree-like, covered by epiblast (figures and , e.g.), and not to be compared to fish gills in structure, or in fact -with- [to] any other gills within the limits of the vertebrata. subsequently (hypoblastic) internal gills (int.g., figure ), strictly homologous with the gills of a fish, appear. then a flap of skin outside the hyoid arch grows back to cover over the gills; this is the operculum (op. in figures and , sheet ), and it finally encloses them in a gill chamber, open only by a pore on the left, which resembles in structure and physiological meaning, but differs evidently very widely in development, from the amphioxus atrium. at this time, the lungs are developing as paired hollow outgrowths on the ventral side of the throat (figure , l.). as the limbs develop, and the tail dwindles, the gill chamber is obliterated. the capillary interruptions of the gills on the branchial arches (aortic arches) are also obliterated. the carotid gland occupies the position of the first of these in the adult. the front branchial arch here, as in all higher vertebrata, becomes the carotid arch; the lingual represents the base of a pre-branchial vessel; the second branchial becomes the aortic arch. the fourth loses its connection with the dorsal aorta, and sends a branch to the developing lung, which becomes the pulmonary artery. the third disappears. a somewhat different account to this is still found in some text-books of the fate of this third branchial arch. balfour would appear to have been of opinion that it gave rise to the cutaneous artery, and that the third and fourth vessels coalesced to form the pulmocutaneous, the fourth arch moving forward so as to arise from the base of the third; and most elementary works follow him. this opinion was strengthened by the fact that in the higher types (reptiles, birds, and mammals) no fourth branchial arch was observed, and the apparent third, becomes the pulmonary. but it has since been shown that a transitory third arch appears and disappears in these types. section . the origin of the renal organ and duct has very considerable controversial interest.* in figure , sheet , a diagrammatic cross-section, of an embryo is shown. i. is the intestine, coe. the coelom, s.c. the spinal cord; n.c. the notochord, surrounded by n.s., the notochordal sheath, ao. is the dorsal aorta. in the masses of somatic mesoblast on either side, a longitudinal canal appears, which, in the torpedo, a fish related to the dog-fish, and in the rabbit, and possibly in all other cases, is epiblastic in origin. this is the segmental duct, which persists, apparently, as the wolffian duct (w.d.). ventral to this appears a parallel canal, the mullerian duct (m.d.), which is often described as being split off from the segmental duct, but which is, very probably, an independent structure in the frog. a number of tubuli, at first metamerically arranged, now appear, each opening, on the one hand, into the coelom by a ciliated mouth, the nephrostome (n.s.), and on the other into the segmental duct. these tubuli are the segmental tubes or nephridia. there grows out from the aorta, towards each, a bunch, of bloodvessels, the glomerulus (compare section , rabbit). these tubuli ultimately become, in part, the renal tubuli, so that the primitive kidney stretches, at first, along the length of the body cavity from the region, of the gill-slits backward. the anterior part of the kidney, called the pronephros, disappears in the later larval stages. internal to the kidney on either side there has appeared a longitudinal ridge, the genital ridge (g.r.), which gives rise to testes or ovary, as the case may be. * in the discussion whether the vertebrata have arisen from some ancestral type, like the earthworm, metamerically segmented, and of fairly high organization, or from a much lower form, possibly even from a coelenterate. such a discussion is entirely outside the scope of the book, though its mention is necessary to explain the importance given to these organs. section . the student should now compare the figures on sheet . in the male, tubular connections are established between the testes and the middle part of the primitive kidney (mesonephros). these connections are the vasa efferentia (v.e.), and the mesonephros is now equivalent to the epididymis of the rabbit. the wolffian duct is the urogenital duct of the adult, and the mullerian duct is entirely absorbed, or remains, more or less, in exceptional cases. in the female, the mullerian duct increases greatly in length-- so that at sexual maturity its white coils appear thicker and longer than the intestine-- and becomes the oviduct; the wolffian duct is the ureter, and the mesonephros is not perverted in function from its primary renal duty. section . tabulating these facts-- in the adult male: pronephros disappears. the mullerian duct (? = pronephric duct) disappears. mesonephros = epididymis; its duct, the urogenital. metanephros and duct, not clearly marked off from mesonephros. (compare dog-fish, section .) in the adult female: pronephros disappears. the mullerian duct, the oviduct. mesonephros and metanephros, the kidney, and their unseparated ducts, the ureters. section . hermaphrodism (i.e., cases of common sex) is occasionally found among frogs; the testis produces ova in places, and the mullerian duct is retained and functional. the ciliated nephrostomata remain open to a late stage of development in the frog, and in many amphibia throughout life. their connection with the renal tubuli is, however, lost. section . the alimentary canal is, at first, a straight tube. its disproportionate increase in length throws it into a spiral in the tadpole (int. figure ), and accounts for its coiling in the frog. the liver and other digestive glands are first formed, like the lungs, as hollow outgrowths, and their lining is therefore hypoblastic. the greatest relative length of intestine is found in the tadpole, which, being a purely vegetable feeder, must needs effect the maximum amount of preparatory change in its food. _the development of the fowl_ section . the frog has an ovum with a moderate allowance of yolk, but the quantity is only sufficient to start the little animal a part of its way towards the adult state. the fowl, on the contrary, has an enormous ovum, gorged excessively, with yolk, and as a consequence the chick is almost perfected when it is hatched. the so-called yolk, the yellow of an egg, is the ovum proper; around that is a coating of white albumen, in a shell membrane and a shell. at either end of the yolk (figure , y.) twisted strands of albuminous matter, the chalazae (ch.) keep the yolk in place. the animal pole is a small grey protoplasmic area, the germinal area (g.a.), on the yolk. section . we pointed out that the presence of the yolk in the frog's egg led to a difference in the size of the cells at the animal and vegetable poles. the late f.m. balfour, borrowing a mathematical technicality, suggested that the rate of segmentation in any part of an ovum varies inversely with the amount of yolk. in the fowl's egg, except just at the germinal area, the active protoplasm is at a minimum, the inert yolk at a maximum; the ratio of yolk to protoplasm is practically infinity, and the yolk therefore does not segment at all. the yolk has diluted the active protoplasm so much as to render its influence inappreciable. the germinal area segments, and lies upon the yolk which has defeated the efforts of its small mingling of protoplasm to divide. such a type of segmentation in which only part of the ovum segments is called meroblastic. if we compare this with the typical blastosphere of the lower type, we see that it is, as it were, flattened out on the yolk. this stage is shown in section in the lower figure of figure . b.d., the blastoderm, is from this point of view, a part of the ripped and flattened blastosphere, spread out on the yolk; s.c. is the segmentation cavity, and y. the yolk. section . there is no open invagination of an archenteron in the fowl, as in the frog--, the gastrula, like the blastosphere, stage is also masked. but, in the hinder region of the germinal area, a thick mass of cells, grows inward and forward, and, appearing in the dorsal view of the egg as a white streak, is called the primitive streak (p.s.). by a comparison of the figures of frog and fowl the student will easily perceive the complete correspondence of the position of this with the blastopore of the frog. the relation of the two will be easily understood if we compare the fowl's archenteron to a glove-finger under pressure-- its cavity is obliterated-- and the frog's to the glove-finger blown out. the tension of the protoplasm, straining over the enormous yolk, answers to the pressure. the gastrula in the fowl is solid. the primitive streak is, in fact, the scar of a closed blastopore. as we should expect from this view of its homology, at the primitive streak, the three embryonic layers are continuous and indistinguishable (figure ). elsewhere in the blastoderm they are distinctly separate. just as the yolk cells of the frog form the ventral wall of the intestine, so nuclei appear along the upper side of the yolk of the fowl, where some protoplasm still exists, and give rise to the ventral hypoblastic cells. by conceiving a gradually increasing amount of yolk in the hypoblastic cells in the ventral side of the archenteron, the substantial identity of the gastrula stage in the three types, which at first appear so strikingly different, will be perceived. carry figures and of the frog one step further by increasing the size of the shaded yolk and leaving it unsegmented, and instead of ar. in show a solid mass of cells, and the condition of things in the fowl would at once be rendered. section . figure a of the fowl will conveniently serve for comparison with figure of the frog. the inturning of the medullary groove is entirely similar in the two cases. the mesoblast appears as solid mesoblastic somites. in the section above figure this layer is shown as having split into somatopleur (so.) and splanchnopleur (spch.). figure answers to figure of the frog, and figure is a later stage, in which the medullary groove is beginning to close at its middle part. the clear club-shaped area around the embryo (a.p.) is the area pellucida; the larger area without this is the area opaca (a.o.), in which the first bloodvessels arise by a running together and a specialization of cells. the entire germinal area grows steadily at its edges to creep over and enclose the yolk. section . so far, the essential differences between the development of fowl and frog, the meroblastic segmentation, absence of a typical gastrula, and the primitive streak, seem comprehensible on the theory that such differences are due to the presence of an enormous amount of yolk. another difference that appears later is that, while the tadpole has an efficient pronephros, the fowl, which has no larval (free imperfect) stages in its life history, has the merest indication of such a structure. section . another striking contrast, due to, or connected with, this plethora of yolk, is the differentiation of a yolk sac (= umbilical vesicle) and the development of two new structures, the amnion and allantois, in the fowl. if the student will compare figure of the frog, he will see that the developing tadpole encloses in its abdomen all the yolk provided for it. this is a physical impossibility in the fowl. in the fowl (figure , sheet ) the enormous yolk (y.) lies outside of the embryo, and, as the cells of the germinal area grow slowly over it, umbilical bloodvessels are developed to absorb and carry the material to the embryo. in the case of an embryo sinking in upon, as it absorbs, this mass of nutritive material, a necessity for some respiratory structure is evident. from the hinder end of the fowl's intestine, in a position corresponding to the so-called, urinary bladder of the frog, a solid outgrowth, the allantois, which speedily becomes hollow, appears. early stages are shown in figures and , sheet (al.); while the same thing is shown more diagrammatically on sheet , figure (all.). this becomes at last a great hollow sac, which is applied closely to the porous shell, and the extent of which will be appreciated by looking at figure , sheet , where the allantois is shaded. allantoic bloodvessels ramify thickly over its walls, and aeration occurs through the permeable shell. section . the nature of the amnion will be understood by following figures b, , and on sheet . the three embryonic layers are indicated by broken lines, dots, and black lines, just as they are in the frog diagrams. not only is the embryo slowly pinched off from the yolk sac (y.s.), but, as the yolk is absorbed beneath it, and it grows in size, it sinks into the space thus made, the extra-embryonic somatopleur and epiblast rise up round it as two folds, which are seen closing in , and closed in , over the dorsal side of the young chick. in this way a cavity, a., lined by epiblast, and called the amniotic cavity, is formed. dorsal to this, in , comes a space lined by somatic mesoblast, and continuous with p.p., the pleuro-peritoneal cavity, or body cavity of the embryo. outside this, again, is a layer, of somatopleur internally and epiblast externally, the false amnion (f.a.), which is continuous with the serous membrane (s.m.) enclosing the rest of the egg. the student should, carefully copy these diagrams, with coloured pencils or inks for the different layers, and should compare them with the more realistic renderings of figures , , and , sheet . section . the heart in the fowl appears first as a pair of vessels, which unite to form a straight trunk in the median line, as the flattened-out embryo closes in from the yolk. the way in which this straight trunk is thrown, first of all, into the s shape of the fish heart, and then gradually assumes the adult form, is indicated roughly by figure . in one respect the development of the heart does not follow the lines one would expect. since, between the fish and the higher form comes the condition of such an animal as the frog, in which the auricles are divided, while there is only one ventricle, we might expect a stage in which the developing chick's heart would have one ventricle, and a septum between the auricles. but, as a matter of fact, the ventricles in fowl and rabbit are separated first, and the separation of the auricles follows, and is barely complete at birth. section . two vitelline veins from the yolk sac (v.v.) flow into the heart from behind, as shown in figure . a later more complete and more diagrammatic figure of the circulation is seen in figure . at first there are two anterior cardinal (a.c.), and two posterior cardinal veins (p.c.) uniting to form cuvierian sinuses (c.s.) that open into the heart just as in the dog-fish. but later the inferior cava is developed and extends backward, the posterior cardinals atrophy, the cuvierian sinuses become the superior cavae, and the anterior cardinals the internal jugular veins. the vitelline veins (v.v.) flow, at first, uninterruptedly through the liver to the inferior cava, but, as development proceeds, a capillary system is established in the liver, and the through communication, the ductus venosus, is reduced-- at last-- completely. bearing in mind that the yolk is outside the body in the fowl and inside it in the frog, the vitelline veins of the former have a considerable resemblance in position, and in their relation to the portal vein, to a portion of the single anterior abdominal vein. blood is taken out to the allantois, however, by the arteries of the latter type. section . five aortic arches are generally stated to appear altogether in the fowl, but not simultaneously. the first two, the mandibular and the hyoid vascular arches, early disappear, and are not comparable to any in the frog. the third is the first branchial arch, and, like the corresponding arch in the frog, forms the carotid artery; the second branchial is the aortic arch; and what has hitherto been regarded as the third (the fifth arch, i.e.) the pulmonary artery. a transitory arch, it is now known, however, appears between the second branchial and the last, and it is therefore the fourth branchial arch which is the pulmonary, just as it is in the frog. section . blood, it may be mentioned, first appears in the area vasculosa, the outer portion of the area opaca. embryonic cells send out processes, and so become multipolar; the processes of adjacent cells coalesce. the nucleus divides, and empty spaces appear in the substance of each of the cells. in this way, the cavities of the smaller vessels and capillaries are formed, and the products of the internal divisions of the cells become the corpuscles within the vessels. the red blood corpuscles of the rabbit, it may be added, are nucleated for a considerable portion of embryonic life. larger vessels and the heart are burrowed, as it were, out of masses of mesoblast cells. the course of the blood in the embryo is by the veins to the right auricle, thence through the imperfection of the auricular septum already alluded to, into the left auricle. then the left ventricle, aortic arches (for the future pulmonary artery is in communication by a part presently blocked, the ductus arterious, with the systemic aorta), arteries, capillaries, veins. the liver capillary system and the pulmonary system only become inserted upon the circulation at a comparatively late stage. section . with the exception of the reduction of the pronephros, what has been said of the development of the frog's nervous system, renal and reproductive organs, and skeleton, applies sufficiently to the fowl for our present purposes. the entire separation of wolffian and mullerian ducts from the very beginning of development is here beyond all question (vide section ). but the notochord in the fowl is not so distinctly connected with the hypoblast, and so distinct from the mesoblast, as it is in the lower type, and no gills, internal or external, are ever developed. the gill slits occur with a modification due to the slitting and flattening out of the embryo, already insisted upon; for, whereas in the tadpole they may be described as perforations, in the fowl they appear as four notches between ingrowing processes that are endeavouring to meet in the middle line. _the development of the rabbit_ section . the early development of the rabbit is apt to puzzle students a little at first. we have an ovum practically free from yolk (alecithal), and, therefore, we find it dividing completely and almost equally. we naturally assume, from what we have learnt, that the next stages will be the formation of a hollow blastosphere, invagination, a gastrula forming mesoblast by hollow outgrowths from the archenteron, and so on. there is no yolk here to substitute epiboly (section ) for invagination, nor to obliterate the archenteron and the blastopore through its pressure. yet none of these things we have anticipated occur! we find solid mesoblastic somites, we find primitive streak, allantois and amnion, features we have just been explaining as the consequence of an excess of yolk in the egg. we even find a yolk sac with no yolk in it. section . a solid mass of cells is formed at the beginning, called a morula, figure . in this we are able to distinguish rather smaller outer layer cells (o.l.c.), and rather larger inner layer cells (i.l.c.), but these cells, in their later development, do not answer at all to the two primitive layers of the gastrula, and the name of van beneden's blastopore (v.b.b.), for a point where the outer layer of cells is incomplete over the inner, only commemorates the authorship of a misnomer. the uniformity, or agreement, in the development of our other vertebrate types is apparently departed from here. {illustration: development section .