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 PEOPLE'S 
 BOOKS 
 
 ZOOLOGY 
 
ZOOLOGY 
 
 THE STUDY OF ANIMAL LIFE 
 BY E. W. MAcBRIDE, F.R.S. 
 
 LONDON: T. C. & E. C. JACK 
 67 LONG ACRE, W.C., AND EDINBURGH 
 NEW YORK: DODGE PUBLISHING CO. 
 
BIOLOGY 
 LIBRARY 
 
CONTENTS 
 
 CHAPTER I 
 
 PAGE 
 THE SCOPE OP ZOOLOGY 5 
 
 Origin of the term zoology Origin of the ideas of "life" 
 of *' animal " and " plant " Modern scientific definitions 
 of these terms. 
 
 CHAPTER II 
 
 THE PRACTICAL IMPORTANCE OF ZOOLOGY 8 
 
 Similarity of the bodies of animals and men, and its bearing 
 on medical science Importance of zoology on account 
 of its bearing on food supply The bearing of zoology 
 on social problems. 
 
 CHAPTER III 
 
 THE LIVING SUBSTANCE . . . . . .14 
 
 The seat of life in animals The manifestations of life The 
 nature of contraction The nucleus and its functions 
 Secretion Definition of the term " cell " Definitions 
 of metabolism, respiration and excretion, &c. Bio- 
 chemistry Chemical nature of proteid. 
 
 CHAPTER IV 
 
 THE VARIOUS KINDS OF CELLS : THE TISSUES . . 22 
 
 Epithelial cells Glandular cells Transitions between epi- 
 thelial cells and muscular and nervous cells Skeletal 
 cells Reproductive cells Definition of tissue Skeletal 
 tissue Muscular tissue Glandular tissue, 
 iii 
 
iv CONTENTS 
 
 CHAPTER V 
 
 THE CLASSIFICATION OF ANIMALS .... 
 
 Definition of phylum Description of the more important 
 phyla Porifera Ccelenterata Platyhelminthes Ne- 
 mertinea Nematoda Annelida Arthropoda Mollusca 
 Brachiopoda Polyzoa Echinodermata Vertebrata. 
 
 CHAPTER VI 
 
 THE ORIGIN OF SPECIES . . 
 
 Evidence that new species are continually coming into being 
 Darwin's theory of their mode of origin Mendel's 
 researches into the laws of heredity Bearing of these 
 researches on Darwin's theory. 
 
 CHAPTER VII 
 
 THE CONSEQUENCES OF DARWIN'S THEORY. THE INTER- 
 PRETATION OF DEVELOPMENT .... 
 
 The recapitulatory element in development Mutual relations 
 of embryo and larva Relation of migration to the 
 formation of new species. 
 
 CHAPTER VIII 
 
 THE BEARING OF ZOOLOGY ON THE QUESTIONS OF 
 HUMAN ORIGIN AND THE FUTURE DESTINY OF 
 THE RACE . 
 
 The origin of man from apes The decisive factors in the 
 evolution of man Outlook for the future of the British 
 
 APPENDIX 91 
 
 INDEX 93 
 
ZOOLOGY 
 
 CHAPTER I 
 
 THE SCOPE OF ZOOLOGY. 
 
 THE word zoology is derived from the Greek word zoon, which 
 meant "animal," and from the termination -logia, which 
 signified a discourse. The name therefore, as might be ex- 
 pected, denotes the science of animals, and is therefore 
 narrower in its scope than the science of biology, which deals 
 with everything living and of which the science of zoology 
 forms a subdivision. 
 
 We see, therefore, that at the outset we are faced with two 
 fundamental questions, viz. (1) What is life ? and (2) What 
 is an animal ? To no two questions is it more difficult 
 to give a precise answer than to these. If with Herbert 
 Spencer we define life "as the continuous adjustment of 
 internal to external relations, " who is the wiser, or who has 
 any better conception of life as a result of the definition ? 
 
 The better way of approaching the problem is the histori- 
 cal one, and instead of asking "What is life V' or "What is 
 an animal ] " let us rather ask how the ideas embodied in 
 the terms " life " and " animal " took their origin. 
 
 The word "animal" is derived from the same root as the 
 Latin word " anima." This originally denoted simply " wind " 
 or " breeze." It was then used to denote " breath/' i.e. the 
 air sucked into and ejected from the lungs in respiration. 
 Breath was recognised by the ancients as the universal 
 concomitant of life and activity in man and animals, and 
 indeed it was regarded by them as the essential principle 
 or cause of animal life. So the word animal signified in the 
 
 5 
 
6 ZOOLOGY 
 
 first instance rv a breathing, being," but the significance of the 
 word was soon extended so as to include all beings which 
 actively moved, such as insects, even if they did not obvi- 
 ously breathe air. The wider idea of life was connected by 
 the ancients with the idea of growth, and the proper name 
 for the science of life would be Physiology, a word com- 
 pounded of the termination -logia and the word phiisis, 
 which signified growth in the widest sense. The good old 
 word physiology had however been used for so long in con- 
 nection with medicine to denote the science of the life of man 
 that Huxley thought it necessary to coin a new word, Biology, 
 to denote life in its widest sense. The etymology of the 
 term is bad Greek, for bios never denoted " life " in the 
 general sense in that language. However, all confusion of 
 thought was avoided by the new term, and we must forgive 
 its faulty derivation. As the idea of animal is that of a 
 moving being, so the idea of life is that of something that 
 grows. These ideas were taken over from primitive times 
 by that orderly careful type of thinking characteristic of our 
 own day, and denominated science ; by it they have been 
 recast, because not everything that moves is an animal for 
 instance, a volcanic bomb or a shooting star is not included 
 in the term nor is everything that grows living for instance, 
 a snowball rolling down a snowy hill or a crystal suspended 
 in its mother liquor both grow, but neither is regarded as 
 alive. 
 
 Life is defined for science as the state or condition of a 
 being which grows and which reproduces itself. The growth 
 postulated is however very different from that of the snow- 
 ball or of the crystal. In these two cases increase in size 
 results from the addition of new matter, which is laid on in 
 successive layers round the original matter which persists as 
 the core. But in the case of a being which has life or, as 
 we term it, a living being or organism growth results from 
 the intercalation of new material between the interstices of the 
 finest particles of the old: this is termed growth by intus- 
 susception. In this way the child grows to manhood ; so that 
 to take the face, for instance, although the absolute size of the 
 features is more than doubled, their general proportions relative 
 to one another remain the same. By reproduction is meant 
 
THE SCOPE OF ZOOLOGY 7 
 
 the formation of new living beings by the cutting off of small 
 portions, termed germs, from pre-existing living beings, which 
 then by growth come to resemble the beings from which they 
 took their origin. Only in this way are new living beings 
 formed ; new life can only arise in connection with pre-existing 
 life, and reproduction is only a special case of growth. It is a 
 marvellous thought to reflect that although living beings are 
 continually dying, i.e. being turned into dead matter which 
 neither moves nor grows, yet dead matter can only be con- 
 verted into a living matter into a part, that is, of a living 
 being through being eaten or assimilated by that being. 
 
 An animal is defined, for the purposes of science, as a living 
 being which moves and which requires for its sustenance food 
 containing peculiarly complicated chemical compounds known 
 as prbteids; these in the majority of cases are taken in the 
 solid form and digested, i.e. melted inside the animal's body. 
 Plants, on the other hand, are defined as living beings which 
 can only take their food in the form of liquids and gases, but 
 which require only simple chemical compounds for their sus- 
 tenance, and which do not move except so far as movement 
 is involved in the process of growth. But in practice these 
 definitions are not rigidly adhered to. The reason for this 
 is that both plants and animals can be divided into groups, 
 the members of which exhibit the closest resemblance to one 
 another, and a particular living being is reckoned as animal or 
 plant according to the general characteristics of the group 
 to which it belongs, even should its own individual peculiarities 
 be in some respects irreconcilable with the general definitions 
 of animal or plant. Thus the sensitive plant droops its leaves 
 and shuts up its leaflets when touched, just as the snail draws in 
 its tentacles and retires into its shell when it is irritated ; and yet 
 the sensitive plant is reckoned a plant, because in all its char- 
 acters it closely agrees with the plants of the Pea order. The 
 carnivorous plants form hollow cups at the tips of their leaves, 
 which entrap insects by means of slanting hairs and other con- 
 trivances. These insects die and their bodies are digested, and 
 the resultant materials are absorbed by the surrounding tissues 
 of the plant which act like the walls of the stomach of an 
 animal. Nevertheless we reckon these carnivorous plants as 
 plants, because in all the details of their structure wood, 
 
8 ZOOLOGY 
 
 leaves, flowers, &c. they closely resemble ordinary plants. 
 Similarly there are some minute living beings which are green 
 and which can live on quite simple chemical compounds like 
 plants, but which are nevertheless reckoned as animals because 
 of their likeness to other living things which are undoubtedly 
 animals. It must be admitted, however, that there is a great 
 deal more doubt about the true position of these "plant- 
 animals" than there is about the position of the "animal- 
 plants" referred to above a doubt which is emphasized by 
 the fact that the same group of beings is lectured on by 
 professors of both botany and zoology, and described in text- 
 books which deal with both sciences. In many cases where 
 animals of some size have a green colour and are apparently 
 able to subsist on simple chemical substances, this appearance 
 has been shown to be due to the fact that their bodies 
 are the home of multitudes of minute plants, which grow in 
 them and give them their colour by shining through the more 
 or less transparent substance of the body, but which sooner or 
 later are digested by the animals in which they live and serve 
 as their food.. The only plant-animals which are left un- 
 accounted for are extremely minute microscopic forms, in the 
 case of many of which, the question of whether they shall be 
 ranked as plants or animals can only be decided by a careful 
 balance of the evidence for and against. 
 
 For all practical purposes the definition of an animal as a 
 living thing or, to use the shorter and more convenient term, 
 an organism which can move and take in solid food which 
 must contain proteid is a good definition, and certainly ex- 
 presses the idea which rises in the mind of a scientific man 
 when he names the word animal. 
 
 CHAPTER II 
 
 THE PRACTICAL IMPORTANCE OF ZOOLOGY. 
 
 HAVING in the previous chapter given the reader some idea of 
 what the science of zoology is about, we propose in the present 
 one to attempt to give him some conception of the enormous 
 
PRACTICAL IMPORTANCE OF ZOOLOGY 9 
 
 practical importance of the subject. We use the word practical 
 in its widest sense : not only in the sense of importance in 
 helping man to make his living, but of importance in aiding 
 man to understand his relationship to this wonderful universe 
 in which he finds himself. Indeed it may be said that a 
 rational conception of his relationship to the universe is just 
 what distinguishes the civilised man from the savage. Since 
 he was able to think at all, man has formed conceptions of his 
 relations to the universe which may be termed working hypo- 
 theses. These still survive amongst large sections of mankind, 
 and even amongst many who consider themselves civilised, as 
 beliefs in magic and other superstitions, but they are not based 
 on data which have been thoroughly examined and tested. It is 
 the great characteristic of science that it will not accept any 
 data except those which have been subjected to the most 
 searching examination, and experience has shown that by the 
 way of science alone, and not by the ways of magic and ancient 
 religion, is it possible to gain any mastery over Nature. 
 
 Now the first reason why zoology is such an eminently 
 practical science is that we are ourselves animals. Our bodies 
 are made of fundamentally the same stuffs as their bodies and 
 are subject to the same laws. Whether the animus or spirit 
 in us, which knows and feels, differs in kind or only in degree 
 from the animus in them, is a question which, strictly speaking, 
 lies outside the province of zoology altogether. The science 
 which should attempt to deal with questions like these is 
 termed Comparative Psychology. In virtue of possessing the 
 power of speaking a power possessed by no other animal 
 man is able to explore and make the acquaintance of the 
 animus which resides in each of his fellow-men ; but this 
 avenue into the inner consciousness of animals is closed, and no 
 other exists which can lead the inquirer very far. Zoology, 
 then, concerns itself with what can be directly observed about 
 animals, viz. the structure of their bodies and their behaviour, 
 without trying to pry very much into what they know and 
 feel. 
 
 But the study of the structure of the bodies and of the 
 behaviour of animals throws an enormous amount of light on 
 the working of the parts of the human body. The only other 
 way of finding out the uses of the parts of the human body is 
 
10 ZOOLOGY 
 
 to observe what happens when one part is thrown out of gear, 
 but this method is subject to serious limitations. When one 
 part is diseased, as St. Paul has said, all the other parts suffer 
 with it, i.e. do not act normally, and it is sometimes difficult 
 to separate the effects due to the absence of the working of a 
 particular part from the effects due to the abnormal working 
 of the other parts. But in the various groups of animals 
 which are most nearly allied to man we have bodies made 
 up of the same parts as our own bodies ; but these parts are 
 of different relative sizes, and we find that when one part is 
 greatly enlarged a particular kind of behaviour is exaggerated, 
 and that when it is relatively much diminished certain ele- 
 ments of behaviour tend to disappear ; and we can reason with 
 complete confidence from the exaggerated structure to the corre- 
 sponding behaviour, and yet we are dealing with organisms in 
 which all the parts are working harmoniously together. To 
 give a very simple instance : there is a band of fibres connect- 
 ing our two eyes beneath the brain which is called the optic 
 chiasma. It is believed that the function of this is to focus 
 the two eyes on a single object so that it can be seen distinctly. 
 This belief is borne out by two facts : first, in ordinary fish 
 which have more or less blade-like bodies the optic chiasma is 
 absent : this is what is to be expected if our view as to the 
 function of the chiasma is correct, for such animals could by 
 no possibility focus the eyes which are on opposite sides of 
 their bodies on the same object ; secondly, in insectivorous birds 
 the optic chiasma is extremely large, and this again is what is 
 to be expected, for such birds which pursue their minute prey 
 on the wing have need of extremely accurate focussing, and 
 owing to the speed at which they travel must rapidly alter their 
 focus as they approach their prey. For the education of any 
 man who wishes to deal in an intelligent and progressive 
 manner, and not merely in a traditional manner with the 
 ailments of the human body a preliminary training in 
 zoology would appear to be absolutely necessary. 
 
 But the study of zoology is of enormous importance on 
 many other grounds. A large part of our food is furnished 
 by the bodies of animals. Proteids, which form a necessary 
 part of our sustenance, can be obtained most directly from 
 this source. A few semi-civilised races are exclusively plant- 
 
PRACTICAL IMPORTANCE OF ZOOLOGY 11 
 
 eaters, but the leading dominating races of mankind who are 
 in the van of the march of culture have always been flesh- 
 eaters. Primitive man also was a hunter and a flesh-eater, 
 and civilised man in moments of relaxation throws off the 
 shackles of civilisation and becomes a hunter again. Now the 
 scientific breeding and care of cattle is a branch of applied 
 zoology, and one that is growing in importance. So long as 
 one was concerned merely with the raising of cattle in old 
 civilised countries like our own, in which through centuries 
 of trial and error traditional rules had been hammered out, 
 the agriculturist got on fairly well without zoology, but when 
 cattle-breeding was begun in new countries where conditions are 
 widely different from home conditions, then the necessity of 
 a knowledge of zoological science became clamant. Frozen 
 meat imported into the London docks was found to be 
 marked by circular scars, caused by the presence of a worm 
 which lay coiled up therein, and the question arose, was such 
 meat dangerous for human consumption or not] Only the 
 zoologist could answer this question, and the answer he gave 
 was that the eggs of this worm pass through a necessary part 
 of their development in the body of a fly which is not found 
 in Great Britain, and hence that the meat infested by these 
 worms could be consumed with impunity. Again, the sea 
 furnishes us with a large part of our animal food. In 1897 
 it was reckoned that the produce of the Canadian fisheries, im- 
 perfectly exploited though these were, exceeded in value the 
 famous Manitoba wheat-harvest. The question, whether we 
 are exhausting our fisheries or not, has been raised again and 
 again. When the steam trawler with its improved fishing 
 gear was introduced, a cry arose from the longshore fishermen 
 that their new rivals were ruining fisheries by destroying the 
 spawn. The zoologist was the only person who could settle 
 this dispute, and he did so by showing that the spawn of 
 most of the valuable fishes floated and was quite out of the 
 reach of the trawls of the new fishing-boats. In the future 
 we confidently anticipate that zoology will be more and 
 more applied to the scientific exploitation of the " harvest of 
 the sea." 
 
 But in our opinion the greatest value of zoology is to be 
 found in another aspect of its teaching. As we study the 
 
12 ZOOLOGY 
 
 lives of animals more closely we become impressed with the 
 tremendous competition which goes on between different kinds 
 of animals. If all the young born in any one species were 
 to survive and grow up, that species would in a very few 
 years overrun the entire surface of the globe. In the case of 
 pheasants it has been calculated that if all the eggs which a 
 pheasant laid were to be hatched and all the young chicks 
 were to survive to maturity and lay the normal number of 
 eggs in turn, a period of eighteen years would suffice to pro- 
 duce from the offspring of a single pair a crowd of pheasants 
 which, if placed side by side touching each other, would cover 
 the entire surface of the globe including the sea, and even 
 then a large number would be left over. But there are many 
 species of animals far more prolific than the pheasant; the 
 cod, for instance, lays nine million eggs in one season, and the 
 only thing which prevents all the food material in the sea 
 being made into young cod in the course of a year or two 
 is the enormous destruction which the young cod are subject 
 to at the hands of their innumerable enemies. 
 
 We may make the matter a little clearer to ourselves if 
 we compare the earth to a huge prairie of dry grass and com- 
 pare each species of animal to a fire which has been lighted in 
 it. Each fire tends to spread outwards so as to overrun the 
 whole prairie, but each is limited in the possibilities of its 
 spread by the spread of its neighbours. One may notice on 
 the banks of the Clyde an oyster- catcher busily gaining its 
 living. This is a bird which feeds largely on the shellfish 
 which are exposed at low tide. The question rises in one's 
 mind, why are not all the rocks denuded of their molluscan 
 inhabitants by this bird? The only answer is that the 
 nestlings of the oyster-catcher are in turn destroyed by some 
 enemy, and that oyster-catchers are thus prevented from 
 becoming inconveniently numerous. It sometimes occurs that 
 a species of animal reaches a new country where its enemies 
 are too feeble to seriously thin its numbers. Then an enor- 
 mous multiplication occurs, and the species spreads in such 
 a manner as to threaten to crush out all other forms of life, 
 If it finally succeeded in doing this it would no doubt bring 
 its own existence to an abrupt termination, for it would 
 exhaust its own food supply. The English rabbit transported 
 
PRACTICAL IMPORTANCE OF ZOOLOGY 13 
 
 to Australia increased at such a rate as to make pasturage 
 well-nigh impossible, so that an enormous sum of money has 
 now to be spent annually in order to keep it in check. 
 
 Now these facts have a double interest for us. First, 
 civilised man for his own use insists on rearing enormous 
 herds of a few kinds of domesticated animals and enormous 
 crops of a few species of cultivated plants. Such thickly 
 massed crowds of individuals belonging to a single species are 
 rare in Nature, and are as stubble to the flame if once invaded 
 by a successful enemy. If an enemy like the boll-weevil gains 
 an entrance to his cotton-fields, how shall man protect himself 
 against it? The answer which one of the best living ento- 
 mologists gives to this question is as follows : " The only way 
 in which a problem of this kind can be attacked is by a careful 
 study of the habits and life-history of the insect enemy ; such 
 a study will be sure to reveal some weak point in the armour 
 of the foe, and it is there that we must attack him." A know- 
 ledge of zoology is essential, therefore, for the proper protection 
 of our crops and herds, and indeed how could it be otherwise 1 
 Are not we ourselves a species of animals maintaining our 
 right to existence by a hard struggle against our competitors *? 
 And it may be added that these competitors are just as ready 
 to attack our own bodies as they are to devour our domesticated 
 animals and plants. Some of them, which have accustomed 
 themselves to live in our blood, run riot there and bring pain 
 and death in their train. Only by a careful study of their life- 
 history is it possible for us to guard ourselves against them. 
 By means of such studies the American biologists have made 
 the Isthmus of Panama, once a graveyard for Europeans, one 
 of the healthiest districts in America. 
 
 The second interest which the competition between species 
 has for the thoughtful man is its bearing on the history of the 
 human race itself. There is no doubt that for tens of 
 thousands of years the numbers of mankind were kept down 
 by internal and external enemies, just as are the numbers of 
 the oyster-catcher. But in the last two thousand years, and 
 more notably in the last two hundred, and most notably of all 
 in the last fifty, man has been learning to know and conquer 
 his enemies. The result of this has been that population 
 amongst civilised nations has begun to increase at an alarming 
 
14 ZOOLOGY 
 
 rate, and all sorts of problems have sprung up in consequence. 
 The nature of these problems is clear, and their occurrence 
 seems inevitable to the man who has had a zoological training, 
 but it is far from clear to many of our so-called thinkers who 
 have not had this advantage, otherwise we should not hear 
 proposals put forward by Fabian societies and similar organi- 
 sations that the State should undertake the care and nurture of 
 the babies of the poor, thus relieving the least competent 
 members of the State of the last deterrent to reckless repro- 
 duction. If we are going to try to prevent Nature's method 
 of choosing the best, viz. the elimination of the poorest and 
 weakest, we must surely substitute for it our own selection, 
 else we shall suffer the fate which befalls half-hearted meddlers 
 in great affairs. 
 
 CHAPTER III 
 
 THE LIVING SUBSTANCE. 
 
 HAVING gained some conception of the nature and bearing of 
 zoological science, we may now proceed to examine some of the 
 special problems with which it deals. In a book of such 
 limited size as the present volume it would be futile to attempt 
 to cover the whole extent of the science : the most that can be 
 done is to try to select a few samples of the kind of question 
 that zoologists have tried to solve. 
 
 All life expresses itself in movement, either, as in the case 
 of plants, in the movement due to growth or, as in the case of 
 animals, in the r^ovement of fully grown parts, which return 
 after their movement to their former condition Now if we 
 leave aside for the present the movement due to growth and 
 confine our attention solely to the characteristic animal move- 
 ment of adult parts, the first question which confronts us is 
 this : How is this movement produced? If we examine small 
 fragments or very thin slices of the part of the body which is 
 the seat of movement, we shall always find clear fibres, which 
 when movement occurs grow shorter and thicker. These fibres, 
 which are termed muscle fibres, are the direct cause of move- 
 ment in all the higher animals. In the lower animals we 
 
THE LIVING SUBSTANCE 
 
 15 
 
 find irregular masses of clear substance which throw out 
 
 tongues in a peculiar way and flow first in one direction and 
 
 then in another. The simplest of all animals, the proteus 
 
 animalcule Amoeba, consists entirely of a small lump of this 
 
 substance, which moves by flowing out in tongues in the way 
 
 described. These movements are consequently termed 
 
 amoeboid, but they are however of fundamentally the same 
 
 character as the contraction of muscle fibres, since the flowing 
 
 .i due to the contraction of one part squeezing out the rest. 
 
 We often find also, both in simple and complex animals, 
 
 projecting hair-like structures termed cilia or flagella, which 
 
 vibrate rhythmically and 
 
 propel the whole animal 
 
 long if it is small, and 
 
 .ropel the surrounding 
 
 duid in a current past 
 
 he animal if the latter 
 
 3 larger. But the vibra- 
 
 ion of these cilia and 
 
 Lagella has also been 
 
 hown to be due to the 
 
 ; .Iternate contraction of 
 
 :iarrow slips on each side 
 
 of the vibrating structure; F IG . i._ An Amoeba progressing. The 
 
 .0 that we may sum up direction of movement is towards 
 
 t he matter thus : all the ri ght ; n, the nucleus. 
 
 { tnimal movement is due 
 
 o contraction in some portion of the moving parts whereby 
 ihat portion becomes shorter and thicker. 
 
 If we could thoroughly explain how this contraction is 
 rought about, we should have penetrated much more deeply 
 into the mystery of life than it has so far been possible to do. 
 But although a complete explanation is not yet forthcoming, 
 ertain broad facts in connection with moving parts may be 
 mphasized The clear substance which is the seat of con- 
 raction is always surrounded by a skin consisting of a thin 
 lastic membrane. When we have to deal with amoeboid 
 novement, this skin is so thin and delicate that it is constantly 
 >eing burst and then formed again round the clear substance 
 vhich has been extruded through the break. This substance 
 
16 ZOOLOGY 
 
 seems to be of a fluid, or at least a semi-fluid character, and the 
 act of contraction seems to be accompanied by an increase in 
 volume in this substance, for the fibre in shortening its length 
 increases, as we have seen, its diameter and gains in volume 
 on the whole a fact familiar to every schoolboy who " raises " 
 his biceps muscle. In the most highly developed contracting 
 fibres, such as are found in the flesh of our own bodies and that of 
 the highest animals, we meet with a phenomenon known as the 
 cross-striping of the fibre. Every contractile fibre when at 
 rest is found to be crossed by a series of dark plates, placed at 
 regular intervals from one another like the sleepers on a railway 
 line. Between these plates the fibre is made up of clear semi- 
 fluid substance. When contraction occurs the clear substance 
 increases in quantity, and appears to be imbibed by the dark 
 
 FIG. 2. Part of a cross-striped muscle fibre: aw, 
 anisotropic substance ; is, isotropic substance in a 
 single fibril ; /, a fibril ; n, nuclei. 
 
 plates which swell and at the same time become less dark in 
 colour. The dark plates are known by their behaviour to- 
 wards light to be solid, and their purpose seems to be by 
 exercising an attraction on the fluid substance to increase the 
 rapidity of the contraction. It is known that cross-striped 
 fibres contract much more rapidly than plain ones, and the 
 striping is best marked in the fibres which move the wings of 
 insects ; and these muscles must in many cases contract and 
 relax many hundreds of times in a second. 
 
 The increase in volume then of the semi-fluid substances 
 seems to be a proximate cause, if not the proximate cause, in 
 producing contraction. Now the only way in which we can 
 conceive of this increase in volume being brought about is by 
 chemical decomposition. When a match is applied to gun- 
 powder contained in a gun-barrel an enormous increase in 
 volume takes place, owing to the fact that the materials of the 
 
THE LIVING SUBSTANCE 17 
 
 gunpowder oxidise, i.e. unite with the oxygen of the air so as 
 to produce great quantities of highly elastic gas. Now we 
 find that if we remove all oxygen from the neighbourhood of 
 the contracting parts of an animal, they very soon cease to 
 be capable of contraction. They are paralysed ; and it seems a 
 plausible suggestion that the chemical decomposition connected 
 with contraction is of the nature of an oxidation. 
 
 Besides its manifestation in the form of movement, life gives 
 evidence of its presence in secretion. By secretion is meant 
 the continuous production of substances, not themselves living, 
 which subserve some secondary use for the animal. Thus the 
 hairs which clothe our bodies are composed of dead dry horny 
 substance, but they are of inestimable value in protecting our 
 bodies against undue loss of heat. But secretions may be 
 fluid : the digestive juice which reduces our solid food to the 
 liquid form, and thus renders it capable of mingling with our 
 living substance and contributing to its " growth by intus-sus- 
 ception," is such a secretion. The point which we wish to 
 emphasize at this juncture is that whenever we penetrate to 
 the point where secretion is being actively carried on we find 
 masses of clear semi-fluid substance essentially similar to 
 that which we found in contractile fibres, and that it is by the 
 continual decomposition of this substance that the secretion is 
 produced. Similarly, if we investigate any quickly growing 
 part we find the same clear semi-fluid substance in a word, 
 all manifestations of life are associated with this substance, 
 or, putting it differently, this substance is the living substance, 
 and the body of an animal consists essentially of this substance 
 and of its secretions. 
 
 This marvellous " living substance " is termed protoplasm, 
 and since in it is hidden the secret of life we may look at it 
 a little more closely. It is not a simple uniform substance, 
 but a mixture of substances. In every mass of living proto- 
 plasm we may detect a nucleus or several nuclei. The nucleus 
 is usually a rather compact body of an oval or spherical shape, 
 but in exceptional cases it may take the form of a rod, or even 
 of a spiral thread or it may be represented by a cloud of 
 granules. In the vast majority of cases, however, it is an 
 oval or spherical body. Its distinguishing feature is that it 
 contains a substance called chromatin, which has a remarkable 
 
18 ZOOLOGY 
 
 power of absorbing staining fluids such as carmine, hsema- 
 toxylin (which is an extract of logwood), &c. Now in very 
 small animals it sometimes happens that there is only one 
 nucleus present ; it is then possible to cut the body of such an 
 animal into two parts, one of which includes the nucleus and 
 the other does not. The part devoid of a nucleus can live for 
 some time and carry out movements, but it is unable to digest 
 or assimilate food, and in a short time it dies. We conclude, 
 therefore, that the production of the substances which melt 
 solid food (i.e. digestion), and the building up of the di- 
 gested food into new living material (i.e. assimilation), are 
 functions which can be carried out only by the aid of the 
 chromatin. If we apply the term protoplasm to the living 
 substance taken as a whole, and call the body in which 
 the chromatin is stored the nucleus, then the rest of the 
 living substance is termed cytoplasm. Broadly speaking, 
 the amount of nuclear material present bears a more or less 
 fixed proportion to the amount of cytoplasm surrounding it, 
 and it may take the form of a single-branched nucleus or 
 of a number of smaller rounded nuclei. Now there are some 
 animals so small that a single nucleus suffices for all their 
 needs ; but in most animals there are many nuclei, and in all 
 the higher groups of animals (termed collectively the Metazoa) 
 there is an area of cytoplasm surrounding each nucleus which is 
 marked off from the rest of the cytoplasm by a thin membrane. 
 Such an area is termed a cell, and when the living substance of 
 an animal is thus divided up it is said to possess cellular 
 structure. The cells may be compared to the bricks in the 
 wall of a house. Animals devoid of cellular structure are 
 termed Protozoa (lit. first-animals). Most of these are of 
 microscopic size, and very many have only one nucleus. Such 
 animals have been compared to a single cell of the body of 
 a higher animal and have been termed unicellular, but the 
 comparison is misleading. They are closely allied to other 
 Protozoa of slightly larger size and which possess numerous 
 nuclei, and it is quite wrong to compare the tiny sun-animal- 
 cule of our ditches with its two hundred nuclei to a single cell 
 of a higher animal. The proper term to apply to Protozoa is 
 non-cellular. A most distinguished American biologist has 
 recently shown what the meaning of the cellular structure is. 
 
THE LIVING SUBSTANCE 19 
 
 We have seen that life amongst animals manifests itself in 
 various ways, viz. by movement and by different kinds of 
 secretion. Now each of these different ways implies that 
 a different kind of chemical decomposition is going on in the 
 cell, and the membranes separating the cells from one another 
 derive their importance from the fact that they regulate the 
 passage of materials in and out of the cells. Each cell is in 
 fact a chemical factory, and the possession of cellular structure 
 by an animal is really a sign of the number and variety of 
 chemical processes going on in it. We have so far alluded 
 chiefly to the decomposition or breaking down of complex 
 chemical substance, which occurs in the cell whether the 
 result of this be movement or secretion ; but, as we have 
 already hinted, a concurrent process of building up new living 
 substance out of the food is also going on, else life would soon 
 come to an end. This building up, which as we have seen 
 is effected by the aid of the chromatin, is termed anabolism, 
 whilst the decomposition is termed catabolism, and metabolism 
 is used to denote the sum total of all the chemical processes 
 occurring in the cell. 
 
 Neither anabolism nor catabolism can take place in the 
 absence of oxygen, which must be constantly supplied to 
 the living substance, as it is constantly being used up in the 
 process of oxidation, which constitutes the principal item in the 
 process of decomposition and is also an element in the processes 
 of anabolism. The absorption of this necessary oxygen by the 
 living substance is termed respiration. To sum up, we shall 
 form the justest conception of the activities of living substance, 
 if we regard it as in a continual state of burning. Life is 
 a fire, as Buddha said long ago, and the scriptural story of the 
 bush which burned and was not consumed exactly expresses 
 the modern conception of the nature of life. 
 
 If we have once grasped this conception, certain conclusions 
 will at once suggest themselves. The ordinary coal-fire 
 burning in our grates would soon be extinguished if the ashes 
 which result from the oxidation of the coal were not con- 
 stantly removed. So it is with life : the products of decom- 
 position, when they are so far broken down that they cannot 
 be used again to build up new protoplasm by the addition 
 of materials froin the, food, must be removed, or else they 
 
20 ZOOLOGY 
 
 will soon put a stop to life. Such products are termed 
 excreta, and the process of getting rid of them is known as 
 excretion. The commonest substance which is excreted is 
 the gas known familiarly but incorrectly as carbonic acid. 
 This gas is very soluble in water, and as all living substance 
 is bathed in water or some other fluid (i.e. blood) largely 
 composed of water, it can be seen that ever opportunity is 
 given for the removal of this compound. Lastly, when an 
 animal takes in solid food and proceeds to digest it, there 
 often remains a residue which resists the digestive juice. 
 This residue is termed faeces, and the process of getting rid 
 of it is called defecation. 
 
 Since oxygen can only be taken into the living substances 
 and the poisonous excreta got rid of by the process of diffusion, 
 it follows that living substance can never be accumulated in 
 large masses, but can only exist in the form of small granules, 
 or of thin plates presenting relatively large surfaces to a 
 circumambient fluid of some kind. By diffusion we mean the 
 process by which two liquids or two gases become commingled, 
 in consequence of the motion of the molecules of which they 
 are made up. In the case of liquids this is always a slow 
 process ; and a solid mass of protoplasm a quarter of an inch 
 in diameter, if such a thing existed, could no more be living 
 throughout its whole extent than a block of coal could be 
 burning throughout its whole extent. In both cases com- 
 bustion could only exist at the surface of the respective 
 masses. 
 
 The task of finding out the physical and chemical structure 
 of living substance is one which ought properly to form part 
 of zoology, but one which, on account of its difficulty and the 
 amount of concentrated effort which it requires, has caused 
 the growth of a subsidiary science devoted to its solution alone. 
 This science is termed Biochemistry. We may briefly indi- 
 cate some of the points which the devotees of this young 
 science have already gained in their struggle with the mystery 
 of mysteries. Since life is a fire, and since this fire requires 
 the constant diffusion of oxygen into the living substance and 
 of carbonic acid out of it, living substance must be a fluid, since 
 only in fluids and gases can diffusion exist. All living substance 
 contains large quantities of water and all life is suspended by 
 
THE LIVING SUBSTANCE 21 
 
 drought, hence it is almost certain that the living substance is a 
 thici solution of certain substances in water. It has been found 
 that when solutions of complicated chemical substances, of such 
 substances as gelatine for instance, are made, the solution may 
 be perfectly transparent and yet refuse to pass through filter 
 paper. This is attributed by biochemists to the circumstance 
 that the molecules of such substances are arranged in groups 
 which are of enormous size compared to the molecules of such 
 simple substances as common salt, &c. They suppose that 
 these complicated molecules are in fact bigger than the pores 
 in the paper. "When the molecules reach an extreme size, the 
 solution ceases to be absolutely transparent and appears slightly 
 cloudy, and such a mixture we can hardly call it a solution 
 is termed an emulsion. Living substance is then supposed to 
 be such a thick solution, or in some cases an emulsion. But 
 what kind of substances form the stuff which is dissolved or sus- 
 pended in the water ] To such a question no perfectly satis- 
 factory answer can be given, for when we begin to analyse 
 living substance chemically, we kill it and then our results 
 give us the composition of dead and not of living material. 
 
 But we may perhaps assume that when living substance is 
 killed by gentle heating its composition is not very much 
 changed. It then forms what is called proteid, which, as 
 we have learned, forms a necessary constituent in the food 
 of all animals. By treatment with solutions of salt and 
 by boiling with dilute acids, proteid can be split up into 
 still simpler substances, and by a continuance of this treatment 
 on the products of its decomposition we at last reach com- 
 paratively simple compounds whose chemical structure can be 
 recognised. These compounds contain carbon, hydrogen, 
 nitrogen and oxygen, and are of the class called amino-acids. 
 The great characteristic of an acid is that it can combine with 
 another kind of chemical compound called a base to form a 
 neutral compound termed a salt. The acid has as it were a 
 hand, by which it can grasp the hand of another compound. 
 An amino-acid is an acid which is also at the same time a 
 base in a word, it has two hands, viz. an acid hand by 
 which it can grasp a base, and a basic hand by which it can 
 grasp an acid. The base to which it unites itself may be the 
 basic portion of another amino-acid, and the acid which it 
 
22 ZOOLOGY 
 
 grasps by the basic hand may be the acid portion of still another 
 amiiio-acid. We see that in this way ammo-acids can be 
 linked together in long chains, and when proteid is broken up 
 by boiling it with acid, what happens is that the links in 
 these chains become separated from one another. Now an 
 acid can be di- or even tri- basic that is to say, that it may 
 have two or even three acid hands instead of one, and so we 
 see that there are possibilities of infinite complication in the 
 structure of these compound amino-acids. But we know that 
 proteid contains sulphur as well as the four elements named 
 above; there must be therefore a sulphur-containing group 
 attached somewhere in the amino-acid chain, and chromatin 
 is known to be a compound of proteid with nucleic acid, which 
 itself is a compound of phosphoric acid. We need not pursue 
 the subject further ; enough has been said to show that if the 
 living substance be not widely different in structure from dead 
 proteid, and if proteid be composed of enormously long amino- 
 acid chains, then, even if every different kind of animal and 
 plant has a different kind of living substance, there is enough 
 possibility of variation in the amino-acid chain to account for 
 them all. So far, this is by far the most plausible theory of 
 the composition of living substance which has yet been put 
 forward; but the working out of this theory in detail will 
 consume many decades of experiments, and it is on this work 
 in detail that its final verification depends. The best and final 
 proof of its truth would be the building up of proteid out 
 of simple substances in the laboratory. Some steps in this 
 direction have been taken, but at every step it is as if one were 
 following a road which continually forked and there were no sign- 
 post to guide one as to which fork one should follow. 
 
 CHAPTER IV 
 
 THE VARIOUS KINDS OF CELLS I THE TISSUES. 
 
 WE have seen that the living substance in the bodies of the 
 larger animals is divided into cells. Now these cells are 
 never all alike, but differ from one another in size, shape, 
 
VARIOUS KINDS OF CELLS: TISSUES 23 
 
 and function. Nevertheless all these various sizes and shapes 
 can be classified under a few leading forms, and a knowledge 
 of these forms is a necessary prerequisite for the understand- 
 ing of the language in which the bodies of the various kinds 
 of animals are described. Indeed these forms may be com- 
 pared to the alphabet of a language in which the various 
 kinds of animals are sentences of different kinds. It is the 
 necessity of learning alphabets of this kind which makes the 
 study of zoology and of other sciences appear so dull and 
 technical to the beginner. Once the alphabet has been thor- 
 oughly assimilated, zoology becomes intensely interesting. The 
 schoolboy struggling with his declensions and his vocabulary 
 
 FIG. 3. Diagrammatic representation of simple 
 columnar epithelium containing gland cells and 
 sense cells ; cut, cuticle ; gl, gland cell ; n, 
 nuclei ; s, sense cell. 
 
 finds learning Latin dull work, but the finished classical 
 scholar finds the reading of the ancient authors stimulating, 
 elevating, and fascinating. 
 
 The simplest kind of cell is a columnar one with outer 
 and inner terminal faces, and with a varying number of sides 
 which fit against the sides of similar cells, so that the whole 
 together constitutes a pavement, or as it is termed in zoology 
 an epithelium (Gr. epi, upon ; thelion, a tile). Such " pave- 
 ments " form the external covering of the body in very many 
 animals; they also line the stomachs and intestines of all 
 animals which possess them. The individual cells are termed 
 epithelial cells, and their height varies very much in relation 
 to their diameter. The height may be so small that the cell 
 appears like a flat tile, or it may be so great that the cell 
 is a pillar. Where the matter has been most closely investi- 
 
24 ZOOLOGY 
 
 gated it is found that each cell is united to its neighbours 
 by little bars or bridges of living substance traversing the 
 cell membrane. When we are dealing with an external 
 epithelium, the normal state of affairs is that each cell secretes 
 on its external face a horny or gelatinous membrane termed 
 the cuticle, which protects the living material from the 
 assaults of external enemies, whilst by its internal face it 
 absorbs nourishment from the fluids of the body. In the 
 case of an internal epithelium matters are rather different. 
 The internal face gives rise to a secretion which helps to 
 digest the food which has been swallowed, and through this 
 same face the products of digestion are absorbed, whilst through 
 the basal face of the cell these same products are given off 
 into the internal fluids of the body. In the highest animals, 
 however, the functions of producing the digestive juice and 
 of absorption are not performed by the same cells. Some 
 are specialised for producing the digestive secretions, whilst 
 others merely absorb. Such secretion-producing cells are 
 called glandular cells : they are generally arranged together 
 in groups, and such a group is termed a gland. It generally 
 happens that the cells forming a gland are arranged so as 
 to surround a blind pocket-like outgrowth of the stomach or 
 intestine. This pocket-like outgrowth may become branched, 
 and a huge glandular mass may be the result of repeated 
 branching. The human liver, which is the largest organ 
 in the body, is an extreme instance of a large branched 
 gland. 
 
 In the case of many land animals, where the outer surface of 
 the body has to be constantly lubricated in order to prevent 
 its drying up, the outer epithelium develops specially glandular 
 cells which sometimes are arranged so as to line glandular 
 pockets. The glands which produce sweat or perspiration in 
 ourselves are examples of this modification. But it must be 
 remembered that the thin membrane which these epithelial 
 cells can produce on their outer surfaces is a secretion resulting 
 from the decomposition of the living substance, and that no 
 line can be drawn between such a secretion and a fluid one like 
 our own perspiration for example, because every intermediate 
 grade of secretion is known. So that after all a glandular cell 
 is merely an epithelial cell in which the function of secretion 
 
VARIOUS KINDS OF CELLS: TISSUES 25 
 
 is emphasized and in which the secretion which is produced is 
 of a fluid character. 
 
 We must now pass to the consideration of the cells by which 
 contraction is carried out. In these, of course, a considerable 
 portion of the living substance has been modified into contrac- 
 tile fibres, the structure of which has already been described 
 (Chap. Ill) ; indeed these cells are often called muscular fibres. 
 But a muscular fibre and a muscular fibril (Fig. 2) are not 
 identical terms. A muscular fibre is a cell the cytoplasm of 
 which is usually made up of many fibrils, and in addition of a 
 certain amount of residual unmodified cytoplasm in which the 
 nucleus is situated. When the fibrils are plain or unstriped, the 
 cell is spindle-shaped and the nucleus remains single. When, 
 however, the fibrils are cross-striped, the fibre often becomes 
 
 FlG. 4. Diagram showing gradual change of epithelial cells 
 into muscle cell /, the basal tails of the cells in which 
 contractile fibrils appear. 
 
 enormously lengthened, and the nucleus divides and gives rise to 
 many daughter nuclei, which are scattered along the length of the 
 fibre (see Fig. 2), each embedded in a little nest of unmodified 
 protoplasm. Now it is a most interesting fact that as we 
 examine a large series of animals we can find every intermediate 
 stage between a muscular cell and an epithelial cell Thus in 
 certain sponges, which may be regarded as the lowest Metazoa, 
 the cells forming the outer epithelium contract when irritated. 
 When relaxed they are like flat tiles, but they gradually change 
 into the shape of pillars as they contract, but this contraction 
 is a slow process. If we ascend one step higher in the scale of 
 Metazoan animals, and examine the sea-anemones, we find that 
 in them the inner epithelium has the power of contraction, but 
 in this case we find that each contractile epithelial cell has its 
 base produced into a fibre, the cytoplasm in which is changed 
 into muscular fibrils. The power of contraction is confined to this 
 elongated base. Within the group of the sea-anemones every 
 
26 ZOOLOGY 
 
 stage can be found between a cell which has a large epithelial 
 portion wedged in between neighbouring epithelial cells, and a 
 cell in which this portion is reduced to a mere button, ' and 
 which consists mainly of the fibre. Such a cell is almost un- 
 distinguishable from a simple spindle-shaped muscular fibre. 
 Contraction in the sea-anemones is far more rapid than in the 
 sponges; and this is due to the fact that definite muscular 
 fibrils or myonemes are developed in the one case and not in 
 the other. 
 
 We conclude, therefore, that epithelial cells are the funda- 
 mental type of cell, and that glandular and muscular cells are 
 derived and modified types. But epithelial cells can undergo 
 other modifications which are equally important and interest- 
 ing. An animal reacts i.e. alters its behaviour in conse- 
 quence of changes in its surroundings. These changes such 
 as changes of temperature, the impact of foreign bodies, and of 
 light and sound, &c. impress the animal through its outer 
 epithelium, and we find, therefore, that certain cells of this 
 epithelium are specially modified to receive and transmit these 
 impressions. Such cells are termed sensory cells (see Fig. 3). 
 In the simplest variety of sensory cells each possesses one or more 
 sense hairs projecting from its outer surface, which can be 
 made to vibrate by sound, or which when pressed on act as levers, 
 to transmit pressure into the interior of the cell. The visual cell, 
 which is affected by light and which forms the essential element 
 in all organs of sight, possesses a clear glassy rod instead of a 
 stiff hair. How the light irritates the visual cell is not known, 
 but it is known that another kind of cell is required before the 
 sensation of light can be received. This cell is a pigment- 
 cell, and its characteristic is that it secretes granules of an 
 intensely deeply-coloured substance termed pigment. Visual 
 cells and pigment cells are placed either side by side or end to 
 end. The suggestion has been made that the intensely rapid 
 vibrations which constitute the waves of light decompose the 
 pigment and produce some active chemical substance which 
 irritates the glassy rod, but this is mere conjecture. The cells 
 which are sensitive to odours have delicate hairs like cilia 
 (see p. 15) projecting from their surfaces, but we have as yet 
 no idea how the infinitesimal particles which constitute the 
 substance of a scent affect these hairs. 
 