} section . as the egg develops, however, we are astonished to find an increasing resemblance to that of the fowl. a split occurs at one point between outer layer and inner layer cells, and the space resulting (y in figure ) is filled by an increasing amount of fluid, and rapidly enlarges, so that presently we have the state of affairs shown in , in which the inner layer cells are gathered together at one point on the surface of the ovum, and constitute the germinal area. if, with hubrecht, we regard the outer layer cells as an egg membrane, there is a curious parallelism between this egg and the fowl's the fluid y representing the yolk; and the inner layer cells the cells of the fowl's germinal area. at any rate, the subsequent development goes far to justify such a view. the inner cells split into epi-, meso-, and hypo-blast, like the blastoderm in the fowl; there is a primitive streak and no blastopore; an amnion arises; the yolk sac, small and full of serous fluid, is cut off just as the enormous yolk of the fowl is cut off; and an allantois arises in the same way. there is no need to give special diagrams-- figures , b, , and of the fowl will do in all respects, except proportion, for the development of the rabbit. the differences are such as we may account for, not on the supposition that the rabbit's ovum never had any yolk, but that an abundant yolk has been withdrawn from it. the nutrition of the embryo by yolk has been superseded by some better method. the supposition that the rabbit is descended from ancestors which, like the birds and reptiles, laid eggs with huge quantities of yolk, meets every circumstance of the case. section . but the allantois and yolk sac of the rabbit, though they correspond in development, differ entirely in function from the similar organs of the fowl. the yolk sac is of the very smallest nutritive value; instead of being the sole source of food, its contents scarcely avail the young rabbit at all as nourishment. its presence in development is difficult to account for except on the supposition, that it was once of far greater importance. at an early stage, the outgrowing allantois, pushing in front of it the serous membrane, is closely applied to the lining of the mother's uterus. the maternal uterus and the embryonic allantois send out finger-like processes into each other which interlock, and the tissue between the abundant bloodvessels in them thins down to such an extent that nutritive material, peptones and carbohydrates, and oxygen also, diffuse freely through it from mother to foetus,* and carbon dioxide, water, and urea from the foetus to the mother. the structure thus formed by the union of the wall of the maternal uterus, allantois, and the intermediate structures is called the placenta. through its intermediation, the young rabbit becomes, as it were, rooted and parasitic on the mother, and utilizes her organs for its own alimentation, respiration, and excretion. it gives off co , h o, and urea, by the placenta, and it receives o and elaborated food material through the same organ. this is the better method that has superseded the yolk. * the embryo. section . in its later development, the general facts already enunciated with regard to the organs of frog and fowl hold, and where frog and fowl are stated to differ, the rabbit follows the fowl. in the circulation the left fourth vascular arch (second branchial) gives rise to the aortic arch; in the right the corresponding arch disappears, except so much of it as remains as the innominate artery. the azygos vein (chapter ) -is a vestige of- [is derived from] the right posterior cardinal sinus. both pulmonary arteries in the rabbit are derived from the left sixth vascular arch (= fourth branchial). compare section . the allantois altogether disappears in the adult fowl; in the adult mammal a portion of its hollow stalk remains as the urinary bladder, and the point where it left the body is marked by the umbilicus or navel. the umbilical arteries become the small hypogastric arteries on either side of the urinary bladder. there is no trace of a pronephros at all in the rabbit. section . we may note here the development of the eye. this is shown in figure , sheet . a hollow cup-shaped vesicle from the brain grows out towards an at first hollow cellular ingrowth from the epidermis. the cavity within the wall of the cup derived from the brain is obliterated, [and the stalk withers,] the cup becomes the retina, and -its stalk- [thence fibres grow back to the brain to form] the optic nerve. the cellular ingrowth is the lens. the remainder of the eye-structures are of mesoblastic origin, except the superficial epithelium of the cornea. the retinal cup is not complete at first along the ventral line, so that the rim of the cup, viewed as in figure , r., is horseshoe shaped. -hence the optic nerve differs from other nerves in being primitively hollow.- in all other sense organs, as, for instance, the olfactory sacs and the ears, the percipient epithelium is derived, from the epiblast directly, and not indirectly through the nervous system. these remarks apply to all vertebrate types. section . the supposition, that the general characters of the rabbit's ovum were stamped upon it as an heritage from a period when the ancestors of the mammals were egg-laying reptiles, is strengthened by the fact that the two lowest and most reptile-like of all the mammalia, the duck-billed platypus and the echidna, have been shown to depart from the distinctive mammalian character, and to lay eggs. and, in further confirmation of this supposition, we find, in tracing the mammals and reptiles back through the geological record, that in the permian and triassic rocks there occur central forms which combine, in a most remarkable way, reptilian and mammalian characteristics. section . in conclusion, we would earnestly recommend the student to see more of embryological fact than what is given him here. it is seeing and thinking, much more than reading, which will enable him to clothe the bare terms and phrases of embryology with coherent knowledge. in howes' atlas of biology there is a much fuller series of figures of the frog's development than can be given here, and they are drawn by an abler hand than mine can pretend to be. there is also an atlas d'embryologie, by mathias duval, that makes the study of the fowl's development entertaining and altogether delightful. such complete series as these are, from the nature of the case, impossible with the rabbit. many students who take up the subject of biology do so only as an accessory to more extended work in other departments of science. to such, practical work in embryology is either altogether impossible, or only possibly to a very limited extent. the time it will consume is much greater, and the intellectual result is likely to be far less than the study of such plates as we have named. _the theory of evolution_ section . we have now considered our types, both from the standpoint of adult anatomy and from embryological data; and we have seen through the vertebrate series a common structure underlying wide diversity in external appearance and detailed anatomy. we have seen a certain intermediateness of structure in the frog, as compared with the rabbit and dog-fish, notably in the skull and skeleton, in the circulation, in the ear, and in the reduced myomeres; and we have seen that the rabbit passes in these respects, and in others, through dog-fish- and frog-like stages in its development, and this alone would be quite sufficient to suggest that the similarities of structure are due to other causes than a primordial adaptation to certain conditions of life. section . it has been suggested by very excellent people that these resemblances are due to some unexplained necessity of adherence to type, as though, the power that they assume created these animals originally, as they are now, coupled creative ability with a plentiful lack of ideas, and so perforce repeated itself with impotent variations. on the other hand, we have the supposition that these are "family likenesses," and the marks of a common ancestry. this is the opinion now accepted by all zoologists of repute. section . it must not be for a moment imagined that it is implied that rabbits are descended from frogs, or frogs from dog-fish, but that these three forms are remote cousins, derived from some ancient and far simpler progenitor. but since both rabbit and frog pass through phases like the adult condition of the dog-fish, it seems probable that the dog-fish has remained more like the primordial form than these two, and similarly, the frog than the rabbit. section . hence we may infer that the mammals were the last of the three groups, of which we have taken types, to appear upon the earth, and that the fishes preceded, the amphibia. workers in an entirely independent province, that of palaeontology, completely endorse this supposition. the first vertebrata to appear in the fossil history of the world are fishes; fish spines and placoid scales (compare dog-fish) appear in the ordovician rocks. in the coal measures come the amphibia; and in the permo-triassic strata, reptile-like mammals. in the devonian rocks, which come between the silurian and the coal measures, we find very plentiful remains of certain fish called the dipnoi, of which group three genera still survive; they display, in numberless features of their anatomy, transitional characters between true fish and amphibia. similarly, in the permian come mammal-like reptiles, that point also downward to the amphibia. we find, therefore, the story told by the ovum written also in the rocks. section . now, when this fact of a common ancestry is considered, it becomes necessary to explain how this gradual change of animal forms may have been brought about. section . two subcontrary propositions hold of the young of any animal. it resembles in many points its parent. it differs in many points from its parent. the general scheme of structure and the greater lines of feature are parental, inherited; there are also novel and unique details that mark the individual. the first fact is the law of inheritance; the second, of variation. section . now the parent or parents, since they live and breed, must be more or less, but sufficiently, adapted to their conditions of living-- more or less fitted to the needs of life. the variation in the young animal will be one of three kinds: it will fit the animal still better to the conditions under which its kind live, or it will be a change for the worse, or it is possible to imagine that the variation-- as in the colour variations of domesticated cats-- will affect its prospects in life very little. in the first case, the probability is that the new animal will get on in life, and breed, and multiply above the average; in the second, it is probable that, in the competition for food and other amenities of life, the disadvantage, whatever it is, under which the animal suffers will shorten its career, and abbreviate the tale of its offspring; while, in the third case, an average career may be expected. hence, disregarding accidents, which may be eliminated from the problem by taking many cases, there is a continual tendency among the members of a species of animals in favour of the proportionate increase of the individuals most completely adapted to the conditions under which the species lives. that is, while the conditions remain unchanged, the animals, considered as one group, are continually more highly perfected to live under those conditions. and under changed conditions the specific form will also change. section . the idea of this process of change may be perhaps rendered more vivid by giving an imaginary concrete instance of its working. in the jungles of india, which preserve a state of things which has existed for immemorial years, we find the tiger, his stripes simulating jungle reeds, his noiseless approach learnt from nature in countless millions of lessons of success and failure, his perfectly powerful claws and execution methods; and, living in the same jungle, and with him as one of the conditions of life, are small deer, alert, swift, light of build, inconspicuous of colour, sharp of hearing, keen-eyed, keen-scented-- because any downward variation from these attributes means swift and certain death. to capture the deer is a condition, of the tiger's life, to escape the tiger a condition of the deer's; and they play a great contest under these conditions, with life as the stake. the most alert deer almost always escape; the least so, perish. section . but conditions may alter. for instance, while most of these deer still live in the jungle with tigers, over a considerable area of their habitat, some change may be at work that thins the jungle, destroys the tigers in it, and brings in, let us say, wolves, as an enemy to the deer, instead of tigers. now, against the wolves, which do not creep, but hunt noisily, and which do not spring suddenly upon prey, but follow by scent, and run it down in packs, keen eyes, sharp ears, acute perceptions, will be far less important than endurance in running. the deer, under the new conditions, will need coarser and more powerful limbs, and a larger chest; it will be an advantage to be rough and big, instead, of frail and inconspicuous, and the ears and eyes need not be so large. the old refinements will mean weakness and death; any variation along the line of size and coarseness will be advantageous. slight and delicate deer will be continually being killed, rougher and stronger deer continually escaping. and so gradually, under the new circumstances, if they are not sufficient to exterminate the species, the finer characteristics will be eliminated, and a new variety of our old jungle deer will arise, and, if the separation and contrast of the conditions is sufficiently great and permanent, we may, at last, in the course of ages, get a new kind of deer specifically different in its limbs, body, sense organs, colour, and instincts, from the deer that live in the jungle. and these latter will, on their side, be still continually more perfected to the jungle life they are leading. section . take a wider range of time and vaster changes of condition than this, and it becomes possible to imagine how the social cattle-- with their united front against an enemy, fierce onslaught, and their general adaptation to prairie life-- have differentiated from the ancestors of the slight and timid deer; how the patient camel, with his storage hump, water storage, and feet padded against hot sand, has been moulded by the necessity of desert life from the same ancestral form. and so we may work back, and link these forms, and other purely vegetarian feeders, with remoter cousins, the ancestral hogs. working in this way, we presently get a glimpse of a possible yet remoter connection of all these hoofed and mainly vegetarian animals, with certain "central types" that carry us across to the omnivorous, and, in some cases, almost entirely vegetarian bears, and to the great and prosperous family of clawed, meat-eaters. and thus we elucidate, at last, a thread of blood relationship between the, at present, strongly contrasted and antagonistic deer and tiger, and passing thence into still wider generalizations, it would be possible to connect the rabbit playing in the sunshine, with the frog in the ditch, the dog-fish in the sea-waters and the lancelet in the sand. for the transition from dog-fish to rabbit differs from the transition from one species of deer to another only in magnitude: it is an affair of vast epochs instead merely of thousands of years. section . it would, however, be beyond the design of this book to carry our demonstration of the credibility of a common ancestry of animals still further back. but we may point out here that it is not a theory, based merely upon one set of facts, but one singularly rich in confirmation. we can construct, on purely anatomical grounds, a theoretical pedigree. now the independent study of embryology suggests exactly the same pedigree, and the entirely independent testimony of palaeontology is precisely in harmony with the already confirmed theory arrived at in this way. section . it is in the demonstration of this wonderful unity in life, only the more confirmed the more exhaustive our analysis becomes, that the educational value and human interest of biology chiefly lies. in the place of disconnected species of animals, arbitrarily created, and a belief in the settled inexplicable, the student finds an enlightening realization of uniform and active causes beneath an apparent diversity. and the world is not made and dead like a cardboard model or a child's toy, but a living equilibrium; and every day and every hour, every living thing is being weighed in the balance and found sufficient or wanting. our little book is the merest beginning in zoology; we have stated one or two groups of facts and made one or two suggestions. the great things of the science of darwin, huxley, wallace, and balfour remain mainly untold. in the book of nature there are written, for instance, the triumphs of survival, the tragedy of death and extinction, the tragi-comedy of degradation and inheritance, the gruesome lesson of parasitism, and the political satire of colonial organisms. zoology is, indeed, a philosophy and a literature to those who can read its symbols. in the contemplation of beauty of form and of mechanical beauty, and in the intellectual delight of tracing and elucidating relationships and criticising appearances, there is also for many a great reward in zoological study. with an increasing knowledge of the facts of the form of life, there gradually appears to the student the realization of an entire unity shaped out by their countless, and often beautiful, diversity. and at last, in the place of the manifoldness of a fair or a marine store, the student of science perceives the infinite variety of one consistent and comprehensive being-- a realization to which no other study leads him at present so surely. to the student who feels inclined to amplify this brief outline of vertebrate anatomy, we may mention the following books: wiedersheim's and parker's vertebrates, huxley's anatomy of the vertebrata, flower's osteology of the mammalia, wallace's distribution, nicholson and lyddeker's palaeontology (volume ), the summaries in rolleston's forms of animal life (where a bibliography will be found), and balfour's embryology. but reading without practical work is a dull and unprofitable method of study. _questions on embryology_ [all these questions were actually set at london university examinations.] {in both editions.} . describe the changes in the egg-cell which precede fertilization; describe the process of fertilization and the formation of the primary cell-layers, as exhibited, in three of the animal types known to you. what is the notochord, and how is it developed in the frog? . describe the early stages in the development of the egg of the fowl as far as the closure of the neural groove. how do you account for the primitive streak? . describe the cleavage and the surface appearances of the egg of the frog and of the rabbit, up to the time when the first gill-slits appear in the embryo. give illustrative diagrams of what you describe. . describe the structure and cleavage of the ovum (a) of the frog, (b) of the fowl, and (c) of the rabbit. (d) explain as far as possible the differences in the cleavage of these three eggs. (e) point out how the embryo is nourished in each case, and (f) describe the constitution of the placenta in the rabbit. . (a) what are the protovertebrae? (b) how does the notochord originate in the frog? (c) how are the vertebrae laid down in the tadpole? (d) describe the vertebral column of the adult frog. (e) in what important respects do the centra of the vertebrae of the frog, the dog-fish, and the rabbit differ from one another? . give an account of the more important features in the development of the frog. . what temporary organs are developed in the embryo frog which are absent from the embryo bird and mammal, and what in the two latter which are absent from the former? . draw diagrams, with the parts named, of the heart and great arteries of the frog, giving descriptions only in so far as is necessary to explain your diagrams; trace the development of these structures in the tadpole; point out particularly in which of the embryonic visceral (branchial) arches the great arteries of the adult run. . trace the history of the post-oral gill-slits and their accompanying cartilaginous bars and vascular arches in the frog, fowl, and rabbit. . give a short account, with illustrative figures, of the mode of formation of the primary germinal layers in amphioxus and in the frog. what explanation can you give of the differences between the two cases? . give a short account, with diagrammatic figures, of the principal changes which occur in the circulatory and respiratory organs during the metamorphosis of the tadpole into the frog. . how do protozoa differ from higher animals (metazoa) as regards (a) structure, (b) reproduction? compare the process of fission in an amoeba with the segmentation of the ovum in amphioxus, pointing out the resemblances and differences between the two cases. -miscellaneous questions_ [most of these questions were actually set at the biological examinations of london university.] {in both editions.} . describe (a) the digestive, (b) the circulatory, (c) the excretory, and (d) the reproductive organs of the amphioxus. . describe the stomach and intestines of the dog-fish and rabbit, and point out in what way their differences are connected with diet. . describe the mechanism of respiration in the adult frog, and contrast it with that of the tadpole. . give an account of the structure of the epidermis and its outgrowths in the frog and the rabbit. . describe the organs of circulation (heart and main arteries and veins) and respiration in the frog in its mature and immature states. . give a brief account of the physiology of respiration. describe fully the means by which respiration is effected in the following animals:-- frog, amphioxus, rabbit, and dog-fish. . describe the minute structure of the blood of the rabbit, frog, and amphioxus. . describe and illustrate by means of sketches the chief points of difference between the skeleton of the rabbit as a typical mammal, and that of the common frog as a typical amphibian. . (a) explain what is meant by the term "central nervous system." (b) describe the tissue elements which enter into its composition. (c) explain, as far as you can, the function of each structure described. (d) how is the central nervous system developed in the frog, and (e) in the rabbit? (f) what conclusions may be drawn from the facts stated as to the origin of the central nervous system in evolution? . give an account of the structure (including histology) and of the functions of the spinal cord and spinal nerves of the frog. . give a description of the minute structure and chemical characters of the following tissues as seen in the frog:-- cartilage, bone, muscle. from which of the primary cell-layers of the embryo are they respectively developed? . what substance is excreted by the renal organ of a frog, and what relation does this substance bear to the general life of the organism? describe the parts by which similar excretion is believed to be effected in amoeba, hydra, earthworm, mussel, and lobster. . describe, with illustrative sketches, the structure of the connective tissue, cartilage, and muscular tissue of a frog. also describe the structure of the muscular tissue of the lobster and snail. . give in account of the more important features in the development of the frog. . describe and compare the structure of the renal organs in a frog and a rabbit. . give an account of the structure of the genito-urinary organs of the frog. compare these organs of the frog with those of the dog-fish and of the rabbit. distinguish in each case the conditions of the two sexes, and describe briefly the microscopic structure and development of the ova and of the spermatozoa. . describe, with diagrams, the arrangement of the urinary and generative organs in the male of (a) the rabbit, (b) the dog-fish, and (c) the frog; (d) point out the most important differences between them. . (a) describe the structure of the ovarian egg of the rabbit, (b) and of the pigeon, (c) and of the frog; (d) from what part of the embryo do they originate? (e) what is the structure and origin of the ovarian follicle in the rabbit, and (f) of the ovarian stroma? (g) what is the "granulosa" and what the "zona pellucida"? . describe the pre-segmentation changes, mode of impregnation, and early stages of development in the ovum of the frog, as far as the closure of the neural canal. . illustrate, with diagrams, from the structure of typical organisms, the principle of repetition of similar parts. -note on making comparisons_ students preparing for examinations are frequently troubled by "comparison" questions. tabulation is often recommended, but we are inclined to favour a rather more flexible plan of marking off differences and resemblances. in tabulation a considerable loss of time is occasioned by writing down the features of both the things compared, and this is a serious consideration for the examinee. we advise him therefore, first, if he possibly can, to draw side by side and in corresponding positions the two things under consideration, and then, going over them in a methodical way, to state simply the difference between each homologous part. we append as examples three test answers actually submitted (with figures) in "correspondence" work:-- . compare the brain of the frog with that of the rabbit. in the frog's fore-brain-- the olfactory lobes are fused in the middle line. there is no corpus callosum, nor is there a middle commissure to the third ventricle. the cerebral hemispheres are not convoluted, and, looked at from the dorsal aspect, do not hide the thalamencephalon and mid-brain. the pineal gland lies in the cranial wall and not deeply between the hemispheres, and its stalk is longer and tilts forward. in the mid-brain-- the optic lobes are two, instead of being corpora quadrigemina, and hollow. in the hind-brain-- the cerebellum is a very small transverse band, and has no lateral parts. the medulla is relatively larger. there are no spinal accessory nor hypoglossal nerves to the brain. . compare the vertebrae of dog-fish, rabbit, and frog. the centra of the dog-fish are -opistho- [amphi]-coelous (i.e., hollow at either end). the centra of the rabbit are flat-faced. the centra of the frog are procoelous (hollow in front). the notochord persists between the centra in the dog-fish and rabbit, within the centra in frog. the centra of the rabbit have epiphyses, absent in the dogfish and frog. the transverse processes of the rabbit typically bear ribs. short ribs occur in the dog-fish, but their homology with those of the rabbit is doubtful. the frog has no ribs. the interneural plates are peculiar to the dog-fish in this comparison. . compare the skull of the dog with that of the frog. the brain case-- of the frog is a cylindrical box, from which the otic capsules project conspicuously on either side. it contains only two ossifications in its cartilaginous substance (the sphen-ethmoid and the ex-occipital), being protected by the membrane bones, the parieto-frontals above and the parasphenoid below. in the mammal it is enormously inflated, and the otic capsules are imbedded in its wall. there are supra- and basi- as well as ex-occipital bones; the para-sphenoid is (? entirely) gone, and its place is taken by the basi- and pre-sphenoids, and the lateral walls contain fresh paired ossifications, the ali- and orbito-sphenoids-- all cartilage bones. the sphenethmoid is perhaps represented in part by the ethmoid. as a result of the inflation of the brain-case, the squamosal, which slopes downward and outward in the frog, and overlies the cartilaginous suspensorium (quadrate cartilage), has become a constituent of the brain-case wall, and slopes downwardly and in. jaw suspension-- the point of attachment of the jaw has shifted outward, and the original suspensorial cartilage (the quadrate) has taken on a new and minor function as the incus of the middle ear-- the squamosal superseding it as the suspensory part. lower jaw-- distinct bones in the frog; one mass in the dog. otic capsule-- position as specified. one centre of ossification in the frog forming pro-otic; several fuse together and form periotic of the dog. there is no bulla and no external ear in the frog. palate-- in the frog the posterior nares open into the front of the mouth. in the dog the maxillae and palatines send plates down and in (the palatine plates) to cut off a nasal passage from the rest of the buccal chamber, and carry the posterior nares back to the pharynx, thus cutting the vomers off from the mouth roof. the pterygoids in the dog are much reduced, and do not reach back to the suspensorium. the frog has no lachrymal bone. -syllabus of practical work_ we would impress upon the student at the outset the importance of some preliminary reading before dissection is undertaken. no one would dream of attempting to explore a deserted city without some previous study of maps and guide-books, but we find again and again students undertaking to explore the complicated anatomy of a vertebrated animal without the slightest, or only the slightest, preparatory reading. this is entirely a mistake. a student should be familiar with the nomenclature of the structures he contemplates examining, he should have some idea of their mutual relations and functions, or his attention will inevitably be diverted by the difficulty of new names and physiological questionings to the neglect of his dissection, and that careful observation of form and mutual position which is the essential object of dissection. on the other hand, it is equally necessary-- perhaps more so-- to warn students against the bookish fallacy, and to assure them of the absolute impossibility of realizing biological facts from reading alone. practical work can alone confirm and complete the knowledge to which the text-book is the guide. in scientific teaching it may sometimes be convenient for the thought to precede the thing, but until the thing has been dealt with the knowledge gained is an unsatisfactory and unstable possession. for such dissection as the subject-matter of this book requires, the following appliances will be needed:-- (a) two or three scalpels of various sizes. (b) scissors, which must taper gradually, have straight blades, and be pointed at the ends, and which must bite right up to the tips (or they are useless). two pairs, small and large, are advisable. (c) forceps, which must hold firmly, and meet truly at the points. (d) two needles set in wooden handles. (e) an ordinary watchmaker's eye-glass is very helpful, but not indispensable. (f) a dissecting dish-- an ordinary pie dish will do-- into which melted paraffin wax has been poured, to the depth of, say, three-quarters of an inch, and allowed to solidify. (this wax may be blackened by mixture with lampblack. if the wax floats up at any time, it can, of course, be remelted. or it may be loaded with lead.) (g) a rough table or board (for the rabbit and dog-fish). (h) blanket pins, and ordinary pins. (i) a pickle or other wide-mouthed jar, and some common, methylated spirit. (j) a microscope, with low power of inch or / inch, and high power / inch or / inch. glass slips and cover glasses, and a bottle of very weak ( per cent.) solution of salt. animals for dissection may be obtained from the recognised dealers, who usually advertise in such scientific periodicals as nature, natural science, and knowledge. sinel (naturalist, jersey) is the most satisfactory dealer in dog-fish in our experience; bolton (malvern) will supply amphioxus through the post; frogs and rabbits may be obtained anywhere. the tame variety of rabbit is quite satisfactory for the purpose of dissection. the following notes may possibly be of some use to the student; they follow the lines of work arranged by the author for the evening classes of the university tutorial college, classes considerably restricted as regards time, when compared with ordinary laboratory workers. most of the sections below occupied about three hours, but for a student working alone they are more likely to take four or five, and even then it is not probable that they will be so satisfactory as if performed under skilled supervision. there are many points extremely difficult to convey verbally which are elucidated at once by actual demonstration upon a specimen. each of these dissections should be repeated, and it is well if a different condition of the type is selected for the repetition-- an old one if the first specimen was immature, a female if the first was a male. -the rabbit_ may be killed by chloroform, or potassium cyanide, or drowned. it may also be readily suffocated with house-hold gas. it should be killed immediately before use, as otherwise the gastric juice attacks the wall of the stomach, and the dissection is, in consequence, rendered extremely disagreeable. a very young rabbit is unsatisfactory as regards the genitalia, but otherwise there is no objection to a little one, and it has this advantage-- that it may be immersed more conveniently under water, in a large pie dish, for purposes of fine dissection. the external features of the animal should be examined: eyelids, whiskers and teeth, toes, anus, perineal space on either side of the same, urogenital opening, and position of the ribs, vertebral column, and limb girdles beneath the skin should be made out. then the animal should be pinned out through the legs, the ventral surface uppermost, the skin opened up along the middle line from pelvic girdle to symphyses of jaw; separated from the body wall below by means of