VARIOUS KINDS OF CELLS: TISSUES 27 
 
 In order that the impressions which reach the sensory cells 
 may affect the animals' behaviour, they must be transmitted to 
 the contractile and secretory cells by which that behaviour is 
 manifested. With doubtful exceptions this transmission is 
 never direct, but the impressions received by many sense cells 
 are gathered into a central cell termed a nerve cell or neuron, 
 and from it are transmitted to the glandular and contractile 
 cells. A nerve cell has a receiving and a transmitting end. 
 The receiving end usually consists of a group of root-like out- 
 growths termed receptive dendrites, which come into close 
 contact with the transmitting end of another nerve cell or with 
 the base of a sense cell. The transmitting end is a long fibre 
 
 FIG. 5. Diagrammatic figure of a neuron or nerve cell in con- 
 nection with a simple muscle cell : ax, axon ; dend. r, re- 
 ceptive dendrites ; dend. t, terminal dendrites ; m, muscle 
 cell. 
 
 termed an axon, which eventually ends in a tuft of branches, 
 called terminal dendrites, which are in close contact with the 
 receiving end of another nerve cell, or in contact with a muscle 
 cell or a gland cell. Now the axon may branch, and each branch 
 terminates in the same manner as the main stem ; the dendrites 
 of the receptive end of the nerve cell may also be in contact 
 with many sense cells. We thus see that the great function of 
 a nerve cell is to add together impressions received by many 
 sense cells and to distribute them to definite contractile and 
 glandular cells. In this way it comes about that the animaPs 
 response to irritation does not consist of a group of isolated 
 jerks, as it would if each sense cell communicated its impulse 
 directly to a muscular cell, but takes the form of an orderly 
 reaction of the whole body designed either to bring it nearer 
 
28 ZOOLOGY 
 
 agreeable objects or to remove it farther from disagreeable ones 
 in a word, the function of the nervous system is not to 
 originate but to collect, add together, and distribute. 
 
 When the nerve cells of such creatures as jelly-fish are studied 
 under the high power of the microscope, every intermediate 
 stage between an ordinary sensory epithelial cell and a nerve 
 cell can be seen. We find some cells with receptive and 
 transmitting ends whose bodies are still wedged between neigh- 
 bouring epithelial cells : and no doubt is left in our minds that 
 a nerve cell, like a muscular cell, a sense cell, and a gland cell, 
 must be regarded as nothing more than a modification of an 
 ordinary epithelial cell. We have now to consider two types 
 of cell which cannot be regarded as modifications of epithelial 
 
 FlG. 6. Diagram showing gradual change of an epithelial 
 cell into a nerve cell: ax, axons; sh, sense-hairs of 
 incipient nerve cell. 
 
 cells : these are (1) the cells of the blood and internal skeleton, 
 and (2) the reproductive cells. 
 
 It must seem strange to the ordinary reader to associate 
 blood and skeleton. That they are closely associated is one 
 of the curious and unexpected discoveries of scientific zoology. 
 The simplest type of internal skeleton is a more or less firm 
 secretion separating two layers of epithelium ; such a skeleton 
 is found in many simple polyp-like creatures which adhere to 
 the seaweeds on our coast. But the character of this " base- 
 ment membrane," as it is termed, varies indefinitely from a 
 thin elastic membrane to a thick gelatinous semi-flnid mass 
 such as is found in jelly-fish. When the skeleton takes on 
 the latter form we find in it free cells, which are capable of 
 slow " amoeboid " movement. These cells have been budded 
 off from the epithelial cells, but they are of quite a different 
 
VARIOUS KINDS OF CELLS: TISSUES 29 
 
 character from these. They have the form of little lumps of 
 protoplasm from which finger-like or hair-like outgrowths 
 radiate in all directions, and these outgrowths seem to be of 
 the same nature as the outflows of cytoplasm by which simple 
 animals like Amoeba progress (see p. 15). But in many 
 cases these cells appear to become stationary and to join 
 each other by the tips of their outgrowths so as to form a 
 
 FiG. 7. Diagrammatic representation of the formation of 
 connective tissue. There is a network of cells joined by 
 processes the cytoplasm of which is dotted. Connective 
 tissue fibres are indicated by heavy lines ; n, nuclei of 
 the cells. 
 
 network. Along the sides of these connected outgrowths there 
 are deposited tough fibres, and in this way a network of fibres 
 is produced (Fig. 7). Now such a network of fibres is the 
 fundamental basis of the construction of our own bones, tendons, 
 and sinews. In our blood we have cells like those just de- 
 scribed, viz. the celebrated white corpuscles of which so much 
 has been written in late years, but the "skeletal substance'' 
 in which they move is the fluid of the blood. Nevertheless 
 even in this fluid there lies latent the power of forming fibres 
 
30 ZOOLOGY 
 
 of essentially the same character as the fibres of tendon, and 
 this power is called into activity in case of a wound when 
 the outflowing blood is exposed to the air. Then a tangled 
 mass of fibres is formed which is called a clot. 
 
 The reproductive cells are those which are cast off from 
 the parent, and which by growth give rise to a new organism. 
 In Protozoa they are of very many kinds, but both amongst 
 Protozoa and amongst Metazoa they may be divided into two 
 categories, viz. those that must unite with one another in pairs 
 before they can develop ; and those which are able to develop 
 
 FIG. 8. Reproductive cells: A, ovum; B, spermatozoon; 
 A, head of spermatozoon; n, nucleus of ovum; nl, 
 nucleolus of ovum ; t, tail of spermatozoon. 
 
 directly without such preliminary union. The former variety 
 are termed gametes, from the Greek word game, which signi- 
 fies a marriage ; the latter are termed asexual spores. In the 
 overwhelming majority of the Metazoa only gametes are pro- 
 duced, and these gametes are invariably of two kinds, viz. 
 comparatively large motionless gametes termed eggs, and small 
 actively moving gametes termed spermatozoa. The egg or 
 ovum is usually a rounded cell, with a considerable amount of 
 reserve food material termed yolk stored up into its cylo- 
 plasm : it has a large nucleus which takes the unusual form 
 of a rounded bladder with a small solid sphere inside it termed 
 the nucleolus. The spermatozoon consists of a minute head 
 
VARIOUS KINDS OF CELLS: TISSUES 31 
 
 which is practically nothing but a small dense nucleus and a 
 nbratile tail which is really a cilium or flagellum. When 
 the union of the two kinds of gametes, or, as it is termed, their 
 conjugation, takes place, the spermatozoon plunges into the 
 egg, breaking off its tail at the egg's surface. The head of the 
 spermatozoon, which, as we have seen, is a nucleus, then travels 
 towards the nucleus of the egg and unites with it. The com- 
 pound cell which results from the union of these two gametes 
 
 termed a zygote, and the zygote, after a longer or shorter 
 Interval, proceeds to divide into a mass of cells termed the 
 embryo, which, through a variety of changes termed develop- 
 ment or ontogeny, become converted into an organism re- 
 sembling the parent. The two kinds of gamete are usually 
 produced by two different kinds of individuals ; that which 
 produces the eggs being termed the female, and that which 
 produces the spermatozoon being termed the male. In some 
 groups of animals, however notably in those that lead some- 
 what isolated lives and have little power of locomotion both 
 kinds of gametes are produced by the same individual. Such 
 an individual is termed an hermaphrodite. 
 
 The difference between male and female is termed a dif- 
 ference of sex. 
 
 Having surveyed the principal types of all, we must now 
 take up the consideration of the manner in which they are 
 combined to make up what are termed the tissues of the 
 animal. Out of the tissues in turn the organs of the 
 animal are constructed. The word organ is used to denote 
 a part of an animal's body which is adapted to perform a 
 particular office. Thus a man's leg is a locomotor organ, the 
 eye a visual organ, and so on. 
 
 The word tissue is derived from a French word meaning a 
 piece of cloth, and indeed it is sometimes used in English in 
 that sense. In zoology it was first applied to the network 
 of cell and fibres which constitutes the basis of the skeleton. 
 This network is termed connective tissue, and the inter- 
 crossing fibres of which it is largely constituted do resemble 
 the threads which form the warp and woof of a piece of cloth. 
 Connective tissue surrounds every organ of the body and 
 separates the different organs from one another. In fact, as 
 Huxley has said, if we could imagine all the organs of an 
 
 o 
 
32 ZOOLOGY 
 
 animal's body to be dissolved away, a complete cast of each 
 in connective tissue would remain. When the ground sub- 
 stance of connective tissue in which fibres and cells lie 
 becomes impregnated with calcareous salts, it becomes changed 
 into what is called bone. 
 
 Though "tissue" is a very suitable word to describe the 
 aggregation of cells and their products which constitute 
 the skeleton, it is by no means an accurate description of 
 aggregates of epithelial cells. Nevertheless it is applied in 
 zoology to denote all aggregates of similar cells, and so we speak of 
 epithelial, nervous, muscular, and glandular tissues. Epithelial 
 tisues consist of thin sheets of epithelial cells, in most cases 
 only one cell thick. In many cases it has been shown that 
 each epithelial cell is connected with its neighbours by bars 
 and bridges of cytoplasm. Where the "tissue" is more 
 than one layer thick it is usually found that only the inner- 
 most layer is in an actively growing healthy condition. The 
 other layers are in a decadent or dying state, and their 
 cytoplasm is being gradually converted into dead secretion. 
 A splendid example of this can be seen in the layers of cells 
 which make up the outer skin or " epidermis " of our bodies. 
 A moment's reflection will show us why this must be so. 
 All protoplasm, in order to be in vigorous life, must be in 
 immediate contact with a nutrient oxygen-carrying medium, 
 and when there are several layers of cells only one can be 
 so situated. 
 
 Muscular tissue consists of bundles of muscle fibres which 
 in the vast majority of cases run parallel with one another, 
 so that the forces occasioned by their contraction act in the 
 same direction. Such a bundle is termed a muscle, it is 
 surrounded by a sheath of connective tissue termed a fascia, 
 and the individual fibres constituting the muscle are likewise 
 separated from one another by connective tissue which penetrates 
 between them. Only in the case of the muscular tissue which 
 makes up the heart of man and the higher animals has it 
 been ascertained that the muscular fibres are connected with 
 one another by bridges of protoplasm. 
 
 Nervous tissue consists primarily of bunches or streaks 
 of nerve cells placed near each other, but not entering into 
 connection with one another. In many cases the axons of many 
 
THE CLASSIFICATION OF ANIMALS 33 
 
 nerves or neurons run parallel with one another, forming a 
 bundle which is called a nerve. A single axon enclosed in 
 its special sheath, the origin of which is doubtful, is termed 
 a nerve fibre. The bodies of the neurons are arranged in 
 many cases in clumps which are termed ganglia (Gr. ganglion, 
 a knot), and the bands of nerve fibres connecting them are 
 termed commissures. 
 
 We have already seen that gland cells are in the vast 
 majority of cases modified epithelial cells, and it therefore 
 follows that glandular tissue is merely modified epithelial 
 tissue. In most cases, as we have seen, glandular tissue is 
 arranged in the form of layers of epithelial cells, forming 
 the walls of tubes. A bundle of such tubes surrounded and 
 separated from one another by connective tissue would be a 
 typical example of glandular tissue. Finally, the reproductive 
 cells themselves are very often arranged in the form of tubes, 
 and a mass of such tubes, with their supporting connective 
 tissue, constitutes a reproductive organ, and might be termed 
 reproductive tissue^ 
 
 CHAPTER V 
 
 THE CLASSIFICATION OF ANIMALS. 
 
 IT is common knowledge that there are an enormous number 
 of sorts of animals in the world. Every one knows also that 
 some of these sorts resemble each other closely, whilst others 
 are widely distinct. Thus, for instance, a missel-thrush and 
 a song-thrush are closely similar to one another; whilst there 
 is apparently little or no resemblance between a star-fish and 
 an elephant. When the matter is looked into closely, it is 
 found that it is possible to arrange all animals in groups, 
 the members of which agree in the common structural plan 
 on which their bodies are built. Such groups are termed 
 Phyla, from the Greek word signifying primarily leaf or shoot, 
 and secondarily stock or race. The Protozoa, or animals 
 devoid of cellular structure, constitute one such phylum ; but 
 the Metazoa consist of many phyla. It would take too long 
 
34 ZOOLOGY 
 
 to go into the characters of all these phyla, but the general 
 features of the most important must be pointed out. We 
 begin with the two simplest, the Porifera or Sponges, and the 
 
 Ccelenterata or Polyps. The 
 animals included in both these 
 groups consist of a series of 
 branched tubes whose walls are 
 made up of two layers of cells 
 separated from one another by 
 a secretion of semi-fluid consis- 
 tency known as the jelly. 
 This jelly often contains cells, 
 which have wandered inwards 
 from the outer layer, and which 
 in many cases secrete needles 
 of flinty or calcareous sub- 
 stance! This jelly, with the 
 cells which it contains, is at 
 once the simplest form of 
 skeleton, and the beginning 
 of the blood system. In 
 these groups the individual 
 is a little difficult to define. 
 In its simplest form it is a 
 single tube with a terminal 
 opening. When this tube 
 branches we regard this as 
 a kind of budding or asexual 
 FIG. 9. 4, diagrammatic longi- reproduction. The branches 
 tudinal section of a simple may separate completely from 
 Sponge. The collar-cells and the parent, but they may 
 spicules are represented in rem am attached to it and form 
 black; B, a single collar-cell . ,. . , , f , . , 
 enlarged ; col in A , collar-cell ; an individual of a higher order. 
 col in B, collar-cell; o, oscu- In both phyla one end of 
 lum ; P, pore ; spic, spicule. the tube opens directly or 
 
 indirectly to the exterior by a 
 
 wide opening. In the Porifera or Sponges this opening is 
 only used for casting out the undigested remnants of the 
 food, and is termed the osculum ; whilst the walls of the 
 tube are perforated by a multitude of minute pores (whence 
 
THE CLASSIFICATION OF ANIMALS 35 
 
 the name of the group) through which water carrying the 
 food enters. The cells forming the outer layer of the sponge 
 are flattened, but, as we have already seen (p. 25), are capable, 
 in some cases at any rate, of changing their shape i.e. of con- 
 traction. The cells forming the inner layer in the simplest 
 sponges, and portions of the inner layer in all sponges, are very 
 peculiar and characteristic. They are termed collar-cells, and 
 each has a free end facing the cavity of the sponge, and a basal 
 end projecting into the jelly. Projecting from the free end there 
 is a flagellum, by the vibration of which the current of water 
 is produced which enters through the pores and brings food 
 to the animal. Surrounding the flagellum is a funnel-shaped 
 collar, apparently composed of stiff cytoplasm, inside which 
 the current apparently forms a little eddy, and in consequence 
 of this eddy food particles are directed inwards and engulfed 
 by the collar-cells. The common bath-sponge is the dried 
 and cleaned skeleton of one of the more complicated sponges. 
 In practically all sponges the skeleton consists of flinty or 
 calcareous needles in some these are bouud together into 
 ropes by a gelatinous substance termed spongin. In the bath- 
 sponge the ropes of spongin are there, but the needles or 
 spicules have disappeared. On this depends the usefulness 
 of the sponge, for were we to use an ordinary species of sponge 
 we should lacerate our faces every time we performed our 
 ablutions. 
 
 The Ccelenterata a phylum which includes polyps, jelly- 
 fish, sea-anemones, and coral-forming animals have, like the 
 Porifera, only a single opening leading into the system of 
 tubes of which their bodies are built up ; but this opening 
 is used both for taking in food (ingestion) as well as for 
 ejecting the undigested material. The walls of the tubes 
 are, like those which make up the bodies of Porifera, com- 
 posed of two layers of cells, the outer of which is termed 
 ectoderm (i.e. outside skin) and the inner, endoderm (i.e. 
 inside skin), but the tubal walls are not perforated by pores, 
 nor does the inner layer consist of collar-cells, nor do the Ccelen- 
 terata obtain their food by causing currents in the water. 
 On the contrary, the terminal opening of each tube is used 
 for taking in food, and is termed the mouth. It is typically 
 surrounded by a ring of tentacles, by means of which prey 
 
36 ZOOLOGY 
 
 is captured and pushed through the mouth into the interior 
 of the tube, where it is digested. These tentacles are muscular 
 prolongations of the body capable of exceedingly quick move- 
 ment, and containing in their outer cells numerous stinging- 
 cells or cnidoblasts. These stinging-cells are just as char- 
 acteristic of Coelenterata as the collar-cells are of Porifera. 
 Each cell has a little projecting process or sense-hair for 
 perceiving stimuli: whilst embedded in its cytoplasm is a 
 little bag, termed the nematocyst, which contains a clear 
 fluid, and one end of which is tucked inwards into its 
 interior like the turned-in toe of a stocking. This end is 
 very long, and is termed the thread. When the animal 
 is irritated, the cells forming the outer layer of the body 
 contract, and squeeze the stinging cell. As a consequence the 
 nematocyst of the stinging-cell is squirted out like a seed from 
 a ripe orange, and the in turned end or thread of the nematocyst, 
 is also turned inside out. This thread is coated with a 
 poisonous substance, and it penetrates the bodies of the 
 animals on which the Ccelenterate preys, and stuns and kills 
 them. In most cases the thread is incapable of penetrating 
 the human skin, but in the case of some of the larger jelly-fish 
 it can pierce even this, and then the sufferer experiences a 
 burning pain. 
 
 "Coral" is a general name given to the hard skeleton 
 of Coelenterata when this is present : it is formed in many 
 different ways. The beautiful red coral, which is used to 
 form bracelets and necklaces, consists of needles of calcareous 
 matter cemented together; these needles are formed just 
 like the spicules of sponges by cells which have migrated into 
 the jelly from the outer layer of cells. The coral, which 
 forms the masses of which coral reefs mainly consist, appears 
 to be formed as an external exudation from the cells of the 
 external layer. In this way cups or thecse are formed, in 
 which the bodies of the Coelenterate individuals are contained. 
 To the fanciful eyes of the ancients these thecse resembled 
 flowers, and so the legend arose that the branches of coral 
 were a subaqueous flowering plant, which became transformed 
 into stone when it was exposed to the air. 
 
 Next in degree of complication of structure to the Coelen- 
 terata comes the phylum of Platyhelminthes or Flat-worms. 
 
THE CLASSIFICATION OF ANIMALS 37 
 
 The word " Worm " is a merely popular term which is used 
 to denote any wriggling creature of simple structure, and 
 amongst the so - called 
 " Worms " are included 
 members of many different 
 phyla. The Flat- worms 
 agree with Ccelenterates 
 in possessing a mouth which 
 is used both for ingesting 
 food and for ejecting waste 
 material, and in having as 
 the basis of their structure 
 a ramified set of tubes lined 
 by an internal layer of cells 
 and contained within an 
 outer layer or skin. Bu-t 
 between skin and internal 
 layer is contained a mass of 
 cells which is differentiated 
 into definite muscular and 
 nervous tissues to a degree 
 which never occurs in any 
 Ccelenterate ; and in par- 
 ticular there are aggrega- 
 tions of nervous matter con- 
 stituting two large ganglia 
 termed the brain, and 
 two cords or bands along 
 the sides of the body. We 
 are thus presented with a FIG. 10. A, diagrammatic longitu- 
 
 definite concentration of 
 
 nervous matter which is 
 
 termed a central nervous 
 
 system; and such a system, 
 
 consisting of the brain and 
 
 the two cords, is found in 
 
 no Coelenterate and is 
 
 characteristic of all Platy- 
 
 helminthes. The phylum Platyhelminthes includes many 
 
 kinds of animals which live in the interior of other animals, 
 
 dinal section of a simple Coelen- 
 terate with a bud attached ; B, 
 a stinging-cell (cnidoblast) with 
 a nematocyst inside ; (7, a dis- 
 charged nematocyst ; b, bud ; ect, 
 ectoderm ; end, endoderm ; j, 
 jelly ; n, nucleus ; nem, nemato- 
 cyst ; o, mouth ; s.h., sense-hair; 
 t, tentacle. 
 
38 
 
 ZOOLOGY 
 
 and derive their nourishment from the tissues of the animals 
 which they infest. Such predatory forms are termed 
 parasites; and the unfortunate animals which have to 
 tolerate their attacks are termed hosts. Some of the parasitic 
 Platyhelminthes cause disease and death to their hosts, in parti- 
 cular the liver-fluke, which lives in the liver of sheep, has in 
 certain years inflicted losses amounting to millions of pounds on 
 the British farmers. This animal possesses suckers, by means of 
 which it adheres to the animal which it infests. Other para- 
 sitic Platyhelminthes find their home in the intestines of man 
 and the higher animals these, the 
 so-called tape-worms, live on the 
 digested food of their hosts, and have 
 in consequence no digestive apparatus 
 of their own : they consist merely of 
 a skin, enclosing nerves, muscles, and 
 reproductive cells the inner set of 
 tubes lined with digestive cells, so 
 characteristic of other Platyhelminthes, 
 being totally absent. These creatures 
 cause only a minor amount of incon- 
 venience to their hosts when they 
 
 T? AT are f u ^ g rown > but many of them, 
 
 IG View7d from bl when ?% and immature, infest the 
 low ; 0, branches of flesh and internal organs of animals and 
 vs, cause disease and death. These parasites 
 have to take their chance of their first 
 host being devoured by a carnivorous 
 
 animal : if this occurs, they complete their development in the 
 intestine of the devourer, and become adult. One tape-worm, 
 in its immature stage, infests the brain of domestic animals, and 
 causes the disease called staggers another attacks the liver 
 of domestic animals and of man himself, and causes a dangerous 
 tumour which is often fatal. 
 
 Another phylum whose members are termed " worms " is 
 the Nemertinea. In these animals we also find a type of 
 structure which consists of a skin and internal digestive tube, 
 and between them a mass of muscular tissue*. There is 
 likewise a central nervous system consisting of two large 
 ganglia termed a brain and two lateral cords. But in one 
 
 gut; o, mouth; 
 sucker. 
 