the handle of a scalpel, and turned back; and then the abdominal wall should be cut into and two flaps pinned back to expose its contents. note the xiphisternum. the caecum and colon will be recognised (section ); the stomach, the right and left central, and left lateral lobes of the liver will probably be apparent; and the urinary bladder (especially if distended) in the middle line behind. without any further dissection, but simply by turning the parts over, all the structures of the abdomen in figure , sheet , will be identified. seek especially for and note particularly, the gall bladder, bile duct, and portal vein, pancreatic duct, sacculus rotundus, vermiform appendix, ureters (by pulling urinary bladder forward), genital ducts (looping over ureters), spleen, kidneys, and adrenals. the vena cava inferior is seen dorsally. the genital duct guides the student to the genital gland; if the subject is a male, the testes may be exposed by dissection, or by pulling the vas deferens gently the scrotal sac will be turned inside out, and the testes brought into view. the ovary lies exposed without dissection posterior to the kidney. examine all this carefully, and make small sketches of points of interest-- the duodenal loop and the pyloric end of the stomach, for instance; the meeting of colon, caecum, and sacculus rotundus again; or the urinary bladder and adjacent parts. note the dorsal aorta and vena cava and their connexions behind. (compare figure of circulation.) cut through pelvic girdle, and remove one hind leg, to see bladder and genital ducts better (compare sheet ). wash away any blood that may flow. turn all the intestines over to the animal's right, and see the dorsal aorta and vena cava inferior of the abdomen, the inferior mesenteric artery, and the spermatic (or ovarian) artery (compare, of course, with figure in book). in front, immediately dorsal to the spleen, is a variable quantity of lymphoidal tissue, which must be very carefully cleared to see the superior mesenteric and coeliac arteries. separate spigelian lobe from stomach, and look for vagus nerve descending by oesophagus, solar plexus around the superior mesenteric artery, and thrown up very distinctly by the purple vena cava inferior beneath, and the splanchnic nerve. to see the abdominal sympathetic behind, gently remove the peritoneum that lies on either side of the aorta; blood-vessels will be seen running in between the vertebral bodies, and the sympathetic chain, with its ganglia, made out very distinctly, as it runs across them longitudinally. now cut oesophagus just in front of stomach, and cut the rectum, cut through the mesentery supporting the intestine, and remove and unravel alimentary canal; cut open, wash out, and examine caecum and stomach. bleeding to a considerable extent is inevitable, chiefly from the portal vein. the liver had better remain if the same rabbit is to serve for the second dissection. second dissection.-- skin front of thorax and neck. note subclavian veins running out to fore limbs-- avoid cutting these. cut through ribs and remove front of thorax, to expose its contents; cut up middle line of neck, and clear off small muscle bands, to expose bloodvessels; pick away carefully whatever is left of thymus gland; make out structure of heart and blood-vessels, as described, in chapter ; note larynx and trachea. now proceed to the examination of the nerves of this region. see phrenic nerve, by vena cava inferior, and between heart and lungs, and sympathetic, running over the heads of the ribs. by the common carotids will be found the large white vagus nerve, the greyish sympathetic, and a small branch of x., the depressor. make out branches of x. named in text. the big white cervical spinal nerves will be evident dorsally. clear forward into the angle between the jaw and the bulla tympani, to see xii. and xi.; ix. will be found, lying deeper, dorsal to the carotid artery and body of the hyoid. compare with figure given of this. skin the cheek, and see vii. running over it. cut through malar and remove it; cut through lower jaw-bone and turn it back, to see the third branch of the fifth nerve on its inner side; examine the muscles of eyeball, and remove it, to expose the first and second branches of v.-- the latter is especially deep within orbit. remove, open, wash out, and examine the heart. shave off the dorsal wall of cranium, to expose hemispheres of brain, and then put the head in strong spirit for a week or so. with a second rabbit, this dissection may advantageously be varied by removing the lower jaw, cutting -up- [through] soft palate, and observing openings of the eustachian tubes. [the tonsils (on the ventral side of the soft palate) must not confused with these.] the heart should also be cut out, washed out and examined (compare sections , .) third dissection.-- (before this is performed the mammalian skull should have been studied and examined.) take the head of a rabbit, the brain of which has been hardened by spirit, and carefully remove cranium; be particularly careful in picking away the periotic bone, on account of the flocculi of cerebellum. it is difficult to avoid injury to the pituitary body embedded in the basisphenoid bone. examine with the help of sheet . make the sections there indicated. -the frog_ may be killed by drowning in dilute methylated spirit, or by chloroform. take a recently-killed frog, and examine a drop of its blood, spread out on a glass slip, under the microscope; compare it with your own. before using the high power, put a cover glass over the object, of course. scrape the roof of the mouth of the frog gently, to obtain ciliated epithelium; and mount in very weak salt solution-- the cilia will still be active. squamous epithelium may be seen by the student similarly scraping the interior of his own cheek. take a piece of muscle from one of the frog's limbs, tease out with needles upon a glass slip, and examine. to see the striations clearly, the high power will be needed. compare a piece of muscle from the wall of the alimentary canal. similarly examine nerve and connective tissue. first dissection.-- pin out the frog in a dissecting dish, ventral surface uppermost, and cover with water. open up the skin along the mid-ventral line. note the large sub-cutaneous lymph spaces, the pelvic and pectoral girdles, and the anterior abdominal vein. cut into the body cavity on one side of this latter, cut across in front of where the vein dips down to liver, and peel the body wall away from it. the xiphisternum will probably be cut in this operation. in early spring the females are greatly distended with ova, and the greater portion of the ovary may, with advantage, be removed. the oviduct is dead white then, and larger and much more in evidence than the (pinkish) intestine even. turn over the viscera, and compare with sheet ; one lung is often found greatly inflated, and then projects back into the body cavity; the stomach is, in some cases, pushed forward and hidden behind the shoulder girdle. observe the allantoic bladder, the spleen, gall bladder, portal vein, and pancreas. by squeezing the gall bladder gently, the bile duct will be injected with bile, and will be apparent if the stomach is turned over. the oesophagus, just in front of the stomach, should be cut through, and the rectum, and the mesentery and alimentary canal supported by it, removed. this will expose the urogenital organs. (vide figures given.) these vary greatly, especially in the females, at different seasons. the condition figured would be seen in late autumn, or winter. in spring females are often found copulating with males, and then the ovary itself is inconspicuous, while the lower part of the oviduct is enormously distended with ova, so as to be mistaken sometimes for the ovary by those who fail to note that the ova are enclosed by a thin semi-transparent skin (wall of oviduct). the vena cava inferior is seen between the kidneys and the renal portal vein beside the ureter. cutting through the mesentery supporting the kidney laterally, the dorsal aorta is exposed, and on either side of it the sympathetic chain and rami communicantes, often tinged with black pigment. this black pigment is a frequent but variable feature of the frog's anatomy, and usually dapples or blackens the testes, and also sometimes darkens the otherwise pale pink arteries. behind the kidneys the sciatic plexus also becomes visible. careful drawings should be made. cut off the head of the frog, shave off top of brain case, and put the head in strong spirit. second dissection.-- a fresh frog is required. pin out under water as before, and open up body cavity. now carefully remove the muscle from the ventral portion of the shoulder girdle, to expose the clavicles and coracoids. cut away xiphisternum, and then cut through clavicles and coracoids on either side, and remove ventral part of shoulder girdle, to expose the heart. open out the cut portions of body wall and pin. the veins going towards the heart should now, with a little examination, be evident. make out the external jugular, the innominate, and its two branches, and the pulmo-cutaneous and vena cava superior. clear by carefully picking away any shreds of semi-transparent tissue. make out, by feeling, the position of the hyoid body, and of its anterior cornua. note the hypoglossal nerve (first spinal) running ventral to this, and the ninth cranial nerve, running parallel to it but dorsal to the hyoid-- hidden therefore by the hyoid, and reappearing in front. the vagus may also be made out less distinctly, running "postero-ventrally" towards the heart. by clearing the muscle by the rumus of the jaw, vii. may be seen, and the third branch of v., running across the jaw at about the middle of its length. pick off the thin transparent pericardium from the heart very carefully, and proceed to cut away all the veins made out. the truncus arteriosus may then be followed up as it branches. note all the branches shown in the figures in this book. the precise position of the vessels will vary to a certain extent with the attitude in which the frog is pinned. the cutaneous artery will prevent the student following up the aortic arch until it is cut; then the arch may be followed round until it meets its fellow to form the dorsal aorta. note the sympathetic again. make careful drawings of all this. cut off lower jaw, and note posterior nares and eustachian openings. if time allows, remove the heart, and examine by cutting open and washing. (compare, section ) remove eyeball, to see the first and second branches of the fifth nerve, and the vidian (i.e. palatial) branch of the seventh. third dissection.-- read the account of the frog's skull carefully. take the head of a recently killed frog and drop into boiling water for a minute. then pick off, very carefully, muscle, connective tissue, nerves, and etc., to clear the cranio-facial apparatus; examine the bones, compare with figures given in this book, and draw. take the head, which has been in spirit a fortnight or so, pick away cranium, and compare brain with figures given. examine ventricles, by taking sections, after drawings have been made. -the dog-fish- first dissection.-- examine external characters, nasal grooves-- no internal nares-- fins, spiracle, scales passing over lips, and cloaca. cut off tail below the cloacal opening. the males are distinguished by the large claspers along the inner edge of the pelvic fin. open up body cavity. usually this is in a terrible mess in the fish supplied by dealers, through the post-mortem digestion of the stomach. wash out all this under a stream of water from a tap or water-bottle. frequently the testes are washed out of the male in this operation and ova from the loose ovaries in the female. now compare with figure given in this book, allowing for the collapse of the stomach, if it has occurred. cut through the oesophagus and rectum, and remove alimentary canal from body; cut open and wash out the intestine, and examine spiral valve. now make a careful examination of the cloaca and its apertures, and dissect away the peritoneum hiding the kidney. in the female find the opening of the oviducts in front of the liver. remove liver, and cut off body now behind pectoral fin. before throwing tail and hinder part of body away, note the myotomes of body wall, the notochord and vertebral body, neural canal, and, in the tail, the haemal canal. [(see {section the dog-fish})] {lines from first edition only.} -the relation of the vertebral bodies to the notochord may be very well seen by taking successive slices, about one-tenth of an inch thick, through the vertebral body. the cartilage is hard and semi-transparent, the notochord jelly-like, least at the centres of the centra, and at a maximum intervertebrally.- [the notochord is a soft jelly.] cut away the ventral part of the pectoral girdle, to open pericardium. with a seeker, make out the pericardio peritoneal opening. cut into the sinus venous, and run seekers into the cuvierian and hepatic sinuses. [cut open the cuvierian and posterior cardinal sinuses, and run seekers into their affluents.] dissect along the truncus arteriosus to afferent branchials. [cut away the heart and oesophagus; run a seeker up the dorsal aorta and cut along it from the ventral side to subclavian and efferent branchial arteries.] skin the top of the head. note, while doing this, the yellow, jelly-like sense-tubuli beneath the skin. shave off top of brain-case, and leave the head in spirit for a week or so. second dissection.-- place the head with the ventral side downward, skin all the dorsal surface as yet unskinned. refer to book for precise position of the anterior cardinal sinus, and then cut down through body wall into this just over gill slits. the tenth nerve will become visible, with its "slit" branches athwart the floor of the sinus. clear to make this more evident, and make out its lateral line and visceral branches, and the ninth nerve. {lines from second edition only.} [the pharyngo-branchials may be felt beneath the sinus. run a seeker from the dorsal aorta to the efferent branchials.] proceed now to orbit, and, without any dissection beyond the removal of skin, make out recti and oblique muscles of eyeball, and the optic, third and fourth nerves. cut through these structures carefully and remove, exposing nerves seven, and five, as described and figured in the text. examine the otic capsule by taking successive slices through it to show the labyrinth of the ear. -remove the dorsal wall of the skull to obtain a dorsal view of brain. if this is sufficiently hard, examine it; if not, return it to spirit for a more convenient occasion.- [examine brain.] -amphioxus_ two specimens of this type should be obtained. it should be examined entire by the naked eye and with the low power of the microscope. immersion, in glycerine will render it more transparent; or it may be cleared with oil of cloves, put up temporarily in that, or permanently in canada balsam. one specimen should then be pinned out in the dissecting dish, ventral side uppermost, and the atrium opened to expose liver and pharynx. a part of the pharynx may be examined with the low power to see the form of the gill slits. the second specimen should be soaked in turpentine for some time, and then dropped into melted paraffin wax. transverse sections may then be cut with a razor, the paraffin wax removed from these by solution in turpentine, the turpentine in its turn dissolved out by alcohol, and the sections, after immersion in oil of cloves, may be transferred to canada balsam for examination and preservation. this work should not be attempted until some practical histological work has been done in botany, and it may be altogether avoided by the purchase of stained and mounted sections. -development_ laboratory work in this portion of the science is not usually undertaken by elementary students of biology, but the reader will probably find it helpful, in the realization of the facts given in this book, to look out for frog spawn, in february and march, and to catch and examine tadpoles of various sizes. a small dissecting dish may be made by pouring melted paraffin wax into one of those shallow china pots chemists use for cold-cream, and tadpoles may be pinned out with entemologists' pins and dissected with needles. but this is a work of supererogation. partially incubated hen's eggs may be obtained at a small cost almost anywhere, and the later stages profitably examined and dissected under warm water. for a clear understanding of the allantois and amnion, this last is almost indispensable. a few microscopic slides of sections of embryonic chicks should also be compared with our rough diagrams. -{key for dissection sheets, and abbreviations.}_ sheet figure . main facts of the rabbit's anatomy (diagrammatic). an., anus. a.ao., arch of the aorta. au., auricle. a.r., ad-renal body. br., brain. b.d., bile duct. brch., bronchus. cd.st., cardiac end of stomach. co., colon. cae., caecum. ddnm., duodenum. d.ao., dorsal aorta. dia., diaphragm. ep., epiglottis. g.d., genital duct (either sex). il., ileum. in.art., innominate artery. k., kidney. lg., lung. lv., liver. l., larynx. l.s.c., [l.c.c.] left common carotid artery. m., mouth. na., nasal passage. oes., oesophagus. p.v., pyloric valve. p.d., pancreatic duct. pt., peritoneal cavity. r., rectum. st., stomach. [stm., sternum.] s.r., sacculus rotundus. s.c., spinal cord. tr., trachea. ur., ureter. ur.b., urinary bladder. v.b., a vertebral body. v.ap., vermiform appendix. v.v., [v.p.] velum palatium. v., ventricle of heart. v.c.i., vena cava inferior. figure . the liver (diagrammatic). g.b., the gall bladder. r.l., r.c., l.l.., l.c., right lateral and central, and left lateral and central, lobes respectively. sp., the spigelian lobe (fits into angle of stomach and oesophagus). {illustration: diagram sheet .} sheet figure . the rabbit's circulation (see footnote to section ). (throughout l. indicates left, r. right. vessels without r. or l. prefixed are median.) -[* the figure is inaccurate at one point; l.c.c. should spring from the base of inn. see sheet .]- {first edition only text} ao.a., aortic arch. au., auricle. az.v., (p.c. in figure ), azygos vein. c.c., common carotid. c.il.a., common iliac artery. coe.a., coeliac artery. d.ao., dorsal aorta. e.il.v., external iliac vein. e.ju., external jugular vein. f., femoral artery. h.v., hepatic vein. inn., innominate artery. in.j., internal jugular vein. i.il.a., internal iliac artery. i.il.v., internal iliac vein. k., kidney. lv., liver. l.g.v., lienogastric vein (portal). m.v., mesenteric (portal system). p.m.a., posterior mesenteric artery. p.v., main portal vein. p.a. pulmonary artery. r., rectum. r.a., renal artery. r.v., renal vein. s.v., and a., spermatic (or ovarian) vein and artery (to genital organ). s.mes.a., superior mesenteric artery. s.-cl.a., subclavian artery. s.-cl.v., subclavian vein. v.c.s., vena cava superior. v.c.i., vena cava inferior. v. or vn., ventricle. figure . figure of circulation (simplified) illustrating certain points in development to be referred to later. figure . respiration. see text, section . figure . blood. see text, section . {illustration: diagram sheet .} sheet histological figures, . {no numbers i., or ii.} figure iii. an amoeba.-- n., nucleus. ns., necleolus. c.v., contractile vacuole. figure iv. embryonic tissue from the blastoderm of a chick. figure v. columnar epithelium.-- g.c. , g.c. , g.c. , successive phases in the development of a goblet cell. figure vi. g.end., is geminating endothelium; the cells divide and apparently drop off to become white corpuscles in the lymph current. sq.end., squamous endothelium from the mesentery. sq.ep., squamous epithelium (from the mucous membrane within the cheek). st., are opening (stomata) communicating between the lymphatics in the mesentery and the peritoneal (coelomic) space. figure vii. ciliated epithelium from the roof of the frog's mouth. figure viii. forms of glands.-- g.ep., is a gastric gland from the stomach; trs., below, is cross section. this is one of the simplest types of gland. s.g., a sweat gland, is also a simple tube, but convoluted below. r.g., is a racemose gland, such as the pancreas, brunner's or the salivary glands. the kidney, we shall see later, is simply an aggregate of branching tubuli (sheet ). figure ix. a duodenal villus.-- lac., the lacteal. v., the vein. figure x.a. diagram of liver structure.-- b.d., the inter-lobular bile duct. h.a., the hepatic artery, bringing blood to oxygenate and nourish the liver tissue, and similarly distributed. h.v., the hepatic vein taking blood from the liver to the heart, its twigs commencing in the lobuli (intra-lobular). lb. lb., lobuli. p.v., the portal vein bringing blood, from which substances are to be elaborated, into the liver, and breaking up between the lobuli (inter-lobular). figure x.b. a diagram of the appearance of an injected liver lobule as seen in section under the microscope. {illustration: diagram sheet .} sheet histological diagrams, . figure xi. a blood capillary. white corpuscles are migrating through the walls into the tissues (compare section ). figure xii. hyaline cartilage (section ). figure xiii. c.c., connective tissue corpuscle. w.i.f., white inelastic fibres. y.e.f., yellow elastic fibres. figure xiv. botryoidal tissue (section ). figure xv. development of a fat drop.-- f.d., fat drop, in a connective tissue corpuscle; c.c., in the formation of adipose tissue (section ). figure xvi. diagrammic cross section of a long bone.-- b.c., bone corpuscle in a lacuna. h.v., haversian vessel (in the haversian canal) surrounded by concentric lamellae of bone, c.l., and together with these and zones of bone corpuscles, called a haversian system. i.l., inner lamellae. m.c., medullary canal full of yellow marrow. o.l., outer lamellae. p.o., periosteum. figure xvii. to illustrate bone development (section ). figure xviii. dentition of rabbit, incisors / , canine / , premolar / , molar / . {illustration: diagram sheet .} sheet . diagram of the rabbit's bones. to be compared with the real things. d and d' show the fore and hind limbs, to illustrate their homology. d is in the embryonic position. the radius and tibia are, at an early stage in development, on the anterior edge of their respective limbs; the ulna and fibula, posterior; the former are spoken of as preaxial in position, the latter as postaxial. but in the adult the humerus is twisted so that the proximal end of the radius lies at the outer side of the elbow, whence it crosses the ulna, so that its distal end is inside, while the femur is also twisted round, so that the entire tibia is internal. figures and . -limbs.-- a.c., acetabulum. acr., acromion. as., astragulus. c., carpus. ca., calcaneum. co., coracoid. [coty., cotyloid bone.] fb., fibula. fe., femur. g., glenoid cavity (for head of humerus). hd., head of femur. hum., humerus. i., ilium. is., ischium. m.c., meta-carpals. na., navicular. o., olecranon process of ulna. o.f., olfactory fossa. pb., pubis. r., radius. u., ulna. figure . -sternum.-- mb., manubrium. r ., r ., and etc., sternal ribs. st., sternebrae. xi., xiphisternum. figure . vertebrae.-- at., atlas. ax., axis. c., [b.] centrum. c.v., caudal vertebra. c.v., [cer.v.] cervical vertebra. ep., epiphysis. f.r., fused rib (in cervical vertebrae). l.v., lumbar vertebra. m., metapophysis (of lumbar vertebra). n.a., neural arch. n.s., neural spine. r., rib. s.v., sacral vertebra. [t.v., thoracic.] tr.p., transverse process. v.a.c., vertebrarterial canal. z., zygapophysis. {illustration: diagram sheet .} sheet . the skull of canis.*-- . dorsal. . ventral. . right lateral aspect. . section a little to the left of the nasal septum. . lower jaw (smaller) . hyoid apparatus. {lines from first edition only.} -*a fox in this case. the skull is quite like that of a dog, but it has the advantage of more distinct sutures between the bones.- a.n., anterior nares. a.s., ali-sphenoid. b.h., body of the hyoid. b.o., basi-occipital. b.sp., basi-sphenoid. c., condyle of the skull. {c. , c. , canines.} c.f., condylar foramen (for xii.). c.h., cerato-hyal. e.f., eustachian foramen. e.h., epihal. -e.n., or a.n., the anterior nares.- e.o., exoccipital. eth., ethmoid. e.t., ethmo-turbinal. f., frontal. f.l.a., foramen lacerum anterius. f.l.m., foramen lacerum medium. f.l.p., foramen lacerum posterius (for ix., x., xi.). f.m., or f.m., foramen magnum. f.o., foramen ovale. f.r., foramen rotundum. {i., incisors.} ju., jugal. m., molars. m.t., maxillo-turbinal. mx., maxilla. na., nasal. n.t., nasal turbinal. o.f., optic foramen. o.s., orbito-sphenoid. p., or pal., palatine. pa., parietal. p.m., pre-maxilla. p.m. , p.m. , premolars. p.n., posterior nares. p.sp., pre-sphenoid. pt., pterygoid. s.h., stylo-hyal. s.m.f., stylo-mastoid foramen (for vii.). s.o., supra-occipital. sq., squamosal. s.t., sectorial tooth. t.h. thyro-hyal. vo., -black line indicating position of- vomer. z.p., zygomatic process of squamosal. {illustration: diagram sheet .} sheet figure . striated muscle fibre (of the rabbit), ruptured to show sarcolemma. e.p., its end plate. k.m., membrane of krause. n., nucleus. nv., nerve. sc., sarcolemma. s.e., sarcous elements. figure . cardiac muscle. figure . unstriated muscle fibres. figure . diagram of the skin. b.v., blood vessel. d., areolar tissue of the dermis (mesoblastic). s.c., stratum corneum, and s.m., stratum mucosum of the epidermis. s.g., sweat gland. t.c., tactile corpuscle. figure . to illustrate kidney structure.-- a.b.v., and e.b.v., afferent and efferent blood-vessels, of which the latter go to break up upon the tubli. b.c., one of bowman's capsules of the cortex; ur.t., the uriniferous tubule running from it into the medulla, where it loops and branches; around it branches a blood-vessel, of which the latter go to break up upon the tubuli. c., cortex. g., glomerulus, a knot of blood-vessels in the capsule. m., medulla. p., pelvis. ur., ureter. the water of the urine is probably filtered off in the capsule, the urea and other salts secreted by the tubuli. {no figure .} figure . the auditory structures of the rabbit (diagram). see text, section . figure . the eye (diagram). see text, section . figure . the retina (diagram). see text, section . {illustration: diagram sheet .} sheet the brain of the rabbit.-- . in median section. . from above, with the top of the right hemisphere sliced off horizontally at the level of the corpus callosum. . a deeper section through the thalamencephalon, corresponding to b in ( ). . under-view of the brain. . diagram referred to in the text and for comparison with sheet , b., and sheet , . {figures - .} ar., arrow in the iter. a.c., the anterior commissure, a thickening of the anterior wall of the third ventricle. c.c., corpus callosum. c. cb., crura cerebri. c.h., cerebral hemispheres. c.q., corpora quadrigemina. f.cbm. (right), flocculus of the cerebellum. l.h., left cerebral hemisphere (=ch.). l.l., lateral lobe of cerebellum. m.c., middle commissure. m.o., medulla oblongata. op., optic nerve. o.l., olfactory lobe. o.th., (right), optic thalamus. p.c., posterior commissure (thickening of postero-dorsal wall of the third ventricle). p.g., pineal gland. pt., pituitary body. p.v., pons varolii. s.c., thin roof of the fourth ventricle. v.cbm., vermis of cerebrum. v.l., lateral ventricle. {figure .} nerves.-- i., olfactory. ii., optic. iii., oculo-motor. iv., patheticus. v., trigeminal. vi., abducens. vii., facial (portio dura). viii., auditory (portio mollis). ix., gustatory (glossopharyngeal. x., pneumogastric or vagus. xi., spinal accessory. xii., hygoglossal. figure . the spinal cord in section.-- c.c., the central canal. d.f., the dorsal fissure. d.n., the dorsal nerve root; g., its ganglion. v.f., the ventral fissure. v.n., the ventral nerve root. note that in figure the central canal is continuous with the fourth ventricle. figure . histological elements.-- g.c., multipolar ganglion cell. n., nucleus of a medullated nerve. a.c., its axis fibre. s.s., (sheath of schwann), medullary sheath interrupted at intervals by n.r., the nodes of ranvier. n.m.f., a non-medullated fibre. {illustration: diagram sheet .} sheet . -the nerves of the rabbit_. figure i. rough sketch of dissection of the neck from the left ventral aspect.-- the bands of muscle between hyoid, mandible, and sternum, and the thymus gland carefully cleared. lr., is the larynx, and b., the balla. s.m.g., the right sub-maxillary gland (the left has been removed). the nerves are numbered. l.r.l.n., [r.r.l.n.] is the left recurrent laryngeal looping under that solid connection between the pulmonary artery (p.a.) and ao., the aortic arch, which was an open tube in the embryo, the ductus arteriosus. hy., is the hyoid with its posterior cornua. ph.n., is the phrenic nerve. r.r.l.n., [l.r.l.n.] is the right recurrent looping under the sub-clavian. s.c.g., is the super or cervical ganglion of the sympathetic (sym.); s.l.n., is the left superior laryngeal, and g. the left depressor branch of x. z., is the ramus descendens noni of the twelfth nerve. in early development the heart lay just beneath the pharynx in the position of the larynx (compare dog-fish and frog); as the neck elongated, the heart shifted back with its vessels, and so the long loop of the recurrent laryngeal comes to be drawn out in this singular way. figure ii. diagram of orbit to show v. orbit-nasal, v. the maxillary, and v. the mandibular branch of v. in order to show these in dissection, the malar must be cut away, and the eye and glands of the orbit removed. s.r., e.r. [p.r.], i.r., and a.r., cut ends of the superior, external (or posterior), inferior, and anterior (or internal) recti muscles. s.o., and i.o., the superior and inferior obliques. figure iii. general diagram of the rabbit's cranial nerves. figure iv. rough sketch of dissection of the nerves and blood-vessels dorsal to stomach.-- the stomach turned over to the animal's right, the spigelian liver lobe cleared from the oesophagus, the mesentery supporting spleen and hiding solar plexus picked off, and the mesentery hiding sympathetic cleared. coe.art., coeliac artery, and s.m.a., superior mesenteric artery. coe.g. coeliac, and s.m.g., superior mesenteric ganglion. the two together form the solar plexus. l.abd.sym., left abdominal sympathetic (in the actual dissection, the right would also be visible). l.a.r., left adrenal. l.sp.n., left splanchnic nerve. r.art., renal artery. r.v., renal vein. st., the stomach, and sp., the spleen. x., the vagus on oes., the oesophagus. {illustration: diagram sheet .} sheet . -reproductive organs of the rabbit_. figure . the male. figure . the female organs. (the symbols below the figures indicate the sex.) pb., is the pubic symphysis [which has been] cut through. r., the rectum, with r.g., the rectal gland, and a., the anus. t., the tail. r.ur., the right ureter. l.ur., the left ureter. ur.b., the urinary bladder. in the male ep., the epididymis. p., the penis. pp., the prepuce. scr., the scrotal sac, containing these; r.v.d., the right vas deferens. t., is the testis. u.m., the uterus masculinus. in the female c.ut, the left cornu uteri. f.t., the left fallopian tube. ov., is the ovary, with a graafian follicle, g.f. v., the vagina. v.b., the vestibule. figure . diagram of ovary with stages in the development of a graafian follicle , , , , , see text, section . the arrow indicates the changes in position of the developing follicles. {illustration: diagram sheet .} sheet . figure . general dissection of frog (male). figure . the heart and great vessels laid open. figure . the circulatory system from the side. figure . blood. {n., nucleus.} r.c., red corpuscle (oval and nucleated). w.c., white corpuscle small figure of frog in left-hand corner is to show position of heel, h. reference letters. all.b., allantoic bladder (= urinary bladder). c.ad., corpus adiposum. cl.c., cut end of the right clavicle. d., duodenum. g.b., gall bladder. il., ileum. k., kidney. l.au., left auricle. l.g., lung. l.int., large intestine. l.s.v., longitudino-spiral valve. l.v., liv., liver. pan., pancreas. r.au., right auricle. sp., spleen. st., stomach. t., testis. t.a., truncus arteriosus. ur., urogenital duct. v., ventricle of heart. arteries (white). ao., aorta. c.a., carotid arch. c.g. [c.gl.], carotid gland. coe., coeliac. cu., -and pa.",- cutaneous. d.ao., dorsal aorta. e.c., lingual artery. [i.c., internal carotid.] l.a.a., left aortic arch. pa., and p., pulmonary. p.c. [p.cu.], pulmo-cutaneous. r.a.a., right aortic arch. [s.cl., sub-clavian.] t.a., truncus arteriosus. veins of the caval system -(black)-. b.v., brachial (from fore limb). e.j., external jugular. h.v., hepatic vein. i.j., internal jugular. [in.v., innominate vein.] l.v.c.s., left vena cava superior. p.v., cutaneous vein. [s.cl.v., sub-clavian vein] s.s.r., sub-scapular vein. v.c.i., vena cava inferior. veins of the portal and renal portal systems -(shaded)-. a.ad., and a.ab.v., anterior abdominal vein. b.v., and p.v., united are called the sub-clavian vein. l.fm., left femoral. l.p., left pelvic. l.r.p., (and r.p.) left renal portal. l.sc., left sciatic. p.v., portal vein. -(the anterior abdominal is coloured black in figure .)- the cutaneous artery in the above figures is turned back. in dissection it will be found to lie over and hide the dorsal-ward sweep of the aortic arch. {illustration: diagram sheet .} sheet . figure . upper view of the frog's brain. figure . under view of the same. figure . the same-- median section. figure . the distribution of the frog's nerves. compare sheet , figure iii. the shaded part in is the -otic capsule- [tympanum]. the hyoid apparatus is roughly represented in black to show its relation to ix. (dorsal to it) and sp. (ventral). compare {nerves} ix and xii in sheet . the nerves are numbered. cb., the cerebellum. c.h., cerebral hemispheres. f.t., filum terminale. g.tr., ganglion on the fifth nerve. l.t., lamina terminalis. mb., mid-brain. md., medulla oblongata. o.l., optic lobes. pin., pineal gland. pit., pituitary body. r.h., olfactory lobes (rhinencephalon). th.c., thalamencephalon. sp. , first spinal nerve. sp. , , brachial plexus to fore limb. figure . the spinal column (and pelvic girdle) of the frog. figure b. vertebrae. figure . the pectoral girdle and limb, dorsal view. figure . the pelvic girdle and right limb from the side. (l.h. shows the position of the right lymph hearts-- they are paired.) as., astragalus. b., body. c., calcar (?= a sixth digit). cal., calcaneum. cl., clavicle overlying a procoracoid cartilage. co., coracoid. f., fibula. [fe., femur.] h., humerus. il., ilium. is., ischium. o.st., omosternum. pu., pubis. r., radius. sc., scapula. s.sc., supra-scapula. s.v., sacral vertebra. t., tibia. t.p., transverse process. ul., ulna. ur., urostyle. x., xiphisternum. z., zygapophysis. , , and etc., first, second, and etc., digits. d. and d'. are simplified diagrams of the limbs for comparison with the similar ones of the rabbit. in each girdle we have a dorsal ossification (scapula, ilium) and two ventral parts (pubis and procoracoid cartilage, ischium and coracoid), and at the meeting-place of the three in each case the proximal bone of the limb (humerus, femur) articulates. {illustration: diagram sheet .} sheet . -urogenital organs of the frog_. figure . the male. figure . the female. the oviduct removed on the animal's left, and the ovary on its right. organs common to both sexes.-- al.b., allantoic bladder. c.ad., corpus adiposum. cl., cloaca. int., intestine. k., kidney. lg., (dotted outline of) lung. oes., oesophagus. r.p.v., renal portal vein. st., stomach. in the male.-- t., testis. v.e., vasa efferentia. u.g.d., urogenital duct. p., prostate gland. in the female.