\ 
 
 THE CLASSIFICATION OF ANIMALS 39 
 
 point there is an enormous advance over the structure ex- 
 hibited by the Platyhelminthes ; the digestive tube has two 
 openings, one in front called the mouth, used for taking in food, 
 and one behind termed the anus, for rejecting undigested material 
 The Nemertinea are further "distinguished 
 by possessing a kind of protrusible trunk at 
 the front end of their bodies, which when 
 not in use is retracted into a sheath. 
 This trunk is termed the proboscis, and 
 it can be rapidly shot out and rolled 
 round the victims on which they prey; 
 for the Nemertinea are active predatory 
 animals. 
 
 Another phylum whose members are 
 termed " worms " is that of the Nematoda 
 or Thread-worms. These animals are in 
 the great majority of cases parasites ; they 
 are practically ubiquitous, and are of enor- 
 mous economic and medical importance. 
 Like the Nemertinea, and indeed all the 
 other groups which we shall still have to 
 consider, they possess both mouth and anus. 
 Their leading feature is the extreme simpli- 
 city of their anatomy. The digestive tube 
 is simple and straight, without complica- 
 tions. The muscular system consists of a 
 single layer of longitudinal muscles ; outside ^ 12 __ A 
 these comes the skin, which consists of a 
 layer of cytoplasm with scattered nuclei, in 
 which separate cells cannot be distinguished. 
 This skin gives rise to the most character- 
 istic feature of the group, viz. a thick, smooth, 
 glistening cuticle, i.e. a sheath of dead 
 secretion which is produced by the transformation of the cyto- 
 plasm of the skin. This cuticle is very elastic, and, as the 
 young animal grows in length, it stretches ; but its elasticity 
 has limits it is eventually burst and cast off, and the animal 
 then grows rapidly in length, and forms a new cuticle by 
 renewed secretion. This process, which is termed moulting, 
 or ecdysis, occurs four or five times in the animal's life. 
 
 mertine worm 
 viewed from 
 upper surface; 
 pr, proboscis ; 
 sh, sheath of 
 proboscis. 
 
40 
 
 ZOOLOGY 
 
 Between the muscular layer and the digestive tube or gut 
 there exists a space, or rather a set of inter-communicating 
 spaces, which we may term the body-cavity. They are filled 
 with a clear fluid which surges about as the animal wriggles ; 
 in this fluid are contained the products of digestion which 
 have been exuded from the digestive cells, and the whole thing 
 is the prototype of the " blood " of the higher animals. In 
 order to distinguish it from body-cavities of a different kind, 
 this type of body-cavity, the simplest and the first we have 
 encountered, is termed a primary body-cavity, or a blood 
 body- cavity or haemocoele. Nematoda vary in size from 
 
 FIG. 13. A Nema- 
 tode worm; o, 
 position of 
 mouth. 
 
 7HUSC 
 
 FIG. 14. Diagrammatic cross- 
 section of a Nematode worm ; 
 cut, cuticle ; g, gut ; A, hsemo- 
 ccele or primary body-cavity ; 
 muse, muscle cells. 
 
 minute microscopic "worms" to creatures a yard long. 
 These last, the dreaded Guinea Worms of the tropics, infest 
 the skin and underlying tissues of men and animals, and 
 cause dangerous sores and ulcers. It has been suggested 
 that they may have been the " fiery flying serpents " which 
 attacked the wandering Israelites. The only safe way to 
 extract them is to tie their protruding ends to a stick, and then 
 slowly and gently to wind them round this. If sudden extrac- 
 tion be attempted, the worm breaks, and the portion left in the 
 sufferer dies and decays, and causes poisoning and death. It 
 is therefore possible that the famous "serpent on the pole" 
 exhibited by Moses was an object-lesson to his followers as to 
 
THE CLASSIFICATION OF ANIMALS 41 
 
 how the scourge was to be dealt with. Many thread-worms 
 infest the flesh of the pig, and one microscopic form escapes 
 when this flesh is eaten and takes up its abode in the intestine 
 of the flesh-consumer. Thence it burrows through the walls 
 of his stomach into his flesh, causing in its passage the severe 
 and often fatal disease called Trichinosis, which has wrought 
 great havoc in Germany, where pig-flesh is often eaten in 
 an imperfectly cooked condition. 
 
 Still another phylum of " worms " exists, which are termed 
 Annelida or Ringed- worms. This group 
 includes our familiar earthworms, and the 
 innumerable forms of marine worms, and, 
 last but not least, the leeches. The lead- 
 ing feature in this group is the repetition 
 of similar parts or organs in a line one 
 behind the other. This kind of repetition 
 plays a great part in the architecture of 
 the bodies of the higher animals, and is 
 known as metamerism. Thus, if we 
 look at the body of the earthworm, we 
 can see that it is divided by a series of 
 constrictions into a set of rings. Each 
 ring is termed a " somite," and in each 
 we find embedded eight bristles. Each 
 somite contains a pair of ganglia of the FIG. 15. Upper view 
 central nervous system. The nervous & 
 system is indeed one of the most charac- repeated bristles; 
 teristic features of the Annelida. It con- som, a somite, 
 sists of a pair of ganglia, closely con- 
 nective, lying above the digestive tube in front and termed the 
 brain ; and a series of pairs of closely connected ganglia lying 
 beneath the gut one pair, as we have already said, being 
 situated in each somite. These lower ganglia are connected with 
 one another by longitudinal bands of nervous matter or commis- 
 sures. The whole knotted chain is termed the ventral nerve- 
 cord, and this cord is connected with the brain by a " nerve- 
 collar " which embraces the digestive tube in front. The under 
 side of an animal is called " ventral " (Lat. venter, the belly) ; 
 the upper side is termed " dorsal " (Lat. dorsum, the back). In 
 the Annelida we meet for the first time with a secondary body- 
 
42 ZOOLOGY 
 
 cavity or ccelom. This coelom consists of a series of ring-shaped 
 cavities, one in each somite, surrounding the gut, and separating 
 it from the skin. A rough model of the earthworm's body could 
 be made by taking a piece of gas-pipe to represent the gut, 
 and slipping over it a series of hollow rubber rings and then 
 enveloping the whole in a linen casing. The linen casing 
 would represent the skin, and the rubber rings the coelom. 
 As this model indicates, the coslom is not merely a series of 
 gaps intervening between other organs, but consists in reality 
 
 coe 
 
 Fig. 16. Diagrammatic cross-section of an Annelid 
 worm ; br, bristles ; coe, coelom or secondary 
 body-cavity ; g, gut ; A, blood-vessels repre- 
 senting the hgemoccele ; I, longitudinal muscles ; 
 o.c., outer circular muscles; p.p., parietal peri- 
 toneum ; v.m. , visceral muscles ; v .n., ventral 
 nerve cord; v.p. t visceral peritoneum. 
 
 of a series of sacs, each with its own inner and outer wall. The 
 outer wall abuts on the skin, and is known as the parietal 
 peritoneum, whilst the inner wall is pressed against the gut 
 and is termed the visceral peritoneum. As the baby worm 
 grows, cells belonging to both walls become transformed into 
 muscular fibres. Those belonging to the parietal layer are 
 arranged longitudinally, and form the parietal or body 
 musculature, whilst those belonging to the visceral layer form 
 a series oi bands encircling the gut, and are termed visceral 
 muscles. In addition there are outer circular muscles derived 
 
THE CLASSIFICATION OF ANIMALS 43 
 
 from the cells of the skin which lie outside the longitudinal 
 muscles. One purpose of the coelom is to allow the gut and the 
 skin to be moved independently of one another ; whereas in 
 the " worms " which we have heretofore considered, the whole 
 body moves in one piece, so to speak, an Annelid might 
 almost be considered as an animal within an animal, for the 
 motions of the gut go on independently of those of the outer 
 skin. The muscles of the gut undergo contraction one after 
 the other in such a way as to cause a wave of contraction to 
 pass along the tube. This wave motion is called peristalsis, 
 and its effect is to push the food along the tube. Similar 
 peristalsis, it is true, takes place in the outer circular muscles, 
 and its effect is to push the watery fluid contained in the 
 coelom along from segment to segment, since the ccelomic sacs 
 communicate with each other by means of small holes beneath 
 the gut. This "outer peristalsis" lengthens the worm, and 
 enables it to push its front end onwards. When progress 
 has thus been effected, the front end is fixed in the earth by 
 its bristles, and the body is then drawn up by the contraction 
 of the longitudinal muscles ; the hind end is then fixed in the 
 ground, 4 and by a repetition of the outer peristalsis the advance 
 of the front end is repeated ; so the worm moves. It must be 
 borne in mind that a primary body-cavity or series of blood- 
 spaces exists in Annelida as well as the secondary body-cavity. 
 This primary body-cavity takes the form of a series of branched 
 tubes which contain a red fluid, and which are distributed to 
 all the organs of the body. This red fluid is, of course, the 
 " blood " of the worm. Nearly all worms of the phylum 
 Annelida are burrowers at some stage of their existence; 
 the majority burrow in the sand and mud at the bottom of the 
 sea, and only rise to the surface when they have ripened their 
 reproductive cells and are about to discharge them. At such 
 times they often appear in countless numbers, and furnish an 
 important source of food-supply to the natives of the Pacific 
 islands, who call them " Palolo." These reproductive cells are 
 produced from the walls of the ccelomic sacs, and in some few 
 cases are only discharged by the breaking of the body ; but in 
 most cases definite openings, termed reproductive pores or 
 coalomiducts, exist, which allow of the discharge of the re- 
 productive cells. No annelid stands convicted of causing 
 
44 ZOOLOGY 
 
 harm to man, his domestic animals, or his crops; on the 
 contrary, as Darwin has shown, the earthworm acts as nature's 
 ploughman, since it eats its way into the earth, and, after having 
 rilled itself with clay, returns to the surface and voids there 
 the clay which it has brought from the depths. The marine 
 worms form an indispensable food-supply for our edible fish. 
 
 When we leave the creatures usually termed "worms" 
 in order to survey the higher groups of animals, we naturally 
 begin with the phylum of the Arthropoda, since the animals 
 belonging to this group show a good deal of general re- 
 semblance to the Annelida. They show a division of the 
 body into successive rings or somites, and a very similar 
 central nervous system to that possessed by the Annelida, 
 consisting of a brain, collar, and ventral nerve-cord. The 
 great characteristic of the Arthropoda is the existence of a 
 thick external cuticle produced by the cells of the skin. A 
 very thin and delicate cuticle of this kind is found amongst 
 Annelida, but in Arthropoda it becomes so thick and hard as 
 to constitute a veritable armour; and as the animal grows 
 this cuticle is constantly shed and re-formed. Moulting or 
 " ecdysis " occurs much oftener than amongst Nematoda, 
 because the cuticle is not expansible. Encased in such an 
 armour, motion is only possible to the Arthropoda through 
 the existence of folds in the skin where the cuticle remains 
 soft and flexible, and which act as hinges. These flexible 
 spots are termed "arthrodial membranes," and the hard 
 places " sclerites." More or fewer of the somites of the body 
 carry paired outgrowths or "limbs," and the characteristic 
 Arthropod way of moving is not by wriggling or peristalsis 
 like an Annelid, but may best be described as a process of 
 rowing itself along by these limbs if it be an inhabitant of 
 water, or of walking on them if it be a land-dweller. Some 
 of these limbs in the neighbourhood of the mouth are generally 
 modified for crushing the food, and are termed " gnathites " or 
 jaws. One of the most important internal differences between 
 Annelids and Arthropoda is the suppression of the secondary 
 body-cavity or ccelom in Arthropoda and the enlargement of 
 the primary body-cavity into a wide series of spaces. The ccelom 
 is usually large and conspicuous in the embryos of Arthropoda, 
 but as the animal grows it fails to keep pace with the growth 
 
THE CLASSIFICATION OF ANIMALS 45 
 
 of other parts, and in the adult is represented by small sac-like 
 cavities, from the walls of which the reproductive cells arise. 
 
 The Arthropoda are an enormous phylum, and include more 
 than half a million kinds of animal, of which accurate descrip- 
 tions have been published. They affect and limit the life of 
 
 FIG. 17. A water- flea as an example of Crustacea 
 viewed from below ; at 1 , first antennae ; at*, 
 second antennae ; gn, gnathites ; I, walking 
 legs ; o, mouth. 
 
 man more than any other group of the animal kingdom. 
 Leaving out of account minor divisions, the Arthropoda are 
 subdivided into three main groups. We have first of all 
 the Crustacea, which are almost all aquatic, and which have 
 the first two pairs of limbs, which are situated in front 
 of the mouth, transformed into delicate feelers or antennae, 
 and the following two pairs, and often others, transformed into 
 
46 
 
 ZOOLOGY 
 
 jaws (gnathites). Then come the Arachnida, in which the 
 first pair of limbs is a small pair of claws termed chelicerse, 
 and this is the only pair which is in front of the mouth ; the 
 following pairs of limbs may have their bases slightly broadened 
 and roughened so as to assist in chewing, but they are also 
 
 used for walking. Most 
 Arachnida are terres- 
 trial, but there are some 
 most interesting aquatic 
 forms. Finally, we have 
 to consider the Insecta, 
 in which there is also only 
 one pair of limbs in front 
 of the mouth, but this 
 pair are converted into 
 feelers or antennae. Three 
 pairs of limbs are con- 
 verted into jaws, and the 
 region of the body to 
 which jaws and feelers 
 are attached is termed the 
 head, and is separated 
 from the rest by a thin 
 flexible region called the 
 neck. Behind the neck 
 comes a region to which 
 three pairs of long limbs 
 Fia. 18. A scorpion as an example of used for walking are 
 Arachnida viewed from below ; ch, attached : this is the 
 chelicerss; g, chewing processes 
 
 thorax, and it consists 
 
 borne at the bases of I, the walking /. ,, .^ Q ^4. a TU^ 
 
 legs ;o, mouth. > of three segments. The 
 
 last two of these ID 
 many cases bear flat 
 
 outgrowths used for flying, which are termed " wings." 
 Following the thorax comes a region consisting of nine or 
 ten segments devoid of appendages, which is termed the 
 abdomen. The word " insect," which literally means "cut 
 into," refers to the striking differentiation of the body into 
 these three regions. The Insecta breathe by means of air 
 tubes which ramify through their bodies, and they are typically 
 
THE CLASSIFICATION OF ANIMALS 47 
 
 denizens of the earth and air. The few that descend to water 
 take their air-tubes with them, and lead an existence which 
 may be more aptly compared to that of a diver than to that 
 of a purely aquatic animal like a fish. The group Insecta 
 include all the myriad forms of flies, beetles, ants, bees, wasps, 
 
 fw. 
 
 FIG. 19. A beetle viewed from below as an example 
 of Insecta ; at, single pair of antennae ; abd, 
 abdomen; f.w., fore wing ; gri 1 , gn z ,yn*, the three 
 pairs of gnathites; h.w. y hind wing; A, head; J, 
 walking legs ; ih t thorax. 
 
 butterflies, moths, bugs, locusts, grasshoppers, &c. Spiders 
 and mites, although popularly termed insects, really belong 
 to the Arachnida. 
 
 The Crustacea include all our crabs, lobsters, shrimps, cray- 
 fish, and the minnte "water-fleas" which are found in our 
 streams aud ponds, as well as the myriads of similar forms 
 which are fouud in the sea. Speaking broadly, the Crustacea 
 are never harmful to man : some of them, indeed, are directly 
 
48 ZOOLOGY 
 
 useful in furnishing food. Everyone knows how universally 
 shrimps are consumed, especially by the poor, and on the 
 coast of a small province like New Brunswick there are 
 800 institutions for "canning" lobsters. But the principal 
 importance to man of the Crustacea lies in the fact that 
 they form a great proportion of the food of fish, and the 
 shoals of fish whose erratic movements cause such distress 
 to fishermen are often determined in their wandering by the 
 presence or absence of swarms of Crustacea. In contrast with 
 the harmlessness of Crustacea lies the harmfulness of Insecta. 
 Amongst the few directly useful insects are the honey bee, 
 the silkworm, moth, and the cochineal insect. Honey is 
 food stored up in a cell made of an exudation of the insect's 
 skin called wax. Its primary use is the nourishment of the 
 developing young of the insect. Silk is a material secreted 
 by the grub or caterpillar of a moth, and is formed as threads 
 which are woven into a case or cocoon which shelters the 
 insect during a critical period in its growth. Cochineal is 
 a stuff contained in the bodies of the insects, and produced 
 by them from the juices of the plants on which they live. 
 
 The only insects which may be said to be indirectly useful 
 are carnivorous insects, which destroy other insects, and such 
 insects as feed on the pollen and honey of flowers and thus 
 inadvertently carry pollen from one flower to another and so 
 effect the fertilisation of the egg-cell of the plant. This 
 fertilisation is necessary of course for the ripening of the fruit 
 and its contained seed. Insects are, in fact, man's great 
 competitors for the enjoyment of the fruits of the earth, 
 and when their numbers increase unduly man runs a great 
 risk of starving. Blood-sucking insects render life almost 
 intolerable in many regions of the tropics, and it has been 
 recently proved that, in addition to depleting the strength 
 of their victims by constant drafts on their blood, they 
 implant in them the germs of parasitic protozoa, which 
 run riot in their blood and cause fever and death. The 
 problem of the successful exploitation of the tropics, which 
 are the naturally richest regions of the earth, is largely a 
 problem of how to fight and conquer the insect life which 
 luxuriates there. It is little wonder that the study of insects 
 has acquired an importance which has raised It to the rank 
 
THE CLASSIFICATION OF ANIMALS 49 
 
 of a distinct science. It is termed Entomology, and it is 
 in this branch that the practical importance of zoological 
 science to the life of man has been most effectively demon- 
 strated. Arachnida are very few in numbers compared to 
 insects, but most of them are distinctly harmfnl to man, from 
 the scorpion with its stinging tail, which lies in wait for the 
 bare-footed inhabitant of the tropics, to the mite popularly 
 termed the harvest bug, which burrows under the skin of 
 the reaper's legs as he mows the corn, and causes painful 
 boils. The spider perhaps merits tolerance as a destroyer 
 of insects. 
 
 Next to the great phylum of Arthropoda comes the phylum 
 of Mollusca, familiarly 
 known to all as shell-fish. \ 
 
 This phylum includes also ^^ ^^ , , A v v _ 5 
 the octopus and cuttlefish, 
 and amongst the latter are 
 included some of the largest 
 animals living. Like the 
 Arthropoda, the Mollusca 
 are characterised by their 
 
 cuticle; but whereas ^^r^J^i^S.^ 
 amongst Arthropoda the 
 cuticle envelops the whole 
 body like an armour, the 
 cuticle of Mollusca, termed 
 the shell, is produced only 
 
 by a limited region of the skin termed the mantle. The 
 rest of the skin remains soft, is in most cases covered 
 with cilia, and is provided with numerous glandular cells 
 which produce mucus or slime. It is to this last cir- 
 cumstance that the group owes its name, for Mollusca is 
 derived from the Latin mottis, soft. The mantle or shell- 
 bearing region usually projects as a free flap, termed the 
 mantle-lobe or mantle-fold, from the surface of the body, 
 and the rest of the body can usually be withdrawn within the 
 embrace of the mantle and overlying shell, and so the animal 
 is protected against its enemies. In contrast with the limbs 
 of Arthropoda, the organ of locomotion of Mollusca is a thick 
 muscular mass on the under surface of the body, termed the 
 
 as an example of the division of 
 Mollusca called Pelecypoda ; /, 
 foot ; m, mantle ; o, o 1 , two pos- 
 terior openings between joined 
 mantle-flaps ; ,, shell. 
 
50 ZOOLOGY 
 
 foot, by the aid of which creeping movements are carried out. 
 So far as the internal organs are concerned, Mollusca are 
 remarkable for the absence of metamerism. Both primary 
 and secondary body-cavities are developed; but the former 
 includes most of the spaces which intervene between the skin 
 and the gut, whilst the latter forms the genital sacs and a 
 space called the pericardium which surrounds the heart. In 
 this last feature lies another contrast between Mollusca and 
 Arthropoda, for in the latter group there is also a space 
 called the pericardium surrounding the heart ; but this space 
 in Arthropoda is part of the primary body-cavity, and the 
 heart opens into it by open slits called ostia. Mollusca are 
 
 FlG. 21. Side view of a living snail as an example 
 of the division of Mollusca called Gastropoda ; 
 /, foot the dotted line, as in Fig. 20, shows the 
 upper limit of the muscular portion ; m, mantle ; 
 o, opening between mantle and body ; s, shell. 
 
 divided into groups according to the size and shape of what 
 we may term the dominating organs, i.e. the mantle, shell, 
 and foot. Thus in one large division the mantle is repre- 
 sented by two flaps situated to the right and left of the 
 animal's body, and the shell consists of stiff right and left 
 valves united by a median horny flexible piece termed the 
 hinge. This is so arranged that when it is uncompressed it 
 holds the two valves of the shell apart from one another \ but 
 when the animal is alive and is alarmed, it can withdraw the 
 foot and lower part of the body within the embrace of the 
 valves, and can then hold the two valves tightly pressed 
 together by the contraction of a powerful muscle. The foot 
 in this group is shaped like an axe or wedge, and the animal 
 when alive slowly ploughs its way through mud or gravel. 
 