-- adr., adrenal. f.t., fallopian tube (anterior part of oviduct). * its opening. o.d., oviduct (letters on [the opening] -uterine portion-). ov., ovary. ur., ureter. (this would be the condition about midwinter.) in march o.d. will be either enormously distended with eggs, or large, flabby, and empty, and ov. will be small and brownish, without any large eggs; the ovary gradually recovers its size through the summer. figure . spermatozoa attached to the parent cell (g.e.) from the lining epithelium of the testis, and one free. fl., the flagellum. {illustration: diagram sheet .} sheet -skull structure and development of the frog_. figure . i., ii., early and late stages of the tadpole's chindrocranium. diagrammatic. figure . dorsal view of a young frog's cranium-- the membrane bones removed. diagrammatic. figures and . dorsal and ventral views, respectively, of the frog's skull-- the lower jaw removed. figure . side view of the frog's skull. figure . median section of the brain case. figure . the hyoid apparatus. figure . i., ii., iii., progressive stages of the tadpole's skull from the side. after w. k. parker. figure . f., side and hind views of the frog's skull. d., the same of the dog. roughly diagrammatic. n.b.-- in all cartilage is dotted, cartilage bone cross-barred, and membrane bone, white. in figure , pt., should be cross-barred; and in , th.h. plain. a.c., anterior cornu of hyoid [(= ch.)] -not lettered, in {figure} -. a.o., antorbital cartilage. ar., angulo-splenial -(on frog section , for articulare read -angulo-splenial_)-. -b., parachordal part of brain box-. b.c., brain case. b.h., body of hyoid. b.r., branchial arches. ch = a.c. c.t., cornua trabeculi. d., dentary. e., eye. e.n., external nares. e.o., exoccipital bone. f., fenestra (membranous part of cranial wall). -f.p., fronto-parietal.- h.m., hyomandibular cleft = eustachian tube and ear drum. mb., mandible. [m.c., meckel's cartilage.] m.mk., mento-meckelian bone. m.p., mouth passage. mx., maxilla. n.c., notochord. n.o., nasal organ. n.p., nasal passage. ot., or o.c., otic (auditory) capsule. pal., palatine bone. pal., hard palate of mammal. p.c., parachordal. p.f., [parieto-frontal] -see f.p.- p.m., premaxilla. p.n., internal nares. p.o., prootic bone. p.p., palato-pterygoid cartilage. psph., parasphenoid bone. pt., pterygoid bone. q., quadrate cartilage. q.j., quadrato-jugal. s.e., sphenethmoid bone. sq., squamosal. t., trabecular part of brain box. t.c., trabecula. th.h., thyrohyal. {illustration: diagram sheet .} sheet figure . dissection of -male- [female] dog-fish to show alimentary canal, the pericardium also being opened and the cloaca slit up. [above is also seen the dorsal view of the head.] figure . the pelvic girdle and fin skeleton [of a male]. {no figure , in first edition.} figure . the spiral valve in the colon. {figure , in second edition.} a.p., abdominal pore. aur., -auricle- [atrium] of heart. b.d., bile duct. b.pt., basi-pterygium. -cl., clasper.- cl.c., -its- [the] supporting cartilage [of the clasper]. co., colon. d'dnm., duodenum. e., the eye. g.bl., gall bladder. g.s., gill slits. l.lv., left lobe of liver. m.lv., middle lobe of liver. olf., olfactory opening. [pan., pancreas.] pcd., pericardial wall. pel.g., the pelvic girdle. p.p., arrow through pericardio-peritoneal canal. r.g., rectal gland. [r.liv., right lobe.] sp., spiracle. spl., spleen. st., the stomach. s.v., sinus venosus. u.g.p., uro-genital pore. v., ventricle. {illustration: diagram sheet .} sheet . figure . circulation of the dog-fish. figure . simplified and more typical fish circulation, in which the posterior cardinals have not coalesced in the median line. the cuvierian veins = the vena cava superior of the higher type; the posterior cardinal is represented by the azygos vein in the rabbit. compare sheet , figure , and sheet , figure . figure . side view of the pericardium. a.br., afferent branchial artery. a.c.s., anterior cardinal sinus (= internal jugular vein). au., atrium (auricle) (= the two auricles of higher forms). b.a., bulbus arteriosus. c.a., conus arterious. cd. a., caudal artery. cd.v., caudal vein. c.s., cuvierian sinus. d.a., dorsal aorta. e., eye. e.br., efferent branchial arteries. g.s., in position of gill slits. h.br.a., hypobranchial artery. h.s., hepatic sinus. [i.j.s., inferior jugular sinus (= external jugular vein).] k., kidney. l.v., lateral vein. [oe.s., ventral wall of oesophagus.] p.c.c., pericardial cavity. p.c.s., posterior cardinal sinus. p.p.c., pericardio-peritoneal canal. p.v., portal vein. r.p.v., reno-portal vein. s.c.v., subclavian vein. vn., ventricle. -v.s.v., inferior (= external) jugular vein-. figure . skeleton of pectoral limb, and girdle.-- g., the girdle (also in figure ). m.p., meso-pterygium. mt.p., meta-pterygium. p.p., pro-pterygium. sc., its dorsal portion. {illustration: diagram sheet .} sheet -the uro-genital organs of the dog-fish_. figure . the female, the oviduct of the left side cut away, -and an egg case in the oviduct.- figure . the male. the rectum is removed in both cases, and the silvery peritoneum dissected off from the kidneys. figure . a generalized diagram of the uro-genital organs.-- all references in text. ms., the mesonephros, is the epididymis in the male, and is reduced in the female; ms.d., its duct, is the vas deferens in the male, and persists only as the urinary receptacle in the female. mt. and mt.d., the metanephros and metanephric duct, become the functional kidney and ureter in both sexes. g. is the gonad (reproductive gland), and m.l. the animal's middle line (median plane). -ps.-, [pr.,] the pronephros, is never developed in the dog-fish; p.d., its supposed duct, is the oviduct of the female, and is suppressed in the male. {illustration: diagram sheet .} sheet . figure . the dog-fish brain, dorsal view. figure . median section of the same. to the right a more diagrammatic figure. the nerves are numbered:-- [br , br , br , br branches of x forking over the second to the fifth gillslit.] cb., cerebellum. h.s.c., horizontal semi-circular canal of ear, exposed by the slicing down of the otic mass. [lat., lateral-line branch of x.] m.o., medulla oblongata. oph., ophthalmic nerve (v. +vii. ). op.l., optic lobe. pit., pituitary body. pr.c., prosencephalon (cerebral hemisphere). rh., olfactory lobe (rhinencephalon). r.t., -its- restiform tracts [of medulla]. -st-. [s.p.g.], stalk of the pineal gland. th., thalamencephalon. th.c., thalamencephalon. -ut., the utriculus, seen through the semi-transparent cartilage-. vid., the vidian branch of vii. [visc., visceral branch of x.] figure . diagram of the ear of a fish. the structure of this is easily made out by clearing otic capsule and cutting slices of the cartilage in the dog-fish (e.g., figure , h.s.c.). amp., their ampullae. a.v.c., p.v.c., h.c., anterior, posterior, horizontal canal respectively. [amp., the ampullae.] d.e., the ductus endo-lymphaticus. -sac., the sacculus; c., a small outgrowth of the latter, corresponding to the rabbit's cochlea-. -ut., the utriculus-. figure . the cranium and branchial bars of a dog-fish. the groove in the otic capsule connects the orbital and anterior cardinal sinuses. a.c.s., position of the anterior cardinal sinus (dotted outline). c., the vertebral centra. c.b., the cerato-branchial. c.h., the cerato-hyal. e.b., epi-branchial. ex.b., extra-branchial. h.m., the hyo-mandibular. i.n.p., inter-neural plate. m.c., meckel's (lower jaw) bar. na.c., the nasal capsule. n.p., neural plate. n.s., neural spine. ot.c., the otic capsule. ph.b., the pharyngo-branchial. p.pt., the palato-pterygoid bar (upper jaw bar). p.s., pre-spiracular ligament, containing a cartilaginous nodule. r., rib. sp., the position of the spiracle. figure . diagrams of a vertebral centrum.-- for reference letters, see text (section ). {no figure , in first edition.} [figure . diagram for comparison with figure iii., sheet .] {illustration: diagram sheet .} sheet . figure . amphioxus, seen from the right side. a----b shows the natural size. the animal is supposed to be clarified, and mounted in some highly refracting medium, so that it is practically transparent; i., ii., iii., and etc., refer to the section figured on sheet . figure . amphioxus, general dissection. (slightly altered from a figure by professor e. r. lankester.) the ventral atrial wall is removed. the pharynx cut away from the dorsal body-wall, and with the true ventral body-wall turned over to the (animal's) right. the arrow a., a., passes through anus to intestine; b., b., is thrust through the atrial pore to the atrial cavity. note coe., the body cavity. references to the two figures. an., anus. at., atrial cavity. at.w., atrial wall. at.p., atrial pore. a.d., anterior dilatata of nervous system. b.w., body-wall. b.t.l., brown tubes of lankester. c.f., ciliated funnel. coe., coelome. c.ao., cardiac aorta. d.ao., dorsal aorta (paired). d.ao'., dorsal aorta median. g., gonads (male or female genital gland). hep., hepatic vein. in., intestine. i.w., intestine wall. lv., liver. m.f., median fin. n.c., notochord. p.v., portal vein. ph., pharynx. -p.s.-, [e.s.] pigment spot ("eye spot"). s.c., spinal cord. {illustration: diagram sheet .} sheet -sections of amphioxus_. the roman numerals indicate the corresponding region in figure , sheet . the lettering is identical; but note, in addition; br.c., branchial canal. c.f., ciliated funnel. d.c.c., dorsal coelomic canal. end., endostyle. ep., epipleur. e.s., eye spot. h.p., hypopharyngeal grove. h.vn., for hepatic vein. o.c., oral cavity (or hood). {illustration: diagram sheet .} sheet . -phases in the development of amphioxus_. figures , , , . phases in segmentation. figure . the blastosphere. figure . the gastrula in section, anterior end to the right. figure . i. dorsal view post gastrula stage. figure . ii. diagrammatic section of the same in the position indicated by the transverse line in , i. figure . diagrammatic section of a later stage. coe.p., the coelomic pouches. n.c., the notochord. n.p., the neural plate. figure .i. still later section. figure . ii. diagrammatic view of late embryo. figures , , illustrate the formation of the atrium as a median ventral invagination, at. {illustration: diagram sheet .} sheet . -the development of the frog_. these diagrams must be studied with the text. they should be compared with the corresponding ones of amphioxus as indicated below. figures , , . stages in segmentation (compare , , of {sheet } amphioxus). figure . blastosphere stage (compare , amphioxus). this, on a smaller scale. the cells on the ventral side are so much larger because distended with yolk. figure . gastrula stage in section (compare , amphioxus). the frog on a smaller scale than amphioxus. figure . dorsal view of gastrula (compare , amphioxus). figure . part of a transverse section of developing tadpole, corresponding to figure of amphioxus. figures and . diagrammatic longitudinal sections of tadpoles (compare . ii. of amphioxus). y. represents a mass of yolk cells. figure . side view of young tadpole, showing external gills (e.g.) and suckers (s.). note the ventral bulging due to the yolk. figure . ventral view of a later tadpole. op., the operculum. int., coiling intestine. figure . head of still later tadpole in horizontal section to show atrial chamber formed by operculum. int.g., internal gills. l., developing lungs. figure . diagrammatic cross-section of the mid-dorsal part of an embryonic vertebrate. ao., aorta. b.c., bowman's capsule. coe., coelom. d.g., ganglion on dorsal root of spinal nerve. gl., -its branch- [arteriole] to form glomerulus. g.r., genital ridge. i., intestine. m.d., mullerian duct. ns. [nst.], nephrostome. n.c., notochord; -n.s.-, [n.sh.] its sheath. s.c., neural canal. w.d., wolffian duct. {illustration: diagram sheet .} sheet . -the development of the fowl_. figure . diagram of the early ovum. the section below is a small portion of the blastodermic area. b.d., blastoderm. y., the undivided yolk. s.c., the segmentation between the blastoderm and yolk. compare s.c. in {sheet} , {figure} . figure . area pellucida about the sixteenth hour. the figure below is the central part of the section indicated by the transverse line, and showing the primitive streak (p.s.). figure . area pellucida about the twenty-first hour. two sections through a and b below. figure . about the twenty-fifth hour; surface view; longitudinal section to right and transverse above. figure b. diagrammatic rendering of same stage (compare figure of frog and .ii. amphioxus). this will be most clearly understood if the reader look at sheet , {figure} , and imagine y. enormously increased, and the embryo sinking into it. epiblast, ep., -line of dashes- [black line]. mesoblast, dotted. hypoblast, -black- [line of dashes]. pp., the pleuro-peritoneal cavity. figure and illustrate formation of amnion (a.) and allantois (all.). is about the fourth day. {illustration: diagram sheet .} sheet . -the development of the fowl_. figure . chick about the -fifth- [third] day. at this stage the chick lies on its left side in the yolk. [for lettering of blood vessels, see ( ) below.] i., the intestine. u.v., the yolk sac. v.v., the vitelline veins. al., the allantois. figure . chick about sixth day. figure . development of heart. figure . development of the eye. figure . chick about the sixteenth day. a.m. is the amnion surrounding the embryo. note particularly how the allantois (al.) has spread over surface of shell and how the yolk sac is shrivelled. figure . figures to illustrate the relative function and importance of allantois and yolk sac in bird and mammal. in the fowl, however, the blood-vessels of the allantois also probably absorb the albumen of the egg, and may excrete urea into the egg-space. figure . simplified figure of the embryonic circulation, for comparison with the similar figures annexed to dog-fish and rabbit. {lines from second edition only.} [a.c., anterior cardinal. ao., aorta. br , sixth aortis arch (fourth branchial). c.s. cuvierian sinus. h., the heart. i.c., inferior cava. p.c., posterior cardinal vein. tr.a., truncus arteriosus. v.v., vitelline vein.] figure . chick on the nineteenth day. {illustration: diagram sheet .} biology by edmund beecher wilson professor of zoology columbia university new york the columbia university press biology a lecture delivered at columbia university in the series on science, philosophy and art november , biology by edmund beecher wilson professor of zoology columbia university new york the columbia university press copyright, , by the columbia university press. set up, and published march, . biology i must at the outset remark that among the many sciences that are occupied with the study of the living world there is no one that may properly lay exclusive claim to the name of biology. the word does not, in fact, denote any particular science but is a generic term applied to a large group of biological sciences all of which alike are concerned with the phenomena of life. to present in a single address, even in rudimentary outline, the specific results of these sciences is obviously an impossible task, and one that i have no intention of attempting. i shall offer no more than a kind of preface or introduction to those who will speak after me on the biological sciences of physiology, botany and zoology; and i shall confine it to what seem to me the most essential and characteristic of the general problems towards which all lines of biological inquiry must sooner or later converge. it is the general aim of the biological sciences to learn something of the order of nature in the living world. perhaps it is not amiss to remark that the biologist may not hope to solve the ultimate problems of life any more than the chemist and physicist may hope to penetrate the final mysteries of existence in the non-living world. what he can do is to observe, compare and experiment with phenomena, to resolve more complex phenomena into simpler components, and to this extent, as he says, to "explain" them; but he knows in advance that his explanations will never be in the full sense of the word final or complete. investigation can do no more than push forward the limits of knowledge. the task of the biologist is a double one. his more immediate effort is to inquire into the nature of the existing organism, to ascertain in what measure the complex phenomena of life as they now appear are capable of resolution into simpler factors or components, and to determine as far as he can what is the relation of these factors to other natural phenomena. it is often practically convenient to consider the organism as presenting two different aspects--a structural or morphological one, and a functional or physiological--and biologists often call themselves accordingly morphologists or physiologists. morphological investigation has in the past largely followed the method of observation and comparison, physiological investigation that of experiment; but it is one of the best signs of progress that in recent years the fact has come clearly into view that morphology and physiology are really inseparable, and in consequence the distinctions between them, in respect both to subject matter and to method, have largely disappeared in a greater community of aim. morphology and physiology alike were profoundly transformed by the introduction into biological studies of the genetic or historical point of view by darwin, who did more than any other to establish the fact, suspected by many earlier naturalists, that existing vital phenomena are the outcome of a definite process of evolution; and it was he who first fully brought home to us how defective and one-sided is our view of the organism so long as we do not consider it as a product of the past. it is the second and perhaps greater task of the biologist to study the organism from the historical point of view, considering it as the product of a continuous process of evolution that has been in operation since life began. in its widest scope this genetic inquiry involves not only the evolution of higher forms from lower ones, but also the still larger question of the primordial relation of living things to the non-living world. here is involved the possibility so strikingly expressed many years ago by tyndall in that eloquent passage in the belfast address, where he declared himself driven by an intellectual necessity to cross the boundary line of the experimental evidence and to discern in non-living matter, as he said, the promise and potency of every form and quality of terrestrial life. this intellectual necessity was created by a conviction of the continuity and consistency of natural phenomena, which is almost inseparable from the scientific attitude towards nature. but tyndall's words stood after all for a confession of faith, not for a statement of fact; and they soared far above the _terra firma_ of the actual evidence. at the present day we too may find ourselves logically driven to the view that living things first arose as a product of non-living matter. we must fully recognize the extraordinary progress that has been made by the chemist in the artificial synthesis of compounds formerly known only as the direct products of living protoplasm. but it must also be admitted that we are still wholly without evidence of the origin of any living thing, at any period of the earth's history, save from some other living thing; and after more than two centuries redi's aphorism _omne vivum e vivo_ retains to-day its full force. it is my impression therefore that the time has not yet come when hypotheses regarding a different origin of life can be considered as practically useful. if i have the temerity to ask your attention to the fundamental problem towards which all lines of biological inquiry sooner or later lead us it is not with the delusion that i can contribute anything new to the prolonged discussions and controversies to which it has given rise. i desire only to indicate in what way it affects the practical efforts of biologists to gain a better understanding of the living organism, whether regarded as a group of existing phenomena or as a product of the evolutionary process; and i shall speak of it, not in any abstract or speculative way, but from the standpoint of the working naturalist. the problem of which i speak is that of organic mechanism and its relation to that of organic adaptation. how in general are the phenomena of life related to those of the non-living world? how far can we profitably employ the hypothesis that the living body is essentially an automaton or machine, a configuration of material particles, which, like an engine or a piece of clockwork, owes its mode of operation to its physical and chemical construction? it is not open to doubt that the living body _is_ a machine. it is a complex chemical engine that applies the energy of the food-stuffs to the performance of the work of life. but is it something more than a machine? if we may imagine the physico-chemical analysis of the body to be carried through to the very end, may we expect to find at last an unknown something that transcends such analysis and is neither a form of physical energy nor anything given in the physical or chemical configuration of the body? shall we find anything corresponding to the usual popular conception--which was also along the view of physiologists--that the body is "animated" by a specific "vital principle," or "vital force," a dominating "archæus" that exists only in the realm of organic nature? if such a principle exists, then the mechanistic hypothesis fails and the fundamental problem of biology becomes a problem _sui generis_. in its bearing on man's place in nature this question is one of the most momentous with which natural science has to deal, and it has occupied the attention of thinking men in every age. i cannot trace its history, but it will be worth our while to place side by side the words of three of the great leaders of modern scientific and philosophic thought. the saying has been attributed to descartes, "give me matter and i will construct the world"--meaning by this the living world as well as the non-living; but descartes specifically excepted the human mind. i do not know whether the great french philosopher actually used these particular words, but they express the essence of the mechanistic hypothesis that he adopted. kant utterly repudiated such a conception in the following well known passage: "it is quite certain that we cannot become adequately acquainted with organized creatures and their hidden potentialities by means of the merely mechanical principles of nature, much less can we explain them; and this is so certain that we may boldly assert that it is absurd for man even to make such an attempt or to hope that a newton may one day arise who will make the production of a blade of grass comprehensible to us according to natural laws that have not been ordered by design. such an insight we must absolutely deny to man." still, in another place kant admitted that the facts of comparative anatomy give us "a ray of hope, however faint, that something may be accomplished by the aid of the principle of the mechanism of nature, without which there can be no science in general." it is interesting to turn from this to the bold and aggressive assertion of huxley: "living matter differs from other matter in degree and not in kind, the microcosm repeats the macrocosm; and one chain of causation connects the nebulous origin of suns and planetary systems with the protoplasmic foundations of life and organization." do not expect me to decide where such learned doctors disagree; but i will at this point venture on one comment which may sound the key-note of this address. perhaps we shall find that in the long run and in the large sense kant was right; but it is certain that to-day we know very much more about the formation of the living body, whether a blade of grass or a man, than did the naturalists of kant's time; and for better or for worse the human mind seems to be so constituted that it will continue its efforts to explain such matters, however difficult they may seem to be. but i return to our more specific inquiry with the remark that the history of physiology in the past two hundred years has been the history of a progressive restriction of the notion of a "vital force" or "vital principle" within narrower and narrower limits, until at present it may seem to many physiologists that no room for it remains within the limits of our biological philosophy. one after another the vital activities have been shown to be in greater or less degree explicable or comprehensible considered as physico-chemical operations of various degrees of complexity. every physiologist will maintain that we cannot name one of these activities, not even thought, that is not carried on by a physical mechanism. he will maintain further that in most cases the vital actions are not merely accompanied by physico-chemical operations but actually consist of them; and he may go so far as definitely to maintain that we have no evidence that life itself can be regarded as anything more than their sum total. he is able to bring forward cogent evidence that all modes of vital activity are carried on by means of energy that is set free in protoplasm or its products by means of definite chemical processes collectively known as metabolism. when the matter is reduced to its lowest terms, life, as thus viewed, seems to have its root in chemical change; and we can understand how an eminent german physiologist offers us a definition or characterization of life that runs: "the life-process consists in the metabolism of proteids." i ask your particular attention to this definition since i now wish to contrast with it another and very different one. i shall introduce it to your attention by asking a very simple question. we may admit that digestion, for example, is a purely chemical operation, and one that may be exactly imitated outside the living body in a glass flask. my question is, how does it come to pass that an animal has a stomach?--and, pursuing the inquiry, how does it happen that the human stomach is practically incapable of digesting cellulose, while the stomachs of some lower animals, such as the goat, readily digest this substance? the earlier naturalists, such as linnaeus, cuvier or agassiz, were ready with a reply which seemed so simple, adequate and final that the plodding modern naturalist cannot repress a feeling of envy. in their view plants and animals are made as they were originally created, each according to its kind. the biologist of to-day views the matter differently; and i shall give his answer in the form in which i now and then make it to a student who may chance to ask why an insect has six legs and a spider eight, or why a yellowbird is yellow and a bluebird blue. the answer is: "for the same reason that the elephant has a trunk." i trust that a certain rugged pedagogical virtue in this reply may atone for its lack of elegance. the elephant has a trunk, as the insect has six legs, for the reason that such is the specific nature of the animal; and we may assert with a degree of probability that amounts to practical certainty that this specific nature is the outcome of a definite evolutionary process, the nature and causes of which it is our tremendous task to determine to such extent as we may be able. but this does not yet touch the most essential side of the problem. what is most significant is that the clumsy, short-necked elephant has been endowed--"by nature," as we say--with precisely such an organ, the trunk, as he needs to compensate for his lack of flexibility and agility in other respects. if we are asked _why_ the elephant has a trunk, we must answer because the animal needs it. but does such a reply in itself explain the fact? evidently not. the question which science must seek to answer, is _how_ came the elephant to have a trunk; and we do not properly answer it by saying that it has developed in the course of evolution. it has been well said that even the most complete knowledge of the genealogy of plants and animals would give us no more than an ancestral portrait-gallery. we must determine the causes and conditions that have cooperated to produce this particular result if our answer is to constitute a true scientific explanation. and evidently he who adopts the machine-theory as a general interpretation of vital phenomena must make clear to us how the machine was built before we can admit the validity of his theory, even in a single case. our apparently simple question as to why the animal has a stomach has thus revealed to us the full magnitude of the task with which the mechanist is confronted; and it has brought us to that part of our problem that is concerned with the nature and origin of organic adaptations. without tarrying to attempt a definition of adaptation i will only emphasize the fact that many of the great naturalists, from aristotle onward, have recognized the purposeful or design-like quality of vital phenomena as their most essential and fundamental characteristic. herbert spencer defined life as the continuous _adjustment_ of internal relations to external relations. it is one of the best that has been given, though i am not sure that professor brooks has not improved upon it when he says that life is "response to the order of nature." this seems a long way from the definition of verworn, heretofore cited, as the "metabolism of proteids." to this brooks opposes the telling epigram: "the essence of life is not protoplasm but purpose." without attempting adequately to illustrate the nature of organic adaptations, i will direct your attention to what seems to me one of their most striking features regarded from the mechanistic position. this is the fact that adaptations so often run counter to direct or obvious mechanical conditions. nature is crammed with devices to protect and maintain the organism against the stress of the environment. some of these are given in the obvious structure of the organism, such as the tendrils by means of which the climbing plant sustains itself against the action of gravity or the winds, the protective shell of the snail, the protective colors and shapes of animals, and the like. any structural feature that is useful because of its construction is a structural adaptation; and when such adaptations are given the mechanist has for the most part a relatively easy task in his interpretation. he has a far more difficult knot to disentangle in the case of the so-called functional adaptations, where the organism modifies its activities (and often also its structure) in response to changed conditions. the nature of these phenomena may be illustrated by a few examples so chosen as to form a progressive series. if a spot on the skin be rubbed for some time the first result is a direct and obviously mechanical one; the skin is worn away. but if the rubbing be continued long enough, and is not too severe, an indirect effect is produced that is precisely the opposite of the initial direct one; the skin is replaced, becomes thicker than before, and a callus is produced that protects the spot from further injury. the healing of a wound involves a similar action. again, remove one kidney or one lung and the remaining one will in time enlarge to assume, as far as it is able, the functions of both. if the leg of a salamander or a lobster be amputated, the wound not only heals but a new leg is regenerated in place of that which has been lost. if a flatworm be cut in two, the front piece grows out a new tail, the hind piece a new head, and two perfect worms result. finally, it has been found in certain cases, including animals as highly organized as salamanders, that if the egg be separated into two parts at an early period of development each part develops into a perfect embryo animal of half the usual size, and a pair of twins results. in each of these cases the astonishing fact is that a mechanical injury sets up in the organism a complicated adaptive response in the form of operations which in the end counteract the initial mechanical effect. it is no doubt true that somewhat similar self-adjustments or responses may be said to take place in certain non-living mechanical systems, such as the spinning top or the gyroscope; but those that occur in the living body are of such general occurrence, of such complexity and variety, and of so design-like a quality, that they may fairly be regarded as among the most characteristic of the vital activities. it is precisely this characteristic of many vital phenomena that renders their accurate analysis so difficult and complex a task; and it is largely for this reason that the biological sciences, as a whole, still stand far behind the physical sciences, both in precision and in completeness of analysis. what is the actual working attitude of naturalists towards the general problem that i have endeavored to outline? it would be a piece of presumption for me to speak for the body of working biologists, and i will therefore speak for only one of them. it is my own conviction that whatever be the difficulties that the mechanistic hypothesis has to face, it has established itself as the most useful working hypothesis that we can at present employ. i do not mean to assert that it is adequate, or even true. i believe only that we should make use of it as a working program, because the history of biological research proves it to have been a more effective and fruitful means of advancing knowledge than the vitalistic hypothesis. we should therefore continue to employ it for this purpose until it is clearly shown to be untenable. whether we must in the end adopt it will depend on whether it proves the simplest hypothesis in the large sense, the one most in harmony with our knowledge of nature in general. if such is the outcome, we shall be bound by a deeply lying instinct that is almost a law of our intellectual being to accept it, as we have accepted the copernican system rather than the ptolemaic. i believe i am right in saying that the attitude i have indicated as a more or less personal one is also that of the body of working biologists, though there are some conspicuous exceptions. in endeavoring to illustrate how this question actually affects research i will offer two illustrative cases, one of which may indicate the fruitfulness of the mechanistic conception in the analysis of complex and apparently mysterious phenomena, the other the nature of the difficulties that have in recent years led to attempts to re-establish the vitalistic view. the first example is given by the so-called law or principle of mendel in heredity. the principle revealed by mendel's wonderful discovery is not shown in all the phenomena of heredity and is probably of more or less limited application. it possesses however a profound significance because it gives almost a demonstration that a definite, and perhaps a relatively simple, mechanism must lie behind the phenomena of heredity in general. hereditary characters that conform to this law undergo combinations, disassociations and recombinations which in certain way suggest those that take place in chemical reactions; and like the latter they conform to definite quantitative rules that are capable of arithmetical formulation. this analogy must not be pressed too far; for chemical reactions are individually definite and fixed, while those of the hereditary characters involve a fortuitous element of such a nature that the numerical result is not fixed or constant in the individual case but follows the law of probability in the aggregate of individuals. nevertheless, it is possible, and has already become the custom, to designate the hereditary organization by symbols or formulas that resemble those of the chemist in that they imply the _quantitative_ results of heredity that follow the union of compounds of known composition. quantitative prediction--not precisely accurate, but in accordance with the