THE CLASSIFICATION OF ANIMALS 51 
 
 The scientific name Pelecypoda (Gr. pelekus, an axe) is derived 
 from this circumstance, whilst the popular name bivalve has 
 reference to the shell. In another group the mantle is a 
 circular flap surrounding the body and the shell is at first a 
 cap, but as the animal grows both mantle edge and shell 
 become lengthened and deepened, and this process goes on 
 unevenly, so that the cap becomes twisted into a spiral. The 
 foot has a flat surface, by means of which the animal crawls 
 over the rocks and stones on which it lives. Here again the 
 scientific name Gastropoda (Ut. belly-foot) is derived from 
 the shape of the foot, whilst the popular name univalve 
 refers to the shape of the shell. Lastly, in the Cuttle-fish the 
 
 FIG. 22. Side view of a cuttie-fish swimming as an ex- 
 ample of the division of Mollusca called Cephalopoda ; 
 ff, fore-foot produced into arms bearing suckers ; hf, 
 hind-foot or funnel the dotted line shows upper 
 edge of foot ; m, mantle ; s, concealed shell. 
 
 mantle is also a circular flap surrounding the body, but the 
 shell, primarily, as in Gastropoda, a cap, is usually concealed 
 in the mantle which grows up at its sides and closes over it. 
 The foot is in two divisions, the fore -foot and the hind-foot. 
 The former has grown up so as to surround the front part of 
 the body like a collar, and the edges of this collar are drawn 
 out into strap-like muscular arms beset with suckers, which 
 are most formidable weapons for attack and defence. With 
 these arms it seizes and securely holds its prey ; and its mouth 
 is armed with a strong beak with which it tears the prey to 
 pieces. The hind-foot consists of a vibrating muscular tube 
 termed the funnel through which a strong stream of water is 
 driven backwards, by the recoil of which the animal is driven 
 forwards. The scientific name Cephalopod (head-foot) which 
 
52 ZOOLOGY 
 
 is applied to this division of Mollusca is derived from the 
 shape of the fore-foot. The bivalves include all the shellfish 
 known as mussels, clams, scallops, and oysters. Amongst the 
 univalves we reckon the land and sea snails, the periwinkles, 
 whelks, top-shells, and cowries of our shores, whilst the 
 Cephalopoda include the rare tropical Pearly Nautilus 
 the only living form belonging to this group which has a 
 spirally coiled external shell the squid, the calamary, and the 
 octopus which haunts the submarine holes in rocks all round 
 our coasts. To the same group belongs the enormous squid 
 of Nova Scotia, in which the body is ten feet long and the 
 "arms" (i.e. the divisions of the fore-foot) may be 40 feet 
 long. The greater number of the Mollusca are neither 
 harmful nor useful to man, but a few are used for food. The 
 most prized and in every way the most valuable is the oyster 
 a bivalve in which the muscular foot is absent, and which 
 on that account is much more edible than its allies ; but the 
 mussel (Mytilus), the cockle (Cardium), and the scallop 
 (Pecten) are extensively eaten by the poorer classes. In 
 America the hard-shelled clam (Venus mercenaria) and the 
 soft-shelled clam (Myd) are esteemed as delicacies. Amongst 
 univalves , the periwinkle (LiUorina) is the only one which is 
 used for food in this country, but in France the snail is 
 consumed, and is actually bred for consumption since it is 
 regarded as a delicacy. Amongst cuttle-fish the common 
 squid (Sepia), the calamary (Loligo), and the octopus are all 
 used for food in Italy : but the secondary usefulness of 
 Mollusca as bait to capture the valuable edible fish far ex- 
 ceeds their direct value as food. In this country countless 
 thousands of the Limpet (Patella), perhaps our commonest 
 univalve, are used for this purpose. In North America the 
 soft-shelled clam (Mya) and various forms of cuttle-fish 
 (Loligo, Ommastrephes, &c.) are used. The struggle to 
 obtain these valuable forms of bait has already led to inter- 
 national complications between the British Colony of New- 
 foundland and the United States of America and has also led 
 to bad blood between the provinces of the Dominion of 
 Canada, since the people of New Brunswick have accused 
 those of Nova Scotia of ravaging their clam-beds. 
 
 Next to the Mollusca we must devote a few words to the 
 
THE CLASSIFICATION OF ANIMALS 53 
 
 phylum Bracttiopoda. These are represented by few species 
 at the present time ; all are inhabitants of the sea, and nearly 
 all of them live at the bottom of comparatively deep water. 
 They are of no economic importance whatever. But in past 
 time, as evidenced by their fossil remains, they must have 
 swarmed in the seas, and in certain places on the coast of the 
 West of Ireland every stone which is lifted from the beach 
 contains a fossil Brachiopod. They are therefore familiar to 
 all collectors of fossils, and since their bodies were protected by 
 two shelly valves they present a considerable resemblance to 
 bivalve Mollusca ; and it is somewhat of a shock to learn that 
 
 FiG. 23. A, shell of Brachiopod viewed from above to com- 
 pare with J5, the shell of Pelecypod or bivalve Mollusc 
 viewed from the side ; d, upper and anterior valve of 
 Brachiopod shell ; v, lower and posterior valve. 
 
 the scientific zoologist regards them as members of a distinct 
 phylum with no relation to the Mollusca whatever. An ex 
 animation of tne living forms shows that the two valves of the 
 Brachiopoda are upper or dorsal and lower or ventral, not right 
 and left as in Mollusca j that there is no foot, but that a stalk 
 arises from the posterior part of the body by which the animal 
 is permanently anchored to the rock on which it lives. Further, 
 the secondary body-cavity is exceedingly wide and spacious, 
 whilst the primary body-cavity or blood-system is very small 
 and difficult to detect. In fact the internal organisation of a 
 Brachiopod has more resemblance to that of an Annelid than 
 to that of a Mollusc ; and in this connection it may be men- 
 tioned that in the edges of the mantle-folds of Brachiopods 
 
54 ZOOLOGY 
 
 there are embedded bristles similar to those found in the skin 
 of most Annelida. 
 
 Another phylum which deserves a brief mention is that 
 of the Polyzoa. These are small animals of almost micro- 
 scopic size, which like the Ccelenterata form colonies by budding. 
 They are however widely different from Ccelenterata, to the 
 colonies of which their colonies present some external resem- 
 blance. Each individual of a Polyzoan colony has, it is true, 
 a mouth surrounded by tentacles, but it possesses a complete 
 alimentary canal with mouth and anus surrounded by a well- 
 developed secondary body-cavity. The body of each individual 
 of the colony consists of two portions : a part in which the 
 
 mouth and anus are situated, 
 in which the skin is soft, and 
 alower part in which the skin 
 secretes a hard cuticle. The 
 soft part is called the poly- 
 pide, the hard part the zoce- 
 cium. The polypide can be 
 withdrawn within the zoce- 
 cium by being turned outside 
 FIG. 24. Diagram showing two in when the animal is alarmed, 
 individuals of Polyzoon colony The masses of zooecia form 
 A 9 with extended, and B, with , r , rl 
 
 retracted, polypide; a, knus; dellcate lace-like encrust* 
 o, mouth ; c, coelom. tions on the inner side of old 
 
 shells, &c. The remains of 
 
 the zooecia of extinct Polyzoa are amongst the commonest of 
 fossils. 
 
 We now turn to a phylum which includes much more 
 familiar animals than the Polyzoa. The name of this phylum 
 is the Echinodermata, and the animals included in it are the 
 star-fish, sea-urchins, and brittle-stars of our coasts. Two 
 other less familiar groups likewise belong to the Echinodermata, 
 viz. the delicate feather-stars and the sausage-shaped sea- 
 cucumbers. All are inhabitants of the sea, and all are 
 characterised by having their organs arranged in radiate 
 symmetry around the mouth. By radiate symmetry is 
 meant that, instead of the organs being in pairs, as they are 
 in ourselves and in most of the phyla which we have here- 
 tofore discussed, they are arranged in Echinodermata like the 
 
THE CLASSIFICATION OF ANIMALS 55 
 
 spokes of a wheel round the hub. In Echinodermata there is 
 a complete digestive tube, surrounded by a spacious secondary 
 body cavity : of the primary body-cavity there are only traces. 
 Surrounding the mouth there is a ring-shaped tube containing 
 a watery fluid : this tube, which is in reality a special section 
 of the ccelom, is termed the water vascular system or 
 hydrocoele. It is perhaps the most characteristic organ of 
 
 FIG. 25. Under view of common star-fish as an 
 example of Echinodermata; o, mouth; sp, 
 spines ; t, tube-feet. 
 
 the Echinodermata. It gives off a number (usually five) of 
 branches which radiate outwards. In star-fish, brittle-stars, 
 and feather-stars these radiating tubes are supported by 
 arm-like outgrowths of the body ; but in the globular or disc- 
 like sea-urchins and in the sausage-shaped sea-cucumbers they 
 run back over the surface of the body like meridians of longi- 
 tude on a school globe. These tubes in turn give off to right 
 and left, paired outgrowths which project externally as tentacles, 
 and which form the sensory organs of the animal. They 
 
56 ZOOLOGY 
 
 are termed "tube-feet." In most star-fish and many sea- 
 urchins and sea-cucumbers the tips of these tube-feet are 
 terminated by circular discs, which act like suckers and enable 
 the animal to adhere to the surface over which it is passing, 
 and the animal literally walks on them. From the ring-canal 
 of the hydrocoele, in the interval between two of the radiating 
 canals, there arises a vertical canal termed the stone-canal 
 which terminates in a plate pierced with pores on the upper 
 surfaces of the animal. This plate is called the madreporite : 
 its pores are lined by ciliated cells, and its purpose is to 
 
 FlG. 26. Upper view of common star-fish as an 
 example of Echinodermata ; a, anus ; madre- 
 porite ; sp, spines. 
 
 introduce fresh supplies of water into the hydrocoele. Next 
 to the water- vascular system, the skeleton of Echinodermata 
 constitutes their most distinctive feature. In Sponges and in 
 some Ccelenterata the skeleton consists, as we have already 
 seen, of needles of calcareous or flinty matter embedded in the 
 jelly which intravenes between skin and digestive tube, but 
 in all the other phyla which we have so far discussed and 
 in most Ccelenterata the skeleton is of the nature of an 
 exudation or secretion from the external surface of the skin. 
 In Echinodermata, on the contrary, the skeleton consists of 
 calcareous plates embedded in the jelly beneath the skin, 
 
THE CLASSIFICATION OF ANIMALS 57 
 
 but outside the outer wall of the coelom. These plates have 
 resulted from the calcification of the ground-substance of con- 
 nective tissue, i.e. from the deposition in it of salts of lime. 
 As a consequence these plates are porous ; they are indeed 
 scaffoldings rather than solid masses of lime, and in their 
 interstices we find the characteristic amoeboid cells of con- 
 nective tissue. In consequence of possessing such a skeleton, 
 Echinodermata are amongst the commonest of fossils. Their 
 economic importance is extremely slight. The star-fish feed 
 on bivalve mollusca, whose valves they forcibly wrench apart 
 by pulling on them with their suckers ; they then turn their 
 stomachs inside out and digest their victims alive. Star-fish, 
 in consequence, are harmful to oyster-beds. The poorer classes 
 in Italy eat the genital organs of sea-urchins when they are 
 ripe, and to some extent this occurs in our own country also, 
 and to this circumstance the common sea-urchin owes its name 
 of Echinus esculentus, i.e. the edible sea-urchin. In the 
 Malayan Archipelago several kinds of sea-cucumber are fished 
 for and their dried bodies, known as Trepang, are sold to the 
 Chinese who use them for making soup. 
 
 The last phylum which we have to consider and infinitely 
 the most important is that of the Vertebrata, to which we 
 ourselves belong. Metazoa are indeed often divided into 
 Vertebrata and Invertebrata, but this is an unscientific classi- 
 fication and obscures the fact that the Invertebrata which we 
 have been discussing consist of many distinct phyla ; whilst all 
 the animals classed as Vertebrata exhibit the same fundamental 
 type of structure and constitute a single phylum. The chief 
 characteristics of Vertebrata are three, viz. : (1) There is an 
 internal skeleton known as the backbone which runs the 
 whole length of the animal, beneath the central nervous system, 
 but above the digestive tube or gut; (2) the central nervous 
 system is situated near the upper or " dorsal " surface of the 
 animal, and has the form of a tube running along the length of 
 the animal ; (3) the front part of the digestive tube communi- 
 cates with the exterior by means of clefts or pores. In aquatic 
 forms these clefts serve to get rid of the surplus water which 
 the animal swallows with its food ; in these they are termed 
 gill-clefts, but in land forms they exist only in the very young 
 animal and close up when it becomes adult. The word 
 
58 ZOOLOGY 
 
 vertebra, which literally signifies something turned in a lathe, 
 is applied to the disc-like segments of which the backbone of 
 the higher Vertebrata is made up, but in all Vertebrata in the 
 early stages of development, and in the simpler forms through- 
 out life, the " backbone " consists of a gelatinous string termed 
 the notochord (lit. back-string). The vertebrae are formed 
 later round this string, and it is then more or less absorbed. 
 The hollow nerve-tube has in the vast majority of cases a great 
 hollow expansion at its front end termed the brain. In the 
 higher Vertebrata the walls of this expansion become very 
 thick and its cavity small. The Vertebrata include ourselves 
 and the animals which most resemble us, viz. the warm-blooded 
 hairy quadrupeds or mammals; but they also include the 
 birds, reptiles, amphibians, and fishes, and moreover a group 
 of forms which the older naturalists classed as Invertebrata, 
 viz. the sea-squirts or Ascidians. These last are most 
 wonderful forms. When young they lead an active life, swim- 
 ming gaily in the sea, and in this stage of their existence they 
 possess a notochord, a hollow brain, a tubular nerve-cord, and 
 a pair of gill-slits. They soon, however, attach themselves to 
 a rock and undergo rapid degeneration. The nerve-cord and 
 notochord disappear, the brain becomes a small solid ganglion, 
 but the single pair of gill-slits become enormously enlarged and 
 converted into a trellis- work by the growth of partitions across 
 them. The bars forming these partitions carry cilia, which 
 serve to drive the water out which passes inwards by the 
 mouth. The animal in fact converts itself into a vast filtering 
 apparatus just as does a Sponge, only that the current is in 
 the opposite direction. The Sea-squirt becomes enveloped in a 
 gelatinous external skeleton, exuded by the skin and containing 
 a substance akin to paper, and no one looking at a Sea-squirt 
 adhering to a rock, and much resembling in outer appearance a 
 Sponge, would imagine that it had any affinity whatever with 
 a Vertebrate (Fig. 27). 
 
 The more normal Vertebrata which retain the backbone 
 throughout life are divided, as we have seen, into Fishes, 
 Amphibians, Reptiles, Birds, and Mammals. These five 
 groups have, however, bonds which unite some of them more 
 closely with one another than with the rest. Thus Fish are 
 separated from the other four groups by the circumstance 
 
FlG. 27. A, side view of Ascidian larva ; E, side view of adult 
 Ascidian cut open ; a (in A ), position of future anus ; (in B\ 
 anus ; br (in A}, hollow brain vesicle ; (in B\ ganglion remnant 
 of larval brain ; fix, pagillas by which the larva attaches 
 itself ; g (in A), single gill-pore on one side; (in j?), trellis-work 
 on side of the adult gullet which develops out of the gill-pore of 
 larva ; m, dotted lines indicating outlines of muscle cells which 
 flank the tail on either side ; nerv., spinal cord of larva ; not., 
 notochord or rudimentary backbone of larva. 
 
60 ZOOLOGY 
 
 that their limbs are in the form of fins supported by spines 
 termed fin-rays, whereas the limbs of all the rest are con- 
 structed on the same principle as the human limb. As the 
 late Sir William Dawson has said, there is more resemblance 
 between the limb of the lowest newt and the limb of man than 
 between the limb of the newt and the fin of a fish. The Keptiles, 
 Birds, and Mammals are bracketed together as Amniota, be- 
 cause in the development of the embryo a portion of the egg 
 forms a hood-like structure ensheathing the body of the embryo. 
 This hood, termed the amnion, or popularly the caul, is cast 
 off at birth, and the young animal enters on its free life in a 
 form closely resembling the adult. Amphibia, a subdivision 
 which includes frogs, toads, and newts, are distinguished from 
 Amniota not only by the total absence of an amnion, but by 
 the fact that they begin their free life as little fish with 
 fins which only gradually change into four-footed creatures. 
 Amphibia are often popularly confused with Keptilia, since a 
 newt is in external form rather like a lizard. It is well to 
 remember, therefore, that the skin of a true Reptile is dry 
 and scaly, and that its toes terminate in nails, whereas an 
 Amphibian is nailless and has a moist slimy skin. Reptiles, 
 Birds, and Mammals are at once distinguished from one another 
 by the character of their skins. In all three groups the skin 
 produces horny structures which serve as a protection for it. 
 The epithelium forming the skin is several layers thick, and 
 the cells forming the outer layers dry up and give rise to the 
 horny structures mentioned. In Reptiles these are scales not 
 to be confounded with the scales of fish, which are not formed 
 in the epithelium but in the connective tissue beneath the skin, 
 and the epithelium is continued over them. In Birds the 
 horny structures are feathers, and in Mammals hairs. The 
 wing of a Bird is of course only the arm and hand of the animal. 
 In the hand only the thumb and two fingers are developed, 
 and to the back of these fingers and to the forearm specially 
 long feathers are attached which form a supporting membrane 
 like the plane of an aeroplane. The Bat has also a wing 
 which is formed from the hand, but in this case the supporting 
 membrane of the wing consists of a web of skin stretched 
 between the elongated fingers. 
 
 Of the enormous economic importance of the Vertebrata 
 
THE CLASSIFICATION OF ANIMALS 61 
 
 it is hardly necessary to speak. One has only to remember 
 that our tables are supplied by fish, domestic poultry, and 
 domestic mammalia, and that all other groups of the animal 
 kingdom form an almost negligible source of food-supply com- 
 pared to these. In tropical countries, Reptilia in the form 
 of lizards and snakes are also consumed, and, as everyone 
 knows, the epicures of the City of London consume the flesh 
 of the marine turtle (another Reptile) at their banquets. Even 
 Amphibia do not escape the attention of man ; the limbs of the 
 larger kinds of Frog are esteemed a delicacy in many countries 
 in Canada the large Bull-Frog is hunted almost to extinction. 
 
 In the earliest days of mankind the large carnivorous 
 Mammalia were amongst man's most serious enemies, and even 
 yet in tropical countries, among backward races of men, the 
 lion and the tiger levy a large annual toll of victims. The 
 poisonous snakes were another group of formidable enemies to 
 mankind, and the dread of them is reflected in our mythologies. 
 But where civilised man holds sway such enemies are rapidly 
 overcome and entirely exterminated. The rate of reproduction 
 of the higher Vertebrata is slow, and they are unable to recover 
 from serious and continued losses. Thus hardly a decade 
 passes that some kind of beast or bird does not become extinct ; 
 and however much the scientific zoologist may deplore this 
 process, it is absolutely inevitable and must go on. Just as 
 the Mammalia drove out the large Reptilia which preceded 
 them, and whose colossal remains astonish the geologist, so 
 Man is gradually exterminating all Mammalia and all birds, 
 except those he finds a use for and those which are sufficiently 
 inconspicuous to escape his attention. Only the fish of the sea 
 with their enormous powers of reproduction seem to defy his 
 efforts. 
 
 We have now passed in rapid review the most important 
 divisions of the animal kingdom. Each primary division or 
 phylum includes many different kinds of animals and is sub- 
 divided again and again. Thus phyla are divided into classes, 
 classes into orders, orders into tribes, tribes into families, 
 families into genera, and genera into species. The species is 
 thus the unit of classification. It includes all animals, indis- 
 tinguishable from one another by any constant mark (apart 
 from the differences due to age and sex), which conjugate freely 
 
62 ZOOLOGY 
 
 with one another and produce fertile offspring. When 
 members of two different species are forcibly crossed with one 
 another, in most cases no offspring at all are produced; in 
 a few cases some young are produced which are themselves in- 
 capable of reproduction. Such young are termed hybrids ; a 
 familiar example of these is the mule, which is the result of a 
 cross between the horse and the ass. In the very rare cases 
 where the hybrids are capable of reproduction they exhibit an 
 unstable heredity, and their offspring tend to revert to the 
 father or mother species. 
 
 Whilst^ so far as we know, the species is the same thing in 
 whatever group of the animal kingdom it is found, this is not 
 true of the higher divisions of the system. A genus of reptiles 
 is a different thing from a genus of singing-birds. In fact all 
 the higher denominations are merely names for more or less 
 convenient bundles of species which resemble one another more 
 or less closely, and the number of grades which we require in 
 classifying a group of animals depends on the thoroughness of 
 our analysis. It is often necessary to introduce intermediate 
 grades of classification between order and tribe, family and 
 genus, and so on ; this is done by introducing the prefix sub-. 
 Thus we speak of sub-genus, sub-order, &c., but these grades are 
 only used if the species within a genus, for instance, fall into 
 groups, the members of which more closely resemble one 
 another than they do the other species belonging to the same 
 genus ; and often what one authority calls a sub-genus, another 
 makes a genus, and so on. What the meaning of the system , 
 of classification may be, in a word, what is the origin of 
 species? is one of the most profound and most interesting 
 questions in zoology, and this we shall discuss in the next 
 chapter. 
 
 CHAPTER VI 
 
 THE ORIGIN OF SPECIES. 
 
 WE have just learnt in the last chapter that the species is the 
 real basis of classification, and that higher grades of classifica- 
 tion are only bundles of species. The question instantly occurs 
 
THE ORIGIN OB' SPECIES 63 
 
 to one's mind, how did all these different species come into 
 ?ing 1 This may be said to be one of the two root-questions 
 oi" biology; the other is, what is the chemical and physical 
 mature of living substance? 
 
 To the question of the origin of species, Linna3us, who was 
 first to define what was meant by the term, answered : " There 
 are as many species as the Infinite Being in the beginning 
 created." Now this answer is deserving of respect and careful 
 examination. The object of all science is to determine the 
 laws of Nature that is to say, those constant modes cf action 
 which, as far as our experience goes, characterise all natural 
 phenomena. Having determined these laws, it is the further 
 task of science to calculate forward to what will be the cumu- 
 lative result of these laws in the future, and to calculate back 
 as to what has been the condition of affairs in the past. Now, 
 as we have already seen, living things, whether plants or animals, 
 originate only from pre-existing plants or animals, but if we 
 interrogate the astronomer or the geologist as to the condition 
 of the globe on which we live, he will inform us that it is a 
 'ooling body, and that at a certain period in past time it must 
 have been a glowing white-hot body like our present sun. At 
 such a temperature living matter could not have existed, since 
 any living matter exposed to a temperature at all approaching 
 this would be resolved into its ultimate elements. We are 
 therefore led to the conclusion that life had a definite begin- 
 ning on our globe, but as to how that beginning was caused 
 Science cannot in the least help us. No process which we 
 can study now will lead to the formation of living out of life- 
 less matter ; and therefore it comes to much the same thing 
 whether we say that living matter owed its origin to some 
 natural process which does not now occur, and about which we 
 know nothing, or that it owed its origin to supernatural process. 
 
 Now if life originated in some way which we can never 
 know scientifically, there is no a priori reason why all the 
 species of animals with which we are acquainted should not have 
 originated at the same time. The chemist, until very recently 
 at any rate, assumed that the eighty odd elements with which 
 he was acquainted had existed from the beginning of time ; 
 and although he now knows that some of the heavier of these 
 are undergoing a slow secular process of decomposition, which 
 
64 ZOOLOGY 
 
 he can neither initiate or arrest, yet he is still left with a 
 large number which by no known process can be changed into 
 anything else. The remains of animals preserved in the rocks 
 teach us, however, that many species which formerly existed an 
 the earth have become extinct ; and there is a general belief 
 among geologists that none of the species now existing on the 
 earth were living when the materials making up the first 
 fossiliferous rocks were laid down as sand, mud, and gravel at 
 the bottom of seas, lakes, and rivers. This belief is based on 
 the experience that if we examine successively older and older 
 rocks, the remains of existing species become scarcer and scarcer 
 and finally disappear, whilst those of extinct species increase in 
 number. This line of argument is, however, not conclusive, 
 because although the presence of the remains of a species in a 
 certain rock proves that the species lived at the time the rock 
 was being formed, yet its absence does not prove that it was 
 not then living. Only a small proportion of the species living 
 at any one time were actually preserved as fossils ; and of the 
 fossils which actually exist, probably only a small proportion 
 have yet been discovered. Every few years fossils of some 
 type of animal are discovered at an horizon far lower than the 
 lowest at which they had been previously supposed to exist. 
 