law of probability--has thus become possible to the biological experimenter on heredity. i will give one example of such a prediction made by professor cuénot in experimenting on the heredity of color in mice (see the following table). the experiment extended through three generations. of the four grandparents three were pure white albinos, identical in outward appearance, but of different hereditary capacity, while the fourth was a pure black mouse. the first pair of grandparents consisted of an albino of gray ancestry, ag, and one of black ancestry, ab. the second pair consisted of an albino of yellow ancestry, ay, and a black mouse, cb. the result of the first union, ag x ab is to produce again pure white mice of the composition agab. the second union, ay x cb is to produce mice that appear pure _yellow_, and have the formula aycb. what, now, will be the result of uniting the two forms thus produced--_i.e._ agab × aycb? cuénot's prediction was that they should yield eight different kinds of mice, of which four should be white, two yellow, one black and one gray. the actual aggregate result of such unions, repeatedly performed, compared with the theoretic expectation, is shown in the foregoing table. as will be seen, the correspondence, though close, is not absolutely exact, yet is near enough to prove the validity of the principle on which the prediction was based, and we may be certain that had a much larger number of these mice been reared the correspondence would have been still closer. i have purposely selected a somewhat complicated example, and time will not admit of a full explanation of the manner in which this particular result was reached. i will however attempt to give an indication of the general mendelian principle by means of which predictions of this kind are made. this principle appears in its simplest form in the behavior of two contrasting characters of the same general type--for instance two colors, such as gray and white in mice. if two animals, which show respectively two such characters are bred together, only one of the characters (known as the "dominant") appears in the offspring, while the other (known as the "recessive") disappears from view. in the next generation, obtained by breeding these hybrids together, both characters appear separately and in a definite ratio, there being in the long run three individuals that show the dominant character to one that shows the recessive. thus, in the case of gray and white mice, the first cross is always gray, while the next generation includes three grays to one white. this is the fundamental mendelian ratio for a single pair of characters; and from it may readily be deduced the more complicated combinations that appear when two or more pairs of characters are considered together. such combinations appear in definite series, the nature of which may be worked out by a simple method of binomial expansion. by the use of this principle astonishingly accurate numerical predictions may be made, even of rather complex combinations; and furthermore, new combinations may be, and have been, artificially produced, the number, character and hereditary capacity of which are known in advance. the fundamental ratio for a single pair of characters is explained by a very simple assumption. when a dominant and a recessive character are associated in a hybrid, the two must undergo in some sense a disjunction or separation in the formation of the germ-cells of the hybrid. this takes place in a quite definite way, exactly half the germ-cells in each sex receiving the potentiality of the dominant character, the other half the potentiality of the recessive. this is roughly expressed by saying that the germ-cells are no longer hybrid, like the body in which they arise, but bear one character or the other; and although in a technical sense this is probably not precisely accurate, it will sufficiently answer our purpose. if, now, it be assumed that fertilization takes place fortuitously--that is that union is equally probable between germ-cells bearing the same character and those bearing opposite characters,--the observed numerical ratio in the following generation follows according to the law of probability. thus is explained both the fortuitous element that differentiates these cases from exact chemical combinations, and the definite numerical relations that appear in the aggregate of individuals. grandparents ag (white) ab (white) ay (white) cb (black) | | | | +---------+ +-----------+ | | parents agab (white) aycb (yellow) | | +----------------------+ | observed calculated {agay} {abay} (white) {agab} offspring ---------------{abab} { {agcy} (yellow) {abcy} { {abcb (black) {agcb (gray) ---- ---- now, the point that i desire to emphasize is that one or two very simple mechanistic assumptions give a luminously clear explanation of the behavior of the hereditary characters according to mendel's law, and at one stroke bring order out of the chaos in which facts of this kind at first sight seem to be. not less significant is the fact that direct microscopical investigation is actually revealing in the germ-cells a physical mechanism that seems adequate to explain the disjunction of characters on which mendel's law depends; and this mechanism probably gives us also at least a key to the long standing riddle of the determination and heredity of sex. these phenomena are therefore becoming intelligible from the mechanistic point of view. from any other they appear as an insoluble enigma. when such progress as this is being made, have we not a right to believe that we are employing a useful working hypothesis? but let us now turn to a second example that will illustrate a class of phenomena which have thus far almost wholly eluded all attempts to explain them. the one that i select is at present one of the most enigmatical cases known, namely, the regeneration of the lens of the eye in the tadpoles of salamanders. if the lens be removed from the eye of a young tadpole, the animal proceeds to manufacture a new one to take its place, and the eye becomes as perfect as before. that such a process should take place at all is remarkable enough; but from a technical point of view this is not the extraordinary feature of the case. what fills the embryologist with astonishment is the fact that the new lens is not formed in the same way or from the same material as the old one. in the normal development of the tadpole from the egg, as in all other vertebrate animals, the lens is formed from the outer skin or ectoderm of the head. in the replacement of the lens after removal it arises from the cells of the iris, which form the edge of the optic cup, and this originates in the embryo not from the outer skin but as an outgrowth from the brain. as far as we can see, neither the animal itself nor any of its ancestors can have had experience of such a process. how, then, can such a power have been acquired, and how does it inhere in the structure of the organism? if the process of repair be due to some kind of intelligent action, as some naturalists have supposed, why should not the higher animals and man possess a similar useful capacity? to these questions biology can at present give no reply. in the face of such a case the mechanist must simply confess himself for the time being brought to a standstill; and there are some able naturalists who have in recent years argued that by the very nature of the case such phenomena are incapable of a rational explanation along the lines of a physico-chemical or mechanistic analysis. these writers have urged, accordingly, that we must postulate in the living organism some form of controlling or regulating agency which does not lie in its physico-chemical configuration and is not a form of physical energy--something that may be akin to a form of intelligence (conscious or unconscious), and to which the physical energies are in some fashion subject. to this supposed factor in the vital processes have been applied such terms as the "entelechy" (from aristotle), or the "psychoid"; and some writers have even employed the word "soul" in this sense--though this technical and limited use of the word should not be confounded with the more usual and general one with which we are familiar. views of this kind represent a return, in some measure, to earlier vitalistic conceptions, but differ from the latter in that they are an outcome of definite and exact experimental work. they are therefore often spoken of collectively as "neo-vitalism." it is not my purpose to enter upon a detailed critique of this doctrine. to me it seems not to be science, but either a kind of metaphysics or an act of faith. i must own to complete inability to see how our scientific understanding of the matter is in any way advanced by applying such names as "entelechy" or "psychoid" to the unknown factors of the vital activities. they are words that have been written into certain spaces that are otherwise blank in our record of knowledge, and as far as i can see no more than this. it is my impression that we shall do better as investigators of natural phenomena frankly to admit that they stand for matters that we do not yet understand, and continue our efforts to make them known. and have we any other way of doing this than by observation, experiment, comparison and the resolution of more complex phenomena into simpler components? i say again, with all possible emphasis, that the mechanistic hypothesis or machine-theory of living beings is not fully established, that it _may_ not be adequate or even true; yet i can only believe that until every other possibility has realty been exhausted scientific biologists should hold fast to the working program that has created the sciences of biology. the vitalistic hypothesis may be held, and is held, as a matter of faith; but we cannot call it science without misuse of the word. when we turn, finally, to the genetic or historical part of our task, we find ourselves confronted with precisely the same general problem as in case of the existing organism. biological investigators have long since ceased to regard the fact of organic evolution as open to serious discussion. the transmutation of species is not an hypothesis or assumption, it is a fact accurately observed in our laboratories; and the theory of evolution is only questioned in the same very general way in which all the great generalizations of science are held open to modification as knowledge advances. but it is a very large question what has caused and determined evolution. here, too, the fundamental problem is, how far the process may be mechanically explicable or comprehensible, how far it is susceptible of formulation in physico-chemical or mechanistic terms. the most essential part of this problem relates to the origin of organic adaptations, the production of the fit. with kant, cuvier and linnaeus believed this problem scientifically insoluble. lamarck attempted to find a solution in his theory of the inheritance of the effects of use, disuse and other "acquired characters"; but his theory was insecurely based and also begged the question, since the power of adaptation through which use, disuse and the like produce their effects is precisely that which must be explained. darwin believed he had found a partial solution in his theory of natural selection, and he was hailed by haeckel as the biological newton who had set at naught the _obiter dictum_ of kant. but darwin himself did not consider natural selection as an adequate explanation, since he called to its aid the subsidiary hypotheses of sexual selection and the inheritance of acquired characters. if i correctly judge, the first of these hypotheses must be considered as of limited application if it is not seriously discredited, while the second can at best receive the scotch verdict, not proven. in any case, natural selection must fight its own battles. latter day biologists have come to see clearly that the inadequacy of natural selection lies in its failure to explain the origin of the fit; and darwin himself recognized clearly enough that it is not an originative or creative principle. it is only a condition of survival, and hence a condition of progress. but whether we conceive with darwin that selection has acted mainly upon slight individual variations, or with devries that it has operated with larger and more stable mutations, any adequate general theory of evolution must explain the origin of the fit. now, under the theory of natural selection, pure and simple, adaptation or fitness has a merely casual or accidental character. in itself the fit has no more significance than the unfit. it is only one out of many possibilities of change, and evolution by natural selection resolves itself into a series of lucky accidents. for agassiz or cuvier the fit is that which was designed to fit. for natural selection, pure and simple, the fit is that which happens to fit. i, for one, am unable to find a logical flaw in this conception of the fit; and perhaps we may be forced to accept it as sufficient. but i believe that naturalists do not yet rest content with it. darwin himself was repeatedly brought to a standstill, not merely by specific difficulties in the application of his theory, but also by a certain instinctive or temperamental dissatisfaction with such a general conclusion as the one i have indicated; and many able naturalists feel the same difficulty to-day. whether this be justified or not, it is undoubtedly the fact that few working naturalists feel convinced that the problem of organic evolution has been fully solved. one of the questions with which research is seriously engaged is whether variations or mutations are indeterminate, as darwin on the whole believed, or whether they may be in greater or less degree determinate, proceeding along definite lines as if impelled by a _vis a tergo_. the theory of "orthogenesis," proposed by naegeli and eimer, makes the latter assumption; and it has found a considerable number of adherents among recent biological investigators, including some of our own colleagues, who have made important contributions to the investigation of this fundamental question. it is too soon to venture a prediction as to the ultimate result. that evolution has been orthogenetic in the case of certain groups, seems to be well established, but many difficulties stand in the way of its acceptance as a general principle of explanation. the uncertainty that still hangs over this question and that of the heredity of acquired characters bears witness to the unsettled state of opinion regarding the whole problem, and to the inadequacy of the attempts thus far made to find its consistent and adequate solution. here, too, accordingly, we find ourselves confronted with wide gaps in our knowledge which open the way to vitalistic or transcendental theories of development. i think we should resist the temptation to seek such refuge. it is more than probable that there are factors of evolution still unknown. we can but seek for them. nothing is more certain than that life and the evolution of life are natural phenomena. we must approach them, and as far as i can see must attempt to analyze them, by the same methods that are employed in the study of other natural phenomena. the student of nature can do no more than strive towards the truth. when he does not find the whole truth there is but one gospel for his salvation--still to strive towards the truth. he knows that each forward step on the highway of discovery will bring to view a new horizon of regions still unknown. it will be an ill day for science when it can find no more fields to conquer. and so, if you ask whether i look to a day when we shall know the whole truth in regard to organic mechanism and organic evolution, i answer: no! but let us go forward. columbia university press a series of twenty-two lectures descriptive in untechnical language of the achievements in science, philosophy and art, and indicating the present status of these subjects as concepts of human knowledge, are being delivered at columbia university, during the academic year - , by various professors chosen to represent the several departments of instruction. mathematics, by cassius jackson keyser, _adrain professor of mathematics_. physics, by ernest fox nichols, _professor of experimental physics_. chemistry, by charles f. chandler, _professor of chemistry_. astronomy, by harold jacoby, _rutherfurd professor of astronomy_. geology, by james furman kemp. _professor of geology_. biology, by edmund b. wilson, _professor of zoology_. physiology, by frederic s. lee, _professor of physiology_. botany, by herbert maule richards, _professor of botany_. zoology, by henry e. crampton, _professor of zoology_. anthropology, by franz boas. _professor of anthropology_. archaeology, by james rignall wheeler, _professor of greek archaeology and art_. history, by james harvey robinson, _professor of history_. economics, by henry rogers seager, _professor of political economy_. politics, by charles a. beard, _adjunct professor of politics_. jurisprudence, by munroe smith, _professor of roman law and comparative jurisprudence_. sociology, by franklin henry giddings, _professor of sociology_. philosophy, by nicholas murray butler. _president of the university_. psychology, by robert s. woodworth, _adjunct professor of psychology_. metaphysics, by frederick j.e. woodbridge, _johnsonian professor of philosophy_. ethics, by john dewey, _professor of philosophy_. philology, by a.v.w. jackson, _professor of indo-iranian languages_. literature, by harry thurston peck, _anthon professor of the latin language and literature_. these lectures are published by the columbia university press separately in pamphlet form, at the uniform price of twenty-five cents, by mail twenty-eight cents. orders will be taken for the separate pamphlets, or for the whole series. address the columbia university press columbia university, new york * * * * *