 There are, however, independent grounds for believing that 
 the species of animals which we now find living have not been 
 in existence since the beginning of life on the globe, but have 
 arisen gradually as time went on through the operation of 
 natural processes. These grounds are fully set forth in the 
 immortal work entitled The Origin of Species, by Charles 
 Darwin, a work which everyone interested in biology should 
 read and re-read, and to which it is hoped that the present 
 book may serve as an introduction. Darwin's work ought to 
 be regarded as the Sacred Book of every student of zoology, 
 for no work has done more to revolutionise the whole out- 
 look of man on the universe than this one. 
 
 A very brief resume* of the principal lines of argument 
 employed by Darwin may be given here. We find, Darwin 
 says, that if a species be closely examined it is found to be 
 divisible into different " races." These races are sections of the 
 species consisting of slightly different types of individual con- 
 fined to distinct areas within the single area to which the whole 
 
THE ORIGIN OF SPECIES 65 
 
 species is confined, and which is termed its area of distribution. 
 The members of different races will interbreed freely with one 
 another and produce fertile young ; nevertheless, in some cases 
 at any rate, the offspring tend to revert to the maternal and 
 paternal types, and in this way we are reminded of the rare cases 
 where two species will interbreed and produce fertile offspring. 
 Further, in most cases he reminds us, we are in great ignorance 
 as to whether two distinct types of animal or plant will or will 
 not unite with each other so as to produce fertile offspring 
 In few cases has the actual experiment been tried, and the 
 vast majority of distinctions between species which closely 
 resemble one another rest merely on the exhibition by all the 
 individuals of each species of some constant mark or character 
 which those forming the other species do not possess. So it 
 comes about that what one good authority regards as merely 
 distinct races another regards as distinct species. Now no one 
 has ever suggested that all the different races into which each 
 species is divided have existed since the beginning of life on 
 this globe; they are universally admitted to have originated 
 through changes produced in members of the mother species by 
 the different conditions to which they have been subjected in 
 different regions. The strong suspicion is therefore aroused 
 that distinctions between species themselves are of the same 
 nature as distinctions between races only carried to a further 
 extent, i.e. that races are in fact only species in the making. 
 This suspicion would rise to a certainty if we could show 
 that there is on the one hand some natural process going 
 on which tends to make the individuals of the same species 
 differ from one another, and if we could further show that 
 the property of sterility between two races when crossed could 
 gradually appear. These are the two propositions which Darwin 
 endeavoured to prove. He pointed out that no two members 
 of the same brood of chickens, no two puppies of the same 
 litter, were exactly alike, and that the differences between them 
 were often inheritable. He asserted that breeders of domestic 
 animals produced new " strains," i.e. new races of animals, 
 by selecting from the litters of their stock those individuals 
 which exhibited some particular trait which they wish to 
 perpetuate. When such individuals were mated together, they 
 often produced offspring in which the desired trait appeared 
 
66 ZOOLOGY 
 
 in greater intensity; and by a continuance of such selective 
 mating a race differing markedly from the parent race was 
 produced. Thus, for instance, two breeders of sheep who had 
 different ideals as to the type of sheep which they desired, 
 produced in the course of years two flocks which differed 
 markedly from one another, so that the veriest tyro could 
 distinguish them. The origin of the principal breeds of 
 domestic animal is lost in the mist of primeval history. 
 In the case of the pigeon and of the dog they differ so much 
 from one another that it has been often believed by breeders 
 that they must have originated from distinct species ; neverthe- 
 less Darwin showed by convincing argument that this cannot 
 have been the case with the breeds of pigeon, but that they 
 must all have arisen by the mating together of selected in- 
 dividuals belonging to one species. Thus he argued that the 
 principal breeds, as for instance the pouter with its enormous 
 crop, and the fantail with its colossal tail, exhibit features which 
 are shown by no wild species of pigeon whatever. They differ 
 from the wild rock-pigeon in characters sufficiently important 
 to distinguish not merely species but genera from one another. 
 Nevertheless they interbreed freely with the rock-pigeon, 
 and have clearly been derived from that species. The ques- 
 tion then arises, whether if man by "artificial selection "" can 
 create forms so different from one another as the fantail 
 pigeon and the pouter pigeon, any process of natural selection 
 exists which in course of time could cause the offspring 
 of the same ancestor to differ as widely from each other as 
 existing species do? lhat such a process does exist is the 
 great point of Darwin's book. He lays stress on what we 
 have already pointed out in this work, viz. that every species 
 of animal tends to multiply itself so rapidly that it would 
 soon overrun the globe unless it were checked, but that never- 
 theless the numbers of any wild species at present living do 
 not vary much from year to year. This means that its num- 
 bers must be subject to constant depletion through the dangers 
 to which they are exposed. Many individuals are devoured 
 by enemies, many die from starvation during periods of 
 scarcity, or from cold or heat during extremes of temperature, 
 and the race will ultimately be propagated by those members 
 which are best fitted to overcome and survive these troubles. 
 
THE ORIGIN OF SPECIES 67 
 
 If external conditions remain the same, the species will remain 
 fairly stable in its character ; all the weaklings will be weeded 
 out, and as time goes on an ever higher standard of fitness 
 will be found amongst the survivors. But if conditions change 
 for the whole species or for a part of it which has wandered 
 into a new environment then a different standard of excel- 
 lence will be required in the survivors, and a new race differing 
 in character from the parent race will slowly come into being 
 through the mating together of survivors possessing slightly 
 different qualities from those called for by the old conditions. 
 In this way as a species spreads it will slowly break up into 
 daughter species, distributed over different regions of the globe, 
 or distributed into different situations in the same region 
 of the globe. These daughter species in course of time will 
 come to differ from one another so much that they will be 
 assigned to different genera, and by a continuance of this 
 process the vast complexity of classes, orders, tribes, families, 
 genera, and species within the limits of a phylum will come 
 about. All the members of a phylum are descendants of a 
 single species, but Darwin hesitated to assert that such 
 different phyla as Arthropoda, Mollusca, and Vertebrata are 
 descendants of a common type of ancestor. He rather inclined 
 to the belief that if life apppeared on this globe by a process 
 which we know nothing about, it may have appeared simul- 
 taneously in several different forms whose descendants remain 
 as the different phyla to-day. It has been the task of suc- 
 ceeding zoologists to show that even phyla so distinct as 
 Arthropoda and Mollusca afford evidence, when closely ex- 
 amined, of having been derived at some inconceivably remote 
 epoch from common ancestors. But in order to complete 
 Darwin's task it was necessary that he should show that the 
 constitutional differences between species which express them- 
 selves in their inability to breed together are paralleled by 
 minor differences of the same kind between breeds. Thie 
 is a part of the work in which he was less successful than 
 he was in demonstrating the existence of natural selection. 
 But he showed that sterility can exist between different forms 
 of the same species. Thus among plants there is the familiar 
 example of the primrose, in which there are two forms of 
 flower, one with tall stamens and a short pistil, and the other 
 
68 ZOOLOGY 
 
 with short stamens and a tall pistil. If pollen be taken from 
 the short stamens and placed on the short pistil, many 
 vigorous seeds will be produced; and the same result will 
 follow if pollen from long stamens be placed on a long pistil. 
 But if pollen from short stamens be placed on a long pistil, 
 or if pollen from long stamens be applied to a short pistil, 
 then few and feeble seeds are produced : the result is therefore 
 exactly the same as if two distinct species had been crossed. 
 Darwin also showed that whereas in the majority of cases 
 two distinct species were absolutely sterile when crossed with 
 one another, yet this is by no means always the case ; that 
 in fact sterility is a question of degree, and that it is to be 
 regarded as a concomitant of divergence of structure. 
 
 Darwin's teaching evoked the most violent opposition 
 because it was in sharp contrast with previous views on the 
 Universe, but amongst all the opponents of his views no 
 one could gainsay the fact of the tremendous slaughter which 
 goes on amongst wild animals ; this fact was previously not 
 denied, but conveniently ignored, and no doubt much irritation 
 was caused by the manner in which this neglected fact was 
 forced into the foreground of the controversy. The sole 
 question at issue, then, is whether this slaughter subserves 
 the welfare of the species or not, and most people will on 
 reflection be inclined to welcome Darwin's view that it sub- 
 serves a useful end. 
 
 It is obvious that Darwin's theory leaves entirely open the 
 question of what causes those differences between individuals 
 out of which natural selection gradually piles up the divergences 
 of structure which separate species from species and genus 
 from genus. These differences are summed up under the 
 general title of Variation. In a later but less well-known 
 work, entitled The .Variation of Animals and Plants under 
 Domestication, Darwin collected together all the evidence he 
 could muster which could throw light on the origin of varia- 
 tions. This work is the grandest which he produced ; to 
 compile it was really the goal of his life, and the Origin of 
 Species was intended by him to be merely a preliminary 
 account of his work which the urgency of his friends forced 
 him into writing, when it appeared probable that his ideas 
 would be anticipated by another naturalist. Unfortunately 
 
THE ORIGIN OF SPECIES 69 
 
 the vogue of the " preliminary account " was so enormous that 
 it completely overshadowed the full work of which it was 
 meant to be the herald. The general conclusion at which 
 Darwin arrived is that variations, so far as they are inheritable, 
 are due to external conditions acting directly on the germ- 
 cells, and that one external condition which seems to have 
 had a great deal of influence has been the presence of an 
 abundance of food. The efforts of biologists, both zoologists 
 and botanists, since Darwin's time, have been in large measure 
 directed to extending and confirming his work, and a great 
 deal of research has been done with a view to finding out 
 the nature and cause of variations. There is a popular 
 idea abroad that one result of these researches has been to 
 discredit Darwin's theory. It cannot be too strongly stated 
 that such is not the case, and a brief statement of what has 
 actually been accomplished will therefore be in place. At the 
 same time that Darwin published his Origin of Species, 
 Gregor Mendel, an Augustinian monk, was carrying on experi- 
 ments in crossing different varieties of peas, and he arrived at 
 a conception of the laws governing inheritance when two races 
 of the same species were crossed, which has been called Men- 
 delism and which has been largely confirmed by the work of 
 subsequent workers. But these laws only apply to cases 
 where the two races or varieties can be separated from one 
 another by one or more clearly definable characters. Mendel 
 himself expressly stated that he refused to have anything to 
 do with differentiating characters which were of a " more or 
 less " description. Now Darwin was well aware that amongst 
 the domesticated animals such sharply -marked varieties 
 popularly known as "sports" did occur, but for various 
 weighty reasons he did not consider that they had been of 
 importance in the building up of new species ; the kind of 
 variation which he believed has been " laid hold of by natural 
 selection " was precisely that kind of " more or less " difference, 
 i.e. slight differences in the proportion of parts of one animal 
 as compared with those of another, which Mendel left out of 
 account. If this view be correct, Mendel's views, while 
 perfectly justified with regard to the subject-matter to which 
 they were applied, have no relevance to the point at issue. 
 Mendel showed that the hybrid produced by crossing two 
 
70 ZOOLOGY 
 
 well-marked races gives rise to offspring whose nature can 
 best be accounted for by supposing that the hybrid produces 
 two kinds of germ-cell of each sex in equal numbers, and that 
 one type of germ-cell carries the differentiating character belong- 
 ing to the paternal race, and the other type the character 
 proper to the maternal race. When, however, two germ-cells 
 carrying different characters unite in sexual union, the resultant 
 offspring does not usually show a blend of the two characters, 
 but one of the characters alone appears ; this is termed the 
 dominant character; whereas the other, whilst equally im- 
 planted in the constitution of the hybrid, does not affect its 
 visible anatomy or appearance, and is therefore termed the 
 recessive character. Of course this law finds its most 
 obvious application in the character of the original hybrid 
 produced by the first cross, which usually favours father or 
 mother, but not both. The parent not represented in the 
 visible structure of the child makes, however, its presence felt 
 when that child comes to form germ-cells and produce offspring, 
 for in some of the grandchildren the features of the " recessive " 
 parent come forth again from obscurity. According to this 
 discovery of Mendel's it results that when two races cross, the 
 offspring of the hybrid gradually breaks up into the parental 
 races again and the hybrids become relatively less numerous 
 at each generation. Such a discovery, therefore, while it shows 
 how two races occupying different but contiguous territories may 
 keep distinct in spite of crossing, has no bearing whatever on 
 the formation of new species. But the question now arises, 
 what will happen when two races which differ from one 
 another in two differentiating characters are crossed? The 
 answer is that half the germ-cells of the first hybrid will con- 
 tain one of each pair of differentiating characters and half will 
 contain the other member of each pair, but that those germ-cells 
 which contain one character of one pair are not in every case 
 identical with those which contain one character of the other 
 pair. It is exactly as if one had a number of balls in a bag 
 and if one were to draw half of them out and paint them red, 
 leaving half of them uncoloured, and if one were then to put the 
 painted balls back again and shake the whole lot up together 
 and then draw out half the number again and paint black dots 
 on them. Half the number of balls would thus receive black 
 
THE ORIGIN OF SPECIES 71 
 
 dots and half would be without, but some of those which had 
 black dots would be red and some would be uncoloured, and some 
 of those which had no black dots would be red and some would 
 be uncoloured. There would thus be four kinds of balls, viz. : 
 (1) red with black dots, (2) red plain, (3) uncoloured with black 
 dots, (4) uncoloured plain. Thus every time that members of 
 two races distinguished from one another by two differentiating 
 characters are crossed with one another, four kinds of germ-cells 
 are produced ; two of these varieties of germ-cell are the same 
 as the germ-cells borne by the two parental races, but two carry 
 new combinations of characters, and all these germ-cells if they 
 unite with others of the same kind produce offspring with stable 
 characters. Thus by crossing two such races two new stable 
 varieties can be created. If two races are crossed which differ 
 from one another in three definite characters, the hybrid pro- 
 duces six (i.e. 3x2) different types of germ-cells ; if in four 
 characters, eight (4 x 2) different kinds of germ-cells. It will 
 thus be seen that by crossing two races differing in a large 
 number of characters a very large number of new stable com- 
 binations can be formed. This discovery is of enormous practical 
 importance, because it enables breeders to combine the desirable 
 qualities found into different races and to produce a new 
 stable variety bearing all the wished-for qualities. But there 
 is grave reason to doubt whether any such process has played a 
 part in the formation of natural species. Bateson, the most 
 brilliant member of the Mendelian school, has shown that in 
 most cases which are closely examined the differentiating 
 character separating two races consists in the presence of one 
 race of some substance or quality which is absent in the other 
 race. The albino, or white mouse, for instance, is deficient in 
 a substance necessary for the production of colour which the 
 ordinary mouse possesses. Nearly all the differences between 
 domestic races with which the Mendelian biologists have ex- 
 perimented are of this character. The wild race, or original 
 type of the species from which the domestic race has been 
 derived, is characterised by the possession of all the differ- 
 entiating characters ; the domestic race by the absence of one 
 or more of them. All domestic races experimented with are 
 therefore in a sense cripples, and their differences from the 
 parent species of a different character from those which usually 
 
72 ZOOLOGY 
 
 separate wild species from one another. The more enthusi- 
 astic followers of Mendel strive to prove that the only inherit- 
 able kind of variation which they term a mutation is of 
 the kind produced from a new type of germ-cell which has 
 arisen from such a new combination of inheritable char- 
 acters as that alluded to above. Variations which consist in 
 small differences in size and proportions are termed by them 
 fluctuations. These fluctuations are due, according to them, 
 to different conditions of nourishment acting on the same type 
 of germ-cell, and such fluctuations are not inheritable. Accord- 
 ing to some of them, a natural species consists of a number of 
 different races and each race is made up of individuals pro- 
 ducing the same type of germ-cell. These races are termed 
 elementary species; they are continually intercrossing and 
 producing varied combinations, and it is, according to these 
 writers, the variety of forms thus produced that has been 
 mistaken by Darwin for continuous variability. 
 
 Now this view, if accepted, so far from helping us to under- 
 stand the formation of natural species, brings us up in effect 
 against a blank wall. How are we to account for the differ- 
 ences between the races or elementary species ? how did they 
 originate ? To such questions the followers of Mendel have been 
 able to give no answer. It is true that a leading botanist 
 named de Vries, one of those who brought Mendel's work 
 into full recognition, has put forward the view that natural 
 species at intervals of time give rise to new "elementary 
 species." The only instance of this, however, which he has 
 been able to discover is open to grave suspicion. This is the 
 case of an evening primrose which he found growing in 
 Holland. But this plant is an introduced American form : 
 it is strongly suspected of being a hybrid, and the supposed 
 production of new varieties may be nothing more than the 
 formation of new types of germ-cell with new combinations 
 of the parental qualities such as we have just described in 
 giving an account of Mendel's doctrine. 
 
 To sum up : the broad general considerations adduced by 
 Darwin afford an overwhelming weight of evidence that new 
 species have been and are being produced by the modification 
 of old species : and the only theory which gives any probable 
 account of how this can happen is his. Changed external 
 
DARWIN'S THEORY 73 
 
 conditions affecting the germ-cells and slightly affecting their 
 hereditary potentialities this is Darwin's account of the cause 
 of variation, especially of that kind of variation out of which 
 the differences between species have been built up ; and no 
 argument has yet been advanced to invalidate this view. It 
 must be remembered, however, that the experimental proof 
 of this view would require a period of time greatly exceeding 
 the span of a single human life, whereas the number of types 
 of germ-cells produced by a hybrid is a matter which can be 
 determined by experiments lasting only a few years. The 
 evidence in favour of Darwin's view must therefore for a long 
 time remain indirect ; but whoever will take the trouble to 
 compare the small wild horse of Northern Europe and Asia 
 pictures of which were scratched by primitive man on flint 
 and specimens of which can be seen in the London Zoological 
 Gardens, with the noble dray-horses which drag heavy loads 
 in the London streets, will form some conception of what may 
 be accomplished in thousands of years by steadily mating 
 together the strongest and best of a species. 
 
 CHAPTER VII 
 
 THE CONSEQUENCES OF DARWIN'S THEORY. THE 
 INTERPRETATION OF DEVELOPMENT. 
 
 IN the preceding chapter we have given a brief sketch of 
 Darwin's theory of evolution and of the reasons for believing 
 it to be true. We have now to examine some of the conse- 
 quences which must follow if the theory be accepted. The 
 first of these consequences is that greater or less likeness 
 between two species of animals is prima facie evidence of 
 nearer or more distant blood relationship, and that the proper 
 classification of animals is one which most adequately expresses 
 this relationship. But animals in their growth from the germ 
 until they reach the adult condition pass through a great 
 number of stages in which they may present quite different 
 appearances, and the study of these stages is termed Embry- 
 ology. The most familiar example of this is found in the 
 life-history of the frog. As every one knows, the frog when 
 
74 ZOOLOGY 
 
 young lives in water and strongly resembles a fish, and is 
 then known as a tadpole. If the tadpole be carefully ex- 
 amined it will be found to present strong resemblances in 
 structure to an archaic type of fish known as the lung-fish, and 
 represented now only by three types, one confined to the rivers 
 of Australia, one to the rivers of Central Africa, and one to the 
 rivers of South America. Like the tadpoles these fish possess 
 both lungs and gills, and in the arrangement of their blood- 
 vessels and in the structure of their brains there is likewise a 
 strong resemblance to tadpoles. If we interpret these resem- 
 blances as evidence of blood-affinity we shall be bound to con- 
 clude that frogs have been developed out of lung-fish. Now 
 it happens that there is corroborative evidence from fossils to 
 support this conclusion. In the rocks known as Devonian or Old 
 Red Sandstone there are numerous remains of fish. Many of 
 these, according to the opinion of the best experts, were allied to 
 the lung-fish. In the Carboniferous rocks which lie above 
 the Devonian, in which the coal-seams are found, abundant 
 remains of very primitive frog-like and newt-like creatures have 
 been discovered, and there seems to be a strong probability 
 that frogs and newts, that is, Amphibia, were evolved out of 
 creatures like lung-fish in the interval between the periods 
 represented by these two sets of rocks. But if this con- 
 clusion be accepted the consequence follows that the frog, 
 in its growth from the egg to the adult condition, gives a 
 summary of the history of the race of frogs in their age- 
 long progress from the condition of fish to the condition 
 of land animals. Now this case of the tadpole and the 
 frog is only one of numerous instances of the same sort of 
 thing. Thus, for instance, the oyster is an exception among 
 bivalves, in that it possesses no foot i.e. no axe-like median 
 muscular mass attached to the underside of its body. But 
 the American oyster, when about |~inch long, has such a ( foot, 
 which it loses as it grows up. When young it moves about, 
 but when older it stays in one place. There are small salt- 
 water shrimps belonging to the class Crustacea, which when 
 young swim about freely and resemble other species of the 
 same order, but which when older attach themselves to the 
 gills and mouth of fish and degenerate into shapeless parasites 
 whom no one would suspect of belonging to the class Crustacea 
 
DARWIN'S THEORY 75 
 
 if their younger stages were unknown. No naturalist has any 
 doubt that these parasites are the descendants of forms which 
 once swam freely about like other Crustacea, and if so, then 
 undoubtedly they recapitulate in their development the history 
 of the race. 
 
 We might add to the list indefinitely, but one other 
 example must suffice. The beautiful feather-star, one of our 
 less-known Echinodermata, which can be dredged in shallow 
 water all round our coasts, possesses when adult a mere blunt 
 knob in the middle of the back from which rooting processes 
 originate ; it can hold on for a time to a given spot by these 
 processes, and it can also let go and swim about by move- 
 ments of its delicate arms until it finds a more suitable place. 
 When young, however, the feather-star has a delicate stalk 
 springing from the middle of the back, and by this stalk it is 
 permanently anchored to one spot. Now the Carboniferous 
 limestone in some parts of England is in large measure made 
 up of the broken fragments of the stalks of extinct feather- 
 stars, and it is practically certain that amongst these ancient 
 stalked feather-stars the ancestors of our present feather-stars 
 are to be found. Here again the life-history of the individual 
 recapitulates that of the race. Since this principle is found 
 to hold where we have independent evidence as to what the 
 ancestry of a given form has been, it seems justifiable to 
 conclude that it underlies the development of an animal from 
 the egg in all cases, since it seems a reasonable supposition to 
 hold that this development is the same sort of thing wherever 
 it occurs. Hence Haeckel formulated what he called "The 
 fundamental law of biogenesis" in the words which we 
 have just used, viz. : " The individual in its development 
 recapitulates the history of the race." When once the idea 
 was grasped that life-history in general had this meaning, 
 an immensely increased interest was awakened in its study 
 and elucidation, and this interest has lasted in scarcely 
 diminished degree till the present day. The field of research 
 is certainly limitless ; there are at least one million distinct 
 species of animals living at the present day, each with its 
 distinct type of life-history in few cases is the complete life- 
 history known, in a good many more cases only bits and 
 scraps of this history have been made out, but in the vast 
 
76 ZOOLOGY 
 
 majority of cases nothing at all of it is known. The 
 recapitulatory interpretation of life-history was pursued with 
 such fervour during the latter half of last century that it 
 provoked a reaction, and many naturalists now speak 
 slightingly of this theory. The fact is that although re- 
 capitulation is the main factor in determining the course of a 
 development, other factors have co-operated in producing the 
 result, so that, as the late Professor Balfour said, the life- 
 history is like an old damaged record with pages missed out, 
 with other pages blurred, and with intercalation of false 
 matter. How does the historian proceed when he tries to 
 decipher such a document? He compares it carefully with 
 all the other documents of the same age which he can obtain, 
 and the true is separated from the false by the consistency of 
 the one and the inconsistency of the other. So the re- 
 capitulatory interpretation of life-history must be based on 
 Comparative Embryology, i.e. a large number of life-histories 
 must be deciphered and compared with one another before the 
 recapitulatory interpretation can be applied with any confidence. 
 But this is a slow process, and the ardent young naturalist 
 who deciphers a new life-history is usually burning with 
 impatience to apply the recapitulatory theory to it at once. 
 Hence we can easily understand that most contradictory results 
 have been obtained from the applications of this theory, 
 and hence the reaction against it. The line of research most 
 in favour at present is termed Developmental Mechanics or 
 Experimental Embryology. The object of this study is to 
 explain the developmental processes which go on in an egg, on 
 the basis of its own structure, without calling in the aid of 
 the recapitulatory theory at all. The experimenter cuts off 
 parts of the egg under observation or displaces the mutual 
 relations of the parts by pressure, &c., and then observes the 
 result. In this way he hopes to find out what are the 
 potencies of the various parts of the egg and how these parts 
 influence each other. But the progress of this research has 
 only brought out the fact that no form of egg is capable of 
 explanation by reference to its own structure alone. We find 
 that eggs differ widely from one another in their constitution. 
 In some cases, as in Worms and Molluscs, if a portion of the 
 egg be cut off the creature which develops from it will have 
 
DARWIN'S THEORY 77 
 
 some important part of its anatomy missing. In other cases, 
 as in some Echinoderinata, the egg can be cut into eight pieces 
 and each piece if it lives will develop into a creature just like 
 the normal form, only of proportionately reduced size. Why 
 the eggs of one form are constituted in one way, and those of 
 another in another way can only be explained on the principle 
 of heredity ; on the ground of the affinities of the parent form 
 and its position in the animal kingdom. 
 
 Developmental Mechanics is in fact an adjunct and Dot a 
 rival to Comparative Embryology. By its means we analyse 
 an egg into its essential factors far better than we could by 
 looking at it through the microscope or even by cutting a 
 thin section of it. But no developmental mechanics will 
 enable us to explain why the feather-star loses its stalk, or 
 the young oyster its foot ; to give any rational account of 
 such occurrences we require the recapitulatory theory. If 
 then recapitulation of ancestral history should be the main 
 factor in determining development the question arises, what 
 are the other factors which modify it? The elucidation of 
 the secondary factors is an inquiry on which comparatively 
 little work has been done and on which, it is to be feared, 
 no very clear conceptions exist in the minds of many natural- 
 ists. A few of the more obvious may be briefly alluded to 
 here. The young of animals exist under two phases, termed 
 respectively the larval and the embryonic. In practically 
 all life-histories both phases occur : the life-history starts 
 by being embryonic and becomes larval before the adult stage 
 is reached. In the larval phase the young animal moves 
 actively about and seeks its own food; in the embryonic 
 phase, on the contrary, it is supplied with food either in 
 the form of yolk-grains embedded in its tissues or in the 
 form of secretions from the maternal womb; and it lives a 
 sheltered life either within an egg-shell or within the womb. 
 All the clearest and most striking evidences for recapitulation 
 occur in the larval phase : and we have every right to 
 assume that the larval type of development is primary and 
 that the embryonic phase is a secondary modification of it 
 The digestive tube of a larva must be in full functional activity, 
 ready to receive and digest food ; but that of an embryo may 
 be, and often is, a solid string or mass of cells packed with 
 
78 ZOOLOGY 
 
 yolk. The limbs of a larva must be functional those of an 
 embryo are often functionless stumps. Then again in both 
 phases of life-history, but most markedly in the embryonic, 
 we find a tendency towards the dislocation of development, 
 by which phrase is meant the precocious development of 
 organs which are of importance to the adult, so that the 
 body of an embryo at one time may represent two periods 
 of ancestral history at the same time, each affecting a 
 different group of organs. This indeed seems to be one 
 of the most frequent factors in modifying development. 
 Lastly, the circumstances of the larva may undergo secondary 
 changes whilst those of the adult remain relatively the same : 
 in this case special larval modifications may arise which may 
 obscure the ancestral type; thus in the majority of cases 
 insect-larvae live in very special surroundings to which they 
 are specially adapted. 
 
 If we then accept the theory of recapitulation as having the 
 great balance of probability in its favour, we may now reflect 
 on what this theory implies as to the course which has been 
 pursued by evolution. According to this theory a larva 
 primarily represents an ancestral stage in the history of the 
 species, and the larval circumstances are a picture of the 
 ancestral environment. When the larva changes into the 
 adult, or to use the technical term, metamorphoses, it deserts 
 the ancestral or larval environment and enters into the modern 
 and adult one. We seem therefore driven to conclude that in 
 past time a new species was made out of an old one by the 
 migration of some of its members into a new environment 
 and their consequent exposure to new influences amongst which 
 the most potent may have been a new kind of food. Now in 
 Chapter II it has been pointed out that every species tends to 
 reproduce itself at such a rate that if unchecked it would soon 
 overrun the world : it was indeed compared to a fire in a dry 
 prairie, which is always striving to spread at its borders. This 
 comparison appears to be increasingly justified the more the 
 circumstances of the life of species is looked into. It has been 
 shown that for many species there is a mother country, in 
 other words, a suitable environment where each flourishes and 
 multiplies so that in this country it is swarming. The surplus 
 population spreads outwards from the borders of the country 
 
DARWIN^S THEORY 79 
 
 seeking room. In most cases, alas ! the colonists perish, but 
 once in a long while they manage to establish themselves and 
 thus a new race comes into being. The incursion into new 
 territory seems to occur in the majority of cases in the 
 adolescent stage; the younger stages of growth being passed 
 through in the mother country : these younger stages then con- 
 stitute a larval phase. Now in response to the new environ- 
 ment the structure of the latest stages becomes altered and 
 that adult structure is capable of alteration under the stress of 
 new circumstances is a well-known fact but we are bound to 
 assume that this change of structure has in time become trans- 
 ferred from the body (soma) to the germ-cells, so that these 
 now have acquired the tendency to develop the new type of 
 structure at earlier and earlier periods in the life-history and 
 quite independently of the stimulus of the new environment. 
 Thus the recapitulatory theory, which is an inevitable deduc- 
 tion from the general theory of evolution, is calculated to 
 throw in turn considerable light on the nature and origin of 
 inheritable variation. The tendency of the Mendelian school 
 to regard the properties of the germ-cells as being as unalter- 
 able as those of the chemical atoms is certainly in direct 
 contradiction to the impression gained by a comprehensive 
 survey of the life-histories of animals. 
 
 If we use the weapon which the recapitulatory interpretation 
 of enbryology puts in our hands, and explore what the actual 
 course of evolution within the great phyla has been, we seem 
 to see that in each phylum Nature has striven after the elabo- 
 ration of some organ or group of organs which have been 
 dominating factors in the life of members of the phylum. To 
 take the most familiar instances : amongst carnivorous mammals 
 the teeth and claws are the all-important things, and so we 
 find in tigers the great eye-teeth enlarged into formidable 
 spears, whilst the back teeth are reduced to one or two pairs 
 of powerful scissors ; the claws are bent back when not in use 
 so that their points may not be worn down, and the animal 
 walks on the hairy second joints of its fingers and toes (the 
 well-known " furry " paw of the cat). Various kinds of cat- 
 like animals are classified by the degree of perfection attained 
 by teeth and claws. 
 
 Amongst hoofed animals or Ungulata the teeth, hoofs, and 
 
80 ZOOLOGY 
 
 horns are likewise the dominating features of development. The 
 teeth, with the cunning alternations of harder and softer material 
 on their elaborate grinding surfaces, are indispensable imple- 
 ments in grinding into impalpable powder the hard and gritty 
 food on which the Ungulata live ; the hoofs are arrangements to 
 permit of long-continued running, and the horns are the final 
 defence of the animal against attack. The perfection of these 
 three sets of organs varies from group to group, and hence they 
 are made the basis of classification. This list could be ex- 
 tended indefinitely. In bivalve mollusca the gill, clothed 
 with ciliated epithelium which wafts the food-bearing current 
 of water to the mouth, and the prolongation of the mantle 
 termed the siphon (see Fig. 20), which enables the animal, 
 though buried in mud, to maintain connection with the vital 
 air-containing sea water above, are the dominating organs, and 
 classification is based on them. Among insects the modified 
 limbs round the mouth termed "jaws," or gnathites by the 
 aid of which they obtain and masticate their food, and the 
 wings which bear them from place to place are the basis 
 of classification. 
 
 If the question be put, how can any but the most perfected 
 organisations survive until the present day] the answer is 
 twofold. First, competition and natural selection are not 
 equally severe in all areas. In small secluded areas less 
 perfect forms of life may survive for millions of years after their- 
 congeners have been eliminated everywhere else on the globe. 
 Thus in New Zealand the Tuatara, a primitive form of reptile, 
 still survives : its nearest allies swarmed all over Europe, Asia, 
 and Africa in the Triassic period. Again, all perfection in an 
 organ implies specialisation in function ; and the power oi 
 carrying out one function in perfection implies more or less the 
 giving up of the power of using the perfected organ for any 
 other purpose. Less perfection in structure may be compensated, 
 for by greater power of adaptability to various uses as the life 
 of the animal may demand. 
 
 The paw of the dog is a much less perfect instrument than' 
 the paw of the cat for striking prey, but the paw of the dog* 
 can be used for long-continued running, whereas the cat is 8 
 only capable of a few short dashes at a time. The wild coii-r 
 gener of the dog, the wolf, can run down and tire out his pre$v 
 
FUTURE DESTINY OF THE RACE 81 
 
 but the lion or tiger must surprise its prey and seize it with 
 a sudden spring ; such animals are incapable of catching it by 
 a straightforward pursuit. 
 
 The study, however, of the relations of animals to their 
 environment and to each other constitutes the lifelong fascina- 
 tion of the study of Zoology : it is indeed like a great book 
 of fairy-stories, wherein every new page contains fresh surprises 
 the probing into it affords a constant source of pleasure 
 which, as Sir Ray Lankester says, persists when all other 
 pleasures seem vain. 
 
 Before, however, we can enjoy this pleasure or appreciate 
 
 the marvels which Nature spreads before the discerning eye, 
 
 we must attain a fair knowledge of the main types of structure 
 
 met with in the animal kingdom, and this knowledge it is of 
 
 course utterly impossible to impart within the limits of a 
 
 book like this. It is not to be attained, and therein it 
 
 resembles all valuable things, without a certain amount of 
 
 drudgery ; we must begin by knowing before we can compare. 
 
 All scientific zoology must be based on the detailed study of 
 
 I types. To this study of types, each considered as an isolated 
 
 I problem, Huxley gave the name Biology, and on. this subject 
 
 ; he wrote his Elementary Biology, the first text-book in 
 
 { which the anatomy of certain types was carefully described. 
 
 I Many good text-books on the subject have been written since, 
 
 ind a list of the most suitable will be found in the Appendix. 
 
 CHAPTER VIII 
 
 THE BEARING OF ZOOLOGY ON THE QUESTIONS OF HUMAN 
 ORIGIN AND THE FUTURE DESTINY OF THE RACE. 
 
 IN the last chapter we were occupied in discussing the relation- 
 ships of animals to one another, and the light which has been 
 thrown on these relationships by the work of Darwin. Inter- 
 esting as it is to unravel the relationship of one group of 
 animals to another group, it is not half so interesting as to 
 determine the relationships and history of the human race. 
 TLe bearing of the doctrine of evolution, therefore, on the origin 
 
82 ZOOLOGY 
 
 and future of the human race has always been a topic of 
 absorbing interest. The most superficial examination will 
 convince us that our bodies are constructed on the same general 
 plan as those of all other Vertebrata, and that all the argu- 
 ments used in favour of the blood-relationship of various groups 
 of Vertebrata with one another, apply equally strongly in 
 favour of the kinship of the human race with the higher 
 Vertebrata. We are typical Mammalia with a covering of 
 hair protecting our skins, a four-chambered heart and an 
 apparatus of sweat glands to regulate our temperature, and the 
 division of the Mammalia to which we show most resemblance 
 and with which we should undoubtedly be classed by an 
 intelligent visitor from Mars is the order of the Primates or 
 Monkeys. In fact, at the present day it may be confidently 
 asserted that there is no intelligent naturalist who is not con- 
 vinced that the human race is descended from monkey-like 
 ancestors ; and the quibbling objection which is sometimes 
 raised, that our ancestors could not have been identical with 
 any race of monkeys existing now, is entirely beside the point. 
 Our ancestors may have so closely resembled a race of monkeys 
 still living that they would have been included in the same 
 species. Yet it would in nowise follow that human beings 
 should in that case be in process of evolution now ; for, as we 
 have seen, a new species arises out of an old one through 
 spreading into a new environment : now the environment into 
 which an incipient new race of men would spread is fully 
 occupied by the present human race, and there is thus no room 
 for the development of any other race of man-like creatures 
 from living monkeys. 
 
 When we come to examine where the environments of 
 monkeys and men differ, we find that much light is thrown 
 on the factors which led to the development of mankind. 
 Monkeys are tree-loving animals ; without exception the lives 
 of all species are passed in climbing from branch to branch, 
 and their descents to ground are quite exceptional, and 
 this applies equally to those monkeys like the Gorilla and 
 Chimpanzee, which most closely resemble man, as to those 
 lower races like the Spider-monkeys and Marmosets of 
 Brazil, which least resemble the human species. The human 
 race, in contrast with the monkeys, are Ground-Apes 
 
FUTURE DESTINY OF THE RACE 83 
 
 which have deserted the trees and taken to the open country 
 for their means of subsistence. It follows that once they 
 had successfully adapted themselves to new conditions they 
 were enabled to spread all over the globe, while their 
 backward cousins whom they left in the forests are doomed 
 to undergo diminution in numbers and final extinction as the 
 forests disappear. But there is no serious question that forests 
 have been gradually disappearing for millions of years, and 
 with the attainment of the weapons of civilisation by the 
 human race their disappearance has been hastened. The 
 reason for the prehistoric disappearance of forests has been the 
 evolution and spread of hoofed animals. The hoofed animals 
 gradually destroy forests by tearing out and eating the seedlings 
 of the trees, and every great stretch of natural open country is 
 related to the existence of one or more wide-ranging species of 
 hoofed animals. Thus, to take one example, the prairies and 
 the bison of North America were not disconnected phenomena : 
 the bison was the cause of the existence of the prairies. Now 
 the evolution of hoofed animals seems to have reached a climax 
 towards the end of the warm Miocene period, which was suc- 
 ceeded by the period of snow and icefields extending to the 
 middle of France which is known as the Glacial period. It is a 
 geological and not a zoological question as to what was the cause 
 of the mild period and of the succeeding icy one ; but when 
 we find evidence from the remains of plants in shale that 
 during the warm period the magnolia flowered in Greenland, 
 and when we find that later our own hills were covered with 
 snow and our valleys occupied with glaciers, we have no doubt 
 of the facts, and these are all that we require for, our purpose. 
 When, therefore, herds of wild oxen and deer roamed over 
 grassy plains occupying the site of our islands plains that 
 were then continuous with the great plain of Northern Europe 
 the forests must have become more and more restricted, and 
 somewhere in the northern continent it is impossible to say 
 where an enterprising race of monkeys began to seek its liveli- 
 hood on the ground, and so the human race was born. The 
 species of monkeys living at present are pre-eminently fruit- 
 eaters, but some of the more active, like the Gibbon, do hunt 
 down and catch the smaller mammals and birds, and thus enjoy 
 a mixed diet. The first traces of man's presence on earth con- 
 
84 ZOOLOGY 
 
 sist in simple tools made of sharpened stones and of charred 
 fragments of bones of the larger hoofed animals, which indubit- 
 ably prove him to have been a hunter. But how could a 
 climbing animal whose natural food was nuts ever become a 
 match for the powerful and fleet Ungulatal If we examine 
 the anatomical differences between ourselves and the highest 
 monkeys, we find that they consist mainly in two features : 
 (1) the feet and legs are converted into pillars capable of sup- 
 porting the body without aid from the arms, and (2) the brain 
 has become greatly enlarged. The first change converted a 
 climbing into a running animal ; hints of this change are seen 
 in the behaviour of the Gibbon to-day. This animal alone 
 among monkeys is able to run on the ground without aid from 
 its arms, and, as we have seen, it hunts down small animals, for 
 which purpose Gibbons associate together in herds. Now 
 here we have the essence of the evolution of man. To hunt 
 down large and powerful animals it was necessary that primi- 
 tive man should associate in bands and it was also necessary 
 that he should be able to run swiftly. As an isolated animal, man 
 was no match for his fierce and powerful competitors. His 
 strength was as nothing to that of the woolly elephant or the 
 wild ox, to say nothing of the cave-bear or cave-lion ; only when 
 acting together in a tribe was he able to maintain himself. 
 The evolution of man is therefore the evolution of society. 
 
 From the few scraps of human bones which have been 
 recovered from pre-glacial and mid-glacial days, we seem to 
 be warranted in concluding that the erect posture had been 
 attained and the consequent modification of the foot and leg 
 had been accomplished long before glacial times. Some of 
 the oldest skulls, however, show in their projecting brows and 
 flat brain-cases unmistakable resemblances to those of the 
 apes. One fragment named Pithecanthropus literally, " Ape- 
 man " found in Java, seems to indicate a size of brain exactly 
 intermediate between that of the highest ape and that of the 
 lowest men living to-day. The brain, like every other organ 
 of the body, grows in proportion to the demands made upon 
 it; in a word, with the evolution of society. The brain 
 seems to have attained its present size before the end of 
 the glacial period; at least the brains of the extinct race 
 called Neanderthal man have now been proved to have been 
 
FUTURE DESTINY OF THE RACE 85 
 
 as large as those of modern Europeans. The shape of the 
 brain-case of Neanderthal man, however, indicates that the 
 same parts of the brain were not so highly developed as in 
 modern man; it seems as if the part of the brain devoted 
 to motor activity, i.e. the part which gave out the impulses 
 to the powerful limb-muscles, was more developed than the 
 part concerned with reflective thinking. Now the purpose of 
 society is to perform common actions for the general good ; and 
 the whole of human history since the glacial epoch has 
 consisted in the development of this co-operative spirit. 
 Necessary actions in the lower animals are secured by ap- 
 propriate instincts : thus a cat when attacking prey strikes 
 it with the fore-paw, whereas a dog seizes it with his jaws. 
 The upholding of the co-operative spirit in man is in 
 similar manner maintained by his moral instincts i.e. 
 his feeling that he ought to do certain things regardless 
 of consequences. The main things which morality ensures 
 are that man shall uphold his tribal brother and that he 
 shall be loyal to his leader. This latter quality is just 
 as important as the former; a tribe without loyalty to its 
 leader, like the army imagined by a modern dramatist, 
 "Where the soldier would be man enough to strike his 
 officer in the face" when he gave an order, would have 
 been speedily removed by natural selection. 
 
 The struggle for existence, then, was not between individual 
 men as such, but between tribes, and this is where philosophers 
 like Nietsche who talk of the " superman " appear to us to go 
 so far astray. They picture the struggle for existence as a 
 struggle between individuals; they imagine mankind as pro- 
 gressing by the appearance of " supermen " of great size, 
 strength, intelligence, and ruthlessness, who will pursue their 
 own ends regardless of their weaker brethren. Such a view 
 is radically false. Mankind progresses by the appearance of 
 individuals in whom the instincts of co-operation and loyalty are 
 more strongly developed. Originally, no doubt, the tribes kept 
 each to their own hunting-grounds just as packs of wolves 
 do now, but as mankind wandered into new areas and got 
 broken up by different vicissitudes into races or incipient 
 species, then some of these races would multiply beyond the 
 resources of their food-supply and would invade the territory 
 
86 ZOOLOGY 
 
 of another tribe, and so inter- tribal warfare would begin. Just 
 such a history has been experienced on a small scale in the 
 island of Tasmania. This island when discovered by Euro- 
 peans was inhabited by a primitive race of men, who used 
 the same tools as did the inhabitants of Great Britain in 
 mid-glacial times. They seem to have lived on fairly good 
 terms with one another, and inter-tribal wars were rare. But 
 when the coast was occupied by Europeans for sheep-farming, 
 the natives who had lived there were displaced ; they invaded 
 the grounds of the tribes who lived in the interior, and a 
 period of internecine warfare ensued, with the result that in 
 fifty years the population was exterminated. There is evidence 
 to show that in mid-glacial times England and Europe were 
 inhabited by a race akin to the Bushmen ; these were dis- 
 placed by tribes with higher culture, and only a small perish- 
 ing remnant of this race survives to-day in South Africa. 
 Still later, when the ice age was passing away, our caves were 
 the retreat of a race akin to the Esquimaux. A few thousands 
 of this race survive in the farthest North ; they are fast dis- 
 appearing as they come in contact with higher races. They 
 are mercilessly slaughtered by Red Indians, and Europeans 
 unwittingly contribute to their destruction by the diseases 
 which they spread. 
 
 As the essence of morality consists in co-operation and 
 loyalty, so the essence of reason and intelligence is the adapta- 
 tion of new means to ends ; it is this which as frequently as 
 superior tribal virtue has given the victory to one tribe over 
 another. It is the opinion of some most competent authorities 
 that the victory of what are called the Neolithic peoples of 
 those who had the art of making axes of polished stone over 
 the peoples living in Europe at the close of the glacial age, 
 was due to the Neolithic discovery of the art of tying a stone 
 knife to a wooden handle and thus making an axe of it. It 
 was the mastery of the horse that enabled the Semitic popu- 
 lations of the East to overrun all Northern Africa and Southern 
 Europe, and to replace the Cross by the Crescent. On the 
 other hand, the Roman Empire was founded not on superiority 
 of knowledge or of inventive capacity on the part of the con- 
 querors, but on superior discipline that is, on tribal loyalty. 
 
 If, then, the essential feature in human history has been the 
 
FUTURE DESTINY OF THE RACE 87 
 
 victory of one tribe over another, and if this victory has been 
 mainly dependent on two factors tribal morality and inventive 
 capacity it becomes a vital question to us all to see how far 
 these necessary factors are embodied in our own tribe, and 
 what consequently is the outlook for its continued success. 
 
 We have seen that the two elements in tribal morality 
 are faithfulness to the tribesmen and loyalty to the leader. 
 But these virtues demand for their healthy development that 
 the tribesmen and leader should know and trust each other. 
 As all are aware, however, for the last eight thousand years 
 there has been a constant tendency to amalgamate tribes into 
 larger wholes, and so to form kingdoms and empires. The 
 mutual knowledge demanded for tribal loyalty tends to be diluted 
 in this process, and so the tie of affection is correspondingly 
 weakened. Many sijch combinations have resisted the dis- 
 integrating forces for a time and then gone down before the 
 assault of tribes in whom the tribal loyalty was strong and 
 vigorous. Thus did the Koman Empire fall before the attacks 
 of Northern tribes ; and the feudal system which succeeded it 
 is only tribalism under another name. 
 
 The British army is founded on the tribal spirit. It is 
 called the regimental spirit ; but that is only another name for 
 the same thing. The regiment primarily not the fatherland 
 is what the soldier lives and dies for. But the army forms 
 only a small proportion of the millions who make up the 
 amalgamation known as the British Empire, and the danger of 
 putting power in the hands of thousands who render no corre- 
 sponding service for the privilege is one which is constantly 
 overlooked by politicians but it is a mistake which will one 
 day have to be paid for. 
 
 The difficulty is to combine the intense loyalty shown by 
 the members of a regiment to one another with the enormous 
 extent of modern kingdoms. This difficulty was in former 
 days overcome by religion. The biological function of religion 
 has always been to uphold the sense of duty, without which 
 the tribe perishes. The original form of religious belief is the 
 continual spiritual presence of a dead leader who watches 
 over the tribe, to whom all must be loyal. Primitive 
 Christianity was a belief of this kind, and on it our modern 
 civilisation is built. In modern times, however, it is 
 
88 ZOOLOGY 
 
 doubtful how far beliefs of this kind can fulfil their original 
 function. Their driving force is continually being weakened 
 by scientific teaching, and unless we should acquire a leader 
 who should re-state in modern language the essential parts of 
 old teaching so as to bind the nation together in a new unity, 
 not much is to be hpoed for from that quarter. 
 
 If, then, our condition is far from satisfactory on the side of 
 loyalty, let us look at it from the point of view of adaptation 
 and invention. Are we superior to others in adaptation of 
 means to ends ? There is no question that we were superior. 
 But superiority is a relative term. A superiority based on 
 rule of thumb, on the following out of rules gained by hard 
 experience, is an excellent thing so long as no one finds out a 
 better way of accomplishing our tasks. But a superiority 
 based on the systematic encouragement of scientific research 
 and the immediate appropriation of its latest results is a far 
 better thing, and this is the method adopted by Germany and 
 Japan. This method, it seems to us, is bound to win in the 
 long run, just as surely as the Neolithic axe prevailed over the 
 ancient chipped flint-knife, or as the bronze weapon in turn 
 conquered the axe. 
 
 There are, however, still other considerations to be weighed. 
 A species of animals living under natural conditions is kept in 
 a state of health and vigour by natural selection. All weak 
 and diseased specimens are ruthlessly eliminated, and only the 
 strong and active reproduce their kind. Under modern con- 
 ditions of civilisation, the effort is made to preserve all, however 
 diseased and imperfect, alive, and to oppose no check to their 
 reproduction. The practical result of this is, that the most 
 reckless and criminal members of society reproduce early and 
 often, and impose the burden of the care of a worthless progeny 
 on the thrifty members of society, who in consequence limit 
 their own reproduction. From whatever point of view this 
 state of affairs is viewed it must cause apprehension. In the 
 so-called good old days, when plagues and wars periodically 
 wiped out large numbers of the population, only the most 
 vigorous survived, and the quality of the race was not im- 
 paired. If we go still further back to the palmy days of the 
 warlike tribes who founded the city-states of Greece and Rome, 
 we learn that sick and weakly infants were not allowed to live. 
 
FUTURE DESTINY OF THE RACE 89 
 
 In fact it may be broadly stated that in all primitive tribes 
 which occupy a limited area, care is taken not to have more 
 babies than are required. 
 
 It is not seriously suggested that infanticide should be 
 practised in a modern Christian state ; only that means should 
 be taken to render it difficult or impossible for the unfit to 
 breed. A beginning has been made by Eugenic societies in 
 stirring up public opinion on the subject ; but no way could be 
 devised in which public opinion could be more effectively 
 stirred up than by making a fair knowledge of zoology a 
 necessary element in modern education. 
 
 The lessons which the Eugenist seeks to enforce are written 
 in flame across every page of zoology : the wiping out of less 
 perfectly developed and less adaptive tribes by better equipped 
 ones is going on daily under our very eyes. If this sort of 
 mental pabulum were supplied to those who are likely to 
 become our public men and leaders instead of the exclusively 
 classical education on which the last generation has been reared, 
 the Eugenist would not preach to deaf ears, and there is at 
 least a chance that social and economic questions would be 
 fairly faced instead of being merely tinkered with. It is 
 easy to put from one's mind general talk about natural 
 selection if one has had a non-scientific education, for the term 
 probably does not convey a definite image to the mind, but to 
 the man with even a very moderate zoological training it 
 means something infinitely too important to be put on one side. 
 Let us hope that a time is coming when a knowledge of the 
 laws of life will be thought to be at least as important as 
 acquaintance with the rules of the grammar of dead languages. 
 
 It would be futile to close a book like this without some 
 reference to the objection that zoology deals with man, as if 
 he were body without soul. The answer to this objection is 
 twofold. First , zoology, as a natural science which deals with 
 phenomena perceptible by the senses, must deal with man as it 
 deals with other animals, as if they were bodies only. Whether 
 man's behaviour is entirely explicable as the outcome of the 
 chemical energy of his body is a question which lies entirely 
 outside the province of zoology. Secondly, all that zoology 
 asserts concerning man is not that he is merely body, but 
 that he and the animals are akin. It may be that in the 
 
90 ZOOLOGY 
 
 behaviour of the higher animals there are elements also which 
 are not explicable on any purely materialistic theory, but if 
 this be so the best way to define them will be to push 
 materialistic explanation as far as ever it will go, so that its 
 limits will be clearly seen. Whether the non-material element 
 in man can survive the body or not is a question of enormous 
 interest, but one to which, naturally, zoology can give no 
 answer ; but if ever a satisfactoiy answer to it be found, it will 
 only be as the result of the same kind of slow, patient, perse- 
 vering research as that by which the natural sciences have 
 been built up such research as, amid much discouragement, 
 is being undertaken by Sir Oliver Lodge and his friends. In 
 the meantime, however, the doctrine of evolution with its con- 
 sequence, the kinship of man and animals, can throw welcome 
 light on some old moral problems. One of these is the nature 
 and origin of evil. Evil in man may be briefly defined as 
 want of self-control and want of subordination to law, i.e. 
 to tribal morality. Now the organ of self-control is the brain, 
 especially the frontal portion of it. This part has been slowly 
 developed through thousands of years by natural selection. 
 Savage tribes who are the last remnants of primitive types 
 of man have less self-control than civilised man. Tribal 
 morality or subordination to law is also a quality which has 
 been slowly and painfully evolved. Now in other types of 
 animals in which an important organ has been the dominating 
 factor in evolution, we find that races are sometimes de- 
 veloped in which this organ is degenerate. At every step 
 of the upward progress some step back. Thus we find 
 wingless insects, toothless quadrupeds, &c. Evil is therefore 
 the reversion to a previous state of evolution, and the so-called 
 original wickedness in children is a repetition of an ancestral 
 stage of the race as truly a larval stage as the tadpole stage 
 of the frog. 
 
 Huxley has compared Life to a game of chess in which all 
 must take part, but in which the Player of the other side is hidden 
 from us. This Player, he says, never forgives offences, but 
 exacts the full penalty for each. The final and deepest object 
 of zoological science is to learn some, at least, of the rules of 
 the game. 
 
APPENDIX 
 
 LIST OF BOOKS RECOMMENDED FOR THE FURTHER STUDY 
 OF ZOOLOGY TO THE READERS OF THIS BOOK 
 
 The works of Darwin are alluded to in the text. This list is 
 arranged in order of complexity, beginning with the most 
 elementary text-books. 
 
 Lesson in Elementary Biology. T. Jeffrey Parker. 
 
 A Junior Course of Practical Zoology. A. Milnes Marshall and 
 C. Herbert Hurst. 
 
 The Frog: An Introduction to Anatomy, Histology, and Embry- 
 ology. A. Milnes Marshall. 
 
 An Introduction to the Study of the Comparative Anatomy of 
 Animals. G. C. Bourne. 
 
 Text-Book of Zoology. J. Arthur Thomson. 
 
 Text-Book of Zoology. A. E. Shipley and E. W. MacBride. 
 
 Vertebrate Embryology. A. Milnes Marshall. 
 
 Students 1 Text-Book of Zoology. A. Sedgwick. 
 
 Text-Book of Comparative ' Embryology. Vol. I, " Invertebrata 
 and Protochordata" (in the press). E. W. MacBride. 
 
 The Cambridge Natural History, Vols. I-X. 
 
 91 
 
INDEX 
 
 Adaptation, 79 
 
 Aixmiota, 60 
 
 Amoeba, 15 
 
 Annelida, 41 
 
 Arachnida, 46, 49 
 
 Arthropoda, 44 
 
 Ascidians, 58 
 
 Biochemistry, 20 
 
 Biogenesis, 75 
 
 Biology, 5, 6 
 
 Birds, 58, 60 
 
 Bone, 32 
 
 Brachiopoda, 53 
 
 Cell, the, 18 
 
 Cells, various kinds of, 22 
 
 Cephalopoda, 51, 52 
 
 Chromatin, 17 
 
 Classification of animals, 33-62 
 
 Cnidoblasts, 36 
 
 Ccelenterata, 34, 35 
 
 Coral, 36 
 
 Crustacea, 45, 47 
 
 Cytoplasm, 18 
 
 Darwinism, 64 et sea. 
 
 Developmental mechanics, 76 
 
 Digestion, 18 
 
 Ecdysis, 39, 44 
 
 Echinodermata, 54 
 
 Elementary species, 72 
 
 Embryo, the, 31 
 
 Embryology, comparative and experi- 
 mental, 73, 76 
 
 Excretion, 20 
 
 Fibres, muscular, 14 
 
 Fibrils, 25 
 
 Fishes, 58 
 
 Flat worms, 36 
 
 Frog, life-history of, 73, 74 
 
 Gametes, 30 
 
 Gastropoda, 51 
 
 Growth, 6, 7 
 
 Guinea worm, 40 
 
 Hennaphroditefl, 31 
 
 Hybrids, 62 
 
 Insects, 46, 48 
 
 Life, defined, 6 
 origin of, 63 
 
 Liver fluke, 38 
 
 Mammals, 58, 60, 61 
 
 Man's place in the unirerse, 81 et 
 seq. 
 
 Mendelisra, 69 et seq. 
 
 Metabolism, 19 
 Metamorphosis, 78 
 Metazoa, 18 
 Mollusca, 49 
 Movement, 14 et seq. 
 Muscle fibres, 14, 25 
 Muscular fibrils, 25 
 
 tissue, 32 
 Mutations, 72 
 Natural selection, 66 
 Nematocysts, 36 
 Nemertinea, 38 
 Nerve cells, 27 
 
 tissue, 32 
 Nucleus, 17, 18 
 
 Origin of species, 62 et seq. 
 
 Pelecypoda, 51 
 
 Phyla of animals, 33 
 
 Plants, defined, 7 
 
 Platyhelminthes, 36 
 
 Polyps, 34 
 
 Polyzoa, 54 
 
 Porifera, 34 
 
 Primates, 82 
 
 Proteids, as animal food, 8, 10, 21 
 
 Protoplasm, 17 
 
 Protozoa, 18 
 
 Recapitulation theory, 75 et seq. 
 
 Reproduction, 7 
 
 Reproductive or germ cells, 7, 30, 79 
 
 Reptiles, 58, 60, 61 
 
 Respiration, 19 
 
 Sea-squirts, 58 
 
 Secretion, 17 
 
 Sex, 31 
 
 Shellfish, 49 
 
 Spermatozoa, 30, 31 
 
 Sponges, 34 
 
 Stinging cells, 36 
 
 Tape-worms, 38 
 
 Thread-worms, 39 
 
 Tissue, kinds of, 31-33 
 
 Trichinosis, 41 
 
 Variation, 68 
 
 Vertebrates, 57 
 
 Worms. Se* Flat- worms, Nemertiuea, 
 
 Thread-worms, Annelida 
 Zoology, bearing on human origin and 
 
 future of race, 81 
 
 practical importance of, 8 
 
 scope of, 6 
 Zygote, a, 31 
 
 I/I3 
 
 Printed by BALLAXTYNK, HANSON <5r Co. 
 Edinburgh 6* London 
 
THE PEOPLE'S BOOKS 
 
 THE FIRST HUNDRED VOLUMES 
 
 The volumes issued (Spring 1913) are marked with an asterisk 
 
 SCIENCE 
 
 *i. The Foundations of Science . . By W, C. D. Whetham, F.R.S. 
 
 *2. Embryology The Beginnings of Life By Prof. Gerald Leighton, M.D. 
 3. Biology The Science of Life . . By Prof. W. D. Henderson. M.A. 
 
 1 4 . Zoology: The Study of Animal Life By Prof. E. W. MacBride, F.R.S. 
 
 5. Botany; The Modern Study of Plants By M. C. Slopes, D.Sc., Ph.D. 
 
 6. Bacteriology By W. E. Carnegie Dickson, M.D. 
 
 7. The Structure of the Earth . . By the Rev. T. G. Bonney, F.R.S. 
 
 *8. Evolution By E. S. Goodrich, M.A. , F.R.S. 
 
 9. Darwin By Prof. W. Garstang, M.A., D.Sc. 
 
 * 10. Heredity By J. A. S. Watson, B.Sc. 
 
 * 1 1. Inorganic Chemistry .... By Prof. E. C. C. Baly, F.R.S. 
 ' '2. Organic Chemistry .... By Prof. J. B. Cohen, B.Sc., B\R.S. 
 13. The Principles of Electricity . . By Norman R. Campbell, M.A. 
 J 14. Radiation . By P. Phillips, D.Sc. 
 
 5. The Science of the Stars . . . By E. W. Maunder, F.R.A.S. 
 
 16. Light, according to Modern Science By P. Phillips, D.Sc. 
 *i 7. Weather-Science By R. G. K. Lempfert, M.A. 
 
 18. Hypnotism By Alice Hutchison, M.D. 
 
 *i9. The Baby: A Mother's Book . . By a University Woman. 
 
 20. Youth and Sex Dangers and Safe-/ By Mary Scharlieb,M.D., M.S., and 
 guards for Boys and Girls . . \ G. E. C. Pritchard, M.A., M.D. 
 *2t. Motherhood-A Wife's Handbook . By H. S. Davidson, F.R.C.S.E. 
 
 *22. Lord Kelvin By A. Russell, M. A., D.Sc. 
 
 *23. Huxley . Hy Professor G. Leighton, M.D. 
 
 24.Sir W. Huggins and Spectroscopic/By E.W. Maunder, F.R. A. S., of the 
 
 Astronomy \ Royal Observatory, Greenwich. 
 
 *62. Practical Astronomy . . . . By H. Macpherson, Jr., F.R.A.S. 
 
 *6 3 . Aviation (*X *$ffk F ' Walker R ' N " 
 
 * 6 4 . Navigation By W/Hail, R.N., B.A. 
 
 *6s. Pond Life By E. C. Ash, M.R.A.C. 
 
 *66. Dietetics By Alex. Bryce, M.D., D.P.H. 
 
 *94 The Nature of Mathematics . . By P. G. B. Jourdain, M.A. 
 
 95. Applications of Electricity ... By Alex. Ogilvie, B.Sc. 
 
 06. The Small Garden . . . . By A. Cecil Bartlett. 
 
 97. The Care of the Teeth . . . By J. A. Young, L.D.S. 
 
 * 9 8. Atlas of the World .... By J. Bartholomew, F.R. G.S. 
 
 PHILOSOPHY AND RELIGION 
 
 25. The Meaning of Philosophy . . By T. Loveday, M.A. 
 
 26. Henri Bergson By H. Wiidon Carr. 
 
 * 27. Psychology By H. J. Watt, M.A., Ph.D. 
 
 28. Ethics ByCanon Rashdall, D.Litt.,F.B.A. 
 
 29. Kant's Philosophy . . . . By A. D. Lindsay, M.A. 
 
 30. The Teaching of Plato . . . By A. D. Lindsay, M.A. 
 
 *6 7 . Aristotle By Prof. A. E. Taylor, M. A., F. B.A. 
 
 *68, Nietzsche By M. A. Mugge, Ph.D. 
 
 69. Eucken By A. J. Jones, M. A., B.Sc., Ph. D. 
 
 70. The B Experimental Psychology of j By c w . Valentine , B . A . 
 
 71. The Problem of Truth . . . By H. Wiidon Carr. 
 
 "' Ge of e ide B a e iis k m ley I *? Philosophy } By G. Dawes Hicks, Litt.D. 
 
 31. Buddhism ...... By Prof. T. W. Rhys Davids, F. B.A. 
 
 *32. Roman Catholicism . . . . By H. B. Coxon. 
 
 33. The Oxford Movement ... By Wilfrid P. Ward. 
 *34- The Bible in the Light of the Higher/ By Rev. W. F. Adeney, M.A., and 
 Criticism . . ( Rev. Prof. W. H. Bennett, Litt.D. 
 
 35- Cardinal Newman By Wilfrid Meynell. 
 
 *7 2 - The Church of England . . By Rev. Canon Masterman. 
 
 73. Anglo-Catholicism . . . . Bv A. E. Manning Foster. 
 
PHILOSOPHY AND RELIGION (continued) 
 *74- The Free Churches .... By Rev. Edward Shillito, M.A. 
 
 75- Judaism By Ephraim Levine, B.A. 
 
 *76. Theosophy By Annie Besant. 
 
 HISTORY 
 
 *36. The Growth of Freedom . . . By H. W. Ncvinson. 
 
 37- gisraarck . By Prof. F. M. Powicke, M.A. 
 
 * 38. Oliver Cromwell By Hilda Johnstone, M.A. 
 
 *39. Mary Queen of Scots . . . . By E. O'Neill, M.A. 
 
 '40. Cecil Rhodes By Ian Colvin. 
 
 *4i. Julius Caesar By Hilary Hardinge. 
 
 History of England 
 
 42. England in the Making . . . By Prof. F. J.C.Hearnshaw.LL.I 
 
 *43. England in the Middle Ages . . By E. O'Neill, M.A. 
 
 44. The Monarchy and the People . . By W. T. Waugh, M.A. 
 
 45. The Industrial Revolution . . . By A. Jones, M.A. 
 
 46. Empire and Democracy . . . By G. S. Veitch, M.A. 
 
 *6r. Home Rule By L. G. Redmond Howard. 
 
 77. Nelson By H. W. Wilson. 
 
 * 7 8. Wellington and Waterloo ... By Major G. W. Red way. 
 
 100. A History of Greece . . . . By E. Fearenside, B.A. 
 
 101. Luther and the Reformation . . By L. D. Agate, M.A. 
 
 102. The Discovery of the New World . By F. B. Kirkman, B.A 
 "103. Turkey and the Eastern Question . By John Macdonald. 
 
 104. A History of Architecture . .By Mrs. Arthur Bell. 
 
 SOCIAL AND ECONOMIC 
 
 * 47 . Women's Suffrage . . . . By M. G. Fawcett, LL.D. 
 
 48. The Working of the British System) R p rt r p, v f A 
 
 of Government to-day . . . J By Prof ' Ramsa y Muir > * 
 
 49. An Introduction to Economic Science By Prof. H. O. Meredith, M.A. 
 
 50. Socialism By F. B. Kirkman, B.A. 
 
 79. Socialist Theories in the Middle Ages By Rev. B. Jarrett, O.?., M.A. 
 
 *8o. Syndicalism By J. H. Harley, M.A. 
 
 81. Labour and Wages . . . . By H. M. Hallsworth, M.A., B.S 
 
 * 82. Co-operation By Joseph Clayton. 
 
 *8s. Insurance as Investment . . . By W. A. Robertson, F.F. A. 
 
 *92. The Training of the Child . . . By G. Spiller. 
 
 105. Trade Unions By Joseph Clayton. 
 
 *io6. Everyday Law By J. J. Adams. 
 
 LETTERS 
 
 * 5 i. Shakespeare By Prof. C. H. Herford ^itt.D. 
 
 '52. Wordsworth By Rosaline Masson. 
 
 *53. Pure Gold A Choice of Lyrics and \ g y jj c O'Neill 
 
 *54. Francis Bacon By Prof. A. R. Skemp, M.A. 
 
 *55. The Brontes By Flora Masson. 
 
 *S6. Carlyle By the Rev. L. MacLean Watt. 
 
 *57. Dante By A. G. Ferrers Howell. 
 
 58. Ruskin By A. Blyth Webster, M.A. 
 
 59. Common Faults in Writing English By Prof. A. R. Skemp, M.A. 
 *6o. A Dictionary of Synonyms ... By Austin K. Gray, B.A. 
 
 84. Classical Dictionary . . . . By A. E. Stirling. 
 
 *8s. History of English Literature . . By A. Comr ton-Rickett. 
 
 86. Browning. By Prof. A." R. Skemp, M.A. 
 
 87. Charles Lamb By Flora Masson. 
 
 83. Goethe By Prof. C. H. Herford, Litt.D. 
 
 89. Balzac By Frank Harris. 
 
 90. Rousseau By H. Sacber. 
 
 91. Ibsen By Hilary Hardinge. 
 
 *93. Tennyson By Aaron Watson. 
 
 107. R. L. Stevenson By Rosaline Masson. 
 
 108. Shelley By Sydney Waterlow. 
 
 109. William Morris By A. Bfcrth WqfetM-, NjTA. 
 
 LONDON AND EDINBURGH : T. C. & E. C. JACK 
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